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WO1998004953A1 - Cristaux liquides a gamme de gris - Google Patents

Cristaux liquides a gamme de gris Download PDF

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Publication number
WO1998004953A1
WO1998004953A1 PCT/GB1997/001943 GB9701943W WO9804953A1 WO 1998004953 A1 WO1998004953 A1 WO 1998004953A1 GB 9701943 W GB9701943 W GB 9701943W WO 9804953 A1 WO9804953 A1 WO 9804953A1
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WIPO (PCT)
Prior art keywords
nucleation
liquid crystal
nucleation points
points
pixels
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Ceased
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PCT/GB1997/001943
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English (en)
Inventor
Carl Vernon Brown
Guy Peter Bryan-Brown
Martin Stuart Bancroft
Paul Edward Dunn
John Clifford Jones
Damien Gerard Mcdonnell
Simon David Haslam
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UK Secretary of State for Defence
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UK Secretary of State for Defence
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Publication of WO1998004953A1 publication Critical patent/WO1998004953A1/fr
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Ceased legal-status Critical Current

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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1343—Electrodes
    • G02F1/13439—Electrodes characterised by their electrical, optical, physical properties; materials therefor; method of making
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1337—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/137—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering
    • G02F1/139—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent
    • G02F1/1391—Bistable or multi-stable liquid crystal cells
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/137—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering
    • G02F1/139—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent
    • G02F1/141—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent using ferroelectric liquid crystals
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2203/00—Function characteristic
    • G02F2203/30—Gray scale

Definitions

  • This invention relates to liquid crystal device with greyscale, particularly bistable devices such as ferroelectric liquid crystal devices, bistable nematic and cholesteric devices and spatial light modulators etc.
  • Liquid crystal display devices are well known. They typically comprise a liquid crystal cell formed by a thin layer of a liquid crystal material held between two glass walls. These walls carry transparent electrodes which apply an electric field across the liquid crystal layer to cause a reorientation of the molecules of liquid crystal material.
  • the liquid crystal molecules in many displays adopt one of two states of molecular arrangement. Information is displayed by areas of liquid crystal material in one state contrasting with areas in the other state.
  • One known display is formed as a matrix of pixels or display elements produced at the intersections between column electrodes on one wall and line (or row) electrodes on the other wall. The display is often addressed in a multiplex manner by applying voltages to successive line and column electrodes.
  • Liquid crystal materials are of three basic types, nematic. cholesteric, and smectic each having a distinctive molecular arrangement.
  • the present invention particularly, but not exclusively, concerns ferroelectric smectic liquid crystal materials.
  • Devices using this material form the surface stabilised ferroelectric liquid crystal (SSFLC) device.
  • SSFLC surface stabilised ferroelectric liquid crystal
  • These devices can show bistability, i.e. the liquid crystal molecules, more correctly the molecular director, adopt one of two aligned states on switching by positive and negative voltage pulses and remain in the switched state after removal of the voltage.
  • the two states can appear as dark (black) and light (white) areas on a display. This bistable behaviour depends upon the surface alignment properties and chirality of the material.
  • bistability property together with the fast switching speed, makes SSFLC devices suitable'for large displays with a large number of pixels or display elements. Such . ⁇ > .
  • ferroelectric displays are described for example in:- N A Clark and S T Lagerwall.
  • bistable devices it is more accurate to call such movement latched or latching where latching is defined as switching with bistability.
  • bistable devices liquid crystal molecules may move under an applied field but unless they remain (or latch) in the new state on removal of applied field, they return to an original state as with monostable devices such as conventional twisted nematic devices. Nevertheless the term switch is often used in bistable devices although the term latch is more appropriate.
  • Temporal dither involves switching a pixel to black for a fraction of a frame time and white for the remainder. Providing the switching speed is above a flicker threshold (e.g. above about 35Hz), a user's eye integrates over a period of time and sees an intermediate grey whose value depends upon the ratio of black to white time.
  • Spatial dither involves dividing each pixel into individually switchable subpixels which may be of different size; each subpixel is sufficiently small at normal viewing distances that subpixels can not be distinguished individually.
  • Analogue greyscale may be described as partly switching a material to a condition lying between fully ON and OFF.
  • a characteristic of SSFLCs is that they switch on receipt of a pulse of suitable voltage amplitude and length of time of application, i.e. pulse width, termed a voltage time product V.t. Additionally when a voltage is applied to electrodes, not all molecules move at the same time.
  • nucleation points are microscopic points within a cell where there is a local change in field and or elastic strain energy variations and or local order parameter variations to either promote or inhibit the latching process.
  • Strength of nucleation points is the change (relative to a uniform case with no nucleation points) in threshold voltage time product required to latch at that nucleation point; strength relates to the magnetude of the effect on the voltage time product.
  • a liquid crystal device cell comprises:
  • nucleation points on the surface of at least one wall, the nucleation points being of a variable strength within at least some of the pixels.
  • the device may also include means for distinguishing between two switched (latched) states of the liquid crystal material.
  • the nucleation points may be provided by small localised surface features such as spheres or projections or spikes or indents or islands on the alignment treatment.
  • the spheres may be of metallic, semi insulating, semi conducting, or insulating material.
  • the alignment treatment may be a rubbing of the cell wall, a rubbing of a polymer layer on the walls, an obliquely evaporated alignment layer, a grating structure formed in a layer of a photoresist material, a grating structure embossed in a plastic layer, a photo-aligning layers, or other topological alignment structure.
  • the variation in strength of nucleation point may be achieved in three ways:
  • the variation in strength of nucleation points may be achieved by photolithographic techniques, or mechanical damage e.g. rubbing of a surface by a cloth, or rolling by a spiked roller, or embossing.
  • the density of nucleation points may vary from a few points per 10 ⁇ m to numerous points per lO ⁇ m .
  • the nucleation points may vary on average between one per 1 O ⁇ m to a thousand or more per 1 O ⁇ T. density being measured as average distance between points.
  • the density may vary uniformly or in a non-linear manner across a pixel, or it may vary differently along an alignment direction to that orthogonal to the alignment direction.
  • the dimension of the localised features may also vary in a similar manner.
  • the electrodes may be arranged to provide sub-pixellation. i.e. a pixel area is divided into a plurality of sub pixels of similar or different areas.
  • the variation of nucleation points may be the same on adjacent pixels or sub pixels, or may vary on adjacent pixels or sub pixels. Areas of the cell between electrodes may have a different density of nucleation points to that within the pixel areas, e.g. the inter pixel gaps may have few nucleation points.
  • the variation in strength of nucleation points may be arranged to be the same or different in different areas of a large display so that overall uniformity of greyscale is obtained.
  • the strength may be different in different areas to compensate for variations in voltage over long electrodes.
  • the liquid crystal material may be a chiral smectic material, e.g. a chiral smectic C* and the device is a surface stabilised ferroelectric liquid crystal device, an electroclinic device with a smectic A material, an antiferroelectric device, a ferrielectric device, or may be a nematic. or long pitch cholesteric material with or without an amount of a dye and'the device is a nematic type device.
  • the present invention exploits the fact that certain features within a cell act as nucleation points in the switching process and the liquid crystal at these nucleation points will switch at a lower applied voltage than that of the surrounding liquid crystal material. Furthermore, the strength of these nucleation points may be varied so that different small areas within a pixel will start to switch at different voltage time products.
  • a number of mechanisms may be responsible for this effect: elastic deformations and variations in the effective field applied as a consequence of the switching voltage; nucleation mechanisms, e.g. disruption of the smectic layer structure giving a localised change in the elastic strain energy, e.g. local variation of electric field such as by dielectric refraction effects or combination of these; and also effective local thickness change of the cell.
  • nucleation mechanisms e.g. disruption of the smectic layer structure giving a localised change in the elastic strain energy, e.g. local variation of electric field such as by dielectric refraction effects or combination of these
  • effective local thickness change of the cell If a switching voltage of sufficient magnitude is applied for a very short time then only the liquid crystal in the immediate vicinity of the nucleation points may change state and small latched domains will result. If the switching voltage is applied for a longer duration then these domains may grow, or at different values of voltage and time domains of different size may be spontaneously created. When these isolated domains within a pixel where
  • Figures "la. lb, are plan and section views of a liquid crystal display device;
  • Figure 2a shows a graph of switching time against applied voltage for a typical ferroelectric liquid crystal device (FLCD),
  • Figure 2b shows a graph of optical transmission against voltage for a given pulse width applied to a typical FLCD
  • Figure 3 shows a diagrammatic view of a portion of the wall of a FCLD
  • Figure 4 shows the result of an atomic force microscopy (AFM) analysis of a portion of rubbed polyimide alignment layer for a FLCD
  • Figure 5 shows the result of AFM analysis of a portion of rubbed polyimide alignment layer to a larger scale than that shown in figure 4
  • Figure 6 shows the result of AFM analysis of a portion of nylon material
  • Figure 7 shows the result of AFM analysis of a portion of nylon material to a larger scale than that shown in figure 6,
  • Figure 8 shows the result of AFM analysis of a portion of nylon alignment layer for use in a FLCD
  • Figure 9 shows a perspective view of AFM analysis of a portion of polyimide alignment layer
  • Figure 10 shows a perspective view of AFM analysis of a portion of nylon material
  • Figure 11 shows a graph of switching time ⁇ against applied voltage for a FLCD having a polyimide alignment layer
  • Figure 12 shows a graph of switching time ⁇ against applied voltage for a FLCD having a nylon alignment layer
  • Figure 13 shows manufacn ⁇ ring steps of an FLCD.
  • the cell 1 shown in Figures 1, 2 comprises two glass walls, 2, 3, spaced about 0.5- 6 ⁇ m apart by a spacer ring 4 and/or distributed spacers. Electrode structures 5, 6 of transparent indium tin oxide are formed on the inner face of both walls. These electrodes may be of conventional line (x) and column (y) shape, seven segment, or an r- ⁇ display. A layer 7 of liquid crystal material is contained between the walls 2, 3 and spacer ring 4.
  • Polarisers 8, 9 are arranged in front of and behind the cell 1. The alignment of the optical axis of the polarisers 8, 9 are arranged to maximise contrast and brightness of the display; i.e. approximately crossed polarisers with one optical axis along one switched molecular direction.
  • a d.c. voltage source 10 supplies power through control logic 1 1 to driver circuits 12, 13 connected to the electrode structures 5, 6, by wire leads 14, 15.
  • the device may operate in a transmissive or reflective mode. In the former light passing through the device e.g. from a tungsten bulb 16 is selectively transmitted or blocked to form the desired display. In the reflective mode a mirror 17 is placed behind the second polariser 9 to reflect ambient light back through the cell 1 and two polarisers. By making the mirror 17 partly reflecting the device may be operated both in a transmissive and reflective mode with one or two polarisers.
  • the walls 2, 3 Prior to assembly the walls 2, 3 are surface treated e.g. by spinning on a thin layer 18, 19 of a polymer such as a polyamide or polyimide or a nylon, drying and where appropriate curing; then buffing with a soft cloth (e.g. rayon) in a single direction Rl, R2.
  • a polymer such as a polyamide or polyimide or a nylon
  • buffing with a soft cloth e.g. rayon
  • This known treatment provides a surface alignment for liquid crystal molecules.
  • the molecules (as measured in the nematic phase) align themselves along the rubbing direction Rl, R2, and at an angle of about 0° to 15° to the surface depending upon the polymer used and its subsequent treatment: see article by S Kuniyasu et al, Japanese J of Applied Physics vol 27. NO 5, May 1988, pp827-829.
  • surface alignment may be provided by the known process of obliquely evaporating e.g.. silicon monoxide onto the cell walls or by grating structures.
  • Grating structures are described for example in WO95/22075, and may be formed photolithographically, with or without polarised light, or by embossing of a plastics layer.
  • profiling an insulating layer alignment may be provided by profiling the electrodes to give both alignment and control of nucleation points.
  • the electrodes may be formed directly on the cell walls or coated onto profile walls of the cell or profiled plastic layers.
  • Suitable liquid crystal materials include catalogue references SCE 8, ZLI-5014-000, available from Merck Ltd, those listed in PCT/GB88/0I004. WO 89/05025. and:- 19.6% CM8 (49% CCI + 51% CC4) + 80.4% Hi
  • CC4 CH 2 C * HC 2 H 5 (0) ⁇ 0)-COO O)-CH 2 C * H C 2H 5
  • Figure 2a shows a graph of switching time ⁇ against applied voltage V for a typical ferroelectric device.
  • a ferroelectric material switches on receiving a voltage time product of suitable value.
  • a voltage time product above the curve results in switching whilst a voltage time product value below the curve will not result in switching.
  • the shape and position of the switching curve depends upon the shape of the applied addressing waveforms. These double switching curves allow greater discrimination between pixels required to switch and those that are not required to switch when a display is multiplex addressed. Pixels required to switch are supplied with a waveform of a shape where the lower curve applies whilst other pixels not required to switch receive a waveform of a shape where the upper curve applies. Both sets of pixels receive a voltage time product that may be above the lower curve, i.e. in the shaded area, but only those pixels receiving the switching shape of waveform will switch.
  • the lower solid curve (100%) gives the minimum time and voltage product required to latch all of the molecules (more correctly the directors) within a pixel into the other state.
  • the broken line (0%) beneath it gives the time and voltage product at which the directors in a pixel will ju£t start to latch.
  • the following discussion will concentrate on the partial switching region of the switching curve, i.e. the portion between the 100% and 0% curves.
  • the partial switching region of the non-switching curve NS may, however, be controlled in an analogous manner.
  • the region of the switching characteristic between the 0% and the 100% curves is utilised.
  • the width of this region is controlled to provide the desired number of grey scales.
  • the width of this partial switching region should be wide. This contrasts with a device without grey scale where the device is required to switch - H -
  • the width of the partial switching region can be made as narrow as possible.
  • the surface of an insulating alignment layer may be profiled with indents or holes of varying depths. Within these indents liquid crystal molecules will switch at a different voltage time product than at the upper part of the alignment layer. For example with some materials, the deeper the indent the higher the electric field at its bottom, and therefore the lower the switching voltage time product at this indent. Conversely, the higher a bump on the alignment surface, the lower the electric field, and the higher the switching voltage time product. Therefore by varying the depths and or number of indents and bumps per unit area (their density) the width of the partial switching region can be varied.
  • FIG. 2b The effect of increasing trie width of the partial switching region is shown in figure 2b.
  • the figure 2b assumes a single pulse width (time of applied voltage) and shows variation of light transmission against applied voltage.
  • An idealised switching curve for a device without greyscale is shown in continuous line. At a certain voltage all molecules switch states. In practice this does not occur and switching occurs over a relatively narrow voltage range shown in chain dotted lines. If the partial switching region is made wide, then some parts of the pixel will switch at a voltage below the normal value whilst others will require a higher voltage to cause switching.
  • the slope and shape of the transmission volta ⁇ e curve can be made lower as shown in dotted line.
  • the illustrated curve could be shifted either to the left or right, or the curve may be made non-linear etc.
  • the shape of the dotted switching curve shows that it is possible to obtain different transmission levels, i.e. greyscales, with different voltages.
  • a similar curve is obtained for a fixed voltage and variable pulse width (address time).
  • Figures 3 to 10 show the effects of different alignment treatments on spacing etc. of nucleation points on a cell wall.
  • Figure 3 shows a diagrammatic representation of a portion of the cell in Figure lb to an enlarged scale.
  • the figure shows a transparent wall 13 and an electrode layer 7 thereon.
  • a barrier layer 19a intended to isolate the electrode 6 from the liquid crystal 7 and above the barrier layer 19a is an alignment layer 19b.
  • the barrier layer 19a and the alignment layer 19b may comprise a single layer 19.
  • the alignment layer 19b in this example comprises rubbed polyimide, for example PI32 (Ciba Geigy). All of the approximate dimensions on the figure are in nanometres. Two observations can be made quickly; the electrode layer 7 is very uneven and the alignment layer 19, while being less uneven than the electrode layer 7 still exhibits some considerable discontinuities.
  • a pixel will typically measure 300 ⁇ m by lOO ⁇ m and a FLCD will typically be 1.5 to 4 ⁇ m in thickness. It will be appreciated by considering figure 3 that certain pixels will have noticeable discontinuities within their boundary such as the rubbing marks and nano-islands shown while other pixels will have rather fewer and smaller discontinuities. These discontinuities cause elastic deformations in the liquid crystal device which permits some areas of the liquid crystal device to change state before other areas, thus providing nucleation points.
  • the roughness of the underlying ITO electrode layer will lead to local variations in the electric field and therefore play an important role in the width of the partial switching region. Where it is desired that the partial switching region be narrow (not as in the present invention), electrode layer should have uniform roughness.
  • Electrode layer increases with the thickness of the electrode layer because it is provided by sputtering. However, this will also increase the size and the density of large spikes. By increasing the length of the sputtering process and polishing the electrode layer to remove these spikes, a more uniformly rough surface is provided.
  • An added benefit of an electrode layer provided in this manner is that a shallower barrier layer may be provided which will reduce the voltage drop across such a layer.
  • FIG 4 shows atomic force microscopy (AFM) results for a rubbed polyimide alignment layer over an area of 5 ⁇ m by 5 ⁇ m.
  • AFM measures the topology of a surface, in other words the degree of roughness.
  • the degree of roughness is depicted by a grey scale and the key is shown on the right hand side of the figure. It can be seen that there is only around lnm of RMS roughness over a distance of 1 ⁇ m.
  • a pair of striations are shown running the length of the AFM plot. These are the rubbing marks which are a consequence of the polyimide layer being rubbed to align the directors of the liquid crystal material.
  • the rubbing marks are widely spaced and are typically several microns or tens of microns apart. While some areas of the layer are comparatively rough and would be expected to provide effective nucleation points for changing the state of a liquid crystal in use, large areas of the AFM result show very little variation in roughness. This means that switching domains would have to grow over larger distances in order to meet one another, increasing the size of the partial switching region.
  • Figure 5 shows AFM results for a 1 ⁇ m square section of a rubbed polyimide layer on a larger scale than those shown in figure 4. Parts of the rubbing marks are again visible in this figure. It can be seen that the rubbing marks provide a high density and reasonably uniform distribution of nucleation points. However, over the remainder of the figure the distribution of such points is extremely varied with some clusters very close together but large areas with no significant topological variation. In addition, most of the potential nucleation points are also small in area.
  • Figure 6 shows AFM results for a 5 ⁇ m square sample of nylon 6,6 which has not been subjected to a rubbing step.
  • the scale of the AFM in this figure is somewhat larger than that of figures 4 and 5 and is shown at the right of the figure.
  • the area of nylon analysed is equal to that of polyimide whose results are shown in figure 4.
  • the roughness is typically 5nm over a 1 ⁇ m distance.
  • Figure 7 shows AFM results for the nylon 6,6 sample over the same area as that used in figure 5. It can be seen from this figure that the potential nucleation points are not only more consistently spaced apart from one another but they are generally of a larger size. It can be seen that the scale of this figure differs slightly from that of figure 6 but the roughness of the surface of the material is still approximately double that of the rubbed polyimide as well as being more uniform.
  • Figure 8 shows AFM results for a sample of rubbed nylon 6,6 over the same area as those shown in figures 4 and 6 but having a different scale. This larger scale is required to cover the combined effects of the intrinsic roughness of the - 15 -
  • Figures 9 and 10 respectively show AFM results in a perspective view for a sample of rubbed polyimide and unrubbed nylon 6,6 over an area of 1 ⁇ m square. Again the topology of the nylon shows not only a greater consistency of roughness but also a greater amplitude of the roughness. Figure 10 shows probably more clearly than the plan-view AFM results the consistent periodic structure of nylon which will be shown to provide a uniform density of nucleation points.
  • Figure 11 shows a series of curves which are equivalent to the curve S sketched in figure 1.
  • the voltage of the applied signal is shown along the X axis and the duration of the signal along the Y axis.
  • a common alternative of 10% and 90% of optical transmission is used in these examples.
  • Three sets of curves are shown in the figure which correspond to different levels of AC stabilisation.
  • AC stabilisation is a technique well known in the use of FLCDs in which an AC voltage of moderate amplitude is applied to each of the pixels of the device constantly when it is not desired to change the state of the pixel. The reasons behind this, and the benefits of so doing are well known to those skilled in the art and do not directly concern the present invention.
  • the curves represent (in descending order):
  • the switching process was observed microscopically and the device with a polyimide aligning layer had few switching nucleation points.
  • the 10% of optical transmission switching level corresponded with just a few domains of switched liquid crystal.
  • Figure 12 represents corresponding results for a FLCD using a rubbed nylon 6,6 alignment layer, the curves in this figure correspond with those identified above (also in descending order). By comparison of the curves shown in figures 11 and 12, it can be seen that the partial switching region of the FLCD using the nylon alignment layer is significantly narrower than that of the rubbed polyimide layer.
  • the present invention further provides for a nucleation site layer or layers to be processed to increase or decrease their nucleation point density prior to inclusion in a ferroelectric liquid crystal device.
  • the invention further provides that the roughness can be electrical in nature rather than partly or purely topological.
  • One way of achieving this is for the electrode layer to be arranged to provide nucleation points by increasing the local electric field. Further, extra material may be used to provide nucleation points in either of the alignment layer or the electrode layer.
  • Figure 13 shows diagrams corresponding to two steps in the manufacture of one example of a liquid crystal device in accordance with the present invention.
  • Figure 13(a) shows a production step to be performed after the substrate has been provided with an electrode layer and a barrier layer.
  • the substrate 12 is arranged to rotate at high speed while a solvent and monomer mixture is spun-down onto the surface thereof. This process is already known to be used to provide an alignment layer.
  • the monomer is polymerised using a known technique, for example ultraviolet irradiation.
  • a mask M of any suitable material is provided on top of the layer and a number of photo-defined holes (or bumps) are provided by photolithography P.
  • the holes (or bumps) may be provided by direct laser lithography.
  • the surface would then contain small photoresist spots separated by regions of the original polymer. Rubbing to provide the alignment for the liquid crystal material is then carried out after applying the dots to the surface although these steps may be reversed.
  • Current patterning techniques would allow dots to be defined with diameters of less than 300nm and technologies are becoming available which will allow lOOnm features to be defined. Varying the depths (heights) or densities of the features in a controlled manner allows the partial switching region to be controlled spatially either from pixel to pixel or within a pixel.
  • This second step shown in figure 13 may be replaced by other treatments such as plasma etching, acid etching or exposure to a combination of ozone and ultraviolet light.
  • plasma etching acid etching
  • ultraviolet light ultraviolet light
  • another technique is to add further material to provide discontinuities in the surface thereof.
  • One such material which has been found to be effective is gold beads having a nominal diameter of 6nm. These beads provide a degree of roughness to the surface of the alignment layer that is comparable to that provided by the nylon alignment layer discussed above.
  • the density of the beads can be controlled by varying the number applied to a given area of the alignment layer.
  • Another material that has been found to be successful are .5 ⁇ m plastics beads. While these beads extend approximately one third of the distance across the device they have not been found to cause adverse effects in terms of the normal operation of the device.
  • Another benefit of using added material in the alignment layer of a FLCD is that the process of adding the material disrupts the alignment layer itself, thus generating further nucleation points.
  • metallic spheres may also be applied to the electrode layer although the size required will be greater than that used in the alignment layer.
  • the sputtering process by which the electrode layer is traditionally produced may be carried out for an extended period to provide a significantly rougher electrode layer. This does tend to result in a number of particular peaks which are t oo high relative to the remainder of the layer. These peaks are then removed by polishing the electrode layer before the barrier layer and/or alignment are added to the wall structure.
  • One suitable technique is ion beam milling and this may be applied to an alignment layer before or after it is rubbed to provide the alignment for the liquid crystal.

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Mathematical Physics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Liquid Crystal (AREA)

Abstract

Un dispositif à cristaux liquides est formé de deux parois cellulaires espacées contenant une couche d'un matériau à cristaux liquides. Des électrodes en rangées et colonnes sur les parois cellulaires, par exemple, forment une matrice de pixels adressables dont l'ensemble présente des informations. Le dispositif a une capacité de gamme de gris, c'est-à-dire, le matériau à cristaux liquides que le dispositif contient peut se verrouiller sur un certain nombre de positions allant d'une position entièrement de MARCHE à une position entièrement d'ARRET. Pour obtenir une gamme de gris, au minimum une paroi cellulaire est munie d'une pluralité de points de nucléation dont la densité varie à l'intérieur au moins de quelques-uns des pixels d'affichage. Un point de nucléation est un point microscopique à l'intérieur d'une cellule dans laquelle il y a un changement local de champ et/ou des variations de l'énergie de déformation élastique et/ou des variations des paramètres d'ordre locaux qui favorisent ou inhibent le processus de verrouillage. La densité des points de nucléation correspond au changement (par rapport à une cellule uniforme dépourvue de points de nucléation) du produit de la tension de seuil par le temps nécessaire pour verrouiller audit point de nucléation. La densité se raporte à la magnitude de l'effet sur le produit de la tension par le temps. La pluralité de points de nucléation peut être obtenue par frottement d'un polymère ou par détourage d'une couche de polymère, par exemple, au moyen d'une technique de photolitographie ou de gauffrage, ou par façonnage d'une surface de la structure d'une électrode.
PCT/GB1997/001943 1996-07-26 1997-07-17 Cristaux liquides a gamme de gris Ceased WO1998004953A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB9615817.5 1996-07-26
GBGB9615817.5A GB9615817D0 (en) 1996-07-26 1996-07-26 Liquid crystal device with greyscale

Publications (1)

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WO1998004953A1 true WO1998004953A1 (fr) 1998-02-05

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Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2338076A (en) * 1998-06-03 1999-12-08 Sharp Kk Manufacture of liquid crystal alignment layer having areas with different alignment properties
GB2373062A (en) * 2001-03-06 2002-09-11 Sharp Kk Nematic Liquid Crystal Devices
EP1421574A4 (fr) * 2001-01-12 2005-06-01 Visible Tech Knowledgy Llc Etiquette electronique intelligente utilisant de l'encre electronique
US7019795B2 (en) 2001-06-20 2006-03-28 Zbd Displays Ltd Liquid crystal device operable in two modes and method of operating the same
EP1674923A3 (fr) * 2004-10-18 2006-08-09 Hewlett-Packard Development Company, L.P. Dispositif d'affichage avec capabilité en niveaux de gris
US7796038B2 (en) 2006-06-12 2010-09-14 Intelleflex Corporation RFID sensor tag with manual modes and functions
US8470411B2 (en) 2002-10-17 2013-06-25 Zbd Displays Ltd. Liquid crystal alignment layer
US8830072B2 (en) 2006-06-12 2014-09-09 Intelleflex Corporation RF systems and methods for providing visual, tactile, and electronic indicators of an alarm condition

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0240010A1 (fr) * 1986-04-02 1987-10-07 Canon Kabushiki Kaisha Dispositif de modulation optique
US4861143A (en) * 1985-08-08 1989-08-29 Semiconductor Energy Laboratory Co., Ltd. Liquid crystal display capable of displaying grey tone
JPH03276126A (ja) * 1990-03-27 1991-12-06 Matsushita Electric Ind Co Ltd 強誘電性液晶パネル
EP0584963A2 (fr) * 1992-07-30 1994-03-02 Canon Kabushiki Kaisha Dispositif d'affichage à cristal liquide
EP0595219A2 (fr) * 1992-10-24 1994-05-04 Sony Corporation Dispositif à cristal liquide ferro-électrique

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4861143A (en) * 1985-08-08 1989-08-29 Semiconductor Energy Laboratory Co., Ltd. Liquid crystal display capable of displaying grey tone
EP0240010A1 (fr) * 1986-04-02 1987-10-07 Canon Kabushiki Kaisha Dispositif de modulation optique
JPH03276126A (ja) * 1990-03-27 1991-12-06 Matsushita Electric Ind Co Ltd 強誘電性液晶パネル
EP0584963A2 (fr) * 1992-07-30 1994-03-02 Canon Kabushiki Kaisha Dispositif d'affichage à cristal liquide
EP0595219A2 (fr) * 1992-10-24 1994-05-04 Sony Corporation Dispositif à cristal liquide ferro-électrique

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Title
PATENT ABSTRACTS OF JAPAN vol. 016, no. 096 (P - 1322) 10 March 1992 (1992-03-10) *

Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2338076A (en) * 1998-06-03 1999-12-08 Sharp Kk Manufacture of liquid crystal alignment layer having areas with different alignment properties
GB2338076B (en) * 1998-06-03 2002-02-20 Sharp Kk Liquid crystal device manufacturing methods
US6559919B1 (en) 1998-06-03 2003-05-06 Sharp Kabushiki Kaisha Liquid crystal device manufacturing methods of controlling a partial switching
EP1421574A4 (fr) * 2001-01-12 2005-06-01 Visible Tech Knowledgy Llc Etiquette electronique intelligente utilisant de l'encre electronique
GB2373062A (en) * 2001-03-06 2002-09-11 Sharp Kk Nematic Liquid Crystal Devices
US7259818B2 (en) 2001-03-06 2007-08-21 Sharp Kabushiki Kaisha Nematic liquid crystal devices comprising surface protrusions formed of anisotropic material for nucleation of an active region
US7019795B2 (en) 2001-06-20 2006-03-28 Zbd Displays Ltd Liquid crystal device operable in two modes and method of operating the same
US8470411B2 (en) 2002-10-17 2013-06-25 Zbd Displays Ltd. Liquid crystal alignment layer
EP1674923A3 (fr) * 2004-10-18 2006-08-09 Hewlett-Packard Development Company, L.P. Dispositif d'affichage avec capabilité en niveaux de gris
US8599326B2 (en) 2004-10-18 2013-12-03 Hewlett-Packard Development Company, L.P. Method of manufacturing a display device with greyscale capability
US7796038B2 (en) 2006-06-12 2010-09-14 Intelleflex Corporation RFID sensor tag with manual modes and functions
US8830072B2 (en) 2006-06-12 2014-09-09 Intelleflex Corporation RF systems and methods for providing visual, tactile, and electronic indicators of an alarm condition

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