Evaluation of W-Si Films Formed by Plasma CVD

Abstract

Refractory metals and their silicides are the major candidates for low resistivity interconnect material for replacing polysilicon. Their films are commonly deposited by co-sputtering or evaporation method. In this paper, tungsten-silicide films are prepared by plasma CVD and their properties are studied by means of X-ray diffraction, Auger electron spectroscopy (AES), and Rutherford back-scattering (RBS). The film properties depend on souce gas ratio (SiH₄ to WF₆), discharge frequency and annealing temperature.
When the source gas ratio is small (SiH₄/WF₆=1), pure tungsten films are obtained, whereas tungsten-silicide films are obtained, when the ratio is large (SiH₄/WF₆=4). By means of RBS, the ratio of Si/W is found to increases from 0.1 to 12 with the source gas ratio. When the ratio of Si/W=0.1, the resistivity decreases to 1×10^-5 ohm-cm after annealing. The decreasing rate depends on deposit frequency and it is found the difference is due to the structural difference of the film.
It is found by X-ray diffraction analysis that the films deposited with small gas ratio had peaks reflecting surface orientations, most intensive of which are W (200) and W (110) for those of 50 kHz and 13.56 MHz, respectively. When the source gas ratio is large, however, it shows the peaks typical of bulk tunsten at as-deposited. After annealing, only films deposited at 50 kHz show tungsten-silicide peaks.