Pad surface roughness

During the course of the experiment, pad surface roughness was measured by contact profilometry. Removal rate was found to be directly proportional to pad roughness (Fig. 11). Microscopic examination of the pad surface clearly showed the progressive smoothing away of the upper pad asperities (Figs. 12a-12d). No evidence of shear deformation of the asperities was... 

A number of researchers have indicated the importance of pad surface roughness in determining polish rate and planari- (8,16,25) Pqj. example, Renteln and Coniff have demonstrated that polish rate increases with pad roughness. In addition, they argue that because of the increased porosity at the pad surface, the surface will exhibit a modulus lower than the bulk modulus. Mechanical behavior of the pad may then be modeled by considering the pad as two separate layers whose moduli add in parallel. 

M.R. Oliver, R.E. Schmidt, and M. Robinson, CMP Pad Surface Roughness and CMP Removal Rate, Intel internal study, unpublished. 

Conditioner effectiveness The effectiveness of a conditioner can be defined as the inverse of time it takes to regenerate the surface roughness of the pad from its glazed condition, which is brought about during polishing. Figure 4.19 is a... 

Note, however, that for a given point on the surface, abrasion is not a continuous process, but rather occurs as discrete events. Because the pad has some surface roughness (on the order of 10-100 pm peak-to-peak) the pad is not in continuous contact with each point on the wafer. This concept will become more evident in the discussion of Section 4.5.1. In between the reformation of the abrasion events, the boundary layers reform. Between the time of the abrasion event and boundary layers, however, the surface is left highly exposed and reacts quickly with the slurry chemicals. As we shall see, surface reactions between abrasion events can have a strong influence on the CMP process, particularly in the case of tungsten CMP. 

In addition to between wafer conditioning, pad break-in techniques are often performed before the pad is first used. Pad break-in often involves the same techniques employed for conditioning, along with running dummy wafers to stabilize the pad performance before running product wafers. In addition to setting the surface roughness, pad break-in is also likely to stabilize other pad properties such as water absorption. 

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