What is a controlling factor of the strength of thin films?

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A deposition of thin films on a substrate and etching process are a principal method in order to fabricate micro machine or nano machine. A construction of film has been changed from a single-layer film to.a multi-layer film to make a three dimensional machine structure. It has been cleared that a large.scale of residual stresses is formed in these thin film structure and these residual stresses will cause a dimensional change in the structure through fabricating stages. The strength of materials constructing a structure is also important and must be investigated. Stress migration is one of the factors that weaken a film structure in a way that an atomic migration due to stress change in the films control a strength of the film. There is a possibility that nano-size and multi-layer film structure increase the strength of films.This program investigated a thermal stress behavior in Al films and Cu films deposited on silicon substrates by synchrotron radiation. The main results are as follows:(1) Thermal stresses in a film, 10, 20 and 50 nm in thickness, deposited on a thermally oxidized silicon film changed in a linear way during thermal cycling from room temperature to 300 deg in C. Al film is strengthened by down sizing to nano-size thickness.(2) Stresses in a 100 nm thick Cu layer of Cu/A1N multi-layer film behaves non-linear change in the first heating cycle but after that stresses behave in a linear way in cooling and heating stages. Cu films strengthen by constructing a multi-layer structure with AlN film.