Understanding Smart Sensors - Nomads.usp

44 Understanding Smart SensorsThe first metal is deposited on a substrate using a conformable insulatorpatterned on an anode, as shown in Figure 2.17(a), to produce the partial layerof Figure 2.17(b). A second metal is blanket deposited over the first,Figure 2.17(c), and then the entire structure is mechanically planarized to aprecise thickness and flatness, as shown in Figure 2.17(d). The process can berepeated for multiple layers. The final structure, such as Figure 2.17(e), is thenetched to produce a three-dimensional device, Figure 2.17(f ). The sacrificialmetal is analogous to the sacrificial material in surface micromachining, exceptit is fully conductive. Therefore, an arbitrary patterning layer, including onewith an overhanging structure, can be electrodeposited over it. The low processtemperature makes it a candidate for additional CMOS circuit integration.2.7 SummaryThe trends in the sensing industry for micromachined structures include thefollowing:• Improved accuracy and resolution;• Extended operating range of existing sensors;• The development of higher operating temperature sensors;• More complex structures using advanced micromachining technologyand new materials;• The addition of integrated circuits;• Cost reduction through process improvements and die shrinking.More complex structures will enable new parameters to be sensed andcombined measurements to be performed. The continued merging of electronicsand various micromachining techniques promises to be a key factor forsmart sensing.References[1] Bryzek, J., K. Peterson, and W. McCulley, “Micromachines on the March,” IEEESpectrum, May 1994, pp. 20–31.[2] http://mems.eeap.cwru.edu/SiC.[3] Howe, R. T., et al., “Silicon Micromechanics: Sensors and Actuators on a Chip,” IEEESpectrum, July 1990, pp. 29–35.