A 31-in. FHD AM-OLED Display using Amorphous IGZO TFTs and RGB FMM

Chih-Yu Su *, Wen-Hui Li*, Long-Qiang Shi*, Xiao-Wen Lv*, Kai-Yuan Ko*, Ya-Wei Liu*, Jin-Chuan Li*, Sai-Chang Liu *, Chih-Yuan Tseng*, Yi-Fan Wang*, Chang-Cheng Lo*

* Shenzhen China Star Optoelectronics Technology Co., Ltd, Guangdong, China Supported by Shenzhen Peacock Plan

Abstract A 31-inch FHD (1920x1080) AM-OLED TV was developed by using a-IGZO TFTs. Co-planar type of a-IGZO TFT was fabricated at Gen 4.5 glass substrate with IGZO (1:1:1) AC sputtering system for AM-OLED. We also used Fine Metal Mask (FMM) for R,G,B sub-pixel arrangement.

Author Keywords AM-OLED , IGZO , TFTs , FMM

1. Introduction Due to the successful and superior image quality of AM-OLED application in mobile displays, more and more research focused on the market and application in large size display like TV. Compared with traditional LCD technology, AM-OLED have several advantages such as high image contrast due to the lower black level luminescence, self-emission nature with wider viewing angle performance, more power saving, faster response time, thinner, and lighter form-factor[1]. Among backplane technologies, amorphous oxide semiconductor (AOS) thin film transistors (TFTs) are potential alternatives to hydrogenated amorphous Si and organic TFTs, due to their high mobility, low off-state leakage, and large area deposition even at room temperature (RT)[2-3]. Several display companies have joined the development of oxide thin film transistor on LCD [4-7] and OLED TV [8-10], especially a-IGZO. Most of the researches on a-IGZO deposition are using DC-sputtering or RF-sputtering. In this work, a Gen 4.5 AC sputtering system was introduced for a-IGZO deposition and Fine Metal Mask (FMM) technology was adapted to define the pixel on bottom emission AM-OLED.

2. Oxide TFT manufacturing process Figure 1 shows the coplanar structure of a-IGZO TFT. Glass with sizes of 730mm x 920mm x 0.5mm was used as the substrate material in Gen 4.5 system. The direct current (DC) sputtering was employed to prepare gate electrode consisting of 3300A-thick aluminum (Al) and 600A-thick molybdenum (Mo). Then, a 2000A-thick SiOx film, which was deposited by plasma enhanced chemical vapor deposition (PECVD) with TEOS at 380℃, was introduced as gate insulator (GI) layer. On the top of GI, the source and drain electrodes were also deposited by DC sputtering. Different from the gate metal layer, the second metal layer consists of Mo/Al/Mo film with the thickness of 200A, 2500A and 300A, respectively. Particularly, we repaired the GI interface by plasma treatment (Cl2, F and O2) with source/drain photo resist as hard mask. Then, a 600A-thick a-IGZO active layer was deposited by alternating current (AC) sputtering system. IGZO targets are made from powder of In2O3, Ga2O3, and ZnO through grounding, molding, and sintering process. And the target is composed from In2O3:Ga2O3:ZnO =1:1:1 mol% .

After IGZO deposition, wet etching process with C2H2O4 acid was adopted for patterning. Then the element was passivated by a SiOx layer. In the depositing of passivation layer, PECVD was operated at 200℃ with different process gases (SiH4, N2O or TEOS). In the next step, the samples were annealed at 350℃ for 1 hr in air. Finally, A 750A-thick ITO layer was deposited as pixel electrode after the formation of via holes and electrical characteristics of the TFTs were measured with Keithley 4200 at room temperature. The stress characteristics of the a-IGZO TFTs were measured at room temperature for 5000sec. The gate voltage was biased at Vg=+30 for positive-bias-temperature-stress (PBTS) test. The morphologies and thickness were examined by SEM.

Figure 1. a-IGZO AM-OLED structure

3. Results and Discussion 3.1 Pixel design The pixel schematic of 31-inch bottom-emission AM-OLED is 2T1C as shown in Figure 2 where the transistors are all N-type (coplanar IGZO TFT). We use the simplest and basic structure because it has the largest adjustment feasibility for driving TFT. The resolution is 1920 x 1080 with pixel size of 351um x 351um and with about 72 ppi. The specification of the 31-inch AM-OLED is listed in Table1.

Figure 2. 2T1C pixel scheme of 31-inch AM-OLED

Regarding the optical performance, the maximum brightness is

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150 nits with polarizer, and NTSC is 73%. The brightness of L0 grey level is 0.001, making contrast ratio up to 100,000. With 8 bits source IC, we have 256 grey levels each color that produce 16.7 million colors. Besides, the source IC has 3-gamma output which can solve the different data voltage (vgs of TFT) of RGB maximum brightness problem and fine tune the white color gamma to 2.2.

Item Content Panel Size 31” Resolution 1920 x 1080

Sub-pixel Pitch 117m x 351m Pixel Density 72ppi

Brightness 150nits Gray Level RGB 8-bit

Contrast Ratio >100,000:1 Color Gamut NTSC 73%

Table 1. 31-inch AM-OLED Specification

3.2 Array structure In order to minimize the defects in semiconductor interface produced during the S/D metal sputtering, plasma treatment was introduced to improve the device performance. Figure 3 shows the transfer characteristics of the a-IGZO TFT with plasma treatment at room temperature. The device with CF4+Cl2 plasma exhibits more excellent performance. The mobility of a-IGZO TFT can reach 11.03cm2/V.s and the sub threshold swing (S.S) can be as low as 0.17V/decade. Moreover, the threshold voltage (Vth) is 0.74V and Ion/Ioff ratio is about 6E108 with extremely low off-current about 10-12~10-13A. The characteristics parameters of a-IGZO TFT with plasma treatment were listed in Table 2 for clearness. In contrast, without plasma repairement, the GI layer would suffer unavoidable damage from etching during source and drain deposition, which could result in irregular and terrible interface for next process. Eventually, the defect in the GI/IGZO interface affects the TFT characteristics negatively and degraded the subthreshold swing.

Gate Voltage (V)-30 -20 -10 0 10 20 30

Dra

in C

urre

nt (A

)

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

VD=0.1VVD=10V

Figure 3. Transfer characteristics of a-IGZO TFT

Table 2. a-IGZO TFT characteristics parameters Figure 4 shows the transfer characteristics of the device with GI treatment under positive-bias-temperature-stress (PBTS). The stress bias voltage was Vg=30V and Vd=0V with 5000sec at room temperature. The Vth of the device after stress has a positive shift of 1.5V, and the subthreshold swing (S.S) has no significant change, which could be attributed to carrier trapping at channel/gate-insulator interface [11][12].

Gate Voltage (V)-30 -20 -10 0 10 20 30

Dra

in C

urre

nt (A

)

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

T = 0 secT = 10 secT = 50 secT = 100 secT = 300 secT = 500 secT = 1000 secT = 1500 secT = 2000 secT = 2500 secT = 3000 secT = 3500 secT = 4000 sec T = 4500 sec T = 5000 sec

Figure 4. Transfer characteristics of a-IGZO TFT

under PBTS

3.3 OLED structure The OLED was defined by fine metal mask technology (FMM) and encapsulated by dam & fill method. It has a device structure as shown in Figure 5(a). For the purpose of achieving a better performance, the thickness of each functional layer was optimized by software simulation and device verification. The Figure 5(b) reveals the lightened states of material test (MT) chips.

Figure 5(a)

Mobility S.S Vth Ion/Ioff ratio 11.03cm2/Vs 0.17V/decade 0.74V 6.8E108

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Figure 5(b)

Figure 5. (a)Device structure of OLED stack, (b) lightened picture of MT chips

The current-voltage-luminance (I-V-L) properties of MT chips were measured by IVL testing system, and the datum was selected when the brightness was 1000nits for red and blue, and 3000nits for green. The R/G/B pixels all have high current efficiency, and the operation voltages are low as shown in Table 3. Figure 6 shows a FHD (1920x1080), 31-inch AM-OLED prototype with co-planar a-IGZO TFT. Maximum brightness is around 150nits with polarizer, and color gamut is 73% (NTSC).

Table3. OLED device property

Figure 6. Image of 31-inch FHD AM-OLED display

4. Conclusion In this article, we demonstrated a 31-inch HD AM-OLED TV driven by a-IGZO TFTs and side-by-side FMM OLED evaporation technology. It is the first and the largest AM-OLED TV application in China so far. For uniformities of plasma distribution and enlargement, the multi-cathode equipment and AC power supplies were introduced for a-IGZO sputtering. Uniform discharge over a large area, abnormal discharge reducing could be achieved for future production.

5. Acknowledgements The authors would like to thank other team members belong to MPD department in TD Centre and supported by Shenzhen Peacock Plan.

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