Refractory Metals and Silicides

In the previous chapter, it was shown that it is possible to deposit quality tungsten films by CVD for either selective or blanket applications at moderate temperatures. As-deposited resistivities are low, so post-deposition high-temperature anneals are not essential. 

As research on thermal CVD of tungsten proceeded, parallel studies were being done on plasma-enhanced depositions.16 At the present, commercial application of thermal CVD tungsten has begun, but all the problems have not been solved. Therefore, it will be useful to review some of the PECVD of tungsten studies as well as similar research for other refractory metals or their sili-cides. 

In effect, the hydrogen serves to scavenge any free fluorine in the plasma, thereby promoting the deposition reaction. 

If SiH4 is added to the reactive gas mixture, then tungsten silicide can be deposited.18 In this case. He was used as a diluent, along with the WF6 and SiH4, and deposition was carried out in a parallel-plate, cold-wall plasma 

For films close to stoichiometric, WSi2, the resistivity even after annealing is 100 ju 2-cm. Since these films were amorphous, both before and after annealing, the reduction in resistivity must have been due to outdiffusion of hydrogen and fluorine. The difference, then, between the observed resistivity ( 100 /ifl-cm) and the expected value ( 50 ju 2-cm) may be due to lack of proper crystallization. 

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