Surface Oxide Effects on Static Fatigue of Polysilicon MEMS
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Surface Oxide Effects on Static Fatigue of Polysilicon MEMS H. Kahn*, R. Ballarini**, and A.H. Heuer* *Dept. of Materials Science and Engineering, **Dept. of Civil Engineering Case Western Reserve University Cleveland, OH 44106, U.S.A. ABSTRACT Static fatigue – crack growth causing delayed failure under constant stress and involving stress corrosion cracking – was investigated in polysilicon MEMS using two different surfacemicromachined devices. One exploited residual tensile stresses to create stress concentrations at micromachined notches, and the other involved a single-edge notched beam specimen integrated with an electrostatic comb-drive microactuator. Tests with both devices revealed that polysilicon is not susceptible to static fatigue in humid environments. However, when a relatively thick (45 to 140 nm) surface oxide was thermally grown on the microactuator devices, the specimens demonstrated delayed fracture in a humid ambient, presumably due to static fatigue of the surface SiO2. The stress intensities at the resulting cracks in the SiO2 were then sufficient to cause catastrophic crack propagation through the polysilicon specimens. The implications of our data on the issue of fatigue in polysilicon is discussed. INTRODUCTION Polysilicon deposited by low-pressure chemical vapor deposition (LPCVD) is the most commonly used structural material for surface micromachined MEMS devices. The mechanical reliability of polysilicon, including its fatigue resistance, is therefore of great interest to MEMS designers. Static fatigue – delayed fracture under a constant applied stress – is a manifestation of stress corrosion cracking; it has never been documented in silicon (with the exception of one report [1] which has not been successfully repeated). However, dynamic fatigue – delayed fracture under applied cyclic stresses – is well documented in silicon [2-6]. We have previously shown [2] that for low-cycle dynamic fatigue of undoped polysilicon using notched specimens, the ambient (air or vacuum) does not affect the fatigue strength; however, the load ratio, R, the ratio of the minimum to the maximum stress in the fatigue cycle, has a strong influence on the fatigue strength. This is shown clearly by the data in Fig. 1. We also demonstrated [2] that for high-cycle dynamic fatigue, the ambient does have an effect on fatigue lifetimes; specimens tested in air (105 Pa) failed significantly faster than those tested in vacuum (10 Pa). These results imply that dynamic fatigue in polysilicon occurs through subcritical crack growth due to mechanically-induced damage (such mechanically-induced microcracking has previously been reported in brittle ceramics [7]), and that for high-cycle fatigue testing, the air exacerbates this effect, possibly due to the growth of a surface oxide on newly formed subcritical cracks which increases wedging effects. After release of our devices by etching the sacrificial oxide in HF [2,3], the undoped polysilicon grows a room temperature surface oxide with a thickness on the order of 2 n
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