Alcohol-Assisted Deposition of Copper Films from Supercritical Carbon Dioxide Albertina Caban ˜ as, Xiaoying Shan, and James J. Watkins* Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts, 01003 Received December 20, 2002 Device quality Cu films were deposited from solutions of bis(2,2,6,6-tetramethyl-3,5- heptanedionate) copper(II) [Cu(tmhd) 2 ] in supercritical CO 2 (scCO 2 ) using alcohols as reducing agents in a cold wall, high-pressure reactor. At 270 °C and pressures between 200 and 230 bar, deposition of copper by the reduction of Cu(tmhd) 2 with ethanol was selective for catalytic surfaces such as Co and Ni over the native oxide of Si wafers or TiN. At 300 °C and above, depositions proceeded readily on all surfaces studied. Secondary ion mass spectroscopy indicated that Cu films are remarkably pure; carbon and oxygen contamination were on the order of 0.1% or less. Resistivities of the films were approximately 2 µΩ-cm. Reduction of Cu(thmd) 2 with primary alcohols including methanol, 1-propanol, and 1-butanol proceeded readily to yield copper films on Co substrates at 270 °C. Sterically hindered alcohols were less effective at the same conditions. Deposition with 2-butanol required higher alcohol concentrations while attempted depositions with 2-propanol were not successful. Reaction mechanisms consistent with these observations are discussed. Introduction Cu is the preferred material for interconnect struc- tures in integrated circuits due to its low resistivity and superior electromigration resistance. 1 As device dimen- sions recede below 90 nm, techniques that yield high purity, void-free deposits in narrow, high aspect ratio features must be developed. Recently, we reported a new technique called chemical fluid deposition (CFD) that can satisfy these requirements. 2-4 CFD involves the chemical reduction of soluble organometallic compounds in supercritical fluids (SCFs) to yield the corresponding metals. 5 Typically, deposition is initiated upon the addition of H 2 . Supercritical CO 2 (scCO 2 ) is an attractive medium for the depositions because it is nonflammable and nontoxic and has convenient critical parameters (T c ) 31 °C, P c ) 73.8 bar). 6 Moreover, CO 2 technology is under development in other applications in the micro- electronic industry including photoresist drying, 7 de- velopment, 8,9 and stripping. 10-12 Nevertheless, other SCFs can be employed for CFD. 4 In addition to Cu, we have deposited Pt, Pd, Au, Ni, and other metal films using appropriate precursors and reducing agents from CO 2 . 3,13-16 The advantages of CFD over conventional deposition techniques are a consequence of the unique properties of supercritical fluids, which lie intermediate to those of liquids and gases. 6 SCFs, including carbon dioxide, can exhibit densities that approach or exceed those of liquids. Consequently, a number of organometallic compounds exhibit significant solubility in CO 2 . 17,18 Precursor transport and reduction in solution in CFD offer significant advantages compared to vapor phase techniques such as chemical vapor deposition (CVD). First, precursor concentrations are several orders of magnitude greater than those used in CVD, which reduces mass transport limitations and promotes step coverage and feature fill. Second, transport in solution eliminates volatility as a precursor design constraint. For example, fluorine contamination resulting from the use of common Cu CVD precusors such as bis(1,1,1,5,5,5- hexafluoroacetylacetonate) copper(II) [Cu(hfac) 2 ] has * To whom correspondence should be addressed. E-mail: watkins@ ecs.umass.edu. (1) Kodas, T. T.; Hampden-Smith, M. J. The Chemistry of Metal CVD; VCH: Weinheim, 1994. (2) Blackburn, J. M.; Caban ˜ as, A.; Zong, Y.; Quinn, J. D.; Watkins, J. J. In Advanced Metallization Conference (AMC), Montreal, Canada, 2001; Mckerrow, A. J., Shacham-Diamand, Y., Zaima, S., Ohba, T., Eds.; Materials Research Society: Warrendale, PA, 2001; pp 177-183. (3) Blackburn, J. M.; Long, D. P.; Caban ˜ as, A.; Watkins, J. J. Science 2001, 294, 141-145. (4) Caban ˜ as, A.; Blackburn, J. M.; Watkins, J. J. Microelectron. Eng. 2002, 64, 53-61. (5) Watkins, J. J.; McCarthy, T. J Method of Chemically Depositing Material onto a Substrate. U.S. Patent 5,789,027, 1998. (6) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice; Butterworth: Boston, 1986. (7) Goldfarb, D. L.; de Pablo, J. J.; Nealey, P. F.; Simons, J. P.; Moreau, W. M.; Angelopoulos, M. J. Vac. Sci. Technol., B 2000, 18, 3313-3317. (8) Sundararajan, N.; Yang, S.; Ogino, K.; Valiyaveettil, S.; Wang, J.; Zhou, X.; Ober, C. K. Chem. Mater. 2000, 12, 41-48. (9) Chem. Eng. News 1998, 76, 33-33. (10) Bok, E.; Kelch, D.; Schumacher, K. S. 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