Piezoresistive feedback for decreased response time of MEMS thermal actuators (2006)

Feedback control is commonly used in positioning systems to improve dynamic response, disturbance rejection, accuracy, and repeatability. Similar benefits can be expected for microelectromechanical systems (MEMS) that are used for positioning applications. Sensing at the micro level poses significant challenges. Most of these challenges are associated with the small size of the devices and the small motions and forces which are of interest. In many situations, applying the macro system paradigm, where the sensor is a component that is added to the system, leads to unacceptable results. At the macro level, sensors are typically small relative to the systems they monitor. At the micro level, the sensing component of a MEMS device is often as large as or larger than the other mechanical components of the device. In many cases the sensor system cannot be considered part of the MEMS device, as is the case with a laser dopler vibrometer, which is a desk-top system. For MEMS to have the significant impact that has been projected, on-chip systems must be developed. At the micro level, the task of measurement is especially challenging. Because of the small forces and motions involved, the measurement process can corrupt the measurement or inhibit the proper function of the MEMS device. For this reason non-contact position sensing methods, such as those based on optics, have proven to be effective in the research environment. Another measurement challenge at the micro level is overcoming the influence of extraneous noise. Producing a sufficiently large signal that results in a satisfactory signal-to-noise ratio is difficult when sensing such small physical phenomena. The level of degradation imposed by measurement and signal-to-noise ratio are two critical factors in the design and implementation of sensors at the micro level. The potential for sensors to be integrated directly into the mechanisms of compliant devices provides a means to overcome some of the critical sensing challenges that exist in many applications. Rather than thinking of sensors as an add-on component (as is done in the macro world), the function of a compliant micro mechanism could be designed to incorporate and utilize the transduction properties of polysilicon to produce signals corresponding to the state of the device. The sensor is not an add-on, but rather an integral part of the mechanism. Measurements of both position and force could be generated from the piezoresistive properties of a polysilicon compliant microdevice. The concept of sensors intrinsic to compliant mechanisms is powerful and enabling for MEMS. There are numerous potential applications that could benefit from the functionality provided by this sensing concept. One application is the feedback control of the thermal inplane microactuator (TIM) shown in Figure 1. The TIM operates by utilizing ohmic heating and thermal expansion. As a voltage difference is applied across the bond pads, current flows through the slender legs and center shuttle of the TIM. The high current density in the legs causes ohmic heating and thermal expansion of the legs. Because the legs join the shuttle at a slight inclination angle, the thermal expansion of the legs results in linear translation of the shuttle. Because the legs of the TIM undergo significant strain as they deflect, their resistivity changes significantly. Preliminary experiments have shown that this resistivity change is measurable and can be used to indicate the position of the TIM.