Subcritical Crack Growth in Single-crystal Silicon Using Micromachined Specimens
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R.S. Iyerb) and R.H. Dauskardt Dept. of Materials Science and Engineering, Stanford University, Stanford, California 94305
T.W. Kenny Dept. of Mechanical Engineering, Stanford University, Stanford, California 94305 (Received 20 November 2001; accepted 7 January 2002)
A micromachined specimen with a test section only 150-m thick was developed for investigating subcritical crack growth in silicon. Crack growth rates in the range 10−4–10−10 m/s were measured as a function of applied stress intensity (v–K curves) during tests in humid air and dry nitrogen lasting up to 24 h. The fracture toughness, KIc of {110} silicon was also measured at 1.15 ± 0.08 MPa m1/2. While some evidence MPa-m1/2 of subcritical crack growth appeared to occur in the region 0.9 KIc < K < 0.98 KIc, the extremely high crack growth exponent (n > 100) and the high ratio of the apparent stress corrosion threshold, KIscc, to the fracture toughness, KIscc/KIc > 0.9, suggests that no clear evidence exists for a stress corrosion process in silicon exposed to humid air. I. INTRODUCTION
Miniaturized structures and sensors fabricated using semiconductor processing techniques are collectively referred to as micro electro mechanical systems (MEMS). Single-crystal silicon is often used as either the substrate material or the structural material in MEMS devices. Many of these devices, such as automotive sensors, are intended for use over long durations in hostile environments where they may be exposed to significant fatigue loading and potentially corrosive species. As a result, there is an increasing need for understanding the mechanical behavior of silicon during such long-term exposure, particularly with respect to potential subcritical crack-growth processes associated with mechanical fatigue crack growth or moisture-assisted stress corrosion cracking. Indeed, the existence of fatigue damage in MEMS structures has been reported in several recent studies,1–3 although the process does not appear to be related to fatigue crack growth but rather to the initiation of fatigue damage.4 The absence of fatigue crack growth is not surprising in single-crystal brittle solids which, due to the low mobility of dislocations and resulting lack of plasticity, do not typically exhibit mechanical cyclic fatigue crack
a)
Address all correspondence to this author. Present address: Sensant Corporation, 14470 Doolittle Dr., San Leandro, CA 94577. e-mail: [email protected] b) Present address: Applied Materials Corp., Santa Clara, CA 95054. J. Mater. Res., Vol. 17, No. 3, Mar 2002
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growth similar to crystalline metals and polycrystalline ceramics.5,6 In brittle materials like glasses7 and singlecrystal ceramics,8 subcritical crack propagation is rarely accelerated by mechanical fatigue loading but results from an environmentally assisted stress corrosion crackgrowth process.9 Surprisingly, however, the existence of a moisture-assisted subcritical crack-growth process has not been unambiguously established
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