Chemical mechanical planarization is employed in different planarization applications during device fabrication. Most important are oxide planarization for shallow trench isolation (STI) and Damascene processing. These applications will be described in some detail to provide practical examples of CMP.
The use of copper presented serious challenges to device manufacturers. Copper cannot be etched using conventional, halide-based dry etching processes since the copper halide products are not volatile and hence cannot be pumped away by a vacuum system. This means that the conventional technology for patterning metal lines could not be used with copper. Additionally, copper does not adhere well to dielectric material and copper atoms are very mobile in SiO2. Direct deposition of copper on insulating oxide layers thus presented problems in achieving stable wiring structures and in terms of contamination of the insulating oxide (producing increased leakage).
The Damascene (and Dual Damascene) process, first introduced by IBM in the early 1990s, was a unique, additive processing technique developed to address the challenges presented by the shift to copper wiring in microelectronic circuits. It gets its name from metal inlay techniques developed in Damascus in the Middle East. The process eliminates the need to dry etch copper by using chemical mechanical polishing instead and it employs special barrier layers to prevent diffusion of the copper into the oxide insulating layers. The first step in the process is the formation of the wiring pattern as etched lines in a dielectric layer (the top structure in Figure 6). A barrier layer of, for example, TiN, TaN, or TiW is then deposited on the dielectric layer to act as a barrier between the copper and the insulating dielectric. A thin seed layer of copper is deposited on the barrier, typically using PVD methods, followed by the electrodeposition of a thick copper layer. Chemical mechanical planarization is then used to remove the excess copper leaving only the metal in the etched lines behind. Several sophisticated variations on the Damascene process have been developed, but these simple steps suffice to describe the basics of how the process works.