Thermal Expansion and Residual Strain in AlN Thin Film Sensor Materials

Aluminum nitride films are attractive materials for high temperature sensor devices because of their stable piezoelectric and semiconducting properties up to 800^oC in air, above which the films degrade by oxidation. In this work, epitaxial AlN (002) films were grown at 930^oC on c-cut sapphire substrates by N₂-plasma-assisted Al evaporation and were characterized up to 1000^oC in air using an X-ray diffraction (XRD) sample hot stage. The coefficient of thermal expansion (CTE) and homogeneous strain were determined from accurate measurements of the c-axis lattice parameter for AlN and sapphire vs. thermal processing up to 800^oC, and film oxidation was evaluated above 800^oC from the relative decrease in the AlN (002) XRD intensity and degradation of the (002) pole figure. AlN films grown to 200nm thickness have c-axis compressive strain, but this residual strain can be relieved by thermal cycling between 25-700^oC. The CTE value of 2.5x10^-6/^oC for the as-deposited films decreased as the film strain was released and is below the value of 5.3x10^-6/^oC reported for bulk AlN. Understanding the level of residual film strain and thermal expansion matching across the relevant interfaces is key for preventing delamination, cracking, and failure of sensor packaging or sensor function itself.