Direct Micro Metal Patterning on Plastic Substrates by Electrohydrodynamic Jet Printing for Flexible Electronic Applications

Direct printing is an attractive technology for the patterning of nanomaterials that enables low-cost fabrication of micro/nano-sized electrical devices through minimal use of materials with non-vacuum environment. Although direct printing opens new border for the patterning technology, its resolution has been highly restricted mainly due to its solution-based properties. When the solution contains much liquid, it spreads out on the substrate after printing. While less liquid solution causes clogging at the oriﬁce. In this study, electrohydrodynamic (EHD) jet printer is employed to use both low and high content of nanomaterial solutions: low viscosity nanoink and high viscosity nanopaste. Meanwhile, there are several considerable factors including ink property for operating EHD jet printer such as substrates, voltage condition and working speed. Therefore, parametric studies are conducted to gain ﬁne patterning. As a results, EHD jet printed silver conductive line at high resolution is achieved on various plastic substrates for ﬂexible device applications.©TheAuthor(s) 2015.

As an alternative to the conventional photolithography in the microelectronic manufacturing process, direct printing has been widely interested. The merit of the direct printing technology is known as on-demand digital material deposition & patterning, which leads to process cost reduction, use of no vacuum and mask, compatibility with various substrates and minimization of material wastes. [1][2][3][4] During last few decades, various direct printing technologies including roll-toroll (R2R), gravure printing, screen printing, dip-pen, inkjet printing and three dimensional printing have been developed for electronics patterning of sensor, light emitting diode (LED), radio frequency (RF) tags, flexible display and other micro/nano-sized components. [5][6][7][8][9][10][11] Recently, nanomaterials have been used in various applications due to its particular electrical, chemical, optical and physical properties. [12][13][14][15] Therefore, a lot of efforts have been devoted for implementation of the nanomaterials into industrial manufacturing processes. Especially in direct printing, metal nanoparticles such as gold, silver and copper are often required to be synthesized in a form of nanoink, paste or suspension since the nano-sized metal materials provide a significant benefit: low annealing temperature owing to its high surface to volume ratio. [16][17][18][19] Nevertheless, direct printing techniques with nanomaterial solutions have several inherent drawbacks such as coffee stain effect, unclear edge sharpness and limitation on the minimum line width (typically >50 μm) because the process and features are strongly dependent on solution property. [20][21][22] For example, inkjet printing usually uses only low viscosity solution to successfully generate droplet and prevent the nozzle clogging. When volume of the droplet gets smaller, droplet flows up to the surface of the capillary nozzle due to surface tension of the solution and its capillary force. Subsequently, undesirable printing such as separated line from satellite droplets, irregular-sized dot printings are observed. At the same time, printing process using micro volume of droplet leads slower working speed, subsequently, results in lower throughputs.
In this study, fine metal patterning with metal nanoink solution is achieved by electrohydrodynamic (EHD) jet printer at high working speed through the combinations of metal nanoparticle ink at high content and micro-sized nozzle. The EHD jet printer enables much z E-mail: paul@kimm.re.kr; maxko@snu.ac.kr smaller feature size of printing through the use of high viscosity solution (>20,000 cPs) as it is specialized in operating of high electrical field that makes micro-sized volume of droplet printed to the substrate from the tip of capillary nozzle. [23][24][25] In the previous studies, individual dot printing, 26 sophisticated process, 27 high annealing temperature with hydrogen gas condition, 28 and printed only on the rigid substrate [26][27][28][29] are conducted. However, fine line patterning is obtained in this study through extensive study for an optimum printing conditions. Furthermore, parametric studies are carried out to determine the comprehensive relationship between working speed and voltage condition on various substrates. As a result, micro metal patterning is successively accomplished on various plastic substrates without aid of conventional photolithography. Finally, 12 × 25 transistor source/drain pads were successfully printed for the large-scale flexible electronic applications.
EHD printing.-Electohydrodynamic (eNano printer) jet printer system produced by EnJet was used in this study. Schematic 1 depicts the EHD system which are mainly composed of three parts; XYZ stage movement, syringe pump controller and AC/DC power supplier. Moving speed of the XY stage varied from 10 3 to 10 5 μm/s. The default distance of Z axis from the XY stage set for 100-150 μm. The stage accuracy is measured ±3-5 μm on XY axis and ±7-10 μm on Z axis. Temperature of the stage maintains at 50 • C for The glass capillary nozzle fabricated by glass pipette puling machine (Shutter Instrument, P-97) for 25 μm, 50 μm and 100 μm of diameter. 25 μm sized glass capillary is adjusted for low viscosity Ag nanoink only due to clogging problem for the high viscosity Ag nanopaste. The printed substrate is finalized with convection oven at 90 • C for 10 min.
There are several critical factors for operating EHD jet printer. First of all, EHD jet printer uses strong electric field that is governed by electrical columbic interaction. Thus, it is crucial to establish and maintain an appropriate voltage condition for both AC/DC, switching period and spacing between the XY stage and the tip of the capillary needle. Otherwise, there is a possibility to initiate an undesirable electric spark or spray-like jetting. Another important parameter is XY stage movement speed. When the stage movement is operated at extremely fast speed, dot patterns are produced whereas wide lines are formed by slow stage movement. Accordingly, balanced injection rate is also a key element for proper printing. Too low injection rate leads to insufficient solution transport that results in disconnected line whereas excessive injection rate causes an over flow which brings unexpected large drops. Fourth, sustaining moderate temperature of the stage is another necessary aspect for printing quality. High temperature of stage accelerates solution vaporization for uniform printing as well as nozzle clogging. Finally, printing quality largely depends on the matching between nanomaterial solution and substrate. Therefore, finding an optimum condition is essential for successful EHD jet printer operation.

Results and Discussion
For examination of standard printing features, Ag nanoink and Ag nanopaste are initially printed onto a typical slide glass substrate and a PVP-coated glass. In common cases, silver layer has poor adhesion on the glass and easily exfoliated out by small friction, whereas the PVP layer exhibits better adhesion between silver and glass substrate. Prior to print onto glass substrate, plasma treatment is carried out under ambient condition in order to obtain uncontaminated and clean surface by removing particles and organic contaminants. However, this treatment causes an unnecessary hydrophilic surface which contributes to widening and spreading of printed nanoink. Fig. 1 represents the results of printed line pattern onto conventional slide glass and PVP-coated glass by using low and high viscosity nanoink. For the low viscosity Ag nanoink, line width roughness (LWR) was increased up to 30-90 μm as shown in Fig. 1a. It is obvious that ink solution flowed over the substrate after jetting owing to hydrophilic property. In contrast, Fig. 1b exhibits relatively straight line width of 75-80 μm is printed on PVP-coated glass. When high viscosity Ag nanopaste is used, straight but wide lines of 110 μm in width on the slide glass substrate and narrower lines of 40-50 μm  in width are attained on the slide glass and PVP-coated glass respectively, shown in Fig. 1c and 1d. Both cases show that printed lines are much wider than the nozzle diameter. In here, PVP coating provides adhesion layer as well as sufficient surface roughness to prevent spreading of the nanoink.
EHD printing on various plastic substrates is demonstrated for further flexible electronics applications. Fig. 2 displays printed line patterns on the PET and PEN film. Fig. 2a shows slightly twisted line width of 70-75 μm that represents the ink drops were partly influenced by induced AC pulse switching during EHD printing. Besides, unclear edge and marginally imbalance line was produced by Ag nanoink on the PEN film where line width of 40-45 μm in Fig.  2b. Printed line width of 50-55 μm on PET film by Ag nanopaste shown in Fig. 2c presents decent printing quality of line patterning with moderate reproducibility. Similar result is obtained on the PEN film for the line width of 20-25 μm in Fig. 2d. These results indicate that the plastic substrate is better than other glass substrates in terms of line patterning. Furthermore, printing tendency between two materials each having different viscosity exhibits minor differences. When individual droplets of the low viscosity Ag nanoink are continuously dropped from the nozzle, the droplets eventually form a connected line. These tiny drops are influenced by nearby the electrical pulse switching which result in a winding line. Also, the line width is always larger than nozzle diameter. On the other hand, when uninterrupted sticky stream such as high viscosity Ag nanopaste is directly transferred to the target substrate, straight and relatively uniform lines are produced. In this case, the line width is typically the same as or even smaller than the nozzle diameter, on the contrary to nanoink. Fig. 3 illustrates the relationship between induced AC voltage for EHD printing and corresponding line width varied from at different nanomaterial solution and substrates. For the reliability, at least, 10 times of the experiments with the same condition are carried out. In 2) possibly due to EHD uses the electrical field that pulls the solution to drop off. 30 Because of the ink solution contains more liquid content that is easier being pulled out from the capillary nozzle, paste initially requires much more voltage than that of ink for jetting (average >0.3 kV). [31][32] The line width can be controlled not only by the induced voltage but moving speed of XY stage (working speed). Fig. 4 shows the relationship between working speed and its corresponding line width using Ag nanopaste on the plastic substrates. The experiment is performed to print a simple maze pattern, then one point of the printed line is randomly selected and measured. This single experiment is repeated for more than 10 times to obtain consistency result. Working speed varied from 2 × 10 3 to 5 × 10 4 μm/sec for PVP, 10 3 to 2 × 10 4 μm/sec for PET and 5 × 10 3 to 5 × 10 4 μm/sec for PEN, respectively. When working speed slower than 5 × 10 3 in PVP, 10 3 in PET and 5 × 10 3 in PEN, wider line width (over 100 μm) occurs. On the contrary, faster working speed at 5 × 10 4 in PVP, 2 × 10 4 in PET and 5 × 10 4 in PEN causes dot printing or disconnected lines with the Ag nanopaste. The size of printed lines can be minimized down to approximately 45 μm on the PVP layer with 5 × 10 4 μm/sec, 50 μm on the PET film with 10 4 μm/sec and 20 μm on the PEN with 5 × 10 5 μm/sec, respectively. However, on the PVP and PET substrate, unsta-   ble line width was observed when the printed width approaches to the ∼45 μm and ∼50 μm. Therefore, a stabilized condition for the reproducible printed line width on the plastic substrate is ∼50 μm on PVP, ∼55 μm on PET and ∼25 μm on PEN. The results are summarized and presented in Table I. The printed line on the PEN film shows narrower line width than other plastic substrate possibly due to the surface energy. The surface energy of the polymer (PVP: 45-46, PET: 42-44, and PEN: [30][31][32][33] indicates PEN has significant smaller surface energy than PVP and PET. 33 Typically, high surface energy between liquid (solution) and solid (substrate) brings more wetting which results in low contact angle, hydrophilic surface. Therefore, printed line on the PEN film tends to achieve narrow line width compared to the other plastic substrate. For the pattern printing, PET film was employed to demonstrate the large-scale productivity as well as practical allowance such as the film price and moderate working speed for manufacturing. Fig. 5 represents printed 12 × 25 pad array on the PET film for flexible application. The single size of printed source/drain pad is approximately 0.97 mm × 0.27 mm. Only few lines (less than 4%) had poor printing quality that were line connected due to wrong printing conditions like ink spreading after printing. Final printed patterns have resistivity approximately 21.1 μOhm · cm which comparable with bulk silver and the patterns by EHD was successfully demonstrated for the further applications at the development stage.

Conclusions
In this study, conductive line patterning by EHD printing system is demonstrated for the flexible electronics applications. The printing characteristics are strongly influenced by substrate, ink property, induced voltage and working speed. Low and high viscosity Ag nanomaterial solution is selected and tested on the various substrates. Unlike normal cases, hydrophilic surface such as plasma treated glass leads a negative effect due to the ink spreading. Low viscosity nanoink re-quires lower AC voltage for EHD printing, but results in wider line width than high viscosity nanopaste. Thus, nanopaste with high viscosity helps to reduce coffee-stain problem and to gain fine printing. Likewise, fast working speed brings uniform and fine printing. As results, for the Ag nanopaste, approximately 20 μm of minimum line width is achieved on the PEN film. While at the same time, 50 μm of minimum line width is produced on PET film. For the large-scale production, condition for 55 μm line patterning on the PET is selected and 12 × 25 array of conductive lines are printed at ambient condition.