IMAGE-REVERSIBLE DRY-FILM PHOTORESISTS

Abstract: A dry-film photoresist is described which comprises a film of a light-sensitive image-reversible photoresist composition supported on a removable backing sheet and, optionally, having a removable cover sheet extending substantially completely over the exposed surface of the film. The photoresist composition employed in the film comprises a mixture of at least one phenolic resin, a 1,2-quinonediazide sensitizer and, optionally, at least one organic solubilizing agent containing phenolic and/or carboxylic acid groups and present in the mixture in an amount to promote the solubility of the mixture in alkaline developer. The dry-film photoresist can be used to produce either positive or negative resists on a substrate. The dry-film photoresist is laminated to the substrate and then exposed imagewise to actinic radiation. To produce a positive image the exposed film is developed using an alkaline developer. To produce a negative image the exposed film is first baked to a temperature and for a time sufficient to render the exposed portions of the photoresist insoluble in alkaline developer and is then developed, optionally after flood exposure to actinic radiation, using an alkaline developer to remove the unwanted portions of the photoresist.

Description: IMAGE-REVERSIBLE DRY-FILM PHOTORESISTS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to photoresist systems and is more particularly concerned with dry-film light- sensitive photoresist systems which can be employed to produce both positive and negative images depending on the post-imaging steps employed. 2. Description of the Prior Art Photoresist systems which give rise to positive resist images and photoresist systems which give rise to negative resist images are well-known in the art. How¬ ever, the two types of system are generally very differ¬ ent from each other in composition and behavior. Thus, positive resist systems generally comprise a base-soluble polymer such as a novolak resin and a photosensitizer containing diazo and keto groups on adjacent positions in a benzenoid structure. Exposure of the composition to actinic radiation serves to convert the diazo-keto con- figuration of the sensitizer to a carboxyl group thereby rendering the exposed composition soluble in alkaline developers. In contrast, negative resist systems general¬ ly comprise a polymer in association with a sensitizer which initiates cross-linking of the polymer upon expo- sure to actinic radiation. The exposed image is thereby rendered insoluble in developer solvents and is developed by dissolving away the unexposed portions of the resist system. More recently, photoresist systems have been described which are capable of being processed in the negative as well as the positive modes. Illustratively, Kaplan et al. U.S. Patent 4,007,047 describes a process by which a negative image can be produced using a system based on an alkali-soluble resin and a diazo-ketone sensi¬ tizer which normally is employed in the positive mode. In order to produce a negative resist image using such a system a layer of the composition is exposed imagewise to actinic radiation to convert the diazo-keto sensitizer to an alkali soluble carboxylic acid, the exposed image is treated with hot mild aqueous acid (which treatment appar¬ ently converts the alkali soluble form of the sensitizer to an alkali-insoluble form by decarboxylation) , and then the layer so treated is subject to a blanket exposure which converts the sensitizer in the previously unexposed portions of the layer to the carboxylic acid form. The negative image is then developed using an alkaline developer which removes the originally unexposed portions of the photosensitive layer which have been rendered alkali soluble by the blanket exposure. Stah'lhofen U.S. Patent 4,581,321 describes- a varia¬ tion of the above process in which the material in the exposed image layer is rendered insoluble in alkaline developer by a post exposure bake of the image after which the previously unexposed portions of the photosensi¬ tive layer are blanket exposed to radiation as in the previous process and the negative image developed using alkaline developers. A hexamethylolmelamine ether is included in the composition employed in the process and its function is said to be that of a cross-linker for the " polymeric resin in the composition. The cross-linking takes place during the post-exposure bake and is said to be catalyzed by the acid generated in the portion of the photosensitive layer which has been exposed to the actinic radiation. Chiong et al., IBM Technical Disclosure Bulletin 27, No. 1A, June 1984 teach a modification of the above type of process in which the post-exposure bake is avoided. The imagewise exposure step is carried out using deep-UV radiation and is followed immediately. without a post-bake step, by blanket exposure using near- UV light. Spak et al. Seventh International Technical Confer¬ ence on Photopolymers, Ellenville, New York, October, 1985, pp. 247-269 describe the properties of, and theore¬ tical background relating to operation of, a typical image reversal system based on a diazo keto sensitizer and a novolak resin, which system is available commer¬ cially under the name A2 R 5214. The description given by Spak et al. of the procedure for use of this system in the negative mode corresponds to that set forth in the aforesaid '321 patent with the exception that no mention is specifically made of the need for a second blanket exposure prior to development of the negative image. In pending application Serial Number 67,732 filed June 26, 1987, there is described an improved image- reversible photoresist system which does not require a blanket exposure of the unexposed portions of the system prior to development of the image in the negative mode. This result is achieved principally by the inclusion in the composition of sufficient quantities of one or more organic solubilizing agents to render the total composi¬ tion soluble in alkaline developer prior to exposure to actinic radiation. Ruckert et al U.S. Patent 4,093,464 describes a dry-film photoresist which can be used in the positive mode only and which employs as the photosensitive layer a mixture of an alkali-soluble phenolic resin, an o-naphtho- quinone diazide sensitizer and an acrylic resin. The present invention provides a dry-film photo¬ resist which can be utilized in both the positive and negative image modes. SUMMARY OF THE INVENTION It is an object of the invention to provide an image-reversible photoresist system in the form of a dry-film. It is another object of the invention to provide a dry-film image-reversible photoresist system which does not require a blanket exposure of the unexposed portions of the system prior to development of the image in the negative mode. These objects, and other objects which will become apparent from the description which follows, are achieved by the compositions and process of the invention. Thus the invention, in one embodiment, provides a dry-film photoresist which comprises a film of a light-sensitive image reversible photoresist composition supported on a removable backing sheet. The composition used to prepare the film can be any of the image reversible photo¬ sensitive compositions known in the art of which those discussed hereinbefore are typical. Thus the composi¬ tions generally comprise a mixture of at least one phenolic resin, a 1,2-quinonediazide sensitizer and, optionally, at least one organic solubilizing . agent containing phenolic and or carboxylic acid groups. The solubilizing agent, if used, is present in the mixture in an amount sufficient to promote the solubility of said composition in alkaline developer. The dry-film photo¬ resist of the invention may also comprise a removable cover sheet extending substantially completely over the otherwise exposed surface of the film of light-sensitive image reversible photoresist composition. The invention also comprises processes for produc¬ ing either positive or negative resist images on a sub¬ strate utilizing the dry-film resists of the invention. Thus a positive resist image is produced by laminating a dry-film resist of the invention to a substrate, exposing the photoresist imagewise to actinic radiation, removing the backing sheet from the photoresist film and dissolv¬ ing the exposed portions of the photoresist in an alka- line developer. A negative resist image is produced by laminating the dry-film resist of the invention to a sub¬ strate and exposing the photoresist imagewise to actinic radiation in the same manner but thereafter heating the photoresist to a temperature and for a time sufficient to render the exposed portions of the photoresist insoluble in alkaline developer. The negative image is then developed, optionally after a blanket exposure to actinic radiation, by dissolving the unwanted portions of the photoresist in an alkaline developer. The backing sheet originally present on the dry-film photoresist of the invention can be removed either before or after the heat- ing step. The term "image reversible photoresist composition" means a light-sensitive composition which can be employed in either the positive or negative image mode. The dry-film photoresists of the invention can be employed to produce resist images on a wide variety of substrates for a wide variety of purposes. Thus the resist images can be produced on metallic substrates such as zinc, aluminum, copper and the like to produce litho¬ graphic printing plates or for use as etch resists in the gravure and or photoengraving arts. The images can be produced on non-metallic substrates such as reinforced epoxy resins, silicon wafers in the fabrication of printed circuit boards or in the manufacture of semi¬ conductors. DETAILED DESCRIPTION OF THE INVENTION The use of dry-film photoresists, as opposed to the use of liquid photoresist systems which are applied to substrates as liquid coatings which must then be dried by removal of solvent, is characterized by a number of advan- tages well-known in the art. The avoidance of the sol¬ vent removal step is, of course, one principal advantage. Another advantage lies in the much greater thickness of the film of photoresist which can be applied to a sub¬ strate using the dry-film in contrast to that which can be achieved using a liquid photoresist. This is parti¬ cularly important, for example, in the fabrication of printed circuit boards since the greater thickness of the resist image permits a greater thickness build-up of electroless and electrolytic copper and the like between the walls of the resist without lateral growth of the circuitry which is being formed. Similarly, in the production of printed circuit boards in which there are through--holes, the use of a dry-film resist has the advantage that it can be used to tent over the holes and thereby preserve the integrity of the copper-plated through-hole walls during the etching of copper from those areas of the board not protected by the resist image. In contrast, a liquid photoresist cannot be used to tent over through-holes. Further, if through-holes become flooded during application of the liquid resist, the subsequent removal of the resist from the holes can present a problem. The use of a dry-film resist also ensures that the film of resist applied to a substrate is substantially uniform in thickness whereas the application of a liquid photoresist as a coating on a substrate followed by removal of solvent can give rise to films which may not have uniform thickness with consequent variations in the thickness of the images produced from the film. The dry-film photoresists of the invention are produced using techniques well-known in the art, the novel feature being the image reversible nature of the light-sensitive photoresist composition employed to produce the film. Thus, for example, the dry-film photoresists of the invention are produced by depositing a film of the photoresist composition on a backing sheet. The latter is fabricated from a material which does not form a permanent bond with the photoresist composition either before or after the latter has been exposed to actinic radiation. The most commonly employed backing sheet is fabricated from the polyethylene terephthalate which is available under the trademark MYLAR but other polyesters and like materials can be employed, if desired. Advantageously the backing sheet has a thickness of about 2 to about 10 mils and is sufficiently flexible to facilitate removal from the photoresist film by peeling at the appropriate moment in the production of the photoresist image layer using the dry-films of the invention. The application of the light-sensitive photoresist composition to the backing sheet can be carried out using conventional coating techniques such as roller coating and the like which are capable of applying a coating of uniform thickness using draw-bars and like devices. As will be discussed more fully below, the photoresist com¬ position is advantageously formulated so that the solvent content is kept to the minimum necessary to permit the coating operation to take place without difficulty. When the coating operation has been completed the layer of photosensitive composition deposited on the backing sheet is treated to remove the solvent in any conventional manner. Generally speaking this step is achieved by heating the coated substrate at a temperature less than that which would result ' in decomposition of the sensi¬ tizer present in the composition (as discussed more fully below) . A temperature of the order of about 90 to 110°C is usually satisfactory and reduced pressures can be employed if desired in order to facilitate removal of the solvent. The actual thickness of the photosensitive layer which it is desirable to produce in the above manner will depend upon the ultimate use to which the dry-film photoresist is to be put. In general the thick¬ ness of the photoresist layer is of the order of about 0.05 to about 2 mils but such thicknesses are not criti¬ cal and higher or lower thicknesses can be employed in particular instances. In an optional feature of the dry-film photoresists of the invention the exposed surface of the film of photo- resist, which has been produced on the backing sheet in the manner described above, is covered with a sheet of protective film to prevent contamination of the film during storage or handling prior to use of the dry-film. The sheet of protective film is fabricated from materials such as polyolefins, of which polyethylene and polypropy¬ lene are examples, and. like polymers which do not form a permanent bond with the photoresist material. Such sheets can be peeled from the surface of the photoresist material prior to use of the dry-film in the formation of resist images using techniques which will be described below. The thickness of the protective cover sheet is advantageously of the order of about 2 to about 10 mils but the actual thickness is not critical and higher or lower thicknesses can be employed if desired. As set forth above, the novel feature of the dry- film photoresists of the invention lies in the nature of the photoresist compositions which are employed in the production thereof. Thus the photoresist compositions are such that the dry-film produced therefrom can be used in either the positive or negative mode i.e., possess image reversible properties. As set forth above any of the image reversible photosensitive compositions known in the art can be employed. Those described in U.S. Patent 4,581,321 and by Chiong et al, supra and Spak et al, supra are typical. In a preferred embodiment the photo¬ sensitive compositions employed in the dry-film photo¬ resists are those described in the aforesaid pending application Serial No. 67,732 since these compositions do not require the blanket exposure of the previously unexposed portions of the resist prior to development. The light-sensitive image reversible photoresist compositions employed to fabricate the dry-film resists of the invention in the broadest embodiment thereof com- prise a phenolic resin, a 1,2-quinonediazide sensitizer, and, optionally, at least one organic solubilizing agent. By "organic solubilizing agent" is meant an organic compound, ' and/or a mixture of two or more such compounds, which serves to render soluble in alkaline developers the total combined components of the photo- sensitive compositions when the latter are present in the areas which have not been exposed to actinic radiation but have been subjected to a post-image bake as herein¬ after discussed. The solubilizing agents employed are compatible with the other components of the compositions, i.e., do not enter into chemical reaction therewith or interfere in any way with the desired operation of the photosensitive composition other than to render it soluble in the unexposed state in alkaline developers, and are stable on storage in admixture with the other components. The extent to which the solubilizing agents render the compositions of the invention alkali soluble is considered adequate if the total composition, without exposure to ' actinic radiation but after being subjected to a post-image bake, is completely soluble in conven- tional alkaline developers in a reasonable length of time, i.e., up to about five minutes but preferably from 1 to 2 minutes in the case of a layer of the composition having an average thickness up to about 50 microns. Illustrative of such organic solubilizing agents are compounds which contain phenolic and of carboxylic groups. The latter compounds are inclusive of organic compounds which contain (a) at least one phenolic group and preferably two or more phenolic groups in a benzenoid ring or rings or (b) at least one, and preferably two or more, carboxylic acid groups or (c) at least one phenolic group and at least one carboxylic group in the molecule. Illustrative of such compounds are phenolic compounds such as mono, di, and trihydroxybenzophenone, resorcinol, 1,3,4-xylenol, hydroquinone, catechol, pyrogallol, phloro- glucinol, polymeric phenolic resins such as poly(vinyl- phenol) , orcinol, 1,2,4-benzenetriol, 4-methylcatechol, 2-methyl-resorcinol, 2,2' 4,4'-tetrahydroxybenzophenone, picric acid, coniferyl alcohol, and the like; weak carboxylic acids such as abietic aciά, cinna ic acid, 9-anthroic acid, 3-methyladipic acid, pimelic acid, 1-methyl-1-eyelohexane carboxylic acid, linolenic acid, and the- like, and compounds containing both carboxylic and phenolic groups such as salicylic, protocatechuic, m-hγdroxybenzoic, vanillic, p-hydroxybenzoic, 2-hydroxy- 3-naphthoic, gallic, 3-hydroxy-4-methoxycinnamic, 4-hydroxy-3-methoxycinnamic, 3-hydroxy-4-methoxymandelic, 4-hydroxy-3-m thoxymandelic, homovanillic, p-hydroxyman- delic acids and the like. Preferred compounds for use in the compositions of the invention are poly(vinylphenols) , polyhydroxybenzo- phenone, vinyl acetate/crotonic acid copolymers, and mixture of two or more such compounds. As set forth above the amount of the organic solu- ^ ' bilizing agent employed, singly or in mixtures of two or more, in the compositions of the invention is at least such that, when employed as films of thickness up to about 50 microns, the total composition without exposure to actinic radiation is rendered completely soluble in a reasonable length of time in conventional alkaline developers such as aqueous sodium or potassium hydroxide solution, sodium meta-silicate, sodium orthophosphate, sodium hydrogen phosphate, and the like. The actual amount of the organic solubilizing agent or agents which is necessary to achieve the desired rate of solubilization of the composition will depend princi- pally, in any given instance, upon the particular mode, i.e., positive or negative image, in which the dry-film obtained from the composition is to be employed. Thus, as discussed more fully below, in the positive mode of opera¬ tion the exposed image and the unexposed photoresist com- position are both soluble to different degrees in aqueous alkaline developer. In order to develop an image under these circumstances it is clearly necessary that the rate of dissolution of the exposed image in the developer must be significantly greater than the rate of dissolution of the unexposed material in the same developer. This difference in rates is generally referred to as develop¬ ment contrast and can be measured in any given instance simply by measuring the percentage of the film loss in the areas which remain after the more soluble areas have been removed entirely. In the case of the compositions which are to be used as dry-film resists in the positive mode the development contrast is advantageously such that the percentage of film thickness remaining in the unexposed areas, when the exposed area has been completely removed, can be as high as about 50 percent of the original thick¬ ness. Preferably the amount of film thickness remaining in the unexposed areas is of the order of about 70 per¬ cent of the original thickness. The concentration of the organic ' solubilizing agent or agents, which is present in the above compositions in order to achieve development contrast in the above range will obviously depend to some degree on the particular solubilizing agent or agents employed but can be determined readily in any given instance by a process of trial and error. In general the total amount of such agent or agents employed in the com¬ positions is of the order of about 1 to about 20 percent by weight based on weight of phenolic resin present in the compositions. When the compositions are employed in dry-film resists in the negative mode the requirement that there be development contrast still holds but in this case, as will be discussed in more detail below, the exposed image has been rendered substantially insoluble in the alkaline developers. Accordingly, in this mode, it is only necessary that the amount of organic solubilizing agent or agents be sufficient to render the unexposed areas of the composition completely soluble in conventional a_D___r— line developers within a reasonable period of time, advarr:— tageously about 10 seconds to about 5 minutes and prefer-— ably about 1 minute for a thickness of film of the orce___r of about 1.25 to 50 microns. The concentration αf tires above solubilizing agent or agents necessary to achieves such a degree of solubility can be readily determined ;L__n any given instance by a process of trial and error. 3_m general the amount of solubilizing agent, or the toca l amount if two or more agents are used, is of the order about 2 to about 20 percent by weight based on weight phenolic resin present in the total composition. Any of the phenolic resins conventionally employesd in positive photoresist compositions can be employed ϊm the compositions employed in preparing the dry-ftTΪTTΠ resists of the invention. Such resins are generally pre— pared by acid condensation of formaldehyde and phenol err an alkyl-substituted phenol under conditions described-., for example, in Chemistry and Application of Phenoiirc Resins, Knop et al. , Chapter 4, Springer Verlag, δfesw York, 1979. A particularly preferred group of resins frrr use in the compositions of the invention is the group erf novolak resins derived from m-cresol alone or frαn ≤a mixture of p-cresol and like phenols in which m-cresol __ΞS the major component, i.e., is present in an amocτττrt greater than 50 percent and preferably greater than abatrrt 90 percent by weight. Advantageously the resinss employed, including the preferred group, have averagee molecular weights in the range of about 600 to about 16(10.0 and preferably from about 800 to about 1500. The 1,2-quinonediazide sensitizers employed in ____=s compositions used to prepare the dry-film resists of * ~ '- ~~ - =- invention can be any of those conventionally employed im positive photoresist compositions available in the a__rr_. Such sensitizers comprise the esters and amides erf naphthoquinone-(l,2)-diazide-(2)-4-sulfonic acid -rτ-_ ή naphthoquinone-(l,2)-diazide-(2)-5-sulfonic acid. The esters are generally those derived from polyhydric phenols such as 2,3,4-trihydroxybenzophenone, 2,4-dihy- droxybenzophenone, 4-decanoyl-resorcinol, 4,4-bis(4- hydroxyphenyl)valeric acid butyl ester and the like or mixtures thereof. A preferred group of such esters are the esters of naphthoquinone-(l,2)-diazide-(2)-4-sulfonic acid with 2,3,4-trihydroxybenzophenone. The amides of the above two acids which can be employed in the compositions are those derived from long chain aliphatic primary amines or from aromatic primary amines. The proportion of such sensitizers employed in the compositions used to prepare the dry-film resists of the invention is advantageously of the order of about 2 per- cent to about 20 percent by weight and preferably about 2 to about 8 percent by weight, based on weight of total constituents' of the compositions other than solvents. • The optimum proportion to employ in any given instance can be determined readily by a process of trial and error. Similarly, the proportion of phenolic resin employed in the above compositions is advantageously of the order of about 2 percent to about 45 percent by weight, and preferably about 2 to about 20 percent by weight, based on weight of total constituents of the composition other than solvents. The above compositions generally also comprise a solvent or a mixture of solvents. The solvent or solvents employed are generally selected based on com- patibility of the photoresist and like considerations. Illustrative of solvents employed alone or admixture in the compositions of the invention are ketones such as methylethylketone, methylisopropylketone, diethylketone and the like; chlorinated hydrocarbons such as trichloro- ethylene, 1,1,1-trichloroethane and the like; aliphatic alcohols such as ethanol, n-propyl alcohol, n-butyl alcohol, n-hexyl alcohol and the like; aliphatic esters such as n-butyl acetate, n-hexyl acetate, cellosolve acetate and the like; glycol ethers such as ethylene gly- col monomethyl ether, propylene glycol monomethyl ether and esters thereof such as the acetates, propionates and the like; and aromatic hydrocarbons such as toluene, xylene and the like. A preferred group of solvents are the aliphatic alcohols, aliphatic esters, glycol ether esters and aromatic hydrocarbons and mixtures of two or more such solvents. The amount of solvent or mixture of solvents is advantageously restricted to the amount which is just necessary to permit the compositions to be applied as uniform coatings to the support sheet in forma¬ tion of the dry-film resist. In general, the amount of solvent ranges from about 35 percent to about 85 percent by weight of the total composition. The above compositions may also comprise other optional components conventionally employed in the art such as plasticizers, surfactants, adhesion-promoters, finely-divided pigments and the like. In a particular embodiment the compositions also comprise one or more cross-linking agents such as hexamethylolmelamine alkyl ethers, 2,6-bis(hydroxymethyl)-4-methylphenol, resoles, epoxy resins, and the like. Such cross-linking agents may be employed in a concentration of about 0.5 to about 20 percent by weight, and preferably in a concentration of about 1 to about 10 percent by weight, based on total components other than solvents present in the composi¬ tions. The optimum concentration for use in any given instance can be determined readily by a process of trial and error. As set forth above the dry-film resists of the invention are capable of producing either positive or negative images, i.e., exhibit image reversible pro¬ perties, depending upon the particular treatment adopted in preparing the photoresist image. The first step in the production of an image, whether it be positive or negative, involves laminating the dry-film resist to the substrate on which the photoresist image is to be formed. The surface of the substrate has generally been cleaned and pretreated using conventional methods which will differ depending on the nature of the substrate being used in any given instance. The lamination is accom¬ plished using methods and apparatus commonly employed in the art of applying dry-film resists to substrates. Illustratively, the dry-film resist with the backing sheet attached but the cover sheet, if any, removed is applied to the substrate with the exposed surface of the resist in contact with the substrate and lamination is effected using heat and pressure. The lamination is generally effected using appropriate equipment available in the art. Illustrative of such equipment is a DuPont HRL laminator available from the Riston Products Division of DuPont, Wilmington, Delaware. The temperature employ¬ ed in the lamination is less than that which would initiate decomposition of the sensitizer in the resist and is advantageously of the order of about 100 to about 135°C. The pressure applied in the lamination is advan¬ tageously of the order of about 5 to about 20 psi. The dry-film resist can be applied to one side only of the substrate if desired but in certain applications, such as the formation of printed circuit boards, resists are frequently applied to both sides of the substrate either simultaneously or sequentially. The laminated dry-film resist and substrate, with the backing sheet of the resist still in place, is then subjected to imagewise exposure to activating radiation. One common means of carrying out this step to achieve either positive or negative images is to expose the photoresist via a photomask to actinic radiation such as ultraviolet light. In order to produce a positive image the backing sheet is then peeled off and the resist image is developed by exposure, using immersion, spraying and like techniques, to the action of alkaline developers which dissolve out the exposed portions of the photo¬ resist. Any of the alkaline developers employed in the art can be used for this purpose. Illustrative of such developers are aqueous sodium or potassium hydroxide, sodium eta-silicate, sodium orthophosphate, sodium hydrogen phosphate and the like. In order to produce a negative image after the above exposure step the photoresist, optionally with the backing sheet still in place, is subjected to a post- exposure bake by heating to a temperature and for a time sufficient to render the exposed portions of the photo¬ resist insoluble in alkaline developer. The bake temperature necessary to achieve this result is generally within the range of about 115°C to about 140°C and preferably within the range of about 120°C to about 135°C. The temperature is chosen prefer¬ ably to avoid any significant decomposition of the sensi¬ tizer present in the unexposed portions of the light- sensitive layer. The time required to achieve the desired result, i.e., insolubilization of the exposed image, will vary depending upon the particular composi¬ tion which is being employed and the heating mode used. In general the time required is of the order of about 5 to about 60 minutes in an oven and 1 to 3 minutes when a hot plate is used. The optimum time required in any given instance can be readily determined by a process of trial and error. The process which takes place in the exposed image portions of the photosensitive layer during the exposure and subsequent post-exposure baking is believed to be as follows. The exposure of the sensitizer to actinic radi¬ ation is believed to result in conversion of the diazo- keto function of the sensitizer to an indene carboxylic acid illustrated schematically as followst During the post-exposure bake the acid function of the indene carboxylic acid produced in situ as shown above is believed to serve to catalyze cross-linking of the novo- lak resin in the exposed image portions of the photo¬ sensitive layer. The cross-linking is facilitated by the presence of the cross-linking agent or agents in the photosensitive compositions of the dry-film resists of the invention. The unexposed portions of the photo- sensitive layer do not undergo cross-linking since no carboxylic acid has been produced in situ therein during imagewise exposure of the • layer to actinic radiation. Accordingly, where a solubilizing agent is present in the composition in a preferred embodiment of the latter, the unexposed portions of the photosensitive layer remain soluble in alkaline developer whereas the exposed por¬ tions become insoluble due to the exposure followed by post exposure baking. Hence in this particular embodi¬ ment, the negative image is produced by dissolving out the unexposed portions of the layer using alkaline developers as hereinbefore described. Where the photosensitive composition of the dry-film resist does not contain a solubilizing agent as defined hereinabσve, or where the amount of solubilizing agent is insufficient to render the unexposed portions readily soluble in alkaline developer, the photosensitive layer is subjected to a second, blanket exposure to actinic radiation thereby rendering the previously unexposed portions of the photoresist soluble in alkaline developer. Thereafter the negative image is developed using alkaline developer to remove the unwanted portions of the resist which have been rendered soluble by the blanket exposure. In case of the production of negative images the step of exposure of the photoresist to actinic radiation via a photomask can be replaced by a so-called direct- write method of production of the image. In the latter method the photoresist, optionally with the backing sheet still in place, is flood-exposed briefly to actinic radia- tion such as ultraviolet light for a time sufficient to cause significant conversion of the diazo sensitizer to indene carboxylic acid as discussed above. The required image pattern is traced using an electron beam the heat generated by which causes cross-linking of the resist to occur in the areas traced by the beam. The negative image is then developed as before using alkaline develo¬ pers to dissolve out those areas of the resist which have not been exposed to the electron beam. In an alternative direct-write method of generating a negative image pattern the latter is traced directly in the resist layer using a thermal laser which accomplishes the localized heating and cross-linking in a single step without the need for an initial flood exposure to effect generation of the indene carboxylic acid. The following examples illustrate the dry-film resists of the invention and the method of utilizing the same and show the best mode known to the inventors of carrying out the invention but are not to be construed as limiting. Example 1 This Example illustrates the preparation of a dry- film photoresist in accordance with the invention and its use in the preparation of a negative resist image. A photoresist composition was prepared by admixing the following components in the stated proportions by weight. Component Parts By Weight novolak resin 1 20.1 poly(vinyl phenol) 2 4.2 sensitizer^ 6.0 2,6-bis(hydroxymethyl) -p-cresol 3.0 tricresylphosphate 4.4 acrylic copolymer 4 3.1 vinylacetate/crotonic acid copolymer^ 4.2 propylene glycol mono-methyl ether 37.7 dipropylene glycol methyl ether acetate 12.6 propylene glycol methyl ether acetate 2.1 dipropylene glycol methyl ether 2.1 sudan blue 0.5 100.0 Footnotes 1. acid catalyzed formaldehyde-m-cresol-p-cresol con¬ densate, average molecular weight = 1000 2. Maruzen resin; average m.wt. = 5000:Maruzen Oil Co. 3. ester of naphthoquinone-(l,2)-diazide-( 2)-4- sulfonic acid with trihydroxybenzophenone 4. ACRYLRON MFP-F: Synthron Inc.; wetting/leveling agent 5. Mowilith Ct. 5 : American Hoechst Corporation An aliquot of the above composition was coated on to a 1 mil. thick sheet (6" x 6") of electrical grade - MYLAR [Cadillac Plastic and Chemical Company, Linden, N.J.] without pretreatment of the latter. The thickness of the coating was controlled using a draw-bar in a Baker Film Applicator. The resulting coating was dried with a hot air gun to leave a film of photoresist of substan¬ tially uniform thickness of 1 mil. The dry-film resist so obtained was then laminated to a previously chemically cleaned copper clad reinforced epoxy-resin substrate (6" x 6"). The lamination was effected using a DuPont HRL Laminator (Riston Division of DuPont) at a laminating temperature of 110°C and a board feed rate of 0.5 meters/minute and a pressure of 15 psi. The resulting laminated board was cooled to ambient temperature and the photoresist with MYLAR backing sheet still in place was then exposed imagewise to ultraviolet light in a broad band exposure mode (150 mJ/cm 2 ) using an ORC HMW-6N exposure unit (ORC Manufacturing Company, Limited). After exposure the coated board was baked at 125°C for 25 minutes and then immersed for approximately 30 seconds in a commercially available aqueous alkaline developer [MF62A; MacDermid, Inc., Waterbury, Ct. ] . After rinsing the board with water and drying the resulting negative image was observed to be well-defined. Example 2 This Example illustrates the preparation of a dry- film photoresist in accordance with the invention and its use in the preparation of a positive photoresist image. The dry-film photoresist was prepared using the same photoresist composition and procedure described in Example 1. The dry-film resist was laminated to a pre¬ viously chemically cleaned copper clad reinforced epoxy resin substrate (6" x 6") using the procedure and equip¬ ment described in Example 1. The resulting laminated board was cooled to ambient temperature and the photo¬ resist layer with MYLAR backing sheet still in place was then exposed imagewise to ultraviolet light in a broad band exposure mode (circa 1000 mJ/cm 2 ) using an ORC exposure unit. After exposure the image on the coated board was developed by immersing the board for about 5 minutes in a 70 percent by volume solution of a commer¬ cially available aqueous developer [MF 62 : MacDermid, Inc.]. The resulting board was rinsed with water and dried. The positive image so produced was observed to be well-defined. Example 3 This Example illustrates the preparation of a dry- film photoresist in accordance with the invention and its use in the preparation of a negative photoresist image. A photoresist composition was prepared by admixing the following components in the stated proportions by weight. Component Parts by Weight novolak resin^ 3.6 poly(vinyl phenol) 2 20.5 sensitizer 3 2.6 2,6-bis(hydroxymethyl) -p-cresol 2.5 acrylic copolymer 4 1.9 vinylacetate/crotonic acid copolymer 5 8.5 abietic acid 2.1 trihydroxybenzophenone 3.1 propylene glycol monomethyl ether 35.7 dipropylene glycol methyl ether acetate 10.7 propylene glycol methyl ether acetate 4.2 dipropylene glycol methyl ether 4.2 sudan blue 0.4 100.0 Footnotes 1. acid catalyzed formaldehyde-p-σresol condensate_ average mol. wt. = 1000. 2. Same as Example 1. 3. Same as Example 1. 4. Same as Example 1. 5. Same as Example 1. . . . - The photoresist composition so prepared was used to prepare a dry-film photoresist on a MYLAR film backing sheet using the same procedure and equipment as that employed in Example 1. The dry-film resist was laminated to a previously chemically cleaned copper clad reinforced epoxy resin substrate (6" x 6") using the procedure and equipment described in Example 1. The resulting lami- nated board was cooled to ambient temperature and the photoresist layer with MYLAR backing sheet still in place was then exposed imagewise to ultraviolet light under the same conditions as those described in Example 1. After exposure the board was baked at 125°C for 25 minutes and then immersed for about 5 minutes in a 25 percent by volume aqueous solution of a commercially available alka¬ line developer [D89: acDermid Inc.]. The board was rinsed with water and dried. The resulting negative image was observed to be well-defined. Example 4 This Example illustrates the preparation of a dry-film photoresist in accordance with the invention and its use in the preparation of a negative resist image. A photoresist composition was prepared by admixing the following components in the stated proportions by weight. Component Parts by Weight novolak resin^ 15.7 poly(vinylphenol) 2 3.3 sensitizer 3 4.7 2,6-bis(hydroxymethyl)-p-cresol 2.5 tricresylphosphate 11.4 acrylic copolymer 4 2.4 vinyl acetate/crotonic acid copolymer^ 10.1 propylene glycol monomethyl ether 29.5 dipropylene glycol methyl ether acetate 9.8 propylene glycol methyl ether acetate 5.0 dipropylene glycol methyl ether 5.0 sudan blue 0.6 100.0 Footnotes The photoresist composition so prepared was used to prepare a dry-film photoresist on a MYLAR film backing sheet using the same procedure and equipment as that employed in Example 1. Using the procedures and equip¬ ment described in Example 1 the dry-film resist was lami¬ nated to a previously chemically cleaned copper clad rein¬ forced epoxy resin substrate (6" x 6") and the photo¬ resist layer with the MYLAR backing sheet still in place was exposed imagewise to ultraviolet light. After exposure the board was baked at 120°C for 30 minutes and then immersed for approximately 30 seconds in an alkaline developer bath having the same composition as that described in Example 1. The board was rinsed with water and dried. The resulting negative image was observed to be well defined. Example 5 This Example illustrates the preparation of a dry- film photoresist in accordance with the invention and its use in the preparation of a positive photoresist image. A dry-film photoresist was prepared using the pro¬ cedure and equipment described in Example 1 but using a photoresist composition prepared as described in Example 4. The resulting dry-film resist was laminated to a pre- viously chemically cleaned copper clad reinforced epoxy resin substrate (6" x 6") using the procedure and equip¬ ment described in Example 1. The laminated board was cooled and exposed imagewise to ultraviolet light in a broad band contact exposure mode (circa 1000 mJ/cm 2 ) Zh using an ORC exposure unit. After exposure, the coated board was developed by immersion for about 5 minutes in an 8 percent by volume aqueous solution of a commercially available alkaline developer [D89 : MacDermid Inc.]. The board was then rinsed with water and dried. The result¬ ing positive image was observed to be well-defined. Example 6 This Example illustrates the preparation of a dry- film photoresist in accordance with the invention and its use in the preparation of a positive photoresist image. A photoresist composition was prepared by admixing the following components in the stated proportions by weight. Component Parts by Weight novolak resin 1 23.2 sensitizer 2 5.8 2,6-bis(hydroxymethyl)-p-cresol 2.9 butyl benzyl phthalate 3 8.1 acrylic copolymer 4 2.9 vinylacetate/crotonic acid copσlymer^ 4.3 propylene glycol monomethyl ether 36.1 dipropylene glycol methyl ether acetate 12.0 propylene glycol methyl ether acetate 2.1 dipropylene glycol methyl ether 2.1 sudan blue 0.5 100.0 Footnotes: 1. Same as Example 1. 2. Same as Example 1. 3. Santicizer 160 : Monsanto Company. 4. Same as Example 1. 5. Same as Example 1. 23 Using the photoresist composition so prepared a dry- film photoresist in accordance with the invention was pre¬ pared using the procedure and equipment described in Exam¬ ple 1. The resulting dry-film resist with MYLAR backing sheet was laminated to a previously chemically cleaned copper ' clad reinforced epoxy resin substrate (6" x 6") using the procedure and equipment -described in Example 1. The laminated board was cooled and exposed imagewise, with the MYLAR backing sheet still in place, to ultra- violet light in a broad band contact exposure mode (circa 1000 mJ/cm 2 ) for a period of 4 minutes using an ORC exposure unit. After exposure, the image on the board was developed by immersing the board for about 5 minutes in a 15 percent by volume aqueous solution of a commer- cially available alkaline developer (D89 : MacDermid Inc.). The board was finally rinsed and dried. The resulting image was observed to be well-defined. Example 7 This Example illustrates the preparation of a dry- film resist in accordance with the invention and its use in the formation of a negative resist image layer. A photoresist composition was prepared using the components and proportions shown in Example 6. This composition was used to prepare a dry-film photoresist of the invention using the procedure and equipment described in Example 1. The resulting dry-film photoresist with MYLAR backing sheet was laminated to a previously cleaned copper clad reinforced epoxy resin substrate (6"x 6") using the procedure and equipment described in Example 1. The laminated board was cooled and exposed imagewise, with the MYLAR backing sheet still in place, to ultra¬ violet light in a broad band contact exposure mode (circa 300 mJ/cm 2 ) using an ORC exposure unit. After exposure, the MYLAR backing sheet was peeled off and the coated board was baked at 120°C for 20 minutes. The board was cooled and immersed for approximately 30 seconds in a commercially available aqueous alkaline developer [MF62A : MacDermid Inc. ] . The board was finally rinsed with water and dried. The resulting negative image was observed to be well-defined. Example 8 A dry-film photoresist in accordance with the invention was prepared and used to form a negative photo¬ resist image layer by repeating the procedure described in Example 7 but making the following changes. The image¬ wise exposure was carried out for 36 seconds at circa 150 mJ/cm 2 . The baking step was carried out at 120°C for 5 minutes and was followed by a flood exposure for 4 minutes using ultraviolet light (circa 1420 mJ/cm 2 ). Developing of the image was carried out using a commer¬ cially available developer (D89 : MacDermid Inc.] which had been diluted with an equal amount by volume .of water. The resulting negative image was well-defined. Example 9 A dry-film photoresist in accordance with the invention was prepared and used to form a negative photo¬ resist image layer by repeating the procedure described in Example 7 but making the following changes. The image¬ wise exposure was carried out for 36 seconds at circa 150 mJ/cm 2 . The baking step was carried out at 120°C for 5 minutes and developing was carried out using a commer¬ cially available developer [D89 : MacDermid Inc. ] at full strength. Example 10 A dry-film photoresist in accordance with the invention having a MYLAR backing sheet is prepared using the same photoresist composition and the same procedures and equipment as described in Example 1. A cover sheet of polyethylene film of 1 mil is applied to the exposed surface of the resist layer and the resulting product is maintained in storage at ambient temperature until required for use in the preparation of a photoresist image layer. At that time the polyethylene cover sheet is peeled from the " photoresist layer before laminating the latter, with the MYLAR backing sheet still in place, to a substrate and producing the required photoresist image using the procedures and equipment described in Example 1.

Claims: What is claimed is: 1. A dry-film photoresist comprising: a film of a light-sensitive image reversible photoresist composition supported on a removable backing sheet; said composition comprising, in admixture, (a) at least one phenolic resin, (b) a 1,2-quinone- diazide sensitizer and (c) a cross-linking agent. 2. A dry-film photoresist in accordance with Claim 1 which also comprises (c) at least one organic solubiliz¬ ing agent containing phenolic and or carboxylic acid groups said agent being present in an amount sufficient to render said compositions soluble in alkaline developer. 3. A dry-film photoresist in accordance with Claim 1 which also comprises a removable cover sheet extending substantially completely over the otherwise exposed surface of said film of light sensitive image reversible photoresist composition. 4. A dry-film photoresist in accordance with Claim 2 wherein said component (c) comprises a poly(vinylphenol) . 5. A dry-film photoresist in accordance with Claim 2 wherein said component (c) comprises a vinylacetate/ crotonic acid copolymer. 6. A dry-film photoresist in accordance with Claim 1 wherein said component (a) comprises a novolak resin derived from m-cresol or a mixture of m-cresol and p-cresol. 7. A dry-film photoresist in accordance with Claim 1 wherein said component (b) comprises an ester or an amide of naphthoquinone-(l,2)-diazo-(2)-4-sulfonic acid or naphthoquinone-(1,2.)-diazo-(2)-5-sulfonic acid. 8. A dry-film photoresist in accordance with Claim 7 wherein said ester is an ester of a polyhydric phenol. 9. A dry-film photoresist in accordance with Claim 8 wherein said polyhydric phenol is a trihydroxybenzo- phenone. 10. A dry-film photoresist in accordance with Claim 7 wherein said amide is derived from a ' primary or secondary amine. 11. A dry-film photoresist in accordance with Claim 1 wherein said cross-linking agent comprises 2,6-bis(hydro- xymethyl)-4-methylphenol. 12. A dry-film photoresist in accordance with Claim 1 wherein said light-sensitive image reversible photoresist composition also comprises a plasticizer. 13. A dry-film photoresist in accordance with Claim 12 wherein said plasticizer comprises tricresylphosphate. 14. A dry-film photoresist in accordance with Claim 1 wherein said removable backing sheet comprises a sheet of mylar film. 15. A dry-film photoresist in accordance with Claim 2 wherein said removable cover sheet comprises a sheet of polyolefin film. 16. A dry-film photoresist in accordance with Claim 15 wherein said polyolefin is polyethylene. 17. A dry-film photoresist comprising: a film of a light-sensitive image reversible photoresist composition supported on a sheet of Mylar film and having its exposed surface covered with a sheet of polyolefin film; said composition comprising, in admixture, (a) at least one novolak resin derived from m-cresol or a mixture of m-cresol and p-cresol; ; (b) an ester of naphthoquinone-(l,2)-diazo- (2)-4- or 5-sulfonic acid; and (c) a cross-linking agent. 18. A dry-film photoresist in accordance with Claim 17 wherein said component (b) is an ester of said acid with trihydroxy-benzophenone. 19. A dry-film photoresist in accordance with Claim 17 wherein said cross-linking agent comprises 2,6-bis(hydro- xymethyl)-4-methylphenol. 20. A dry-film photoresist in accordance with Claim 17 which also comprises at least one organic solubilizing agent containing phenolic and or carboxylic acid groups said agent being present in an amount sufficient to render said compositions soluble in alkaline developer. 21. A dry-film photoresist in accordance with Claim 20 wherein said solubilizing agent is selected from polγ(vinylphenol) , vinyl acetate/crotonic acid copoly- mers, polyhydroxybenzophenone and mixtures thereof. 22. A dry-film photoresist in accordance with Claim 17 wherein said light-sensitive image reversible photoresist composition also comprises a plasticizer. 23. A dry-film photoresist in accordance with Claim 22 wherein said plasticizer comprises tricresyl phosphate. - 24. A process for producing a negative resist image on a substrate which process comprises the steps of: laminating to said substrate a dry-film photoresist in accordance with Claim 1; exposing said photoresist imagewise to actinic radiation; heating said dry-film photoresist, after said exposure, to a temperature and for a time suffi¬ cient to render the exposed portions of said photo- resist insoluble in alkaline developer; subjecting said so treated photoresist to a blanket exposure to actinic radiation; removing the backing sheet either before or after the heating and or blanket exposure step; and thereafter dissolving the unwanted portions of the photoresist in an alkaline developer. 25. A process for producing a positive resist image on a substrate which process comprises the steps of: laminating to said substrate a dry-film photoresist in accordance with Claim 1; exposing said photoresist imagewise to actinic radiation; removing the backing sheet from the photo¬ resist film; and thereafter dissolving the exposed portions of the photoresist in an alkaline developer. 26. A process for producing a negative resist image on a substrate which process comprises the steps of: laminating to said substrate a dry-film photoresist in accordance with Claim 2; exposing said photoresist imagewise to actinic radiation; heating said dry-film photoresist, after said exposure, to a temperature and for a time sufficient to render the exposed portions of said photoresist insoluble in alkaline developer; removing the backing sheet either before or after the heating step; and thereafter dissolving the unwanted portions of the photoresist in an alkaline developer.

Inventors: ARONHIME MARC T, BEN-SHUSHAN GIORA, GAL CHAVA, JOHN J GRUNWALD, ARONHIME, MARC, T., BEN-SHUSHAN, GIORA, GAL, CHAVA, JOHN, J., GRUNWALD
Applicants: MACDERMID INC, MACDERMID, INCORPORATED
Application year: 1988
Main classification: G03C1
IPC codes: G03F7