Method for Fabrication of Electroscopic Display Devices and Transmissive Display Devices Fabricated Thereby

The present invention relates to display devices and, more particularly, to a novel method for fabricating electroscopic display devices and to transmissive electroscopic display devices fabricated thereby.

It is highly desirable to provide flat panel matrix displays. Such displays often employ a matrix array of light valve elements. Heretofore, various liquid crystal effects have been suggested and utilized for these light valves, albeit with certain tradeoffs having to be made, whereby high brightness, high contrast, fast response and matrix addressability are not often all achieved in a single display. Recently, a reflective electroscopic display has been described by T. S. te Velde at the 1980 Society for Information Display Symposium. This form of display (described in some detail hereinbelow) is known only in a reflective display form. Techniques for fabricating the micro-mechanical plates required by such a display in simple fashion and with high yield, are presently not available. It is not only highly desirable to provide transmissive electroscopic display devices, but to also provide a method for fabricating the micro-mechanical microscopic plates in simple and high-yield fashion.

In accordance with one aspect of the invention, a member comprised of a moveable perforated metal plate, having spring members attached between the periphery thereof and a fixed mounting portion for mounting upon the surface of a substrate, is fabricated by sequentially depositing upon the substrate surface at least one layer of an etchable material, over-deposited with a layer of conductive material from which the member is to be formed. The outline of the member, and the placement of apertures therethrough, is defined by a previously deposited photo-resist layer. The photo-resist is then removed and portions of the at least one etchable layer are etched away to provide a thin apertured plate, suspended above the substrate surface by the remaining, unetched portions of the at least one layer. The plate/spring/support member-bearing substrate is utilizable in either reflective or transmissive displays.

In accordance with another aspect of the present invention, a transmissive electroscopic display device is provided by utilizing the above-described structure with a front substrate having at least one transparent electrode upon the interior surface thereof, and positioned spaced from, but facing, the electroscopic member-bearing rear substrate. The fixed portion of the plate member includes reflective elements positioned to align with the apertures in the moveable plate, when the plate is adjacent to the rear substrate surface.

Accordingly, it is one object of the present invention to provide a novel method for fabricating at least one micro-mechanical electroscopic plate member upon a substrate surface.

It is another object of the present invention to provide novel transmissive electroscopic displays.

These and other objects of the present invention will become apparent upon consideration of the following detailed description, when read in conjunction with the drawings.

FIG. 1 is a sectional side view of a portion of a prior art reflective electroscopic display device;

FIG. 2 is a section side view of a portion of a transmissive electroscopic display device in accordance with the present invention;

FIG. 2a is a partially sectionalized top view of a portion of the display of FIG. 2, illustrating the relationship between the moveable and nonmoveable rear electrode plate member portions, in accordance with the principles of the present invention;

FIGS. 3a-3d are sequential side views illustrating the method for fabricating the micro-mechanical electroscopic display members upon a display substrate, in accordance with the principles of the present invention; and

FIG. 4 is a partially-sectioned plan view of another preferred embodiment of a transmissive electroscopic display device in accordance with the principles of the present invention.

Referring initially to FIG. 1, the reflective electroscopic display 10, described by te Velde, utilizes an transparent front substrate 11, spaced from a rear substrate 12. Substantially all of the interior surface 11a of the front substrate supports a transparent electrode 14. The interior-facing surface 12a of the rear substrate supports a conductive electroscopic plate portion 16, having a plurality of apertures 16a therethrough, and a substantially complimentary fixed electrode portion 18, having support portions 18a fixed to substrate surface 12a and other portions 18b which are coincident with apertures 16a in the electroscopic plate when the plate is in the rest position, substantially coplanar with fixed portions 18. A portion of the conductive material layer is so formed as to provide spring arms 20 (shown relatively schematically in FIG. 1) between fixed support portions 18a and the periphery of the moveable plate 16. A potential source 22 is electrically connectable, via switch means 24, between the continuous front electrode 14 and the continuous, conductive rear electrode formed of at least moveable plate portion 16.

The "camera obscura" reflective mode of operation requires a highly transmissive medium 26 between the substrates, a highly absorbant plate surface 16b and highly-reflective fixed portions 18b. In the "off" condition (with switch 24 open, as at the left of FIG. 1) the entering light ray 28a passes through reflective front substrate 11 and front electrode 14, through the transmissive fluid 26 and is reflected only from the relatively large rear fixed member 18; light infringing upon moveable plate 16 portions is substantially absorbed. The light beam 28b, reflected only from portions 18b, passes back through transmission medium 26 and the transparent front electrode 14 and front substrate 11, and is visible to an observer stationed in front of substrate 11, albeit with relatively low intensity (due to the ratio of reflector area of portions 18b to absorber area of plate surfaces 16b). In the "on" condition, switch 24' is closed, and the electrostatic force between front electrode 14 and moveable plate portion 16 causes plate 16 to be attracted upwardly to the front electrode. Entering light ray 30a is either absorbed at the dark front surface 16b of the moveable plate portion, or passes through one of apertures 16a. The position of apertures 16a are so located that any light ray 30a passing through an aperture 16a and then reflected from a stationary portion 18b will impinge upon a solid portion of moveable plate 16, whereby light is not reflected from the "on" portion of the display (at the right-hand portion in the illustration thereof).

In the "electroscopic fluid display" mode, media 26 supports a multiplicity of light-absorbing pigment particles 26a. In the "off" condition (left portion of FIG. 1) the entering light ray 32 is substantially absorbed by the pigmentation particles and is not reflected. In the "on" (right-hand) portion of the display, entering light ray 34a travels a very small distance through the pigmented media 26, and is thus relatively unattenuated, upon arrival at the upper surface 16b of the moveable plate member portion 16. This surface 16b is highly reflective, in the electroscopic fluid display, and reflects the impingent light ray to be viewable as a light ray 34b by an observer situated in front of the display.

Referring to FIGS. 2 and 2a, in accordance with one aspect of the present invention, a transmissive electroscopic display 40 includes a front transparent substrate 41, having an interior surface 41a spaced from and substantially parallel to the interior substrate 43a of a substantially transparent rear substrate 43. At least one substantially continuous conductive electrode 45 is fabricated upon front substrate interior surface 41a. A conductive electrode structure 57 is fabricated upon rear substrate interior surface 43a, and includes support portions 58 fixedly attached to substrate surface 43a, and joined via spring arm sections 59 to a micro-mechanical moveable plate portion 60. In the normal resting position, as shown at the left-hand portion of FIG. 2, moveable portion 60 is substantially coplanar with the fixed support portions 58 and spring arm portions 59. A large portion, e.g. approximately 50%, of the surface area of moveable plate 60 has a pattern of apertures 60a formed therethrough, with complimentary shaped electrode portions 61 protruding through the apertures 60a, in the rest position. Portions 61 are fixedly attached to rear substrate surface 43a and have dimensions slightly smaller than the corresponding dimensions of apertures 60a, whereby, when plate 60 is normally resting coplanar with stationary support portions 58 and electrode portions 61, substantially all of apertures 60a are filled by electrode portions 61. It should be understood that portions 61 may also lie under apertures 60a, or that portions 61 may be slightly larger than apertures 60a whereby, at rest, apertures 60a are totally blocked. Advantageously, at least the bottom plate surface 60b and the top electrode portion surfaces 61a are highly reflective.

In operation, a light source (not shown) is positioned behind the rear substrate exterior surface 43b, and provides light rays obliquely impingent thereon. When the display cell is in the "off" condition, with plate 60 resting adjacent to rear substrate surface 43a and substantially coplanar with support portions 58 and electrode portions 61 (as may occur when switch 24, in series with a potential source 22, is open-circuited), a light ray 65a will be transmitted through the substantially transparent rear substrate 43, but will be substantially blocked and reflected, as light ray 65b, at the electrode structure 57 surface closest to the rear substrate. The display portion defined by that cell is viewable as a dark area. In the "on" condition, as when closed switch 24' connects potential source 22 between front electrode 45 and the conductive rear electrode structure 57, the electrostatic force counteracts the pull of spring arms 59 and plate 60 moves to a position adjacent to the interior surface of electrode 45. An obliquely-entering light ray 67a passes through the substantially transparent rear substrate 43 and through one of the openings 57a formed between support portions 58 and electrode portions 61 (and occupied in the "off" condition by a solid portion of plate 60). Entering light ray 67a passes through the cell and is reflected at the plate rear surface 60b, as a reflected light ray 67b. The reflected light ray 67b travels back through the interior of the cell and either exits through another electrode aperture 57a and the rear substrate, or is reflected at the front surface 61a of one of electrode portions 61, as a double-reflected light ray 67c. The double-reflected light ray 67c traverses the interior of the cell and, due to the positioning of the electrode portions 61 and the solid portions of plate 60, passes through a plate aperture 60a, the substantially transparent front electrode 45 and the substantially transparent front substrate 41, emerging from the front of the cell as a transmitted light ray viewable by an observer stationed in front of cell 40. It should be understood that operation with a multiplicity of reflections, between plate rear surface 60b and portion 61a surfaces, is possible before the reflected ray is transmitted through the front substrate. It should also be understood that the display may be a matrix display, with a plurality of front electrode 41 stripes aligned in a first direction, and a plurality of lines of plates 60, with each plate line extending in a second direction substantially orthogonal to the first direction.

Referring now to FIGS. 3a-3d, in accordance with another aspect of the present invention, the support-spring arm-moveable plate electrode of an electroscopic display (of either the reflective or transmissive type), is fabricated upon a substantially transparent, e.g. glass and the like, substrate 70. A thin layer 72 of a first etchable conductive material, such as chrome and the like, is fabricated to a first thickness T.sub.1, e.g. on the order of 40 nanometers, upon substrate surface 70a. A second layer 74 of an etchable conductive material, which is preferably of a material different from the material of first layer 72, and etched by a substance different than the substance utilized for etching the material of first layer 72, is fabricated upon that surface 72a of the first layer furthest from substrate 70. Illustratively, second layer 74 is fabricated of copper, to a thickness T.sub.2 on the order of 200 nanometers. Layers 72 and/or 74 may be fabricated by sputtering, evaporation and the like processes. Preferably, the first layer 72 is formed of a material providing increased adhesion for the material of the second layer 74 to the material of the underlying substrate 70. A side view of the structure thus far formed is illustrated in FIG. 3a.

Thereafter, a patterned layer 76 of a photo-resistive material is fabricated upon second layer surface 74a (FIG. 3b). The photo-resistive material layer 76 is patterned with apertures 76a positioned to define the placement of the solid portions of the support-spring arm-plate electrode 57 (see FIGS. 2 and 2a). Thus, photo-resistive layer 76 is the negative of the desired spring plate pattern. Thereafter, a third layer 78 of a conductive material, such as nickel and the like, is fabricated upon the non-photo-resist-bearing portions of the second layer surface 74a. Illustratively, third layer 78 may be fabricated of nickel, to a thickness T.sub.3 on the order of 100 nanometers, by electro-plating in a nickel sulfamate bath, to provide a low-stress nickel film layer 78.

Thereafter, photo-resist layer 76 is removed (FIG. 3c) and the positive plate pattern of the conductive material of layer 78 exists atop the first and second layers 72 and 74. The rear substrate-electrode assembly is immersed in a bath of a material selected to (a) etch at least the material of second layer 74 and (b) have substantially no effect on layer 78. The etching time is controlled to be long enough so that there is sufficient etching and undercutting of second layer 74 to remove the portions of that layer underneath the plate portions 78a, but insufficient to remove portions of layer 74 underneath the larger layer portions 78b which will form support portions 58 (see FIG. 2a). Thereafter, a different etchant bath is used to remove the material of layer 72, in the same manner, whereby all of the first and second layer material underneath plate portions 78a are removed, and a "pillar" of first and second layer material remains beneath the electrode support portions 78b, attaching those portions to the substrate surface 70a (FIG. 3d). Electrode portions 61 may, for example, then be fabricated by masking the surface of electrode 78 and evaporating the portions 61 through the resulting apertures in plate 78. The resulting cantilevered spring arm-plate portion (supported by the remaining deposits of layers 72 and 74) and the associated rear substrate may then be assembled with a front glass substrate, having one or more transparent front electrodes, overcoated with an insulating layer, thereon. In addition, spacers and the like may be used in manner known to the display (and particularly the liquid crystal display cell) art, to complete the display. The cell interior volume 40a may be under vacuum, or filled with a gas, or a clear or colored liquid, all as desired for the end display.

Referring now to FIG. 4, an alternative support-spring arm-plate member 57' utilizes plate 60' rotation, in the direction of arrow A, to provide rotational movement of sector-shaped apertures 60a', when plate 60' is subjected to electrostatic attraction and movement towards the front electrode (not shown). In this embodiment, the plate has a substantially circular periphery 60d', with a first end of each of a plurality of arcuate spring arms 59' being attached to an associated point equally spaced about the plate periphery. The remaining end of each spring arm is attached to an associated one of a like number of support portions 58', themselves attached to the surface 43a' of rear substrate 43'. Stationary portions 61' may also be sector-shaped and, if such portions have a highly-reflective surface 61a', may be exposed only when the sector-shaped plate apertures 60a' are rotated, in the direction of arrow A, responsive to the electrostatic force, to adjacent positions, as e.g. the position of sector aperture 60a". The sector-shaped reflective portions 61' will be covered by the solid portions of circular plate 60", in the "off," or rest, position of the transmissive display thus formed.

While several embodiments of my novel transmissive electroscopic display device, and a presently preferred method for fabricating the support-spring arm-plate member for an electroscopic display of either the reflective or transmissive type, have been described in detail herein, many modifications and variations will now become apparent to those skilled in the art. It is my intent, therefore, to be limited only by the scope of the appending claims, and not by the details disclosed herein.