Micro-patterning of aluminium-induced crystalline-silicon through reactive ion etching to fabricate pressure-sensing devices

Flexibility and miniaturization are crucial towards building ‘smart skin’, which has the potential to perform real-time sensing of several stimuli together. A low-temperature process using aluminum-induced crystallization (AIC) has been explored for the fabrication of piezoresistive polycrystalline silicon. Low temperature allows deposition of AIC-Silicon on flexible polyimide (Kapton) substrate at a lower thermal budget. In this work, AIC-Silicon with a high gauge factor of 42 has been synthesized over the Kapton substrate. Further, it has been patterned to form a device as per the design using two different processes – one with photolithography, lift off and Al wet etching, and another with photolithography along with reactive ion etching (RIE). Device is designed in the form of Wheatstone bridge consisting of four piezo-resistors made of polycrystalline silicon film obtained by AIC. While using RIE, with a DC bias of -25V applied to the substrate electrode and using sulphur hexafluoride gas flow rate of 10 sccm, high etch selectivity of 7.8 is obtained for AIC-Si with respect to the photoresist. Devices patterned with RIE demonstrated low line edge roughness (within 1 micron), leading to a variation in resistance values of the four resistors in the Wheatstone bridge to within 19.3%. On the contrary, devices patterned without RIE showed line edge roughness up to 10 microns which caused a variation of resistance values up to 93.8%. Subsequently the array of four sensors was fabricated using the RIE process flow.