Electrochemical Cu Nanoparticle Deposition on TaSiN Diffusion Barrier Films Jingye Li, a Shaoxin You, Matthew J. O’Keefe, and Thomas J. O’Keefe * Department of Materials Science and Engineering and Materials Research Center, University of Missouri-Rolla, Rolla, Missouri 65401, USA High-density Cu nanoparticles were spontaneously deposited on TaSiN diffusion barrier layers using organic solutions. These activated surfaces were then plated with Cu using electroless deposition. The organic deposition solution was composed of conventional solvent extractants that are very poor electrolytic conductors but can sustain short range spontaneous reactions. The process proceeds by an electrochemical displacement mechanism and effective Cu seed layers could be obtained at 35°C in 10 to 20 s. Additives consisting of low formula weight organics were used to enhance the Cu nanoparticle deposition. Other operating procedures, such as substrate etching, solution concentration, additives, agitation, and deposition time, were evaluated to deter- mine their effects on particle density, morphology, and uniformity. The Cu-seeded TaSiN surfaces were then built up using a standard electroless Cu process. A continuous, pore free, smooth, and adherent Cu film was attained after 2 min of electroless deposition. © 2006 The Electrochemical Society. DOI: 10.1149/1.2335616 All rights reserved. Manuscript submitted April 19, 2006; revised manuscript received June 14, 2006. Available electronically August 21, 2006. Copper metallization is increasing in use on silicon integrated circuits because of low bulk resistivity, potentially higher resistance to electromigration and stress-induced voiding, and acceptability for subsequent deposition by electrochemical and chemical vapor depo- sition CVD. 1,2 However, there are several problems associated with copper metallization, including poor adhesion to interlevel di- electrics and movement through the dielectric under field acceleration. 3-5 Moreover, Cu has a high rate of diffusion in silicon and silicides, and forms Cu-Si compounds at temperatures as low as 200°C, resulting in degradation of device characteristics. 6,7 Hence, an effective diffusion barrier between the copper conductive layer and the low dielectric constant interlayer is a prerequisite for Cu metallization. Various materials have been evaluated as diffusion barriers be- tween the Cu metallization and the dielectric and Si substrate. Re- fractory metals are an attractive class of materials because of their high thermal stability and good electrical conductivity. However, the films are susceptible to failure either because the barrier and copper react to form a high resistivity intermetallic or copper interacts with the substrate after diffusing through barriers via grain boundaries. Ternary amorphous metallic thin films consisting of a transition metal, a nonmetal Si or B component and nitrogen have been extensively explored as potential candidates. 8-10 Amorphous Ta-Si-N is one of the effective barrier materials because it does not react with copper, lacks fast diffusion paths, and has a high crystal- lization temperature. 11,12 Copper can be deposited on the barrier layers by physical vapor deposition PVD, chemical vapor deposition CVD, and electro- lytic or electroless deposition. Among these techniques, electroless deposition is attractive in the microelectronics and semiconductor industries due to high selectivity, excellent throwing power, good trench-filling capability, and because no electrical contacts need to be made on the wafers during deposition. Due to the difficulties in depositing Cu directly onto most diffu- sion barriers, an activation step to seed the inert barrier layer is employed. Copper can be deposited by a number of techniques, including CVD, 15 PVD, 16 and electrochemical deposition. Of these techniques, electrochemical deposition is a viable candidate due to its inherent advantages in filling high aspect ratio features, as well as low processing cost. Palladium is a proven seed material to initiate the autocatalytic reaction of subsequent electroless copper deposi- tion. However, Pd reduces the stability of the electroless Cu plating bath and the formation of CuPd alloys increases the resistivity of electroless Cu deposits. 13,14 In this research, Cu was chosen as the activating seed material on TaSiN for the subsequent electroless copper deposition. A novel process for depositing nanoparticle metal activation lay- ers on barrier films for subsequent electroless or electrolytic copper deposition has been under development. 10,17 The process is based on an electrochemical displacement mechanism in which the more noble metal ions in the organic solution are reduced to metal nano- particle crystals at the less noble solid metal substrate. It is similar to immersion plating, but conventional solvent extraction organics, loaded with the metal ion to be deposited, are used instead of an aqueous solvent. The process is unique because the organic solvents used in the plating bath are poor electrolytic conductors and strong polarizers and can sustain only microscale, or even nanoscale, local- ized reactions. Previous research showed that both palladium and copper can be successfully seeded onto various blank and patterned Ti, TiN, TiSiN, Ta, and TaN barrier films using the novel organic medium. 10,17 In this study, the influence of processing parameters, such as deposition time, agitation, additives, and Cu ion concentration on the nanoparticle morphology and density were determined. The overall effectiveness of the seed layer was determined by evaluating the quality of the electroless Cu film formed on the TaSiN surface. Experimental The major process steps involved in forming the Cu coatings on the TaSiN substrate included deposition solution preparation, sub- strate pretreatment, deposition of catalyzing Cu seed crystals, and Cu buildup by electroless deposition. Deposition solution.— The deposition solution was prepared by loading the Cu ion from an aqueous CuSO 4 solution with a concen- tration of 15 g/L. This aqueous solution was mixed with an equal volume of organic extraction solution consisting of 30 vol % D2EHPA di-2-ethylhexy phosphoric acid D, 20 vol % TBP tri- n-butyl phosphate T, and 50 vol % kerosene K30D20T50K to transfer the Cu ion from the aqueous phase. In some cases the rela- tive amount of D to T to K was changed, but unless otherwise indicated 30D20T50K was used. After agitating the mixture for 10 min to extract the Cu 2+ ion into the organic phase the solution was settled, separated, and filtered with silicone treated paper. The aqueous solution was analyzed using atomic absorption spectros- copy to determine, by difference, the copper concentration loaded into the organic phase. The Cu concentration in the organic phase was around 1000 ppm, and was diluted to provide the desired con- centration in a range from 10 to 300 ppm. A 2 vol % HF aqueous solution 50% concentration was added into the deposition solution for in situ activation of the substrate * Electrochemical Society Active Member. a Present address: CookSon Electronics, Enthone Inc., Orange, CT 06477. Journal of The Electrochemical Society, 153 10 C722-C727 2006 0013-4651/2006/15310/C722/6/$20.00 © The Electrochemical Society C722