Uncongruent laser ablation and electroless

Uncongruent laser ablation and electroless metallization of SiC G. A. Shafeev, L. Bellard, J.M. Themlin, C. FauquetBen Ammar, A. Cros et al. Citation: Appl. Phys. Lett. 68, 773 (1996); doi: 10.1063/1.116738 View online: http://dx.doi.org/10.1063/1.116738 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v68/i6 Published by the AIP Publishing LLC.

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Uncongruent laser ablation and electroless metallization of SiC G. A. Shafeev General Physics Institute of Russian Academy of Sciences, 38, Vavilov strasse, 117942, Moscow, Russia

L. Bellard, J.-M. Themlin, C. Fauquet-Ben Ammar, A. Cros, and W. Marinea) URA CNRS 783, Groupe de Physique des Etats Condense´s, Faculte´ des Sciences de Luminy, Case 901, F-13288 Marseille, France

~Received 9 August 1995; accepted for publication 2 December 1995! Experimental results on the ablation of a SiC single crystal in air under irradiation with a XeCl excimer laser beam are presented. XPS analysis reveals the Si enrichment of the ablated areas under an energy density of the laser beam of 1 to 3.5 J/cm2 and the formation of SiOx in the ablated areas. The nonstoichiometric ablation of SiC allows one to activate selectively the SiC surface for metal deposition from an electroless plating solution. Both Ni and Cu deposits show a good adherence to the SiC surface ~up to 0.5 N/mm2!. Similar results are observed with SiC ceramics with a better adherence of the electroless metal deposit. The adherence of the deposit to the ablated SiC samples increases upon HF treatment. © 1996 American Institute of Physics. @S0003-6951~96!04906-3#

With its extreme thermal, mechanical, and electronic properties, SiC is an attractive material for high-temperature electronics. The permanently increasing field of applications of SiC in various electronic devices requires the elaboration of alternative methods of surface treatment. In particular, laser-assisted processing is a promising technique for tailoring solid surfaces properties. Recently, excimer laser machining of SiC has been reported, combining a laser irradiation of SiC monocrystal with further mechanical removal of the modified surface layer.1 In this letter, we show that laser ablation is an interesting technique for SiC processing leading to mm-sized features. We have mostly investigated the excimer laser ablation of SiC in air. XPS and AFM measurements were made to characterize the ablated surface of a SiC single crystal. The irradiated SiC was subsequently immersed into either Cu or Ni electroless plating solution to realize area-selective metallization. The mechanism of laser activation of SiC is discussed in relationship with similar activation processes of some other dielectrics, such as SiO2, Al2O3, and diamond.2– 4 Compensated SiC single crystal samples doped with nitrogen ~1017 – 1018 cm 23! with their c axes perpendicular to the surface were used in the experiments. The irradiation of the samples was carried out in air with a UV XeCl excimer laser ~l5308 nm, FWHF of pulse duration: 20 ns! at normal incidence. Prior to irradiation, the SiC samples were dipped for 30 min in a 30% HF water solution to dissolve the native oxide layer. The morphology of laser-ablated parts of the samples was studied with the help of an atomic force microscope ~AFM!. The XPS measurements were carried out using unmonochromatized Al K a radiation at 1486.6 eV. The spectra were taken at an intermediate electron takeoff angle of 20° ~spectrometer axis with respect to the surface normal!. The composition of both Cu and Ni electroless baths is given elsewhere.5 The Cu electroless deposition was carried out at room temperature ~pH512! while Ni was deposited at a!

Author to whom correspondence should be addressed. Electronic mail: [email protected]

Appl. Phys. Lett. 68 (6), 5 February 1996

100 °C and pH55. The adherence of the deposits was measured under a tensile stress test. Figure 1 shows the C 1s and Si 2 p XPS core level spectra of a SiC single crystal. Two peaks at binding energies of 281.8 and 284.4 eV are, respectively, attributed to carbon in SiC and to carbon contamination from air exposure.6 The Si 2 p peak is centered at 99.8 eV, corresponding to silicon in SiC. The binding energy of the SiC peaks has been found to depend strongly on the history of the sample ~temperature of annealing, band bending, etc.!. However, the 182.0 eV energy difference found here for SiC between the C 1s and Si 2 p is in good agreement with the value previously reported by other authors.6,7 Figure 2~a! shows the AFM view of the initial SiC single crystal surface. The surface is rather flat even if some defects can be noticed. Excimer laser irradiation of SiC samples in air at laser fluences of 1–3.5 J/cm2 results in the decomposition of SiC. The XPS data on the laser-irradiated area reveal a large incorporation of oxygen, a widening and shift of the Si 2 p peak towards higher binding energy, and a pronounced charging effect due to the presence of Si oxide. The carbon atoms detected on the surface are partially oxidized. It is noteworthy that after laser irradiation, the silicon-to-carbon

FIG. 1. Core level spectra of C 1s ~a! and Si 2 p ~b! of SiC before ~bottom! and after ~top! ablation in air with two laser pulses of a XeCl excimer laser at energy density 3.0 J/cm2 followed by 3 h dipping in HF.

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FIG. 2. AFM view of ~a! a virgin c-SiC surface; ~b! ablated with an excimer XeCl laser at energy density 3 J/cm2; ~c! and dipped into HF for 30 min. Note the drastic increase of surface roughness of the ablated SiC surface.

concentration ratio deduced from the peak areas and corrected by the atomic sensitivity factor has increased by a factor of 2, the Si concentration remaining roughly constant. This effect can be explained by the preferential photoablation of carbon and/or the superficial growth of Si oxide. These results are consistent with the recently reported observations for the ablation of SiC with an excimer KrF laser.1 Laser ablation of SiC is accompanied by a drastic increase of surface roughness. The topology of the crystal surface after the ablation with an excimer XeCl laser at an energy density of 3 J/cm2 with 50% overlap between subsequent laser shots is shown in Fig. 2~b!. The characteristic scale of the surface 774

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roughness is about 150–200 nm and presents smooth, rounded hillocks rather than regular sharp edges. This may indicate that the surface has remelted during the laser pulse. The irradiated SiC samples promote the selective electroless deposition of both Cu and Ni from the corresponding solutions on the ablated areas only. The best spatial resolution of the deposition is about 10 mm, which is limited by the laser spot size. The catalytic activity of the ablated samples is stable with time. Many months after sample irradiation, electroless metal deposition is still successful. These observations are similar to those on laser-assisted activation of Al2O3 and diamond;3,4 in particular, these laser-generated catalytic sites are nonmetallic, and stable upon acid treatment. Moreover, the metal deposit can be dissolved in acid and deposited many times on the ablated areas. Even a rapid heating of the sample in air up to 850 °C does not affect the efficiency of metal deposition. The rate of metal deposition under multishot ablation does not significantly depend on the number of laser shots. However, a single shot irradiation of the SiC sample at 3 J/cm2 energy density does not cause any activation for metal plating. It remains possible to induce a catalytic activity for electroless metal plating by dipping the sample into HF. We have recently stressed the role played by the microroughness of silica for a good quality electroless metal deposit.2 Apparently, the difference between the single- and multishot activation is due to a partial damage of the SiOx film. Indeed, the adherence of the metal deposit on the ablated samples is influenced by a treatment in HF before electroless deposition. The adherence of Cu to SiC ablated in multishot regime is rather poor: 0.1–0.2 N/mm2. The treatment of the samples in HF enhances the adherence of Cu deposit up to 0.5 N/mm2. This effect can be related to the small-scale modulation of the relief of the ablated areas. Indeed, as shown in Fig. 2~c! this treatment does not modify much the large-scale roughness while the specific surface of the ablated area increases. The ablated SiC samples further treated in HF also show a higher rate of metal reduction from electroless solution than the as-irradiated ones. XPS results presented in Fig. 1 show the presence of Si–Si bonds on the Si 2 p line at 99.2 eV and a wide C 1s spectrum at 286.5 eV due to oxidized carbon species at the surface.8 A wide Si 2p peak is also observed at 103.2 eV due to the remaining Si oxide that has not been etched by HF after 3 h dipping. This oxide can be attributed to either very porous oxide that is not entirely removed by 3 h dipping in HF, or to native oxide formed during air exposure. A large set of experiments was also carried out on SiC ceramics. The effects of laser irradiation on the XPS spectra described for the SiC single crystal were even more pronounced on these substrates. The preferential ablation of C, the SiOx formation, and the catalytic activity for electroless metal plating were observed on these highly heterogeneous surfaces. The ablated areas were more porous than those in a single crystal SiC since more than 48 h of HF treatment was necessary to dissolve the oxide layer entirely. The adherence of the electroless metal deposit to the ablated SiC ceramics was higher than on single crystalline SiC. For instance, the adherence of an Ni deposit annealed at 300 °C is as high as 9.5 N/mm2. Shafeev et al.


The ability of the ablated SiC surface to reduce metals from the electroless plating solutions is presumably due to the modification of the chemical composition of the crystal surface as evidenced from XPS spectra. The electroless metal deposition proceeds on the elemental Si as well as on the Si oxide formed here under strong nonequilibrium conditions. The catalytic activity of the ablated substoichiometric Si oxide for electroless metal plating has been reported recently.9 The electroless metal deposition on SiC material unexposed to laser light does not occur normally in the conditions of this study. As for other wide band gap dielectrics, the density of electronic states in the gap of SiC in the vicinity of the electrochemical potential of metal reduction is not sufficient to reduce the metal from the electroless solution. However, it is possible to induce, at least temporarily, a sufficient density of states that makes the electroless deposition possible. This may be accomplished through either the microroughness, inducing a high density of surface states, or a mechanical stress that induces a band gap narrowing and the widening of the valence band up to the potential of metal reduction.10 The relevance of the mechanical stresses to the activation of SiC for electroless plating can be illustrated by the following experiment. The scratching of a SiC sample dipped into the Cu electroless bath with a diamond results in the deposition of Cu onto the indented ~scratched! areas. In this instance, however, the metal can be deposited only once since the band bending takes place during the indentation. In summary, the chemical modifications on a SiC single crystal surface have been studied by XPS and AFM after ablation in air with XeCl excimer laser radiation. SiC de-

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composition and Si oxidation are observed. We have shown that the ablated parts of SiC promote the deposition of Cu and Ni from the corresponding electroless solutions through a buffer SiOx layer. The adherence of the metal deposit to SiC can be largely improved by processing the ablated areas in HF. The electroless deposition of metals on SiC can be mechanically induced as well by the indentation of the surface. However, complete understanding of the SiC activation for electroless plating via laser ablation as well as the physical and chemical properties of the SiC/electroless metal interface requires further investigation. We sincerely acknowledge the help of D. Pailharey in AFM measurements. One of the authors ~G.A.S.! is grateful to International Science Foundation ~Soros Fund! for support, Grant No. MLZ 000. 1

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