Ferroelectric active matrix displays with wide operating...

Details Ferroelectric active matrix displays with wide operating... Ferroelectric active matrix displays with wide operating...

2001-09-24

2004-06-08

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

With viewing layer of specified composition

C252S299610, C252S299620, C252S299630, C252S299670, C544S298000, C544S303000, C544S333000, C546S001000, C548S136000

Reexamination Certificate

active

06746731

ABSTRACT:

Replacement of the cathode ray tube with a flat panel screen requires a display technology which simultaneously makes it possible to achieve a high resolution, i.e. more than 1 000 lines, a high brightness (>200 Cd/m
2
), a high contrast (>100:1), a high frame rate (>60 Hz), an adequate color representation (>16 million), a large image format (>40 cm), a low power consumption and a wide viewing angle, at low production costs. At present, there is 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, desktop monitors etc. Use is made here of the technologies STN (supertwisted nematics), AM-TN (active matrix—twisted nematics), AM-IPS (active matrix—in-plane switching) and AM-MVA (active matrix—multidomain vertically aligned), which are described in detail in the relevant literature (see, for example, T. Tsukuda, TFT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach 1996, ISBN 2-919875-01-9 and the references cited therein; SID Symposium 1997, ISSN-0097-966X and the references cited therein). Furthermore, mention should be made of the technologies PDP (plasma display panel), PALC (plasma addressed liquid crystal), ELD (electro-luminescent display), FED (field emission display) etc., which are also explained in the above-cited SID report.
Clark and Lagerwall (U.S. Pat. No. 4,376,924) were able to show that the use of ferroelectric liquid crystals (FLC) in very thin cells results in opto-electrical switching or display elements which have response times which are faster by a factor of up to 1 000 compared with conventional TN (twisted nematic) cells (see, for example, EP-A 0 032 362). Owing to this and other favorable properties, for example the possibility of bistable switching and the fact that the contrast is virtually independent of the viewing angle, FLCs are basically suitable for areas of application such as computer displays and TV 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 tilted or orthogonal 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. The desired phase should be stable over the broadest possible temperature range to ensure that the display has a broad operating range.
The individual pixels of an LC display are usually arranged in an x,y matrix formed by the arrangement of a series of electrodes (conductor tracks) along the rows and a series of electrodes along the columns on the upper or lower side of the display. The points of interception of the horizontal (row) electrodes and the vertical (column) electrodes form, addressable pixels.
This pixel arrangement 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 display production and consequently lower production costs, but the disadvantage that passive addressing can only be carried out line by line, which results in the addressing time for the entire screen with N lines being N times the line addressing time. For usual line addressing times of about 50 microseconds, this means a screen addressing time of about 60 milliseconds in, for example, the HDTV (high definition TV, 1152 lines) standard, i.e. a maximum frame rate of about 16 Hz, too slow for displaying moving images. In addition, display of gray shades is often difficult. At the FLC conference in Brest, France (Jul. 20-24, 1997, see Abstract Book 6th International Conference on Ferroelectric Liquid Crystals, Brest/France), a passive FLC display with digital gray shades was shown by Mizutani et al., in which each of the RGB pixels (RGB=red, green, blue) was divided into sub-pixels, allowing the display of gray shades in digital form through partial switching. Using three basic colors (red, green, blue), N gray shades result in 3
N
colors. The disadvantage of this method is the considerable increase in the number of screen drivers necessary and thus in the costs (in the case of the display shown in Brest, three times as many drivers were necessary as in a standard FLC display without digital gray shades).
In so-called active-matrix technology (AMLCD), a nonstructured 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 elements can 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, for example, T. Tsukuda, TFT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach 1996, ISBN 2-919875-01-9, and the references 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 alignment 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, i.e. the excessively slow response 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, for example in WO 97/12355 or Ferroelectric 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 used a combination of the so-called “quasi-bookshelf geometry” (QBG) of an FLC and a TFT (thin-film transistor) active matrix to simultaneously achieve high response speed, gray shades and high transmission. However, the QBG is not stable over a broad temperature range, since the temperature dependence of the smectic layer thickness disrupts or rotates the field-induced layer structure. Moreover, Hartmann utilizes an FLC material having a spontaneous polarization of more than 20 nC/cm
2
, which, for pixels having realistic dimensions of, for example, 0.01 mm
2
, leads to high electrical charges (at saturation, Q=2 AP, A=pixel area, P=spontaneous polarization). With low-cost amorphous silicon TFTs, for example, these high charges cannot reach the pixel in the course of the opening time of the TFT. For these reasons, this technology has not been pursued any further to date.
While Hartmann utilizes the charge-controlled bistability to display a virtually continuous gray scale, Nito et al. have suggested a monostable FLC geometry (Journal of the SID, 1/2, 1993, pp. 163-169) in which the FLC material is aligned by means of relatively high voltages such that only a single stable position results from which a number of intermediate states are generated by application of an electric field via a thin-film transistor. These intermediate states correspond to a number of different brightness values (gray shades) when the cell geometry is matched between crossed polarizers.
The disadvantage of the paper by Nito et al. is the occurrence of a streaky texture which limits the contrast and brightness of this cell (see FIG. 8 of the abovementioned citation). While it is possible to correct the disadvantageous streaky texture by treatment with a high electric voltage (20-50 V) in the nematic or cholesteric phase (see page 168 of the abovementioned citation), such a field

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