Laser based hybrid inkjet printing of nanoink for flexible electronics Seung Hwan Koa, Yeonho Choia, Costas P. Grigoropoulos*a and Jaewon Chungb, Nicole R. Bieric, Tae-youl Choic, Cedric Dockendorfc, Dimos Poulikakosc a Laser Thermal Lab., Dept. of Mechanical Eng., Univ. of California, Berkeley; b Dept. of Mechanical Eng., Korea Univ., Seoul, South Korea; c Lab. of Thermodynamics in Emerging Technologies, Dep. of Mechanical and Process Eng., ETH Zurich, Switzerland ABSTRACT Many applications require delivery of small quantities of functional materials into locations on a substrate in the form of liquid solution. Consequently, interest in nongraphical inkjet printing is growing. In addition, higher resolution for printing flexible electronics is becoming more critical to enhance the performance of printing electronics. Since the resolution of inkjet process is limited by the nozzle size and the statistical variation of droplet flight and spreading phenomena, hybrid inkjet printing has emerged as an attractive processing method. In this work, surface monolayer protected gold nanoparticle was printed in a liquid solution form and cured by laser irradiation to fabricate electrically conductive microlines on glass or polymer substrate at a reduced temperature. Continuous laser curing enabled local heating and the morphology could be controlled as well. Thermal penetration into the substrate could be minimized by using pulsed laser beam. Nanoparticle film was effectively removed by applying femtosecond laser, so that small feature size was obtained. Printing on a heated substrate has advantages over room temperature printing. The solvent evaporates soon after contact, so that a thick layer can be deposited with high jetting frequency. The rapid liquid evaporation also eliminated uneven wetting problems and the smaller feature size was obtained. Keywords: Flexible electronics, nanoparticle, gold, inkjet, laser, ablation, sinter, droplet, impact, evaporation

1. INTRODUCTION There are many applications that require the delivery of small quantities of functional materials into locations on a substrate in the form of liquid solution. Inkjet printing technique satisfies these requirements and interest for nongraphical inkjet printing is growing. Especially, in many electronic devices, such as large area displays, RFID (Radio Frequency Identification), adaptive distributed antennas, etc, cost is a primary consideration determining commercial success and marketability. Corresponding to these requirements, direct writing (additive) process using drop on demand printing technology has gained significant interest as an alternative approach to conventional, subtractive integrated circuit (IC) processes [1-4]. In addition, in large area electronic devices using flexible polymeric material as a substrate, it is important to fabricate low resistance conductors because the signal has to propagate a long distance and RC delays should be minimized. The processing temperature is also an issue, since flexible polymeric materials possess low glass transition or melting temperature. In conjunction with these requirements, metal nanoparticle processing has a great potential owing to remarkable size dependent thermophysical properties. Specific to this paper are gold nanosized particles smaller than 5 to 7nm of diameter, which has a significant depression of melting temperatures compared to the melting temperature of bulk gold (1063°C) [5]. It is noted that nanoparticle used in inkjet printing is often protected by organic surface monolayer to prevent agglomeration and to be dissolved into the organic solvent [6, 7]. Especially, in two phase chemical method [8], surface monolayer is applied during nanoparticle synthesis process, so that the final size distribution of nanoparticles can be controlled by the concentration of surface monolayer. Combining the above two characteristics, several studies related to printing and curing of nanoink using a hot plate or a curing oven have been carried out [9-11]. Since nanoparticles melt at low temperature, there is great potential for producing microconductors at low processing temperature suitable for polymeric substrate. *

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However, the limited resolution of inkjet printing compared to IC process is an issue. The minimum inkjet printed line width is determined by the nozzle size (≈ the droplet diameter) and the additional spreading after droplet impact. The line edge resolution is therefore limited by the statistical variation of droplet flight and spreading phenomena. To overcome this disadvantage of inkjet printing, the jetting head with a small nozzle size (several micrometers) and sufficient jetting power is under development by several major manufacturers. At the same time, hybrid inkjet printing has also gained interests. Hybrid inkjet printing can be either in a pre process or post process sequence. In the surface energy patterning technique [12], high line edge resolution is obtained by spreading ink on the pre-patterned hydrophilic area by photolithography, etc. As a post process, laser can be locally irradiated to define small feature on preprinted film either by curing [13-16] or ablation. Since laser based hybrid printing is a data driven process (i.e. does not require mask process such as surface patterning technique), it can be more compatible to direct inkjet writing. In this work, surface monolayer protected gold nanoparticle was printed in a liquid solution form and cured to make electrically conductive microlines on glass or polymer substrate at a reduced temperature. Subsequently, laser was locally irradiated to define small feature on preprinted film either by curing [6-9] or ablation. In section 2.1, a focused beam continuous Ar ion laser scanned the printed nanoink line with a significant amount of solvent on glass substrate. In this process, the focused laser beam was mostly absorbed in the sintered gold film. Heat conduction through the glass substrate dominated the evaporation of solvent, Marangoni flow and nanoparticle deposition. By focusing the laser beam tightly and controlling laser beam intensity, a small portion of printed nanoparticle ink line was cured. Uncured nanoink could be washed away, the enabling small feature size which can not be achieved by inkjet printing alone. In section 2.2, printed nanoparticle film on polyimide film was cured with the pulsed irradiation of temporally modulated Ar ion laser using AOM (acousto optic modulator). Here, pulsed laser was used to minimize thermal penetration depth. In section 2.3, nanoparticle film on glass and polymer substrate was effectively removed by applying femtosecond laser, to define small feature size. In addition, printing on a heated substrate is shown to possess several advantages over room temperature printing. The solvent evaporates soon after contact, so that thicker layers can be deposited with high jetting frequency. The rapid liquid evaporation also eliminated uneven wetting problems and smaller feature size was obtained [18]. In section 3, issues related to substrate heating and inkjet printing are discussed.

2. LASER BASED HYBRID INKJET PRINTING 2.1 Nanoparticle deposition using continuous laser beam The employed gold nanoparticle ink is composed of 3-7nm average size gold particles (30% weight, 1.9% volume) protected by organic surface monolayer in a toluene solvent [6] After generating stable droplets of 46µm diameter (i.e. 51pL) at 30Hz using the modified drop-on-demand printing system [14], a continuous line was printed on the microscope soda lime glass slide by moving an automatic translation stage at 2mm/s. The width of the deposited line was measured at about 125µm.. Ar ion laser beam(514nm) was irradiated at the center of a printed line with 45°of incidence angle. The glass substrate is placed on a translation stage and in-situ images were taken by a fixed up-right microscope. A long working objective lens (20 X) was used and a filter eliminated reflected Argon laser from the curing line. Figure 1(a) shows an in-situ micrograph of the laser curing process at 100mW of incident laser power and 0.2mm/s of translation speed. The focused beam waist (1/e2) along the minor axis that is perpendicular to the printed line is 27µm and the beam waist along the major axis is 38µm. A highly reflective gold layer begins to form near the evaporation interface contact line. The toluene evaporates due to laser radiation absorption in the sintered gold layer and the subsequent thermal diffusion toward the evaporation interface through the glass substrate. Due to thermocapillarity, ink was displaced ahead and around the scanning laser spot forming a U-shaped convex ink meniscus. Gold nanoparticles are mainly deposited right at the evaporation interface contact line. Neglecting thermal losses due to radiation, convection and evaporation, the calculated temperature profiles of the substrate induced by scanning a continuous Gaussian elliptic laser beam [18] were employed to provide approximate quantitative information on the temperature field. The isotherms in Fig. 1(a) show that the region bounded by the evaporation interface contact line is only slightly narrower than the isotherm at 110°C which is the evaporation temperature of toluene under atmospheric pressure. Atomic force microscope (AFM) images in Fig. 1(b) depict the morphology of cured nanoparticle after washing the leftover toluene solvent. This morphology resembles a “volcano” cross section.

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Assuming uniform nanoparticle deposition rate per unit arc length at the approximately semi-circular evaporation interface contact line, the thickness of the deposited gold should be proportional to the arc length per unit length in the y-direction, which results in a “volcano” cross section. Experiments were carried out using tighter focused beams. In these cases, nanoparticle ink was mostly displaced around the laser spot due to smaller beam waist (i.e. bigger curvature of 110°C isotherm). Fig. 2a) shows gold lines cured by a single laser beam pass at 200mW of incident laser power, 2mm/s of translation speed and 2µm of beam waist radius. The gold layer in the center region is strongly melted and coalesced due to the high laser intensity. This region appears dark in an optical reflection image due to the formation of distinct aggregates (droplets) of gold that was confirmed by AFM imaging. At 20mW of incident laser power, the width of a cured gold line is as small as 8µm in Fig. 2b). While the current basic study is focused on the understanding of the phenomena involved, it is a matter of parametric optimization to enhance both scanning speed and smaller line width. 2.2 Nanoparticle curing using pulsed irradiation of continuous laser beam In the experiment, a line of nanoink on polyimide film printed at room temperature was heated so as to evaporate the solvent. Pulsed Argon laser (514nm) irradiation was then applied using an acousto-optic modulator at the incidence angle of 45°, while the temperature of polyimide film was maintained at 200°C in order to decrease the required laser pulse energy. Since the solvent is evaporated by heating the substrate at a moderate temperature before irradiation of the pulsed laser, thermocapillary flow was not observed. In addition, the pulsed laser can effectively minimize the heataffected zone. The same experiment setup explained in section 2.1 was used to observe the laser curing process. Since the focused beam waist (1/e2) of 30µm is smaller than the width of the printed line (about 160µm), the pulsed laser spot had to scan several times parallel to the line to cure the entire width of the printed nanoink line (refer to inset picture in

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Fig. 3 (a)). It is noted that the nanoink spread more on polyimide film than on glass substrate, which resulted in a wider printed line. In addition, the morphology of deposited nanoparticles printed at room temperature and cured at 200°C shows the double ridge pattern. More detail will be presented in section 3. Experiments were carried out varying the laser power, pulse duration (tpulse), frequency, beam waist, scanning speed, and scanning gap (dgap). Figure 3 (a) shows the results of a pulsed laser cured printed line at different scanning gaps. In this experiment, the laser frequency was set at 100Hz allowing a sufficiently long time interval between pulses for the heated area to cool down to near the original substrate temperature of 200°C. As the scanning gap increases, the resistance per length, RL increases (Fig. 3(a)). When the scanning gap is larger than the diameter of the focused beam (60µm), dark lines appear indicating low curing level. These lines are separated by a distance matching the scanning gap (see reflection image in Fig. 3(a)). Fig. 3(b) depicts a pulsed laser cured line for varying laser pulse duration, tpulse. As tpulse increases up to 40µs, the resistance per length, RL, decreases to 7.5Ω/mm. It is noted that the same printed nanoink lines were cured by substrate heating. The resistance per length of 7.5Ω/mm was obtained at the curing temperature of 400°C. In addition, the resistivity at the curing temperature of 400°C is 4-5 times higher than the bulk value. At 50µs, RL increases due to delamination of gold layer which corresponds to the dark spot in the inset picture in Fig. 3 (b). Note that the polyimide film did not show a significant deformation even at tpulse=50µs. Therefore, the limiting factor for obtaining lower resistance (i.e. more complete curing of nanoparticles) is not the deformation of the polymer film, but the gold film delamination (peeling off). Upon pulsed laser irradiation, it appears that rapid expansion of the trapped vapor bulges and occasionally explodes the gold film which resulted in the delamination of the gold film. Though not shown here, similar curing effect could be obtained at the scanning speed of 20mm/s with 30µs pulse by increasing the laser frequency to about 3000Hz. 2.3 Nanoparticle ablation using irradiation of femtosecond laser Besides the additive process to deposit the functional material at the right place with modified inkjet system, the subtractive process to remove some parts of the deposited line by additive process can be achieved by ultrashort pulse laser ablation. Because both the influence of heat conduction within the material and screening of the incident laser light are strongly diminished with picosecond pulses and can be ignored with femtosecond pulses, material removal is very localized and requires less energy[19]. In the experiment, gold nanoparticle ink which is composed of 1-3nm average size gold particles (10% weight) protected by organic surface monolayer in alpha-terpineol solvent [7] was used. Alpha-terpineol is in the solid state at room temperature and needs to be heated above 50°C for proper jetting. To maintain 50°C at the jetting head, substrate was heated up to 100°C on vacuum chuck heater to transfer thermal energy via heat conduction across the air gap. This substrate heating has two advantages: firstly, heating effect can minimize the so-called coffee stain effect which produces sombrero shaped material deposition after solvent evaporation, and secondarily, the viscosity of ink can be shifted to the optimum value for stable jetting. Lines of nanoink were printed on glass, polyimide and PET film with the

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same modified drop-on-demand printing system as is used in other experiments. The average line width was 70~140µm. Femtosecond laser (83fs full width half maximum(FWHM ), 800nm wavelength, 1mJ maximum pulse energy) was irradiated. Ablation were performed at pulse energy at 0.15 µJ and 100 to 1000Hz without damage to the polymer substrate. Figure 4 shows femtosecond laser ablated gold lines on glass substrate at 0.15~0.2µJ of incident laser energy with 0.25mm/s translation stage speed and 100Hz laser pulse. Lines with 1.5~20µm width and 2~4µm trenches could be fabricated. Besides glass, PET(poly(ethylene terephthalate)) and PI(polyimide) film were used as substrate as in Figure 5. The same features could be reproduced on polymer substrate with 0.12~0.15µJ of incident laser energy combined with 0.25mm/s translation stage speed and 100Hz laser pulse. Available ablation window for PI is smaller than that of PET due to its dark brown color. This femtosecond laser ablation could be used to trim gold lines to achieve more sophisticated gold line feature.

3. INKJET PRINTING ON HEATED SUBSTRATE Printing on a heated substrate has advantages over room temperature printing. As opposed to printing on paper or textile which absorbs the ink, material deposition in nanoink printing on glass or polymer requires the evaporation of solvent. Therefore, the substrate heating can evaporate the solvent soon after contact, so that a relatively thick layer can be deposited with high jetting frequency while maintaining a small lateral feature size. Figure 6 shows the evaporation time for pure toluene and terpineol droplets at different substrate temperature. As the substrate temperature increases above the respective boiling temperature, the evaporation time decreases to 3-5ms and higher substrate temperature results in Leidenfrost phenomenon causing droplet rebound. Even though the total evaporation time is about 3-5ms, a significant amount of solvent is evaporated within about 1ms. Therefore, thick layer could be deposited with 1kHz jetting frequency while maintaining a small feature size. In addition, the solvent evaporates before spreading by surface tension occurs, so that smaller printed spot can be obtained. Figure 7 shows in situ image of droplet impact and spreading taken from the side with a slight angle. Images were obtained using flash photography by synchronizing the

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piezo signal for the droplet generation and illumination flash light. During the impact inertia dominant period up to 5060µs, the evaluation of droplet impact and recoil manifested by the shape of impacted droplet is identical and not dependent on the substrate temperature. During the surface tension dominant period after 50-60µs, while droplet spreads continuously at 150°C substrate temperature, the solvent evaporates faster at 250°C which results in smaller spot size. Fig. 8 shows the morphology of the deposited nanoparticle at different substrate temperatures. At low substrate temperature (100°C), most nanoparticles are deposited at the edge of the droplet, which is often called as “coffee stain problem.” Rapid evaporation at higher substrate temperature results in relatively uniform nanoparticle deposition and also eliminates uneven wetting in response to minute contamination of the substrate. Toluene Boiling point

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However, there are limitations in printing on a heated substrate. For stable droplet generation and impact (which is directly related to the spatial resolution), the nozzle to substrate distance should be maintained less than 2-3mm. This causes temperature rise in the nozzle (and corresponding decrease in both viscosity and surface tension of fluid) due to conductive heat diffusion from the hot substrate through air [20], changing jetting conditions, such as jetting velocity, droplet size, etc. In addition, unless droplet is jetted in a continuous mode, the solvent evaporates in the nozzle, increasing the possibility of partial or complete nanoparticle clogging. Therefore, integration of the active cooling system into jetting head to control the nozzle temperature, would be very beneficial to minimize the above problems.

4. CONCLUSIONS In this work, surface monolayer protected gold nanoparticle was printed in a liquid solution form and cured by laser irradiation to form electrically conductive microlines on glass or polymer substrate at a reduced temperature. Continuous laser curing enabled local heating and small feature size that could not be obtained by inkjet printing and substrate heating was demonstrated. Due to Gaussian profile of the focused laser beam, the deposited nanoparticles may have spatially different curing level. To this end, deposited nanoparticle line can be additionally cured using pulsed laser beam. Thermal penetration into the substrate could be minimized by using pulsed laser beam. In addition, nanoparticle film was effectively removed by applying femtosecond laser without damage in polymeric substrate, so that small feature size was obtained. In summary, as a post process, laser can be effectively used to define small feature on preprinted film either by curing or ablation. In addition, printing on a heated substrate has advantages over room temperature printing. The solvent evaporates soon after contact, so that thicker layer can be deposited with high jetting frequency. The rapid liquid evaporation also eliminates uneven wetting problems, yielding the smaller feature size. However, the jetting environment also changes as the substrate temperature is elevated. Therefore, it would be beneficial if the temperature at the nozzle could be kept low using an active cooling system in jetting head.

5. ACKNOWLEDGMENTS The authors wish to thank to Professor Vivek Subramanian of the Department of Electrical Engineering and

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Computer Sciences, University of California, Berkeley for valuable discussions. Financial support to the University of California, Berkeley by the U.S. National Science Foundation under Grant CTS-0417563 and to the Swiss Federal Institute of Technology in Zurich by the Swiss National Science Foundation under grant No. 2000-063580.00 is gratefully acknowledged.

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