Electrothermal Bimorph MEMS Sensor for Helium Detection

This article reports a method for detecting helium in atmospheric air based on the thermal conductivity of helium using a bimorph microstructure. The bimorph structure is designed to deflect less with helium’s presence than air under Jole heating’s application. The COMSOL Multiphysics finite element software was used to design and verify the operation of the structure. Based on the simulation, Si/SiO2 bimorph microcantilever structures with trenches were fabricated and experimentally tested to detect the presence of helium. The detection difference due to the presence of helium measured using a digital hologram microscope (DHM) was 30 nm, which closely matched what we reported in the simulation. Further simulation results were conducted to vary some design parameters in the sensor, showing that the sensor performance can be improved significantly to around 15 times. The best-explored design parameters were found to be a cantilever with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5~\mu $ </tex-math></inline-formula>m thickness and a gas gap thickness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2~\mu $ </tex-math></inline-formula>m. This article provides proof of concept for developing a capacitive electrothermal MEMS helium sensor that can be used in many critical applications, such as monitoring nuclear waste stored in pressurized helium dry casks.