Monostable ferroelectric active-matrix display

Details Monostable ferroelectric active-matrix display Monostable ferroelectric active-matrix display

2001-03-06

2004-03-09

Parker, Kenneth (Department: 2871)

Liquid crystal cells, elements and systems

With specified nonchemical characteristic of liquid crystal...

Within smectic phase

C349S191000

Reexamination Certificate

active

06704086

ABSTRACT:

Replacing the cathode ray tube (picture tube) by a flat screen requires a display technology which simultaneously makes it possible to achieve a high picture resolution, i.e. more than 1000 lines, a high picture brightness (>200 Cd/m
2
), a high contrast (>100:1), a high frame frequency (>60 Hz), an adequate color representation (>16 million colors), a large picture format (>40 cm screen diagonal), a low power consumption and a wide viewing angle and, moreover, can be produced cost-effectively. Until now, there has been no technology which fully satisfies all these features simultaneously.
Many manufacturers have developed screens which are based on nematic liquid crystals and have been used in recent years in the field of notebook PCs, personal digital assistants and desktop monitors. Use is made here of the technologies STN (Supertwisted Nematics), AM-TN (Active Matrix—Twisted Nematics), AM-IPS (Active Matrix—In Plane Switching), AM-MVA (Active Matrix—Multidomain Vertically Aligned), which are extensively described in the literature, see e.g. T. Tsukuda, TFT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach 1996, ISBN 2-919875-01-9 and the literature cited therein; SID Symposium 1997, ISSN-0097-966X, pages 7 to 10, 15 to 18, 47 to 51, 213 to 216, 383 to 386, 397 to 404, and the literature cited therein. Furthermore, use is made of the technologies PDP (Plasma Display Panel), PALC (Plasma Addressed Liquid Crystal), ELD (Electro Luminescent Display) and FED (Field Emission Display), which are likewise explained in the SID report cited above.
Clark and Lagerwall (U.S. Pat. No. 4,367,924) have been able to show that the use of ferroelectric liquid crystals (FLCs) in very thin cells results in opto-electrical switching or display elements which have switching times which are faster by a factor of up to 1000 compared with conventional TN (“twisted nematic”) cells, also see EP-A 0 032 362. On the basis of this and other favorable properties, e.g. the possibility of bistable switching and the fact that the contrast is virtually independent of the viewing angle, FLCs are fundamentally suitable for areas of application such as computer displays and television sets, as shown by a monitor marketed in Japan by Canon since May 1995.
The use of FLCs in electro-optical or fully optical components requires either compounds which form smectic phases and are themselves optically active, or the induction of ferroelectric smectic phases by doping compounds which, although forming such smectic phases, are not themselves optically active, with optically active compounds. In this case, the desired phase should be stable over the broadest possible temperature range.
The individual pixels of an LC display are usually arranged in an x-y matrix formed by the arrangement of a respective series of electrodes (conductor tracks) along the rows and columns on the lower or upper side of the display. The points of intersection of the horizontal (row) and vertical (column) electrodes form addressable pixels.
This arrangement of the pixels is usually referred to as a passive matrix. For addressing, various multiplex schemes have been developed, as described for example in Displays 1993, vol. 14, No. 2, pp 86-93 and Kontakte 1993 (2), pp. 3-14. Passive matrix addressing has the advantage of simpler production of the display and associated low production costs, but the disadvantage that passive addressing can only ever be effected line by line, which results in the addressing time for the entire screen with N lines being N times the line addressing time. For customary line addressing times of approximately 50 microseconds, this means a screen addressing time of approximately 60 milliseconds in e.g. the HDTV standard (High Definition TV, 1152 lines), i.e. a maximum frame frequency of approximately 16 Hz. The latter frequency is too low for the representation of moving images. In addition, the representation of gray shades is difficult. On the occasion of the FLC Conference in Brest, France (Jul. 20-24, 1997, see Abstract Book 6th International Conference on Ferroelectric Liquid Crystals, Brest/France), Mizutani et al. presented a passive FLC display with digital gray shades in which each of the RGB pixels (RGB=red, green, blue) was subdivided into sub-pixels, thereby allowing the representation of gray shades in digital form by means of partial switching. With N gray shades using three primary colors (red, green, blue), 3
N
colors are produced. The disadvantage of this method is the considerable increase in the number of required screen drivers and thus in the costs. In the case of a screen shown at Brest, three times as many drivers were required as in the case of a normal FLC display without digital gray shades.
In so-called active matrix technology (AMLCD), a non structured substrate is usually combined with an active matrix substrate. An electrically non linear, element, for example a thin-film transistor, is integrated into each pixel of the active matrix substrate. The non linear element may also be diodes, metal-insulator-metal and similar elements, which are advantageously produced by thin-film processes and are described in the relevant literature, see e.g. T. Tsukuda, TFT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach 1996, ISBN 2-919875-01-9, and the literature cited therein. Active matrix LCDs are usually operated with nematic liquid crystals in TN (twisted nematics), ECB (electrically controlled birefringence), VA (vertically aligned) or IPS (in plane switching) mode. In each case, the active matrix generates an electric field of individual strength on each pixel, producing a change in orientation and thus a change in birefringence, which is in turn visible in polarized light. A severe disadvantage of these processes is the poor video capability caused by the excessively long switching times of nematic liquid crystals.
For this and other reasons, liquid crystal displays based on a combination of ferroelectric liquid crystal materials and active matrix elements have been proposed, see e.g. WO 97/12355 or Ferroelectrics 1996, 179, 141-152, W.J.A.M. Hartmann, IEEE Trans. Electron. Devices 1989, 36, (9; Pt. 1), 1895-9, and Dissertation Eindhoven, the Netherlands 1990.
Hartmann utilized a combination of the so-called “quasi-bookshelf geometry” (QBG) of FLC and a TFT (Thin-Film Transistor) active matrix and simultaneously achieved a high switching speed, gray shades and high transmission. However, the QBG is not stable over a wide temperature range, since the temperature dependence of the smectic layer thickness disrupts or rotates the field-induced layer structure. Furthermore, Hartmann utilizes an FLC material having a spontaneous polarization of more than 20 nC/cm
2
, which, in the case of pixels having realistic dimensions of e.g. 0.01 mm
2
area, results in large electric charges (at saturation, Q=2 A P, A=pixel area, P=spontaneous polarization) which, e.g. using amorphous silicon TFTs that can be produced cost-effectively, cannot reach the pixel during the opening time of the TFT. For these reasons, this technology has not been further pursued hitherto.
While Hartmann utilizes the charge-controlled bistability to display a virtually continuous gray scale, Nito et al. have proposed a monostable FLC geometry, see Journal of the SID, 1/2, 1993, pages 163-169, in which the FLC material is oriented with the aid of comparatively high voltages in such a way that only one stable position results, from which a number of intermediate states are then generated by application of an electric field via a thin-film transistor. These intermediate states correspond to a number of different brightness levels (gray shades) when the cell geometry is matched between crossed polarizers.
One disadvantage of this procedure, however, is the occurrence of a streaky texture in the display, which limits the contrast and brightness of this cell (see FIG. 8 in the abovementioned citation). Although the disadvantageous

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