hydrogen reduction process has been found to be problematic at temperatures of approximately 450 u00b0 c and below for example at temperatures of 435 u00b0 c and below the time averaged deposition rate on air exposed tin falls dramatically as illustrated in fig1 by data points 11 and approximated by curve 10 this figure illustrates this temperature dependence as a plot of the log of the deposition rate versus the reciprocal of the temperature in degrees kelvin the slope of such a curve is proportional to a characteristic 34 activation energy 34 e a for a chemical process or reaction in comparison the deposition rates for the chemical vapor deposition of tungsten onto silicon substrates exhibit a better behaved temperature dependence tungsten deposition rates onto a silicon wafer coated in the same manner as the tin wafer discussed above are represented by the diamond shaped data points of 12 which are approximated by curve 18 in fig1 one indication of the onset of the poor nucleation of tungsten onto a tin surface is a very large variation of film thickness across the tin surface of a wafer as reported by m iwasaki et al in tungsten and other refractory metals for vlsi ulsi applications vol v page 187 s s.