Phase composition and microstructure of polycrystalline and epitaxial HfN_x layers grown on oxidized Si(001) and MgO(001) by reactive magnetron sputter deposition

H.-S. Seo, J.G. Wen, J.E. Greene, and I. Petrov

Materials Science Department and Frederick Seitz Materials Research Laboratory, University of Illinois 104 South Goodwin Ave., Urbana, IL 61801

Abstract

The mononitrides of group IVB metals Ti, Zr, and Hf have been used in hard coating, wear-resistant, and diffusion barrier applications because of their excellent mechanical and electrical properties. Unlike the Ti-N, where the TiN is the terminal phase, there are several conflicting reports of existence N-rich phases in the Hf-N system. Here, we report the results of a systematic investigation of the ultrahigh vacuum reactive magnetron sputtering of Hf as a function of the N₂ fraction f_N2 in mixed Ar/N₂ discharges together with the phase composition and microstructure of HfN_x layers grown on MgO(001) and oxidized Si(001). We find a wide single-phase field corresponding to the growth of B1-NaCl-structure HfN_x layers with x ranging from 1.0 (f_N2 = 0.04) to 1.3 (f_N2 = 0.1) on oxidized Si(001) substrates at T_s = 400 °C, and their resistivity values also increase from 99 mW-cm at x = 1.0 to 488 mW-cm at x =1.3. HfN_x layers with x > 1.3 are mixture of B1-NaCl HfN_x and a new N-rich phase, with their resistivity values rapidly increasing to 3´10⁵ mW-cm at x = 1.7 for films grown in pure N₂ ambient.

HfN_x layers grown on MgO(001) at T_s = 700 °C are epitaxial, exhibiting a cube-on-cube relationship: (001)_HfN||(001)_MgO with [100]_HfN||[100]_MgO. HfN_x with 1.17 ≤ x ≤ 1.3 were entirely single crystalline while film with 1 < x < 1.17 develop polycrystalline columns which nucleated at varying film thicknesses. The relaxed lattice constant of HfN_x(001) layers decreases linearly from 0.4526 nm with x = 1.17 to 0.4511 with x = 1.3. Epitaxial B1-NaCl-structure HfN_x films with x = 1.17 exhibited resistivity value of 24 mW-cm and hardness (H) and elastic modulus (E) value, determined by nanoindentation, of 23 and 399 GPa, respectively. Higher nitrogen contents led to increase in r and decrease in H and E.