Characterization of a surface micromachined pressure sensor array

A surface micromachined pressure sensor array has been designed and fabricated. The sensors are based upon deformable, silicon nitride diaphragms with polysilicon piezoresistors. Absolute pressure is detected by virtue of reference pressure cavities underneath the diaphragms. For this type of sensor, design tradeoffs must be made among allowable diaphragm deflection, diaphragm size, and desirable pressure ranges. Several fabrication issues were observed and addressed. Offset voltage, sensitivity, and nonlinearity of 100 µm diameter sensors were measured. MDL 1 . While the principal objective of the project was to develop a robust, wide-range pressure sensor technology, many specific goals were put forth. The required pressure range of the sensor array is 1-10 atm (absolute), with frequency responses from DC to 2 MHz. Most of the processing equipment used for micromechanics at the MDL is shared with a 0.5 µm CMOS line. The requirement for 100% CMOS compatibility was to guarantee that sensor lots would not contaminate equipment, and also to allow integration with CMOS circuitry if necessary. Conceivable electronic functions are: signal conditioning, calibration, temperature compensation, analog to digital conversion, or network communication protocol. Finally, the desired operating environments were gaseous (including air), liquid, and severe (e.g. corrosive). Operation in all of these media provides enhanced flexibility of applications. 2. EXPERIMENTAL 2.1 Sensor Design The two principal design considerations for a diaphragm-base d pressure sensor are diaphragm geometry and piezoresistor size and placement. These piezoresistors act as strain gauges that change their resistance value according to the deflected state of the diaphragm. This sensor design utilizes both circumferentially and radially oriented piezoresistors, which have opposite resistance changes. This feature can be used to increase the overall sensitivity of the device. Details of piezoresistor size and placement are described elsewhere 2,3 The geometry of the diaphragm effects the mechanical deflection under applied pressure and, hence, the sensitivity and range of the sensor. A circular geometry was chosen for the pressure sensor. Circular symmetry lends itself to analytical solutions of plate theory. The deflection at the center of an edge-clamped plate due to a uniform applied pressure is given by * Under contract from the Center for High Technology Materials, The University of New Mexico, Albuquerque, NM 87131.