TFT Backplane Technologies for AMLCD and AMOLED Applications

Transcription

1 Journal of the Korean Physical Society, Vol. 54, No. 1, January 2009, pp TFT Backplane Technologies for AMLCD and AMOLED Applications Jae Beom Choi, Young Jin Chang, Cheol Ho Park, Beom Rak Choi and Hyo Seok Kim OLED Lab., Samsung Electronics Co., Gyeonggi Kee Chan Park Department of Electronic Engineering, Konkuk University, Seoul (Received 24 January 2008) We thoroughly investigated low-temperature polycrystalline silicon (LTPS) thin-lm transistor (TFT) backplane technologies based on (1) a melt-mediated crystallization process with laser systems, (2) a solid phase crystallization process with advanced annealing systems and (3) a singlecrystalline Si layer transferred onto a large glass substrate for at-panel-display applications. Extensive micro-structural analyses of the silicon lms, comparison of the TFT performances and evaluation of the image quality of the displays enabled us to choose the competitive technologies for large-area active-matrix liquid-crystal display (AMLCD) and active-matrix organic light-emitting diode (AMOLED) applications. PACS numbers: Pg, Cw, Cy, Fq, Jt Keywords: Low-temperature polycrystalline silicon (LTPS), Excimer laser annealing (ELA), Sequential lateral solidication (SLS), Solid phase crystallization (SPC), Nanocap-assisted crystallization (NAC), Silicon on glass (SiOG) I. INTRODUCTION Large-area active-matrix liquid crystal display (AMLCD) TVs are based on amorphous silicon (a- Si) thin-lm-transistor (TFT) backplanes. However, small-area AMLCDs for premium mobile devices are based on low-temperature polycrystalline silicon (LTPS) TFT backplanes because the conventional a-si TFT backplane cannot meet specications such as high aperture ratio and low power consumption. In addition, active-matrix organic light-emitting diode (AMOLED) displays with LTPS TFT backplanes have been adopted not only for the mobile devices but also for TVs with diagonal sizes larger than 10 inches. Unlike the AMLCD which utilize an a-si:h TFT backplane for large size and a LTPS backplane for small size, there are still many research activities to nd the optimum TFT backplane technology for a high-quality AMOLED display. There are four major TFT backplane technologies for AMOLEDs: (1) a-si:h TFTs, (2) LTPS TFTs obtained by using a melt-mediated crystallization process, (3) LTPS TFTs obtained by using a solid phase crystallization process and (4) a single crystalline silicon layer transferred onto a glass substrate. Among these, the a-si:h TFT technology is best established for mass production and the displays with a-si:h TFT backplanes show excellent uniformity over large areas [1]. However keechan@konkuk.ac.kr; Fax: the threshold voltage shift under continuous positive bias stress is a critical limitation to AMOLED applications [2]. In this paper, we will review the other three dierent technologies to obtain a suitable backplane for both active-matrix display applications. II. MELT-MEDIATED CRYSTALLIZATION PROCESS The melt-mediated crystallization process includes (1) melting of the precursor a-si lm and (2) subsequent solidication of the liquid a-si, resulting in a polycrystalline silicon (poly-si) lm of various microstructures, depending on the process parameters. The phase transformation scenarios of the melt-mediated crystallization process include (1) partial melting, where only the surface of the a-si lm is melted and solidication takes place in the vertical direction from the bottom, resulting in small grains, (2) near complete melting, where the a-si lm is melted to the bottom with a small number of unmelted Si clusters remaining discontinuously and lateral solidication takes place producing grains much larger than the lm thickness and (3) complete melting, where the entire a-si lm is melted and nucleation-triggered solidication starts under super-cooled condition, again resulting in ne grains. Among the various types of laser systems that can be used for the melt-mediated crystallization process, the -549-

2 -550- Journal of the Korean Physical Society, Vol. 54, No. 1, January 2009 Fig. 1. TS-SLS process: (a) laser irradiation and (b) resulting microstructure. excimer laser annealing (ELA) system has been widely utilized for the LTPS TFT backplanes for AMLCDs [3]. The crystallization is carried out by scanning narrow laser pulses (e.g., 465-mm long and 0.5-mm wide) over the a-si lm on a large glass substrate. The process window of the laser energy density in the near-completemelting condition is rather narrow; thus, the resulting microstructure of the poly-si material is sensitive to uctuations in the laser energy. Therefore, the ELA system should have highly uniform laser intensity pro le, both the long and the short axes and shot-to-shot consistency. In particular, even a small variation in the laser intensity can be easily perceived in case of the AMOLED display because the brightness is directly associated with the current owing through the driving transistor. The mura in the scanning direction is associated with the nonuniformity of the laser intensity along the long axis and the mura perpendicular to the scanning direction is associated with the shot-to-shot nonuniformity of the laser intensity. On the other hand, the sequential lateral solidi cation (SLS) process with a patterned laser beamlet has also been utilized for AMLCD production [4, 5]. The SLS process is composed of (1) laser irradiation through a patterned mask to provide an abrupt temperature prole at the edge of the irradiated area, leading to controlled super lateral growth (C-SLG), (2) translation of the substrate by a precisely controlled distance and (3) repetition of (1) and (2) resulting in complete crystallization of the lm. Compared to the conventional ELA systems, the advantage of the SLS process is (1) a wider process window in laser intensity, (2) controllability of the grain size and (3) scalability of the substrate. However, the process time of the original SLS process is several times longer than that of the ELA. To improve the throughput of the SLS processes to be even higher than ELA, the two-shot (TS) SLS process has been developed and is being utilized in the mass production as Fig. 2. (a) 300 VGA AMLCD and (b) 1400 WXGA AMOLED display fabricated on the TS-SLS TFT backplane. the most competitive technique [6]. Figure 1 illustrates the TS-SLS process including (a) the laser irradiation scheme and (b) the resulting microstructure. \L" is the line width of the open area on the mask, \S" is the space between the open areas and the grain size is determined as (L + S)/2. With the LTPS TFT backplanes obtained by using the TS-SLS process, we could make high-quality AMLCD products and a 1400 WXGA (1280 RGB 768) AMOLED display without any mura associated with the laser crystallization process, as shown in Figure 2. We adopted a voltage-addressed compensation circuit with six TFTs and a capacitor in each pixel for the 1400 AMOLED display. III. SOLID PHASE CRYSTALLIZATION (SPC) The solid phase crystallization (SPC) process is the simplest and the lowest-cost process to obtain poly-si lms on large-area glass substrates. In this process, unlike the melt-mediated crystallization process, the a-si lm is directly transformed to the crystalline structure via nucleation and grain growth process at a temperature around 600 C. The SPC process can be catego-

3 TFT Backplane Technologies for AMLCD and AMOLED { Jae Beom Choi et al Fig UXGA AMOLED display based on the NAC TFT backplane. Fig qvga AMOLED display employing the SPC TFT backplane. rized into two groups: (1) simple SPC process and (2) metal-induced-crystallization (MIC) process where the crystallization temperature is reduced to below 500 C by employing metal catalysts. As for the process equipment, magnetic-eld-aided rapid thermal annealing may be utilized in addition to a conventional furnace. The magnetic eld induces an eddy current in the heated a- Si lm and, thus, produces a poly-si lm in a reduced process time [7]. The simple SPC process does not require any additional process other than the thermal annealing. The precursor material and the annealing temperature determine the average grain size and the crystallinity of the completed lm. Since the scale of crystalline irregularity in the SPC poly-si is much smaller than the device dimension, SPC TFTs have uniform characteristics over a large area [8,9]. As shown in Figure 3, we could obtain a qvga (240 RGB 320) AMOLED display by using the SPC TFT backplanes without any compensation circuit in the pixel. For AMOLED TV applications, a carrier mobility of 1 cm 2 /V s is high enough to drive the OLED current in each pixel because the TFT channel width can be expanded to hundreds of micrometers for a pixel density below 100 ppi (pixels per inch) to meet the current requirements, which is the case in a normal TV. However, the crystallinity must be increased by reducing the defect density in the SPC poly-si lm in order to improve the mobility up to a level that can be used for high-performance AMLCDs because more and more circuits need to be integrated on recent value-added display panels. In addition, the process temperature should be further lowered below 550 C in order to prevent the glass warpage problem frequently observed in large glass substrates. In the MIC process, metal catalysts are utilized to promote the crystallization process at reduced temperature. For example, when Ni is used as the catalyst material, the Ni atoms in/on the a-si lm can form nickel silicide at temperatures lower than the intrinsic crystallization temperature (600 C) [10, 11] and the NiSi 2 propagates through the a-si matrix, leaving a needleshaped crystalline Si region even at 484 C [12]. Since the individual Si grains obtained when using the MIC process tend to have textures with a certain orientation, further treatment can provide better structured poly-si lms with reduced defect density at the grain boundaries [13,14]. With improved crystallinity, the MIC TFTs can have a mobility higher than that of the SPC TFTs [12, 15{20]. However, the leakage current associated with Ni contamination is rather higher compared with that of the simple SPC TFTs. In order to solve the Ni contamination problem, a gettering process, in which the Ni atoms in the crystallized Si lm diuse to the phosphorus-implanted region during the dopant activation process and a nanocap-assisted crystallization (NAC) process [21], in which a thin SiO 2 capping layer is formed on top of the a-si lm prior to the deposition of Ni in order to control the Ni contents, have been developed. With the LTPS TFT backplanes obtained by using the NAC process, we fabricated a UXGA (1600 RGB 1200) AMLCD as shown in Figure 4. In order to exploit these MIC materials for AMOLED applications, we found that the nonuniformity of the grain size and the nonuniformity of the Ni distribution along the domain boundaries should be controlled. Although the uniformity and the current-driving capability of the SPC, including MIC, TFTs are sucient as AMOLED backplanes, the stability is not satisfactory

4 -552- Journal of the Korean Physical Society, Vol. 54, No. 1, January 2009 Fig. 5. Schematic diagram of the SiOG process comprising (a) hydrogen implantation, (b) bonding, (c) separation and (d) thinning of the Si wafer transferred on the glass substrate. due to high trap density. The poor stability is revealed as hysteresis in the TFT characteristics and causes image sticking in the display [22,23]. The microcrystalline silicon (c-si) TFTs that have recently attracted much attention also suer from the same instability problem, though they are much better than the a-si TFTs [8,24]. Poor stability due to the interface states is observed even for the badly fabricated ELA LTPS TFTs [25]. However it is remarkably improved by using a SLS backplane owing to the higher crystallinity that is characteristic of the SLS process. IV. SINGLE CRYSTALLINE SILICON Finally, we investigated the feasibility of utilizing single crystalline Si materials on large glass substrates for active matrix display applications. Since the variation in the TFT performance is detrimental to the image quality of the AMOLED, there have been attempts to avoid the grain boundary-related problems by transferring singlecrystalline Si layers to the glass substrates [26]. Figure 5 shows a schematic diagram of the silicon-onglass (SiOG) process developed by Corning Inc. [27]. The SiOG process consists of (1) hydrogen implantation to the Si wafers to form the separation zone (Figure 5(a)), (2) anodic bonding of the hydrogen-implanted wafers to the glass substrate, resulting in strong SiO x bonding between them (Figure 5(b)), (3) separation of the wafers, leaving a thin silicon layer on the glass substrate (Figure 5(c)) and (4) thinning the Si layer through chemical and mechanical polishing (Figure 5(d)). The thickness of the Si layer can be controlled within a standard deviation of several nanometers and the maximum process temperature does not exceed 400 C. Figure 6 shows a (320 RGB 240) AMLCD and a (240 RGB 320) AMOLED display fabricated by using the SiOG process. F The advantages of the SiOG process are (1) the absence of troublesome grain boundaries, which enables us to make a uniform AMOLED display free from the mura associated with the grain boundaries, (2) a lower defect density compared with Fig. 6. (a) qvga AMLCD and (b) qvga AMOLED display based on the SiOG TFT backplane. the polycrystalline material which leads us to fabricate an AMLCD with a high level of monolithic circuit integration and (3) a simplied TFT process with cost competitiveness over the conventional LTPS process. The ultimate goal of the SiOG technology is to obtain highquality AMOLED displays without any compensation circuit in the pixel and fully-integrated AMLCDs. V. SUMMARY We reviewed the TFT backplane technologies for high-performance active-matrix at panel display applications. Although the melt-mediated crystallization process with a laser system is widely utilized in the mass production of AMLCDs, the laser-related device's nonuniformity should be further improved to fabricate AMOLED displays with enhanced production yield. Solid phase crystallization may be an alternative for large-area AMOLED displays if the stability of the TFT is improved. For the present, the laser-annealed LTPS

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