Design And Fabrication Of SU-8Micro-force Sensor Based On Metal Piezoresistive Strain Gauge

Micro-force sensor is a kind of indispensable sensor in the field of microelectro mechanical systems (MEMS). However, there are relatively few sensors for micro force detecting among the existing sensors. Furthermore, in order to prevent operation from damaging the object and complete the operation quickly, force needs to be measured in the process of microassembry and micromanipulation. This paper presents the design of a polymer micro-force sensor and novel processing technologies are adopted to make it. At last some related tests are done.SU-8is chosen as the substrate material for the micro-force sensor, copper as the electrode and the piezoresistance. Based on the strain theory of the resistance strain gauge, performance of the SU-8micro-force sensor is in comparison with that of the semiconductor piezoresistive micro-force sensor. SU-8micro-force sensor has a typical structure of double-layer cantilever. The specific structure size of the sensor is identified according to the size of measured object and explored process experience. And five kinds of micro-force sensors with different metal width are designed. As a negative photoresist, SU-8has good corrosion resistance and biological compatibility that makes the SU-8micro-force sensor more extensive in the scope of application.In order to make the SU-8micro-force sensor, two kinds of fabrication process are investigated in this paper. One is the lift-off process, which begins with the processing of a SU-8film as the basal layer of the micro-force sensor, and then makes the structure opposite to metal layer by spin-coating and lithography, at last sputters copper and processes the lift-off technology. The other is the wet etching process, including processing a SU-8film, sputtering metal on it and patterning the masking layer structure which is same as metal layer for wet etching using spin-coating and lithography. Finally SU-8micro-force sensor is obtained after releasing process.The sensor circuits are designed to facilitate the use of the SU-8micro-force sensor, including the Wheatstone bridge circuit, three-stage amplifying filter circuit and the power supply circuit. Resistance variation is converted into voltage variation by Wheatstone bridge circuit. And the μV-leveled signal is amplified with filter processing to mV-leveled one using the three-stage amplifying filter circuit. Power supply circuit mainly provides±12V voltage for the three-stage amplifying filter circuit.For the structure features of the SU-8micro-force sensor and the measurement needs, this paper puts up two sets of device. One set is the calibration system of the micro-force sensor, which is used for the deflection sensitivity measurement and the force sensitivity calibration. The results show that the combination property of micro-force sensor with10μm width is the best, and the force sensitivity is0.55mV/μN. The other set is clamping force measuring device, and the measuring object is jaw normally close561microgripper which is designed by this research group. Using the micro-force sensor of10μm width, we successfully measure the clamping forces of different jaw and compare the measured values with the simulation values.