Multi-nozzle electrohydrodynamic inkjet printing of ... - Springer Link

Appl Phys A (2011) 104:1113–1120 DOI 10.1007/s00339-011-6386-0

Multi-nozzle electrohydrodynamic inkjet printing of silver colloidal solution for the fabrication of electrically functional microstructures Arshad Khan · Khalid Rahman · Myung-Taek Hyun · Dong-Soo Kim · Kyung-Hyun Choi

Received: 3 January 2011 / Accepted: 17 March 2011 / Published online: 7 April 2011 © Springer-Verlag 2011

Abstract In this paper, multi-nozzle electrohydrodynamic (EHD) inkjet printing of a colloidal solution containing silver nanoparticles in a fully controlled fashion is reported. For minimizing interaction, i.e. cross-talk, between neighboring jets, the distance between the nozzles was optimized numerically by investigating the magnitude of the electric field strength around the tip of each nozzle. A multinozzle EHD inkjet printing head consisting of three nozzles was fabricated and successfully tested by simultaneously printing electrically conductive lines of a colloidal solution containing silver nanoparticles onto a glass substrate. The printed results show electrical resistivity of 5.05 × 10−8 m, which is almost three times larger than that of bulk silver. These conductive microtracks demonstrate the feasibility of the multi-nozzle EHD inkjet printing process for industrial fabrication of microelectronic devices.

A. Khan · K. Rahman · M.-T. Hyun · K.-H. Choi () School of Mechanical Engineering, Jeju National University, Jeju 690-756, South Korea e-mail: [email protected] Fax: +82-64-7523174 A. Khan e-mail: [email protected] K. Rahman e-mail: [email protected] M.-T. Hyun e-mail: [email protected] D.-S. Kim Korea Institute of Machinery and Materials (KIMM), 104, Sinseongno, Yuseong-Gu, Daejeon 305-343, South Korea e-mail: [email protected]

1 Introduction Development of inkjet printing technology for rapid prototyping of electronic microstructures or patterns has attracted much attention in recent years because of its low cost, noncontact mask-free deposition of a wide range of materials on a variety of substrates [1, 2]. A number of inkjet printers based on different actuation mechanisms like thermal, piezoelectric and aerosol are widely used in the electronics industry [3]. To maintain high printing throughput in these printers, a multi-nozzle printing process has been investigated by many researchers over the years. Multi-nozzle piezoelectric inkjet printing heads typically consisting of tens, hundreds or even thousands [4] of separate nozzles were fabricated and successfully used for microelectronic device fabrication. In order to decrease the required number of piezoelectric elements in these denser inkjet arrays, bulk piezoelectric materials were used which covered large areas and also achieved enough actuation strength for uniform drop generation [5]. Wang and Bokor fabricated a multinozzle thermal inkjet printing head by silicon micromachining technology using a dense array of thermal bubble inkjet devices made on a single silicon wafer [6]. Optomec, Inc. developed a commercially available multi-nozzle aerosol inkjet printing head in order to meet the needs of photovoltaic manufacturers. The printing head is closely coupled with a series of atomizers to insure efficient distribution of inks through the print head manifold [7]. The above-mentioned inkjet printers are already commercialized; however, due to various intrinsic problems of nozzle blockage and overheating of organic materials in these existing printing devices, a new direct printing process based on electrohydrodynamic (EHD) atomization is the focal research topic for many industrial and academic researchers [8]. The EHD inkjet printing process is considered

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to be a promising alternative to piezoelectric and thermal based inkjet printing techniques because of its unique feature of generating very small jets or droplets as compared to the nozzle orifice [9]. In the EHD inkjet printing technique, a colloidal solution consisting of solid nanoparticles dispersed in a solvent is supplied through a nozzle where it is charged to a sufficiently high electrical potential, so that its meniscus forms the shape of a stable cone known as a Taylor cone [10], whose pinnacle emits a much smaller jet as compared to the delivery nozzle. The generated colloidal jet is then impinged onto a substrate where only the nanoparticles remain after the sintering process [11]. Several researchers demonstrated the feasibility of the EHD inkjet printing process in manufacturing of microelectronic devices by direct fabrication of high-resolution printed metal interconnect electrodes [9], collectors for printed solar cells [12] and electrodes for thin-film transistors (TFTs) [13, 14] with critical dimensions much smaller than that of the printing nozzle. Though direct deposition by the EHD inkjet printing process offers various advantages in fine patterning, the low production speed, i.e. low throughput, of EHD inkjet printing is a severe drawback that has hampered its possible widespread applications in the electronics industry. In order to surmount this drawback and attain a high production efficiency EHD inkjet printing process for industrial production of printed displays, printed circuit boards, printed TFTs and printed solar cells, a multi-nozzle EHD inkjet printing process has been primarily investigated by a few scientists [15–18]. Most of these studies have focused on making a stable cone jet and reducing the positioning error of the ejected jet. However, due to the interaction (cross-talk) between the electrically charged neighboring jets, a well-controlled and reproducible multi-nozzle EHD inkjet printing process has not been reported. Silver is one of the most important materials in electronic microstructures because of its high conductivity, chemical stability and resistance to surface oxidation [19]. A number of research groups have focused on direct printing of a colloidal solution containing silver nanoparticles for fabrication of electrically functional microstructures [20, 21] and microtracks [22–24] using a single-nozzle EHD inkjet printing head. However, to our knowledge, printing of silver nanoparticles by a multi-nozzle EHD inkjet printing process has not been reported in the literature. In this study, to overcome the limitation of low throughput of the EHD inkjet printing process, a multi-nozzle EHD inkjet printing head consisting of three nozzles is fabricated and successfully tested by printing simultaneously conductive lines of silver colloidal ink onto a glass substrate. The distance between the nozzles was optimized numerically by investigating the symmetry of the electric field strength around the tip of each nozzle. Generation of an axisymmetric Taylor cone at the tip of each nozzle and the deposited

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patterns demonstrate a well-controlled multi-nozzle EHD inkjet printing process. The low electrical resistivity of conductive microtracks illustrates the feasibility of the multinozzle EHD inkjet printing process for the industrial fabrication of microelectronic devices.

2 Experimental details 2.1 Fabrication of multi-nozzle EHD inkjet printing head The multi-nozzle EHD inkjet printing head presented in this work consists of three parts, i.e. a poly-di-methyl siloxane (PDMS) holder, glass capillaries and copper electrodes. The procedure followed for individual part fabrication and assembling of these parts into a complete multi-nozzle EHD inkjet printing head is given below. The PDMS holder was fabricated in-house by using the standard soft lithography technique. The selection of PDMS for the capillaries’ holder is primarily motivated by its low cost, dielectric disposition, hydrophobic nature, desirable optical properties and low thermal expansion constant. Figure 1a shows the simplified fabrication steps involved in mold preparation. Three 4-mm-long and three 3-mm-long stainless steel circular rods each having 1.5-mm outer diameter were partially inserted through small drilled holes into two rectangular poly- methyl meth-acrylate (PMMA) plates of sizes 15.5 mm × 6.5 mm and 14.5 mm × 5.5 mm, respectively. The plates (with inserted rods) were then assembled to form an open-shaped mold. The PDMS mixture consisted of a Sylgard 184 silicone elastomer base and a curing agent (Dow Corning, Midland) that were mixed in a ratio of ten parts base to one part curing agent by mass. The prepolymer mixture was poured onto the mold and degassed at 5–10 Pa in a desiccator with a mechanical vacuum pump (KH201, Koino) for 1 h to remove any air bubbles in the mixture and to insure complete mixing of the two parts. The PDMS prepolymer was then cured for 1 h at 100°C on a hot plate. After curing, the PDMS replicas were peeled off from the mold leaving three L-shaped (having three 1.5-mm inlet and three 1.5-mm outlet) flow-ready channels. Since the stainless steel rods used to mold the channels were very smooth, the process of removing them from the cured PDMS did not introduce imperfections from tearing or rubbing. The schematic of the final PDMS holder is shown in Fig. 1b. Three glass capillary tubes of 1.5-mm outer diameter and 0.75-mm inner diameter (BF 150, Sutter Instrument) were pulled and micronozzles were formed, each with a sharp tip of 100-µm inner diameter and ≈140–170 µm outer diameter by using a micropipette puller (P-97, Sutter Instrument). These glass capillaries were then inserted in the outlet channels of the PDMS holder. Finally, three copper electrodes having an outer diameter of 500 µm were inserted from the top of the PDMS holder. The complete schematic of the multi-nozzle EHD inkjet printing head is shown in Fig. 1c.

Multi-nozzle EHD inkjet printing of silver colloidal ink for the fabrication of electrically functional microstructures

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Fig. 1 Schematic illustration of the multi-nozzle EHD inkjet printing head fabrication process: (a) Simplified fabrication steps of mold preparation, (b) resulting PDMS holder having L-shaped channels for

ink supply and clasping of glass capillaries, (c) complete multi-nozzle EHD inkjet printing head

2.2 Experimental set-up

Table 1 Physical properties of colloidal ink used for experiment Property

Figure 2a and b show a schematic diagram and a photograph of the laboratory-developed E-jet printing rig, respectively. The multi-nozzle EHD printing head was held in a holder made of acrylic. In order to supply voltage to each nozzle, all the three nozzles were connected to a voltage distributor (HM10-5, Hanmac) through copper electrodes which were further connected to a high-voltage power supply (10/40 A, Trek). The inlet channels of the PDMS holder were connected to a multi-channel pump (IP-RS232, Ismatec) through Teflon tubes for individual ink supply to each nozzle. A copper plate was placed on an X–Y moving stage 800 µm below the nozzle exits as ground/counter electrode. A glass substrate having 500-µm thickness was placed on top of the ground plate. The behavior of the jetting was monitored with the help of a high-speed camera (×3, 11×, 5000 fps, Motion Pro). A high-voltage power supply, multi-channel pump, high-speed camera, moving stage (for substrate motion) and the head holder (for adjustment of the distance between nozzles and ground plate) were connected through a hardware system (PXI-1042Q, National Instruments), controlled by laboratory-developed software based on LabVIEW. Moreover, the amplitude of the applied voltage was also controlled with the help of this customized software.

Value

Resistivity ( cm)

1.051E+5

Viscosity (Pa s)

0.039

Surface tension (N/m)

0.005

Silver nanoparticle weight (%)

39

Average particle size (nm)

30

For deposition, a commercial colloidal solution (properties are shown in Table 1) containing silver nanoparticles (npk-020, NPK.CO) was used. After deposition the printed samples were subsequently cured at 250°C for 1 h. The width of the printed lines was analyzed by an optical microscope (BX 41, Olympus). Surface characterization of the printed lines was performed using X-ray diffraction (D/MAX 2200 PC, Rigaku) and a scanning electron microscope (JSM-7600F, Jeol) while the thickness (crosssectional areas) of the printed lines was measured by an atomic force microscope (AFM-100, EM4SYS). The electrical properties of the printed lines were characterized by a home-made four-point probe method using a nanovoltmeter (2182A, Keithley) and a current source (6221, Keithley).

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Fig. 2 (a) Schematic of experimental set-up. (b) Photograph of EHD inkjet printing rig used for experiments

(a)

(b)

3 Results and discussion 3.1 Axisymmetric microjet generation In EHD inkjet printing, the jet emanates from that point of the liquid meniscus where the electric charge density, i.e. the electric field strength, has the highest magnitude [8]. In case of single-nozzle EHD inkjet printing the jet emanates from the center point of the meniscus, since it is the point of maximum electric field strength. However, in case of multinozzle EHD inkjet printing, due to the cross-talk, i.e. repulsive force, between the neighboring jets, the point of highest electric field strength becomes slightly off center especially at peripheral nozzles, resulting in emission of outward-tilted jets [25]. There are two factors that have a dominant effect on the cross-talk between the neighboring jets. One is the distance between the adjacent nozzles of the EHD inkjet printing head; in general, the cross-talk increases with the decrease in the distance between the adjacent nozzles [26], and the other is the material composition of the printing nozzles. Generally, the use of metallic nozzles strengthens the

repulsive force between the neighboring jets as compared to nozzles made of a dielectric material [17]. To minimize the cross-talk between the neighboring jets, the multi-nozzle inkjet printing head was completely fabricated from dielectric materials, i.e. glass and PDMS. Moreover, to find the minimum distance at which the adjacent jets experience no cross-talk, numerically simulated values of electric field strength at various points of the meniscus of each nozzle have been investigated by commercially available finite element software COMSOL (3.5a, Comsol Inc.). Figure 3a shows the schematic of the Finite Element Analysis (FEA) model used for electric field simulation of the multi-nozzle EHD inkjet printing head. Six FEA models were developed having the same configuration (Fig. 3a) but with different nozzle-to-nozzle distances, i.e. 500 µm, 1 mm, 1.5 mm, 2 mm, 2.5 mm and 3 mm. Simulations were run by applying a dc voltage of 3.5 kV at the boundaries of each electrode. The parameters used for simulation are summarized in Table 2. Numerical values of electric field strength at seven different points, i.e. A, B, C, D, E, F and G (as shown


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Fig. 3 (a) Schematic of the FEA model used for electric field simulation. (b) Simulation in COMSOL 3.5a of the electric field lines for the case of 3-mm nozzle-to-nozzle distance

(a)

(b)

Table 2 Details of electric field simulation parameters used for analysis Parameter

Type/Value

Model

2D electrostatic

Solver

UMFPACK

Mesh

Triangular

Number of mesh elements

57184

Number of boundary elements

2508

Average element area ratio

2.9E-5

Relative tolerance

1.0E-6

in Fig. 3a), around the meniscus of each nozzle have been calculated for each model. It is observed from the electric field simulation results that for all models having nozzleto-nozzle distances less than 3 mm, the point of maximum electric field strength is not at the center of the meniscus for peripheral nozzles. For comparison, the resulting values of electric field strength around the tip of each nozzle of 2.5mm nozzle-to-nozzle distance and 3-mm nozzle-to-nozzle

distance are listed in Tables 3 and 4, respectively. The data in Table 3 shows that at the meniscus of nozzle 1, the electric field strength has maximum value at point ‘C’ while at the meniscus of nozzle 3, the electric field strength has maximum value at point ‘E’. Since both point ‘C’ and point ‘E’ are not the central points of the meniscus, the emanated jets will be off center and will fall away from the targeted position on the substrate. On the other hand, it is evident from the data in Table 4 that in case of 3-mm nozzle-to-nozzle distance the electric field strength has maximum value at point ‘D’ (center point) of each nozzle, which means that the jets will emanate from the center of the meniscus of each nozzle. The resulting electric field values for the case of 3-mm nozzle-to-nozzle distance are also shown in Fig. 3b. Following the simulated results, the multi-nozzle printing head consisting of three nozzles and having 3-mm nozzle-to-nozzle distance has been fabricated. Due to dielectric properties of glass and optimized nozzle-to-nozzle distance (3 mm), cross-talk between the adjacent jets had not been observed. Figure 4 shows high speed camera images of a stable meniscus formed at the tip

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Table 3 Simulated electric field (kV/m) values around the meniscus of each nozzle at different points at an applied voltage of 3.5 kV and nozzle-to-nozzle gap of 2.5 mm Nozzle/Point

A

B

C

D

E

F

G

Nozzle 1

315

530

740

610

570

325

240

Nozzle 2

230

295

510

700

510

290

230

Nozzle 3

235

320

550

600

735

520

310

Table 4 Simulated electric field (kV/m) values around the meniscus of each nozzle at different points at an applied voltage of 3.5 kV and nozzle-to-nozzle gap of 3 mm Nozzle/Point

A

B

C

D

E

F

G

Nozzle 1

260

320

550

730

545

310

245

Nozzle 2

240

300

520

710

520

295

240

Nozzle 3

250

315

540

735

550

320

260

(a)

Fig. 4 Photographs of axisymmetric liquid menisci established at the nozzle tips of multi-nozzle EHD inkjet printing head

of individual nozzles of the multi-nozzle EHD inkjet printing head. Axisymmetric Taylor cones were formed at the tip of each nozzle, i.e. nozzle 1, nozzle 2 and nozzle 3, whose pinnacle emitted very small jets (10 ± 2 µm) at an applied dc voltage and flow rate of 3.5 kV and 20 µl/h, respectively. 3.2 Printed results on glass substrate Controlling the morphology of the printed conductive pattern plays an important role to determine its electrical conductivity and mechanical adhesion property. Furthermore, the conductive patterns with a non-uniform surface are generally unsuitable for use in multilayer interconnect structures, since overlying different components are prone to pin holes due to the poor coverage of the numerous ridges and valleys [27]. Experimental parameters such as ink flow rate, applied voltage and traveling time of the jet, i.e. nozzle to substrate distance, have a great effect on the morphology of printed patterns. Therefore, these parameters need to be optimized in order to get printed lines of uniform microstructure [28]. Optimizing the experimental parameters, printing was performed by applying a dc voltage of 3.5 kV and a flow rate of 20 µl/h to each nozzle. The nozzle to substrate distance was set at 300 µm while the substrate speed was kept

(b) Fig. 5 (a) High-zoom static camera and optical microscope image of continuous silver lines printed on glass substrate by multi-nozzle EHD inkjet printing technique. The bottom inset shows the average line width of 140 µm. (b) X-ray diffraction (XRD) spectrum of the printed line. (c) SEM image of continuous silver track deposited by multi-nozzle EHD inkjet printing process. The bottom inset shows the three-dimensionally interconnected silver nanoparticles

constant at 10 mm/s. Figure 5a shows the high zoom static camera and optical microscope image of continuous silver tracks simultaneously printed by three nozzles on a glass substrate without any defects such as bulges or coffee-ring effects [29]. Since the substrate, i.e. glass, has a hydrophilic surface, the deposited jets were able to spread out so that


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(a)

(c) Fig. 5 (Continued)

the width of the lines became larger than the original sizes of the generated jets (10 ± 2 µm). The average line width of the printed lines is 140 µm with a standard deviation of 5 µm. Figure 5b shows the X-ray diffraction (XRD) spectrum of a typical continuous track printed by the multi-nozzle EHD inkjet printing process. The diffraction peaks correspond to the (111), (200), (220) and (311) planes, respectively. Since the phases in Fig. 5b show the existence of silver only, it is confirmed that the material of the printed line consisted of silver nanoparticles only. Moreover, the scanning electron microscopy (SEM) images of a typical printed line as shown in Fig. 5c also illustrate that the nanoparticles are three-dimensionally interconnected with each other, which favorably affects the electrical conductivity. 3.3 Electrical characterization of the printed lines After sintering at 250°C, for 1 h at a constant heating rate of 2°C/min, the resistance R of the printed silver lines was measured by using a standard four-point method. A linear ohmic I –V curve (shown in Fig. 6a) was obtained by measuring the voltage drop V across a 1.5-mm-long sample of printed line by applying a current I of different intensities (10 µA, 20 µA, 50 µA, 75 µA and 100 µA). The electrical resistivity ρ of the printed line was then calculated from the resistance R, the length L and the cross-sectional area A of the line using ρ = R • A/L. The cross-sectional area was determined by numerical integration of the measured profile from the AFM image of the printed line shown

(b) Fig. 6 (a) I –V curve of the printed lines. (b) AFM image and cross– sectional profile of a typical silver line printed by multi-nozzle EHD inkjet printing process

in Fig. 6b. The value of the resistivity was calculated to be 5.05 × 10−8 m, which is almost three times higher as compared to bulk silver resistivity at room temperature (1.59 × 10−8 m [30]). These electrically conductive silver tracks can be used in fabrication of many microelectronic applications such as metallization in printed circuit boards (PCBs) and radio frequency identification (RFID) tags, collectors for organic light emitting diodes (OLEDs) and so-

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lar cells and electrodes for thin-film-transistor (TFT) circuits etc.

4 Conclusions This work demonstrates well-controlled and reproducible multi-nozzle EHD inkjet printing of colloidal silver ink. In order to minimize the interaction, i.e. cross-talk, between the neighboring jets, the distance between the nozzles was optimized numerically by investigating the symmetry of the electric field strength around the tip of each nozzle. Uniform and straight colloidal jets of diameter 10 ± 2 µm have been generated from the tip of each nozzle; however, due to the hydrophilic nature of the glass substrate the generated jets spread after deposition resulting in larger sizes than the original sizes of the generated jets. The average width and resistivity of the printed tracks are 140 ± 5 µm and 5.05 × 10−8 m, respectively. The resulting conductive microtracks demonstrate the feasibility of multi-nozzle EHD inkjet printing in industrial fabrication of microelectronic devices. Acknowledgements This study is supported by the Ministry of Knowledge Economy, South Korea through the project ‘Strategic Technology Development Project’.

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