The Design of Multilayered Polysilicon for MOEMS Applications
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The Design of Multilayered Polysilicon for MOEMS Applications D. Sherman*¶, H. Kahn*, S.M. Phillips**, R. Ballarini***, and A.H. Heuer* *Department of Materials Science and Engineering, **Department of Electrical Engineering and Computer Science, ***Department of Civil Engineering, Case Western Reserve University, Cleveland, Ohio 44106 USA ¶ Dept. of Materials Engineering, Technion, Haifa 32000, Israel ABSTRACT A rigorous analysis of a multilayered polysilicon laminated system, constructed by alternating deposition of low-pressure chemical vapor deposition (LPCVD) polysilicon at two different temperatures is presented. Different residual deformation fields are generated in these polysilicon thin films as a function of fabrication temperatures, due to different crystallization behavior at the two temperatures. The combination of the two layers, however, enables precise control of the radius of curvature of released structures, provided the material properties are well defined. We describe a new method, which combines experimental and numerical procedures, to define the material properties, which are responsible for the residual stresses as a function of layer thickness, as well as a procedure to design the desired curvature of a multilayered Microoptical-electromechanical system (MOEMS) device. A linear deformation field is assumed. It is shown that precise design of the thicknesses of the individual layer is a prerequisite for controlled curvature. The procedure we have developed predicts the curvature of multilayered polysilicon systems with good precision. INTRODUCTION The MultiPoly™ process has been developed for producing polysilicon films with nearzero stress, near-zero stress gradients, and very low surface roughness, with a maximum process temperature of 615°C [1]. The process involves deposition of alternate tensile fine-grained and compressive columnar layers of polysilicon, which are formed at growth temperatures of 570°C and 615°C, respectively [2-9]. The deformation of the multilayered system after releasing affects its functionality. If flat multilayer is desired, any curvature arisen from the internal state variables (strain or stress) may be crucial for the desired application. On the other hand, if a certain curvature is required and planed, any deviation from the chosen curvature reduces the efficiency of the device. The aim of this investigation is twofold: (i) to formulate the driving force for the residual stresses in a single, thin layer generated during fabrication of that layer, and (ii) to develop a tool for designing a flat or controlled curvature multilayered device. STRESS ANALYSIS OF THE MULTILAYERED POLYSILICON The problem is that of a multilayered polysilicon laminate, free of external loading. The driving force for internal stresses may be the result of several reasons: mismatch in thermal expansion coefficients, mismatch in lattice parameters, changes in the density of an individual layer, dopants density, etc. We postulate that the residual stresses in both polysilicon layers are the
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