WO2000014603A1

WO2000014603A1 - Process for preparing a color filter or overlay

Info

Publication number: WO2000014603A1
Application number: PCT/US1999/020292
Authority: WO
Inventors: Harris R. Miller
Original assignee: Polaroid Corp
Current assignee: Polaroid Corp
Priority date: 1998-09-04
Filing date: 1999-09-03
Publication date: 2000-03-16
Anticipated expiration: 2001-03-04

Abstract

A pattern of filter or overlay elements are formed on a substrate (typically an image sensor or display device) using a positive photoresist composition (preferably as described in US-A-4 808 501) comprising a resin and a thermally activated cross-linking agent. After exposure, the photoresist layer is heated to a temperature sufficient to activate the cross-linking agent, thereby cross-linking the exposed layer. The presence of the cross-linking agent reduces or eliminates yellowing which may otherwise occur during the conventional thermal stabilization of the exposed photoresist, and also increases the resistance of the filter or overlay elements to solvents present in photoresist compositions used to form additional sets of filter or overlay elements and to other thermal treatments which may be required during manufacture of a device.

Description

PROCESS FOR PREPARING A COLOR FILTER OR OVERLAY

Attention is directed to commonly-owned International Application PCT US99/XXXXX (Agent's reference 8343PCT(DJC)), filed simultaneously herewith, which describes and claims a process for forming a filter. This process is generally similar to that of the present invention, but uses a photoresist composition comprising a diazo compound, which sensitizes a novolak resin to actinic radiation in a first wavelength range, and an acid generator which, upon exposure to radiation in this range, generates an acid. The resultant acid contributes to solubilizing the resin in the exposed areas of the photoresist composition and thus decreases the exposure required to achieve formation of the filter elements.

This invention relates to a process for forming a filter or overlay on a substrate; this process is primarily, although not exclusively, intended for forming filters or overlays on solid state imagers and color display devices, for example charge coupled device (CCD) image sensors, complementary metal oxide semiconductor (CMOS) image sensors, liquid crystal displays (LCD's), and plasma screen display devices. The process may also be used to form filters or overlays on other types of solid state circuits.

The term "filter" or "filter layer" is used herein to mean the whole layer placed upon a substrate to control the passage of electromagnetic radiation to or from this substrate; this filter is typically comprised of more than one color. The term "filter element" is used to refer to a single physically continuous element of the filter of the same color throughout; such a filter element may be a dot or a stripe or have a different physical form. The term "set of filter elements" refers to a plurality of filter elements of the same color physically separated from one another. The term "having color" is used to mean "modulating at least a portion of electromagnetic radiation of a particular wavelength by transmission, absorption, diffraction, refraction, fluorescence or phosphorescence", and does not necessarily refer only to visible radiation, but can also refer to wavelengths well beyond the reach of the human eye; thus, the filters formed by the present process may pass only predetermined infra-red or ultra-violet wavelengths, even though such filter elements appear opaque or transparent to the human eye. The modulation of electromagnetic radiation by a filter element is generally fixed by the absorption of a dye or by diffraction of a coating, However, the portion of the wavelength range that the filter element modulates can itself be modulated by external means, such as by applying an electrical field.

To obtain a color image using solid state imagers such as charge coupled devices or CMOS images sensors, optical filters in a multicolor stripe or mosaic array are employed; in many cases, these filters are formed directly upon the photosensitive surface of the solid state imager. Similarly, in color liquid crystal display devices, optical filters in a multicolor stripe or mosaic array are provided to control the color of the light which is reflected from, or transmitted through, the "light gate" provided by each individual liquid crystal pixel. Both these types of filters are normally provided with elements having two or three differing colors. For example, a two color filter may have yellow and cyan elements which overlap in part, with the overlap area providing, in effect, a green element. A three color filter will typically have red, green and blue, or cyan, magenta and yellow elements.

A number of processes are described in the art for preparing such filters. For example, US-A-4 239 842 describes a process for producing a color filter array by depositing successively on a semi-conductive layer, such as a charge coupled device, a sub-coat, a polymeric mordant, and a photoresist. The photoresist layer is exposed and developed to form a mask. A dye is then heat transferred through the apertures in the photoresist into the polymeric mordant. Finally, the photoresist is stripped.

US-A-4 808 501 describes a process for forming a color filter on a support, such as a charge coupled device, by (a) forming a layer on a support with a composition comprising a positive photoresist and a dye, the dye being soluble in the solvent and the polymer of the photoresist; (b) exposing predetermined portions of the layer to radiation adapted to increase the solubility of the coating in the exposed areas; (c) developing the exposed areas to form a pattern of filter elements; and (d) repeating these steps with a different color dye in the composition; wherein the dye constitutes in excess of 10% by weight, dry basis of the composition, is substantially non-absorptive in the exposure wavelength of the composition, and provides predetermined absorptive characteristics for the specified filter element and the dye possesses substantially the same polarity as the composition. In practice, after the exposed areas have been developed, the patterned photoresist must be baked, typically at 140-150°C for 3 to 5 hours, to stabilize the filter elements.

US-A-5 059 500 describes a process for forming a filter using differential reactive ion etching techniques. This process comprises: providing on the substrate a layer of an absorber material having predetermined absorption and transmission characteristics; providing a layer of a barrier material superposed on the layer of absorber material, the barrier material being more susceptible to reactive ion etching than the absorber material under a first set of etching conditions, but resistant to reactive ion etching under a second set of etching conditions under which the absorber material is susceptible to etching; providing a layer of a photoresist material superposed on the layer of barrier material; patternwise exposing the layer of photoresist material and developing the exposed layer to remove either the exposed or non exposed regions thereof, thereby to bare the regions of the barrier layer underlying the removed regions of the photoresist material, the remaining regions of the photoresist material being resistant to reactive ion etching under said first set of etching conditions but susceptible to reactive ion etching under said second set of etching conditions; reactive ion etching the coated substrate under said first set of etching conditions, thereby etching away the bared regions of the barrier layer and baring selected regions parts of the absorber layer, but not etching away the remaining regions of the photoresist material nor substantially etching away the bared regions of the absorber layer; and reactive ion etching the coated substrate under said second set of etching conditions, thereby etching away the remaining regions of the photoresist layer and the bared regions of the absorber layer, and thereby forming a filter on the substrate. The processes described in US-A-4 808 501 and US-A-5 059 500 give excellent results. However, the process of US-A-5 059 500 requires the use of reactive ion etching equipment. The process of US-A-4 808 501 requires three separate, time-consuming baking steps and imposes a number of stringent requirements upon the dye. As discussed in this patent, the dye must be sufficiently soluble in the photoresist resin that the relatively concentrated dye solution required for the process can be achieved, without the dye tending to precipitate out, either during the formation of the filter, or during the long service life (of the order of several years) of the solid state imager. Furthermore, the dye must be sufficiently stable in solution and sufficiently stable in the filter elements to withstand, without unacceptable color loss, the thermal or radiant-exposure treatments which are normally required to stabilize the filter elements of each color before the next color is applied; if this thermal or radiant-exposure treatment is omitted, the solvent used to deposit the second layer of photoresist redissolves the first set of filter elements, thereby deforming the filter elements, reducing the selectivity of the filter, promoting cross-talk between the various color channels of light passing through the filter, increasing the color noise and reducing the resolution of the filter. The dye must also not interfere with development of the exposed areas of the photoresist. Finally, the dye must be substantially non-absorptive in the exposure wavelength of the composition. This combination of requirements greatly limits the choice of dyes which can be used in the process of US-A-4 808 501, and thus limits the minimum exposure wavelength and hence the minimum feature size, filter element size, or sensor resolution. In particular, the requirement that the dye withstand a thermal stabilization treatment eliminates numerous dyes from being used in the process, and increases the concentrations of other dyes which must be used, since many dyes which can survive a thermal stabilization undergo significant color loss during this step. A thermal stabilization treatment also creates other problems, especially reflowing and yellowing of the photoresist, which distorts and discolors the filter elements, thus reducing the resolution and sensitivity of the device, and reduce production yields by rendering certain filters unacceptable. US-A-5 667 920 describes a modification of the process of US-A-

4 808 501 in which, after formation of a first set of filter elements, the substrate on which the filter elements are formed is treated with a silylation compound having at least two functional groups, this silylation compound being capable of cross-linking the photoresist used to form that filter elements, and of promoting adhesion of the photoresist to the substrate. In a preferred form of this process, the substrate is also treated with the silylation compound prior to the formation of the first set of filter elements. The silylation treatment reduces or eliminates the aforementioned high temperature baking steps which are otherwise required to stabilize one set of filter elements before the substrate is subjected to the process required to form a second set of filter elements, and thus enables a wider variety of dyes to be used in the process. The reduction or elimination of the baking steps also reduces, or in some cases eliminates the problems caused by yellowing of the photoresist. However, despite the silylation treatment, some yellowing of the photoresist can still occur. This residual yellowing can be significantly reduced by exposing the photoresist to light prior to silylation, but unfortunately such light exposure greatly reduces the stabilizing action of many silylating compounds.

It has now been found that the aforementioned yellowing and reflowing problem can be reduced or eliminated by incorporating into the photoresist composition a thermally activated cross-linking agent capable of cross- linking the resin used in the photoresist. Incorporating the cross-linking agent also enables a filter to be formed without requiring the equipment needed to deliver the silylating compound under vacuum, as required by US-A-5 667 920. Similar advantages can be gained in a process for producing a colorless overlay on a substrate. Accordingly, this invention provides a process for forming filter or overlay elements on a substrate. This process is generally similar to those of the aforementioned US-A-4 808 501 and US-A-5 667 920 in that it comprises: forming on the substrate an adherent layer of a positive photoresist composition comprising a resin; imagewise exposing the adherent layer of photoresist composition to actinic radiation, thereby increasing the solubility of the layer in the exposed areas; and removing the exposed areas of the adherent layer of photoresist composition to form a pattern of elements. However, the process of the present invention is characterized in that the photoresist composition comprises a thermally activated cross-linking agent capable of cross-linking the resin, and is also characterized in that the formed elements are heated to a temperature sufficient to activate the cross-linking agent, thereby cross-linking the resin in the elements.

This invention also provides an article having a surface bearing a pattern of filter elements, these filter elements comprising a positive photoresist and a dye. The article is characterized in that the photoresist is cross-linked with a thermally activated cross-linking agent.

As will readily be apparent to those skilled in the art of forming filters on solid state imagers and similar devices, the process of the invention has two principal variants. In the first variant, a photoresist composition (preferably a novolak resin) containing a dye is used, so that the filter elements are formed already dyed. In the second variant, the dye is omitted from the photoresist composition and transparent overlay elements are produced. This second variant may be used to form transparent "filter elements" or a transparent barrier coating, such as a protective or planarizing layer. The transparent overlay elements may, of course, subsequently be dyed to form colored filter elements.

In both variants of the process an adherent layer of the photoresist composition is formed on the substrate, and exposed and developed in any conventional manner to form filter or overlay elements. These elements are then heated to a temperature sufficient to activate the cross-linking agent, thus causing cross-linking of the resin in the elements. The cross-linking agent should be chosen so that it is not activated during the heating step (usually called the "post-apply bake" or "PAB") required to remove the solvent during the formation of the adherent layer of photoresist composition on the substrate, nor during the heating step (usually called the "post-exposure bake" or "PEB") which is normally carried out between the exposure and development steps of the process. After the exposure and development steps have been completed, the cross-linking agent is raised to a temperature greater than those of the PAB and PEB, and then rapidly cures the elements, thus enabling stabilized elements to be produced without degradation of the photoresist's sensitivity to the exposing radiation and without significant yellowing of the elements. In addition, the stabilization renders the elements impervious to attack by solvent present in subsequent photoresist compositions used to form additional sets of elements or other components during later stages of the process for forming the final device. The stabilization also simplifies reworking of subsequent sets of elements without affecting earlier sets of elements. Finally, the stabilization ensures that the elements can tolerate higher temperatures commonly used to package semiconductor devices.

Although other types of cross-linking agents may be used, the present process is desirably carried out using a melamine derivative as the cross-linking agent; a specific preferred cross-linking agent is hexamethoxymethylolmelamine. This cross-linking agent does not activate at temperatures around 90-95°C which are typically used in PAB and PEB, but does activate when heated to about 145°C in a convection oven for approximately 15 minutes, or on a hot plate for 2 minutes, conditions to which many substrates, such as solid state imagers and liquid crystal display components, can be exposed without damage. The optimum amount of any specific cross-linking agent may readily be determined empirically; however, the cross-linking agent is preferably used in an amount of from 0.1 to 10 per cent by weight, preferably 0.5 to 2 per cent by weight of the photoresist composition (proportions of materials in the photoresist composition mentioned herein are proportions by dry weight, i.e., excluding the solvent(s) present in the composition). If desired, the photoresist composition containing the cross-linking agent may be exposed to actinic radiation after development but prior to the thermal treatment which activates the cross-linking agent; such exposure is sometimes useful in further reducing the risk of yellowing of the photoresist. The present invention can be, and preferably is, used in the process described in the aforementioned International Application No. PCT/US99/XXXXX, in which the photoresist composition comprises a diazo compound which sensitizes the resin to the actinic radiation used for the exposure, and an acid generator which, upon exposure to the actinic radiation used, generates an acid. In this form of the present process, a wide variety of acid generators can be employed, provided of course that the specific acid generator used generates acid when exposed to the radiation employed, and provided that the acid generator does not cause unwanted interactions with the other components of the photoresist composition. Various types of acid generators are known to those skilled in microlithography; see, for example, Thompson, L.F., Willson, C.G. and Bowden, M. J., Microlithography (2d

Edn.), American Chemical Society, Washington DC (1994), at 217, 262-263. Types of acid generators which may be useful in the present process include triazines (for example, l,3,5-trø(trichloromethyl)-.syrø-triazine), diaryliodonium salts (for example, diphenyliodonium hexafluoroantimonate), triarylsulfonium salts (for example triphenylsulfonium hexafluoroantimonate) o-nitrobenzyl esters (for example, the trifluoromethanesulfonic acid ester of o-nitrobenzyl alcohol), phloroglucinol sulfonates (for example, the methanesulfonic acid triester of phloroglucinol), bisphenols and derivatives thereof (for example "Bromobisphenol A", 2,2-b/5(4-bromophenyl)propane), hydroxamic acid esters (for example, the trifluoromethanesulfonic acid ester) and diazosulfonates (for example, the compound of formula Ph-SO₂-C(=N )-C(=O)-Ph, where Ph represents a phenyl group). A specific preferred acid generator for use in the present process is Bisphenol A (2,2-bts(4-hydroxyphenyl)propane).

It is desirable to keep the proportion of the diazo sensitizer in the photoresist composition low in order to avoid yellowing of the photoresist layer in the unexposed areas. On the other hand, if the proportion of sensitizer is reduced too far, the sensitizer will not be effective in reducing the solubility of the photoresist resin in the unexposed areas (i.e., may not sufficiently inhibit dissolution of the photoresist in the aqueous alkaline developer) and may thus degrade the filter or overlay elements during the development step. Although the optimum proportion of sensitizer will vary with the exact sensitizer employed, in general it is desirable that the diazo sensitizer comprise from 10 to 30 percent by weight of the photoresist composition (excluding any solvent present therein); desirably, the diazo sensitizer comprises about 21 percent by weight of the adherent layer. The optimum proportion of acid generator in the photoresist composition can readily be determined empirically. As will be apparent to persons skilled in microlithography, too small a proportion of acid generator will result in insufficient acceleration of the breakdown on the photoresist resin in exposed areas, and a longer-than-optimum exposure time being required. On the other hand, increasing the proportion of acid generator beyond the optimum results in little or no additional acceleration of the breakdown on the photoresist resin and may have various disadvantages, for example by reducing the stability of the filter elements. In general, it is preferred that the acid generator comprise from 0.2 to 10 percent by weight of the photoresist composition (excluding any solvent present therein); desirably, the acid generator comprises from 0.3 to 3 percent by weight of the photoresist composition.

Dyes which may be used in the first variant of the present process include metal phthalocyanine dyes, especially copper phthalocyanine dyes, xanthene, phenazine, azo and organo-chrome complex dyes. Specific preferred dyes of the copper phthalocyanine, xanthene and phenazine types are described in the

Example below.

Apart from the presence of the cross-linking agent, the preferred components and conditions for use in the first variant of the process of the present invention are the same or similar to those used in the process of the aforementioned US-A-4 808 501. Thus, desirably the photoresist composition contains a large proportion of the dye, preferably from 10 to 50 per cent by weight.

Any dye and cross-linking agent employed in the present invention must of course be soluble in the same solvent as the photoresist resin. Optionally, a second solvent may be employed to facilitate dissolution of the dye and the other components. Examples of suitable solvents for use in the photoresist compositions include dimethyl sulfoxide, dimethyl formamide, n-butyl acetate, 2-ethoxyethyl acetate, ethoxyethyl propionate, xylenes, ethyl benzene, propylene glycol methyl ether (l-methoxy-2-propanol), and combinations thereof. The dye, if present in the initial photoresist composition, must also be thermally stable and light stable, so that during the useful life of the product and during the processing and heat-stabilization step (see below), the pre-dyed color will be sustained. It is also necessary that the dye completely solubilizes in the resin so that crystallization into a separate phase will not occur upon drying of the photoresist composition. To achieve this compatibility, the dye is desirably selected to have the same polarity as that of the photoresist resin so that the dye will mimic the bulk properties of the resin. The concentration of dye in the photoresist composition is selected with respect to the desired optical density of the filter elements. Thus, the concentration of the dye must be such to provide predetermined absorption and transmission filtering characteristics for the desired filter element. As stated above, desirably the dye constitutes in excess of 10% up to about 50% by weight, dry basis, of the photoresist composition. It should also be understood that the term "dye", as used herein, is intended to refer to combinations of one or more dyes as well as single dyes and also includes, as well as dyes in the visible region, near infrared and fluorescent dyes. By employing large amounts of dye, i.e., in excess of 10% by weight, dry basis, filters having good light transmission characteristics are obtained without the need for very thick filter elements. Thus, the filter elements produced by the present process can be 1.2 to 2 micrometers or less in thickness.

Typically, the process of the invention will be carried out in the conventional manner which will be familiar to those skilled in the manufacture of solid state imagers. The solid state imager, liquid crystal display or other substrate on which the filter is to be formed, is typically vapor deposited or spin coated with an adherent layer and then dried. Next, the photoresist composition is spin coated over the adherent layer, and heated (the PAB step) to a temperature sufficient to ensure rapid evaporation of the solvent (typically to 90-110°C) and cause the formation of the adherent layer of photoresist. The coated substrate will then be exposed to the actinic radiation, and then normally heat treated again to remove the standing waves and improve the sidewall angle of the filter elements (the PEB step). The coated substrate is then developed by treatment with a solvent, or etched with a gas plasma, which removes the exposed areas of the layer; the solvent is typically an aqueous alkaline solution, for example a solution of a quaternary ammonium hydroxide. Finally, the developed substrate is washed with a solvent, typically deionized water, to remove all traces of the developing agent.

Following these development and rinsing steps, the filter or overlay elements formed are stabilized by baking to a temperature sufficient to activate the cross-linking agent, as already described. Optionally, the filter or overlay elements may also be silylated in accordance with US-A-5 667 920, prior to the formation of any further elements on the substrate.

As is well known to those skilled in preparing filters, a full color filter requires the formation of filter elements having at least two, and usually three, different colors; three color filters typically have red, green and blue, or, cyan, magenta and yellow filter elements. Each of the different colors of filter elements requires a separate processing cycle including formation of a photoresist layer, exposure and development. It will be appreciated that when the present process is employed in the manufacture of a full color filter, not all of the filter elements need be formed by the present process. For example, when a filter having red, green and blue filter elements is to be manufactured, the green and blue filter elements might be produced by the process of the present invention, while the red filter elements might be produced by the process of US-A-5 667 920. Similarly, when a filter having yellow, cyan and magenta filter elements is to be manufactured, the yellow and cyan filter elements might be produced by the process of the present invention, while the magenta filter elements might be produced by a different process. Furthermore, each color can be formed by a combination of two or more dyes in the photoresist composition, or by layering or overlapping two or more filter elements. For example, red can be formed by incorporating red dye in a photoresist composition, incorporating magenta and yellows dyes in a photoresist composition, or by overlaying a magenta filter element over a yellow filter element. Similarly, a fluorescing dye may absorb radiation in the ultra-violet region and emit this radiation in the visible region, this visible radiation in turn being modulated by another dye which transmits part of the visible spectrum. Both the fluorescing dye and visible dye can be incorporated in the same photoresist composition or in two or more separate elements.

The following Example is now given, though by way of illustration only, to show details of particularly preferred reagents, conditions and techniques used in the process and composition of the present invention. The dyes used in this Example are as follows: Dye A: A copper phthalocyanine dye of the formula:

Dye B: A xanthene dye of the formula:

Dye C: A phenazine dye of the formula:

Dye D: Orasol Yellow 2GLN, available commercially from Ciba Specialty

Chemicals Corporation, 4050 Premier Drive, High Point, North Carolina 27265, United States of America.

Dye E: Orasol Red B, also available commercially from Ciba Specialty

Chemicals Corporation.

Example Red, blue and green photoresist compositions were prepared having the following compositions: Red photoresist composition

Blue photoresist composition

This material comprises approximately 8.5 percent by weight diazo sensitizer and approximately 30 percent by weight total solids. Green photoresist composition

The PR1-2000S1 photoresist is available from Futurrex, Inc., 44-50 Clifton Street,

Newton, New Jersey 07860, United States of America. The OCG 825 50cs photoresist is available from OCG Microelectronics Materials, West Patterson, New Jersey, United States of America, and is a novolak resin containing a diazonaphthoquinone sensitizer. Cymel 303 resin is available commercially from Cytex Industries, Inc., South Cherry Street, Wallingford, Connecticut, United States of America.

A silicon wafer was pretreated in a vacuum oven with a hexamethyl- disilizane adhesion layer and was spin coated with the red photoresist composition at a spin speed of 3000 rpm., and then baked on a hot plate at 90°C for 90 seconds to produce an adherent layer of dried photoresist composition 1.2 μm thick. The coated surface of the wafer was then imagewise exposed to 400 mJ cm^" of 365 nm ultra-violet radiation, and then again baked on the hot plate at 90°C for 90 seconds. The red photoresist layer was developed with a 0.132 M (1.4 % w/w) aqueous solution of tetramethylammonium hydroxide (TMAH) for 180 seconds at 22°C, then rinsed with deionized water for 10 seconds and air dried to form a set of red filter elements on the silicon wafer. These red filter elements were then stabilized by baking the wafers on a hot plate for 3 minutes at 145°C. The wafer bearing the red filter elements was next spin coated with the blue photoresist composition at a spin speed of 3000 φm., and then baked on the hot plate at 90°C for 90 seconds to produce an adherent layer of dried photoresist composition 1.2 μm thick. The coated surface of the wafer was then imagewise exposed to 450 mJ cm^"2 of 365 nm ultra-violet radiation, and developed with a 0.132 M (1.4 % w/w) aqueous solution of TMAH for 90 seconds at 22°C, then rinsed with deionized water for 10 seconds and air dried to form a set of blue filter elements on the silicon wafer. These blue filter elements were then stabilized by baking the wafers on a hot plate for 3 minutes at 145°C.

The wafer bearing both the red and blue filter elements was next spin coated with the green photoresist composition a spin speed of 3000 rpm., and then baked on the hot plate at 90°C for 90 seconds to produce an adherent layer of dried photoresist composition 1.2 μm thick. The coated surface of the wafer was then imagewise exposed to 800 mJ cm^"2 of 365 nm ultra-violet radiation, and developed with a 0.132 M (1.4 % w/w) aqueous solution of TMAH for 90 seconds at 22°C, then rinsed with deionized water for 10 seconds and air dried to form a set of green filter elements on the silicon wafer. These green filter elements were then stabilized by placing the wafers in a convection oven for 30 minutes at 145°C.

From the foregoing, it will be seen that the present process a process for forming filter or overlay elements which overcomes the yellowing and reflowing problems encountered in prior art processes. The present process can be practiced as a modification of that described in US-A-4 808 501, and can be carried out using materials which are readily available commercially and using commercial apparatus well known to those skilled in the art of microlithography.

Claims

l b

CLAIMS 1. A process for forming filter or overlay elements on a substrate, which process comprises: forming on the substrate an adherent layer of a positive photoresist composition comprising a resin; imagewise exposing the adherent layer of photoresist composition to actinic radiation, thereby increasing the solubility of the layer in the exposed areas; and removing the exposed areas of the adherent layer of photoresist composition to form a pattern of elements, the process being characterized in that the photoresist composition comprises a thermally activated cross-linking agent capable of cross-linking the resin, and further characterized in that the formed elements are heated to a temperature sufficient to activate the cross-linking agent, thereby cross-linking the resin in the elements.

2. A process according to claim 1 characterized in that the photoresist composition further comprises a dye so that the elements formed are dyed filter elements.

3. A process according to claim 2 characterized in that the resin is a novolak resin.

4. A process according to claim 1 characterized in that the photoresist composition is essentially free from dye so that the elements formed are undyed overlay elements.

5. A process according to claim 4 characterized in that, after the heating step, the formed overlay elements are dyed to form dyed filter elements.

6. A process any one of the preceding claims characterized in that the cross-linking agent is a melamine derivative.

7. A process according to claim 6 characterized in that the melamine derivative is hexamethoxymethylolmelamine.

8. A process any one of the preceding claims characterized in that the cross-linking agent is present in an amount of from 0.1 to 10 per cent by weight of the adherent layer of photoresist composition.

9. A process any one of the preceding claims characterized in that the photoresist composition further comprises a diazo compound which sensitizes the resin to the actinic radiation used for the exposure, and an acid generator which, upon exposure to the actinic radiation used, generates an acid.

10. A process according to claim 9 characterized in that the acid generator comprises any one or more of a triazine, a diaryliodonium salt, a triarylsulfonium salt, an ort rø-nitrobenzyl ester, phloroglucinol sulfonate, a hydroxamic acid ester, a diazosulfonate and bisphenol A.

11. A process according to claim 9 or 10 characterized in that the diazo compound comprises from 10 to 30 percent by weight of the photoresist composition., and/or the acid generator comprises from 0.2 to 10 percent by weight of the photoresist composition.

12. A process according to claim 2 or 3 characterized in that the dye is a metal phthalocyanine dye.

13. A process according to any one of claims 2, 3 and 12 characterized in that the dye comprises from 10 to 50 percent by weight of the adherent layer of photoresist composition.

14. A process according to any one of claims 2, 3, 12 and 13 characterized in that, after formation of the filter elements, a second adherent layer of dye-containing photoresist composition is formed on the substrate, and imagewise exposed to actinic radiation, and one of the exposed and unexposed area of the second adherent layer of photoresist is removed to form a second pattern of filter elements, the second pattern of filter elements having a color different from the earlier- formed pattern of filter elements.

15. A process any one of the preceding claims characterized in that the substrate a solid state imager or a liquid crystal display or component thereof.

16. An article bearing a pattern of filter elements, the filter elements comprising a positive photoresist and a dye, characterized in that the photoresist is cross-linked with a thermally activated cross-linking agent.

17. An article according to claim 16 characterized in that the cross-linking agent is a melamine derivative.

18. An article according to claim 17 characterized in that the melamine derivative is hexamethoxymethylolmelamine.

19. An article according to any one of claims 16 to 18 which is a solid state imager.