- Email: [email protected]
Selective surface modification of ceramics with laser radiation
Applied Surface Science 109r110 Ž1997. 242–248
Selective surface modification of ceramics with laser radiation B. Stolz a
a,)
, G. Backes a , A. Gillner b, E.W. Kreutz
a
Lehrstuhl fur Technischen Hochschule Aachen, Steinbachstr. 15, D-52074 Aachen, Germany ¨ Lasertechnik, der Rheinisch-Westfalischen ¨ b Fraunhofer Institut fur ¨ Lasertechnik, Steinbachstr. 15, D-52074 Aachen, Germany Received 4 June 1996; accepted 24 October 1996
Abstract Laser-assisted processes that enable the direct fabrication of conducting structures and contacts on ceramic substrate materials ŽAlN, Al 2 O 3 . through modification are presented. These can serve as an alternative to conventional methods of thick film, thin film and chemical metallization. A method for fabrication of conductive structures on Al 2 O 3 with CO 2 laser irradiation Ž l s 10.6 m m. is developed and a corresponding procedure for AlN with excimer laser irradiation Ž l s 248 nm. is examined. The changes of resistance in the modified areas dependent on the processing variables are investigated and correlated to the results of a chemical analysis by means of electron beam microprobe.
1. Introduction Ceramic components have become part of the state of the art in connection technology. Due to the steady advances in semiconductor technology and the increased performance achievement of computing systems by miniaturization of the functional devices ceramic substrates have to meet increasing demands: the substrate and packaging materials have to provide mechanical protection and moreover they have to fulfill different thermal and electrical requirements in order to realize the possibilities of the semiconductor technology in efficient way. A comparison between the demands made on the ceramic substrate and the material properties reveals that AlN and Al 2 O 3 are especially suitable as substrate mate-
)
Corresponding author. Tel.: q49-241-8906410; fax: q49241-89061211; e-mail: [email protected].
rials w1x. Metallization is important for the connection between substrate and chip. Metallization systems used for global interconnects have to meet a lot of different requirements, e.g. a certain contact resistance to nq and pq contacts, no degradation due to electromigration, corrosion resistance and sufficient adherence between interconnects and dielectrics Žsuch as oxides and nitrides used for insulating different levels of metals.. Aluminum satisfies most of the required conditions and has become a widely used material in IC circuits w2x. By means of laser-assisted processes for metallization of ceramics based on thermally activated mechanisms, conducting and adhesive structures are to be produced at the surface of dielectric substrates, aiming at applications in microtechnology. Applications in communication technology, in medical sectors and in sensor technology as well show the technical and economic potential of microsystems. Historically conditioned by the development in mi-
0169-4332r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 6 . 0 0 9 1 6 - 6
B. Stolz et al.r Applied Surface Science 109 r 110 (1997) 242–248
croelectronics, the majority of these systems are based on silicon. In order to ensure an acceptance of microsystems in multiple technical fields, cost-effective fabrication processes should be available for as many materials as possible. Laser-assisted processes enable the application of conducting structures on non-conducting substrates in a direct and more simplified way, i.e. with a reduced number of processing steps, than the conventional methods of thick film, thin film and chemical metallization methods w3x. In order to produce conducting structures the substrate material is irradiated for a sufficient processing time with laser radiation of such intensity that in the irradiated zones a modification of the insulating base material results, leading to a lowering of the electrical resistance. With regard to the demands placed on substrate materials various electrical and thermal specifications must be met. Al 2 O 3 fulfills the requirement of a high electrical resistivity Ž r ) 10 14 V cm. which minimizes loss currents through the carrier material that would otherwise impair the function of the electronic device. In contrast AlN Ž r s 10 11 V cm. shows a significantly lower resistivity. Concerning the demand for dielectric losses as low as possible in order to realize high switching frequencies Al 2 O 3 and AlN are nearly equally suitable with Al 2 O 3 being nearer to the optimum value. With respect to the required thermal conductivity Žideally ) 100 WrmK for efficient heat conduction. AlN is better suited than Al 2 O 3 . Also with respect to the thermal expansion coefficient, AlN presents a better-suited value for a compound with silicon. The requirement of high mechanical strength, which is important for large substrates, for reduced substrate thicknesses, and for enhanced application of automated handling systems, is approximately met by Al 2 O 3 with a bending strength of about 400–450 Nrmm2 , being higher than that of AlN. If cost is considered, Al 2 O 3 will be favored for average applications and mass production, whereas the use of AlN will more and more be limited to high performance applications. These combinations of properties make the insulating materials Al 2 O 3 and AlN well-suited substrate materials for integrated circuit ŽIC.-carriers w1,4x. The direct metallization of dielectrics is achieved by two fundamentally different processes: laser-induced modification and laser alloying. The modifica-
243
tion of the dielectrics takes place in the interaction zone heated by the absorbed optical energy. The possibility of modifying the dielectric Al 2 O 3 into a conducting aluminum compound by irradiation is fundamentally examined and for AlN the process of metallization is investigated. Depending on the type of ceramic the reactions during modification consist predominantly of deoxidation, decarbonization or denitriding processes. The influence of the processing variables on the decrease of the electrical resistance is investigated for different dielectrics ŽAl 2 O 3 , AlN.. These resulting resistance values are correlated to the results of chemical composition of the samples analyzed by electron beam microprobe investigations ŽEBMA.. The analysis with EBMA was carried out by the GFE ŽGesellschaft fur ¨ Elektronenmikroskopie, organization for electron microscopy, Aachen, Germany.. The signal used for the EBMA-analysis is characteristic X-ray-radiation and its intensity and the relative error of EBMA-measurement amounts to 0.5%.
2. Experimental details 2.1. Modification of Al 2 O3 For the modification of Al 2 O 3 , the CO 2 laser radiation Žcw, l s 10.6 m m, PL s 1.5 kW. is focussed 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 liquid bath of ethanol ŽC 2 H 5 OH.
Fig. 1. Process of modification of Al 2 O 3 .
244
B. Stolz et al.r Applied Surface Science 109 r 110 (1997) 242–248
2.2. Modification of AlN
Fig. 2. Resistance dependent on laser power for Al 2 O 3 .
during laser treatment and is covered to a depth of about 2 mm by ŽFig. 1.. The trace geometry is determined by the processing variables such as laser power, processing velocity, focal length and spot size. The processing variables velocity Žchanged in a range between 10 mmrmin and 3000 mmrmin. and laser power Žfrom 25 W to 1000 W, which corresponds to intensities between 2 = 10 4 to 5 = 10 5 Wrcm2 . are varied. The resulting values of resistance in the modified areas are measured with a simple ohmmeter method and could not be compared to the measurement of a four-point-probe-technique, because the underlying resistances were out of the measuring range of the four-point measuring technique being available.
The modification of AlN with excimer laser radiation ŽKrF, l s 248 nm. is carried out by mask projection processing, wherein the substrate surface is positioned in the focal plane and the trace geometry is essentially determined by the dimension of the mask being used. The substrate is located in a vacuum chamber on a motor-driven x–y-manipulator. The energy density is controlled by a beam splitter and the resonator high voltage. The processing variables speed Žin the range from 0.1 to 20 mmrs. energy density Žfrom 1.5 to 6 Jrcm2 . repetition rate Žfrom 3 to 120 Hz., number of passes and ambient atmosphere are varied. The values of resistance in the modified traces were determined by four-point probe technique and simple ohmmeter measurement as well. A comparison of both techniques by calculating the resistivity into resistances showed that the values determined by the four-point probe technique are about 10% higher than the values measured with the ohmmeter.
3. Results 3.1. Results for modification of Al 2 O3 The selective modification is investigated for sintered and slurry cast alumina of different purity and
Fig. 3. Resistance of modified Al 2 O 3 dependent on processing velocity.
B. Stolz et al.r Applied Surface Science 109 r 110 (1997) 242–248
Fig. 4. Al 2 O 3 processed with CO 2-laser-radiation without coating and with ethanol Žtrace on the left side: processing without coating leads only to ablation; the other trace covered with ethanol during laser-treatment consists of a conducting modification; the same processing parameters were used for the fabrication of both traces: laser-power P s 25 W, processing velocity Õ s 50 mmrmin, focal length f s 200 mm..
powder quality. By irradiation ŽSection 2.1. with intensities in the range from 2 = 10 4 to 5 = 10 5 Wrcm2 values of resistance per unit length between the ends of the modified traces are attained in the range of k Vrcm to M Vrcm. The resulting resistance values present a clear dependence on the intensity and the processing velocity, as is shown in Fig. 2. The resistivity of the base material is higher than 10 14 V cm and thus the lowering of the resistance down to the above-mentioned values means a reduction of more than 10 orders of magnitude. The processing velocity has an essential influence on the trace geometry and moreover a significant influence on the attainable conductivity values which cannot only be traced back to the geometry factor: in case of lower powers being used, i.e. 30–50 W, the resulting conductivity changes increase with decreasing processing velocities ŽFig. 3.. If the process is carried out without ethanol there only occurs an ablation of the base material as can be seen in Fig. 4. The results from the chemical analysis of the modified samples ŽEBMA. are not yet sufficiently revealing with respect to the evaluation of the mechanisms being responsible for the emerged resistance reduction. The results of the analysis reveal an increase of the Al-percentage in the modified conducting areas compared to the base material of merely
245
Fig. 5. REM picture of the modified area in the finegrained base Al 2 O 3 .
0.16 wt% Ž0.15 at%. and corresponding low decreases in the O-percentage of 0.13 wt% and in the C-percentage of 0.03 wt. The microstructure in the modified zone consists of sternlike crystallites corresponding to the strongly directed solidification ŽFig. 5.. In the transition zone only a few detachings can be seen eventually caused by the thermal shock or lattice mismatch. In opposite traces produced by using other processing variables, i.e. higher laser power, don’t even adhere macroscopically to the substrate.
Fig. 6. Photography of modified traces in AlN processed with excimer-laser-radiation Ž l s 248 nm. pulse energy Ep s 200 mJ, energy densities in the range 2–7 Jrcm2 , processing velocity Õ s 0.5 mmrs, frequency f s10 Hz.
246
B. Stolz et al.r Applied Surface Science 109 r 110 (1997) 242–248
Fig. 7. Resistance and wt% of Al for AlN-substrate dependent on velocity.
3.2. Results for modification of AlN Because some of the traces cannot be detected microscopically the trace depth of the modified structures in AlN ŽFig. 6. is calculated by the results from EBMA investigations for two different energies Ž5 kV and 15 kV. and thus volumina of different magnitude included in the measurement. A multilayer model consisting of three layers Žbase material AlN, modified material supposed to contain Al, N and O, sputter layer of Au. in combination with the
results of the analysis for different penetration depth is used to determine the trace thickness w5x. The resulting values show no consistent dependence on the processing variables but are in the range from 100 to 115 nm. The processing velocity influences the modified traces: the attainable reduction of resistance can be enhanced by increasing processing velocity ŽFig. 7.. This tendency is confirmed by the resistance values resulting for different repetition rates Ž3–120 Hz. being used. A comparison to the results of the
Fig. 8. Wt% of Al for AlN-substrate dependent on processing velocity measured by using different energies.
B. Stolz et al.r Applied Surface Science 109 r 110 (1997) 242–248
EBMA-investigation from the measurement with 5 kV presents a corresponding increase of the Al-percentage with increasing velocity for all repetition rates being chosen as shown in Fig. 7. The results from the measurement with 15 kV don’t reveal this kind of correlation but show a course being independent of velocity ŽFig. 8. because in this case, due to the higher penetration depth into the material below the modified region, the measured results are considerably determined and nearly dominated by the composition of the base material.
4. Discussion The influence on the resulting values of conductivity is not exclusively conditioned by the changes in trace geometry. In case of laser powers in the range of 30–50 W the attained conductivity increases with decreasing velocity which hints at a process that claims time for the formation of the conducting modification. If the base material is processed without ethanol as coating the treatment only leads to ablation in the substrate material without production of conducting areas ŽFig. 4.. This reveals that on one hand the ethanol coating effects a reduced absorption of the energy by the substrate and on the other hand it is, due to its function as a possible reactant, indispensable to enable the selective modification of the insulating into a conducting material. The fundamental mechanisms and the reactions caused by the ethanol, however, are still to be clarified. It has still to be investigated whether the ethanol fulfills the function of a usual reaction partner or catalyst or if it inhibits reactions that suppress the formation of a conducting compounds. The assumption, however, that a pure carbonization, causing the emerged conductivity, took place in the modified traces is weakened by the habitus of the microstructure in the modified zone ŽFig. 5.. The REM photography presents a directly solidified microstructure in the fine-grained base material. Results from chemical analysis that could determine an increase of the Al-percentage merely within the measurement inaccuracy oppose to the assumption of a so-called metallization as explanation for the arisen conductivity. The following conception
247
could eventually explain the observed phenomena: although the chemical composition has hardly changed a drastic change in the conductivity is effected which could lead to the conclusion that not an absolute change in composition is necessary but that a change in the mainly ionic bonding character caused by the modification enables the formation of rather small conducting areas. Developing this idea, namely that these conducting areas of extreme low expansion, i.e. in the range of nanoparticles, could contact each other and enable a current by forming a kind of net could supply a, however hardly secured, explanation concerning the constitution in the modified traces and the related conductivity.
5. Summary Laser-assisted processes for the fabrication of conductive structures and contacts on ceramic substrate materials ŽAlN, Al 2 O 3 . through modification are described that enable the direct fabrication of conducting traces. These can serve as an alternative to conventional methods of thick film, thin film and chemical metallization. Metallization is important for the connection between substrate and chip. Due to their material properties the dielectrics AlN and Al 2 O 3 are especially suitable substrate materials meeting most of the requirements placed on substrates ŽSection 1.. A process for fabrication of conductive structures on Al 2 O 3 with CO 2 laser irradiation Ž l s 10.6 m m. is presented and a corresponding procedure for AlN with excimer laser irradiation Ž l s 248 nm. is examined. The changes of resistance in the modified areas dependent on the processing variables are investigated. These resulting resistance changes are correlated to the changes of the chemical composition in the modified areas analyzed by means of EBMA. Al 2 O 3 shows an increase in the attainable resistance lowering with decreasing substrate velocity whereas AlN presents an opposite dependence. The modified traces in AlN, fabricated with increasing processing velocity, correlate in respect to the changes in the chemical composition, i.e. increase of Al-percentage by weight, with a reduction of the resistance values.
248
B. Stolz et al.r Applied Surface Science 109 r 110 (1997) 242–248
References w1x Technische Keramik, T. Quiel, Handbuch, 2.Ausgabe, Vulkan Verlag, Essen, 1990, p. 124–137. w2x Aluminum-based metallurgy for global interconnects, Dipankar Pramanik MRS bulletin, Nov. 1995, Volume XX, No. 11, p. 57.
w3x Funktionswerkstoffe der Elektrotechnik und Elektronik, ISBN 3-342-00524-6, 1993. w4x IEEE Trans. Components, Hybrids and Manuf. Technology, USA June 1985, CHMT-8, Nr. 2, p. 247–253. w5x Dr. rer.nat. Karduck, G.f.e. Aachen, private communication, to be published.
Selective surface modification of ceramics with laser radiation B. Stolz a
a,)
, G. Backes a , A. Gillner b, E.W. Kreutz
a
Lehrstuhl fur Technischen Hochschule Aachen, Steinbachstr. 15, D-52074 Aachen, Germany ¨ Lasertechnik, der Rheinisch-Westfalischen ¨ b Fraunhofer Institut fur ¨ Lasertechnik, Steinbachstr. 15, D-52074 Aachen, Germany Received 4 June 1996; accepted 24 October 1996
Abstract Laser-assisted processes that enable the direct fabrication of conducting structures and contacts on ceramic substrate materials ŽAlN, Al 2 O 3 . through modification are presented. These can serve as an alternative to conventional methods of thick film, thin film and chemical metallization. A method for fabrication of conductive structures on Al 2 O 3 with CO 2 laser irradiation Ž l s 10.6 m m. is developed and a corresponding procedure for AlN with excimer laser irradiation Ž l s 248 nm. is examined. The changes of resistance in the modified areas dependent on the processing variables are investigated and correlated to the results of a chemical analysis by means of electron beam microprobe.
1. Introduction Ceramic components have become part of the state of the art in connection technology. Due to the steady advances in semiconductor technology and the increased performance achievement of computing systems by miniaturization of the functional devices ceramic substrates have to meet increasing demands: the substrate and packaging materials have to provide mechanical protection and moreover they have to fulfill different thermal and electrical requirements in order to realize the possibilities of the semiconductor technology in efficient way. A comparison between the demands made on the ceramic substrate and the material properties reveals that AlN and Al 2 O 3 are especially suitable as substrate mate-
)
Corresponding author. Tel.: q49-241-8906410; fax: q49241-89061211; e-mail: [email protected].
rials w1x. Metallization is important for the connection between substrate and chip. Metallization systems used for global interconnects have to meet a lot of different requirements, e.g. a certain contact resistance to nq and pq contacts, no degradation due to electromigration, corrosion resistance and sufficient adherence between interconnects and dielectrics Žsuch as oxides and nitrides used for insulating different levels of metals.. Aluminum satisfies most of the required conditions and has become a widely used material in IC circuits w2x. By means of laser-assisted processes for metallization of ceramics based on thermally activated mechanisms, conducting and adhesive structures are to be produced at the surface of dielectric substrates, aiming at applications in microtechnology. Applications in communication technology, in medical sectors and in sensor technology as well show the technical and economic potential of microsystems. Historically conditioned by the development in mi-
0169-4332r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 6 . 0 0 9 1 6 - 6
B. Stolz et al.r Applied Surface Science 109 r 110 (1997) 242–248
croelectronics, the majority of these systems are based on silicon. In order to ensure an acceptance of microsystems in multiple technical fields, cost-effective fabrication processes should be available for as many materials as possible. Laser-assisted processes enable the application of conducting structures on non-conducting substrates in a direct and more simplified way, i.e. with a reduced number of processing steps, than the conventional methods of thick film, thin film and chemical metallization methods w3x. In order to produce conducting structures the substrate material is irradiated for a sufficient processing time with laser radiation of such intensity that in the irradiated zones a modification of the insulating base material results, leading to a lowering of the electrical resistance. With regard to the demands placed on substrate materials various electrical and thermal specifications must be met. Al 2 O 3 fulfills the requirement of a high electrical resistivity Ž r ) 10 14 V cm. which minimizes loss currents through the carrier material that would otherwise impair the function of the electronic device. In contrast AlN Ž r s 10 11 V cm. shows a significantly lower resistivity. Concerning the demand for dielectric losses as low as possible in order to realize high switching frequencies Al 2 O 3 and AlN are nearly equally suitable with Al 2 O 3 being nearer to the optimum value. With respect to the required thermal conductivity Žideally ) 100 WrmK for efficient heat conduction. AlN is better suited than Al 2 O 3 . Also with respect to the thermal expansion coefficient, AlN presents a better-suited value for a compound with silicon. The requirement of high mechanical strength, which is important for large substrates, for reduced substrate thicknesses, and for enhanced application of automated handling systems, is approximately met by Al 2 O 3 with a bending strength of about 400–450 Nrmm2 , being higher than that of AlN. If cost is considered, Al 2 O 3 will be favored for average applications and mass production, whereas the use of AlN will more and more be limited to high performance applications. These combinations of properties make the insulating materials Al 2 O 3 and AlN well-suited substrate materials for integrated circuit ŽIC.-carriers w1,4x. The direct metallization of dielectrics is achieved by two fundamentally different processes: laser-induced modification and laser alloying. The modifica-
243
tion of the dielectrics takes place in the interaction zone heated by the absorbed optical energy. The possibility of modifying the dielectric Al 2 O 3 into a conducting aluminum compound by irradiation is fundamentally examined and for AlN the process of metallization is investigated. Depending on the type of ceramic the reactions during modification consist predominantly of deoxidation, decarbonization or denitriding processes. The influence of the processing variables on the decrease of the electrical resistance is investigated for different dielectrics ŽAl 2 O 3 , AlN.. These resulting resistance values are correlated to the results of chemical composition of the samples analyzed by electron beam microprobe investigations ŽEBMA.. The analysis with EBMA was carried out by the GFE ŽGesellschaft fur ¨ Elektronenmikroskopie, organization for electron microscopy, Aachen, Germany.. The signal used for the EBMA-analysis is characteristic X-ray-radiation and its intensity and the relative error of EBMA-measurement amounts to 0.5%.
2. Experimental details 2.1. Modification of Al 2 O3 For the modification of Al 2 O 3 , the CO 2 laser radiation Žcw, l s 10.6 m m, PL s 1.5 kW. is focussed 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 liquid bath of ethanol ŽC 2 H 5 OH.
Fig. 1. Process of modification of Al 2 O 3 .
244
B. Stolz et al.r Applied Surface Science 109 r 110 (1997) 242–248
2.2. Modification of AlN
Fig. 2. Resistance dependent on laser power for Al 2 O 3 .
during laser treatment and is covered to a depth of about 2 mm by ŽFig. 1.. The trace geometry is determined by the processing variables such as laser power, processing velocity, focal length and spot size. The processing variables velocity Žchanged in a range between 10 mmrmin and 3000 mmrmin. and laser power Žfrom 25 W to 1000 W, which corresponds to intensities between 2 = 10 4 to 5 = 10 5 Wrcm2 . are varied. The resulting values of resistance in the modified areas are measured with a simple ohmmeter method and could not be compared to the measurement of a four-point-probe-technique, because the underlying resistances were out of the measuring range of the four-point measuring technique being available.
The modification of AlN with excimer laser radiation ŽKrF, l s 248 nm. is carried out by mask projection processing, wherein the substrate surface is positioned in the focal plane and the trace geometry is essentially determined by the dimension of the mask being used. The substrate is located in a vacuum chamber on a motor-driven x–y-manipulator. The energy density is controlled by a beam splitter and the resonator high voltage. The processing variables speed Žin the range from 0.1 to 20 mmrs. energy density Žfrom 1.5 to 6 Jrcm2 . repetition rate Žfrom 3 to 120 Hz., number of passes and ambient atmosphere are varied. The values of resistance in the modified traces were determined by four-point probe technique and simple ohmmeter measurement as well. A comparison of both techniques by calculating the resistivity into resistances showed that the values determined by the four-point probe technique are about 10% higher than the values measured with the ohmmeter.
3. Results 3.1. Results for modification of Al 2 O3 The selective modification is investigated for sintered and slurry cast alumina of different purity and
Fig. 3. Resistance of modified Al 2 O 3 dependent on processing velocity.
B. Stolz et al.r Applied Surface Science 109 r 110 (1997) 242–248
Fig. 4. Al 2 O 3 processed with CO 2-laser-radiation without coating and with ethanol Žtrace on the left side: processing without coating leads only to ablation; the other trace covered with ethanol during laser-treatment consists of a conducting modification; the same processing parameters were used for the fabrication of both traces: laser-power P s 25 W, processing velocity Õ s 50 mmrmin, focal length f s 200 mm..
powder quality. By irradiation ŽSection 2.1. with intensities in the range from 2 = 10 4 to 5 = 10 5 Wrcm2 values of resistance per unit length between the ends of the modified traces are attained in the range of k Vrcm to M Vrcm. The resulting resistance values present a clear dependence on the intensity and the processing velocity, as is shown in Fig. 2. The resistivity of the base material is higher than 10 14 V cm and thus the lowering of the resistance down to the above-mentioned values means a reduction of more than 10 orders of magnitude. The processing velocity has an essential influence on the trace geometry and moreover a significant influence on the attainable conductivity values which cannot only be traced back to the geometry factor: in case of lower powers being used, i.e. 30–50 W, the resulting conductivity changes increase with decreasing processing velocities ŽFig. 3.. If the process is carried out without ethanol there only occurs an ablation of the base material as can be seen in Fig. 4. The results from the chemical analysis of the modified samples ŽEBMA. are not yet sufficiently revealing with respect to the evaluation of the mechanisms being responsible for the emerged resistance reduction. The results of the analysis reveal an increase of the Al-percentage in the modified conducting areas compared to the base material of merely
245
Fig. 5. REM picture of the modified area in the finegrained base Al 2 O 3 .
0.16 wt% Ž0.15 at%. and corresponding low decreases in the O-percentage of 0.13 wt% and in the C-percentage of 0.03 wt. The microstructure in the modified zone consists of sternlike crystallites corresponding to the strongly directed solidification ŽFig. 5.. In the transition zone only a few detachings can be seen eventually caused by the thermal shock or lattice mismatch. In opposite traces produced by using other processing variables, i.e. higher laser power, don’t even adhere macroscopically to the substrate.
Fig. 6. Photography of modified traces in AlN processed with excimer-laser-radiation Ž l s 248 nm. pulse energy Ep s 200 mJ, energy densities in the range 2–7 Jrcm2 , processing velocity Õ s 0.5 mmrs, frequency f s10 Hz.
246
B. Stolz et al.r Applied Surface Science 109 r 110 (1997) 242–248
Fig. 7. Resistance and wt% of Al for AlN-substrate dependent on velocity.
3.2. Results for modification of AlN Because some of the traces cannot be detected microscopically the trace depth of the modified structures in AlN ŽFig. 6. is calculated by the results from EBMA investigations for two different energies Ž5 kV and 15 kV. and thus volumina of different magnitude included in the measurement. A multilayer model consisting of three layers Žbase material AlN, modified material supposed to contain Al, N and O, sputter layer of Au. in combination with the
results of the analysis for different penetration depth is used to determine the trace thickness w5x. The resulting values show no consistent dependence on the processing variables but are in the range from 100 to 115 nm. The processing velocity influences the modified traces: the attainable reduction of resistance can be enhanced by increasing processing velocity ŽFig. 7.. This tendency is confirmed by the resistance values resulting for different repetition rates Ž3–120 Hz. being used. A comparison to the results of the
Fig. 8. Wt% of Al for AlN-substrate dependent on processing velocity measured by using different energies.
B. Stolz et al.r Applied Surface Science 109 r 110 (1997) 242–248
EBMA-investigation from the measurement with 5 kV presents a corresponding increase of the Al-percentage with increasing velocity for all repetition rates being chosen as shown in Fig. 7. The results from the measurement with 15 kV don’t reveal this kind of correlation but show a course being independent of velocity ŽFig. 8. because in this case, due to the higher penetration depth into the material below the modified region, the measured results are considerably determined and nearly dominated by the composition of the base material.
4. Discussion The influence on the resulting values of conductivity is not exclusively conditioned by the changes in trace geometry. In case of laser powers in the range of 30–50 W the attained conductivity increases with decreasing velocity which hints at a process that claims time for the formation of the conducting modification. If the base material is processed without ethanol as coating the treatment only leads to ablation in the substrate material without production of conducting areas ŽFig. 4.. This reveals that on one hand the ethanol coating effects a reduced absorption of the energy by the substrate and on the other hand it is, due to its function as a possible reactant, indispensable to enable the selective modification of the insulating into a conducting material. The fundamental mechanisms and the reactions caused by the ethanol, however, are still to be clarified. It has still to be investigated whether the ethanol fulfills the function of a usual reaction partner or catalyst or if it inhibits reactions that suppress the formation of a conducting compounds. The assumption, however, that a pure carbonization, causing the emerged conductivity, took place in the modified traces is weakened by the habitus of the microstructure in the modified zone ŽFig. 5.. The REM photography presents a directly solidified microstructure in the fine-grained base material. Results from chemical analysis that could determine an increase of the Al-percentage merely within the measurement inaccuracy oppose to the assumption of a so-called metallization as explanation for the arisen conductivity. The following conception
247
could eventually explain the observed phenomena: although the chemical composition has hardly changed a drastic change in the conductivity is effected which could lead to the conclusion that not an absolute change in composition is necessary but that a change in the mainly ionic bonding character caused by the modification enables the formation of rather small conducting areas. Developing this idea, namely that these conducting areas of extreme low expansion, i.e. in the range of nanoparticles, could contact each other and enable a current by forming a kind of net could supply a, however hardly secured, explanation concerning the constitution in the modified traces and the related conductivity.
5. Summary Laser-assisted processes for the fabrication of conductive structures and contacts on ceramic substrate materials ŽAlN, Al 2 O 3 . through modification are described that enable the direct fabrication of conducting traces. These can serve as an alternative to conventional methods of thick film, thin film and chemical metallization. Metallization is important for the connection between substrate and chip. Due to their material properties the dielectrics AlN and Al 2 O 3 are especially suitable substrate materials meeting most of the requirements placed on substrates ŽSection 1.. A process for fabrication of conductive structures on Al 2 O 3 with CO 2 laser irradiation Ž l s 10.6 m m. is presented and a corresponding procedure for AlN with excimer laser irradiation Ž l s 248 nm. is examined. The changes of resistance in the modified areas dependent on the processing variables are investigated. These resulting resistance changes are correlated to the changes of the chemical composition in the modified areas analyzed by means of EBMA. Al 2 O 3 shows an increase in the attainable resistance lowering with decreasing substrate velocity whereas AlN presents an opposite dependence. The modified traces in AlN, fabricated with increasing processing velocity, correlate in respect to the changes in the chemical composition, i.e. increase of Al-percentage by weight, with a reduction of the resistance values.
248
B. Stolz et al.r Applied Surface Science 109 r 110 (1997) 242–248
References w1x Technische Keramik, T. Quiel, Handbuch, 2.Ausgabe, Vulkan Verlag, Essen, 1990, p. 124–137. w2x Aluminum-based metallurgy for global interconnects, Dipankar Pramanik MRS bulletin, Nov. 1995, Volume XX, No. 11, p. 57.
w3x Funktionswerkstoffe der Elektrotechnik und Elektronik, ISBN 3-342-00524-6, 1993. w4x IEEE Trans. Components, Hybrids and Manuf. Technology, USA June 1985, CHMT-8, Nr. 2, p. 247–253. w5x Dr. rer.nat. Karduck, G.f.e. Aachen, private communication, to be published.