Resistance to cracking of a stretchable semiconductor: Speed of crack propagation for varying energy release rate
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Resistance to cracking of a stretchable semiconductor: Speed of crack propagation for varying energy release rate Sheng Liu, Hee C. Lim, Min Qu, John F. Federici, Gordon A. Thomas Department of Physics, NJIT, Newark, NJ 07102 Helena Gleskova, and Sigurd Wagner Department of Electrical Engineering, Princeton University, Princeton, NJ ABSTRACT We have measured and calculated the propagation velocity of successive cracks in a single sample of amorphous SiNx as a function of energy release rate. We have obtained the conditions for controlled, repetitive crack formation by using a substrate of compliant plastic that survives the cracking of a thin film formed on it. We have recorded the crack velocity curves using high-speed micro-photography using dark field illumination. Under uniform, uniaxial tensile strain, the films crack in an array of essentially straight, parallel lines, if the increase of the strain density is slow. We find reasonable agreement in the comparison of theory and experiment and find a linear relationship between the initial velocity and energy release rate threshold. Consequently, in cases where the theoretical agreement with the data is reasonable, the successive cracks show velocity curves that scale with each other.
INTRODUCTION Flexible electronic circuits using amorphous, hydrogenated Si (α-Si:H) to fabricate thin film transistors, flexible sensors and other devices are of growing technological importance [1-3]. An important design of these flexible circuits uses a highly compliant, plastic substrate, such as Kapton-E, a buffer layer of amorphous SiNx, and the active layer of α-Si:H with other materials for electrical connections. The buffer layer, amorphous SiNx is important because the active layer, α-Si:H, has inadequate adhesion to the plastic substrate. These cracking studies delineate the region of applied stress under which the flexible circuits can operate without damage due to mechanical challenges including linear tensile strain and bending. For simplicity, in this paper we study the system with only one deposited film, Kapton- SiNx. The thicknesses of Kapton and SiNx are 51 µm and 1 µm, respectively. EXPERIMENTAL DETAILS We stretched the films (uniformly clamping the ends and thus shortening the exposed length to 65mm, width 4.5mm, ratio about 14:1) while flattening them with a supporting plate. We moved two translation stages synchronously in opposite directions at a controlled rate of vs=10-4 cm/s to apply stress while leaving the strip's center stationary under a microscope. The images are recorded of the same region with a microscope and a high-speed digital camera (136 frames/sec) with dark field illumination directed at a small angle to the applied stress direction. Because of the compliant substrate, the sample remains intact after the first crack forms, so that we can
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observe a sequence of cracks. The subsequent cracks form parallel to the first within our measurement accuracy (with a few exceptions), and images of sequences of cracks on SiNx thin film a
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