Diffusivity of Moisture in Thin Films

Absorption of moisture by thin films produces a volume increase which changes the film stress toward compression. This effect is the basis of a new technique for measuring water diffusivity. The diffusion coefficient is determined by a least squares fit of the stress data to the diffusion equation, with appropriate boundary conditions. This technique provides a fast, quantitative supplement to the traditional bias-temperature-humidity stress testing. The differences in diffusivities of common passivations are substantial; phosphosilicate glasses are orders of magnitude higher than plasma nitrides. Diffusivities for several passivation materials are given.