Flatness & Nanotopography -- Role in CMP
The interaction between nanotopography upon film removal uniformity in CMP has been under extensive investigation by Boning and co-workers and Tamura et al. The primary effect of oxide uniformity removal is due to the hardness of the CMP pad. The fundamental concept is very simple: soft polishing pads conform to local topology variations (i.e. nanotopography) while hard pad does not. Figure 3 (below), adapted from Boning et al., illustrates this principle. Typically, a wafer has a characteristic nanotopography length (NL shown in the top illustration of Figure 3). The soft pad will conform over the nanotopography and maintain a uniform film. The hard pad will not conform to the nanotopography and produce an non-uniform film with high spots on the wafer surface having a thinner film and low spots having a thicker film. Traditionally soft pads have been used for film removal CMP. However, with the need for better planarization, because of more layers, smaller CD and for multi-function logic devices which have several different areas of varying pattern densities, stiff pads are required. To some extent, the effect of nanotopography can be minimized by using polishing additives, such as ceria particles11. Nonetheless wafer nanotopography becomes increasingly important.
To understand the influence of nanotopography in CMP film removal uniformity, the concept of planarization length should be considered. The planarization length (PL) is the spatial length at which polishing cannot reduce the step height of a feature in the film thickness. This is illustrated in Figure 4 (below). The important aspect to consider is when PL is less than NL the film uniformity is maintained. When PL is more than NL one finds non-uniform film removal. Two typical examples are shown in Figure 3. The CMP process and the film uniformity specifications may be considered to determine the level of nanotopography required.

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