Measurement and characterisation of displacement and temperature of polymer based electrothermal microgrippers

Abstract

This paper presents our work on measurement and characterization of electrothermal microgrippers for micromanipulation and microassembly applications. The SU-8 based microgrippers were designed with embedded microheaters in the actuation structures of the grippers to improve thermal efficiency and to reduce the undesirable out of plane movement of the gripper tips. Electrothermal testing and characterization have been conducted to determine the displacement between the grippers’ tips under an applied voltage. A measurement method based on an image tracking approach was used to measure gripper displacement. It has been shown that a displacement of 11 and 8 µm can be obtained at the actuation voltage of 0.65 and 0.7 V for two different designs. The microheaters were also used as a sensor to find out the heater temperature. This has been done by measuring the resistance change at the applied voltage. The temperature coefficient of resistance (TCR) was determined independently and then used to calculate the heater temperature based on the measured resistance change. The values of TCR of the thin film chromium/gold microheaters were determined to be 0.00136 and 0.00149/°C at 20 °C for the two different gripper designs which are significantly less than the value of the bulk gold material which is 0.0034/°C. The results show that it is important to determine the TCR of the thin film microheaters for accurate calculation of the heater temperature at an applied actuation voltage. In addition, the resistivity of the metal layer has been calculated and it was between 6.2 and 6.8 μΩ cm. this value is much larger than 2.44 μΩ cm for the bulk gold value. For the fabricated thin gold film based microheaters, it was found the ratio of the increase in the resistivity is the same as the ratio of decrease in the TCR value.