Effects of Nano-scale Colloidal Abrasive Particle Size on SiO 2 by Chemical Mechanical Polishing
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Effects of Nano-scale Colloidal Abrasive Particle Size on SiO2 by Chemical Mechanical Polishing Chunhong Zhou*, Lei Shan*, S.H. Ng*, Robert Hight*, Andrew.J. Paszkowski** and S. Danyluk* *The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405 **Chemical Products Corporation (dba Precision Colloids, LLC), Cartersville, GA 30120 Abstract This paper reports on the effect of colloidal abrasive particle size in the polishing of thermally grown silicon dioxide on 100mm diameter, P-type, (100), single crystal silicon wafers. The abrasive particle sizes were varied in six (6) slurries with pH values of 10.97 ± 0.08. The abrasive sizes were 10, 20, 50, 80, 110 and 140nm in diameter, and the slurry contained 30 weight percent abrasives. The experimental results indicate that the material removal rate (MRR) varies with the volume of the particle size. Results also confirm that there exists an optimum abrasive particle size with respect to material removal rate and surface finish. For a pad surface roughness of 5.2µm (Ra), the slurry containing 80nm particles resulted in the highest material removal rate and best surface finish. A nano-film model based on the pad roughness is used to explain the results. 1. Introduction Chemical Mechanical Polishing (CMP) is a critical technology in the manufacture of semiconductor devices. CMP is used for planarization and process control of shallow trench isolation, metal gates, Cu-based interconnects and low “k” dielectrics [1,2]. CMP is done by “sliding” a silicon wafer of a partially processed IC on a polyurethane pad flooded by an abrasive slurry. In spite of the wide use of CMP, there is still little knowledge about the fundamentals of material removal by abrasive slurries. The accepted mechanism for CMP was derived from glass polishing [3] and was based on empirical results. One fundamental issue is the influence of abrasive particle size on the material removal rate and surface finish. Some researchers found that the polishing rate increases with both particle size and concentration [4,5] while others claimed that a decreased particle size leads to a higher polishing rate [6,7]. Cook [8] proposed that particle size had no effect on polishing rate. Mahajan et al. [9] proposed that the CMP process was based on two removal mechanisms: an indentation-based mechanism which dominates material removal for large particles, and a contact area based mechanism which dominates for small (sub- micron) partic les. This paper presents the experimental results of polishing SiO 2 with slurries containing different abrasive particle sizes. A nano- film model based on the pad roughness has been developed that explains the experimental results.
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2. Experimental Investigation Six slurries, manufactured by the Chemical Products Corporation (CPC), each with a different mean abrasive size, were used to polish 100mm diameter, (100), p-type polished silicon wafers that contained a thickness of 1.3 µ m thermally grown oxide. The silica particles
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