Abrasive-free Copper Chemical Mechanical Polishing in an Orbital Polisher

Transcription

1 Abrasive-free Copper Chemical Mechanical Polishing in an Orbital Polisher Qingjun Qin Advisor: Professor R. Shankar Subramanian Center for Advanced Materials Processing (CAMP) Department of Chemical & Biomolecular Engineering Clarkson University Potsdam, New York 13699

2 Outline Objective Background Experimental Work Model development Results and Discussion Conclusions Suggested future work Acknowledgements

3 Objective To obtain a fundamental understanding of abrasive-free copper Chemical Mechanical Polishing in an orbital tool from both experimental and theoretical perspectives To develop a model accommodating slurry fluid mechanics, chemical reaction at the wafer surface, and mechanical removal by pad asperities to predict removal rate and radial non-uniformity of removal rates on the wafer surface To compare predicted removal rate and WIWNU with experimental data obtained from a SpeedFam-IPEC 676

4 Background Special features of orbital tool Axis of wafer rotation Wafer Pad Center line of platen/pad Orbit circle of the pad center Wafer Pad Platen Slurry Flow Reference: Oliver et al. (24)

5 Slurry (liquid) composition: Experimental Work Experiment conditions Hydrogen Peroxide - 5. wt%; Glycine -.2,.5, and 1. wt%; ph=5.5 Slurry Flow Rate: 2 ml/min Pad: IC (XY grooved hard pad) Pressure: 4 psi Pad Orbit Speed:, 2, and 3 RPM Sample: blanket copper wafer Polisher: SpeedFam-IPEC AvantGaard 676 CMP System Cu film thickness measurement: Prometrix Omni RS 35E

6 Experimental Work Glycine conc. & pad orbit speed Impact Slurry Flow Rate = 2ml/min Pressure = 4 psi H 2 O 2 Concentration = 5. wt% Pad Orbit Speed (rpm) Glycine Concentration (wt%) Flow Rate=2 ml/min Pressure = 4 psi Hydrogen peroxide Conc. = 5. wt% Glycine Conc. (wt%) Pad Orbit Speed (RPM) 4 The removal rate increases with both glycine concentration and pad orbit speed

7 Experimental Work Removal rate on radial positions ml/min 1. wt% Glycine 3 RPM Wafer Center First Circle Second Circle Third Circle Orbit Speed (RPM) ml/min 1. wt% Glycine 4 psi Edge Wafer Center Edge Positions Positions There is a radial non-uniformity of the removal rates on the wafer surface The removal rate is symmetric about the wafer center with higher values in the central and edge areas, but lower values in other regions on the wafer surface

8 Model Development Main idea to predict removal rate.127 Yi () Yi () Yi () Xi () Xi () Xi ().127 cos( w ) ocos( o ) ( ) sin ( ) x = d + ρ β + ω t R θ + ω t y = ρsin β + ω t R θ + ω t w o o Yi () l dm R = f C + f C + V V da Ng N hi m i j i= 1 i j= 1 j m j 3 ( ) ( ) R m Xi () dm3 ( C ) = f + da

9 Model Development Three functional regions on the pad region-iii region-i region-ii ρ+r o ρ-r o region-iii region-i region-ii η = r + Ro ρ η π 1 = arccos 2rRo

10 Model Development Assumptions & Simplifications The slurry flow rate in a pad groove is proportional to the pressure difference between its two ends Q = K ( P 2 -P 1 ); The constant K is assumed to be the same for all pad grooves; The kinetics of copper removal rate as a function of the glycine concentration at the wafer surface (Zhang & Subramanian, 21) ( ) ( /( )) R = f C = C + C moles m s reaction glycine glycine glycine The pressure underneath all the holes in the pad is the same, and is treated as an unknown constant; The resistance to mass transfer in the liquid is neglected; and Glycine is assumed to be consumed instantaneously by reaction with copper ions to form a complex.

11 Model Development Slurry fluid mechanics y i, j+1 pad P hij (, ) x i-1, j i, j i+1, j z Rubber pad backer Q = KΔP i, j-1 steel pad backer Q + q = Q in source out P d K nπ b 2 2 sinh 128a 1 ab 2a 2a = πμl n= 1,3,... n nπ n π nπ b cosh 2a P + P + P + P 4P = i, j+ 1 i+ 1, j i 1, j i, j 1 i, j q Pi, j+ 1 + Pi+ 1, j+ Pi 1, j+ Pi, j 1 4Pi, j= K (, i j) P q P i, j hij (, ) d= Kh q (, i j) = Q total Reference: Shah and London (1978)

12 Model Development Pressure Distribution Pressures near the pad center are much higher than those in the pad edge area The pressure drops fast near the pad edge area Slurry flow rates in each groove can be obtained by using this pressure distribution and Q = K ( P 2 -P 1 )

13 Model Development Glycine concentration in a pad groove C B = ( ) ( +Δ ) = 2 ( ) A exp( 2bWL QAB ) CA exp( 2bWL QAB ) 1 L' =ηl C ( ) QAB C x C x x Wdxf C x

14 Model Development Computation of glycine concentration distribution C 1 C 3 C 1 C 2 C 1 C1 C2 C 2 C Calculation of the glycine concentrations at all intersections from 61 known concentrations at holes is an iterative computation

15 Model Development Glycine concentration distribution Glycine is consumed rapidly near the pad center On average, glycine concentration is larger at the edge than in the pad central area

16 Model Development Average glycine concentration in path annulus Glycine Concentration (mol/m 3 ) 15 C = mol/m 3 C = 66.6 mol/m 3 5 C = 26.6 mol/m Radial Position (m) C g = i i C A gi A g g

17 Model Development Mechanical removal by pad asperities Mechanical Removal Rate (nm/min) 15 5 Glycine concentration (wt%) Pad Orbit Speed (rpm) Mechanical Removal Rate (nm/min) 15 5 Pad orbit speed (rpm) Glycine Concentration (wt%) R = R f C ( ) mech expt model ( ) R= f C + C V model 2.16 model wp

18 Results and Discussion Overall Removal Rate Pad orbit speed (rpm) rpm rpm 3 rpm Glycine Concentration (wt%) Glycine concentration (wt%) wt%.5 wt%.2 wt% Pad Orbit Speed (rpm) The overall removal rate increase linearly with pad orbit speed but non-linearly with glycine concentration The increasing slope is attributed to a synergistic action between the chemical reaction and the mechanical removal

19 Results and Discussion Radial Variation of Removal Rate on Wafer Surface wafer center Sites (a) wafer center Sites (b) wafer center Sites (c) liquid flow rate = 2 ml/min pressure = 4. psi H 2 O 2 concentration = 5. wt% glycine concentration = 1. wt% (a) rpm (b) 2 rpm (c) 3 rpm

20 Results and Discussion Radial Variation of Removal Rate on Wafer Surface wafer center 15 5 wafer center Sites (a) Sites (b) wafer center Sites (c) liquid flow rate = 2 ml/min pressure = 4. psi H 2 O 2 concentration = 5. wt% glycine concentration =.5 wt% (a) rpm (b) 2 rpm (c) 3 rpm

21 Results and Discussion Radial Variation of Removal Rate on Wafer Surface wafer center Sites (a) wafer center Sites (b) wafer center Sites (c) liquid flow rate = 2 ml/min pressure = 4. psi H 2 O 2 concentration = 5. wt% glycine concentration =.2 wt% (a) rpm (b) 2 rpm (c) 3 rpm

22 Conclusions The discrepancy between the experimentally measured removal rate and the predicted chemical removal is attributed to mechanical removal of the reacted film by the pad asperities on the mesas The overall removal rate of copper from the wafer surface depends approximately linearly on the pad orbit speed and non-linearly on the glycine concentration There appears to be a synergy between the chemical action and mechanical action during CMP, which reinforces material removal and can be enhanced by increasing the glycine concentration

23 Conclusions (Continued) The shape of the radial variation of the removal rate depends on the nominal glycine concentration in the experiment, but is independent of the nominal glycine concentration or the pad orbit speed in the modeling For glycine concentrations of.5 wt% and 1. wt%, in most cases, the agreement between the predicted radial variation and the experimental radial variation is good near the but not good in the central region of the wafer Other factors can influence the radial variation of the removal rates

24 Suggested Future Work The constant K in the assumption Q = K ( P 2 -P 1 ) will be different for different pad grooves, and needs to be calculated individually Mass transport in the groove should be accommodated For an accurate prediction of the radial non-uniformity of the removal rate on the wafer surface, the incoming-wafer film uniformity, down-force, wafer curvature, backside pressure, wafer-to-retaining-ring protrusion, retaining ring pressure, pad conditioning, etc. should be considered The mechanical aspects of removal in abrasive-free polishing are worth pursuing in-depth A better understanding of the slurry delivery system is needed to improve the accuracy of the predictions from the model

25 Acknowledgements Professor R. Shankar Subramanian for his guidance throughout this research program both personally and professionally Novellus Systems, Inc. (SpeedFam/IPEC) and Rohm and Haas Electronic Materials CMP Technologies for supplies and advice New York State Office of Science, Technology, and Academic Research (NYSTAR) for financial support Professor Yuzhuo Li and members of his group for their help with the experiments