Properties of reactively sputtered tungsten films in nitrogen and oxygen

Abstract

Tungsten films reactively sputtered in nitrogen or oxygen were characterized by measurements of electrical resistivity and microstructure. The deposition rate of W–N films is slightly reduced with increasing nitrogen partial pressure accompanied by an increase in electrical resistivity. For example, the nominal resistivity of pure tungsten films increases from 11 to 16 μΩ cm as the nitrogen partial pressure increases to 17%. Transmission electron microscopy shows that films are polycrystalline tungsten with bcc structure and typical grain size range of 200 to 300 nm. Upon annealing at high temperature above 700 °C, the effects of nitrogen are gradually diminishing. For example, the high values of resistivity are annealed out and approach that of pure tungsten film after annealing at 1000 °C for 30 min. Annealing does not change the film thickness nor the grain size, although films prepared in nitrogen partial pressure of greater than 7% show precipitates with typical size of 20 nm. The effects of oxygen partial pressure are much more pronounced. The electrical resistivity increases very sharply above 4% oxygen partial pressure. At 12.5%, the resistivity exceeds 400 μΩ cm. Transmission electron microscopy shows bcc tungsten structure in films deposited with oxygen partial pressure up to 6%, and amorphous structure for 12.5%. The average grain size decreases from 100 to 50 nm as the oxygen partial pressure increases from 2% to 6%. Upon annealing two significant changes occur: (1) the amorphous tungsten (for 12.5% oxygen) transforms to polycrystalline structure and (2) all samples show precipitation of WO2 phase. The grain size of these films is not affected by annealing, although a large drop in resistivity is obtained.