PROCEEDINGS OF THE IEEE, VOL. 61, NO. 7, JULY 1973 829

A Hybrid Liquid-Crystal Display with a Small Number

of Interconnections

CEES J. GERRITSMA AND JEAN H. J. LORTEYE

Abstracf-A nmnerical display cell (nine digits of seven seg- ments) with nematic liquid crystals cau be driven in a dynamic or a static mode. The static mode combines a flicker-free display with optimum contrast, but it has the drawback of a large number of interconnections (63). This paper describes a method to reduce this number to only four: a 63-b buffer memory in the form of an IzL shift register is attached directly to the glass plate carrying the elec- trode pattern. The required transparent electrode patterns provided with metallized contacts are prepared by means of thin-film tech- niques.

A I. INTRODUCTION

COMMON liquid-crystal display configuration con- sists of a transparent nematic liquid-crystal layer between two glass plates provided with transparent

electrode patterns. The thickness of this layer is of the order of 10-20 pm. In its simplest version, a numerical display consists of a number of digits, each provided with 7-segment electrodes. By energizing some of the seven segments each numeral can be made visible using one of the electrooptic effects of nematic liquid crystals. At the moment, the most promising effects for display application are the following: 1) dynamic scattering, 2) field induced birefringence, and 3) alignment of a twisted nematic. The power consumption of these effects is extremely low. The first electrooptic effect is induced by an electric current with a density of the order of 10 pA/cmZ at 20 V. Both other effects are induced by an electric field a t thresholds round about 5 V. The field effects use current densities less than 1 pA/cmZ. Since application of dc fields leads to polarization of the electrodes and to electrochemical reactions, ac drive is preferred& all cases. Dynamic scattering is only observed at frequencies below a cutoff frequency fc (conduction regime), while field in- duced birefringence is optimal at frequencies above fc (di- electric regime). In most practical cases the cutoff frequency is of the order of 1 kHz. For twisted nematic displays the contrast is frequency independent; in practice, low-frequency driving is used for these devices.

In connection with display applications, the response times of the electrooptic effects are also important. The rise and decay times of all three effects depend on material prop- erties, on the thicknesses of the layer, and on the magnitudes of the applied fields. Both switching times are rather long; for 20-pm layers and applied voltages of 20 V, the rise and decay times are of the order of 10 and 100 ms, respectively, and do not differ significantly for the three effects,

In contrast with dynamic scattering, both field effects require the use of polarizers in combination with the display cell. However, apart from this, the cell construction and driving circuits are identical for the effects mentioned.

Manuscript received January 1.5, 1973. The authors are with Philips Research Laboratories, N. V. Philips’

Gloeilarnpenfabrieken, Eindhoven, The Netherlands.

In this paper some of the driving aspects for a 9-digit numerical display will be discussed. There are two modes of operation for such a register display, the static mode where the appropriate segments of the 9 digits are energized con- tinuously and the dynamic mode where the 9 digits are energized successively. The static mode has the advantage of full contrast and flicker-free image and the disadvantage of a large number of connections to the individual segments. The dynamic mode requires a small number of interconnections with the accompanying disadvantages of lower contrast due to a reduced duty cycle and the presence of flicker. In this paper we describe a hybrid display device in which part of the driving circuitry is integrated and fixed directly onto the glass plate carrying the electrode pattern, thus combining the advantage of high contrast and a flicker-free image with a small number of interconnections. In the first part attention is paid to the driving circuitry while the second part is de- voted to a description of the thin-film technique used to prepare transparent electrode patterns with metallized contacts.

11. DRIVING LIQUID-CRYSTAL DISPLAYS A . The Static X o d e of Operation

From the block diagram of the static mode of operation (Fig. 1) i t can be seen that each segment of the display is connected to one stage of a 63-b shift register. The informa- tion to be displayed can be presented directly to the char- acter generator or, as is shown in the diagram, temporarily stored in the buffer store. The information is transferred to a 7-segment code by the character generator and shifted in series into the 63-b shift register by means of the parallel-to- series converter. The synchronization is obtained by a trig- gerable clock and two frequency dividers. The information in the buffer store has to be available only once to update the 63-b shift register which takes only a few microseconds. Then the clock is stopped and the shift register data are displayed by the liquid crystal until new information is available in the store and the clock has been reactivated by a start pulse. In the static mode of operation there is one common back electrode on the rear glass plate.

B . The Dynamic Mode of Operation In a dynamic display where each digit has its own back

electrode, the digits are selected successively by a voltage between the segments and the appropriate back electrode (Fig. 2) . This means that every back electrode has to be driven separately by a one-out-of-nine decoder. On the glass plate carrying the digit electrode pattern, the corresponding segments of the 7-segment format are interconnected for all 9 digits. These 7 segments are directly driven by the char- acter generator which transfers the information of the first place of the buffer store into the 7-segment code. The syn-

830 PROCEEDINGS OF THE IEEE, JULY 1973

- c m m m 2 Y 2 O " 8 +1.5 V

t

char a m

Pig. 1. B l d diagram of a statically driven numerical display. 3 *l.S v

6 6 -2ov ,b) '20'0"

Fig. 3. Circuit diagram of a dc (a) and an ac (b) driven liquid-crystal dispky. SR represents 1 b of the shift register of Fig. 1 .

Fig. 2. . Block diagram of a dynamically driven numerical display.

chronization of this dynamic display is obtained by a con- tinuous clock generator which recycles the data of the buffer store in such a way that the character generator energizes the right segments for the 9 digits. In the same sequence as the recycling buffer store, a 9 counter controls the decoder for the 9-digit electrodes.

Regarding the electronics necessary for static and dy- namic mode of operation, we conclude that the amount of circuitry is comparable for both driving methods. However, due to the absence of a sharp threshold voltage for the dy- namic scattering effect, the dynamic mode of operation can only be realized with a high-frequency inhibiting voltage on the nonselected digits [4], [SI. This requires more complex circuitry.

111. THE HYBRID LIQUID-CRYSTAL DISPLAY A . The Driving Circuits

In the dynamic mode of operation, there are 7 connections to the segments on the front glass plate and 9 leads to the digit electrodes on the rear glass plate. For the static mode of operation (see Fig. l), the 63 interconnections between the segments and the 63 stages of the shift registers can be reduced to 4 external leads by integrating the shift register and connecting the register to the bonding pads on the dis- play plate. In this way the integrated shift register is part of the hybrid liquid-crystal display. The external connec- tions to the hybrid display are an information input lead, a clock pulse lead, and two leads for the voltage supply to the integrated circuits. Because drive circuit and display are combined in one unit, the advantage of a low-power con- sumption of the liquid-crystal display must not be destroyed by the power consumption of the integrated circuit. From a point of view of low power dissipation, low supply voltage, high breakdown voltage, and easy processing, the 5-mask integrated injection logic (PL) developed by Hart and Slob

[6] is used as integration technology for the 63-b shift register. The-circuit diagram of a dc and an ac driver stage for liquid-crystal displays is shown in Fig. 3. The dc driver stage consists of 2 cascaded p-n-p transistors. The current injected into the first transistor can, depending on the state of the shift register stage, flow to the substrate or through the second transistor and the liquid-crystal display element to the negative supply voltage. Due to current losses in the p-n-p transistor and the current sink in the shift register the current through the driver stage is a few times the average current through the display.

The ac drive circuit is more complex and consists of a bidirectional transistor switch T , in series with each segment of the display. In the case of dynamic scattering, the supply voltage across the display and the transistor switch multi- vibrates at a frequency of 100 Hz between 0 and -20 V on one electrode and its complementary voltage of - 20 and 0 V on the other electrode. Thus an ac current flows through the liquid-crystal display. An improved current source with transistors TI and Tz enables the voltage to swing between 0 and - 20 V on the base of the transistor switch T,. Whether the current source is on or off depends on the state of the shift register stage. For the other electrooptic effects, both frequency and voltage are adjusted for optimal contrast.

B. The Cell Construction . . . .

The electrode patterns for a 9-digit hybrid liquid-crystal display cell are shown in Fig. 4. A glass plate with dimen- sions 83 by 46 by 2 mm [Fig. 4(a)] carries the digit pattern of transparent tin-oxide electrodes, an interconnection pat- tern of 100-pm-wide leads of SnO2, and contacts for the 12L shift register, distributed over three IC's. The latter contacts, as well as the four external connection contacts for buffer supply voltage (1.5 V), for the clock pulse, and for the in- formation, are metallized. The common transparent back electrode pattern [Fig. 4(b)] is applied to an 85 by 27 by 1 mm glass substrate. In the following paragraphs, details are given of the preparation of these electrode patterns.

GERRITSMA AKD LORTEYE: HYBRID LIQTID-CRSSTAL DISPLAS 83 I

SnOZ

A I

lbl Ni

lCTTrrTrrTI IW

Fig. 1. (a) The 9-digit i-segment electrode pattern. (b! The common back electrode pattern. The marks (0 and X ) are used to fix the position of the electrode patterns in the sandwich cell.

glass AI

(a ) AI

IC1 SnOt

Fig. 5 . Preparation of SnOn electrode patterns (schematic).

Since tin oxide is chemically very stable, an indirect method is used to etch a pattern in SnOn layers. This method is briefly described below. The glass substrate is covered with a 0.25-pm-thick aluminum layer by evaporation [Fig. 5(a)]. A negative print of the final SnOz pattern is now etched into the aluminum layer using standard photolithographic tech- niques [Fig. 5(b)]. The aluminum pattern is then covered with a 0.1-pm layer of SnO,. This layer is obtained by spray- ing a solution of 20 wt yo SnC14 in butyl-acetate onto the substrate a t a temperature of about 450°C [Fig. 5(c)]. Finally, the aluminum is removed by underetching with an alkaline solution. \\.'hen the A1 dissolves, the upper layer of SnOt covering the A1 is also removed. The SnOn remains where i t is in direct contact with the glass substrate [Fig. 5(d)]. This method is used to make the. electrode pattern of Fig. 4(b). The digit pattern with metallized contacts is prepsred in a n analogous. .way. First, the desired negative AI pattern of the digit pattern, interconnection pattern, and contacts are formed on the glass substrate [Fig. 6(a)]. After the SnOs layer [Fig. 6(b)] is sprayed, the lower p w t of the glass plate carrying the contact pattern is completelyawered with a uniform layer of Ni/Cr and Ni [Fig. 6(c)]. Both layers are about 0.2 pm thick and are vacuum deposited. Again the SnOt, Ni/Cr, and S i layers can be removed locally by dis- solving the aluminum bottom layer i n an alkaline solution [Fig. 6(d)]. I n this way the contacts are metallized. The contacts are made solderable with soldering flux and subse- quent dipping into liquid-tin solder. The solder adheres only to the metallized parts of the glass substrate [Fig. 6(e)]. Then the glass plates are sealed together to form a sandwich cell by the use of 20-pm Mylar spacers to assure a constant layer thickness for the nematic liquid crystal. The cell is filled in vacuum with the nematic material a t a temperature above the clearing point of the material used. After filling, the cell is hermetically sealed with epoxy resin. Finally, the

Cr 2

N I K r

alass

le1

Fig. 6. Preparation of S n G electrode patterns with metallized contacts (schematic).

Fig. 7. Photograph of a statically driven hybrid liquid-crystal display incorporating integrated shift registers and drivers (dynamic scatter- ing effect).

63-b shift register, consisting of 3 in-series-connected inte- grated circuits with each circuit controlling the segments of 3 digits, is soldered onto the glass plate. On the photograph of the hybrid display device (Fig. 7) , the 3 integrated circuits housed imthree separate packages and the 3 pin--head carbon resistors to adjust the current into the circuits are clearly visible. There are 4 connections to the shift register and one lead to the common back electrode. With the technique described in this section, encapsulated IC's were attached on to the glass substrate by reflow.saidering. However, i t is also possible to utilize unpacked IC's. I n that case, the connec- tions of the IC's to the display can be made by much cheaper methods, using either solder bumps or beam leads. \Vith solder bumps, both conventional soldering as well as pres- sure-soldering techniques can be used in combination with a properly metallized contact. I n principle, the technique pre- viously described can .also be used to prepare other metal- lizations.

\Then the shift register isincorporated in the display, a static display is obtained with the advantages of full con- trast and flicker-free image, combined with a reduction in the number of interconnections to 4. In this way there is full freedom to extend the number of digits beyond 9.

ACKNOWLEDGMEST The authors wish to thank C. hl. Hart, A. Slob, and

A. Schmitz for supplying the 12L circuits; B. Symersky for valuable discussions on metallization techniques; and A. M . van Boxtel and F. van Veggel for their important contribu- tions in the preparation of the electrode patterns.

832 PROCEEEINGS OF THE IEEE, VOL. 61, NO. 7, JULY 1973

REFERENCES [ l ] G. H. Heilmeier, L. A, , Zanoni, and L. A . Barton, “Dynamic scat-

tering in nematic liquid crystals,” A p p l . Phys. Lett., vol. 13, pp. 46-47, 1968.

[2] M . F. Schiekel and K. F. Fahrenschon, “Deformation of nematic

Phys. Lett., vol. 19, pp. 391-393, 1971. liquid crystals with vertical orientation in electric fields,” A p p l .

thin nematic films,” J . A p p l . Phys., vol. 43, pp. 2029-2037, 1972. R. A. Soref and M. J. Rafuse, “Electrically controlled birefringence of

display,” Electron. Lett., vol. 7, pp. 699-700, 1972. G. Assouline, 11. Hareng, and E. Leiba, “Two-color liquid crystal

F. J. Kahn, “Electric-field-induced deformation of nematic liquid

crystals: Tunable birefringence,” A p p l . Phys. Lett., vol. 20, pp. 199-201, 1972.

[3] AI. Schadt and W. Helfrich, “Voltage-dependent optical activity of a twisted nematicliquid crystal,” A p p l . Phys. Left . , vol. 18, pp. 127-128, 1972.

[4] P. J. Wild and J. Nehring, “Turn-on time reduction and contrast enhancement in matrix-addressed liquid-crystal light valves,” A p p l .

[SI C. R. Stein and R. A. Kashnov, “ A two-frequency coincidence ad- Phys. Lett., vol. 19, pp. 335-336, 1971.

dressing scheme for nematic liquid crystal displays,” A p p l . Phys. Lett., vol. 19, pp. 343-345, 1971.

[6] K. Hart and A . Slob, “Integrated injection logic: A new approach t3 LSI,” IEEE J. Solid-State Circuits. vol. SC-7, pp. 346-351, Oct. 1972.

Electrophoretic Image Display (EPID) Panel

Abstract-A new reflective-type electrophoretic image display (EPID) panel is described which is based on electrophoresis. In the EPID panel, a suspension is mainly composed of pigment particles and a suspending liquid. A dc voltage applied across a suspension layer changes the reflective color of the suspension as a result of the electrophoretic migration of the particles. The precipitation of the particles in the suspension is prevented by making the densities of the pigment particles and the suspending liquid equal. Color com- binations of both the pigment particles and the suspending liquid can be used to achieve a desired color display. A reversal between the colors of the displayed pattern and its background can be ob- tained by changing the polarities of the applied dc voltage. The EPID panel has a memory function because of the deposition of the pigment particles on the electrode surface. A reflective contrast ratio of 40: 1 at 75 V dc is demonstrated with experimental EPID panels. Rise and fall times of the panels are 20 ms and 10 ms, respectively, at 100 V ac. Power dissipation of the panel is about 7.5 mW for dis- playing a numeral “8” (107 by 59 mmz).

I . ISTRODUCTIOS L E C T R O P H O R E S I S is the movement of charged pig-

ment particles suspended in a liquid under the influ- ence of an electric field. This phenomenon, fir>t ob-

served by Reuss in 1809 for clay particles suspended in water [ l ] , is widely applied in the fields of science and industry a t prese’nt, for example, analysis or separation of high molecular compounds, such as proteins, and electrophoretic deposition and liquid development in electrophotography [ 2 ] , [3]. How- ever, no attention has been paid to the application of electro- phoresis to display devices. In order to apply the electro- phoretic phenomenon to display devices, it is necessary to

hlanuscript received February 7, 1973. The authors are with the Wireless Research Laboratory, Alatsushita

Electric Industrial Company, Ltd., Osaka, 571, Japan,

overcome some peculiar problems caused by using a suspen- sion. I n a suspension, an electrical double layer is generally formed at the interface between the particle and the liquid, and the particles are either positively or negatively charged. In t he case of a suspension with a nonaqueous liquid, the particles are charged mainly as a result of dissociation of sur- face groups or adsorption of ionic surfactants [4]. The charges or adsorbed polymeric molecules on the surface of the particles contribute to avoid the precipitation of the particles in a sus- pension [SI, but do not seem to be sufficiently effective in a practical suspension, especially for use in display devices. Furthermore, for the purpose of the display, the particles in the suspension are required to repeat the electrophoretic migration between electrodes by changing the polarities of the applied voltage. I n the suspension under a high electric field, the following phenomena often occur within a relatively short time: sticking of the particles on the electrode surface, change and deterioration i n the charging characteristic of the parti- cles, irreversible electrochemical reaction with the electrodes, and electrolytic decomposition. The suspension for the dis- play devices must therefore have both static and operational stability. This paper describes principle, construction, and characteristics of a new reflective-type electrophoretic image display (EPID) panel that is based 011 electrophoresis.

11. PRIXCIPLE A N D COSSTRUCTIOK O F THE E P I D PANEL T h e E P I D panel utilizes, as a basic mechanism, the elec-

trophoretic migration of charged pigment particles in a sus- pension. The suspension, mainly composed of pigment parti- cles and a suspending liquid, is sandwiched between a pair of electrodes, one of which is transparent. h suspension layer is usually from 2 5 to 100 pm in thickness. An applied dc electric