Optimization of Reverse Offset Roll to Plate (RO-R2P) Through Contact Angle

Transcription

1 Optimization of Reverse Offset Roll to Plate (RO-R2P) Through Contact Angle Ji-Hyun Hwang, Kayna Lee Mendoza, Kyungdeok Jang, Seyeon Hwang, Namsoo Kim * Department of Metallurgy and Materials Engineering, The University of Texas at El Paso * nkim@utep.edu Abstract- The application of 5 µm nano-metal ink pattern has taken center stage for the field of cellular phones, organic solar cell, and so on due to its invisible property and high electrical conductivity. While the non-contact printing, such as ink jet and silk screen printing method, cannot print less than 40 um width pattern, reverse offset printing can control the accuracy of pattern width up to maximum 5 µm. However, due to its direct printing, fracture of final substrate is the biggest problem at the high pressure in the process. Therefore, the following rapid prototyping focuses on optimization of reverse offset printing based on low pressure and optimizing the contact angle as the primary parameters in achieving a high-resolution, 5 µm pattern. This paper proposes the sequence of contact angle (θ) based on the reverse offset printing process as: θ (Stage 1) > θ (Roll) > θ (Stage 2), θ (Stage 3). Through the experiment, a contact angle differential should be 5.4 θ 6.4 between roll and Stage 1, θ 10.1, between roll and Stage 2, and θ 13.4 between roll and Stage 3. Whereas the final pattern is not printed or non-continuous when the differential of contact is out of the proposed range, continuous high resolution of 5 µm pattern could be printed when the contact angle range and sequence are optimized as proposed method. Finally, the continuous and high resolution of 5 µm pattern is printed on the Kapton PV9101 when M1 is used for material of roll. Therefore, the continuous high resolution of 5 µm pattern could be produced by optimizing the range of contact angle in the reverse offset system. Keywords- Contact Angle; Reverse Offset; Roll to Plate A. Reverse Offset Printing I. INTRODUCTION Transparent electrodes, such as Indium Tin Oxide (ITO) and Indium Zinc Oxide (IZO), are widely used for displays and solar cells due to their transparency [1-3]. However, such transparent electrodes are faulted by their high cost and low electrical conductivity. The cheaper and more conductive solution to transparent electrode is printed metal ink [4-6]. Even though metal ink is not transparent, when printed at a width of less than 5 µm, the conductive patterns are optically transparent, or invisible to normal human sight. Thus, printed metal inks can replace transparent electrode in the market due to the lower cost and greater conductivity. However, for printed metal ink to be successful, more precise and consistent micro-fabrication technology is required. High-resolution, fine patterns can be more successfully achieved through reverse offset printing, compared to other direct printing processes such as ink-jet, screen printing, gravure printing which are unable to print a pattern width less than 10 µm due to low accuracy and residual material [7, 8]. Reverse offset printing, however, minimizes the error rate, and the residual material can be reduced by using cliché, which is a kind of engraved printing plate, during the operation. In addition, because the reverse offset printing is a direct printing method, which means that printing roll and printing stages directly contact each other like a stamp, the method could accomplish high resolution of conductive pattern rather than non-contact printing method, such as ink jet and silk screen method. For successful printing process, rapid prototyping, focusing rapid product by developing and optimizing the manufacturing process, is essential because it is directly connected to a reducing time of market and cutting cost of product development [9, 10]. For rapid prototyping on the printing, the parameters for reverse offset printing should be optimized, such as the speed and pressure of the roll [11]. Traditionally, high pressure has been used to achieve high-resolution, however, the high pressure limits the substrate selection because the direct contact printing method tends to break more rigid substrates as well as the intagliated printing plates, such as glass cliché [12]. According to previous research, the glass substrate for AZO (Al: ZnO) film had µm length of crack at 0.98 N [13]. As a solution, the proposed solution is focused contact angle with relatively low pressure (0.5 N) optimized for reverse offset printing in order to manufacture high-resolution, fine patterns using metal ink. B. Contact Angle A droplet (liquid) on substrate (solid) forms a certain shape when solid, liquid, and vapor phase are at thermodynamic equilibrium, and contact angle can be determined by inside angle of the droplet between liquid/vapor interface and solid substrate. Contact angle shows wettability of a solid surface via liquid [14]. Roll to plate, reverse offset system is proceeded by contacting each stage and roll directly; the contact angle can determine how much the liquid (silver ink) absorbs more between the materials for roll or each stage. Therefore, the high resolution of fine pattern can be printed if the contact angle data are applied on reverse offset printing. In this paper, 10 different materials are used for every stage and roll in reverse offset printing, and the optimum sequence is determined through the result of contact angle for 5 µm pattern

2 A. Contact Angle Measurement II. EXPERIMENTS The contact angle between silver ink (silverjet DGH ink for Reverse offset, Advanced Nano Products Co., Ltd.) and 10 different materials (M1, M2, M3, M4, Glass Cliché, PET, Katpon PV 9101 (Polyimide, DuPont Inc.), Kapton PV 9102 (Polyimide, DuPont Inc.), Silicon wafer (Sigma Aldrich), Glass (Fisher Scientific)) is measured using a goniometer with DROP image Advanced (Model 250 Std G/T: rame-hart). The sessile drop method is used for measuring the contact angle, and the results of contact angle are mean values based on three redundant experiments. Each droplet is 10 µl. B. Reverse Offset Printing Figure 1 is a schematic of the reverse offset printing process. As shown in Figure 1, the ink transfer substrate, which is spin-coated in silver ink (18 seconds in 6,000 rpm), is placed on the first stage. The silver ink is then transferred to Cliché on the second stage by roll and the embossed parts on the Cliché takes silver ink from the roll. Then the remaining ink on the roll (pattern) is printed on the final substrate in the third stage. Pressure and speed of the rolls are maintained at 0.5 N and 2 cm/s respectively. Fig. 1 Schematic image of reverse offset printing process (IPen Co.) C. Optical Microscope, Scanning Electron Microscope and Atomic Force Microscope (AFM) An optical microscope (SV-35, SOMETECH Co.) was used for taking the pattern images at low magnification ( 100), and a scanning electron microscope (TM-1000 Tabletop Microscope, HITACHI) was used for identifying the width of the patterns at high magnification ( 10,000). Additionally, the heights of the patterns were measured using an atomic force microscope (AFM, NT-MDT Co.). A. Contact Angle III. RESULT AND DISCUSSION Figure 2 is the result of the contact angle between the silver ink and the 10 substrate materials. Among the materials, M4 had the biggest contact angle between the silver ink (θ=30.5 ) while PET, Kapton PV9101, and Kapton PV9102 have the smallest contact angle (θ=0 ). Thus, the silver ink adsorbed more into the PET, Kapton PV9101, and Kapton PV9102 than the M4 material. Fig. 2 Image and data of contact angle between silver ink and 10 different materials

3 As mentioned above, the silver ink was transferred from Stage 1 to Stage 3 in sequence through the roll. During the roll with ink contact on cliché directly, ink remains on the embossed part of the cliché, and the other ink remains on the roll as fine pattern. In other words, the contact angle of cliché should be smaller than the roll when the two materials contact directly. Also, the contact angle of the final substrate should be smaller than the roll when printing the fine pattern on final substrate from the roll. Finally, the following sequence of contact angle (Eq. 1) should be applied on the reverse offset system to get a high resolution of the fine pattern. According to Equation 1, M4 is placed on Stage 1 and M1, M2, and M3 are selected as material for the roll. Cliché should be placed on Stage 2 and the other materials, glass, silicon wafer, PET, Kapton PV9101, and Kapton PV9102, are placed on Stage 3 for final substrate. Following this setting, Figure 3 is the result of contact angle differential between roll and each stage. θ (Stage 1) > θ (Roll) > θ (Stage 2), θ (Stage 3) (1) When M2 is used for the roll, the differential contact angle between the roll and Stage 1(M4) is 2.1, and the differential between the roll and Stage 2 (glass cliché) is For the roll and Stage 3, the differential is 19.4 between the roll/glass and 21.1 between the roll/silicon wafer. And the differential is 28.5 between the roll/pet, Kapton PV9101, and Kapton PV9102 respectively. When M3 is used for the roll, the contact angle differential between the roll/stage 1 is 5.4, and roll/stage 2 is Also, the differential between roll/glass and roll/silicon wafer are 16.1 and 17.7 respectively. For PET, Kapton PV9101, and Kapton PV9102 substrates, 25.1 is the contact angle differential between the roll. When M1 is used for the roll, the contact angle differential between roll/stage 1 is 6.4, and roll/stage 2 is Also, the differential between roll/glass and roll/silicon wafer are 15.1 and 16.7 respectively. For PET, Kapton PV9101, and Kapton PV9102, has 13.4 as contact angle differential between the roll. Fig. 3 Contact angle differentials between materials for roll and materials for each stage B. Ink Transfer Mechanism Focused on Roll/Stage 2 (Cliché) Figure 4 is the resulting 5 µm patterns according to the different material types of roll (M2, M3, and M1) and the final substrates, which are glass, silicon wafer, PET, Kapton PV9101, and Kapton PV9102. (a)-(e) in the figure are the results of the 5µm pattern when M2 (θ=28.5 ) is used for the roll. As shown, no patterns were printed on the final glass, silicon wafer, PET, Kapton PV9101, and Kapton PV9102 substrates. During the printing, most of silver ink remained on Stage 1 (M4) due to the low contact angle differential ( θ=2.1 ) between Stage 1 (M4; Wa=46.7 mn/m) and the roll (M2). Also, during the transfer of the silver ink from roll to glass cliché, most of ink remained on the glass cliché due to the large contact angle differential between the roll and glass cliché( θ=14.1 ). Therefore, no patterns can be seen on the final substrates. (f)-(j) in Figure 4 are the results of the 5 µm pattern when M3 (θ=25.1 ) was used for the roll. As shown in the figures, no pattern was printed on the silicon wafer. Parts of the 5 um pattern was printed on the glass, PET, Kapton PV9101, and Kapton PV9102, but the patterns are not continuous. The contact angle differential between Stage 1 (M4) and the roll (M3) was bigger than when M2 was used for the roll ( θ=5.4 ), so the transfer from Stage 1 to roll proceeded without any problems. However, during the transfer of the silver ink from the roll to glass cliché, most of the ink remained on the glass cliché due to the large contact angle differential between the roll and glass cliché ( θ =11.0 ). Therefore, the contact angle differential between the roll and glass cliché was not appropriate to produce a continuous, high-resolution pattern. (k)-(o) in Figure 4 are the results of the 5 µm pattern when M1 (θ=24.1 ) was used for the roll. As shown in the figure, the final patterns are cleaner and more continuous than (f)-(j), with the exception of the glass and silicon wafer. When M1 was

4 used for the roll, the proper contact angle differential ( θ=10.1 ) allowed most of the silver ink to be absorbed by the embossed part of the glass cliché, and the rest of the silver ink (the fine pattern) remained on the roll during that stage. The result patterns on the glass and silicon wafer are not continuous, with considerable residual ink left on the silicon wafer. However, the 5 µm patterns on the PET, Kapton PV9101, and KaptonPV9102 substrates, which have a higher contact angle than the two previously mentioned substrates, are clean and continuous. Therefore, the PET, Kapton PV9101, and Kapton PV9102, which have a small contact angle differential between the roll, are proved to be promising materials for producing a precise and consistent 5 µm pattern. Fig. 5 5µm patterns ( 100 magnification) using M2: (a) Glass (b) Silicon wafer (c) PET, (d) Kapton PV 9101 (e) Kapton PV9102,M3: (f) Glass (g) Silicon wafer (h) PET, (i) Kapton PV 9101 (j) Kapton PV9102, and M1: (k) Glass (l) Silicon wafer (m) PET, (n) Kapton PV 9101 (o) Kapton PV9102PET for Roll in Reverse offset system. C. Ink Transfer Mechanism Focused on Roll/Stage 3 (Final Substrate) The 5 µm pattern on the three different substrates (PET, Kapton PV9101, Kapton PV9102) received further analysis due to their continuity. Figure 5 shows the results of the pattern for Figure 4 (m)-(o), at 10,000 magnification using the SEM, and the pattern height using the AFM. (a)-(c) in Figure 5 are the patterns for PET, Kapton PV9101, and Kapton PV9102 respectively when M1 was used for the roll with the pattern being 5 µm on the substrates. On the PET substrate, the pattern achieved a pattern resolution of 5 µm and 1.6 µm of uniform height, but residual silver ink around the pattern can be observed. On the Kapton PV9101 material, the pattern has a relatively lower resolution than the patterns on PET and Kapton PV9101, but the pattern has 5 µm of uniform width without any residual silver ink and 1.2 µm of uniform height. On Kapton PV9102, the pattern has the highest resolution without any residual silver ink, but the height of the pattern is not uniform, which is 1.5 µm- 2.3 µm of height. Also the width of the pattern on Kapton PV9102 is 6 µm, which is the thickest width among the pattern on three different substrates

5 Fig. 5 Result of 5 um pattern on three different substrates when M1 is used for roll (Top: SEM 10,000 magnifications, middle: 3D results of AFM (a) PET, (b) Kapton PV 9101, and (c) Kapton PV 910, bottom: 2D height results of AFM Therefore, when M1 is used for the roll, PET, Kapton PV9101 and Kapton PV9102 are suitable substrates for producing the most continuous, high resolution 5 µm pattern among the five different substrates. The glass and silicon wafer do not seem appropriate as these substrates have a higher contact angle than the PET, Kapton PV9101, and Kapton PV9102. Among PET, Kapton PV9101 and Kapton PV9102, the conductive pattern on Kapton PV9101 had the most uniform width and height. Therefore, Kapton PV9101 is the most suitable substrate for producing a high-resolution, fine pattern using this printing method. All three steps are important to produce a high-resolution, fine pattern, but this paper concludes that the optimization of contact angle differential between roll and Stage 3 is the most important factor. Even though the contact angle differential between roll/stage 1 and roll/stage 2 is optimized, the final pattern could not be printed on substrate if the contact angle differential between roll/stage 3 is not optimized as image of (k) and (l) Figure 4 that is result of pattern on glass and silicon wafer. IV. CONCLUSION Because the high pressure of direct printing process causes fracture or deformation of final substrate and glass cliché in reverse offset of R2P, this paper shows the optimizing process of reverse offset printing by controlling the sequence and range of contact angle at the low pressure (0.5 N) to print a continuous and high-resolution 5 µm pattern. The results of contact angle are applied to the reverse offset system based on optimized sequences like Equation 1. The silver ink could be transferred from Stage 1 to roll when the contact angle differential between roll/stage 1 is bigger than 5.4 (M2) or smaller than 6.4 (M1). Also the roll could take ink from the cliché to form the high resolution pattern when the contact angle differential between the roll/glass cliché is smaller than At the final stage, the continuous high-resolution, fine pattern could not be printed on silicon wafer and glass, which have a 9.3 and 8.3 of contact angle differential respectively, although the differential between roll/glass cliché is optimized. On the other hand, the continuous 5 µm pattern could be printed on PET, Kapton PV9101, and Kapton PV9102, which have 13.4 of contact angle differential. Therefore, the contact angle differential between the roll and each stage should be optimized as 5.4 θ(roll/stage1) 6.4, θ(roll/stage2) 10.1, and θ(roll/stage 3) Based on the contact angle differential, the final 5 µm is printed with a uniform height and high resolution when M1 is used for roll and Kapton PV9101 is used for final substrate. Likewise, the continuous and high resolution of 5 µm pattern could be printed at the low pressure by changing the sequence and range of contact angle in order to supplement fracture of material. Therefore, the higher performance of solar cell and touch screen panel with high resolution could be produced if the sequence and range of contact angel is matched as final conclusion

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