Cleaning and Passivation of Copper Surface Using N2H4, and Self-Assembled Monolayers for Area-Selective Atomic Layer Deposition (AS-ALD) Applications
Authors: Hwang Su Min , Kim Jin-Hyun , Kim Sang Woo , Jung Yong Chan , Veyan Jean-Francois , Alvarez Daniel , Spiegelman Jeff , Choi Rino , Ahn Jinho , Kim Jiyoung 
Published: December 2020

Copper is widely used in the semiconductor industry as interconnects due to its low resistivity, high resistance to electromigration, low temperature coefficient of resistance, and good thermal stability (1). As advanced nanoelectronics has moved towards the sub-5 nm node technology and beyond, the back-end of line interconnects process is scaled down to smaller pitch sizes with a lot of challenges (2,3). Recently, area-selective atomic layer deposition (AS-ALD) by locally passivating the surface has garnered attention as it can lead to a paradigm shift from today’s top-down VLSI fabrication by not only reducing the number of processing steps but also by alleviating key challenges associated with lithography and layer alignment at the sub-5 nm node (4). However, there are still several challenging factors to implement them in the process node successfully. Despite the enormous scientific effort in recent years, lack of surface science during cleaning and passivation of Cu surfaces impede the development of AS-ALD. Specifically, the initial surface condition of Cu films can affect not only a cleaning surface (i.e., adventitious contaminants and CuxO) but also further passivation process using a self-assembled monolayer (SAMs). Recently, the reduction of the Cu surface using vapor-phase N2H4 has been reported due to its higher reduction capability (4,5).
Herein, the effect of the initial surface condition of Cu samples on both cleaning and passivation of the surfaces was investigated. Electroplated Cu films were pretreated using anhydrous N2H4 for vapor phase surface cleaning. Reflectance absorption infrared spectroscopy (RAIRS) with ALD capability was employed to elucidate the surface chemistry. During surface cleaning, N2H4 reduces the surface oxide (Cu2O) to metallic copper as well as remove adventitious surface contaminants (e.g., –CHx, –CO3, and –OH). In the case of cleaning with CH3COOH, the Cu surface reduces the surface oxide (Cu2O) to metallic copper, importantly the copper acetate which might be the intermediate material was formed after cleaning. After pretreatment, each Cu sample was immersed into 1 mM octadecanethiols (ODTs) in ethanol for 20 hours, then ALD of AlOx using TMA and H2O was performed at 120 oC. The N2H4-treated Cu sample shows better physical and chemical stability during ALD process, resulting in good selectivity compared to the SAMs on the as-is Cu. The detailed experimental results will be presented.
This work is supported by Rasirc Inc. by providing the anhydrous N2H4. This work was also partly supported by the Fostering Global Talents for Innovative Growth Program (No. P0008750) through KIAT and MOTIE. J. Ahn and J. Kim also acknowledge partial financial supports by Brain Pool Program through National Research Foundation by the Ministry of Science and ICT in Korea (Grant #: 2019H1D3A2A01101691).

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