The buckling of compressively prestressed square membranes with built-in edges is investigated experimentally and analyzed theoretically. The buckling depends weakly on Poisson's ratio and essentially is a function of the reduced prestrain, " 0 = " 0 a 2 =h 2 , where " 0 is the physical prestrain, a is the width, and h is the thickness of the membrane. As " 0 becomes increasingly negative, the membrane undergoes two symmetry breaking buckling transitions. Beyond the first transition occurring at " cr1 , the buckling profile has all the reflection and rotation symmetries of a square. The reflection symmetries are lost through a second instability transition at "cr2: The bifurcation points, "cr1 and "cr2; and buckling profiles were calculated using analytical energy minimization and nonlinear finite-element simulation. Both methods agree. The buckling of micromachined plasma-enhanced chemical vapor deposition silicon nitride membranes on a silicon wafer is interpreted in terms of the theoretical results. Good matching between measured and calculated buckling profiles is found. The extracted strain values are consistent irrespective of the size and buckling mode of the membranes. From the average strain across the wafer " 0 = 03:50 2 10 04 and complementary wafer curvature measurements, a Young's modulus of 130 GPa is deduced. Methods for the straightforward extraction of " 0 from experimental center deflections of buckled square membranes are described. [443]
We report a novel method to measure Poisson's ratio v of compressively prestressed thin films. The method exploits a v-dependent mechanical instability transition of buckled long rectangular membranes under differential pressure. This allows to determine v with high accuracy, using a physical model of the plate instability. The method was used to extract v = 0.253±0.017 from 0.704±0.003 μim thick PECVD silicon nitride films.
This paper reports recent advances in surface micromachining by Sacrificial ALuminum Etching (SALE).This method is applied to standard CMOS substrates in a single-mask, or even maskless, post-processing scheme. Underetching distances of several hundred microns are feasible. Design issues and technological steps such as protection of contact pads, etching, rinsing, and drying are discussed. The combination of SALE with other micromachining techniques, e.g., silicon hulk micromachining or material deposition is reported. New devices include thermal, capacitive, and mechanical structures for thermal microfluidics, the measurement of pressures and inertial forces, and the evaluation of mechanical thin film properties.
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