GB2028529A

GB2028529A - Liquid crystal displays

Info

Publication number: GB2028529A
Application number: GB7925693A
Authority: GB
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1978-08-21
Filing date: 1979-07-24
Publication date: 1980-03-05
Also published as: JPS5536897A; FR2434444A1; DE2933312A1

Abstract

A reflective liquid crystal display cell includes a reflector-electrode positioned within the cell to eliminate parallax and improve display brightness. The reflector is of the diffuse type and is fabricated by grinding and etching a rear glass substrate 52 of a cell prior to deposition of a highly reflective material 54 upon the interior substrate surface. The reflector surface 54a includes a random array of spherical dome-like formations substantially confining reflected light to a cone having a half-angle less than the critical angle of the liquid crystal layer of the cell. <IMAGE>

Description

SPECIFICATION Liquid crystal displays The present invention relates to information displays, and more particularly, to a novel method for fabricating a reflector element for use in a liquid crystal display device and to the device fabricated therewith.

Liquid crystal displays are highly desirable due to the relatively low power consumption thereof.

In many applications, reflective liquid crystal displays are particularly advantageous in that ambient light is reflected to provide visibility of the display, whereby an additional, power-consuming light source is not required. A typical display cell consists of a pair of glass plates having transparent conductive electrodes on their facing surfaces and a layer of liquid crystal material filling the volume between the electrodes. The liquid crystal material may act as a host for a guest dichroic dye dissolved therein. The liquid crystal layer may allow selective absorption and transmission of light in the cell by use of a cholesteric-nematic phase change effect responsive to changes in the amplitude of an electric field formed between the electrodes.

Other light-absorption/light-transmission effects may be equally utilized, as known to the art.

Typically, a diffuse reflector will be placed behind the rear glass substrate to complete the reflective cell. It is known that this form of cell construction leads to both additional absorption and reflection, in the rear glass substrate of the cell, and to severe shadowing or parallax caused by the separation of the reflective surface from the nearest surface of the absorbing liquid crystal layer. Attempts have been made to reduce the undesirable absorption and reflection of the rear glass substrate by placing the reflector on the substrate surface.

closest to the liquid crystal layer, such as described in U.S. Patent 3,963,312, issued June 15, 1976. However, an additional element is typically required between the liquid crystal layer and reflector surfaces, whereby the shadowing or parallax effects remain due to the separation between these elements. It is thus desirable to provide a reflective liquid crystal display having reduced parallax and also having the light reflected therefrom confined over some viewing angle relative to the normal to the display surface, to increase the brightness of the display.

In accordance with the invention, a reflective liquid crystal display cell having a pair of substantially transparent substrates and a layer of liquid crystal therebetween, has a planar, typically segmented electrode interposed between the front substrate and the front surface of the liquid crystal material and has a reflective coating of a conductive material deposited upon an etched and ground inner surface of the rear substrate. The conductive coating acts as both a rear electrode and as a diffuse reflector directly in contact with the rear surface of the liquid crystal layer.

Segments can be formed into the conductive reflecting layer to provide segmented electrodes adjacent to the liquid crystal layer near the surface and still maintain a uniform appearance of the background, while proper selection of the grinding material and etching solutions and times causes a surface texture, comprising an array of spherical dome-like formations, to be formed upon the reflective surface, to limit the reflected light within the cell to a relatively narrow cone disposed about the normal to the display surface, whereby increased display brightness is achieved.

In one preferred embodiment, the rear substrate is formed of a glass material and is ground with 5-50 micron diameter abrasive particies prior to etching in a 10% hydrofluoric acid solution for about 30 minutes to provide the desired surface. A highly reflective layer of aluminum or silver, having a thickness on the order of 1,000 angstroms, is deposited upon the ground-and-etched surface to form the reflecting electrode. Thin lines may be etched through the conductive layer to form isolated segment electrodes therein.

In the accompanying drawings, Figure 1 is a sectional side view of a prior art reflective liquid crystal display cell; Figure 1 a is a somewhat schematic side view-of the prior art display cell, and illustrating light transmission and reflection therein; Figure 2 is a sectional side view of a liquid crystal display cell in accordance with the principles of the present invention; Figure 2a is somewhat schematic side view of the liquid crystal display cell of Figure 2, and illustrating light transmission and reflection therein; and Figures 3a-3c are a series of side views and Figure 3d is a perspective view, illustrating the method of fabrication of the diffuse reflectorbearing rear substrate of the display cell, and in accordance with the principles of the present invention.

Referring initially to Figures 1 and 1 a, a prior art reflective liquid crystal display cell 10 includes a front substrate 1 formed of a substantially transparent material, such as glass and the like. A conductive, substantially transparent electrode 12 is fabricated upon the interior surface of substrate 11 and is in abutment with a layer of liquid crystal material 14. Front electrode 12 may have various continuous grooves and apertures 1 2a-formed therein to separate the conductive portions of the electrode into segment electrode portions and background electrode portions, in manner known to the art, and as required for the display of desired symbols, characters and other indicia.

A second planar, conductive and substantially transparent electrode 1 5 is placed parallel to planar front electrode 12 and in abutment with the rear surface of the layer of liquid crystal material.

A member 18, which may be of an insulative material, possibly having a polarization effect, is interposed between rear electrode 1 5 and a reflector member 20, serving as the rear support for the cell. It will be seen that the parallax and the attenuation of light passing through insulative member 18 is proportional to the thickness thereof, and that removal of member 1 8 is desirable, in the ideal case. It is also known to provide the interior surface 20a of the reflective member with a diffuse surface to provide some degree of light scattering. Suitable gasket means 22 are utilized to contain the liquid crystal material between the front and rear electrodes.

In operation, assuming that the liquid crystal material layer 1 4 is designed for use of the cholesteric-nematic effect and has a guest dichroic dye dissolved within the host liquid crystal material, the axes 24a of the elongated dye molecules align perpendicular to the parallel planar electrodes surfaces when a switch 26 is closed to couple an AC signal source 27, of suitable frequency and magnitude, between the electrodes. A ray 30 of light entering the cell has random polarization, with polarization vectors 30a and 30b being disposed orthogonal to each other and to the direction of travel of ray 30. The large mismatch in refractive 11 and substantially transparent electrode 1 2 with relatively little attenuation therein.As the polarization vectors are orthogonal to the direction of alignment of the axes of (gr 1.5) substrate of 11 and the surrounding air 31 (n 1.0) causes substantial refraction at the interface therebetween (substrate front surface 1 1 at. The refracted ray 32 passes through the substrate and substantially transparent electrode 1 2 with relatively little attenuation therein. As the polarization vectors are orthogonal to the direction of alignment of the axes of dye molecules 24a, there is relatively little absorption of the ray during transmission through liquid crystal layer 14. The refractive indices of the liquid crystal material, the electrodes and the substrate are reasonable watched and relatively little refraction occurs at the interfaces therebetween.The ray is now transmitted through rear electrode 1 5 and insulative layer 18 and is reflected at the front surface 20a of the reflector.

The ray 32, in layer 18, strikes reflective surface 20a at some angle and is reflective therefrom as a ray 34 at another angle between zero and 900 relative to the normal 35 to the plane of reflected surface 20a, due to the random distribution of scattering angle from the diffuse reflective surface.

Ray 34 is transmitted through layer 18, electrode 15, liquid crystal layer 14, electrode 12 and front substrate 11 to arrive at the interface between the front substrate and surrounding air 31 at some angle 6 to the normal 37 to the front substrate surface 11 a. Due to the random distribution of reflection angles encountered by ray 32, the angle 6may be greater than the critical angle (typically on the order of 400) of the liquid crystal cell, whereby lightray 34 is totally reflected at the front surface of the'front electrode and must again traverse, as lightray 38, the thickness of the cell before impinging upon the diffuse ray surface 20a and undergoing a second reflection.The rereflected ray 40 is again sequentially transmitted through layer 18, electrode 15, liquid crystal layer 14, electrode 1 2 and front substrate 11. If the angle 6 of ray 40 with the normal 37' to the liquid crystal cell front surface 11 a is less than the critical angle, the light is transmitted and refracted across the front electrode-air interface and emerges from the front of the display as a light ray 46 having polarization vectors 46a and 46b orthogonal to each other and to the direction of travel of light ray 46.However, if the new angle 6' is again greater than the critical angle, the rereflected ray 40 is again totally internally reflected in the liquid crystal layer and must undergo at least one additional reflection before emerging from the front surface of the display. As passage through each of the front and rear substrates, front and rear electrodes and the liquid crystal layer has a certain amount of partial absorption associated therewith, the magnitude of light reflected by the display is diminished and the brightness of the display is decreased.In an extreme case, with a relatively roughly ground diffuse reflecting surface 20a, wherein there is a random distribution of reflection angles, about 75% of the light strking their reflection surface is scattered such that the encounter angles 6 are greater than the critical angle, whereby the light must undergo a plurality of reflections prior to emergence from the cell; if the transmission in a reflecting area (i.e. an area having the axes 24a of the absorbing molecules parallel to the direction of light travel) is about 70%, as found in typical liquid crystal layers, the result is that the brightness of the cell is only about 60% of the brightness obtained if light is not reflected beyond the critical angie.

If switch 26 is open, whereby an electric field does not exist between electrodes 12 and 1 5, the axes 24b of the host liquid crystal molecules and guest dichroic dye molecules assume a homogenus orientation parallel to the planes of the facing interior surfaces of the electrodes. The entering light ray 30' now has its orthogonal polarization vectors 30a' and 30b' parallel to the elongated molecules, whereby substantial absorption of light occurs and substantially no light is reflected by the display.

Referring now to Figures 2 and 2a, a reflective display cell 50 utilizes a front substrate 11, a layer 14 of liquid crystal material (illustrated as having dichroic dye molecules dissolved therein), and a substantially transparent (possibly segmented) conductive electrode 12 sandwiched therebetween. A rear substrate 52 has a surface 52a thereof, closest to the liquid layer, with a textured surface and supporting a layer 54 of a conductive and reflective material thereon and in abutment with a rear surface of liquid crystal layer 1 4. Suitable gasket means 22 are again utilized between the front and rear cell members to confine liquid crystal layer 14.

The reflector layer 54 is characterized by having a surface 54a, in contact with the liquid crystal layer, which refiects substantially all light impingent thereon at an angle less than some angle , with respect to the normal 55 to the plane of reflector surface, which angle (p is less than the critical angle for the liquid crystal layer. Thus, in operation and with switch 26 closed, the field applied across the liquid crystal layer (responsive to AC excitation source 27) aligns the axes 24a' of the dichroic dye modules in the homeotropic condition, i.e. substantially perpendicular to the planes of the front electrode 1 2 and the conductive reflector 54.An entering light ray 60, having polarization vectors 60a and 60b orthogonal to each'other and to the direction of travel, is refracted at the front substrate-air interface and then passes through the front substrate, front electrode and liquid crystal layer to impinge as a light ray 61, upon the diffuse reflector surface 54a. The reflected light ray 62 is within a cone formed about normal 55 and bounded by sides forming angles no greater than angle (p therewith. The reflected light ray is transmitted through layer 14, electrode 12 and substrate 11 to strike the interface between front substrate and the surrounding air at an angle P which is less than the critical angle.The ray is transmitted across the interface and is refracted thereat to emerge from the viewable surface of the display as light ray 64, having its polarization vector 64a and 64b. As the polarization vectors 60a, 60b, 64a and 64b are all orthogonal to the direction of elongation of dye modules 24a'7 relatively little light is absorbed in the liquid crystal layer during the single reflection of the light ray within the cell; ray 64 emerges with substantially greater brightness than is obtained in prior art display cells. Typically, the brightness of a transmitted ray, undergoing several reflections prior to emergence, in a prior art cell is only about 60% of the brightness of the ray emerging from cell 50, with all other conditions being substantially identical.

If switch 26 is open, the liquid crystal material and the guest dichroic dye molecules dissolved therein are caused to align in the homogeneous condition, with the elongated dye molecule axes 24b' parallel to the electrode surfaces, whereby a light ray 66 impingent upon the cell has its orthogonal polarization vectors 66a and 66b parallel to the dye molecule axes 24b' and is substantially absorbed therein, providing contrasting areas.

Referring now to Figs. 3a-3d, one preferred method is described for fabricating a diffuse reflecting layer 54 upon the rear substrate 52. The substrate 52, preferably of glass and the like materials, and of a relatively great thickness (such as to provide stability and uniform cell thickness in large area display cells) is initially provided with a substantially flat front surface 52' (Fig. 3a). The front surface of the substrate is ground to provide a roughened front surface 52" having gouges and cracks 70 across the entire surface thereof (Fig.

3b). We prefer to grind a glass substrate 52 with an aluminum oxide abrasive, having particles of diameter in the range between about 5 and 50 microns, with the abrasive maintained on a glass lapping plate.

The ground plate is then etched (Fig. 3c) in a 10% hydrofluoric acid solution for about 30 minutes at room temperature, to produce the ground-and-etched substrate 52 having the roughened surface 52a thereon. A relatively thin layer 54 of a conductive reflecting material, such as aluminum, silver, and the like, is then deposited, as by evaporation and the like, upon surface 52a of the glass substrate.

Advantageously, reflective, conductive layer 54 has a thickness of about 1,000 angstroms. The resulting reflective surface 54a is formed with a random array of spherical dome-like formations 75 (Fig. 3d) for producing the light-scattering diffuse reflective surface. Typically, the random array of dome-like formations of surface 54a will produce a 50% intensity scattering cone with halfangle of about 300 and with substantially no light scattering beyond a cone characterized by an angle ç of about 400.It should be understood that the maximum angle (p of the light scattering cone is dependent upon the etching time interval utilized; an etching time interval exceeding the optimum etching time interval by more than about 25% produces a reflective surface 54a which is too specular, while an etching time interval shorter than the optimal time interval by about 25% produces a reflector surface which is too diffuse and which has substantial proportions of light scattered therefrom at angles well beyond the critical angle of the liquid crystal layer.

As the reflective layer 54 is formed of a conductive material, the entire surface 54a thereof acts as the rear electrode, directly in contact with the rear surface of liquid crystal layer 14. The rear electrode is easily segmented by forming grooves 78 (Fig. 2) completely therethrough to the insulative substrate 52; the grooves may be of relatively fine dimensions, as the reflective layer 54 is relatively thin.

Claims

1. A reflective liquid crystal display cell comprising a layer of liquid crystal material sandwiched between a pair of electrodes, the interface between one of the electrodes and the liquid crystal material diffusely reflecting light transmitted through the layer of liquid crystal material.

2. A display cell according to Claim 1 in which the diffusely reflecting interface is provided by a random array of spherical dome-like formations on the surface of the electrode.

3. A display cell according to Claim 2 in which the diffusely reflecting electrode is supported on a substrate of insulative material.

4. A display cell.according to Claim 3 wherein the substrate is formed of glass.

5. A display cell according to any one of the preceding claims in which the diffusely reflecting electrode is formed of a metallic material.

6. A display cell according to Claim 5 wherein the metallic material is silver or aluminium.

7. A display cell according to any one of the preceding claims in which the diffusely reflecting electrode has a thickness of about 1 ,000 angstroms.

8. A display cell according to any one of the preceding claims in which the diffusely reflecting electrode has at least one groove formed completely therethrough to define a segment electrode for forming indicia in the display cell.

9. A display cell according to any one of the preceding claims in which the diffusely reflecting electrode comprises a layer of reflective material deposited upon a ground and etched substrate surface.

10. A reflective liquid crystal display cell, comprising: a layer of liquid crystal material having first and second opposed surfaces;an electrode substantially in abutment with the first surface of said layer; a conductive reflector member having a surface formed with a random array of spherical dome-like formations, said surface being substantially in abutment with the second surface of said layer; and means coupled between said electrode and said conductive reflective member for causing said liquid crystal layer to selectively transmit and absorb light passing therethrough.

11. A method for fabricating a reflector member, comprising grinding a surface of a substrate; etching the ground surface of the substrate; and depositing a layer of reflective material upon the ground and etched surface to form a light diffusing surface having a random array of spherical dome-like formations.

12. A method according to Claim 11 wherein the substrate is formed of glass and the grinding step is performed with an abrasive having particle diameters in the range between 5 microns and 50 microns.

13. A method according to Claim 11 or Claim 1 2 wherein the etching includes providing a 10% solution of hydrofloric acid; and etching the ground plate for about thirty minutes at room temperature.

14. A method according to any one of the Claims 11-13 wherein the reflective material is deposited by eyaporating the reflective material onto the surface of the substrate.

15. A method according to Claim 14 wherein the reflective material is silver or aluminum.

16. A method according to any one of the Claims 11-15 wherein the reflective material is deposited to a thickness of about 1,000 a ngstroms.

17. A display cell according to Claim 1 and substantially as herein described with reference to Figs. 2 and 3 of the accompanying drawings.

18. A method according to Claims 11 and substantially as herein described with reference to Fig. 3 of the accompanying drawings.