Phase transformation of thin sputter-deposited tungsten films at room temperature

Thin films of W have application in. semiconductor interconnect structures as diffusion barriers and potentially as seed layers for electroplating. Thin W films have been deposited by, sputtering [physical vapor deposition (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 muOmegacm. 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 (< 20 degreesC) show beta-phase peaks, which then transform to alpha in 10-20 h at 25 degreesC. 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. (C) 2002 American Vacuum Society.