The invention relates to a reflective liquid crystal display comprising at least a) a stack of layers comprising a substrate with a reflective electrode and an alignment layer, b) a stack of layers comprising a transmissive substrate with a transmissive electrode and an alignment layer, and c) comprised between stacks a) and b) a liquid crystal layer. The method further relates to a method for enhancing the (light stability) lifetime of said display.

A subclass of reflective liquid crystal displays (hereinafter abbreviated as liquid crystal on silicon: LCoS) is based on the concept of using a small (and therefore low cost) LCD display, to project a large (and therefore high value) image in a rear-projection TV. LCoS displays are known in the art, for instance in J. A. Shimizu"Single Panel Reflective LCD Projector", Proceedings of SPIE, vol. 3634, p. 197-206,1999. The inevitable consequence of this concept is the extremely high light load on the LCoS panel (currently about 30"suns"). As turns out, this high intensity illumination leads to loss of alignment [W.

Oepts et al. , Eurodisplay'02 Conference, paper 11.2] and therefore limits the display's lifetime.

In JP 2001-348531 and JP 2002-229027 displays were disclosed having an ion getter film-forming coating liquid or ion adsorbing fine particles contained in a transparent electrode protective film. These displays are of another type, i. e. displays wherein both electrodes are of the transmissive type. These displays are used as direct view displays, in contrast to LCoS displays that are used as projection-displays. Projection-displays are subjected to extremely high light-intensity dosages (at least 100 to 1000 times more than conventional direct view displays). After prolonged exposure to intense illumination an LCoS display will not longer function and becomes black. The problems inherently present in LCoS displays, i. e. the limited lifetime under prolonged exposure, do not occur in such displays and therefore have never been mentioned therein.

According to the state of art for achieving alignment in LCoS displays either organic or inorganic alignment layers are used, either rubbed, obliquely evaporated, or ion milled to give the desired surface anisotropy. For instance, the conventional approach is spinning/printing a polyimide alignment layer and using a rubbing cloth to give alignment, as

is currently used in production. Another commercially used method is obliquely evaporating SiOx layers to give alignment.

These methods, as tested in lifetime experiments, give insufficient lifetime (in combination with standard twisted nematic liquid crystal materials). Chemical analyses of these layers suggest photochemical cleavage of liquid crystal molecules. The resulting anionic remnants (fluoride and chloride ions) penetrate into the alignment layer surface and attach thereto. In addition the affected liquid crystal cores are found to deposit on top of the alignment layer surfaces. This phenomenon was found in cells with organic and inorganic alignment layers, resulting in loss of the alignment until the display stops functioning.

It is an object of the invention to provide a solution to the problem of limited lifetime of LCoS displays in projection applications.

To this end the invention pertains to a reflective liquid crystal display comprising at least a) a stack of layers comprising a substrate with a reflective electrode which is superposed by a porous auxiliary layer comprising at least one of a metal oxide, silicon oxide, metalo-organic compound, and organo-silicon oxide, and an alignment layer, b) a stack of layers comprising a transmissive substrate with a transmissive electrode, which is superposed by a porous auxiliary layer comprising at least one of a metal oxide, silicon oxide, metalo-organic compound, and organo-silicon oxide, and an alignment layer, and c) comprised between stacks a) and b) a liquid crystal layer.

These porous auxiliary layers can either be fabricated between the conventional alignment layers and electrodes, or alternatively can serve as dual-function layers, combining its lifetime-extending properties with alignment properties.

These porous auxiliary layers avoid or at least reduce the building-up of water and/or anions like fluoride, chloride, and the like in the critical alignment interfaces and liquid crystal bulk. Thereby the deposition is reduced and the liquid crystal alignment preserved, resulting in at least two to three times longer lifetimes of the liquid crystal device.

Transparent porous auxiliary layers are applied between one or both (conventional) alignment layers (for instance 40 nm rubbed polyimide layers) and the electrodes. These porous auxiliary layers should have a porous structure. Preferably, the pores have a diameter smaller than 10 nm, and more preferably are between 0.5 and 10 nm.

Suitable materials for these layers are porous metal oxides, e. g. prepared from wet-chemical precursor solutions (for instance copolymerized metal alkoxides). The thickness of the porous auxiliary layers is generally 10-500 nm, preferably 80-200 nm. The porous auxiliary layers may be made of metal oxides, such as Ti02, Si02, Zr02, and other metal oxides. As an

example a porous auxiliary layer of 100 nm thickness was made containing 60 % Ti02, 30 % Si02 and 10 % Zr02. The porous auxiliary layers may advantageously comprise nanoparticles, for instance metal oxides such as Sb205, Bi203, or the corresponding partly hydrolyzed oxides. These nanoparticles are added to the porous auxiliary layer in amounts that preferably range from 0. 01 to 5 %, more preferably from 0.1 to 1.5 %. The porous auxiliary layers can be made by conventional methods such as spin-coating from solution, followed by curing, which might comprise UV-exposure and/or thermal annealing. One example uses post-baking at temperatures above 150° C (for instance about 300° C) during 1 min to 3 h, for instance about 30 min.

The substrate is preferably a silicon-containing layer, for instance a silicon substrate or a glass layer containing amorphous, polycrystalline or microcrystalline silicon.

The alignment layers are conventional alignment layers such as made of organic materials such as polyimide, polyamide, pva, mylar, etc. , and inorganic materials such as diamond-like carbon, silicon oxide, silicon nitride, aluminum oxide, and silicon carbide. The alignment layers are provided by spin-coating or are anisotropically obtained by sputtering or evaporation. Orientation is performed in the conventional manner, such as a rubbing process, photo-alignment, or ion bombardment. Ion bombardment is a method of choice when the porous auxiliary layer also acts as the alignment layer.

The transmissive electrode is preferably ITO, but other materials such as (doped) tin oxides, zinc oxides, such as indium zinc oxide of aluminum zinc oxide, can also be used.

The reflective electrode is preferably aluminum, but other reflective materials such as silver, copper, platinum, and gold, or alloys containing these metals can also be used.

It is also possible to use multilayer interference stacks.

During the operation the LCoS device is exposed to high intensity illumination. As a consequence, it is believed that liquid crystal molecules may be photochemically cleaved. These ions and the affected LC (liquid crystal) cores may attach to the alignment layers, which then lose their alignment capabilities. XPS analyses of exposed alignment layers have confirmed the presence of fluoride and chloride atoms and organic deposits originating from the LC.

The result of the presence of this kind of porous auxiliary layers is presented in Fig. 1. In Fig. 1 the accelerated lifetime (@25 Mlux UHP illumination, 50° C) measurements of LCoS test cells are plotted. The points indicate the reference lifetimes of cells without porous auxiliary layers. The error bars indicate two lifetime measurements of

cells with 100 nm porous auxiliary layers with nanoparticles in both porous auxiliary layers, and a single lifetime measurement of a 100 nm porous auxiliary layer without nanoparticles.

The figure shows that the lifetime is increased by a factor 2 to 3. In this specific experiment the porous auxiliary layers consisted of 60 % Ti02, 30 % Si02 and 10 % ZrO2. These layers are fabricated by spin-coating liquid containing precursors, followed by UV curing (6000 mJ/cm2 at 365 nm) and thermal annealing (30 min at 300° C). The resulting layers were 100 nm thick and had porosities of about 19 %. The nanoparticles used were Sb205 with average diameter of 30 nm and in concentration of about 0.5 %.

In another embodiment the porous auxiliary layer simultaneously functions as an alignment layer. This can be achieved by application of a similar porous auxiliary layer as in the previous embodiment, but after application to the display performing an alignment generating process to this porous auxiliary layer.

This alignment generating process can be any of the known processes like conventional rubbing, but particularly interesting is ion bombardment (see for instance P.

Chaudhari et al, Atomic-beam alignment of inorganic materials for LCDs, Nature 411,56-59 (2001)).

The invention therefore also pertains to a method for enhancing the (light stability) lifetime of a reflective display comprising a reflective electrode, a transmissive electrode, each superposed by an alignment layer, and a liquid crystal layer comprised between the alignment layers, by applying a porous auxiliary layer comprising at least one of a metal oxide, silicon oxide, metallo-organic compound, and organo-silicon oxide between the electrodes and the alignment layer.