WO2000014602A1

WO2000014602A1 - Process for forming a color filter

Info

Publication number: WO2000014602A1
Application number: PCT/US1999/020291
Authority: WO
Inventors: Harris R. Miller
Original assignee: Polaroid Corporation
Priority date: 1998-09-04
Filing date: 1999-09-03
Publication date: 2000-03-16

Abstract

When forming color filters (for example, on solid state imagers or liquid crystal displays) using the process of US-A-4 808 901, in which a positive photoresist composition, containing a novolak resin, a diazo compound which sensitizes the resin to radiation, and a dye, is exposed and developed to form filter elements, very long exposure times are required when the dye obscures the radiation (typically 365 nm, mercury I-line radiation) used for exposure. By adding an acid generator, which generates acid upon exposure to the radiation used to expose the photoresist, exposure times can be substantially reduced without requiring the use of an amount of diazo compound which produces undesirable yellowing in the filter elements.

Description

PROCESS FOR FORMING A COLOR

Attention is directed to commonly-owned International Application PCT/US99/XXXXX (Agent's reference 8342PCT), 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 thermally activated cross-linking agent. After exposure and development of the photoresist to form filter elements, the photoresist is heated to a temperature sufficient to activate the cross-linking agent, thus cross-linking the resin in the filter elements, and stabilizing these elements without yellowing. The entire disclosure of this copending application is herein incorporated by reference.

This invention relates to a process for forming a filter on a substrate; this process is primarily, although not exclusively, intended for forming filters 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 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 filter arrays. In particular, 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 a preferred form of this process, the photoresist resin is a novolak (polyphenol-formaldehyde) resin, which is sensitized to ultra-violet and blue visible radiation with a diazo compound, typically a substituted naphthoquinone diazide compound. Such diazo sensitizers have the advantage that they not only accelerate the breakdown of the photoresist resin in the exposed areas of the adherent layer, but also inhibit the solubility of the photoresist resin in unexposed areas. 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.

This process requires that the dye which is incorporated into the photoresist to form the color image be substantially transparent to the exposing wavelength. This imposes a significant restriction upon the selection of dyes which can be used in the photoresist composition for this process. In particular, certain cyan (blue-green) and yellow dyes which are suitable for use in the process when the exposing wavelength is 436 nm (the "G" mercury emission line) or longer, are no longer suitable for use in the process when the exposing wavelength is 365 nm or shorter because of their absorptions at this shorter wavelength. As is well known to those skilled in semiconductor manufacturing, there is a constant effort to reduce the size of features on semiconductor devices in order to reduce the size and cost of the devices themselves, and reducing the size of features requires a corresponding reduction in the exposing wavelength; features as small as 0.5 μm must be made with exposing wavelengths of 365 nm or shorter. Accordingly, the inability to use the aforementioned dyes with 365 nm or shorter wavelengths presents a serious obstacle to their use in modern semiconductor manufacturing processes.

US-A-5 268 245 describes a process which is generally similar to that of the aforementioned US-A-4 808 501 but which uses a photoresist composition comprising a photoresist resin and a thermochromic dye, this dye being substantially transparent to the radiation used for exposure of the photoresist resin but capable, upon heating, of undergoing a thermally induced color change, which renders it absorptive of such radiation. The photoresist composition is exposed and developed to form filter elements while the thermochromic dye is in its nonabsorptive form (so that no additional exposure time is required), and thereafter the filter elements are heated to cause the dye to undergo its color change, thus causing the filter elements to assume their final color and thus become absorptive of the radiation used for the exposure. This process gives good results, but is restricted to the use of thermochromic dyes, and there are numerous dyes which are desirable for use in filter elements which are not, and cannot conveniently be made, thermochromic.

It might at first glance appear that the lengthy exposure times required to carry out the process of US-A-4 808 501 with dyes which absorb the radiation used for exposure at 365 nm could be reduced by increasing the proportion of diazo sensitizer used with the preferred novolak resin. However, increasing the proportion of sensitizer does not necessarily increase the sensitivity or reduce the exposure time, but does cause formation of a yellow color within the unexposed regions of the photoresist layer which eventually form the filter elements, and obviously the formation of such a yellow color is highly undesirable, since it reduces the amount of blue light "seen" by the imager, resulting in an image sensor which is less sensitive to blue light.

It has now been found that the lengthy exposure times to 365 nm radiation required to carry out the preferred process of US-A-4 808 501 with dyes which absorb the radiation used for the exposure can be substantially reduced, without causing yellowing of the filter elements, by including with the novolak resin and diazo sensitizer an acid generator which produces acid upon exposure to the radiation used to expose the layer of photoresist composition.

Accordingly, this invention provides a process for forming a filter on a substrate. This process is generally similar to that of the aforementioned US-A-

4 808 501 in that it comprises forming on a substrate an adherent layer of a dye- containing positive photoresist composition comprising a novolak resin, a diazo compound which sensitizes the resin to actinic radiation of a first wavelength range, and a dye which absorbs in the first wavelength range; imagewise exposing the adherent layer of photoresist composition to actinic radiation of the first wavelength range; and removing the exposed areas of the adherent layer of photoresist composition to form a pattern of filter elements. However, the process of the present invention is characterized in that the photoresist composition comprises an acid generator which, upon exposure to actinic radiation of the first wavelength range, generates an acid.

This invention also provides a dye-containing photoresist composition for use in this process. This composition comprises a novolak resin; a diazo compound which sensitizes the resin to actinic radiation of a first wavelength range; a dye which absorbs in the first wavelength range; and a solvent in which the novolak resin, diazo compound, dye and acid generator are dissolved. The composition of the present invention is characterized in that it comprises an acid generator which, upon exposure to actinic radiation of the first wavelength range, generates an acid.

As already indicated, in the process of the present invention there is included in the photoresist composition, in addition to the novolak resin, the diazo sensitizer and the dye, an acid generator which, upon exposure to the radiation of the first wavelength range used for exposure, generates an acid. This acid contributes to solubilizing the novolak resin in the exposed areas of the layer of photoresist composition, thus increasing the rate of dissolution of these exposed areas in the dilute aqueous alkaline developing solutions typically used to develop photoresists and thus decreases the exposure required to achieve formation of the filter elements. In some cases, the amount of 365 nm exposure required can be reduced by about 50 per cent.

As will be apparent to those skilled in the art of using photoresists, it is not essential in the present process that the sensitizer and the acid generator absorb at exactly the same wavelength. In some cases, the process may be carried out using a broadband source, such as an ultra-violet fluorescent tube or broad band mercury lamp, and in such cases the sensitizer may absorb at one wavelength and the acid generator at a different wavelength within the first wavelength range (a term which is used herein with its conventional meaning as regards ultra-violet radiation, namely a band of adjacent wavelengths not exceeding 50 nm). In other cases, the exposure may be carried out using essentially monochromatic radiation, and in such cases it is of course essential that both the sensitizer and the acid generator absorb the same, single wavelength of radiation used. Although any type of ionizing radiation may be used in the present process, desirably, the first wavelength range falls within the near ultra-violet region of 190 to 400 nm, and in an especially preferred embodiment of the present process, the exposure is effected using radiation having a wavelength of 365 nm. The preferred diazo sensitizer for use in the present process is esterified 2,1,5 naphthoquinone diazosulfonochloride. Other useful sensitizers include diazoquinones such as diazonaphthoquinone 5-arylsulfonates and 4- sulfonates, 1.3-diacyl-2-diazoquinones and diazopiperidinedione.

One class of dyes which give filter elements having very stable colors but which have substantial near ultra-violet absorption, and which are unusable in the unmodified process of US-A-4 808 501, are metal phthalocyanine dyes, especially copper phthalocyanine dyes. Other metal complex dyes, such as chrome and cobalt complexes also have strong absorption at 365 nm. As illustrated in the Example below, dyes of these types can be successfully used in the process of the present invention.

A wide variety of acid generators can be employed in the present process, 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 micro- lithography; 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-tπ trichloromethyl)-^w-triazine), diaryl- iodonium salts (for example, diphenyliodonium hexafluoroantimonate), triaryl- sulfonium 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-Z>7s(4-bromophenyi)propane), hydroxamic acid esters (for example, the trifluoromethanesulfonic acid ester) and diazosulfonates (for example, the compound of formula Ph-SO -C(=N₂)-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- ots(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 cause improper formation of the filter 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 photoresist composition.

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 an 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 introducing unwanted absorptions in 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. Apart from the presence of the acid generator, the preferred components and conditions for use in the process and composition of the present invention are the same or similar to those used in the process of 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 (excluding the solvent). The dye, sensitizer and acid generator 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 must also be thermally stable and light stable, that is, so that during processing or during the heat-stabilization step (see below), the pre-dyed color will be sustained. Since the product is a filter it should have good long term stability. It is also necessary that the dye interact sufficiently with the resin whereby 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 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 the 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 "post-apply bake") to a temperature sufficient to ensure rapid evaporation of the solvent (typically to around 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 "post- exposure bake"). 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 elements formed are stabilized by baking or by silylation in accordance with US-A-5 667 920, prior to the formation of any further filter 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, since not all of the dyes used may have significant absorption at the 365 nm or other wavelength of the exposing radiation. 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 a different process not requiring the use of an acid generator, for example that described in 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 the process described in US-A-4 808 501. 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: Axanthene dye of the formula:

Dye C: Aphenazine 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

*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^"2 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 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 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 allows the use, in a process similar to that of the aforementioned US-A-4 808 501, of dyes which absorb significantly at the wavelength used for the exposure, without requiring excessive exposure and bake times. The present process 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. The present process avoids the costly and complicated problem of designing a specific dye molecule having the visual absorption desired in the filter elements and an ultraviolet absorption which does not interfere with the exposure step of the process. Instead, in the present process the absorption of the dye at the wavelength used for the exposure is in effect overcome by "amplification" of the exposing radiation, this amplification being achieved by incorporating the acid generator.

Claims

1. A process for forming a filter on a substrate, which process comprises: forming on a substrate an adherent layer of a dye-containing positive photoresist composition comprising a novolak resin, a diazo compound which sensitizes the resin to actinic radiation of a first wavelength range, and a dye which absorbs in the first wavelength range; imagewise exposing the adherent layer of photoresist composition to actinic radiation of the first wavelength range; and removing the exposed areas of the adherent layer of photoresist composition to form a pattern of filter elements, characterized in that the photoresist composition further comprises an acid generator which, upon exposure to actinic radiation of the first wavelength range, generates an acid

2. A process according to claim 1 characterized in that the diazo compound is a diazonaphthoquinone.

3. A process according to either of the preceding claims characterized in that the dye is a metal phthalocyanine dye.

4. A process any one of the preceding claims 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.

5. A process any one of the preceding claims characterized in that the diazo compound comprises from 10 to 30 percent by weight of the photoresist composition.

6. A process according to any one of the preceding claims characterized in that the acid generator comprises from 0.2 to 10 percent by weight of the photoresist composition.

7. A process according to claim 6 characterized in that the acid generator comprises from 0.3 to 3 percent by weight of the photoresist composition.

8. A process any one of the preceding claims characterized in that the dye comprises from 10 to 50 percent by weight of the photoresist composition.

9. A process any one of the preceding claims characterized in that the first wavelength range falls within the range of 190 to 400 nm.

10. A process any one of the preceding claims characterized in that, after the imagewise exposure, the adherent layer of photoresist composition is heated to a temperature sufficient to stabilize the photoresist before the exposed areas of the adherent layer are removed.

11. A process any one of the preceding claims characterized in that the substrate is a solid state imager.

12. A dye-containing photoresist composition for use in forming a filter, the composition comprising: a novolak resin; a diazo compound which sensitizes the resin to actinic radiation of a first wavelength range; a dye which absorbs in the first wavelength range; and a solvent in which the novolak resin, diazo compound, dye and acid generator are dissolved the composition being characterized by an acid generator which, upon exposure to actinic radiation of the first wavelength range, generates an acid;.

13. A composition according to claim 12 characterized in that the components thereof are as defined in any one of claims 2 to 8.