A micro Fabry–Perot sensor for nano-lateral displacement sensing with enhanced sensitivity and pressure resistance

Abstract

This paper proposes a novel sensitivity/signal enhancement method for a nano-displacement sensor fabricated by polymer MEMS technology and operated by Fabry–Perot interferometry. The surface roughness of the floating element fabricated by UV lithography on SU-8 photoresist is better than 5 nm (Ra value) on 35 μm×35 μm area and can be served as reflection mirror. Silicon oil with refractive index 1.406 is filled into the sensor cavity as an index matching medium for signal and sensitivity enhancement as well as a buffer material for pressure resistance and vibration reduction. With silicon oil filling, the sensor sensitivity can be improved by 1.85 times and signal intensity is increased by 16.4 dB, respectively. The minimum detectable displacement and shear stress have been demonstrated to be 10 nm and 0.33 Pa, respectively. The sensitivity of displacement and shear stress sensing are of 0.1249 nm/nm (wavelength shift/floating element displacement) and 6.825 nm/Pa (wavelength shift/shear stress), respectively. Besides, the signal spectrum shifts have been tested within 1 nm under static pressure from 1 to 6 atm or acoustic vibration from 1 Hz to 5 kHz, because of the incompressibility and damping effect of the silicon oil. However, the temperature dependency and hysteresis of the sensor due to the thermal effect need to be improved or compensated for practical applications in the future.