Applications of Polysilicon Films in Microsensors and Microactuators
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APPLICATIONS OF POLYSILICON FILMS IN MICROSENSORS AND MICROACTUATORS Roger T. Howe
Berkeley Sensor & Actuator Center Dept. of Electrical Engineering and Computer Sciences and the Electronics Research Laboratory University of California Berkeley, California 94720 ABSTRACT
This paper reviews the applications of polysilicon in microsensor technology and in the emerging field of microfabricated actuators. Polysilicon is an attractive material for thin-film strain gauges, and is also used to fabricate micromechanical structures. The need for better understanding of the non-electrical properties of polysilicon, such as Young's modulus, residual strain, ultimate strength, friction, and wear, is emphasized. INTRODUCTION
Silicon microstructure technology has developed rapidly over the past decade. The cornerstone of this technology has been orientation-dependent chemical etching of the silicon substrate, in conjunction with masking films and various etch-stopping techniques. This extension of silicon integrated-circuit technology has been used to make a variety of sensing structures, such as silicon diaphragms (for pressure) and cantilever beams (for acceleration) [1,2]. Since silicon is piezoresistive, the deformation of silicon microstructures can be readily detected by means of embedded diffused strain gauges. Polycrystalline silicon (polysilicon) is attracting growing interest as both a piezoresistive thin film [31 and as a microstructural material [4,5]. The use of CVD polysilicon rather than diffused stain gauges has the advantages of dielectric rather than junction isolation and improved temperature stability. Microstructures can be fabricated from patterned polysilicon thin films by etching an underlying sacrificial oxide layer in hydrofluoric acid. This simple planar process does not require orientation-dependent silicon etchants. Recently, techniques have been developed for sealing polysilicon structures after etching of the sacrificial layer, enabling fabrication of polysilicon diaphragm pressure sensors [5]. This paper reviews the applications of polysilicon to microsensor technology, beginning with a discussion of the mechanical properties of polysilicon. Although the electrical properties of polysilicon are have been exhaustively studied, relatively little is known about such properties as Young's modulus, Poisson's ratio, residual strain, and ultimate strength. These mechanical properties and their dependence on the fabrication process are essential to microsensor design. The several micromachining processes which incorporate polysilicon are described. Selected applications of polysilicon thin films to microsensors, as well as to the embryonic field of microactuators, are discussed. Finally, the directions for future research in these unconventional applications of polysilicon are considered. POLYSILICON MATERIAL PROPERTIES
The electrical properties of polysilicon have been thoroughly studied. Of particular interest for microsensor applications is the temperature variation of resistivity. Lightly doped po
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