On the thermal expansion coefficients of thin films

The coefficient of thermal expansion (CTE) is an important mechanical property for thin film materials. There are several problems that arise from the thermal expansion effect; for example, the mismatch of thermal expansion between the thin films and the underlying substrate may lead to residual stresses in the thin films. On the other hand, the thermal expansion effect can be exploited to drive microactuators. The CTEs of Al and Ti thin films were determined in the present study using the bilayer microcatilever technique. The contribution of this paper is to demonstrate the variation of the thin film CTE with the film thickness. The CTE of the Al thin film changes from 18.23 x 10(-6)/degrees C to 29.97 x 10(-6)/degrees C, when the film thickness increases from 0.3 to 1.7 mu m. The CTE of the Ti thin film changes from 21.21 x 10(-6)/degrees C to 9.04 x 10(-6)/degrees C, when the film thickness increases from 0.1 to 0.3 mu m. Further, the concept that thin film CTE may be influenced by the defects in thin film materials is proposed. Thus, a desired thin film CTE can be obtained by tuning the film thickness as well as the deposition conditions. (C) 2000 Elsevier Science S.A. All rights reserved.