Stepped-etching for preserving critical dimensions in through-wafer deep reactive ion etching of thick silicon

Abstract

This paper presents the experimental investigation of stepped deep reactive ion etching (DRIE) process in order to minimize critical-dimension (CD) variations due to local heating observed in through-wafer etch of 100µm-thick, high aspect ratio silicon microstructures that are suspended over glass substrate. Classical methods of cooling the substrate, using a heat-sink layer, or increasing the thickness of sidewall passivation in general turns out to be insufficient for preventing excessive damage in critical dimensions during deep etches. Alternatively, stepped etching is evaluated for improving the CD variation in deep through-wafer etch. Preliminary results indicate that the CD variation improves from +2.56µm to +0.55µm for a 2µm-wide and 100µm-deep capacitive comb finger gap, by using 7 successive DRIE steps with 10min etch and 20min interrupt periods, compared to a single 70min DRIE without any interrupt.