Measurement of Thin-Film Stress, Stiffness, and Strength Using an Enhanced Membrane Pressure-Bulge Technique
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Measurement of Thin-Film Stress, Stiffness, and Strength Using an Enhanced Membrane Pressure-Bulge Technique Aaron J. Chalekian, Roxann L. Engelstad, and Edward G. Lovell Computational Mechanics Center, University of Wisconsin-Madison 1513 University Ave., Madison, WI 53706 U.S.A. ABSTRACT Accurate mechanical properties of thin films are essential for viable design and fabrication of semiconductor devices and microelectromechanical systems. Relevant properties of thin films such as intrinsic stress, biaxial modulus, and fracture strength can be significantly different than their corresponding bulk values, and much more difficult to measure. However, such data can be obtained from the pressure-deflection response of clamped freestanding membranes, i.e., the socalled pressure-bulge test. Experimental challenges include membrane leakage prevention, ensuring proper structural boundary conditions, and accurately measuring applied pressure and transverse displacements simultaneously. In addition to these issues, most previously-developed pressure-bulge instruments rely on vacuum pump loadings. Such tools are limited by the oneatmosphere differential pressure over the membrane, which is inadequate for burst testing of high-strength films. Consequently, an enhanced pressure-bulge tool has been developed and will be described in this paper. It incorporates positive pressure to overcome the one-atmosphere load limitation, improved edge constraints, and the ability to test an array of membrane windows across a single substrate. INTRODUCTION In-plane gradients of intrinsic stress and material properties such as biaxial modulus and fracture strength were determined by first depositing thin films on silicon wafers, which were subsequently back-etched to create arrays of freestanding membranes. Deflection due to an applied pressure difference was accurately mapped across individual membranes using a scanning laser vibrometer which allows for assessing boundary conditions and locating the point of maximum deflection. The load-deflection response provides the prestress and biaxial modulus, window by window, to characterize gradients across a thin film. Likewise, by identifying burst pressure and maximum deflection for each window, fracture strength can be determined using correlations with analytical and finite element modeling. This technique was also directly applied to the analysis of electron-projection lithography masks, which are designed and fabricated with a wafer grillage supporting an array of thin freestanding silicon membranes. Typical membranes are 1.0 to 4.0 mm square, with a thickness of 2.0 µm. This paper describes the use of a new pressure bulge tool to identify in-plane stress gradients of the silicon film, as well as its burst strength. EXPERIMENTAL DETAILS Traditional vacuum-based pressure bulge tools, are limited to a one-atmosphere differential pressure across the membrane. To eliminate this restriction, a positive pressure tool was designed and fabricated to properly support the sample while applying
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