EFFECTS OF TRANSPORT AND ADDITIVES ON ELECTROLESS COPPER PLATING By 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. 2 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 3 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 4 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) 5 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) 6 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) 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’ and ‘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) 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’ and ‘glyoxylic acid only’ half systems. The predicted mixed potential and the predicted current 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