Silicon wafers, mechanical polishing

Zhao, Y. W. and Chang, L., AMicro-Contact, and Wear Model for Chemical-Mechanical Polishing of Silicon Wafers, Wear, Vol. 252,2002, pp. 220-226. 

Polishing uses abrasive compounds and mechanical action to remove contaminants. Although used extensively in preparing glass substrates for chromium masks and in obtaining the desired flatness of silicon wafers, it is seldom, if ever, used once the silicon wafer enters a process sequence. 

Mullany B, Byrne G. The effect of slurry viscosity on chemical-mechanical polishing of silicon wafers. J Mater Process Technol 2003 132 28-34. 

Zhao Y, Chang L. A micro-contact and wear model for chemical-mechanical polishing of silicon wafers. Wear 2002 252 220-226. 

One of the first and die most widely used CMP process, aside from the final step in the preparation of silicon wafers, is oxide CMP for back-end planarization after the initial oxide ILD deposition and between metal levels. As a result, oxide CMP is the most mature process, with the most fundamental studies having been performed in this area. Indeed, much of our understanding of the CMP of metals and other materials is derived from our understanding of oxide CMP. This chapter first presents the current understanding of the oxide CMP fundamentals. The discussion includes the mechanisms of both mato-ial removal and surface planarization. The second part of the chapter is devoted to the practice of oxide CMP, including reported results on planarization and polish rate performance of oxide CMP processes in industry. In addition, process integration, cost of ownership, manufacturability, and yield issues will be discussed. 

This paper has summarized some of the results related to the fluid pressure distribution between a hard (steel) surface and a soft compliant (polyurethane) surface. The results are pertinent to the mechanical aspects of chemical mechanical polishing of silicon. The results show that the model can predict a subambient pressure. This is due to the fact that the leading edge of the wafer squeegees the slurry, and the stress distribution beneath the wafer is non-uniform. 

Alkaline slurries of precipitated silicas are used as polishing agents for semiconductor silicon wafers. The mechanism of polishing is generally believed to be chemical-mechanical the high pH of the slurry leads to oxidation of the silicon surface, followed by mechanical removal of the oxidized layer under the action of the silica particles and the polishing pad. In recent years the use of precipitated silicas in this application has largely been supplanted by the use of silica sols. 

As depicted in Pigs. 9.11 and 9.12, the BM investigations reveal broad, mechanically stiffened interphases in the EP adjacent to all the metal films. The v level of these interphases is almost the same but the position of the peak maximum and the half-width at half-maximum (HWHM) depend on the kind of metal. The EP interphases on aluminum and copper are almost twice as wide as on magnesium and gold. In contrast to the polished metals considered above, the adhesion strength is good enough to maintain the EP-metal joint in the mechanical test. The samples fail between the metal film and the silicon wafer. 

Silicon-Silicon Wafer Bonding A clean and polished silicon wafer can be bonded to another silicon wafer at room temperature [3]. Two bonding mechanisms exists (1) hydrophilic silicon surfaces will allow hydrogen bonds to form between two wafers with thin silicon dioxide layers and (2) hydrophobic silicon stufaces will allow van der Waals interactions between two clean silicon surfaces. Thermal annealing at high temperature (>1,000 °C) is then required to permanently bond the silicon wafers. 

Kang et al. [6] studied the effect of slurry pH during polishing of silicon wafer and polycrystalline silicon films using abrasive-free and silica slurries to understand and compare the polishing mechanism of sUicon. They noticed that the poly-Si CMP process was strongly influenced by mechanical factors however, bare silicon wafer polishing was influenced more by chemical effects. 

On the positive side, tribocorrosion phenomena can be used as a manufacturing process, such as in the chemical mechanical polishing (CMP) of silicon wafers. An enhanced material removal for CMP processes has been achieved using electrochemical techniques [2]. The coupling of mechanical and environmental effects can also create surfaces of specific reaction layers on materials which could inhibit corrosion and/or wear. Examples of this are self-lubricating and/or self-healing surface layers [3]. 

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