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On the origin of selectivity and anisotropy loss during microstructure release etch

The origin of the loss of Si anisotropy and selectivity to barrier metals during the release etch process of a CMOS-MEMS fabrication flow is investigated experimentally using in situ infrared imaging. The release etch is performed in an inductively coupled plasma etch chamber fitted with a CaF2 window to maximize IR transmission. Suspended disk test structures of varying plan area, thickness, transverse suspension cross-section and suspension length are etched at various bias voltages using a SF6/O2/C4F8 time-multiplexed, Bosch-type, anisotropic DRIE process followed by an O2 polymer removal process and an isotropic SF6 process. Temperatures up to 150 °C are reported for suspended disk test structures during the isotropic SF6 process while the temperature rise during the anisotropic DRIE and polymer removal processes is less than 7 °C. A physical model based on the power balance of the suspended disk test structures is developed and used to extract the proportion of etch heat absorbed by the structures, which is determined to be 0.26. The model predicts that exothermic heat of reaction of Si with neutral F species is the dominant source of heat and is supported by the observed temperature trends.