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Direct writing of inkjet-printed short channel organic thin film transistors
Organic Electronics 51 (2017) 485–489
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Direct writing of inkjet-printed short channel organic thin film transistors ∗
Ta-Ya Chu , Zhiyi Zhang, Afshin Dadvand, Christophe Py, Stephen Lang, Ye Tao
T
∗∗
Advanced Electronics and Photonics Research Centre, National Research Council Canada, 1200 Montreal Road, Ottawa, ON, K1A 0R6, Canada
A R T I C L E I N F O
A B S T R A C T
Keywords: Printed electronics OTFTs Inkjet
A direct-writing fabrication process for fully inkjet-printed short-channel organic thin-film transistors (OTFTs) has been developed. Channels as narrow as 800 nm between two printed Ag electrodes were achieved by printing a special Ag ink on an SU-8 interlayer, which can be partially dissolved by the solvents used in the Ag ink. The ridge formed along the printed Ag line edges due to redistribution of the interlayer material during the drying process limits the ink spread, and separates neighboring printed lines, and is the key to defining an ultranarrow channel for transistor fabrication. The short-channel OTFTs fabricated using this technique have demonstrated well-defined linear and saturation regimes. An extracted mobility of 0.27 cm2/Vs with an on/off ratio of 105 was obtained at a driving voltage of −12 V. The excellent performance of these devices demonstrates the potential of this technique in fabrication of short-channel devices using standard printing technologies.
1. Introduction Flexible electronics has attracted a lot of attention from both academic and industrial research communities in the last decade because of its flexibility, light weight and potential application in wearable devices. Solution processable organic thin-film transistors (OTFT) are one of the basic components in electronic circuits. Their mechanical flexibility and printability are advantageous for use in flexible electronics. Over the last two decades, the performance of OTFTs has been improved significantly, especially in the charge carrier mobility and device air stability. K. Nakayama et al. reported high performance short channel OTFTs with thermally evaporated small molecule materials and lithographically patterned/wet etched Au electrodes [1]; Zschieschang et al. achieved a 0.5 μm short channel small molecule OTFT by a fivemask vacuum deposition process [2]. In many research labs, the fabrication of solution processable polymer OTFTs generally relies on spincoating and quite often also uses photolithography patterning processes. Although using vacuum deposition, photolithography, and etching process can increase the patterning resolution and device uniformity, it will also increase the fabrication cost and limit the substrate size and scalability. Different printing techniques, such as gravure, flexography, screen, inkjet, and blade coating, have been previously reported for the fabrication of all-printed OTFT devices [3–8]. One of the challenges for all-printed transistors is how to obtain a short channel length by a direct writing/printing process without using photolithography. The minimum channel length that can be obtained ∗
by conventional printing methods is around 20 μm. Beyond that, the possibility of short-circuits between the electrodes increases, leading to a high failure rate in device fabrication. Sirringhaus et al. demonstrated a printed 5-μm channel by using a photolithographically patterned polyimide strip as an ink bank between the two electrodes [9]. Another method introduced by Sirringhaus's group used a fluorinated layer coated on PEDOT:PSS to separate the electrodes and achieved sub-100 nm channel length [10]. However, process variation and printing registration accuracy pose a major challenge for large-scale production. A channel length of 2 μm between two electrodes was achieved by using microcontact printing [11], or by selectively modifying the surface energy with UV irradiation through a photomask [12]. In inkjet printing, one approach is to reduce the size of ink droplets to minimize ink spread. Electrostatic inkjet technology uses a very small nozzle of 0.5 femtoliter, and can achieve a printed Ag line width of 1 μm and a channel length of 1 μm [13]. However the resistance of the printed Ag line is too high and multiple printing passes are needed to reach a sufficient conductivity for electronics application. Direct writing has a significant advantage in simplifying the fabrication process for large area electronics. The droplet size of most commercially available and highly reliable inkjet-printing heads is usually in the range of picoliters (pl) to tens of pl. A directly printed gap of 10 μm can be achieved by optimizing the substrate surface energy and carefully controlling the drop spacing [14]. Recently, Wei Xu et al. achieved a 2 μm short channel by a direct writing method on PVAmodified PET substrate with printed conductive PEDOT:PSS electrodes.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (T.-Y. Chu), [email protected] (Y. Tao).
∗∗
http://dx.doi.org/10.1016/j.orgel.2017.09.047 Received 16 August 2017; Received in revised form 11 September 2017; Accepted 27 September 2017 Available online 29 September 2017 1566-1199/ Crown Copyright © 2017 Published by Elsevier B.V. All rights reserved.
Organic Electronics 51 (2017) 485–489
T.-Y. Chu et al.
They demonstrated a mobility of 0.64 × 10−3 cm2/Vs and an on/off ratio of 103 in their all-printed transistor [15]. In this work, a minimum gap of 0.8 μm was obtained between printed Ag electrodes by an innovative direct writing method. Allprinted OTFTs with a short channel length of 3 μm and a channel width of 4 mm were fabricated. A mobility of 0.27 cm2/Vs was extracted from the output curve with an on/off ratio of over 105. The Ag ink used in this experiment contains ethanol - a solvent of high vapor pressure, and the ability to partially dissolve the SU-8 based interlayer. The interaction between ink and interlayer during the ink drying process leads to a redistribution of the interlayer material, forming an ink bank on the edges of printed lines which plays an important role in separating two neighboring lines and limiting ink spread.
2. Experiments We invented a method of printing metal lines to form highly reproducible ultra-narrow-gaps for the fabrication of all-printed short channel transistors [16]. When DMP-3000 Model Fluid, a water based carbon testing ink from Fuji Dimatix, was mixed with ethanol at a volume ratio of 1:1, its surface energy was drastically reduced, and the lines printed on SU-8 surface using this ink could no longer hold their shapes, but instead formed large drops, due to de-wetting. However, two neighboring drops did not merge together and a gap of around 0.3 μm was observed between the two drops, as shown in Supplemental Fig. S1. This paper details our findings and the investigation of the phenomenon. Since ethanol can partially dissolve SU-8, a drop of ethanol evaporating from an SU-8 surface will leave behind a rimmed dimple on the SU-8 surface. In some sense, this is similar to the wellknown coffee ring effect where a radial capillary flow brings the particles contained in the solution to the pinned edge of the droplet. The main difference is that in our case, the SU-8 was gradually dissolved by the ethanol drop, and the surface underneath the droplet became dimpled, as shown in the illustration of Fig. 1 (a). As demonstrated by the modified model ink, ethanol contained in an ink mixture can partially dissolve SU-8, generate ink banks/ridges on the surface, and prevent neighboring lines/drops from merging together. The same principle was applied in our experiment to print metal lines with ultra-narrow gap for OTFTs application. A commercial Ag ink, Sunjet EMD 5603, which contains 25 to 40% ethanol, was selected for printing the source and drain electrodes on SU-8 surfaces. The height of resulting ink bank/ridge along the edges of the Ag lines is proportional to the degree of crosslinking of the SU-8. When the spin coated SU-8 (Microchem SU-8 2002) was exposed to UV light for 0 sec, 4 sec and 10 sec, and soft baked at 95 °C for 7 min, the contact angles on the SU-8 surface were 63°, 67° and 86° respectively. A contact angle of 94° was observed when the SU-8 had UV exposure for 10 sec, with a soft bake at 95 °C for 7 min, and a hard baked at 140 °C for 10 min. The corresponding ridge heights of these samples are 20 nm, 7 nm, 3 nm,
Fig. 2. (a) ZYGO 3D image and (b) SEM cross-section of printed two Ag lines on the uncrosslinked SU-8.
and 2 nm respectively as measured using a ZYGO 3D optical profiler, as shown in Fig. 1(b). For a fixed pattern design, reducing the drop spacing will increase the ink quantity printed in the same area, thus increase the line width, and reduce the gap spacing. If the drop spacing is too small, two neighboring lines may eventually merge together by spill over the ink bank. When printing the Ag ink on an uncrosslinked SU-8 surface, without UV exposure, a minimum gap of 0.8 μm was achieved between two printed Ag lines by carefully adjusting the printing parameters such as drop spacing and number of pixels between two lines, as shown in Fig. 2. No contact or short-circuit was found between the two lines over the entire length of 2.5 mm. When printing Ag lines on a UV exposed and hard baked SU-8 Fig. 1. (a) An illustration of solvent interaction between ink and interlayer material and (b) the ridge height versus the contact angle measured from the different degrees of crosslinking. Spin coated SU-8 exposed to UV light for 0 sec, 4 sec and 10 sec and soft baked at 95 °C for 7 min results in contact angles of 63°, 67° and 86°. The fully crosslinked SU-8 had extra hard baking at 140° for 10 min and produced a contact angle of 94°.
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Organic Electronics 51 (2017) 485–489
T.-Y. Chu et al.
Fig. 3. Printed Ag lines with a short channel length of 3 μm measured by ZYGO (a) slope-X-map and (b) 3D image. Allprinted OTFT measured by SEM for (c) top-view image and (d) cross-section image.
Fig. 4. The (a) output and (b) transfer curve of the all-printed OTFT with a channel length of 3 μm and channel width of 4 mm. Solid lines are fitted data in the transfer curve for the mobility and threshold voltage extraction.
mobility and threshold voltage (Vth) from the transfer curve, using a square root plot of Ids versus Vgs, a threshold voltage of −5.8 V and a mobility of 3 × 10−3 cm2/Vs were obtained, as shown in Fig. 4(b). However, the simulated curves based on the extracted mobility and threshold voltage from the transfer curve show a very poor agreement with the experimental data in the output characteristics as shown in Supplemental Fig. S2. It indicates that extracting mobility from the transfer curve is not very reliable in this case. C. Liu et al. introduced a G-function method to extract mobility from the output conductance [17,18]. Using this method, the obtained mobility is gate bias dependent, and its value changed from 0.16 to 0.27 cm2/Vs in our all-printed transistors when the gate voltage varied from −8 V to −12 V, as shown in Fig. 5. Using the same dielectric and semiconductor materials, a mobility of 0.86 cm2/Vs was obtained for 3-layer-printed OTFTs when lithographed Au was used to fabricate the source and drain electrodes [19]. Therefore, the mobility extracted by using G-function method is more reliable in this work. The printing processes were conducted with a Dimatix DMP 5000 inkjet printer using a Dimatix 10 pl print head. The Ag ink EMD 5603 Ag was purchased from Sun Chemical, and inkjet printed on the SU-8 interlayer which was spin coated on the PET substrate. The resistivity of
surface, Ag ink will dewet and form ink beads due to the low surface energy of crosslinked SU-8. By applying an air plasma treatment on the crosslinked SU-8 substrate at 25W for 30 sec, a channel of 3 μm long and 4 mm wide was obtained by direct printing, as shown in Fig. 3. The x-axis slope image from the 3D profiler shows a significant edge effect. Very uniform and smooth edges on the printed lines, over 4 mm in length, have been observed by both optical microscopy and SEM. Conjugated p-type organic semiconductor, poly(4-vinylphenol) (PVP) based dielectric (Xerox xdi-d1.2), and a Ag top gate were inkjet-printed in sequence on top of the channel to form a bottom contact top gated OTFT, as shown in the SEM cross-section image in Fig. 3. The sample was tilted at 45° for the cross-section SEM measurement and the thickness of the dielectric layer was measured to be 350 nm. Over 80% of the all-printed transistors with a 3 μm channel length fabricated using this method were functional without any gate leakage current issue. Fig. 4 shows the output and transfer curves of a typical all-printed transistor. The output curves show an excellent saturation characteristic. Over 1 μA of on-current, and an on-off ratio of 105 have been obtained, and a pinch-off voltage of 1.0 V was observed when the gate voltage was set at −12 V. When we used the standard MOSFET model to extract the saturated 487
Organic Electronics 51 (2017) 485–489
T.-Y. Chu et al.
Fig. 5. (a) Plot of transconductance versus Vds for the G-function mobility extraction, (b) the extracted mobility from 0.16 cm2/Vs to 0.27 cm2/Vs were obtained when the gate voltage increased from -8V to −12V for the all-printed OTFT.
Ag was measured at ∼50 × 10−8 Ω m after a thermally anneal at 140 °C for 20 min. For the fabrication of all-printed bottom-contact top gated OTFTs, the conjugated p-type organic semiconductor material, DPP-TT (N-alkyl diketopyrrolo-pyrrole dithienylthieno[3,2-b] thiophene) was dissolved in orthodichlorobenzene (5 mg/ml) and filtered through a 1 μm PVDF filter, and inkjet-printed to cover the channel area with the cartridge heated to 40 °C and the print platen kept at room temperature. Samples were kept in a nitrogen box at room temperature overnight. Then the PVP based dielectric, Xerox xdi-d1.2, was inkjetprinted on top of the semiconductor layer using a drop spacing of 20 μm with three pixels line, with both the print platen and nozzle head kept at room temperature. A smooth dielectric layer with a thickness of 350 nm and roughness of ∼1.0 nm was obtained by using the coffee ring effect method [19]. Then the samples were treated by air plasma at 25 W for 10 sec before printing the top Ag gate in order to allow the Ag ink wet properly on the crosslinked PVP surface. In all thermal annealing processes, the oven was first ramped up from 80 °C to 140 °C over 20 min with the samples inside; for Ag and semiconductor layers, the oven was kept at 140 °C for another 20 min; for dielectric layers the oven was kept at 140 °C for 40 min.
sensitivity of the device. However, the rough Ag surface observed on uncrosslinked SU-8 may cause high leakage currents when used as bottom electrodes in top gate configuration OTFTs. Using a thicker high-K dielectric layer printed on the source and drain electrodes may overcome this problem. To obtain narrow channels and smoother bottom electrodes for OTFT applications, a fine-tuning is required in order to minimize the dissolution of SU-8, to have a smoother Ag surface, but still to produce a proper ink bank to block ink spread. This was achieved by printing Ag ink on a crosslinked SU-8 surface, because the SU-8 films exposed to UV light and hard baked at 140 °C for 10 min still contains a small percentage of epoxy groups that are not crosslinked and can be partially dissolved by the solvent. Source and drain electrodes with very smooth surfaces, roughness Ra∼6 nm, are presented in Fig. 3. Prior to printing, a plasma pre-treatment is required for the cross-linked SU-8 in order to increase the surface energy, so the Ag ink can wet properly on the SU-8 surface. We also use another polymer material, PVP, instead of SU-8 as an interlayer on PET substrate. A short channel of 4 μm has also achieved by controlling the crosslink of PVP. Wei Xu et al. reported 2 μm short channels of all-printed OTFTs by using PEDOT:PSS ink on a PVA-modified PET substrate. A mobility of 0.64 × 10−3 cm2/Vs with an on/off ratio of 103 was obtained [15]. In our work, the short channel is attributed not only to the partial dissolution of the interlayer, but also to the fast solvent evaporation that redistributes the interlayer material to form ink banks. We obtained a standard variation of 47% in turn-on current, and estimated a similar order of magnitude variation in hole mobility at −12 V from 22 all-printed OTFTs, where channel length is around 3 μm. It is difficult to measure the variations in channel lengths at this scale using optical microscopy. Therefore, the variation in turnon current and calculated mobility could be caused by variations in the channel length, the dielectric layer thickness and the morphology of semiconductor layer. Well defined linear and saturation regime have been obtained from our all-printed OTFTs, remarkable flat saturation currents with the relatively small pinch-off voltages are noticed, as shown in Fig. 4(a). Lack of saturation characteristic in OTFT output curves is usually observed in short-channel OTFTs, caused by the field-dependent mobility and shortchannel effect aggravated by high operating voltage [20]. It is a surprise that non-saturation characteristic was not observed in our allprinted OTFTs. L. Mariucci et al. reported the current spreading effect in fully printed OTFTs with Schotty type source and drain electrodes. A large Schottky barrier between source electrode and semiconductor layer may result in the pinch-off of the channel at the source end [21]. In our work, the surface work function of the printed Ag was measured as 4.37 eV by Ultraviolet Photoelectron Spectroscopy (UPS), and the highest occupied molecular orbital (HOMO) energy level of the
3. Discussion It is well known that many polymer substrates/surfaces can be dissolved by some solvents, and result in different degrees of swelling and material redistribution. The dissolving/swelling effect is usually a drawback for many microfabrication processes. We have demonstrated in this work that by controlling the interaction between ink and substrate/interlayer, a very small and reproducible gap can be produced between two printed Ag lines. SU-8 is a commonly used negative photoresist in the semiconductor industry. Uncrosslinked SU-8 thin films are soluble and can be removed by solvents. Exposure to UV light will initiate polymerization in SU-8 to form a crosslinked SU-8 thin film. The degree of cross-linking is dependent on the UV dosage (intensity and time) and thermal annealing temperature. Therefore, when the ethanol-containing Ag ink is printed on an uncrosslinked SU-8 film/ substrate, it dissolves some SU-8 underneath and causes a significant redistribution of SU-8 due to the capillary flow toward the edges during the ink drying process, resulting in ridged edges along the printed lines, as illustrated in Fig. 1(a). Because the SU-8 ridges formed along the edges of the printed lines function well as an ink bank to keep ink from spreading, a reproducible 0.8 μm gap between two printed Ag lines has been achieved by the direct writing method. This direct writing of a small gap between two electrodes could be used in humidity or temperature sensors with active material deposited between two electrodes; the smaller the gap between the electrodes, the larger the 488
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[5] W.J. Hyun, E.B. Secor, G.A. Rojas, M.C. Hersam, L.F. Francis, C.D. Frisbie, Allprinted, foldable organic thin-film transistors on glassine paper, Adv. Mater. 27 (2015) 7058–7064. [6] D. Tobjörk, N.J. Kaihovirta, T. Mäkelä, F.S. Pettersson, R. Österbacka, All-printed low-voltage organic transistors, Org. Electron. 9 (2008) 931–935. [7] K. Fukuda, Y. Takeda, Y. Yoshimura, R. Shiwaku, L.T. Tran, T. Sekine, M. Mizukami, D. Dumaki, S. Tokito, Fully-printed high-performance organic thinfilm transistors and circuitry on one-micron-thick polymer films, Nat. Commun. 5 (2014) 4147. [8] Linrun Feng, Chen Jiang, Hanbin Ma, Xiaojun Guo, Arokia Nathan, All ink-jet printed low-voltage organic field-effect transistors on flexible substrate, Org. Electron. 38 (2016) 186–192. [9] H. Sirringhaus, T. Kawase, R.H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E.P. Woo, High-resolution inkjet printing of all-polymer transistor circuits, Science 290 (2000) 2123–2126. [10] C.W. Sele, T. von Werne, R.H. Friend, H. Sirringhaus, Self-aligned inkjet printing with sub-hundred-nanometer resolution, Adv. Mater. 17 (2005) 997–1001. [11] J.A. Rogers, Z. Bao, A. Makhija, P. Braun, Printing process suitable for reel-to-reel production of high-performance organic transistors and circuits, Adv. Mater. 11 (1999) 741–745. [12] K. Suzuki, K. Yutani, M. Nakashima, K. Kameyama, All-printed organic TFT backplanes for flexible electronics paper, J. Imaging Soc. Jpn. 50 (2011) 142–147. [13] T. Yokota, T. Sekitani, Y. Kato, K. Kuribara, Ute Zschieschang, Hagen Klauk, Tatsuya Yamamoto, Kazuo Takimiya, Hirokazu Kuwabara, Masaaki Ikeda, Takao Someya, Low-voltage organic transistor with subfemtoliter inkjet sourcedrain contacts, MRS Commun. 1 (2011) 3–6. [14] W. Tang, Y. Chen, J. Zhao, S. Chen, L. Feng, X. Guo, Inkjet printing narrow fine Ag lines on surface modified polymeric films, Proc. 8th IEEE Int. Conf. NEMS, 2013, pp. 1171–1174. [15] W. Xu, Z. Hu, H. Liu, L. Lan, J. Peng, J. Wang, Y. Cao, Flexible all-organic, allsolution processed thin film transistor array with ultrashort channel, Sci. Rep. 6 (2016) 29055. [16] T.Y. Chu, C. Py, Y. Tao, Z. Zhang, A. Dadvand, Method of Printing Ultranarrow-Gap Lines. PCT International Patent Application WO2017004703. [17] C. Liu, T. Minari, Y. Xu, Y.Y. Noh, Direct and quantitative understanding of the nonOhmic contact resistance in organic and oxide thin-film transistors, Org. Elecron. 27 (2015) 253–258. [18] C. Liu, G. Huseynova, Y. Xu, D.X. Long, W.T. Park, X. Liu, T. Minari, Y.Y. Noh, Universal diffusion-limited injection and the hook effect in organic thin-film transistors, Sci. Rep. 6 (2016) 29811. [19] N. Graddage, T.Y. Chu, H. Ding, Y. Tao, Inkjet printed thin and uniform dielectrics for capacitors and organic thin film transistors enabled by the coffee ring effect, Org. Electron. 29 (2016) 114–119. [20] S. Locci, M. Morana, E. Orgiu, A. Bonfiglio, P. Lugli, Modeling of short-channel effects in organic thin-film transistors, IEEE Trans. Elec. Devices 55 (2008) 2561–2567. [21] L. Mariucci, M. Rapisarda, A. Valletta, S. Jacob, M. Benwadih, G. Fortunato, Current spreading effects in fully printed p-channel organic thin film transistors with Schottky source-drain contacts, Org. Electron. 14 (2013) 86–93. [22] Y. Li, S.P. Singh, P. Sonar, A high mobility P-Type DPP-thieno[3,2-b]thiophene copolymer for organic thin-film transistors, Adv. Mater. 22 (2010) 4862–4866. [23] L. Jun, Y. Zhao, H.S. Tan, Y. Guo, C.A. Di, G. Yu, Y. Liu, M. Lin, S.H. Lim, Y. Zhou, H. Su, B.S. Ong, A stable solution-processed polymer semiconductor with record high-mobility for printed transistors, Sci. Rep. 2 (2012) 754.
semiconductor material DPP-TT [22,23] (N-alkyl diketopyrrolo-pyrrole dithienylthieno[3,2-b] thiophene) is 5.4 eV, therefore in our case the injection barrier is estimated to be ∼1.03 eV. Therefore, the low pinchoff voltage observed in our all-printed OTFTs could be attributed to the large Schottky barrier between source electrode and semiconductor layer. The outstanding saturation characteristic along with a relatively low pinch-off voltage of all-printed transistors shows their potential for use in some specific circuit design. Further investigation is in progress. 4. Conclusion In summary, we have demonstrated that the redistribution of the interlayer material due to the ink solvent/SU-8 interaction can form SU8 ridges along the printed lines, which provides a way to precisely control the gap between the lines. This result clearly demonstrates the importance of solvent effects at interfaces in the printable electronics. Acknowledgement The authors gratefully acknowledge the technical support from Mr. Jeff Fraser, Mr. Eric Estwick, Ms. Raluca Movileanu, Mr. Hiroshi Fukutani and Mr. Craig Storey. This work was performed as part of the Printable Electronics program at the National Research Council of Canada. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.orgel.2017.09.047. References [1] K. Nakayama, M. Uno, T. Uemura, N. Namba, Y. Kanaoka, T. Kato, M. Katayama, C. Mitsui, T. Okamoto, J. Takeya, High-mobility organic transistors with wet-etchpatterned top electrodes: a novel patterning method for fine-pitch integration of organic devices, Adv. Mater. Interfaces 1 (2014) 1300124. [2] U. Zschieschang, U. Kraft, R. Rödel, H. Klauk, Submicron-channel-length organic thin-film transistors on flexible substrates, IEEE Nano. Mater. and Dev. Conference (NMDC), 2016, http://dx.doi.org/10.1109/NMDC.2016.7777140. [3] A.F. Vornbrock, D. Sung, H. Kang, R. Kitsomboonloha, V. Subramanian, Fully gravure and ink-jet printed high speed pBTTT organic thin film transistors, Org. Electron. 11 (2010) 2037–2044. [4] A. Pierre, M. Sadeghi, M.M. Payne, A. Facchetti, J.E. Anthony, A.C. Arias, Allprinted flexible organic transistors enabled by surface tension-guided blade coating, Adv. Mater. 26 (2014) 5722–5727.
489
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Direct writing of inkjet-printed short channel organic thin film transistors ∗
Ta-Ya Chu , Zhiyi Zhang, Afshin Dadvand, Christophe Py, Stephen Lang, Ye Tao
T
∗∗
Advanced Electronics and Photonics Research Centre, National Research Council Canada, 1200 Montreal Road, Ottawa, ON, K1A 0R6, Canada
A R T I C L E I N F O
A B S T R A C T
Keywords: Printed electronics OTFTs Inkjet
A direct-writing fabrication process for fully inkjet-printed short-channel organic thin-film transistors (OTFTs) has been developed. Channels as narrow as 800 nm between two printed Ag electrodes were achieved by printing a special Ag ink on an SU-8 interlayer, which can be partially dissolved by the solvents used in the Ag ink. The ridge formed along the printed Ag line edges due to redistribution of the interlayer material during the drying process limits the ink spread, and separates neighboring printed lines, and is the key to defining an ultranarrow channel for transistor fabrication. The short-channel OTFTs fabricated using this technique have demonstrated well-defined linear and saturation regimes. An extracted mobility of 0.27 cm2/Vs with an on/off ratio of 105 was obtained at a driving voltage of −12 V. The excellent performance of these devices demonstrates the potential of this technique in fabrication of short-channel devices using standard printing technologies.
1. Introduction Flexible electronics has attracted a lot of attention from both academic and industrial research communities in the last decade because of its flexibility, light weight and potential application in wearable devices. Solution processable organic thin-film transistors (OTFT) are one of the basic components in electronic circuits. Their mechanical flexibility and printability are advantageous for use in flexible electronics. Over the last two decades, the performance of OTFTs has been improved significantly, especially in the charge carrier mobility and device air stability. K. Nakayama et al. reported high performance short channel OTFTs with thermally evaporated small molecule materials and lithographically patterned/wet etched Au electrodes [1]; Zschieschang et al. achieved a 0.5 μm short channel small molecule OTFT by a fivemask vacuum deposition process [2]. In many research labs, the fabrication of solution processable polymer OTFTs generally relies on spincoating and quite often also uses photolithography patterning processes. Although using vacuum deposition, photolithography, and etching process can increase the patterning resolution and device uniformity, it will also increase the fabrication cost and limit the substrate size and scalability. Different printing techniques, such as gravure, flexography, screen, inkjet, and blade coating, have been previously reported for the fabrication of all-printed OTFT devices [3–8]. One of the challenges for all-printed transistors is how to obtain a short channel length by a direct writing/printing process without using photolithography. The minimum channel length that can be obtained ∗
by conventional printing methods is around 20 μm. Beyond that, the possibility of short-circuits between the electrodes increases, leading to a high failure rate in device fabrication. Sirringhaus et al. demonstrated a printed 5-μm channel by using a photolithographically patterned polyimide strip as an ink bank between the two electrodes [9]. Another method introduced by Sirringhaus's group used a fluorinated layer coated on PEDOT:PSS to separate the electrodes and achieved sub-100 nm channel length [10]. However, process variation and printing registration accuracy pose a major challenge for large-scale production. A channel length of 2 μm between two electrodes was achieved by using microcontact printing [11], or by selectively modifying the surface energy with UV irradiation through a photomask [12]. In inkjet printing, one approach is to reduce the size of ink droplets to minimize ink spread. Electrostatic inkjet technology uses a very small nozzle of 0.5 femtoliter, and can achieve a printed Ag line width of 1 μm and a channel length of 1 μm [13]. However the resistance of the printed Ag line is too high and multiple printing passes are needed to reach a sufficient conductivity for electronics application. Direct writing has a significant advantage in simplifying the fabrication process for large area electronics. The droplet size of most commercially available and highly reliable inkjet-printing heads is usually in the range of picoliters (pl) to tens of pl. A directly printed gap of 10 μm can be achieved by optimizing the substrate surface energy and carefully controlling the drop spacing [14]. Recently, Wei Xu et al. achieved a 2 μm short channel by a direct writing method on PVAmodified PET substrate with printed conductive PEDOT:PSS electrodes.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (T.-Y. Chu), [email protected] (Y. Tao).
∗∗
http://dx.doi.org/10.1016/j.orgel.2017.09.047 Received 16 August 2017; Received in revised form 11 September 2017; Accepted 27 September 2017 Available online 29 September 2017 1566-1199/ Crown Copyright © 2017 Published by Elsevier B.V. All rights reserved.
Organic Electronics 51 (2017) 485–489
T.-Y. Chu et al.
They demonstrated a mobility of 0.64 × 10−3 cm2/Vs and an on/off ratio of 103 in their all-printed transistor [15]. In this work, a minimum gap of 0.8 μm was obtained between printed Ag electrodes by an innovative direct writing method. Allprinted OTFTs with a short channel length of 3 μm and a channel width of 4 mm were fabricated. A mobility of 0.27 cm2/Vs was extracted from the output curve with an on/off ratio of over 105. The Ag ink used in this experiment contains ethanol - a solvent of high vapor pressure, and the ability to partially dissolve the SU-8 based interlayer. The interaction between ink and interlayer during the ink drying process leads to a redistribution of the interlayer material, forming an ink bank on the edges of printed lines which plays an important role in separating two neighboring lines and limiting ink spread.
2. Experiments We invented a method of printing metal lines to form highly reproducible ultra-narrow-gaps for the fabrication of all-printed short channel transistors [16]. When DMP-3000 Model Fluid, a water based carbon testing ink from Fuji Dimatix, was mixed with ethanol at a volume ratio of 1:1, its surface energy was drastically reduced, and the lines printed on SU-8 surface using this ink could no longer hold their shapes, but instead formed large drops, due to de-wetting. However, two neighboring drops did not merge together and a gap of around 0.3 μm was observed between the two drops, as shown in Supplemental Fig. S1. This paper details our findings and the investigation of the phenomenon. Since ethanol can partially dissolve SU-8, a drop of ethanol evaporating from an SU-8 surface will leave behind a rimmed dimple on the SU-8 surface. In some sense, this is similar to the wellknown coffee ring effect where a radial capillary flow brings the particles contained in the solution to the pinned edge of the droplet. The main difference is that in our case, the SU-8 was gradually dissolved by the ethanol drop, and the surface underneath the droplet became dimpled, as shown in the illustration of Fig. 1 (a). As demonstrated by the modified model ink, ethanol contained in an ink mixture can partially dissolve SU-8, generate ink banks/ridges on the surface, and prevent neighboring lines/drops from merging together. The same principle was applied in our experiment to print metal lines with ultra-narrow gap for OTFTs application. A commercial Ag ink, Sunjet EMD 5603, which contains 25 to 40% ethanol, was selected for printing the source and drain electrodes on SU-8 surfaces. The height of resulting ink bank/ridge along the edges of the Ag lines is proportional to the degree of crosslinking of the SU-8. When the spin coated SU-8 (Microchem SU-8 2002) was exposed to UV light for 0 sec, 4 sec and 10 sec, and soft baked at 95 °C for 7 min, the contact angles on the SU-8 surface were 63°, 67° and 86° respectively. A contact angle of 94° was observed when the SU-8 had UV exposure for 10 sec, with a soft bake at 95 °C for 7 min, and a hard baked at 140 °C for 10 min. The corresponding ridge heights of these samples are 20 nm, 7 nm, 3 nm,
Fig. 2. (a) ZYGO 3D image and (b) SEM cross-section of printed two Ag lines on the uncrosslinked SU-8.
and 2 nm respectively as measured using a ZYGO 3D optical profiler, as shown in Fig. 1(b). For a fixed pattern design, reducing the drop spacing will increase the ink quantity printed in the same area, thus increase the line width, and reduce the gap spacing. If the drop spacing is too small, two neighboring lines may eventually merge together by spill over the ink bank. When printing the Ag ink on an uncrosslinked SU-8 surface, without UV exposure, a minimum gap of 0.8 μm was achieved between two printed Ag lines by carefully adjusting the printing parameters such as drop spacing and number of pixels between two lines, as shown in Fig. 2. No contact or short-circuit was found between the two lines over the entire length of 2.5 mm. When printing Ag lines on a UV exposed and hard baked SU-8 Fig. 1. (a) An illustration of solvent interaction between ink and interlayer material and (b) the ridge height versus the contact angle measured from the different degrees of crosslinking. Spin coated SU-8 exposed to UV light for 0 sec, 4 sec and 10 sec and soft baked at 95 °C for 7 min results in contact angles of 63°, 67° and 86°. The fully crosslinked SU-8 had extra hard baking at 140° for 10 min and produced a contact angle of 94°.
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Fig. 3. Printed Ag lines with a short channel length of 3 μm measured by ZYGO (a) slope-X-map and (b) 3D image. Allprinted OTFT measured by SEM for (c) top-view image and (d) cross-section image.
Fig. 4. The (a) output and (b) transfer curve of the all-printed OTFT with a channel length of 3 μm and channel width of 4 mm. Solid lines are fitted data in the transfer curve for the mobility and threshold voltage extraction.
mobility and threshold voltage (Vth) from the transfer curve, using a square root plot of Ids versus Vgs, a threshold voltage of −5.8 V and a mobility of 3 × 10−3 cm2/Vs were obtained, as shown in Fig. 4(b). However, the simulated curves based on the extracted mobility and threshold voltage from the transfer curve show a very poor agreement with the experimental data in the output characteristics as shown in Supplemental Fig. S2. It indicates that extracting mobility from the transfer curve is not very reliable in this case. C. Liu et al. introduced a G-function method to extract mobility from the output conductance [17,18]. Using this method, the obtained mobility is gate bias dependent, and its value changed from 0.16 to 0.27 cm2/Vs in our all-printed transistors when the gate voltage varied from −8 V to −12 V, as shown in Fig. 5. Using the same dielectric and semiconductor materials, a mobility of 0.86 cm2/Vs was obtained for 3-layer-printed OTFTs when lithographed Au was used to fabricate the source and drain electrodes [19]. Therefore, the mobility extracted by using G-function method is more reliable in this work. The printing processes were conducted with a Dimatix DMP 5000 inkjet printer using a Dimatix 10 pl print head. The Ag ink EMD 5603 Ag was purchased from Sun Chemical, and inkjet printed on the SU-8 interlayer which was spin coated on the PET substrate. The resistivity of
surface, Ag ink will dewet and form ink beads due to the low surface energy of crosslinked SU-8. By applying an air plasma treatment on the crosslinked SU-8 substrate at 25W for 30 sec, a channel of 3 μm long and 4 mm wide was obtained by direct printing, as shown in Fig. 3. The x-axis slope image from the 3D profiler shows a significant edge effect. Very uniform and smooth edges on the printed lines, over 4 mm in length, have been observed by both optical microscopy and SEM. Conjugated p-type organic semiconductor, poly(4-vinylphenol) (PVP) based dielectric (Xerox xdi-d1.2), and a Ag top gate were inkjet-printed in sequence on top of the channel to form a bottom contact top gated OTFT, as shown in the SEM cross-section image in Fig. 3. The sample was tilted at 45° for the cross-section SEM measurement and the thickness of the dielectric layer was measured to be 350 nm. Over 80% of the all-printed transistors with a 3 μm channel length fabricated using this method were functional without any gate leakage current issue. Fig. 4 shows the output and transfer curves of a typical all-printed transistor. The output curves show an excellent saturation characteristic. Over 1 μA of on-current, and an on-off ratio of 105 have been obtained, and a pinch-off voltage of 1.0 V was observed when the gate voltage was set at −12 V. When we used the standard MOSFET model to extract the saturated 487
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Fig. 5. (a) Plot of transconductance versus Vds for the G-function mobility extraction, (b) the extracted mobility from 0.16 cm2/Vs to 0.27 cm2/Vs were obtained when the gate voltage increased from -8V to −12V for the all-printed OTFT.
Ag was measured at ∼50 × 10−8 Ω m after a thermally anneal at 140 °C for 20 min. For the fabrication of all-printed bottom-contact top gated OTFTs, the conjugated p-type organic semiconductor material, DPP-TT (N-alkyl diketopyrrolo-pyrrole dithienylthieno[3,2-b] thiophene) was dissolved in orthodichlorobenzene (5 mg/ml) and filtered through a 1 μm PVDF filter, and inkjet-printed to cover the channel area with the cartridge heated to 40 °C and the print platen kept at room temperature. Samples were kept in a nitrogen box at room temperature overnight. Then the PVP based dielectric, Xerox xdi-d1.2, was inkjetprinted on top of the semiconductor layer using a drop spacing of 20 μm with three pixels line, with both the print platen and nozzle head kept at room temperature. A smooth dielectric layer with a thickness of 350 nm and roughness of ∼1.0 nm was obtained by using the coffee ring effect method [19]. Then the samples were treated by air plasma at 25 W for 10 sec before printing the top Ag gate in order to allow the Ag ink wet properly on the crosslinked PVP surface. In all thermal annealing processes, the oven was first ramped up from 80 °C to 140 °C over 20 min with the samples inside; for Ag and semiconductor layers, the oven was kept at 140 °C for another 20 min; for dielectric layers the oven was kept at 140 °C for 40 min.
sensitivity of the device. However, the rough Ag surface observed on uncrosslinked SU-8 may cause high leakage currents when used as bottom electrodes in top gate configuration OTFTs. Using a thicker high-K dielectric layer printed on the source and drain electrodes may overcome this problem. To obtain narrow channels and smoother bottom electrodes for OTFT applications, a fine-tuning is required in order to minimize the dissolution of SU-8, to have a smoother Ag surface, but still to produce a proper ink bank to block ink spread. This was achieved by printing Ag ink on a crosslinked SU-8 surface, because the SU-8 films exposed to UV light and hard baked at 140 °C for 10 min still contains a small percentage of epoxy groups that are not crosslinked and can be partially dissolved by the solvent. Source and drain electrodes with very smooth surfaces, roughness Ra∼6 nm, are presented in Fig. 3. Prior to printing, a plasma pre-treatment is required for the cross-linked SU-8 in order to increase the surface energy, so the Ag ink can wet properly on the SU-8 surface. We also use another polymer material, PVP, instead of SU-8 as an interlayer on PET substrate. A short channel of 4 μm has also achieved by controlling the crosslink of PVP. Wei Xu et al. reported 2 μm short channels of all-printed OTFTs by using PEDOT:PSS ink on a PVA-modified PET substrate. A mobility of 0.64 × 10−3 cm2/Vs with an on/off ratio of 103 was obtained [15]. In our work, the short channel is attributed not only to the partial dissolution of the interlayer, but also to the fast solvent evaporation that redistributes the interlayer material to form ink banks. We obtained a standard variation of 47% in turn-on current, and estimated a similar order of magnitude variation in hole mobility at −12 V from 22 all-printed OTFTs, where channel length is around 3 μm. It is difficult to measure the variations in channel lengths at this scale using optical microscopy. Therefore, the variation in turnon current and calculated mobility could be caused by variations in the channel length, the dielectric layer thickness and the morphology of semiconductor layer. Well defined linear and saturation regime have been obtained from our all-printed OTFTs, remarkable flat saturation currents with the relatively small pinch-off voltages are noticed, as shown in Fig. 4(a). Lack of saturation characteristic in OTFT output curves is usually observed in short-channel OTFTs, caused by the field-dependent mobility and shortchannel effect aggravated by high operating voltage [20]. It is a surprise that non-saturation characteristic was not observed in our allprinted OTFTs. L. Mariucci et al. reported the current spreading effect in fully printed OTFTs with Schotty type source and drain electrodes. A large Schottky barrier between source electrode and semiconductor layer may result in the pinch-off of the channel at the source end [21]. In our work, the surface work function of the printed Ag was measured as 4.37 eV by Ultraviolet Photoelectron Spectroscopy (UPS), and the highest occupied molecular orbital (HOMO) energy level of the
3. Discussion It is well known that many polymer substrates/surfaces can be dissolved by some solvents, and result in different degrees of swelling and material redistribution. The dissolving/swelling effect is usually a drawback for many microfabrication processes. We have demonstrated in this work that by controlling the interaction between ink and substrate/interlayer, a very small and reproducible gap can be produced between two printed Ag lines. SU-8 is a commonly used negative photoresist in the semiconductor industry. Uncrosslinked SU-8 thin films are soluble and can be removed by solvents. Exposure to UV light will initiate polymerization in SU-8 to form a crosslinked SU-8 thin film. The degree of cross-linking is dependent on the UV dosage (intensity and time) and thermal annealing temperature. Therefore, when the ethanol-containing Ag ink is printed on an uncrosslinked SU-8 film/ substrate, it dissolves some SU-8 underneath and causes a significant redistribution of SU-8 due to the capillary flow toward the edges during the ink drying process, resulting in ridged edges along the printed lines, as illustrated in Fig. 1(a). Because the SU-8 ridges formed along the edges of the printed lines function well as an ink bank to keep ink from spreading, a reproducible 0.8 μm gap between two printed Ag lines has been achieved by the direct writing method. This direct writing of a small gap between two electrodes could be used in humidity or temperature sensors with active material deposited between two electrodes; the smaller the gap between the electrodes, the larger the 488
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[5] W.J. Hyun, E.B. Secor, G.A. Rojas, M.C. Hersam, L.F. Francis, C.D. Frisbie, Allprinted, foldable organic thin-film transistors on glassine paper, Adv. Mater. 27 (2015) 7058–7064. [6] D. Tobjörk, N.J. Kaihovirta, T. Mäkelä, F.S. Pettersson, R. Österbacka, All-printed low-voltage organic transistors, Org. Electron. 9 (2008) 931–935. [7] K. Fukuda, Y. Takeda, Y. Yoshimura, R. Shiwaku, L.T. Tran, T. Sekine, M. Mizukami, D. Dumaki, S. Tokito, Fully-printed high-performance organic thinfilm transistors and circuitry on one-micron-thick polymer films, Nat. Commun. 5 (2014) 4147. [8] Linrun Feng, Chen Jiang, Hanbin Ma, Xiaojun Guo, Arokia Nathan, All ink-jet printed low-voltage organic field-effect transistors on flexible substrate, Org. Electron. 38 (2016) 186–192. [9] H. Sirringhaus, T. Kawase, R.H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E.P. Woo, High-resolution inkjet printing of all-polymer transistor circuits, Science 290 (2000) 2123–2126. [10] C.W. Sele, T. von Werne, R.H. Friend, H. Sirringhaus, Self-aligned inkjet printing with sub-hundred-nanometer resolution, Adv. Mater. 17 (2005) 997–1001. [11] J.A. Rogers, Z. Bao, A. Makhija, P. Braun, Printing process suitable for reel-to-reel production of high-performance organic transistors and circuits, Adv. Mater. 11 (1999) 741–745. [12] K. Suzuki, K. Yutani, M. Nakashima, K. Kameyama, All-printed organic TFT backplanes for flexible electronics paper, J. Imaging Soc. Jpn. 50 (2011) 142–147. [13] T. Yokota, T. Sekitani, Y. Kato, K. Kuribara, Ute Zschieschang, Hagen Klauk, Tatsuya Yamamoto, Kazuo Takimiya, Hirokazu Kuwabara, Masaaki Ikeda, Takao Someya, Low-voltage organic transistor with subfemtoliter inkjet sourcedrain contacts, MRS Commun. 1 (2011) 3–6. [14] W. Tang, Y. Chen, J. Zhao, S. Chen, L. Feng, X. Guo, Inkjet printing narrow fine Ag lines on surface modified polymeric films, Proc. 8th IEEE Int. Conf. NEMS, 2013, pp. 1171–1174. [15] W. Xu, Z. Hu, H. Liu, L. Lan, J. Peng, J. Wang, Y. Cao, Flexible all-organic, allsolution processed thin film transistor array with ultrashort channel, Sci. Rep. 6 (2016) 29055. [16] T.Y. Chu, C. Py, Y. Tao, Z. Zhang, A. Dadvand, Method of Printing Ultranarrow-Gap Lines. PCT International Patent Application WO2017004703. [17] C. Liu, T. Minari, Y. Xu, Y.Y. Noh, Direct and quantitative understanding of the nonOhmic contact resistance in organic and oxide thin-film transistors, Org. Elecron. 27 (2015) 253–258. [18] C. Liu, G. Huseynova, Y. Xu, D.X. Long, W.T. Park, X. Liu, T. Minari, Y.Y. Noh, Universal diffusion-limited injection and the hook effect in organic thin-film transistors, Sci. Rep. 6 (2016) 29811. [19] N. Graddage, T.Y. Chu, H. Ding, Y. Tao, Inkjet printed thin and uniform dielectrics for capacitors and organic thin film transistors enabled by the coffee ring effect, Org. Electron. 29 (2016) 114–119. [20] S. Locci, M. Morana, E. Orgiu, A. Bonfiglio, P. Lugli, Modeling of short-channel effects in organic thin-film transistors, IEEE Trans. Elec. Devices 55 (2008) 2561–2567. [21] L. Mariucci, M. Rapisarda, A. Valletta, S. Jacob, M. Benwadih, G. Fortunato, Current spreading effects in fully printed p-channel organic thin film transistors with Schottky source-drain contacts, Org. Electron. 14 (2013) 86–93. [22] Y. Li, S.P. Singh, P. Sonar, A high mobility P-Type DPP-thieno[3,2-b]thiophene copolymer for organic thin-film transistors, Adv. Mater. 22 (2010) 4862–4866. [23] L. Jun, Y. Zhao, H.S. Tan, Y. Guo, C.A. Di, G. Yu, Y. Liu, M. Lin, S.H. Lim, Y. Zhou, H. Su, B.S. Ong, A stable solution-processed polymer semiconductor with record high-mobility for printed transistors, Sci. Rep. 2 (2012) 754.
semiconductor material DPP-TT [22,23] (N-alkyl diketopyrrolo-pyrrole dithienylthieno[3,2-b] thiophene) is 5.4 eV, therefore in our case the injection barrier is estimated to be ∼1.03 eV. Therefore, the low pinchoff voltage observed in our all-printed OTFTs could be attributed to the large Schottky barrier between source electrode and semiconductor layer. The outstanding saturation characteristic along with a relatively low pinch-off voltage of all-printed transistors shows their potential for use in some specific circuit design. Further investigation is in progress. 4. Conclusion In summary, we have demonstrated that the redistribution of the interlayer material due to the ink solvent/SU-8 interaction can form SU8 ridges along the printed lines, which provides a way to precisely control the gap between the lines. This result clearly demonstrates the importance of solvent effects at interfaces in the printable electronics. Acknowledgement The authors gratefully acknowledge the technical support from Mr. Jeff Fraser, Mr. Eric Estwick, Ms. Raluca Movileanu, Mr. Hiroshi Fukutani and Mr. Craig Storey. This work was performed as part of the Printable Electronics program at the National Research Council of Canada. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.orgel.2017.09.047. References [1] K. Nakayama, M. Uno, T. Uemura, N. Namba, Y. Kanaoka, T. Kato, M. Katayama, C. Mitsui, T. Okamoto, J. Takeya, High-mobility organic transistors with wet-etchpatterned top electrodes: a novel patterning method for fine-pitch integration of organic devices, Adv. Mater. Interfaces 1 (2014) 1300124. [2] U. Zschieschang, U. Kraft, R. Rödel, H. Klauk, Submicron-channel-length organic thin-film transistors on flexible substrates, IEEE Nano. Mater. and Dev. Conference (NMDC), 2016, http://dx.doi.org/10.1109/NMDC.2016.7777140. [3] A.F. Vornbrock, D. Sung, H. Kang, R. Kitsomboonloha, V. Subramanian, Fully gravure and ink-jet printed high speed pBTTT organic thin film transistors, Org. Electron. 11 (2010) 2037–2044. [4] A. Pierre, M. Sadeghi, M.M. Payne, A. Facchetti, J.E. Anthony, A.C. Arias, Allprinted flexible organic transistors enabled by surface tension-guided blade coating, Adv. Mater. 26 (2014) 5722–5727.
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