Influence of deep reactive ion etching process parameters on etch selectivity and anisotropy in stacked silicon substrates for fabrication of comb-type MEMS capacitive accelerometer

This paper presents etching optimization of comb structure formation in small silicon substrates mounted on a large silicon wafer (150 mm diameter) using the S1818 photoresist adhesive layer. A series of deep reactive ion etching (DRIE) experiments are done by varying the etching conditions and tweaking the baking schedule of the photoresist adhesive layer to get uniform vertical arrays of comb-fingers. The S1818 photoresist is also used as a masking layer (2.5 μm thick) for patterning a 16 μm deep vertical comb structure using the Bosch process-based DRIE. The optimized RF powers of etching (2200 W) and deposition (1900 W) cycles of the DRIE yielded minimal damage on the masking layer. The optimized soft-bake temperature (90 °C for 1 min) of the photoresist adhesive layer helps in achieving desired comb structure with smooth vertical sidewalls (scallops: <50 nm). The silicon etch rate in the optimized condition is found to be 3.5 (± 0.2) µm/ min with an etch selectivity of 20. The prime reason for achieving the smooth vertical comb-walls is the efficient RF power generated by heat dissipation through the moisture of the solvent (1-methoxy-2-propanol-acetate) present in the photoresist adhesive layer during the DRIE process. Finally, a bulk-micromachined comb-type MEMS accelerometer structure is successfully fabricated in a small silicon substrate after optimizing the smooth vertical comb-walls using DRIE. The fabricated accelerometer exhibited a 60 mV/g scale-factor sensitivity with 120 Hz 3dB bandwidth.