Liquid crystal device

Details Liquid crystal device Liquid crystal device

2000-11-14

2003-09-09

Ton, Toan (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S089000

Reexamination Certificate

active

06618113

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal device (LCD). Such LCDs may be used in reflective or transmissive configurations.
2. Description of the Related Art
FIG. 1
of the accompanying drawings illustrates a known type of reflective LCD having a passive matrix addressing arrangement. The device comprises mirror electrodes
1
in the form of parallel stripes extending vertically in FIG.
1
. The mirror electrodes
1
are disposed above a rear light absorber
2
which is visible in the view from above of
FIG. 1
in the gaps between the mirror electrodes
1
. Transparent electrodes, such as indium tin oxide (ITO) electrodes
3
are disposed above the liquid crystal layer and take the form of elongate parallel electrodes which extend horizontally in FIG.
1
. The gaps between the electrodes
3
result in unaddressed regions
4
which are disposed above the reflective mirror electrodes
1
. Thus, light passing through the regions
4
is reflected and is not absorbed by the rear absorber
2
.
The LCD shown in
FIG. 1
is of the ferroelectric liquid crystal (FLC) type in which the liquid crystal layer acts as a retarder with in-plane optic axes switching. A fixed quarter wave plate with its optic axis oriented at 75° is disposed between the FLC layer and the mirror electrodes. Picture elements (pixels) in the white state, such as 5, have their optic axes switched to an angle of −7.5° whereas pixels such as 6 in the black state have their optic axes switched to an angle of +15°. The unaddressed regions
4
are not subjected to an applied addressing field between the electrodes
1
and the electrodes
3
and thus adopt an undefined state. This state corresponds to a non-black state so that incident light on the regions
4
is at least partly reflected and is visible to a viewer of the display. This results in a reduction of the contrast ratio.
A known technique for avoiding this problem is disclosed in D. G. McDonnell et al, “An Ultra-High-Resolution Ferroelectric Liquid-Crystal Video Display”, Digest of Technical Papers, Society for Information Display International Symposium, 1993, p 654. According to this technique, the liquid crystal in the inter-pixel gaps or regions such as
4
in
FIG. 1
to switched by fringing fields into a controlled state. However, it is not always possible to achieve such switching, for example because of limitations in addressing, device configuration and material characteristics or because the inter-pixel gap contains a non-switchable spacing element.
In cases where it is not possible to switch the inter-pixel gaps, it is known to provide a black matrix for masking such regions, for example as disclosed in Koden et al, “Key Technologies for the &tgr;-V
min
Mode FLCD”, IDW 1997, p 269 and in D. E. Castleberry et al, “A One Mega-Pixel Colour a-Si TFT Liquid-Crystal Display”, Digest of Technical Papers, Society for Information Display International Symposium, 1988, p 232. However, this increases the processing steps and hence the cost of making such displays and decreases the fill factor of the display.
It is also known to provide cell spacing by means of patterned structures of uniform height but this also reduces the contrast ratio of LCDs. For example, Koden et al (as mentioned above) describes the use of continuous spacer walls to achieve mechanical stability in smectic devices and Colgan et al “On-chip metallisation layers for reflective light valves”, IBM Journal of Research and Development, vol. 42 no. 3 1998 discloses the use of silicon dioxide spacer posts at the corners of pixels.
FIG. 2
of the accompanying drawings illustrates the arrangement of such known devices with spacer walls
7
disposed in the gaps between the upper electrodes
3
. The spacers
7
are transparent and are made of substantially isotropic material. Incident light is therefore reflected by the underlying portions of the mirror electrodes
1
and this again reduces the contrast ratio of such LCDs.
It Is known for the spacer walls
7
to be disposed between the gaps in the mirror electrodes
1
as illustrated in
FIGS. 3 and 4
of the accompanying drawings. However, any Inaccuracies in positioning and size of the spacer walls
7
results in a reduction of contrast ratio. For example, as shown in
FIG. 3
at
8
, the spacer walls
7
are misaligned with the gaps between the mirror electrodes
1
and are therefore skewed so as to overlap the partially reflective regions of the pixels. Translational errors may also occur and result in the spacer wall
7
overlapping the partially reflective regions. As shown in
FIG. 4
at
9
, if the spacer walls
7
are too wide, they will again overlap the mirror electrodes
1
. Light passing through the spacer walls
7
and striking the overlapping portions of the mirror electrodes
1
is reflected back out of the LCD and results in a reduction in contrast ratio.
The spacers
7
may be made of a black polymer material, for example as disclosed in D. E. Castleberry et al (as mentioned above) and in C. M. Healer et al. “Pigment-Dispersed Organic Black-matrix Photoresists For LCD Colour Filters”, SID 1995 Digest, p 446. However, in order to avoid compromising the contrast ratio of the LCD, such a material would have to have sufficient light-absorbing properties and, in practice, there is some reduction in contrast ratio. Also, the spacers are disposed within the liquid crystal layer and the dyes or pigments used in such materials can contaminate the liquid crystal. This compromises the alignment quality and switching behaviour of the LCD.
The presence of the spacers
7
in the liquid crystal layer can also cause “pinning” of the liquid crystal molecules adjacent the spacers so that the liquid crystal adjacent the spacers is not substantially affected by the applied addressing field. As described hereinafter, the liquid crystal may be pinned in the black or white state and this reduces contrast ratio and aperture ratio of the device. It is thus necessary to provide a black matrix which is wider than the spacers
7
so as to mask such pinned white states. This increases the number of production process steps and hence the cost of such devices while substantially decreasing the fill factor of such devices.
SUMMARY OF THE INVENTION
According to the invention, there is provided a liquid crystal device comprising a liquid crystal layer containing a plurality of pixels separated by inter-pixel gaps, each of the pixels having a first optical state resulting in maximum light attenuation, characterised by at least one spacer disposed in the inter-pixel gaps and having substantially the same optical property as the first pixel optical state.
The at least one spacer may comprise a plurality of pillars.
The at least one spacer may comprise a plurality of walls. The walls may be continuous. The walls may enclose the pixels. The wells may fill the inter-pixel gaps.
The optical property may comprise changing the polarisation of light. The optical property may comprise retardation with a predetermined optic axis orientation. The device may comprise a linear polariser for transmitting light with a first direction of linear polarisation, a reflector, a half waveplate disposed between the polariser and the reflector and a quarter waveplate disposed between the half waveplate and the reflector, the liquid crystal layer comprising one or both of the half waveplate and the quarter waveplate.
The liquid crystal layer may be of the in-plane switching type and the at least one spacer may have the same retardation as the liquid crystal layer and an optic axis oriented in the same direction as the liquid crystal layer in the first optical state. The liquid crystal of the layer may be a smectic liquid crystal. The liquid crystal may be a ferroelectric liquid crystal.
The liquid crystal layer may comprise the half waveplate whose optic axis is switchable between −7.5° and +15° to the first direction, the at least one spacer may have an optic axis at +15° to the first direction, and t

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