Chemical mechanical polishing with fixed abrasives using different subpads to optimize wafer uniformity

Abstract

Chemical mechanical polishing (CMP) processes are widely used in the semiconductor industry and are conventionally carried out using abrasive slurry and a polishing pad. In an alternative procedure, called ‘slurry free CMP’, the abrasive particles are embedded in the pad material (‘fixed abrasives’). A microreplicated resin layer of pyramids filled with the abrasives is placed on top of a rigid polycarbonate layer and a resilient foam sublayer. Instead of slurry, only DI-water or a basic solution is applied. Drying in of the slurry and glazing of the pad is not possible and pad conditioning is not required. Experiments were carried out for slurry free CMP to optimize the with-in-wafer non-uniformity (WIW-NU), removal rate and planarization of oxide ILD. Results of this optimization are compared with our best slurry process. It is shown that the fixed abrasives process is superior to the slurry process with regard to planarization: the rate of planarization is almost three times faster for the slurry free process. This enables further process optimization, such as the use of a thinner pre-CMP oxide layer. Besides higher planarization rates, better with-in-die non-uniformity (WID-NU) can be obtained by adjusting the subpad construction of these ‘slurry free’ pads. The flexibility of the pad construction makes it possible to optimize the WID-NU and WIW-NU. Stiffer pads give lower WID-NU, but for the stiffest pad the WIW-NU will slightly increase. However, compared to the slurry process both WID-NU and WIW-NU are better.