(Invited) Present and Future of LTPS Technology

Abstract

Japan Display Inc. has developed excellent quality TFT-LCD displays based on precise LTPS technology and progressive LCD-cell technology [1]. Their advantages are less than half the power consumption and a contrast ratio of 2000:1 using unique RGBW technology, photoalignment technology, and an in-cell touch panel technology with 1mmΦstylus input simultaneously. We have recently developed two high resolution displays with more than 500-ppi which one can realize higher response rate with higher frame rate driving and another can reduce the power consumption by 30% with a lower frame rate driving. One is the 17.3-inches 8K4K display with 550ppi and 120-Hz frame rate driving and another is the 8-inches 4K2K with 510-ppi with 30-Hz frame rate driving. We adopted two technologies for the LTPS backplanes, one was a 1.5-µm high resolution process, and the other was an increase of the pixel capacitance. Figure 1 shows the cross sectional SEM image of a poly-Si layer. I-line photolithography was adopted and the dry etching condition of the poly-Si patterning process was optimized in order to obtain poly-Si islands 1.5-µm wide with a good taper angle. In addition, a thinner insulator for the storage capacitance Cs was fabricated almost by half to increase the CSvalue with care in terms of coverage over other layers. The adoption of a 1.5-µm short channel LTPS process and thinner insulator thickness can contribute two benefits. One is a decrease in load capacitance which is considerably increased as the number of pixels increase up to 8K4K. Another is a decrease in feed through voltage ΔVth which is described by equation (1), which causes cross talk especially on higher resolution display. The developed 17.3-inches 8K4K LTPS-LCD with 510-ppi resolution in collaboration with NHK is shown in Figure 2 [2]. We realized uniform transmittance distribution and sufficiently small cross talk much less than 2% on this prototype display which attributes its small load capacitance and small feed through voltage. We also developed 8-inches 4K2K display with 510-ppi. By adopting the same LTPS technologies, we also realized 30-Hz frame rate driving with acceptable flicker level which is defined as the range of the maximum and minimum liminance. We could decrease the total power consumption for drinving by 32% adopting 30-Hz frame rate driving as shown in Figure 3. Reference: [1] T. Kaneko, et al., "Innovative 5-inch FHD and 7-inch WQXGA Displays for Next Generation Smartphones and Tablets", Proc. SID'13, 2013, pp693-296. [2] K. Mochizuki, et al., "A 510-ppi 8K4K LTPS TFT-LCD with 120-Hz Frame Rate Driving" SID'16, Digest paper, 67.2, 2016 Figure 1