Stress and microstructure in tungsten sputtered thin films

Abstract

Tungsten films, 0.05–1 μm thick, were deposited on silicon wafers by rf magnetron sputtering. Induced film stresses were investigated as a function of substrate temperature and background pressure in connection with microstructural observations and limited compositional analysis. Homologous substrate temperatures Ts/Tm, where Tm is the melting point of tungsten (3683 K), ranged from 0.08 to 0.15. Argon pressures investigated ranged from 1 to 7 Pa. This corresponds to the low-temperature part of Thornton’s microstructure model, within zones I and T. Two regions were distinguished for increasing values of the homologous temperature and/or decreasing values of argon pressure: (i) The first region, observed for low substrate temperature and high argon pressure, was a voided zone I microstructure with small grain size (200–400 Å). Film stresses increased from zero towards tensile values as voids disappeared. (ii) The second region, observed for higher values of Ts/Tm and/or low pAr, was a denser zone T structure, with a bimodal grain distribution (<3000 Å). After reaching a maximum tensile value σ=2×103 MPa, corresponding to the transition from zone I to zone T microstructure, the stress decreased abruptly to a constant value σ=−2.7×103 MPa. All stress values were found to be quite independent of thickness in the range from 0.05 to 1 μm. Even though β metastable phase is usually observed for zone I deposits, a quasipure α film was obtained at low temperature and high pressure (within zone I) under especially clean conditions. Independent of phase composition (α or β phase) the voided zone I was observed to be highly reactive in air. The shift with time toward compressive stress for the zone I structure was attributed to oxygen absorption in the pores. For 3-month-old films, a high oxygen content of ∼20% was measured by nuclear reaction analysis. In the denser zone T films without voids, no change in film stresses was observed over time, and oxygen content was negligible (<1%) after 3 months.