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Laser-driven high-resolution patterning of indium tin oxide thin film for electronic device
ARTICLE IN PRESS Optics and Lasers in Engineering 48 (2010) 816–820
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Laser-driven high-resolution patterning of indium tin oxide thin film for electronic device Hyunkwon Shin, Boyeon Sim, Myeongkyu Lee Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea
a r t i c l e in f o
a b s t r a c t
Article history: Received 2 January 2010 Received in revised form 16 February 2010 Accepted 16 February 2010 Available online 7 March 2010
We here introduce a laser-driven process to pattern transparent thin films on transparent substrates. This method utilizes a pre-patterned metal film as the dynamic release layer and the transparent thin film is selectively removed by a thermo-elastic force laser-induced in the underlying metal layer. Highfidelity indium tin oxide (ITO) thin film patterns were fabricated on plastic and glass substrates using a pulsed Nd:YAG laser. Tens of square centimeters could be patterned with several pulse shots. We fabricated a pentacene thin film transistor with ITO source and drain electrodes and observed a very low off-current level. This tells that the channel area between ITO electrodes was completely etched out by this laser-driven process. Combined with the absence of photoresist and chemical etching steps, this method provides a simple high-resolution route to pattern transparent thin films over large areas at low temperatures. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Patterning Indium tin oxide Laser
1. Introduction Transparent electronics [1–4] is an emerging technology for the next generation of a wide range of optoelectronic devices, where the key components are wide band-gap oxide semiconductors. Indium tin oixde (ITO) is an unusual material that is both electrically conductive and visually transparent [5,6]. Thus, ITO films are widely used as transparent electrodes in a variety of devices such as liquid crystal displays, touch screen panels, organic light emitting diodes, and solar cells. Simple ITO coatings in the unpatterned state are often used as passive components in many devices. High-resolution patterning is essential, however, if they are to be employed as the electrode or interconnect lines for transparent devices. The ITO pattern is typicaly fabricated by photolithography and wet etching with acidic solutions. Since a fully transparent device would need the use of oxide materials not only for electrodes but also for dielectric and channel layers, it is an important issue to secure sufficient etching selectivity among similar oxides. Moreover, ITO patterns should be fabricated on plastics for flexible devices, which reveal weak stability against chemical and thermal processes associated with the conventional lithographic technique. In this respect, there has been a fervant search for nonlithographic methods to fabricate ITO patterns on transparent substrates. Laser-direct patterning of ITO films has long been
Corresponding author.
E-mail address: [email protected] (M. Lee). 0143-8166/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2010.02.008
studied by different groups [7–14]. This process is based on the pulsed laser-induced photoablation. Since a very high energy is required to directly ablate a transparent film, the laser beam should be tightly focused and the patterning is carried out by scanning it. This serial or ‘‘spot by spot’’ fashion has a fundamental limitation in speedily fabricating high-resolution patterns of variant geometry and feature size. In addition, it is extremely difficult to obtain a clear pattern without damaging the underlying layer by this laser-direct photoablation. A more advanced method has been suggested, which is to ablate a photoresist layer and then use it as a deposition mask for ITO film [15]. The final ITO pattern is created by lift-off process. It consists of fewer steps than the conventional lithographic process because the development and etching of photoresist layer are not required. As polymers can be ablated with a lower laser energy than ITO, a potential damage to other constituent materials is less probable with this process. However, its scalability and applicability to actual devices have not been presented. We have recently shown that metal thin films (Al, Ag, and Au) evaporated on transparent substrates can be directly patterned by a spatially modulated pulsed Nd:YAG laser beam at 1064 nm incident from the backside of the substrate [16]. This method utilizes a pulsed laser-induced thermo-elastic force exerting on the film which plays a role to detach it from the substrate. The thermo-elastic force is proportional to the rate of temperature increase [17–20], not determined by the absolute magnitude of temperature rise. This makes parallel patterning possible with an ultrashort pulse even when the total pulse energy is not so high. Deposited metal films were polycrystalline with nano-sized
ARTICLE IN PRESS H. Shin et al. / Optics and Lasers in Engineering 48 (2010) 816–820
grains and thus localized etching of the material might be possible with shearing along the weakly bonded grain boundary regions. We also found that metal thin films can be directly photoetched by a laser beam irradiating the film surface [21]. Our approach is to utilize a thin metal pattern as the dynamic release layer to fabricate ITO pattern. This method uses the same types of optical modulation as in lithography, but does not require any photoresist or chemical etching steps. Therefore, the overall process steps are significantly reduced compared to the conventional lithography. Since this is a parallel process utilizing a spatially modulated laser beam, it can provide much higher resolution and throughput over the laser-direct photoablation. We here discuss the resolution and scalability of this laser-induced dynamic release process for patterning transparent ITO films, along with its feasibility for electronic devices.
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patterned with several pulse shots. Fig. 2(a) shows a photographic image of Al pattern of ‘‘The Birth of Venus’’ formed on polyethylene terephthalate (PET) substrate, where a black and white ‘‘The Birth of Venus’’ painting printed on a transparency sheet (polyester) by a desktop computer was used as the photomask. The ITO pattern fabricated using this Al pattern is given in Fig. 2(b). It was hardly seen to the naked eyes at a
2. Experimental procedure Fig. 1 shows the schematic of ITO patterning process. First, a metal thin film deposited on transparent substrate is directly patterned by a spatially modulated laser beam as described above and then an ITO film is deposited onto the patterned metal film. When a uniform laser beam is incident, a thermo-elastic force, caused by rapid thermal expansion resulting from the pulsed laser irradiation, will be exerted on the metal film to detach it from the substrate. If this force is strong enough, the ITO film located above the releasing metal layer can be removed together, leaving a patterned structure on the substrate. In this illumination step, the beam can be made incident either from the substrate side (back-side illumination) or the film side (front-side illumination) because both of the substrate and ITO are transparent and a very thin metal film (o 20 nm) is employed as the absorption layer. In this work, we used a pulsed Nd:YAG laser (wavelength¼1064 nm, pulse width¼6 ns, repetition rate¼10 Hz, maximum average power¼8.5 W) as the laser source. An output beam of 0.9 cm diameter was expanded by a beam expander ( 3) before spatial modulation and a single pulse was used in the patterning process. All metal films were thermally evaporated at room temperature. The thickness was monitored during deposition with a microbalance and calibrated using an optical profiler. 150-nm-thick ITO films were deposited by DC magnetron sputtering using a 10 wt% SnO2-doped In2O3 target. Deposition was performed without heating the substrate under a working pressure of 6.6 10 3 Torr and gas flow rates of 90 SCCM Ar and 0.1 SCCM O2. The sheet resistance, measured using a 4-probe, was 84 O/sq. Since the available maximum beam diameter was 2.7 cm, the sample was moved after the incidence of each pulse in order to pattern over the areas with larger lateral dimension than this value. Tens of square centimeters could be
Fig. 2. (a) Photographic image of an Al thin film pattern formed on PET substrate. (b) PET substrate containing an ITO pattern fabricated using the Al pattern given in ‘‘(a)’’ as the dynamic release layer. It was not easily seen to the naked eyes but was recognizable when viewed with a black background (inset).
Fig. 1. Schematic of the ITO patterning process.
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H. Shin et al. / Optics and Lasers in Engineering 48 (2010) 816–820
glimpse but was recognizable when viewed under a black backgound (inset of Fig. 2(b)).
3. Results and discussion The sequence of ITO patterning was well envisaged when a holographically patterned Al film was used as the absoprtion layer, as seen in Fig. 3. Due to an upward force from the embedded Al layer, the ITO film was initially cracked along the boundaries which would be the edges of the final pattern. With increasing pulse energy, some parts of the film began to be detached together with the underlying Al film. Further increase in the pulse energy completely removed the ITO film located above the Al layer, leaving only the areas directly touching the substrate. No edge peeling or cracking was observed in the patterned ITO film. This indicates that the ITO film has relatively weak cohesion compared to its adhesion to the substrate. For a highly cohesive film (e.g., single-crystalline film), it would be very difficult to fabricate a pattern without the edge peeling [16]. A pulse energy density of 35 mJ/cm2 was required to detach a 150-nm-thick ITO film together with a 10 nm Al layer. The threshold energy density for a single Al layer of 10 nm thickness was 30 mJ/cm2. The small difference indicates that the incident laser energy is mostly
used up to induce a thermo-elastic force in the metal film to detach it from the substrate, with a very small portion used to break the internal bonds of the ITO film. In the meanwhile, the overall quality of fabricated ITO patterns became poor when the Al layer was thinner than 5 nm. This is attributable to the fact that the substrate surface is not completely covered by the metal film at this thickness due to the island-like growth mode. Other metal films could also be used as the dynamic releasing layer. Fig. 4(a) shows the optical profiler images of patterned Au and ITO films. It can be seen that the fabricated ITO patterns have inversion images to the used Au patterns. Considering the mechanism of this dynamic release process, the fidelity and quality of the final pattern may be dependent on the feature size of a used metal layer. In the back-side illumination scheme using a photomask, the metal film is away from the mask by the substrate thickness even though the mask is made in direct contact with the substrate. This imposes a limitation on the minimum feature size to be patterned, because the film should be located within the near-field diffraction region from the mask in order to have a high-fidelity image transfer. Here, we used 188mm-thick PET substrates and could obtain metal and ITO patterns with feature sizes of 10 mm when the mask-used contact mode was adopted. Thinner substrates might enable finer structures, but there exists a limit in reducing the substrate thickness. In
Fig. 3. Scanning electron microscopy (SEM) images showing the sequence of ITO patterning. A 10-nm-thick Al film was patterned by three-beam interference and then, a 150-nm-thick ITO film was uniformly deposited over the patterned Al layer. The images were taken after the film was illuminated with a single uniform beam of slightly different pulse energy density. Scale bar is 10 mm.
Fig. 4. (a) Optical profiler images of patterned Au (top) and ITO (bottom) films and (b) ITO patterns created using holographically patterned Al layers. The height profile was measured along the marked line.
ARTICLE IN PRESS H. Shin et al. / Optics and Lasers in Engineering 48 (2010) 816–820
order to estimate how small features can be achievable with this method, 15-nm-thick Al films were patterned by two-beam and three-beam interferences. The interference profile was generated using a single refracting prism made of fused silica (refractive index ¼1.45 at 1064 nm). A prism of isosceles-triangle shape (apex angle¼ 1651) was used for two-beam interference and a prism of trigonal-pyramid shape, for three-beam interference. A laser beam was made incident onto the bottom surface of the prism and this made the simultaneous split and recombination of the beam possible with a single prism. After being deposited on these holographically fabricated Al patterns, 150-nm-thick ITO films were illuminated by a uniform laser beam. The optical profiler images of obtained ITO patterns are given in Fig. 4(b). Fairly sharp-edged trenches of a few mm size were obtained. ITO patterns could be fabricated on glass substrate as well as PET (Fig. 5). To ascertain the feasibility of this process for electronic devices, a pentacene thin film transistor (TFT) has been fabricated using patterned ITO source and drain electrodes. A heavily doped p-type Si wafer was used as the gate electrode and a thermally grown SiO2 layer (200 nm thick), as the gate dielectric. 150-nmthick ITO source and drain electrodes were formed on the SiO2/Si substrate. A 10-nm-thick Al film was pre-deposited and used as the releasing layer to pattern ITO electrodes. Since the substrate is opaque, it was directly photoetched by irradiating the film surface with a modulated laser beam and the ITO pattern was also created by the front-side illumination scheme. Finally, a 50-nm-thick pentacene semiconductor layer was deposited over the electrodes by thermal evaporation. The devices has a channel length of L¼57 mm and a channel width of W¼865 mm. Fig. 6 shows the measured TFT characteristics. The calculated mobility was 3.81 10 3 cm2V 1 s 1 and the current on/off ratio was higher than 104, with an off-current of 9.9 10 11 A. According to the literature [22–25], the characteristics of pentacene TFTs with ITO electrodes are much dependent on the TFT structure, dielectric layer, W/L ratio, and surface treatment. The reported values are in the ranges of 10 1–10 4 cm2V 1 s 1 (mobility), 102–105(on/off ratio), and 10 7–10 11 A (off-current level). The relatively low mobility is attributed to our source/drain structure which is bottom-contact type. A fairly high on/off ratio and a low offcurrent level observed in the fabricated device tell that the channel area between ITO electrodes was completely etched out by this laser-driven process. Incomplete removal would result in a current leakage and thus an additional chemical or physical etching step would be necessary. During ITO patterning, the substrate is also exposed to the laser beam. However, no damage
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Fig. 6. Characteristics of a pentacene-TFT with ITO source and drain electrodes patterned by the laser-driven process (channel length L ¼57 mm; channel width W¼ 865 mm). (a) Drain current (Id) vs. gate voltage (Vg) biased at drain voltage (Vd)¼ 40 V and (b) Id vs. Vd relation.
to the substrate was observed and the damage threshold of Si was found much higher than the used pulse energy density.
4. Conclusion
Fig. 5. ITO patterns formed on glass (left, 120 mm thick) and PET (right) substrates. Scale bars are 200 mm.
We here introduce a laser-driven process to pattern transparent thin films on transparent substrates. This method utilizes a pre-patterned metal film as the dynamic release layer and the transparent thin film is selectively removed by a thermo-elastic force laser-induced in the underlying metal layer. High-fidelity ITO thin film patterns were fabricated on PET and glass substrates over several square centimeters by a single Nd:YAG laser pulse. The feasibility of this process for electronic devices has been demonstrated with the fabrication of TFTs using patterned ITO electrodes. Combined with the absence of photoresist and chemical etching steps, the method presented here provides a simple high-resolution route to pattern transparent thin films for electronic devices.
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Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (grant number: 2009-0081142) and by Seoul R&BD Program (ST090854). References [1] Nomura J, Ohta H, Takagi A, Kamiya T, Hirano M, Hosono H. Roomtempearature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors. Nature 2004;432:488–92. [2] Nomura K, Ohta H, Ueda K, Kamiya T, Hirano M, Hosono H. Thin-film transistor fabricated in single-crystalline transparent oxide semiconductor. Science 2003;300:1269–72. [3] Masuda S, Kitamura K, Okumura Y, Miyatake S, Tabata H, Kawai T. Transparent thin film transistors using ZnO as an active channel layer and their electrical properties. J Appl Phys 2003;93:1624–30. [4] Hoffman R, Norris B, Wagner J. ZnO-based transparent thin film transistors. Appl Phys Lett 2003;82:733–5. [5] Holland L. Vacuum Deposition of thin films. London: Chapman and Hall; 1970. [6] Chopra K, Major S, Pandya D. Transparent conductors-A status review. Thin Solid Films 1983;102:1–46. [7] Yavas O, Takai M. High-speed maskless laser patterning of thin films for giant microelectronics. Jpn J Appl Phys 1999;38:7131–4. [8] Xu M, Li J, Lilge L, Herman P. F2-laser patterning of indium tin oxide (ITO) thin film on glass substrate. Appl Phys A 2006;85:7–10. [9] Park M, Chon B, Kim H, Jeong S, Kim D, Lee J, Chu H, Kim H. Ultrafast laser ablation of indium tin oxide thin films for organic light-emiting diode application. Opt Lasers in Eng 2006;44:138–46. [10] Chen M, Chen Y, Hsiao W, Gu Z. Laser direct write patterning technique of indium tin oxide film. Thin Solid Films 2007;515:8515–8. [11] Kim K, Kwon S, Mok H, Tak T. Laser direct patterning of indium tin oxide layer for plasma display panel bus electrode. Jpn J Appl Phys 2007;46: 4282–5.
[12] Choi H, Farson D, Bovatsek J, Arai A, Ashkenasi D. Direct-write patterning of indium-tin-oxide film by high pulse repetition frequency femtosecond laser ablation. Appl Opt 2007;46:5792–9. [13] Raciukaitis G, Brikas M, Gedvilas M, Rakickas T. Patterning of indium-tin oxide on glass with picosecond lasers. Appl Surf Sci 2007;253:6570–4. [14] Li Z, Cho E, Kwon S. A new laser direct etching method of indium tin oxide electrode for application to alternative current plasma display panel. Appl Surf Sci 2009;255:9843–6. [15] Chae J, Appasamy S, Jain K. Patterning of indium tin oxide by projection photoablation abd lift-off process for fabrication of flat-panel displays. Appl. Phys. Lett. 2007;90:261102–13. [16] Yoo H, Shin H, Sim B, Kim S, Lee M. Parallelized laser-direct patterning of nanocrystalline metal thin films by use of a pulsed laser-induced thermoelastic force. Nanotechnology 2009;20:245301–6. [17] Shin H, Kim H, Lee H, Yoo H, Kim J, Kim H, Lee M. Photoresist-free lithographic patterning of solution-processed nanostructured metal thin films. Adv Mater 2008;20:3457–61. [18] Arronte M, Neves P, Vilar R. Modeling of laser cleaning of metallic particulate contaminants from silicon surfaces. J Appl Phys 2002;92:6973–82. [19] Lee J, Curran C, Watkins K. Laser removal of copper particles from silicon wafers using UV, visible, and IR radiation. Appl Phys A 2001;73:219–24. [20] Vereecke G, Rohr E, Heyns M. Laser-assisted removal of particles from silicon wafers. J Appl Phys 1999;85:3837–43. [21] Lee H, Shin H, Jeong Y, Moon J, Lee M. Laser-direct photoetching of metal thin film for the electrode of transistor. Appl Phys Lett 2009;95. 071104-1-3. [22] Kang S, Noh M, Park D, Kim H, Kim S, Koo B, Kang I, Whang C. Geometric effect of channel on device performance in pentacene thin-film transistor. Jpn J Appl Phys 2004;43:7718–21. [23] Ichikawa M, You L, Tatemichi S, Koyama T, Taniguchi Y. Organic thin-film transistors with conductive metal-oxides as source-drain electrodes. Jpn J Appl Phys 2006;45:L1171–4. [24] Han C, Shin H, Park J, Han M, Pang H, Kim K, Chung I, Pyo S, Lee D, Kim Y. Organic thin film transistors with indium tin oxide bottom electrode. Phys. Scr. T 2006;126:41–4. [25] Lee H, Cho J, Kim W, Lee J, Cho K. Effects of physical treatment of ITO electrodes on the electrical properties of pentacene thin-film transistors. Electrochem Sol State Lett 2007;10:H239–42.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Laser-driven high-resolution patterning of indium tin oxide thin film for electronic device Hyunkwon Shin, Boyeon Sim, Myeongkyu Lee Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea
a r t i c l e in f o
a b s t r a c t
Article history: Received 2 January 2010 Received in revised form 16 February 2010 Accepted 16 February 2010 Available online 7 March 2010
We here introduce a laser-driven process to pattern transparent thin films on transparent substrates. This method utilizes a pre-patterned metal film as the dynamic release layer and the transparent thin film is selectively removed by a thermo-elastic force laser-induced in the underlying metal layer. Highfidelity indium tin oxide (ITO) thin film patterns were fabricated on plastic and glass substrates using a pulsed Nd:YAG laser. Tens of square centimeters could be patterned with several pulse shots. We fabricated a pentacene thin film transistor with ITO source and drain electrodes and observed a very low off-current level. This tells that the channel area between ITO electrodes was completely etched out by this laser-driven process. Combined with the absence of photoresist and chemical etching steps, this method provides a simple high-resolution route to pattern transparent thin films over large areas at low temperatures. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Patterning Indium tin oxide Laser
1. Introduction Transparent electronics [1–4] is an emerging technology for the next generation of a wide range of optoelectronic devices, where the key components are wide band-gap oxide semiconductors. Indium tin oixde (ITO) is an unusual material that is both electrically conductive and visually transparent [5,6]. Thus, ITO films are widely used as transparent electrodes in a variety of devices such as liquid crystal displays, touch screen panels, organic light emitting diodes, and solar cells. Simple ITO coatings in the unpatterned state are often used as passive components in many devices. High-resolution patterning is essential, however, if they are to be employed as the electrode or interconnect lines for transparent devices. The ITO pattern is typicaly fabricated by photolithography and wet etching with acidic solutions. Since a fully transparent device would need the use of oxide materials not only for electrodes but also for dielectric and channel layers, it is an important issue to secure sufficient etching selectivity among similar oxides. Moreover, ITO patterns should be fabricated on plastics for flexible devices, which reveal weak stability against chemical and thermal processes associated with the conventional lithographic technique. In this respect, there has been a fervant search for nonlithographic methods to fabricate ITO patterns on transparent substrates. Laser-direct patterning of ITO films has long been
Corresponding author.
E-mail address: [email protected] (M. Lee). 0143-8166/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2010.02.008
studied by different groups [7–14]. This process is based on the pulsed laser-induced photoablation. Since a very high energy is required to directly ablate a transparent film, the laser beam should be tightly focused and the patterning is carried out by scanning it. This serial or ‘‘spot by spot’’ fashion has a fundamental limitation in speedily fabricating high-resolution patterns of variant geometry and feature size. In addition, it is extremely difficult to obtain a clear pattern without damaging the underlying layer by this laser-direct photoablation. A more advanced method has been suggested, which is to ablate a photoresist layer and then use it as a deposition mask for ITO film [15]. The final ITO pattern is created by lift-off process. It consists of fewer steps than the conventional lithographic process because the development and etching of photoresist layer are not required. As polymers can be ablated with a lower laser energy than ITO, a potential damage to other constituent materials is less probable with this process. However, its scalability and applicability to actual devices have not been presented. We have recently shown that metal thin films (Al, Ag, and Au) evaporated on transparent substrates can be directly patterned by a spatially modulated pulsed Nd:YAG laser beam at 1064 nm incident from the backside of the substrate [16]. This method utilizes a pulsed laser-induced thermo-elastic force exerting on the film which plays a role to detach it from the substrate. The thermo-elastic force is proportional to the rate of temperature increase [17–20], not determined by the absolute magnitude of temperature rise. This makes parallel patterning possible with an ultrashort pulse even when the total pulse energy is not so high. Deposited metal films were polycrystalline with nano-sized
ARTICLE IN PRESS H. Shin et al. / Optics and Lasers in Engineering 48 (2010) 816–820
grains and thus localized etching of the material might be possible with shearing along the weakly bonded grain boundary regions. We also found that metal thin films can be directly photoetched by a laser beam irradiating the film surface [21]. Our approach is to utilize a thin metal pattern as the dynamic release layer to fabricate ITO pattern. This method uses the same types of optical modulation as in lithography, but does not require any photoresist or chemical etching steps. Therefore, the overall process steps are significantly reduced compared to the conventional lithography. Since this is a parallel process utilizing a spatially modulated laser beam, it can provide much higher resolution and throughput over the laser-direct photoablation. We here discuss the resolution and scalability of this laser-induced dynamic release process for patterning transparent ITO films, along with its feasibility for electronic devices.
817
patterned with several pulse shots. Fig. 2(a) shows a photographic image of Al pattern of ‘‘The Birth of Venus’’ formed on polyethylene terephthalate (PET) substrate, where a black and white ‘‘The Birth of Venus’’ painting printed on a transparency sheet (polyester) by a desktop computer was used as the photomask. The ITO pattern fabricated using this Al pattern is given in Fig. 2(b). It was hardly seen to the naked eyes at a
2. Experimental procedure Fig. 1 shows the schematic of ITO patterning process. First, a metal thin film deposited on transparent substrate is directly patterned by a spatially modulated laser beam as described above and then an ITO film is deposited onto the patterned metal film. When a uniform laser beam is incident, a thermo-elastic force, caused by rapid thermal expansion resulting from the pulsed laser irradiation, will be exerted on the metal film to detach it from the substrate. If this force is strong enough, the ITO film located above the releasing metal layer can be removed together, leaving a patterned structure on the substrate. In this illumination step, the beam can be made incident either from the substrate side (back-side illumination) or the film side (front-side illumination) because both of the substrate and ITO are transparent and a very thin metal film (o 20 nm) is employed as the absorption layer. In this work, we used a pulsed Nd:YAG laser (wavelength¼1064 nm, pulse width¼6 ns, repetition rate¼10 Hz, maximum average power¼8.5 W) as the laser source. An output beam of 0.9 cm diameter was expanded by a beam expander ( 3) before spatial modulation and a single pulse was used in the patterning process. All metal films were thermally evaporated at room temperature. The thickness was monitored during deposition with a microbalance and calibrated using an optical profiler. 150-nm-thick ITO films were deposited by DC magnetron sputtering using a 10 wt% SnO2-doped In2O3 target. Deposition was performed without heating the substrate under a working pressure of 6.6 10 3 Torr and gas flow rates of 90 SCCM Ar and 0.1 SCCM O2. The sheet resistance, measured using a 4-probe, was 84 O/sq. Since the available maximum beam diameter was 2.7 cm, the sample was moved after the incidence of each pulse in order to pattern over the areas with larger lateral dimension than this value. Tens of square centimeters could be
Fig. 2. (a) Photographic image of an Al thin film pattern formed on PET substrate. (b) PET substrate containing an ITO pattern fabricated using the Al pattern given in ‘‘(a)’’ as the dynamic release layer. It was not easily seen to the naked eyes but was recognizable when viewed with a black background (inset).
Fig. 1. Schematic of the ITO patterning process.
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glimpse but was recognizable when viewed under a black backgound (inset of Fig. 2(b)).
3. Results and discussion The sequence of ITO patterning was well envisaged when a holographically patterned Al film was used as the absoprtion layer, as seen in Fig. 3. Due to an upward force from the embedded Al layer, the ITO film was initially cracked along the boundaries which would be the edges of the final pattern. With increasing pulse energy, some parts of the film began to be detached together with the underlying Al film. Further increase in the pulse energy completely removed the ITO film located above the Al layer, leaving only the areas directly touching the substrate. No edge peeling or cracking was observed in the patterned ITO film. This indicates that the ITO film has relatively weak cohesion compared to its adhesion to the substrate. For a highly cohesive film (e.g., single-crystalline film), it would be very difficult to fabricate a pattern without the edge peeling [16]. A pulse energy density of 35 mJ/cm2 was required to detach a 150-nm-thick ITO film together with a 10 nm Al layer. The threshold energy density for a single Al layer of 10 nm thickness was 30 mJ/cm2. The small difference indicates that the incident laser energy is mostly
used up to induce a thermo-elastic force in the metal film to detach it from the substrate, with a very small portion used to break the internal bonds of the ITO film. In the meanwhile, the overall quality of fabricated ITO patterns became poor when the Al layer was thinner than 5 nm. This is attributable to the fact that the substrate surface is not completely covered by the metal film at this thickness due to the island-like growth mode. Other metal films could also be used as the dynamic releasing layer. Fig. 4(a) shows the optical profiler images of patterned Au and ITO films. It can be seen that the fabricated ITO patterns have inversion images to the used Au patterns. Considering the mechanism of this dynamic release process, the fidelity and quality of the final pattern may be dependent on the feature size of a used metal layer. In the back-side illumination scheme using a photomask, the metal film is away from the mask by the substrate thickness even though the mask is made in direct contact with the substrate. This imposes a limitation on the minimum feature size to be patterned, because the film should be located within the near-field diffraction region from the mask in order to have a high-fidelity image transfer. Here, we used 188mm-thick PET substrates and could obtain metal and ITO patterns with feature sizes of 10 mm when the mask-used contact mode was adopted. Thinner substrates might enable finer structures, but there exists a limit in reducing the substrate thickness. In
Fig. 3. Scanning electron microscopy (SEM) images showing the sequence of ITO patterning. A 10-nm-thick Al film was patterned by three-beam interference and then, a 150-nm-thick ITO film was uniformly deposited over the patterned Al layer. The images were taken after the film was illuminated with a single uniform beam of slightly different pulse energy density. Scale bar is 10 mm.
Fig. 4. (a) Optical profiler images of patterned Au (top) and ITO (bottom) films and (b) ITO patterns created using holographically patterned Al layers. The height profile was measured along the marked line.
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order to estimate how small features can be achievable with this method, 15-nm-thick Al films were patterned by two-beam and three-beam interferences. The interference profile was generated using a single refracting prism made of fused silica (refractive index ¼1.45 at 1064 nm). A prism of isosceles-triangle shape (apex angle¼ 1651) was used for two-beam interference and a prism of trigonal-pyramid shape, for three-beam interference. A laser beam was made incident onto the bottom surface of the prism and this made the simultaneous split and recombination of the beam possible with a single prism. After being deposited on these holographically fabricated Al patterns, 150-nm-thick ITO films were illuminated by a uniform laser beam. The optical profiler images of obtained ITO patterns are given in Fig. 4(b). Fairly sharp-edged trenches of a few mm size were obtained. ITO patterns could be fabricated on glass substrate as well as PET (Fig. 5). To ascertain the feasibility of this process for electronic devices, a pentacene thin film transistor (TFT) has been fabricated using patterned ITO source and drain electrodes. A heavily doped p-type Si wafer was used as the gate electrode and a thermally grown SiO2 layer (200 nm thick), as the gate dielectric. 150-nmthick ITO source and drain electrodes were formed on the SiO2/Si substrate. A 10-nm-thick Al film was pre-deposited and used as the releasing layer to pattern ITO electrodes. Since the substrate is opaque, it was directly photoetched by irradiating the film surface with a modulated laser beam and the ITO pattern was also created by the front-side illumination scheme. Finally, a 50-nm-thick pentacene semiconductor layer was deposited over the electrodes by thermal evaporation. The devices has a channel length of L¼57 mm and a channel width of W¼865 mm. Fig. 6 shows the measured TFT characteristics. The calculated mobility was 3.81 10 3 cm2V 1 s 1 and the current on/off ratio was higher than 104, with an off-current of 9.9 10 11 A. According to the literature [22–25], the characteristics of pentacene TFTs with ITO electrodes are much dependent on the TFT structure, dielectric layer, W/L ratio, and surface treatment. The reported values are in the ranges of 10 1–10 4 cm2V 1 s 1 (mobility), 102–105(on/off ratio), and 10 7–10 11 A (off-current level). The relatively low mobility is attributed to our source/drain structure which is bottom-contact type. A fairly high on/off ratio and a low offcurrent level observed in the fabricated device tell that the channel area between ITO electrodes was completely etched out by this laser-driven process. Incomplete removal would result in a current leakage and thus an additional chemical or physical etching step would be necessary. During ITO patterning, the substrate is also exposed to the laser beam. However, no damage
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Fig. 6. Characteristics of a pentacene-TFT with ITO source and drain electrodes patterned by the laser-driven process (channel length L ¼57 mm; channel width W¼ 865 mm). (a) Drain current (Id) vs. gate voltage (Vg) biased at drain voltage (Vd)¼ 40 V and (b) Id vs. Vd relation.
to the substrate was observed and the damage threshold of Si was found much higher than the used pulse energy density.
4. Conclusion
Fig. 5. ITO patterns formed on glass (left, 120 mm thick) and PET (right) substrates. Scale bars are 200 mm.
We here introduce a laser-driven process to pattern transparent thin films on transparent substrates. This method utilizes a pre-patterned metal film as the dynamic release layer and the transparent thin film is selectively removed by a thermo-elastic force laser-induced in the underlying metal layer. High-fidelity ITO thin film patterns were fabricated on PET and glass substrates over several square centimeters by a single Nd:YAG laser pulse. The feasibility of this process for electronic devices has been demonstrated with the fabrication of TFTs using patterned ITO electrodes. Combined with the absence of photoresist and chemical etching steps, the method presented here provides a simple high-resolution route to pattern transparent thin films for electronic devices.
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