Poly‐Si Thin‐Film Transistors on Polyimide Substrate for 1 mm Diameter Rollable Active‐Matrix Organic Light‐Emitting Diode Display

Abstract

Recently, flexible displays based on active-matrix organic lightemitting diode (AMOLED) on polyimide (PI) substrate are of increasing interest for use in foldable displays. PI substrates are not only thin and light, [1,2] but also could be processed at relatively high temperature (≥450 C). To achieve a flexible AMOLED display, thin-film transistor (TFT) array should be manufactured on a plastic substrate. [3][4][5][6][7][8][9][10][11] Popular semiconductor materials used for TFTs are amorphous silicon (a-Si), amorphous indium-gallium-zinc-oxide (a-IGZO), and polycrystalline Si (poly-Si) on PI substrate. Due to its low-temperature process on large glass substrates such as 10.5G (2.2 m  2.5 m), the a-Si:H has been used as a TFT switching element. However, it cannot be used for AMOLED TFT backplanes because of its threshold voltage (V TH ) shift during OLED operation. [12,13] Note that a-IGZO TFT has at least one order of magnitude higher mobility compared to a-Si:H TFT even though the TFT process temperature is similar. The a-IGZO TFT can be manufactured by using G10.5 glass so that manufacturing cost of oxide TFT backplanes for OLED can be much lower than that of low-temperature poly-Si (LTPS) TFT. Therefore, intensive studies on amorphous oxide semiconductors are being carried out for TFT backplanes of OLED displays. However, all smartphone OLED displays are still based on LTPS TFT backplanes because of its high mobility and excellent stability. The LTPS TFTs on PI substrate with high mobility and high stability are very suitable for mobile application. [14] The effects of mechanical strain and electrical reliability of LTPS TFTs have been previously reported (show in Table 1) with the conclusion that the mechanical stability is inferior to oxide TFTs. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Therefore, finding a solution to overcome the performance degradation during mechanical stress is important for more flexible and foldable AMOLED displays.In this article, to improve the reliability under mechanical stress, flexible LTPS TFTs were fabricated using three kinds of interlayers (ITLs): inorganic (SiO 2 /SiN x ), hybrid (SiN x /PI), and organic (PI) layers. Reliability test of the LTPS TFTs is performed using bending and rolling machines until 10 000 cycles. Using low Young's modulus organic material (PI) ITL, the change in electrical performance under mechanical strain could be reduced significantly. Especially, rolling test with rolling radius of 1 mm was examined until 10 000 cycles. The LTPS TFT with inorganic ITL shows the crack generation after rolling with 1 mm radius. However, the performance of the LTPS TFTs with hybrid ITL

Electrical-performance and reliability improvement of flexible low-temperature polycrystalline silicon thin-film transistors via post-annealing process

“…At least ten devices for each annealing condition were measured to obtain the statistical data. As a reference device, Device A exhibited a V th of −2.36 V, a µ FE of 76.7 cm 2 (V•s) −1 , an SS of 0.44 V dec −1 , and electrical properties comparable to previously reported results [3,7]. The V th and µ FE seem to have relatively large deviation values.…”

Carrier transport mechanisms in polycrystalline semiconductors: from grain boundary physics to device performance

“…3c). 54 Similar behavior is seen in ZnSnO 3 thin films, where VRH dominates below 100 K. The extracted density of states (B10 19 eV À1 cm À3 ) corroborates this hopping framework. 77 In oxidized nanocrystalline PbTe(In), hopping conduction prevails below 150 K, attributed to enhanced carrier localization along inversion channels at oxidized grain surfaces.…”

Mechanically stable, flexible, and stretchable metal–insulator–semiconductor‐ferroelectric thin‐film transistors for micro‐LED display applications

“…The proposed FE‐TFT can incorporate conventional SiO 2 as the gate insulator (GI), which offers advantages for a large bandgap, smooth surface, and low leakage current density 22 . Note that the mechanical stability of SiO 2 gate insulator has been confirmed from our previous work withstanding a 1 mm bending radius 23 . This can lead to more robust operation under mechanical stress.…”

“…22 Note that the mechanical stability of SiO 2 gate insulator has been confirmed from our previous work withstanding a 1 mm bending radius. 23 This can lead to more robust operation under mechanical stress. Table 1 summarizes the reported fluorite-based flexible FE-TFTs.…”