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The 66th Japan Society of Applied Physics (JSAP) Spring Meeting 2019 (March 9-12)


The 66th Japan Society of Applied Physics (JSAP) Spring Meeting 2019
New manufacturing processes for organic-TFT are easy to notice

March 9-12, gThe 66th Japan Society of Applied Physics (JSAP) Spring Meeting 2019h was convened in Tokyo Institute of Technology. In this article, some notice oral lectures are picked up among organic-TFT and CNT-TFT technologies.

Organic-TFT can be driven at low-voltage by use of hydrophobic insulator

Recently, if a hydrophobic insulation film with ultra-low surface energy is used as a gate insulator, it's considered to be easy to drive at low voltage and stable driving because desirable boundary between gate insulator and semiconductor layer is formed. The research group of University of Tokyo and National Institute of Advanced Industrial Science and Technology (AIST) have researched that whether or not this low-voltage and stable driving can be realized in case of using a solution-processed organic semiconductor.


Fig 1. (a) Transfer characteristics with two different gate dielectric. Gate dielectric layer is (a) Cytop (red curve) and SiO2 (blue curve), respectively. (b) Switching characteristics of TFTs with different thickness of Cytop layers on silica substrates.1)

In this experiment, a perfluoro polymergCytophand a small molecular and polymer organic semiconductor were used as a gate insulation film and a semiconductor film respectively. Concretely, ‡@Cytop was coated on silicon wafer substrate without SiO2 oxide film, ‡AAg source/drain were printed by the SuPR-NaP method, ‡BOrganic semiconductor film was coated by the push-coating method. And also, a reference device with hydrophilic SiO2 insulation film and evaporated Au source/drain was prepared.

As concerns polymer device using DA polymer, the device with cytop was driven at low-voltage. Furthermore, hysterisis characteristic and bias stress stability were superior, and also, Vth shift was minimized.

On the other hand, a small molecular device was manufactured by making use of enhanced process of wettable property effectively. It's reason why a low small molecular semiconductor-BTBT-Cnh cannot be coated on Cytop film directly because of its hydrophobic property. Device characteristics were almost same as that of the polymer device. In short, if cytop is used, enhancement of characteristics was confirmed to be obtained in the solution-processed device, as with the last deposited device.

Vth is controlled by O2 plasma treatment

Kobe University reported that Vth of the bottom-contact organic transistor could be controlled by O2 plasma treatment. The research group succeeded to control Vth of the top-contact device by O2 plasma treatment of SiO2 insulator film in the last time. In this time, they tried to adopt a bottom-contact device, which is a general structure and easy to manufacture.


Fig 2. Drain current VS gate voltage characteristics of pentacene TFTs2)

Firstly, source/drain electrodes were formed by deposition of Au/Au0.95Ni0.05 films and photolithography processes. And then, UV ozone treatment was done for 15 minutes, the next, O2 plasma treatment was done for 0 - 180 sec, and surface treatment was by making use of hexamethyldisilazane (HMDS) and pentafluorobenzenethiol (PFBT). Finally, pentacene organic semiconductor film was evaporated at 45 nm thickness. By the way, channel length and channel width are 4ƒÊm and 1mm respectively.

Figure 2 shows transfer characteristic. Vth was changed lineally by O2 plasma treatment time. Trend of Vth and carrier mobility against O2 plasma time was almost same as that of the top-contact device. As a result, in the bottom-contact device with short channel, Vth can be controlled within a few V while maintaining carrier mobility.

Single crystal organic film on hydrophilic substrate is transferred to hydrophobic insulator

With respect to manufacturing process of organic-TFT, some new processes were reported as effective methods for the continuous edge casting method, which was possible to obtain single crystal organic semiconductor. Firstly, the research group of University of Tokyo, OPERANDO-OIL, Japan Science and Technology Agency (JST), and National Institute for Materials Science (NIMS) reported a new transfer process for single crystal organic semiconductor film to a hydrophobic insulation film.


Fig 3. (a) Chemical structure of Cn-DNBDT-NW, (b) the electron diffraction pattern of the exfoliated thin film, (c) schematic illustration of the transfer method, (d) transistor characteristic of the transferred-film-based OFET in saturation regime.3)

As above, if a hydrophobic insulation film is used as a gate insulator, device characteristics of organic-TFT is expected to be enhanced, however, it's difficult to coat an organic semiconductor film on gate insulation film directly because of its surface energy. Therefore, the research group tried to transfer single crystal semiconductor film to hydrophobic gCytoph insulation film by use of transfer method. By the way, in the continuous edge casting method, an organic semiconductor liquid is supplied to a glass blade continuously while moving the substrate at X direction. As a result, a single crystal film is deposited by optimization of volatilization time of solvent in this liquid.

In this experiment, a p type organic semiconductor gCn-DNBDT-NW (figure 3 - (a))h film coated on a native mica substrate (hydrophilic property) by the continuous edge casting method. The next, if it's dipped into water, single crystal organic film is isolated from the mica substrate, as a result, it becomes to be free-standing state. It's reason why difference of surface energy between mica and organic semiconductor film is large, as a result, water penetrates into boundary easily. As figure 3 - (b), after isolation, organic semiconductor keeps being single crystal state. On the other hand, a hydrophobic gate insulation film Cytop was coated on silicon wafer with SiO2 film. It's adhered to the mica substrate, and then, water is dropped, as a result, single crystal organic film is transferred to Cytop film by separation of the mica substrate. Finally, Au source/drain are formed by the mask-through evaporation method.

Figure 3 - (d) shows device characteristics. Its carrier mobility was very high value same as 12 cm2/Vs, which is highest as organic-TFTs. And also, hysterisis was not almost observed. Furthermore, while a glass substrate is used instead of the mica substrate, this separation and transfer process can be adopted.

S/D with dopant is formed by lift-off method

And, almost same research group (University of Tokyo, OPERANDO-OIL, JST, NIMS, and Pi-Crystal) proposed a damage-free lithography process as effective method for the continuous edge casting method. In this process, operation speed is increased by forming a dopant on boundary between source/drain and organic semiconductor film.

Figure 4 shows process flow. First of all, Cn-DNBDT-NW single crystal film is coated by the continuous edge casting method. The next, Cytop insulation film is@coated, and then, its surface is modified by O2 ashing. Subsequently, a photoresist is coated and patterned by the photolithography, and then, Cytop layer is etched by the wet-etching method. Finally, F8-TCNQ dopant and Au are evaporated without mask, and they are lifted-off by making use of fluorine series solvent. As a result, F8-TCNQ is formed on source/drain only.


Fig 5. Cut-off frequency measurement.4)


Fig 4. Process flow4)

Contact resistance of the pilot-produced device with channel length 50 ƒÊm was extremely low same as approximate 20 ƒ¶¥cm. On the other hand, carrier mobility of 1.5 ƒÊm channel device was relatively high same as 2.9 cm2/Vs. Furthermore, as figure 5, cut-off frequency was 38 MHz, which was maximum value for organic-TFT.

Process steps and time are reduced by doping SAM into organic semiconductor ink

As concerns printing process, JNC PETROCHEMICAL proposed an organic semiconductor ink with doped self assemble monolayer (SAM) material and indicated that contact resistance of electrode and organic semiconductor and manufacturing process cost could be reduced by making use of this material.

As you know, if insulation layer and electrode are chemically modified by SAM material, carrier injection property is expected to be enhanced. The research group found that if pentafluorothiophenol (PFBT) is used as SAM, modification effect of Au electrode is extremely increased. It's reason why carrier injection height is lowered by reducing difference of ionization energy among company's organic semiconductor (5.44 eV) and modified Au (4.97 eV ¨ 5.26 eV).


Fig 6. OFET fabrication process. Blue:Traditional method, Red:New method5)

However, it's necessary to treat by SAM for enormous time in general. For this reason, the research group doped SAM material into organic semiconductor ink, in order to reduce process steps. Composition ratio of organic semiconductor, polystyrene, and SAM are 0.5 wt%C0.2 wt%, 0.1 wt% respectively. PFBT, benzenethiol (BT), and perfluorodecanethiol (F-DT) were used as SAM material. As figure 6, organic semiconductor layer and SAM on Au electrode can be formed at the same time by the spin-coating method, as a result, process time and organic solvent can be reduced.

If PFBT mix ink is used, carrier mobility of device was 0.21 cm2/Vs, which was almost same as that of reference device (Au was modified by PFBT previously). By contrast, in case of doping BT, device was not driven, and in case of doping F-DT, its film was not formed on Au electrode because of strong hydrophobic property.

Uniform channel is formed by reverse offset printing method

By contrast, the research group of Yamagata University and Mitsubishi Chemical reported that uniform channel length between source/drain electrode could be fabricated by the reverse offset printing method.


Fig 7. Microscopic image of the printed source/drain electrodes and histogram of channel length.6)

In the reverse offset printing method, firstly, an ink is supplied to whole area of blanket roll, and then, unnecessary ink is transferred to intaglio brief, on the other hand, remained ink on blanket roll is transferred to the substrate. Compared to the ink-jet printing method, precision of line width is higher, and also, printing result is not easy to be influenced by state of underground. For this reason, variability of TFT characteristics is expected to be minimized, too.
In manufacturing process of pilot-produced device, Nano size Ag ink was printed as source/drain electrodes by the reverse offset printing method. And, DA type polymer was used as an organic semiconductor material.

While channel length of 10 ~ 10 array was measured, in all channel lengths (5 ƒÊm, 10 ƒÊm, 25 ƒÊm, 50 ƒÊm, and 70 ƒÊm), high uniformity same as } 1ƒÊm and under was obtained. Carrier mobility was 0.05 cm2/Vs (average), ON/OFF current ratio was approximate 106, and also, their repeatability was superior, too. Furthermore, variability in characteristics was greatly suppressed.

Carrier mobility of CNT-TFT was greatly increased same as that of LTPS-TFT

As regards CNT-TFT, the research group of Toray and National Institute of Advanced Industrial Science and Technology (AIST) reported that carrier mobility of CNT-TFT was increased same level as that of LTPS-TFT.


Fig 8. Transfer characteristics of TFTs7)

If purity of semiconductor type CNT in bulk-SWCNT is increased, variability of@diameter is decreased, and length is elongated, characteristics of CNT-TFT are enhanced. In this time, the research group furthermore devised an original extraction method of semiconductor type CNT ggel column methodh, for example, optimization of pH of dispersant in CNT dispersion liquid.

As a result, elimination ratio of metal type CNT was increased because of decrease of absorption of metal type CNT in 700 nm neighborhood. And also, as figure 8, transfer characteristics was enhanced in proportion to purity of semiconductor type CNT. It's reason why semiconductor characteristics of CNT network is improved by decrease of metal type CNT, which is impossible to control by gate voltage. As a result, carrier mobility was increased from 118 cm2/Vs to 155 cm2/Vs. As you know, it means that its carrier mobility is superior to that of organic-TFT and oxide-TFT, which are competing TFT.

Reference
1)Kitahara, et.al.FLow-voltage and Stable Operation of Solution-Processed Organic Transistors Based on Ultrahydrophobic Carrier Transport Interface, The 66th JSAP Spring Meeting, 2019, 10-325 (2019.3)
2)Fujita, et.al.FThreshold Voltage Control for Bottom-Contact Organic Transistors by Oxygen Plasma Treatment, The 66th JSAP Spring Meeting, 2019, 10-089 (2019.3)
3)Makita, et.al.FA Technique of Transferring Solution Processed Organic Semiconductor Thin Film onto Highly Hydrophobic Substrate and its Application to Field-Effect Transistors, The 66th JSAP Spring Meeting, 2019, 10-324 (2019.3)
4)Yamamura, et.al.FHigh-Speed Organic Single-Crystal Transistor by Damage-Free Lithography Process, The 66th JSAP Spring Meeting, 2019, 10-327 (2019.3)
5)Nakahara, et.al.FElectron-withdrawing SAM-material-added Organic Semiconducting Ink to Reduce Fabrication Process, The 66th JSAP Spring Meeting, 2019, 10-337 (2019.3)
6)Takeda, et.al.FEvaluation of variations in characteristics of solution processed organic transistors using printed electrodes with uniform channel length, The 66th JSAP Spring Meeting, 2019, 10-225 (2019.3)
7)Hirai, et.al.FTFT performance improvement by highly purified semiconducting SWCNTs, The 66th JSAP Spring Meeting, 2019, 10-224 (2019.3)


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