The invention relates to the manufacture of thin-film electronic devices, particularly but not exclusively liquid crystal displays. In particular, the invention relates to a method of forming a diffusely reflective pixel electrode for use in such a display, and to photomasks for use in the manufacture of such devices.

A pixel electrode as used in a thin film transistor (TFT) active matrix liquid crystal display (AMLCD) is usually coated with a layer of reflective metal that distributes incident light over the display's viewing area. In order to reduce mirroring in the viewing area, these pixel electrodes are generally formed with an irregular upper surface to provide diffuse reflection that disperses light throughout the required viewing area.

A known process for producing this irregular upper surface is described in US 2002/0093604 A1. A first stage in the production of a diffusely reflective TFT AMLCD pixel electrode involves applying a coating of organic material to a pre-prepared TFT plate by means such as spinning or screen-printing. This layer may consist of a material such as polyimide, acrylic, or photoresist.

In a second stage, the organic material is patterned. This may be achieved using a process such as conventional photolithography in which a photo mask is placed over the TFT plate, and UV light is directed through the mask and is used to pattern a layer of photosensitive organic material. The mask has opaque regions that are formed from solid chrome, and regions that are transparent to UV light. Consequently, regions of the organic material that are exposed to the UV light are, once developed, partly removed, the thickness of the organic material removed being dependant on the UV intensity and exposure time used. An array of steeply sided islands is formed in the organic material covering the TFT plate by the photolithography.

In a third stage of the process, a semi-planarising organic layer is applied over the entire plate to form a topography suitable for a pixel electrode, as the steeply sided islands do not provide appropriately diffuse reflection.

This semi-planarising layer is then removed from the TFT contact pad areas in a fourth stage involving photolithography. Finally, a highly reflective metal layer is deposited over the upper layer to form the diffusely reflective pixel electrode.

The present invention seeks to reduce the number of stages of processing required to produce diffusely reflective pixel electrodes such as those found in TFT-AMLCDs.

According to the invention, from one aspect there is provided a method of forming a diffusely reflective pixel electrode for a liquid-crystal display, comprising applying a photosensitive material to a thin-film device plate of the display, and using a grey-tone mask in a photolithographic process to produce a curved surface topography for the diffusely reflective pixel electrode.

The grey-tone mask may have one or more grey-tone areas of opaque material configured in at least one periodic pattern. The opaque material may be arranged into squares, each square having sides between 0.4 to 2, um in length.

According to the invention from another aspect there is provided a grey- tone mask which may have two or more parallel transparent slits.

Advantageous features in accordance with the present invention are set out in the appended Claims. These and others will now be illustrated in specific embodiments of the invention, by way of example, now to be described with reference to the accompanying drawings, in which: Figure 1a illustrates a plan view of a pixel array as found in a TFT- AMLCD, at a stage during its production; Figure 1 b illustrates a small section of the plan view of Figure 1 a ; Figure 1c illustrates a cross sectional view of the pixel electrode of Figure 1 b taken along the line AB;

Figure 2 illustrates the processing steps involved in a known method of manufacture for diffusely reflective pixel electrodes for use in liquid crystal displays; Figure 3 illustrates a plan view of a small area of a photo mask which could be used in the photolithographic stage of the process of Figure 2, namely between Figures 2a and 2b; Figure 4 illustrates an embodiment of the process described by the current invention for the reduced steps required in the production of diffusely reflective pixel electrodes for use in liquid crystal displays; Figure 5 illustrates a plan view of a small area of a photo mask that could be used in the photolithographic stage of the process of Figure 4, namely between Figures 4a and 4b ; Figure 6 illustrates three grey-tone masks suitable for use in one embodiment of the current invention; Figure 7 illustrates a plan view of a small area of a photo mask, incorporating the double slit as of Figure 8, which could be used in the photolithographic stage of a process such as described in Figure 4; Figure 8 illustrates the Fraunhofer diffraction pattern generated by light penetrating a double slit ; and Figure 9 illustrates a cross section of the TFT plate after a photolithographic process such as described in Figure 4 has been carried out using the photo mask of Figure 7.

Figure 1 a is a plan view of a prior art AMLCD at a stage in the process of its manufacture, with an enlarged section illustrated in Figure 1b. Each pixel 1 has numerous bumps 2 patterned onto its surface via a known process involving conventional photolithography. This process is illustrated in the cross sectional Figures 2a to 2d. The photosensitive organic material 3, which could be photoresist, polyimide or acrylic, is applied to the TFT plate 4 using a known process such as spin coating or screen-printing which results in a plate with the cross section as shown in Figure 2a. A conventional photo mask 5, shown in Figure 3, having both UV-transparent 6 and UV-opaque 7 areas is

then placed in registration with the TFT plate and UV light is directed through the mask and onto the photoresist to pattern the photoresist. The opaque regions 7 may have a random feature size, w, which is to the order of 8, um to 10 ; im with a random spacing of 12, um to 14Rm. Using a mask 5 having the required pattern of opaque regions, which could be formed in chrome on a quartz plate, results in the cross sectional view 2b also shown as 1c, which can be seen to correspond to a cross section taken along line AB of Figure 1 b.

The remaining photoresist 2 can be seen in Figure 2b to be in the form of steep bumps which, when covered with a reflective metal layer, would not provide the appropriately diffuse reflection required for the LCD. Accordingly, a semi-planarising organic layer 8 is then applied to provide the cross section of Figure 2c, which is more suitable for diffuse reflection. A further photolithographic processing stage is then carried out in order to remove the semi-planarising organic layer 8 from the TFT contact pad area (neither this process nor the contact pad area are shown in Figures). A highly reflective layer 9, which could be aluminium or silver, is then applied to the TFT plate using a method such as sputtering. This results in the TFT plate of Figure 2d, which now has the diffusely reflective properties required for use in a liquid crystal display.

The invention seeks to reduce the number of processing stages in the production of the pixel electrode by introducing the use of a grey-tone mask in the photolithographic stage. Grey-tone masks are photo masks that achieve a greyscale effect when used in conjunction with an optical system. Such a mask could be created using a pattern of UV transparent apertures on a chrome-on-quartz plate.

As illustrated in Figure 4a, a thick layer of photosensitive material 10, such as photosensitive acrylic, is applied to the TFT plate 4. In one example this may be a positive tone photodefinable polyimide as produced by HD MicroSystems, for example HD-8001. This may be applied using a known process such as spin coating, which would preferably be performed at between 500 and 3000 rpm, or screen- printing, to produce a thickness which may be 2. 5, um or greater. A

photo mask 11, as shown in Figure 5, is then used in a photolithographic stage in a similar way to that previously described, however, this mask also includes a variety of fine checkerboard or similar grey-tone patterns between the opaque blocks 12. Examples of these grey-tone patterns covering 50%, 40% and 25% of the mask area are illustrated in Figures 6a, 6b and 6c respectively. Each of these comprises a checkerboard arrangement of UV-opaque regions 14, which could be formed from chrome on an otherwise UV-transparent quartz plate 15. The opaque regions 14 in one such mask could have a feature size, s, of 1. 5) mi, with the spacing between such features 14 adjusted to produce the desired percentage penetration of UV light.

The grey-tone regions described could be applied in the mask at the points shown in Figure 5 as 6a, 6b and 6c, producing, following a photolithographic stage, the cross section of Figure 4b. At the same time, transparent regions on the mask can be used to expose the TFT electrode contact pads in the same photolithographic stage. By way of example, the area shown as T on the photo mask 11 is such a UV- transparent region which, on Figure 4b, can be seen to completely expose the TFT plate 4 where an electrode contact pad may be located.

Other dimensions of the mask, namely d and w as shown in Figure 5, are similar to those of the known mask of Figure 3, as previously described.

The photosensitive material layer 10 is exposed to UV light through the photo mask 11, and the exposed photosensitive material is then developed using, for example, common aqueous positive photoresist developer. The photosensitive material layer is patterned by this process as shown in figure 4b.

As a result of using the different grey-tone masks of Figure 6, it can be seen that the depth of the remaining photosensitive material in Figure 4b is greater in the region 17 relating to the 6a (50%) mask than the region 18 relating to the 6b (40%) mask, due to the extra UV exposure of the photosensitive material at 18. Similarly, the depth of the region 19 of the material is greater than the region 18 relating to the 6c

(25%) mask. Having this multi-depth feature to the pixel electrode is beneficial as it aids the diffusing effect of the pixel.

In addition to this multi-depth feature of the photosensitive material, the corners of the profile are rounded due to diffraction at the border between the opaque and the grey-tone mask regions. This diffraction causes a dispersion of the UV-light which causes a softening of the definition between different regions of the mask. These aspects provide the irregular and smooth surface topography which, when coated with a highly reflective metal layer, such as aluminium or silver, provides the required diffuse reflection for the LCD. The final TFT plate is shown in Figure 4c, with the applied reflective metal layer 21. Using a grey-tone diffraction mask and thus removing the need for a semi-planarising organic layer 8, has resulted in several of the time consuming and expensive stages in the production process of the pixel electrode becoming unnecessary.

In a further aspect of the invention, the grey-tone mask is arranged in a manner such as illustrated in Figure 7. A course line 22, which could measure approximately 1. 5Lm across, of opaque chrome now circumscribes, at a small distance of approximately 1. 5Fm, the opaque blocks 23 that are used to define each of the bumps of the pixel electrode. This results in slits of approximately 1. 51lm of transparent mask area 24 and 25 remaining between the blocks 23 and the chrome line 22. As a result, the double slits either side of the chrome line 22 propagate the Fraunhofer diffraction pattern, an illustration of which is shown in Figure 8. It can be seen that the light intensity profile caused by light penetrating the double slits has a curved intensity profile.

Figure 9 depicts a cross section taken of the TFT plate that has been exposed to UV light that has passed through the mask of Figure 7, the cross section being taken across a line on the TFT plate beneath the line AB of the mask 27. Shown in Figure 9, similarly to the cross section of Figure 4b, the rounded topography necessary for the diffractive surface of the pixel electrode has been produced in one processing stage. This smooth surface is a result

of the double slits on the mask that cause the Fraunhofer diffraction pattern with a curved light intensity profile. Additionally, the electrode contact pads can also be removed during the same photolithographic processing stage with a similar UV-transparent region to that shown in Figure 5 as T. This method of producing diffusely reflective pixel electrodes removes the need for several of the expensive and time consuming steps of the know manufacturing process.

From reading the present disclosure, other variations and modifications will be apparent to persons skilled in the art. Such variations and modifications may involve equivalent and other features which are already known in the design, manufacture and use of photo masks and which may be used instead of or in addition to features already described herein.