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

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 100 mum-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 +2.56 mum to +0.55 mum for a 2 mum-wide and 100 mum-deep capacitive comb finger gap, by using 7 successive DRIE steps with 10 min etch and 20 min interrupt periods, compared to a single 70 min DRIE without any interrupt.