Galvanic Corrosion Between Copper and Tantalum under CMP Conditions Subramanian Tamilmani, a Wayne Huang, b and Srini Raghavan * ,z Department of Materials Science and Engineering, The University of Arizona, Tucson, Arizona, USA Chemical mechanical planarization CMP has emerged as the most viable method to planarize copper thin films during fabrication of integrated circuits. The final stage of copper CMP requires the simultaneous polishing of copper and the barrier metal, where the metals are prone to galvanic corrosion due to exposure to slurry. In this study, the extent of galvanic corrosion between copper and tantalum was estimated using electrochemical polarization measurements. A novel setup was designed to make direct mea- surement of the galvanic current between copper and tantalum and was successfully used to measure galvanic current in two different chemical systems. Galvanic corrosion current values obtained from polarization and direct measurements are compared and their implications during barrier polishing are discussed. © 2006 The Electrochemical Society. DOI: 10.1149/1.2170583 All rights reserved. Manuscript submitted July 11, 2005; revised manuscript received December 12, 2005. Available electronically February 24, 2006. Chemical mechanical planarization or polishing CMP of cop- per is now routinely used for the formation of copper interconnect structures. In a CMP process, planarization of metal and dielectric areas is achieved by polishing a wafer with uneven topography on a polymeric pad held by a rotating platen using a colloidal slurry consisting of submicrometer-sized abrasive particles. Chemicals in the slurry, depending on their nature, play the role of oxidizer, slurry stabilizer, metal ion complexant, or corrosion inhibitor. In the abrasive-free polishing AFP process, the polishing medium con- sists of only chemicals and no particles. 1,2 A typical copper deposition and CMP process involves various stages. Initially, copper is electrodeposited in vias and trenches cre- ated in a dielectric layer such as SiO 2 . Prior to electrodeposition, a thin diffusion barrier layer such as Ta and a copper seed layer are deposited in the trenches and vias. Copper electrodeposition fills the trenches and vias and leaves an overabundance of copper. The ex- cess copper is first removed by CMP process. The next step is to remove the barrier layer and stop on the dielectric layer. An addi- tional overpolish step is often included to ensure all the copper and barrier metal is cleared from the dielectric surface. During the re- moval of the barrier layer it is important that the removal rate of copper is significantly reduced. When all steps are successfully com- pleted, the resulting structure would contain copper vias or lines in a dielectric matrix. During the polishing of the bulk copper, removal rates as high as 6000–8000 Å/min have been obtained using various chemistries. The removal of the bulk copper exposes the underlying tantalum barrier in the field areas. In the second polishing step, copper and tantalum have to be ideally removed at 1:1 selectivity to obtain a planarized surface at the end of the CMP process. Because copper and tantalum are in direct electrical contact during the second pol- ishing step, galvanic corrosion between these materials is likely. The nature and extent of such galvanic corrosion is a strong function of the slurry chemistry. Hydrogen peroxide is the most common oxidant in slurries used for copper CMP. Copper removal rate in these slurries depends on the peroxide level, 3-5 and removal rates as high as 5000 Å/min have been reported. One disadvantage of using hydrogen peroxide is its tendency to decompose, resulting in lower oxidizing strength. In order to maintain the peroxide concentration, titration of additional hydrogen peroxide is often required, which leads to increased pro- cess costs. Hydroxylammonium salts are being considered as alter- natives to hydrogen peroxide because of their higher pot life. 6-8 Hydroxylamine salts are stable for as long as several months with less than 1% degradation. 9 Several studies have shown that the re- moval rates of copper in hydroxylamine-based slurries show a maxi- mum at a pH in the neighborhood of 6. 9,10 In the hydroxylamine system, the redox potential can be controlled by varying the free amine to salt ratio. The literature contains several reports on galvanic corrosion in copper CMP. Brusic et al. have predicted the likelihood of galvanic corrosion between Ta and Cu immersed in aqueous solutions at different pH values, under static conditions, using polarization curves. 11 Tai et al. investigated the extent of galvanic corrosion between Cu and four different barrier materials, Ta, W, WN, and TaN. 12 The galvanic corrosion density followed the order Cu/W  Cu/WN  Cu/TaN  Cu/Ta and were in the range 32– 2 A/cm 2 . Unfortunately, this reference does not provide any de- tails on the chemistry and pH of the system. Direct measurement of galvanic corrosion current density between copper and various bar- rier metals, including Ta, in a variety of chemical systems has been reported in literature. 13,14 In all the above cases the extent of gal- vanic corrosion was either estimated from electrochemical polariza- tion of individual metals or measured directly under static non abra- sion conditions. Because Ta passivates rapidly in many chemistries, direct measurement must be carried out during simultaneous abra- sion of samples. The objective of the study reported in this paper was to develop a method to measure the galvanic corrosion between copper and tantalum while both materials are under abrasion by a pad in the presence of peroxide or hydroxylamine-based slurry. The measured galvanic corrosion rates have been compared to those estimated from polarization curves. Materials and Methods Electroplated copper films of thickness 16 kÅ were used in the experiments. These films were plated on a stack structure of physical vapor deposited Cu  1000 A /Ta  500 Å /SiO 2  1000 A /Si. Tan- talum samples  2000 Å were prepared by physical-vapor deposi- tion on SiO 2 /Si wafers. Two different chemical systems were used in the abrasion experiments. The first type contained 0.5 M hy- droxylamine and the pH of this chemical system was varied by adding sulfuric acid. The second type contained 1.2 M hydrogen peroxide; potassium hydroxide was used to adjust the pH of this chemistry. All chemicals used in the experiments were of microelec- tronic grade. All abrasion experiments were performed on a spe- cially designed laboratory-scale electrochemical polisher EC-AC tool diagrammed in Fig. 1. The electrochemical polisher is designed to polish or abrade metal films and at the same time perform elec- trochemical experiments on those films. A typical abrasion experiment was conducted as follows. A diced copper-plated wafer sample  3  3 cm was placed on a circular copper plate as shown in Fig. 1. The sample was initially spin- coated with photoresist to prevent static etching of the unabraded area of the sample during abrasion experiments. In cases where * Electrochemical Society Active Member. a Present address: Intel Corporation, Santa Clara, California, USA. b Present address: Micron Technology, Boise, Idaho, USA. z E-mail: srini@email.arizona.edu Journal of The Electrochemical Society, 153 4 F53-F59 2006 0013-4651/2006/1534/F53/7/$20.00 © The Electrochemical Society F53