WO2005116736A1

WO2005116736A1 - Method of forming a film for a colour filter of a transflective liquid crystal display

Info

Publication number: WO2005116736A1
Application number: PCT/IB2005/051649
Authority: WO
Inventors: Paulus C. Duineveld; Dirk J. Broer; Peter J. Slikkerveer; Ronald Van Rijswijk
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-05-28
Filing date: 2005-05-20
Publication date: 2005-12-08

Abstract

The present invention relates to a method for forming on a substrate of a transflective liquid crystal display a film suitable to act as a color filter for a sub-pixel using an arrangement which exploits flow effects (Marangoni flow) to achieve a desired film profile.

Description

METHOD OF FORMING A FILM FOR A COLOUR FILTER OF A TRANSFLECTIVE LIQUID CRYSTAL DISPLAY

The present invention relates to a method for forming on a substrate of a transflective liquid crystal display a film suitable to act as a color filter for a sub-pixel. The present invention also relates to a transflective, e.g. liquid crystal, display device comprising a color filter in the form of a film. Such a film may, for example, comprise an array of RGB patterns, where for each pixel, there is an R, a G and a B filter for respective subpixels.

A transflective display is one in which either reflective display or transmissive display can be selected as required. Thus a transflective liquid crystal display can be used in both a transmissive and/or reflective mode. In the reflective mode, external light is reflected by a reflective layer and passes to the liquid crystal layer. In the transmissive mode, light from a backlight or the like is transmitted through the reflective layer and passes to the liquid crystal layer. In order to perform this dual function, it will be apparent that a part of each sub- pixel must be reflective and a part of each sub-pixel must be transmissive. Alternatively, the whole pixel could be transflective, i.e. partly transmissive and partly reflective, although this would not necessarily lead to optimum performance. In order to display a color, light before passing to the liquid crystal layer or light after passing through the liquid crystal layer passes through a color filter and the appropriate wavelength range is selected to display the color. Referring to Fig. 1 of the drawings, there is illustrated schematically an idealized sub-pixel 100 comprising a reflective portion 102 defined by a reflector 104, transmissive portions 106 and a color filter 108. The color filter 108 in the reflective portion 102 is ideally much thinner (of the order of a factor of two) than in the transmissive portions 106 because light 110 crosses the color filter material 108 twice in the reflective portion 102 and once in the transmissive portions 106. In other words, in the reflective mode, the light passes through the color filter twice and in the transmissive mode only once. The reflected light "sees" double the layer thickness which gives a higher color saturation and higher absorbance. If a subpixel is divided into two parts then ideally the reflective part of the color filter should be thinner than the transmissive part, as illustrated. A general drawback of color display in a transflective liquid crystal device is that although the optical path length of light passing through the color filter in the reflective mode differs from the optical path length of light passing through the color filter in the transmissive mode, a conventional color filter has the same color density in the reflective mode and in the transmissive mode. Thus the light density in a region of the color filter corresponding to the optical path during reflection must be set differently from that in a region of the color filter corresponding to the optical path during transmission.

In the arrangement described in US Patent Application No. 2003/0113639A1, this is achieved by providing a color filter comprising a plurality of dot-shaped films formed on a substrate. Each film consists of a first film and a second film having different color properties, wherein the thickness ratio of the first film to the second film changes with one dot-shaped color film. Thus, for example, the first film may be concave (wherein the center defines a dip relative to the edges) and the second film has a complementary convex shape. The films may be deposited by means of respective inkjet printing steps. A conventional production technique for making color filters is photolithography. However, photolithography is not particularly suitable for producing a coating of varying thickness. One option would be to perform an additional photolithographic step for each color. However this makes photolithography a very expensive production technique. Inkjet inks used in piezo-electric or thermal inkjet printing typically comprise a colorant and a vehicle itself often containing water and relatively low surface tension liquids. It has been proposed that color films for color filters be produced using an inkjet printing technique. In general, the color film of the color filter is formed using a material in which a colorant composed of a pigment, a dye and a natural pigment or the like is dissolved in an appropriate solvent. The light density and consequently the appearance of color during color display can be changed by changing the quantity of the colorant in the film material.

Although it has been possible to adjust the appearance of color in a transflective liquid crystal device using this method, it has been difficult to obtain the desired appearance simultaneously in the reflective display mode and in the transmissive display mode. The present invention seeks to improve the formation of a film suitable to act as a color filter for a sub-pixel on the substrate of a transflective liquid crystal display by exploiting (in a film forming step) an arrangement in which a suspension exhibits flow effects leading to a film with a desirable profile. Thus viewed from one aspect the present invention provides a method for forming a film suitable to act as a color filter for a sub-pixel on a substrate of a transflective e.g. liquid crystal, display, the method comprising:

(a) obtaining the substrate with a surface boundary defining the sub-pixel; (b) applying a volume of a liquid suspension onto the substrate within the boundary, wherein the suspension comprises a colorant and one or more liquid carriers; and (c) allowing or causing the droplet to dry by evaporation of the one or more liquid carriers to form the film wherein said one or more liquid carriers are selected to cause the colorant to migrate to the boundary during step (c). The method of the invention in a single application step advantageously forms a film with a profile which may be exploited as a color filter for the sub-pixel. The present invention also extends to a transflective display device (e.g. liquid crystal) comprising a substrate, a plurality of sub-pixels defined by respective surface boundaries, and a color filter in respect of said sub-pixels, said color filter being in the form of a film formed on said substrate in accordance with the above-mentioned method. In a preferred embodiment, the boundary defines the sub-pixel such that a first part of the sub-pixel is reflective and a second part of the sub-pixel is transmissive. The characteristics of the steps of the method of the invention may be judiciously chosen to ensure that the profile of the film coincides in a desirable manner with the first part and the second part of the sub-pixel. Preferably the profile of the film is such that the thickness of a portion of the film coinciding with the second part of the sub-pixel exceeds the thickness of a portion of the film coinciding with the first part of the sub-pixel. Particularly preferably the thickness of the film coinciding with the second part of the sub-pixel is greater than the thickness of the film coinciding with the first part of the sub-pixel by a factor of between 1.5 and 2.5, more preferably between 1.75 and 2.25, especially preferably 1.9 to 2.1, most especially preferably of about 2 or more. Preferably the second part of the sub-pixel is at or near to the boundary and the first part of the sub-pixel is remote from the boundary. In a preferred embodiment, the boundary comprises a barrier of closed form, which defines the sub-pixel. The barrier may be introduced onto a substrate in accordance with standard practices such as lithography (e.g. photolithography). In a preferred embodiment, step (b) comprises: ink-jet printing the volume of liquid onto the substrate within the boundary (e.g. between the first barrier and the second barrier). The colorant may be natural or synthetic and may be one or more pigments or dyes (for example acting as a paint or ink). Preferably the colorant is one or more pigments. These may be chosen for their desired color characteristics. Typically the particle size of the pigment is 1 micron or less. Suitable pigments will be familiar to those skilled in the art and many are commercially available. They include amino anthraquinones such as CI pigment red 177 which may be used in an amount in the range 3-10wt%, isolindolines such as CI pigment yellow 139 which may be used in an amount in the range l-15wt% (preferably l-5wt% or 5- 15wt%) and copper based complexes which may be used in an amount in the range 3-15wt% (for example copper halogenated phthalocyanine complexes such as CI pigment green 36 which may be used in an amount in the range 5-15wt% and copper phthalocyanine complexes such as CI pigment blue 156 which may be used in an amount in the range 3- 10wt%). The surface of the volume of liquid will, due to the relatively small dimensions, always have a curved profile, as the surface tension force is dominant over gravity. The surface tension of the liquid, in combination with the surface properties of the barrier material, will influence the wetting of the barrier material. Thus, in a preferred embodiment, the one or more liquid carriers are selected so that the droplet has a tendency towards wetting the boundary. Without wishing to be bound by theory, it is understood that during evaporation, the liquid which favors wetting the boundary whilst having a tendency towards a curved profile causes the one or more liquid carriers and therefore the colorant to migrate towards the boundary. Thus by virtue of this embodiment of the method according to the invention, the dry film is relatively thick near the boundary and is relatively thin remote from the boundary (e.g. in the middle). In a preferred embodiment, the profile of the film is essentially concave. In a preferred embodiment the one or more liquid carriers are selected to create a differential surface tension (e.g. a Marangoni stress). This embodiment advantageously exploits a phenomenon known as Marangoni flow in which migration of colorant is advantageously driven by surface tension gradients. In a preferred embodiment, the one or more liquid carriers are selected to have a rate of evaporation which is faster (e.g. several orders of magnitude faster) than diffusion of colorant in the one or more liquid carriers. In a preferred embodiment, the one or more liquid carriers are selected to have a differential rate of evaporation. At least one (e.g. two) of the one or more liquid carriers may be an ester. Typically the ester (or esters) may be present in the range 10-60wt%. Preferably the or each ester is an alkoxyalkyl alkylate, particularly preferably an alkoxyalkyl acetate, more preferably an alkoxypropyl acetate (e.g. a Cι-6-alkoxypropyl acetate such as a Cμ- alkoxypropyl acetate) or a Cι-6-alkoxyalkyl acetate (e.g. a Cι ^-alkoxyalkyl acetate). Specifically preferred are methoxypropylacetate (preferably present in an amount in the range 40-60wt%) and ethoxypropylacetate (preferably present in an amount in the range 10- 20wt%). One of the one or more liquid carriers may be an alcohol or ketone. Preferred is a ketone such as a Cι-6-alkanone. Typically the ketone is present in the range 10-20wt%. Specifically preferred is cyclohexanone. In a preferred embodiment, the one or more liquid carriers are two or more liquid carriers. Preferably one of the two or more liquid carriers has the lowest rate of evaporation and the highest surface tension of the two or more liquid carriers. Preferably a first liquid carrier of the two or more liquid carriers is a ketone (e.g. as defined hereinabove, preferably cyclohexanone). Particularly preferably a first liquid carrier of the two or more liquid carriers is a ketone (e.g. as defined hereinabove) and a second liquid carrier of the two or more liquid carriers is an ester (e.g. as defined hereinabove). Preferably the second liquid carrier is methoxypropylacetate or ethoxypropylacetate and the first liquid carrier is cyclohexanone. In a preferred embodiment, the one or more liquid carriers are three or more liquid carriers (e.g. three liquid carriers). Preferably a first liquid carrier of the three or more liquid carriers is a ketone (e.g. as defined hereinabove, preferably cyclohexanone).

Particularly preferably a first liquid carrier of the three or more liquid carriers is a ketone (e.g. as defined hereinabove, preferably cyclohexanone), a second liquid carrier of the three or more liquid carriers is a first ester (e.g. as defined hereinabove) and a third liquid carrier of the three or more liquid carriers is a second ester (e.g. as defined hereinabove). Preferably the first ester is methoxypropylacetate and the second ester is ethoxypropylacetate. The terms alkyl, alkan- or alkoxy used herein may refer to linear or branched, substituted or unsubstituted, cyclic or acyclic or saturated or unsaturated groups. The evaporation rate of some specific compositions within the context of the present invention might be quite fast, such that because "open time" of the nozzle in an inkjet printer i.e. the time that no droplets are generated, is not very long, and due to evaporation of the solvent, the viscosity close to the nozzle increases which may prevent the droplet generation when the inkjet printer is started again. Preferably, therefore, the evaporation rate of the total inkjet ink might be decreased somewhat, without influencing the relative effect of the difference in evaporation rate and surface tension between the different solvents in the ink. Therefore, in the end, the ink composition for use in the method of the present invention can be optimized, such that the solvents used may have some higher boiling points, or vapor pressures. 1 -Methoxy-2propylacetate has a vapor pressure of 930 Pa @ RT, whereas for inkjet printing, this vapor pressure is preferably a factor of 2 or more lower. The higher evaporation rate is favorable for a spin coating process. The suspension may further comprise a film-forming additive such as a binder (e.g. one or more polymeric or resinous binders). A preferred polymeric binder is a methacrylate derivative copolymer (for example a copolymer of methacrylic acid and benzylmethacrylate). Typically the polymeric binder is present in an amount in the range 5- 15wt%. The suspension may further comprise a monomer. A preferred monomer is a polyfunctional acrylate, particularly preferably a hexa-acrylate such as a hexaester of dipentaerythritol with acrylic acid. Typically the monomer is present in an amount in the range 3-13wt%. The suspension may further comprise an initiator such as a triazine or a derivative thereof. Preferred is a polyhaloalkyl substituted triazine derivative. Typically the initiator is present in an amount in the range 0-1 wt%. The viscosity of the suspension is typically about 5 to 20cP. Suitable suspensions may be available commercially such as COLOUR

MOSAIC^R CR-7001 (red), COLOUR MOSAIC^R CG-7001 (green) and COLOUR MOSAIC^R CB-7001 (blue) available from Arch Co. Ltd. Other additives familiar to those skilled in the art may be added as desired such as driers and viscosity enhancing agents, and it will be particularly appreciated that the solvents can be varied so as, for example, to decrease the evaporation rate of the solvent for use in, for example, an inkjet printing process. A degree of control over the characteristics (e.g. profile) of the color filter may be usefully exercised by changing the characteristics (e.g. components) of the suspension. For example, higher levels of binder (or viscosity enhancing additives) may be used to increase the viscosity of the suspension and slow the migration of colorant thereby leading to a more uniform (i.e. flatter) film. The thickness of the film may be varied by changing the total volume of suspension (e.g. by changing the distance between printed droplets). The sub-pixel may be 300 microns or less in diameter e.g. in the range 66-266 microns. The barrier height is typically in the range 1 to 5 microns. The substrate in step (c) may be allowed to dry under ambient conditions (e.g. in the open air). Alternatively the substrate may be heated. Typically the substrate is raised to an elevated temperature less than about 75°C. The substrate may be a suitable material familiar to those skilled in the art. The surface of the substrate may be treated or untreated. Any such treatment would be for the purpose of changing the wetting properties of the liquid with the substrate and barrier material. For example, the substrate may be pre-treated with O₂ plasma followed by a CF₄ plasma. Also, but less preferably, a UV or ozone treatment may be used. Viewed from a further aspect the present invention provides a transflective liquid crystal display comprising at least one sub-pixel with a color filter exhibiting a thickness distribution substantially as is illustrated in Fig. 2. Preferably the color filter exhibits a substantially concave profile. These and other aspects of the present invention will be apparent from, and elucidated with reference to, the embodiments described herein.

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which: Fig. 1 is a schematic illustration of an idealized sub-pixel of a transflective display according to the prior art; Fig. 2 is a sketched illustration of a printed sub-pixel (Fig. 2b) according to an embodiment of the invention and of the thickness distribution (Fig. 2a) of the ink-jet printed color filter; Fig. 3 is a cross-sectional schematic illustration of the distribution of the suspension shortly after step (A) of an embodiment of the invention; and Fig. 4 is a schematic illustration of the migration of liquid carrier and colorant to the edge of a sub-pixel during evaporation of the liquid carrier.

Example Barriers defining a sub-pixel were constructed on a substrate having thereon a partially inorganic/organic coating, i.e. (:in this specific example) 18%C, 38%O and 43%Si. On this coating, the organic barriers were applied by means of standard photolithography. The substrate was treated with O₂ plasma and then with a CF₄ plasma. This is to let the liquid wet the surface in between the barriers, but prevents wetting of the top of the barriers. The suspension used in this Example was a red material known as COLOUR MOSAIC^R CR-7001 from Arch Chemicals, Inc and is optimized for spin coating. Generally speaking, the printing step involves exploiting alignment marks on the substrate. The position of the barriers relative to the alignment marks is known from the photolithographic fabrication and the position of each sub-pixel on the substrate is known from the mask design. The substrate is aligned (which gives the position of the sub-pixels) and droplets are printed between the barriers and the liquid fills the complete pixel (i.e. this step is self- aligned). Thus the process is fully self-aligned. The suspension was ink-jet printed in a sub-pixel of width 266 microns between barriers 50 microns wide and 3 microns high. The reflective portion of the pixel was positioned in the middle of the sub-pixel with the transmissive portion at the edge. Droplets had a volume of about 90-100 picolitre and were printed in the pixel at a distance of 65 microns between individual droplets. Due to the O₂ and CF₄ treatment the top of the barriers were not wetted. The suspension was dried using a special effect known as the "coffee stain" effect which is illustrated schematically in Fig. 3 for a cross-section of the sub-pixel in which the droplet had just landed and the suspension had achieved its equilibrium curved shape. The effect is attributable to contact angle hysteresis (i.e. the difference between advancing and receding contact angle). In Fig. 3, there is a continuous distribution of colorant in the suspension. It should be noted that the thickness of the wet film exceeded the thickness of the dry film because 90-95% of the suspension is liquid. During evaporation of the liquid, the colorant migrates to the edge of the sub- pixel because the liquid is pinned to the barrier (i.e. the receding contact angle is zero) which means that the liquid has a tendency not to dewet the barrier during evaporation. Due to the fact that surface tension is the dominant force, the liquid has an overriding tendency to maintain its curved shape which can only be achieved by the migration of liquid to the barrier (carrying the colorant with it). The migration of the liquid and colorant to the barrier is illustrated schematically in Fig. 4. In accordance with this embodiment of the invention, this effect is enhanced because at the barrier where the liquid evaporates much more quickly, there is a greater proportion of cyclohexanone as this has the lowest evaporation rate and the largest surface tension of the three liquid carriers. This creates a surface tension difference (a Marangoni stress) over the sub-pixel which provides extra drive behind the liquid flow to the barrier. A difference in concentration would normally lead to diffusion of colorant particles. However for the suspension used in this embodiment, the diffusion effect was negligible as the evaporation rate of the liquid carriers is very fast and diffusion has no time to evenly redistribute the colorant particles. A sketched illustration of a printed sub-pixel according to the invention and the thickness distribution of the ink-jet printed color filter is given in Fig. 2. The dry film thickness varied desirably between 1 and 2μm. When a flat uniform color filter is desired in a sub-pixel, the liquid characteristics are altered at several points in order to prevent flow of particles to the barrier. Firstly the viscosity of the liquid may increase with increasing concentration (e.g. by adding more binder). Alternatively the liquid carriers may be changed to reduce or eliminate Marangoni stress. It should be noted that the above-mentioned embodiment illustrates rather than limits the invention and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises" and the like does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A method for forming a film suitable to act as a color filter for a sub-pixel on a substrate of a transflective display, the method comprising:

(a) obtaining the substrate with a surface boundary defining the sub-pixel;

(b) applying a volume of a liquid suspension onto the substrate within the boundary, wherein the suspension comprises a colorant and one or more liquid carriers; and

(c) allowing or causing the droplet to dry by evaporation of the one or more liquid carriers to form the film; wherein said one or more liquid carriers are selected to cause the colorant to migrate to the boundary during step (c).

2. A method as claimed in claim 1 wherein the boundary defines the sub-pixel such that a first part of the sub-pixel can be used in a reflective mode and a second part of the sub-pixel is transmissive.

3. A method as claimed in claim 1 or 2 wherein the profile of the film is such that the thickness of a portion of the film coinciding with the second part of the sub-pixel exceeds the thickness of a portion of the film coinciding with the first part of the sub-pixel.

4. A method as claimed in claim 3 wherein the thickness of the film coinciding with the second part of the sub-pixel is greater than the thickness of the film coinciding with the first part of the sub-pixel by a factor of between 1.5 and 2.5, more preferably 1.75 and 2.25, especially preferably 1.9 to 2.1 , most especially preferably of about 2.

5. A method as claimed in claim 3 or 4 wherein the second part of the sub-pixel is at or near to the boundary and the first part of the sub-pixel is remote from the boundary.

6. A method as claimed in any preceding claim wherein the boundary comprises a substantially closed barrier which defines the sub-pixel.

7. A method as claimed in any preceding claim wherein step (b) comprises: ink-jet printing the liquid onto the substrate within the boundary.

8. A method as claimed in any preceding claim wherein the one or more liquid carriers are selected so that the droplet has a tendency towards wetting the boundary.

9. A method as claimed in any preceding claim wherein the one or more liquid carriers are selected to create a differential surface tension.

10. A method as claimed in any preceding claim wherein the one or more liquid carriers are selected to have a rate of evaporation which is faster than diffusion of colorant in the one or more liquid carriers.

11. A method as claimed in any preceding claim wherein the one or more liquid carriers are selected to have a differential rate of evaporation.

12. A method as claimed in any preceding claim wherein at least one of the one or more liquid carriers is an ester.

13. A method as claimed in any preceding claim wherein one of the one or more liquid carriers is a ketone.

14. A method as claimed in any preceding claim wherein said suspension comprises two or more liquid carriers.

15. A method as claimed in claim 14 wherein one of the two or more liquid carriers has the lowest rate of evaporation and the highest surface tension of the two or more liquid carriers.

16. A method as claimed in claim 14 or 15 wherein a first liquid carrier of the two or more liquid carriers is a ketone.

17. A method as claimed in any of claims 14 to 16 wherein a first liquid carrier of the two or more liquid carriers is a ketone and a second liquid carrier of the two or more liquid carriers is an ester.

18. A method as claimed in any preceding claim wherein the one or more liquid carriers are three or more liquid carriers.

19. A method as claimed in claim 18, wherein a first liquid carrier of the three or more liquid carriers is a ketone.

20. A method as claimed in claim 18 or 19, wherein a first liquid carrier of the three or more liquid carriers is a ketone, a second liquid carrier of the three or more liquid carriers is a first ester and a third liquid carrier of the three or more liquid carriers is a second ester.

21. A transflective liquid crystal display comprising at least one sub-pixel with a color filter exhibiting a thickness distribution substantially as is illustrated in Fig. 2.

22. A transflective display device comprising a substrate, a plurality of sub-pixels defined by respective surface boundaries, and a color filter in respect of said sub-pixels, said color filter being in the form of a film formed on said substrate in accordance with the method of any one of claims 1 to 21.