Past and future technologies of information displays

Past and future technologies of information displays Kouji Suzuki SED Incorporated, Hiratsuka, Japan Research and Development Center, Toshiba Corporation, Kawasaki, Japan Abstract A concise summary is given of the history of display technology and new display devices coming in the near future. CRT display including CRT projection display was for a long time the only device capable of exhibiting moving picture images applicable to TV receivers. To improve bulky and heavy structure of CRT, FPDs have been developed and are available in the market nowadays. Typical FPDs, namely LCD and PDP, have been applied to TV sets. Especially, LCD has many application fields, because of its size variation and high-resolution capability. However, picture quality of current FPD are still inferior to that of the CRT, and new types of flat panel display with high picture quality under development will come into the market in the near future. Some examples are OLED and FED. The presentation will also introduce key technologies that realize those displays. Introduction During the 20 th century, electronic displays have been widely used in the world, as the development of TV broadcast and information technology. Information displays are essential for human society and we further expect that new displays would open new products in future. Typical displays widely used today are CRT (Cathode ray tube), LCD (liquid crystal display), PDP (Plasma display panel) and a projection display. Among them CRT was the only information display for half of the century, although its bulky and heavy structure limited enlarging screen area of CRT less than 40 inches in diagonal size. After long time effort to develop FPD (Flat panel display), dot-matrix LCD (Liquid crystal display) came into the market in 1980s. Late 1990s, color PDP (Plasma display panel) was realized for TV use after 30 years of research and development. Major projection display was a CRT projector for a long time, however projectors with a new display engine using LCD or DMD (digital micro-mirror device) have been increasing recently. Nowadays, the display market is occupied by those four major displays, and share of FPD grows year by year. In this situation, research and development on new information displays is still continued aiming for either higher picture quality or new application. Classification We have many types of displays including under development stage. Those displays are classified as shown in Fig.1 in view point of operational principle. There are two types, a direct view display and a projection one. Direct-view Emissive CRT (cathode ray tube) PDP (plasma display panel) OLED (organic light-emmiting display) Passive-matrix OLED Active-matrix OLED FED (field emission display) Spindt-type emitter CNT(carbon nanotube) emitter SCE (surface-conduction emitter) ELD (electro-luminescent display) VFD (vacuum florescent display) Non-emissive LCD (liquid crystal display) Passive-matrix LCD Active-matrix LCD TN (twist nematic mode) IPS (in-plane mode) MVA (multi vertically aligned ) OCB(optically compensated bend) EPD (electro-phoretic display) Projection display Rear projector Front projector Fig.1. Classification of information Displays The direct-view displays are classified into emissive displays and non-emissive ones. The non emissive displays like an LCD needs light source and has capability of a reflective display. In view points of driving method, the direct view displays are categorized into a simple-matrix display and an active-matrix display (AM-display). AM-display needs a switching device in each pixel and provides a memory function, 0-7803-9269-8/05/$20.00 (c) 2005 IEEE

which prevents the deterioration of picture quality even for higher resolution display. The detail is introduced latter. The projection displays are classified into a rear projector and a front projector. The performance of projection displays depends on not only the display engine but also on the optical system including a screen. While the CRT projector has been the most popular one for a long time, the LCD and the DMD projector gradually increases because of high-resolution image. In this paper the direct displays are discussed. Display resolution, that defined by the number of pixels, is one of the important specification of a display and is standardized as shown in Table 1. For TV case, the conventional NTSC TV system closes to VGA and SVGA class, and a high-definition (HD) TV system uses XGA or FHD (Full HD) format. 1960.As the cost/performance of CRT advanced, TV broadcast spread rapidly throughout the world. Front glass Frit seal Fennel glass S hadow m ask P hosphor layer Electron gun H o t c ath o de Electron beam Deflection york Fig.2 Cross-sectional view of a color CRT Table 1. Typical display formats Format Pixels(H x V) Numb. of pixels VGA 680 x 480 307,200 SGVA 800 x 600 480,000 XGA 1024 x 768 786,432 SXGA 1280 x 1024 1,310,720 UXGA 1600 x 1200 1,920,000 QXGA 2048 x 1536 3,145,728 QSXGA 2560 x 2048 5,242,880 HDTV 1920 x 1080 2,073,600 CRT (Cathode Ray Tube) Since the invention of CRT by W. K. Braun in 1897, CRT was developed for application to a TV receiver, because the CRT was the only available candidate in 1920s. After success of displaying image on CRT screen by electron-beam scanning method by K. Takayanagi in 1927 and Zworykin (RCA) in 1928, local TV broadcasting started in Germany, 1935, and in England, 1936. Commercial TV broadcasting based on standardized TV system began in USA, France and USSR in 1945 using monochrome TV, and expanded gradually in the world. We have three TV standard nowadays, NTSC (National Television System Committee), PAL (Phase Alternation Line) and SECOM (Sequential Couleur a Memorire), which were established until 1967. Color TV broadcasting started in the USA in 1954, and in Japan in Figure 2 shows a cross-section of a conventional color CRT, which consists of three major parts, an electron-gun unit, a phosphor coated front glass and a magnetic deflection yoke. The electron gun generates an electron beam emitted from a cathode with a heating filament and controls electron beam by several grid electrodes, which modulate beam current and focus and accelerate with high voltage of around 30kV. The deflection yoke scans beam as to cover a whole screen. Then electron beam passing through the hole in the shadow-mask impinges the phosphor layer. Photoemission occurs by cathode luminescence in principle. Actually, there are three beams, and three phosphors corresponding to R, G and B colors. Luminance efficiency in white is approximately 3-4 (lm/w) as CRT system with peak brightness of about 500 cd/m2. Performance of CRT has been improved for more than 50 years. The early CRT had very low brightness less than 10 cd/m2. History of R&D has been focused to improve brightness. Related important technologies are as follows. In 1946, RCA group invented a high luminance screen structure called MB (metal back) where the phosphor layer was covered by thin Al film, which functioned as an anode electrode as well as a reflection film. In 1950, RCA group also invented a shadow-mask for color CRT, which shaped the beam narrower and prevented color. In 1961, Roland introduced BM (black matrix) into screen structure, which significantly reduced reflection of outer light and enhanced the contrast ratio in a bright room. In 1964, red phosphor material of YOS:Eu was introduced and improved luminance efficiency. In 1975, RCA

developed color filters, which were formed between phosphor and a substrate to enhance contrast ratio. As brightness and contrast ratio was a trade-off relation, after sufficient brightness was achieved, the extra margin of luminance was turned to contrast improvement. We can see the great contributions of RCA in CRT technology. In regarding to production, Japanese companies produced large share in the world after introducing technologies from RCA. However, center of production sites has now moved to Korea, China and Taiwan. Market size has now almost saturated and has begun to decrease due to the increased position of FPD TVs. strength. Color image is created by color filters formed on the substrate. A BM layer is also formed to prevent light transmitance at the boundary region of each pixel electrode. The conventional LCD structure is shown in Fig.4, where screen area is arranged with pixels in x-y matrix and each pixel consists of a TFT (thin-film transistor) and a pixel electrode. This type of LCD is called an active-matrix (AM) LCD and operates like a analog dynamic memory. The advantage of AM-LCD is that the LC layer is stably driven during hold periode. If the on/off current ratio of TFT is suficient, LCD with a large number of pixels can be realized without deterioration of picture quality. LCD (Liquid Crystal Display) LCD has many application fields compared with other displays, since it has a thin and flat structure and good legibility. LC molecule has a rod-like shape and shows anisotropic electrical, optical and mechanical properties between longtudinal and anti-longtudinal directions of the molecule. This anisotopic nature is utirized for display operation. In 1963, R. Williams at RCA, discovered that LC alignment was controllable with electric field, and in 1968, G. Heilmeier, also at RCA, developed a prototype display with dynamic scattering mode. In 1971, M. Schadt developed a TN (twisted nematic) LC. TNLC is basically the same LC mode in use today. Fig.4. Schematic structure of a color TFT-LCD Fig.3. Cross-sectional view of LC cell Figure 3 shows a cross sectional view of TN mode LCD. LC molecules are aligned palarel to the surface with changing molecule direction 90 degrees from the bottom to the top substrate under no electric field. Under this condition, the polarized incident light is passing through the panel. When electric field is applied, molecules bigin to tilt along to the field direction and the transmitance decreases depending on field Fig.5. Equivalent circuit of one pixel in TFT-LCD The concept of the active-matrix display was proposed by B. J. Lechner (RCA) in 1971. TFT was achieved by Weimer (RCA) using CdSe in 1949. In 1973, T.P. Brody (Westinghouse) demonstrated a CdSe TFT AM-LCD. However Cd-Se TFT was insufficient in on/off current ratio which was an essential requirement to realize a high resolution LCD. In 1979, the first a-si (amorphous silicon) TFT was presented by P.G. LeComber (Dundee Univ.) and showed

large on/off ratio more than 6 orders of magnitude. A 3-in.-diagonal a-si TFT-LCD was demonstrated by Toshiba, in 1982, and a 1.5-in. high-temperature poly-si (HTPS) TFT-LCD by S. Morozumi (Seiko-Epson), in 1983. In 1986, T. Samesima (Sony) developed ELA (excimer laser anneal) low-temperature p-si (LTPS) TFT on a glass substrate. Field effect mobility was larger than two orders of magnitude compared with that of a-si TFT, which was suitable for integration of driver circuits. In 1997, a 12.1-in. LTPS LCD with XGA format with integrated peripheral circuits was developed by Toshiba. To meet TV application, both slow response and narrow viewing angle had to be improved. To expand viewing angle, three LC modes were developed. In 1992, G. Baur proposed IPS (in-plane switching) LC mode with wide viewing angle, where electric field were applied through electrodes on the same substrate and LC molecule moved paralel to substrate. In 1998, A. Takeda developed MVA (multi- domain vertical alignment) LCD, where pixel were devided into several regions and LC molecules were aligned in the vertial direction on the substrate. In 1993, T. Miyashita introduced OCB (optically compensated birefrigence) mode, which exibited wide viewing angle as well as fast response time less than 10 msec. However, TFT-LCDs still need the improvement of response time and dark level in brightness for large-area TV application. formed in one substrate. Another important factor for eficient productivity is to minimize the number of process steps. This depends on the structure of TFTs shown in Fig.6. Typical numbers of photo-exposure processes for a-si TFTs ranges 5 to 7, and 5 to 9 for ELA LTPS TFTs. PDP (Plasma Display Panel) PDP is an emissive display utilizing gas discharge. In 1956, B. Findeisen demonstrated a DC type PDP. A dot-matrix DC PDP needed a primary discharge to continue sequential discharges for brightness control. In 1966, D. Bitzer (Univ. of Illinois) invented AC PDP with Ne gas and exhibited a neon-orange monochrome display. His group also proposed a color PDP, where Xe gas discharge was adopted for emitting UV light to excite phosphors. Fig. 7 Drive scheme of AC-PDP Fig. 6 TFT Structure; (a) a-si TFT, (b) LTPS-TFT Production of a-si TFT-LCDs started in 1987 with diagonal size of 2 inches. Since 1990, LCD production has scaled up year by year along with PC market growth. One point to enhance productivity was the size of a glass substrate. The first generation used a 300 X 400 mm2 glass in 1990, however, the recent eighth generation uses substrate size of 2200 x 2200 mm2, where 8 45-in.-diagonal LCDs could be Figure 7 shows principle of AC-PDP operation. Space for gas discharge is surrounded with the bottom and the top substrates and the barrier ribs of approximately 100um height. The driving scheme consists of three periods, a writing period to accumulate charges on the surface of dielectric layer, a sustaining period to continue discharge, and an erasing period to neutralize surface charge. The number of discharges controls brightness during the sustaining period. A schematic structure of a conventional color AC-PDP is shown in Fig.8. On the bottom substrate, data electrodes formed in vertical direction, protection insulation layer and phosphor layer are formed with the barrier ribs defining pixel space. On the top substrate, sustaining ITO electrodes formed in the horizontal direction, protection insulation layer and evaporated MgO layer are formed. Discharge space is filled with He-Xe gas. Although monochrome PDP with Ne gas discharge was applied for the computer display 25 years ago, the development of a color PDP took a time for improvement of

discharge damage on phosphors or the electrodes. These failures were resolved by the invention of three-electrode configuration with a protection dielectric layer in 1983. In 1989, Shinoda (Fujitsu) developed a 23-in.-diagonal color AC-PDP. In 1997, the first product came into the market with 42-in.-diagonal color AC-PDP. In 2004, the largest 102-in.-diaonal color AC-PDP was demonstrated. Fig. 8. Structure of a color AC PDP Technical issues of current color AC-PDP are low brightness or large power consumption. Luminance efficiency of the products ranges 1 to 2 (lm/w) today. R&D are continued to improve the efficiency on the discharge gas, a cell structure and driving method that enables recycling waste power of charge/discharge on the electrode lines. Displays in Future Those disadvantages are improving gradually. However, OLED and FED have capability to overcome the disadvantages of the current TV displays. OLED display has the similar structure with TFT-LCD. Light emission layer is formed on pixel electrode instead of LC layer and formed by either vacuum deposition of monomer materials or ink-jet printing of polymer materials. Light emitting layer is a pn diode formed with organic materials. AM drive allows OLED to operate in a small current for longer lifetime. The advantage of OLED is not only emissive display but further thin and light-weight compared to LCDs, since OLED does not need a back light unit. Technical issues of OLED are lifetime of organic materials and improvement of pixel circuits for gray scale control especially in low luminance image. In addition, further development of process technology related to OLD layers is needed in viewpoint of uniformity on a large substrate. Some small OLEDs have already come into the market, however, the target application of OLED is TV. FED is a thin CRT, which has a cold emitter at each pixel and an emitted electron beam excites phosphor. A field emitter is the key device and several types have been developed. Typical examples are Spindt type emitter based on thin film process, CNT (carbon nono-tube) emitters, and MIM (Metal-insulator-metal) emitters. Among them, SCE (Surface conduction electron emitter) shows excellent characteristics and comprises the SED (Surface conduction electron emitter display). The applications for FPDs have expanded - especially for LCDs featuring thin light-weight structure and high resolution capability and are now adopted for not only for TVs but also for PCs, automobile and portable equipment. In this situation, the upcoming displays would take one of following two approaches for being marketed successfully. The first type must compete with the current displays in cost-performance. The second type would have a novel function to create a new application. Typical examples for the first type of display are OLED (organic light emitting display) and FED (field emission display). In large-area TV application, higher picture quality is still desired. LCD TV is insufficient in dark brightness, typically 1 cd/m2, and response time, which depends on LC mode, but is typically 10 msec. PDP TV lacks brightness of full white, which is limited by drive system to suppress power consumption. Fig.9. Operational principal of an SED Figure 9 shows operational principal of SED. On the rear glass plate each pixel contains a field emitter, which is consists of thin PdO film of about 10nm thickness with narrow gap in the order of nm. Emitters operate at about teens volts and flows electron current between nanogap. On the face glass

plate red, green and blue phosphors are patterned by screen print. Thin Al metal layer is deposited on the phosphor layer and is applied high voltage of 10kV. Several percents of emitted electrons are reached to the faceplate and excite phosphors. Therefore light emission is the same as that of CRT and SED exhibits the similar picture quality to CRT. Figure.10 shows a schematic structure of an SED. An electron emitter at each pixel is connected between a scan line and a data line. Scan pulses of negative voltage are applied to the scan lines sequentially and select the emitters on the scan line. Simultaneously, data signal pulses of positive voltage are applied to data lines. Sum of the scan line and the data line voltages are applied to the emitter and electrons are emitted for the selected line. The faceplate and the rear plate are supported by spacers with the space of about 2 mm. Inside the panel is kept in vacuum condition.. Fig.10 Schematic structure of an SED panel Figure 11 shows the typical current-voltage characteristics of the emitter under high anode voltage of 10kV. Drive voltage across the emitter is 18.9 V for emission condition and 9.3 V for non-emission condition. Corresponding on/off current is sufficient enough for matrix driving. The contrast ratio in dark room exceeds 100,000, which is quite large compared to those of LCD of 300-500 and PDP of 3,000. Advantages of SED are fast response time of less than 1 msec, low power consumption due to high luminance efficacy of phosphor, wide color gamut and high contrast ratio. A 36-in.-diagonal SED with XGA format has been successfully developed and will come into the market in near future As for the second type of the future display with a novel function, typical examples are as follows. A portable sheet display with mechanical flexibility and ultra low power would be a new type of display. Flexible display has been demonstrated using reflective TFT-LCD, EPD (electro-phoretic display) and OLED, however as a color display the whiteness is insufficient for TFT-LCD and EPD, while OLED consumes power consumption. Another example is SOP (system on panel), which contains not only display but also many circuits on the same substrate. In 2004, Semiconductor Energy Lab. and Sharp fabricated 2.2-in. LTPS LCD with integration of not only peripheral circuits but also 8bit CPU, a graphic controler, audio circuits, DC converters and memory circuits. The proto type display was occupaied a lot of area for circuits. Fine submicron design rule are necessary to achieve a compact SOP, while, the display area could be made with a few micron design rule. Conclusion Brief history and operational principle of information displays are presented. Some candidates for the future display are introduced in conjunction to the current technical status. Author believes that new technologies will continuously developed and as a results great advancement of display performance would be achieved. We also expect a breakthrough technology that will create new applications. Fig. 11 Current-voltage characteristics of SCE and luminance under the anode voltage of 10 kv