An array of SiNx islands on a PI substrate stretched in uniaxial tension. (a) Schematic of the in-situ tensile test system under an optical microscope; (b) and (c) Micrographs of 500 nm thick SiNx islands 20 μm in size, subjected to (b) 2.12%, (c) 3.04% strain. The ligament length, d, is measured as a function of applied strain as indicated in the figure.
The delaminated lengths of the stretched islands are measured experimentally as a function of applied strain. The delaminated length is defined as the island size minus the ligament length (2a = L − d).
A residual stress in the film can be accompanied with an applied strain in the substrate. (a) The interface stress intensity factors in mode I and mode II loading, K|, K‖, are affected by both residual stress in the film and applied strain in the substrate. (b) The interface stress intensity factors K| and K‖ are zero because there is no strain mismatch across the interface. (c) The interface stress intensity factors in this state are same as in state (a).
Plane strain models of (a) symmetric, (b) asymmetric debonding propagation. S/L = 1.5 is fixed in all the experiments and simulations. (c) The normalized energy release rate is plotted as a function of the normalized debonded length. Symmetric and asymmetric debondings show almost the same energy release rate when the ligament size is the same.
FEM results of (a) normalized energy release rate and (b) mode angle as a function of debonded length for symmetric debonding. The energy release rates for convergent debonding (Eq. (4) ) are shown by the dashed lines.
Combining Figs. 2 and 5 , (a) the interfacial toughness between SiNx islands and polyimide substrate and (b) the mode angle are plotted as a function of applied strain for various island thicknesses and island sizes. The open symbols in (a) are calculated from Eq. (4) and the filled symbols are calculated from the computed energy release rate. Very consistent interfacial toughness and mode angle are obtained regardless of island size, island thickness, and applied strain.
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