Yield improvement via minimisation of step height non-uniformity in chemical mechanical planarisation (CMP) with pressure and velocity as control variables

Obtaining local and global planarities is one of the prime criteria in dielectric and metal planarisations. Although chemical mechanical planarisation (CMP) helps us achieve these criteria in constant pattern density surfaces, the same is not true for variable pattern density surfaces. This results in formation of global step heights across the die. This paper provides open loop control algorithms for obtaining planarity across a die containing variations in pattern densities. Zonal pressure and/or velocity variations are used as control variables for this purpose. Based on the variation of pattern density and surface heights across the die, the surfaces are separated into zones and the pressure and velocity in individual zones are varied spatially and temporally. It is observed from simulations that the proposed algorithm can significantly improve the local and global planarities. The zonal pressure control improves the upper surface uniformity, whereas zonal velocity control significantly increases step height uniformity. The advantageous features of these two schemes are then combined to simultaneously maximise upper surface uniformity, and minimise step heights across the entire die. Work is currently in progress on physical realisation of these algorithms.