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EL device pad-printed on a curved surface

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2010 J. Micromech. Microeng. 20 015016

(http://iopscience.iop.org/0960-1317/20/1/015016)

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IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 20 (2010) 015016 (10pp) doi:10.1088/0960-1317/20/1/015016

EL device pad-printed on a curved surfaceTaik-Min Lee1, Shin Hur, Jae-Hyun Kim and Hyun-Cheol Choi

Nano Machinery Research Division, Korea Institute of Machinery & Materials, Daejeon, 305-343, Korea

E-mail: taikmin@kimm.re.kr

Received 29 September 2009, in final form 28 October 2009Published 2 December 2009Online at stacks.iop.org/JMM/20/015016

AbstractThis paper is unique in that the electro-luminescence (EL) display device is fabricated on acurved surface using the pad-printing method. The precision of the pad-printing process isexplored to verify whether it can be used for micro patterning. The minimum pattern size andpattern distortion, which is caused by use of the pad, were tested and simulated. The minimalpattern was found to be 35 μm wide and 2.4 μm thick. Pattern distortion when pad-printingon a flat surface, caused by the deformation of the silicon pad, was less than 5 μm. Numericalanalysis shows how to estimate pattern distortion when pad-printing on a curved surface. Theproposed EL display device consists of five layers, namely a bottom electrode, dielectric layer,phosphor, transparent electrode and a bus electrode. The ink of each layer was reformulatedwith solvents and the pad-printing conditions were controlled. A PEN film was used first inorder to realize the pad-printing process condition of each layer. Finally, the EL display devicewas printed onto a dish with a radius of curvature of 80 mm. The luminance was 180 cd m−2.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Electro-luminescence devices (ELDs), which produce visiblelight by exposing a substance to an electric field, havebeen particularly useful in applications where ruggedness,speed, brightness, high contrast and a wide angle of visionare required. Electro-luminescence was first observed andfabricated from silicon carbide (SiC) by Henry Joseph Roundin 1907 [1]. Georges Destriau, the man who discovered thepowder ACEL, published a report on the emission of lightfrom zinc sulfide (ZnS) powders [2]. The first thin-film ELstructures were fabricated in the late 1950s by Vlasenko andPopkov [3]. ELDs were revitalized in the 1980s and haveadvanced further still in recent years, especially with regardto their use in the back light unit of liquid crystal displays(LCDs), lighting devices and advertising boards, all of whichis due to the success of their commercialization, includingthe manufacturing methods and the development of materials.This paper presents an ELD which is patterned on a curvedsurface by the pad-printing method.

Recently, in spite of their inferior resolution and limitedscope of application, various micro-printing technologies havecome under the spotlight due to their fast manufacturing time,suitability for mass production and low-cost competitiveness

1 Author to whom any correspondence should be addressed.

when compared to existing lithographic processes formanufacturing certain parts used in the manufacture of displayunits, electronic papers, RF-ID information devices and soforth [4]. The photolithography and electroplating methodsrequire acid washes, a substantially larger amount of materialand a long manufacturing time. The abbreviation ‘PEMS’stands for printed electromechanical system, which can befabricated by means of various printing technologies. Passiveand active components in 2D or 3D such as conducting lines,resistors, capacitors, inductors and TFT [5], which are printedwith functional materials, can be classified into this category.PEMS products also include assemblies of printed passive andactive components, such as RFID tags, E-paper displays, solar-cell devices and printed sensors. Such PEMS products havebeen commercially available in the display sector for sometime, including plasma display panels (PDP) and LCDs.

There are various types of printing methods for PEMSdevices, which have the advantage of one-step directpatterning, including the inkjet, gravure, screen, flexo, offsetand pad-printing methods [6, 7].

In 2000, Seiko-Epson [8] fabricated a prototype of a colorfilter using the inkjet method. Research on inkjet printingsystems for display parts is currently spread between mostof the major LCD and PDP manufacturers, such as Samsung[9, 10], Sharp [11], LG [12], DNP [13], Chunghwa PictureTubes [14], etc. Even though the inkjet is regarded as one

0960-1317/10/015016+10$30.00 1 © 2010 IOP Publishing Ltd Printed in the UK

J. Micromech. Microeng. 20 (2010) 015016 T-M Lee et al

of the most powerful processes for non-contact directpatterning, it still has many problems. An inkjet-printedpattern is obtained from the complex relationships betweenseveral components, i.e. the inkjet head, inkjet system, ink,pattern geometry, substrate, environment and so on. Each ofthese components also has a lot of parameters and a numberof issues which need to be resolved. Conventional inkjetprint heads for industrial applications—such as conductingelectrodes—use low-viscosity (10 cP) functional ink undera room-temperature environment. The patterns printed byinkjet systems require such post-processes as drying, curingand sintering. The volume and final shape of the pattern arealtered dramatically after processing [15]. The ink dryingcondition should be controlled subtly in order to obtain a flatsurface geometry. Therefore, the fabrication of the displayparts via an inkjet system is not that easy or cheap. Indeed,the cost of the equipment remains expensive [16].

The screen printing method is one of the most reliable andeasily accessible direct patterning methods for PEMS devices.Thanks to the availability of silver particle paste and lead-freesolder paste, the electrode printing and packaging processesare now widely used. This has a great advantage in termsof cost and productivity. Screen printing equipment is lessexpensive than the inkjet system and is also simpler than theinkjet process, which requires thousands of inkjet nozzles,and the gravure offset process, which entails a second inktransferring process. In order to print patterns precisely by thescreen printing method, one of the most critical requirementsis to maintain the same printing conditions over the entirearea, including the off-contact speed, mesh tension and shearrate of paste, which is virtually impossible. These printingconditions depend on having a precisely ground squeegee,adequate ink formulation, precise printing process control,screens composed of fine mesh and precise screen printers[17–19]. The screen printing method cannot make patternson a curved surface, since it uses a screen mesh and a flatsqueegee.

The gravure offset printing method, which has beenadopted by major PDP electrode and PDP EMI mesh filtermanufacturers such as Samsung and LG, is based on directink transferal between the two plates [20–22]. Gravure offsetprinting transfers ink from the plate cylinder to the blanketcylinder and then to a substrate such as glass, paper or flexiblefilm. Since this printing process uses relatively high viscosityink, the volume and shape of the final pattern scarcely changeafter post-processing. If the gravure offset printing unit iscombined with a roll-to-roll web providing system, it is verycost-effective for the mass production of flexible electronicdevices such as RFID tags, E-paper displays and solar-celldevices. However, despite its many advantages, the gravureoffset printing method too cannot make patterns on a curvedsurface, since it uses a cylinder-type engraved roller andblanket.

There are two principal ways for fabricating printedpatterns on a curved surface [23]. The first way involvesthe fabrication of patterns on a flat substrate, followedby deformation into its final shape [24]. This approachallows for the use of well-established patterning techniques

prior to substrate deformation. Excessive strain during thedeformation process may cause damage to the various devicelayers, limiting its applicability to a narrow class of materialsand to surfaces with only limited deformation [24]. The otherway involves patterning directly on a pre-shaped substrate suchthat the patterns are not subject to deformation-induced strain.This latter way is attractive because it allows for the realizationof high performance devices over a nearly unlimited rangeof surface geometries. However, it presumes the availabilityof reliable patterning techniques for three-dimensional (3D)surfaces [23]. Although various techniques for the generationof textured or patterned 3D surfaces have been reported[25–27], it is very hard to find the full realization of conformalelectronics on an arbitrarily curved surface.

The pad-printing method is one of the best printingprocesses for realizing a printed electronic device on a curvedsurface. The pad-printing method is similar to the gravureoffset printing method in that the ink moves from a plate withengraved grooves to a substrate via a pad, which is made ofsilicon rubber. Since the pad is soft, the pad-printing processcan be applied to the curved substrate. Ink is inserted into theengraved grooves via a doctoring process and then transferredin turn. In this paper, we present a pad-printing process ofconsiderable precision. In order to utilize the pad-printingprocess for micro-patterning, minimal pattern size and patterndistortion—which is caused by use of the pad—should beclarified.

An electro-luminescence (EL) display device was finallypad-printed onto a dish, which has a radius of curvature of80 mm. The characteristics of the formulated inks and pad-printing process for fabricating the EL display device werealso investigated.

2. Precision of micro pad-printing

2.1. Precision test of pad-printing

Pad-printing is a combined technology consisting of a pad,engraved plate, substrate and ink. Figure 1 shows a simpleschematic diagram of the pad-printing process. The engravedplate is doctor-bladed with ink, the silicon pad proceeds towardthe plate, the ink is transferred onto the pad, the pad proceedstoward the curved surface and the ink on the pad is finallytransferred onto the surface [11].

Although there is the conceptual possibility of patterningto within several microns in the pad-printing process, fromour pad-printing experiments using the engraved plate withvarious pattern widths, it was found that the minimum patternwidth of pad-printing is several tens of microns. It is highlydependent on the rheology of the ink. Figure 2 shows a pad-printed pattern measuring 35 μm in width, which was theminimum size used in our experiment. The patterned thicknesswas 2.4 μm.

In order to inspect the pattern distortion of pad-printingcaused by the deformation of the silicon pad, concentric circleswhose diameters were increased by 2 mm were pad-printed ona flat surface, as shown in figure 3. The interval between thepatterns, which is supposed to be 2 mm, was measured with

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J. Micromech. Microeng. 20 (2010) 015016 T-M Lee et al

(a) (b)

(c) (d)

(e) ( f )

Figure 1. Schematic diagram of the pad-printing process: (a) doctor-blading; (b) pad approaching the engraved pattern; (c) ink beingtransferred onto the pad; (d) moving toward the substrate; (e) pad approaching the substrate; (f ) ink being transferred onto the substrate.

Figure 2. Pad-printed line pattern with a width of 35 μm and a depth of 2.4 μm.

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Table 1. Mechanical properties of the pad materials.

Ogden parameters

Pad materials μ (MPa) α Average elongation at break (%)

XR-3003 μ1 = 0.005 06, μ2 = 0.448 α1 = 7.32, α2 = −4.80 188XR-3005 μ1 = 0.0131, μ2 = 0.463 α1 = 7.44, α2 = −3.78 197

Figure 3. Concentric circle patterns for measuring pattern distortionwith their diameters increased by 2 mm.

Figure 4. Pattern distortion error caused by deformation of thesilicon pad (measured at intervals of 2 mm from the center of thepad).

an optical microscope. Figure 4 shows the distortion errorsin the measured pattern. The pattern distortion is less than5 μm on a flat surface. These error values fall just within themeasurement error due to the limitations of the resolution ofthe optical microscope. If the substrate is a curved surface,then the pattern distortion will depend on the curvature of thesurface.

2.2. Mechanical characterization of the pad

Two pad materials (polydimethylsiloxane (PDMS) pad, madeby Dow Corning Korea, commercial name: XR-3003 and XR-3005) were selected. Dumbbell-shaped specimens were cut

0.0 0.5 1.0 1.5 2.0 2.50

1

2

3

4

5 XR-3003 XR-3005

Eng

inee

ring

Str

ess

(M

Pa)

Engineering Strain

Figure 5. Measured stress–strain curves of the pad materials,XR-3003 and XR-3005.

from a sheet of each pad material and uniaxially loaded oneby one onto a UTM (Universal Testing Machine) at a speedof 1 mm s−1. A mechanical extensometer was employed toaccurately measure the strain, and two pneumatic grips wereutilized to hold both ends of a specimen while minimizing theslippage caused by large deformation.

The measured stress–strain curves are shown in figure 5.The XR-3005 was observed to be stiffer and stronger than theXR-3003, while the elongations of the XR-3005 were observedto be larger than those of the XR-3003. A slight slippage orstress drop was observed for the stress–strain curve of the XR-3005 above 2.5 MPa, while no such drop was observed withthe XR-3003. The stress–strain behavior of the pad materialscan be represented by the Ogden strain energy potential forhyper-elastic behavior by

U =2∑

i=1

(2μi

α2i

) (λ

α11 + λ

α22 + λ

α33 − 3

). (1)

Here, λi represents the deviatoric principal stretches and μi

and αi are the materials parameters. The mechanical propertiesof both pad materials are summarized in table 1.

2.3. Numerical modeling of pad-printing

For modeling and analyzing the pad-printing process,ABAQUS, a nonlinear finite element analysis tool was used.Figure 6 shows the two kinds of finite element models (FEM).

The first model, a quarter-sized model for a PDMS padlocated on a flat gravure plate, was fabricated in order toinvestigate pad deformation during the transference of inkfrom the flat gravure plate to the PDMS pad. The real shape

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(a) (b)

(c) (d )

Figure 6. Finite element model of the PDMS pad (XR-3003), flat plate and curved substrate: (a) FEM model for deforming the PDMS padonto the flat plate; (b) FEM model for deforming the PDMS pad onto the curved substrate; (c) the shape of the PDMS pad deformed onto aflat surface; (d) the shape of the PDMS pad deformed onto a curved surface with a radius of curvature of 80 mm.

of the PDMS pad is a hemisphere with a diameter of 100 mm.The meshes of the PDMS pad were generated into linearhexahedron-shaped elements with eight nodes (C3D8H). Forthe generation of a mesh for the flat gravure plate, a rigidbody element with 4-node-type (R3D4) meshes was used.The second model, as shown in figure 6(b), was employedto investigate pad deformation during the transference of inkfrom the PDMS pad to the substrate with a curved surface.The geometry of the substrate is a hemisphere with a diameterof 160 mm. The meshes of the PDMS pad were the same asthose of the first model, and R3D4-type meshes were used forthe substrate modeling.

Symmetrical boundary conditions were applied to eachcross-section of the PDMS pad. The degree of freedom(DOF) of the reference point when applied to the PDMS padwas restricted, except in the vertical direction. For the flatgravure plate and substrate, the fixed boundary condition wasapplied. The initial gap distance between the PDMS padand the bottom parts—including the flat gravure plate andsubstrate—was 1 mm. Contact modeling between the PDMSpad and the bottom parts can be established by defining thecontact pair between the R3D4 and C3D8H elements. Thematerial properties of the PDMS pad for Ogden strain energypotential were obtained from the experiment, as described insection 2.2. A numerical deformation analysis was carried outuntil the PDMS pad moved 15 mm downward.

Figures 6(c) and (d) show the deformation results,which were pressed onto the flat plate and curved substrate,respectively, when the PDMS pad moved 12 mm downward.

Figure 7 shows the nodes located on the quarter of thePDMS pad surface and taken from among the entirety of thenodes made for the numerical analysis. Because the printedarea was less than 50 mm, even though the shape of the padis a hemisphere with a diameter of 100 mm, the nodes locatedwithin 25 mm of the center were extracted. The non-deformedpad, as shown in figure 7(a), was first pressed toward the flatgravure plate for picking up ink. Figures 7(b) and (c) show thenodes located on the PDMS pad pressed onto a flat surface.After picking the ink up, the shape of the pad returned tothe non-deformed shape shown in figure 7(a). To transferthe ink to the curved surface, the pad underwent a secondpressing toward the curved surface with a radius of curvatureof 80 mm. Figure 7(d) shows the nodes dislocated by thesecond deformation. Since the ink originally located on theflat gravure plate was transferred onto the curved substrate,the pattern distortion error caused by the pad-printing ontothe curved surface can be estimated, after all, if wecompare the dislocations of the nodes between figures 7(c)and (d). The pattern distortion error is shown in figure 8. Ata close distance from the center of the pad, the dislocationerror is less than + 50 μm. As the distance from the centerlengthens, the error increases rapidly. The maximum error is

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Figure 7. Surface nodes of a quarter-sized PDMS pad: (a) non-deformed pad; (b) deformed pad which is half-pressed onto a flat surface;(c) deformed pad which is almost pressed onto a flat surface; (d) deformed pad which is almost pressed onto a curved surface with a radiusof curvature of 80 mm.

more than –200 μm. This is because the inclination of thesubstrate is nearly flat around the center and the inclinationincreases as the distance from the center lengthens. However,at locations where the distance is more than 20 mm, the errordoes not increase. Since this location is close to the spot wherethe pad does not make contact with the substrate, the effect ofpad deformation does not increase.

Using this numerical analysis method, any patterndistortion error caused by a substrate with various curvedshapes can be estimated, and we can also minimize the effectof pattern distortion, if this result is utilized in the patterndesign process reversely.

3. EL display device pad-printed on a curved surface

3.1. Pad-printing process for an EL display device

For efficient pad-printing, the ink of each layer should beformulated properly and the printing conditions should becontrolled. In order to fill an engraved groove with ink, the

viscosity of the ink should be low. After the ink has beentransferred to the pad, from the viewpoint of the ink beingtransferred from the pad to a substrate, the ink on the padshould become more viscous. Namely, the ink should containa volatile solvent which induces a change in its viscosity. Inthis research, we used commercially available screen printinginks for each layer. Each of the screen printing inks has itsown solvent which is unique to its manufacturer and unknownto consumers. Therefore, we tried to remove the existingsolvents by putting the inks into a sonicator to volatilize thesolvents. After this treatment, several kinds of solvents suchas ethyl acetate and BK were mixed into the inks to control theviscosity, and a thickening agent such as BYK-4xx was addedto increase the viscosity of the inks. Table 2 shows the initialviscosity of ink for each layer. The viscosities were measuredwith a Brookfield DV-II+ Pro.

The EL display device consists of five layers, as shownin figure 9 namely, a bottom electrode, a dielectric layer,phosphor, a transparent electrode and a bus electrode [28].Each layer has its own thickness specification. However, when

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Figure 8. Pattern distortion error caused by deforming onto acurved surface.

Figure 9. Design and the fabrication process of the EL display lampprinted on a curved surface.

using the pad-printing method, the printed thickness is usuallylow, at around 1 μm. To increase the thickness, the pad-printing process was carried out repeatedly. If the thicknessis required to be lower, the weight percentage of the solventshould be increased further still.

For the electrodes—such as the bottom electrode,transparent electrode and bus electrode—we had to considertwo major points: electrical conductivity and adhesion

Figure 10. EL display lamp pad-printed on a PEN film.

Table 2. Viscosity of ink for each layer

Ink Viscosity (cPs) Remarks

Silver 7000–7500 Screen print inkCarbon 34 000 Screen print inkDielectric 4139 BK treatedPhosphor 2076 Screen print inkTransparent 4511 BK treated

between layers. Generally, the higher the electricalconductivity, the brighter the EL display device in the sameoperating condition. For the bottom electrode, silver inkand carbon ink were used for printing on a PEN film anda ceramic substrate, respectively. One-off pad-printing withsilver ink results in a pattern height of 1.4 μm. To obtainsufficient conductivity, in the case of carbon ink, we printedfive times, obtaining a pattern height of 5.77 μm. Since theelectrical conductivity of an electrode printed with silver ink ismuch higher than that of an electrode printed with carbon ink,the choice of silver ink for the bottom electrode can reducethe number of printings. Conversely, however, the adhesiveproperty of carbon ink is greatly superior to that of silver ink.In particular, adhesiveness between the ceramic dish and thesilver ink is so poor that a cured pattern printed with silver inkis easily effaced by rubbing with a finger. As such, carbon inkwas chosen to print the bottom electrode onto the dish.

In the case of dielectric and phosphor ink printing, thick,uniform layer printing is critical, since such defects as apin hole, non-uniformity or a thin layer can cause a shortcircuit at high voltage. In this research, we duplicated thepad-printing five times, and the thicknesses of the dielectricand phosphor layers were found to be 4.6 μm and 2.2 μm,respectively.

Table 3. Pad-printing process conditions for each layer.

PatternLayer Printing conditions height (μm) Remarks

Bottom electrode One printing: 4 min curing at 120 ◦C 1.4 1.1 � cm−1

Dielectric Five printings: 2 min drying at 120 ◦C 4.6Phosphor Five printings: 2 min drying at 120 ◦C 2.2Transparent electrode Five printings: 3 min drying at 120 ◦C 3.5 8–20 k� cm−1

Bus electrode Five printings: 4 min curing at 120 ◦C 5.77 2–6 k� cm−1

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(a)

(b)

Figure 11. EL display lamp pad-printed on a dish with a radius of curvature of 80 mm: (a) off state; (b) on state (ac 200 V, 1 kHz).

For the transparent layer, ink made of PEDOT-PSS wasused. The conductivity of the PEDOT-PSS ink pattern is two tothree times lower than that of the carbon ink pattern. Therefore,we duplicated the pad-printing several times, and the finalthickness of the transparent layer was 3.5 μm.

The required characteristic of the bus electrode is almostthe same as the bottom electrode. Since the bus electrode isprinted onto the top of the transparent electrode, the adhesivequality is only a little different from that of the bottom electrodeprinted onto the ceramic. From the experiment, we alsofound that the adhesion between carbon ink and the transparentelectrode was much better than the adhesiveness of silver ink.Carbon ink was chosen for the bus electrode. The printingprocess condition for each layer is described in table 3.

3.2. Pad-printed EL display device

Figures 10 and 11 show the fabrication results of the pad-printed EL display device. The PEN film was used firstin order to realize the pad-printing process condition ofeach layer. The pad-printed EL display device fabricatedon the PEN film is flexible up to a radius of curvature of5 mm. Since the curing temperature of all layers was 120 ◦C,it was also possible to use a PET film. As shown infigures 10 and 11, to align each layer, we used two seriesof alignment markers located on both sides of the ELdisplay device. The phosphor layer had a pattern of ‘FLEXPEMS’. The EL display device was driven at ac 200 V of1 kHz. As shown in figure 11, the EL display device was

Figure 12. CIE coordinates of the EL display lamp pad-printed onthe dish.

finally printed on a standard ceramic dish with a radiusof curvature of 80 mm and a shiny surface. Due to thedifficulty encountered when printing the first layer onto thissurface, a buffer layer had been considered for enhancing the

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adhesiveness. However, instead of inserting a buffer layer,carbon ink with strong adhesive properties was adopted. Thefabricated EL display device was turned on at ac 200 V of1 kHz. The CIE X and Y coordinates of the EL display devicewere 0.172 and 0.319, respectively, as shown in figure 12, andthe luminance was 180 cd m−2; i.e. sufficient brightness forindoor illumination. The CIE coordinates and the degree ofluminance were measured using a CS-100A made by KonicaMinolta.

4. Conclusion

Even though such printing methods as the gravure, screen,flexo, inkjet and pad-printing methods have the advantageof one-step direct patterning, it is very difficult to print anelectrical device onto a curved surface. This paper is uniquein that the proposed EL display device is fabricated on a curvedsurface by the pad-printing method. We have also investigatedthe pad-printing process for the fabrication of electrical devicesas follows.

(1) The minimum pattern for pad-printing was found to be35 μm wide and 2.4 μm thick. Pattern distortionwhen pad-printing on a flat surface, which is caused bydeformation of the silicon pad, was measured at intervalsof 2 mm from the center of the pad. Pattern distortionon a flat surface was found to be less than 5 μm, whichmeans, as one of the micro-patterning processes, that thepad-printing method can be used to create precise 35 μmwide patterns with a location error of just 5 μm.

(2) Pattern distortion during pad-printing onto a curvedsurface varies according to its surface geometry. Weconducted a numerical analysis of pad-printing on acurved surface. By using this kind of numerical method,any distortion error caused by the various curved surfacescan be estimated and compensated.

The EL display lamp was finally printed onto a dish,with a radius of curvature of 80 mm. The EL display lampconsisted of five layers, namely a bottom electrode, dielectriclayer, phosphor, transparent electrode and a bus electrode.The ink of each layer was reformulated with solvents andthe pad-printing condition was controlled. The EL displaylamp was driven at ac 200 V of 1 kHz. The luminance was180 cd m−2.

Consequently, it is concluded that the EL display devicewas fabricated on a curved surface using the pad-printingmethod, which has great potential as a technique of microfabrication onto various shapes. It is expected to be applicablein many industrial fields where it is necessary to make patternson curved surfaces, such as direct circuit printing on a cellularphone surface, various advertising goods, display parts andso on.

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