The Determination of Mechanical Parameters and Residual Stresses for Thin Films Using Micro-Cantilever Beams
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THE DETERMINATION OF MECHANICAL PARAMETERS AND RESIDUAL STRESSES FOR THIN FILMS USING MICRO-CANTILEVER BEAMS S. HONG, T. P. WEIHS, J. C. BRAVMAN AND W. D. NIX Dept. of Materials Science and Engineering, Stanford University, Stanford, CA. 94305 ABSTRACT A method for determining mechanical parameters and residual stresses for thin films is described. Multi-layer cantilever beams (LPCVD SiNx/thermal SiO 2 ) are fabricated utilizing standard IC processing technologies and micromachining of silicon. The elastic response of the beams to imposed deflections is then measured using a Nanoindenter, a sub-micron hardness testing machine. The elastic constants of the nitride films are calculated from the force vs. deflection slope and known elastic constants of the thermal Si0 2 and silicon. By measuring the curvature of the multi-layer cantilever beams with a scanning electron microscope after successive etching of the LPCVD nitride films, average and differential stresses in the films were calculated. INTRODUCTION LPCVD (low pressure chemical vapor deposition) SiNx films are widely used in silicon semiconductor devices as a diffusion barrier and as passivation layers. The structure of thin LPCVD SiNx over thermally grown SiO 2 is particularly important in the fabrication of most MOS devices, since this structure is essential for the LOCOS process. The residual stress in the nitride film is one of the main factors responsible for the generation of point and line defects in the silicon substrate during/after the bird's beak formation. The ratio of the thicknesses in the nitride and oxide layers is critical to the defect formation [1]. To gain a better understanding of the mechanism of defect generation, the stress state and elastic constant of the thin nitride films should be known. In this paper, a new test method which can determine both the elastic constants and residual stresses of very thin films will be described. The technique is an extension of the single-layer cantilever bending technique [2], and makes use of multi-layer cantilever beam structures. Thin films for which elastic constants are to be determined (LPCVD SiNx in this paper) are deposited on well characterized films, which can sustain large elastic deformations. Thermal SiO 2 ideal for this application [3]. The elastic parameters of the top layer can be evaluated without the yielding of the bottom reference film. Furthermore, the bottom layers can withstand the large stresses that develop in the beams due to significant growth or thermal mismatches between the top and bottom films. This allows the residual stresses to be calculated with a simple elastic model. After the fabrication of double-layer cantilever beams, the elastic constants of the nitride films are calculated from the force-deflection slope and known elastic constant of the thermal SiO 2 . The curvature of this double-layer beam provides information on the residual stresses in the films. This test method is useful for measuring films that are either too thin (less than 0.5 gm thick) to test as s
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