Thin Solid Films 519 (2011) 6853–6857

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Effect of UV/ozone treatment on interactions between ink-jet printed Cu patternsand polyimide substrates

Young-In Lee a, Yong-Sung Goo a, Kun-Jae Lee a, Yun-Gu Hwang a, Younghun Byun b, Hee Jung Park c,Deok-Yong Park d, Nosang V. Myung e, Yong-Ho Choa a,⁎a Department of Fine Chemical Engineering, Hanyang University, Ansan 426-791, Republic of Koreab Advanced Materials Laboratory, Samsung Advanced Institute of Technology, Yongin 446-712, Republic of Koreac Materials Research Laboratory, Advanced Institute of Technology, Yongin 446-712, Republic of Koread Department of Applied Materials Engineering, Hanbat National University, Daejeon 305-719, Republic of Koreae Department of Chemical and Environmental Engineering and Center for Nanoscale Science and Engineering, University of California-Riverside, Riverside, CA 92521, USA

⁎ Corresponding author. Tel.: +82 11 9629 7446; faxE-mail address: choa15@hanyang.ac.kr (Y.-H. Choa)

0040-6090/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tsf.2011.04.050

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Available online 22 April 2011

Keywords:UV/ozoneInk-jet printingWettabilityPolyimide filmSurface modification

In this study, we apply UV/ozone treatment to modify the surface properties of polyimide substrate andinvestigate the effects of UV/ozone treatment on surface properties of polyimide and the morphologies of ink-jet printed Cu patterns. Wettability of polyimide surface is enhanced by UV/ozone treatment as confirmed bycontact angle analysis and XPS results. The change in wettability affected droplet sizes and morphologies ofink-jet-printed lines. The average droplet size increased from 26.8 μmon untreated polyimide to 42.5 μmafterUV/ozone treatment. Furthermore, irregular patterns of coexisting beads and short lines were observed inuntreated polyimide film, despite minimal overlap of dots. In contrast, patterns that were ink-jet printed onpolyimide film that was modified by UV/ozone for 1 h showed continuous lines with straight edges.

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1. Introduction

In recent years, interest in ink-jet printed electronics has increasedbecause ink-jet printing is simpler and cheaper than traditionalphotolithography, in which the creation of patterns requires vacuumprocessing, chemical deposition, and etching [1–3]. Ink-jet printingenables the creation of flexible devices at low cost and has beenadopted for a variety of flexible electronic devices such as thin-filmtransistors, radio frequency identification tags, and printed circuitboards [4–7].

Many factors such as ink, substrate, and jetting method affect theoutcomes of ink-jet printing. In particular, appropriate interactionsbetween ink and substrate are important to successfully formcontinuous lines with the desired width and shape on substrates[8]. Polyimide film is one of the most widely used polymers inelectronics because of its mechanical, thermal, electrical, and chemicalproperties. Polyimide film typically has a hydrophobic surface due toits low surface energy. When water based ink is printed onto apolyimide film surface, each ink droplet has a high contact angle.Although small droplets with high contact angles are advantageousfor obtaining narrow lines, it is difficult to make continuous lines ofuniform shape due to the problem of wettability. Therefore, it is

necessary to modify the surface properties of polyimide film fromhydrophobic to hydrophilic to enhance wettability.

Various methods including corona discharge, ultraviolet irradia-tion, electron bombardment, and plasma treatments have been usedin order to modify the surfaces of substrates [9–11]. Among these,oxygen plasma is very effective because it generates a complexmixture of high energy photons, electrons, ions, radicals and excitedspecies that can modify the surface of substrates to be hydrophilic.However, although oxygen plasma treatment is brief compared withother methods, the surface suddenly changes from hydrophobic tohydrophilic because of the high energy plasma. Thus, it is difficult tocontrol the change to hydrophilicity and meet the contact anglerequirement. Moreover, oxygen plasma treatment is not cost-effectivebecause a vacuum process is needed to generate the oxygen plasma.

Ozone gas is a reactive gas that is used in industrial processes suchas preparation of very clean semiconductor silicon wafer surfaces andsurface modification of various substrates in the manufacture ofmicroelectronics devices, in the form of ozone-water and ozone gas[12]. Ozone treatment is a very simple process in comparison withother surface modification processes because ozone consists ofoxygen molecules with three oxygen atoms and energetic UVradiation and does not require a vacuum. Wettable surfaces ofpolyimide were recently obtained using this treatment [12–14].However, to the best of our knowledge, no studies have applied ozonetreatment to ink-jet printing or investigated the relationship betweenink and polyimide substrates that are treated by ozone.

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In this study, we apply UV/ozone treatment to modify the surfaceproperties of polyimide substrates and investigate the effects of UV/ozone treatment on polyimide surfaces and the morphologies of ink-jet printed Cu patterns. We studied the contact angle of Cu ioncomplex ink that was synthesized bymodified electrolysis and changein contact angle as a function of UV/ozone treatment time. In addition,Cu ion complex ink was ink-jet printed onto polyimide filmsfabricated with and without UV/ozone treatment. The morphologiesof ink-jet printed patterns with controlled dot spacing wereinvestigated before and after annealing.

2. Experimental details

Cu ion complex inks were synthesized by a process informed bythe principle of electrolysis. In previous work, we reported thesynthesis process and mechanism of Cu ion complex ink and theeffects of the complex agent on the characteristics of a Cu conductivepattern [15]. Briefly, the process involves Cu plates (2.5×5×0.1 cm3,Aldrich) and formic acids (HCOOH, Junsei Chemical, 99.5%) as theelectrode and complex agent for forming the Cu ion complex,respectively. Formic acid (40 mM) was added to 500 ml ammoniawater to obtain an electrolyte solution. After the Cu electrodes weredipped in the electrolyte solution, a direct voltage of 40 V was appliedbetween the electrodes to generate Cu ions and synthesize Cu ioncomplex ink. The Cu ion complex solution was evaporated for 30 minto increase the Cu concentration in the ink and ultimately Cu ioncomplex ink with a concentration of 15 wt.% was obtained.

Modification of the surfaces of polyimide films (Kolon LV 200,50 μm thickness), was conducted using a commercial UV/ozonecleaner (Model#: 42, Jelight Inc.). This equipment has a high intensitylow pressure mercury vapor UV grid lamp, and generates UV-emissions at 185 nm and 254 nm wavelengths. Polyimide film wasplaced in the ozone exposure chamber with a gas mixture of air andozone and an ozone gas concentration of 1000 ppm. The distancebetween the UV-source and the film was 6 mm. The UV/ozonetreatment of polyimide film was carried out for 30, 60, 90, or 120 min.

Ink-jet printing of Cu ion complex ink was performed using apiezoelectric ink-jet printer (DMP 2831, Fujifilm Dimatix Inc.)equipped with multiple nozzles 25 μm in diameter onto a polyimidefilm that was treated by UV/ozone for 1 h and an untreated controlpolyimide film. Dot and line patterns were formed by ink-jet printing.The spaces between dots for making line patterns were set at 10, 20,30, and 40 μm by controlling the ink-jet printer. The printed patternswere annealed under H2 at 250 °C for 1 h using a tube furnace.

The contact angles of Cu ion complex ink were measured using acontact angle analyzer (Phoenix 300 plus, SEO) at room temperatureto evaluate the wettability of the polyimide films. A Cu ion complexink droplet of 2–3 μl was placed on the polyimide film surface, and the

Fig. 1. Graph of contact angles and images of Cu ion complex ink drop

steady state contact angles and droplet images were recorded within30 s. X-ray photoelectron spectroscopy (XPS; ESCA 2000, VG Micro-tech) analyses were performed to investigate chemical bonds in thepolyimide film surfaces before and after UV/ozone treatment. Inaddition, the ratios of carbon, nitrogen, and oxygen were evaluatedusing XPS spectra. The roughness of polyimide films was character-ized by Atomic ForceMicroscopy (AFM; XE 100, PSIA). The shapes andsizes of dot and line patterns that were ink jet-printed on polyimidefilms were analyzed by optical microscopy (OM; BX51M, Olympus)and an image analysis program. After annealing at 250 °C to reduce Cuion complex to bulk Cu, the shapes and sizes of the patterns wereobserved by OM.

3. Result and discussion

Fig. 1 shows the contact angle of a Cu ion complex ink and DI waterdroplet on polyimide film as a function of UV/ozone treatment time.The initial value of the contact angle for a bare polyimide film was 57°and decreased to 40° and 32° after UV/ozone treatment times of30 min and 1 h, respectively. The decrease in contact angle was alsoobserved when the DI water was used as test solution for measuringcontact angle. This shows that the polyimide film surface can bemodified from hydrophobic to hydrophilic by UV/ozone treatment.The contact angles did not decline further after exposure to UV/ozonefor more than 1 h. In order to further decrease the contact angle,ozone concentration and polyimide film surface temperature could beincreased. However, very low contact angles are unsuitable forforming the desired lines. Therefore, in this study, the exposureperiod was fixed at 1 h.

The improvement in wettability of polyimide films can beexplained by changes in the chemical bonds of the polyimide filmsurfaces. Molecular oxygen is dissociated to atomic oxygen by the185 nm wavelength generated by UV emission. Ozone is created bythe combination of molecular oxygen with generated atomic oxygen.The wavelength of 254 nm creates activated atomic oxygen from theozone and also breaks the C\C and C\H bonds of polyimide. Thegenerated atomic oxygen and activated one are combined with thebroken bonds of polyimide, and thus the hydrophilic groups can begenerated. XPS analysis was carried out to investigate the chemicalbonding states of the polyimide film surfaces. Fig. 2(A–C) shows theXPS spectra and ratios of carbon and oxygen on polyimide filmsurfaces before and after exposure to ozone gas for 1 h. The XPScarbon peak at 285 eV decreased and that at 288 eV increased,indicating a decrease in the number of C\C and C\H bonds andadditional generation of hydrophilic groups such as C\O, C_O, andCOO. Evidence that the bonds related to oxygen increased can be seenin the increase in the peak of oxygen, as shown in Fig. 2(B). Thechange in the chemical bonding state is consistent with calculated

lets on polyimide film as a function of UV/ozone treatment time.

Fig. 2. XPS spectra of (A) carbon, (B) oxygen, (C) ratios calculated by XPS spectra and (D) AFM results on polyimide film surface before and after exposure to ozone gas for 1 h.

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concentrations of carbon and oxygen by peak area for the differentpeaks (Fig. 2(C)). Fig. 2(D) shows surface images and root meansquare surface roughness (Ra) of polyimide films before and afterexposure to ozone gas for 1 h. After UV/ozone treatment, the surfaceroughness of polyimide film slightly increased compared with that ofnon-treated polyimide film. The wettability of the polyimide filmsurface is expected to enhance as a result of the increase in surfaceroughness as well as the change of chemical bond because the changeof wettability has the same tendency as the increase in surfaceroughness.

In ink-jet printing, wettability is an important factor to determinehow ink droplet size is related to the morphologies and widths oftraces. Fig. 3 shows an array of ink-jet-printed single dots and theirsizes on polyimide film that was untreated (control) and treated byUV/ozone for 1 h. In both cases, dots with a uniform diameter, shape,

Fig. 3. Arrays of single dots that were ink-jet-printed under a selected ink-jet printing conditisize distribution graph.

and distance between dots were successfully formed. It was observedthat the droplets formed on polyimide films modified by UV/ozonewere more spread out in comparison with droplets printed onuntreated polyimide surfaces. The average droplet size on barepolyimide film was 26.8 μm and increased to 42.5 μm on polyimidefilms treated by UV/ozone. Modifying the polyimide films bytreatment with UV/ozone enhanced wettability by making thesurfaces more hydrophilic.

It is necessary to overlap droplets to make continuous lines usingink-jet printing. Therefore, the distances between dots must benarrower than the diameters of the dots.When drops of ink are placedon a solid surface and connected to form lines, the cohesive forcesbetween droplets minimize the total surface energy. At the same time,adhesive forces between droplets and solid surfaces work to maintaincontact between the liquids and the solid surfaces. In other words, the

on on polyimide film that was (A) untreated and (B) treated by UV/ozone for 1 h and dot

Fig. 4. OM images of ink-jet-printed lines on polyimide film that was (A–D) untreated and (E–H) treated by UV/ozone using a dot spacing of (A, E) 10, (B, F) 20,(C, G) 30, and(D, H) 40 μm.

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morphologies of lines that are formed by the overlap of dots aredetermined by adhesive and cohesive forces. Therefore, it is importantto balance the two forces by increasing or decreasing wettability.

Fig. 4 shows OM images of an ink-jet-printed line on untreatedpolyimide films using dot spacings of 10, 20, 30, and 40 μm. In thecases of 30 and 40 μm droplet inter-spacing, droplets were notconnected to each other because the interspacing distances betweendroplets were larger than the diameters of single droplets (Fig. 4(C,D)). Bead necklace patterns are observed in Fig. 4 despite the overlapof droplets (Fig. 4(A, B)). A decrease in the diameter of the beads maybe observedwith increasing dot spacing. The average diameters of thebeads were approximately 65 μm (dot spacing: 10 μm) and 40 μm(dot spacing: 20 μm). Morphological features of ink-jet-printedpatterns on polyimide films treated by UV/ozone for 1 h are shownin Fig. 4(E–H). The morphologies of the patterns are smoother thanthose of patterns on untreated polyimide, and the formation of beadsin the patterns is obstructed. A continuous line with a flat surface wasformed when the droplet inter-spacing was 30 μm (Fig. 4(G)).

Fig. 5 shows OM images of the patterns given in Fig. 4 after heattreatment in H2 at 250 °C for 1 h. The color of the ink-jet printedpatternmarkedly changed to the color of bulk Cu after heat treatment,and it was confirmed that the Cu ion complex ink was converted to Cuduring the change in color. In particular, special changes inmorphology were not observed compared with patterns before heattreatment.

Fig. 5. OM images of ink-jet-printed lines on polyimide film thatwas (A–D) untreated and (E–Hafter heat treatment in H2 at 250 °C for 1 h.

The differences in the morphologies of patterns before and afterUV/ozone treatment can be explained by increases in wettability afterUV/ozone treatment. If the wettability, which corresponds to anadhesion force between droplets and substrates, is low, overlappeddroplets aggregate because the cohesive forces between ink dropletsare higher than the adhesion force. This occurs when using untreatedpolyimide films. Therefore, despite minimizing the overlapped size of3 μm (Fig. 4(b)), a bead necklace pattern was formed. On the otherhand, after increasing wettability using UV/ozone treatment, thecohesive force and adhesive force were counterpoised for a specificinterspacing distance between droplets and a line was successfullycreated using interspacing distance between droplets of 30 μm. Fromthese results, we determined that proper enhancement of wettabilityis necessary to form continuous patterns with straight edges and thedesired diameters when ink-jet printing on polyimide film usingwater based ink. In addition, UV/ozone treatment is an effectivemethod for modifying polyimide films to be more hydrophilic.

4. Conclusion

In this study, polyimide film surfaces were successfully modified tobe more hydrophilic by UV/ozone treatment. This changed themorphologies of ink-jet printed Cu patterns after treatment. Enhancedwettability was confirmed by decreases in the contact angles of the Cuion complex from 57° to 32° after the exposure of polyimide to ozone

) treated byUV/ozone using a dot spacing of (A, E) 10, (B, F) 20,(C, G) 30, and (D, H) 40 μm

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gas for 1 h. An increase in oxygen concentrations at the polyimide filmsurfaces observed byXPS spectra indicated thatwettability increased bymodifying the chemical bonding of the polyimide film surface throughUV/ozone treatment. The changes in the chemical bonding statestrongly affected the morphologies of the ink-jet printed patterns. Inuntreated polyimide films, irregular patterns of coexisting beads andshort lines were observed despite minimal overlap due to low adhesionforces between ink and substrate. On the other hand, patterns thatwereink-jet printed on polyimide films that were modified by UV/ozone for1 hwere continuous lineswith straight edges because the adhesion andcohesive forceswere balanced by enhancing the adhesion force throughUV/ozone treatment. In conclusion, we determined that UV/ozonetreatment was effective for modifying polyimide films, and patternswith the desired morphology were successfully formed on polyimidefilms after applying UV/ozone treatment.

Acknowledgement

This work was supported by Grant No. 10031768 from the Ministryof Knowledge Economy (MKE) and Fundamental R&D Program for Core

TechnologyofMaterials funded by theMinistry of Knowledge Economy,Republic of Korea.

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