X-Ray Photoelectron Spectroscopy Estimation of Cobalt Seed Layer Reactivity Toward Air Exposure: A Challenge?
Authors: Etcheberry Arnaud , Lakhdari Amine , Caillard Louis , Suhr Dominique , Thiam Mikailou , Raynal Frederic , Simon Nathalie , Gonçalves Anne-Marie , Mevellec Vincent , Frégnaux Mathieu 
Published: May 2020

For the last several generations of CMOS technology, copper has been used widely as an interconnect material in the back end of the line (BEOL). [1] [2] [3] As the current nodes technology are decreasing continuously, several copper scaling challenges appeared such as the Cu barrier thinning, the copper electromigration... [4] [5] Hence, cobalt and ruthenium are proposed as replacement materials. Recently, Co has made inroads in microelectronics, specifically on-chip metallization, serving as a capping layer for Cu to diminish electromigration but also provide a better conductivity for line width inferior to 10 nm [6] [7] [8].
In this very challenging context, the control of Co surface oxidation process is strategic. Conformal Co seed characteristics (i.e electronic conductivity), preceding the following filling chemistry, are partially related to its surface oxide. Its understanding, then its control, will be determinant to access to a perfect and reliable filling of the nodes. To collect quantitative information, we carefully investigate the behavior of a pure Co metallic surface which is exposed to air oxidation. For this purpose, we have used a 400nm thick Co layer electrochemically plated using Kari© aveni acidic chemistry. The Co layer is electrochemically deposited on Co seed (3nm) /Ta/TaN barrier/Tetraethyl orthosilicate (TEOS). The latter, with its native oxide, is introduced in ultra-high vacuum (UHV) of a XPS Nexsa ThermoFisher spectrometer. Then, a totally deoxidized Co surface is obtained with a argon beam sputtering. The initial free oxide bulk metallic Co is then checked by XPS. Starting from this surface, we explore again by XPS, consequences of different air exposure times using the entrance prepation chamber of the spectrometer. According to the time air-exposure, the kinetic of the oxide growth mechanism can be evaluated. Our work shows clearly that the metallic Co surface evolves very quickly to an ultra-thin (<nm) oxide capping layer whose thickness will increase slowly, progressively. However, the metallic signal of the buried metallic Co is always present even for very long-time air-exposure. These results indicate a phenomenological trend over the oxide thickness limit. In our paper, these kinetic studies which are very reproducible in time and spectral modifications will be analyzed as all the different Co oxide phases thanks to the Co2p and O1s spectra from XPS (Figure.1).
Moreover, a simulation of behaviors of this oxide surface coverage immersed in acidic solution will be proposed to understand how an ultra-thin Co seed layer interacts in Co filling solution. For both cases (air-exposure and cobalt filling solution) the results comparison and a discussion will be proposed .
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Figure 1