Subcritical Crack Growth in Single-crystal Silicon Using Micromachined Specimens

Abstract

A micromachined specimen with a test section only 150-μm 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.

Mechanisms for Fatigue of Micron‐Scale Silicon Structural Films

“…High-resolution infrared imaging of the fatigue specimen revealed only minimal temperature changes (~1K) during testing, which strongly implied that the enhanced notch root oxidation was not thermally induced but mechanical in origin [11] . As discussed below, since the cracking processes occurs within the oxide layer, this mechanism is consistent with the fact that bulk silicon is not susceptible to environmentally-induced cracking in air [9,16,28,34] . This image shows stable cracks, ~50 nm in length, in the native oxide formed during cyclic fatigue loading; testing of this sample was interrupted after 3.56 × 10 9 cycles at a stress amplitude σ a = 2.51 GPa.…”

“…In bending fatigue tests, the number of cycles to failure increased from 10 Specimen height is 7.6 mm with a length of 12 mm. [28] Dauskardt, Kenny, Fitzgerald, and coworkers investigated subcritical crack growth in precracked, 150 µm thick, single-crystal silicon specimens [28,29] . A discrete step-like crackgrowth process was observed under monotonic loading.…”

“…It is thus unlikely that this mechanism contributes significantly to the fatigue of silicon thin films. Furthermore, arguments based on the role of compressive loading fail to account for observations of fatigue failure in silicon films under cyclic tension loading (i.e., at R ≥ 0) [13,[21][22][23][25][26][27][28][29][33][34][35] .…”

“…Based on a wide spectrum of studies (listed in Tables 1 and 2) involving a range of different testing methods (described in the Appendix), stress-lifetime data (Figures 1, 10) for thin-film silicon show relatively consistent trends, specifically that stress amplitudes as low as half the (single-cycle) fracture stress can cause delayed fatigue failure, typically after 10 11 cycles or more [11,24,31,33,48] . Such fatigue failure has been reported for various modes of cyclic loading, specifically for fully reversed cyclic loading (R = -1) [4,[11][12][13][14][15]18,19,24,[30][31][32][33][34][35][41][42][43]48,49,[52][53][54]59] and tensile loading with a positive mean stress (R ≥ 0) [13,[21][22][23][25][26][27][28][29][33][34][35]40,[55]…”

“…More importantly, the frequency of loading does not appear to influence the fatigue life [26,27,[55][56][57] . These results suggest that the mechanism(s) responsible for such thin-film silicon fatigue must include both time-independent and cycle-dependent contributions, and that the phenomenon is unlikely to be the sole result of environmentally-induced cracking [28,29,34] , as this would lead to lifetimes in terms of time (and not in terms of cycles) which are frequencyindependent [58] .…”