A novel thermal switch and variable capacitance implement by CMOS MEMS process approaching in micro electrostatic converter

This paper focuses on implementing a novel thermal switch and variable capacitance design by using commercially available CMOS MEMS process which can approach in a micro electrostatic converter system. In this system, there are two major parts. First is the variable capacitance, and the second is the thermal switch. In the variable capacitance, it implement by UMC 0.18 μm one-poly seven-metal (1P7M) CMOS MEMS process. In the post process, the silicon-oxidation has been released and the gap between two metal layers filled with PDMS (Polydimethylsiloxane). Filling with PDMS is to significantly increase Cmax.In the thermal switch design, there are two novel designs in this switch: first, the soft contact structure and post-processing fabrication; second, using residual stress to achieve large structural deformation and a new design of thermal switch. To create the soft contact structure, residual stress effect has been utilized to make different bending curvatures. According to the experiments, the layer metal 1 has the largest residual stress [1] effect that can achieve the largest deflection in z-axis. Because the residual stress of the layer metal 1 is negative, the structure will bend down after release, the largest contact area which has been set up to get the lowest contact miss ability. In the post-processing fabrication, 0.3 μm thickness gold will be patterned on the contact tips. Due to gold, rather than Aluminum, has no oxidation issue, it has more reliability on preventing the problem of oxidation than Aluminum. In the new thermal actuator design, we design a novel folded-flexure [2] and [3] with the electro-thermal excitation to turn the switch on or off. In the prototype, the device size is 500 μm × 400 μm and the gap between two contact pads is 9 μm in off-state.Depending on the experimental results, the Cmax is 19.22 nF, and the Cmin is 10.65 nF in variable capacitance. The switch can work stably at 3 V, and the working temperature and operating bandwidth are individually 20–200 °C. The natural frequency of the switch is 42.9 kHz and the response time is 14.28 Hz.

► First is the variable capacitance, and the second is the thermal switch.
► The soft contact structure and post-processing fabrication.
► 0.3 μm thickness gold will be patterned in the post-processing fabrication.
► A novel folded-flexure with the electro-thermal excitation have been designed.