治疗中结合ELA技术提高AMOLED显示器中图像质量-指导essay_ESSAY

治疗中结合ELA技术提高AMOLED显示器中图像质量-留学生论文
Treatments in ELA Process to Improve the Image Quality in AMOLED Displays

Kang-Chia Peng, Yao Peng and Min-Che Ho

LTPS Dept., New FPD Tech Division, R & D Center, TFT Business Unit

Chung-Hwa Picture Tubes, LTD., Bade City, Taoyuan, R. O. C.

摘要 Abstract

Supplementary treatments combined with the standard Excimer Laser Annealing (ELA) process are introduced to poly-Si recrystallization.The surface condition of the poly silicon film becomes more uniform and smoother after these treatments. The variation of the current that drives AMOLED is reduced from 30% to 12%. ELA过程中所造成的条纹已大大降低。最后，AMOLED的图像质量大幅度提高。The streak mura caused by ELA process has been greatly reduced as well. Finally, the image quality of AMOLED is highly improved. http://www.ukassignment.org/dxessay/

简介 1. Introduction

为了追求更薄，更轻，更快的响应，更宽广的可视角度，低功耗和成本更低的显示器，AMOLED作为下一代的主流显示已经提供了很好的解决方案。To pursue a thinner, lighter, faster-response, wider viewing-angle, lower-power consumption and lower-cost displays, AMOLED has provided an excellent solution as the mainstream displays of the next generation. In order to drive AMOLED, as usual, a-Si and LTPS TFT are applied. Compared with a-Si, LTPS has more stable and reliable characteristics. Commonly, OLEDs are driven by current, in order to get a uniform display image, the current passing through the driving TFT must be kept as stable and

uniform as well. However, due to the non-uniform crystallization in the ELA process, it results in a disorder boundary distribution of poly silicon (p-Si), thus it’s hard to manage the gray-scale well to display the smooth colors. Besides, we can easily observe the image defects visually, for example, the streak mura caused by the

laser line beam overlapped scanning.

Generally, when silicon is at the solid state, its density is smaller than that at the liquid state. During the annealing process, the last location changing from the liquid phase to the solid phase is at the boundary, and the extruding phenomenon will happen here due to the change of density. Reducing the protrusion can reach for better electrical characteristics [1]. In order to minimize or eliminate the streak mura, we try to flatten the protrusion and repair the defects among the grain boundaries by applying BOE

wet etching and two-laser processes.

Here we adapted a pixel structure with two transistors and one capacitor (2T1C) of AMOLED. With the non-compensated circuit of threshold voltage, Vth, the image improvement contributed by the applied methods can be clearly defined.

背板的制造 2. Fabrication of Backplane

TFT和电容器电路 2.1 2 TFTs and 1 Capacitors Circuits

A pixel structure with two transistors and one capacitor is applied in this study. In order to observe the results more definitely, we use such devices so that the visual image defects can be distinguished individually. And also, the other uncertain factors caused by the compensation circuits can be excluded. Figure 1 shows the 2T1C structure used in this experiment.

Figure 1. The 2T1C pixel structure.

2.2 Streak Mura

When the laser light irradiates onto the amorphous silicon (a-Si) film on glass substrates, the energy is transferred into the manner of heat from light. Meanwhile the heat is absorbed by the film and causes the melt-nucleation-grain growth processes. The grain growth step can be extended by way of continued heat supplying, i.e., another shot or longer pulse duration. For present methods, the overlapped laser line beam scanning is the most common one.

However, due to the energy variation between each shot of the pulse laser, the crystallization process is unique everywhere on the substrate [2]. This means TFT expresses non-uniform performance vertically and horizontally. If such overlapped scanning method is applied, the visible line mura will appear when the panel is turned on. Figure 2 and figure 3 illustrates the way of excimer laser scanning and streak mura caused by ELA,respectively.

Figure 2. Illustration of laser beam scanning.

light

Figure 3. Streak mura caused by ELA

2.3 Treatments in ELA Process

In this experiment, chemical solutions such as ozone water, DHF(Diluted HF), BOE (Buffer Oxide Etcher) and 2 times of ELA process are used to modify the surface roughness of p-Si. The procedure is shown as figure 4. Prior to the first ELA process, the foreign matters and native oxide on the surface of the a-Si are removed by ozone water and DHF. Following that the first ELA turning a-Si into p-Si is performed. And then, we apply BOE to modify the surface of the p-Si formed by the first ELA process.

After that, the second ELA process is applied. The TFT is manufactured according to the regular top gate process steps.

Figure 4. Process flow of this treatment

3. Protrusion Reduction

The protrusions, which appear after ELA process, are high and uneven at boundaries. It further leads to non-uniform electrical characteristics and poor image quality. Figure 5 shows the results under three various treatments. Figure 5(a) shows the result of the standard ELA process, figure 5(b) shows the result of applying ozone water and DHF cleaning process before ELA process, and figure 5(c) shows the result of applying ozone water and DHF cleaning followed by the first ELA process, then treated with

BOE, finally applying the second ELA process, respectively.

(a)

(b)

(c)

Figure 5. Tilt surface images of the poly-silicon under (a) Standard ELA process. (b) The pre-cleaning and ELA Process. (c) ELA pre-cleaning, the first ELA, BOE etching

and the second ELA. Visually Comparing figure 5(a) with 5(b), it shows that the protrusions have been reduced slightly by pre-ELA cleaning process. In figure 5(c), the protrusions have been greatly reduced which means the stepped cleaning and etching treatment described here is effective for evenly removing protrusions.

电性能 4. Electrical Property

AMOLED的是由电流驱动，这意味着OLED是对电流的变化敏感的，从而使稳定的电流供应是保证AMOLED的图像质量的关键。AMOLED is driven by the current, it means OLED is sensitive to the current variation, so that the steady current supply is critical for the image quality of the AMOLED. Figures below are the Id-Vg linear plots of standard ELA process and the one combined with treatments, respectively.

Ozone water and DHF

cleaning

First ELA

Second ELA

BOE etching

Figure 6. Id-Vg linear plots with (a) Standard ELA process. (b) Treatments combined with standard ELA process. Within the operation voltage, for example when Vg= -4V is

applied. One can see that, in figure 6(b), the variation of the current is smaller than that of figure 6(a). The variation of the current is 30% in figure 6(a) and 12% in figure 6(b). With the comparison discussed above, it indicates that the variation of the current within the operation voltage is more consistent when the treatments are applied with standard ELA process. The evidence shows that the protrusion at the boundaries is the major impact of varying the driving current. Therefore, when the protrusions are

reduced, the electrical characteristics will become more consistent and uniform.

实验结果 5. Experimental Results

The following pictures are the images of 2.2” LTPS CMOS AMOLED displays. We compared the images using green color with three different grey levels (Low, medium and high grey level).

From figure 7, we can see that the streak mura is reduced apparently in low grey level. While in medium and high grey level, the streak mura is hardly observed, figure 8(b), and there is no streak mura, figure 9(b).

In comparisons mentioned above, all AMOLED images with the standard ELA process display serious streak mura. When the

(a)

(b)

Figure 7. 2.2” AMOLED images at low grey level with (a)

Standard ELA process. (b) Treatments with ELA process.

(a)

(b)

Figure 8. 2.2”AMOLED images at medium grey level with

(a) Standard ELA process. (b) Treatments with ELA process.

1698 • SID 06 DIGEST

53.4 / K.-C. Peng

(a)

(b)

Figure 9. 2.2”AMOLED images at high grey level with (a) Standard ELA process. (b) Treatments with ELA process.ELA combined with treatments are applied, we can see that a great improvement has been achieved.

结论 6. Conclusion

AMOLED panels manufactured by the current pulse laser annealing method express an imperfect image. 从过程的角度来看，湿法工艺和ELA治疗已经使-Si膜的表面粗糙度降低。From the process point of view, a wet process and a second ELA treatment has been proposed to planarize the surface roughness of p-Si film. TFT的漏极电流的变化被最小化，从30％降至至12％。条纹已被明显抑制，并显示的图像质量也有所改善。The variation of the TFT drain current has been minimized from 30% to 12%. The streak mura has been suppressed significantly and he quality of display image has been improved also.#p#分页标题#e#

7. References

[1] Y. C. Cheng, Y. R. Liu, J. X. Lin, C. L. Chen, J. F. Chang,

Y. F. Wu, Y. H. Yeh, C. Y. Sheu and S. W. Chang

“Fabrication of extremely low polycrystalline silicon and its

correlation to device performance” P216, SID 2003.

[2] J. W. Hamer and A. Yamamoto, G. Rajeswaran and S. A.

Van Slyke “Mass production of full-color AMOLED

displays” P1906, SID 2005.