NANO-STRUCTURED SILVER COATING ON COPPER PREPARED IN AN ETHANOL-BASED SOLUTION

CHINA PARTICUOLOGY Vol 3, No 5. 271-274. 2005NANO-STRUCTURED SILVER COATING ON COPPERPREPARED IN AN ETHANOL-BASED SOLUTIONZheliang Wei, Dian Tang,and Thomas O KeefeResearch, Fuzhou University, Fuzhou 350002, P. R. ChinaMaterials Research Center, University of Missouri-Rolla, Rolla, MO 65409, USAautHortowhomcorrespondenceshouldbeaddressedE-mail:diantang@yahoo.comAbstract Recently, silver as an electrochemical deposit on copper substrate has been attracting much attention inthe microelectronics field. To deposit nano scale silver particles on copper, immersion plating using cyanide -based bathsis commonly used In this study, non-toxic ethanol was used as the plating solution. Sputtered copper samples wereimmersed in an ethanol-based solution containing 0. 2 g-L silver for silver deposition. The silver deposits were charac-terized by a field emission scanning electron microscope(FE-SEM), an energy dispersive X-ray spectroscope(EDS)and an atomic force microscope(AFM). It was found that the deposited particles are metallic silver. After 3 s immersion,fine particles whose diameters were around 6 nm had covered about 40% of the surface of the copper substrate. After10 s immersion, the copper surface was completely covered by silver particles, the diameters of which have increased toabout 10-15 nm. After the whole surface was covered, a dense and smooth silver coating was obtainedKeywords Ag, Cu, nano-particle, ethanol1 Introductionnano-scale structure of the silver deposits prepared werecharacterized by a field emission scanning electron mi-For its high electrical and thermal conductivities, silver croscope(FE-SEM)and an atom force microscope(AFM)as been one of the most important noble metals used inthe electrical industries in the last century(Kudryk et al1984: Benner et aL., 1991). However, the development of2. Experimentalsilver application has been hampered due to certain unde- 2.1 Sample preparationsirable phenomena such as "electrochemical migrationCopper was deposited on silicon wafer, 1 mm thick andminum and copper as the principal materials for electronic 90 mm in diameter, using Discovery 18 type DC-RF mag-conduction or interconnection purposes( orczynski, 1998: netron sputtering system, under vacuum, at about 1.0x10.Tang et al., 2003). Especially in the semiconductor indus- Torr, and a sputtering power of 300 W. The copper depos-tries, aluminum is used as the first-generation- material for iting was conducted at 36.5 V with argon flowing at a ratemetallization, followed by copper. Due to the solution of the of 26 sccm. After 20 min, a shiny copper coating was de-shortcomings of silver such as metallization, scientists posited on the silicon wafer substratehave begun to reconsider the use of silver(Zhu et al., 2000: An immersion-plating bath was made by dissolving silverTen Kortenaar et al., 2001; Moffat et al., 2002: Baker et al, nitrite in ethanol to make a concentrated ethanol-based2003). Immersion-Ag(I-Ag)as a printed circuit board(PCB) solution containing 0.2 g-L of silver, to which citric acidsurface finish began to be used on telecommunication was added to the amount of 0.5g.Lcircuit pack assemblies some years ago(Wenger et al, The sputtered copper samples for the experiment were2000), primarily due to the need for a flat, solderable sur- 1 cmx1 cm, which were degreased and cleaned by ace-face that provides reliable, high-yield solder assembly tone for 1 min. After drying with compressed air, the sam-through multiple thermal excursions. Now, interest in I-Ag ples were immersed in the immersion-plating bath mairon copper or printed circuit board substrates has grown tained at a temperature of 25C. The plating was con-dramatically(Arra et al., 2004). Publications describing the ducted with ultrasonic vibration in a 28X ULTRAsonik inprocesses for the preparation of I-Ag are however limited strument for time spans of 0, 3, 10 and 60to aqueous baths(Shimizu Ohkubo, 2002; Hirsch& 2.2 Instrumental testsRosenstein, 1996). Trials to get better I-Ag coatings andsearches for different deposition methods have thus be- The morphologies of the deposits were studied using acome keen topics(Ritchie& Robertson, 1997; Huang et al., Hitachio0 field emission scanning electron micro2004). a dense and uniform metallic silver coating con- Scope中国煤化工 pensive X-ray spesisting of nano-particles is most suitable for PCB finishing. troscopyIn this study, an I-Ag on sputtered copper substrate was paratusCNMHgs were also ob-prepared using an ethanol-based plating solution. It is still served using a Nanascope ml system atom force micro-unknown, however, if the microstructure of the I-Ag so scope made by Digital Instruments operating in tappingprepared is reasonable. In this paper, the species and the mode. Cantilevers with integrated silicon tips were em-272CHINA PARTICUOLOGY Vol 3, No 5. 2005ployed and all images were acquired at a scan rate of 1 Hz After 3 s immersion plating( Fig. 1(b), the sample surand a resolution of 512512 pixels. To obtain representa. face remains shiny, but its color has changed a bit. We cantive images of the surfaces, wide areas(typically 12 um) see that fine silver particles have already deposited on thewere first scanned at different parts of the samples before top flat surface of the copper grains. Compared with thethe scan size was reduced to 1 umi um or 2 umx2 um to blank copper surface, the copper grain boundaries becomeobtain the final imageindistinct at some areas, apparently because of the inroadof silver particle3. Results and DiscussionAfter 10 s plating(Fig. 1(c). the color of the surface hastotally changed to silver shiny. The copper grains were no3.1 SEM analysislonger in sight, having been covered completely by silver,Some representative images for the morphology of silvewhich is composed of fine particles and clusters of particlesdeposits after different plating time are shown in Fig. 1. on top of the sample surface. Some holes can still be seenThe image of the morphology of the sputtered copper bein the silver coating, apparently caused by former coppergrain boundariesfore plating is also presented. Because the copper sub- The sample surfaces prepared after 30 s or 60 s platingboth looked silver shiny to the naked eye. The SEM picturecopper looks shiny like a mirror. However, as we can see, of the 60 s sample( Fig. 1(d)shows a dense silver coatingthe surface of the sputtered copper is composed of manygrains, mostly between 80 nm and 120 nm in size, and thecovering the surface with a grain size much smaller thanwidths of the copper grain boundaries are around those of the sputtered copper and filling almost all the10-30nm(Fig.1(a)holes on the underlying copper grain boundaries.400nm(r题Fig. 1 FE-SEM images of the surface morphology of the samples after silver plating for(a)0 s: (b)3 s: (c)10 s; (d)60 s3.2 EDS analysisAfter 10 s plating, the color of the sample changes fromthat of copper to silver shiny, and the percentage of silverThe EDS results in Table 1 show that after 3 s immersion reaches its highest level without further increase with timein silver solution, silver can be detected on the surface, It indicates that silver deposition on copper is a rapidalthough the silver particles are small and scattered to process, much like what happens in Ag immersion usingmake their presence just noticeable. The amount of oxy- aqueous solution, and further that the deposition rate isgen on the surfaces varies little with silver plating time, high at first and stops completely after some time. Thisindicating that the deposited species is metallic silverphenomenon, known as blocking deposit, producessmooth, coherent and adherent silver coating(RitchieTable 1 EDS results for surfaces before and after immersion in silver Robertson, 1997 ). The SEM images shown above verifysolutioncoh中国煤化工results in very smooth,The maximum amountPlating time/sofCNMHGin about 10 s plating,y Uudniyoo wiui p nun y ng deposition up to 60 s94.0However, the coating of the 60 s sample looks denser,113286.1410.2110000possibly due to the re-dissolution of the smaller silver par-ticles that have deposited earlierWei, Tang OKeefe: Nano-structured Silver Coating on Copper2733.3 AFM analysiscles, measuring about 10 nm, could be observed clearly toFigure 2(b)is the AFM image of the morphology of silverhave covered the depressions on the surface of the sub-deposit after 3s immersion plating, showing many highstrate. It should be emphasized that the original copperlands on the surface, sized around 100 nm, much similar to grain boundary network is no longer in sight, possibly tothe underlying copper grains before silver deposition ashave been eroded into wider or more spacious grooves asshown in Fig. 2(a). It is obvious that the silver highlands silver immersion goes on. Silver immersion is a chemicalgrow from the protrusions of the copper substrate, ac-placement reaction: as silver reduces and deposits oncounting for a coverage of about 40%. The sizes of the the surface, copper corrodes and dissolves into the platinggrains measure around 6 nmbath. The other reason might be from the deposition effectsfor silver immersion has its superconformal coverage feaTang et aL., 2005), by which silver particles could fillevery narrow space on the copper surface. As a result, the键Whatever the cause, it is interesting to note that using thisethanol-based bath the 20-40be filled by fine silver particles. Because the immersiondeposited silver particles are very fine, they can be used asa good seeding layer in microelectronics, e.g., for the45 nm trenched chip1000m4. Conclusions(1)Immersion silver on sputtered copper has been pre-pared in an ethanol-based silver nitrite solution. Eperimental results indicate that the deposit consists ofmetallic silver particles. The deposition process is veryfast: in 3 s, the silver coverage is about 40%; in 10 s,silver particles have covered the entire copper sur(2)It is observed that the silver deposit consists of verysmall silver particles. After 3 s plating, the surface iscovered by 6 nm silver particles, and when the wholesurface is covered, the silver particles have grown to10-15nn( 3)It is shown that the silver coating is smooth and dense0after 10 s plating, and even denser and smoother after60 s, when the network of the original copper grainboundaries is no longer visible, having been filled in byfine nano-scale silver particlesAcknowledgmentThis work is part of Research Project numbered 20011011Theauthors are grateful to the financial support of the Department ofScience and Technology of Fujian Province, P. R. ChinaReferencesArra, M, Shangguan, D, Xie, D, Sundelin, J, Lepisto, T.& Ris-tolainen, E(2004). 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