Combining Experimental and DFT Investigation of the Mechanism Involved in Thermal Etching of Titanium Nitride Using Alternate Exposures of NbF₅ and CCl₄, or CCl₄ Only

Thermally activated chemical vapor-phase etching of titanium nitride (TiN) is studied by utilizing either alternate exposures of niobium pentafluoride (NbF5) and carbon tetrachloride (CCl4) or by using CCl4 alone. Nitrogen (N-2) gas purge steps are carried out in between every reactant exposure. Titanium nitride is etched in a non-self-limiting way by NbF5-CCl4 based binary chemistry or by CCl4 at temperatures between 370 and 460 degrees C. Spectroscopic ellipsometry and a weight balance are used to calculate the etch per cycle. For the binary chemistry, an etch per cycle of approximate to 0.8 angstrom is obtained for 0.5 and 3 s long exposures of NbF5 and CCl4, respectively at 460 degrees C. On the contrary, under the same conditions, the etch process with CCl4 alone gives an etch per cycle of about 0.5 angstrom. In the CCl4-only etch process, the thickness of TiN films removed at 460 degrees C varies linearly with the number of etch cycles. Furthermore, CCl4 alone is able to etch TiN selectively over other materials such as Al2O3, SiO2, and Si3N4. X-ray photoelectron spectroscopy and bright field transmission electron microscopy are used for studying the post-etch surfaces. To understand possible reaction products and energetics, first-principles calculations are carried out with density functional theory. From thermochemical analysis of possible reaction models, it is found that NbF5 alone cannot etch TiN while CCl4 alone can etch it at high temperatures. The predicted byproducts of the reaction between the CCl4 gas molecules and TiN surface are TiCl3 and ClCN. Similarly, TiF4, NbFCl3, and ClCN are predicted to be the likely products when TiN is exposed to both NbF5 and CCl4. A more favorable etch reaction is predicted when TiN is exposed to both NbF5 and CCl4 (Delta G = -2.7 eV at 640 K) as compared to exposure to CCl4 only (Delta G = -2 eV at 640 K) process. This indicates that an enhanced etch rate is possible when TiN is exposed alternately to both NbF5 and CCl4, which is in close agreement with the experimental results.