Highly Conformal Thin Films of Tungsten Nitride Prepared by Atomic Layer Deposition from a Novel Precursor

Highly uniform, smooth, and conformal coatings of tungsten nitride were synthesized by atomic layer deposition (ALD) from vapors of a novel precursor, bis(tert-butylimido)-bis(dimethylamido)tungsten, (BuN)2(Me2N)2W, and ammonia at low substrate temperatures (250-350 °C). This tungsten precursor is a low-viscosity, noncorrosive liquid with sufficient volatility at room temperature to be a vapor source for ALD. These vapors were alternately pulsed into a heated reactor, yielding up to 0.1 nm of tungsten nitride film for every cycle, with no initial delay or induction period. The films were uniform in thickness along the 20-cm length of the deposition zone, as determined by scanning electron microscopy. Successful depositions were carried out on all substrates tested, including silicon, glass, quartz, glassy carbon, stainless steel, aluminum, gold, and copper. The films are shiny, silvercolored, and electrically conducting. All of the films showed good adhesion to the substrates, were acid-resistant, and did not oxidize over time. The stoichiometry of the WN films was determined to be 1:1 by Rutherford backscattering spectrometry. The films were amorphous as-deposited, as shown by X-ray diffraction and high-resolution transmission electron microscopy. 100% step coverage was obtained inside holes with aspect ratios greater than 200:1. Annealing for 30 min at temperatures above 725 °C converted the WN to pure, polycrystalline tungsten metal. WN films as thin as 1.5 nm proved to be good barriers to diffusion of copper for temperatures up to 600 °C. ALD of copper onto the surface of the WN produced strongly adherent copper films that could be used as “seed” layers for chemical vapor deposition (CVD) or electrodeposition of thicker copper coatings.