The Phase Transformation of Thin Sputter-Deposited Tungsten Films at Room Temperature

Thin films of W have been deposited by sputtering (PVD) at near-room temperature, using Ar as
the working gas, for evaluation of the electrical and structural properties of the films in the
thickness range of 3 to 150 nm. Films deposited at 45 nm or greater thickness are composed of
alpha (bcc) phase (only) with an electrical resistivity of 12 micro-Ohm-cm. Films deposited at
thicknesses below 5 nm are mostly beta (A15 cubic) phase as-deposited with significantly higher
resistivity, which is due partly to the phase and partly to electron-surface scattering (the ‘size
effect’). In the thickness range of 5 to 45 nm, the as-deposited films are mostly beta-phase and
undergo transformation to the alpha phase at room temperature in tens of hours to several days.
The resistivity also declines concurrently, as much as 70%. The exact mechanism driving the
phase transformation is unclear, but is expected to be due to energy stored at grain boundaries
and at the film-substrate interface coupled with the metastable nature of the beta phase. The
transition is thermally driven and can be enhanced by heating or slowed by cooling. The effective
activation energy for the phase transformation measured by the modified Kissenger method for
the change in sheet resistance, the disappearance of the W-beta peaks, or the appearance of the
W-alpha peaks has an average value of 1.1 +/- 0.2 eV. It does not appear that there is any critical
thickness for the transition. However, since the deposition process for PVD-W is intrinsically
hot, thicker films are effectively deposited at higher temperatures, which drives the beta-to-alpha
transition to completion. Thicker films deposited cold (<20C) show beta-phase peaks, which then
transform to alpha in 10-20 hours at 25C. In addition, the scaling of the resistivity of the
resultant alpha W with thickness suggests an electron scattering mean-free-path of 10-12 nm,
much below the reported 41 nm (1).