Copper is the metal of choice serving as interconnect material in advanced submicron multilevel technologies, because its low resistivity and high electromigration resistance. A mayor disadvantage of copper for this application is that it corrodes, especially in aqueous environments containing oxidants. It has been reported that self assembled monolayers (SAMs) of alkanethiols acts as barrier films that retard the oxidation of Cu. However, the tendency of copper to oxidize rapidly at the defects of the modified surface when exposed to air makes the experimental measurement not so reproducible. In order to manage with this problem, in this work we studied for the first time the oxidative adsorption of 1-octanethiol and 1,8-octanedithiol under potential control on polycrystalline Cu surfaces.
A surface pre-treatment which originates a clean, plane and reproducible Cu substrate was optimised. The SAMs were formed by two different approaches. The first consist in expose the Cu electrode in a 5mM solution of thiols in hexanes for one hour. The second one consist in place the substrate electrode into an electrochemical cell and polarize it at an adequate negative potential in a 0.05 M solution of Na2SO4 and inject enough thiols to obtain a micromolar concentration.
The electronic properties of the monolayers formed were studied by appropriate electrochemical techniques such as Cyclic Voltametry (CV) and Electrochemical Impedance Spectroscopy (EIS). Structural and mechanical properties were investigated trough AFM and STM. For the immersion method it was found that monolayers of 1-octanethiol were more compact and stable than those formed with 1,8-octaneditiol, in contrast with previous results reported for Au substrates[1]. For the second method (oxidative adsorption), the cathodic potential at which the monolayers were formed was varied in order to choose the best conditions for the formation of the film. Interfacial capacity and CV measurements indicated that monolayers formed under potential control are more compact and prevent better copper from corrosion than those formed by the immersion method.
Since oxygen molecules permeate through SAMs based on simple alkanethiols the next step is to use this methodology to study multilayers of 1-8octanedithiols and Cu2+.
[1] - M. L. Carot, M. J. Esplandiu, F. P. Cometto, E. M. Patrito and V. A. Macagno;
J. Electroanal.Chem., in press.
