Evaluation of film biaxial modulus and coefficient of thermal expansion from thermoelastic film stress measurements

Abstract

The so-called “double-substrate technique,” a popular method for determining thermoelastic properties of films, involves deposition of a film on two (or more) substrates of differing coefficient of thermal expansion (CTE) and measuring the elastic stress response to thermal cycling to determine the film biaxial modulus and CTE. However, the substrate CTE variation with temperature is often neglected. As an unintended consequence, the calculated film properties will depend on the temperature range used during thermal cycling and data analysis. An analysis technique that allows the film CTE to vary with temperature is proposed, ensuring that the calculated film properties are independent of the temperature range used during thermal cycling. The technique is demonstrated for a NiFe (permalloy) film electroplated on Al2O3–TiC, Si, and glass substrates.