Metal and Ceramic Thin Film Growth by Reaction of Alkali Metals with Metal Halides: A New Route for Low Temperature CVD

Jay H. Hendricks*, Maria I. Aquino, Michael R. Zachariah+,
National Institute of Standards and Technology, Process Measurements Division, Gaithersburg, MD 20899

A new low temperature CVD method for the growth of metal and ceramic thin films has been demonstrated. This method involves the use of a low pressure coflow diffusion flame reactor to react alkali metal vapor with metal halide vapor. The reaction chemistry is described by the following general equation: (mn)Na + nMXm ------> (M)n + (nm)NaX, where Na is sodium vapor (or another alkali metal) and MXm is a metal-halide (M = metal or other element such as Si, or C, and X = halogen atom, and m and n are integers). In this reaction, the alkali metal strips halogen from the metal halide. The metal is then free to grow into a thin film on a substrate placed in the reaction zone. Metal nitride or metal oxide ceramic films are easily formed by the introduction of nitrogen or oxygen gases into the reactor. Previously, this technique has been successfully employed for the production of nanoparticles and as an industrial synthesis route for the reduction of bulk metals from their metal-halides. Now, for the first time, this reaction chemistry has been demonstrated to be a viable technique for thin film growth as well. Using the precursors of sodium metal vapor, titanium tetrachloride (the limiting reagent), and either Ar or N2 gases, salt-free titanium (Ti), titanium nitride (TiN), and titanium silicide (Ti3Si5, TiSi2) thin films have been grown on copper and silicon substrates. Guided by theoretical modeling calculations, reactant concentrations were adjusted to prevent gas-phase particle nucleation and growth. Using this technique, we have produced salt-free titanium and titanium nitride thin films on copper substrates heated to 600 oC. This temperature is considerably lower than the 900oC to 1200 oC required for conventional thermal CVD of titanium. A composite salt/Ti film was grown on a silicon wafer at 260 oC, while at 600 oC, a salt-free titanium silicide thin film was produced. The quality and composition of the thin films were analyzed by Scanning Electron Microscopy (SEM), Energy Dispersive Spectrometry (EDS), X-Ray Diffraction (XRD), Raman spectroscopy, and Transmission Electron Spectrometry (TEM).