Passive ferro-electric liquid crystal display device and method of manufacturing such a device.

0001The invention relates to a liquid crystal device comprising a ferro-electric liquid crystal medium between two supporting plates provided with electrodes, which device has an adjustable light transmissivity.

0002Such a device is suitable, for example as a light shutter.

0003The invention also relates to a display device comprising a ferro-electric liquid crystalline medium between two supporting plates, the first of which comprises a system of row electrodes and the second comprises a system of column electrodes which define pixels at the crossings of row and column electrodes.

0004Such a device is suitable for passively driven matrices having large numbers of lines. Since the contrast is maintained, the device is very suitable for LCD TV, but also for datagraphic applications.

0005A display device of this type is described in EP 0,214,857. In this device the desired transmission states (grey scales) are obtained by switching on a pixel in n successive fields during a weighted period, dependent on the desired transmission value. This means that a picture memory is required and notably in video applications the ferro-electric liquid crystalline medium must switch n times as fast. Moreover, during the second and subsequent selected fields the pixel is never completely switched on, which leads to loss of brightness.

0006One of the objects of the invention is to eliminate these drawbacks by providing a display device in which grey scales can be realised with easily realisable signals on row as well as column electrodes, while the data voltages on the column electrodes can be kept so low that possible crosstalk does not affect or hardly affects the electro-optical transmission value (grey scale) of the display medium. It is another object of the invention to provide a display device in which the use of picture memories is not necessary, while there is less loss of brightness.

0007It is a more general object of the invention to provide a ferro-electric liquid crystal device having a faint transmission/voltage characteristic.

0008The invention is based, <u style="single">inter alia</u>, on the recognition that the threshold voltage in the transmission/voltage characteristic is also determined by the angle of the smectic layers of the ferro-electric molecules with respect to the supporting plates.

0009In addition it is based on the recognition that it is possible, notably in a display device, to give the transmission/voltage characteristic a relatively large transition range for a given pulse width (for example, equal to one line period or a part thereof). The range of data voltages used may be limited to substantially the width of this transition range.

0010A liquid crystal device according to the invention is characterized in that, viewed across the surface of the device, the angle of the smectic layers of the ferro-electric liquid crystal molecules exhibits a spread with respect to at least one of the supporting plates.

0011In a display device according to the invention the angle of the smectic layers of the ferro-electric liquid crystal molecules exhibits a spread with respect to at least one of the supporting plates at least at the area of a part of a row electrode or of a column electrode, defining a pixel as viewed across the part defining the pixel.

0012Due to the spread of this angle, which is preferably distributed uniformly, threshold voltages of local transmission/voltage characteristics are uniformly distributed across the pixel, as viewed on a microscale, which distribution, as viewed on a macroscale, becomes manifest in a transmission/voltage characteristic having a faint slope whose transition range has a width of at least 0.5 Volt. The width of the transition range is understood to mean the voltage range within which the transmission increases or decreases from 10% to 90% of its maximum value.

0013The spread on a microscale can be obtained by a spread in surface roughness. The presence of spacers may play a role in this case. A preferred embodiment of a device according to the invention is therefore characterized in that it comprises at least 10 spacers per pixel.

0014More generally it is found that the creation of areas having a higher threshold voltage (and hence the said spread of the threshold voltage across different areas) is often related to the occurrence of so-called "needle defects". At the area of these needle defects (which are not only introduced by spacer particles but also, for example by a fast cooling of the liquid crystal material or by a rubbing treatment of the orienting surface) the areas with different angles of the smectic layers, or threshold voltages are produced by a treatment in an electric field with a high field strength. The length of the needle defects, which may detrimentally influence the transmission/voltage characteristic, can be reduced by a treatment in an electric field with a low field strength.

0015When manufacturing a device according to the invention it is therefore subjected to at least one or more low-frequency electric field treatments at a low field strength, preceded, if necessary, by a treatment at a high field strength.

0016Low-frequency is herein understood to mean a frequency of 0.1-200 Hz. The duration of the treatment is of the order of 0.5-2 minutes and, likewise as the field strength, it is also dependent on the liquid crystal material used.

0017It has been found that these treatments make the device less sensitive to spreads in the distance between the two supporting plates, because the field strength variations play a role in acquiring said spreads, both during manufacture and during use.

0018The relevant spread can also be obtained by subjecting the pixels after filling to a local thermal treatment (for example, laser irradiation) while traversing the smectic A-smectic C trajectory.

0019The spread may also be obtained mechanically, chemically, magnetically or in another process; in this respect it may be advantageous to give the device a pretreatment introducing inhomogeneities.

0020In order to reduce the influence of previous picture contents as much as possible in the device thus obtained, it is preferably adapted in such a way that a desired transmission state is reached from an extreme transmission state during line selection.

0021The invention will now be described in greater detail by way of example with reference to the accompanying drawings in which <ul id="ul0001" list-style="none"><li>Fig. 1 is a diagrammatic cross-section of a part of a device according to the invention,</li><li>Fig. 2 shows the molecular positions associated with two extreme positions in Fig. 1, while</li><li>Fig. 3 shows diagrammatically a pixel of the device,</li><li>Fig. 4 shows the transmission/voltage characteristics of the pixels of Fig. 3 and the transmission/voltage characteristic of the entire pixel, and</li><li>Fig. 5 shows a further modification.</li></ul>

0022Fig. 1 shows diagrammatically a part of a surface-­stabilized ferro-electric display device 1 according to the invention. Row and column electrodes or other electrodes (not shown) of conducting materials such as, for example, indium-tin-oxide (ITO) are provided on the two supporting plates 2 and 3 of, for example glass or quartz. The device also comprises polarizers which are not shown.

0023An optimum switching effect between a light-transmissive and a light-absorbing state is obtained if in both switching states the liquid crystal molecules are located, as it were, in parallel layers 6 perpendicular to the supporting plates (the so-called "bookshelf geometry") (situation a in Fig. 1). Such a surface-stabilized ferro-­electric liquid crystal device can be obtained with a small thickness of the layer of liquid crystal material 5 and suitable orientation layers 4 on the supporting plates 2 and 3. The orientation layers 4 of, for example polyimide, polyamide or polyvinyl alcohol are subjected to a suitable rubbing treatment whereafter the liquid crystal material (in this example ZLI 3654 of the firm of Merck) is introduced in the isotropic phase into the display device, whereafter it is slowly cooled down <u style="single">via</u> the nematic phase and the smectic A phase to the smectic C phase. After cooling, the layers 6 in a device according to the invention do not have the completely perpendicular structure, but as a result of interactions of the liquid crystal molecules with the orienting layers 4 they have a kinked or "chevron" structure. These layers 6′, 6˝ and 6 extend at an angle δ at the location of the supporting plates 2, 3 with respect to the normal on the supporting plates 2 and 3.

0024The molecules of the ferro-electric material, whose directors move in principle along the surface of a cone with its axis perpendicular to the layers 6, are differently aligned along the orienting layer 4 dependent on the angle δ in a surface-stabilized structure. This will be explained in greater detail with reference to Fig. 2 showing that the liquid crystal molecules are anchored for a layer 6 (δ = 0) with all molecules in a layer perpendicular to the supporting plate 3, and for a layer 6′ which is aligned in such a way that the axis of the cone along which the molecules can move extends at an angle δ with respect to the supporting plate 3. Due to the action of the orienting layer 4, the possible positions of the molecules (director) on the cone are limited in both situations in the fieldless state to the positions (a) and (b) in the layer 6, and (a′) and (b′) in the layer 6′. Optically, this means that the two states have different switching angles 2ϑ1 and 2ϑ2. When switching between two optical states, the directors flip over, for example from position a(a′) to position b(b′).

0025The angle ϑ can be optically determined by determining the value of ϑp for minimum intensity in transmission. (ϑp: angle of one of two mutually perpendicular polarizers with respect to the rubbing direction). In that case, ϑ = ϑp.

0026In a measuring arrangement, areas in a surface-­stabilized ferro-electric display device with different angles ϑ (or different angles δ) were mutually compared on their switching behaviour. For measuring areas having a surface of approximately 1 µm², inter alia, the switching times, the optical switching angles and the transmission/voltage characteristics were measured. When reducing the angle δ (or increasing the angle ϑ) a longer switching time is found at a fixed pulse amplitude, which may be explained by the fact that the director during switching must travel a longer path across the switching cone, as is shown in Fig. 2. Also the coupling of the electric field and the permanent polarization of the director may play a role in this respect. On the other hand, a reduction of the angle δ upon measurement with a fixed pulse duration was accompanied by an increase of the threshold voltage.

0027Fig. 3 shows (partly or not) a pixel 7 which is subdivided into sub-areas 8a ..., 8b ...., 8c ..., 8d .... at which the value of the angle δ decreases from 8a to 8d. The sub-areas may be mutually arranged in different manners, as is shown in Figs. 3a and 3b. A more arbitrary distribution is also possible.

0028Fig. 4 shows the transmission/voltage characteristics 9a, 9b, 9c, 9d associated with the sub-areas 8a, 8b, 8c, 8d, measured at a fixed pulse duration, and the transmission/voltage characteristic associated with the overall area.

0029It is apparent from the Figure that the distribution of sub-areas having steep transmission/voltage characteristics and different threshold voltages across a pixel leads to a transmission/voltage characteristic having a wide transition range Ca less steep slope) for the entire pixel, which provides the possibility of adjusting grey scales. The curve 10 can be explained from the fact that for a uniform distribution of the sub-areas 8 the areas with a low threshold voltage switch first at an increasing voltage, subsequently those with a slightly higher voltage, etc. The width of the transmission/voltage characteristic 10 may thereby be adjusted at a value of approximately 0.5-10 Volts. To mitigate the detrimental influence of previous picture contents as much as possible, a pixel is brought to an extreme state before it is driven, as is described in greater detail in NL-A 870340 (PHN 12.352). Other drive schemes, such as the so-called "AC stabilisation" method, are alternatively possible.

0030The variation in the angle δ (or in the apex angle of the "chevron") can be obtained in different manners. When cooling the ferro-electric liquid crystal material from the isotropic phase to the smectic C phase, so-called needle defects are usually produced. These needle defects are produced generally at the location of inhomogeneities such as, for example, spacer spheres and are comparable with so-called "zigzag defects" at which the angle δ within the defect is opposed to that outside it.

0031An electric voltage treatment at a low frequency, in the relevant example a square-wave voltage of ± 2 V, 20 Hz for 2 minutes causes these needle defects to shrink to minimum dimensions (< 10 µm). In a subsequent treatment at a higher voltage (± 10 V, 20 Hz, 1 minute) areas with different δ will grow, generally starting at one of the remainders of the needle defects. A uniform distribution of such areas can thus be obtained by increasing the number of needle defects, for example, by a large number of spacer particles (> 10 per pixel which may be uniformly distributed, for example, by means of photolithographic methods or other methods), accelerated cooling or a special rubbing treatment (notably a low-energy rubbing treatment).

0032It is found that a larger threshold voltage spread of the areas and hence a faint transmission/voltage characteristic of the device can be obtained by subjecting the devices subsequently to treatments with different electric voltages, for example, first ± 12.5 V, ± 15 V, ± 17.5 V, ± 20 V, all at 20 Hz and for 1 minute. By variation of these process parameters (voltage variation, frequency, etc.) the threshold voltage and the slope of the transmission/voltage characteristic can be adjusted, dependent on the ferro-electric material and the thickness of the liquid crystal (approximately 1.6 µm in this case).

0033The threshold voltage spread can also be introduced by firstly giving the device a low-frequency alternating voltage treatment at a fairly high voltage (in the example of ZLI 3654, for example, ± 15 V, 20 Hz, for 1 minute) so that many extra regions with a varying δ are produced on the sides of the needle defects, and by subsequently reducing the still present needle defects for a large part by means of a treatment at a low electric voltage (± 2 V, 20 Hz, 2 minutes). Areas with different δ are then distributed across a pixel in such a way that a faint transmission/voltage curve is obtained.

0034If necessary, the treatment may be continued by means of an extra treatment at a higher voltage (approx. 25 V, approx. 30 V) for 1 to 5 minutes. Although the distribution over different angles δ now becomes slightly narrower (closer to 0^o), so that a less wide transition range in the transmission/voltage curve is obtained, the maximum transmission is higher which results in a better contrast. Moreover, the device is now less sensitive to crosstalk.

0035The variation of the angle δ can also be obtained by varying the rubbing pressure during rubbing of the orientation layer 4, or by using a spread in the rubbing energy.

0036The orientation layer 4 may also be composed of, for example polymer types which cannot be fully mixed, or certain reactions (for example, curing of the orientation layer) can be influenced chemically or thermally or through irradiation in different manners. The orientation layer may also be obtained by (oblique) vapour deposition of silicon oxide or by other known techniques.

0037In addition, a combination of an electric field treatment and a chemical treatment of the liquid crystal material may be considered whereby this variation can be obtained, notably treatments influencing the viscosity and the elastic constants and/or the spontaneous polarization.

0038An electrical, magnetical or mechanical treatment may also be considered, by which the smectic layers are deformed, or a thermal treatment of temporary heating or cooling the sub-areas or of providing a thermal gradient.

0039A combination of said methods is alternatively possible.

0040Instead of being used in a matrix-oriented display device, a device according to the invention may alternatively be used in other applications where a faint transmission/voltage curve is desired, for example, in dashboard display devices.

0041Although in the example described only smectic layers having a so-called "chevron structure" are used, the layers 6 need not necessarily have a kink but may alternatively be planar (layers 6ⁱⁱ in Fig. 5), with the angles of the layers 6ⁱⁱⁱ exhibiting a spread with respect to the supporting plates. Moreover, the kink of the "chevron" need not necessarily be right in the centre of the smectic layer (see 6^iv in Fig. 5).