Thermal Expansion of Low-pressure Chemical Vapor Deposition Polysilicon Films

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R. Ballarini Department of Civil Engineering, Case Western Reserve University, Cleveland, Ohio 44106

A.H. Heuer Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106 (Received 31 January 2002; accepted 1 May 2002)

Polysilicon films were deposited using low-pressure chemical vapor deposition (LPCVD) onto oxidized silicon substrates, after which substrate curvature as a function of temperature was measured. The curvatures changed with temperature, implying that the thermal expansion of LPCVD polysilicon differs from that of the single crystal silicon substrate. Further, polysilicon films with tensile residual stresses displayed an increased thermal expansion, while polysilicon films with compressive residual stresses displayed a decreased thermal expansion. Following high temperature annealing, the residual stresses of the polysilicon films were reduced to near zero, and the thermal expansion of the polysilicon films matched that of the single crystal substrate. The apparent change in thermal expansion coefficient due to residual stress was much larger than predicted theoretically.

I. INTRODUCTION

The microstructure of low-pressure chemical vapor deposition (LPCVD) polysilicon films varies markedly with deposition conditions, particularly the deposition temperature. Films are amorphous at the lowest growth temperatures (lower than approximately 560 °C), display fine (approximately 0.1-␮m diameter) ellipsoidally shaped grains at intermediate temperatures (approximately 560 °C to approximately 600 °C), and contain columnar (110)-textured grains with a thin finegrained nucleation layer at the substrate interface at higher temperatures (>600 °C).1,2 The fine-grained microstructure results from the homogeneous nucleation and growth of silicon crystallites within an as-deposited amorphous silicon film. In this “amorphous” regime, the deposition rate is just slightly greater than the crystallization rate. The columnar microstructure seen at the higher growth temperatures results from the formation of crystalline silicon films as-deposited, with growth being fastest in the 〈110〉 directions. The residual stresses in these films are also sensitive to the deposition temperatures. The amorphous and the columnar films display compressive residual stresses, while the fine-grained films contain tensile stresses. The origin of the tensile stress in the fine-grained polysilicon arises from the slight volume decrease which accompanies the J. Mater. Res., Vol. 17, No. 7, Jul 2002

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crystallization of the as-deposited amorphous material. The origins of the compressive stresses in the amorphous and columnar films are not entirely understood, but excess (interstitial) atoms incorporated during growth is one possible explanation. For polysilicon films deposited onto silicon substrates, thermal contributions to the residual stresses, i.e., thermal expansion mismatch, have not been previously considered, because the thermal expansions