EP1097398A1

EP1097398A1 - Display device with low energy consumption

Info

Publication number: EP1097398A1
Application number: EP99931341A
Authority: EP
Inventors: Jean-Frédéric Clerc
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1998-07-15
Filing date: 1999-07-13
Publication date: 2001-05-09
Also published as: JP2002520673A; FR2781305A1; WO2000004412A1

Abstract

The invention concerns a display device comprising: two transparent or reflecting walls (2, 4), one at least of the walls consisting of a flexible film or a thin deformable layer; a light-absorbing liquid (6) inserted between said two walls; means for varying the distance between the two walls.

Description

VERY LOW CONSUMPTION DISPLAY DEVICE

Technical field and prior art

The invention relates to a display device using a liquid capable of absorbing light, this liquid being introduced between two walls. Such a display device can be applied to many fields; it can in particular be matrix.

Are known display devices using liquid crystal (LCD, ^"Liquid Crystal Display"), including reflective type devices "Guest Host", that is to say dichroic dye. In this type of device, a modulation of the attenuation or absorption coefficient α is carried out, as a function of an applied electric field. For example, when the field is applied, the absorption coefficient O _ON has a value lower than the absorption coefficient OC _OFF in the absence of a field. The thickness of the liquid crystal layer is constant.

The use of this property in "Guest Host" liquid crystals cannot be carried out uniformly in all directions. This results in a reduction in visual contrast when looking at the display with certain angles of incidence.

It will also be noted that these devices do not simultaneously present a strong contrast with a strong transmission. This is due to the fact that the dye used must be oriented by the liquid crystal, and that the absorption is anisotropic (α _// > α ±). The effectiveness of the dye is thereby reduced by about half. Statement of the invention

The problem therefore arises of making a new type of display device. In particular, there is also the problem of producing a display device making it possible to use isotropic dyes.

The subject of the invention is a display device comprising transparent or reflecting walls, at least one of which is a film or a layer or a thin layer, and an absorbent or very absorbent liquid comprised between the walls. The walls delimit a volume or an area in which; -: OR where the layer of liquid is contained or trapped. Means are further provided for varying the thickness of the layer of liquid between the two walls or for varying the distance between the two walls or for varying the shape of the volume or of the area in which the liquid is contained or imprisoned.

The absorbent or very absorbent nature of the liquid is relative to a certain spectral range. This liquid can be an organic dye or a pigment, for example particles in solution in a liquid such as particles of anthraquinone and / or azoic in an aqueous solution.

The variation in distance or thickness or volume causes a variation in absorption through the layer, therefore of transmission or reflection, but the absorption coefficient of the liquid remains constant.

The device comprises at least one central zone, called the display zone, and at least one lateral zone, called the liquid reserve zone. In addition, it may further comprise, for each lateral zone, means for deforming a part of one of the two walls consisting of a flexible film or of a thin deformable layer, so as to increase the volume available for the liquid. between the deformed wall and the other wall.

Thus, according to the invention, is the liquid contained in a constant deformable volume between two walls, at least one of which is deformable. The optical absorption is proportional to the product of the thickness by the absorption coefficient. If the thickness changes the optical absorption changes, whatever the angle of observation.

The thickness variation between the walls can be obtained by applying an electromechanical force on at least one of them.

The means for actuating the walls may for example be electromagnetic or electrostatic. For reasons of conservation of the volume of liquid, it may be stored, when the distance or the thickness or the volume between the walls is reduced, in one or more lateral reserve zones. For questions of dynamics, we prefer miniature storage areas and close to a display area called display area.

A micro-suction system allows the creation of such storage micro-zones. Such a system has a particularly advantageous hysteresis effect for the intended application, and which makes it possible to physically separate the "actuator" part from the "fluid" part. The micro-suction system is, for elsewhere, associated with control means, for example electrodes.

Brief description of the figures

In any case, the characteristics and advantages of the invention will appear better in the light of the description which follows. This description relates to the exemplary embodiments, given by way of explanation and without limitation, with reference to the appended drawings in which:

- Figures 1A and 1B illustrate the operating principle of a display device according to the invention, - Figures 2A and 2B represent an embodiment of the invention,

FIG. 3 represents the distance between the electrodes of the control means of a device according to the invention, as a function of the voltage applied between these electrodes,

- Figures 4A to 4F show stages in the production of a device according to the invention.

Detailed description of embodiments of the invention

A display according to the invention implements a variation in volume or thickness of a layer, or of a pocket, containing an absorbent or very absorbent liquid 6 (FIG. 1A). A variation in thickness causes a variation in absorption through the layer

6, therefore transmission or reflection. There may for example be a double optical path after return by a mirror located under layer 4. The liquid layer 6 is between walls 2, 4, which are transparent or reflective depending on the operating mode chosen. In transmissive mode, the two walls are transparent, in reflective mode, at least one of the walls is transparent and the other is reflective (that is to say associated either directly or indirectly with a reflective layer). At least one of these layers, or walls, can undergo translation relative to the other wall. It is for example a flexible film or a thin layer.

In FIG. 1A, the walls 2, ^" 4 are made of a transparent material. They feel represented in the rest state: the thickness E which separates them is quite large, and there is therefore a strong attenuation of a radius 8 which passes through the wall 2 and the liquid layer 6.

In the activated state ("ON") the thickness e of the liquid layer is reduced to a lower value and the beam 8 can pass through the display device while being less attenuated.

The law that governs the absorption of light is exponential. If we denote by α the attenuation or absorption coefficient of the material 6 and by x the distance to be crossed by a light beam, we have:

or :

where I denotes the intensity of the beam with respect to its initial intensity I ₀ . We can see that if 1 μm of dense dye allows only 10% of the light to pass, 0.1 μm of the same dye will let about 81% of the light pass:

if So: ^τ 0 ± _— e-0.2302 _— _Q O ₁ I ₉ Ό.

10

The variation of the thickness between the walls 2,

4 can be obtained by applying an electromechanical force to at least one of them. _. The activation means can be electromagnetic: or electrostatic.

We will describe below a principle of micro-suction cups, which allows the creation of storage microzones around the display area of the display device, each display area corresponding to an image point.

Such a device is shown in FIGS. 2A and 2B (respectively: "OFF" state and "ON" state).

This device comprises first and second transparent walls 2, 4. In other applications one or the other of these walls may be reflective. According to one embodiment, the wall 2 is made of glass or plastic and the wall 4 is made of plastic or consists of a mineral layer (for example: silicon nitride).

This device also comprises a first and a second electrode 10, 16 located against the wall 4. It also comprises a third and a fourth electrode 12, 14 located respectively with regard to each of the electrodes 10, 16. These electrodes can for example be made of metal, for example aluminum.

The electrodes 10, 16 are each located in a compartment 18, 19, which can optionally be maintained at reduced pressure (that is to say less than atmospheric pressure or at 1 bar, for example of the order of a few millibars or 1 millibar). The spaces 18, 19 are separated by a transparent spacer film 20 which can have the same composition as the film or the layer 4. In fact, the spaces 18, 19 can be produced in this film 20.

A seal 24 delimits each image point. Such a seal consists for example of a photosensitive organic resin, such as those used for masking in microelectronics.

Finally, the entire device rests on a substrate 22 (plastic or glass), which may possibly be a mirror in a reflective operating mode of the device. The electrodes 12, 14 being interposed between the film 20 and the substrate 22.

When applying a voltage to the electrodes 10, 12 and 14, 16, the display area 30 of the display device, which constitutes the image point (FIG. 2B), has its thickness reduced to a low value, depending on the operating principle of the device explained above in connection with FIGS. 1A and 1B.

FIG. 3 represents the evolution of the distance d between the electrodes 10, 12 (and 14, 16) as a function of the application of a voltage between these same electrodes. The voltage scale can be divided into three zones A, B, C. The device is in "OFF" mode when the electrodes 10 and 12 (respectively 14 and 16) are subjected to a voltage in region A. The device is in operating mode ("ON") when the electrodes are subjected to a voltage contained in region C. The device is in the "OFF" or "ON" position in storage mode when the electrodes are subjected to a voltage included in region B. In other words, the device has no change in state. Thus, the structure of the invention has a bistable operation.

In the mode shown in FIG. 2B, it can be seen that the application of a voltage to the electrodes ^' 10, 12, 14, 16 leads to the deformation of the wall 4 in the zones to which the electrodes are fixed. This results in the formation of zones 38, 40 for storing the fluid 6. The latter is therefore attracted to the lateral parts of the display device, hence a reduction in the thickness of the display zone 30. The zone designated in FIG. 2B by the reference 32 therefore becomes a weak absorption zone, delimited by two zones 34, 36 for concentrating the fluid.

Thus, a deformation system of the wall 4 is produced, of the "suction cup" type.

According to a particular embodiment, the film, or the wall, 2 can have a thickness of between a few μm and 100 μm, while the film, or the wall 4, has a thickness of between 1 μm and a few hundred nm. As for the dye, its thickness varies from approximately 1 μm ("OFF") to approximately 100 nm ("ON" state).

The electrodes 10, 12, 14, 16 can have a typical thickness of the order of 100 n. Spaces 18, 19 have a thickness of between a few μm and a few tens of μm.

The zone 20 has a thickness equal to that of the spaces 18, 19 increased by the sum of the thicknesses of the electrodes 10, 12 (or 14, 16).

The substrate 22 can have a thickness of the order of 1 mm. As for the seal 24, it can typically have a width of 10 μm and a height equal to the thickness of the liquid 6 in the "OFF" state of the device.

The applied voltages are between a few volts and a few hundred volts, for example 300 volts, depending on the stiffness and the size of the walls 2, 4. The surface occupied by the "microwells" is preferably as small as possible relative to the surface of the image point, since the transmission is limited by the surface of these microvents (the lateral zone is approximately 1/5 to 1/3 lower than the display zone)

FIGS. 4A to 4F represent a method of manufacturing a device according to the invention.

In a first step (FIG. 4A), the walls 50, 52 of silica are deposited and etched. In the middle of the etched area is deposited, and etched, a titanium layer 54. Optionally, an insulating layer 56 can be deposited over the titanium layer, to avoid short circuits. In a second step (FIG. 4B) a sacrificial layer 58 of tungsten is deposited, over the electrode 54. This layer is then etched (FIG. 4C) so as to then be able to produce a second titanium electrode 62, partially or completely integrated (FIG. 4D). A nitride membrane 64 is then deposited, with a thickness of around 500 n. The electrodes 54, 62 have a thickness of approximately 100 nm by providing (for example, by etching) one or more access zones to the layer 58 (these zones are preferably outside the lateral zones).

Then withdraws tungsten by chemical etching in the liquid phase then patching access areas by vacuum deposition, for example ^'silica. The final thickness E of the chamber 56 thus formed is approximately a few μm, and the diameter φ of the chamber is between a few tens and a few hundred μm.

One thus proceeds to the production of a "micro-cup", the operating mode of which has been given above in connection with FIGS. 2B.

A matrix structure of the display areas separated by lateral areas can be produced in the same way according to the invention.

Claims

1. Display device comprising:

- two transparent or reflecting walls (2, 4), at least one of the walls consisting of a flexible film or of a thin deformable layer,

- a light absorbing liquid (6) contained in a constant deformable volume between these two walls,

- Means (10, 12, 14, 16, 18, 19) for varying the distance between the two walls.

2. Device according to claim 1, the liquid comprising an organic dye or a pigment in solution.

3. Device according to claim 1 or 2, the means for varying the distance between the two walls being electromagnetic.

4. Device according to claim 1 or 2, the means for varying the distance between the two walls being electrostatic.

5. Device according to one of the preceding claims comprising at least one central zone (30), said display zone, and at least one lateral zone (34, 36), said liquid reserve zone.

6. Device according to one of the preceding claims, further comprising for each lateral zone means (10, 12, 14, 16, 18, 19) for deforming a part of one of the two walls consisting of a flexible film or a thin deformable layer, so as to increase the volume available for the liquid between the deformed wall (4) and the other wall (2).

7. Device according to claim 6, the means for deforming part of one of the two walls comprising at least one assembly consisting of first and second electrodes (10, 12, 14, 16), separated by a variable distance d.

8. Device according to claim 7, the first electrode (10, 16) being mechanically linked to the wall consisting of a flexible film or a thin deformable layer and being located in a chamber (18, 19) inside from which it can move.

9. Device according to claim 8, the second electrode (12, 14) being located opposite the first electrode, but under the chamber (18, 19) inside which the first electrode (10, 16) can be move.