SnÕPd Catalyzation and Electroless Cu Deposition on TaN Diffusion Barrier Layers Hong-Hui Hsu, Ching-Wei Teng, Su-Jien Lin,* ,z and Jien-Wei Yeh Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan The catalyzation of TaN/SiO 2 /Si substrates was carried out by immersion in SnCl 2 /HCl and PdCl 2 /HCl solutions for electroless Cu deposition. The sizes and morphologies of the catalytic sites on the TaN layers were found to be a function of catalyzation conditions, including solution temperature, immersion time, and the surface oxides. The appropriate formula for catalyzation was obtained by considering both the quality and efficiency. The catalytic sites were composed of Sn and Pd, and the ratio of Sn/Pd was about 1.3. During electroless Cu deposition on the catalyzed TaN/SiO 2 /Si substrates, Cu nuclei first formed at the catalytic sites in the early stage, gradually agglomerated into dense islands, and finally merged to continuous deposition films. The Cu films were uniformly and smoothly deposited with a surface roughness of 6.2 nm under a film thickness of 210 nm. The lowest electrical resistivity of the Cu films was 2.5  cm, and the residual resistivity contributed to the participation of Sn-Pd catalyst and internal defects. Good gap-filling capability of electroless Cu deposition on Sn/Pd catalyzed, patterned substrates exhibited its high potential to act as a seed layer for Cu electrodeposition and even to completely fill submicrometer gaps in ultralarge-scale integrated metallization. © 2002 The Electrochemical Society. DOI: 10.1149/1.1436084 All rights reserved. Manuscript submitted July 5, 2001; revised manuscript received September 24, 2001. Available electronically January 29, 2002. Metallization is a critical issue in the production of ultralarge- scale integrated ULSI circuits. As the size of the devices scales down and chip density highly increases, copper Cu has been pro- posed as the most reliable interconnect material to replace aluminum because of its significant advantages of low electrical resistivity, low power dissipation, and high resistance to electromigration. 1,2 Re- cently, Cu deposition by electrochemical methods has received great attention, since high-quality Cu films can be easily obtained at a low deposition temperature and by low tool cost. 3,4 Electroless copper deposition has excellent step-coverage capability for high-aspect- ratio A.R. gaps and can be used either to produce the seed layer for copper electrodeposition or to fill the fine gaps directly. 5-7 Besides, due to the high selectivity, the low processing temperatures, the low cost of raw materials and equipment, and the feasibility, 8 it becomes attractive and is under continuous investigation. However, some problems associated with Cu metallization must be solved, especially, the easy diffusion of Cu into SiO 2 and Si and its poor adhesion to dielectric layers. Therefore, for the successful integration of Cu metallization with integrated circuit IC pro- cesses, proper diffusion barrier layers of refractory metals and their nitrides are required to be placed between Cu and either the dielec- tric layers or the Si substrate to prevent the diffusion of Cu and to improve interfacial adhesion. Tantalum nitride TaN, recognized as one of the most promising diffusion barriers for Cu, not only pro- vides high thermal stability, but also has characteristics such as ac- ceptable conductivity and the chemical inactivity with Cu. 9,10 In addition, TaN does not react with Cu to form intermetallic com- pounds at elevated temperatures. 9 As a consequence, the study of the metallization scheme of electroless Cu deposition on TaN diffusion barrier is interesting and necessary. Electroless metal deposition is a process of autocatalytic nucle- ation and growth, and needs a seed to initiate the reaction. Tantalum nitride, however, is not catalytic to electroless deposition and has to be catalyzed prior to the deposition of electroless Cu. The catalyza- tion of substrate materials using sensitization and activation by im- mersion in SnCl 2 /HCl and PdCl 2 /HCl solutions has been widely studied. 11,12 Sard investigated the catalyzation behavior of carbon substrate and found that after sensitization and activation process, particle-like structures with a size less than 20 Å were present on the substrate, and these particles tended to agglomerate to dense clumps which are an order of magnitude larger in size. 11 Marton and Schlesinger observed that the catalytic sites on the SnCl 2 /PdCl 2 -treated dielectric substrates glass, quartz, and mica, etc. were less than 30 Å diam. 12 Until now, little information about the sensitization and activation of TaN diffusion barriers were pub- lished, although this procedure plays an important role in electroless Cu deposition on TaN. Therefore, in this paper, the catalyzation of TaN layers which were sputtered on SiO 2 /Si substrates was studied. It was carried out by immersion in SnCl 2 /HCl and PdCl 2 /HCl solutions, followed by electroless Cu deposition of the TaN/SiO 2 /Si substrates. The surface morphologies and chemical compositions of the catalyzed substrates and deposited Cu films were examined. The experimental param- eters including solution concentration, immersion time, and wet etching of the native oxides on TaN layers were modified to clarify the catalyzation behavior. The surface roughness, thickness, purity, and electrical resistivity of the electrolessly deposited Cu films were characterized. In addition, patterned TaN/SiO 2 /Si substrates with various sizes of gaps were also catalyzed and then Cu deposited to evaluate the step-coverage capability and gap-filling ability of the electroless Cu deposition. Experimental The substrates used in this study were thermally oxidized 150 mm Si100 wafers, followed by dc magnetron sputtering of 500 Å thick TaN diffusion barrier layers. For patterned substrates, the trenches with a size of 0.4-0.55 m and the vias with a size of 0.13-0.18 m were formed in the thermal oxide layers using stan- dard photolithography and plasma etching processes, then the TaN layers were dc magnetron sputtered onto the patterned oxide sur- faces. These TaN/SiO 2 /Si substrates were degreased and cleaned in acetone for 10 min under ultrasonic vibration and rinsed in deion- ized DI water. Since TaN was not catalytic to electroless Cu depo- sition, the surface activation of the substrates was subsequently car- ried out before Cu deposition. In the catalyzation process, TaN was first wet etched by a BOE buffered oxide etchant, 34.9% NH 4 F, 7.2% HF, and water/HNO 3 solution for 1 min to remove the native oxides, followed by immersion of the etched substrates in a sensiti- zation solution 10 g/L SnCl 2 and 40 mL/L HCl and an activation solution 0.25 g/L PdCl 2 and 2.5 mL/L HCl for 1-7 min at room temperature. Catalyzation at 60°C was also tried to evaluate the effect of temperature on the activation, and some substrates were not wet etched but directly sensitized and activated for comparison to study the influence of native oxides on catalyzation. After each of the sensitizing and activating steps, an identical procedure of 20 s DI water rinse was done to remove the residual chemical solutions. The activated TaN/SiO 2 /Si substrates were then electrolessly Cu depos- ited at a temperature of 60°C. The composition of electroless Cu deposition solution was listed in Table I. * Electrochemical Society Active Member. z E-mail: sjlin@mse.nthu.edu.tw Journal of The Electrochemical Society, 149 3 C143-C149 2002 0013-4651/2002/1493/C143/7/$7.00 © The Electrochemical Society, Inc. C143 Downloaded 19 Dec 2010 to 140.114.66.106. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp