Chunhong Zhou

Influence of Colloidal Abrasive Size on Material Removal Rate and Surface Finish in SiO2 Chemical Mechanical Polishing

This paper addresses the influence of nano-scale abrasive particle size in the polishing of thermally-grown silicon dioxide on 100 mm diameter, p-type, (100), single crystal silicon wafers. The abrasive particles are incorporated in a chemical slurry, which is used in chemical mechanical polishing (CMP). Polishing (material removal) rate was measured with six (6) slurries, each with a different mean abrasive particle diameter of 10, 20, 50, 80, 110 and 140 nm. The experimental results indicate that the material removal rate (MRR) is related to the particle size. Results confirm that there exists an optimum abrasive size (80 nm) with respect to material removal rate and surface finish, for a given set of experimental conditions. The variation of the MRR vs. particle size (on a log-log plot) varies as d3.4. In addition, the surface polished with 10 nm and 20 nm abrasives was stained by the slurries. For a pad surface roughness of 5.2 μm (Ra), the slurry containing the 80 nm particles resulted in the highest material removal rate and best surface finish. The effects of polishing conditions and abrasive concentrations are also investigated in this paper. The nano-film polishing model that relates the particle size to the roughness of the pad can explain the polishing mechanism. Presented at the 56th Annual Meeting in Orlando, Florida May 20–24, 2001

Mechanical interactions and their effects on chemical mechanical polishing

Mechanical interactions, such as contact stress and fluid pressure are of extreme importance in silicon wafer polishing, especially for the wafer-scale planarity of the finished surfaces. In this paper, the measurements of interfacial fluid pressure and friction, as well as their dependence on some major process variables, are presented. A nonuniform subambient fluid pressure was measured, and the resulting wafer/pad contact stress, obtained by combining the effects of both applied normal load and interfacial fluid pressure, is determined. An analytical model was developed to predict the magnitude and distribution of the interfacial fluid pressure. The results of polishing experiments show good evidence of the effects of this subambient fluid pressure on with in-wafer nonuniformity (WIWNU). By properly designing the polishing process variables, the fluid pressure may be tailored, and a relatively uniform material removal can be achieved.

Pad Soaking Effect on Interfacial Fluid Pressure Measurements During CMP

Pad Soaking Effect on Interfacial Fluid Pressure Measurements During CMP Prior work has shown that there exist a sub-ambient fluid pressure at the interface between a rigid flat and the polishing pad during chemical mechanical polishing (CMP). This sub-ambient fluid pressure can have a significant impact on the polishing process since its magnitude may be similar to the applied load, depending on conditions. Further results have shown that there is a relationship between pad soaking time and the magnitude of this sub-ambient fluid pressure. This paper addresses measurements of the pad soaking time versus the magnitude of the sub-ambient interfacial fluid pressure. Experiments utilized a Rodel IC1000 polishing pad made of foamed polyurethane with average void size of 30 to 50 microns. Pad soaking tests indicated that the weight of the pad increased with soaking time due to water absorption. There is a high rate of water absorption initially before the pad becomes saturated and the mass of the pad stabilizes. It is also observed that the pad material is impermeable to water and most of the water penetrated only the topmost layer of voids in the material. These experiments suggest that the water progressively softens the top layers of the pad during the soaking and causes the sub-ambient fluid pressure to increase in magnitude. A model of the sub-ambient fluid pressure increasing as the elastic modulus of the pad decreases is also suggested.

Effects of Nano-scale Colloidal Abrasive Particle Size on SiO 2 by Chemical Mechanical Polishing

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.

Nanoparticulate and Interfacial Mechanics in Confined Geometries Typical of Chemical-Mechanical Planarization

Chemical-mechanical planarization (CMP), a surface preparation process used widely in integrated circuits manufacture, is currently the leading nanoscale manufacturing process worldwide, with an annual economic impact well in excess of

Fluid pressure and its effects on chemical mechanical polishing

1 billion. Originally developed for glass polishing, CMP is used by the microelectronics industry to create silicon, silicon oxide, tungsten and copper surfaces with average roughnesses of O(10 mm). The process typically involves shearing a dilute abrasive silica or ceria nanoparticle-laden “slurry” between a compliant rough surface (the “pad”) and the surface to be polished (the “wafer”). The composition of the slurry can greatly affect material removal rates. Despite its importance, however, a lot still remains to be discovered about the fundamental mechanisms involved in this process. A multidisciplinary effort at Georgia Tech has focused upon the interfacial mechanics of this process and how nanoparticles chemomechanically wear SiO2 , Si and Cu surfaces. It has been found, for example, that the wear rate of dielectric varies approximately as the particle diameter. The entrapment of particles at the asperity/dielectric interface is thought to produce the polishing, but the exact nature of this interaction is still unknown. An evanescent-wave visualization technique has therefore been developed to visualize the dynamics of fluorescent 300–500 nm diameter colloidal silica and polystyrene particles within a particle diameter of the “wafer” surface in a simplified model pad-wafer geometry. The technique has been used for the first time to the authors’ knowledge to directly measure the velocity and concentration of the interfacial particles—which presumably interact with and wear the wafer. Although the pad speeds in these studies are much lower than those encountered in the actual CMP process, the initial results suggest that there is negligible “slip” between the particle and fluid phase velocities at the wafer surface. The number of particles at the wafer surface appears, however, to be strongly affected by particle properties, including particle density and size.Copyright