Physicochemical properties in tungsten films deposited by radio-frequency magnetron sputtering

The microstructure, reflectivity, resistivity, and oxygen contamination of thin tungsten films (<250 nm) deposited by rf magnetron sputtering on silicon substrates are reported. Three structural stages, depending on the operating conditions, are found to occur during the deposition: (i) In the range of the rf applied power density studied (<6 W cm-2), an amorphous structure is always observed when the film thickness does not exceed ∼80 nm. The resistivity remains at a fairly high level (≅1.3 μΩ m). (ii) Upon further deposition and if the power density is <0.6 W cm-2, the β-W phase is detected. (iii) A thermally activated transformation of the β-W phase into pure α--W occurs for a critical temperature ∼150 0C during depositions carried out at higher powers(≥1 W cm-2). The resulting β-W or α-W films consist of small grains (5--20 nm) which present a low dislocation density. Resistivity and reflectivity are mainly related to the oxygen content of the films. When the rf deposition power density is low (≤0.6 W cm-2), both high oxygen contamination levels (>10 at. %), large resistivity (>>1 μΩ m), and low reflectivity (<48%) are concurrently observed, whatever the thickness. The oxygen contamination is controlled by the deposition rate and by the substrate temperature. Long-run depositions carried out at high power densities (>2 W cm-2) exhibit a very low contamination degree (<1 at. %)