Stress and Microstructure in LPCVD Polycrystalline Silicon Films: Experimental Results and Closed Form Modeling of Stres
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STRESS AND MICROSTRUCTURE IN LPCVD POLYCRYSTALLINE SILICON FILMS: EXPERIMENTAL RESULTS AND CLOSED FORM MODELING OF STRESSES P. KRULEVITCHt, G.C. JOHNSONt, and R.T. HOWE* tBerkeley Sensor & Actuator Center. Department of Mechanical Engineering Sensor & Actuator Center, Department of Electrical Engineering and Computer Sciences University of California, Berkeley, CA 94720
"Berkeley
ABSTRACT Characterization of undoped polycrystalline silicon films indicates that correlations exist between stress and microstructure. Films of thickness between 0.5-3.6 .im were deposited onto Si0 2-covered single crystal silicon wafers between 605 and 700*C using low pressure chemical vapor deposition (LPCVD). The average in-plane film stress and the stress gradient through the film thickness were determined from wafer curvature measurements, and film microstructure was studied with cross-sectional TEM. Films deposited near 605*C exhibit overall tensile stresses that result from an amorphous to crystalline phase change. At deposition temperatures exceeding 630*C, a columnar grain structure evolves out of a transition region of small grains at the Si0 2 interface. The columnar films are compressive, with the source of compression linked to the region of small grains. Stress is modeled using a closed form solution that considers a linearly elastic contracting ellipsoidal inclusion near the surface of a half space. Several applications of the stress model are discussed. 1.0 INTRODUCTION The newly developing field of micro-electro-mechanical systems (MEMS) employs micron scale thin film members that are totally or partially freed from the substrate. Upon release, the large stresses typically present in thin films often cause device failure by instability, curling, or fracture. In addition, stress affects device performance by, for example, altering the response of resonant microstructures [1] and thin film diaphrams [2]. Polycrystalline silicon (polysilicon) is commonly used for MEMS, and is the thin film considered here. The origin of large tensile and compressive intrinsic stresses is discussed in relation to the evolution of the polysilicon microstructure. Undoped polysilicon films deposited by low pressure chemical vapor deposition (LPCVD) near 600°C consist of more or less equi-axed grains and are tensile, due to an amorphous to crystalline solid state transformation that occurs during the film deposition. At deposition temperatures exceeding 630°C, the grains are columnar and the stress is compressive. The source of compression is still uncertain, but is shown to be related to a transition layer of small grains at the film/substrate interface. Stress in the tensile films is modeled using a closed form solution by Sco and Mura [3] for a uniformly contracting ellipsoidal inclusion near the surface of a half space. By making the ellipsoid's axis normal to the surface very small relative to the other axes, the ellipsoid resembles a contracting film on top of a semi-infinite substrate. The advantage of a closed form solution is the ease
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