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 DisplaysKang-Chia Peng, Yao Peng and Min-Che HoLTPS Dept., New FPD Tech Division, R & D Center, TFT Business UnitChung-Hwa Picture Tubes, LTD., Bade City, Taoyuan, R. O. C.摘要 AbstractSupplementary treatments combined with the standard ExcimerLaser Annealing (ELA) process are introduced to poly-Si recrystallization.The surface condition of the poly silicon filmbecomes more uniform and smoother after these treatments. Thevariation of the current that drives AMOLED is reduced from30% 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 beengreatly reduced as well. Finally, the image quality of AMOLEDis 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 hasprovided an excellent solution as the mainstream displays of thenext generation. In order to drive AMOLED, as usual, a-Si andLTPS TFT are applied. Compared with a-Si, LTPS has morestable and reliable characteristics. Commonly, OLEDs are drivenby current, in order to get a uniform display image, the currentpassing through the driving TFT must be kept as stable anduniform as well. However, due to the non-uniform crystallizationin the ELA process, it results in a disorder boundary distributionof poly silicon (p-Si), thus it’s hard to manage the gray-scale wellto display the smooth colors. Besides, we can easily observe theimage defects visually, for example, the streak mura caused by thelaser line beam overlapped scanning.Generally, when silicon is at the solid state, its density is smallerthan that at the liquid state. During the annealing process, the lastlocation changing from the liquid phase to the solid phase is at theboundary, and the extruding phenomenon will happen here due tothe change of density. Reducing the protrusion can reach forbetter electrical characteristics . In order to minimize oreliminate the streak mura, we try to flatten the protrusion andrepair the defects among the grain boundaries by applying BOEwet etching and two-laser processes.Here we adapted a pixel structure with two transistors and onecapacitor (2T1C) of AMOLED. With the non-compensated circuitof threshold voltage, Vth, the image improvement contributed bythe applied methods can be clearly defined.背板的制造 2. Fabrication of BackplaneTFT和电容器电路 2.1 2 TFTs and 1 Capacitors CircuitsA pixel structure with two transistors and one capacitor is appliedin this study. In order to observe the results more definitely, weuse such devices so that the visual image defects can bedistinguished individually. And also, the other uncertain factorscaused by the compensation circuits can be excluded. Figure 1shows the 2T1C structure used in this experiment.Figure 1. The 2T1C pixel structure.2.2 Streak MuraWhen the laser light irradiates onto the amorphous silicon (a-Si)film on glass substrates, the energy is transferred into the mannerof heat from light. Meanwhile the heat is absorbed by the film andcauses the melt-nucleation-grain growth processes. The graingrowth 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 thepulse laser, the crystallization process is unique everywhere on thesubstrate . This means TFT expresses non-uniformperformance vertically and horizontally. If such overlappedscanning method is applied, the visible line mura will appearwhen the panel is turned on. Figure 2 and figure 3 illustrates theway of excimer laser scanning and streak mura caused by ELA,respectively.Figure 2. Illustration of laser beam scanning.lightISSN0006-0966X/06/3 1696 • SID 06 DIGEST 702-1696-$1.00+.00 © 2006 SIDFigure 3. Streak mura caused by ELA2.3 Treatments in ELA ProcessIn this experiment, chemical solutions such as ozone water, DHF(Diluted HF), BOE (Buffer Oxide Etcher) and 2 times of ELAprocess are used to modify the surface roughness of p-Si. Theprocedure is shown as figure 4. Prior to the first ELA process, theforeign matters and native oxide on the surface of the a-Si areremoved by ozone water and DHF. Following that the first ELAturning a-Si into p-Si is performed. And then, we apply BOE tomodify the surface of the p-Si formed by the first ELA process.After that, the second ELA process is applied. The TFT ismanufactured according to the regular top gate process steps.Figure 4. Process flow of this treatment3. Protrusion ReductionThe protrusions, which appear after ELA process, are high anduneven at boundaries. It further leads to non-uniform electricalcharacteristics and poor image quality. Figure 5 shows the resultsunder three various treatments. Figure 5(a) shows the result of thestandard ELA process, figure 5(b) shows the result of applyingozone water and DHF cleaning process before ELA process, andfigure 5(c) shows the result of applying ozone water and DHF cleaning followed by the first ELA process, then treated withBOE, 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 ELAProcess. (c) ELA pre-cleaning, the first ELA, BOE etchingand the second ELA.Visually Comparing figure 5(a) with 5(b), it shows that theprotrusions have been reduced slightly by pre-ELA cleaningprocess. In figure 5(c), the protrusions have been greatly reducedwhich means the stepped cleaning and etching treatment describedhere is effective for evenly removing protrusions.电性能 4. Electrical PropertyAMOLED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 tothe current variation, so that the steady current supply is criticalfor the image quality of the AMOLED. Figures below are the Id-Vg linear plots of standard ELA process and the one combinedwith treatments, respectively.Ozone water and DHFcleaningFirst ELASecond ELABOE etchingFigure 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 isapplied. One can see that, in figure 6(b), the variation of thecurrent is smaller than that of figure 6(a). The variation of thecurrent is 30% in figure 6(a) and 12% in figure 6(b). With thecomparison discussed above, it indicates that the variation of thecurrent within the operation voltage is more consistent when thetreatments are applied with standard ELA process. The evidenceshows that the protrusion at the boundaries is the major impact ofvarying the driving current. Therefore, when the protrusions arereduced, the electrical characteristics will become more consistentand uniform.实验结果 5. Experimental ResultsThe following pictures are the images of 2.2” LTPS CMOSAMOLED displays. We compared the images using green colorwith three different grey levels (Low, medium and high greylevel).From figure 7, we can see that the streak mura is reducedapparently in low grey level. While in medium and high greylevel, the streak mura is hardly observed, figure 8(b), and there isno streak mura, figure 9(b).In comparisons mentioned above, all AMOLED images with thestandard 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 DIGEST53.4 / 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 agreat improvement has been achieved.结论 6. ConclusionAMOLED panels manufactured by the current pulse laserannealing 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 processpoint of view, a wet process and a second ELA treatment has beenproposed 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。Thevariation of the TFT drain current has been minimized from 30%to 12%. The streak mura has been suppressed significantly and hequality of display image has been improved also.#p#分页标题#e#7. References 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 itscorrelation to device performance” P216, SID 2003. J. W. Hamer and A. Yamamoto, G. Rajeswaran and S. A.Van Slyke “Mass production of full-color AMOLEDdisplays” P1906, SID 2005.