TiN reactive sputter deposition studied as a function of the pumping speed

Abstract

TiN is an extremely important barrier material in present day integrated circuits. The most common deposition method is reactive sputter deposition from a titanium target in a argon –nitrogen gas mixture. In reactive sputter deposition of TiN a fraction of the nitrogen feed gas is incorporated in the film. This consumption might lead to non-uniform deposition for large wafersizes and/or low pumping speeds. For relatively low pumping speeds, or a large fraction of incorporated nitrogen, it was expected that the film uniformity over the wafer would deteriorate. TiN was deposited at fixed partial pressures of argon and nitrogen. The pumping speed was varied over one order of magnitude. Even at the lowest pumping speed tested, i.e. at 84% consumption of the supplied nitrogen, the uniformity did not deteriorate. The process proved to be insensitive to the pumping speed. We propose that the explanation of this insensitivity is due to the high reactivity of Ti atoms on the target to N atoms, while the chance for additional nitrogen atoms to be adsorbed at the nitrided target is very small. The target is operated in the poisoned mode which means that the target surface consists of TiN. Calculations showed that the number of N atoms striking the target is about 80 times larger than the flux of Ti atoms sputtered from the target. The sticking coefficient of N2 on Ti is high, therefore the chance for the target to recover its TiN surface after a nitrogen atom or a TiN fragment is sputtered off is high. On the other hand the sticking coefficient of nitrogen on TiN is very low. An increase in partial pressure of nitrogen will not result in extra ‘adsorbed nitrogen’ on the target. Therefore the ratio of titanium to nitrogen atoms removed from the target by the ion bombardment is insensitive to the partial pressure of the nitrogen and hence to the pumping speed.