Area-Selective Chemical Vapor Deposition of Thin Films: Suppression of Nucleation on Oxides by Amine Adsorption

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Advanced microelectronic chips, for both civilian and military applications, should be faster, perform more data operations, and consume less electrical power. To meet these goals, the size of future devices will be reduced, e.g., the active width of a transistor will be less than 10 nanometers (0.4 millionths of an inch). Conventional chip fabrication processes involve multiple steps in which a thin layer of material, such as metal, is deposited over the entire area, then a pattern corresponding to the devices is formed over that layer, and unwanted material is removed by chemical etching. However, at such small size the patterning step becomes significantly difficult and costly because the pattern must align precisely with the partially fabricated device below it. In response, this project develops an approach that obviates the need for many of the patterning and etching steps: a chemical vapor deposition route is developed such that a layer of material deposits selectively on top of already-existing pattern features, but does not deposit on surfaces where it is not wanted. It can also enable the fabrication of new, three-dimensional transistor structures and other types of nanoscale device structures beyond microelectronics. Thus, the results of the project boost the manufacturing capabilities and economic competitiveness of the U.S. high technology industry critical to national prosperity and needed for our national defense systems. The scientific and technical results are reported at major professional conferences, in the peer-reviewed literature, and by seminars at major microelectronic industries. The project also supports the development of a diverse U.S. workforce through training of graduate student research assistants, who perform this research as the central focus of their dissertation. Area-selective deposition of thin metal films by chemical vapor deposition occurs when a suitable amine molecule, such as ammonia, is added to the precursor feedstock, such as a metal carbonyl molecule. The metal film, such as cobalt or ruthenium, then deposits on conductive substrate features but not on oxide surfaces such as silica, titania, alumina, or magnesia during the time required to grow the desired thickness of metal film. The role of the amine is to suppress metal nucleation on oxide surfaces, especially those with acidic hydroxyl groups, on which metal islands would otherwise form. This project tests mechanistic hypotheses concerning the interaction of the amine with the oxide surface and with adsorbed precursor, including the roles of site-blocking and conversion of the surface from acidic to basic character. Experimental variables include modification of different oxide surfaces, the use of different precursor-amine combinations, growth temperature, and partial pressures. Analytical methods are in-situ Fourier transform IR spectroscopy to detect the identity of adsorbates, and spectroscopic ellipsometry to detect the onset of nucleation, and ex-situ microstructural probes of adsorbate bonding and quantity. The scientific determination of underlying mechanisms will enable the reliable use of this approach for advanced device fabrication, as well as its extension to other precursors, amines and surfaces.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.