ARTICLE pubs.acs.org/JPCC
Direct Photopatternable OrganicInorganic Hybrid Materials as a Low Dielectric Constant Passivation Layer for Thin Film Transistor Liquid Crystal Displays Yangho Jung,† Tae Hoon Yeo,†,‡ Wooseok Yang,† Youngwoo Kim,† Kyoohee Woo,† and Jooho Moon*,† † ‡
Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea Electronic Materials Business Division, Dongjin Semichem Co., Ltd., 625-3 Yodang-Ri, Yanggam-Myeon, Hwaseong-Gun, Gyeonggi-do 445-935, Korea ABSTRACT: Direct photopatternable and highly cross-linked organosiloxane-based organicinorganic hybrid materials (Ltpassimer) were synthesized using a solgel reaction of phenyltrimethoxysilane and tetraethyl orthosilicate as a low dielectric constant passivation layer for thin film transistor (TFT) liquid crystal displays (LCDs). Addition of hexa(methoxymethyl)melamine into the synthesized hybrid material resin minimizes the polar silanol group in Lt-passimer film by enhancing the cross-linking reaction and improves the thermal stability. The highly cross-linked siloxane network and low concentration of silanol groups cause the Lt-passimer film to have low dielectric constant (k ∼ 2.8) and excellent electrical insulation. Compared with the conventional photosensitive acryl film (k ∼ 3.2), our Ltpassimer films are optically transparent and have a relatively low outgassing and water vapor permeability, which proves the feasibility of Lt-passimer as a low dielectric constant passivation layer in high aperture ratio TFT-LCDs.
1. INTRODUCTION Active matrix-liquid crystal displays (AM-LCDs) have been widely used in various application fields from small screens such as in cellular phones and notebooks to large flat panel displays as in smart television and large-scale outdoor screen. Remarkable advances have been made to improve the performance and the fabrication throughput of thin film transistors (TFTs) in LCDs applications. However, the issues of contrast ratio, resistance capacitance (RC) time delay, pixel resolution, and transmittance (or brightness) still become critical as the LCD panel size enlarges. Especially, the transmittance of the LCD panel must be improved to achieve both high brightness and better resolution for full high-definition (HD) televisions. For this purpose, the aperture ratio (or fill factor) of the TFT-LCD panel should be maximized by eliminating inactive area and minimizing the misalignment with the color filter layer.1,2 Recent AM-LCDs widely utilize the inverted-staggered back channel-etched (BCE) hydrogenated amorphous-silicon (a-Si: H) TFTs in which silicon nitride (SiNx) film as a gate insulator, hydrogenated amorphous-silicon film as an active layer, and phosphorus-doped silicon (n+ a-Si:H) film as a source/drain contact layer are involved. In conventional BCE a-Si:H TFTs, SiNx deposited by plasma-enhanced chemical vapor deposition (PECVD) has been also served as a passivation layer for the TFTs to protect the back channel from damage and contamination during the subsequent processing, as shown in Figure 1a. The SiNx passivation layer should have uniform thickness to make a conformal contact with the protruding TFT structure and good electrical properties. r 2011 American Chemical Society
However, the high dielectric constant (k ∼ 7) of the conventional SiNx can cause capacitance coupling between the indium tin oxide (ITO) pixel electrode and gate/source lines, leading to degradation of the TFTs performance. To minimize the capacitance coupling, the region which is overlapped by the pixel electrodes and gate/source lines need to be minimized as much as possible. This requirement sets the size of ITO pixel electrode and inactive region around the ITO pixel electrode is covered by a black matrix (BM), which consequently impairs the aperture ratio and the resolution of the LCD panel.35 To overcome this limitation, solution processed low-k materials with high transmittance and good planarization capability have been proposed.69 The TFT-LCD panels containing thick (∼3 μm) low dielectric constant (k) passivation layer allows the enlargement of pixel electrode over the gate and data lines with decreased RC time delay and improved aperture ratio as shown in Figure 1b. Various low-k organic materials including benzocyclobutene (k ∼ 2.7) and polyimide (k ∼ 2.8) have been investigated for this purpose.10,11 However, these materials are unsuitable for the passivation layer due to their poor ion-blocking property, nonphotopatternability, and high-temperature annealing. Ideal low-k passivation materials must be highly transparent, photopatternable, and low temperature processable (94% transmittance in the range of visible wavelength (300800 nm). Compared with Lt-passimer, 200 nm thick SiNx film, which was deposited by PECVD at 350 °C, has lower transmittance of
