Thermally-actuated cantilever beam for achieving large in-plane mechanical deflections

Abstract

The design, finite-element analysis, and experimental performance evaluation of a microelectromechanical systems (MEMS) device known as a thermally-actuated beam is presented. A MEMS polysilicon thermally-actuated beam is a device that uses resistive (Joule) heating to generate thermal expansion and movement. To be a useful MEMS device, a thermally-actuated beam needs to produce incremental in-plane mechanical beam tip deflections that span 0–10 μm, while generating force magnitudes on the order of 10 μN. The thermally-actuated beam design was accomplished with the L-Edit software program, and the devices were fabricated using the Multi-User Microelectromechanical Systems (MEMS) Process (MUMPs) foundry at the Microelectronics Center of North Carolina (MCNC). Finite-element modeling analysis was accomplished with the IntelliCAD computer program. This CAD software incorporates an MCNC fabrication process description file that generates a 3-D solid model of the thermal beam. The resulting thermo- and electro-mechanical finite-element analyses predicted beam tip deflections and forces consistent with experimental observations. For example, when the drive voltage was varied between 0 and 6.5 V DC (corresponding to currents spanning 0–4.5 mA), tip deflections on the order of 0–13 μm were observed and calculated. When the ‘hot’ arm's temperature was modeled to be 200°C (Joule heating), the resulting beam tip deflection was calculated to be 4.55 μm. The resonant frequency associated with in-plane motion, without damping, was calculated to be 75.16 kHz. The average resonant frequency measured in ambient air was 69.73 kHz. The average tip force generated by the thermal beam was measured to be 8.5 μN. A relative measure of the reliability of the thermal beam was established to be greater than 3 million cycles when continuously operated with a 30 Hz, 3-volt amplitude square wave with a 1.5-V DC offset.