In Situ Characterization of Cleaning and Passivation of Cu Surface for Applications to Area Selective Atomic Layer Deposition

With continuous scaling down towards the sub-3 nm node technology and beyond, the back-end of line interconnects process has confronted several challenges. To circumvent issues, area-selective atomic layer deposition (AS-ALD) is one of the promising techniques with reducing the number of process steps and by alleviating key challenges associated with lithography and layer alignment at the sub-5 nm node.^1–3 However, despite the significant scientific efforts in recent years, lack of fundamental understanding of surface impede the development of AS-ALD. Specifically, <i>in-situ</i>characterization of the metal surface during cleaning and passivation with self-assembled monolayers (SAMs) has been rarely reported.<br/>Herein, post-CMP Cu substrates were cleaned with glacial acetic acid (CH₃COOH), followed by passivation using a 1 mM of octadecanethiols (ODTs) solution for 20 h. An <i>in-situ</i> reflectance absorption infrared spectroscopy (RAIRS) system equipped with an ALD chamber were employed to elucidate the surface chemistry, stability of ODTs during ALD of AlO_x process using TMA and H₂O at 120 ^oC. During surface cleaning with CH₃COOH, adventitious surface contaminants (e.g., –CH_x, –CO₃, and –OH) were removed, and most importantly, the surface oxide (Cu₂O) were reduced to metallic copper by forming copper acetate. In the <i>ex-situ</i> XPS and RAIRS, the SAMs on the CH₃COOH-treated Cu sample exhibits poor selectivity of ALD-AlO_x compared to the SAMs on the as-is Cu, suggesting that the residual copper acetate on the surface can affect the chemisorption of ODTs during passivation. On the other hand, subsequent UHV annealing treatment (~10^-8 Torr at 75 ^oC) after CH₃COOH cleaning of the Cu sample can effectively reduce the copper acetate and residual contaminants on the surface, which can improve not only ODTs quality in the passivation process but also the increase of nucleation delay during the consecutive ALD process. With additional UHV annealing process, nucleation delay of ALD-AlO_x was improved from 2 to 15 ALD cycles. The detailed experimental results will be presented.<br/>The authors acknowledge Lam Research Foundation for the partial financial support and Rasic Inc. for providing the Brute N₂H₄ as well as their partial support.<br/>¹ N.F.W. Thissen et al., 2D Mater. <b>4</b>, (2017)<br/>² L.F. Pena et al., ACS Appl. Mater. Interfaces <b>10</b>, 38610 (2018)<br/>³ M. He et al., J. Electrochem. Soc. <b>160</b>, D3040 (2013)