Abrasive Effects in Oxide Chemical Mechanical Polishing
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ABSTRACT In this study, we have characterized the effects of abrasive properties, primarily particle size, on the Chemical Mechanical Polishing (CMP) of oxide films. Sol-gel silica particles with very narrow size distributions were used for preparing the polishing slurries. The results indicate that as particle size increases, there is a transition in the mechanism of material removal from a surface area based mechanism to an indentation-based mechanism. In addition, the surface morphology of the polished samples was characterized, with the results showing that particles larger than 0.5 pim are detrimental to the quality of the SiO 2 surface. INTRODUCTION Chemical Mechanical Polishing (CMP) has become the industry-wide standard for achieving global planarization of metal and dielectric films in Multilevel Metallization schemes [1]. Oxide CMP, in particular, has been integrated most successfully into the manufacturing environment. However, the fundamental processes underlying the polishing process have yet to be fully understood. Most of our knowledge in this regard has been derived from previous studies on glass polishing [2], [3]. Those results may not be applicable under the conditions of present-day CMP, which utilizes extremely small (submicron sized) abrasives to polish thin metal and oxide films. In addition, the effect of abrasive size on polishing rate and surface smoothness has not been fully understood. Different researchers have obtained different (and
often contradictory) conclusions as to how particle size influences polishing properties of a slurry. Jairath et al. [4] observed that polish rate increased with both particle size and concentration. Xie and Bhushan [5] obtained similar results for polishing of copper and ferrite with diamond and alumina abrasives. However, Cook [2] and Sivaram [6] proposed that polish rate is independent of abrasive size, and Izumitani [3] observed that decreased abrasive size led to higher polish rates for optical glasses. Recent studies conducted by us [7] on particle size effects in Tungsten CMP have shown that polish rate increases as particle size is reduced. In this paper, the results of a systematic study on the effect of particle size on polish rate and surface roughness in silica CMP will be presented. EXPERIMENT Sol-gel silica particles of four different sizes, viz. 0.2 jam, 0.5 jim, 1.0 lam and 1.5 Pm were obtained from Geltech® Corporation. Particle size analysis on the slurries was carried out using a Honeywell Microtrac® UPA 150 particle size analyzer, which utilizes the dynamic light scattering technique. In addition, Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) was also used for determining particle size and shape. Polishing experiments were carried out p-type Silicon wafers on which a 1.5 pm thick Si0 2 layer was deposited by PECVD. Slurries were prepared by dispersing the silica particles in DI 27
Mat. Res. Soc. Symp. Proc. Vol. 566 ©2000 Materials Research Society
water, using an ultrasonic probe to break up agglomer
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