Water diffusion coefficient measurements in deposited silica coatings by the substrate curvature method

The diffusion of water in silica coatings deposited by evaporation and physical vapor deposition (sputtering) is studied using the substrate curvature measurement technique. The diffusion of water into the coatings induced a swelling, which in turn caused bending (curvature) of the silicon substrate. The curvature change was measured in situ during a humidity increase from 0% to 95% at room temperature. The diffusivity of water in the sputtered silica coating was measured to be 10 × 10−12 cm2/s and achieved equilibrium in about 10 min. The diffusion of water in the evaporated silica coating achieved equilibrium in about 2 min. Because the coatings exhibited very short equilibration times, the impact of a non-instantaneous humidity change on the calculated diffusion coefficients was also examined.