Surface conductivity modification of ceramics with laser radiation

Surface and Coatings Technology 112 (1999) 394–400

Surface conductivity modification of ceramics with laser radiation B. Stolz *, R. Poprawe Fraunhofer Institut fu¨r Lasertechnik, Rheinisch-Westfa¨lische Technische Hochschule Aachen, Steinbachstr. 15, D-52074 Aachen, Germany

Abstract Laser-assisted processes that enable the direct modification of the surface conductivity of dielectric ceramics by cw- and pulsedlaser irradiation are presented: a method for fabrication of conductive structures on Al O with CO laser irradiation (l=10.6 mm, 2 3 2 cw) is developed, and the feasibility is proved for Al O of a different purity. A corresponding procedure for AlN with excimer 2 3 laser irradiation (l=248 nm, t=25 ns) is systematically examined, concerning its dependence on the processing variables. In order to characterize the modification, the laser-treated specimens are analysed with respect to electrical resistivity (by four-point- and two-point-probe techniques), chemical composition [Auger-electron (AES ), X-ray-photoelectron spectroscopy ( XPS ) and electron beam microprobe analysis (EBMA)], structure (micro-Raman spectroscopy) and morphology (microscopy). © 1999 Elsevier Science S.A. All rights reserved. Keywords: Alumina; Aluminium nitride; Laser radiation; Resistivity modification

1. Introduction Ceramic components and substrates, as a part of the state-of-the-art in microelectronics (e.g. IC fabrication) and electronic packaging, have to meet increasing demands. The substrate and packaging materials have to provide mechanical protection and, moreover, they have to fulfil different thermal and electrical requirements. A comparison between the demands on the ceramic substrate and the material properties ( Table 1) reveals that AlN and Al O are well-suited substrate 2 3 materials for integrated circuit (IC ) carriers [1,2]. Due to the combination of the material properties described in the following, Al O and AlN are chosen 2 3 to investigate the feasibility of laser-assisted writing of conductive structures by surface modification. Al O 2 3 fulfils the requirement of a high electrical resistivity (r>1014 V cm), which minimizes loss currents through the carrier material that would otherwise impair the function of the electronic device. In contrast, AlN (r=1011 V · cm) shows a significantly lower resistivity. Concerning the demand for dielectric losses to be as low as possible in order to realize high switching frequencies, Al O and AlN are almost equally qualified, with 2 3 Al O being nearer to the optimum value. With res2 3 * Corresponding author. Tel: +49 241 89 06 410; Fax: +49 241 89 06 121; e-mail: [email protected]

pect to the required thermal conductivity (ideally >100 W mK−1 for efficient heat conduction), AlN is more suitable than Al O . Concerning the thermal 2 3 expansion coefficient, the value of AlN fits better for a junction with silicon [1,3]. In processes known as laser-induced surface activation [4,5] the activated ceramic samples are immersed in an electroless bath after laser irradiation in which the modified areas promote the metal deposition from the bath containing metal ions and a reducing agent. In the case of Al O , microscopic inspection revealed that 2 3 deposition starts at the domains consisting of c-configurated Al O , which might be explained by the sub2 3 stoichiometric character of the c-Al O (i.e. a spinel in 2 3 which a third of the regular metal sites are vacant) compared to the base material consisting of a-Al O 2 3 [6 ]. The enlarged presence of periodically distributed vacancies and the emerging dislocation microstructure as a consequence of the laser treatment are considered to promote the deposition of the metal ions from the plating solution. The deposited ions growing to metallic isles join together during immersion by an autocatalytic procedure, thus forming coherent conductive structures in which the conductivity can be presumed to be restricted to the deposited metal layer. In contrast to this type of activation, the laser treatment performed here aims at a selective modification, directly resulting in a macroscopically measurable

0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 73 9 - 7

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B. Stolz, R. Poprawe / Surface and Coatings Technology 112 (1999) 394–400 Table 1 Material properties of AlN and Al O 2 3

Thermal conductivity ( W mK−1) Thermal expansion coefficient (RT–200°C ) (10−6 K−1) Electric resistivity (V · cm) Permittivity, e (1 MHz) r Dielectric loss, tan h (1 MHz) (103) Dielectric strength ( KV mm−1) Flexural strength, s (N mm−2) B Elasticity modulus (N mm−2) Price referring to Al O =1 2 3

change of resistivity in the interaction zone. Laserassisted processes enable the production of conductive and adhesive structures on non-conductive substrates in a direct and flexible way, i.e. with a reduced number of processing steps. They can serve as an alternative to conventional methods of thick-film- and thin-film metallization and as a base for further conventional or chemical metallization (e.g. electroless plating). The demands on metallization systems for the connection between substrate and chip are described elsewhere [7]. In this work, the feasibility of modifying the dielectric Al O into a conductive aluminum compound by CO 2 3 2 laser radiation is examined and proved for different types of Al O ranging from a highly pure material 2 3 (>99.8% Al O ) to qualities of low purity (95% 2 3 Al O ), which will become important for future applica2 3 tion aspects. Possible reactions between the Al O sub2 3 strate and ethanol used as processing environment are discussed. For AlN, the process of modification with excimer laser irradiation and the influence of the processing variables on the decrease of the electrical resistivity are investigated. The processing variables mainly governing the modification process are fluence, ambient atmosphere and effective pulse number (which is defined as the number of laser-pulses that a certain area unit has ‘‘seen’’ during treatment or the product of the frequency and the length of the laser spot size in the processing direction divided by the processing velocity, respectively).

2. Experimental 2.1. Modification of Al O 2 3 The modification of Al O is performed by CO laser 2 3 2 radiation (cw, l=10.6 mm, P =1.5 kW ) that is focussed L by a beam-guiding optical system with a focal length of 200 mm on the substrate, which is positioned on a motor-driven x–y manipulator. The substrate is located in a bath of liquid ethanol (C H OH ) during laser 2 5 treatment. A more detailed description of the experimen-

AlN

Al O 2 3

Requirement*

140–150 3.5 ≥1011 8.6–8.8 0.5–1.0 >20 250–350 310 9

20–50 6.5 >1014 9.5 0.2–0.3 >25 400–450 300–400 1

>100 3–4 ≥1014 <4 <0.1 >20 >500

tal set-up is given elsewhere [8]. The processing variables are mainly the velocity (varied in a range between 10 and 3000 mm min−1) and laser power (varied from 25 W to 1 kW, corresponding to intensities between 2×104 and 5×105 W cm−2). The initial height of the ethanol layer is alternated during the irradiation due to evaporation and deformation of the liquid surface by recoil pressure of the emerging vapors. 2.2. Modification of AlN The modification of AlN substrates (Hoechst) with excimer laser radiation ( KrF, l=248 nm) is carried out by mask projection processing, wherein the substrate surface is positioned in the focal plane on a motordriven x–y manipulator [8]. The processing variables are effective pulse number (1–140) (defined in Section 1), fluence (1.5–6 J cm−2), surrounding atmosphere and repetition rate (3–120 Hz). 2.3. Surface analysis The resistivity in the laser-modified traces is determined by a four-point- and/or two-point-probe technique. The four-point-probe technique available cannot be applied for Al O (due to the measuring range) and 2 3 moreover requires the film thickness as input data. Since the actual depth of the modified traces in AlN cannot be determined microscopically, the measured values of resistivity refer to an assumed thickness of 100 nm. Calculations considering the two-point measurements reveal a thickness of about 90 nm in the case of AlN. XPS- and AES investigations are performed in a conventional ESCA-lab [spatial resolution of 1.6 mm2 ( XPS ) and 100 mm2 (AES ), with an energy resolution of 1.0 eV ( XPS) and 1.5 eV (AES)]. AES is well-suited for analyzing the chemical modifications because the chemical state of Al (metallic or in a nitride or an oxide) can be resolved unambiguously due to the different energy of the LVV transition for the different bonding states. Micro-Raman measurements are performed using the wavelength l=488 nm for excitation (Ar+-ion laser)

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with a spatial resolution of 1 mm and a spectral resolution of 2 cm−1.

3. Results and discussion 3.1. Modification of Al O 2 3 Laser treatment only succeeds in the formation of conductive traces if ethanol (C H OH ) is used as the 2 5 process environment during irradiation [Fig. 1(a)]. The microstructure in the laser-modified zone consists of columnar crystallites corresponding to the strongly directed solidification, which can be easily distinguished from the uniform polyeder-habitus in the base material

consisting of a-Al O [Fig. 1(b)]. In the transition zone, 2 3 several detachments can be seen, possibly caused by thermal shock or lattice mismatch. A higher laser power (>50 W ) leads to poor adhesion of the traces to the substrate. The best adhesion is generally achieved on samples of a lower purity, or samples that were sintered at a lower temperature or produced by slurry casting. The use of other carbon-containing liquids [e.g. methanol (CH OH )] did not lead to surface conductance 3 changes; nor did the use of ethanol on other binary or ternary ceramics. This contradicts the assumption of a mere carbon deposition in the modified traces by thermal decomposition of ethanol. A possible catalytic effect of Al O to the conversion of ethanol by pyrolysis should 2 3 be taken into account since the catalytic properties of

Fig. 1. Traces resulting from laser treatment (P =25 W, v=25 mm min−1, I=2.0×104 W cm−2) for different process environment: (a) airablation, L ethanolmodification; and (b) REM picture of the modified area in the finegrained base material.

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alumina are reported to be important for the ethylene synthesis, starting with ethanol. The observed higher activity (for a subsequent Cu-deposition process) of Al O processed with CO -cw laser radiation compared 2 3 2 to the activity resulting from UV laser treatment is verified by shorter incubation times for metal deposition from plating solutions and can probably be traced back to synergetic reactions between the modified material containing amorphous carbon and lattice defects, which will be described elsewhere [9]. The resulting resistances per unit length of the conductive traces in the different Al O base material investi2 3 gated (different purity, sintered or slurry casted ) depend strongly on the processing velocity ( Fig. 2). The lowest achieved resistance corresponds to a resistivity of 0.5 V · cm, which means a reduction of more than 14 orders of magnitude compared to the base material (r>1014 V · cm). The XPS investigations (spectra not shown) of the base material show peaks characteristic for Al, C, N and O with a positive shift of about +4.0 eV, which can be explained by electrical charging effects due to the insulating base material [10]. The Al 2p and O 1s core level binding energies are characteristic of Al O . In the laser-treated regions, all core-level 2 3 peaks show a double-peak structure. This suggests a reduced electrical charging (and a reduced binding energy shift) in at least a part of the modified surface. There is no indication of the Al0 (metallic) valence state within the XPS sampling depth (10 nm). Any metallic aluminum if produced through the treatment at all reoxidizes in contact with oxygen from the atmosphere. The Raman spectra of the base material show peaks characteristic of Al O that disappear after laser treat2 3 ment [11]. In the conductive traces, additional peaks are observed at positions that are correlated to amorphous carbon in its sp2-hybridized form, which is pre-

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sumed to be at least partly responsible for the observed conductivity. However, the C 1s signal of the XPS investigation is comparable in the treated and untreated regions, thus contradicting the assumption of a complete carbonization being responsible for the conductivity. At the present state, the investigations do not allow an unambiguous statement concerning the modification processes and the resulting composition. In order to discuss the emerging conductivity, there are probably several factors to be considered, proceeding on the assumption that the modified material consists of a compound or mixture of phases: e.g. a carbon formation stemming from the ethanol, a simultaneous partial conversion of the a-Al O into c-Al O (cubic, spinel with 2 3 2 3 1/9 of the metal atom sites in the spinel lattice vacant), thus presenting an offstoichiometric Al O [12] or a 2 3 conversion in b-Al O (hexagonal unit cell [12]), which 2 3 is known to be an ionic conductor [13]. 3.2. Modification of AlN The laser-induced modification leads to bright metallike traces in which two phases can be clearly distinguished: dark grains are surrounded by a brighter matrix phase (in optical microscopy; Fig. 3). The dark grains turned out to be Yttrium-enriched precipitations by EBMA analysis. Y compounds (e.g. Y O , YF ) are 2 3 3 used as sinter additives in order to reach a phase melting at temperatures as low as possible without strongly influencing the heat conductivity. The achieved resistivity decreases with increasing effective pulse number uniformly for the frequency range investigated ( Fig. 4). The effective pulse number turned out to be the parameter mainly determining the modification degree. The distribution of elements in the treated areas (at.%), as calculated from the AES results, reveals

Fig. 2. Resistance per length as a function of the processing velocity for different Al O qualities (P =30 W, F =0.125 mm2, 2 3 L L I=2.4×104 W cm−2).

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Fig. 3. Comparison between (a) base and (b) modified AlN surface.

an increase in Al and O (about 10% Al and 15% O for F=3.0 J cm−2 in air) compared to the base material. In order to avoid any spurious effects by contamination to air before analyzing the modified sample, in-situ experiments were performed. According to the relation of peak intensities, the increase in metallic Al can be estimated as 20% (for F=1.2 J cm−2 in vacuum p=10−6 mbar), which is valid for the assumption that the chemical composition is homogeneous in the information depth of 10 nm. The Al increase (however far away from the Al content in Al alloys) is thought to be one of the reasons for the emerging conductivity, whereas the O increase can be explained by a direct reaction between Al and O as soon as Al is formed due to the high affinity between

these elements. The Raman spectra of the base material show peaks characteristic of AlN [14], whose intensities are strongly weakened in the modified areas. In the dark grains, the peak intensities of AlN are less weakened than in the bright grains, which can be traced back to (1) a higher Al content in the bright grains, as proved by EBMA analysis and Al not being a Raman-active element, and (2) an alternated transmissivity in dark and bright grains for the Ar+-laser wavelength. Further systematic Raman investigations as a function of the effective pulse number reveal AlN peaks of continuously decreasing intensity with increasing effective pulse number [Fig. 4(b)]. This is in good agreement with the resistivity measurements showing a decreasing resistivity with increasing effective pulse number.

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Fig. 4. (a) Resistivity of modified AlN as a function of the effective pulse number for different repetition rates (vacuum: n=10 Hz, F=3.5 J cm−2) and (b) Raman spectra of AlN as a function of the effective pulse number (vacuum; n=10 Hz, F=3.5 J cm−2).

Remarkably, optical microscopy reveals grain boundaries hinting at twin formation in the resolidified surface [Fig. 3(b)], which is quite untypical for Al because of its high stacking-fault energy, but might be promoted by the high cooling rate after each laser pulse and the rapid solidification. Nevertheless, the rather high fraction of N in the conducting traces might be explained by an additional formation of a N-enriched secondary phase (probably on a nanometer scale), which, however, could not be proved by analytic methods.

4. Summary Laser-assisted processes for direct conductivity modification on ceramic substrate materials (AlN, Al O ) are 2 3

presented. The resulting changes in conductivity are correlated to the changes of the chemical composition in the modified areas. The treatment of Al O surfaces 2 3 in an ethanol environment with cw-CO -laser radiation 2 leads to the formation of conductive traces with a lowest resistivity of 0.5 V · cm. Amorphous C emerges in the treated areas supposed to partly cause the conductivity. The modification of AlN with pulsed excimer laser radiation leads to the formation of conductive, metallike traces. The resistivity (in the range of 300–50 mV · cm dependent on processing variables) decreases continuously with increasing effective pulse number corresponding to a higher amount of energy absorbed, which leads to a higher degree of modification until the modification process saturates when a balance between modification and evaporization is reached. The constitution of the

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modification in AlN is considered to be a network of conductive areas, which are not thought to consist predominantly of metallic aluminum (due to the lack of an unambigious analytic evidence) but of at least one further conductive modification synthesized by the special non-equilibrium treatment conditions.

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[4] Pedrazza, Mater. Res. Soc. Symp. Proc. Vol. 39 1996 Material Research Society. [5] Shafeev, Adv. Mater. Opt. Elect. 2 (1993) 183. [6 ] Laude, Kolev, Brunel, Deleter, Appl. Surf. Sci. 86 (1995) 368–381. [7] Dipankar, Pramanik, Aluminium-Based Metallurgy for Global Interconnects, MRS Bull., Vol. XX, No. 11, November 1995. [8] B. Stolz, E.W. Kreutz, Appl. Surf. Sci. 109–110 (1997) 242–248. [9] B. Stolz, G.A. Shafeev, A.V. Simakin, E.W. Kreutz, R. Poprawe, Electroless Cu Plating of Alumina Treated With cw CO2 Laser Radiation, SPIE Proc. 5th Conf. Smart Structures, San-Diego, CA, March, to be published. [10] Chastain ( Ed.), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Physical Electronics Division. [11] Nyquist, Putzig, Leugers, Handbook of Infrared and Raman Spectra of Inorganic Compounds and Organic Salts, Academic Press, San Diego, CA, 1997. [12] Pearson, Handbook of Lattice Spacing and Structures of Metals, International Series of Monographs on Metal Physics and Physical Metallurgy. [13] Illschner, Werkstoffwissenschaften, Springer, Berlin, 1990, p. 150. [14] Brafman, Langyel, Mitra, Solid State Commun. 6 (1986) 523