Transport Phenomena in Tungsten LPCVD in a Single‐Wafer Reactor

Abstract

The influence of the WF~ concentration on the growth rate in tungsten LPCVD from WF6 and H2 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 Petit, whereas for WF6 inlet pressures below Pcr,t 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 Petit 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 Pcrit. It is also shown that thermal diffusion phenomena in coldwall reactors are very important. As a result, WF6 concentrations at the wafer surface will al~vays be significantly lower than the inlet concentration.

Development of a spatially controllable chemical vapor deposition reactor with combinatorial processing capabilities

“…The gas phase reactions associated with these deposition processes are negligible due to low reactor pressure during the process operation (Arora and Pollard 1991;Kleijn et al 1991), therefore, the deposition rate is determined only by surface reaction and chemical species transport rates in the gas/wafer surface interface and through the deposited film itself.…”

“…While W CVD deposition mechanisms and reactor systems have been studied extensively (e.g., Arora and Pollard 1991;Kleijn 2000;Kleijn et al 1991;Kleijn and Werner 1993), open process development issues remain. Even in the simplest case of blanket deposition using H 2 and WF 6 , the reactant/reducing gas ratio and WF 6 gas concentration result in two operational degrees of freedom that force a tradeoff between film conformality and deposition rate, where the latter is compounded by reactor design factors that determine where the transition from reaction rate to mass-transfer limited operation takes place.…”

Simulation‐based design and experimental evaluation of a spatially controllable CVD reactor

“…Thin films of tungsten can be deposited in either a selective or blanket tungsten deposition mode. The gas‐phase reactions associated with these deposition processes are negligible as a result of low reactor pressure during the process operation (Arora and Pollard,1991; Kleijn et al,1991); thus the deposition rate is determined only by surface reaction and chemical species transport rates in the gas/wafer surface interface and through the deposited film itself. The overall reaction of tungsten deposition by hydrogen reduction on a silicon wafer substrate begins with surface reactions by Si reduction during the film nucleation step; the reaction is in the range of deposition temperature used in our experiments (Leusink et al,1992).…”

“…W CVD, used to form vertical interconnects in very large scale integration (VLSI) and ultralarge scale integration (ULSI) circuits, represents a relatively mature, but commercially important, manufacturing process (Ireland,1997), and so was chosen for evaluating the engineering design of the programmable reactor. Although W CVD deposition mechanisms and reactor systems have been studied extensively (for example, Arora and Pollard,1991; Kleijn,2000; Kleijn and Werner,1993; Kleijn et al,1991), open process development issues remain. Even in the simplest case of blanket deposition using H 2 and WF 6 , the reactant/reducing gas ratio and WF 6 gas concentration result in two operational degrees of freedom that force a trade‐off between film conformality and deposition rate, where the latter is compounded by reactor design factors that determine where the transition from reaction rate to mass‐transfer limited operation takes place.…”

Multiscale modeling in chemical vapor deposition processes: Coupling reactor scale with feature scale computations

“…The thermal diffusion coefficients are calculated by an empirical relation built in FLUENT accounting for the Soret effect which causes heavy molecules to diffuse less rapidly towards a heated surface than light molecules. For the binary diffusion coefficients, the viscosity, and the thermal conductivity, Chapman-Enskog formulas (Bird et al, 2002;FLUENT, 2006b) are used; the Lennard-Jones parameters used are summarized in Table 1 ( Kleijn et al, 1991). Regarding the mixture properties, mass weighted mixing rules are used for calculating the viscosity and thermal conductivity.…”