- Email: [email protected]
Single-shot, high repetition rate metallic pattern transfer
Microelectronic Engineering 50 (2000) 541–546 www.elsevier.nl / locate / mee
Single-shot, high repetition rate metallic pattern transfer *, W. Groß, A. Menschig ¨ R. Bahnisch ¨ Luft- und Raumfahrt ( DLR), Institut f ur ¨ Technische Physik, Pfaffenwaldring 38 – 40, Deutsches Zentrum f ur D-70569 Stuttgart, Germany Abstract A flexible high-speed fabrication of relatively thick (about 1 mm) and large (about 120 3 120 mm) thin film metal pads with a laser-induced forward transfer technique using femtosecond laser pulses (fs-LIFT) will be discussed. Possible applications are thickening of thin film contact pads for wire bonding, the deposition of solder pads or the frequency or electrical resistance tuning of discrete devices. The use of ultrashort laser pulses instead of nanosecond laser pulses reduces the melting problem, increases the quality and the adhesion of the transferred metal pads and allows the transfer of complete disks out of thin films ( , 1 mm thickness). The combination of ultrashort laser pulses with a pre-structuring of the metal film improves the geometry of the pads and increases the film disk thickness ( . 1 mm). As a example the transfer of 0.76- and 1.8 mm thick gold / tin disks with a single laser pulse are presented. 2000 Elsevier Science B.V. All rights reserved. Keywords: Ultrashort laser pulses; Laser-induced forward transfer (LIFT); Structured metallization; Contact pads; Solder pads
1. Introduction The thickening of thin film contact pads during the semiconductor and micro-system device fabrication often is necessary. Pure metal (e.g. gold or aluminium) or solder (e.g. AuSn) pads with a thickness of some microns are required for wire or die / flip-chip bonding, respectively. The standard processes use lithography in combination with evaporation, sputtering or galvanic techniques for the deposition. All these technologies gather such problems as cross-contamination and disposal or recycling of solvents and plating baths. A high-speed local deposition process with a large flexibility in the formation of metal patterns can be an interesting alternative, especially if the technology can be scaled to any substrate size and used also for modifications of already diced devices. For small and medium scale production, a maskless technique will also save time and cost for mask fabrication. To simplify the integration in existing production sequences a new process should deposit the metal pads without any contamination problem. *Corresponding author. Fax: 1 49-711-6862-555. ¨ E-mail address: [email protected] (R. Bahnisch) 0167-9317 / 00 / $ – see front matter PII: S0167-9317( 99 )00325-1
2000 Elsevier Science B.V. All rights reserved.
542
¨ et al. / Microelectronic Engineering 50 (2000) 541 – 546 R. Bahnisch
Fig. 1. Scheme of the fs-LIFT arrangement.
The laser-induced forward transfer (fs-LIFT) process is a local and inherent clean technique to generate pads of different metals and different sizes [1,2]. The fs-LIFT uses ultrashort laser pulses to pull off thin disks from a transparent support and deposit them onto a substrate. The laser beam is focussed through the carrier to the support / metal interface (Fig. 1). During the laser pulse, energy is deposited within the laser spot size into the support / film interface. The material at the interface is evaporated and the expansion of the material accelerates the non-evaporated part of the metal film towards a substrate. The LIFT with nanosecond pulses is a well-known technology to generate structured metallizations onto substrates [3–5]. The formation of high quality patterns is difficult to control, especially due to melting effects [6,7]. The use of femtosecond laser pulses to overcome the melting problems is well known from micromachining applications [8,9]. In this paper we will show the transfer of Au / Sn disks using ultrashort laser pulses. To improve the geometry and to increase the thickness of the pad some films on the support will be pre-structured.
2. Experimental set-up For the experiments we use ultrashort pulses from a Ti:sapphire laser (Clark-MXR CPA1000; wavelength, 775 nm) with a repetition rate of 1 kHz. The pulse width and the pulse energy can be varied electronically between 0.1 and 8 ps and 0 and 0.5 mJ, respectively. The laser system is modified to allow computer-controlled continuous, pulse train and single-pulse operation. The optical set-up projects the image of a pinhole (5.0 mm diameter) via an achromatic lens (focal length, 200 mm) or a diffractive optical element (DOE) onto the workpiece with a reduction factor of about 40. To adjust the image of the pinhole, the imaging lens is mounted on a motorized z-translation stage, whereas the workpiece is mounted on motorised x–y translation stages under full computer
¨ et al. / Microelectronic Engineering 50 (2000) 541 – 546 R. Bahnisch
543
control. The workpiece, the lens and the translation stages are in a vacuum chamber ( p , 10 24 mbar) to avoid deterioration of the high-intensity laser beam through non-linear effects in air and for debris reduction on the surface during the laser pre-patterning process. The experiments were carried out using a transparent glass carrier with a metal film and a aluminium frame as a spacer sandwiched to a substrate. The metal film consists of Au / Sn (80 / 20, wt.%) with 0.76 or 1.8 mm thickness. The spacer is a thermally evaporated aluminium frame with a thickness of approximately twice the thickness of the metal film. Most of the results are for the transfer to a Si substrate, but other substrates, e.g. glass, also show good adhesion. The thicker metal films were pre-structured. This means a decrease of the film thickness at the circumference of the pad by removing material with the ultrashort pulsed laser beam in a standard patterning arrangement [2].
3. Experimental results First we carried out experiments for the transfer of Au / Sn disks from a 0.76 mm thick film with different pulse lengths (0.1, 1 and 3.5 ps), energies (0.05, 0.1 and 0.15 mJ) and number of pulses (one, two, four, eight, 10). The metal film was not pre-structured and the image of the pinhole was projected onto the workpiece. With an pulse energy of 0.05 mJ a disk never was transferred completely. The difference in the appearance of the transferred disks at 0.10 and 0.15 mJ pulse energies was not significant. For pulses with a length . 3 ps, one pulse is sufficient to deposit a pad. For smaller pulse length, two or more pulses were necessary. For example, Fig. 2 shows a Au / Sn (80:20, wt.%) solder pad (0.76 mm thick, 0.12 mm diameter) which was transferred to a Si substrate by a single 3.5 ps pulse with a pulse energy of 0.1 mJ. Experiments with an unstructured 1.8 mm Au / Sn film fail. The transferred disks were not complete and very inhomogeneous. So, to deposit complete Au / Sn pads with a thickness of 1.8 mm we reduced the shear force with the film pre-structure. Also we used a DOE to generate a flat-top intensity profile. The laser parameters to pre-structure the metal film shown in Fig. 3 were 0.1 ps pulse length, 0.15 mJ
Fig. 2. A 120 mm diameter Au / Sn pad with a thickness of 760 nm transferred with a single 3.5 ps pulse.
544
¨ et al. / Microelectronic Engineering 50 (2000) 541 – 546 R. Bahnisch
Fig. 3. Part of a laser pre-structured Au / Sn Film (1.8 mm thick).
pulse energy and a speed of 200 mm / s of the x–y positioning stages. In Fig. 4, a 1.8 mm thick Au / Sn (80:20, wt.%) solder pad transferred from a glass carrier to a Si substrate is shown as a optical photograph. To transfer this pad the laser pulse had a duration of 0.1 ps and a energy of 0.22 mJ. The picture demonstrates that the geometry of the transferred pad is defined mainly by the pre-structuring.
Fig. 4. Au / Sn pad with a thickness of 1.8 mm transferred with a single 0.1 ps pulse; top view with a optical microscope.
¨ et al. / Microelectronic Engineering 50 (2000) 541 – 546 R. Bahnisch
545
4. Discussion During the laser pulse, energy is deposited within the laser spot size times the thermal penetration depth of the electrons at the support / film interface. In the regime of ultrashort pulses the peak power density is so high (TW/ cm 2 ) and the time scale so short that the electrons and the phonons are not in equilibrium [10,11]. Therefore, the conventional heat flow theory [6] is not applicable for the process description. Since the penetration depth is a few nanometers, the energy is deposited in a small volume. Only the material at the metal / glass interface reaches a state of superheated matter or hot dense plasma. Also, the increase of the temperature is so fast that only the material within the penetration depth is influenced. For example a 1.3 ps laser pulse with 0.15 mJ pulse energy generates, after 3 ps within a gold film, a temperature up to 4000 K (phonon temperature) and a pressure of about 17 GPa. The force generated by this pressure must overcome the shear force in the surrounding solid film and accelerate the thin disk towards the substrate. The shear force in the 0.76 mm Au / Sn film is lower than in the 1.8 mm Au / Sn film. So, for the 0.76 mm film the produced pressure is high enough to propel the disk out of a complete film. For the unstructured 1.8 mm film the shear force in the rest of the solid film prevents the detachment of the film. To generate thick pads, the shear force must be decreased or eliminated. This is done by removing material at the circumference of the pad. This also defines the geometry of the transferred pad more precisely than the area of the laser beam profile.
5. Conclusion We have shown the single-shot transfer of disks out of a metallic film with ultrashort laser pulses. The deposition of a variety of metals to different substrates was discussed. The transferred Au / Sn disks show a good adhesion to the silicon substrate and a well-defined pad geometry for thicknesses up to 1.8 mm. This new technology is a local, flexible, fast and inherent clean method to deposit material from a support to a substrate. The technology can be scaled to any substrate size and also be used for the modification of already diced devices.
Acknowledgements ¨ Bildung, Wissenschaft, This work was financially supported in part by the Bundesministerium fur Forschung und Technologie under contract number 13N7047. The authors are indebted to the Siemens ¨ AG for the good co-operation and the support of materials. The authors thank Dr Bernd Huttner for the discussion and some calculations.
References [1] I. Zergioti, S. Mailis, N.A. Vainos, P. Pagakonstantinou, C. Kalpouzous, C.P. Grigoropoulos, C. Fotakis, Microdeposition of metal and oxide structures using ultrashort laser pulses, Appl. Phys. A66 (1998) 579–582.
546
¨ et al. / Microelectronic Engineering 50 (2000) 541 – 546 R. Bahnisch
¨ [2] R. Bahnisch, W. Groß, A. Menschig, Femtosecond laser based technology for fast development of micromechanical devices, emrs 1998, Sensors and Actuators A 74 (1999) 31–34. [3] J. Bohandy, B.F. Kim, F.J. Adrian, Metal deposition from a supported metal film using an excimer laser, J. Appl. Phys. 60 (1986) 1538–1539. [4] R.J. Baseman, N.M. Froberg, J.C. Andreshak, Z. Schlesinger, Minimum fluence for laser blow-off of thin gold films at 248 and 532 nm, Appl. Phys. Lett. 56 (1990) 1412–1414. [5] R.J. Baseman, N.M. Froberg, Time-resolved transmission of thin gold films during blow-off, Appl. Phys. Lett. 55 (1989) 1841–1843. [6] F.J. Adrian, J. Bohandy, B.F. Kim, A.N. Jette, A study of the mechanism of metal deposition by laser-induced forward transfer process, J. Vac. Sci. Technol. B5 (1987) 1490–1494. [7] V. Schultze, M. Wagner, Blow-off of aluminium films, Appl. Phys. A53 (1991) 241–248. ¨ ¨ [8] H.W. Bergmann, H. Junge, M. Wilfert (Eds.), Prazise optische Bearbeitung von Festkorpern, VDI-Verlag, Handbuch¨ reihe Bd. 5, ISBN 3-18-40 1599-8, Dusseldorf, 1996. [9] X. Liu, D. Du, G. Mourou, Laser ablation and micromachining with ultrashort laser pulses, IEEE J. Quantum Electron. 33 (1997) 1706. ¨ [10] B. Huttner, G.C. Rohr, On the theory of ps and sub-ps laser pulse interaction with metals; I. Surface temperature, Appl. Surf. Sci. 103 (1996) 269–274. ¨ [11] B. Huttner, G.C. Rohr, On the theory of ps and sub-ps laser pulse interaction with metals II. Spatial temperature distribution, Appl. Surf. Sci. 126 (1998) 129–135.
Single-shot, high repetition rate metallic pattern transfer *, W. Groß, A. Menschig ¨ R. Bahnisch ¨ Luft- und Raumfahrt ( DLR), Institut f ur ¨ Technische Physik, Pfaffenwaldring 38 – 40, Deutsches Zentrum f ur D-70569 Stuttgart, Germany Abstract A flexible high-speed fabrication of relatively thick (about 1 mm) and large (about 120 3 120 mm) thin film metal pads with a laser-induced forward transfer technique using femtosecond laser pulses (fs-LIFT) will be discussed. Possible applications are thickening of thin film contact pads for wire bonding, the deposition of solder pads or the frequency or electrical resistance tuning of discrete devices. The use of ultrashort laser pulses instead of nanosecond laser pulses reduces the melting problem, increases the quality and the adhesion of the transferred metal pads and allows the transfer of complete disks out of thin films ( , 1 mm thickness). The combination of ultrashort laser pulses with a pre-structuring of the metal film improves the geometry of the pads and increases the film disk thickness ( . 1 mm). As a example the transfer of 0.76- and 1.8 mm thick gold / tin disks with a single laser pulse are presented. 2000 Elsevier Science B.V. All rights reserved. Keywords: Ultrashort laser pulses; Laser-induced forward transfer (LIFT); Structured metallization; Contact pads; Solder pads
1. Introduction The thickening of thin film contact pads during the semiconductor and micro-system device fabrication often is necessary. Pure metal (e.g. gold or aluminium) or solder (e.g. AuSn) pads with a thickness of some microns are required for wire or die / flip-chip bonding, respectively. The standard processes use lithography in combination with evaporation, sputtering or galvanic techniques for the deposition. All these technologies gather such problems as cross-contamination and disposal or recycling of solvents and plating baths. A high-speed local deposition process with a large flexibility in the formation of metal patterns can be an interesting alternative, especially if the technology can be scaled to any substrate size and used also for modifications of already diced devices. For small and medium scale production, a maskless technique will also save time and cost for mask fabrication. To simplify the integration in existing production sequences a new process should deposit the metal pads without any contamination problem. *Corresponding author. Fax: 1 49-711-6862-555. ¨ E-mail address: [email protected] (R. Bahnisch) 0167-9317 / 00 / $ – see front matter PII: S0167-9317( 99 )00325-1
2000 Elsevier Science B.V. All rights reserved.
542
¨ et al. / Microelectronic Engineering 50 (2000) 541 – 546 R. Bahnisch
Fig. 1. Scheme of the fs-LIFT arrangement.
The laser-induced forward transfer (fs-LIFT) process is a local and inherent clean technique to generate pads of different metals and different sizes [1,2]. The fs-LIFT uses ultrashort laser pulses to pull off thin disks from a transparent support and deposit them onto a substrate. The laser beam is focussed through the carrier to the support / metal interface (Fig. 1). During the laser pulse, energy is deposited within the laser spot size into the support / film interface. The material at the interface is evaporated and the expansion of the material accelerates the non-evaporated part of the metal film towards a substrate. The LIFT with nanosecond pulses is a well-known technology to generate structured metallizations onto substrates [3–5]. The formation of high quality patterns is difficult to control, especially due to melting effects [6,7]. The use of femtosecond laser pulses to overcome the melting problems is well known from micromachining applications [8,9]. In this paper we will show the transfer of Au / Sn disks using ultrashort laser pulses. To improve the geometry and to increase the thickness of the pad some films on the support will be pre-structured.
2. Experimental set-up For the experiments we use ultrashort pulses from a Ti:sapphire laser (Clark-MXR CPA1000; wavelength, 775 nm) with a repetition rate of 1 kHz. The pulse width and the pulse energy can be varied electronically between 0.1 and 8 ps and 0 and 0.5 mJ, respectively. The laser system is modified to allow computer-controlled continuous, pulse train and single-pulse operation. The optical set-up projects the image of a pinhole (5.0 mm diameter) via an achromatic lens (focal length, 200 mm) or a diffractive optical element (DOE) onto the workpiece with a reduction factor of about 40. To adjust the image of the pinhole, the imaging lens is mounted on a motorized z-translation stage, whereas the workpiece is mounted on motorised x–y translation stages under full computer
¨ et al. / Microelectronic Engineering 50 (2000) 541 – 546 R. Bahnisch
543
control. The workpiece, the lens and the translation stages are in a vacuum chamber ( p , 10 24 mbar) to avoid deterioration of the high-intensity laser beam through non-linear effects in air and for debris reduction on the surface during the laser pre-patterning process. The experiments were carried out using a transparent glass carrier with a metal film and a aluminium frame as a spacer sandwiched to a substrate. The metal film consists of Au / Sn (80 / 20, wt.%) with 0.76 or 1.8 mm thickness. The spacer is a thermally evaporated aluminium frame with a thickness of approximately twice the thickness of the metal film. Most of the results are for the transfer to a Si substrate, but other substrates, e.g. glass, also show good adhesion. The thicker metal films were pre-structured. This means a decrease of the film thickness at the circumference of the pad by removing material with the ultrashort pulsed laser beam in a standard patterning arrangement [2].
3. Experimental results First we carried out experiments for the transfer of Au / Sn disks from a 0.76 mm thick film with different pulse lengths (0.1, 1 and 3.5 ps), energies (0.05, 0.1 and 0.15 mJ) and number of pulses (one, two, four, eight, 10). The metal film was not pre-structured and the image of the pinhole was projected onto the workpiece. With an pulse energy of 0.05 mJ a disk never was transferred completely. The difference in the appearance of the transferred disks at 0.10 and 0.15 mJ pulse energies was not significant. For pulses with a length . 3 ps, one pulse is sufficient to deposit a pad. For smaller pulse length, two or more pulses were necessary. For example, Fig. 2 shows a Au / Sn (80:20, wt.%) solder pad (0.76 mm thick, 0.12 mm diameter) which was transferred to a Si substrate by a single 3.5 ps pulse with a pulse energy of 0.1 mJ. Experiments with an unstructured 1.8 mm Au / Sn film fail. The transferred disks were not complete and very inhomogeneous. So, to deposit complete Au / Sn pads with a thickness of 1.8 mm we reduced the shear force with the film pre-structure. Also we used a DOE to generate a flat-top intensity profile. The laser parameters to pre-structure the metal film shown in Fig. 3 were 0.1 ps pulse length, 0.15 mJ
Fig. 2. A 120 mm diameter Au / Sn pad with a thickness of 760 nm transferred with a single 3.5 ps pulse.
544
¨ et al. / Microelectronic Engineering 50 (2000) 541 – 546 R. Bahnisch
Fig. 3. Part of a laser pre-structured Au / Sn Film (1.8 mm thick).
pulse energy and a speed of 200 mm / s of the x–y positioning stages. In Fig. 4, a 1.8 mm thick Au / Sn (80:20, wt.%) solder pad transferred from a glass carrier to a Si substrate is shown as a optical photograph. To transfer this pad the laser pulse had a duration of 0.1 ps and a energy of 0.22 mJ. The picture demonstrates that the geometry of the transferred pad is defined mainly by the pre-structuring.
Fig. 4. Au / Sn pad with a thickness of 1.8 mm transferred with a single 0.1 ps pulse; top view with a optical microscope.
¨ et al. / Microelectronic Engineering 50 (2000) 541 – 546 R. Bahnisch
545
4. Discussion During the laser pulse, energy is deposited within the laser spot size times the thermal penetration depth of the electrons at the support / film interface. In the regime of ultrashort pulses the peak power density is so high (TW/ cm 2 ) and the time scale so short that the electrons and the phonons are not in equilibrium [10,11]. Therefore, the conventional heat flow theory [6] is not applicable for the process description. Since the penetration depth is a few nanometers, the energy is deposited in a small volume. Only the material at the metal / glass interface reaches a state of superheated matter or hot dense plasma. Also, the increase of the temperature is so fast that only the material within the penetration depth is influenced. For example a 1.3 ps laser pulse with 0.15 mJ pulse energy generates, after 3 ps within a gold film, a temperature up to 4000 K (phonon temperature) and a pressure of about 17 GPa. The force generated by this pressure must overcome the shear force in the surrounding solid film and accelerate the thin disk towards the substrate. The shear force in the 0.76 mm Au / Sn film is lower than in the 1.8 mm Au / Sn film. So, for the 0.76 mm film the produced pressure is high enough to propel the disk out of a complete film. For the unstructured 1.8 mm film the shear force in the rest of the solid film prevents the detachment of the film. To generate thick pads, the shear force must be decreased or eliminated. This is done by removing material at the circumference of the pad. This also defines the geometry of the transferred pad more precisely than the area of the laser beam profile.
5. Conclusion We have shown the single-shot transfer of disks out of a metallic film with ultrashort laser pulses. The deposition of a variety of metals to different substrates was discussed. The transferred Au / Sn disks show a good adhesion to the silicon substrate and a well-defined pad geometry for thicknesses up to 1.8 mm. This new technology is a local, flexible, fast and inherent clean method to deposit material from a support to a substrate. The technology can be scaled to any substrate size and also be used for the modification of already diced devices.
Acknowledgements ¨ Bildung, Wissenschaft, This work was financially supported in part by the Bundesministerium fur Forschung und Technologie under contract number 13N7047. The authors are indebted to the Siemens ¨ AG for the good co-operation and the support of materials. The authors thank Dr Bernd Huttner for the discussion and some calculations.
References [1] I. Zergioti, S. Mailis, N.A. Vainos, P. Pagakonstantinou, C. Kalpouzous, C.P. Grigoropoulos, C. Fotakis, Microdeposition of metal and oxide structures using ultrashort laser pulses, Appl. Phys. A66 (1998) 579–582.
546
¨ et al. / Microelectronic Engineering 50 (2000) 541 – 546 R. Bahnisch
¨ [2] R. Bahnisch, W. Groß, A. Menschig, Femtosecond laser based technology for fast development of micromechanical devices, emrs 1998, Sensors and Actuators A 74 (1999) 31–34. [3] J. Bohandy, B.F. Kim, F.J. Adrian, Metal deposition from a supported metal film using an excimer laser, J. Appl. Phys. 60 (1986) 1538–1539. [4] R.J. Baseman, N.M. Froberg, J.C. Andreshak, Z. Schlesinger, Minimum fluence for laser blow-off of thin gold films at 248 and 532 nm, Appl. Phys. Lett. 56 (1990) 1412–1414. [5] R.J. Baseman, N.M. Froberg, Time-resolved transmission of thin gold films during blow-off, Appl. Phys. Lett. 55 (1989) 1841–1843. [6] F.J. Adrian, J. Bohandy, B.F. Kim, A.N. Jette, A study of the mechanism of metal deposition by laser-induced forward transfer process, J. Vac. Sci. Technol. B5 (1987) 1490–1494. [7] V. Schultze, M. Wagner, Blow-off of aluminium films, Appl. Phys. A53 (1991) 241–248. ¨ ¨ [8] H.W. Bergmann, H. Junge, M. Wilfert (Eds.), Prazise optische Bearbeitung von Festkorpern, VDI-Verlag, Handbuch¨ reihe Bd. 5, ISBN 3-18-40 1599-8, Dusseldorf, 1996. [9] X. Liu, D. Du, G. Mourou, Laser ablation and micromachining with ultrashort laser pulses, IEEE J. Quantum Electron. 33 (1997) 1706. ¨ [10] B. Huttner, G.C. Rohr, On the theory of ps and sub-ps laser pulse interaction with metals; I. Surface temperature, Appl. Surf. Sci. 103 (1996) 269–274. ¨ [11] B. Huttner, G.C. Rohr, On the theory of ps and sub-ps laser pulse interaction with metals II. Spatial temperature distribution, Appl. Surf. Sci. 126 (1998) 129–135.