Coffee-Ring Defined Short Channels for Inkjet-Printed Metal OxideThin-Film TransistorsYuzhi Li, Linfeng Lan,* Peng Xiao, Sheng Sun, Zhenguo Lin, Wei Song, Erlong Song, Peixiong Gao,Weijing Wu, and Junbiao Peng*

State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

*S Supporting Information

ABSTRACT: Short-channel electronic devices several micro-meters in length are difficult to implement by direct inkjetprinting due to the limitation of position accuracy of the commoninkjet printer system and the spread of functional ink onsubstrates. In this report, metal oxide thin-film transistors (TFTs)with channel lengths of 3.5 ± 0.7 μm were successfully fabricatedwith a common inkjet printer without any photolithography steps.Hydrophobic CYTOP coffee stripes, made by inkjet-printing andplasma-treating processes, were utilized to define the channel areaof TFTs with channel lengths as short as ∼3.5 μm by dewettingthe inks of the source/drain (S/D) precursors. Furthermore, byintroduction of an ultrathin layer of PVA to modify the S/Dsurfaces, the spreading of precursor ink of the InOx semi-conductor layer was well-controlled. The inkjet-printed short-channel TFTs exhibited a maximum mobility of 4.9 cm2 V−1 s−1 and an on/off ratio of larger than 109. This approach offabricating short-channel TFTs by inkjet printing will promote the large-area fabrication of short-channel TFTs in a cost-effectivemanner.

KEYWORDS: coffee-ring effect, hydrophobic, inkjet printing, metal oxide, thin-film transistors

1. INTRODUCTION

Inkjet printing is an attractive technique used to fabricatevarious electronic devices, including thin-film transistors(TFTs), organic light-emitting diodes (OLEDs), radiofrequency identification (RFID) tags, etc.1−6 As a drop-on-demand patterning technique, inkjet printing has beenrecognized as a versatile method which has the advantages oflow material waste, maskless patterning, and scalability to large-area fabrication.7 In the past few years, inkjet-printed TFTsbased on organic and inorganic semiconductors have beenintensively studied.8−11 Among these kinds of TFTs, metaloxide (MO) TFTs are intrinsically superior to the organic onesin terms of mobility and stability,12,13 and relevant studies basedon inkjet printing methods have grown continuously.To date, most reports on inkjet-printed MO TFTs focus on

the selection of oxide component14,15 preparation of inks13,16

and elimination of coffee rings.17,18 The resolution of theprinted pattern, limited by the droplet size, positional accuracyof the inkjet printer, and the spread of ink on substrates, isgenerally tens of micrometers. However, the backplanes of theflat panel displays (FPDs) generally require TFTs with channellength of less than 10 μm,19 which is far beyond the resolutionof most printing methods. Therefore, realization of inkjet-printed short-channel TFTs is of great importance forapplications in FPDs.

Several methods have been proposed to achieve shortchannel length by controlling the ink spreading. Tang et al.20

introduced a cross-linked poly(vinyl alcohol) (PVA) layer tomodify the substrate, which controlled the spreading of silverink and realized all solution-processed organic TFTs withchannel length down to 20 μm, but the channel length variety(nonuniformity) was too large due to the positional inaccuracyof the inkjet printer (Dimatix, DMP-2831). Sekitani et al.21

printed Ag nanoink directly on the organic semiconductorlayers to manufacture p-channel and n-channel organic TFTswith channel lengths as short as 1 μm using a subfemtoliterinkjet printer with a high positioning accuracy. However, thehigh cost and difficulty of large-size fabrication of the high-accuracy inkjet printer are the two main problems for itsapplications in mass production. Different from the directprinting approach, Sirringhaus et al.22 demonstrated short-channel organic TFTs with submicrometer lengths based onsurface-energy-assisted inkjet printing. In this approach, ahydrophobic stripe was predefined onto a hydrophilic substrate,and the deposited functional ink was able to spontaneously splitto form a narrow gap due to the dewetting of ink on thepattern. The whole process could be performed using common

Received: June 14, 2016Accepted: July 15, 2016Published: July 15, 2016

Research Article

www.acsami.org

© 2016 American Chemical Society 19643 DOI: 10.1021/acsami.6b07204ACS Appl. Mater. Interfaces 2016, 8, 19643−19648

inkjet printing system with a relative low accuracy. However,the preparation of hydrophobic stripes required photo-lithography or electron-beam lithography processes, whichincreased the manufacturing complexity and cost.In this report, short-channel (3.5 ± 0.7 μm) MO TFTs

fabricated with a common inkjet printer (Dimatix, DMP-2800)without any photolithography steps were demonstrated. Thereproducible, well-controlled hydrophobic stripes were ob-tained from the printed CYTOP line pattern by plasmatreatment and used as surface-energy patterns to define ITOsource/drain (S/D) electrodes by dewetting the printed ink.Furthermore, by introduction of an ultrathin layer of PVA tomodify the S/D surfaces, the spreading of precursor ink of InOxon the substrate was well-controlled, and continuous InOxsemiconducting films were successfully printed. The inkjet-printed short-channel TFTs exhibited a maximum mobility of4.9 cm2 V−1 s−1. This novel approach of fabricating short-channel TFTs by inkjet printing will promote the large-areafabrication of short-channel TFTs in a cost-effective manner.

2. EXPERIMENTAL PROCEDURE2.1. Ink and Solution Preparation. CYTOP ink was prepared by

mixing solute (Asahi Glass, CTL-107MK) and solvent (Asahi Glass,CT-SOLV180) with a volume ratio of 1:10, and the ink was stirred atroom temperature for 12 h. The ITO ink was prepared by dissolving0.4874 g In(NO3)3·xH2O (Aldrich, 99.99%) and 0.0406 g SnCl2·xH2O(Aldrich, ≥99.995%) into a solvent containing 4 mL of 2-methoxyethanol (Shanghai Aladdin Bio-Chem. Technology Co.,99.8%) and 2 mL of ethylene glycol (Shanghai Aladdin Bio-Chem.Technology Co., ≥99%). The ink was stirred vigorously at 50 °C for12 h. The InOx ink with concentration of 0.2 M was prepared bydissolving In(NO3)3·xH2O into a mixed solvent with a volumetricratio of 50% deionized water and 50% ethylene glycol, and the ink wasstirred at room temperature for 12 h. PVA solution was prepared bydissolving 5 mg poly(vinyl alcohol) (Alfa Aesar, 98−99% hydrolyzedPVA) in 5 mL of deionized water, and the solution was stirredvigorously at 80 °C for 12 h. The inks and solution were all filteredthrough a 0.22 μm syringe filter before use. The surface tension andviscosity of the inks are summarized in Table S1.2.2. Fabrication Processes. For the bottom-gate TFTs, the

bottom-contact structure is more feasible than the top-contactstructure in the inkjet printing process.12 Therefore, bottom-gatebottom-contact architecture was employed to fabricate InOx TFTs.First, an aluminum:neodymium (Al:Nd) alloy film with 300 nmthickness was deposited by DC sputtering. Then, a 200 nm-thickanodic AlOx:Nd insulator layer (38 nF/cm2) was formed by anodizingthe Al:Nd alloy. The anodizing process and the preparation of

anodizing electrolyte were performed following the literature.23 ADimatix (DMP-2800) piezo-inkjet printer with a 10 pL cartridge wasused to print layers with the desired pattern. The droplets of CYTOPink with drop spacing of 40 μm were printed onto the AlOx:Nd surfacepreheated to 37 °C to form line patterns with obvious coffee stripes onthe edges (Figure 1a). The distance between the adjacent CYTOPlines was 160 μm. Then, the CYTOP lines were treated with oxygenplasma for 2.5 min to remove CYTOP at the center of the lines,leaving the coffee stripes on the substrate to define the channel lengthof the TFTs (Figure 1b). The polarizing microscope image (FigureS1) showed that the reproducibility of the CYTOP coffee stripes wasgood. Subsequently, perpendicular CYTOP lines with spacing of 280μm were printed onto the predefined coffee stripes to define thechannel width of the TFTs (Figure 1c). Next, the substrate wasannealed at 120 °C in air for 10 min. The droplets of ITO ink withdrop spacing of 20 μm were printed onto the array of the rectangularCYTOP framework. The narrow hydrophobic CYTOP coffee stripessplit the printed droplets into isolated droplets (Figure 1d). Then, thearray of ITO was dried at 50 °C for 5 min and then hard annealed at350 °C for 1 h in air to form ITO electrode pairs as the S/D electrodes(Figure 1e). Meanwhile, the CYTOP decomposed under the hardannealing conditions, which was confirmed by the polarizingmicroscope images shown in Figure S2. Before the InOx layers wereprinted, the substrate was modified with an ultrathin PVA layer byspin-coating PVA solution at 3000 rpm for 40 s (Figure 1f). The InOxprecursor was printed onto the channel areas with drop spacing of 40μm (Figure 1g) and then annealed at 300 °C for 1 h to remove PVAand form semiconducting channels (Figure 1h). More detailedparameters of inkjet printing such as waveform are shown in Figure S3.

2.3. Characterization. Film thickness and surface morphologieswere measured using a Dektak 150 surface profiler (VeecoInstruments, Inc.) and an atomic force microscope (Bruker,Multimode 8). The optical microscopy images were obtained from aNikon Eclipse E600 POL with a DXM1200F digital camera. Thewetting properties of various film surfaces were characterized bycontact-angle measurement using contact-angle analyzer (BiolinScientific, Theta Lite 101). The viscosity of various inks was measuredby a viscometer (Brookfield, DV-I+). The TFT characterizations weremeasured using a Keithley 4200-SCS semiconductor parameteranalyzer and a probe station in ambient atmosphere. The mobility(μsat) in the saturation region of TFTs was calculated using thefollowing equation:

μ= −I

W C

LV V

2( )DS

sat iGS th

2(1)

where Ci is the unit-area capacitance of the insulator, Vth is thethreshold voltage obtained by fitting the saturation region of IDS

1/2 vsVGS plots and extrapolating fitted line to IDS = 0, and W and L are thechannel width and length, respectively.

Figure 1. (a) Printing CYTOP line pattern. (b) CYTOP coffee stripes. (c) Printing perpendicular CYTOP onto predefined coffee stripes. (d)Printing ITO ink onto the array of rectangular CYTOP framework. (e) ITO electrode pairs. (f) Modification of substrate with ultrathin PVA layer.(g) Printing InOx ink onto channel areas. (h) Array of InOx TFTs.

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3. RESULTS AND DISCUSSIONFigures 2a and b show the surface profile of an as-printed singleCYTOP line. The CYTOP line exhibited two parallel coffee

stripes with a height of dozens of nanometers and a width of∼3.1 μm, while the average thickness of CYTOP between thetwo parallel coffee stripes was only several nanometers. Thisphenomenon is regarded as the coffee-ring effect, which is oftenobserved in dried coffee drops.24 When the inkjet-printeddroplet reaches on the substrate, it spreads out and evaporatessimultaneously. Since the evaporation rate on the edge regionof drop is higher than that on the center region, inducingoutward capillary fluid flow, the solute is carried to the edge ofthe drop by the solvent migration to replenish the evaporationloss at the edge. The concentration of the inks and the substratetemperature are crucial for the coffee ring formation. Figure 3shows the plots of the CYTOP coffee stripe width and heightand the distance between adjacent coffee stripes versussubstrate temperature for different concentration ink. With anincrease in CYTOP solute concentration, the height and widthof the coffee stripes increased along with a decrease in thedistance between the two parallel coffee stripes. When thesubstrate temperature increased, the height of the coffee linesincreased and the distance between the two parallel coffeestripes decreased as the drop evaporation rate increased.Meanwhile, the width of the coffee stripes exhibited only asmall change.The coffee-ring effect would inevitably occur for the films

deposited by inkjet printing, and intensive efforts have beendevoted to eliminate the effect.25−27 On the other hand, thereare several reports utilizing the coffee-ring effect to preparefunctional materials or/and fabricate electronic devices.28−32

Herein, the thin CYTOP layer between the two parallel coffeestripes can be removed easily by plasma treatment, leaving thecoffee stripes on the substrate (Figure S4). The residual coffeestripes were still hydrophobic after plasma treatment with a

water contact angle above 110° (Figure S5). Therefore, thehydrophobic plasma-treated CYTOP coffee stripes can be usedas surface-energy patterns to define electrode pairs with narrowgap. Figures 4b and c show the surface profile of a pair of ITOelectrodes. The ITO films with thickness of ∼76 nm(resistivity: ∼2.4 × 10−1 Ω·cm) were free of coffee rings, andthe gap between the ITO electrode pair was about 3.3 μm,which also confirmed by the AFM image (Figure 4a). Thenarrow gap formation was ascribed to the dewetting of ink onthe hydrophobic coffee-lines. For the surface-energy barriers ofa monolayer, neglecting gravity and volume changes of thedroplet before and after dewetting, the occurrence of dewettingof the ink should meet the following equation:22

βΔ≥

−L

S21

1 cos (2)

where L is the width of the hydrophobic stripe, 2ΔS is theincrease in the surface area of a droplet in hydrophilic regionsdue to the appearance of curved edges on both sides of thehydrophobic stripe, and β is the contact angle on thehydrophobic surface. Because ΔS decreases as the thicknessof the droplet decreases, the dewetting takes place when thedroplet is below a critical thickness. Herein, the hydrophobicCYTOP coffee stripes were dozens of nanometers in thickness,which means that the critical thickness for the occurrence ofdewetting on the CYTOP coffee stripes is higher than that onthe monolayer surface-energy barriers without changing theother parameters. The increased critical thickness is good forobtaining thick conductive films to improve sheet conductance.In other words, the hydrophobic coffee stripes with a properthickness are in favor of dewetting of the inks and thus avoidingshorting between the electrode pair, as indicated in Figure S6.For the fabrication of bottom-gate bottom-contact InOx

TFTs, the spreading of InOx ink on both the ITO andAlOx:Nd substrates should be investigated. Figure 5a shows theas-printed InOx layer on the substrates of bare AlOx:Nd, PVA/AlOx:Nd, bare ITO, and PVA/ITO from up to down. It can beseen that the ink tended to form isolated droplets on both bareAlOx:Nd and bare ITO surfaces, and it could not form acontinuous line by reducing drop spacing, which was ascribedto the relatively low surface energy. The droplet on the surfacewith low surface energy tended to move the predepositeddroplet forward to form a larger droplet, which prevented thecontinuous line pattern formation. To solve the problem, a thinPVA layer was introduced to modify the surface of ITO andAlOx:Nd, and continuous line could be formed by setting thedrop spacing to 40 μm. The PVA layer effectively increased thesurface energy and could pin the droplet on the substrate stablybecause of the dissolubility of PVA in the solvent of the droplet.

Figure 2. (a) Surface profile of the as-printed single CYTOP line and(b) its zoom-in profile of a coffee stripe.

Figure 3. Plots of (a) CYTOP coffee stripe width, (b) coffee stripe height, and (c) the distance between adjacent coffee stripes versus substratetemperature for different concentrations of ink.

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The printed short-channel InOx TFT arrays fabricated usingthe method mentioned above were shown in Figure 5b.Figure 6 shows the output and transfer electrical character-

istics of the inkjet-printed InOx TFT. The device showed a

highest μsat of 4.9 cm2 V−1 s−1, a Vth of −8.4 V, a subthresholdswing (SS) of 0.3 V dec−1, and an on/off ratio of 2.7 × 109. Therelatively low mobility of the short-channel TFTs compared tolong-channel ones was mainly ascribed to contact resistance.33

It is known that the total resistance (Rt) between source anddrain electrodes can be defined as the sum of the source/draincontact resistance (Rc) and the channel resistance (Rch):

34

= +R R Rt c ch (3)

As the channel length reduces, Rch reduces, while Rc remainsunchanged. Therefore, Rc plays a more important role in Rt asthe channel length becomes shorter and shorter. As a result, thecalculated mobilities based on eq 1 decrease.To evaluate the device uniformity of printed TFTs, 50

devices were fabricated. Figure 7a shows the statisticaldistributions of channel lengths. The channel lengths showed

a good uniformity of 3.5 ± 0.7 μm. It is worth noting that thechannel length was larger than the width of CYTOP coffeestripes (Figure 3a, 2.95 ± 0.35 μm). This indicates that, afterdewetting from CYTOP coffee stripes, the three-phase contactline of ITO droplets cannot pin on the substrate but move awayfrom the coffee stripes. The InOx TFTs exhibited mobilities of3.65 ± 1.25 cm2 V−1 s−1 (Figure 7b). These results indicate thatthe method of fabrication of short channel TFTs based oncommon inkjet printer system is promising.

4. CONCLUSIONIn summary, short-channel InOx TFT arrays with channellengths of 3.5 ± 0.7 μm were fabricated using the inkjetprinting method. Different from the short-channel fabricatingapproaches reported elsewhere, ultrafine hydrophobic CYTOPcoffee stripes, obtained by inkjet-printing and plasma-treatingprocesses without any photolithography steps, were used todefine the channel length of the TFTs. To the best of ourknowledge, it is the first time that MO-TFTs were fabricatedwith a channel length below 10 μm through the use of acommon inkjet printer without any photolithography steps.Additionally, PVA was first introduced to modify the surface ofITO and AlOx:Nd to regulate the wetting property and controlthe spread of droplets of the InOx ink. This work provides aroute toward fabricating short-channel MO TFTs in a cost-effective manner.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b07204.

Figure 4. (a) AFM image of the ITO electrode pair. (b) Surface profile of the ITO electrode pair and (c) its zoom-in profile in channel region.

Figure 5. (a) Inkjet-printed line pattern of InOx ink on the substratesof bare AlOx:Nd, PVA/AlOx:Nd, bare ITO, and PVA/ITO from up todown. Contact angle images on the right are the ink solvent on thecorresponding substrates (bar = 50 μm). (b) Polarizing microscopeimage of printed InOx TFTs with a scale bar 200 μm length.

Figure 6. (a) Typical output and (b) transfer characteristics of theprinted InOx TFT.

Figure 7. Statistical distributions of (a) channel lengths and (b)saturated mobilities for 50 devices. The channel length was measuredby surface profile before the deposition of InOx semiconducting layer.

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Polarizing microscope image of CYTOP coffee-line array(Figure S1), polarizing microscope images of CYTOPcoffee stripes before and after annealing (Figure S2),jetting waveform and corresponding parameters (FigureS3), surface profile of CYTOP line after treatment ofplasma (Figure S4),water contact angle on CYTOP films(Figure S5), plot of current versus voltage betweenelectrode pairs (Figure S6), and surface tension andviscosity of prepared inks (Table S1) (PDF)

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: lanlinfeng@scut.edu.cn.*E-mail: psjbpeng@scut.edu.cn.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors are grateful to the National “863” Project of China(Grant 2014AA033002), the National “973” Project of China(Grant 2015CB655000), the National Natural ScienceFoundation of China (Grants 61204087 and 51173049), thePearl River S&T Nova Program of Guangzhou (Grant2014J2200053), and the Guangdong Province Science andTechnology Plan (Grants 2013B010403004, 2014B010105008,2014B090916002, 2015B090914003, and 2016B090906002)

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