Basics-EPD-DISPLAY

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-24, NO. 7, JULY, 1977

Electrophoretic Display Technology ANDREW L. DALISA

Abstract-The electrophoretic image display (EPID) is a passive display technique based upon the scattering and absorption of ambient light. This paper will review the present status of EPID technology with emphasis on developments at Philips Laboratories. The nature and requirements of the colloidal suspension Le., the pigment particles suspended in a dyed nonaqueous liquid, that constitutes the working medium of an EPID device will be discussed. A description of the optical, electrical, and hydrodynamic characteristics of EPID devices will be presented. Fabrication methods, device degradation modes, and the status of x-y addressing techniques will be described.

T I. INTRODUCTION

H E electrophoretic image display (EPID) is a passive display concept based upon the transport of

charged. pigment particles in a colloidal suspension. The charged pigment particles are transported by means of an applied electric field; this phenomenon is commonly known as electrophoresis.

The first use of electrophoresis for recording or displaying information was the establishment of liquid development processes for electrophotographic applications [l]. In these processes a plate containing an electrostatic c:harge image of the desired information is immersed in a colloidal suspension of charged toner particles of opposite polarity. The toner particles are transported to and deposited on the charge pattern due to electrostatic at- traction. The resulting image of toner particles was then made permanent by a fixing process. The use of an electrophoretic process as a reversible display technique was first reported by Evans et al. [2]. In 1974, Ota e t al. [3] described the implementation and preliminary characteristics of an electrophoretic display device that has be- come known as EPID. Based upon this initial work and that of others [4]-[6], the EPID concept is a promising new passive display technique with distinctive appearance, inherent memory, and low power consumption. Currently, this technology is under investigation by several laboratories. This paper presents a review of the status of the development of EPID technology.

11. THE ELECTROPHORETIC IMAGE DISPLAY (EPID) CONCEPT

EPID is a passive display concept based upon electrophoresis in a highly,stable colloidal suspension. The colloidal suspension consists of pigment particlesdispersed

March 8, 1977.

10510.

Manuscript received December 9,1976; revised February 14,1977, and

The author is with the Philips Laboratories, Briarcliff Manor, NY

COLOR OF

D Y E 1 1 4 OBSERVER

827

TRANSPARENT ELECTRODE

-+ELECTRODE SEGMENT

SUBSTRATE

CHARGED PIGMENT P A R T I C L E S ( - )

DYED SUSPENDING MEDIUM

-ELECTRODE SEGMENT

I---- SPACER / S E A L

, G L A S S P L A T E

Fig. 1. Schematic of simple EPID cell.

in a dyed nonaqueous suspending liquid of contrasting color. The pigment particles are submicron in size and electrically charged to the same polarity.

A simple EPID cell consists of a thin layer (-50 pm) of colloidal suspension sandwiched between the transparent electrode surfaces of two glass plates. Fig. 1 shows a cross section of a simple cell in which the particles are assumed to be charged negatively. Information can be displayed on this cell by segmenting one of the transparent electrodes, using standard photolithographic techniques. If one electrode segment is connected to a voltage source of positive polarity, and the other segment connected to a voltage source of negative polarity, the pigment will be driven to opposite sides of the cell, as shown in the figure. In the region of the cell in which the pigment has been packed on the front electrode, the color of the pigment will be seen by the observer. In the region in which the pigment is on the rear electrode, the ambient room light is absorbed and scattered by the dyed liquid and the color of the dye is observed. By proper selection of the pigment and dye a variety of color combinations are possible, including black and white. Due to the passive nature of the display, the near Lambertian scattering of the pigment, and the strong absorption of the dyed liquid, EPID devices exhibit excellent contrast over a wide range of viewing angles and

828 - .

ambient light levels. If the polarity of the applied voltages are reversed, the position of the pigment and henc? the color tone will be reversed. For typical EPID deviceil, the pigment can be transported across the cell in approximately 20 ms with 50 V applied to a cell that is 50 pm thick. The average current level during operation is less than 0.1 pA/cm2. In addition, when the applied voltage is removed, the pigment remains on the electrodes, giving this dtwice inherent memory. In the next section, a brief introduction to the basic nature of colloidal systems will be discutmed. In Section IV, a description of the requirements and composition of EPID suspensions will be presented.

111. THE ELECTRICAL DOUBLE LAYER IN COLLolDS

Although a colloidal suspension is electrically neutral on a macroscopic scale, a microscopic picture would show the particles to have a net charge on their surfaces, M hile the neighboring liquid has a net charge of opposite polmity [7]. The charges may arise in a variety of ways, e.g., by adsorption of anionic or cationic surfactants; by dissccia- tion of ionogenic groups on the particles surface; 0:' by preferential adsorption of ions present in the liquid. ':?his surface charge is balanced by an equal amount of c h r g e of opposite polarity in the liquid phase. Due to coulonhbic forces, these counter ions are attracted to the surface of the particles. However, since the ions in the liquid are subjact to diffusion forces arising from their normal thermal :no- tion, the distribution of the counter ions in the surrounding liquid is diffuse rather than sharp. The distribution of the surface and counter-ion charges is called the electrieal double layer. The portion of the electrostatic potential that appears between a shear plane near the surface of the particle and the bulk of the liquid is termed the zeta potential 5: The theoretical interpretation of all electrokimtic effects in colloids is based upon this parameter; howeyrer, its measurement and interpretation in nonaqueous E ~ S -

tems is very difficult [SI.

IV. THE COLLOIDAL SUSPENSION

The colloidal suspension is the active and most crucial part of an EPID device. It consists of a suspending liquid, a soluble dye, stabilizing agents, and the submicron ~ i g - ment particles. The composition of the suspension determines to a large degree the lifetime, contrast, and response times of the device. In an ideal suspension the pigment would neither settle nor float in the suspending liquid i t . , it would be sedimentationally stable.

Secondly, the individually dispersed pigment particles would remain separate and not bunch together or itg- glomerate under all operating conditions. This require- ment is called colloidal stability [9]. In addition, all the constituents of the suspension must be chemically stakle; i.e., compatible with each other and with the other max- rials that are present in the EPID cell such as electroc.es and seals.

Some of the general considerations in the selection of suitable suspension constituents are discussed below. A

IEEE TRANSACTIONS ON ELECTRON DEVICES, JULY 1977

TABLE I

Pigments

Organics Inorganics Optical characteristics

excellent fair-good Solubility (swelling

excellent good

Surface charging fair-qood good

Specific gravity excellent

good good Chemical stability very poor

and softening)

J- 1 TABLE I1

C o n c G - Suspension DY-1 Material t ra t . ion t t r t .%)

Solvents Perchloroethylene/Xylen 78.3%/18.5% Pi gmen t Dairylide yellow

(American Cyanimid)

Sudan Red 4B (GAE') 0 .20%

Stabilizer OLOA-370 (Chevron)

major consideration in the selection of a suspending liquid is that it contributes to high electrophoretic mobility. The electrophoretic mobility of a particle in a suspending liquid is given by [7]

p = c(/6.rrq. (1)

Hence, liquids with the highest t/v ratio should be chosen for fast display operation. This choice must be consistent with the other requirements on the suspending liquid, such as wide temperature range, low toxicity, excellent chemical stability, high specific gravity, chemical inertness, and high resistivity (>lo12 D - cm).

The dye in the suspension must be selected based upon the following requirements: solubility in suspending liquid, chemical stability, chemical compatibility with suspension constituents, and high optical density at the portion of the optical spectrum reflected by the pigment.

The particles used in EPID devices can be either organic or inorganic pigments. The general requirements on the selection of the pigments are: acceptable optical characteristics (e.g., scattering power, color, opacity, etc.), inso- lubility in the liquids, little swelling or softening, allows good surface charging, chemically stable, and a specific gravity that can be matched by a suitable suspending liquid. Table I shows how the organic and inorganic pigment generally compare for EPID applications.

The stabilizing agents used in EPID suspensions are of critical importance but their interactions with the pigment surface, e.g., the charging mechanisms are very complex and very poorly understood. In general, effective stabilizing materials for a given pigment are determined by empirical testing guided by the relative Bronsted acidity or basicity of the pigment surface and stabilizer. The role of the stabilizer in producing an acceptable suspension will be discussed in Section IV-B-1.

An example of a useful EPID suspension designated as DY-1, is shown in Table 11. In a simple two-electrode test cell, DY-1 would have a typical operating life of lo7

DALISA: E:LECTROPHORETIC DISPLAY TECHNOLOGY 829

switches, toN and toFF of 30-40 ms a t a color contrast of 98 using the 1964 CIE standard. DY-1 typically has 1011 Q cm resistivity after switching.

A. Sedimentat ion Stabi l i ty

The most direct method for achieving sedimentational stability is to insure that the pigment and suspending liquid have equal specific gravities. This can generally be accomplished by using a suspending liquid that is com- posed of two liquids, one of relatively high specific gravity and one of lower specific gravity. The two are mixed such that the resulting specific gravity matches that of the partic1e;s. Although this matching can in general be achieved at only one temperature (e.g., room temperature), the slight mismatch at other temperatures has not proved to be a problem.

B. Colloidal Stability

Colloidal suspensions are inherently unstable systems, since the large free-surface energy of a suspension is re- duced when the particles bunch together, i.e., agglomerate. The attractive force between particles that is responsible for agglomeration is called the London dispersion force. Th.e term “dispersion force” is employed not because this force is present in colloidal dispersions but rather to indicate that it is related to the same electron fluctuations that give rise to the dispersion of the index of refraction.

1 ) Electrostatic Stabilization: To stabilize a colloidal system against agglomeration, a repulsive force between particles must be developed to counteract the London dispersion force. The repulsive force can be provided by charging the pigment particles to produce electrostatic repulsion. Verwey and Overbeek [7] calculated the repulsive potential energy for two charged colloid particles surroulnded by their double layers and established a quantitative theory of electrostatic stabilization of colloid systems.

The colloid stability requirements on a suspension for an EPID device are extremely strenuous. Many suspensions which do not separate for many months in a test tube with no applied field will be agglomerated by the electric field after just a few switching operations in an EPID cell.

The difficulty of achieving stability in an EPID cell can be appreciated by understanding that the optical contrast of the (device is achieved by compressing the pigment onto the electrode. Under the applied electric field, particles are forced into close proximity and the probability of ag- gl.omeration is greatly increased. In addition, the particle concentration on the electrode is increased by as much as an order of magnitude over the concentration in the bulk suspension, and it has been found that the stability of the suspension decreases rapidly as the particle concentration increases [lo] due to increased screening of the overlapped double layers. However, even though the stability requirements are strenuous, EPID suspensions have been developed that can sustain more than 108 switching op-

erations and still continue to operate with satisfactory appearance.

C. Chemical Stabili ty

It is necessary that the EPID suspension exhibit a high degree of chemical stability in quiesient as well as during operating conditions, Chemical reactions in the suspension can adversely affect operating lifetime, response speed, and contrast. Therefore, the choice of the suspension materials must emphasize their chemical compatibility and general inertness. Areas of major concern are: solubility of pigments in the solvents, photo- or electrodegradation of the dyes andlor pigment, electrode reactions, and, since the dyes and stabilizers are surface-active materials, there is the possibility of antagonistic or competitive interactions. In general, standard purification procedures are employed to remove contamination (e.g., HzO) from all constituents. In addition, the chemical structure of the dyes, pigments, and stabilizers can be analyzed to determine and avoid possible chemical problems.

D. Preparation of Suspensions

Suspensions are prepared by ballmilling in capped glass vials, using stainless steel or ceramic grinding balls. The procedure is to first dissolve the stabilizer in the suspending liquid. The pigment is then added, and the mix- ture is ballmilled for several hours. The ballmilling is used to break up the highly agglomerated dry pigment powder into individual particles and allow the exposed surfaces to interact with the stabilizer. The dye is then added and ballmilling is continued for an additional hour. Routine testing of suspensions are done in simple test cells and measurements of the contrast, response times, conduc- tivity, switched charge, and lifetime are performed. The lifetime is tested by applying a repetitive square-wave signal to the test cell at a frequency of 1/2 Hz. End of life is based on an evaluation of the cells’ appearance. This evaluation includes the contrast, color uniformity, and the distribution of pigment on the electrode. For simple test cells, the main degradation mechanisms are agglomeration and clustering. Agglomeration is first observed as a fine granularity in the color of the cell, as shown in Fig. 2(b). This granularity becomes more noticeable with continued operation. Clustering as shown in Fig. 2(c) is generally a more coarse redistribution of pigment that can form var- ious complex pigment distributions and has been related to hydrodynamic instability in the cell. Clustering will be discussed in more detail in Section V-C.

V. DEVICE CHARACTERISTICS

A. Optical Characteristics

There are three principal device parameters that determine the contrast of an EPID device. The first such parameter is the composition of the suspension. Consider a suspension that consists of clear fluid and dispersed pigment. Without dye there would, of course, be no con-

IEEE TRANSACTIONS ON ELECTRON DEVICES, JULY 1977

2 0 v 5 v

Fig. 2. Photomicrograph in transmitted light of (a) a stable suspension, (b) a suspension in which agglomeration has begun, and (c) a suspension exhibiting a typical cluster pattern (magnification = 40X).

trast. As dye is added to the suspension, pigment on the rear electTode starts being obscured and the contras:; increases. A t a certain concentration of dye the contrast will peak and then begin to decrease as the dye concentra ;ion increases. The decrease starts when the opacity of the dye starts reducing the brightness of the ON state, i.e., pigncent on the viewers side of cell, more than it improves the darkness of the OFF state, i.e., pigment on the rear side of the cell. Fig. 3 shows the dependence of the contrast r#ltio on the concentrations of dye and pigment for three oiler- ating voltages.

The second principal device parameter that determines the contrast is the cell thickness. Fig. 4 shows that for a given suspension composition and applied electric fidd, as the cell is increased in thickness the contrast would first increase and then level off when essentially all the light that enters the cell and scatters off the pigment on the xear

5 5 -

c 4 L

0

a a

a In

3 - z ,o 2 -

2 - 30 3 - 4 0 4 - 45

0- I I I 1 1 I I I / 2 4 6 8 1 0 2 4 6 8 1 0 2 4 6 8

DYE CONCENTRATION ( m g l m l )

Fig. 3. Dependence of contrast ratio on the concentrations of dye and pigment in a typical EPID suspension, for three different operating voltages.

; 4 5 [

- 4 / E - I V /pm

I I I I I 4 I 1

25 5 0 75 100 125 I50 I75 200 C E L L T H I C K N E S S ( p m 1

Fig. 4. Contrast ratio versus thickness of EPID cell for a given suspension a t applied electric fields of 1 V/pm and 0.5 V/pm.

electrode is absorbed in the layer of dyed liquid. However, by decreasing the dye concentration and further increasing the cell thickness higher contrast would be available, since the brightness of the pigment on the front electrode would improve.

The third device parameter that determines the contrast is the applied electric field. Consider an EPID cell in the ON state. The brightness of this cell for a given suspension composition and cell thickness is dependent upon the density of pigment particles on the electrode. The effect of the applied electric field which packs the pigment layer is shown in Fig. 5.

A distinctive feature of the EPID concept is its high contrast over a wide range of viewing angles. Fig. 6 shows a comparison of the contrast merit factor [ll] for several display media. An EPID device with a suspension of white pigment and black dye greatly surpasses the performance of the other media and approaches that of black ink on white paper as a passive display [5] . Now consider a voltage pulse applied to the cell with the polarity, magnitude, and duration necessary to transport and pack all the pigment onto the rear electrode. Observation of the pigment layer through a microscope as it leaves the front electrode and moves through the dyed liquid, indicates that the side of the layer facing the observer recedes as a rather continuous

DALISA ELECTROPHORETIC DISPLAY TECHNOLOGY 83 1

I I I , J IO 20 30 40 50

APPLIED VOLTAGE ( VOLTS1 Fig. 5. Brightness of pigment layer for a typical suspension as a function

of applied voltage.

v) 10-

V I E W I N G A N G L E Fig. 6. Contrast merit factor for several passive display media.

sheet of pigment. A very simplified model of the fall time of the EPID cell, i.e., the time to go from ON to OFF, has been developed [12] for a continuous sheet of scattering particles moving into an opaque liquid. It is assumed that the ch;uge in this system resides on the pigment alone. The dyed liquid has an absorbtion coefficient a at the wave- length of the incident light (I) on the cell. From [12] the rise time and fall time are given by

I& - exp [ - 2 a ( ~ - xz(t))] (2)

1!-jkF - exp [+a x z ( t ) ] . (3)

Fig. 7 !3hows the fit between the experimental data and the theory for VO = 25 and 50 V, L = 50 X cm, and a = 332 cm-l.

After the particles have moved away from the electrode they travel a t a velocity given by

0 I I I I 0 10 20 30 40

t(rns) Fig. 7. Fit of fall time data with the theoretical model for applied volt-

ages of 25 and 50 V.

50 \/OLTS

~ l l l ~ l l l ~

0 40 80 msec

Fig. 8. Typical rise time of the optical signal from an EPID cell with 50-V pulse applied to a 50-fim-thick cell.

The time necessary for the first few 1,ayers of pigment to transit the cell is approximately

From the slope of experimental data for t~ransit time versus voltage, the zeta potential can be determined. A typical value is 90 mV.

The rise time for a typical cell is shown in Fig. 8. The brightness of the ON state is strongly affected by the packing of the pigment, and the rise time shows a long saturation time that, is believed to be related to this packing process. The side of the pigment layer that approaches the observer is not a dense sh.eet, but a rather porous and diffuse distribution of pigment. For these reasons it is expected that this simplified model will not be satisfactory for the rise time. Additional work on characterizing the dynamics of the pigment layer and the packing process is necessary to describe this response.

B. Electrical Characteristic

The suspending liquids employed in the suspensions are highly insulating, nonaqueous liquids. Aqueous liquids are not suitable due to the unavoidable hydrolysis at the electrodes. In addition, highly insulating fluids are chosen to reduce the parasitic current which consumes power and controls electrochemical reaction.

832 IEEE TRANSACTIONS ON ELECTRON DEVICES, JULY 1977

z ,050

a 3 0

0 n

,025 '

SUSPEN DY - SlON

I

I I I I I 1 I 0 20 40 60 80 100 120

APPLIED VOLTAGE Fig. 9. Current (dc) versus applied voltage for a typical EPID cell..

A typical electrophoretic suspension has a currenut- voltage behavior that is essentially ohmic, as shown in Fig. 9. When the potential difference across a 50-pm-thick twt cell is 50 Vdc, the corresponding current density is abcut 0.10 pA/cm2, corresponding t o a suspension resistivity of approximately 1011 cm. The switched charge (not i n - cluding the parasitic current) for a typical suspensim (DY-1) is about 0.1 pC/cm2. The typical capacitance of a simple EPID cell is approximately 240 pF for a cell thickness of 50 pm and an area of 6 cm2.

C . Hydrodynamics

An important but somewhat unexpected result of thle device-characterization work has been the emergence of the role of hydrodynamic phenomena in the operation of EPID devices.

1 ) Pigment Migration: The first hydrodynamic effcct of importance is the redistribution of pigment after many switching operations. The redistribution or pigment n ~ i - gration first appears as a small area (several millimet;c!~rs in size) of different color due to the local variation in the thickness of the pigment layer. The cause of this migration is the interaction of electrical and fluid forces on the particles in the EPID cell.

During the switching of an EPID cell, the moving p4k- ticles set the fluid in motion and the moving fluid, in twm, perturbs the motion of the particles. Hence, as the pigment on a typical segment electrode is switched back and for%, the ensuing fluid motion will cause pigment to migr:.te laterally until some particles move outside the boundar .es of the segment electrode. These pigment particles accu- mulate in the surrounding background-electrode regi.m of the cell where there may be no applied electric field or a constant electric field, and, in general, they will not ! b e -

turn to the segment. It is possible to greatly reduce pigmt: nt migration by switching the pigment on the segment and background electrodes such that the color tone is peric d- ically reversed. Since all the pigment in the cell is set in motion in this switching mode, the pigment distributim

D. Device Construction and Reliability

EPID test cells and prototype devices are fabricated using commercially available indium oxide coated glass. The requiremets on the seals of an EPID cell are: 1) high resistance to the organic solvents in the suspension (i.e., insoluble and little or no swelling or softening); 2) chemically inert; 3) electrically insulating; 4) low permeability to moisture or solvent vapor; 5) good tensile and shear strength. Initially, several types of epoxy seals were eval- uated with moderate success. At present, a proprietary sealing technique is being employed which uses a sheet of polymer material that simultaneously provides accurate spacing and parallelism of the two glass plates, as well as excellent strength and hermeticity. The polymer sheet is sandwiched between the glass plates, and the seal is formed by applying moderate heat and pressure for several min- utes. The resulting seal has a tensile strength of 4-5 kg/cm2 and a shear strength of 30 kg/cm2. It can withstand boiling water for over 500 h. No leaks have been developed in cells that have been subjected to a temperature range of -50"

DA.LISA ELECTROPHORETIC DISPLAY TECHNOLOGY 833

to -tlOOaC, as well as to repetitive thermal shocks (0'- 100OC). These cells have drilled fill holes sealed with Teflon balls and epoxy.

Principal failure modes for EPID devices include: agglomeration, clustering, pigment migration, leaks, and electrochemicaI effects (e.g., hydrolysis, bleaching of dye, etc.), In simple, two-electrode test cells, agglomeration is the primary failure mode, while in practical devices the primary failure mode is pigment migration. Currently, the operatilng lifetime of simple EPID devices has reached los switches and they are still. in operation. It is expected that this will be extended to lo9 switches. For practical devices (numerics, etc.), the operating life can be about 5 X lo7, and substantial improvement is expected to be possible.

Up to the present, only limited tests of environmental effects have been performed. A preliminary result of temperature testing indicates that a storage temperature range of -40' t o 100°C for EPID cells with DY-1 is practical. The operating temperature range is believed to be -10" to 100°C (with little noticeable change in the switching speed). Although no forma! vibration tests have been performed, preliminary tests as well as practical ex- perience has indicated that the pigment is not dislodged from thLe electrodes due to normal vibration and shocks.

VI. X - Y ADDRESSING

For rnultidigit or alphanumeric displays, the required number of electrical leads can seriously increase the cost and reduce the reliability of the device. A standard method for reducing the number of leads and interconnections for display techniques that exhibit a voltage threshold is to sandwitch the display medium between orthogonally ar- ranged row and column electrodes. However, the optical response of a standard EPID device does not have a voltage threshold.

One solution to this problem would be to develop an EPID suspension which exhibits such a threshold. Inves- tigation of a wide variety of surface active materials has resulted in a class of stabilizers that do indeed possess threshold properties [15]. At present, it appears that these suspeneions have significantly poorer operating lifetimes than standard suspensions.

Although desirable, a switching threshold is not necessary for x-y addressing an EPID display. A novel technique, called Multilevel Voltage Selective Addressing, has been developed [16] that employs eight voltage levels, simultaneously addresses all row and column leads of a standard 5 X 7 alphanumeric character, and requires no threshold properties in the suspension or in the device. This technique has been implemented in a simple alphanumeric display and an EPID bar graph, but it is not considered suitable for large matrix arrays.

Recently a more effective x-y addressing technique has been demonstrated in prototype devices. This technique is baseld upon the implementation of a control-grid elec-

TABLE I11 Characteristics of EPID Displays

Appearance : excellent contrast over very wide viewing angles.

Color Capability : many color combinations including

Speed : approximately 20 msec at 50 volts

Lifetime : > lo8 switches or 8500 hours 7 Voltage : 15-50 volts

Power conswnpS1on : < 5 pw/cm2

Memory : = 100 hrs.

Matrix Addressing : Con-krol a r rd +e&, u( UHdrr dcqc1oprnm-k;

black/white.

3 TTL or CMOS compatible and addressing at 1 msec/l.ine

Resolution : with abogc addressing scheme: = 5 Isn*5/nrn

Temperature : operation -1OoC-70"C, Storage: -40°C

- 1 o o o c

Size : very flexible (< 1 inch2 - > 100 ft2)

cost : expected to be low cost; l o w materials cost and simple fabrication.

trode [5]. Preliminary results indicate that a sharp, well- defined threshold in the switching of a.n EPID cell is achieved, which could enable the x-y addressing of very large matrix arrays (e.g,, 2000 X 2000 elements). The addressing of the matrix can be achieved using TTL or CMOS logic; once addressed, the particles are transported across the cell with a dc voltage of approximately 50 V. The line-to-line addressing time can be less than 1 ms. Further development of this control grid technique is being actively pursued, and it is believed that it will provide a practical and effective method for x-y addressing EPID devices. A detailed paper describing these results will soon be published.

VII. APPLICATIONS

The display applications for EPID technology can be assessed by considering the expected device characteristics as shown in Table 111.

VIII. CONCLUSIONS

The EPID is a highly promising passive display technology. It possesses a distinctive appearance, practical response times, inherent memory, and lolw cost of fabrication. This technology is young in comparison to LCD's or LED's, and much work remains to be done in characterization and understanding of the device as well as in its development towards a successful product. However, the present state of EPID development indicates that in addition to its desirable display characteristics, this display technique will provide practical operating lifetimes and permit x-y addressing with low-voltage logic. With these features and capabilities, a wide range of potential applications can be considered. These range from simple bi-

834 IEEE TRANSACTIONS ON ELECTRON DEVICES, JULY 1977

stable status indicators to large-scale (e.g,, 2000 X 2UOO- element) x-y addressed panels capable of displaying a page of textual information.

ACKNOWLEDGMENT

The author wishes to thank B. Fitzhenry, R. Lieberl: ~ P. Murau, S. Quon, and B. Singer for their helpful discus- sions; J. Jacco, A. Monahan, and R. White for their ex1:el- lent technical assistance; and J. Kostelec for his sup- port.

REFERENCES K. A. Metcalfe and R. J. Wright, “Fine grain development in xero- graphy,” J. Sci. Instrum. vol. 33, p. 194, 1956. P. F. Evans et al., “Color display device,” U. S. Patent 3 612 758 1971. I. Ota, J. Ohnishi, and M. Yoshiyama, “Electrophoretic dis day pane!-EPID,” Proc. IEEE, vol. 61, p. 832,1973. R. T. Blunt and G. M. Garner, “Electrophoretic display resear :R,” Plessey Co. Ltd., Ann. Res. Rep., 1973; also d. F. Lewis, “Electro- phoretic displays,” Nonemissive Electrooptic Displays, A. R. Krletz and F. K. Von Willisen, Ed. New York: Plenum Press, 19Jfi, p. 223. A. L. Dalisa and R. A. Delano, “Recent progress in electropholetic displays,” presented at the Int. Symp. Society for Inforfnat~lon

Display, San Diego, CA, 1974. [6] D. Vance, “Influence of material properties on optical characteristics

of electrophoretic displays,” presented at the Biennial Display Conf., New York, NY, 1976.

[7] E. J. Verwey and J. Overbeek, Theory of the Stability of Lyophobic Colloids. Amsterdam: Elsevier, 1948.

[8] G. D. Parfitt, Dispersions of Powders in Liquids. New York Wiley, 1973.

[9] J. van der Minne and P. H. Hermanie, “Electrophoresis measurements in benzene-Correlation with stability,” J. Colloid Sci., vol. 8, p. 38,1953.

[lo] W, Albers and J. Th. Overbeek, “Stability of emulsions of water in oi1,”J. Colloid Sci. vol. 14, p. 570,1959.

[11] A. L. Dalisa and R. J. Seymour, “Convolution scattering model for ferroelectric ceramics and other display media,” Proc. IEEE, vol. 61, p. 981, 1973.

[I21 W. S. Quon, “Optical switching times in electrophoretic image displays,” presented at the Biennial Display Conf., New York, NY, 1976.

[I31 P. Murau, B. Fitzhenry, A. L. Dalisa, R. Liebert, and B. Singer, “Sources of colloidal suspension instabilities in electrophoretic displays,” presented at the Int. Electron Device Meet., Washington, DC, 1976.

[14] S. Chandrasekhar, Hydrodynamic and Hydromagnetic Stability. Oxford: Oxford Univ. Press, 1961, p. 9.

[15] A. L. Dalisa, B. Singer, and S. Ross, “Characterization of electrophoretic displays,” presented at the Device Research Conf. Salt Lake City, UT, 1976.

[16] W. S. Quon, “Multilevel voltage select (MLVS)--A novel technique to X- Y address an electrophoretic display,” IEEE Trans. Electron Deuices, to be published.

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Basics-EPD-DISPLAY