Measurement of oxide growth stress on thin iron films

Abstract

Thin film stress arises from a constrained volume change when a film is attached to a substrate. The Pilling–Bedworth theory for anionic-diffusion-controlled oxidation predicts large compressive oxide stresses. To our knowledge, we report the first dynamic stress measurement of an evaporated 50 to 150 nm Fe film oxidized at 250 to 350 °C under 6.6×10−2 Pa (5×10−4 Torr) O2 in a LN2 trapped, oil-diffusion-pumped vacuum system. Away from interfaces, the Auger MVV transition peaks at 44 and 51 eV are characteristic of Fe3O4. Comparisons of EXAFS results with spectra of iron oxide standards indicate only the presence of Fe3O4. Type 302 stainless steel substrates are end clamped, with the free end connected to a Cahn microbalance. Stresses are calculated from substrate dimensions, film thickness, and the force exerted by the vacuum microbalance necessary to restore the substrate’s free end to its initial position. The stresses measured are large, but contrary to most oxides, are tensile. Typically, a 50 nm film oxidized at 300 °C under 6.6×10−2 Pa flowing O2 developed a maximum tensile stress of about 1010 dynes/cm2.