Journal of Electroanalytical Chemistry 462 (1999) 259–263

Short Communication

Electroless copper plating on a glass substrate coated with ZnO filmunder UV illumination

Y.A. Yang a, Y.B. Wei a, B.H. Loo b, J.N. Yao a,*a Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101, People’s Republic of China

b Department of Chemistry, Uni6ersity of Alabama, Hunts6ille, AL 35899, USA

Received 15 September 1998; received in revised form 15 October 1998

Abstract

A new method of electroless copper plating on ZnO-coated glass substrates has been developed. The method used UVillumination to initiate the plating process in place of a palladium chloride solution activator. The copper layer obtained wassmooth, conductive and adhered to the substrate well. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Electroless plating; Copper; ZnO; Glass substrates; UV illumination

1. Introduction

Electroless plating on insulators has attracted muchattention due to its applications in printed circuitboards (PCBs). Insulator substrates used include plas-tics [1], glass [2,3], ceramics [4,5], etc. Conventionalelectroless plating involves several steps, namely, sur-face etching, sensitization, activation and plating. Ingeneral, tin chloride solution is used as a sensitizerwhereas palladium chloride solution is used as an acti-vator, and the following reaction takes place on asubstrate,

Sn2+ +Pd2+�Sn4+ +Pd0 (1)

Under the catalytic action of palladium, metal cationsare deposited into a metal layer by capturing electronsfurnished by a reducing agent. Adhesion strength andlateral resolution of the metal layer, among otherthings, are properties to be improved on.

ZnO is an important semiconducting material. Thinfilms of ZnO have found application in many areas,including piezoelectric transducers [6], photovoltaics[7,8], chemical sensors [9,10], and optical waveguides[11]. Recently, a new method of electroless copperplating on a thin ZnO film was developed [12]. The thinZnO film was coated on a glass substrate by spraypyrolysis [13,14] of zinc acetate+ethanol solution. Inthis method, no etching or sensitization of the substratesurfaces was required. The Cu layer plated by theelectroless technique adheres rather strongly to thesubstrate, with an adhesion strength exceeding 2.5 kgmm−2. However, palladium chloride solution was stillrequired to initiate the plating process, implying highcost and waste disposal problems for the palladiumcompounds. In the present work, we describe a simpler,yet environmentally cleaner, method of electroless cop-per plating on glass substrates coated with ZnO films.In lieu of a chemical activator, a UV light source isused. We also explore a different method of preparingZnO films on glass substrates by vacuum evaporationof ZnO powder [15].

* Corresponding author. Fax: +86-10-64879375; e-mail:jnyao@ipc.ac.cn.

0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.PII: S 0 0 2 2 -0728 (98 )00405 -7

Y.A. Yang et al. / Journal of Electroanalytical Chemistry 462 (1999) 259–263Y.A. Yang et al. / Journal of Electroanalytical Chemistry 462 (1999) 259–263260

2. Experimental

The precursor ZnO powder was evaporated onto aclean glass slide substrate in a vacuum chamber main-tained at 1×10−3 Pa. The freshly evaporated film wasblack, but after annealing at 500 for 1 h, the filmbecame almost transparent and its transmittance washigher than 80%.

To initiate electroless Cu plating, the ZnO-coatedglass substrate was immersed in a plating bath andilluminated with a UV light. A 400W high pressure Hglamp was used as the light source. After 30–60 sillumination, the light source was turned off. Duringthe electroless plating process, the bath was agitatedlightly to mix the solution, and to remove hydrogen gasbubbles from the ZnO surface.

Distilled deionized water was used to make the plat-ing bath solution, which contained the following ingre-dients: copper sulfate pentahydrate, 15 g l−1; potassiumsodium tartrate, 15 g l−1; EDTA, disodium salt, 20 gl−1; sodium hydroxide, 14 g l−1; formaldehyde, 15 gl−1; potassium ferrocyanide, 0.15 g l−1; PEG 400, 0.5 gl−1. The pH of the plating bath, measured with a REXPHS-3C pH meter, was adjusted to 12.5.

The transmittance of ZnO films on glass substrateswas measured with a Shimadzu UV-1601PC doublebeam UV–vis spectrophotometer. The crystalline struc-ture of the ZnO films and copper layers was studiedwith an X-ray diffractometer (RIGAKU RINT 2000system) using a Cu–Ka source. The identity of thecopper layers was verified by an X-ray photoelectronspectrometer (VSW HA150) at Beijing Synchronic Ra-diation Laboratory, and the morphology and cross-sec-tions of the copper layers were investigated on ascanning electron microscope (Jeol-35CF).

3. Results and discussion

3.1. Electroless copper deposition

Freshly evaporated ZnO films appeared black be-cause some ZnO decomposed during evaporation,forming elemental zinc. If a drop of dilute HCl orH2SO4 solution was added to the film surface, hydrogengas bubbling could be observed, due to the followingreaction:

Zn+2H+�Zn2+ +H2(g) (2)

Annealing freshly evaporated ZnO films at high tem-peratures effectively converted Zn into ZnO. Fig. 1shows the transmittance spectra of ZnO films withvarious annealing temperatures. Although the spectrashown have not been corrected for the backgroundabsorption of the glass substrate, it is clear that theZnO film was opaque or semitransparent (curves a–c)

Fig. 1. Transmittance spectra of ZnO films coated on glass substratesannealed at (a) 300°C; (b) 400°C; (c) 450°C; (d) 500°C; (e) 600°C for1 h.

when annealed at temperatures lower than 500°C. Thefilm became transparent (curve d) when annealed at500°C. However, no further improvement in the trans-mittance was attained with annealing temperatures over500°C (curve e). Hence, for all of the experimentsdescribed below, ZnO-coated glass substrates were an-nealed at 500°C, the optimal annealing temperature.

The XRD pattern (Fig. 2) of the ZnO film annealedat 500°C shows three strong peaks, identified as (100),(002) and (101), indicating that the annealed samplewas polycrystalline. The SEM photograph revealed thatthe ZnO film was about 1.5 mm thick, with a grain sizeless than 0.1 mm. In comparison, the ZnO film preparedby spray pyrolysis [12] was only 0.8 mm thick with agrain size of 0.2 mm. The adhesion strength of thedeposited Cu layer was determined and found to in-crease with the thickness of ZnO film [16]. Hence, the

Fig. 2. XRD pattern of the ZnO film annealed at 500°C for 1 h.

Y.A. Yang et al. / Journal of Electroanalytical Chemistry 462 (1999) 259–263Y.A. Yang et al. / Journal of Electroanalytical Chemistry 462 (1999) 259–263 261

vacuum evaporated ZnO films could provide strongeradhesion at the same grain size.

No Cu deposition occurred when the ZnO-coatedglass substrate was immersed in the plating bath with-out initial UV illumination. However, as soon as theUV illumination was turned on for about 30 s, a thinblack layer was formed on the surface. After some time,this thin layer was covered with a bright layer (theblack and bright copper layers are later identified byXPS as Cu layers, and hereafter they are referred to asthe black and bright copper layers). After the appear-ance of the black copper, electroless deposition wouldstill proceed with the final formation of a bright copperlayer even in the absence of continued UV illumination.No copper deposition on glass surfaces without ZnOcoatings was observed under any conditions. From theabove results, the following conclusions can be drawn:(1) electroless copper plating is initiated only by UVillumination, and it occurs only on a ZnO-coated glasssurface; (2) both the black copper and the freshlydeposited bright copper have some catalytic activity insustaining the deposition process in the absence of UVillumination, once the deposition has been initiated.

The first conclusion can be easily understood becauseZnO is a semiconductor with a bandgap Eg of 3.2 eV[17], which is accessible only by UV radiation. Harbourand Hair [18] have investigated the reactions on thinZnO films under UV illumination, and they can bedescribed as follows:

ZnO+h6�ZnO*+h+ +e− (3)

h+ +H2O�OH+H+ (4)

or h+ +OH−�OH (5)

In the electroless plating bath, the following reactionswill continue [19]:

OH+HCHO�HCOOH+H (6)

H+OH−�H2O+e− (7)

Cu2+ +2e−�Cu0 (8)

When ZnO was excited by light with energy greaterthan its bandgap Eg, electron-hole pairs were formedaccording to Eq. (3). The holes underwent reactions (4)or (5), generating OH species, which then reacted withformaldehyde, a component in the plating bath, toform formic acid and H (Eq. (6)). The generated Hatom continued to react with OH− and formed H2O,supplying an electron to the valence band of ZnO (Eq.(7)). On the other hand, electrons from the conductionband reacted with Cu2+ ions, forming elemental Cu(Eq. (8)). The photo-reactions (Eqs. (3)–(8)) proceededrapidly, so copper cations were reduced to fairly smallcopper grains.

The black copper had a strong catalytic action, andonce it was formed on the ZnO surface, it could sustain

Fig. 3. A scheme illustrating a sequence of events that shows electro-less Cu deposition on ZnO-coated glass substrate.

electroless copper plating. At this stage, UV light nolonger had any effect on electroless plating. This wasbecause the UV light could not penetrate the blackcopper layer and reach the underlying ZnO film. As theCu layer grew thicker and thicker, it appeared bright incolor. Hydrogen gas evolution was also observed tooccur. Hence, catalysis by the copper layer can bedescribed by the reactions,

2HCHO+4OH−�2HCOO− +2H2O+H2(g)+2e−

(9)

Cu2+ +2e−�Cu0 (10)

which took place in lieu of reactions (4–8). Formalde-hyde reacted with hydroxide ions on the copper surface,producing electrons which then reduced Cu2+ ions toelemental Cu [20]. From the above analysis, electrolesscopper plating can be visualized as a three-stage pro-cess: the first is the photo-reaction of the semiconduct-ing ZnO film with UV illumination (Eq. (3)), the secondis the formation of the black copper layer (Eqs. (4)–(8)), and the third is the formation of a thicker brightcopper layer (Eqs. (9) and (10)).

To observe the catalytic action of the black copperlayer, the following sequence of experiments was car-ried out, as depicted in Fig. 3. One half of the ZnO-coated substrate was covered (called covered region,c-R) from UV illumination, while the other half wasnot covered (uncovered region, u-R). When the u-Rwas illuminated in the plating bath, a thin layer of theblack copper was formed (state 1 in Fig. 3). At thistime, the UV illumination was stopped and the cover ofthe c-R removed, with the substrate still remained inthe bath (state 2). After a few seconds, it was observedthat the black copper expanded into the c-R, at a speedof about 1 mm min−1 (state 3). After about 20 min, allof the ZnO film was covered with a bright copper layer(state 4).

Y.A. Yang et al. / Journal of Electroanalytical Chemistry 462 (1999) 259–263Y.A. Yang et al. / Journal of Electroanalytical Chemistry 462 (1999) 259–263262

In state 3 of Fig. 3, both the black and bright coppercoexisted on the ZnO film, and they were viewed fromthe edge side of the glass slide substrate with SEM. Fig.4A–C shows the SEM photographs of the regions A, Band C in Fig. 3, respectively. Fig. 4A reveals that thethickness of the ZnO film was about 1.5 mm, and that

of the black copper was about 1 mm. Fig. 4B shows thatthe grain size of the bright copper was 2–3 mm. After60 min of plating, the ZnO film surface was covered bya bright copper layer of about 10 mm thick (Fig. 4C).The growth of copper in the vertical direction, asshown in Fig. 3, under the catalytic action of the brightcopper was about 0.15 mm min−1, which was aboutone-seventh of that of the black copper. Thus, it isconcluded that catalytic power of the black copper wasbetter than that of the bright copper.

3.2. Characterization of the black and bright layers

The XPS spectra of the black and bright layers gavepeaks at 932.5 and 952. eV. They agreed with the values932.4 (2P3/2) and 952.2 eV (2P1/2), reported previouslyfor Cu [21]. Hence, both the black and bright layers areidentified as Cu layers.

During the initial deposition, very small grains (B1mm) of copper were formed on the ZnO surface. As aresult, they absorbed most of the light and the filmappeared dark. As the Cu layer grew in thickness,smaller grains of Cu aggregated to form larger grains,which reflected most of the light. Hence, the Cu layerappeared bright.

Fig. 5a shows the morphology of the bright copperlayer prepared by electroless plating with UV illumina-tion. The grains were uniform and were mostly spheri-cal. Their size was about 1.5–2.5 mm. The XRD patternof the bright copper layer, shown in Fig. 5b, is inaccordance with that of standard pure copper-surfacecubic crystals [22]. In addition, the conductivity coeffi-cient of the Cu layer was found to be 5.7×107 S m−1,which was nearly the same as that of a standard Cusample, 6.0×107 S m−1 [23].

To see how well the copper layer adhered to the ZnOfilm, the following three experiments were performed,and the results indicated that the electroless platedcopper layers adhered strongly to the substrates:1. The electroless plated Cu layers were kept at 200°C

for 12 h and at 500°C for 8 h. In both cases, noblistering in the copper layers was observed;

2. The copper layers were pulled with adhesive tapesfor 50 times, no damage to the copper layers wasobserved;

3. The copper layers were pulled with weights attachedto the layers with an epoxy. When the weightsreached 2 kg mm−2, the epoxy separated from thecopper layer. However, the copper layers remainedintact.

4. Conclusions

A novel method of electroless copper plating on glasssubstrates coated with ZnO films has been developed.Fig. 4. SEM photographs of the regions A, B, C in state 3 of Fig. 3.

Y.A. Yang et al. / Journal of Electroanalytical Chemistry 462 (1999) 259–263Y.A. Yang et al. / Journal of Electroanalytical Chemistry 462 (1999) 259–263 263

Fig. 5. (a) SEM photograph of the bright copper, and (b) its corresponding XRD pattern.

The vacuum-evaporated ZnO films were first annealedat 500°C, the optimal annealing temperature, and theannealed films were polycrystalline. The electroless cop-per plating was initiated when ZnO-coated glass sub-strates were illuminated with a UV light. The electrolesscopper plating process consisted of three stages: (1) thephoto-reactions on the ZnO film; (2) the formation ofan initial finer grain black copper layer; and (3) theformation of a coarser grain bright copper layer. Theblack copper layer had a strong catalytic activity, andonce it was formed on ZnO film, it would sustainfurther electroless copper plating in the absence of UVillumination. The bright copper layer was producedafter electroless plating had proceeded for some time. Itwas smooth and conductive, and adhered to the sub-strate well. All measurements indicated that the copperlayer fabricated in this manner may be suitable for usein PCBs.

Acknowledgements

The authors are grateful to Professor Fengqin Liu,Drs Rexi Kui, Haijie Qian and Lin Guo for XPSexperiments. This work was supported by the NationalScience Foundation of China and Chinese Academy ofSciences.

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