Abrasive wear polishing

Wear is one of the most important parameters in evaluating the CMP process. Wear in CMP is evaluated as the material removal rate (MRR). The primary wear mechanisms that occur in CMP are adhesive wear, abrasive wear, electrochemical wear, tribochemical wear, and fatigue wear on both wafer and pad surfaces. In this chapter, we will first introduce basic wear concepts. We will then discuss wear in polishing and in conditioning. Throughout the text, we show examples of CMP failure due to wear. 

The governing principle of pad conditioning is to introduce friction between the polishing pad and the diamond disc, which characterizes a two-body abrasive wear mechanism. As illustrated in Figure 13.3, the diamond abrasives embedded on the disc create microscopic cuts or furrows on the pad surface to continually regenerate new pad surface and asperities. At the same time, they remove the glazed or accumulated particles on the polishing pad surface. 

As conditioning is primarily considered as a mechanical process characterized by a two-body abrasive wear mechanism [8], the classical Preston equation [62], originally used to model polishing of glass, has been widely used to describe material removal (polishing) rate in [61]. Considering the similarity between wafer—pad interaction and pad—conditioner interaction, the Preston s equation has been adopted by many to model pad wear caused by conditioning. The Preston equation states that MRR is proportional to the applied pressure P and the relative velocity V between the wafer and the pad and Kp is a constant, called Preston s coefficient. 

Abrasive wear Aluminum oxide PA Slush-pump piston tods, polish rod... 

In 1996, Liu et al. [129] analyzed the wear mechanism based on the rolling kinematics of abrasive particles between the pad and wafer. They summarized that the kinetics of polishing are (1) material removal rate is dependent on the real contact area between the slurry particle and the wafer surface. The real contact area is related to the applied pressure, the curvature, and Young s modulus of the slurry... 

Xie, Y. and Bhushan, B., "Effects of Particle Size, Polishing Pad and Contact Pressure in Free Abrasive Polishing, Wear, Vol. 200,1996, pp. 281-295. 

Larsen, B. J. and Liang, H., "Probable Role of Abrasion in Chemo-Mechanical Polishing of Tungsten, Wear, Vol. 233-235,1999, pp. 647-654. 

Ahmadi, G. and Xia, X., "A Model for Mechanical Wear and Abrasive Particle Adhesion During the Chemical Mechanical Polishing Process," Journal of the Electrochemical Society, Vol. 148, No. 3,2001, pp. G99-G109. 

Minch s method is designed for determination of the abrasive power of powders and polishing micropowders, grain-size 160-3 (am. The test outfit miniaturized when compared to Bohme s disc, shows an average of three times lower wear of the glass plate for the same abrasive action time and the same grain size (A. Szymanski, 1972). 

Ahmadiz G, Xia X. A model for mechanical wear and abrasive particle adhesion during the chemical mechanical polishing process. J Electrochem Soc 2001 148(3) G99-G109. 

In the extreme case where the load is supported entirely by solid-solid contact, frictional wear will be at a maximum. However, the transport of slurry under the wafer will be poor, resulting in a limited amount of chemical activity and little lubrication effect. Under such conditions elevated temperatures would be expected and mechanical abrasion would dominate. As a consequence, the polished surface is likely to be severely damaged. 

Section 4.4.2 further separates the polishing mode of abrasion into two submodes, that of Hertzian indentation based wear and fluid-based wear. The difference between these two polishing modes is the nature of the slurry fluid layer between the pad and the wafer. This area of CMP is still poorly understood, yet has important implications as to the removal mechanisms of CMP. 

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