2000-Atmosphere bulk-type dual-cavity MEMS pressure sensor

Abstract

Many industrial, automotive, and oil exploration applications demand the measurement of pressure higher than 100 MPa. However, conventional diaphragm-type MEMS pressure sensors are not ideal for such applications. The representative MEMS high-pressure sensors are based on the piezoresistive principle and involves bonding of heterogeneous materials which are burdened by the inherent difference between the coefficient of thermal expansion of dissimilar materials. An all-silicon bulk-type single-cavity sensor that does not require fragile deformable diaphragm-like micro-structures has been previously developed and characterized up to 200 MPa. Incorporating a half Wheatstone bridge consisting of two sensing piezoresistors and two relatively constant reference resistors to measure the biaxial compression induced by a hydrostatic pressure load, the sensor exhibits a sensitivity of 79 μV/V/MPa. However, the sensitivity of such bulk-type, single-cavity design was significantly hindered by tthe bulging of the surface on which the piezoresistors were constructed. To solve this, a bulk-type, dual-cavity pressure sensor with an improved construction and a new layout design of the piezoresistors was proposed and realized. The dual-cavity construction was expected to eliminate the surface bulging and, moreover, induce stress amplification to further enhance pressure sensitivity. The measurement results shows that the dual-cavity sensor exhibits a ~2.5-times higher sensitivity than the single-cavity version. To further improve the performance of the dual-cavity sensor, a sensor incorporating stress modifying trenches was developed. With active piezoresistors aligned to orientations with stress components exhibiting opposite signs, a fully active Wheatstone bridge can be realized. This leads to a significant increase in the sensitivity of the pressure sensor to 513 μV/V/MPa over the same pressure range, which is 6.5 times the sensitivity for the initial single-cavity design. Besides the improvement of mechanical design, the electrical approaches were also explored. A monolithic integrated high-pressure MEMS pressure sensor with integrated circuits built by metal oxide thin film transistors as proposed and realized, yielding a voltage gain of ~2.8 compared with the raw output of the MEMS pressure sensor. The result demonstrates the potential of a TFT application-specific integrated circuit (ASIC) to be monolithically integrated with the MEMS device for boosting the pressure sensitivity and other signal-conditioning applications.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Man Hoi WONG (Supervisor)