We report a method for producing thin, completely continuous and highly conductive copper films conformally inside very narrow holes with aspect ratios over 35: 1 by atomic layer deposition (ALD). Pure copper thin films were grown from a novel copper (I) amidinate precursor, copper (I) N, N'-di-sec-butylacetamidinate, and molecular hydrogen gas as the reducing agent. This copper precursor is a liquid during vaporization because its melting point (77 degrees C) is lower than its vaporization temperature (90-120 degrees C). Thus, the transport of the precursor vapor is very reproducible and controllable. Carbon and oxygen impurities were below 1 atom %. The growth per cycle varied from 1.5-2 angstrom/cycle on SiO2 or Si3N4 surfaces but was only 0.1-0.5 angstrom/cycle on metallic Ru, Cu, and Co surfaces. On oxide surfaces, copper atoms form isolated copper crystallites that merge into rough polycrystalline films after more deposition cycles. On Ru and Co metal surfaces ALD Cu nucleates densely, forming smooth and strongly adherent films that are continuous even for films as thin as 4 atomic layers. With 4 nm Cu deposited on 2 nm Ru substrates, the sheet resistance is below 50 Omega/rectangle, which is low enough for making seed layers for electroplating Cu interconnect wires. (c) 2006 The Electrochemical Society.