Experimental Investigation on Wafer Polishing of Oxide and Copper Layers

Abstract

Polishing, in particular chemical-mechanical polishing (CMP), is a critical technology for the planarization of wafers. This paper investigates, via experiments, and compares the performance of CMP process with different process parameters for wafers with silicon-dioxide (SiO2) layer and for wafers with copper (Cu) layer. Polishing pressure (P), speed (V), and back pressure (BP) are used as process parameters in this study. Different pads and slurries are also experimented for copper layer as its properties are different from that of conventional oxide layer. Material removal rate (RR) and non-uniformity (NU) are used as indices to measure the performance. Experimental data on oxide layers show RR increases as P and V increase but NU gets worse at the same time. This condition can be improved, for both oxide and copper layers, with suitable BP. Experiments on copper CMP using slurry with abrasives show that RR increases with higher P and V. While NU gets worse with higher P, it can be reduced as V increases using a soft pad. Better NU can be obtained using soft pad though RR is lower in this case. For abrasive-free polishing of copper layer, RR, though relatively lower compared to CMP with regular slurry, is unstable using hard pad despite that NU becomes better at higher P. NU of polished wafer is best at certain pressure but becomes worse at low pressure for hard pad and at high P for soft pad. It is also observed that NU of AFP can be improved with BP and softer pad. Soft pad gives better polishing quality and performance though RR is lower than that using slurry with abrasives.