PASSIVE DISPLAY DEVICE
The invention relates to a passive display device comprising a first and a second supporting plate of which at 5 least the first supporting plate is transparent, first and second electrodes on the facing surface of the first and second supporting plates respectively, at least the first electrodes being transparent, third electrodes which comprise an apertured display part which is secured to 10 one of the supporting plates by means of a number of resilient elements, and which third electrodes can be moved between the first and second electrodes by electrostatic forces, and further comprising an opaque liquid between the supporting plates the colour of which 15 liquid contrasts with the colour of the side of the third electrodes facing the first supporting plate. The invention also relates to a method of manufacturing such a device. An opaque liquid is to be understood to mean herein a liquid the depth of penetration of light in which 20 is smaller than the distance between the supporting plates.
Such a passive display device is known from Netherlands Patent Application No. 7510103 and is used, for example, for displaying alphanumeric information. If 25 the third electrodes are present on the side of the second electrodes, the colour of the opaque liquid is observed through the transparent first supporting plate. If the third electrodes are present on the side of the first electrodes, however, the colour of the third electrodes con- 30 trasting with the liquid is observed. The third electrodes which are connected to one of the supporting plates by means of a number of resilient elements can move between the supporting plates by applying a voltage on the first, second and third electrodes. The occurring 35 resilient forces are negligible with respect to the electrostatic forces. The third electrodes are electrically insulated from the first and second electrodes by an insulating layer which is provided on the first and second electrodes. In the case in which the first and second 40 electrodes are kept at a direct voltage of +V and —V respectively, or at an alternating voltage having an effective value V, and a variable voltage Vg is applied to the third electrodes, the electrostatic forces acting on the third electrodes are such that the third electrodes 45 can assume only two stable positions at the first supporting plate or the second supporting plate. When a third electrode is at one of the supporting plates, the voltage Vg at the third electrode, dependent on the thickness of the insulating layer, may decrease to substantially +V 50 or —V before it flips over to the other supporting plate. As a result of this bistable character the display device has a very large threshold voltage and a memory. These properties make it possible to realize large matrix display devices. In such a matrix display device the first 55 electrodes, for example, form the row electrodes and the second electrodes form the column electrodes of the display device and all third electrodes are electrically interconnected. The manufacture of the movable third electrodes is carried out with a so-called undercutting 60 technique. In this technique a layer is provided on an intermediate layer in which layer the pattern of third electrodes with resilient elements and apertures in the display part is etched. The material of the intermediate layer is then etched away via the edges and the aper- 65 tures in the display part. This is continued until only the resilient elements are still connected to the substrate by means of a pillar. In this manner it is possible to make
small resiliently connected electrodes which are very flat and are substantially free from mechanical stresses. In this manner third electrodes having an area of 0.5x0.5 mm2 have been made with apertures of 4 ftm diameter and a pitch of 20 jam. A display device having such third electrodes showed a switching time of 25 msec at a distance between the supporting plates of 25 jum arid at control voltages of 30 V.
However, the known display device suffers from the disadvantage that with smaller third electrodes a considerable loss of contrast occurs and the control characteristic becomes asymmetrical.
In the known display device the resilient elements with which the third electrodes are connected to one of the supporting plates are situated beside and in the same plane as the apertured display part. As a result of this, area is lost for the actual display operation. The minimum possible area of the resilient elements is determined by the resolving power of the photo-etching methods used in manufacturing the third electrodes. This has for its result that when third electrodes become smaller the resilient elements occupy an ever increasing part of the area of a third electrode and the display part forms an ever smaller part of the area of a third electrode. When electrodes become smaller, the so-called whiteness, that is to say the effective reflecting area of a third electrode and hence also the contrast of the observed picture decreases.
In third electrodes having an area of approximately 0.5 X0.5 mm2 the resilient forces occurring as a result of the resilient elements are small with respect to the electrostatic forces. When third electrodes become smaller, the overall electrostatic forces decrease, whereas as a result of the decreasing size of the resilient elements, the resilient forces increase considerably. In the case of smaller electrodes the resilient forces are hence no longer negligible. The result of the comparatively large resilient forces is that an asymmetric control characteristic is obtained which is much less ideal for matrix control.
It is hence the object of the invention to provide a display device having small third electrodes with which pictures having a high contrast can be observed. For that purpose, a display device of a kind mentioned in the opening paragraph is characterized according to the invention in that the resilient elements of the third electrodes are present below the display parts of the third electrodes at the side remote from the first supporting plate. As a result of this the entire area of a third electrode can be used as a display part. As a result of this construction whiteness has become independent of the size of the third electrodes. As a result of this, smaller third electrodes than before can be manufactured while substantially maintaining contrast. Since the resilient elements are provided below the display part, the fill area below the display elements may be used in designing the resilient elements. As a result of this larger freedom of design, very small spring constants can easily be realized so that the occurring resilient forces are negligible with respect to the electrostatic forces even in small third electrodes. Furthermore, more resilient elements can be provided below the display part than is strictly necessary, which increases the reliability (redundance) of the display device. Moreover, the accurate photolithographic processes, as used in the case in which the resilient elements are situated in the same plane as the display part, are not necessary for the manufacture of the resilient elements.