Floating Carriers—The Use of Hydrostatic Pressure

The second definition of the purpose of a carrier was to remove the overburden of material above a surface above the device plane. For present purposes, we define the device level as the boundary between the material one wishes to remove and the material one wants to keep. It is not necessarily planar, and it moves up with each layer. For oxide CMP, this layer lies within the topmost film layer. For metal CMP, this surface is defined by the topmost surface of the dielectric into which lines and vias are etched for a damascene process. This definition must accommodate a wafer with a modest amount of bow, tilt, warp, and total thickness variation. Furthermore, it must accommodate very modest amounts of bow, warp, tilt. 

This approach has proved to be so successful that it has spawned a cottage industry of carrier designs based on the use of hydrostatic pressure. With some of these inventions a backing plate is present, with others [32] 

All of these approaches are intended to address global planarization issues. We close this section with a brief note about an alternative approach that focuses on achieving planarization at the die level. The distributed polish head [37], extends the concept of hydrostatic pressure by integrating polish blocks in the carrier design to minimize the locally the spatial variation in the polish. As shown in Fig. 9, each polish block makes contact with several die, each of which may contain regions of high and low pattern density. 

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