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ScienceDirect Procedia CIRP 42 (2016) 62 – 66
18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII)
Influence of Pulse duration on Processing Characteristics of Transparent Conductive Film Containing Silver Nanowires by ns Pulsed Fiber Laser Masafumi Oshitaa, Norio Nishia,b, Yasuhiro Okamotoa,*, Akira Okadaa, Togo Shinonagaa, Tomokazu Sakagawab a Okayama University, 3-1-1 Tsusimanaka, Kita-ku, Okayama 700-8530, Japan Kataoka Corporation, 140 Kuzetsukiyamacho, Minami-ku, Kyoto 601-8203,Japan
b
* Corresponding author. Tel.: +81-86-251-8039 ; fax: +81-86-251-8266. E-mail address:
[email protected]
Abstract Processing characteristics of transparent conductive film containing silver nanowires by ns pulsed fiber lasers with various pulse duration were experimentally investigated in the view-points of insulating state and surface quality. The laser fluence achieving the insulation state became smaller with decreasing the pulse duration. Appearance of removed trace area of silver nanowires was varied by the pulse duration, and the removal area ratio of removed trace area to the whole one was evaluated to discuss the surface quality. Small removal area ratio and good insulation state could be performed at a wide processing window at long pulse duration of 200ns.
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining XVIII). (ISEM XVIII) Keywords: Transparent conductive film containing silver nanowires; ns pulsed fiber laser; Pulse duration; Insulation; Selective removal
1. Introduction Recently, transparent conductive film has been widely for liquid crystal panel and organic applied electroluminescence, and its applications has increased with decreasing the size of displays and mobile devices [1]. In addition, the diversification of optoelectronic technologies requires high refractive index and high flexibility to transparent conductive film. Nowadays, ITO (Indium Tin Oxide) films have been widely used for electrodes such as flat panel display and solar panel. However, there are some problems in the cost and the sustainable supplying of material, since ITO contains indium as a rare metal. In contrast, since silver nanowires have good transparency and conductivity, transparent conductive film containing silver nanowires is expected as an alternative material to ITO film [2]. In order to use transparent conductive film as the electrodes, selective removal of various layers formed on the substrate with precise and high throughput process is essential to enhance these products. Laser beam processing have been adopted as the
removal of transparent conductive film, since high-speed and high-quality removal can be expected without a mechanical contact [3]. It is reported that the insulation state can be obtained across the laser scanned line on transparent conductive film containing silver nanowires, and the favorable results in electric insulation state were obtained by an ultrashort pulsed laser [4][5]. In contrast, acceptable electric insulation state could be also obtained by a ns pulsed laser, and the achievement of good quality removal results with ns pulsed laser has an advantage of cost reduction and low load of maintenance. However, ns pulsed laser processing partly includes the thermal removal, and the pulse duration in ns range has an influence on removal characteristics. Since the suitable pulse duration was not clarified yet in ns range, silver nanowires were selectively removed by using the ns pulsed laser with various pulse duration in this study. The effect of pulse duration on surface quality and insulation state of transparent conductive film containing silver nanowires was experimentally investigated by ns pulsed lasers.
2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.189
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2. Experimental setup 2.1. Laser irradiation method Three types of ns pulsed fiber laser were used in this experiment to use various pulse duration. Main specifications of these laser oscillator are shown in Table 1. The pulse duration of 30ns, 50ns, and 100ns were obtained from one ns pulsed fiber laser. Other two lasers generated “120ns and 160ns”, and 200ns, respectively. The center emission wavelength and the pulse repetition rate were 1060nm and 50kHz, for all laser oscillators, respectively. Fig.1 shows the schematic diagram of experimental setup and optical setup after the optical fiber. Fig.2 shows the beam modes for various pulse duration. The square-shaped top-hat intensity distribution of laser beam was obtained by using the optical fiber (Core size: 50Pm square), since the laser beam emitted from the laser oscillators has Gaussian mode distribution. In the case of pulse duration of 200ns, the laser beam from the square-shaped optical fiber was focused on the specimen by using a collimation lens of f1c=100mm and a focusing lens of f1f=60mm in focal length. In this optical setup, 30Pm square spot with top-hat intensity distribution was obtained at the focusing plane. As for pulse duration of 30ns, 50ns, 100ns, 120ns and 160ns, the laser beam was focused on the specimen by using the combination of collimation lens and focusing lens of f2f=f2c=100mm in focal length. Top-hat intensity distribution was obtained as 50Pm square spot at the focusing plane. The optical setup with pulse duration of 200ns was different from that with other pulse duration, since specifications of this laser oscillator required a short focal length in order to obtain the homogeneous top-hat beam mode. Hence, 30Pm square spot was used in the case of 200ns pulse duration. Although spot sizes were different in pulse duration, the same laser fluence was used in all pulse duration by controlling the pulse energy. Transparent conductive film containing silver nanowires was mounted with the jig on X-Y-Z stage in air. The laser spot was scanned on the surface of the specimen by controlling the feed rate of stage. The feed rate was determined by combination of spot size and pulse repetition rate to obtain individual irradiated spot. Straight processing line was formed at the overlap rate of 0%.
Table 1. Main experimental conditions
Wavelength
1060nm
Pulse duration
30,50,100,120,160,200ns
Fluence
0.4-12.2J/cm2
Pulse repetition rate
50kHz
Fig.1. Schematic diagram of experimental setup㻌
High㻌
Low㻌 Fig.2. Beam modes for various pulse duration
Silver nanowires coating㻌
PET㻌 3Pm㻌 (a) Surface㻌
1Pm㻌 (b) Cross section㻌
2.2. Transparent conductive film containing silver nanowires Transparent conductive film containing silver nanowires was used as a specimen, and it consists of two layers, which are acryl overcoat layer containing the silver nanowires and PET (Polyethylene terephthalate) of base material. Fig.3 shows the surface and the cross section of specimen observed by SEM (Scanning electron microscope). The thickness of the overcoat layer containing silver nanowires is approximately 2Pm, and the total thickness of the specimen including PET substrate is 120Pm. The silver nanowires with diameter of approximately 80nm and length of approximately 15Pm exist in the whole area of the overcoat layer. Since these silver nanowires are contacted each other, transparent conductive film containing silver nanowires has the conductivity and high transmissivity
Fig.3. Appearance of transparent conductive film containing silver nanowires
at the range of visible wavelength. The optical absorption rate of the specimen is approximately 1.5% at the wavelength of 1060nm by the measurement with a spectrometer. 3. Evaluation method of electric insulation state 3.1. Measurement of electric resistance The electrical resistance crossing over a scanning line was measured by using an insulation resistance tester (HG561H, SANWA ELECTRIC INSTRUMENT CO., LTD.) in order to
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Test probe㻌
10mm㻌
Probe head㻌 Prob Jig㻌 Scanning line㻌
Specimen㻌 Fig.4. Insulation resistance measuring apparatus㻌
evaluate the insulation characteristics of the specimen processed by laser beam scanning. The specimen was set with the jig as shown in Fig.4. The distance between the electrode tips of probes was 10mm, and scanning line was put on the center of probe tips. In this study, electric resistance was measured at voltage of 25V, when the usage in touch panels was assumed. When the electric resistance showed more than 21M:, it was defined that scanning line kept the electric insulation state.
(a) Pulse duration tp=30ns
3.2. Evaluation of electric insulation Fig.5 shows the insulation characteristics crossing over a scanning line for various laser fluences, when the number of laser irradiation per one point was N=1 (no overlap of laser spot) at pulse duration of 30ns and 200ns. On the one hand, in the case of 30ns pulse duration, electric insulation could be achieved reliability more than laser fluence of 0.5J/cm2. On the other hand, in pulse duration of 200ns, electric insulation could be achieved reliability more than laser fluence of 2.0J/cm2, while electric insulation could not be kept at low laser fluence. These results indicate that the boundary of laser fluence to obtain the stable electric insulation at pulse duration of 30ns and 200ns were 0.5J/cm2 and 2.0J/cm2, respectively.
(b) Pulse duration tp=200ns Fig.5. Insulation characteristics at pulse duration 30ns and 200ns
4. Removal characteristics and its appearance 4.1 Removal appearance of silver nanowires The electric insulation is assumed by removal of silver nanowires in overcoat layer with laser beam irradiation on transparent conductive film containing silver nanowires. The traces after the removal of the silver nanowires appeared on the specimen surface. Fig.6 shows a SEM photographs of the specimen surface before and after laser irradiation at the same location. The removal parts, which were caused by spouting of silver nanowires from the overcoat layer were confirmed at the dense parts and the intersections of the silver nanowires. It was reported that the intensity of laser beam increased at the intersection of silver nanowires [6]. Therefore, it is considered that temperature increased at the intersections of the silver nanowires, and the silver nanowires were spouted from these locations.
1Pm㻌
Pulse repetition rate: 50kHz, Pulse duration: 200ns, 㻌 Wavelength: 1060nm, Lens combination: Achromatic lens㻌 Fig.6. Surface appearance of specimen before and after laser irradiation at the same location
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The removed trace form of silver nanowires were varied with laser fluence and pulse duration. Fig.7 shows the SEM photographs of specimen surface after the laser irradiation for various laser fluences. At low laser fluence of 0.8J/cm2, the silver nanowires remained in the overcoat layer because of insufficient removal of the silver nanowires. At middle laser fluence of 1.6J/cm2, the silver nanowires were partially removed with spouting out from overcoat layer due to temperature rise of silver nanowires. In contrast, the traces formed by the removal of silver nanowires became larger at high laser fluence of 4.0J/cm2. Moreover, the linear removal state of silver nanowires was obtained. As mentioned above, the removed trace forms of silver nanowires were classified into mainly three types, such as the state of the partial remains by the incomplete removal of silver nanowires, the dotted removal state, and the linear removal state.
Pulse repetition rate: 50kHz, Pulse duration: 100ns, 㻌 Wavelength: 1060nm, Lens combination: Achromatic lens
㻌
Fig.7. Removal forms of silver nanowires far various duration㻌
4.2 Removal area ratio in surface of specimen Fig.8 shows the surface appearance of the specimen for various pulse duration under the same laser fluence condition. The removed trace of silver nanowires on the specimen surface was varied by the pulse duration even under the same laser fluence condition. The removal area ratio was defined to discuss the influence of pulse duration on the surface quality. SEM photographs were converted in binary images as shown in Fig.9. In the binary images, removal area SR caused by spouting of silver nanowires from the overcoat layer were depicted with white areas, and non-removal area SNR were depicted with black areas. Removal area ratio RRA was calculated by dividing the removal area SR with total measurement area (SR+SNR). A large removal area ratio indicates a wide damaged area caused by the removal of silver nanowires, and a small removal area ratio leads to a small difference of surface appearance before and after laser irradiation process. Fig.10 shows the relationships between removal area ratio and laser fluence for various pulse duration. Removal area ratio increased with increasing the laser fluence for all pulse duration. Removal area ratio with long pulse duration was smaller than that with short pulse duration even under the same laser fluence condition. Since the peak power was small at long pulse duration even under the same laser fluence condition, the temperature rise of silver nanowires became gradual. Therefore, removal area ratio became small, since the removal of silver nanowires was mainly occurred at the contact portion of silver nanowires near the surface. In contrast, removal area ratio became large in the case of short pulse duration. The temperature rise of silver nanowires became steep at short pulse duration, since the peak power was large under the same laser fluence condition. The temperature rise of silver nanowires increased uniformly, and the removed traces showed linear shapes due to simultaneous evaporation of silver nanowires. Therefore, it is considered that removal area ratio was varied with the pulse duration under the same laser fluence condition. Ultrashort pulsed lasers can minimize heat affected zone, but removed trace appearance of silver nanowires was similar to short pulse duration of ns range due to steep temperature rise [5]. In contrast, ns pulsed laser of 200ns pulse duration has a
3Pm 3 Pm㻌
Pulse repetition rate: 50kHz, Fluence: 1.6J/cm / 2, 㻌 Wavelength: 1060nm, Lens combination: Achromatic lens㻌 Fig.8. Appearance of specimen surface after laser irradiation with various pulse duration
((a)) SEM photograph h h
(b) Binary i image i 㻌
Fig.9. Measurement of removal area of silver nanowires 㻌
Fig.10. Removal area ratio of silver nanowires far various duration㻌
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(a) Pulse duration tp=30ns
(b) Pulse duration tp=200ns
Fig.11. Insulation characteristics and of removal area ratio for various laser fluence
possibility to reduce removal area ratio because of its gradual removal of silver nanowires. Since removal area ratio affects the visibility of transparent material which is important characteristic for transparent conductive film, the relationship between removal area ratio and visibility is planned to be investigated in the future. However, it is considered that the defective parts might affect the reliability of products, small removal area ratio would be useful for the manufacturing process of transparent conductive film containing silver nanowires. 4.3 Relationships for electric insulation and removal area ratio Fig.11 shows the electric insulation characteristics and the removal area ratio at pulse duration of 30ns and 200ns. The threshold of laser fluence to achieve electric insulation showed smaller value at pulse duration of 30ns compared with that at pulse duration of 200ns, while the increasing tendency of removal area ratio at pulse duration of 30ns was larger than that at pulse duration of 200ns. The laser fluence to obtain the reliable electric insulation at pulse duration of 200ns was high comparatively, but its removal area ratio was stable with a small value. Although the electric insulation can be achieved at the removal area ratio of 2% or more, the increase rate is steep at pulse duration of 30ns. Therefore, in the transition region from the conductive state to the insulation one crossing over the laser scanning line, removal area ratio at 200ns pulse duration was reliably smaller than that at 30ns pulse duration. As a result, the high-quality processing of silver nanowires with good insulation state can be expected around pulse duration of 200ns. 5. Conclusions In this study, the processing characteristics of transparent conductive film containing silver nanowires by ns pulsed fiber lasers with various pulse duration were experimentally investigated in the view-points of insulating state and surface quality. Main conclusions obtained in this study are as follows.
(1) Electric insulation could be obtained reliably by ns laser scanning processes on transparent conductive film containing silver nanowires. The laser fluence to achieve reliable electric insulation with short pulse duration was smaller than that with long pulse duration. (2) Removed trace form of silver nanowires on the specimen surface were varied by the pulse duration even under the same laser fluence condition, and that were classified into mainly three types, such as the state of the partial remains by the incomplete removal of silver nanowires, the dotted removal state, and the linear removal state. (3) Removed trace form of silver nanowires could be evaluated by removal area ratio of removed trace area to the whole one, and removal area ratio at long pulse duration was smaller than that at short pulse duration under the same laser fluence condition. (4) Longer pulse duration of 200ns showed almost constant small removal area ratio at the boundary region of laser fluence to change conductive state to insulation one, although shorter pulse duration showed drastic change of removal area ratio. References [1] Eugene M, Christopher J S. ITO Thin Film Coatings Obtained from Developed Ceramic Rotary Sputtering Targets, Advanced Processing and Manufacturing Technologies for Structural and Multifunctional Materials III, Vol. 30, John Wiley and Sons; 2009. pp.93-99. [2] Huang H, Huang J. Organic and Hybrid Solar Cells, Springer; 2014, pp.182-189. [3] Exarhos G. Engineering Performance in TCO Films for Energy Applications, Advanced Materials and Concepts for Energy Harvesting, Journal of the Electrochemical Society; 2009. p.29. [4] Henley S J, Cann M, Jurewicz I, Daltonc A, Milneb D. Laser Patterning of Transparent Conductive Metal Nanowire Coatings: Simulation and Experiment, Nanoscale 6; 2014. p.946. [5] Mizukawa T, Ikeno J, Matsuo R, Kunishi Y, Szuki H. Study on laser insulating technique of transparent conductive film containing silver nanowires(2nd Report), Proceedings of the Japan Society for Precision Engineering; 2011. pp.601-602 [6] Spechler A J, Arnold B C. Direct-write pulsed laser processed silver nanowire networks for transparent conducting electrodes, Applied Prhsics A; 2012. pp.25-28.
