Mechanical Properties of Microsensor Materials: How to Deal with the Process Dependences?
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MECHANICAL PROPERTIES OF MICROSENSOR MATERIALS: How to Deal with the Process Dependences? STEPHEN D. SENTURIA Barton L. Weller Professor of Electrical Engineering, Microsystems Technology Laboratories, Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, 02139 ABSTRACT Microsensors are measurement devices fabricated using planar integrated circuit technology together with enhancements generically called "micromachining". It is well known that the thinfilm materials used in microelectronics can have properties which differ from their bulk counterparts. In addition, thin-film materials exhibit residual stress, which is known to be strongly process dependent. The sensor designer must understand how the detailed process used to fabricate a particular device impacts the specific mechanical properties, hence, the expected mechanical device performance. This paper illustrates the problem of process-dependent material properties using examples of residual stress of several materials, and then presents an object-oriented material-property database which is being developed as part of a CAD system for microelectromechanical systems (MEMCAD). INTRODUCTION This paper examines a fundamental problem in the design of microsensors, microactuators, and microelectromechanical components (MEMS). It has been long recognized that the design process has been limited by inadequate knowledge of the mechanical properties of microelectronic materials and by the fact that these properties often depend on the specific process used to fabricate the device [1]. The design process is illustrated schematically in Figure 1. With a design goal in hand, the designer uses either manual drafting or computer-aided design tools (CAD) to create a proposed design. The behavior of this design is then simulated, and the design is revised as needed to reach the desired goal. Prior to final manufacture, the design is verified to the extent possible. In the case of integrated circuits, which has a well-developed CAD environment, simulators exist for the fabrication process, for the electrical behavior of individual devices, and for the circuit performance at various levels - logic, timing, and full waveforms. In addition, design verification aids are available for checking the layout against design rules, verifying that the mask set captures the correct circuit, and estimating the circuit effects of parasitic elements in the final design.
Figure 1. Schematic illustration of the design process.
The ultimate verification, of course, is experiment. The goal of a CAD system is to replace expensive and slow physical experiments with computer modeling and experimentation, even exploring ranges of variation which cannot be readily or controllably accessed in physical experiments. Ideally, the CAD simulation and verification tools are sophisticated and accurate enough Mat. Res. Soc. Symp. Proc. Vol. 239. 01992 Materials Research Society
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to provide reasonable assurance that the physically fabricated devi
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