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

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 WF₆. Our calculated results suggest that the overall reactions of the WF₆ 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 WF₆ with the B-covered TiN surface is attributed to the presence of B-covered surface made by B₂H₆ molecules. As a result, an effect of the B₂H₆ flow serves as a catalyst to decompose WF₆ molecules. Two additional reaction processes right after WF₆ bond dissociation, such as W substitution and BF₃ desorption, were also explored to clearly understand the detailed reactions that can occur by WF₆ 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, BF₃ because BF₃ desorption is an energetically favorable reaction with a low activation energy. Furthermore, we also investigated the effect of H₂ 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 H₂ dissociative reaction and HF desorption can occur sufficiently well under somewhat high temperature and H₂ 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.