Room Temperature Recrystallization of Electroplated Copper Thin Films: Methods and Mechanisms
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Room Temperature Recrystallization of Electroplated Copper Thin Films: Methods and Mechanisms D. Walther1, M. E. Gross1*, K. Evans-Lutterodt1, W. L. Brown1, M. Oh2, S. Merchant2, P. Naresh1 1 Bell Labs, Lucent Technologies, Murray Hill, NJ 07974 2 Bell Labs, Lucent Technologies, Orlando, FL 32819 Abstract We report a comparison of the room temperature recrystallization of electroplated (EP) copper in blanket films as a function of thickness measured by focused ion beam (FIB) microscope images and sheet resistance measurements. Both sets of data show an increase in rate with film thickness from 0.75 µm up to 5 µm, while little recrystallization is observed in films thinner than 0.75 µm. Interestingly, the recrystallization rates from FIB analysis are consistently faster than those from the sheet resistance measurements. These data suggest that the recrystallization is initiated close to the top surface of the EP Cu film, but that in thinner films a high surface-to-volume ratio allows surface inhibition or pinning to retard the transformation. A Johnson-Mehl-AvramiKolmogorov (JMAK) analysis of the two data sets yields unusually high values for the Avrami exponent α of up to 7 for the FIB data, while lower values of around 4 are obtained for the sheet resistance data. X-ray diffraction pole figures of the films have also been collected and correlations between the crystallographic texture, film thickness and recrystallization are discussed. I. Introduction Cu is increasingly replacing Al as the interconnect metal in high performance integrated circuits for increased speed due to higher conductivity and improved reliability due to higher electromigration resistance.1,2 However, understanding the influence of the microstructure and texture of Cu on its electrical performance and reliability is still at an early stage.2,3 Cu electroplating involves a Cu sulfate/sulfuric acid plating bath to which HCl and organic species have been added to achieve the desirable bottom-up fill by suppression of plating at feature corners and regions between the trenches and acceleration within the trenches.4 This major alteration of the Cu surface chemistry and the exchange current density during plating result in fine-grained (0.1-0.2µm) Cu deposits incorporating a fraction of the organic and inorganic additives and a high density of defects and twins.5-7 The ramifications of the use of additives in the plating bath extends beyond the as-plated microstructure to the subsequent secondary recrystallization that proceeds at room temperature to give a final grain size of up to several microns (Figure 1). This remarkable transformation is related to the influence of additives in the plating bath on the microstructure, composition, defect density and/or strain of the plated films.8-17 The influence of topography, radial position, plating bath chemistry, barrier layers, and low temperature cycling on the room temperature recrystallization rate of EP Cu have been reported recently as indirect means of learning something about the mechanisms. As we
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