Oxidation of Sputtered Niobium Nitride Films

Isothermal (440°–540°C) weight change measurements were made in 0.1% in Ar on films of cubic niobium nitride deposited on fused silica or sapphire substrates. The data were fit to various rate laws using a fraction‐reacted from 0.2 to 0.9. The best fitting rate equation over these ranges of temperature and fraction‐reacted was the Avrami equation, but several others fit nearly as well. The Arrhenius parameters derived from these data had an activation energy of 44.3 kcal mol−1and a log pre‐exponential term of 9.26 sec−1. The value of the activation energy was virtually independent of the choice of the best fitting rate law. This apparent activation energy agrees well with that reported for the diffusion of oxygen in . It is suggested that the mechanism is analogous to the oxidation of niobium metal and the rate controlling step is diffusion of oxygen through a thin continuous layer of initial product. Studies of the oxidation showed no significant dependence of reaction rate on . The average weight gain was (weight percent) w/o which leads to a film stoichiometry of based upon a stoichiometric product. X‐ray diffraction above 450°C studies indicated that the product films were the low temperature polymorph of . Below 450°C, the entire product films were amorphous or microcrystalline. In spite of the large lattice expansion associated with this oxidation, scanning electron microscopy revealed that the product films were adherent and continuous without large (>0.1 μm) cracks or pores. Matching the thermal expansion of the substrate and film does not seem important since similar results were obtained with both substrates.