- Email: [email protected]
Characterization of organic material micro-structures transferred by laser in nanosecond and picosecond regimes
Applied Surface Science 255 (2009) 5439–5443
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Characterization of organic material micro-structures transferred by laser in nanosecond and picosecond regimes Ludovic Rapp *, Christophe Cibert, Anne Patricia Alloncle, Philippe Delaporte Laboratoire Lasers, Plasmas et Proce´de´s Photoniques (LP3), UMR 6182 CNRS – Universite´ de la Me´diterrane´e, C. 917, 13288 Marseille Cedex 9, France
A R T I C L E I N F O
A B S T R A C T
Article history:
The laser-induced forward transfer technique has been performed on thin layers of conducting organic materials for applications in plastic micro-electronics. This process is a promising alternative for fabrication of organic electronic components on flexible supports when usual techniques, such like ink-jet printing, cannot be considered. For example, when the organic material has no solubility properties or when complex architectures are needed. Experiments on the influence of pulse duration (nanosecond and picosecond) and wavelength on a large range of fluences have been proceeded using different lasers. An optimization of the process has been carried out by inserting a thin layer of absorbing metallic material between the substrate and the organic film. The advantage of this technique is to preserve organic layers from being damaged by thermal and photochemical effects during the interaction. The morphology and thickness of the deposit have been investigated by optical and scanning electronic microscopy. This experimental study is supplemented by electrical characterization of the deposits. ß 2008 Elsevier B.V. All rights reserved.
Available online 3 August 2008 Keywords: Laser-induced forward transfer Polymer Nanosecond Picosecond
1. Introduction The laser-induced forward transfer (LIFT) technique consists in removing a small piece of a thin layer previously deposited on a transparent substrate and transferring it on another substrate using a pulsed laser (Fig. 1). This simple, single step, direct printing technique offers the ability to make surface micro-patterning or localized deposition of material. Various materials, such as metals [1,2], semiconductors [3], superconductors [4,5] and biological materials [6–9], have been successfully transferred using this process. New conductive polymers used in plastic micro-electronics are regularly synthesized, but numbers of them do not have the solubility properties which would allow their deposition using inkjet techniques. LIFT process gives the possibility to produce electronic components with non-soluble organic materials. In this work, our objective is to study the LIFT process of a conducting organic polymer, the poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS). Its electrical properties and its high stability in time make it a good candidate for the fabrication of the source and drain of organic thin film transistor
* Corresponding author. Tel.: +33 4 91 82 95 13; fax: +33 4 91 82 92 89. E-mail address: [email protected] (L. Rapp). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.165
(OTFT) [10]. The transferred PEDOT:PSS structures must be spatially well defined in order to get a channel length as short as possible and the homogeneity and electrical properties of the material must be kept. 2. Experimental setup LIFT experiments have been carried out using different lasers allowing different conditions of pulse duration and wavelength (Table 1). The setup (Fig. 2) is almost the same for the different lasers. A mechanical shutter placed on the beam axis allows selecting one pulse. The fluence is controlled with calibrated attenuating plates or a polarization device. A mask is used to select a homogeneous part of the beam, which is focused on the thin layer using a converging lens. A receptor substrate (silicon or quartz suprasil) is placed in close contact to the donor, perpendicularly to the beam. The precise positioning of the sample is obtained by micrometric translation (x, y, and z) devices and controlled by imaging the spot with a CCD camera. The transfer is achieved using a single pulse and under atmospheric conditions. During LIFT process, the ablation phase occurs inside the layer to be transferred. The ablation phase is a violent process that typically results in the total or partial disintegration of the donor film during the transfer. Evidently there is an inherent disadvan-
L. Rapp et al. / Applied Surface Science 255 (2009) 5439–5443
5440 Table 1 Lasers used for LIFT experiments Laser
Wavelength (nm)
Pulse duration and energy
Irradiated area dimensions
KrF (EMG 203 MSC Lambda Physics) Nd:YAG (Leopard S10/20 Continuum)
248 355 (3rd harmonic)
35 ns, 1–100 Hz, 0.5 J 50 ps, 10 Hz, 0.036 J
220 mm 340 mm 520 mm 520 mm
high vacuum (BocEdwards Auto306 Vacuum Coating System) on quartz suprasil substrates with various thicknesses (10, 25 and 50 nm). Thin films of PEDOT:PSS have been spin-coated on transparent substrates of quartz suprasil with and without DRL. The samples have then been annealed at 80 8C during 20 min to evaporate the solvent. The PEDOT:PSS thickness is quite homogeneous and varies from 300 nm to 320 nm. Samples have been characterized by optical microscopy using a Zeiss Axiotech with a Nomarski objective and by scanning electronic microscopy (SEM) (JEOL 6320F and JSM-6390). Electrical properties have been characterized using I(y) measurements with a four-point probe Su¨ss Microtec PM5 system. Fig. 1. Principle of LIFT.
3. Results and discussion 3.1. Influence of the pulse duration The material transfer using nano- and sub-nanosecond laser pulses is first studied without absorbent layer in order to determine the optimal conditions of irradiation. The fluence is gradually varied from insufficient conditions to break the donor film to an excessive irradiation. A comparative panel of the effect of fluence on the deposit and the donor layer as a function of pulse duration is shown in Fig. 3.
Fig. 2. Experimental setup.
tage of this method in that the donor film acts as its own propellant and significant damages result from its ablation. Previous study has shown that it is preferable to use a wavelength which is absorbed in the first nanometers of the layer in order to confine theses damages [11]. To solve this problem we insert a sacrificial layer of gold or silver, also called dynamic release layer (DRL). The advantage of this technique is to preserve the polymer from being damaged by thermal and photochemical effects during the interaction. However, it can bring complications in contaminating the deposited material with debris resulting from its ablation. Gold and silver layers have been deposited by thermal evaporation in
3.1.1. Nanosecond regime LIFT of PEDOT:PSS films has been first studied in nanosecond regime (Fig. 3A). To obtain a good deposit (uniform color with our optical system), it is necessary to irradiate the film with fluences higher than 0.09 J/cm2 (Fig. 3A(a)). The shape of the deposit is homogeneous and begins to be reproducible. When fluence increases, thermal effects, characterized by the ejection of micro-droplets, are more and more visible all around the deposit. The amount of ejected particles increases and the deposit is fragmented (appearing of light areas in Fig. 3A(g)). The irradiated area brightness, on the donor substrate, increases with the fluence, meaning that organic material remains, at least till 0.17 J/cm2. 3.1.2. Picosecond regime LIFT results with the 50 ps Nd:YAG laser pulses at 355 nm are presented in Fig. 3B. At this wavelength, the film of PEDOT:PSS is
Fig. 3. Optical microscopy images of the donor (up) and the receiver (down) substrate after transfer. The donor film was made of 300 nm of PEDOT:PSS. Fluence is varied horizontally. (A) PEDOT:PSS irradiated with 35 ns pulse duration. (B) PEDOT:PSS irradiated with 50 ps pulse duration.
L. Rapp et al. / Applied Surface Science 255 (2009) 5439–5443
partially transparent to this incident radiation and it is estimated that only 35% of the incident energy is trapped in the layer. From 0.18 J/cm2, a partial transfer is obtain although it is not homogeneous but formed with ejected fragments (Fig. 3B(a)). The best transfer is achieved at 0.46 J/cm2. Ejected particles are visible all around the deposit at 0.66 J/cm2 (Fig. 3B(g)). The total ablation of the irradiated surface is carried out at a fluence threshold of 0.74 J/cm2. Over 0.74 J/cm2, the surface of the deposit starts to be damaged and the shape is partially destroyed. Some big holes appear in the deposit (Fig. 3B(d)). 3.1.3. Analyze of the deposits SEM visualizations of the transferred PEDOT:PSS surfaces are shown in Fig. 4. The surface structure is very different for the two pulse durations. In nanosecond regime, micrometric holes are visible over the entire surface inducing a high inhomogeneity and a high roughness (Fig. 4A). The absorption of the radiation seems to occur on different hot points, creating some ‘‘micro-bubbles’’ or ‘‘micro-explosions’’ all over the surface (Fig. 4A(b)). In picosecond regime, the structure is much more homogenous (Fig. 4B). The difference in surface structuring is due to the difference in both pulse duration and absorption properties. In nanosecond regime, thermal effects are very important even under UV irradiation. The polymer is very absorbing to the incident radiation and the energy is trapped in a small volume limited by the optical penetration of the beam inside the polymer. During the ablation phase, the polymer is partially vaporized and the ablation products (gas, solid or liquid particles; Fig. 4A(c)) are expelled from the removed layer and give this kind of surface morphology. In picosecond regime, a larger volume is affected by the direct irradiation but thermal effects are limited because of the short pulse duration (Fig. 4B(b)). The edge of the deposit is well defined in a very short range of fluences [11], but in both regimes a lot of debris is visible around the spot.
5441
3.2. Influence of the metallic layer To avoid direct laser damaging, an additional dynamic release layer (DRL) is introduced. Gold and silver layers have been used because of their mechanical properties and their short absorption length (labs) for UV radiation (gold: labs = 13 nm at 248 nm and 15 nm at 355 nm; silver: labs = 15 nm at 248 nm and 51 nm at 355 nm) [12]. We have first analyzed the transfer with a metal thickness of 50 nm in both cases. In picosecond regime, homogeneous deposits with well-defined edges are obtained between 0.18 J/cm2 and 0.60 J/cm2 using gold DRL. Huge metallic debris can appear all around the deposit but they are very easy to remove with a soft air blow. Over 0.40 J/cm2, some thermal effects are visible on the deposit surface. For fluences higher than 0.90 J/cm2, the transferred material is deteriorated. The donor layer is ablated on the irradiated area from 0.18 J/cm2 to 0.30 J/cm2. At higher fluences, the metallic layer is destroyed on a surface widely higher than the spot size under high mechanical stress effect. Close characteristics are observed with a 25 nm layer. For 50 nm of silver, very good deposits are obtained between 0.28 J/cm2 and 0.46 J/cm2. But in this case the thickness of the layer has almost the same value than the absorption length of the beam for this radiation. The energy is transferred inside the metal in a larger volume and, as a consequence, the mechanical stress inducing the ejection of the film is lower. In reducing or increasing the thickness of the layer, no good deposits are obtained. They are inhomogeneous and have no square shape. In nanosecond regime, there is no ablation with 50 nm silver under 0.09 J/cm2. Good deposits are transferred from 0.16 J/cm2 to 0.40 J/cm2 (Fig. 5A(a)). For gold layers the best transfer is obtained when the thickness of the film is twice the absorption length (Fig. 5B) of the material (in this case 25 nm). We have obtained very good results of deposition in a large range of fluences for the two regimes of pulse duration, pointing
Fig. 4. SEM visualizations of PEDOT:PSS surface of the deposit transferred on silicon. (A series) PEDOT:PSS irradiated by an UV radiation (248 nm) with 35 ns pulse duration. (B series) PEDOT:PSS irradiated by an UV radiation (355 nm) with 50 ps pulse duration. (a) Corresponds to surface sample, (b) is a close up of the surface sample, (c) is the edge of the deposit.
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L. Rapp et al. / Applied Surface Science 255 (2009) 5439–5443
Fig. 5. SEM visualizations of PEDOT:PSS deposits (300 nm) using different DRL layers. Nanosecond regime: (A) 50 nm of silver at 0.12 J/cm2. (B) 25 nm of gold at 0.15 J/cm2. Picosecond regime with EDAX analyses: (C) 25 nm of gold at 0.28 J/cm2. (D) 10 nm of gold at 0.65 J/cm2.
out the benefits of using a thin sacrificial release layer. But, SEM visualizations (Fig. 5) revealed a contamination of the deposited material with residual metallic debris. SEM and X-ray dispersive energy analyses (EDAX) show that the contamination is localized in micro- and nanoparticles of gold (Fig. 5B(b) and C(b)) and silver (Fig. 5A(b)) on the surface of the transferred polymer. By reducing the thickness of the metallic layer to 10 nm, we have reduced the range of usable fluences to get a good transfer for both metals. For gold, ejected particles are still present all around the spot and fragments on the top of the deposit are clearly visible (Fig. 5D(a)). EDAX have shown that metallic elements are only present in these fragments, indicating a mixing of PEDOT:PSS and metal (Fig. 5D(b)). 3.3. Electrical properties The electrical properties of the transferred PEDOT:PSS have been characterized. Lines of PEDOT:PSS have been made by juxtaposing deposits (different overlapping values have been tested). The transfer has been realized with samples without DRL layer in picosecond regime at 0.40 J/cm2. Then gold electrodes separated by 250 mm distance have been evaporated onto these lines using a mask. I(y) measurement have been performed at different distances on the line. The different measurements for a same distance show a good reproducibility of the characteristics but point out also the difficulty to keep the electrical contact between two juxtaposed spots. The results for different distances (from 250 mm to 1060 mm) are presented in Fig. 6. The value of the resistance increases with the distance of measurement. From these measurements, an average resistivity of the deposit is estimated to be 5 10 3 V cm. The value of the conductivity associated is 200 S cm 1. This is an interesting resistivity value in comparison with the constructor value 0.01– 0.02 V cm and printing PEDOT:PSS value 13 10 3 V cm [10].
Fig. 6. I(y) measurement of PEDOT:PSS deposits.
4. Conclusion We have reported in this paper LIFT experiments on conducting PEDOT:PSS layers under different conditions of irradiation and laser pulse duration. Polymer layers can be transferred with LIFT technique to obtain micro-structures with a high spatial resolution. The deposits keep a good cohesion during the transfer. We have shown the benefit of using a sacrificial metallic layer and the importance of its thickness. It has been shown that the thickness of the layer is optimized as a function of the absorption properties of the material. The homogeneity and the spatial resolution are really improved but there is a risk of contamination. This contamination
L. Rapp et al. / Applied Surface Science 255 (2009) 5439–5443
is localized on the top of the transferred polymer. That is why using an insulator with absorbing properties could be a good idea in order to optimize the process [13]. This work will be completed by time resolved shadowgraphic imaging of the ejection of PEDOT:PSS with and without DRL in order to obtain a better comprehension of this mechanism [14]. Moreover, experiments using 266 nm picosecond radiation can be performed with no DRL on PEDOT:PSS to take advantage of its high absorption at this wavelength. The transferred film has interesting electrical properties, showing that conducting polymer can be transferred by LIFT process. Therefore, no limitation is expected in miniaturization, which is a necessary requirement.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Acknowledgments The authors would like to acknowledge scientists from the CINaM (Marseille) and the CMP- GC (Gardanne) for their collaboration within the MICROPOLY framework (CIMPACA project) and the ANR program e-PLAST.
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[11] [12] [13] [14]
J. Bohandy, B.F. Kim, F.J. Adrian, A.N. Jette, J. Appl. Phys. 63 (4) (1988) 1158. H. Yamada, T. Sano, T. Nakayama, I. Miyamoto, Appl. Surf. Sci. 197–198 (2002) 411. Z. Toth, T. Szo¨renyi, Appl. Phys. A 52 (1991) 273. E. Fogarassy, C. Fuchs, F. Kerherve, G. Hauchecorne, J. Perriere, J. Appl. Phys. 66 (1) (1989) 457. G. Koundourakis, C. Rockstuhl, D. Papazoglou, A. Klini, I. Zergioti, N.A. Vainos, C. Fotakis, Appl. Phys. Lett. 78 (7) (2001) 868. D.B. Chrisey, A. Pique´, R.A. McGill, J.S. Horwitz, B.R. Ringeisen, D.M. Bubb, P.K. Wu, Chem. Rev. 103 (2003) 553. B. Hopp, T. Smausz, Z. Antal, N. Kresz, Z. Bor, D. Chrisey, J. Appl. Phys. 96 (6) (2004) 3478. P. Serra, M. Colina, J.M. Ferna´ndez-Pradas, L. Sevilla, J.L. Morenza, Appl. Phys. Lett. 85 (9) (2004) 1639. M. Colina, M. Duocastella, J.M. Fernandez-Pradas, P. Serra, J.L. Morenza, J. Appl. Phys. 99 (2006) 084909. M. Barret, Impression par jet de matie`re de transistors organiques sur support souple, The`se de 3e`me cycle soutenue le 12.10.07. B. Thomas, A.P. Alloncle, P. Delaporte, M. Sentis, S. Sanaur, M. Barret, P. Collot, Appl. Surf. Sci. 254 (2007) 1206. Michael Bass (Ed.), Handbook of Opticssecond edition, 1995. R. Fardel, M. Nagel, F. Nu¨esch, T. Lippert, A. Wokaun, Appl. Surf. Sci. 254 (2007) 1322. I. Zergioti, A. Karaiskou, D.G. Papazoglou, C. Fotakis, M. Kapsetaki, D. Kafetzopoulos, Appl. Surf. Sci. 247 (2005) 584.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Characterization of organic material micro-structures transferred by laser in nanosecond and picosecond regimes Ludovic Rapp *, Christophe Cibert, Anne Patricia Alloncle, Philippe Delaporte Laboratoire Lasers, Plasmas et Proce´de´s Photoniques (LP3), UMR 6182 CNRS – Universite´ de la Me´diterrane´e, C. 917, 13288 Marseille Cedex 9, France
A R T I C L E I N F O
A B S T R A C T
Article history:
The laser-induced forward transfer technique has been performed on thin layers of conducting organic materials for applications in plastic micro-electronics. This process is a promising alternative for fabrication of organic electronic components on flexible supports when usual techniques, such like ink-jet printing, cannot be considered. For example, when the organic material has no solubility properties or when complex architectures are needed. Experiments on the influence of pulse duration (nanosecond and picosecond) and wavelength on a large range of fluences have been proceeded using different lasers. An optimization of the process has been carried out by inserting a thin layer of absorbing metallic material between the substrate and the organic film. The advantage of this technique is to preserve organic layers from being damaged by thermal and photochemical effects during the interaction. The morphology and thickness of the deposit have been investigated by optical and scanning electronic microscopy. This experimental study is supplemented by electrical characterization of the deposits. ß 2008 Elsevier B.V. All rights reserved.
Available online 3 August 2008 Keywords: Laser-induced forward transfer Polymer Nanosecond Picosecond
1. Introduction The laser-induced forward transfer (LIFT) technique consists in removing a small piece of a thin layer previously deposited on a transparent substrate and transferring it on another substrate using a pulsed laser (Fig. 1). This simple, single step, direct printing technique offers the ability to make surface micro-patterning or localized deposition of material. Various materials, such as metals [1,2], semiconductors [3], superconductors [4,5] and biological materials [6–9], have been successfully transferred using this process. New conductive polymers used in plastic micro-electronics are regularly synthesized, but numbers of them do not have the solubility properties which would allow their deposition using inkjet techniques. LIFT process gives the possibility to produce electronic components with non-soluble organic materials. In this work, our objective is to study the LIFT process of a conducting organic polymer, the poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS). Its electrical properties and its high stability in time make it a good candidate for the fabrication of the source and drain of organic thin film transistor
* Corresponding author. Tel.: +33 4 91 82 95 13; fax: +33 4 91 82 92 89. E-mail address: [email protected] (L. Rapp). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.165
(OTFT) [10]. The transferred PEDOT:PSS structures must be spatially well defined in order to get a channel length as short as possible and the homogeneity and electrical properties of the material must be kept. 2. Experimental setup LIFT experiments have been carried out using different lasers allowing different conditions of pulse duration and wavelength (Table 1). The setup (Fig. 2) is almost the same for the different lasers. A mechanical shutter placed on the beam axis allows selecting one pulse. The fluence is controlled with calibrated attenuating plates or a polarization device. A mask is used to select a homogeneous part of the beam, which is focused on the thin layer using a converging lens. A receptor substrate (silicon or quartz suprasil) is placed in close contact to the donor, perpendicularly to the beam. The precise positioning of the sample is obtained by micrometric translation (x, y, and z) devices and controlled by imaging the spot with a CCD camera. The transfer is achieved using a single pulse and under atmospheric conditions. During LIFT process, the ablation phase occurs inside the layer to be transferred. The ablation phase is a violent process that typically results in the total or partial disintegration of the donor film during the transfer. Evidently there is an inherent disadvan-
L. Rapp et al. / Applied Surface Science 255 (2009) 5439–5443
5440 Table 1 Lasers used for LIFT experiments Laser
Wavelength (nm)
Pulse duration and energy
Irradiated area dimensions
KrF (EMG 203 MSC Lambda Physics) Nd:YAG (Leopard S10/20 Continuum)
248 355 (3rd harmonic)
35 ns, 1–100 Hz, 0.5 J 50 ps, 10 Hz, 0.036 J
220 mm 340 mm 520 mm 520 mm
high vacuum (BocEdwards Auto306 Vacuum Coating System) on quartz suprasil substrates with various thicknesses (10, 25 and 50 nm). Thin films of PEDOT:PSS have been spin-coated on transparent substrates of quartz suprasil with and without DRL. The samples have then been annealed at 80 8C during 20 min to evaporate the solvent. The PEDOT:PSS thickness is quite homogeneous and varies from 300 nm to 320 nm. Samples have been characterized by optical microscopy using a Zeiss Axiotech with a Nomarski objective and by scanning electronic microscopy (SEM) (JEOL 6320F and JSM-6390). Electrical properties have been characterized using I(y) measurements with a four-point probe Su¨ss Microtec PM5 system. Fig. 1. Principle of LIFT.
3. Results and discussion 3.1. Influence of the pulse duration The material transfer using nano- and sub-nanosecond laser pulses is first studied without absorbent layer in order to determine the optimal conditions of irradiation. The fluence is gradually varied from insufficient conditions to break the donor film to an excessive irradiation. A comparative panel of the effect of fluence on the deposit and the donor layer as a function of pulse duration is shown in Fig. 3.
Fig. 2. Experimental setup.
tage of this method in that the donor film acts as its own propellant and significant damages result from its ablation. Previous study has shown that it is preferable to use a wavelength which is absorbed in the first nanometers of the layer in order to confine theses damages [11]. To solve this problem we insert a sacrificial layer of gold or silver, also called dynamic release layer (DRL). The advantage of this technique is to preserve the polymer from being damaged by thermal and photochemical effects during the interaction. However, it can bring complications in contaminating the deposited material with debris resulting from its ablation. Gold and silver layers have been deposited by thermal evaporation in
3.1.1. Nanosecond regime LIFT of PEDOT:PSS films has been first studied in nanosecond regime (Fig. 3A). To obtain a good deposit (uniform color with our optical system), it is necessary to irradiate the film with fluences higher than 0.09 J/cm2 (Fig. 3A(a)). The shape of the deposit is homogeneous and begins to be reproducible. When fluence increases, thermal effects, characterized by the ejection of micro-droplets, are more and more visible all around the deposit. The amount of ejected particles increases and the deposit is fragmented (appearing of light areas in Fig. 3A(g)). The irradiated area brightness, on the donor substrate, increases with the fluence, meaning that organic material remains, at least till 0.17 J/cm2. 3.1.2. Picosecond regime LIFT results with the 50 ps Nd:YAG laser pulses at 355 nm are presented in Fig. 3B. At this wavelength, the film of PEDOT:PSS is
Fig. 3. Optical microscopy images of the donor (up) and the receiver (down) substrate after transfer. The donor film was made of 300 nm of PEDOT:PSS. Fluence is varied horizontally. (A) PEDOT:PSS irradiated with 35 ns pulse duration. (B) PEDOT:PSS irradiated with 50 ps pulse duration.
L. Rapp et al. / Applied Surface Science 255 (2009) 5439–5443
partially transparent to this incident radiation and it is estimated that only 35% of the incident energy is trapped in the layer. From 0.18 J/cm2, a partial transfer is obtain although it is not homogeneous but formed with ejected fragments (Fig. 3B(a)). The best transfer is achieved at 0.46 J/cm2. Ejected particles are visible all around the deposit at 0.66 J/cm2 (Fig. 3B(g)). The total ablation of the irradiated surface is carried out at a fluence threshold of 0.74 J/cm2. Over 0.74 J/cm2, the surface of the deposit starts to be damaged and the shape is partially destroyed. Some big holes appear in the deposit (Fig. 3B(d)). 3.1.3. Analyze of the deposits SEM visualizations of the transferred PEDOT:PSS surfaces are shown in Fig. 4. The surface structure is very different for the two pulse durations. In nanosecond regime, micrometric holes are visible over the entire surface inducing a high inhomogeneity and a high roughness (Fig. 4A). The absorption of the radiation seems to occur on different hot points, creating some ‘‘micro-bubbles’’ or ‘‘micro-explosions’’ all over the surface (Fig. 4A(b)). In picosecond regime, the structure is much more homogenous (Fig. 4B). The difference in surface structuring is due to the difference in both pulse duration and absorption properties. In nanosecond regime, thermal effects are very important even under UV irradiation. The polymer is very absorbing to the incident radiation and the energy is trapped in a small volume limited by the optical penetration of the beam inside the polymer. During the ablation phase, the polymer is partially vaporized and the ablation products (gas, solid or liquid particles; Fig. 4A(c)) are expelled from the removed layer and give this kind of surface morphology. In picosecond regime, a larger volume is affected by the direct irradiation but thermal effects are limited because of the short pulse duration (Fig. 4B(b)). The edge of the deposit is well defined in a very short range of fluences [11], but in both regimes a lot of debris is visible around the spot.
5441
3.2. Influence of the metallic layer To avoid direct laser damaging, an additional dynamic release layer (DRL) is introduced. Gold and silver layers have been used because of their mechanical properties and their short absorption length (labs) for UV radiation (gold: labs = 13 nm at 248 nm and 15 nm at 355 nm; silver: labs = 15 nm at 248 nm and 51 nm at 355 nm) [12]. We have first analyzed the transfer with a metal thickness of 50 nm in both cases. In picosecond regime, homogeneous deposits with well-defined edges are obtained between 0.18 J/cm2 and 0.60 J/cm2 using gold DRL. Huge metallic debris can appear all around the deposit but they are very easy to remove with a soft air blow. Over 0.40 J/cm2, some thermal effects are visible on the deposit surface. For fluences higher than 0.90 J/cm2, the transferred material is deteriorated. The donor layer is ablated on the irradiated area from 0.18 J/cm2 to 0.30 J/cm2. At higher fluences, the metallic layer is destroyed on a surface widely higher than the spot size under high mechanical stress effect. Close characteristics are observed with a 25 nm layer. For 50 nm of silver, very good deposits are obtained between 0.28 J/cm2 and 0.46 J/cm2. But in this case the thickness of the layer has almost the same value than the absorption length of the beam for this radiation. The energy is transferred inside the metal in a larger volume and, as a consequence, the mechanical stress inducing the ejection of the film is lower. In reducing or increasing the thickness of the layer, no good deposits are obtained. They are inhomogeneous and have no square shape. In nanosecond regime, there is no ablation with 50 nm silver under 0.09 J/cm2. Good deposits are transferred from 0.16 J/cm2 to 0.40 J/cm2 (Fig. 5A(a)). For gold layers the best transfer is obtained when the thickness of the film is twice the absorption length (Fig. 5B) of the material (in this case 25 nm). We have obtained very good results of deposition in a large range of fluences for the two regimes of pulse duration, pointing
Fig. 4. SEM visualizations of PEDOT:PSS surface of the deposit transferred on silicon. (A series) PEDOT:PSS irradiated by an UV radiation (248 nm) with 35 ns pulse duration. (B series) PEDOT:PSS irradiated by an UV radiation (355 nm) with 50 ps pulse duration. (a) Corresponds to surface sample, (b) is a close up of the surface sample, (c) is the edge of the deposit.
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L. Rapp et al. / Applied Surface Science 255 (2009) 5439–5443
Fig. 5. SEM visualizations of PEDOT:PSS deposits (300 nm) using different DRL layers. Nanosecond regime: (A) 50 nm of silver at 0.12 J/cm2. (B) 25 nm of gold at 0.15 J/cm2. Picosecond regime with EDAX analyses: (C) 25 nm of gold at 0.28 J/cm2. (D) 10 nm of gold at 0.65 J/cm2.
out the benefits of using a thin sacrificial release layer. But, SEM visualizations (Fig. 5) revealed a contamination of the deposited material with residual metallic debris. SEM and X-ray dispersive energy analyses (EDAX) show that the contamination is localized in micro- and nanoparticles of gold (Fig. 5B(b) and C(b)) and silver (Fig. 5A(b)) on the surface of the transferred polymer. By reducing the thickness of the metallic layer to 10 nm, we have reduced the range of usable fluences to get a good transfer for both metals. For gold, ejected particles are still present all around the spot and fragments on the top of the deposit are clearly visible (Fig. 5D(a)). EDAX have shown that metallic elements are only present in these fragments, indicating a mixing of PEDOT:PSS and metal (Fig. 5D(b)). 3.3. Electrical properties The electrical properties of the transferred PEDOT:PSS have been characterized. Lines of PEDOT:PSS have been made by juxtaposing deposits (different overlapping values have been tested). The transfer has been realized with samples without DRL layer in picosecond regime at 0.40 J/cm2. Then gold electrodes separated by 250 mm distance have been evaporated onto these lines using a mask. I(y) measurement have been performed at different distances on the line. The different measurements for a same distance show a good reproducibility of the characteristics but point out also the difficulty to keep the electrical contact between two juxtaposed spots. The results for different distances (from 250 mm to 1060 mm) are presented in Fig. 6. The value of the resistance increases with the distance of measurement. From these measurements, an average resistivity of the deposit is estimated to be 5 10 3 V cm. The value of the conductivity associated is 200 S cm 1. This is an interesting resistivity value in comparison with the constructor value 0.01– 0.02 V cm and printing PEDOT:PSS value 13 10 3 V cm [10].
Fig. 6. I(y) measurement of PEDOT:PSS deposits.
4. Conclusion We have reported in this paper LIFT experiments on conducting PEDOT:PSS layers under different conditions of irradiation and laser pulse duration. Polymer layers can be transferred with LIFT technique to obtain micro-structures with a high spatial resolution. The deposits keep a good cohesion during the transfer. We have shown the benefit of using a sacrificial metallic layer and the importance of its thickness. It has been shown that the thickness of the layer is optimized as a function of the absorption properties of the material. The homogeneity and the spatial resolution are really improved but there is a risk of contamination. This contamination
L. Rapp et al. / Applied Surface Science 255 (2009) 5439–5443
is localized on the top of the transferred polymer. That is why using an insulator with absorbing properties could be a good idea in order to optimize the process [13]. This work will be completed by time resolved shadowgraphic imaging of the ejection of PEDOT:PSS with and without DRL in order to obtain a better comprehension of this mechanism [14]. Moreover, experiments using 266 nm picosecond radiation can be performed with no DRL on PEDOT:PSS to take advantage of its high absorption at this wavelength. The transferred film has interesting electrical properties, showing that conducting polymer can be transferred by LIFT process. Therefore, no limitation is expected in miniaturization, which is a necessary requirement.
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Acknowledgments The authors would like to acknowledge scientists from the CINaM (Marseille) and the CMP- GC (Gardanne) for their collaboration within the MICROPOLY framework (CIMPACA project) and the ANR program e-PLAST.
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