Chemical Vapor Deposition (CVD) of Tungsten Nitride for Copper Diffusion Barriers
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CHEMICAL VAPOR DEPOSITION (CVD) OF TUNGSTEN NITRIDE FOR COPPER DIFFUSION BARRIERS Roy G. Gordon, Jeffrey Barton and Seigi Suh Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138
ABSTRACT A new process was developed for deposition of the tungsten nitride at moderate substrate temperatures (350-400 oC). The tungsten source bis(tert-butylamido)bis(tertbutylimido)tungsten, (tBuNH)2(tBuN)2W, reacts with ammonia in a low-pressure CVD reactor. Growth rates ranged from about 0.6 to 4.1 nm per minute. The stoichiometry of the films varied from WN1.2O1.7 to WN1.6O0.25, depending mainly on deposition temperature. The films are amorphous by X-ray diffraction, and smooth by scanning electron microscopy. Step coverage is nearly 100% in vias with an aspect ratio of 6:1 for films deposited at 400 oC or lower. Barriers 45 nm thick resist diffusion of copper up to temperatures of 600 oC. Adhesion is strong to all substrates tested, including silicon, silicon dioxide, soda-lime glass, glassy carbon, aluminum and stainless steel. This new halogen-free process avoids halogen contamination of films and corrosion of equipment. Uniformity of thickness and stoichiometry are readily achieved. This process is a promising method for forming copper diffusion barriers in future generations of microelectronics. INTRODUCTION Electrically conductive barriers are a critical component of the metal interconnections in microelectronics. Although barriers are essential to stabilizing tungsten and aluminum interconnections, the increasing use of copper places even greater demands on the performance of barriers. Because of the high diffusivity of copper, all potential gaps in the barrier, such as grain boundaries, pinholes or cracks must be rigorously excluded. Furthermore, the deposition process for a barrier must be conformal over walls and bottoms of holes (vias) and trenches having increasingly high aspect ratios. Titanium nitride has been the most commonly used barrier material with tungsten and aluminum wiring. Limitations of titanium nitride show up with copper, including leakage along grain boundaries in the polycrystalline TiN and the need for relatively thick barriers that consume too much of the space needed for the low-resistivity copper. Sputtered tantalum nitride normally has an amorphous structure and has been found to be an effective barrier in thinner layers than TiN. However, the step coverage of sputtered tantalum nitride may not be adequate for geometries with higher aspect ratios, and its resistivity is higher than desired. Other transition metal nitrides are promising as potential barriers, including niobium nitride1 and tungsten nitride.2 Tungsten nitride is particularly notable in that it has the lowest electrical resistivity (in single crystal form) of any of the transition metal nitrides. Although tungsten nitride can be successfully deposited by sputtering, it is likely that a CVD method would be needed to produce adequate step coverage. CVD of tungsten nitride has often been carried out using
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