High Temperature Behavior of Polysilicon
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High Temperature Behavior of Polysilicon Chung-Seog Oh, George Coles, and William N. Sharpe, Jr. Department of Mechanical Engineering, Johns Hopkins University ABSTRACT The polysilicon elements of thermal actuators can reach temperatures high enough to cause permanent deformation. A fundamental understanding of the constitutive behavior is necessary for intelligent design and life prediction, but mechanical testing at high temperatures is especially challenging at the micron level. This paper describes techniques for testing freestanding thin-film polysilicon specimens in tension at temperatures up to 700°C. Strain is measured directly on the specimens by laser interferometry from platinum markers. The complete stress-strain curve can be obtained as well as strain versus time for creep tests. Initial results show that polysilicon is ductile at temperatures above 500°C and can have a high creep rate. INTRODUCTION Doped polysilicon has a low enough resistivity to act as a heating element, and the associated thermal expansion makes it attractive as an actuator element. One can observe a glow of the heated element in some applications, which means that the local temperatures are high. One then wonders about the variation in mechanical properties of the material with temperature. This paper describes experimental techniques and procedures for testing polysilicon at high temperatures and presents some initial results. The focus is on the tensile stress-strain curve and tensile creep behavior of polysilicon. There has been little research in this area as the BACKGROUND section shows. Plastic deformation of polysilicon at temperature has been demonstrated, but there are as yet no complete studies of the basic constitutive behavior. Unique test capabilities have been developed at Hopkins and are being refined to permit measurement of the stress-strain curve and creep strain versus time at temperatures up to 800°C; these are described in the TEST METHODS section. Two kinds of polysilicon specimens are used – wide ones 3.5 µm thick and 600 µm wide and narrow ones of the same thickness but 50 µm wide. INITIAL RESULTS are presented for both kinds of tests and serve to both demonstrate the capability of the test methods and identify the temperature range of importance. BACKGROUND Comtois and Bright [1] introduced the lateral thermal actuator consisting of two long Ushaped polysilicon arms through which current passes. Both arms are fixed to the substrate at their ends. One of the arms is narrow, while the other is much wider. When heated, the narrow arm expands but the wide one doesn’t (as much) causing the tip of the actuator to sweep in a
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lateral motion. A 240 µm long actuator can easily deflect 16 µm at its tip; this large range of motion and the relatively large forces generated make this an attractive actuator. One would expect degradation in behavior of these thermal actuators if the heating is severe enough, and Conant and Muller [2] showed that to be the case. Similar polysilicon actuators operating at 6
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