Reference / Paper · 1983
Ise's own account of the VFD: construction of filament, grid, phosphor and envelope, the oxide cathode at 650 C, ZnO:Zn response times, and the brightness curves against filament voltage, anode and grid voltage and duty, from single digits to colour panels.
Vacuum fluorescent displays: from single digits to colour TV K. KIYOZUMI, T. NAKAMURA Physical principles and operational characteristics of various vacuurn fluorescent displays are outlined. A VFD history and details of construction are also given. Displays of various resolutions for different applica- tions are described up to a 256 x 256 dot matrix unit. Uniform brightness and precise switching operation are realized using improved anode and grid electrode construction, anode baseplate processing, and grid configuration and its fixing processes, A scrolling VFD and a MOS FET switching array VFD are introduced. The latter uses an integrated circuit of 241 x 246 elements to produce a picture 23 x 23 mm. Prospects for the development of VF Ds are given in conclusion. Keywords: display devices (computers); vacuum fluorescent displays; graphic displays; Japan. The vacuum fluorescent display tube, based on the combined concept of a triode and a cathode ray tube, was first produced by Ise Electronics Corporation in 1967. Three stages of development have followed since then. The first was the single digital tube using a tubular glass envelope. The second was the multi-digital glass envelope tube capable of displaying six to 13 digits. Both of these make use of ceramic substrates upon which electrodes are constructed, Material cost of these however called for a third generation which could survive competition in the market, which in those days was clearly characterized by two overwhelming needs, for cost competitiveness and for a planar or flat display configuration. The third generation‘ of VED dispenses with the ceramic substrate which its prede- cessors depended upon and achieves the required planar configuration, It makes use of one side of the flat glass vacuum envelope as the substrate, upon which a wiring layer, insulating layer, anode electrodes, phosphor layers, grid mesh, and the rest are mounted by various deposition processes, Another glass plate, the faceplate, over which a set of linear filamentary cathodes is mounted, is sealed to the substrate or the base glass plate leaving a little space between the two. CONSTRUCTION The VFD is essentially a three-element vacuum tube’, part of which is made of transparent material. The envelope encloses a direct heated filament cathode, metallic mesh grids and phosphor-coated anode segments (Fig, 1). The wiring layer, insulating layer and the phosphor-coated anodes are processed by thick film printing. The wiring layer edge protrudes from the envelope, facilitating connec- tions of internal electrodes to external circuits. As the dimensions of the dot anode and dot pitch decrease, a high density of wiring becomes necessary. The only way for thick film printing to cope with this high density of wiring The authors are at the Ise Electronics Corporation, 700 Aza Wada, Ueno-cho, Ise City, Mie, Japan. DISPLAYS, OCTOBER 1983 is in stacking up the wiring into a multilayer structure. Increased processing costs and augmentation of inter- electrode capacitance are inevitable with this technique. These difficulties are overcome using a composite technique on the glass substrate. Here, thin films of fine aluminium wiring are added to the thick film printing? . The composite glass substrate is shown in Fig. 2° . The grid is constructed by photo-etching a stainless foil into a lattice structure or into sheets with hexagonal pores. The cathode is an extremely fine pure tungsten filament coated with alkaline earth carbonates. Activation of the cathode during proces- sing is carried out in the same manner as for conventional electron tubes: (Ba, Sr, Ca) CO; = CO, + (Ba, Sr, Ca) O. Until 1981, the face glass was mostly formed and processed from plate glass, However, in recent years pieces of glass cut from plate glass which are then joined by fritting to form the desired shapes for face glasses have become prevalent. NESA coating Grid Filament Phosphorus Face glass Conductive adhesive Mould resin Frit glass ¢eeaiita acum sir Sn a \] peut pipe seen ~~ _ —_ ae ores Solder CORNER SANTE AOA ES WF MN IAT ET TCR | PANY, Conductive Conductive layer layer Insulation fayer Anode Lead. pin (segment) Glass substrate Fig. 1 Cross section through a vacuum fluorescent display 0141—9382/83/040213—08 $03.00 © 1983 Butterworth & Co (Publishers) Lid 213 Anode dots (aluminium) ISix-pitch 50um Al wiring {nsulating layer (by thick fllm printing) Fig. 2 Cross section showing detail of the composite glass substrate (dimensions not defined are in um) Vv] 4 Vv FF ahi My ih wtp foto he py tT at had to Fig. 3 VFD operation OPERATION The principle of operation of VFDs is shown in Fig. 3. By application of prescribed voltages to each of the electrodes, the cathode will emit electrons at 650°C. The grid located between the cathode and the anode controls the electron flow from the cathode to the anode. The correct positive potential on the grid accelerates as well as diffuses the electrons from the cathode to the anode; the correct negative potential cuts off the electrons to the anode. Since the grid is made of metallic mesh, some of the electrons that strike it are captured. This gives rise to the grid current. Those electrons that pass through the grid mesh arrive at the anode and, by virtue of their kinetic energy, excite the phosphor to emit light, thus causing the anode current to flow, Here, the anode must also have applied to it the correct positive potential, ie the grid and anode should be simultaneously positively charged for the emission of light. CHARACTERISTICS As stated before, the VFD is basically a triode, and is characterized by a variety of properties. These will be described for a typical dot character VFD (DC405A2). Cathode start-up characteristics Figure 4 shows the change in filament current /;, brightness L, anode current ip and grid current J, against filament 214 voltage #;. Each parameter increases as H's is raised. In the case shown, L, i, and i, exhibit saturation beyond Fy = 8 V. If #¢ is fixed at a value lower than 8 V, a variation of bright- ness results from the fluctuation of the power source. If Hy is fixed at an excessive voltage, it is evident that substantial filament power is wasted which is not only ineffective in raising the brightness, but also deteriorates the filament life by unnecessarily raising the temperature. It is most desirable to set Zr at the lowest possible valuc in the saturation region, The standard rating of filament voltage of the DC405A2 is 8.9 V. Anode and grid voltage The anode voltage ep and the grid voltage e, have a preat effect on the brightness. Effective use of the VFD depends to a large extent on the best choice of ey and e, values. Except in rare special cases, ey and e, are assigned equal voltages. As shown in Fig. 5,2, iy and ig do not exhibit saturation with respect to the increase of cp. Brightness L apparently exhibits 4 good increase, but the cathode current i, , which is the sum of Zp and ig, also increases. This inakes it likely to 100 - 150 oS oa I = € 80 w 60 -- 100 < L (%) i, he (mA p-p) on ] 5 6 7 8 9 10 I &; (Vac) Fig. 4 Cathode start-up characteristics (eh = e¢¢ 45 V p-p; duty ratio 1/50) 30 -- 300 20 + 200 3 a ~ < x E ve = ~l a lo F 100 ot oO 6, +& (Vp-p) Fig. 5 Voltage characteristics (Ef = 8.9 V ac; duty ratio 1/50) DISPLAYS, OCTOBER 1983 ry o1z2 #02 40 60 80 100 a r | ————— 4 eae | | a | I an a | Oo 20 40 60 80 100 b Time (us) Fig. 6 Z2nO:Zn phosphor light response: (a) for a square input voltage waveform on the anode and grid (b) 300 200 £(%} itefe) 50 30 L Poot | tt iN | 20 30 50 80 100 200 Reciprocal of duty factor Fig. 7 Brightness against duty factor for various values of ep = &¢ shown (Eg = 8.9 V a c) exceed the limit for stable electron emission heating, which may result in damage to the cathode. The standard rating of ép and é, of DC405A2 is set at 45 Vp-p. Duty factor Ina dynamically driven VFD, duty factor (DF) is considered one of the factors that governs the brightness and quality of the display. The duty factor depends upon the number of digits of the VFD and the blanking time. Figure 6 shows the pulse activation response of a ZnO:Zn phosphor which gives out blue-green light. The rise and fall times, t, and ¢p, are both about 10 ys. As shown in Fig. 7,2 is proportional to DF, which is expressed as DF = ty/T The pulse length ¢, and period of repetition T are shown in Fig. 6. Figures 4 and 5 show results of measurements DISPLAYS. OCTOBER 1983 performed at DF = 1/50 and tp = 100 ys. Cut-off In practice, a VFD must extinguish the light by placing negative potentials on the grid and anode. Figures 8 and 9 show the cut-off characteristics of the grid and anode. Referring to Fig. 3, a voltage E;. is placed on each of the grids or the dot anodes as the cut-off voltage through resistors Rg and Rp. DOT CHARACTER VFDs The ability to increase the size of the displayed information is important for a display device, In the newly developed types of VFDs of 5 x 7 elements, 5 x 7 elements with cursos, 5 x 12 elements, JIS and ASCII bases exist which render possible displays of five letters comprised of upper and lower case alphabets, Katakana and special symbols. Dot.character displays have been developed with one line with 6 to 80 characters up to two to 12 lines with 40 characters per line at various character heights. Currently, 480 is the largest number of characters on a VFD (Fig, 10). Since it has a large number of lead terminals, this VFD is mounted on a special printed circuit board which is designed to be able to be connected to a conventional printed circuit connector. An example of a block diagram of a drive circuit is shown in Fig. 115. DOT MATRIX VFDs Since both segment and dot character displays are parti- tioned between the digits restriction on the quantity and fe) Eveg (VC) & f “10 ! ! L _ ! 10 20 30 40 50 60 70 &, (vide) Fig. 8 Grid cut-off characteristics (Ex = 8.9 V a c) 6 . “5 Saco ¥ dc} -10 ! l i 1 t 10 20 30° 40 50 60 70 Se (V p-p) Fig. 9 Anode cut-off characteristics (Ef = 8.9 Vac: duty ratio 1/50) 216 Fig. 10 480-character display with cursor S }—___\ |# 3 o =o e $ “ x +S 240-character - 5 5 “S85 VFD + wl o nv i} 7 2 z ors 40 grids a a aes) Cc < LE Addrass_bus ddress data bus Driver 40 5x8 bit Shift register Eight-bit Data_¥ CPU Pp) || Is 1 Busy«<— Fig. 11 Example of a VFD driver block diagram variety of display is inevitabie® . An array of picture elements at equal spacings along the vertical and horizontal directions overcomes this limitation, This enables displays of Hiragana, Kanji and other complex letters in addition to the simple alphabets already catered for using smail dot matrix VFDs. Drawings, graphs, graphics and TV images” may now also be displayed. Figure 12 is an example of an image display on 128 x 128 element screen, The dot matrix display has a construction basically similar to that of the element shown in Fig. 1. However, connection of the element anodes and the arrangement of the grids are such as to forma matrix. Driving conditions are also devised to match the anode and grid conditions. Driving voltages on the anodes and grids give rise to light emission at the inter- section point when the same positive potential is applied to each, The anode elements must be made small to realize high resolution, which necessitates a thinning down of the grid width. This in turn leads to insufficient contro! of electron flow from the cathode to the anode which allows the effect of the neighbouring negatively biased grid to extinguish the light on the anode. It is thus necessary to focus as many electrons as possible to the single anode which is supposed to be activated® . Figure 13 shows examples of the connection of the anodes in a dot matrix display. In the first case, which is a single matrix arrangement, neither end of the anode luminesces as mentioned above. In the case of the double matrix however, the anodes are alternately wired in two groups, and the 216 two neighbouring grids that face the dot anode have a positive driving voltage together with the dot anode. This enables the dot anode to luminesce over its whole area ut the required brightness. A half-pitch shift of the grid with regard to the anode dots and simultaneous application of a driving voltage to two neighbouring grids result in an overall luminescence of the dot anode (Fig. 14). The arrangement of the dot anodes and grids is shown in Fig. 15. Agraphic display using this scheme with 256 x 256 elements is shown in Fig. 16. This is a VFD with dots 0.2 mm square. The elements are arranged in the x and y directions at a pitch of 0.4 mm. A block diagram of a 128-element square image display system is shownin Fig. 17. Figure 12 is an example of a displayed picture with 15 gradations of grey scale. SCROLL VFDs The scroll VFD is a dot matrix display with many more elements in the x direction. A unit shift when the display Fig. 12 Example of an image displayed on a 128 X128 element VI'D ~— Cathode © IWS 2 = ~«— Anode ~<— Cathode nL {it way ea Ly “ta ~— Anode Yat-a Fig. 13 Connection of dot anodes of a dot matrix display DISPLAYS. OCTOBER 1983 +— Cathode = ae - *y * Al-a Y Bi-7 Fig. 14 Connection of dot anodes of an improved dot matrix display panes meee 0 x, ! l J ' Xo fh nal xX, | : J : 1 : =~ . t . { ; -- ‘ 4 : 1 X que { . J ell Xai . i 5 x, I J. — —J }Uacuum Fluorescent Display (Citron) Model: DM256x256 one) + hg . Ane ee | | 7 2s os Fig. 16 Example of a graphic display on a 256 X256 element VFD_ DISPLAYS, OCTOBER 1983 pattern is scrolled corresponds to a displacement of a unit pitch of the dot. Repetition of the shift at a constant rate causes the displayed pattern to move from the left to the right. This scrolling display also uses the double matrix, giving a better quality of display. Figure 18 shows a single matrix in which a supposedly active dot anode is seen to illuminate only partially, affected by the potential of the neighbouring grid, while a supposedly inactive anode is seen to illuminate partially, - degrading the quality of display performance. With the double matrix arrangement, however, combined with improvements in the driving system, the whole area of just the active dots luminesces. A bright, high quality display results. Figure 19 shows the dot anode connection and grid arrangement of a scrolling display. Figure 20 shows a section of an animated picture displayed on a 256 x 64 dot matrix display (pitch — 0.65 mm; dot size — 0.4 mm square). The driver circuit for the display is shown in the block diagram of Fig, 21. Figure 22 shows another display, this one 256 x 26 elements. Its character d . ‘i §b Line memories . (serlal to parallel conversion) peGrey Scclewm GSBO-15 ] | l ookenieahentend control I pulse generator — iteliors - St L YorversB | ceil py | | | ---- cpa I Anodes Pa .ei26 Verticat sync, H oy 128x128 put control foes fed a Fo on Dot matri Horizontal sync pulse co Bt! le jot matrix input generator J] is x VFD au | j | ru __ Anodes Fas aiza Y drivers A _t f GSBO-15 aii t———-t Grey scale r ee > Multiproxars ree WU = = - abed Vi bere Line memories . eo 4 Video input ifier o-digital (serial to parallel conversion} CORIVST IOP Fig. 17 Drive circuit to picture display ~ z ~ +— Cathode edee lana aren ran yk en oat i mite s hewe _s + b Fig. 18 Connection of dot anodes of a scroll display 217 a Hal OO 5 a! GLa i oketiniaas | (ORbaaeian | eieee | 28 CO @, 10 aye oo 1, ,' | 2 Nee ond i Gl ! Cos D Toray + OO elute Oil | Ono G9 0; ie 4b Ib Qrao TT 6 6 Gy G 7 =p cy (a? Sig Sige Fig. 19 Arrangement of dot anodes and grids on a scroll display a VFD _ Kes Su Te = s Fig. 20 Example of an animation display on a 256 Line X64 element display of 24 x 24 dots is more than sufficient to delineate the Ming Dynasty Kanji characters shown. In fact a 16 x 16 display proves adequate for practically acceptable Kanji characters. The relatively large number of lead terminals in dot matrix scroll] display panels is accommodated by mounting the panels on specially designed printed circuit boards, which may accept conventional connectors. ACTIVE MATRIX VFDs The number of pixels available on VFDs has increased sufficiently for TV images now to be displayble. However, TV displays depend upon the grid scanning system which cul Oty Se 2 + « ACE fle BEF fl Fig, 22 Examples of Kanji and Hiragana characters displayed ona 256 X26 element VED Double matrix anode wiring Et ; Balla 5 . 1! off Offa a RAM £h 8 £18 at ae Qo refresh ae 3 s ® le ; a g » ROM —_— 7 Is pattern +f tf rs o ad & [oN alls ly e rof V lofie giao 2 nN G G 2 Columns per grid 6 Giog x Ww 4 ROM tN] rr] sla Grid line program / | Pf ee ee ee Driver Elght-bit Address KC _ it i™ latch CLK | ALE S| Shift register . Three-state | Tree-starel, Three~-state fatch latch Control Address out in Data Fig. 21 256 X64 element VFD driver block diagram 218 DISPLAYS. OCTOBER 1983 o 6-4"6% STS c? > FSIS PER SOR) SOIR PSS 2 SSS PSN SCY SKSO » Y ESSE SER ISOOS he SSC SOFOS SOSH ICS SSS OSS OS SIONS S50 fEX? SSCSOSES2S os ESSN tS Ney OOS OOS OOS KX SY Drain Source Fig. 23 Active matrix display operation limits the pixel density and requires a low driving voltage. One possibility for a breakthrough in this area is to use silicon MOS FETs as switching elements®!°, Figure 23 illustrates the basic principle of the switching element. Each pixel consists of a MOS FET processed on a silicon wafer and a phosphor dot deposited on the drain electrode of the FET (in the p channel case). By applying an appropriate combination of voltages to the gate and source electrades, the conductivity of the FET is increased, allowing the positive source voltage to be applied to the phosphor, thereby illuminating it. An array of cathode filaments acts as a source of electrons, together with a single grid electrode extending across the display to draw the electrons from the cathode and cause uniform flooding of the phosphor elements, The semiconductor substrate allows memory and driver functions to be built in. This enables a brightness of about 1 000 cd m~ to be produced at voltages in the range 15 to 20 V. The driving voltage and current appear to satisfy most of the conditions required for the various potential applications. This proved most effective in reducing the number of connecting terminals which had necessarily increased greatly with the increase-of the number of pixels in the grid dynamic driving system. The improved scheme also enhances reliability and cuts the cost of the display module. Furthermore, it lends itself well to microprocessing tech- nology for the realization of extremely high density pixel displays. Experiments have shown that pixels with a pitch of 20 to 30 um are realizable. Characteristics of an experimental tube example are listed in Table 1. Except for the driving voltage and number of pixels, values are approximate. No memory or driving circuit was included internally in the experimental tube. DISPLAYS. OCTOBER 1983 Table 1. Experimental tube characteristics Tube thickness 10 mm Chip size 23 x 23 mm Number of pixels 241 x 246 Pixel pitch 90 pm Luminous area 45 ym diameter Power 1 W (60 Hz; duty factor: 1/300) Driving voltage (V,;) 30 V Maximum brightness 30 cd m~ (60 Hz; duty factor: 1/300) While the monolithic substrate is a must, restrictions imposed upon the display area associated by use of the silicon chip may cause problems, Nevertheless, in applica- tions where a large scale monolithic substrate is not necessary, it is possible to construct a silicon chip having a desired area by patchwork of a number of small area silicon chips as in the case of a hybrid integrated circuit. Colour display on any new type of display (ie not the CRT) harbours difficulties. However, of these the VFD appears to be closest to providing a practical colour display. In particu- lar, in the case of the present active matrix scheme, realiza- tion of an effective duty cycle of 100 per cent is always possible regardless of the duty cycle of the input signal. This should prove extremely advantageous in exciting phosphors of low luminous efficiency — the blue and red. With these features, applications are anticipated in displays of high density, ultra-fine picture images, in complex character displays, such as those for Kanji, and other complex displays. VFD PROSPECTS Since VFDs have been applied as dot matrix character displays, their growth in size has become apparent, The need for increased quantities of displayed information calls for the development and establishment of processing and constructing materials, components and the manufacturing technologies necessary in assembling these into a composite display device. For dot matrix displays, improving the resolution remains the most important problem, This calls for a reduction of the dot pitch and size for the availability of the number of dots withisi a unit area. This relates closely to the grid structure and to the tube fabrication technology which includes the way in which the dot anodes are connected. Problems also remain in the wiring, connection with external circuits, and also in the further development of the driving circuit chips which demand high levels of cooperation with the semiconductor industry. Finally, the development of multi-colour matrix displays promises a vast expansion of VFD applications. References 1 Kiyozumi, K., Masuda, M., Nakamura, T. ‘Flat panel multi-digit fluorescent display’ Soc Inf Disp Dig (1976) 130 2 Morikawa, M., Hattori, M., Shimojo, T. ‘Fabrication of substrate glass for VFD deposited with aluminium thin film’ Ise Electronics Tech Rep 310 219 3 Kasano, K., Masuda, M., Shimojo, T., Kiyozumi, K. ‘A 240- character vacuum fluorescent display and its drive circuitry’ Proc Soe Inf Disp 21 2 (1980) 107 4 Nakamura, T., Kiyozumi, K,, Mito, S. “Vacuum fluorescent display’ in ‘Advances in image pickup and display, Volume 5° edited by B. Kazam (Academic, 1983) 199 5 Iwade, M., Kasano, K. ‘A 240-character vacuum fluorescent display and its drive circuitry’ Ise Electronics Tech Rep 3 30 6 ‘Ise Electronics comes out with novel VFD capable of Kanji displays’ Electron Mater Mag (Japan) (1979) 19 220 7 Iwade, M., Kasano, K., Masuda, M., Nakamura, T. ‘Vacuum fluorescent display for TV video image’ Soc Inf Disp Dig (1981) 136 8 ‘Ise Develops a 128 X128 pixels graphic display VFD’ Nikkei Electron Rep (25 December 1978) 60 9 Uemura, S., Kiyozumi, K. ‘Flat VFD TV display incorporating MOS FET switching array’ JEEF Trans Electron Devices ED-28 6 (1981) 749 10 Uemura, S., Kiyozumi, K. ‘Flat VFD TV display incorporating MOS FET switching array’ J Inst TV Eng Japan ED-477 (1979) IPD 46 DISPLAYS. OCTOBER 1983