Investigation of coefficient of thermal expansion of silver thin film on different substrates using X-ray diffraction

Silver thin films (200 nm) were deposited on two different substrates, SiO₂ and polyethylene naphthalate (PEN) by e-beam evaporation. The thickness of the Ag thin film on both substrates was determined to be 200 nm by Rutherford backscattering spectrometry. X-ray diffraction analysis showed that the coefficient of thermal expansion (CTE) of Ag on PEN (1.9 × 10^{- 5}/°C) was the same as that of bulk Ag (1.9 × 10^{- 5}/°C) while the CTE of Ag on SiO₂ (3.1 × 10^{- 5}/°C) was much larger than that of Ag on PEN. The results of our study indicate that the CTE of the Ag thin film is affected by the underlying substrate. In the case of Ag on PEN, the perpendicular and the lateral CTE were the same (1.9 × 10^{- 5}/°C) because the Ag thin film expanded freely along all directions. In the case of Ag on SiO₂, however, the lateral constraint of the thin film by the substrate caused anisotropic expansion and the apparent perpendicular CTE (3.1 × 10^{- 5}/°C) was much larger than the apparent lateral CTE (0.54 × 10^{- 5}/°C).