A Thermally Actuated Microelectromechanical (MEMS) Device For Measuring Viscosity

Abstract

A thermally actuated non-cantilever-beam micro-electro-mechanical viscosity sensor is presented. The proposed device is based on thermally induced vibrations of a silicon-based membrane and its damping due to the surrounding fluid. This vibration viscometer device utilizes thermal actuation through an in-situ resistive heater and piezoresistive sensing, both of which utilize CMOS compatible materials leading to an inexpensive and reliable system. Due to the nature of the actuation, thermal analysis was performed utilizing PN diodes embedded in the silicon membrane to monitor its temperature. This analysis determined the minimum heater voltage pulse amplitude and time in order to prevent heat loss to the oil under test that would lead to local viscosity changes. In order to study the natural vibration behavior of the complex multilayer membrane that is needed for the proposed sensor, a designed experiment was carried out. In this experiment, the effects of the material composition of the membrane and the size of the actuation heater were studied in detail with respect to their effects on the natural frequency of vibration. To confirm the validity of these measurements, Finite Element Analysis and white-light interferometry were utilized. Further characterization of the natural frequency of vibration of the membranes was carried out at elevated temperatures to explore the effects of temperature. Complex interactions take place among the different layers that compose the membrane structures. Finally, viscosity measurements were performed and compared to standard calibrated oils as well as to motor oils measured on a commercial cone-and-plate viscometer. The experimentally obtained data is compared to theoretical predictions and an empirically-derived model to predict viscosity from vibration measurements is proposed. Frequency correlation to viscosity was shown to be the best indicator for the range of viscosities tested with lower error (+/-5%), than that of quality factor (+/-20%). Further viscosity measurements were taken at elevated temperatures and over long periods of time to explore the device reliability and drift. Finally, further size reduction of the device was explored.

Mechanical Vibrations of Thermally Actuated Silicon Membranes

“…The temperature increase, due to the bias conditions of 20 V for 20 µs, is only 3.3 °C. This confirms the measurements taken with the in-situ diodes presented in [9]. This peak temperature is reached in 35 µs, 10 µs after the heating bias has been removed.…”

“…The results match those predicted by the theory presented in the previous section indicating that the sudden thermal load sets the membrane in motion to vibrate at its natural frequency. The typical energy bias applied to the membrane shows only a slight increase in temperature of the membrane, matching the measurements shown in [9]. The initial response is to move either up or down, depending on the layers of materials on the membrane, and oscillate around this static displacement location.…”

“…As mentioned above, we performed the analysis by assuming a simply supported structure vibrating in a (1,1) mode but our case seems to be between a simply supported and a free membrane. Due to the nature of the microelectronic process used to fabricate the structures [9], the silicon edge is not clamped and is free to slide in the normal direction to the plane. The silicon edge is only weakly constrained in the vertical direction.…”

“…Thermally actuated membranes were fabricated as described in [9]. A schematic representation of such structures is shown in Figure 3.…”

“…A thermally actuated MEMS device has been proposed as a viscosity sensor by the authors in [9]. The proposed device is a silicon-based membrane which is actuated by the sudden application of heat that sets the silicon membrane to vibrate at its natural frequency.…”