Highly Conformal Diffusion Barriers of Amorphous Niobium Nitride
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335 Mat. Res. Soc. Symp. Proc. Vol. 564 © 1999 Materials Research Society
collisions with the side walls of a hole without depositing material. Hence, the molecules are more likely to deposit further within the hole and to give good step coverage. Unfortunately, the observed good step coverage for this process is accompanied by high carbon contamination of the films, high electrical resistivity, and porosity (low density). Better quality titanium nitride films have been obtained by a CVD process for the deposition of titanium nitride from vapors of tetrakis(diethylamido)titanium and ammonia.3 This process operates at substrate temperatures below 400 TC, which are low enough for use in semiconductor microcircuits. The step coverage obtained by this process is adequate for forming barrier layers on microcircuits currently produced with feature sizes around one-quarter of a micron and aspect ratios at least up to4 4:1. The step coverage can be improved by adding trimethylamine to the CVD gas mixture. Another potential problem for the use of titanium nitride in future generations of computer chips is that thinner layers may not be effective diffusion barriers. For features below 0.25 micron, very thin diffusion barriers will be needed so that the barrier material does not take up too much of the space needed for the copper.: Titanium nitride films have a microcrystalline structure. Diffusion of copper through thin titanium nitride barriers might occur along boundaries between the microcrystalline grains. This leakage might be prevented by "stuffing" the grain boundaries with impurities such as oxygen. Amorphous diffusion barriers are expected to perform better than microcrystalline ones because amorphous materials lack intergranular pathways for diffusion. Titanium nitride films can be made amorphous by addition of silicon, but the resistivity increases greatly.6 Amorphous tantalum nitride 7 or niobium nitride8 form the most conductive thin barriers to diffusion of copper that are known. Unfortunately, the sputtering processes commonly used to make amorphous tantalum nitride or niobium nitride do not provide adequate step coverage. Somewhat better step coverage is currently obtained with ionized PVD, but this process may not be successful for future smaller design rules. CVD processes, which might produce better step coverage, were previously reported for niobium nitride 9 and tantalum nitride'° at deposition temperatures above 500 °C, which is too high for most microelectronic applications. Low-Temperature CVD Process for Amorphous Niobium Nitride Barriers This CVD process"' involves the reaction of ammonia gas with the vapors of niobium dialkylamide compounds such as niobium(V) dimethylamide, Nb(NMe 2)5,or niobium(IV) diethylamide, Nb(NEt 2)4. The typical substrate temperatures are below 350 °C, and thus are compatible with silicon semiconductor processing. The carbon content of the deposited films is low (
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