High Mobility Metal Oxide Thin Film Transistors Active-Matrix Organic Light-Emitting Diode Television

Tsung-Hsiang Shih*, Hung-Che Ting, Chih-Lei Chen, Lun Tsai, Chia-Yu Chen, Li-Fong Lin,

Hong-Shen Lin, Lee-Hsun Chang, Yu-Hsin Lin

Large Size OLED Technology, AU Optronics Corporation, No. 1, Li-Shin Rd. 2, Hsinchu Science Park, Hsinchu, Taiwan

Phone: +886-3-5008899-6941 E-mail: thshih@auo.com

We reported large size amorphous Indium-Gallium-Zinc-Oxide and Indium-Tin-Zinc-Oxide thin film transistors active-matrix organic light-emitting diode television development history in AUO. The Gen6 threshold voltage uniformity can lower than 1.0V. Amorphous Indium-Tin-Zinc-Oxide thin film transistors show a higher mobility of 33.2 cm2/VS. Amorphous 56 inches back channel etch type Indium-Tin-Zinc-Oxide active-matrix organic light-emitting diode television is 1st time revealed. Metal-organic chemical vapor deposition system is used to evaluate the new high mobility material. The side-by-side organic light-emitting diode device is realized by fine metal mask. The dam and fill encapsulation method shows a simple process procedure and highly stability. Ink jet printing is adopted to produce a small size full color panel. By the application of compensation pixel circuit, the panel drove by AUO engine shows an excellent characteristic. 1. Introduction

Flat plane display (FPD) using amorphous oxide semiconductors (AOS) have actively developed. TAOS thin film transistors (TFTs) have gained special attention as an attractive alternative to poly-Si TFTs, because they provide much better large area uniformity in terms of their device characteristics, including the threshold voltage (Vth), amorphous phase, and higher field effect mobility are easily obtained.1-4)

For a-IGZO TFTs, its mobility is ~10 cm2/VS. Fukumoto Eri et al. from Sony reported an 8” amorphous indium tin zinc oxide (a-ITZO) panel in IDW 2010.5) ITZO has been reported a higher mobility of >~30 cm2/VS, which is more suit for the next-generation displays.6) High mobility of ITZO TFT induces higher driving current under the same electric field, and more suit for a current driving AMOLED TV usage. In other words, it means a lower driving voltage or smaller device size can be designed to achieve the same driving current. AUO has been demonstrated the 32”, 37”, 65” large size a-IGZO AMLCD TV and 14”, 32”, 65” a-IGZO AMOLED TV. Except traditional a-IGZO, we also report 56” IS/BCE type & 65” a-ITZO AMOLED TV.7-15) 2. Fabrication

Array Structure Figure 1 shows the schematic cross-section of oxide

TFTs, included typical coplanar/IS/BCE type structures. AUO demonstrated the worldwide 1st TV size AMLCD/AMOLED using a-IGZO backplane. Compare to IS type, BCE type backplane can be obtained by different SD electrode or metal oxide material. Cleaned Gen6 (1850*1500mm2) glass was used as the substrate. SiOx/SiNx was used as the gate insulator (GI) layer. Oxide

semiconductor active layer was deposited by a sputtering or metal-organic chemical vapor deposition (MOCVD) system. The MOCVD technology can achieve and balance the mobility and the reliability due to the composition can be adjusted. SiOx/SiNx IS layer and N2O plasma pre-treatment were used. The Al-base or Cu-base metal were used as electrodes. SiOx/SiNx was used as passivation layer (BP/PV). The transfer and output characteristics were measured at room temperature with an Agilent E5270B precision semiconductor parameter analyzer. The stress characteristics of the oxide TFTs were measured at 60 oC for 14.4Ksec in an air/dark box. Gate voltage biased at -15V and Drain voltage biased at +10V for negative bias temperature stress (NBTS) test.

(a) (b) (b)

Figure 1. Schematic cross-section of (a) coplanar, (b) IS and (c) BCE type TFTs.

OLED Structure

We adopt an EL component onto the oxide TFTs substrate to achieve an AMOLED TV, which is combined of hole injection layer (HIL), double hole transporting layer (HTL), emitting layer (EML), electron transporting layer (ETL) with carrier injection function, and a highly reflective cathode structure. Double hole transporting layers are necessary for a good carrier injection into EML, especially for HTL, it further helps to confine the excitons in the emitting layer and extend the EML lifetime.

SP-3Invited

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Full-color OLED Manufacturing Process

Regarding OLED device structure, there are HIL, HTL, EML, ETL, electron-injection layer (EIL), and top electrode (cathode) to be included. Each layer is deposited by plane, line and point sources. The dam and fill (D&F) encapsulation method shows a simple process procedure and high stability. Generally speaking, there are mainly three kinds of methods for OLED full-color patterning evaporation, included 1) RGB selective evaporation, or RGB side-by-side (RGB SBS) deposition; 2) White OLED plus color filter (WOLED+CF); 3) blue OLED plus color change medium (BOLED+CCM). RGB SBS is one of the most important technologies. Three kinds of different color are formed separately by FMM. 3. Results and Discussion

Large-size Array Device Development The high space and color resolution have greatly

enhanced the TV ability of showing details and made TV color closer to the real world. AUO has been demonstrated the 14”, 32”, 65” a-IGZO AMOLED TV. Except traditional a-IGZO, we also report 56” IS/BCE type & 65” a-ITZO AMOLED TV as shown in Fig. 2~7. AUO 56” BCE type a-ITZO AMOLED TV is 1st time revealed. These panels achieve good image quality, such as high contrast ratio, a wide viewing angle, and fast response time, and thin module thickness.

Figure 2. 14” qHD a-IGZO AMOLED TV panel.

Figure 3. 32” FHD a-IGZO AMOLED TV panel.

Figure 4. 65” FHD a-IGZO AMOLED TV panel.

Figure 5. 56” IS type a-ITZO AMOLED TV panel.

Figure 6. 56” BCE type a-ITZO AMOLED TV panel.

Figure 7. 65” FHD a-ITZO AMOLED TV panel.

Figure 8 shows the normalized transfer and output characteristics (inset) of IGZO (black) and ITZO (red) TFTs. The IGZO TFT initial Vth and SS are ~0V and 0.39

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0 1 2 3 4 5100

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(V/decade), respectively, The ITZO TFT initial Vth and SS are ~0V and 0.35 (V/decade), respectively. The initial Vth of a-IGZO and a-ITZO can be positive/negative adjusted by additional process/treatment. The Gen6 Vth range can be lower than 1.0V. The IDS of ITZO TFT is higher than IGZO TFT over 3 times. The mobility of ITZO TFTs can reach 33.2 cm2/VS. It’s ~3.3 times of a-IGZO TFTs.

Figure 9 shows the hysteresis of IGZO & ITZO TFTs. The delta Vth shifts are ~0.033 & ~0.034V, respectively. The characteristic of ITZO TFTs is closed to a-IGZO TFTs and shows the possibility to drive the large size AMOLED TV.

Figure 8. Transfer and output characteristics of ITZO and IGZO TFTs.

Figure 9. Hysteresis of IGZO & ITZO TFTs.

Figure 10 shows the ITZO TFTs NBTS results of w/i and w/o N2O plasma pre-treatment of IS layer. Figure 10 also shows the NBTS comparison between IGZO and ITZO TFTs. The ITZO and IGZO TFTs delta Vth shifts can be improved ~44.4% and ~31.8% after the usage of N2O plasma pre-treatment, respectively. The result implied that the passivation effect of N2O plasma on back channel interface. However, even w/i N2O plasma pre-treatment, IGZO TFT NBTS delta Vth shift still better than ITZO TFT ~43.8%. This maybe attributed from the high mobility

ITZO active layer and photo resistance contamination on the back channel interface.

High mobility of ITZO TFT induces higher driving current under the same BTS condition (electric stress field). Basically, a higher mobility means a lower driving voltage can be used to achieve the some driving current (constant current stress is better).

The other possibility of poor BTS is from the photo resistance contamination. After the patterning of oxide semiconductor active layer, the C related bonds are found by ESCA/XPS. The C=O bond can be detected on both IGZO/ITZO surface and O-C bond is only observed on ITZO. Especially, the percentage of C=O on ITZO surface reaches 57.8%, it’s several times of IGZO. This maybe implied that C is more easily remained on ITZO than on IGZO, and will degrade the BTS performance. Before test, all samples surface are pre-sputtered to lower the air contamination noise.

Figure 10. NBTS comparison between ITZO/IGZO TFTs.

For IGZO grown by MOCVD system, the mobility can reach ~23.1 cm2/VS and SS keep ~0.2V/decade. The mobility higher than 50 cm2/VS is possible. High mobility TAOS TFTs have gained special attention and show the possibility as an attractive alternative to poly-Si TFTs.

Panel Design & Driver The developed compensation pixel circuit is applied.

The pixel circuit can compensate the Vth of the driving transistor and also keep the constant current from the voltage variation of an OLED device. Moreover, the compensation pixel circuit can reduce the influence of the shifts in the TFT and OLED characteristics due to temperature and stress time. AUO OLED engine can get the signal RGB data and AC power from system side.

Panel Performance OLED device with weak-cavity structures were

adopted. For example, 65” panel, the color characteristics of red, green, and blue were (0.661, 0.337) for red, and (0.254, 0.650) for green and (0.140, 0.09) for blue emission

Type IGZO ITZO △Vth 0.033V 0.034V

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1E-13

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spectra. Color gamut of OLED panel is about 83% in CIE 1931 coordinate. In order to meet the TV specification, the OLED devices are modified into in suitable color space and met the sRGB space for the commercial TV product. In HDTV category, sRGB is usually selected in the standardization, so that all display platforms have the capability to reproduce it in sRGB space if it is probably calibration.

By RGB SBS technology, it is possible to optimize the device lifetime, light efficiency, and chromaticity. Low power consumption and color saturation can be obtained. Due to the high resolution demand, the FMM of RGB SBS becomes an important technology and need to develop. We use RGB SBS FMM for the OLED full-color patterning. The FMM of active area (AA) is composed of many divided mask sheets, and then it can be used successfully to monochrome R/G/B color separately w/o color mixing.

-80 -60 -40 -20 0 20 40 60 800.0

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inan

ce R

atio

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LCD OLED

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50% Luminance

Figure. 11 LCD/OLED luminance ratio comparison of

different viewing angle.

Compare to LCD TV, OLED TV shows wider viewing angle. As shown in Fig. 11, we show the LCD TV & OLED TV luminance ratio in the same scale. From the measurement data, OLED TV has 80% luminance within ±60 degree and 50% luminance within ±80 degree. A normal LCD TV has 50% luminance within only ±30 degree. Different from LCD controls the light by liquid crystal, OLED is self-emitting. This characteristic makes users to watch in larger angle with less brightness drop. This means that users can watch OLED TV comfortably at every angle. 4. Conclusions

We reported large size a-IGZO and a-ITZO TFTs AMOLED TV panel development history in AUO. The Gen6 Vth uniformity can lower than 1.0V. A-ITZO TFTs show a higher mobility of 33.2 cm2/VS. AUO 56” BCE a-ITZO AMOLED TV is 1st time revealed. We reported N2O plasma pre-treatment effect on back channel. MOCVD system is used to evaluate the new high mobility material. The SBS OLED device is realized by FMM. The D&F

encapsulation method shows a simple process procedure and highly stability. By the application of compensation pixel circuit, the panel drove by AUO engine shows an excellent characteristic. 5. Acknowledgement

Authors would like to thank AUO RD team and fab members for their cooperation.

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