Out-of-Plane CTE Measurement Method for Freestanding Thin Films

An out-of-plane linear coefficient of thermal expansion (CTE) measurement method was developed to overcome the difficulty in measuring the in-plane deformation of freestanding thin films with a thickness of less than 1 μm. The out-of-plane profile measurement was relatively easy with the added advantage of simplicity, easy specimen preparation, and a simple test setup. White light interferometry was used for determining the out-of-plane thermal deformation as a function of temperature. Two types of supporting substrates, silicon and ZERODUR®, were used to account for the substrate effect on the CTE measurement. Attempts were made to fit the measured profiles using several peak functions and then find the optimal one. The test procedures were demonstrated using a freestanding 530-nm-thick aluminum film that was used in a previous in-plane measurement method. The calculated CTE value of this aluminum film was 23.7 ppm/°C, which was in good agreement with the in-plane measurement result. The out-of-plane CTE measurement method incorporating a zero CTE substrate was shown to be the most convenient and straightforward method.