Laser-Fabricated Piezoresistive Force Sensor with Liquid Immersion Laser Sidewall-Doping

This paper reports on a laser-based fabrication method for a probe-type MEMS force sensor with a sidewall-doped piezoresistive layer. The proposed sensor is a probe-type force sensor designed for in-plane force measurement. A piezoresistive layer is patterned on the sidewall of a supporting beam that holds the probe. Both the sensor structure and the sensing element are fabricated entirely through laser processing. In particular, by irradiating the sidewall with a laser while the device is immersed in a dopant solution, the piezoresistive layer can be patterned in arbitrary shapes extending from the sidewall to the top surface. Using this method, a prototype sensor was fabricated with a beam width of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$200\ \mu \mathrm{m}$</tex> and a chip size of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$12 \times 6\ \text{mm}$</tex> The sensor exhibited a linear resistance response to an applied displacement of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$20\ \mu \mathrm{m}$</tex>. Therefore, the proposed process is applicable not only to the rapid prototyping of MEMS devices without lithography, but also to the fabrication of multiaxial force sensors with high directional independence.