- Email: [email protected]
Nanoimprint assisted inkjet printing to fabricate sub-micron channel organic field effect transistors
Microelectronic Engineering 110 (2013) 292–297
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Nanoimprint assisted inkjet printing to fabricate sub-micron channel organic field effect transistors Lichao Teng a,⇑, Matthias Plötner a, Alexander Türke a, Barbara Adolphi a, Andreas Finn a,b, Robert Kirchner a, Wolf-Joachim Fischer a,b a b
Institute of Semiconductors and Microsystems, Technische Universität Dresden, Germany Fraunhofer Institute for Photonic Microsystems, Dresden, Germany
a r t i c l e
i n f o
Article history: Available online 21 February 2013 Keywords: Organic transistor Nanoimprint Short channel effect Inkjet printing Nanoparticle ink Contact barrier
a b s t r a c t Solution processed poly(3-hexylthiophene) organic field effect transistors with channel lengths down to 750 nm were fabricated by nanoimprint assisted inkjet printing. The nanoimprint lithography was used to define sub-micron channels into a resist because of its high resolution. A silver-containing ink was inkjet-printed onto a pre-patterned resist layer to form a metallic film, which acts as source and drain electrodes after lift-off. This process replaces the expensive vacuum evaporation of gold electrodes. The transistor short channel effect was suppressed successfully by constant field downscaling. However, samples with inkjet-printed silver electrodes have limited current density. They also have lower effective charge mobility due to higher charge injection barrier, as well as the rough metal surface. Gold nanoparticles were added into the silver ink to modify its work function and therefore reduce the contact resistance between electrodes and polymer. This emphasizes the importance of the metal-semiconductor contact especially for short channel organic transistors. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Organic electronics have been widely used in many low-cost, large-area and printable applications such as tags of radiofrequency-identification (RFID) systems, backplanes for matrix displays and sensors [1,2]. The organic field effect transistor (OFET) needs to be low cost and provides high switching speed and sufficient driving current. The cut-off frequency (fc) scales approximately with transistor’s lL-2. The drain current (ID) scales with L-1, where L is the channel length [3,4]. Due to the limited charge carrier mobility, l, for organic semiconductors, shorter channels are required to keep a sufficient fc and ID. For a hole mobility of l = 0.1 cm2/Vs, which is a good value for poly(3-hexylthiophene) (P3HT) [1], sub-micron channels are required to achieve a fc in the MHz range neglecting impacts from other factors such as gate overlap capacitance. Other organic semiconductors, like pentacene (l up to 5 cm2/ Vs) [1,2], the fc in MHz range can be achieved without sub-micron patterning [5]. However, they require expensive vacuum deposition to form the functional layer, which counteracts the cheap production of organic electronics [1,2]. Such short channels provide other benefits such as higher integration density (more
⇑ Corresponding author. Tel.: +49 351 46336414. E-mail address: [email protected] (L. Teng). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.02.027
function/area) and reduce power consumption through lower voltage operation. Many organic semiconductors such as P3HT and organic insulators can be processed in solutions. This feature enables the use of the cheap and high-yield inkjet printing (IJP) to directly fabricate OFETs components such as electrodes or functional layers. Current resolution of commercially available IJP tools is limited to 20–50 lm, depending on the minimum ink droplet volume and the surface proprieties [1,2]. In this work, we demonstrate a technique termed nanoimprint assisted inkjet printing (NIL-IJP) to fabricate sub-micron channel OFETs. The thermally assisted nanoimprint lithography (T-NIL) was used to enhance the IJP resolution. An inkjet printer was applied to print a metal film acting as source and drain electrodes on the pre-patterned resist. It leads to a much lower equipment costs compared to metal evaporation in vacuum. 2. Experiments Source and drain electrodes patterns were fabricated onto a silicon mold (Fig. 1a) with i-line projection lithography (ASML PAS 5500-250 C) and reactive ion etching (RIE). The silicon mold was etched 1 lm deep with an 87° sidewall angle, which is suitable for NIL [6]. The silicon mold surface was modified with a perfluorotrichlorosilane anti-sticking layer to ensure defect-free demolding [7]. To compare devices with different channel lengths, a group of
L. Teng et al. / Microelectronic Engineering 110 (2013) 292–297
Fig. 1. (a) OFETs Si chip (10 mm 10 mm) bonded onto a glass supporter (65 mm 65 mm) with the minimal channel length of 833 nm measured by scanning electron microscope (SEM). (b) Gold source and drain electrodes with a distance of 754 nm after lift-off measured by SEM. The reduced channel dimension comes from the anisotropy of the oxygen plasma etching. (c) OFET with the bottom gate bottom source (S) and drain (D) architecture. The S/D could be vacuum evaporated gold or inkjet-printed silver.
long channel OFETs (L = 2.7 lm, 5 lm and 17.5 lm) was patterned by contact lithography. OFETs were fabricated on highly phosphor-doped silicon wafers (specific resistance 0.08–0.1 X cm) acting as gate electrode. The thermally grown SiO2 (20 nm and 100 nm thick) was applied as gate dielectric. A poly(methylmethacrylate) (PMMA) layer (880 nm thick, molecular weight 120 k) was spin-coated (1000 rpm for 30 s) and baked (140 °C for 2 min) on the SiO2 layer as the photo resist. The T-NIL was done with the 1 lm deep silicon mold, shown in Fig. 1a, and L down to 833 nm. A customized NIL tool, capable of 10 bar maximal pressure was used. The imprint process was performed at 200 °C for 10 min before cooling down to room temperature. Afterwards, the residual resist layer (about 320 nm thick) was measured by a surface profiler (Veeco DEKTAK 8). Following the removal of the residual layer by oxygen plasma RIE, a 5 nm titanium (Ti) film, as the adhesion promoter and a 50 nm gold (Au) film, were deposited by e-beam evaporation. The lift-off was performed by an ultrasonic assisted acetone bath. After that, a minimal channel of 754 nm (Fig. 1b) was generated. The applied oxygen plasma RIE has anisotropy (vertical to lateral) of 8 against the PMMA 120 k resist under the parameters we used. This helped to reduce the channel dimension furthermore. Finally, the P3HT in dichlorobenzene solution (Sepiolid P100, BASF AG) was spin-coated (2000 rpm for 30 s) and baked at 65 °C for 1 h under nitrogen. The OFETs with a bottom gate bottom contact configuration (Fig. 1c) were fabricated. 3. OFETs with gold electrodes The device characterization was conducted in ambient air. Oxygen acts as a dopant for many organic semiconductors because it generates many band gap states [8]. Taylor et al. reported that the polythiophene conductivity increased after exposure to oxygen [9]. Due to high humidity, current saturation is missing in the output characteristics of P3HT-based OFETs [10]. In this work, X-ray photoelectron spectroscopy (XPS) was applied to detect changes
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of the highest occupied molecular orbital (HOMO) due to the air exposure. The fresh spin-coated P3HT film in N2 without any air exposure has a HOMO level of 5.1 eV, which could be the intrinsic value for the material. After a 10 min exposure to the air, (comparable to one electrical measurement duration), the P3HT film was tested immediately and its HOMO level was again 5.1 eV. However, after 10 days storage in a glove box (O2 and H2O concentration below 0.1 %) and the same time in the air, the HOMO level reduced to 4.6 eV and 3.7 eV, respectively. This indicates that an air exposure for about one measurement period (about 10 min) does not cause any changes of P3HT energy levels. Obviously, the absorbed oxygen or water molecules at the surface need a long time before they diffuse into the channel and affect the device performance. Therefore, the test data are reliable under our testing conditions. The electrical measurements were realized with a micro-probe station connected to a precise I-V measure unit. In order to detective the hysteresis effect of the OFETs, a cyclic gate bias sweep from the OFF state to the ON state and then back to OFF state was applied. The VGS has a sweep rate of 4 V/step. Every step has 2 s duration. All the devices with the identical P3HT layer were tested under the same conditions. The performance of the OFET with L = 17.5 lm and W = 1 mm is shown in Fig. 2. The channel length L/gate dielectric thickness (dox = 100 nm) ratio is 175, the Ion/Ioff ratio is above 1100, the threshold voltage (Vth) is 31.7 V and the charge carrier mobility at saturation is 4.4 10 4cm2/Vs. In the ON state (VDS = 36 V and VGS = 4 V), the electrical field over the gate dielectric is Eox = 0.4 MV/cm while EDS = 0.02 MV/cm. This field distribution (Eox/EDS = 20) follows the well-known gradual channel approximation and therefore delivers an excellent long channel behavior. The gate leakage current in the ON state is 0.44 nA while the drain current is 160 nA. The ID–VDS curve (Fig. 2a) shows a very well reproducibility during the whole VGS sweep cycle because of the welldefined gold contacts and the dielectric-polymer interface. In this case, the hysteresis effect is negligible. However, the OFET with L = 650 nm (Fig. 3) shows a typical short channel effect [11–15]. First of all, a super-linear ID increase at the lowest VDS, which indicates a metal-polymer contact barrier [14–16]. It influences the short channel devices more severely because of the reduced channel resistance. Based on the XPS, the ebeam evaporated gold electrodes after lift-off has a work function of 4.6 eV, due to some process residuals left on the gold surface. An increased concentration of carbon atoms (59 %) and oxygen atoms (25 %) was found on the gold surface. Therefore, a P3HT-gold contact offset of 0.5 eV was observed, which might limit the contact resistance. Second, short channel OFET unable to reach saturation In Fig. 3, the device has L = 650 nm, W = 0.27 mm and a 100 nm thick SiO2 (specific capacitance Ci = 3.5 10-4 As/Vm2). The L/dox ratio is 6.5. In the ON state (VDS = 30 V, VGS = 4 V), the Eox is 0.4 MV/ cm, which is even smaller than EDS = 0.46 MV/cm. It means that with decreasing L, the 2-dimensional electrical field distribution in channel deviates from the gradual channel approximation. The gate leakage current is 0.55 nA while the drain current is 1.68 lA at this point. Additionally, the increased EDS always leads to a large bulk current flowing through the entire P3HT film termed space charge limited current (SCLC) according to the Poole–Frenkel effect, which prevents to reach the saturation range [14,15,17–19]. In the OFF state (VDS = 30 V, VGS = 28 V), the large Ioff reduces the Ion/Ioff ratio to 4 and makes the transistor unable to be turned off. The threshold voltage is calculated about 32 V due to the limitation of the dielectric layer. It is comparable with the long channel device (Fig. 2b) due to the similar dielectric interface. The relative large Vth indicates that the SiO2 surface needs to be purified or modified (e.g. by a self-assembling monolayer) in order to
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(a)
Fig. 2. (a) Output and (b) transfer characteristics at VDS = channel.
(b)
36 V of the P3HT-OFET with gold source drain electrodes, 100 nm thick SiO2 layer and 17.5 lm long, 1 mm wide
(a)
Fig. 3. (a) Output and (b) transfer characteristics at VDS = channel.
(b)
30 V of the P3HT-OFET with gold source drain electrodes, 100 nm thick SiO2 layer and 650 nm long, 0.27 mm wide
lower the threshold voltage. The charge carrier mobility is 1.6 10 3 cm2/Vs due to the huge SCLC. In this case, a very small reduction of the ID in the reverse sweep of VGS was observed compared to the forward VGS scan. One possible reason for this tiny hysteresis is the bias stress on the channel induced by the high EDS and Eox [20–22]. Due to the instability of the organic semiconductor structure, the electrical field induced mechanical strains (electrostrictive effect) always causes the reversible hysteresis or even irreversible degradation of the P3HT molecules [23,24]. In Fig. 4, to keep a constant field downscaling, the gate dielectric was thinned from 100 nm to 20 nm (Ci = 1.6 10 3 As/Vm2) and the L/dox ratio was increased to 35. The channel was 750 nm long and 1.35 mm wide. A piranha (mixture of sulfuric acid and hydrogen peroxide) etching for 30 s was conducted to purify the gold surface and its work function was returned to 4.9 eV. The linear increase of ID at the low VDS shows the contact barrier was lowered effectively. The piranha cleaning also removes the residuals left on the SiO2 surface after lift-off. The threshold voltage is lowered to 8 V because of the decreased dox and the cleaned SiO2 surface. Improved saturation was achieved while the SCLC was avoided. The Ion/Ioff ratio was improved to 300. In the ON state (VDS = 10 V and VGS = 12 V), the Eox is 6 MV/cm while the EDS is 0.13 MV/ cm. This Eox/EDS ratio of 45 ensures a long channel performance.
The hole mobility after saturation is 3.8 10 4 cm2/Vs, which is lower than at the 17.5 lm channel OFET with 100 nm SiO2. The gate leakage was measured at 0.8 nA, which is not seriously increased compared to the thick dielectric devices. The ID at this work point (3.7 lA) is 4400 times larger than the gate leakage. The hysteresis effect is negligible because of the purified P3HTSiO2 interface. However, the big-area reliability of thin SiO2 layers is still a long term challenge for OFETs. We have confirmed that although the dielectric strength of 20 nm SiO2 is comparable with the 100 nm film, but its yield is limited.
4. Nanoimprint assisted inkjet printing IJP was performed to replace the expensive gold evaporation. After NIL, the residual layer was removed by oxygen plasma etching. This step made the PMMA as well as the SiO2 surfaces hydrophilic. The ink consisting of a silver neodecanoate saturated solution in xylene (AgNeo) was printed using a commercial IJP-tool (DIMATIX DMP-2830). The ink droplets with a volume of 10 pl began to spread on the PMMA surface and fill up the cavities due to the high surface energy. As a result, an ink film was generated covering the PMMA and the SiO2 surfaces. The solvent in the ink was
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(a)
Fig. 4. (a) Output and (b) transfer characteristics at VDS = channel.
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(b)
10 V of the P3HT-OFET with gold source drain electrodes, 20 nm thin SiO2 layer and 750 nm long, 1.35 mm wide
rapidly evaporated at room temperature once it was printed due to its tiny volume. An AgNeo film with a stable dimension on SiO2 was formed.
Fig. 5. (a) Inkjet-printed AgNeo film changes into Ag film (lines) that covers PMMA and SiO2 after heating at 180 °C. (b) Ag source and drain electrodes after lift-off with an L of about 900 nm measured by SEM. The printed Ag film shows a limited uniformity with a lot of grains. (c) S/D contacts made of AgNeo ink with additional gold nanoparticles. The channel is 912 nm long with an enlarged grain size after a post-heating step at 250 °C for 5 min.
The silver layer in metallic form was generated after heating to 180 °C for 10 min (Fig. 5a). The heating temperature here should be kept low to prevent the movement of PMMA but high enough to convert the AgNeo to metallic silver. At 180 °C, corners of the PMMA patterns were rounded by a certain degree but no flowing effect was observed owning to its high viscosity. Therefore, the channel dimension was kept. Additionally, the shape of the Ag film on the SiO2 surface was maintained once the organic components in the ink were evaporated. After a lift-off, the silver source and drain electrodes were generated on the 20 nm thin SiO2 layer to avoid the short channel effect. The channel length was measured at about 900 nm (Fig. 5b), which has a larger scale compared to Fig. 1b with gold contacts due to the ink shrinkage after the organic components were vaporized. Finally, the semiconductor was spin-coated and the printed OFETs were tested at the same conditions like the devices with gold electrodes. The OFET (L = 900 nm, W = 1.35 mm) with inkjet-printed Ag source and drain electrodes works successfully with a well-defined saturation. This is due to enlarged L/dox ratio of 45 and the Eox/EDS ratio of 42 (Fig. 6). However, in the ON state (VDS = 10 V, VGS = 12 V), the drain current (2 nA) and the hole mobility (7.5 10-8 cm2/Vs) was decreased by almost 3 orders of magnitudes compared to Fig. 4. The Ion/Ioff ratio was kept at 50, the threshold voltage was measured at 4 V, and the gate leakage was 0.5 nA. XPS was used to investigate the P3HT-silver contact. A pure silver surface has a work function of 4.6 eV. Our silver electrodes were generated from an ink, which leaves many organic contaminations and process residuals from lift-off. Such contaminations shift the work function of the printed Ag contacts to 3.6 eV. A huge contact barrier of 1.5 eV to P3HT was generated. Besides the contact offset, the unclean surface and the increased surface roughness (Fig. 6b) of the Ag electrodes also prevent an efficient charge injection from the source into the channel and thus lead to a limited drain current. In Fig. 6, a severe hysteresis effect was observed with lower ID during the reverse VGS sweep. Because the Ag electrodes are not resistant against piranha, the lift-off remains and the ink residuals accumulate on the SiO2 surface and therefore, the interface states could be generated. In the forward scan, many charge carriers could be trapped by these surface states. And if the rate of the release of these trapped charges at the reverse VGS sweep is lower, a reduction of ID is observed [25]. In order to improve the contact performance and increase the hole mobility, we mixed gold nanoparticles into the AgNeo ink to
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(a)
Fig. 6. (a) Output and (b) transfer characteristics at VDS = 1.35 mm wide channel and with hysteresis effect.
(b)
10 V of the P3HT-OFET with inkjet-printed Ag source drain electrodes, 20 nm thin SiO2 layer, 900 nm long,
(a)
(b)
Fig. 7. (a) Output and (b) transfer characteristics at VDS = 10 V of the P3HT-OFET with inkjet-printed Ag + Au nanoparticles source drain electrodes, 20 nm thin SiO2 layer, 912 nm long, 1.35 mm wide channel and with hysteresis effect. The drain current and the charge mobility were improved after a work function modification and a post heating step.
modify its effective work function. After the lift-off step, we performed a post-heating step at 250 °C for 5 min just before P3HT coating with the purpose to remove the organic components from the ink as well as from the lift-off solution. After that, the work function of Ag contacts with Au components was improved from 3.6 eV to 4.1 eV and thus the injection barrier was lowered from 1.5 eV to 1 eV. In Fig. 5c, we found that the grain size of the printed film was enlarged after the post-heating step compared to Fig. 5b, which is believed to benefit the charge transportation in electrodes. In Fig. 7, the OFET with the same configuration but fabricated with a modified ink delivers an improved performance. It has a threshold voltage of 7.7 V, a mobility of 1.4 10-7 cm2/Vs and an Ion/Ioff ratio of 22. In the ON state, the drain current is 2.5 nA and the gate leakage is 0.45 nA. However, the printed Ag film is still much rougher compared to e-beam evaporated Au film in Fig. 1b. The hysteresis effect was still significant due to the impurities on the SiO2 surface as well as the low ID. It emphasizes the importance of a clean dielectric interface.
contacts. This technology leads to a reduced equipment investment as well as a lower operation costs compared to the e-beam gold evaporation in vacuum, especially for the small-lot research uses. The resolution of IJP is improved with nanoimprint as prepatterning method from 20 lm to the sub-micron range. The constant field scaling and a well-adjusted electrode-semiconductor contact can inhibit the short channel effect. This requires an ultrathin and reliable gate dielectric as well as a selected metal with pure surface, especially for sub-micron channel OFETs. The inkjet-printed silver electrodes might be responsible for the limited charge injection due to an enlarged contact offset and limited surface propriety. An optimization of the printed metal and a further downscaling of the channel are required. The P3HT in solvent can also be inkjet-printed to generate organic semiconductor films which may help to save the material compared to spin-coating.
Acknowledgment 5. Summary and outlook Nanoimprint assisted inkjet printing (NIL-IJP) was applied to fabricate 750 nm channel OFETs with printed source drain
The authors acknowledge the funding for this work by the DFG Research Training Group ‘‘Nano- and Biotechnologies for Packaging of Electronic Systems’’ (DFG-1401-2).
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References [1] H. Klauk, Organic Electronics, WILEY-VCH Verlag, 2006, ISBN 3-527-31264-1. [2] C. Wöll, Physical and Chemical Aspects of Organic Electronics, WILEY-VCH Verlag, 2009, ISBN 978-3-527-40810-8. [3] S.M. Sze, Physics of Semiconductor Devices, second ed., John Wiley & Sons, New York, 1981. [4] R.F. Pierret, Semiconductor Device Fundamentals, Addison-Wesley, Reading, 1996. [5] K. Myny, S. Steudel, S. Smout, P. Vicca, F. Furthner, B. van der Putten, A.K. Tripathi, G.H. Gelinck, J. Genoe, W. Dehaene, P. Heremans, Org. Electron. 11 (2010) 1176–1179. [6] J. He, K. Richter, J.W. Bartha, S. Howitz, J. Vac. Sci. Technol. B: Microelectron. Nanometer Struct. 29 (2011) 06FC16. [7] R. Kirchner, L. Teng, B. Lu, B. Adolphi, W.-J. Fischer, Jpn. J. Appl. Phys. 50 (2011) 06GK13. 8pp. [8] J. Manca, L. Goris, E. Kesters, L. Lutsen, T. Martens, K. Haenen, M. Nesladek, O. Sanna, D. Vanderzande, J.D. Haen, L. DeSchepper, Mat. Res. Soc. Symp. Proc. 771 (2003) L12.3.1. [9] D.M. Taylor, H.L. Gomes, A.E. Underhill, S. Edge, P.I. Clemension, J. Appl. Phys. 24 (1991) 2032. [10] S. Hoshino, M. Yoshida, S. Uemura, T. Kodzasa, N. Takada, T. Kamata, K. Yase, J. Appl. Phys. 95 (2004) 5088.
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Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Nanoimprint assisted inkjet printing to fabricate sub-micron channel organic field effect transistors Lichao Teng a,⇑, Matthias Plötner a, Alexander Türke a, Barbara Adolphi a, Andreas Finn a,b, Robert Kirchner a, Wolf-Joachim Fischer a,b a b
Institute of Semiconductors and Microsystems, Technische Universität Dresden, Germany Fraunhofer Institute for Photonic Microsystems, Dresden, Germany
a r t i c l e
i n f o
Article history: Available online 21 February 2013 Keywords: Organic transistor Nanoimprint Short channel effect Inkjet printing Nanoparticle ink Contact barrier
a b s t r a c t Solution processed poly(3-hexylthiophene) organic field effect transistors with channel lengths down to 750 nm were fabricated by nanoimprint assisted inkjet printing. The nanoimprint lithography was used to define sub-micron channels into a resist because of its high resolution. A silver-containing ink was inkjet-printed onto a pre-patterned resist layer to form a metallic film, which acts as source and drain electrodes after lift-off. This process replaces the expensive vacuum evaporation of gold electrodes. The transistor short channel effect was suppressed successfully by constant field downscaling. However, samples with inkjet-printed silver electrodes have limited current density. They also have lower effective charge mobility due to higher charge injection barrier, as well as the rough metal surface. Gold nanoparticles were added into the silver ink to modify its work function and therefore reduce the contact resistance between electrodes and polymer. This emphasizes the importance of the metal-semiconductor contact especially for short channel organic transistors. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Organic electronics have been widely used in many low-cost, large-area and printable applications such as tags of radiofrequency-identification (RFID) systems, backplanes for matrix displays and sensors [1,2]. The organic field effect transistor (OFET) needs to be low cost and provides high switching speed and sufficient driving current. The cut-off frequency (fc) scales approximately with transistor’s lL-2. The drain current (ID) scales with L-1, where L is the channel length [3,4]. Due to the limited charge carrier mobility, l, for organic semiconductors, shorter channels are required to keep a sufficient fc and ID. For a hole mobility of l = 0.1 cm2/Vs, which is a good value for poly(3-hexylthiophene) (P3HT) [1], sub-micron channels are required to achieve a fc in the MHz range neglecting impacts from other factors such as gate overlap capacitance. Other organic semiconductors, like pentacene (l up to 5 cm2/ Vs) [1,2], the fc in MHz range can be achieved without sub-micron patterning [5]. However, they require expensive vacuum deposition to form the functional layer, which counteracts the cheap production of organic electronics [1,2]. Such short channels provide other benefits such as higher integration density (more
⇑ Corresponding author. Tel.: +49 351 46336414. E-mail address: [email protected] (L. Teng). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.02.027
function/area) and reduce power consumption through lower voltage operation. Many organic semiconductors such as P3HT and organic insulators can be processed in solutions. This feature enables the use of the cheap and high-yield inkjet printing (IJP) to directly fabricate OFETs components such as electrodes or functional layers. Current resolution of commercially available IJP tools is limited to 20–50 lm, depending on the minimum ink droplet volume and the surface proprieties [1,2]. In this work, we demonstrate a technique termed nanoimprint assisted inkjet printing (NIL-IJP) to fabricate sub-micron channel OFETs. The thermally assisted nanoimprint lithography (T-NIL) was used to enhance the IJP resolution. An inkjet printer was applied to print a metal film acting as source and drain electrodes on the pre-patterned resist. It leads to a much lower equipment costs compared to metal evaporation in vacuum. 2. Experiments Source and drain electrodes patterns were fabricated onto a silicon mold (Fig. 1a) with i-line projection lithography (ASML PAS 5500-250 C) and reactive ion etching (RIE). The silicon mold was etched 1 lm deep with an 87° sidewall angle, which is suitable for NIL [6]. The silicon mold surface was modified with a perfluorotrichlorosilane anti-sticking layer to ensure defect-free demolding [7]. To compare devices with different channel lengths, a group of
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Fig. 1. (a) OFETs Si chip (10 mm 10 mm) bonded onto a glass supporter (65 mm 65 mm) with the minimal channel length of 833 nm measured by scanning electron microscope (SEM). (b) Gold source and drain electrodes with a distance of 754 nm after lift-off measured by SEM. The reduced channel dimension comes from the anisotropy of the oxygen plasma etching. (c) OFET with the bottom gate bottom source (S) and drain (D) architecture. The S/D could be vacuum evaporated gold or inkjet-printed silver.
long channel OFETs (L = 2.7 lm, 5 lm and 17.5 lm) was patterned by contact lithography. OFETs were fabricated on highly phosphor-doped silicon wafers (specific resistance 0.08–0.1 X cm) acting as gate electrode. The thermally grown SiO2 (20 nm and 100 nm thick) was applied as gate dielectric. A poly(methylmethacrylate) (PMMA) layer (880 nm thick, molecular weight 120 k) was spin-coated (1000 rpm for 30 s) and baked (140 °C for 2 min) on the SiO2 layer as the photo resist. The T-NIL was done with the 1 lm deep silicon mold, shown in Fig. 1a, and L down to 833 nm. A customized NIL tool, capable of 10 bar maximal pressure was used. The imprint process was performed at 200 °C for 10 min before cooling down to room temperature. Afterwards, the residual resist layer (about 320 nm thick) was measured by a surface profiler (Veeco DEKTAK 8). Following the removal of the residual layer by oxygen plasma RIE, a 5 nm titanium (Ti) film, as the adhesion promoter and a 50 nm gold (Au) film, were deposited by e-beam evaporation. The lift-off was performed by an ultrasonic assisted acetone bath. After that, a minimal channel of 754 nm (Fig. 1b) was generated. The applied oxygen plasma RIE has anisotropy (vertical to lateral) of 8 against the PMMA 120 k resist under the parameters we used. This helped to reduce the channel dimension furthermore. Finally, the P3HT in dichlorobenzene solution (Sepiolid P100, BASF AG) was spin-coated (2000 rpm for 30 s) and baked at 65 °C for 1 h under nitrogen. The OFETs with a bottom gate bottom contact configuration (Fig. 1c) were fabricated. 3. OFETs with gold electrodes The device characterization was conducted in ambient air. Oxygen acts as a dopant for many organic semiconductors because it generates many band gap states [8]. Taylor et al. reported that the polythiophene conductivity increased after exposure to oxygen [9]. Due to high humidity, current saturation is missing in the output characteristics of P3HT-based OFETs [10]. In this work, X-ray photoelectron spectroscopy (XPS) was applied to detect changes
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of the highest occupied molecular orbital (HOMO) due to the air exposure. The fresh spin-coated P3HT film in N2 without any air exposure has a HOMO level of 5.1 eV, which could be the intrinsic value for the material. After a 10 min exposure to the air, (comparable to one electrical measurement duration), the P3HT film was tested immediately and its HOMO level was again 5.1 eV. However, after 10 days storage in a glove box (O2 and H2O concentration below 0.1 %) and the same time in the air, the HOMO level reduced to 4.6 eV and 3.7 eV, respectively. This indicates that an air exposure for about one measurement period (about 10 min) does not cause any changes of P3HT energy levels. Obviously, the absorbed oxygen or water molecules at the surface need a long time before they diffuse into the channel and affect the device performance. Therefore, the test data are reliable under our testing conditions. The electrical measurements were realized with a micro-probe station connected to a precise I-V measure unit. In order to detective the hysteresis effect of the OFETs, a cyclic gate bias sweep from the OFF state to the ON state and then back to OFF state was applied. The VGS has a sweep rate of 4 V/step. Every step has 2 s duration. All the devices with the identical P3HT layer were tested under the same conditions. The performance of the OFET with L = 17.5 lm and W = 1 mm is shown in Fig. 2. The channel length L/gate dielectric thickness (dox = 100 nm) ratio is 175, the Ion/Ioff ratio is above 1100, the threshold voltage (Vth) is 31.7 V and the charge carrier mobility at saturation is 4.4 10 4cm2/Vs. In the ON state (VDS = 36 V and VGS = 4 V), the electrical field over the gate dielectric is Eox = 0.4 MV/cm while EDS = 0.02 MV/cm. This field distribution (Eox/EDS = 20) follows the well-known gradual channel approximation and therefore delivers an excellent long channel behavior. The gate leakage current in the ON state is 0.44 nA while the drain current is 160 nA. The ID–VDS curve (Fig. 2a) shows a very well reproducibility during the whole VGS sweep cycle because of the welldefined gold contacts and the dielectric-polymer interface. In this case, the hysteresis effect is negligible. However, the OFET with L = 650 nm (Fig. 3) shows a typical short channel effect [11–15]. First of all, a super-linear ID increase at the lowest VDS, which indicates a metal-polymer contact barrier [14–16]. It influences the short channel devices more severely because of the reduced channel resistance. Based on the XPS, the ebeam evaporated gold electrodes after lift-off has a work function of 4.6 eV, due to some process residuals left on the gold surface. An increased concentration of carbon atoms (59 %) and oxygen atoms (25 %) was found on the gold surface. Therefore, a P3HT-gold contact offset of 0.5 eV was observed, which might limit the contact resistance. Second, short channel OFET unable to reach saturation In Fig. 3, the device has L = 650 nm, W = 0.27 mm and a 100 nm thick SiO2 (specific capacitance Ci = 3.5 10-4 As/Vm2). The L/dox ratio is 6.5. In the ON state (VDS = 30 V, VGS = 4 V), the Eox is 0.4 MV/ cm, which is even smaller than EDS = 0.46 MV/cm. It means that with decreasing L, the 2-dimensional electrical field distribution in channel deviates from the gradual channel approximation. The gate leakage current is 0.55 nA while the drain current is 1.68 lA at this point. Additionally, the increased EDS always leads to a large bulk current flowing through the entire P3HT film termed space charge limited current (SCLC) according to the Poole–Frenkel effect, which prevents to reach the saturation range [14,15,17–19]. In the OFF state (VDS = 30 V, VGS = 28 V), the large Ioff reduces the Ion/Ioff ratio to 4 and makes the transistor unable to be turned off. The threshold voltage is calculated about 32 V due to the limitation of the dielectric layer. It is comparable with the long channel device (Fig. 2b) due to the similar dielectric interface. The relative large Vth indicates that the SiO2 surface needs to be purified or modified (e.g. by a self-assembling monolayer) in order to
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(a)
Fig. 2. (a) Output and (b) transfer characteristics at VDS = channel.
(b)
36 V of the P3HT-OFET with gold source drain electrodes, 100 nm thick SiO2 layer and 17.5 lm long, 1 mm wide
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Fig. 3. (a) Output and (b) transfer characteristics at VDS = channel.
(b)
30 V of the P3HT-OFET with gold source drain electrodes, 100 nm thick SiO2 layer and 650 nm long, 0.27 mm wide
lower the threshold voltage. The charge carrier mobility is 1.6 10 3 cm2/Vs due to the huge SCLC. In this case, a very small reduction of the ID in the reverse sweep of VGS was observed compared to the forward VGS scan. One possible reason for this tiny hysteresis is the bias stress on the channel induced by the high EDS and Eox [20–22]. Due to the instability of the organic semiconductor structure, the electrical field induced mechanical strains (electrostrictive effect) always causes the reversible hysteresis or even irreversible degradation of the P3HT molecules [23,24]. In Fig. 4, to keep a constant field downscaling, the gate dielectric was thinned from 100 nm to 20 nm (Ci = 1.6 10 3 As/Vm2) and the L/dox ratio was increased to 35. The channel was 750 nm long and 1.35 mm wide. A piranha (mixture of sulfuric acid and hydrogen peroxide) etching for 30 s was conducted to purify the gold surface and its work function was returned to 4.9 eV. The linear increase of ID at the low VDS shows the contact barrier was lowered effectively. The piranha cleaning also removes the residuals left on the SiO2 surface after lift-off. The threshold voltage is lowered to 8 V because of the decreased dox and the cleaned SiO2 surface. Improved saturation was achieved while the SCLC was avoided. The Ion/Ioff ratio was improved to 300. In the ON state (VDS = 10 V and VGS = 12 V), the Eox is 6 MV/cm while the EDS is 0.13 MV/ cm. This Eox/EDS ratio of 45 ensures a long channel performance.
The hole mobility after saturation is 3.8 10 4 cm2/Vs, which is lower than at the 17.5 lm channel OFET with 100 nm SiO2. The gate leakage was measured at 0.8 nA, which is not seriously increased compared to the thick dielectric devices. The ID at this work point (3.7 lA) is 4400 times larger than the gate leakage. The hysteresis effect is negligible because of the purified P3HTSiO2 interface. However, the big-area reliability of thin SiO2 layers is still a long term challenge for OFETs. We have confirmed that although the dielectric strength of 20 nm SiO2 is comparable with the 100 nm film, but its yield is limited.
4. Nanoimprint assisted inkjet printing IJP was performed to replace the expensive gold evaporation. After NIL, the residual layer was removed by oxygen plasma etching. This step made the PMMA as well as the SiO2 surfaces hydrophilic. The ink consisting of a silver neodecanoate saturated solution in xylene (AgNeo) was printed using a commercial IJP-tool (DIMATIX DMP-2830). The ink droplets with a volume of 10 pl began to spread on the PMMA surface and fill up the cavities due to the high surface energy. As a result, an ink film was generated covering the PMMA and the SiO2 surfaces. The solvent in the ink was
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(a)
Fig. 4. (a) Output and (b) transfer characteristics at VDS = channel.
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(b)
10 V of the P3HT-OFET with gold source drain electrodes, 20 nm thin SiO2 layer and 750 nm long, 1.35 mm wide
rapidly evaporated at room temperature once it was printed due to its tiny volume. An AgNeo film with a stable dimension on SiO2 was formed.
Fig. 5. (a) Inkjet-printed AgNeo film changes into Ag film (lines) that covers PMMA and SiO2 after heating at 180 °C. (b) Ag source and drain electrodes after lift-off with an L of about 900 nm measured by SEM. The printed Ag film shows a limited uniformity with a lot of grains. (c) S/D contacts made of AgNeo ink with additional gold nanoparticles. The channel is 912 nm long with an enlarged grain size after a post-heating step at 250 °C for 5 min.
The silver layer in metallic form was generated after heating to 180 °C for 10 min (Fig. 5a). The heating temperature here should be kept low to prevent the movement of PMMA but high enough to convert the AgNeo to metallic silver. At 180 °C, corners of the PMMA patterns were rounded by a certain degree but no flowing effect was observed owning to its high viscosity. Therefore, the channel dimension was kept. Additionally, the shape of the Ag film on the SiO2 surface was maintained once the organic components in the ink were evaporated. After a lift-off, the silver source and drain electrodes were generated on the 20 nm thin SiO2 layer to avoid the short channel effect. The channel length was measured at about 900 nm (Fig. 5b), which has a larger scale compared to Fig. 1b with gold contacts due to the ink shrinkage after the organic components were vaporized. Finally, the semiconductor was spin-coated and the printed OFETs were tested at the same conditions like the devices with gold electrodes. The OFET (L = 900 nm, W = 1.35 mm) with inkjet-printed Ag source and drain electrodes works successfully with a well-defined saturation. This is due to enlarged L/dox ratio of 45 and the Eox/EDS ratio of 42 (Fig. 6). However, in the ON state (VDS = 10 V, VGS = 12 V), the drain current (2 nA) and the hole mobility (7.5 10-8 cm2/Vs) was decreased by almost 3 orders of magnitudes compared to Fig. 4. The Ion/Ioff ratio was kept at 50, the threshold voltage was measured at 4 V, and the gate leakage was 0.5 nA. XPS was used to investigate the P3HT-silver contact. A pure silver surface has a work function of 4.6 eV. Our silver electrodes were generated from an ink, which leaves many organic contaminations and process residuals from lift-off. Such contaminations shift the work function of the printed Ag contacts to 3.6 eV. A huge contact barrier of 1.5 eV to P3HT was generated. Besides the contact offset, the unclean surface and the increased surface roughness (Fig. 6b) of the Ag electrodes also prevent an efficient charge injection from the source into the channel and thus lead to a limited drain current. In Fig. 6, a severe hysteresis effect was observed with lower ID during the reverse VGS sweep. Because the Ag electrodes are not resistant against piranha, the lift-off remains and the ink residuals accumulate on the SiO2 surface and therefore, the interface states could be generated. In the forward scan, many charge carriers could be trapped by these surface states. And if the rate of the release of these trapped charges at the reverse VGS sweep is lower, a reduction of ID is observed [25]. In order to improve the contact performance and increase the hole mobility, we mixed gold nanoparticles into the AgNeo ink to
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Fig. 6. (a) Output and (b) transfer characteristics at VDS = 1.35 mm wide channel and with hysteresis effect.
(b)
10 V of the P3HT-OFET with inkjet-printed Ag source drain electrodes, 20 nm thin SiO2 layer, 900 nm long,
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(b)
Fig. 7. (a) Output and (b) transfer characteristics at VDS = 10 V of the P3HT-OFET with inkjet-printed Ag + Au nanoparticles source drain electrodes, 20 nm thin SiO2 layer, 912 nm long, 1.35 mm wide channel and with hysteresis effect. The drain current and the charge mobility were improved after a work function modification and a post heating step.
modify its effective work function. After the lift-off step, we performed a post-heating step at 250 °C for 5 min just before P3HT coating with the purpose to remove the organic components from the ink as well as from the lift-off solution. After that, the work function of Ag contacts with Au components was improved from 3.6 eV to 4.1 eV and thus the injection barrier was lowered from 1.5 eV to 1 eV. In Fig. 5c, we found that the grain size of the printed film was enlarged after the post-heating step compared to Fig. 5b, which is believed to benefit the charge transportation in electrodes. In Fig. 7, the OFET with the same configuration but fabricated with a modified ink delivers an improved performance. It has a threshold voltage of 7.7 V, a mobility of 1.4 10-7 cm2/Vs and an Ion/Ioff ratio of 22. In the ON state, the drain current is 2.5 nA and the gate leakage is 0.45 nA. However, the printed Ag film is still much rougher compared to e-beam evaporated Au film in Fig. 1b. The hysteresis effect was still significant due to the impurities on the SiO2 surface as well as the low ID. It emphasizes the importance of a clean dielectric interface.
contacts. This technology leads to a reduced equipment investment as well as a lower operation costs compared to the e-beam gold evaporation in vacuum, especially for the small-lot research uses. The resolution of IJP is improved with nanoimprint as prepatterning method from 20 lm to the sub-micron range. The constant field scaling and a well-adjusted electrode-semiconductor contact can inhibit the short channel effect. This requires an ultrathin and reliable gate dielectric as well as a selected metal with pure surface, especially for sub-micron channel OFETs. The inkjet-printed silver electrodes might be responsible for the limited charge injection due to an enlarged contact offset and limited surface propriety. An optimization of the printed metal and a further downscaling of the channel are required. The P3HT in solvent can also be inkjet-printed to generate organic semiconductor films which may help to save the material compared to spin-coating.
Acknowledgment 5. Summary and outlook Nanoimprint assisted inkjet printing (NIL-IJP) was applied to fabricate 750 nm channel OFETs with printed source drain
The authors acknowledge the funding for this work by the DFG Research Training Group ‘‘Nano- and Biotechnologies for Packaging of Electronic Systems’’ (DFG-1401-2).
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