The Study of Excimer Laser for Flat Panel Display

• Main Authors: C. N. Chen, 陳兆南
• Other Authors: G. M. Wu
• Format: Others
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/76237615976269152331

博士 === 長庚大學 === 電子工程學研究所 === 96 === In this thesis, excimer laser techniques for fabricating flat panel displays (FPD) are studied. Excimer laser annealing and patterning were widely application on the process of high performance thin film transistors (TFTs). High performance low temperature polycrystalline silicon (LTPS) TFTs were crystallization by line beam excimer laser annealing and sequential lateral solidification (SLS) process in this study. Under optimal process conditions, a array like arrange aligned poly-Si array thin films could be obtained by line beam excimer laser annealing without any extent mask or process. The ELA system optical lens module is limit the poly-Si grain size around 0.3 m and array like arrange. The electrical characteristics of PMOS TFT fabricated using the aligned poly-Si array thin films were nearly independent of device channel direction, in different TFT channel direction, the average value of the field effect mobility, sub-threshold swing (SS), threshold voltage (Vth) and drain ON current (Ion) were varied within 5% average value. LTPS TFTs with such properties are good enough for driving OLED displays. In order to realized system on panel (SOP) which to integrate the memory and controller to be integrated on a glass panel, SLS technology have been proposed to improve the performance and uniformity of LTPS TFTs. For the SLS process, the two shots (2-shots) SLS method was used for this study. The performance of 2 shots SLS LTPS TFT devices mainly depends on the sub-grain crystallinity during solidification instead of the existence of grain boundaries. It is useful for the layout design of circuits on the 2 shots SLS based poly-Si active layer. To enable high reliability of LTPS circuits, the alignment of layout and SLS mask should be considered. The performance of SLS LTPS TFTs is known to be extremely dependent on the grain boundary. The values of field-effect mobility for the TFT channel direction of the 0° angle case were 133.3 cm2/Vs, whereas for 90° angle case were 48.5 cm2/Vs, for Vd = 0.1 V. PMOS TFTs fabricated using the SLS poly-Si thin films mobility were dependence of device channel direction. We report on AMOLED (active matrix organic emitting displays) fabricated using LTPS TFTs backplane substrate which crystallization by conventional line beam excimer laser annealing (ELA) technology. We propose the chemical solution treatments and ELA process to improve the LTPS TFT characteristics uniformity and AMOLED display image quality. We used DHF and O3 solutions treatment before and after ELA process, the electrical characteristics of LTPS TFT were great improved. The surface condition of the poly silicon thin film becomes more uniform and smoother after these treatments. The variation of the current that drives AMOLED is reduced from 30% to 10%. The streak mura caused by ELA process has been greatly reduced as well. Finally, the image quality of AMOLED is highly improved. We developed a nonlithographic patterning method by excimer laser patterning process, which is attractive for use in the fabrication of TFT-LCDs. KrF 248 nm laser projection ablation has been developed for direct patterning of inorganic and organic thin films on glass substrates. The new process provides an alternative method to conventional lithographic patterning, and is capable of reducing the number of process steps and process cost. The organic passivation thin film Polyimides (PI: SE-7492, Nissan Chemical) 2000 nm and inorganic (SiNx) passivation thin film were deposited on MoW/glass substrate. The etch rate of passivation layer by excimer laser photoablation depending on the laser energy density and irradiation shot number. From the relationship between the etch depth and the laser energy density, the laser photoablation thresholds energy density was found to be 200 mJ/cm2 for organic thin film and 1200 mJ/cm2 for inorganic thin films. The field effect mobility (μFE) and average threshold voltage (Vth) of a-Si TFTs device by laser ablation is 0.27 cm2/Vs and 2.5 V. The field effect mobility (μFE) and average threshold voltage (Vth) of a-Si TFTs device by conventional lithographic process is 0.18 cm2/Vs and 3.5 V. The performance of a-Si TFTs fabricated by laser direct patterning process is comparable to that by conventional lithography process. Amorphous silicon TFTs device fabricated with organic passivation layer could have good transfer characteristics. The field effect mobility (μFE) was as high as 0.23 cm2/Vs, and the average threshold voltage was 1.25 V of Vd = 0.1 V. The sub-threshold swing (Vth ) was around 0.646 V/dec. The device characteristics are generally good enough to satisfy the LCD application requirements