Understanding Smart Sensors - Nomads.usp

Micromachining 39Figure 2.13 Monolithic piezoresistive pressure sensor. (Courtesy of Motorola, Inc.)integrated circuitry and mechanical structures on the same silicon wafer [30].The micromachining is performed after the CMOS processing has been completed,which allows mixed electrical and mechanical silicon devices to bedeveloped and even produced in small volumes. With that approach instrumentationand measurement devices can be reduced in both size and cost.High-volume, cost-sensitive applications, such as automobiles or appliances,will require a dedicated wafer-fabrication facility under the direct control of thesensor manufacturer.Another standard 3-mm CMOS process has been combined with anisotropicwet etching at Fraunhofer Institute in Berlin [31]. The parameters ofthe CMOS process were not modified. Six more masking layers were needed tofabricate monolithic accelerometers. The sensor structure was defined in theCMOS processing, and testing of the IC portion was possible prior to themicromachining steps. Both top and backside (with etch stop) anisotropicetching were performed to create the accelerometer structure.A modular CMOS and surface micromachining process was developedat the University of California [32]. A silicon foundry can perform the highly