Overall reaction mechanism for a full atomic layer deposition cycle of W films on TiN surfaces: first-principles study

We investigated the overall ALD reaction mechanism for W deposition on TiN surfaces based on DFT calculation as well as the detailed dissociative reactions of WF(6). Our calculated results suggest that the overall reactions of the WF(6) on the B-covered TiN surfaces are energetically much more favorable than the one on the TiN surfaces, which means that the high reactivity of WF(6) with the B-covered TiN surface is attributed to the presence of B-covered surface made by B(2)H(6) molecules. As a result, an effect of the B(2)H(6) flow serves as a catalyst to decompose WF(6) molecules. Two additional reaction processes right after WF(6) bond dissociation, such as W substitution and BF(3) desorption, were also explored to clearly understand the detailed reactions that can occur by WF(6) flow. At the first additional reaction process, W atoms can be substituted into B site and covered on the TiN surfaces due to the stronger bonding nature of W with the TiN surface than B atoms. At the second additional reaction process, remaining atoms, such as B and F, can be easily desorbed as by-product, that is, BF(3) because BF(3) desorption is an energetically favorable reaction with a low activation energy. Furthermore, we also investigated the effect of H(2) post-treatment on W-covered TiN surface in order to remove residual F adatoms, which are known to cause severe problems that extremely degrade the characteristics of memory devices. It was found that both H(2) dissociative reaction and HF desorption can occur sufficiently well under somewhat high temperature and H(2) ambience, which is confirmed by our DFT results and previously reported experimental results. These results imply that the understanding of the role of gas molecules used for W deposition gives us insight into improving the W ALD process for future memory devices.