Novel strategy for low-temperature, high-rate growth of dense, hard, and stress-free refractory ceramic thin films

Abstract

Growth of fully dense refractory thin films by means of physical vapor deposition (PVD) requires elevated temperatures Ts to ensure sufficient adatom mobilities. Films grown with no external heating are underdense, as demonstrated by the open voids visible in cross-sectional transmission electron microscopy images and by x-ray reflectivity results; thus, the layers exhibit low nanoindentation hardness and elastic modulus values. Ion bombardment of the growing film surface is often used to enhance densification; however, the required ion energies typically extract a steep price in the form of residual rare-gas-ion-induced compressive stress. Here, the authors propose a PVD strategy for the growth of dense, hard, and stress-free refractory thin films at low temperatures; that is, with no external heating. The authors use TiN as a model ceramic materials system and employ hybrid high-power pulsed and dc magnetron co-sputtering (HIPIMS and DCMS) in Ar/N2 mixtures to grow dilute Ti1−xTaxN alloys on Si(001) substrates. The Ta target driven by HIPIMS serves as a pulsed source of energetic Ta+/Ta2+ metal–ions, characterized by in-situ mass and energy spectroscopy, while the Ti target operates in DCMS mode (Ta-HIPIMS/Ti-DCMS) providing a continuous flux of metal atoms to sustain a high deposition rate. Substrate bias Vs is applied in synchronous with the Ta-ion portion of each HIPIMS pulse in order to provide film densification by heavy-ion irradiation (mTa = 180.95 amu versus mTi = 47.88 amu) while minimizing Ar+ bombardment and subsequent trapping in interstitial sites. Since Ta is a film constituent, primarily residing on cation sublattice sites, film stress remains low. Dense Ti0.92Ta0.08N alloy films, 1.8 μm thick, grown with Ts ≤ 120 °C (due to plasma heating) and synchronized bias, Vs = 160 V, exhibit nanoindentation hardness H = 25.9 GPa and elastic modulus E = 497 GPa compared to 13.8 and 318 GPa for underdense Ti-HIPIMS/Ti-DCMS TiN reference layers (Ts < 120 °C) grown with the same Vs, and 7.8 and 248 GPa for DCMS TiN films grown with no applied bias (Ts < 120 °C). Ti0.92Ta0.08N residual stress is low, σ = −0.7 GPa, and essentially equal to that of Ti-HIPIMS/Ti-DCMS TiN films grown with the same substrate bias.