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CHARACTERIZATION OF COPPER ELECTROPLATING AND
ELECTROPOLISHING PROCESSES FOR SEMICONDUCTOR INTERCONNECT
METALLIZATION

by
JULIE MARIE MENDEZ

Submitted in partial fulfillment of the requirements
For the degree of Doctor of Philosophy

Dissertation Advisor: Dr. Uziel Landau Department of Chemical Engineering
CASE WESTERN RESERVE UNIVERSITY

August, 2009

CASE WESTERN RESERVE UNIVERSITY
SCHOOL OF GRADUATE STUDIES

We hereby approve the thesis/dissertation of

Julie Mendez

_____________________________________________________

PhD

candidate for the ______________________degree *.

Uziel Landau

(signed)_______________________________________________
(chair of the committee)

Heidi B. Martin

________________________________________________

Frank Ernst

________________________________________________

Chung-Chiun Liu

________________________________________________ ________________________________________________ ________________________________________________

May 12, 2009

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

TABLE OF CONTENTS

Page Number

List of Tables List of Figures Acknowledgements List of Symbols Abstract
34910 13

  • 1. Introduction
  • 15

15 20 22 24
1.1 Semiconductor Interconnect Metallization – Process Description 1.2 Mechanistic Aspects of Bottom-up Fill 1.3 Electropolishing 1.4 Topics Addressed in the Dissertation

2. Experimental Studies of Copper Electropolishing
2.1 Experimental Procedure
26 29 30 34 34 36 38 40 42 44 49 51 52
2.2 Polarization Studies 2.3 Current Steps
2.3.1 Current Stepped to a Level below Limiting Current 2.3.2 Current Stepped to the Limiting Current Plateau 2.3.3 Effect of Current Density 2.3.4 Effect of Rotation Speed
2.4 Highly Resistive Surface Film 2.5 Electrochemical Impedance Spectroscopy 2.6 Stability of the Film in Presence of Chloride 2.7 Two-Compartment Cell Experiments 2.8 Conclusions

3. A Mechanistic Model for Copper Electropolishing
3.1 Regime I – Buildup of Surface Copper Ion Concentration 3.2 Regime II – Controlling Transport through a Surface Layer 3.3 Model Verification
53 53 57 60 67 67 72
3.3.1 Time Delay Prior to the Onset of the Sharp Potential Increase 3.3.2 Effect of Water Concentration on the Limiting Current
3.4 Conclusions

4. Novel Polyether Suppressors Enabling Copper Metallization of High Aspect Ratio

  • Interconnects
  • 73

75 76 79 89 96 98
4.1 Experimental Procedure 4.2 Results and Discussion
4.2.1 Polarization Data 4.2.2 Modeled Via-fill Ratio 4.2.3 Interaction with the Anti-suppressor
4.3 Conclusions

1
5. Mechanistic Studies of Polyether Adsorption
5.1 Experimental Details
99 101
5.1.1 Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy

  • (ATR-FTIR)
  • 101

103 106 118 125 127
5.1.2 Quartz Crystal Microbalance (QCM)
5.2 ATR-FTIR Studies of PEG Adsorption 5.3 Quartz Crystal Microbalance 5.4 Effect of Cu+ on Copper Deposition 5.5 Conclusions

6. Major Conclusions and Recommendations for Future Work
6.1 Major Conclusions
128 128 128 129 131
6.1.1 Electropolishing 6.1.2 Novel Polyethers Extending Gap-fill Capabilities
6.2 Recommendations for Further Studies

  • Bibliography
  • 135

2

LIST OF TABLES

Page
Number

  • 48
  • Table 2.1. Ohmic and polarization resistances measured by electrochemical

impedance spectroscopy.

  • Table 3.1. Copper electropolishing model parameters. Justification for the
  • 61

estimated values is given in the text.

  • Table 4.1. Polyethers explored in the polarization studies.
  • 77

  • 90
  • Table 4.2. Kinetics parameters fitted to polarization data in Figure 4.1 and

Figure 4.2.

3

LIST OF FIGURES

Page
Number

  • 17
  • Figure 1.1. Schematic detailing the dual Damascene process. Both (a) the

via and (b) the trench are etched into the insulator. (c) A diffusion barrier layer and copper seed layer are both deposited by physical vapor deposition. (d) Copper is electrodeposited to fill the via and trench. (e) The overburden copper is removed by chemical mechanical planarization (CMP).

Figure 1.2. Schematic of the transport and diffusion processes inside the via. (a) PEG adsorbs quickly but is diffusion limited; therefore, it adsorbs primarily at the top of the via sidewalls. SPS diffuses quickly but adsorbs more slowly than PEG and adsorbs primarily at the via bottom. (b) As the fill progresses, the bottom surface contracts. The SPS becomes more concentrated at the bottom, bringing about rapid growth at the via bottom.
19

  • 28
  • Figure 2.1. Schematic of concentration profiles in the two major proposed

mechanisms for copper electropolishing. In mechanism (a), an acceptor species (indicated as water) diffuses towards the anode, complexes there with the discharged cupric ions, and diffuses back (as a complex) toward the bulk solution. According to mechanism (b), the concentration of cupric ions increases at the anode until the solubility limit is reached, at which point, a solid film is formed on the anode. Cupric ions then diffuse towards the bulk across a mass transport boundary layer of thickness δ.

Figure 2.2. Polarization curves for electropolishing of a copper disk electrode at various rotation speeds (A – 50 rpm, B – 100 rpm, C – 200 rpm, D – 400 rpm, E – 600 rpm) in 85 wt% phosphoric acid. The potential was scanned at 10 mV/s.
32

  • 33
  • Figure 2.3. Limiting current density for phosphoric acid solutions of various

water concentrations at 800 rpm. The linear relationship between limiting current density and bulk water concentration has led numerous investigators to associate it with the transport of an acceptor species (water) toward the anode.

Figure 2.4. Potential response to a current pulse at 100 rpm (a) below the limiting current (6.3 mA/cm2), and (b) at the limiting current (19.6 mA/cm2). The current is stepped up to the specified value at 100 s, held at that value for 400 s, and then stepped down to zero at 500 s. Note that the potential scales in (a) and (b) are quite different.
35
4
Figure 2.5. Potential transient responses to current steps from zero to three values on the limiting current plateau. The current was stepped to (A) 18.8 mA/cm2; (B) 17.8 mA/cm2; (C) 17.0 mA/cm2 at 100 s. The disk was rotated at 100 rpm.
39

  • 41
  • Figure 2.6. Potential responses to currents steps from zero to 19.6 mA/cm2

(at the limiting current) for various rotation speeds: A – 90 rpm, B – 100 rpm, C – 110 rpm. The time delay prior to the sharp potential increase shows a strong dependence on the rotation speed.

Figure 2.7. Nyquist plots at various applied potentials below the limiting current at 400 rpm. The ohmic resistance remains approximately constant, but the polarization resistance decreases as the potential is increased.
46

  • 47
  • Figure 2.8. Nyquist plots at an applied potential of 1.3 V vs. copper (at the

limiting current plateau) and various rotation speeds. These measurements indicate an ohmic resistance of approximately 2.8 Ω-cm2 and a polarization resistance between 1.6 and 3.4 Ω-cm2, which decreases with increasing rotation speed.

Figure 2.9. Potential response to current step of 14.2 mA/cm2 (near the limiting current) in 85 wt% H3PO4 solution containing 100 ppm HCl. The potential oscillations suggest the formation and breakdown of a film.
50

  • 54
  • Figure 3.1 Schematics representing (a) Regime I and (b) Regime II of the

proposed model. In Regime I, the concentration at the anode increases until Csat is reached. In Regime II, a flux imbalance leads to the buildup in thickness (x) of a surface film.

Figure 3.2. Comparison of measured and modeled overpotential response to a current pulse in Regime I. The measured data are from Figure 2.4a, while the predicted response is based on the numerical solution of Eqs. [3.1] and [3.5]. The two curves are in reasonable agreement. The small deviation can probably be attributed to the spatial distribution of the copper ions.
63
Figure 3.3. Comparison of the model governing Regime II (Eq. [3.12]) to the experimental potential response displayed in Figure 2.4b (region C-D). Note the highly expanded time scale.
65

  • 66
  • Figure 3.4. Sensitivity analysis for parameters A and M (Eq. [3.12]). The

lines indicate values for these parameters such that the model correlates the data from Figure 2.4b (region C-D) within the indicated percentages.

5
Figure 3.5. The model (Eq. [3.22]) predicts a nearly linear relationship between the limiting current density and the bulk water concentration for copper electrodissolution in phosphoric acid. Also indicated is a linear approximation for Eq. [3.22].
71

  • 80
  • Figure 4.1. Polarization data for solutions containing 0.5 M CuSO4 (pH~2),

70 ppm Cl-, and 100 ppm of the specified polyether, all with molecular mass of approximately 1000 g/mol. The points for polyoxyethylene lauryl ether (diamonds) and polyoxyethylene cetyl ether (circles) fall nearly on top of one another.

Figure 4.2. Polarization data for solutions containing 0.5 M CuSO4 (pH~2), 70 ppm Cl-, and 100 ppm of the specified polyether with molecular mass of approximately 600 g/mol.
82 83 85 86 88
Figure 4.3. Polarization data for additional solutions containing 0.5 M CuSO4 (pH~2), 70 ppm Cl-, and 100 ppm of the specified polyether with molecular mass of approximately 600 g/mol.

Figure 4.4. Polarization data for solutions containing 0.5 M CuSO4 (pH~2), 70 ppm Cl-, and 100 ppm of the specified polyether with molecular mass of approximately 300 g/mol.

Figure 4.5. Polarization data for solutions containing 0.5 M CuSO4 (pH~2), 70 ppm Cl-, and 100 ppm of the specified polyether with molecular mass in the range of approximately 2000 g/mol to 4000 g/mol.

Figure 4.6. Overpotentials at 5 mA/cm2 as a function of the number of ether oxygen atoms in the polyether. The circles correspond to PEG, and the squares are all other polyethers studied (listed in Table 4.1). Although some trend is indicated between increased overpotential and the number of ethereal oxygen atoms, the data spread implies that other factors, including the chemical structure, are important.

Figure 4.7. Bottom-up fill ratio (iB/iSW) as a function of total current simulated for a 300 mm wafer with 15% feature loading (pattern density) for solutions containing one of the following suppressors: PEG 1000, polyoxyethylene lauryl ether, or polyoxyethylated β-naphthol. Significant improvement (3 ~ 4x) is expected by replacing PEG with either of the above listed other polyethers.
93
6
Figure 4.8. SEM cross-sections of vias with aspect ratio close to 10 electroplated with copper after PVD seed deposition.48 Fill quality is compared for copper plating in the presence of two different suppressors: (a) PEG 1000, exhibiting center-line voids due to inferior bottom-up fill rate (Figure 4.7), and (b) polyoxyethylated (POE) β-naphthol, indicating voidfree fill on account of its improved suppression and higher bottom-up fill rate.
95

  • 97
  • Figure 4.9. Voltage transient response at 5 mA/cm2 to SPS injections into

polyether polarized deposition. The solution initially contained 0.5 M CuSO4 (pH~2), 70 ppm Cl-, and 100 ppm of the specified polyether. After steady-state was reached, 10 ppm SPS was injected into the solution. Polyoxyethylated β-naphthol has a similar voltage response to that of PEG.

Figure 5.1. Schematic of ATR-FTIR system (not to scale). The laser beam is introduced from the back, through a thin Si wafer coated with Cu. Only a thin region (~1 μm) at the electrode/electrolyte interface is sampled.
102 105 108
Figure 5.2. Schematic of QCM cell (not to scale). The top electrode is exposed to the solution, while the bottom electrode is in contact with air, serving as the reference.

Figure 5.3. Spectrum for solid PEG relative to ZnSe-air. The most prominent peaks are the C-O-C stretch at 1109 cm-1 and the C-H stretch at 2885 cm-1. Since it is believed that the ethereal oxygen is important in the adsorption of polyethers, the C-O-C stretch peak is of interest.

Figure 5.4. Spectrum for solid cupric sulfate pentahydrate relative to ZnSeair. The most prominent peaks are at 670, 3630, and 3730 cm-1. There is also a peak at 1090 cm-1, which is near the C-O-C stretch peak of PEG at 1100 cm-1.
109 110 112
Figure 5.5. Spectrum for 0.5 M CuSO4 and 70 ppm Cl- on a Cu substrate, relative to water. A prominent sulfate peak is evident at 1100 cm-1, near a peak of interest for PEG. The peaks at 3280 and 1640 cm-1 are due to error in referencing to the spectrum for water on Cu.

Figure 5.6. FTIR spectra for solutions containing 0.5 M CuSO4, 70 ppm Cl-, and 100 ppm PEG 1000 on Cu (bottom) and on Si (top) substrates. The spectra indicate similar behavior on both Si and Cu. The spectra were shifted vertically for clarity.

7
Figure 5.7. Spectra for various solutions on Cu. The solutions contain the specified components in the following concentrations: 0.5 M CuSO4, 70 ppm Cl-, 100 ppm PEG 1000, and 0.67 M Na2SO4. No peak is observed with a solution of 100 ppm PEG and 70 ppm Cl- in water.
114
Figure 5.8. FTIR spectrum for a solution containing 0.5 M CuCl2 and 100 ppm PEG 1000, relative to 0.5 M CuCl2. The C-O-C stretch peak of PEG at 1100 cm-1 can be detected in the absence of sulfate.
115

  • 117
  • Figure 5.9. FTIR spectrum for a solution containing 0.5 M CuSO4, 70 ppm

Cl-, and 100 ppm PEG 1000, relative to 0.5 M CuSO4 and 70 ppm Cl-. Different solutions were introduced using a flow cell. A peak associated with the C-O-C stretch of PEG is evident at 1100 cm-1.

Figure 5.10. Frequency response upon addition of small volumes of 0.5 M CuSO4 to the cell initially containing 0.5 M CuSO4 solution. The arrows indicate times when additional solution was introduced into the cell. The frequency signal stabilizes to nearly the original value, indicating that additional solution does not affect adsorption on the substrate.
119

  • 121
  • Figure 5.11. Frequency response upon addition of Cl- and PEG to a 0.5 M

CuSO4 solution. When 70 ppm Cl- was added to the cell, a change in frequency difference of ~30 Hz, corresponding to ~130 ng/cm2, was observed. When 100 ppm PEG (molecular mass 1000 g/mol) was added to the Cl--containing solution, a similar change in frequency difference of ~30 Hz (~130 ng/cm2) was observed.

Figure 5.12. Frequency response upon addition of PEG and Cl- to a 0.5 M CuSO4 solution. When 100 ppm PEG (molecular mass 1000 g/mol) was added, a small decrease in the frequency difference was observed. When 70 ppm Cl- was added to the PEG-containing solution, an increase in frequency difference of ~50 Hz (~220 ng/cm2) was observed.
123

  • 126
  • Figure 5.13. Overpotential response to injection of 10 ppm Cu+ into

acidified (pH~2) 0.5 M CuSO4 solutions containing the indicated additives. The cathode was polarized at 10 mA/cm2. When Cu+ is added to a solution containing no additives, the overpotential decreases by ~30 mV. No change in overpotential is observed when Cu+ is added to solutions containing PEG or PEG and Cl-.

8

Acknowledgements

I would like to thank Professor Uziel Landau for his guidance in the completion of this dissertation. I would also like to thank the other members of my committee, Professor Heidi Martin, Professor C. C. Liu, and Professor Frank Ernst for helpful comments and suggestions regarding this work.
Funding from the Intel Research Council supported this work. I would like to thank Rohan Akolkar at Intel for his collaboration in this work and for the opportunity to spend a summer working at Intel. I also thank Tatyana Andryushchenko at Intel for contributions to the electropolishing work.
I would like to thank Jim Adolf for helpful suggestions regarding the polyether work and the students in the Landau Group for their helpful comments and support. Professor Heidi Martin assisted with the FTIR measurements. Professor Kathleen Kash provided assistance with electron beam deposition. Professor Jim Burgess and Professor Robert Savinell provided use of QCM equipment. Craig Virnelson and Dan Shelberg assisted with data acquisition for the QCM. Cliff Hayman and the students in the Diamond Lab have helped with various things in the lab. Professor E. Gileadi and Professor J. Newman are acknowledged for helpful discussions regarding electropolishing.
I appreciate the support and encouragement from my parents, sister, and extended family. I am grateful to the friends I have made here in the last four years.
To J.D., my husband, thank you for your understanding and for always being there for me.

9

List of Symbols

film resistance parameter (Eq. [3.11])

A

AB Asupp

B

area at bottom of features area at feature sidewalls and wafer top surface film resistance parameter (Eq. [3.11]) Cu2+ concentration

C

Cb Ce

bulk Cu2+ concentration Cu2+ concentration near the electrode concentration of water

CH2O

CH3PO4 concentration of phosphoric acid

Cm Csat

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    Electrochemical Kinetics of Corrosion and Passivity The basis of a rate expression for an electrochemical process is Faraday’s law: Ita m = nF Where m is the mass reacted, I is the measured current in ampere, t is the time, a is the atomic weight, n the number of electrons transferred and F is the Faraday constant (96500 Cmol-1). Dividing Faraday’s law by the surface area A and the time t leads to an expression for the corrosion rate r: m ia r = = tA nF With the current density i defined as i = I/A. Exchange current density: We consider the reaction for the oxidation/reduction of hydrogen: rf + - 2H + 2e H2 rr 0 + This reaction is in the equilibrium state at the standard half cell potential e (H /H2). This means that the forward reaction rate rf and the reverse reaction rate rr have the same magnitude. This can be written as: i a r = r = 0 f r nF In this case is i0 the exchange current density equivalent to the reversible rate at equilibrium. In other words, while the standard half cell potential e0 is the universal thermodynamic parameter, i0 is the fundamental kinetic parameter of an electrochemical reaction. The exchange current density cannot be calculated. It has to be measured for each system. The following figure shows that the exchange current density for the hydrogen reaction depends strongly on the electrode material, whereas the standard half cell potential remains the same. 1 Electrochemical Polarization: Polarization η is the change in the standard half cell potential e caused by a net surface reaction rate.
  • An Investigation Into Aqueous Titanium

    An Investigation Into Aqueous Titanium

    An Investigation into Aqueous Titanium Speciation Utilising Electrochemical Methods for the Purpose of Implementation into the Sulfate Process for Titanium Dioxide Manufacture Samala Shepherd, BSc. (Hons) Masters of Philosophy in Chemistry University of Newcastle March, 2013 STATEMENT OF ORIGINALITY This thesis contains no material which has been accepted for the award of any other degree or diploma in any university or other tertiary institution and, to the best of my knowledge and belief, contains no material previously published or written by another person, except where due reference has been made in the text. I give consent to this copy of my thesis, when deposited in the University Library**, being made available for loan and photocopying subject to the provisions of the Copyright Act 1968. **Unless an Embargo has been approved for a determined period. Samala L. Shepherd i Acknowledgements There are many people who have helped me and contributed to my work in a number of ways and I’d like to thank them. I’d like to thank BHP Billiton Newcastle Technology Centre for making this project possible. The ARC for support and funding. Dr. Scott Donne for the vast knowledge he provided me with and the friendship and support. Thank you to Carolyn Freeburn, Vicki Thompson, and Stephen Hopkins for the ‘store/equipment room’ when I was in need. Thank you to Dianna Brennan for keeping me supplied. Michael Fitzgerald for his continued support. Last but not least a big thanks to my family, they are stuck with me but bare the burden with smiles and support and I thank them greatly.
  • The PCB Magazine, May 2015

    The PCB Magazine, May 2015

    & ETCHING Electroplating May 2015 PLATING 22 Through-Holes with Different Geometry: A Novel and High-Productivity Process ENEPIG: 44 The Plating Process May 2015 • The PCB Magazine 1 13th ANNUAL MEPTEC MEMS TECHNOLOGY SYMPOSIUM Enabling the Internet of Things: Foundations of MEMS Process, Design, Packaging & Test MEMS based products are key en- ablers in the Internet of Things (IoT) Wednesday, May 20, 2015 revolution. The availability of large numbers of reliable and cost-effec- Holiday Inn San Jose Airport • San Jose, California tive MEMS sensors and actuators KEYNOTE SPEAKER has driven an explosion in the num- DIAMOND SPONSOR ber of IoT applications to reach the Creating a Trillion Sensors Based Future market. The Internet of Things is Dr. Janusz Bryzek pushing MEMS technology to new Chair, TSensors Summits levels of performance and bringing forth new requirements. Sensors are one of the eight exponential technologies enabling growth of goods and services faster than This event will showcase advances growth of demand for them. Exponential technologies enable Exponen- in core technologies that form the tial Organizations (ExO), which demonstrate sales growth to a billion SILVER SPONSOR foundation of the creation of MEMS- dollars in one to three years. New Exponential organizations are expected based products. Experts from the to replace 40% of Fortune 500 companies in the coming decade, in a field will present the latest innova- similar mode to Kodak replacement by Instagram in 2012. Presented will tions in MEMS fabrication processes, an amazing showcase of available sensor based products. packaging, assembly, & test. Insight will be provided as to new technolo- EXHIBITING COMPANIES TO DATE gies, materials and software that will fuel the creation of new devices cou- pled with traditional MEMS technol- ogies to address new markets and new requirements for the Internet of Things.