Application of amorphous oxide TFT to electrophoretic display
Introduction
Transparent oxide TFTs have attracted much attention over the last several years. Various sorts of oxide semiconductors have been reported so far, such as, ZnO [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], MgxZn1−x[17], [18], Zn–Sn–O (ZTO) [19], [20], [21], [22], [23], In–Zn–O (IZO) [24], [25], [26], [27], [28], [29], SnO2[30], [31], Ga2O3[32], In–Ga–O (IGO) [33], In2O3[34], In–Sn–O (ITO) [35], single crystalline InGaO3(ZnO)5 superlattice [36], and amorphous In–Ga–Zn–O (a-IGZO) [37], [38], [39], [40], [41], [42], [43], [44], [45], [46].
One of the most important criteria for choosing appropriate oxide semiconductor was proposed by Hosono et al. [47], [48]. They predicted that amorphous double oxides composed of heavy metal cations with an electronic configuration (n − 1)d10ns0 (n ⩾ 4) are promising candidate for semiconductor. These ns orbitals have large radii, so that there is a large overlap between the adjacent orbitals, which leads to insensitiveness to the distorted metal–oxygen–metal chemical bonds.
Based upon above mentioned criteria, they reported a novel oxide semiconductor a-IGZO [37]. Amazingly, high mobility of 7 cm2/V s and high on/off ratio of more than five orders of magnitude were achieved even at room temperature process. Incorporating Ga ions is crucial in a-IGZO for suppressing excessive carrier generation via oxygen vacancy.
Transparent amorphous oxide semiconductor enjoys three unique features in comparison with conventional a-Si semiconductor, that is, high performance, low process temperature and transparency. High performance of the oxide semiconductor is attractive to current demanding applications, such as organic light emitting diodes (OLED). Oxide semiconductor can be deposited by sputtering technique, which is suitable for large area fabrication [39]. Actually, 3.5 in. top emission OLED display was successfully driven by a-IGZO TFT array [49], [50]. These results encourage us that the oxide TFT is one of the most promising candidates as a backplane for OELD device.
Low process temperature of amorphous oxide TFT is also quite appealing. Low process temperature is fully compatible with polymer substrate. Displays fabricated onto polymer substrate have the advantage of flexible, light, thin and non-fragile properties. Furthermore, flexible display has the ability or possibility to be fabricated by roll-to-roll process, which can dramatically reduce the process cost. We focused attention on the low process temperature of the amorphous oxide TFT and fabricated 2 in. flexible a-IGZO TFT array onto poly-ethylene–naphthalate (PEN) substrate with the pixel number of 80 × 60. After fabrication of TFT array, E Ink imaging film was laminated. We have successfully driven flexible 2 in. display by a-IGZO TFT array [51], [52]. As far as we know, this was the first demonstration of the flexible display driven by oxide based TFT.
Transparency of the oxide semiconductor is also attractive to new applications. We focused attention on the transparent property of oxide semiconductor and proposed novel display structure for color electronic paper [53], [54]. Transparent TFT array was fabricated onto color filter array and subsequently E Ink imaging film was laminated on to the array. In this structure, the TFT array can be fabricated directly onto color filter array, which facilitates the alignment of the color filter and the TFT array remarkably. This display structure can be realized due to the transparent properties of the oxide TFT, because the display was seen through transparent TFT arrays in this structure. As far as we know, this was the first demonstration of the actual application which utilizes the transparent property of the oxide semiconductor.
In this report, we investigate the feasibility of fabricating much higher resolution display by transparent amorphous oxide TFT. Moreover, we explore low-cost fabrication process of oxide based TFT. In conventional TFT fabrication process, vacuum deposition and photolithography processes are employed repeatedly. In order to avoid these costly and time-consuming processes, we propose that some layers can be formed by printing method. Printing process is vacuum-free and direct patterning can be performed, which can cut down the expensive manufacturing equipment cost of vacuum chamber and photolithography machine. Besides that, number of processes can be eliminated markedly. In this article, we attempt to apply screen-printed source and drain electrode to a-IGZO TFT. Characteristic of the TFT will be demonstrated.
Section snippets
E Ink display
Electrophoretic displays have been studied for more than 30 years. Electrophoresis is the translation of charged objects in a fluid in response to an electric field. In earlier days of electrophoretic displays, there were common problems like sticking of particles to each other and on electrode surfaces and migration of particles to electrode edges. Microencapsulated electrophoretic display film, which is called E Ink imaging film, has been developed by E Ink Corporation [55]. What is
Results
In Fig. 2, transfer characteristic of a pixel TFT is shown. High on/off ratio of more than five orders of magnitude and field effect mobility, μFE of 4.3 cm2/V s were achieved. Fig. 3(a) depicts the magnified view of a-IGZO TFT array. Effective size of the TFT array is 4 in. diagonal and the number of the pixel is 640 × 480 (QVGA) with the resolution of 200 ppi. Pixel dimension is 125 × 125 μm. Channel length (L) and channel width (W) of a pixel TFT is L = 20 μm and W = 5 μm. After fabrication of TFT array,
Discussions
Characteristic of a pixel TFT of 4 in. display is comparable to the reported value [37]. Further optimization is needed to improve the characteristic of the TFT, however, these values are good enough to drive E Ink imaging film. In our fabrication process, all the layers were fabricated at room temperature. At room temperature process, quality of the gate insulator layer is far inferior to that of high process temperature. Actually, we observe some hystereses in our TFT. TFT characteristics can
Summary
The applications of a-IGZO TFTs are explored. We have fabricated 4 in. QVGA a-IGZO TFT array and E Ink display was successfully driven by the TFT array. Resolution of the display is 200 ppi and the number of the pixel is 640 × 480 (QVGA). As far as we know, this is the finest resolution display which was driven by oxide based TFT.
Screen-printed source and drain electrodes are applied to a-IGZO TFT. The TFT shows high on/off ratio of more than seven orders of magnitude and the field effect mobility
Acknowledgments
We would like to express our deepest appreciation to Professor Hosono of Tokyo Institute of Technology for providing valuable suggestions. We are thankful to E Ink Corp. for supplying us with E Ink imaging film for this experiment. We would like to thank to Mr T. Nishimoto, Mr O. Kina, Mr K. Imayoshi, M. Tamakoshi, Mr H. Yamada and Mr Y. Takashima, Dr T. Saito, Mr Y. Kokubo of Toppan Printing Co., Ltd., for offering beneficial suggestions.
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