Intrinsic stress in sputtered thin films

Abstract

A review of the literature reveals that the intrinsic stress in sputtered thin films can be tensile or compressive depending on the flux and energy of particles striking the film. Stress data indicates that the normalized momentum P*n=γ√ME (where γ is the energetic particle/atom flux ratio, M the mass, and E the energy) may be the appropriate stress scaling factor. The forward sputtering model predicts a √E dependence. An idealized stress–momentum curve is constructed consisting of three regimes:(1) a region of increasing tensile stress at low P*n, due to a porous microstructure, followed by (2) a sharp transition from tensile to compressive stress at intermediate momentum, accompanied by a conversion to zone T-type microstructure and (3) a saturation region at high momentum, due to plastic flow. For any deposition process the sign and magnitude of the stress depends on P*n, which is a function of several deposition parameters. Calculations indicate that stress reversal in sputtered and ion-assisted evaporated films occurs at normalized momentum P*n of about 5–15×10−23 kg m/s/atom (Pn=0.3–1 √eV/atom).