GB2315876A

GB2315876A - Liquid crystal device

Info

Publication number: GB2315876A
Application number: GB9715839A
Authority: GB
Inventors: Martin Stuart Bancroft; Carl Vernon Brown; Guy Peter Bryan-Brown; Paul Edward Dunn; Simon David Haslam; John Clifford Jones; Alan John Pidduck; Malcolm Orr
Original assignee: UK Secretary of State for Defence; Sharp Corp
Current assignee: UK Secretary of State for Defence; Sharp Corp
Priority date: 1996-07-26
Filing date: 1997-07-25
Publication date: 1998-02-11
Also published as: GB9615732D0; JPH1090652A; GB9715839D0

Abstract

A ferroelectric liquid crystal device comprises polarisers (10,26), transparent cell walls (12,24), electrode structures (14,22) and alignment surfaces (16, 20) containing a liquid crystal (18). The device is arranged into a plurality of pixels. In order to reduce the time taken for the liquid crystal to change state, at least one nucleation site layer is included. Either of the alignment layers or the electrode structures might comprise a nucleation site layer. The purpose of the nucleation site layer is to provide points from which the change of state of the liquid crystal molecules tends to spread out. The nucleation site layer thus allows the state of a liquid crystal device to be changed more quickly. Each pixel of the device is associated with a plurality of nucleation sites. The nucleation site layer may be provided, for example, by treatment of the layer or by adding further particles to it. The nucleation points can be the result of physical and/or electrical disturbances.

Description

Liquid Crvstal Device The present invention relates to a liquid crystal device comprising a plurality of pixels and having a small partial switching region. The invention also relates particularly to a method of controlling the nucleation point density of a ferroelectric liquid crystal device.

One type of bistable liquid crystal device based on ferroelectric liquid crystal materials is the surface stabilised ferroelectric liquid crystal device (SSFLCD). These devices are interesting because they can be switched between two states by DC pulses of alternate sign and exhibit bi-stability. In other words they will remain in a particular state in the absence of a drive voltage until a DC pulse corresponding to the other state is applied.

This is in contrast to a twisted nematic LCD in which a drive signal must continue to be applied to maintain the device in one of its states. SSFLCDs are of particular interest for devices used in multiplexed applications where the level of multiplexing is not restricted by requirements to re-address particular pixels within a very short time period.

One particular feature of SSFLCDs is the partial switching region of the device characteristics. In order to change the state of a SSFLCD it is necessary to apply a voltage signal of particular magnitude and polarity across the device for a finite duration. To some extent these parameters are interchangeable, for example the voltage applied may be decreased in conjunction with an increase in the duration of application and vice versa. Reduction of the voltage and/or the duration of the signal applied across the device is desirable to reduce the expense of driving circuitry, the addressing time or heating effects and so on. However, this can result in partial switching of the device, in other words only a part of the liquid crystal in the relevant area of the device changes state. Where the device is arranged as a display device and the two fully-switched states correspond to black and white respectively, partial switching results in the appearance of grey.

The size of the partial switching region of a typical ferroelectric liquid crystal device causes this 'grey' region to occur too readily.

It is an object of the present invention to provide a bistable liquid crystal device having a reduced partial switching region.

It is a further object of the present invention to provide a method of reducing the partial switching region of a bistable liquid crystal device.

According to a first aspect of the present invention there is provided a liquid crystal device comprising a liquid crystal material contained between a pair of wall structures, each wall structure comprising a respective electrode layer arranged to address a plurality of pixels and a further layer between the electrode layer and the liquid crystal material, wherein at least one of the wall structures includes a nucleation site layer for providing a plurality of nucleation points within each pixel.

According to a second aspect of the present invention there is provided a method of controlling the nucleation point density in a liquid crystal device comprising processing a nucleation site layer by at least one of rubbing, acid etching, photolithography, plasma ashing, embossing, addition of small particles, ion beam milling and exposure to ultraviolet light in combination with ozone.

The nucleation site layer may comprise a specially provided layer or it may comprise one of the layers of the wall structure that also performs another function. For example, the electrode layer may comprise Indium Tin Oxide (ITO) electrode tracks arranged to provide the plurality of nucleation points. The nucleation site layer may comprise a layer that contacts the liquid crystal material, for example a layer of material provided also to align the liquid crystal material. The nucleation site layer may comprise a particular material selected or processed to provide the required density of nucleation points per pixel. Such processing may comprise rubbing, acid etching, photolithography, plasma ashing, addition of small particles, ion beam milling and application of ultraviolet light and ozone in combination. Where at least one of the electrode layers comprises the nucleation site layer, ITO electrodes may be sputtered to provide irregularities in their height that provide the required density of nucleation points by way of local field strength variations.

The device and method of the present invention are based upon an understanding of the effect of the application of an electric field to a ferroelectric liquid crystal device (FLCD) and the switching process that results. A FLCD typically has a plurality of pixels which are defined by the electrode structures arranged on either side of the liquid crystal material. One arrangement is a large area display in which a first plurality of electrodes are arranged on a first wall of the device and extend in a first direction while a second plurality of electrodes are arranged on a second wall and extend at right angles to the plurality of first electrodes. The area at which a first electrode and a second electrode intersect defines a pixel and every pixel within the device can be addressed using known multiplexing techniques. The present invention is equally applicable to other bistable liquid crystal devices, particularly FLCDs, such as seven segment displays and shutter arrangements for use in laser printers and the like.

During the switching of the liquid crystal within an FLCD from one state to the other, not all of the molecules or directors change state simultaneously. The present invention exploits a realisation that certain features of the device act as nucleation points in the switching process and the liquid crystal material in the immediate vicinity of these points will alter its state more readily than parts of the device more distant from these points. Thus, the liquid crystal at these nucleation points will switch at a lower applied voltage time product than that of the surrounding liquid crystal and, while the switching voltage is applied, the domain of the device that changes state grows outwardly from these points. Two mechanisms are expected to be mainly responsible for this effect; elastic deformations and variations in the effective field applied as a consequence of the switching voltage. 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 will change state and small switched domains will result. If the switching voltage is applied for a longer duration then the domain over which the liquid crystal changes state will be greater. When these isolated domains within a pixel where the liquid crystal has changed state all meet one another, the pixel will have fully changed state.

Thus by increasing the density of nucleation points within a FLCD, the size of the partial switching region can be reduced. For a two-state device (whether monochrome or comprising means to provide colour) particularly a multiplexed large area array, speed of switching is likely to be important. Fast switching of such a device between the two states is enhanced by arranging for the device to have a narrow partial switching region. By arranging that the distance between nucleation points in the nucleation site layer is small (in other words the density of the nucleation points is large), the switched domain around each nucleation point does not have to grow very far before full switching of a pixel is achieved. Consequently, a pixel will change state in a shorter period of time thus giving faster switching. Furthermore, since the 0% and 100% curves for both switching and non-switching (Figure 1) are brought closer together, the operating range (hatched region) may be increased.

The invention includes a number of techniques by which the partial switching region of a FLCD is controlled. These include nucleation points provided by selection of materials, treatment of the layers carried on the walls of the device and the addition of material to cause disruptions in these layers. A nucleation site layer may be provided on one wall of the device only or as part of both of the wall structures. When the nucleation site layer comprises an alignment layer it is important to consider the effect of the nucleation points on the alignment layer. The nucleation points should not disrupt the alignment noticeably since this might make the device more susceptible to mechanical damage.

While the invention is described in relation to a display device, it is equally applicable to shutter devices used, for example, in laser printers and plain paper facsimile machines.

The invention will now be described, by way of example, with reference to the accompanying figures, in which: Figure 1 shows a graph of switching time against applied voltage for a typical ferroelectric liquid crystal device, Figure 2 shows a simplified elevational view of a ferroelectric liquid crystal device, Figure 3 shows a diagrammatic cross-sectional view of a portion of the wall of a ferroelectric liquid crystal device, Figure 4 shows the result of an atomic force microscopy (AFM) analysis of a portion of rubbed polyimide alignment layer for a ferroelectric liquid crystal device, 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 selected in accordance with the present invention, 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 in accordance with the invention, 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 selected in accordance with the present invention, Figure 11 shows a graph of switching time T against applied voltage for a FLCD having a polyimide alignment layer, Figure 12 shows a graph of switching time T against applied voltage for a FLCD having a nylon alignment layer, and Figure 13 shows manufacturing steps of an FLCD in accordance with the present invention.

Figure 1 shows a graph of switching time T against applied voltage V for a typical ferroelectric liquid crystal device. The lower pair of curves S (solid and broken lines) relate to the switching signal applied to a pixel and the upper pair of curves NS relate to the non-switching signal. The lower solid curve (100%) gives the minimum time and voltage product required to switch all of 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 just start to switch. The upper curve is of significance when a FLCD is addressed using a multiplex technique and can be used to ensure that undesired switching of pixels does not take place during addressing of pixels in the same row or column of the device. The following discussion will concentrate on the partial switching region of the switching curves. The partial switching region of the nonswitching curve NS may, however, be reduced in an analogous manner.

Figure 2 shows a simplified side elevational view of part of a ferroelectric liquid crystal device. The device comprises a polariser 10, a first transparent cell wall 12 which has an electrode structure 14 on its lower face and an alignment surface 16 immediately below the structure 14. The ferroelectric liquid crystal material 18 is contained between the surface 16 and a further alignment surface 20. Beneath the surface 20 is a further electrode structure 22 and a second transparent cell wall 24. A second polariser, or analyser 26 is arranged beneath the wall 24. The ferroelectric liquid crystal material is also chosen to suit the device application, one example is SCE8 available from Merck, Merck House, Poole, U.K. - now available from Hoechst Aktiengesellscaft, Frankfurt am Main, Germany. The alignment surfaces 16 and 20 will be described in greater detail below.

The manufacture and operation of ferroelectric liquid crystal devices is known and further details of device construction (except insofar as it pertains to the present invention), liquid crystal filling, alignment of the liquid crystal, driving circuitry and so on will not be discussed further.

Figure 3 shows a diagrammatic representation of a portion of the wall structure of a FLCD. The figure shows a transparent substrate 12 and an electrode layer 14 thereon.

Above the electrode layer is a barrier layer intended to isolate the electrode layer from the liquid crystal and above the barrier layer is an alignment layer 16. The barrier layer and the alignment layer may comprise a single layer. The alignment layer 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 is very uneven and the alignment layer, while being less uneven than the electrode layer still exhibits some considerable discontinuities.

The present inventors have appreciated that these discontinuities have a particular effect on the switching performance of the FLCD. A pixel will typically measure 300us by 100 > m and a FLCD will typically be 1500nm 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. Since it is desired that the partial switching region be narrow, this will result from a uniformly rough electrode layer. Surface roughness 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 shallower barrier and alignment layers may be provided which will reduce the voltage drop across such layers.

It can be understood from figure 3 that while the discontinuities arising from the alignment layer and the electrode layer provide potential nucleation points for the switching of the liquid crystal, these discontinuities are so widely spaced and randomly located that there is no control over the switching performance of the device.

Figure 4 shows atomic force microscopy (AFM) results for a rubbed polyimide alignment layer over an area of 5 um by 5 clam. AFM measures the topology of a surface, in other words the degree of roughness. The degree of roughness is depicted by shades of grey 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 Clam. In addition, 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 lum 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 Sum 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 lum distance. By comparison with figure 4 it will be appreciated that the potential nucleation points for this material are higher in number and larger (the material is more rough) and also that the distribution of such points is substantially more even (the roughness is more consistent). It will be shown later with reference to figures 11 and 12 that this has a profound effect on the partial switching region of the device.

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 roughness 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.

We now consider whether the regular topology and potential nucleation point properties of the nylon layer are maintained if the layer is rubbed to provide an alignment layer.

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 nylon and the roughness induced by the rubbing process. Six rubbing lines are visible in the figure running substantially from the top to the bottom of the figure. While the density of potential nucleation points is increased along the rubbing marks, the density of such points away from the rubbing marks is still substantial and evenly distributed.

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 lum 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.

From these results it would be expected that the partial switching region of a FLCD using an alignment layer of nylon would exhibit a smaller partial switching region than one using an alignment layer of rubbed polyimide. Figures 11 and 12 show the results of such a comparative test.

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. Instead of using a measurement of 0% and 100% of optical transmission to provide the results 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. In figure 11, the curves represent (in descending order): 1) Vac=7.5V 90% switched 2) Vac=7.5V 10% switched 3) Vac=5.0V 90% switched 4) Vac=5.0V 10% switched 5) Vac=0V 90% switched 6) Vac=0V 10% switched 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 fairly large 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.

From this experimental switching data taken on nylon and polyimide devices filled with SCE8 it is seen that a typical polyimide cell showed a At of 5.9us at a tm,r, of 42.4 s (At is defined as the time difference between the 10% switching and 90% optical transmission switching points). The corresponding nylon cell showed a At of 2.9us at a tmin of 46.5,us. Hence the partial switching region of the nylon cell is roughly half that of the polyimide cell. Thus a uniform, high density of nucleation points in the alignment layer provides a partial switching region of reduced width. In contrast to the microscopic examination of the switching process in the device having a polyimide alignment layer, the 10% level of optical transmission of the nylon-aligned device was found to correspond with a large number of relatively small switched domains.

As alternatives to selection of a suitable material to form an alignment 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. Once the layer is spun down the monomer is polymerised using a known technique, for example ultraviolet irradiation. Then 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. Alternatively, the holes (or bumps) may be provided by direct laser lithography.

After development, 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 100nm 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.

This second step shown in figure 13 may be replaced by other treatments such as plasma ashing, acid etching or exposure to a combination of ozone and ultraviolet light.

These techniques, which are well known and will not be discussed further here, are suitable for controlling the overall density of nucleation points on the alignment layer.

In an aspect of the invention alternative to treatment of the alignment layer itself, 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 metal, for example 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. In addition, the density of the beads can be controlled by varying the number applied to a given area of the alignment layer. Larger beads may be used, for example beads having a nominal diameter of no more than 100nm. Another material that has been found to be successful are 0.Sum 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.

The addition of metallic spheres may also be applied to the electrode layer although the size required will be greater than that used in the alignment layer. Alternatively, 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 too 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.

Claims

CLAIMS:

1. A liquid crystal device comprising a liquid crystal material contained between a pair of wall structures, each wall structure comprising a respective electrode layer arranged to address a plurality of pixels and a further layer between the electrode layer and the liquid crystal material, wherein at least one of the wall structures includes a nucleation site layer for providing a plurality of nucleation points within each pixel.

2. A liquid crystal device as claimed in claim 1, wherein the nucleation site layer comprises a substantially uniform density of nucleation points.

3. A liquid crystal device as claimed in claim 1 or claim 2, wherein the nucleation site layer is arranged to increase the density of nucleation points.

4. A liquid crystal device as claimed in claim 3, wherein the nucleation site layer is roughened by a process selected from rubbing, acid etching, photolithography, plasma ashing, embossing and exposure to ultraviolet light in combination with ozone.

5. A liquid crystal device as claimed in claim 3, wherein the nucleation site layer comprises added material.

6. A liquid crystal device as claimed in claim 5, wherein the added material comprises at least one of metal spheres and plastics spheres.

7. A liquid crystal device as claimed in claim 6, wherein the added material comprises metal spheres having a diameter of 100nm or less.

8. A liquid crystal device as claimed in claim 6, wherein the added material comprises plastics spheres having a diameter of 500nm or less.

9. A liquid crystal device as claimed in any one of the claims 1 to 8, wherein the nucleation site layer comprises the alignment layer.

10. A liquid crystal device as claimed in any one of the claims 1 to 7, wherein the nucleation site layer comprises the electrode layer.

11. A liquid crystal device as claimed in any one of the claims 1 to 10, wherein both of the wall structures include a nucleation site layer.

12. A liquid crystal device as claimed in any one of the claims 1 to 11, wherein the device comprises a display device.

13. A liquid crystal device as claimed in any one of the claims 1 to 12, wherein the device comprises a ferroelectric liquid crystal device.

14. A method of controlling the nucleation point density in a liquid crystal device comprising processing a nucleation site layer by at least one of rubbing, acid etching, photolithography, plasma ashing, embossing, addition of small particles, ion beam milling and exposure to ultraviolet light in combination with ozone.

15. A method of controlling the nucleation point density in a liquid crystal device as claimed in claim 14, wherein the liquid crystal device comprises a ferroelectric liquid crystal device.

16. A liquid crystal device substantially as hereinbefore described with reference to figures 2 to 13 of the accompanying drawings.

17. A method of controlling a nucleation point density substantially as hereinbefore described with reference to figures 2 to 13 of the accompanying drawings.