J. Phys. Chem. C 2008, 112, 1101-1108
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Oxidation of Polycrystalline Copper Thin Films at Ambient Conditions Ilia Platzman,† Reuven Brener,‡ Hossam Haick,*,†,§ and Rina Tannenbaum*,†,§,⊥ Department of Chemical Engineering, Solid State Institute, and Russell Berrie Nanotechnology Institute, TechnionsIsrael Institute of Technology, Haifa 32000, Israel, and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed: August 30, 2007; In Final Form: October 25, 2007
Qualitative and quantitative studies of the oxidation of polycrystalline copper (Cu) thin films upon exposure to ambient air conditions for long periods (on the order of several months) are reported in this work. Thin films of Cu, prepared by thermal evaporation, were analyzed by means of X-ray photoelectron spectroscopy (XPS) to gain an understanding on the growth mechanism of the surface oxide layer. Analysis of highresolution Cu LMM, Cu2p3/2, and O1s spectra was used to follow the time dependence of individual oxide overlayer thicknesses as well as the overall oxide composite thickness. Transmission electron microscopy (TEM) and spectroscopic ellipsometry (SE) were used to confirm the results obtained from XPS measurements. Three main stages of copper oxide growth were observed: (a) the formation of a Cu2O layer, most likely due to Cu metal ionic transport toward the oxide-oxygen interface, (b) the formation of a Cu(OH)2 metastable overlayer, due to the interactions of Cu ions with hydroxyl groups present at the surface, and (c) the transformation of the Cu(OH)2 metastable phase to a more stable CuO layer. These three stages were found to occur simultaneously and to be mutually dependent on each other. The findings of this study may provide guidance in choosing the optimal conditions to fabricate and store copper-based ultra-large-scale integrated (ULSI) circuits.
1. Introduction The continuous drive for miniaturized devices and the desire to extend current capabilities to address future technological needs are among the factors responsible for the increased interest in copper (Cu) as interconnecting material in ultra-large-scale integration (ULSI) devices. This could be attributed mainly to the high thermal and electrical conductivities and low electromigration resistance of copper, as compared with more traditional interconnection materials, such as gold and aluminum.1-7 However, the formation of an oxide layer on Cu (even at room temperature8-11) is thought to induce trap states at the Cu/Cu oxide interface that can ultimately cause a decrease in its thermal and electrical conductivities,12 as well as a significant degradation in its interconnection capabilities.13-15 These effects become more and more critical with the shrinkage of the device dimensions. Kinetic results of the oxidation of Cu thin films and bulk crystals at elevated temperatures were reported in several studies.16-21 The main outcome of these studies indicated that the oxide produced by the thermal oxidation of Cu at temperatures higher than 270 °C was essentially copper(II) oxide (CuO). This oxide grew according to Wagner’s22 “parabolic rate law”, i.e., a diffusion-controlled oxidation mechanism,16,23 where Cu atoms are assumed to migrate from the metal domain through the oxide layer to the oxide-oxygen interface. This process was * Corresponding authors. E-mail:
[email protected] (R.T.);
[email protected] (H.H.). † Department of Chemical Engineering, TechnionsIsrael Institute of Technology. ‡ Solid State Institute, TechnionsIsrael Institute of Technology. § Russell Berrie Nanotechnology Institute, TechnionsIsrael Institute of Technology. ⊥ Georgia Institute of Technology.
assumed to be driven by the concentration gradient of Cu ions between the metal domain and the oxide-oxygen interface. Conversely, the oxidation mechanism of Cu oxidation at low temperatures still remained unresolved, due to limitations in precise measurements, as films grown under these conditions have, most likely, thin oxide layers on their surface. Cabrera and Mott24,25 modified Wagner’s rate law for the oxidation at low temperatures and proposed that the Cu oxide at low temperatures grows in a homogeneous, uniform, layer-by-layer fashion. The oxide film formation is driven by field-enhanced ionic transport that is accelerated at the initial oxidation stages and attenuated with the increase of the oxide layer thickness. On the basis of this model, it was predicted that the electric field, which induced the migration of the metal cations, ceased playing a role when the oxide film thickness reached values on the order of 10 nm. The theoretical prediction of Cabrera-Mott was confirmed experimentally, however, only with partial accuracy, for a (001) plane in a Cu single crystal.26 The reason for this partial success in the correlation between the theoretical and experimental results was recently attributed to the inhomogeneous oxide growth on the Cu surface,27-30 rather than a homogeneous one as assumed in the Cabrera-Mott model. On the basis of these studies,27-29 it was concluded that the surface oxidation for a (100) plane in a Cu single crystal proceeds in three distinct steps: (a) dissociative adsorption of O2, (b) initial formation of copper(I) oxide (Cu2O), and the appearance of highly elongated islands, and (c) formation of highly corrugated CuO islands. Other results showed that oxidation of crystalline Cu(111) planes is negligible compared to Cu(100) and Cu(110), as Cu(111) planes exhibit higher activation barrier for water dissociation.31,32 Several studies have characterized Cu (single) crystal thin film oxidation at room-temperature conditions9,10 by means of
10.1021/jp076981k CCC: $40.75 © 2008 American Chemical Society Published on Web 01/09/2008
1102 J. Phys. Chem. C, Vol. 112, No. 4, 2008 angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and spectroscopic ellipsometry (SE) and, at temperatures slightly above room temperature17 (from 50 to 150 °C), by means of SE and X-ray diffraction (XRD) techniques. Oxidation was characterized for relatively short times, mostly between a few hours9,17 and 50 days,10 after the preparation of the film. The main outcome of these studies was the observation that a layer of CuO has grown on top of a Cu2O layer. The rate at which the oxide grew followed an inverse-logarithmic rate law17 or logarithmic rate law.10 The growth of the oxide to thicknesses between 1 and 8 nm was dependent on the Cu preparation method as well as on the exposure time to air. Only a few studies have characterized polycrystalline Cu thin film oxidation at room-temperature conditions,33,34 by means of sputter etching technology and XPS measurements. The main outcome of these studies was similar to that of the aforementioned crystalline Cu thin film studies. However, two minor differences were observed. The first lay in the shorter time for CuO layer formation, and the second lay in the formation of CuO only after the complete growth of the Cu2O layer. On the basis of these findings, it was concluded that the growth of the CuO layer was exclusively a terminal process. Unfortunately, none of the aforementioned studies proposed an overall Cu thin film oxidation mechanism, especially not as it pertains to a longertime (>50 days) investigation period. In this paper, we provide useful information for fabrication and storage procedures in ULSI technology in issue of Cu thin film oxidation, without employing any physical (e.g., sputtering) and/or chemical (e.g., wet etching) modification techniques that could change the chemical state of the investigated copper or copper oxide layers. We examined the oxidization behavior and growth mechanism of Cu thin film as a function of long time period exposure to ambient conditions, both qualitatively and quantitatively. X-ray photoelectron spectroscopy was used for the qualitative analysis of the Cu thin film oxidation mechanism. High energy resolution XPS data was used in order to estimate the individual overlayer thickness and the overall composite thickness. Spectroscopic ellipsometry and transmission electron microscopy (TEM) were employed to confirm the XPS results. 2. Experimental Section 2.1. Materials. Aluminum (Al) and Cu metals with >99% purity were purchased from Kurt J. Lesker, U.S.A. Two inch double-side polished silicon (100) wafers with roughness
