16.2: World‐Best Performance LTPS TFTs with Robust Bending Properties on AMOLED Displays

Abstract

This study reported a low temperature polycrystalline silicon (LTPS) thin film transistor (TFT) fabrication process on plastic substrates for flexible display applications. Polycrystalline silicon (poly-Si) films were formed by excimer laser annealing (ELA) method. It was found by ELA thermal simulation that there was around 70 on plastic surface during ELA crystallization process. The excimer laser irradiated film was analyzed by using various spectroscopic methods such as X-ray diffraction, scanning electron microscopy, and atomic force microscopy. Dehydrogenation and activation processes were performed by a conventional LTPS method without causing any plastic substrate distortion. The fabricated poly-Si TFT on a flexible backplane shows a very good performance with field effect mobility of 95.3 cm 2 /Vs, on/off ratio current ratio > 10 8 , and threshold voltage of -1.6 V. Bending tests after a delamination process were also performed with TFT backplane samples. AMOLED panel Bending machineSID 11 DIGEST • 197 16.2 / M. Kim

Polysilicon thin-film transistors on polymer substrates

“…The use of polymer substrates with 250 1CoT M o350 1C appears more attractive for several reasons: (i) it is possible to use many of the conventional processes adopted for LTPS TFT fabrication with minor modifications; (ii) key materials, such as dielectrics, which are fundamentally poorer at low temperatures ( o250 1C), show adequate performance in the T-range 250-350 1C; (iii) ULTPS TFT performance are generally inferior to those fabricated at T4250 1C, as key fabrication steps, such as device hydrogenation, are problematic at very low temperatures [8]. Therefore, more recent activities have focused on the use of PAR [8,17] and PI [8,[26][27][28][29][30][31][32][33] and devices with performance comparable to those fabricated on glass have been demonstrated, with field effect mobility for n-channel devices up to 370 cm 2 /Vs [33] and for p-channel devices up to 124 cm 2 /Vs [31]. In the next sections we will describe in some detail the main steps for the direct fabrication of polysilicon TFTs on polymer substrates, including the process we developed on ultra-thin PI, the electrical characteristics of the devices fabricated on polymer substrates and some of the issues and, finally, the applications.…”

“…To limit the hydrogen content in the a-Si precursor, substrate temperatures 4250 1C during deposition are typically used, confining the choice of polymer substrates to PAR, PI and PNB. Several strategies for laser de-hydrogenation/crystallization have been proposed [8,[27][28][29][30][31][32][33][40][41][42][43]. In general, the a-Si:H film is irradiated with increasing energy densities, in order to progressively heat the film to promote de-hydrogenation, while for higher energy densities the film is crystallized via the melt/ resolidification process.…”

“…In the conventional self-aligned polysilicon TFT structure, the source-drain contact formation is normally obtained by using ion-implantation, auto-registered to the gate electrode, followed by dopant activation, which can be obtained by either thermal [48] or laser [16][17][18][19][20][21][22][23]26,29,30] annealing. While conventional ion implantation or ion-shower doping techniques can be easily applied when using polymer substrates [8,[16][17][18][19][20][21][22][23]26,[29][30][31][32]49], both dopant activation processes are problematic. In fact, thermal annealing requires temperatures (typically T44501C [48]) well above the polymer T M , while laser activation, which takes place after active layer island formation, imply UV irradiation of the polymer substrate.…”

“…Some examples of LTPS technology applications are flexible active-matrix organic light emitting displays (AMOLED) [31,32] and active-matrix liquid crystal displays (AMLCD) [2,8,12], fingerprint sensors [3,13] and smart sensors [58,59] Fig. 16, has a pixel number of 240 Â RGB Â 400, which corresponds to WQVGA resolution.…”

Self-aligned offset gate poly-Si TFTs using photoresist trimming technology

“…It is the superior driving capability and performance stability that makes the poly-Si TFT technology the only one currently used for fabricating active matrix organic light-emitting diode (AM-OLED) display devices in mass-production [3].…”