Electroless metallization of sic after laser ablation

Surfaceand CoatingsTechnology80 (1996)224-228

Electroless metallization of Sic after laser ablation G.A. Shafeeva, L. Bellard b, J.-M. Themlin b, C. Fauquet-BenAmmar b, A. Cros b, W. Marine b** a General Physics Institute of Russia Academy of Sciemes, 38 Vavilov Sty., 117942 Moscow, Russia b URA CNRS 783, Groupe de Physique des Etats Condens&, Famltd des Sciences de Lwniny, Case 901, F-13288 Marseille,

France

Abstract Experimental results are presented on the ablation of Sic ceramics in air under irradiation with XeCl and ArF excimer laser beam. X-Ray photoelectron spectroscopy analysis reveals the enrichment of the ablated areas in Si under laser beam energy densities of l-3.5 J cm-‘. At higher energy densities the ablated areas contain Si oxide. The non-stoichiometric ablation of Sic allows one to activate selectively the Sic surface for metal deposition from electroless plating solution. Both Ni and Cu deposits show excellent adherence to both Sic ceramics and single-crystal Sic. Keywords: Laser ablation; Metallization; Sic; Electroless coating

1. Introduction Sic is an attractive material for high temperature electronics. This requires the elaboration of various Sic

treatments. In particular, laser-assisted processing is a promising technique for tailoring the solid surface. Recently, excimer laser machining of Sic has been reported combining laser irradiation of single-crystal Sic with further mechanical removal of the modified surface layer [ 11. Furthermore, laser ablation seems to be quite an interesting technique for Sic processing leading to micron-sized features. In this paper we present experimental results on the excimer laser ablation of Sic in air. X-Ray photoelectron spectroscopy (XPS) reveals the preferential ablation of C and the formation of an SiO, layer. Atomic force microscopy (AFM) evidences a significant increase in surface roughness. Owing to the incongruent ablation, the irradiated parts of Sic show the ability to reduce metals from various electroless plating solutions. The mechanism of laser activation of Sic is discussed in relation to a similar activation process of some other dielectrics such as SiO,, Al,O, and diamond [ 2-41.

crystals doped with nitrogen ( 1017-1018 cmu3) with the c axis perpendicular to the surface. The irradiation of the samples was carried out in air with either an XeCl (308 nm) or an ArF (193 nm) UV excimer laser. The as-received Sic ceramics have a significant excess of carbon. Prior to irradiation the ceramic samples were thus annealed in air for several hours at 500 “C. The Sic samples were dipped for 30 min in HF to dissolve the native oxide layer. The morphology of laser-ablated parts of the samples was studied by AFM. XPS measurements were carried out using unmonochromatized Al Ka radiation at 1486.6 eV. The spectra were not corrected for eventual charging effects and were taken at an intermediate electron take-off angle of 20” (spectrometer axis with respect to surface normal). The composition of both Cu and Ni electroless baths is given elsewhere [ 51. The Cu electroless deposition was carried out at room temperature (pH 12), while Ni was deposited at 100 “C and pH 5. The adherence of the deposits was measured by a tensile stress test.

3. Results and discussion 2. Experimental details Two types of Sic samples were used, namely hotsintered ceramics and compensated Sic (c-Sic) single * Correspondingauthor. 0257-8972/96/$15.00 0 1996ElsevierScienceS.A.All rightsreserved ssnr

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Figs. 1 and 2 show the C 1s and Si 2p XPS core level spectra of Sic ceramics and single crystal. The peak at a binding energy of 283 eV is attributed to carbon in Sic. The Si 2p peak is centred around 100.5 eV, corresponding to silicon in Sic. Fig. 3(a) shows an AFM image of the initial c-Sic single-crystal surface. One can

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cis

Si 2p hv=l486.6eV

hv=l486.6eV

295

290 285 280 Binding Energy (eV)

275

110

105 100 95 ,Binding Energy (eV)

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Fig. 1. C 1s and Si 2p core level XP spectra Sic ceramics ablated in air with an ArF excimer laser. The shift in both spectra at an energy density of 3.5 J cm-* is due to charging of the SiOz layer. I

CIS hv=l486.6

Irradiated

I

I

I

I

I

Si 2p hv=l486.6 e’

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Irradiated

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I

+ HF

Irradiated

i

290

285

280

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3

I 108

I 106

I 104

I 102

I 100

Binding Energy (eV)

I 98

9

Fig. 2. C 1s and Si 2p core level XP spectra c-Sic ablated in air with an XeCl excimer laser at an energy density of 3.5 J cm-‘.

see that the surface is rather flat, with some defects due to crystal growth. ArF excimer laser irradiation of Sic samples in air at laser fluences of l-3.5 J cme2 results in the preferential ablation of carbon and the oxidation of Si. Figs. 1 and 2 show the Si 2p and C Is core level XPS spectra recorded after irradiation at fluences of 1 and 3.5 J cmv2. For the lower fluence the Si 2p levels characteristic of Sic around 100 eV split into two peaks attributed to

elemental Si (Si-Si covalently bonded atoms) and oxidized Si. The C 1s core level of the characteristic Sic peak near 282.5 eV disappeared completely. The ablation is accompanied by a progressive decrease in carbon concentration for larger fluences. Drastic incorporation of oxygen leads to the formation of a thick SiO, surface layer, as evidenced by its spectroscopic signature on the Si 2p levels around 103 eV and the observed charging effect. These results are consistent with recently reported

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observations on the ablation of Sic with a KrF excimer laser [ 11. The oxide layer is responsible for the change in reflectivity of Sic ceramics in laser-ablated parts, which look brighter than the dark virgin surface. The reflectivity of the samplerecovers after the HF dip which removes silicon oxide. Laser ablation of c-Sic is accompanied by a drastic increasein surfaceroughness.The topology of the crystal surface after ablation with an XeCl excimer laser at an energy density of 3 J cmm2is shown in Fig. 3(b). The characteristic scale of the surface roughness is about 150-200 nm. According to the XPS analysis,this surface is mostly composed of SiO,. Further processing of the ablated surfacein HF does not influence much the largescalesurface roughness (Fig. 3(c)). At the same time the specific surface of the ablated area increasesowing to a small-scalemodulation of the relief. The irradiated Sic samples promote the electroless deposition of both Cu and Ni from their corresponding solutions despite the presence of the SiOz film on the surface. The catalytic activity of the ablated samplesis stable with time. Many months after sample irradiation, electrolessmetal deposition is still successful.Moreover, the metal deposit can be dissolved in acid and deposited many times on the ablated areas. These features are similar to earlier observations on the laser-assistedactivation of Al,O, and diamond [ 3,4]. Metal deposition is only observed on the irradiated parts of the Sic surface. The best spatial resolution of the deposition is about 10 urn. The rate of metal deposition under multishot ablation does not depend significantly on the number of laser shots either on ceramics or c-Sic. However, a single-shot irradiation of a c-Sic sample at an energy density of 3-3.5 J cmB2 at 308nm does not causeany activation for metal plating, though the formation of an SiO, layer is already observed. Dipping the sampleinto HF restoresits catalytic activity for electroless metal plating. Apparently the difference between single-shot and multishot activation is due to partial damage of the SiO, film. We have recently stressedthe role played by the microroughness of silica in good quality electroless metal deposition [a]. This is supported by the dependenceof the adherence of the metal deposit on the pretreatment of the ablated samplesin HF before electrolessdeposition. Generally the ablated Sic samplestreated in HF show a somewhat higher rate of metal reduction from the electroless solution than irradiated ones.Indeed, the Si 2p XPS spectrum of Fig. 2 shows a high degree of SiO, reduction. For ablation with an XeCl excimer laser the corresponding values of adherence strength are 3.1 N mmm2 before and Fig. 3. AFM images of (a) a virgin c-Sic surface, (b) after ablation with an XeCl excimer laser at an energy density of 3 J cmb2 and (c) after dipping in HF for 30 min (c). Note the drastic increase in surface roughness of the ablated Sic.

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5.2 N mm.-’ after the HF treatment for Sic ceramics. The adherence of the metal deposit to laser-ablated c-Sic, in contrast, appears to be affected by the HF treatment. First, as has been mentioned, no deposition is observed after a single-shot ablation. The adherence of Cu to c-Sic ablated in a multishot regime is rather poor, i.e. 0.1-0.2 N mm-‘. Treatment of the samples in HF enhances the adherence of Cu deposit up to 0.5 N mm-‘, This means that the metal deposition on samples with an oxide layer occurs in its pores and the removal of the layer increases the effective area of the contact. Hence the catalytically active parts of the Sic surface are beneath the oxide layer. The higher adherence of metal to HF-treated samples is due also to the smallscale modulation of the relief of ablated areas revealed by the treatment (Fig. 3(c)). Typically the adherence of Ni deposit is slightly higher than the adherence of Cu deposit. This might be explained by the elevated temperature during Ni deposition (100 “C compared with ambient temperature for Cu electroless deposition). Still, the adherence is not significantly affected by the number of laser shots. The adherence of Ni deposit can be further improved by annealing the metallized samples at 300 “C. For instance, the adherence of the annealed Ni deposit to SIC ceramics is 9.5 N mmw2. Let us consider the possible mechanisms of catalytic activity of laser-ablated SIC. Electroless metal deposition is a mixed potential reaction wherein both the oxidation of the reducing agent and the metal reduction take place at the same electrochemical potential of the surface catalytic site [ 51. The deposition is accompanied by electron transfer through the catalytic site whose energy level is close enough to the position of the mixed potential in the absolute energy scale. A sketch of the energy diagram is shown in Fig. 4. The electron current i of the deposition may be written as ice

p(E)p(E,

- E)dE

? A-

‘k

F

al

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where p(E) is the density of electronic states and y(E,) is the mixed potential. The width of y(E,) is much smaller than the gap of Sic and may be approximated by a 6 function, i.e. q(E, - E) = 6(E, - E). In the virgin sample the mixed potential is situated in the gap of Sic where the density of electronic states is zero, so the resulting deposition current is i=O. Laser ablation causes partial amorphization of the Sic surface, as can be evidenced from the broadening of the Si 2p core level spectrum of Sic. This leads to a non-zero density of electronic states in the gap of Sic in the vicinity of the mixed potential of deposition. The resulting current i becomes non-zero and the deposition commences. In turn, the amorphization of the Sic surface is due to the high mechanical stresses in the sample during ablation. The stresses remaining in SIC may lead to a widening of the valence band of Sic up to the level of the mixed potential [6]. This effect also provides a non-zero density of states in the vicinity of the mixed potential, which can also promote metal deposition. The relevance of the mechanical stresses to the activation of Sic for electroless plating can be illustrated via the following example. Scratching the Sic sample dipped into the Cu electroless bath with another piece of Sic results in the deposition of Cu on the indented areas. The deposition is observed even without damaging the surface. In this instance, however, the metal can be deposited on the indented areas only once - no deposition is observed if the initially deposited metal is dissolved in the acid, since the strain in the material is elastic. In summary, the chemical changes in SIC have been studied after its ablation in air with excimer laser radiation (XeCl or ArF). Sic decomposition is observed along with the oxidation of Si. The ablated parts of Sic promote the deposition of Cu and Ni from their corresponding electroless solutions. The adherence of the metal deposit to c-Sic can be greatly improved by processing the ablated areas in HF. The electroless deposition of metals on SIC can also be induced by indentation of the surface. The concrete mechanism of the activation of SIC for electroless plating via its laser ablation requires further investigation.

Acknowledgements

SIC

We sincerely acknowledge the help of D. Pailharey with the AFM measurements. One of the authors (G.A.S.) is grateful to the International Science Foundation (Soros Fund) for support (grant MLZ 000).

References Fig. 4. Sketch of energy diagram of SiC:q,, level of mixed potential of metal deposition; cb, conduction band; vb, valence band.

[l]

Y. Hibi, Y. Enomoto, K. Kikuchi and N. Shikata, Appl. Phys. Len., 66 (1995) 817.

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[5] G. Gutzeit, E.B. Sauberstre and D.R. Turner, in A.K. Graham (ed.), Electroplating Engineering Handbook, Van Nostrand Reinhold, New York, 3rd edn., 1971. [6] J.J. Gihnan, J. Mater. Res., 7(3) (1992) 535.