Effects of Transport and Additives on Electroless Copper Plating

EFFECTS OF TRANSPORT AND ADDITIVES ON

ELECTROLESS COPPER PLATING

RONALD ZESZUT

Dissertation Advisor: Prof. Uziel Landau

DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING

CASE WESTERN RESERVE UNIVERSITY

August, 2017 Case Western Reserve University School of Graduate Studies

We hereby approve the dissertation of Ronald Zeszut candidate for the degree of Doctor of Philosophy

Committee Chair Prof. Uziel Landau

Committee Member Prof. Rohan Akolkar

Committee Member Prof. Robert Savinell

Committee Member Prof. Daniel Scherson

Date of Defense April 27, 2017

*We also certify that written approval has been obtained for any proprietary material contained therein.

Table of Contents

List of Tables………………………………………………………………………………………………………… 5

List of Figures………………………………………………………………………………………………………. 6

Acknowledgements………………………………………………………………………………..……………. 14

List of Symbols……………………………………………………………………………………………………… 15

Abstract………………………………………………………………………………………………..……………… 18

Chapter 1: Introduction……………………………………………………………………………………….. 21

1.1 Overview and Rationale………………………………………………………..……………. 21

1.2 Fabrication of Copper Interconnects for Semiconductor Devices..……… 22

1.3 Electrodeposition of Copper for Feature Fill…………………………..…………… 23

1.4 Electroless Plating………………………………………………………………..…………….. 26

1.5 Screening of Additives for Electroless Feature Fill………………..…………….. 28

1.6 Objectives……………………………………………………………..…………………………….31

1.7 Structure of Thesis……………………………………………………………………..………. 31

Chapter 2: Electroless Plating Model Accounting for Transport and

Concentration Effects……………………………………………………..……………………….. 39

2.1 Experimental Methods…………………………………..…………………………………… 40

2.2 Half-Cell Mixed Potential Analysis…………………..………………………………….. 46

2.3 Complete Electroless System Polarization Curves and

Mixed Potential Analysis…………………………..………………………………………… 50

2.4 Effect of Substrate………………………………………..……………………………………. 53

2.5 Electroless Plating Rate Model………………………………..…………………………. 56

2.6 Hydroxide Surface Concentration……………………………………………………….. 68

2.7 Effect of Transport on the Electroless Plating Rate……………..………………. 71

2.8 Conclusions………………………………………………………………………………………… 76

Chapter 3: Rapid Screening Technique for Additives Providing Bottom-up

Electroless Plating…………………………………………………………………………………….. 80

3.1 Experimental Methods……………………………………………………………………….. 84

3.2 Additives Adsorption and Diffusion in a Feature…………………………………. 86

3.3 Effect of Additive Concentration on Electroless Plating Rate………..…….. 89

3.4 Coupon Plating and Feature Fill Imaging………………………………..…………… 94

3.5 Conclusions……………………………………………………………………………….……….. 96

Chapter 4: Additive Transport, Adsorption, and Inclusion……………………..…………….. 100

4.1 Experimental Methods………………………………………………………..…………….. 102

4.2 Additive Diffusion, Adsorption, and Inclusion……………………………………… 102

4.3 Estimation of the MPS Inclusion Rate Constant………………………..………… 108

4.4 Estimation of the MPS Adsorption Rate Constant…………………..………….. 114

4.5 Polynomial Approximation for Electroless Plating Rate Model…..………. 116

4.6 Effect of Rate Constant Variability…………………………………………….……….. 118

4.7 Model for Electroless Plating Incorporating Additive Effects..…………….. 121

4.8 Three Additive System Analysis……………………………………………………………123

4.9 Conclusions……………………………………………………………………..…………………. 125

Chapter 5: Conclusions and Future Directions………………………..……………………………. 127

Appendix A: Applicability of the Levich Equation to RDE at Low Rotation Speeds… 130

List of Tables

Table 2.1. Baseline Electroless Bath Composition (p. 43)

Table 2.2 Stripping Voltammetry Measurements (p. 45)

Table 2.3. Substrate Pre-plating Effects on Electroless Deposition Rates (p. 54)

Table 2.4. Physical Constants Used in the Levich Equation (p. 60)

Table 2.5. Electrochemical parameters derived from fitting parameters for copper reduction and glyoxylic acid oxidation reactions occurring in the full electroless system

(p. 64)

Table 3.1. List of additives studied (p. 85)

Table 3.2: Plating rate at high and low rotation speeds with SPS and MPS (p. 90)

Table 3.3 Plating rates at high and low rotation speeds for various additive combinations

(p. 92)

Table 3.4 Physical constants for MPS and PPG (p. 93)

Table 3.5 Required rotation rate for feature bottom simulation (p. 93)

Table 4.1 Parameters for polynomial approximation of electroless plating rate (p. 117)

List of Figures

Figure 1.1. Feature fabrication and fill process diagram. (a) Dielectric and etch stop deposition. (b) Trench formation. (c) Barrier and seed layer deposition. (d) Feature fill with plated copper. (p. 23)

Figure 1.2. Schematic of feature fill without additives resulting in a void. Current distribution limitations lead to faster copper plating at the feature top and upper surfaces. As plating continues, the feature top seals with a void still remaining lower in the feature, where plating can no longer occur. (p. 23)

Figure 1.3. Comparison of different sized features. In the large feature (left) the thickness of the PVD copper seed layer (deposits thicker at feature top than bottom) does not significantly interfere with ability to achieve void-free fill. In the small feature

(right) the copper seed layer significantly fills the feature and makes void-free fill difficult. (p. 24)

Figure 1.4. Additive molecule structures. (a) Polyethylene glycol (PEG) (b) Bis-(sodium sulfopropyl)-disulfide (SPS). (p. 25)

Figure 1.5. Additive surface coverage for void-free feature fill. Suppressor molecules adsorb preferentially to the feature top and upper sidewalls. Anti-suppressor molecules adsorb preferentially at the feature bottom. Plating proceeds slowly at the suppressor covered areas, and quickly at the anti-suppressor covered areas. This results in bottom- up fill with no voids. (p. 26)

Figure 2.1. Experimental apparatus diagram. Three electrode cell with a rotating disk working electrode, Cu/CuSO4 reference electrode, and copper foil counter electrode in a jacketed beaker. (p. 41)

Figure 2.2. Electrode tilt angle diagram. The system was tilted approximately 5°. (p. 42)

Figure 2.3. A diagram of the stripping voltammetry procedure. Cu is pre-electroplated onto Pt substrate, measuring the charge passed. Electroless copper is then deposited in the course of the experiments onto the preplated Cu layer. Eventually, all Cu is stripped off, down to bare Pt, while measuring charge passed. Charge difference between the preplating and the stripping steps corresponds to copper deposited in the electroless process. (p. 45)

‘glyoxylic acid only’ half systems. The predicted mixed potential, corresponding to potential where the anodic and cathodic current densities match is at – 0.54 V. The matching predicted current densities are 2 mA/cm2. By contrast, the observed corresponding values for the complete electroless system are quite different: Mixed potential of -0. 44 V and a current density of 5 mA/cm2. (p. 48)

7 densities shift in the opposite directions to the observed corresponding values for the complete electroless system: Mixed potential of -0. 71 V and a current density of 5 mA/cm2. Test parameters are identical to those indicated for Fig. 2.4. (p. 49)

Figure 2.6. External, glyoxylic acid, and copper partial current densities in the full electroless chemistry as a function of the external applied potential. Data taken on a

RDE rotated at 400 rpm. (p. 51)

Figure 2.7. Polarization curves gathered from the full bath and half bath systems at 400 rpm. (p. 52)

Figure 2.8. Electroless deposition thickness at 400 rpm on Pt substrate pre-plated with

Cu from alkaline or acidic chemistries and on Ru-coated wafer substrate. (p. 55)

Figure 2.9. Sample data used for establishing the parameters in the electroless process model. (a) Equivalent copper current density determined from the amount of plated copper and (b) OCV were measured as a function of bulk copper concentration. All data shown with 0.19 M bulk glyoxylic acid concentration, at pH = 12.8 and rotation rate of

400 rpm. (p. 62)

Figure 2.10. (a) Equivalent current density as determined from copper plated in the electroless process, and (b) OCV measurements as a function of copper concentration.

Points show experimental data and the curves correspond to the model (Eqs. 2.22 and

2.23). Process parameters: 0.19 M glyoxylic acid, pH =12.8. RDE rotated at 400 rpm. (p.

65)

Figure 2.11. (a) Equivalent current density as determined from plated copper in the electroless process, and (b) OCV measurements as a function of the glyoxylic acid

8 concentration. Dots indicate the measured data and the curves correspond to the model

(Eqs. 2.22 and 2.23). The process parameters: 0.036 M copper sulfate; pH=12.8; RDE rotated at 400 rpm. (p. 66)

Figure 2.12. (a) Equivalent current density as determined from copper plated in the electroless process, and (b) OCV measurements as a function of the pH. The dots indicate the measured data and the curves correspond to the model (Eqs. 2.22 and

2.23). Process parameters: 0.036 M copper sulfate; 0.19 M glyoxylic acid; RDE rotated at

400 rpm. (p. 67)

Figure 2.13 Transport dependence of the glyoxylic acid oxidation, confirming the presence of hydroxide concentration gradient. Potentiostatic measurement at 0 V vs.

SHE on a Pt rotating disk electrode with saturated calomel reference electrode. The electrolyte consisted of excess (0.19 M) glyoxylic acid (no copper) at a pH of 12 to assure that the hydroxide is the limiting reactant. Initial rotation rate of 400 rpm, decreased to

100 rpm as indicated. 25° tilt angle to remove bubbles. (p. 70)

Figure 2.14. Dependence of the electroless plating reaction rate (in the absence of additives) on transport rates with experimental measurements and the model prediction. Reactants bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8. RDE rotation rate varied from 16 to 400 rpm. (p. 72)

Figure 2.15. Electrode photographs taken in-situ during electroless plating indicating significant bubble coverage at the lower rotation rates. (a) 16 rpm, (b) 100 rpm, and (c)

400 rpm. Electrode was tilted 5° to the horizontal in all experiments. (p. 73)

Figure 2.16. Effect of the rotating disk electrode tilt angel on the electroless plating rate.

Equivalent current density determined from the amount of copper plated in the electroless process at baseline bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8. RDE rotation rate varied from 16 to 400 rpm. Tilt angle varied from 0 to 25°. (p. 74)

Figure 2.17. Electrode photographs taken in-situ during electroless plating indicating significant bubble coverage at the lower tilt angle. (a) 5° and (b) 25° degree. 16 rpm rotation rate was maintained for both experiments. (p. 75)

Figure 2.18. Dependence of the electroless plating reaction rate (in the absence of additives) on transport rates at an electrode tilted 25° to minimize bubble coverage.

Shown are experimental measurements and the model prediction. Reactants bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8. (p. 75)

Figure 3.1. Schematic representation of equivalent boundary layer thicknesses for a cylindrical via. The boundary layer at the top flat surface (flat) is given by the conventional Levich equation. The equivalent boundary layer thickness to the bottom of the via (via) accounts for the external boundary layer, flat, and for the equivalent boundary layer for the diffusion transport to the bottom of the via, eq. The latter accounts for the via depth, L, and for additive adsorption to the sidewalls (p. 88)

Figure 3.2. Effect of PEG 4000 and SPS additives at various concentrations on electroless plating rate. Electroless plating was conducted on a RDE rotated at 100 rpm for 1 hr. The data corresponds to: PEG 4000, SPS, PEG at various concentrations with 0.5 ppm SPS, and PEG at various concentrations with 0.25 ppm SPS. (p. 89)

Figure 3.3. Electroless plating for 5 min at 16 rpm on ruthenium substrate. (a) 0.5 ppm

MPS + PPG 725, (b) 40 ppm dipyridyl, (c) no additives. (p. 86)

Figure 3.4. Electroless copper plating for 5 minutes with 0.25 ppm MPS, 0.25 ppm PPG

725, and 40 ppm dipyridyl. (p. 91)

Figure 3.5. Patterned wafer segment before (a) and after (b) electroless copper deposition. Plating is observed only in areas with trenches (48, 100 nm wide, 150 nm deep) with essentially no plating on the flat, non-patterned surface. The wafer segment, mounted on a shaft rotating at 400 rpm, was plated for 30 s in electroless chemistry with 0.5 ppm MPS + 0.5 ppm PPG 725. The seed layer consisted of Cu PVD. (p.95)

Figure 3.6. SEM image of copper deposited in features 48 nm wide and 150 nm deep. Cu

PVD seed layer on wafer, rotated at 400 rpm for 5 min in full chemistry electroless bath with 0.5 ppm MPS + 0.5 ppm PPG 725 (p. 96)

Figure 4.1. Plated thickness of electroless copper plated in the presence of 0.5 ppm MPS and 0.5 ppm PPG at baseline bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8. RDE rotation rate =16 rpm. (p. 104)

Figure 4.2. Equivalent current density determined from electroless copper plated in the presence of 1.5 ppm MPS at baseline bulk concentrations: 0.036 M copper sulfate, 0.19

M glyoxylic acid. pH = 12.8. RDE rotation rate varied from 16 to 400 rpm. Plating was carried for 30 min. (p. 109)

Figure 4.3. External current density in the full electroless chemistry at -0.56 V vs. SHE

(applied externally). Data taken on a RDE rotated at 50 rpm. 3 ppm MPS was injected at

60 s. (p. 112)

Figure 4.4. Equivalent current density model dependence on additive fractional surface coverage of MPS. Full model (Eq. 4.24) shown as solid lines, polynomial approximations

(Eq. 4.45) shown as points. Modeled at baseline bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8. RDE rotation rate at 16 and 400 rpm. (p. 118)

Figure 4.5. MPS adsorption and inclusion fluxes as a function of fractional additive surface coverage. Rate constants were determined from electroless measurements. Two limiting cases for additive adsorption are indicated: diffusion limit and surface adsorption limit. Baseline bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8. 16 rpm RDE rotation rate. (p. 119)

Figure 4.6. MPS adsorption and inclusion fluxes as a function of fractional additive surface coverage. Applied rate constants were determined from electroless and electroplating measurements. Baseline bulk concentrations: 0.036 M copper sulfate,

0.19 M glyoxylic acid. pH = 12.8. RDE rotated at 16 rpm. (p. 120)

Figure 4.7. Electroless plating rate expressed in terms of equivalent current density as a function of RDE rotation rate. The dots represent experimental plating rate determined from copper plated in the electroless process at baseline bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid, and 1.5 ppm MPS. pH = 12.8. RDE rotation rate varied from 16 to 400 rpm. The curves show the model predictions with rate constants derived from electroless and electroplating measurements. (p. 121)

12 sulfate, 0.19 M glyoxylic acid, and 1.5 ppm MPS. pH = 12.8. RDE rotation rate varied from 16 to 400 rpm. The curve shows the model predictions with averaged rate constants based on data from electroless and electroplating measurements. (p. 122)

Figure 4.9. Electroless plating rate expressed in terms of equivalent current density as a function of RDE rotation rate. The dots represent plating rate determined from copper plated in the electroless process at baseline bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid, and 1.5 ppm MPS, 40 ppm dipyridyl, and 0.25 ppm PPG. pH = 12.8. RDE rotation rate varied from 16 to 400 rpm. The curve shows the model predictions with averaged rate constants based on data from electroless and electroplating measurements. (p. 125)

Figure A.1. Measured limiting current density for an acidic copper solution (0.5 M

CuSO4, 0.1 M H2SO4) for rotation speeds from 400 rpm to 16 rpm (shown as square root of rotation speed). (p. 131)

Acknowledgements

There are so many people whom I would like to thank, as my dissertation was only possible through the help of many. First, I would like to thank my advisor,

Professor Uziel Landau for the opportunity to continue my education and work in his lab, as well as for his guidance, patience, dedication, and deep knowledge that he shared with me. Thank you for your incredible efforts to put me on the path to success.

I would like to thank Atotech for funding my research. Additionally, I am appreciative of the many insightful conversations with the Atotech community both locally, as well as from Albany and Berlin.

I would also like to acknowledge the support of the department of Chemical and

Biomolecular Engineering at Case Western Reserve University including the faculty, staff, and fellow graduate students. Thank you for providing a welcoming and encouraging atmosphere for the past four years. In particular, thank you to the other members of my research group for support and helpful discussions. A special thanks as well to my defense committee and the guidance they provided.

Both before and throughout graduate school, the support of my family has been instrumental. Thank you to my parents, Ronald and June, for your constant support, encouragement, and work you have done to provide me with the educational opportunities that I have been given. Thanks as well to my siblings, Jill, Anthony, and

Mary.

Last and certainly not least, I would like to thank God for the opportunities and talents He has given me. AMGD

List of Symbols

2 AGA Arrhenius rate constant for glyoxylic acid oxidation, mol/(cm s) C Concentration, M D Diffusion coefficient, cm2/s E° Standard Potential, V f Temperature dependent factor, F/RT, V-1 F Faraday’s Constant, 96,485 C/eq G Equivalent current density fitting parameter, mA/cm2 I Current, mA i Current density, mA/cm2

2 i0 Exchange current density, mA/cm

2 ik Kinetic current density, mA/cm

2 iL Limiting current density, mA/cm

3 kA Adsorption rate constant, cm /(mol s)

-1 k-A Desorption rate constant, s

2 kf,GA Glyoxylic acid oxidation rate constant, mol/(cm s)

2 kI Inclusion rate constant, cm /(A s)

Kw Equilibrium constant of water L Feature depth, cm M Operating potential equation fitting parameter for reactants, V n Number of electrons involved in a reaction, eq N Molecular flux, mol/(cm2 s) P Operating potential equation fitting parameter, V Q Dipyridyl suppression factor R Gas constant, 8.314 J/(mol K) or feature width, cm t Time, s T Temperature, K

v Reaction rate, mol/(cm2 s) V Voltage, V

V0 Operating potential, V X Fitting parameter for polynomial current density approximation, mA/cm2 Y Fitting parameter for polynomial current density approximation, mA/cm2 Z Fitting parameter for polynomial current density approximation, mA/cm2

Greek

 Charge transfer coefficient, eq/mol

 Adjusted charge transfer coefficient

 Concentration dependence exponent

 Surface concentration, mol/cm2

 Diffusion boundary layer thickness, cm

 Fractional surface coverage

 Kinematic viscosity, cm2/s

 Rotation rate, rad/s

Subscripts A Related to adsorption -A Related to desorption Add Related to additive b In the bulk solution Cu Related to copper dip Related to dipyridyl eq Equivalent

16 f Forward flat Related to the flat electrode GA Related to glyoxylic acid I Related to inclusion NoAdd Related to the additives-free surface or system OH Related to hydroxide ref Related to reference conditions S Related to conditions at the electrode surface sat Saturated v Related to the via

Effects of Transport and Additives on Electroless Copper Plating

Abstract

RONALD ZESZUT

Metal deposition in electroless plating can be advantageously used to metalize non-conducting substrates and electrically isolated features. This research focuses on the metallization of nanometer-scale interconnects in semiconductor devices, which are rapidly approaching sizes too narrow for electroplating. A number of challenges still exist for the application of electroless plating to feature fill: (i) identifying an additives mixture that provides bottom-up fill in electroless plating; (ii) developing an experimental technique for rapid screening of such additives; (iii) quantification of transport in the electroless system; (iv) a comprehensive, quantitative model for electroless plating rates as a function of the important system parameters must be developed in order to enable predictive design. This research addresses all the above listed items.

A technique for simulation of feature fill by electroless plating on a flat, non- patterned rotating disk electrode (RDE) is presented. Using deposition experiments performed at two different rotation speeds to simulate the feature top and bottom. This technique that provides a rapid and inexpensive method for additives screening, was used to identify promising additives for bottom-up fill. 3-mercaptopropanesulfonic acid

(MPS) was identified as a promising additive for bottom-up fill, with polypropylene

18 glycol (PPG) and 2’-2’-dipyridyl included in an additive mixture to provide a bright and uniform deposit.

A model that provides electroless plating rates and accounts for the reactants and additives concentrations and for the effects of transport, has been developed. The model is based on experimental data and the electrochemical rate equations for both the oxidation and reduction reactions to provide the plating rate and operating potential as a function of the bulk reactants concentrations and the RDE rotation rate.

The additives activity has been accounted for through the determination of their surface concentration, as determined by a balance of their transport, adsorption, and removal by inclusion into the deposited metal. It is shown that MPS is the critical, rate determining suppressor. The effect of dipyridyl is also included to model a multi-additive system. The model predictions match the observed data well, over reactants bulk concentrations, as well as rotation rates in the range of interest.

Chapter 1. Introduction

1.1 Overview and Rationale

Electroless plating is a metallization process where the deposited metal is being reduced from its ionic form by a chemical reducing agent, rather than by electrons supplied through a metallic conductor. As such, electroless plating can be used to metallize non- conducting substrates or electrically isolated regions. A particularly attractive application, as discussed below in more detail, is the metallization of interconnects in ever shrinking semi-conductor devices. Being a chemical rather than an electrochemical process, electroless plating is more sensitive than electroplating to the chemical composition, the temperature, and the transport rates of the chemicals to the reaction site. Since no external current can be measured, determination of the electroless process rate is more difficult and quite critical, yet reaction rate models of the electroless process are sparse, and the dependence of the rates on the transport conditions are typically not available. The focus of this work is to develop such a model.

The developed model is specific to the particular electroless process studied, however, the methodology presented is general and can be applied to other systems as well.

Almost all electroplating and electroless plating systems incorporate special additives mixtures that control the deposit distribution and its properties. In particular, the successful electroplating of surface features, namely, nano-scale vias and trenches in semiconductor interconnect applications, depends on special additives mixture that provide the bottom-up fill1,2. The additives mixtures that have been developed and are used extensively in electroplating are ineffective in electroless plating. Since the atomic

21 level mechanism of the additives mixtures remains unknown, all additives mixtures are found empirically which is a process that is time-consuming and very costly. A second objective of this research is to develop a rapid scanning technique for identifying effective additives for electroless plating, and to utilize the technique to find additives combination that provides bottom-up fill in electroless plating.

1.2 Fabrication of Copper Interconnects for Semiconductor Devices

The trend towards smaller and more powerful electronics devices is made possible by increasing the semiconductor density in integrated circuits3,4. As the semiconductors become smaller, so too, must the copper interconnects which link them. The copper interconnects are fabricated through the ‘dual-damascene’ process, shown schematically in Figure 1.15. This process includes etching of the dielectric layer to form the desired pattern of trenches and vias, followed by deposition of a thin barrier layer, often TaN or TiN. The role of the barrier layer is to minimize diffusion of copper into the dielectric which would cause interconnect failure. On top of the barrier layer, a thin layer of copper is deposited by physical vapor deposition (PVD) to form a conductive seed layer which is required for electrochemical deposition. The interconnect pattern can then be filled by electrodeposition6. This process can then be repeated to build additional layers of interconnects to achieve a three-dimensional structure.

Etch Stop Insulator Trench

a b

Seed Layer

Barrier Plated Layer Copper c d

Figure 1.1. Feature fabrication and fill process diagram. (a) Dielectric and etch stop deposition. (b) Trench formation. (c) Barrier and seed layer deposition. (d) Feature fill with plated copper.

1.3 Electrodeposition of Copper for Feature Fill

As described earlier, electrodeposition of copper is often used to provide the filling of features fabricated through the dual damascene process. However, due to current density limitations, copper preferentially plates at the features top rim, rather than the bottom. This causes the feature to seal at the top before the fill is completed, resulting in a void as shown schematically in Figure 1.2.

As features continue to get smaller, the additional thickness of the PVD copper seed layer required for electrodeposition becomes significant, especially due to its non-

23 conformal deposition profile as shown in Figure 1.57, further increasing the difficulty of achieving void-free fill.

In order to prevent voids, special additive compounds are included in the plating bath which adsorb to the surface and affect ‘bottom-up’ plating1,2,8-10. The two main components of the additives mixture are plating ‘suppressors’ and ‘anti-suppressors’.

The suppressor is typically a fast adsorbing, slow diffusing molecule such as polyethylene glycol (PEG, Figure 1.3a) that adsorbs preferentially at the feature rim and on the top surface, slowing down plating at those locations. Anti-suppressors are typically slow adsorbing, fast diffusing molecules such as bis-(sodium sulfopropyl)- disulfide (SPS, Figure 1.3b) that do not significantly modify the copper plating rate, however, these species adsorb preferentially at the via and trench bottom, blocking those sites from the suppressor adsorption.

PEG SPS Figure 1.4. Additive molecule structures. (a) Polyethylene glycol (PEG) (b) Bis-(sodium sulfopropyl)-disulfide (SPS)

Additionally, the anti-suppressor must adsorb more strongly than the suppressor, such that it can over time displace the adsorbed suppressor on the surface, allowing the plating from the bottom to propagate all the way to the feature top. The behavior and interaction between the suppressor and anti-suppressor in a feature is shown schematically in Figure 1.4. The suppressor molecules can quickly adsorb on the upper plating surfaces at the feature top, thus preventing the anti-suppressor molecules from adsorbing on these surfaces. The predominant coverage of the suppressor significantly slows plating here. However, the fast diffusing anti-suppressor can quickly transport into the feature and adsorb on the feature bottom preventing suppressor adsorption at these sites. This allows plating to continue at approximately its additive-free rate at the feature bottom. The fast rate at the feature bottom allows for bottom-up, void-free fill.

Figure 1.5. Additive surface coverage for void-free feature fill. Suppressor molecules (red) adsorb preferentially to the feature top and upper sidewalls. Anti-suppressor molecules (purple) adsorb preferentially at the feature bottom. Plating proceeds slowly at the suppressor covered areas, and quickly at the anti-suppressor covered areas. This results in bottom-up fill with no voids.

Electrochemical11 and quartz crystal microbalance12 based testing to determine the effectiveness and mechanism of additives interactions have been utilized. Specific techniques for additive screening include injection and co-injection methods1,2,13,14.

Although these additive combinations work effectively in electrolytic plating, the requirement of a conductive seed layer for electroplating makes the electroplating process less suitable for the filling of increasingly small features. The reason for this is that the PVD process for laying down the copper seed layer becomes challenging as the features become very narrow. When the PVD deposited seed itself occupies a significant portion of the feature, typically highly non-uniform fill is generated, being a line-of-sight process.

1.4 Electroless Plating

Electroless plating is a process in which a metal is deposited from its ionic form on a substrate utilizing a chemical reducing agent. Electroless deposition is used in numerous applications requiring a metallic coating for a variety of metals including copper, nickel, gold, silver, and iron15. Unlike electroplating, no external current is required to drive the process. The electron source required for copper reduction is the

26 oxidation of the reducing agent. In the specific process studied here, the reducing agent is glyoxylic acid:

Cu202 e Cu (1.1)

   2CHOCOOH 4 OH  2 HCO2 4  2 HO 2  H 2  2 e (1.2)

The electrons released from the reducing agent directly reduce the copper ion to its metallic form, so there is no external current.

As can be noted in Eq. 1.2, hydroxide is a reactant in the oxidation of glyoxylic acid, so the concentration of hydroxide, and as a result, the pH, must be higher than is present in most electroplating baths to achieve a significant reaction rate for glyoxylic acid. At a pH of 12.8, as in the system studied here, the solubility of copper is extremely low. For this reason, a complexing agent (in this case, ethylenediaminetetraacetic acid,

EDTA) is used to complex the copper and maintain it in solution. These changes to the solution from the acidic, uncomplexed electroplating system to the alkaline, complexed electroless system result in very different additive behavior. On account of the many differences between electroplating and electroless processes, additive systems successfully used in electroplating do not work in electroless systems. For example, SPS, which is an anti-suppressor for electroplating, is a strong suppressor in the electroless system. Consequently, new additives for electroless systems, different from those already identified for electroplating baths, must be found.

There are increased challenges when studying electroless plating as compared to electroplating. For example, the ability to control the current density or the potential of the electrode as in electroplating is not possible in electroless plating. The plating rate

27 and electrode potential in electroless plating are controlled by factors such as the system chemistry and temperature and are not adjustable nor can these be monitored in-situ. As there is no external current, plating rate cannot easily be measured in the manner used in electroplating. These factors make control and monitoring of electroless systems significantly more difficult than electroplating systems.

However, as stated earlier, there are significant advantages to electroless plating as compared to electroplating. The electroless process does not require a conductive seed layer16, so a PVD copper layer is not required. Additionally, electroless plating does not have the current distribution concerns associated with electroplating, which lead to non-uniform plating due to current being focused on the upper corners of a features, as well as the terminal effect17.

1.5 Screening of Additives for Electroless Feature Fill

As discussed above, successful feature-fill requires in most practical applications, bottom-up plating, which is enabled by the additives mixture. In the absence of molecular level comprehensive understanding linking the additives structure to function, the search for effective additives is empirical. This search is more challenging in the electroless system, since current and potential cannot be directly measured, and therefore the classical injection method introduced by Akolkar and Landau7,8 cannot be directly applied. Different methods have been applied to identify additives that promote void-free feature fill in electroless systems. The most common is to subject a patterned coupon to electroless plating, and then image the fill, checking for voids18. Other investigators have first measured the effect of additive concentration on the electroless

28 plating rate then plated and imaged the feature19-23. Clearly, these approaches are extremely time-consuming and costly. Another approach that has been used to study electroless plating is based on characterizing the electrochemical behavior of the separate half-cell reactions of the electroless process, independently24. When studying the separate half-cell reactions, the mixed potential theory is typically being invoked.

The mixed potential theory stipulates that in an electrochemical process with no external current (corrosion processes and electroless plating) the electrode potential will assume a value at which the current associated with the oxidation reaction is equal to the current associated with the reduction reaction as measured in the independent half-cell reactions25. By measuring the polarization curves of the anodic and cathodic reactions separately, the potential at which the two currents are equal and opposite is assumed to correspond to the electrode potential of the electroless process, and the current (equal at this point for both reactions) is considered to correspond to the rate at which the full electroless reaction should occur. This model applies to a few systems, but in many others, the chemistry, substrate, and the process conditions of the full electroless system cause significant deviations from the mixed potential theory. The independent electrochemical characterization of the half-cell reactions can be used along with other techniques to examine the effectiveness of an additives system26-33.

However, as stated, analysis from separate half reaction polarization curves in conjunction with mixed potential theory often does not match the full electroless system behavior and has proven to be unreliable for predicting the effectiveness of additives in the full electroless system. It has been shown that an autocatalytic

29 mechanism exists in the full electroless system that provides faster reaction rates than those observed in the separate half systems for both formaldehyde34-38 and glyoxylic acid39 based electroless processes.

Although some empirical models exist for electroless plating40,41, they do not correlate experimental data over a large range and do not connect their results to theory. A mathematical model has been proposed by Ramasubramanian et al.42, however, it invokes many simplifying assumptions and is not compared to experimental data for confirmation. In another study, the electrochemical parameters of the separate half-cell reactions were estimated from applying a potential such that one of the half reaction rates is approximately zero to study the effect of complexing agents43. In another study, the effects of additives in the electroless system on the charge transfer resistance and the double layer capacitance were studied using impedance spectroscopy44.

There is only very general discussion about the effect of transport on the electroless process45. Although it is well understood that additives, being minor species, are subject to stronger transport limitations than the major reactants (cupric ion and glyoxylic acid), and therefore their distribution on the electrode is likely to be transport controlled, there is little published research attempting to quantify the phenomena. In investigating additives effects in electroplating, a rotating disk electrode (RDE) is a commonly used tool that allows for the precise control and quantification of the transport conditions in the system. However, the RDE has received very little use in studying electroless plating, which is typically studied in systems without well characterized agitation, often involving

30 quartz crystal microbalance which, in its typical configuration, is incompatible the rotating disk electrode system or with other modes of convective flow. The search for additives for electroless plating typically relies on expensive and time consuming

Edisonian testing involving plating an actual patterned substrate, and then imaging the fill with an SEM. Additionally, the literature lacks a definitive answer as to why half-cell reaction analysis using mixed potential theory fails to predict the results of the full electroless system, and how to quantify the deviation.

1.6 Objectives

The objective of this research is to characterize the effects of additives and transport rates on the copper electroless plating process with focus on application of feature filling in microelectronics production. Specifically:

 Develop a transport and concentration based model for the electroless system to

predict plating rate in the absence of additives

 Characterize the effect and behavior of additives in the electroless system

 Develop a rapid screening test for promising additives that will reduce the need

for costly and time-consuming SEM analysis and utilize this test to identify a

promising additive system

1.7 Structure of the Thesis

 Chapter 2 develops an additives free model from Butler-Volmer type

equations dependent upon surface concentrations of the reactants. The

surface concentration of each reactant is dependent upon the transport

conditions in the system.

 Chapter 3 introduces a screening test for additives in an electroless

system for bottom-up fill. The model simulates a feature top and bottom

based on varied rotation speeds on an RDE. MPS is found to be a

promising additive for feature fill, with the addition of PPG and dipyridyl

required for a bright and uniform deposit.

 Chapter 4 extends the model developed in chapter 2 to account for the

effect of additives (identified in chapter 3) in the system. A model for

single additive (MPS) diffusion, adsorption, and inclusion into the metal

deposit is presented. Extension of the model to a three additive system is

discussed.

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Chapter 2: Electroless Plating Model Accounting for Transport

and Concentration Effects

The electroless plating process is not well characterized, even in the absence of plating additives. As stated previously, without the ability to apply a potential or measure an external current, in-situ control is impossible, and consequently, the electroless system determines its own plating rate and operating potential based on thermodynamics, kinetics, and transport parameters. The latter are closely linked to the reactants bulk concentrations, transport effects, and temperature. Electrochemical techniques applied to the characterization of electroless systems have indicated significant differences between the overall electroless reaction and its electrolytic half- reactions1. Attempts to empirically model electroless systems have demonstrated predictive results for specific systems2, but lack fundamental characterization of the system or linkage to the underlying electrochemical and transport processes.

This chapter presents an analysis leading to a model of the electroless process in the absence of additives. The model is extended in chapter 4 to account for the effects of additives. While the model applies specifically to electroless copper deposition using glyoxylic acid as a reducing agent, the approach can be extended to other systems. The analysis consists of the following sections:

 Comparisons of the polarization curves of the full electroless system and the two

half-reaction systems

 Substrate effect on the electroless plating rate

 Development of a semi-empirical model for the electroless plating rate and the

electrode potential. The model is derived from Butler-Volmer type equations for

the oxidation and reduction reactions.

 Accounting for the effect of agitation on reactant transport to the electrode and

on the resultant electroless plating rate

Better characterization of the electroless process in the absence of additives is essential for the subsequent modeling of the role of additives and their effect on plating rate. This chapter seeks to establish an additives-free model as a baseline for comparison with additives-containing systems, as well as provide a methodology for characterization of other electroless deposition systems.

2.1 Experimental Procedure

2.1.1 Experimental Apparatus

A three electrode system was used for electrochemical measurements as shown schematically in Figure 2.1.

Cu/CuSO4 Reference Electrode (when Copper foil required) counter electrode Pt rotating disk electrode embedded in Teflon shield

Figure 2.1. Experimental apparatus diagram. Three electrode cell with a rotating disk working electrode, Cu/CuSO4 reference electrode, and copper foil counter electrode in a jacketed beaker.

In order to characterize the agitation, the substrate for the electroless plating process was mounted on a rotating disk electrode (RDE). The electroless plating substrate (’working electrode’) was, except where noted, a 0.79 cm diameter platinum disk embedded in a Teflon shaft (1.8 cm in diameter). A Pine Instrument Company model ASR2 rotator was used to control rotation rate. For electrochemical experiments, a copper foil approximately 2 cm by 5 cm placed at the beaker edge served as the counter electrode, except where noted. The reference electrode was a copper/copper sulfate electrode consisting of a 30 ml syringe filled with acidic copper sulfate solution

(0.5 M copper sulfate, 0.1 M sulfuric acid), with a copper wire protruding through the syringe top to make contact with the solution inside the syringe. The syringe was connected to the plating solution through a small tube filled with glass wool and a 0.5 M sodium sulfate solution. The reference electrode was placed at the edge of the beaker opposite to the counter electrode. The solution was maintained at 60 °C using a

41 cylindrical jacketed beaker (6.5 cm in diameter and 9 cm deep) and hot water circulator.

Due to hydrogen bubble formation during electroless plating, the beaker was tilted approximately 5° during all experiments, as shown in Figure 2.2, except where indicted otherwise.

Tilt angle

Figure 2.2. Electrode tilt angle diagram. The system was tilted approximately 5°.

A potentiostat (Bio-Logic VSP or Solartron Analytical Modulab) was used for the electrochemical measurements.

2.1.2 Solution Compositions

The ‘baseline’ electroless system used in the experiments, except where otherwise noted, is listed in Table 2.1. The system consisted of copper sulfate, ethylenediaminetetraacetic acid (EDTA), glyoxylic acid, and sodium hydroxide.

Table 2.1. Baseline Electroless Bath Composition

Chemical Concentration [M] Function Source Copper (II) Sulfate 0.036 Copper Source Fisher Scientific EDTA 0.24 Complexing Agent Fisher Scientific Glyoxylic Acid 0.19 Reducing Agent Acros Organics (CHOCOOH) Sodium Hydroxide Adjust pH to 12.8 pH Control Fisher Scientific

Although formaldehyde is commonly used and effective reducing agent for electroless plating, glyoxylic acid was chosen for this work due to the numerous health and environmental concerns associated with formaldehyde. Unlike formaldehyde, glyoxylic acid is a solid at room temperature and does not readily vaporize. For this reason, the health concerns of working with glyoxylic acid are much lower than with formaldehyde3.

2.1.3 Stripping Voltammetry

In the absence of external current, quartz crystal microbalance (QCM) is commonly used to measure the amount of metal deposited in electroless processes.

QCM measurements often require, however, completely stagnant electrolyte. Work has been done to combine a QCM with controlled-flow systems, such as an impinging jet cell4, channel flow cell5, as well as an RDE6. However, those systems are excessively complicated and their precision is limited. Since the objective of the present study is to characterize the effects of electrolyte flow and agitation, which are ever present in practical systems, and due to the excessive complexity of the flow-compatible systems,

QCM measurements were not used. Since the amount of deposited copper was too small for conventional gravimetric analysis, stripping voltammetry technique was used to measure small amounts of the electroless plated copper. Stripping voltammetry requires a substrate made of a metal different from (the deposited) copper. Platinum

43 has been utilized here. Since the initial substrate strongly affects the electroless plating rate, the platinum RDE substrate was preplated in a ‘copper only’ solution, consisting of the baseline electroless bath without the glyoxylic acid, for 5 minutes at -10 mA/cm2 at a rotation rate of 400 rpm and 60 °C in the jacketed beaker. Copper foil was used as a counter electrode and a copper wire served as the reference electrode. This procedure deposited approximately 1.4 m of copper onto the platinum. This copper surface exhibited similar activity to that of electroless plated copper as demonstrated by a constant plating rate from deposition initiation to steady state. The amount of copper deposited on the platinum, during the substrate preparation step, was determined from the amount of charge passed during the pre-plating step as shown in Figure 2.3. On top of this pre-plated copper ‘seed’ layer (of pre-determined amount), electroless deposition was subsequently carried out (throughout the investigation), adding a second layer of copper with yet to be determined mass. In order to determine the amount of plated copper, the electrode was stripped in an acidic copper solution (0.5 M

CuSO4, 0.1 M H2SO4) at a potential of at 0.59 V vs. standard hydrogen electrode (SHE) for 6 minutes at a rotation rate of 400 rpm to remove both the electroless and electroplated seed copper layers down to the bare platinum. The stripping in the acidic solution was carried at room temperature, using a copper foil as the counter electrode, and a copper wire as the reference electrode. The amount of copper stripped was determined from the current passed during stripping. The difference between the charge passed during the preplating and the stripping steps is the charge corresponding

44 to the electrolessly deposited copper layer. The pre-plating and stripping processes are shown schematically in Fig. 2.3

Pre-electroplate Cu Electroless Cu Strip all Cu back to on Pt Disk Plating Pt (0.59 V vs. SHE) (-10 mA/cm2)

There is a small amount of side reaction, most likely dissolved oxygen reduction, which accounts for an unknown amount of charge passed during the preplating and stripping steps. However, this is accounted for by running a ‘blank’ calibration experiment which consists of electroplating and subsequent stripping without electroless plating. The charge difference recorded in the blank experiment, although very small (about 5-20% of the electroless plated amount), was then subtracted from the charge measured in the electroless plating experiments as shown in Table 2.2.

Table 2.2 Stripping Voltammetry Measurements

Condition Pre-Electroplating Stripped Charge Adjusted Charge Charge (C) Charge (C) Difference (C) (C) No Electroless -1.501 1.423 -0.078 0 30 min eless, 1.5 -1.501 4.902 3.402 3.480 ppm MPS, 100 rpm

With this stripping voltammetry technique, electroless plated copper equivalent thickness could be accurately measured down to 50 nm.

2.2.4 Linear Sweep Voltammetry of the Half-Cell Reactions

Linear sweep voltammetry was used to generate polarization curves for each of the half-cell systems. To measure the oxidation rate of glyoxylic acid by itself, a ‘glyoxylic acid only’ bath, consisting of the baseline electroless chemistry without the copper, was used. To measure the reduction rate of copper by itself, a ‘copper only’ bath, consisting of the baseline electroless bath without glyoxylic acid, was used. In each case, the other, nonreactive species, e.g., complexing agents and supporting electrolytes, were included in the bath to better simulate the conditions of the full electroless system.

The linear voltammetry sweep was applied at a rate of 5 mV/s from -0.7 V to 0 V vs. SHE. The experiment was performed on a 0.64 cm diameter polished copper disk embedded in a Teflon shield, which was rotated at 100 rpm. A platinum mesh, approximately 1 cm by 5 cm, was used as the counter electrode.

2.2 Half-Cell Mixed Potential Analysis

The advantage of half-cell reaction measurements using conventional electrochemical techniques is that in the half cell reactions current and potential can be controlled and measured in-situ. This is not the case with the full electroless bath, where external current does not flow, and therefore it is difficult to measure in-situ the reaction rates. One common method for in-situ measurements of electroless plating rates is deposition on a quartz crystal microbalance (QCM), however, as stated earlier, conventional QCM is incompatible with flow.

An easy and tempting way to make in-situ measurements in electroless systems incorporating flow, without the use of QCM, is to measure the polarization curves of the

46 two separate half-cell reactions. Mixed potential theory can then be applied to the half- cell polarization curves to characterize the complete electroless system. Mixed potential theory states that in a system with no external current flow (such as corrosion or electroless deposition processes) the corresponding oxidation and reduction reactions must proceed at the same rate, such that all electrons generated by oxidation are consumed by reduction7. According to mixed potential theory, invoking the assumption that the two separate half-cell reactions occur independently, the potential at which the oxidation and reduction reactions proceed at equal but opposite current density, is the potential and rate at which the reaction will occur in the full electroless system.

However, in the system studied here, the half-cell polarization curves indicate a different mixed potential as well as much lower predicted plating rates than those observed in the full electroless system. As shown in Figure 2.4, the predicted mixed potential is -0.54 V and the predicted current density is about 2.1 mA/cm2, while the actual measured values in the complete electroless system are quite different: -0.44 V for the mixed potential and 5 mA/cm2 for the current density.

5 Measured Plating Rate 4 (Expressed as a Current Density) 3

Predicted Current ] 2 2 Density Glyoxylic Acid 1 Measured OCV 0 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 -1 Mixed Potential

-2 Prediction Current Density [mA/cm Density Current -3

-4 Copper

-5 Potential [V vs. SHE]

Figure 2.4. Comparison of mixed potential predictions to actual electroless system behavior for the copper-glyoxylic acid process. The polarization curves, scanned at 5 mV/s on copper disk electrode rotated at 100 rpm, were measured on ‘Copper only’ (orange) and ‘glyoxylic acid only’ (blue) half systems. The predicted mixed potential, corresponding to potential where the anodic and cathodic current densities match is indicated by the green vertical line at – 0.54 V. The matching predicted current densities are 2 mA/cm2 (purple dots). By contrast, the observed corresponding values for the complete electroless system are quite different: Mixed potential of -0. 44 V (black vertical line) and a current density of 5 mA/cm2 (red dots).

Clearly, the separate half-reactions do not match the behavior of the full system.

Similar discrepancies have been reported in the literature for the same1 and for other8-12 electroless systems. The deviations are most likely due to different thermodynamics

(Gibbs free energy) that depend on system chemical composition, and different electrode kinetics that are influenced, and likely catalyzed by the electroless deposited copper and possibly by reaction by-products.

This same technique (half-cell polarization measurements) was also applied to an additives-containing electroless system. The main goal was to determine if trends and shifts in the predicted mixed potential associated with the presence of additives were consistent with the non-additive system and to find if those trends could be applied to the complete electroless system. Polyethyleneimine, a common additive, was included

(200 ppm) in the system for this purpose. Results are shown in Figure 2.5.

5 Measured Plating 4 Rate (Expressed as Glyoxylic Acid Current Density) with PEI (200

3 ppm)

] 2 2 Predicted Current Mixed Density Potential 1 Measured Prediction OCV 0 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 -1

-2 Copper with PEI

Current Density [mA/cm Density Current (200 ppm) -3

-4 Potential Shift from the 'No Additives' System -5 Potential [V vs. SHE]

Figure 2.5. Comparison of mixed potential predictions to actual electroless system behavior for the copper-glyoxylic acid process in the presence of 200 ppm polyethyleneimine. The polarization curves, correspond to ‘Copper only’ (orange) and ‘glyoxylic acid only’ (blue) half systems. The predicted mixed potential (green vertical line) and the predicted current densities (purple dots) shift in the opposite directions to the observed corresponding values for the complete electroless system: Mixed potential of -0. 71 V (black vertical line) and a current density of 5 mA/cm2 (red dots). Test parameters are identical to those indicated for Fig. 2.4.

In the presence of polyethyleneimine, the observed and predicted potentials shift in opposite directions, and the predicted current density is still less than half of the observed current density.

These observations led to the determination that continued electrochemical testing of half-cell systems in conjunction with application of the mixed potential theory is unlikely to provide straightforward usable conclusions for the studied system.

2.3 Complete Electroless System Polarization Curves and Mixed Potential Analysis

Although polarization curves generated from the half bath systems did not correspond to the full electroless system, polarization curves of the full electroless bath can be obtained and provide important insight into the system behavior. The rate of the two reactions (Eqs. 1.1 and 1.2) in the studied system can be considered in terms of a generated currents associated with the electrons transfer. These two currents are designated as IGA and ICu, and are attributed to the glyoxylic acid oxidation and the copper reduction, respectively. In electroless plating, there is no external current, so the system assumes a potential such that:

IICu GA 0 (2.1)

Notice that per conventional designation, cathodic current, (here ICu) is negative.

However, when a potential or current is superimposed externally on the full electroless system, the sum of the currents is nonzero.

IIICu GA ext (2.2)

Here, Iext is the superimposed external current. Iext can be directly measured as a function of the externally applied constant potential. By generating a series of data points at various potentials, polarization curves can be drawn as shown in Figure 2.6.

Here, the copper deposition current, Icu (blue line in Fig. 2.6) is determined from the deposited copper weight measured by strip voltammetry. The glyoxylic acid current, IGA

(orange curve in Fig. 2.6) can then be determined from the rearranged Eq. 2.2:

IIIGA ext Cu (2.2a)

Glyoxylic Acid Current ] 2 5 Density, iGA 0 Total External Current Density, iext -5 -10 -15 Copper Current -20 Current Density [mA/cm Density Current Density, iCu -25 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 Potential [V vs. SHE]

Figure 2.6. External, glyoxylic acid, and copper partial current densities in the full electroless chemistry as a function of the external applied potential. Data taken on a RDE rotated at 400 rpm. Red circle indicates the predicted ‘pure’ electroless plating operation point (no external current).

The polarization curves generated in the full electroless system were compared to the polarization curves generated in the same manner from the ‘glyoxylic acid only’ and ‘copper only’ systems as shown in Figure 2.7.

10 Glyoxylic Acid, Complete Glyoxylic Acid, Half-Cell

5 System

] 2 0

-5 Electroless System OCV and Copper, Half-Cell Plating Rate (Expressed as -10 Current Density)

-15

Copper, Complete System Current Density [mA/cm Density Current -20

-25 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 Potential [V vs. SHE]

Figure 2.7. Polarization curves gathered from the full bath and half bath systems at 400 rpm. The full electroless bath without an applied current operated at the current density and potential showed in black.

Figure 2.7 indicates a slight enhancement in the glyoxylic acid reaction rate but a considerably higher enhancement in the copper deposition rate for the full system as compared to the half bath. This figure represents the true polarization curves for the full electroless process, accounting for the catalytic effects of the redox reactions happening simultaneously, and for the possibly modified thermodynamics of the complete system.

As expected, the OCV and plating rate of the full electroless system operating as a pure electroless process (without an externally applied potential or current) match the data indicated by the full bath polarization curves very well. As there is no external current in the ‘conventional’ electroless process, charge balance must be maintained, and the two partial redox reactions must proceed at the same rate. This condition, indicated in

Figure 2.7 at about +/- 5 mA/cm2 (black dots) at a potential of -0.42 V vs. SHE, is in

52 excellent agreement with the observed electroless process as indicated in Figure 2.4 which is approximately -0.44 V. The disagreement seen between the full electroless system and the separate half-cell reactions indicates that the redox reactions are not independent and proceed differently when reacting simultaneously on the same substrate.

2.4 Effect of Substrate

One possible cause for the differences between the full and half bath systems is the reactivity of the freshly deposited copper surface and its morphology as generated by plating copper under different processes and conditions. Since, the experimental tests performed here, are often short in duration, the initial substrate, before it is modified by subsequent deposition, may have a large effect. Since electroless deposition, unlike electrodeposition, is not driven by external potential and involves a surface-catalyzed redox reaction, it is expected to be much more susceptible to the surface composition and structure. In exploring the best way to pre-plate the platinum electrode with copper, two different substrate preparation techniques were investigated: pre-plating from an acidic bath (0.5 M copper sulfate, 0.1 M sulfuric acid), and from the alkaline ‘copper only’ chemistry (complete electroless chemistry but without glyoxylic acid). The subsequent electrodeposition plating rates were measured in both systems, with and without additives as shown in Table 2.3.

Table 2.3. Substrate Pre-plating Effects on Electroless Deposition Rates The pre-plated copper thickness has been 1.4 m. Electroless plating was conducted at 60 0C, and pH=12.8, for 5 min.

Pre-plating Electroless Process Rotation Rate Electroless Plated Process pH Additives [ppm] [rpm] Thickness [ m] Acid None 400 0.62 Alkaline None 400 1.1 Acid 0.5 MPS + 0.5 PPG 16 0.11 Alkaline 0.5 MPS + 0.5 PPG 16 0.29

Each experiment consisted of pre-plating for 5 minutes at 400 rpm from either acid or alkaline solution, as listed in Table 2.3. The pre-plated substrates were then plated by the electroless process for 5 minutes at the indicated rotation rate, with or without additives. The plated thicknesses listed in Table 2.3 correspond to the net copper deposited by the electroless process and do not include the pre-electroplated copper. As can be seen, the plated thickness on the different substrates varies substantially.

To determine the preferred pre-plating method, the two pre-plating approaches were also compared to a Ru coated wafer as a substrate. Because Ru is expected to be the substrate on which the electroless process will ultimately be implemented in semiconductor device manufacturing, it is desirable that the pre-plating procedure utilized here produces similar results to those expected when plating on Ru. The comparison of the electroless plating rates on the three substrates (pre-plated Cu from acid, pre-plated Cu from alkaline, and Ru) is shown in Figure 2.8.

3.5

Ru Substrate m]  3

2.5 Alkaline Preplated Cu Substrate 2

1.5

Acid Preplated Cu Electroless Plated Thickness [ Thickness Plated Electroless 0.5 Substrate

0 0 300 600 900 Time [s]

Figure 2.8. Electroless deposition thickness at 400 rpm on Pt substrate pre-plated with Cu from alkaline or acidic chemistries and on Ru-coated wafer substrate.

Fig. 2.8 indicates that the electroless plated thicknesses on the Ru coated wafer closely match the thicknesses on the substrate pre-plated from the alkaline electrolyte.

These rates are quite different from the thicknesses on the substrate pre-plated from the acidic electrolyte. Additionally, it is noted that the plating rate of the substrate pre- plated from the alkaline chemistry and the Ru substrate are approximately constant

(~0.22 m/min), while the plating rate on the substrate pre-plated from the acidic chemistry varies over time: the plating rate for the first 600 seconds on the substrate pre-plated with acidic Cu just slightly exceeds one half of the rate observed in electroless plating on Ru or on the Cu substrate pre-plated from the alkaline chemistry.

However, after about 600 seconds, corresponding to a thickness of about 1.2 m, the

55 plating rate on the acid plated Cu substrate increases, matching the rate on the alkaline

Cu plated and the Ru substrates. It is interesting to note that the substrate effect extends through quite a thick plated layer of over 1.2 m, corresponding to thousands of copper atomic layers. Having a constant plating rate is desirable as this allows for easier comparison across experiments of different time spans, as well as easier application of the results to commercial production. For these reasons, the pre-plating process employing alkaline Cu chemistry was chosen as the baseline for all experiments.

2.5 Electroless Plating Rate Model

To quantify the electroless plating process, a plating rate model has been developed, accounting for the concentration and transport effects of the reactants. The model assumes that both the anodic and cathodic reactions could be modeled independently by a power rate law with an Arrhenius-type rate constant. Although each reaction is modeled independently, the reaction constants were determined from measurements made in the full electroless plating systems containing both glyoxylic acid and copper. Therefore, the interaction between the two simultaneous redox reactions is accounted for, unlike in the half-cell polarization experiments discussed in section 2.2, where the interaction between the redox reactions was unaccounted for.

We consider first the glyoxylic acid oxidation. This reaction can be written as:

   2CHOCOOH 4 OH  2 HCO2 4  2 HO 2  H 2  2 e (1.2)

The forward reaction rate of the glyoxylic acid can be represented in terms of a general rate expression:

GA OH vf,, GA k f GA C GA C OH (2.3)

Here, vf,GA is the forward reaction rate of glyoxylic acid oxidation, kf,GA is the rate constant, C represents the concentration, and the  terms are the exponents for the glyoxylic acid and hydroxide concentration dependence, respectively. The subscripts

‘GA’ and ‘OH’ designate the glyoxylic acid and the hydroxide ion, respectively.

The backward reaction is assumed negligible because there is only miniscule amount of hydrogen (a reactant in the backwards reaction) dissolved in solution, and furthermore, the system potential is significantly anodic to the standard reduction potential of glyoxylic acid. Assuming that the reaction rate constant is of an Arrhenius form,

0 GAf V E GA kAf, GA GA (2.4) and expressing the glyoxylic acid reaction rate in terms of the current density, i v  GA (2.5) f, GA nF

We can substitute Eq. 2.4 and 2.5 into Eq. 2.3 to obtain the glyoxylic acid current density:

 f V E0 GA OH GA GA iGA nFA GA C GA C OH e (2.6) n is the number of electrons transferred, F is Faraday’s number, AGA is a rate constant defined in Eq. 2.4, GA is the charge transfer coefficient for the glyoxylic acid reaction, V

0 is the applied potential, E GA is the standard electrode potential for the glyoxylic acid half reaction (-1.01 V vs. SHE)3, and F f  (2.7) RT

Where R is the gas constant, and T is temperature. Defining the exchange current density for the glyoxylic acid reaction as nFA i  GA (2.8) 0,GAref . GA OH CCGAref,, OHref and introducing arbitrarily selected reference concentrations, yields:

GA OH 0 CCGAs,,   OHs  GAf V E GA iGA i0,GA, ref     e (2.9) CCGA,ref   OH ,ref 

Here, i0,GA,ref is the exchange current density for glyoxylic acid at the selected glyoxylic acid and hydroxide reference concentrations. The reference concentrations presented here are the bulk concentrations of each reactant in the baseline electroless plating system.

An equation for the copper reduction reaction can be similarly derived:

Cu CCus, fCu V E0, Cu iCu i0, Curef ,  e (2.10) CCuref,

The negative sign in Eq. 2.10 designates a cathodic reaction. In electroless plating, charge balance must be preserved, and therefore the glyoxylic acid oxidation current must equal in magnitude the copper reduction current:

iiCu GA (2.11)

By equating the two current density expressions, Eqs. 2.9 and 2.10, the mixed potential,

V0, can be isolated and solved for:

i  CCC      ln0,Curef ,  lnCus,   ln GA, s   ln OHs , i Cu  C  GA  C  OH  C  0,GA,ref   Curef ,   GA, ref   OH, ref EE f Cu0, Cu GA 0,GA V0  Cu GA

(2.12)

Where constants can be grouped such that Eq. 2.12 can be expressed as

VMCMCMCP0Culn Cus ,  GA ln GA, s  OH ln OHs ,   (2.13)

By plugging this voltage back into Eq. 2.10, the copper reaction rate can be expressed as:

*** Cu1   GA   OH  iCu G C Cus,,,  C GAs  C OHs (2.14)

Where  *  Cu (2.15) Cu GA

And

fCu GA 1**EE0,Cu  0,GA 1 * ** Cu  GACu     GA    OH  G i0,Curef ,  i 0,GA, ref e C Curef , C GA, ref C OH, ref (2.16)

Eq. 2.14 is of similar form to an empirical rate law model developed by Donahue et al2.

However, the present model was derived from kinetics and electrochemical principles and therefore the model parameters have electrochemical significance rather than just being empirical fitting parameters. Both * and G are constants that depend only on known parameters and 7 fitting parameters.

Next, the model was extended to account for transport effects. Transport affects the reaction rate through its impact on the reactants’ concentration at the electrode surface13. The surface concentration can be related to the bulk concentration by:

i CCSb1 (2.17) iL

14 Where the limiting current (iL) is given by the Levich equation .

211  362 iLb0.62 nFD C (2.18)

Here, D is diffusion coefficient,  is rotation rate, and  is kinematic viscosity. Each reactant will have its own limiting current, based on the different diffusion coefficients and bulk concentrations. Substituting Eq. 2.17 into Eq. 2.14 yields,

*** Cu1   GA   OH  iCu     i Cu     i Cu   iCu G C bCu,1     C b ,GA  1     C b ,OH  1    (2.19) iLCu,     i L ,GA     i L ,OH  

Table 2.4 indicates the values used for diffusion coefficients and kinematic viscosity.

Table 2.4. Physical Constants Used in the Levich Equation

Property Value Units Reference Kinematic Viscosity 4.74*10-3 cm2/s 15 Copper Diffusion 7.8*10-6 cm2/s Measured Coefficient Glyoxylic Acid Diffusion Coefficient 1.1*10-5 cm2/s 16 (assumed same as glycolic acid) Hydroxide Diffusion 6.1*10-5 cm2/s 17 Coefficient

Reliable data for the diffusion coefficient of glyoxylic acid could not be found or easily measured, as the glyoxylic acid reaction also includes hydroxide as a reactant. Due

60 to the similar structure between glyoxylic and glycolic acid, it was assumed that the diffusion coefficients of both compounds are the same. The values of the diffusion coefficients obtained from the literature were adjusted to account for the temperature prevailing in the electroless system using Eq. 2.20 which is derived from the Einstein relation14. DD 12 (2.20) TT12

After determination of the appropriate physical constants, plating rates and open circuit voltage data was gathered at different reactant concentrations (copper, glyoxylic acid, and hydroxide) in order to determine the fitting parameters for Eq. 2.13 and 2.14. Figure

2.9 shows sample data, where the plating rates have been converted to equivalent copper current densities.

8 7

6 ] 2 5 4

3 [mA/cm 2 1 Equivalent Current Density Current Equivalent 0 0 0.01 0.02 0.03 0.04 0.05 0.06 Bulk Copper Concentration [M] a

-0.2 -0.25 -0.3 -0.35 -0.4 -0.45 -0.5 -0.55

Potential [V vs. SHE] vs. [V Potential -0.6 -0.65 -0.7 0 0.01 0.02 0.03 0.04 0.05 0.06 Bulk Copper Concentration [M] b

In addition to data gathered by varying the copper concentration, glyoxylic acid concentration and pH were varied in separate experiments. In order to identify interactions between the parameters (species concentrations and pH) experiments in which more than one parameter was varied at a time, were conducted as well. The

62 fitting parameters were then determined using the parameter fitting function in Origin software. The current density (Eq. 2.14) and the potential (Eq. 2.13) equations had four fitting parameters each (G,GA*,OH*,Cu(1-*), and L, M, N, P). However, both equations had terms that provided a direct ratio of the  terms for the hydroxide and glyoxylic acid.

OH1.12 GA (2.21)

Because the plating rate (expressed in terms of the current density) data was generally more consistent, the ratio determined by the current density expression, shown in Eq.

2.21 was substituted into the potential expression (Eq. 2.13) before determining the potential fitting parameters. The equations for current density and the potential with the best-fit parameters are:

0.403 0.247 0.308 iCu87.5 CS, Cu  C S,GA  C S,OH (2.22)

V0 0.373  0.033lnCCCS , Cu  0.011ln S,GA  0.012ln S,OH (2.23)

Each of the fitted parameters incorporates a collection of electrochemical constants:

fCu GA 1**EE0,Cu  0,GA 1 * ** Cu  GACu     GA    OH  G i0,Curef ,  i 0,GA, ref e C Curef , C GA, ref C OH, ref 87.5

(2.16a)

Cu 1 * 0.403 (2.24)

GA * 0.247 (2.25)

OH * 0.308 (2.26)

Cu MCu 0.033 (2.27) f Cu GA

GA MGA  0.011 (2.28) f Cu GA

OH MOH  0.012 (2.29) f Cu GA

i0,Curef , lnCu lnCCC Curef,   GA ln GA, ref   OH ln OH, ref  i0,GA,ref CuEE0, Cu GA 0,GA Pf 0.373 Cu GA

(2.30)

After incorporation of the relationship between  values for hydroxide and glyoxylic acid as described above, there were 7 independent fitting parameters and 7 electrochemical parameters, so each electrochemical parameter could be determined.

The parameters and their values are listed in Table 2.5.

Table 2.5. Electrochemical parameters derived from fitting parameters for copper reduction and glyoxylic acid oxidation reactions occurring in the full electroless system

Parameter Units Value Typical Electroplating Value (Acidic pH) Cu Dimensionless 0.72 0.5 GA Dimensionless 0.35 - Cu Dimensionless 1.23 0.6 - 0.7 GA Dimensionless 0.41 - OH Dimensionless 0.46 - 2 i0,GA,ref mA/cm 0.0053 - 2 i0,Cu,ref mA/cm 0.025 1

The electrochemical parameters determined for our system are within a reasonable range of typical copper plating electrochemical parameters except for copper exchange current density. The lower copper exchange current density may be due to the low concentration and complexed form of the copper in the present system.

The model correlates experimental equivalent current density data well over the entire tested range, as shown in Figures 2.10, 2.11, and 2.12. The correlation of the OCV is slightly less tight, however, it still provides good correlation across the broad range tested.

10 9 8 Observed Data

7 ] 2 6 5

4 Model Prediction [mA/cm 3 2

Equivalent Current Density Current Equivalent 1 0 0 0.02 0.04 0.06

a Copper Surface Concentration [M]

-0.3 Observed Data -0.35 -0.4 -0.45 Model Prediction -0.5 -0.55 -0.6 Potential [V vs. SHE] vs. [V Potential -0.65 -0.7 0 0.02 0.04 0.06 b Copper Surface Concentration [M]

Figure 2.10. (a) Equivalent current density as determined from copper plated in the electroless process, and (b) OCV measurements as a function of copper concentration. Points show experimental data and the curves correspond to the model (Eqs. 2.22 and 2.23). Process parameters: 0.19 M glyoxylic acid, pH =12.8. RDE rotated at 400 rpm.

6 Observed Data

] 2 4 Model Prediction

3 [mA/cm 2

Equivalent Current Density Current Equivalent 1

0 0.00 0.10 0.20 0.30 a Glyoxylic Acid Surface Concentration [M]

-0.2

-0.25 Observed Data -0.3

-0.35

-0.4 Potential [V vs. SHE] vs. [V Potential -0.45 Model Prediction -0.5 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Glyoxylic Acid Surface Concentration [M] b

Figure 2.11. (a) Equivalent current density as determined from plated copper in the electroless process, and (b) OCV measurements as a function of the glyoxylic acid concentration. Dots indicate the measured data and the curves correspond to the model (Eqs. 2.22 and 2.23). The process parameters: 0.036 M copper sulfate; pH=12.8; RDE rotated at 400 rpm.

10 ] 2 Model Prediction 9 8 7 Region of 6 interest: pH 11.5- 13 5 4 Observed 3 Data 2 1 0 Equivalent Current Density [mA/cm Density Current Equivalent 10 11 12 13 14 a Surface pH

-0.2 Model Prediction -0.25 Region of interest: pH 11.5-13 -0.3 -0.35 -0.4 -0.45 Observed Data

-0.5 Potential [V vs. SHE] vs. [V Potential -0.55 -0.6 10 11 12 13 14 Surface pH b

As can been seen, there is good agreement between experimental data and the fitted model especially for the model dependence on glyoxylic acid and copper

67 concentrations. There is slightly more variability in the pH correlation, however, the proper trend is present, particularly, within the indicated (shaded) normal operating range. Factors that may reduce the accuracy of the modeled pH effects may include changes in reactant adsorption, structure of the double layer, and copper complexation in solution, among other possible factors18.

2.6 Hydroxide Surface Concentration

In the electroless system studied here, hydroxide is being consumed by the electroless reaction at the electrode surface. Theoretical determination of whether this hydroxide is provided via transport or via water decomposition is difficult. The model presented in section 2.5 invokes the assumption that all reactants must reach the surface via diffusion through the mass transport boundary layer. This assumption may be challenged for the hydroxide ion that may be released by water dissociation right at the electrode instead of being transported from the bulk by diffusion through a hydroxide concentration boundary layer. The water dissociation reaction that may provide the hydroxide directly at the electrode is given by:

Kw  HO2  H  OH (2.31)

Where the equilibrium constant, Kw is given by:

  14 Kw  H  OH  10 (2.32)

This reaction is very fast such that water almost immediately returns to its equilibrium state after any perturbation. Despite the commonality of the phenomena, no explicit studies that address this question were found. (Similarly and surprisingly, no explicit discussion in the literature was found for the analog process of proton replenishment at

68 the cathode during hydrogen evolution which could proceed via proton diffusion from the bulk or by proton release from water decomposition). However, it can be argued that since extracting the hydroxide via water decomposition requires significant energy, about 280 kJ/mol, the alternate transport route for replenishing the hydroxide consumed by the reaction is thermodynamically favored. By the same token, it is reasonable to assume that once the hydroxide concentration at the electrode becomes very low (at high reaction rates), hydroxide replenishment via water decomposition may become predominant. Accordingly, while we expect the potential to shift to more anodic values as the reaction rate increases and the hydroxide at the electrode diminishes, we do not expect to observe a limiting current since hydroxide supply from decomposition of water, which is plentiful (~55 M), will take over.

In the absence of clear theoretical indication, experiments were conducted to determine whether hydroxide concentration gradient is indeed present. Such a gradient would be strongly dependent upon the boundary layer thickness and consequently, the agitation. A ‘glyoxylic acid only’ solution, containing excess glyoxylic acid (0.19 M) and no copper sulfate, was tested, with the pH adjusted to 12 so that the hydroxide would be the limiting reactant, rather than the glyoxylic acid. A potential of 0 V vs. SHE was applied (using a saturated calomel reference electrode) to a Pt disk electrode rotating at

400 rpm. The counter electrode was a Pt mesh. At this potential, glyoxylic acid oxidation with a corresponding lowering of hydroxide ion concentration at the surface (as indicated by Eq. 1.2) is expected. After approximately 50 s, the rotation rate was abruptly decreased to 100 rpm, maintaining all other independent parameters,

69 including the applied potential, constant. A significant decrease in the current density was observed, as shown in Figure 2.13, confirming the strong transport-dependence of the oxidation, hydroxide consuming, process.

2 ]

2 1.8 Rotation rate 1.6 decreased to 1.4 100 rpm 1.2 1 0.8 0.6 0.4

Current Density [mA/cm Density Current 0.2 0 0 20 40 60 80 100 Time [s]

Figure 2.13 Transport dependence of the glyoxylic acid oxidation, confirming the presence of hydroxide concentration gradient. Potentiostatic measurement at 0 V vs. SHE on a Pt rotating disk electrode with SCE reference electrode. The electrolyte consisted of excess (0.19 M) glyoxylic acid (no copper) at a pH of 12 to assure that the hydroxide is the limiting reactant. Initial rotation rate of 400 rpm was decreased to 100 rpm at the indicated point. 25° tilt angle was applied to the RDE system to assure bubbles removal.

Due to the high excess of glyoxylic acid (0.19 M) as compared to hydroxide (0.01

M), the calculated surface concentration of glyoxylic acid changes only minimally and cannot be the cause for the large observed decrease in current density. Additionally, bubble coverage of the surface cannot be responsible for this decrease as the high tilt angle effectively removes bubbles from the electrode surface, as discussed in the following section. The observed decrease in the current density must be due to limited hydroxide transport to the electrode at the lower rotation rate and the associated thicker diffusion boundary layer. Based on Levich equation, we expect that the transport

70 rate to the RDE surface will be proportional to the square root of the rotation speed. A decrease of the rotation speed from 400 to 100 RPM would thus lead to 50% of the original transport rate ([100/400]1/2). The observed decrease was from 1.5 to 1.1 mA/cm2 i.e., a factor of 0.73. The discrepancy between the expected and observed current density decrease is indicative that the process under the given conditions may not be completely under hydroxide transport control and a small amount of hydroxide might still be provided via direct water decomposition. Nonetheless, this experiment validates the assumption that the primary source of hydroxide to the reactive surface is via diffusion from the bulk and that the possible hydroxide released from water is minor.

2.7 Effect of Transport on the Electroless Plating Rate

All the data used to determine the fitting parameters for the model was measured at 400 rpm. In order to validate the model ability to account for transport rates variations, the model was also tested at other rotation rates. In the absence of additives, we expect lower deposition rates at lower rotation rates due to transport limitations which cause lower surface concentration of the reactants. While the electroless deposition process is controlled by kinetics, the kinetics are directly affected by the surface concentration. The plating rate as a function of the rotation speed, as predicted by the model together with observed experimental data, are shown in Figure

2.14.

] 2 5 4 Model Prediction 3

2 Observed 1 Data

Current Density [mA/cm Density Current 0 0 100 200 300 400 Rotation Rate [rpm]

Figure 2.14. Dependence of the electroless plating reaction rate (in the absence of additives) on transport rates. Blue dots indicate experimental measurements; the orange curve is the model prediction. Reactants bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8. RDE rotation rate varied from 16 to 400 rpm. The rotating disk assembly was tilted 5° to lessen the accumulation of bubbles on the electrode. Hydrogen evolution is inherent to the electroless deposition reaction that is being employed, as indicated by Eq. 1.2. Due to limited hydrogen solubility, a portion thereof accumulates as bubbles on the electrode surface. It is expected that at lower rotation rates, where the drag forces are lower, more and larger bubbles will accumulate on the electrode. Photographs of the electrode, taken in-situ during the deposition process, shown in Figure 2.15, confirm this supposition. Although the electrode was tilted at 5° to the horizontal during all experiments to minimize bubble accumulation, significant electrode coverage by bubbles is still observed, particularly at the lower rotation rates.

a c b

Figure 2.15. Electrode photographs taken in-situ during electroless plating indicating significant bubble coverage at the lower rotation rates. (a) 16 rpm, (b) 100 rpm, and (c) 400 rpm. Electrode was tilted 5° to the horizontal in all experiments.

Since the reported equivalent current density is calculated from the total plated amount divided by the geometric disk area, blockage of portions of the disk will lower total plated amount, and the resulting total calculated current density will not reflect the proper reaction rate at the sites not covered by bubbles. To further confirm this assumption, additional experiments, where the electrode tilt angle was varied as indicated in Figure 2.16, were carried out. It is expected that at the higher tilt angles, bubble coverage will be reduced, giving rise to higher observed deposition rates.

7 6 400 rpm

5 ]

2 200 rpm 4

3 100 rpm [mA/cm 2 16 rpm 1 Equivlaent Current Density Current Equivlaent Increasing Tilt Angle 0 0 5 10 15 20 25 30 Tilt Angle [Degrees]

Figure 2.16. Effect of the rotating disk electrode tilt angel on the electroless plating rate. Equivalent current density determined from the amount of copper plated in the electroless process at baseline bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8. RDE rotation rate varied from 16 to 400 rpm. Tilt angle varied from 0 to 25°.

The data displayed in Fig. 2.16 indicates that the plating rates increase significantly with the tilt angle at the low rotation speeds, but do not vary at the high rotation speeds (400 RPM). This suggests a direct link between bubble coverage and plating rates: bubbles are being more effectively removed from the surface at higher tilt angles availing a larger fraction of the electrode for the reaction as shown in Figure 2.17.

At the higher rotation rates, where the bubbles are more effectively swept from the surface by flow, the tilt angel effect is minimal.

a b Figure 2.17. Electrode photographs taken in-situ during electroless plating indicating significant bubble coverage at the lower tilt angle. (a) 5° and (b) 25° degree. 16 rpm rotation rate was maintained for both experiments.

Comparing the current densities at the higher tilt angle to the model predictions shows good agreement, as indicated in Figure 2.18.

] 2 5

4 Observed Model Prediction Data 3

1 Current Density [mA/cm Density Current 0 0 100 200 300 400 Rotation Rate [rpm]

Figure 2.18. Dependence of the electroless plating reaction rate (in the absence of additives) on transport rates at an electrode tilted 25° to minimize bubble coverage. Blue dots indicate experimental measurements; the orange curve is the model prediction. Reactants bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8.

Thus the model is demonstrated to be effective in predicting plating rates accounting for transport effects once bubbles are properly removed or accounted for.

2.8 Conclusions

Greater insight into the additive-free system was achieved:

 The correct polarization curve for the electroless process was determined and

compared to the half-reaction systems

 A strong and long-lasting substrate effect on electroless plating rates was noted.

An alkaline Cu pre-plating process was established to provide results consistent

with those of plating on Ru substrate.

 A model predicting electroless plating rate and OCV as a function of reactant

concentrations and transport rates was derived from basic principles

 Extensive data gathered to determine the appropriate fitting parameters of the

model

 The model was verified and demonstrated to accurately predict the electroless

plating rates as a function of reactant concentrations and transport rates.

The proposed model links electrochemical data to empirical observations. The model has been shown to be accurate in predicting plating rates and operating potentials over a wide range of Cu and glyoxylic acid concentrations, as well as a pH range from 11.5 to 13. This model sets a baseline to which additive containing systems can be compared. Additionally, this work sets forth a methodology and a set of derived equations based on electrochemical principles that may be applicable to numerous other electroless systems. As the understanding of the role of additives in electroless

76 systems continues to advance, this work offers a technique to determine an additives- free baseline for other electroless systems to allow for the advancement of additives understanding in a variety of electroless chemistries.

References 1. L. Yu, L. Guo, R. Preisser, R. Akolkar, "Autocatalysis during electroless copper

deposition using glyoxylic acid as reducing agent", J. Electrochem. Soc., 160,

D3004 (2013).

2. F. M. Donahue, K. L. Wong, R. Bhalla, “Kinetics of electroless copper plating: IV.

Empirical rate law for H2CO-EDTA baths”, J. Electrochem. Soc., 127, 2340-2342.

(1980).

3. J. Darken, “Electroless Copper-an Alternative to Formaldehyde”, Paper B6/2,

presented at Printed Circuit World Conventions-V, Glasgow, June 12–15, (1990).

4. C. Arkam, V. Bouet, C. Gabrielli, G. Maurin, H. Perrot, “Quartz crystal

electrogravimetry with controlled hydrodynamics”, J. Electrochem. Soc., 141,

L103 (1994).

5. C. M. Galvani, A. Graydon, D. J. Riley, D. York, “Electrochemical quartz crystal

microbalance in a channel flow cell: A study of copper dissolution”, J. Phys.

Chem. C, 111, 3669 (2007).

6. P. Kern and D. Landolt, “Design and characterization of a rotating

electrochemical quartz-crystal microbalance electrode”, J. Electrochem. Soc.,

147, 318 (2000).

7. C. Wagner, and W. Traud, “On the interpretation of corrosion processes through

the superposition of electrochemical partial processes and on the potential of

mixed electrodes”, Z. Elektrochem., 44, 391 (1938).

8. K. G. Mishra, R. K. Paramguru, “Kinetics and mechanism of electroless deposition

of copper”, J. Electrochem. Soc., 143, 510 (1996).

9. I. Ohno, O. Wakabayashi, S. Haruyama, “Anodic oxidation of reductants in

electroless plating”, J. Electrochem. Soc., 132, 2323 (1985).

10. H. Wiese and K. G. Wiel “On the mechanism of electroless copper deposition”,

Ber. Bunsenges. Phys. Chem., 91, 619 (1987).

11. I. Ohno, “Electrochemistry of electroless plating”, Materials Science and

Engineering, 146, 33 (1991).

12. I. Ohno and S. Haruyama, “Measurements of the instantaneous rate of

electroless plating by an electrochemical method”, Surface Technology, 13, 1

(1981).

13. F. M. Donahue, “Kinetics of electroless copper plating: III. Mass transport

effects”, J. Electrochem. Soc., 127, 51 (1980).

14. A. J. Bard and L. R. Faulkner, Electrochemical Methods, New York: Wiley. (2001).

15. J. C. Crittenden, R. R. Trussell, D. W. Hand, K. J. Howe, G. Tchobanoglous, MWH’s

Water Treatment: Principles and Design, New York: Wiley. (2012).

16. W. J. Albery, A. R. Greenwood, R. F. Kibble, “Diffusion coefficients of carboxylic

acids”, Trans. Faraday Soc., 63, 360 (1967).

17. W. M. Haynes, CRC Handbook of Chemistry and Physics: A Ready Reference Book

of Chemical and Physical Data, Boca Raton: CRC Press. (2012).

18. J. Duffy, L. Pearson, M. Paunovic, “The Effect of pH on Electroless Copper

Deposition”, J. Electrochem. Soc., 130, 876 (1983).

Chapter 3: Rapid Screening Technique for Additives Providing

Bottom-up Electroless Plating

The search for additives that provide void-free fill in electroless plating is a major research activity that is highly important for implementing electroless plating technology in the semiconductor industry future road-map. Although feature fill by electroplating has been successfully implemented for features larger than 10 nm, the ever decreasing size of semiconductor features necessitates the discovery of new and more effective feature metallization techniques. As stated earlier, prime among those is electroless plating, however, implementation of the technology requires void-free metallization that is typically assured by bottom-up plating, enabled by a special additives mixture. However, the additives combination that is effective in the acidic copper plating environment, does not work in the alkaline electroless process. The numerous studies focused on identifying effective electroless plating additives are based on an Edisonian approach. Small patterned features are plated by the electroless process incorporating the tested additives. The plated sample is then cross-sectioned and imaged using a scanning electron microscope (SEM). The image indicates whether void-free fill has been achieved. Although this method is valuable to verify the proper function of an additives system, it is costly and time-consuming. A rapid and inexpensive additive screening test to identify promising additive combinations for further study and reject ineffective additives would speed-up the search for effective additives while also reducing cost.

One commonly used method is to analyze by electrochemical techniques, monitoring current or voltage, the effect of an additive on two separate ‘half-cell’ systems. This involves measuring the polarization curves for each of the two redox reactions (given e.g. by Eq. 1.1 and 1.2) with no additives, then with varying additives compositions and concentrations. Estimates based on mixed potential then indicate whether or not an additive will be effective in the full electroless system1-7. However, as discussed in section 2.2, measurements made with or without additives in the half-cell reactions often do not correspond to the full electroless system, making this approach a poor testing procedure. Another screening method involves cyclic voltammetry performed in the full electroless chemistry, but at potentials high or low enough for which either the oxidation or reduction reaction does not take place. In these regions, the single, on-going reaction can then be measured directly as total current. By testing a variety of additives using this approach, Paunovic and Arndt were able to identify additives that accelerated either one or both of the oxidation and reduction reactions and showed that these additives also accelerated the electroless system when operated without an applied potential8. Although measured in the full electroless bath, measurements could not be taken at the potential at which electroless normally operates, so only qualitative conclusions could be drawn. Another screening method involves measuring the electroless plating rate on a flat surface in the presence of additives, to determine the effect of additive concentration on plating rate either gravimetrically2-7,9-13 or by QCM1. This method is essential in determining the large-scale effect of the additive, as to whether it is a suppressor or accelerator, and at what

81 concentrations it is most effective. It is also noted in many of these studies that an additive can act as an accelerator at lower concentrations, but a suppressor at higher concentrations. However, all the studies described above, do not provide information about the key issue of whether the additive system is capable of producing bottom-up fill. There is some discussion in few of the studies on how the additives concentration will be lower inside the feature due to depletion and transport limitations, however, this discussion is mostly qualitative, suggesting only that additives exhibiting slower plating at higher concentrations would be more promising for bottom-up fill. Hasegawa et al.14 analyzes the effect of an additive on both the bottom-up plating rate in a feature as well as the sidewall plating rate. By using SEM images of a feature at different electroless plating intervals, it is shown that PEG-4000 causes much faster plating at the feature bottom than at the feature sidewalls, thus providing better understanding of the additives distribution in the feature. Although the method more rigorously investigates the actual plating rates at the feature bottom and sidewalls, it heavily relies on SEM imaging, making the approach costly and time consuming.

The screening method presented in this chapter provides quantitative information on the effect of a single additive or an additive mixture on the electroless plating rate as a function of transport. Unlike in previous studies, the effect of transport is highlighted and quantified. This transport effect is essential for the bottom-up plating process, since the main difference between a recessed feature (via or trench) and the top surface is the different transport rate. Although the mode of plating is different, the adsorption and diffusion processes of the additives will be similar in electroless and

82 electroplating systems. Additive transport and adsorption inside a feature has been modeled in electroplating systems by Akolkar and Landau15,16. This model, which quantifies the transport conditions within a via, is adapted here to the electroless system. Akolkar and Landau also introduced a rapid additive screening technique, based on measuring the transient response of the potential upon additives injection into an

RDE system plating under constant current. This additives injection technique is highly effective and widely used for additives characterization in electrolytic plating where external current and potential can be measured. This additives injection technique is not applicable, however, to electroless plating where no external current or potential can be applied. Instead, we introduce here a technique that is based on simulating on a flat

(non-patterned) rotating disk electrode the transport conditions that prevail on the flat surface and within the feature, and thereby, by conducting two simple plating experiments on a flat RDE substrate, rotated at two different speeds, characterize the bottom-up fill capability of an additives system. This approach allows for the simulation of plating at the feature top, using a faster rotation speed which corresponds to a thinner diffusion boundary layer, as well as simulation of the feature bottom, using a slower rotation speed which corresponds to thicker diffusion boundary layer. Following

Akolkar and Landau’s analysis15,16, this thicker diffusion boundary layer accounts for both the geometrical distance down the feature that must be traveled by the additive, as well as for the effects of the additive adsorption onto the feature sidewall before reaching the feature bottom. By comparing the plating rates observed on the flat RDE

83 under the fast and slow rotation rate experiments, predictions for the plating rates at the feature rim and bottom can be made and feature fill capability can be evaluated.

This chapter presents:

 Adaptation of Akolkar and Landau’s model for use in electroless systems

to predict additive effectiveness

 The effect of additive concentration and transport on electroless plating

rate

 Comparison of plating rates for the same additive bulk concentrations at

different rotation rates

 Effect of additional additives which promote brighter and more uniform

deposit

 Verification of the model via experiments

The newly developed screening method is expected to greatly assist in the search for new additives for bottom-up electroless feature fill. Additionally, a promising additive mixture containing, PPG, MPS, and 2’2’ dipyridyl has been identified which yields a bright and uniform deposit with promising feature fill capability.

3.1 Experimental Methods

A detailed description of the apparatus, electroless chemistry, and the method of stripping voltammetry for weight gain measurement discussed in this chapter, is available in section 2.1. In addition to stripping voltammetry measurements, used for determining the smaller deposit weight gains, some larger amounts of deposited copper were measured by direct weighing on a balance. The RDE substrate was polished with

800 grit sandpaper to a smooth finish prior to the experiments. This method is used where noted.

3.1.1 Additives

A variety of additives were examined in the course of this study, as listed in Table 3.1

Table 3.1. List of additives studied

Chemical Name Source Polyethyleneimine (PEI) Fisher Scientific Bis-(sodium sulfopropyl)-disulfide (SPS) Fisher Scientific Mercaptopropanesulfonic Acid (MPS) Chem-Impex International Polyethylene Glycol (PEG) MW 4000 Fisher Scientific Polypropylene Glycol (PPG) MW 725 Aldrich 2’2’ Dipyridyl Acros Organics

Additives were dissolved in water, at concentrations ranging from 0.5-5 g/L, prior to inclusion in the electroless plating bath. Small amounts of these solutions were then injected into the electroless bath using a pipette to bring the full bath to the desired concentration.

3.1.2 Patterned Coupon and Feature Imaging

The technique and most of the measurements described in this chapter were carried out on a flat RDE substrate, which provided a simple means for determining the deposition rate as a function of transport. However, a few experiments involving patterned coupon plating, mounted on the rotator shaft, were carried out to confirm feature fill. The coupons had a copper or ruthenium substrate and feature widths of

100, 70, and 48 nm. The coupons were mounted onto a specially fabricated holder, with the ruthenium or copper surface electrically connected to the rotator with copper tape.

Kapton tape was then applied to mask the surface leaving a 1.9 by 1.3 cm rectangle

85 exposed. In order to remove oxides from the ruthenium surface before plating, a pre- plating procedure was carried out, consisting of coupon immersion in 50 mM sulfuric acid for 60 s at -0.6 V vs. SHE, followed immediately by DI water rinse and immersion in the electroless plating bath.

Focused ion beam (FIB) milling and scanning electron microscopy were used on the coupon to determine feature fill using an FEI Helios Nanolab 650. A thin platinum layer was deposited onto the surface to protect the underlying features before FIB milling. An area, 28 m by 6 m was milled, starting at 30 V and 14 nA, reducing the current down to 1.6 nA for final polishing. Imaging was done at 2 kV and 25 pA with secondary electron detection.

3.2 Additives Adsorption and Diffusion in a Feature

As discussed earlier, additives for feature fill must promote faster plating at the feature bottom than the feature top. Transport rates to the via rim and top flat surface are given, in terms of the equivalent boundary layer thickness, by the Levich equation16

111 362 flat1.61D  flat  (3.1)

The transport rates to the bottom of the feature are lower than those at the feature rim and top wafer area. This is due to two factors: first, the via depth by itself, designated here as L, presents a longer diffusion path than the top of the wafer due to geometrical considerations. Second, and typically in small vias, more importantly, the additive adsorption on the side-walls along its path to the via bottom, increases, often very substantially, the effective transport resistance. These transport rates can be represented in terms of an effective diffusion length or equivalent mass transport

86 boundary layer thickness. The equivalent boundary layer at a cylindrical feature bottom, subject to diffusing species adsorption on the sidewalls, has been shown by Akolkar and

Landau15 to be:

sat v L (3.2) CRb

Here, v is the equivalent, flat surface diffusion boundary layer thickness of the cylindrical via, sat is the saturation surface concentration of the diffusing and adsorbing species, and L and R are the via depth and its radius, respectively. As shown schematically in Figure 3.1, neglecting dispersion effects at the via opening, which are expected to be minor for narrow features, we find that the equivalent flat boundary layer thickness, eq, for a cylindrical via is:

111 362 sat eq  flat   v 1.61DL    (3.3) CRb

The equivalent rotation speed of the non-patterned rotating disk which would simulate the transport to a via bottom of the given dimensions (L and R) is obtained by determining from the Levich equation the rotation speed that would generate eq as given by Eq. 3.3.

We find:

2 11 1.61D36 eq  (3.4) 111 362 sat 1.61DLflat  CRb

Since in typical cases, v >> f we can obtain as approximation:

21 33 2.59DCR b eq  2 (3.5) Lsat

Diffusion Boundary δflat δeq Layers

δv L

Eq. 3.5, indicates the required rate that an RDE must be rotated at in order to simulate transport to the via bottom in patterned surface. For MPS as a suppressing additive in the electroless chemistry, and a feature that is 500 nm deep and 30 nm wide, this corresponds to a rotation rate of 16 rpm, which was subsequently used as a baseline in many of the experiments. Although exact features of this size were not used, the rotation rate required for simulating transport to the bottom of these vias, on a flat

RDE should indicate promising additives for bottom-up fill of small features. For the flat top and the via rim simulation, 400 rpm was chosen as this represents a relatively fast transport typical to agitated systems.

3.3 Effect of Additive Concentration on Electroless Plating Rate

Polyethylene glycol (PEG) and bis-(sodium sulfopropyl)-disulfide (SPS) were first tested since these are commonly used additives in electroplating. Both species suppress electroless plating, as shown in Figure 3.2. The dots represent calculated deposition rates determined from the mass of copper deposited by electroless plating in one hour on a RDE rotated at 100 rpm.

8 7

SPS hr)]

2 6 5 PEG conc. with 4 0.5 ppm SPS 3 PEG 4000 2 PEG conc. with 0.25 ppm SPS

Plating Rate [mg/(cm Rate Plating 1 0 0 1 2 3 4 5 Concentration [ppm]

Figure 3.2. Effect of PEG 4000 and SPS additives at various concentrations on electroless plating rate. Electroless plating was conducted on a RDE rotated at 100 rpm for 1 hr. The data corresponds to: PEG 4000 (orange), SPS (blue), PEG at various concentrations with 0.5 ppm SPS (gray), PEG at various concentrations with 0.25 ppm SPS (green).

As can be seen, PEG 4000 by itself causes a gradual decrease in the plating rate as its concentration is increased while SPS by itself has little effect at low concentration, but at a threshold concentration of 2 ppm, it shuts down almost completely the electroless plating. PEG in combination with SPS exhibits a synergistic effect, with much greater suppression than either of the two additives independently. The ‘shut-off’ mechanism of SPS (when introduced by itself) indicates a promise for feature fill

89 applications. Because the SPS would be subject to increased transport limitations within the feature, its concentration at the feature bottom would be lower than at the feature top. This would allow for significant plating in the feature bottom, while providing relatively little plating at the feature top. In addition to SPS, a chemical with a similar structure, 3-mercapto-1-propanesulfonic acid (MPS), was tested. The results are shown in Table 3.2.

Table 3.2: Plating rate at high and low rotation speeds with SPS and MPS

 = 400 rpm  = 16 rpm 1 2

Additives Plating rate [mg/hr-cm2] No additives 9.1 6.8 MPS (1.5 ppm) 0.2 6.3 SPS (1.5 ppm) 1.2 4.9

The plating rate measurements here were recorded by weight change following electroless plating for 1 hour on a 0.64 cm diameter polished copper disk, using a balance. The system without additives exhibits the expected trend of higher plating rate at a higher rotation speed, where the transport of the reactants to the electrode is enhanced and surface concentrations are expected to be higher. However, this trend does not support bottom-up fill since it implies that the plating at the bottom of the via will be slower than that at the feature rim, where transport is enhanced. Plating in the presence of either MPS or SPS exhibits the correct trend with slower plating at the higher rotation speed (corresponding to the feature rim or the top surface, while plating at the slow rotation speed, corresponding to the via bottom, is slower. Although SPS

90 and MPS both show a lower plating rate at 400 rpm as compared to 16 rpm, MPS shows the greater difference as a function of the rotating speed, and therefore provides more promise as an additive for feature fill. The deposits with both additives, however, were dark in appearance. For this reason, additives such as PEG and other polyethers were introduced in order to brighten the deposit. PPG proved to be the most effective brightener as it produced the brightest deposit, smoothest surface, and offered the largest variation in plating rates between feature top and bottom, when used in conjunction with MPS, as shown in Table 3.3. This additive mixture of 0.5 ppm MPS and

0.5 ppm PPG shows promise for feature fill. With this additives combination, however, copper was not depositing uniformly, with patches of deposit appearing on the plated surface as shown in Figure 3.3.

a b c

100 m 100 m 100 m

Figure 3.3. Electroless plating for 5 min at 16 rpm on ruthenium substrate. (a) 0.5 ppm MPS + PPG 725, (b) 40 ppm dipyridyl, (c) no additives.

The addition of dipyridyl allowed for uniform, flat, bright deposit with the desired ability of plating much more rapidly at low rotation speed than at high rotation speed as shown in Table 3.3.

Table 3.3 Plating rates at high and low rotation speeds for various additive combinations

 = 400 rpm  = 16 rpm 1 2

2 Additives Plating rate [mg/hr-cm ] No additives 9.1 6.8 1.5 ppm MPS (dark, patchy 0.2 6.3 deposit) 0.5 ppm MPS + 0.5 ppm PPG 0.6 4.2 725 (bright, patchy deposit) 40 ppm dipyridyl + 0.25 ppm MPS + 0.25 ppm PPG 725 0.034 0.32 (bright, uniform deposit)

Plating rates were determined from the mass plated on a polished copper disk with 0.64 cm diameter for 1 hour, except with the dipyridyl containing system which was measured after plating for 5 min using the previously described stripping voltammetry technique. Although with the addition of dipyridyl, the plating rates are lower by an order of magnitude, the ratio between simulated feature top plating rate and simulated feature bottom plating rate is still nearly 10:1 which is very promising.

Although 16 rpm was taken as a standard for representing the thicker diffusion boundary layer for the feature bottom, the actual required rotation rate changes with feature dimensions, as well as with the additive type and concentration as indicated by

Eq. 3.4 or its approximated form, Eq. 3.5. As there are 3 different additives used in this preferred additives combination (dipyridyl, MPS, PPG), each will have its own required rotation rate for each feature size and concentration. Table 3.4 shows the additive properties required for determining the appropriate rotation rate.

Table 3.4 Physical constants for MPS and PPG

Additive Parameter Units Value Reference Diffusion cm2/s 1.2*10-5 18 Coefficient MPS Surface Saturation mol/cm2 8*10-10 18 Concentration Diffusion cm2/s 5.6*10-7 15 Coefficient PPG Surface Saturation mol/cm2 3.3*10-11 19 Concentration

Due to their similar size and structure, PPG is assumed to have the same characteristics as PEG, and dipyridyl is assumed to have similar properties to MPS, as no literature values for either of these were available. Additionally, the diffusion coefficients were adjusted for temperature.

Table 3.5 lists the rotation rate ranges for each additive in this three additive system, for different feature sizes.

Table 3.5 Required rotation rate for feature bottom simulation

Feature RDE RPM for RDE RPM for RDE RPM for dimensions Feature Bottom Feature Bottom Feature Bottom [nm] Simulation with Simulation with Simulation with 40 Depth Width .25 ppm MPS .25 ppm PPG ppm Dipyridyl 500 30 2.6 1.7 409 150 100 97 65 15,000 150 48 46 31 7,300

The concentrations for MPS and PPG were selected such that the required rotation rate for simulation are relatively close. The required rotation rate for dipyridyl, however, is significantly larger. Due to the much higher concentration of dipyridyl, it is

93 assumed that a high concentration of dipyridyl already exists everywhere within the feature, and that MPS and PPG are the two additives mainly causing the large drop in the plating rate. The measured plating rate dependence on rotation rate for electroless plating with the triple additive combination is shown in Figure 3.4.

0.35

hr)] 0.3 2 0.25 0.2 0.15 0.1 0.05

Plating Rate [mg/(cm Rate Plating 0 0 100 200 300 400 Rotation Rate [rpm]

Figure 3.4. Electroless copper plating for 5 minutes with 0.25 ppm MPS, 0.25 ppm PPG 725, and 40 ppm dipyridyl.

For 48 nm wide and 150 nm deep features, with MPS and PPG as additives the via bottom will be simulated by non-patterned RDE rotating at about 40 rpm. Figure 3.4 indicates that at this rotation rate, high plating rate ~0.15 mg/cm2-hr is observed as compared to ~0.035 mg/cm2-hr at 400 rpm. This indicates promise for feature fill.

3.4. Coupon Plating and Feature Fill Imaging

Although good plating was obtained on the non-patterned RDE with the three additive mixture, no plating was seen on the Ru or PVD copper wafer substrate at high rotation speed on the upper surfaces or in features. This may be due to additives suppressing nucleation on these substrates more than on the pre-plated copper

94 substrate. For this reason, a simplified system containing just MPS and PPG (0.5 ppm each) was used for patterned wafer plating. With the wafer segment rotated at 400 rpm, plating was observed only within features on the wafer, but not on the flat upper surfaces as shown in Figure 3.5.

E’less No E’less Plating Plating Plating

a b 50 m

Prior to plating the shown segment was all of uniform, bright color, similar to the central section in Fig. 3.5. After plating for 30 s at a rotations speed of 400 RPM in the presence of 0.5 ppm MPS + 0.5 ppm PPG 725, the non-patterned region of the wafer segment

(central region in Fig. 3.5b) remained at its original appearance, suggesting no or very little plating. However, patterned regions with trenches of 100 nm wide on the left and of 48 nm wide on the right, appear darker, exhibiting appearance typical to slightly rough deposited copper. This indicates that plating only occurred in regions where additive surface concentrations were lower due to diffusion limitations, supporting the model predictions.

Figure 3.6 shows the SEM image of feature fill by the electroless process in the presence of 0.5 ppm MPS + 0.5 ppm PPG 725. The plated features are 48 nm wide and

150 nm deep. Plating was conducted at 400 rpm for 5 min. Good feature fill is noted. A few small voids on the sidewalls are due to seed defects.

100 nm Figure 3.6. SEM image of copper deposited in features 48 nm wide and 150 nm deep. Cu PVD seed layer on wafer, rotated at 400 rpm for 5 min in full chemistry electroless bath with 0.5 ppm MPS + 0.5 ppm PPG 725

3.5 Conclusions

A fast screening method for identifying additives that promote bottom-up fill has been presented.

 Electroplating model by Akolkar and Landau15,16 has been applied to the

electroless system

 MPS has been identified as a suppressor causing plating shut-off at high

concentration

 Promising additive combination of MPS, PPG, and dipyridyl identified which

provides a bright and uniform deposit

 Additive combination of MPS and PPG, identified by the screening method,

provided good feature fill in 48 nm features

The feature fill in the MPS and PPG system confirms, with some caveats, the model. The plating rate measurements made on the flat RDE were on a copper substrate preplated

96 from a basic solution, while the wafer had either a PVD copper or ruthenium substrate on which nucleation with the three additive system did not occur at higher rotation speeds. The technique introduced here provides an effective and fast method to identify promising additive combinations, which can then be subject to further testing to confirm feature fill. This theory-based screening method provides significant improvement over current additive screening methods, which are essentially all

Edisonian.

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Chapter 4: Additive Transport, Adsorption, and Inclusion

Although this research, as well as numerous other investigations, have identified promising additives for feature fill in electroless plating, the presence, stability, and atomic level mechanistic role of the additives on the surface are not well understood.

While determination of the atomic level mechanism of the additives action is beyond the scope of this investigation, determining the surface concentration of the additives on the surface is important for developing a process model, accounting for the additives action. This surface concentration is determined by the balance between the additives arrival at the surface through diffusion and adsorption, and their removal from the surface by desorption and inactivation, due e.g., to incorporation within the deposit. As shown in Chapter 3, the bulk concentration of additives affects the plating rate.

Additionally, a change in rotation rate at the same additive bulk concentration can change the plating rate significantly, so clearly the transport factors of the system are critical to understanding of the presence of additives on the reactive surface. Although increased rotation rate has a suppressing effect by bringing more suppressing additive to the surface, it can also have an accelerating effect as the electroless reactants are also brought to the surface faster. So, understanding the balance between these two opposing effects is essential.

For electroplating systems, Roha and Landau1 have developed a model for additive presence on the surface, based on three fluxes: additive adsorption, desorption, and removal by inclusion within the deposit. A similar approach is adapted here for electroless plating.

100

As MPS is the most effective additive for suppressing plating at higher rotation speeds, this additive was modeled individually. Although dipyridyl and PPG were included in the most promising additive combination as described in Chapter 3, MPS was the additive required in this mixture to produce the desired effect of fast plating at slow rotation speeds and very slow plating at high rotation speeds. Applying experimental results with MPS as the sole additive in the system, rate constants for the adsorption and inclusion fluxes were estimated using two different techniques, one based on electroplating measurements and the other on electroless measurements. The rate constants determined both ways were reasonably close to each other, and a best fit value was found within the range bound by these two. With these estimated values, the fractional surface coverage by the additives and the resulting electroless plating rate could be predicted by applying the additive-free electroless plating model developed in

Chapter 2. The single additive model was also extended to the previously identified three additives system (MPS, PPG, dipyridyl).

This chapter presents a model predicting additive surface coverage and effective current density in the electroless system.

 A mechanism for additive diffusion, adsorption, and inclusion in the deposit for

electroless plating is derived based on the work of Roha and Landau1

 Rate constants for additive inclusion and adsorption of MPS are estimated using

electroless and electroplating methods

 The effects of dipyridyl are combined with the model developed for MPS to

model a multi-additive system

101

 The model for single and multi-additive electroless plating is compared to

observed data, indicating good agreement.

This model, which includes the effects of additive diffusion, adsorption, and inclusion in the deposit, along with the effects of reactant diffusion and surface concentration yields results which are not obvious upon initial consideration of the system, but match the observed data well. With either MPS considered individually or in the multi-additive system, this model provides a framework for which the effects of other additives and additive combinations can be understood and modeled.

4.1 Experimental Methods

Detailed discussion of apparatus, chemistry, and the method of stripping voltammetry for weight gain measurement is provided in section 2.1. Diffusion and adsorption properties of the additives applied here are described in sections 3.1 and 3.3.

4.2 Additive Diffusion, Adsorption, and Inclusion

The adsorption of additives on an electrode changes the kinetics of deposition at that site. When an electrode is partially covered by an additive, the total plating rate (i) can be considered in terms of a weighted average of the current density on the open sites under additives-free kinetics (iNoAdd) and the on the occupied sites under additive covered kinetics (iAdd).  is the fractional surface coverage of the additive.

i iNoAdd1   i Add (4.1)

Experiments with high MPS concentration have shown plating shut down, indicating that the kinetics at MPS covered sites are extremely sluggish, suggesting that plating can be assumed negligible at these sites. In this case, iAdd is zero, which leads to

102

iiNoAdd 1  (4.2)

Eq. 4.2 relates the total macroscopic plating rate to the surface coverage by MPS. It should be noted that the values for total current density and current density on the open sites in Eq. 4.2 are dependent on surface concentration which is dependent upon numerous other factors.

An additive mass balance model for the electrode surface was derived by Roha and Landau1. This model describes additive surface coverage in electroplating but can be extended for use in electroless plating. The model considers three fluxes which control the additive concentration on the electrode surface: adsorption (NA), desorption

(N-A), and inclusion (NI) into the deposit.

Adsorption: NA k A CS, Add  sat  (4.3)

Desorption: NAA k (4.4)

Inclusion: NII ki (4.5)

Here  is the adsorbed additive surface concentration, sat is the adsorbed additive surface concentration at saturation, and kA, k-A, and kI are the adsorption, desorption, and inclusion rate constants, respectively. Langmuir-type adsorption is assumed. Eq. 4.3 indicates that the additive can adsorb only on open sites, given by the difference between sat and  Due to the complete shut down in plating observed with our additive system, it is assumed that desorption is negligible so that the system can, under low current conditions, approach surface saturation, as indicated by experiments.

103

As plating shut down from complete surface coverage has been observed, a mechanism must exist by which plating can continue over long periods. If there were no deactivation of the additives in the system, then over long time periods, the low rotation rate system would achieve the same steady-state plating rate as the high rotation rate system, namely plating shut down. As can be seen from Figure 4.1, at 16 rpm with a combination of MPS and PPG, which has been shown to shut down plating at high rotation speeds, plating continues at a constant non-zero value over a long period of time.

350

300 m]  250 200 150 100

Plated Thickness [ Thickness Plated 50 0 0 10 20 30 40 50 60 Time [min]

Figure 4.1. Thickness of electroless copper plated in the presence of 0.5 ppm MPS and 0.5 ppm PPG at baseline bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8. RDE rotation rate =16 rpm.

This confirms additive deactivation or removal from the surface. Additives have been shown to be included in metal deposits in electroless deposition2, and it is assumed that this removal mechanism exists in this system. Inclusion is assumed to be proportional to current and adsorbed additive concentration.

104

A mass balance on the adsorbed additive on the electrode interface is given by

Eq. 4.6. d N  N  k C    ki  (4.6) dt A I A s, Add sat I

The adsorbed surface concentration can be related to fractional surface coverage.

sat (4.7)

Substituting this relationship into Eq. 4.6 yields d  k C 1   ki  (4.8) satdt A S, Add sat  I sat

Cancelling sat we get, d k C1   ki (4.9) dt A S, Add  I

The concentration of the additive in solution at the interface, Cs,add, is not easily measureable. To solve for the solution surface concentration of additive, additive diffusion to the interface is considered.

CCDb,, Add S Add  N  (4.10) D 

It is assumed that additive diffusion from the bulk proceeds at the same rate as additive adsorption at steady-state, i.e., no accumulation at the interface.

CCDb,, Add S Add  N N   k C 1  (4.11) A D A S, Add sat  

Isolating CS,Add yields an expression for the adsorption flux, NA, based only on constants, known system parameters, and surface coverage.

105

C C  b, Add (4.12) S, Add k 1 1 A sat   D

kCA b, Add sat 1  N  (4.13) A k 1 1 A sat   D

Substituting Eq. 4.12 into Eq. 4.9 yields

d kCA b, Add 1 ki (4.14) k 1 I dt 1 A sat   D

At steady-state, this becomes

kCA b, Add 1 ki (4.15) k 1 I 1 A sat   D

This equation provides the fractional surface coverage, , as a function of constants and known experimental parameters.

Eqs. 4.14 and 4.15 are presented in a general form, but there are two cases where the system can be simplified. The first case is where additive adsorption at the surface is rate limiting and is much slower than diffusion. Under these conditions

CCS,, Add b Add (4.16)

Substituting Cb,Add for CS,Add into Eq. 4.9 yields d k C1   ki (4.17) dt A b, Add  I

This result is the simplified form of the equation when adsorption at the interface is the rate limiting step. This same result is derived from Eq. 4.14 when

106

k 1 1 A sat   D (4.18)

This case occurs in experiments where complete surface coverage is approached, as the

(1-) term will approach zero as full surface coverage is approached. This will necessarily cause the right hand side of equation 4.18 to be much smaller than one.

The other case occurs when adsorption is very fast as compared to diffusion; the latter being rate limiting. Under these conditions

CS, Add 0 (4.19)

As discussed earlier, the rate of additive diffusion is equal to the rate of additive adsorption, which is also equal to the rate of additive inclusion at steady-state.

NNNDAI (4.20)

Specifically looking at the equality between diffusion (Eq. 4.10) and inclusion (Eq. 4.5) yields

CCDb,, Add S Add  ki (4.21)  I

Substituting zero for CS,Add and sat for  into Eq. 4.21 and rearranging yields

CDb, Add kiI  (4.22) sat

This result is the simplified form of the equation when diffusion is rate limiting. This same result is derived from Eq. 4.15 when k 1 1 A sat   D (4.23)

107

This case occurs in experiments at low rotation speed where additive molecules arriving at the electrode interface are almost immediately adsorbed.

With a generalized model, along with the limiting cases determined, values for constants required in these equations must be determined. In the studied system, a three additive mix was identified which shows promise for feature fill, as described in chapter 3. However, MPS by itself also shows promise for feature fill, and was determined to be the additive responsible for the favorable plating rate difference between low and high rotation rate in the additive combination. The dipyridyl and PPG were only required for a brighter, more uniform deposit, although they also contributed to plating suppression. For this reason, the MPS was first modeled by itself in order to simplify the system.

4.3 Estimation of the MPS Inclusion Rate Constant

Electroless plating was carried out with only MPS at different rotation speeds. The data is shown. in Figure 4.2.

108

4.5 4 3.5

] 3 2 2.5 2

[mA/cm 1.5 1

0.5 Equivalent Current Density Current Equivalent 0 0 100 200 300 400 Rotation Rate [rpm]

Figure 4.2. Equivalent current density determined from electroless copper plated in the presence of 1.5 ppm MPS at baseline bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8. RDE rotation rate varied from 16 to 400 rpm. Plating was carried for 30 min.

Electroless plating in the presence of 1.5 ppm MPS (a suppressor) in solution was carried out for 30 min on a substrate pre-plated from an alkaline electrolyte. At low rotation speeds, plating increases with rotation rate, similarly to the additives-free model.

However, in the MPS containing system, plating sharply declines at higher rotation rates to almost complete shut off. Mechanistically, this indicates that the diffusion limitation of the MPS, which is an electroless plating suppressor, plays a significant role. At low rotation speeds, very little MPS reaches the surface, which remains mostly free of suppressor. Increasing agitation allows more reactant to get to the surface, but also more suppressor. At first, the increased reactant surface concentration causes the plating rate to increase, but with very high rotation rate, sufficient additive transport to the surface is achieved such that plating is significantly slowed down. In order to model

109 this data, kI and kA were each estimated in two different ways: with an electroless approach and applying an electroplating approach.

4.3.1 Estimation of Inclusion Constant from Electroless Measurements

In order to determine kI, the electroless data shown in Figure 4.2. was used. Specifically, the low rotation speed conditions slowed suppressor transport to the surface, so the assumption was made that at 16 rpm the adsorption process is diffusion limited, which is the case described by Eq. 4.22. Rearranging Eq. 4.22 to isolate kI yields

CDb, Add kI  (4.23) sati

All required inputs to solve for kI are known except for . To solve for , the model for electroless plating on non-additive covered surfaces from chapter 2 is employed.

0.403 0.247 0.308 iNoAdd87.5 CS, Cu  C S,GA  C S,OH (2.22)

This equation can similarly be expressed in terms of bulk concentration of each reactant, similar to Eq. 2.19.

0.403 0.247 0.308 i     i     i   iNoAdd87.5 C bCu, 1     C b ,GA  1     C bOH ,  1    (4.24) iLCu,,,     i LGA     i LOH  

Substituting Eq. 4.2 into Eq. 4.24 for the iNoAdd term yields

0.403 0.247 0.308 i i     i     i   87.5CCCbCu, 1     b ,GA  1     bOH ,  1    (4.25) 1 iLCu,,,     i LGA     i LOH  

Eq 4.25 allows for the determination of  from the experimental total average current density data from the 16 rpm experiment.

0.63 (4.26)

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With all required inputs experimentally determined or calculated, kI can be determined from Eq. 4.23.

2 k 11 cm I As (4.27)

However, the value for electroless current density, used in estimating  in Eq. 4.25 is an average over 30 minutes and may not accurately reflect the instantaneous effective current density at steady-state. Additionally, this method for estimating kI does not factor in that the additive may affect the initial nucleation, which may have a large impact in electroless plating.

4.3.2 Estimation of the Inclusion Constant from Electroplating Measurements

To obtain measurements for current density (copper plating rate) and fractional surface coverage for estimating kI, a second approach, involving electrolytic experiments was taken. The experiments were carried out in the complete electroless chemistry at a constant potential of -0.56 V (vs. SHE), a value selected such that the glyoxylic acid oxidation rate was negligible, but copper deposition still occurred. At 50 rpm and with no additives in solution, a steady-state current of -4.2 mA/cm2 was observed. However, injection of 3 ppm MPS into the solution, a plating rate of -3.5 mA/cm2 was observed as shown in Figure 4.3.

111

-3 -3.2 Steady-State 3 ppm MPS Current Density -3.4 -3.6 -3.8 -4 -4.2 Steady-State No Additive Current Density -4.4 -4.6

Current Density [mA/cm2] Density Current -4.8 Injection Point -5 0 100 200 300 400 500 600 Time [s]

Figure 4.3. External current density in the full electroless chemistry at -0.56 V vs. SHE (applied externally). Data taken on a RDE rotated at 50 rpm. 3 ppm MPS was injected at 60 s.

These values were used to determine the MPS surface coverage. As MPS causes complete plating shut down, it is assumed that the fraction of the surface covered by additive yields no plating, while the uncovered fraction proceeds at normal kinetics.

However, the change in surface concentration due to changes in plating rate must also be accounted for. Because of this, for electrolytic experiments, the kinetic current densities were compared. The kinetic current density (ik) is defined in Eq. 4.28. 1 1 1  (4.28) i iLk i

This calculation for the kinetic current density uses the limiting current (solved for by the Levich equation3), to account for changes in surface concentration of the reactant. It should be noted that Eq. 4.28 is only valid for steady-state or near steady-state conditions. Based on values calculated and measured from the experiments conducted at 50 rpm (Fig. 4.3), ik could be determined as shown below:

112

1 1 1  (4.29) 4.1 9.4 ik,No Add ,50 rpm

ik,No Add ,50 rpm 7.3 (4.30)

All current densities in Eqs. 4.29 – 4.32 are in units of mA/cm2

For the additives containing conditions, ik could also be determined 1 1 1  (4.31) 3.5 9.4 ik, Add ,50 rpm

ik, Add ,50 rpm 5.6 (4.32) and surface coverage was solved using Eq. 4.33.

i 5.6 1 k, Add ,50 rpm  1   0.27 (4.33) ik,No Add ,50 rpm 7.6

With experimental and calculated results for surface coverage and current density, kI could be isolated and solved for from Eq. 4.23

2 85mol cm 1.9 103  1.2 10 CDb, Add cm s kI  (4.34)  i 10 mol   A  sat 8 10 0.0066cm 0.0034 0.27 cm22   cm 

2 k 47 cm I As (4.35)

This is higher than, but in the same range, as the inclusion constant determined by the electroless method, which was 11 cm2/(A*s). While the electroplating rate constant estimation method side-steps the concerns raised in the electroless approach, namely, time averaged measurements and effects of nucleation, the electroplating approach has its own concerns: the system operates under a different potential and with a

113 suppressed glyoxylic acid reaction. So, both methods invoke assumptions regarding their similarity to the true steady-state electroless conditions, but the similarity of the two determined values for the constants indicate that the true value of the inclusion rate constant should be within the range of these two values.

4.4 Estimation of the MPS Adsorption Rate Constant

The determination of the adsorption rate constant is based on the assumption that under conditions where fractional surface coverage, , approaches 1, the process is no longer limited by diffusion, but instead becomes limited by adsorption, as modeled by Eq. 4.17.

4.4.1 Estimation of Adsorption Rate Constant from Electroless Measurements

Examining Eq. 4.17 as full surface coverage is approached, it can be noted that the adsorption term must always be larger than the inclusion term if full surface coverage is to be reached.

kA C b, Add1 ki I (4.36)

By substituting Eq. 4.2 into Eq. 4.36, the following expression is derived.

kA C b, Add11  ki I NoAdd     (4.37)

At near complete surface coverage, iNoAdd will be approximately constant as the surface concentrations of the reactants will not change significantly when plating is nearly stopped. So the only remaining variable, at near full surface coverage, is the bulk concentration of the suppressor, which, based on the assumptions made, will be approximately the surface additive concentration if the adsorption step is significantly slower than diffusion. Under a certain threshold of the additive concentration, even

114 with infinitely fast transport, the surface will never saturate, and plating will never stop.

When this concentration value at the threshold of plating shut down is used, Eq. 4.37 becomes.

kA C b, Add11  ki I NoAdd     (4.38)

As the values for adsorption and inclusion are approximately equal, the surface does not move toward saturation at high surface coverage. At 1.5 ppm, MPS is shown to be able to stop electroless plating at a high rotation speed (400 rpm), but at a somewhat lower rotation speed (200 rpm) it does not. This indicates that the threshold for plating shut down is just below 1.5 ppm, so it is assumed that 1 ppm is the cut-off concentration.

Isolating kA in Eq. 4.38 yields

kiI NoAdd 1 kiI NoAdd kA  (4.39) CCb,, Add1 b Add

The (1-) term in Eq. 4.36 can be canceled as it appears in the numerator and denominator, and furthermore,  can be assumed to be 1. As full surface coverage is approached, the total plating rate approaches zero. This can be used in Eq. 4.24 to determined iNoAdd. With all required inputs known, kA can be estimated.

3 k 1.1 107 cm A mol s (4.40)

Thus, the adsorption constant using the electroless data has been derived.

4.4.2 Estimation of Adsorption Constant from Electroplating Measurements

An estimate for adsorption rate constant from electroplating data was also attempted. 6 ppm injection of suppressor shut off plating at 400 rpm, however, a 3 ppm injection

115 could not shut off plating under these conditions. Accordingly, 3 ppm was considered as the cutoff concentration for plating shut-off. As total current density is very low as full surface coverage is approached, the copper surface concentration is approximately the same as the bulk. Therefore, current density can be expressed as the surface fraction not covered by additive, (1-), multiplied by the kinetic current density.

ii1  k,No Add ,400 rpm (4.41)

As full surface coverage is approached, ik,NoAdd,400rpm is analogous to iNoAdd in the electroless system, as both represent the current density on active sites with no diffusion limitation of the reactants. Therefore, ik,NoAdd,400rpm can be substituted for iNoAdd in Eq. 4.39.

kiIk1 kiIk kA  (4.42) CCb,, Add1 b Add

Solving for kA from electroplating data yields

3 k 1.8 108 cm A mol s (4.43)

This value for kA is again higher that that determined by electroless measurements.

Understanding the differences between these two sets of constants gives important insight into the system.

4.5 Polynomial Approximation for Electroless Plating Rate Model

The equation for electroless plating rate on open sites (Eq. 4.24) is necessary for using the model developed in this chapter to make predictions in the additives containing

116 system. By substituting Eq. 4.2 into Eq. 4.24 for all total current (i) terms, Eq. 4.44 is derived.

0.403 0.247 0.308 iNoAdd1     i NoAdd 1       i NoAdd  1     iNoAdd87.5 C bCu, 1     C b ,GA  1     C bOH ,  1    iLCu,,,     i LGA     i LOH  

(4.44)

In order to make Eq. 4.44 more tractable, especially when substituted into Eq. 4.14 or

4.15, a polynomial equation was fit to Eq. 4.44 to relate modeled current density and additive surface coverage.

2 iNoAdd  X  Y  Z (4.45)

Here, X, Y, and Z are the polynomial parameters for a given rotation rate, as shown in

Table 4.1.

Table 4.1 Parameters for polynomial approximation of electroless plating rate

Rotation Rate [rpm] X Y Z 16 1.593 1.359 3.503 50 0.701 1.370 4.412 100 0.402 1.204 4.883 200 0.223 0.995 5.274 400 0.120 0.786 5.586

The parameters of the polynomial representation differed based on rotation rate, but at any given rotation rate, from 16 rpm to 400 rpm, the fit of the current to the additive surface coverage was very good as shown in Figure 4.4.

117

7 6

5 400 rpm ] 2 4 3

[mA/cm 16 rpm 2

1 Equivalent Current Density Current Equivalent 0 0 0.2 0.4 0.6 0.8 1 Fractional Additive Surface Coverage

Figure 4.4. Equivalent current density model dependence on additive fractional surface coverage of MPS. Full model (Eq. 4.24) shown as solid lines, polynomial approximations (Eq. 4.45) shown as points. Modeled at baseline bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8. RDE rotation rate at 16 and 400 rpm.

Assuming steady-state for the electroless process, and with the inclusion of the polynomial term for iNoAdd, Eq. 4.15 now becomes

kCA b, Add 1 k1   X 2  Y   Z (4.46) k 1 I     1 A sat   D

Eq. 4.46 can be solved for  numerically based on known constants and experimental parameters.

4.6 Effect of Rate Constant Variability

The values for the rate constants kA and kI, determined in section 4.3 and 4.4, are only estimates, as discussed earlier. The rate of adsorption is described by Eq. 4.13, while the rate of inclusion, with the electroless rate estimated as a second order polynomial is

2 NI k I1   X   Y   Z  sat (4.47)

118

Figure 4.5 shows these rates, using rate constants determined from electroless measurements, as a function of fractional surface coverage at 16 rpm.

15 ] MPS Adsorption 12 MPS Adsorption (Surface (Diffusion Limited)

12 Adsorption Limited)

s)*10 2 9

6 MPS Adsorption 3 MPS Inclusion 0 Additive Flux[mol/(cm Additive 0 0.2 0.4 0.6 0.8 1 Fractional Surface Covereage

Figure 4.5. MPS adsorption (solid orange) and inclusion (blue) fluxes as a function of fractional additive surface coverage. Rate constants were determined from electroless measurements. Two limiting cases for additive adsorption are indicated: diffusion limit (dashed) and surface adsorption limit (dotted). The red circles correspond to expected steady-state conditions. Baseline bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8. 16 rpm RDE rotation rate.

The additive inclusion flux (blue) starts at zero for zero fractional surface coverage, then peaks around half surface coverage, before returning to zero at full surface coverage.

The adsorption flux (solid orange) starts higher, exhibiting its maximal value at zero surface coverage and decreases to zero at full coverage. The adsorption line curvature indicates that the system is moving from diffusion limited to surface adsorption limited, as the surface coverage increases. In the purely diffusion limited case (dashed orange), the line maintains the same value regardless of surface coverage, while the fully surface adsorption limited case (dotted orange) is a line of a constant slope intersecting zero at

=1. The points of intersection between the adsorption and inclusion lines are where

119 the model predicts that steady-state operation is expected. Considering that the system starts at zero surface coverage, the surface coverage will increase until the first intersection, and steady-state is predicted at approximately =0.35. Figure 4.6. shows how the adsorption and inclusion rates change when different values for kI and kA are applied.

] 12 45 MPS Inclusion (E- 40 plate k )

s)*10 I

2 35 30 25 MPS Adsorption 20 MPS Inclusion (E-plate kA) 15 (Eless kI) 10 5 0 MPS Adsorption (Eless kA) Additive Flux[mol/(cm Additive 0 0.2 0.4 0.6 0.8 1 Fractional Surface Covereage

Figure 4.6. MPS adsorption (orange) and inclusion (blue) fluxes as a function of fractional additive surface coverage. Applied rate constants were determined from electroless (Eless) (solid) and electroplating (E-plate) measurements (dashed). Baseline bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid. pH = 12.8. RDE rotated at 16 rpm.

The solid lines indicate the inclusion (blue) and adsorption (orange) fluxes using the rate constants determined from the electroless system. The dotted lines indicate the inclusion and adsorption fluxes applying the constants determined from electroless experiments. As can be seen, an increase in kI causes the magnitude of the inclusion rate to increase. An increase in kA makes the adsorption flux slightly higher, while maintaining more of a constant adsorption rate until rapid drop off near full coverage.

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4.7 Model for Electroless Plating Incorporating Additive Effects

With the rate constants estimated and Eq. 4.46 derived, the model for electroless plating with MPS is compared to the observed data in Figure 4.2. Eq. 4.46 is used to solve the surface coverage for a set of electroless conditions, then the expected equivalent current density can be determined from this surface coverage using Eq. 4.25.

Figure 4.7. shows the model predictions based on the rate constants estimated in sections 4.3 and 4.4 with experimental electroless data.

5 4.5 4

3.5 ] 2 3 2.5

2 [mA/cm 1.5 1

Equivalent Current Density Current Equivalent 0.5 0 0 100 200 300 400 Rotation Rate [rpm]

The curve based on rate constants determined from electroless plating indicates that plating shut off occurs at rotation rates much lower than experimentally observed.

However, the plating rate model based on rate constants determined from electroplating experiments does not indicate plating shut off even at 400 rpm. As both

121 of these methods only provided a rough estimate of the rate constants, a mid-value between that estimated from electroless measurements and that estimated from electroplating measurements for kI was applied in the model. The corresponding kA was solved using the method described in section 4.4.1.

cm2 kI 25 (4.48) V

3 7 cm kA 2.5 10  (4.49) mol s

These rate constants matched fairly well those calculated from electroless data measured at 50, 100 and 200 rpm using the method described in section 4.3.1. Using these averaged values, the model matched the observed data much more closely, as shown in Figure 4.8.

4.5 4 3.5

] 3 2 2.5 2

[mA/cm 1.5 1

0.5 Equivalent Current Density Current Equivalent 0 0 100 200 300 400 Rotation Rate [rpm]

Figure 4.8. Electroless plating rate expressed in terms of equivalent current density as a function of RDE rotation rate. The dots represent plating rate determined from copper plated in the electroless process at baseline bulk concentrations: 0.036 M copper sulfate, 0.19 M glyoxylic acid, and 1.5 ppm MPS. pH = 12.8. RDE rotation rate varied from 16 to 400 rpm. The curve shows the model predictions with averaged rate constants based on data from electroless and electroplating measurements.

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This model, with the averaged rate constants captures the initial increase in plating rate with increased rotation rate, as well as the decrease to full shut off at higher rotation speeds.

4.8 Three Additive System Analysis

With a model developed for MPS acting by itself, further insight into the three additive system can be attained. Out of the three additives used, only MPS caused plating shut off, even when the additives were applied at high concentration. Also, with the very high concentration of dipyridyl as compared to the other two additives, the surface concentration of dipyridyl is expected to be very high. In the model for the three additive system, we assume that the surface is saturated with dipyridyl, except at sites where MPS is adsorbed. It was further assumed that the PPG effect on the plating rate was negligible due to its low concentration. In the dipyridyl covered regions, plating does not occur at a rate equal to additives-free copper surface, but occurs at a lower rate, corresponding to the dipyridyl effect on the copper deposition kinetics. In electroless plating with only dipyridyl (40 ppm) at high rotation speed (400 rpm), it was found that this additive slowed plating from an effective current density of 5.3 mA/cm2 to 0.58 mA/cm2. This decrease to 11% of the original plating value is defined as the dipyridyl suppression coefficient, Q. Accordingly, the model was adjusted so that the plating rate on the uncovered portion of the surface was reduced to 11% of the value that has been assumed earlier for the bare copper surface.

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i 0.58 QDip  0.11 (4.50) iNoAdd 5.3

Substituting this relationship into Eq. 4.46 yields

kCA b, Add 1 kQi (4.51) k 1 I 1 A sat   D

The reduced plating rate must also be accounted for in calculation of the reactant surface concentrations.

i iQNoAdd 1 CCCS b11   b  (4.52) iiLL

Figure 4.9. shows the relationship of plating and rotation rate using the updated model for electroless kinetics on a dipyridyl covered surface.

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0.45 0.4 0.35

] 0.3 2 0.25 0.2

[mA/cm 0.15 0.1

0.05 Equivalent Current Density Current Equivalent 0 0 100 200 300 400 Rotation Rate [rpm]

This model of dipyridyl suppressing the kinetics on the non-MPS occupied surfaces demonstrates a fairly good match with the data, both in terms of the plating rate magnitude and the drop off with increased plating rate. The somewhat larger discrepancy between the model and the data than that observed in Fig. 4.8 (for a single suppressor) may be associated with inaccuracies in fitting constants for the three additives system, and to possibly more complex interaction between the three additives.

4.9 Conclusions

A mechanistic understanding of the behavior of MPS as well as the more complex system consisting of three additives was achieved. Specifically:

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 A framework for additive behavior on the surface was developed

 Rate constants for MPS adsorption and inclusion were estimated using two

different methods, obtaining reasonable agreement

 Best-fit values for the MPS rate constants were determined that resulted in a

model which closely matched experimental data

 Methodology was demonstrated for modelling multi-additive system in

electroless plating.

The proposed additives model is made possible by the development of the additives- free model as described in Chapter 2, but expands it to account for additives adsorbed on the surface. MPS was identified as a promising additive to generate bottom-up fill due to its ability to shut off plating at high rotation speeds. This behavior is explained by this model for electroless plating with additives through a balance between diffusion, adsorption, and inclusion in the deposit. Although applied to MPS here, this framework for additive analysis in electroless systems can be applied to other additives systems as well. Additionally, the three additive system modeling exemplifies how the effects of multiple types of additives can be combined into a single model

References

1. D. Roha, U. Landau, “Mass transport of leveling agents in plating: steady-state

model for blocking additives,” J. Electrochem. Soc., 137, 824 (1990).

2. D. Foulke, O. Kardos, “Current distribution on microprofiles I,” Proc. Am.

Electroplat. Soc., 43, 172 (1956).

3. A. J. Bard and L. R. Faulkner, Electrochemical Methods, New York: Wiley. (2001).

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Chapter 5: Conclusions and Future Directions

Conclusions

The main goals of this work were (i) to gain a better fundamental understanding of the factors controlling plating rate in electroless deposition with and without additives, to enable modeling of the process, and (ii) to develop a method for identifying and characterizing promising additives that promote bottom-up feature fill in electroless deposition with focus on microelectronics applications. These goals have been fully achieved.

A model for electroless plating of copper with glyoxylic acid as the reducing agent has been developed from fundamental electrode kinetics principles. The model accounts for the reactant concentration and the prevailing agitation. The model parameters were derived from experimental measurements. The plating rates predicted by the model were validated across a broad range by experimental measurements. This model has later been extended to account for the introduction of a single suppressor

(MPS) and eventually, for a three additives mixture (MPS, PPG and dipyridyl) at a composition that is expected to produce bottom-up fill. The additives model is based on material balance, accounting for the additive transport, adsorption, and removal via incorporation in the deposit. The model parameters were obtained from both electroless and electroplating experiments. The model provides the electroless plating rate as a function of the agitation and predicts quantitatively the magnitude of the plating rate and the shut-off behavior observed with certain additives at high rotation speeds.

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A screening test, using a flat (non-patterned) disk electrode rotating at two different speeds, one fast and one slow, has been developed to rapidly identify promising additives for bottom-up fill without the need for plating patterned substrates and their subsequent SEM imaging, thereby providing significant time- and cost-saving.

The rotation rates of the RDE are selected such that the fast speed corresponds to the transport conditions prevailing at the via rim and the top, flat surface. The slow speed is selected to represent the transport limitations present at the feature bottom. Without additives, high plating rates are expected and observed at the high speed, and lower rates at the slow rotation speed. However, when an appropriate suppressor, or an appropriate additives mix is present, the plating rates are reversed, with low plating rates observed at the high rotation speed (representing the rim) and high plating rates are observed at the low rotation speed (representing the cavity bottom). The screening method has been applied and a three additives mixture, consisting of 0.25 ppm MPS,

0.25 ppm PPG and 40 ppm dipyridyl, expected to provide bottom-up fill, has been identified.

Throughout this work, the effect of agitation is emphasized and shown to be a major factor in electroless plating with and without additives.

Suggestions for Future Work

The model developed in chapter 2 provides a framework through which other additive systems can be studied. Reducing agents other than glyoxylic acid, and metals other than copper can be modeled in a similar way. Such studies could be carried out and the derived models could be tested. Of particular interest would be to find out if the

128 electrochemical reaction rate constants (exchange current density, charge transfer coefficient, and concentration dependence) would be the same for either copper or glyoxylic acid if paired with a different reducing agent or metal, respectively. The effects of the complexant is also an area for further study. As EDTA is not a direct reactant in the electroless process, its effect was not considered. Accordingly, the effect of changing the concentration of EDTA, and employing other complexants could be explored. Extending the model to account for different temperatures would also be important. Currently the model includes an input for temperature, in the ‘f’ term in the exponent, but this temperature dependence was not tested.

The work in chapter 3 could be expanded by screening and identifying additional additives for the electroless chemistry studied here and for other electroless chemistries. Additional plated feature imaging through SEM and TEM could be done to more widely confirm the dual speed screening test for additional sets of additives.

Chapter 4 can be further explored by trying to better understand the role of each additive in solution. Just as MPS is modeled independently in this work, PPG, dipyridyl, and other additives could be modeled independently. Other techniques for estimation of inclusion and adsorption rates could also be explored, possibly using a QCM for in-situ instantaneous measurements.

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Appendix A: Applicability of the Levich Equation to RDE at Low Rotation Speeds

The use of the Levich equation was central to quantifying the effects of transport in this work. One condition for proper application of the Levich equation is that the effects of natural convection must be negligible as compared to the effects of forced convection by the rotating disk electrode. At very low rotation speeds, the convective transport is reduced and natural convection may become significant and even dominant. The lowest rotation speed used in this work was 16 rpm. It must be verified that the effects of natural convection under these conditions are negligible and that application of the

Levich equation to quantify transport rates at this rotation speed is still appropriate. To confirm this, the limiting current on the RDE was measured in an acidified copper solution (0.5 M CuSO4, 0.1 M H2SO4) for rotation speeds from 400 rpm to 16 rpm as shown in Figure A.1.

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35 30 y = 1.5918x

25 R² = 0.9883 ] 2 20

15 [mA/cm 10

5 Limiting Current Density Current Limiting 0 0 5 10 15 20 1/2 [rpm1/2]

Figure A.1. Measured limiting current density for an acidic copper solution (0.5 M CuSO4, 0.1 M H2SO4) for rotation speeds from 400 rpm to 16 rpm. The linear dependence of the limiting current on the square root of the rotation speed (displayed here in terms of the square root of the rpm) indicates the validity of the Levich equation over the entire tested range.

The linear relationship between limiting current density and the square root of rotation speed is in agreement with the Levich equation and confirms that the Levich equation is valid for measurements made at rotation speeds down to 16 rpm.

131