esearch & Development of Surface-Discharge Color Plasma Display Technologies Tsutae Shinoda Peripheral System Laboratories,Fujitsu Laboratories Ltd. 64, Nishiwaki, Ohkubo-cho, Akashi 674-8555 Japan Abstract Plasma display technologies are reviewed. The technical meaning of two essential technologies of panels structures and ADS sub-field method are discussed and other important technologies for display image is introduced. Current status and future technical tend will be discussed. --- Before success of a 21-in.-diagonal color PDP--- (2) Development of 42-in.-diagonal PDP technologies and display quality improvement. This paper will discuss these technologies developed and future technologies. Development of basic technologies for color PDP Introduction Long awaited exciting age for researchers and engineers who were always with PDPs (PDPs) has opened. In 1980s, a lot of them left from developing PDPs with a disappointment because PDP market reduced. In 1990s, the circumstances were suddenly and largely changed with success of practical color PDP development. In 1992 and 1996, a 21-in-diagonal color PDP[1] and a 42-in.-diagonal PDPs[2] have been put into the markets, respectively and then PDP field were revived with an expectation to become a key display device in multi-media era. The color PDP technologies realized Wall hanging television and large area multi-media displays. The recent development of the PDP technologies is amazing in the field of display image, process technologies and plasma science[3],[4]. We can find some technological jumping steps in the developing process of color PDP as follows. (1) Principle technology has been invented and developed. Although there were many researches to realize color PDP in the early stage of the PDP development, it was not successful because of short operating life due to the phosphor degradation. The author has started to research surface discharge technologies to develop a full color PDP since 1979 and the technology opened a new way to research a full color PDP. There are two important inventions for panel structure and driving technologies. Essential panel structure development The color surface discharge technologies have been developing with new structure of three electrode presented first at SID 84 having an address electrode and two display electrodes[5]. Panel structure was fixed with development of 21-in.-diagonal full color PDP. The target specification of a 21-in.-diagonal panel to be developed was as follows. The number of pixels are 640 horizontally by 480 vertically. A pixel has been composed of three primary colors of blue, red, and green. The luminous target was higher than 150 cd/m2.
Figure 1 compares the color arrangement between a 21-in.-diagonal panel and the three color which was the world first color plasma display products just finished to develop. The difference between them was pixel resolution as that the resolution of the 21-in.-diagonal PDP is about 4 times higher than that of the three color PDP. The sub-pixel pitch between red and green was 0.8 mm. In the case of 21-in.- PDP, a pixel is made with a combination of three sub-pixels, such as red, blue and green colors as shown in the figure. The sub-pixel pitch between neighboring colors is 0.22 mm. We investigated four kinds of panel structures in terms of rib and phosphor structures. Figure 2 shows the cross section of the structures. Figure 2 (a) shows a transmission type panel with single rib structure in which ribs were deposited on only rear glass substrate and phosphor was only on front substrate. A luminance was not sufficiently high because we observed the visible light through the phosphor layers. We investigated three kinds of reflection panel structure in which phosphors were deposited only on the back plate. It was expected to realize high luminance because we could observe both the emitted light on the phosphor surface and the reflected light by the phosphor layer. Figure 2 (b) shows a reflective type panel with double rib structures in which the phosphors are deposited only on the dielectric layer and side wall of the rib on the back plate. The panel however has a disadvantage of high accurate alignment when assembling the two substrate. Figure 2 (c) shows another reflection and single rib structure in which the phosphor is deposited only on the dielectric layer of the rear plate. Figure 2 (d) shows the other reflective and single rib structures in which the phosphors are deposited on both the side wall of the rib and the dielectric layer. The structure of figure 2 (d) has advantages over other structures in terms of luminance and viewing angle which are most important features for large area color PDPs. Practical panel structure Figure 3 show the practical panel structure developed for the 21-in.-diagonal color PDP, respectively. The panel structure is called as the reflection type three-electrode surface discharge color PDPs. Paired parallel display electrode X and display scan electrode Y are formed on the front glass substrate. Each display electrode is composed of a transparent and a narrow bus electrode to emit a luminance effectively. These electrodes are covered by a dielectric layer and a thin MgO layer. On the other rear substrate, striped address electrodes are arranged. Striped barrier ribs are on both side of the address electrodes to separate the adjacent discharge cells and to eliminate the optical cross-talk between them. Three primary color phosphor materials for red, blue, and green colors are deposited in the neighboring channels made by the ribs to cover both of the side wall of the ribs and the dielectric layer. The structure has realized good performances such as a high luminance, a high luminous efficiency and a wide viewing angle. The substrates are assembled each other with about 150 m gap. A Ne and Xe gas mixture is introduced between the gap. The panel structure is the simplest one of conventionally developed color PDPs and the fabrication process is simple enough to mass produce, so it has advantages such as a low cost process, easiness to manufacture large area panels and high resolution panels. Driving technologies(ads method)[6] In the early stage of the color plasma research, we had no way to realize a high speed addressing method to realize a high level gray scale. The author proposed the address-
display-period-separated (ADS) sub-field method to solve the problem. ecently, some papers pointed out that the ADS method has a problem of low display duty with essential miss understanding. Display duty is not the matter. ADS method is the fastest way to address comparing to conventional ways. Figure 4(a) shows the multiple addressing sub-field method (MAS) which was the fastest way of the conventional sub-field methods. A 16 level gray scale for monochrome PDP was achieved with the method. Although this method has so called a 100 % display duty, we abandoned because of slow addressing speed. In the method, eight address pulses are continuously applied to scan lines to address display data. Namely, the first scan line is addressed and then the next scan line which is separate 128k scan lines from the first scan line is addressed without inserting the sustaining pulses. And then each 64k, 32k, 16k, 8k, 4k, and 2k separated scan line is addressed sequentially as well. There is a relationship as shown in the following equation. Tf = N(Tw+mTs+Te), where, Tf, Tw, Ts, Te, N and m are time for a frame, time for write, display pulse width, erase pulse width, number of display electrode pair and number of pulse pair for elemental display pulse set. ADS method is shown in figure 4(b). A picture of one frame is divided into 8 pictures which are called sub-frame. The each sub-frame has isolated address period and display period which are concurrent to all of display area. Figure 5 shows the applied waveform in detail. The address period is performed with reset step and address step. The reset pulses, write and erase pulses, are applied to erase the wall charges accumulated in advanced sub-field, that make a same surface condition of all display cells. The writing pulses are applied between display-scan electrode and address electrodes. The step is repeated from first scan line to 1000th scan line sequentially according to the display data to make a wall charge on dielectric layers. Then, sustaining pulses are applied to all of the display electrodes in the display period. As a result, the discharges in the cells accumulated a wall charges previously are maintained in the following display period. The number of sustaining pulses are decided corresponding to the weight of luminance for each sub-field. In the case of 8 bit sub fields, Tf is shown with the following equation. Tf=8(Te+Tw)+ 256 mts. Table 1 compares the characteristics between ADS and MAS. This shows that the pulse width needed for MAS is too narrow to obtain a sufficient operating voltage margin. The high display frequency drive of MAS has problems of large power consumption and short operating life as a rapid degradation of luminance due to phosphor degradation which depends on the number of discharge. On the contrary, the sustain pulse width of ADS is sufficiently wide to get a wide operating margin. The method has a redundancy to adjust a display frequency for appropriate design of luminance and operating life. The reset step has two function, one is for elimination of the wall charge of discharged cells in the previous sub-field and the other is for supply a charges to ignite a stable discharge in the addressing step. Especially, for AC-PDP, supplying the initial charge is important step. The fast addressing ADS method can keep a time for the reset sufficiently. The other important advantage of the ADS method is that it can offer a special first pulse in the display period, such as wider sustaining pulse. The wide sustain pulse offer a sufficient efficiency of following the addressed wall charges to sustain the discharge in the display period.
High Contrast Technology[7] At the early stage of ADS method, each addressing period had a full write sequence in which full display cells are ignited once and then erased to reset and stabilize the initial condition of addressing step. The back ground light come from the full write sequence was about 4 cd/m2 decreasing the contrast ratio as 50 to 1. So, the display quality was poor on the early 42-in.-diagonal PDP. To improve the problems, a system in which a full write sequence is used only once in a frame was introduced and reduced the background light to 0.5 cd/m2. The sequence improved the contrast ratio in dark room as 400 to 1 which is a sufficient value for usual moving picture. Color Contour Improvement Technologies[8] The color contour problem is due to the unique sub-field method of color PDP. The improvement points for the contour problems are as follows. (1) Division of higher brightness sub-fields to making uniform the brightness level in a frame. (2) Decreasing the change in brightness levels between adjacent frames. (3) Two display modes in which sub-frame arrangement with symmetric relation to each other are placed in stagger. (4) Increasing in the number of sub-fields. With the combination of the improvement methods, the color contour issue is improved in insignificant levels for practical TV use. 42 and 50-in.-diagonal PDPs A large area 42-in. and 50-in.-diagonal PDPs[7] are now available using the technologies described above. The former display's resolution was 852 x 480 wide TV format to target the TV and also public information market. The latter has a 1365 x 768 HDTV format. Figure 6 shows a sample display of the 42-in.-diagonal full-color PDP. The panel has good color purity, a white peak area luminance of 350 cd/m2, a wide viewing angle of more than 160 degrees, and 16.7 million colors. These characteristics are comparable to that of CTs. Future Technologies In terms of the future target of the color PDP technologies it is needed to develop large area technologies, high resolution technologies and low power consumption technologies. The target of the size will be up to 70-in.-diagonal with 1920 x 1024 resolution for full spec HDTV. It needs high speed addressing to realize a progressive addressing. The other application for work station will be accepted for PDPs. A 25-In.-diagonal PDP with 1920 x 1024 resolution has already been reported. The technologies showed feasibility to develop from 20 to 70 high resolution WS and HDTV. Low power consumption display is also expected. To penetrate into more wide commercial society such as home use, low consumption should be realized. High luminous efficiency of the discharge and low power consumption driver system should be developed. I believe that these technologies development will realize an ideal wall hanging TV with a very thin, right weight and low power consumption plasma display like a tablet. eferences [1]S.Yoshikawa, et al,: Full color AC plasma display with 256 gray scale, Japan Display 92, P.605(1992) [2]T.Hirose, et.al., Performance Features of a 42-in.-Diagonal Color Plasm Display, SID 96 Digest, pp.279-282,(1986) [3]M.Sawa, et.al., Direct Observation of VUV ays for Surface-Discharge ac Plasma
Nu m b er o f lin e Displays by Using an Ultra-High-Speed Electronic Camera, SID98 Digest, pp.361-364. [4]H.S.Jeong,et. al., Analysis of He-Xe Discharge Kinetics in an AC-PDP Cell, SID98 Digest, pp.365-368. [5]T. Shinoda and A.Niinuma, : Logically addressable Surface Discharge ac Plasma Display Panels with a New Write Electrode, SID 1984, Digest, pp.172-175,(1984). [6] T. Shinoda, et. al., High Level ray Scale for AC Plasma Display Panels Using Address-Display Period-Separated Sub-Field 0.75mm 2.4 mm 0.8 mm (a) Color arrangement for three color PDP 3.0m m 0.22mm 0.66 mm B 0.66 mm (b) Color arrangement for 21-in.-diagonal full color PDP Fig. 1 Pixel arrangement for color PDP Method Transaction of IEICE C-2, No.3, pp.349-355.(1998) in Japanese. [7]Y.Sano,e. al., High Contrasst 50-in. Color ac Plasma Display with 1365 x 768 Pixels, SID98 Digest, pp.275-278. [8] T. Makino et.al Ashia Display 95,pp.381-384(1995) Discharg e Lowest luminance Address electrode UV Phosphor s Display (a) Transmission electrodes and single rib type Low and narrow angle UV luminance viewing Front plate ear (C) eflection and plate (d eflection and single rib type ) single rib Fig. 2 Cross section of researched type structures (b ) Low luminance and serious alignment UV eflection and double rib type High and wide angle UV luminance viewing 16.7ms Front glass plate Dielectric layer Protecting layer Dielectric layer Address electrode Phosphors(ed,reen,Blue) Transparent electrode Barrier rib ear glass plate Bus electrode Display electrodes Fig.3 Panel structure for color plasma display (a) Multiple addressing sub-field method(mas) F1 F2 F3 F4 F5 F6 F7 F8 Address period Display period Time (b) Address Display period separated sub-field method(a Fig. 4 Comparison of addressing methods
Items ADS MDS Tf 8(NTw+Te)+ 256mTs N(8Tw+mTs+Te) Display Freq. 256m x 60 mn x 60 N=1000 Add. pulse width 1.7 s 1.0 s Sus. pulse width 6.6 s 3.9 s Display freq. 30 khz 120kHz Tf: Time for frame, N: Number of Display/scan lines, Ts: Sustain pulse width, Tw :Write pulse width, Te: Erase pulse width m: Number of pulses in sustain pulse unit Display frequency: Number of sustain pulses for display per sec Table 1 Comparison of ADS and MDS Electrode voltage An Subframe SF Address Period Display Period eset Address Erasing Addressing Sustaining X Yj Yj+1 Writing Scanning Yj+2 Fig.5 Applied waweform of ADS method (1996) Item Display Area Aspect atio Number of Pixels Pixel Pitch Number of Colors Luminance Viewing Angle Power Consumption Weight Performance 920mm x 518mm 16 : 9 852 (,,B) X 480 1.08mm X 1.08mm 16.7 million 350cd / m > 160 degree 300 W max 18Kg Fig. 6 42-in.diagonal color plasma display