TRANSPORT PHENOMENA IN TUNGSTEN LPCVD IN A SINGLE-WAFER REACTOR

The influence of the WF6 concentration on the growth rate in tungsten LPCVD from WF6 and H-2 has been studied both experimentally in a coldwall single-wafer reactor and with the use of a mathematical simulation model, predicting the gas flow, heat transfer, species transport, and chemical reactions in the reactor. Model predictions were in very good agreement with experimental growth rates and uniformities. The growth rate was found to be independent of the WF6 inlet pressure above a certain value P(crit), whereas for WF6 inlet pressures below P(crit) the growth rate decreases linearly with the WF6 inlet pressure. It is shown that this transition is due to mass-transfer limitations rather than a change in the reaction mechanism. The value of P(crit) depends on the reactor geometry and process conditions and may be obtained experimentally or from model simulations as presented in this study. It is shown that large concentration gradients may be present in CVD reactors, even at low reactant conversion rates, and that criteria for "gradientless" reactor operation based on conversion rates are incorrect. We propose a better criterion, based on the value of P(crit). It is also shown that thermal diffusion phenomena in coldwall reactors are very important. As a result, WF6 concentrations at the wafer surface will always be significantly lower than the inlet concentration.