Photoresist with photoactive compound mixtures

Abstract: The invention relates to a positive-acting photoresist composition containing a mixture of photoactive compounds. One component of the mixture is the esterification product of an o-quinonediazide sulfonyl compound with a phenolic resin. Another component of the mixture is the esterification product of an o-quinonediazide sulfonyl compound with a low molecular weight phenol having from one to three aryl groups and from one to three hydroxyl groups. A third photoactive component which may be present in the formulation is the esterification product of an o-quinonediazide sulfonyl compound with a relative high molecular weight, polyhydric, polynuclear phenol.

Description: BACKGROUND OF THE INVENTION I. Introduction This invention relates to new photoresist compositions. More particularly, this invention relates to positive-working photoresist compositions containing a mixture of photoactive compounds. One component of the mixture is the esterification product of an o-quinonediazide sulfonyl compound with a phenolic resin. Another component of the mixture is the esterification product of an o-quinonediazide sulfonyl compound with a low molecular weight phenol having from 1 to 3 aryl rings and from 1 to 3 hydroxyl groups. A third photoactive component may be the esterification product of an o-quinonediazide sulfonyl compound with a relative high molecular weight, polynuclear phenol having two to five aryl rings and four or more hydroxyl groups. II. Description of the Prior Art Photoresist compositions are well known in the art and described in numerous publications including DeForest, Photoresist Materials and Processes , McGraw-Hill Book Company, New York, 1975. Photoresists comprise coatings produced from solution or applied as a dry film which, when exposed to light of the proper wavelength, are chemically altered in their solubility to certain solvents (developers). Two types are known. The negative-acting resist is initially a mixture which is soluble in its developer, but following exposure to activating radiation, becomes insoluble in developer thereby defining a latent image. Positive-acting photoresists work in the opposite fashion, light exposure making the resist soluble in developer. Positive-working photoresists are more expensive than negative-working photoresists but are capable of providing superior image resolution. For example, the positive-working photoresist described above can be developed to yield relief images with a line width as low as one micron or less. In addition, considering the cross-section of a photoresist image, the channels formed in the photoresist by development have square corners and sidewalls with only minimal taper. The positive-working photoresists typically comprise a light-sensitive compound in a film-forming polymer binder. The light-sensitive compounds, or sensitizers as they are often called, most frequently used are esters and amides formed from o-quinone diazide sulfonic and carboxylic acids. These esters and amides are well known in the art and are described by DeForest, supra , pp. 47-55. These light-sensitive compounds, and the methods used to make the same are all well documented and described in prior patents including German Patent No. 865,140 granted February 2, 1953 and U.S. Patent Nos. 2,767,092; 3,046,110; 3,046,112; 3,046,119; 3,046,121; 3,046,122; and 3,106,465. Sulfonic amide sensitizers that have been used in the formulation of positive-acting photoresists are shown in U.S. Patent No. 3,637,384. These materials are formed by the reaction of a suitable diazide of an aromatic sulfonyl chloride with an appropriate resin amine. Methods for the manufacture of these sensitizers and examples of the same are shown in U.S. Patent No. 2,797,213. Other positive-working diazo compounds have been used for specific purposes. For example, a diazo compound used as a positive-working photoresist for deep U.V. lithography is Meldrum's diazo and its analogs as described by Clecak et al., "Technical Disclosure Bulletin," Volume 24, No. 4, September 1981, IBM Corporation, pp. 1907 and 1908, and o-quinone diazide compounds suitable for laser imaging as shown in U.S. Patent No. 4,207,107. The resin binders most frequently used with the o-quinone diazides in commercial practice are the alkali-soluble phenol formaldehyde resins known as the novolak resins. Photoresists using such polymers are illustrated in U.K. Patent No. 1,110,017. These materials are the product of reaction of a phenol with formaldehyde, or a formaldehyde precursor, under conditions whereby a thermoplastic polymer is formed. In the prior art, the above-described positive photoresists using novolak resins as binders are most often used as a mask to protect substrates from chemical etching and photoengraving processes. For example, in a conventional process for the manufacture of printed circuit boards, a copper clad substrate is coated with a layer of a positive-working photoresist, exposed to actinic radiation to form a latent circuit image in the photoresist coating, developed with a liquid developer to form a relief image, and etched with a chemical etchant whereby unwanted copper is removed and copper protected by the photoresist mask is left behind in a circuit pattern. For the manufacture of printed circuit boards, the photoresist must possess chemical resistance, must adhere to the circuit board substrate, and for high density circuits, must be capable of fine-line image resolution. Similar photoresists are also used in the fabrication of semiconductors. As in the manufacture of printed circuits, the photoresist is coated onto the surface of a semiconductor wafer and then imaged and developed. Following development, the wafer is typically etched with an etchant whereby the portions of the wafer bared by the development of the photoresist are dissolved while the portions of the wafer coated with photoresist are protected, thereby defining a circuit pattern. For use in the manufacture of a semiconductor, the photoresist must possess resistance to chemical etchants, must adhere to the surface of the semiconductor wafer, and must be capable of very fine-line image resolution. Recent developments in photoresist technology involve processes where high temperatures are encountered. For example, a recent development in the fabrication of semiconductors substitutes dry plasma etching or wet chemical etching to define a circuit. Plasma etching provides advantages over wet chemical etching in that it offers process simplification and improves dimensional resolution and tolerance. However, the demands on the photoresists are significantly greater when using plasma etching. For both wet etching and plasma etching, the photoresist must adhere to the substrate and must be capable of fine-line image resolution. For plasma etching, in addition to these properties, the photoresist must often be capable of withstanding high temperatures without image deformation and without eroding as plasma etching generates high temperatures at the wafer's surface. The above-described prior art positive-working photoresists provide good resistance to chemical etchants and fine-line image resolution. However, they possess a relatively low glass transition temperature and soften and begin to flow at temperatures of approximately 120°C thereby resulting in image distortion and poor image resolution. Various methods have been used in the prior art to increase the thermal performance of positive-working photoresists. For example, attempts have been made to increase the molecular weight of the novolak resins by chain extension, incorporation of di- and tri-alkyl substituted phenols, and by use of higher molecular weight phenolic reactants. One method reported in the prior art involves increasing the aromaticity of the novolaks by replacing formaldehyde or other alkyl aldehydes used to form novolak resins with an aromatic aldehyde such as benzaldehyde. The substitution of benzaldehyde for formaldehyde is suggested by Hiraoka et al., "Functionally Substituted Novolak Resins, Lithographic Applications, Radiation Chemistry, and Photooxidation," Polymer Press (American Chemical Society, Division of Polymer Chemistry), Volume 25, No. 1, 1984, pp. 322-323. An additional method reported in U.S. Patent No. 5,266,440 involves the use of resin blends or resin additives where one component of the blend has a high glass transition temperature. Initial attempts to substitute aromatic aldehydes for aliphatic aldehydes were relatively unsuccessful because the aromatic aldehydes are substantially less reactive than the aliphatic aldehydes. Consequently, the resins formed by the substitution were relatively low molecular weight oligomers having softening temperatures comparable to those resins formed with aliphatic aldehydes. Relatively higher molecular weight aromatic novolak resins are disclosed in U.S. Patent Nos. 5,216,111 and 5,238,776. In accordance with the procedures set forth in U.S. Patent No. 5,238,776, a first aromatic novolak resin is formed by the condensation of a bis-hydroxymethyl phenol with another reactive phenol in the absence of an aldehyde to form an alternating novolak copolymer. In accordance with the procedures of U.S. Patent No. 5,216,111, a phenol is reacted with an aromatic aldehyde to produce a novolak resin. The aromatic novolak resins produced by either patent can then be chain extended to increase molecular weight and thermal properties by further reaction with an aldehyde which may be an aliphatic or aromatic aldehyde to form a block copolymer; or by reaction with an aldehyde which may be an aliphatic or aromatic aldehyde with another phenol to form a block copolymer; or by reaction with a bishydroxymethylated phenol alone or in combination with an additional reactive phenol. The polymers formed by the reactions described in said patents have glass transition temperatures well in excess of the conventional novolak resins formed by reaction of a phenol with an aliphatic aldehyde. In addition to those efforts directed towards increasing glass transition temperature of novolak resins by increasing their aromaticity, efforts have also been made to provide single component photoresists by combination of the photoactive component of the photoresist with the resin component by esterification of an o-quinone diazide sulfonyl compound with the phenolic hydroxyl group of the novolak resin. An early patent showing this reaction is U.S. Patent No. 3,046,120, granted July 24, 1962, where an o-cresol formaldehyde resin is esterified with a naphthoquinone-(1,2)-diazide-(2)-5-sulfonyl chloride. The resulting product was used for fabrication of printing plates. Additional efforts to combine a photoactive compound with a phenolic resin are reported in U.S. Patent Nos. 3,635,709; 4,123,279; and 4,306,011. It is believed that the photoresist compositions described in each of the above patents have not been fully suitable for current submicron resolution (<1.0 um) requirements. It is further believed that the reason that such photoresists are not fully suitable for commercial use is that photolithographic and other properties of the photoresist are compromised by both the selection of the resin structure and its subsequent esterification with the photoactive compound. For example, the esterification reaction consumes phenolic hydroxyl groups on the resin which groups are necessary for development of a photoresist coating. Therefore, when the hydroxyl groups are consumed, developability of the photoresist coating is compromised unless exposure energy is increased substantially to yield sufficient acidic groups to enable dissolution. Using such materials, residue-free development is difficult. A recent effort to formulate a photoresist comprising the esterification product of a phenolic resin and an o-quinone diazide sulfonyl compound is disclosed in U.S. Patent No. 5,279,918, granted January 18, 1994. The object of this patent is to provide a photoresist composition having resolution higher than conventional photoresists, a "practical sensitivity" and an image having a good pattern profile. This is achieved by condensing o-quinone diazide sulfonyl compound groups with relatively low molecular weight novolak resins with from 40 to 90 percent of the phenolic hydroxyl groups condensed with the o-quinone diazide sulfonyl group. It is believed that the objects of higher resolution and enhanced pattern profile are achieved because of the high concentration of o-quinone o-quinone diazide units on the novolak resin backbone. However, a high concentration of the diazide units substantially increases the cost of the photoresist. Moreover, it is further believed that low molecular weight novolak resins are required to enable development following exposure at a commercially acceptable energy dose because of the substantial number of phenolic hydroxyl groups esterified with the o-quinonediazide sulfonyl halide. This is shown by the experimental data given that "practical sensitivity" as a stated object of the patent means a substantially higher exposure dose than required for conventional photoresists to obtain clean development. In copending U.S. Patent Application Serial No. (Attorney Docket No. 42471) filed contemporaneously herewith and assigned to the same assignee as herein, a photoresist composition is disclosed comprising the esterification product of an o-quinonediazide sulfonyl compound and an aromatic novolak resin with control of the reaction conditions and reactant concentrations whereby a maximum of 20% of the phenolic hydroxyl groups are esterified. By selection of certain aromatic novolak resins for esterification with the o-quinonediazide sulfonyl compound, it was found that the advantages of high image resolution and high glass transition temperature could be retained with limited consumption of phenolic hydroxyl groups through esterification. As a consequence of the limited consumption of hydroxyl groups, the dissolution properties of the photoresist are retained at conventional exposure dosage permitting clean development of imaged photoresist coatings. One disadvantage of a photoresist layer formed from an esterified novolak resin photoresist, whether aromatic or conventional, is a high incidence of standing waves due to thin film interference effects. Consequently, standing wave problems interfere with commercial utility of such materials. SUMMARY OF THE INVENTION In accordance with the subject invention, it has been found that the incidence of standing waves is minimized by use of a photoresist composition comprising a novolak resin having phenolic hydroxyl groups esterified with o-quinonediazide sulfonyl groups and an additional photoactive component that is a the esterification product of o-quinonediazide sulfonyl groups with a relatively low molecular weight, photoactive compound having 1 to 3 aryl groups and from 1 to 3 hydroxyl groups. It has been found that the latter photoactive compound enhances image resolution, reduces photoresist residue after development, and reduces standing waves. The photoresist composition desirably also contains a high molecular weight polyhydric, polynuclear phenol having at least 50 percent of its phenolic hydroxyl groups esterified with an o-quinone diazide sulfonyl compound. Accordingly, the photoresist of the invention comprises a mixture of photoactive compounds where one component of the mixture is the esterification product of an o-quinone diazide sulfonyl compound with a novolak resin and another component of the mixture is the esterification product of an o-quinonediazide sulfonyl compound with a low molecular weight phenol. Though not wishing to be bound by theory, it is believed that enhanced image resolution is due to substantial mobility of the photoactive compound of the low molecular weight phenol permitting substantial diffusion during the post exposure bake thereby enhancing developability of the imaged photoresist coating. DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, the subject invention relates to a photoresist that is the combination of an esterification product of a novolak resin and the esterification product of a low molecular weight phenol, each with a naphthoquinone diazide sulfonyl compound. Image resolution is improved using substantially any novolak resin esterified with an o-quinonediazide sulfonyl acid group admixed with the esterification product of the low molecular weight phenol. However, the esterified aromatic novolak resin of copending U.S. Patent Application Serial No. (Attorney Docket 42471) is preferred for purposes of this invention. For purposes of the invention described herein, the term "aromatic novolak resin", as used herein, refers to a novolak resin formed by condensing a phenol with a functional aromatic reactant capable of reacting with said phenol such as an aromatic aldehyde, a bis-hydroxymethylated bis-hydroxymethylated phenol, or the combination of an aromatic aldehyde and a phenol to form a novolak resin. The term also includes within its scope aromatic novolak resins further reacted with an additional aldehyde or a functional aromatic reactant alone or in combination with additional phenols to form alternating or block copolymers. A method for the formation of an aromatic novolak resin by reaction of a phenol with an aromatic aldehyde is disclosed in U.S. Patent No. 5,216,111. In accordance with the procedure of that patent, the aromatic novolak resins are acid catalyzed condensation products of one or more phenols and an aromatic aldehyde formed by the condensation of the reactants in the presence of a strong organic or mineral acid optionally in the presence of a divalent sulfur compound as a co-catalyst under reaction conditions set forth in the above-referenced patent. The phenol is of the type conventionally used in the formation of novolak resins, such as, for example, phenol itself, the cresols, xylenols, resorcinols, naphthol, bisphenols such as 4,4'-isopropylidenediphenol and mixtures of such phenols. Preferred phenols for purposes of the invention include the cresols, m-cresol being most preferred because it yields a polymer having a narrow molecular weight distribution. The aromatic novolak resin is desirably formed by condensation of the phenol with an aromatic aldehyde. The aromatic aldehyde is preferably one conforming to the following formula: where R is a member selected from the group consisting of halogen, cyano, nitro, carboxyl, alkoxy or alkyl having from 1 to 5 carbon atoms; m is a whole integer ranging between 0 and 2, and n is a whole integer ranging between 0 and 3. Preferred aromatic aldehydes are those where a hydroxyl group is in a position ortho to the carbonyl group. Most preferred aromatic aldehydes are salicylaldehyde, benzaldehyde and mixtures of the two. Other aromatic aldehydes suitable for purposes of the invention include 2-chlorobenzaldehyde, 3-hydroxybenzaldehyde, 4-hydroxybenzaldehyde, 2-methoxybenzaldehyde, 3-nitrobenzaldehyde, etc. Mixtures of aromatic aldehydes may also be used. In a lesser preferred embodiment, the aromatic aldehyde may be mixed with formaldehyde or a formaldehyde precursor such as paraformaldehyde if desired. However, the aromatic aldehyde preferably is in molar excess of formaldehyde, more preferably comprises at least 90% by weight of the aldehyde mixture, and most preferably is the only aldehyde used to form the resin. The aromatic novolak resins are formed by condensing the phenol with the aromatic aldehyde in the presence of an acid. The molar concentration of the aromatic aldehyde may be slightly less than that of the phenol, but unexpectedly, may also be equivalent to or slightly in excess of the phenol without formation of a crosslinked resin. In this respect, the ratio of the phenol to the aromatic aldehyde may vary between about 1.1 to 1.0 and 1.0 to 1.1 Aromatic aldehydes, compared to formaldehyde, are less reactive towards condensation reactions leading to polymerization. However, under more vigorous reaction conditions, aromatic aldehydes can condense with a reactive phenol in the presence of a strong mineral or organic acid such as hydrochloric acid, sulfuric acid, or toluene sulfonic acid. The condensation of the aromatic aldehyde with a reactive phenol results in the formation of polymers that are of high molecular weight typically having a weight average molecular weight in excess of 1500 Daltons and often in excess of 2500 Daltons. Further, with respect to molecular weight, it has been found that the aromatic novolak resins have a narrower molecular weight distribution compared to novolak resins formed using formaldehyde as the condensation reactant. In addition to increased molecular weight, the resins have improved glass transition temperatures generally in excess of 125°C and often in excess of 175°C. The condensation of the aromatic aldehyde with the phenol is conducted by mixing the reactants and catalysts in a dehydrating non-interfering solvent in a reactor and refluxing the mixture at an elevated temperature for a period of time sufficient to form a polymer of at least the weight average molecular weight given above. The reactor may be any conventional condensation reactor equipped with an agitator, means for reflux and distillation, and conventional heat transfer means as required to perform the reaction. In general, a preferred method for carrying out the condensation reaction is to dissolve the aromatic aldehyde and the phenol in an appropriate dehydrating water miscible solvent, then add the acid catalyst and, if used, an ionizable divalent sulfur compound. The resultant mixture is agitated and heated to reflux over a period of time ranging from about 0.2 to 48 hours, during which time, the phenol and the aldehyde condense. The condensation reaction typically involves the initial formation of low molecular weight intermediates which then combine with each other at a later stage in the reaction to form higher molecular weight polymers. Following reflux, excess cater may be removed from the condensate and the condensate may then be subjected to distillation at a temperature of from 130 to 180°C to complete the condensation reaction. Whether or not subjected to distillation and a higher temperature, the resin solution may then be treated to remove undesired impurities and then typically diluted with more solvent and added to excess water to precipitate the resin. The resin is then washed with water and dried at elevated temperature under vacuum. The resin will typically have a glass transition temperature of at least 125°C. An alternative method for formation of the aromatic novolak resin comprises reaction of a reactive phenol with a bis-hydroxymethyl phenol in accordance with the procedures set forth in U.S. Patent No. 5,238,776. The procedure may be conducted in the absence of an aldehyde or in the presence of an aldehyde such as an aromatic aldehyde. In accordance with procedures given in the aforesaid patent, bis-hydroxymethylated phenols that are particularly useful are represented by the following structures: where in each formula as applicable, o is a whole integer equal to 1 or 2, p is an integer equal to 0 or 1; R 2 is hydrogen, an alkyl, an aryl, or a heterocyclic group, but preferably is H; R 3 is hydrogen, an alkyl or an acyl group, but preferably is hydrogen or methyl; R 4 and R 5 are individually hydrogen, halogen, an alkyl group, an alkoxy group having 1 to 4 carbon atoms, a phenoxy group, any aryl group or an arylalkyl group; and R 5 is the same as R 4 and R 6 , and in addition a carboxylic group; and X is a bridging group which may be alkylene having 1 to 3 carbon atoms, -O-, -S-,-SO- or -SO 2 -. The bishydroxymethyl phenols may be formed by the reaction of formaldehyde with a phenol in the presence of a base. Suitable phenols include o-cresol, m-cresol, p-cresol, phenol, 2-phenylphenol, 3-phenylphenol, 4-phenylphenol, 2,3-dimethylphenol, 3,4-dimethylphenol, 3,5-dimethylphenol, 2,3,5-trimethylphenol, 3,4,5-trimethylphenol, p-ethylphenol, p-propylphenol, p-butylphenol, p-nonylphenol, p-tolylphenol, bromophenols, fluorophenols, chlorophenol, trimethylsilylphenol, chloromethylphenols, acylphenols, p-hydroxybenzoic acid, p-nitrophenol, bis(hydroxyphenyl)methane, isopropylidenebisphenol, biphenol, p-cresol formaldehyde dimers and trimers and the like, in the presence of a base. Suitable bases which will promote the reaction include sodium and potassium hydroxide and tetramethyl ammonium hydroxide. The bishydroxymethyl groups of the above phenols are reactive, and in the presence of heat and acid, react with each other or the ring of other reactive phenols to form alternating copolymers. Such phenols include phenol, cresols, dimethylphenols, trimethylphenols, naphthol, biphenols, phenylphenols, bisphenols, bis(hydroxyphenyl)methane, isopropylidenebisphenols, catechols, resorcinols, thiobiphenols, low molecular weight oligomers of phenols and the like, which contain at least two sites in the aromatic ring(s) of sufficient reactivity to undergo facile Friedel-Crafts reactions. The condensation reaction of the reactive phenol with the bishydroxymethylated phenol will sustain polymerization and result in the formation of alternating copolymers. To obtain the desired alternating copolymers, the reaction is conducted in the absence of an aldehyde. If an aldehyde is present in the reaction mixture during the formation of the alternating copolymer, then a polymer is formed that contains segments having both the alternating and random copolymer features. Such a polymer is substantially an alternating block copolymer. Other alternating block copolymer structures may be formed by first reacting the reactive phenol with an aldehyde to form a random structure and then introducing into the reaction mixture the bishydroxymethylated phenolic compound and reacting it with the reactive phenol and the random polymer structure. Alternatively, the bishydroxymethylated phenol is reacted first with the reactive phenol and then an aldehyde is added to the reaction mixture containing unreacted phenol to form the alternating block copolymer. Additional alternating block copolymer structures may be formed by reacting preformed alternating copolymers with preformed random copolymers with either or both an aldehyde and a bishydroxymethylated compound with or without the presence of a reactive phenol. To prepare polymers having utility according to the invention, the reactants are dissolved in an appropriate non-reactive or non-interfering solvent and then catalyst is added. When desired, the same or additional reactants may be added at different stages of the polymerization process. When a volatile reactive phenolic is used in excess, it may also serve as a solvent or cosolvent. The condensation reaction leading to polymer formation can be carried out below, at, or above ambient temperature. It is generally more practical to conduct the reaction at elevated temperature. A suitable acid catalyst may be a mineral acid such as hydrochloric acid, sulfuric acid, phosphoric acid, or an organic acid such as oxalic acid, maleic acid, formic acid, toluenesulfonic acid and the like or acid cation exchange resins such as those having a sulfonic acid group. In certain cases, an inorganic catalyst may also be used. Such catalysts may include compounds of zinc, manganese, magnesium, aluminum, titanium, copper, nickel and cobalt. Upon completion of the condensation reaction, the solvent and unreacted reactive phenolic compound may be removed by volatilization under reduced pressure and elevated temperature. The preferred method of recovering the polymer from the reaction mixture is by precipitation of the polymer into liquid which is a non-solvent for the polymer but a solvent for the reaction solvent, catalyst, and unreacted reactants. If this precipitation approach is used, the polymer is dried at elevated temperatures under reduced pressure. The resulting polymer may be either substantially an alternating copolymer or an alternating block copolymer structure. To form a block copolymer, the aldehyde used includes any aldehyde containing the RCHO functionality, including without limitation, alkyl aldehydes and aromatic aldehydes. Preferred aldehydes include formaldehyde, benzaldehyde, salicylaldehyde, and mixtures thereof. An additional phenol may be included in the reaction mixture such as those identified above. Two differing procedures may be used for formation of an alternating copolymer. The melt process comprises dissolution of the reactive phenol, the bishydroxymethylated phenol, water, solvent and catalyst in a resin reactor followed by gentle heating to about 60 0 C to initiate the reaction. An exothermic condensation reaction ensues. After about 3 to 6 hours of reflux, the volatile components are removed by distillation at ambient pressure. When the temperature of the reaction mixture reaches about 220 0 C, vacuum is applied to remove the unreacted phenol. The resin melt is then poured into a tray and allowed to cool and solidify. In this reaction, the ratio of phenol to bishydroxymethylated phenol is typically 2:1 to about 1:0.9. The catalyst may be any of the acids identified above, the preferred being oxalic and hydrochloric. These are normally used within a range of from about 0.1 to 3.0 percent. The solution process comprises dissolution of the reactive phenol, bishydroxymethylated phenol, solvent and catalyst in a resin reactor followed by gentle heating to reflux. The reaction temperature may vary from 68 to over 170°C depending on the boiling point of the solvent, concentration of reactants, and catalyst in the solvent. Typical reaction times vary from 1 to 24 hours, usually around 5 hours. Suitable solvents for the reaction are the water miscible aprotic or protic type of materials. Some solvent examples are ethanol, diglyme, monomethylether of propylene glycol, propionic acid and dioxane. These solvents may be used alone or in combination. Total monomer concentration in the solvent or solvent mixture is usually in the 5 to 75% weight range relative to the solvent, preferably in the 10 to 30% range. Preferred catalysts are oxalic, hydrochloric and methanesulfonic acid. The effective catalyst concentration necessary to conduct the reaction will depend on the acid strength, temperature and reaction environment. Typical catalyst concentration may vary from 1 to 15% relative to the reactive phenol weight. Once the reaction is completed the solution is slowly added to a non-solvent of the polymer to cause precipitation. The product is then collected, washed and dried. A typical process for forming the alternating block copolymer in which formaldehyde or other lower alkyl aldehyde is used comprises the following steps. The reactive phenol, bishydroxymethylated phenol, water, solvent and catalyst are added to a resin reactor and the mixture gently heated to about 70°C to initiate the reaction forming the alternating copolymer. At about 100°C the aldehyde is slowly added to the reaction mixture to form substantially phenol-methylene polymer segments and to chemically bond these segments to the previously formed alternating copolymer. After about 3 to 6 hours reflux, heat is applied to distill off the water and solvent. When the temperature of the reaction mixtures reaches about 210-220°C, vacuum is applied to remove the unreacted phenol. The resin melt is then poured into a tray and allowed to cool and solidify. This reaction may also be conducted in solution as above. However, hydrochloric acid is not used when formaldehyde is present due to the formation of toxic byproducts. The alternating block copolymers containing aromatic aldehyde, due to high resin glass transition and melt viscosity, are normally made by a solution process. The alternating copolymers and alternating block copolymers made with an aliphatic aldehyde are characterized by enhanced ortho-, ortho-coupling, weight average molecular weight of about 1500 to 5500 Daltons, polydispersity of 1.6 to 4, and a glass transition temperature of about 85 to 150°C. The aromatic novolak resins used to form photoresists by condensation with an o-quinonediazide sulfonyl compound in accordance with this invention are preferably those having a molecular weight of at least 1200 Daltons and more preferably, a molecular weight ranging between 1700 and 3000 Daltons; a glass transition temperature of at least 85 0 C, and preferably from about 130°C to 220°C; a polydispersity (ratio of weight average molecular weight to number average molecular weight) of from 1.6 to 4.0; and a dissolution rate in a standard developer (polymer prior to esterification) of at least 20 nm (200 Angstroms) per second and more preferably, from about 120-350 nm (1200 to 3500 Angstroms) per second. Though aromatic novolak resins are preferred for purposes of this invention, the low molecular weight mononuclear phenol photoactive compounds improve image resolution with conventional alkali-soluble novolak resins esterified with an o-quinonediazide sulfonyl compound where esterification is limited to 20 percent of the phenolic hydroxyl groups. Typical esterified novolak resins are disclosed in U.S. Patent Nos. 3,046,120; 3,635,709; 4,306,011; 4,308,368; and 5,145,763; U.K. published patent application No 2,172,117A; and European published patent application No. 0,110,214A1. In accordance with the invention, an esterified novolak resin is combined with an additional photoactive component that is the esterification product of an o-quinonediazide sulfonyl compound and a low molecular weight phenol having from 1 to 3 aryl groups and from 1 to 3 hydroxyl groups. These photoactive compounds are well known in the art and described in the various publications identified above. The preferred photoactive compounds are those formed by esterification of 2,1,4- and 2,1,5- diazonaphthoquinone diazide sulfonyl chlorides with monomeric phenols having up to three esterifiable hydroxyl groups. Examples of suitable phenols include phenol, the cresols, the xylenols, catechol, resorcinol, cumyl phenol, etc. The photoresist of the invention may also contain an additional photoactive compound that is the esterification product of an o-quinonediazide sulfonyl compound and a high molecular weight polyhydric phenol where an average of at least 50 percent of phenolic hydroxyl groups are esterified with o-quinonediazide sulfonyl groups. Preferred polyhydric phenols have 2 to 5 aryl rings and at least 4 hydroxyl groups. Most preferred polyhydric monomeric phenols conform to the general formula: where each Z independently may be the same or different and each is a member selected from the group consisting essentially of a hydrogen atom, an alkyl group, an aryl group, a halogen atom or a hydroxyl group, provided that at least four of said Z's are hydroxyl and preferably, there is at least one hydroxyl group on each ring; each m is a whole integer varying between 1 and 5; X is a single bond or a member from the group consisting essentially of CO, S, O, SO 2 , and CR 1 R 2 where R 1 and R 2 each is independently a member selected from the group consisting of hydrogen, halogen, hydroxy, alkyl having 1 to 6 carbon atoms, alkoxy having 1 to 6 carbon atoms, and aryl and n is a whole integer varying between 1 and 4. Compounds corresponding to the above formula are known in the art and disclosed in multiple publications including GB-A-935,250; EP-A-0 341 608; US-A-3,188,210; 4,266,000; 4,424,270; 4,555,469; and 5,290,656. Suitable phenols represented by the above formula include 2,2',3,4-tetrahydroxybenzophenone, 2,4,2',4'-tetrahydroxydiphenyl sulfide, 2,2',4,4'-tetrahydroxydinaphthylmethane, 2,3,4,4'-tetrahydroxy-2,2'-methyl-5,5'-di-tert-butyldiphenyl sulfide, 4,4'-dihydroxydiphenyl sulfide, 4,6-bis-(2,4-dihydroxyphenylthio)-resorcinol, 2,4,2',4'-tetrahydroxy-3, 5,3',5'-tetrabromodiphenyl sulfone, 2,4,2',4'-tetrahydroxy-3,5,3',5'-tetrabromobiphenyl and 2,2',4,4'-tetrahydroxy-3,5-dibromobenzophenone. In accordance with the invention, the novolak resins and the high and low molecular weight phenols are esterified with an o-quinonediazide sulfonyl compound. Examples of suitable o-naphthoquinonediazide groups include a 1,2-naphthoquinonediazide-4-sulfonyl group, a 1,2-naphthoquinonediazide-5-sulfonyl group, a 2,1-naphthoquinonediazide-4-sulfonyl group, a 2,1-naphthoquinonediazide-6-sulfonyl group, a 2,1-naphthoquinonediazide-7-sulfonyl group, and a 2,1-naphthoquinonediazide-8-sulfonyl group. These groups may be used alone or in combination as a mixture. Among these groups, a 1,2-naphthoquinonediazide-4-sulfonyl group and a 1,2-naphthoquinonediazide-5-sulfonyl group are particularly preferred. The condensation of the novolak resin or the phenol with the o-quinonediazide sulfonyl halide is carried out in a water miscible, non-interfering or inert solvent. Normally, the material to be esterified and the sulfonyl chloride are dissolved in acetone or dioxane forming an 8 to 25 w/w% solution. The solution temperature is adjusted and a solution of an organic base such as triethylamine (10 to 50 w/w%) is slowly added to drive the reaction to completion. This is followed by a reaction period of about 0.5 to 4 hours at a constant temperature. Normal reaction temperatures may vary from about 20 to 40 0 C. If the salt byproduct forms a precipitate, it is filtered and the filtrate neutralized with a mineral acid such as hydrochloric acid. If appropriate, the solution is filtered and then it is added to a non-solvent such as water to separate the esters from the solvent. After collection and washing, the product is dried under reduced pressure at about 30 to 55 0 C. When a 1,2-naphthoquinonediazide-4-sulfonate is used, the product is imaged at a wavelength of within the range of from 330 to 420 nm. When a 1,2-naphthoquinonediazide-5-sulfonate is used, the photoresist is desirably imaged at a wavelength within the range of 330 to 450 nm. A photoresist is prepared by dissolving the components in a suitable photoresist solvent. Known photoresist solvents include, for example, ether esters such as anisole, ethyl cellosolve acetate, methyl cellosolve acetate, and propylene glycol monomethyl ether acetate; ether alcohols such as ethyl cellosolve, methyl cellosolve, propylene glycol monomethyl ether; carboxylates such as ethyl acetate and butyl amyl acetate; lactones such as butyrolactone; cyclic ketones such as cyclohexanone and 2-heptanone; carboxylates of di-basic acids such as diethyloxylate and diethylmalonate; dicarboxylates of glycols such as ethylene glycol diacetate and propylene glycol diacetate; and hydroxy carboxylates such as ethyl-2-hydroxy propionate (ethyl lactate), and ethyl-3-hydroxy propionate. Solvents may be used alone or in admixture with each other and may be further mixed with non-solvents for one or more of the constituents. Suitable photoresist coating compositions can contain up to 50% by weight solids and preferably from 20 to 40 percent solids. The photoresist of the invention comprises the esterification products of the novolak resin, the esterification product of the low molecular weight mononuclear phenol, and optionally, the esterification product of the higher molecular weight phenol. The concentration of each in the photoresist may vary within broad limits and preferably the concentration ratios of the esterification product of the novolak resin to the esterification products of the low and high molecular weight phenols vary between 1:10 and 10:1 and more preferably, between 1:5 and 5:1. More preferred combinations of esterification products would have a concentration of components as set forth in the following table: Parts by Weight Preferred Most Preferred Esterified Resin 50% - 99.7% 60% - 99.4% Esterified Low MW Phenol Compound 0.3% - 25% 0.3% - 20% Esterified High MW Phenol Compound 0 - 25% 0.3% - 20% Other components may be added to the photoresist coating composition. For example, other phenolic resins such as aromatic novolak resins unreacted with the o-quinone diazide component, conventional novolak resins and polyvinyl phenols may be mixed into the photoresist coating composition. The additional phenolic resins may be used in amounts of from 0 to 25 times the amount to the total of the photoactive components of the resin, preferably in amounts of 1 to 15 times the amount of the photoactive components and more preferably, in amounts of from 1 to 10 times the amount of the photoactive components. Other additives that may be added to the photoresist coating composition include actinic and contrast dyes, anti-striation agents, plasticizers, speed enhancers and the like as would be known to those skilled in the art. The following examples will better illustrate the method for preparation of photoresists in accordance with the invention. GROUP I - Aromatic Resin Formed With Aromatic Aldehyde EXAMPLE 1 An m-Cresol Salicylaldehyde Resin A mixture of 183.2 grams salicylaldehyde, 162.2 grams m-cresol, 5.0 grams 3-mercaptopropionic acid, 1.5 grams of a 50 percent aqueous solution of p-toluenesulfonic acid and 200 ml of glacial acetic acid were charged into a 1-liter reaction vessel equipped with a paddle stirrer, reflux condenser and a nitrogen inlet tube. The mixture was heated to reflux and maintained at reflux for 16 hours. The reaction mixture was slowly poured into 7 liters of deionized water to precipitate the resin. Once the solvent had exchanged with water, the product was collected on a filter, slurried once in warm deionized water for about 30 minutes, again collected, rinsed with water and dried in a vacuum oven under about 125 mm Hg and at about 100°C. About 286 grams of a brown powder was obtained. EXAMPLE 2 An m-Cresol Salicylaldehyde Resin - Additional Example A mixture of 184.1 grams of salicylaldehyde, 162.2 grams of m-cresol, 5.0 grams of 3-mercaptopropionic acid, 1.5 grams of p-toluenesulfonic acid and 200 ml of glacial acetic acid were charged into a 1-liter reactor vessel equipped with a paddle stirrer, reflux condenser and a nitrogen inlet tube. The mixture was heated to reflux and maintained at reflux for 7.5 hours. The reaction mixture was slowly added to a 7-liter flask of deionized water to precipitate the resin. Once the solvent had exchanged with water, the product was collected on a filter, slurried in warm deionized water for 30 minutes, again collected, rinsed with water and dried in a vacuum oven under 125 mm Hg and at about 105° to 110°C. About 297 grams of a brown powder having a weight average molecular weight of 4,138 Daltons and a polydispersity of 2.1 was collected. Due to the nature of the primary and secondary structure of the resin, its glass transition temperature could not be determined from the differential scanning calorimetry melting curve but it can be stated that it possessed a glass transition temperature in excess of 150°C. EXAMPLE 3 An o-Cresol Salicylaldehyde Resin A mixture of 183.6 grams of salicylaldehyde, 162.2 grams of o-cresol, 3.0 grams of 3-mercaptopropionic acid, 1.1 grams of p-toluenesulfonic acid monohydrate and 75 ml of bis(2-methoxyethyl)ether were charged into a 1-liter reaction vessel equipped with a paddle stirrer, reflux condenser and a nitrogen inlet tube. The mixture was heated to reflux and maintained at reflux for 4.0 hours. The more volatile components in the reaction mixture were then removed by distillation at ambient pressure under a constant flow of nitrogen. The mixture temperature was then raised and maintained between 170° and 178°C for two hours to complete the reaction. Upon cooling, the product mixture was diluted first with 150 ml of glacial acetic acid followed by 300 ml of methanol. The solution was slowly added to 3.2 liters of deionized water to precipitate the resin. Once the solvent had exchanged with water, the product was collected on a filter, slurried in warm deionized water, rinsed with water and dried in a vacuum oven under about 125 man Hg and at about 115° to 120°C. About 289 grams of a brown powder having a weight average molecular weight of 19,439 Daltons and a polydispersity of 7.32 was obtained. Due to the nature of the primary and secondary structure of the resin, its glass transition temperature could not be determined from the differential scanning calorimetry melting curve but it can be stated that its glass transition temperature exceeded 150°C. EXAMPLE 4 A p-Cresol Salicylaldehyde Resin A mixture of 183.2 grams of salicylaldehyde, 162.2 grams of p-cresol, 3.0 grams of 3-mercaptopropionic acid, 1.1 grams of p-toluenesulfonic acid monohydrate and 75 ml of bis(2-methoxyethyl)ether were charged into a 1-liter reactor vessel equipped with a paddle stirrer, reflux condenser and a nitrogen inlet tube. The reaction was carried out according to the procedure of Example 3 to yield about 272.5 grams of resin having a weight average molecular weight of 11,622 Daltons and a polydispersity of 7.39. Due to the nature of the primary and secondary structure of the resin, its glass transition temperature could not be determined from the differential scanning calorimetry melting curve but it can be stated that its glass transition temperature exceeded 150°C. EXAMPLE 5 An m-Cresol Salicylaldehyde-Benzaldehyde Resin A mixture of 778.6 grams of m-cresol, 453.1 grams of benzaldehyde, 357.8 grams of salicylaldehyde, 14.5 grams of 3-mercaptopropionic acid, 9.6 grams of p-toluenesulfonic acid monohydrate and 650 ml of propionic acid were charged into a 4-liter reaction vessel equipped with a paddle stirrer, reflux condenser and a nitrogen inlet tube. The mixture was heated to reflux and maintained at reflux for 4 hours. The more volatile components in the reaction mixture were then removed by distillation at ambient pressure under a constant flow of nitrogen. The mixture temperature was then raised and maintained between 141° and 145°C and allowed to reflux for 1 hour to complete the reaction. Upon dilution with 1.6 liters of propionic acid, the resin solution was precipitated into 14 liters of deionized water. The product was collected on a filter, slurried in 14 liters of deionized water, collected by filtration, rinsed with deionized water and dried at 110° to 115°C. About 1,485 grams of a brown powder having a weight average molecular weight of 1,790 Daltons and a polydispersity of 1.95 was obtained having a glass transition temperature of about 177°C. EXAMPLE 6 An m-Cresol Salicylaldehyde-Benzaldehyde Resin An alternative procedure A mixture of 194.8 grams of m-cresol, 113.4 grams of benzaldehyde, 89.5 grams of salicylaldehyde, 1.0 liters of ethanol and 0.1 liters of concentrated hydrochloric acid were charged into a 2 liter 3-necked round bottom flask equipped with a mechanical stirrer, heating mantle, reflux condenser and a nitrogen inlet tube. The mixture was heated to and kept at reflux for about 24 hours. The reaction mixture was transferred to an addition funnel and slowly added into 6 liters of deionized water to precipitate the product. After a 4-hour soak, the precipitate was collected on a Buckner tunnel aided by reduced pressure. The product was rinsed, re-slurried into 6 liters of deionized water and after a 1-hour soak, was again collected on the Buckner filter and rinsed. The resin was partially air dried to remove the bulk of the water and further dried under vacuum at 65 to 68 0 C. The yield was about 360.4 grams of powder having a glass transition temperature of about 158°C and a weight average molecular weight of about 1823 Daltons. EXAMPLES 7 - 29 Miscellaneous Cresol Aromatic Aldehyde Resins Example No. Monomers Mole % Tg. °C. Molecular Weight Average 7 m-cresol 50 152 2,023 benzaldehyde 50 8 m-cresol 50 None 4,138 salicylaldehyde 50 obs. 9 m-cresol 50 164 3,038 benzaldehyde 45 salicylaldehyde 5 10 m-cresol 50 166 2,610 benzaldehyde 40 salicylaldehyde 10 11 m-cresol 50 147 1,962 benzaldehyde 38 salicylaldehyde 12 12 m-cresol 50 nm 2,001 benzaldehyde 36.6 salicylaldehyde 13.3 13 m-cresol 50 nm 2,317 benzaldehyde 36 salicylaldehyde 14 14 m-cresol 50 147 1,999 benzaldehyde 35 salicylaldehyde 15 15 m-cresol 50 161 2,319 benzaldehyde 25 salicylaldehyde 25 16 m-cresol 50 189 2,826 benzaldehyde 17 salicylaldehyde 33 17 o-cresol 50 125 4,328 benzaldehyde 30 salicylaldehyde 20 18 o-cresol 50 132 4,446 benzaldehyde 25 salicylaldehyde 25 19 o-cresol 50 134 6,610 benzaldehyde 20 salicylaldehyde 30 20 m-cresol 43.3 133 1,667 p-cresol 13.3 benzaldehyde 26 salicylaldehyde 24 21 m-cresol 43.3 131 3,041 o-cresol 13.3 benzaldehyde 26 22 m-cresol 33 138 2,007 o-cresol 17 benzaldehyde 30 salicylaldehyde 20 23 m-cresol 25 125 2,343 o-cresol 25 benzaldehyde 30 salicylaldehyde 20 24 m-cresol 17 131 2,564 o-cresol 33 benzaldehyde 30 salicylaldehyde 20 25 o-cresol 50 132 4,446 salicylaldehyde 25 benzaldehyde 25 26 m-cresol 33 138 2,007 o-cresol 17 benzaldehyde 30 salicylaldehyde 20 27 m-cresol 43.3 133 1,667 p-cresol 13.3 benzaldehyde 26 salicylaldehyde 24 28 m-cresol 50 147 1,671 3-hydroxybenzaldehyde 50 29 m-cresol 50 nm 1,451 4-hydroxybenzaldehyde 50 GROUP II - Aromatic Resins Formed From Bis(hydroxymethyl)phenol EXAMPLE 30 An m-Cresol Salicylaldehyde 2,6-bis(hydroxymethyl)-p-Cresol Resin A mixture of 146.0 grams m-cresol, 146.5 grams salicylaldehyde, 25.2 grams 2,6-bis(hydroxymethyl)p-cresol, 5.0 grams 3-mercaptopropionic acid and 1.5 grams p-toluenesulfonic acid monohydrate in 200 ml glacial acetic acid was reacted as described in Example 1 and about 281 grams of resin were obtained. The resin had a glass transition temperature of 233°C and a molecular weight of about 4,085 Daltons with a weight average molecular weight to a number average molecular weight of 2.77. EXAMPLE 31 A Resin Formed From m-Cresol and 2,6 Bis(hydroxymethyl)-p-cresol A 2-L four-neck resin kettle equipped with a stirrer, heating source, thermometer, variable reflux ratio distilling head and a nitrogen inlet tube was charged with 129.8 g m-cresol, 134.5 g 2,6-bis(hydroxymethyl)-p-cresol, 20 ml deionized water, 5.0 g oxalic acid dihydrate and 50 ml ethyl cellosolve acetate. The mixture was heated to about 60°C at which point an exothermic condensation reaction ensued. The reaction was allowed to continue for about 4 hours at reflux temperature. The more volatile components in the reaction mixture were removed by distillation at ambient pressure under a constant flow of nitrogen. When the temperature of the reaction mixture reached about 220°C, a partial vacuum pressure was applied and was gradually increased until a maximum vacuum of 7 mm Hg was achieved and the mixture was at about 228°C. The liquefied resin remaining in the kettle was poured into a tray under nitrogen and allowed to cool and solidify. About 217 g of an alternating resin having a number average molecular weight, M n , of 1640 Daltons and a Tg of 106°C were obtained. EXAMPLE 32 A 2,5-Dimethylphenol - Bishydroxymethyl-p-cresol Resin A mixture of 2,5-dimethylphenol (12.22g, 0.1 mol), 2,6-bis(hydroxymethyl)-p-cresol (11.20 g, 0.0667 mol) and 95 ml of monomethyl ether of propylene glycol was charged into a 250 ml 2-necked round bottom flask equipped with a heating mantel, mechanical stirrer and reflux condenser. Concentrated hydrochloric acid (10ml) was added and the mixture gently heated to reflux and kept at reflux for 5 hours. The solution was then cooled and dropwise added into about 1.8 L of deionized water to precipitate the polymer. The product was collected on a filter, air-dried and then further dried at about 85°C in a vacuum oven. About 19.3 g of a light powder was obtained having a glass transition temperature of about 95°C, weight average molecular weight of about 1570 Daltons and a polydispersity of 1.65. EXAMPLE 33 A 2,3-Dimethylphenol, 2-Methylresorcinol, Bishydroxymethyl-p-cresol Resin A mixture of 2,3-dimethylphenol (7.33 g, 0.06 mol), 2,6-bis(hydroxymethyl)-p-cresol (11.37 g, 0.0677 mol), 2-methylresorcinol (3.25 g, 0.0262 mol) and 95 ml of monomethyl ether of propylene glycol was charged into a 250 ml 2-necked round bottom flask equipped with a heating mantel, mechanical stirrer and reflux condenser. Concentrated hydrochloric acid (10 ml) was added to the flask and the mixture gently heated to reflux and kept at reflux for 5 hours. The solution was then cooled and dropwise added into about 1.8 L of deionized water to precipitate the polymer. The product was collected on a filter, air dried and then further dried at about 85°C in a vacuum oven. About 19.3 g of a light pink powder was obtained having a glass transition temperature of about 126°C, weight average molecular weight of about 2894 Daltons and a polydispersity of 1.91. EXAMPLE 34 A 4,4'-isopropylenebisphenol-Bishydroxymethyl-p-cresol Resin A 1.0 L round-bottom flask equipped with a stirrer, condenser, thermometer and a gas inlet tube was charged with 50.0 g (0.22 mole) of 4,4'-isopropylenebisphenol, 36.8 g (0.22 mole) of bishydroxymethyl-p-cresol and 500 ml of 1,4-dioxane. The reaction mixture was saturated with anhydrous hydrochloric acid and allowed to stand at ambient temperature for about 15 hours. After a portion of the hydrochloric acid was expelled by heating to 95°C the reaction mixture was cooled and then slowly added to about 3.0 L of deionized water. The product layer was separated, washed with water and dried in a vacuum oven at 60°C. The product was then dissolved in 200 ml of acetone, precipitated into about 1.6 L of deionized water, collected on a filter, rinsed with water and again dried under vacuum at 60°C. About 63 g of a tan product was obtained having a glass transition temperature of 88°C. GROUP III - Alternating Block Copolymers Formed By Reaction Of Alternating Copolymer With Aldehyde EXAMPLE 35 A Resin Formed From m-Cresol, 2,6-Bis(hydroxymethyl)-p-cresol and 1-Naphthol A mixture comprised of 72.1 g 1-naphthol, 84.1 g 2,6-bis(hydroxymethyl)-p-cresol, 5.0 g oxalic acid dihydrate, and 150 ml ethyl cellosolve acetate m a 0.5-L resin kettle, equipped as in Example 30, was heated to initiate the condensation reaction substantially forming the 1-naphtholmethylene-p-cresol alternating copolymer. An exothermic reaction resulted, reaching a peak temperature of about 120°C. The reaction mixture was allowed to reflux for about 2 hours, 108.1 g m-cresol and 48.8 g of 36.9 percent formaldehyde were added, and the condensation mixture again allowed to reflux for about 2 hours. The volatiles present in the reaction mixture were removed by distillation and stripping as described in Example 1. About 201 g of a block copolymer having a number average molecular weight, M n , of 1940 Daltons and a Tg of about 121°C were obtained. EXAMPLE 36 An m-Cresol, Salicylaldehyde and 2,6-Bis(hydroxymethyl)-p-cresol Resin A mixture of 146.0 g m-cresol, 25.2 g 2,6-bis(hydroxymethyl)-p-cresol, 5.0 g 3-mercaptopropionic acid and 1.5 g p-toluenesulfonic acid monohydrate in 200 ml glacial acetic acid were charged into a 1-L reaction vessel equipped with a paddle stirrer, reflux condenser and a nitrogen inlet tube. The mixture was heated to reflux and maintained at reflux to allow formation of the alternating copolymer. After 8 hours, 146.5 g salicylaldehyde were added over time to the reaction mixture to form alternating copolymer segments bonded with salicylaldehyde to form the block segments. Reflux was continued for 8 hours. The reaction mixture was slowly poured into 7 L deionized water to precipitate the resin. Once the solvent had exchanged with water, the product was collected on a filter, slurried once in warm deionized water for about 30 minutes, again collected, rinsed with water and dried in a vacuum oven under about 125 man Hg and at about 100° to 105°C. About 281 g of a block copolymer resin having a number average molecular weight, M n , of 1472 Daltons and a Tg of 233°C were obtained. EXAMPLE 37 A Resin Formed From m-Cresol and 2,6-Bis(hydroxymethyl)-p-cresol and Formaldehyde A phenolic resin rich in alternating phenolic copolymer block segments was formed by reacting a mixture of 519.0 g of 99 percent pure m-cresol, 538.2 g of 97 percent pure 2,6-bis(hydroxymethyl)-p-cresol and 20 g oxalic acid dihydrate in a solvent mixture of 40 ml deionized water and 200 ml ethyl cellosolve acetate. The reaction mixture was heated to about 70°C to initiate the reaction forming the alternating copolymer. At 102°C, 32.6 g of 36.9 percent formaldehyde was added in 8 minutes to the reaction mixture to form substantially m-cresol formaldehyde block segments and to chemically bond these segments to the previously formed alternating copolymer. After heating the mixture at reflux for three hours, the amount of heat was increased to remove the water and solvent by distillation. Partial vacuum was applied at about 220°C and gradually increased to 4 man Hg to remove the unreacted monomers. Maximum mixture temperature during vacuum stripping was about 226°C. The copolymer was poured from the reaction kettle and allowed to cool. About 840 g of a block copolymer having a number average molecular weight number, M n , of 1241 Daltons and a Tg of 112°C were obtained. EXAMPLE 38 A m-Cresol, 2,3,5-Trimethylphenol Bishydroxymethyl-p-cresol, Formaldehyde Resin To a 2-L resin kettle equipped with mechanical agitator, heating mantle, reflux condenser with a variable distillation head, thermometer and nitrogen inlet tube a mixture was added comprising 432.6 g of m-cresol, 148.6 g of 2,3,5-trimethylphenol, 463.3 g of bishydroxymethyl-p-cresol, 180 g of propyleneglycol methyl ether acetate, 36 g of water and 9.1 g of oxalic acid dihydrate. The mixture was heated to reflux and 127.9 g of 37.2% formaldehyde was added over a 25 minute period. After 3 hours of reflux, the amount of heat was increased to remove the water and solvent by distillation. Partial vacuum was applied at about 215°C and gradually increased to 0.9 man Hg to remove the unreacted monomers. Once the batch temperature again reached 215°C the resin was poured from the reaction kettle and allowed to cool. About 871 g of resin was obtained having a Tg of 113°C, a weight average molecular weight of 2193 Daltons and a polydispersity of 2.04. GROUP IV - Synthesis of Random Phenolic-Formaldehyde Novolak Resins EXAMPLE 39 A 2 L four-necked resin kettle equipped with a stirrer, heating mantel, thermometer, water cooled condenser and nitrogen inlet/outlet ports was charged with 278.3 g (99%) m-cresol, 35.5 g (99%) p-cresol, 34.3 g (99%) o-cresol, 68.3 g of formalin, 20 g deionized water and 12.0 g of oxalic acid dihydrate. the mixture was gently heated to reflux and while at reflux, 273.3 g of formalin was added over a period of time about one hour. The reaction was then allowed to continue for about four hours at reflux temperature. The more volatile components in the reaction mixture were removed by distillation at ambient pressure under a constant flow of nitrogen. When the temperature of the reaction mixture reached about 220°C, a partial vacuum pressure was applied and was gradually increased until a maximum vacuum of 7 mmHg was achieved. The vacuum was interrupted with nitrogen and the resin melt poured into a tray and cooled under nitrogen. About 516 g of novolak resin having a Tg of about 100°C. EXAMPLES 40 - 46 Following generally the procedure of Example A, additional novolak resins were prepared using various combinations of monomers. the phenolic monomers, their mole ratio, Tg and weight average molecular weight of the polymers formed are set forth in the following table: Example No. Monomers Mole % Tg.°C. Molecular Weight Average 40 m-cresol 39.0 p-cresol 55.5 2,3-Xylenol 5.5 101 5070 41 m-cresol 34.5 p-cresol 55.5 2,3,5-trimethylphenol 10.0 92 2880 42 m-cresol 34.5 p-cresol 55.5 2,5-xylenol 5.5 107 6940 43 m-cresol 39.0 p-cresol 55.5 3.5-xylenol 5.5 101 3880 44 m-cresol 25.0 p-cresol 60.0 2,5-xylenol 15.0 93 3340 45 m-cresol 25.0 p-cresol 50.0 2,5-xylenol 25.0 100 3850 46 m-cresol 42.0 102 5560 p-cresol 58.0 GROUP V - Photoactive Polymers Formed By Esterifying Aromatic Resins with o-Naphthoquinonediazide Group EXAMPLE 47 o-Naphthoquinonediazide Esterified Aromatic Resins A 1-L 3-necked round bottom flask equipped with a water bath, stirrer and thermometer was charged with 700 mL of acetone, 100 g of aromatic resin prepared according to Example 5 and having a dissolution rate of about 56 nm·s -1 (560 A/sec.), and 16.32 g of 1,2-naphthoquinone-(2)-diazide-5-sulfonyl chloride. The mixture was warmed up to about 30°C and a base solution comprising about 7.74 g of triethylamine in 25 mL acetone was slowly added in about 15 minutes. The reaction mixture was then maintained at about 30°C for about 2 hours to complete the esterification reaction. After this period, the excess base was neutralized with hydrochloric acid and the solution slowly added to about 7 L of deionized water to precipitate the product. The product was collected using a Buchner funnel assisted by a partial vacuum, rinsed with water and reslurried in 3 L of water. Again, the product was collected with the funnel, rinsed with water and partially air dried on the filter. Final drying was achieved at about 40 to 50°C under vacuum. About 110 g of esterified aromatic resin containing about 8.5 percent 1,2-naphthoquinone-(2)-diazide-5-sulfonic acid ester groups was obtained. EXAMPLES 48 - 53 Misc. Esterified Aromatic Resins Under similar reaction conditions other 1,2-naphthoquinone-(2)-diazide-5-sulfonic acid esterified aromatic resins (Examples 40-43) of the invention were prepared. Table 1 sets out the precursor resin used and resulting percent esterification. Example Resin Precursor Synthesis Procedure according to Precursor Resin Dissolution Rate, nm s -1 Ester % 48 Example 6 207,3 12 49 Example 38 20,9 15 50 Example 32 172,2 12 51 Example 37 4,6 7.5 52 Example 39 n/a 6.25 53 Example 46 82,6 10 GROUP VI - Formulation and Performance of Photoresist Made In Accordance With Invention EXAMPLES 54 to 69 The following examples illustrate the preparation and use of photoresists containing the esterified resins in accordance with the invention. Examples 54 to 69 demonstrate positive-working photoresist compositions containing the novel esterified resins of this invention. The photoresist compositions were evaluated by spin coating onto four-inch silicon wafers using either a GCA or SVG wafer coater-hot plate track to give approximately a 9700A ± 25A film of resist and baked at 85° to 90°C for 60 seconds. Initial average film thickness was measured with a Nanometrics 215 instrument. The films were irradiated using a GCA XLS, 0.55 NA i-line stepper and then subjected to a post-exposure bake of 115°C for 60 seconds. The exposed resists were developed with MF701 or MF703 developer (a Shipley Co. product) at 21°C for 60 seconds using a single puddle process. Scanning electron microscopy was used to analyze the resist films in the exposed and unexposed regions and to determine performance characteristics. EXAMPLE 54 Resist Composition: Ester of Example 48 7.667 g Resin of Example 38 11.695 g PAC 1.515 g PAC 1.122 g Solvent 75.360 g Performance: Photospeed 81. mJ/cm 2 Resolution 0.330 µm Wall angle at 0.35 µm 88.0° Depth of Focus at 0.35 µm 0.75 µm EXAMPLE 55 Resist Composition: Ester of Example 49 4.473 g Resin of Example 38 14.890 g PAC 1.515 g PAC 1.122 g Solvent 75.360 g Performance: Photospeed 124. mJ/cm 2 Resolution 0.335 µm Wall angle at 0.35 µm 87.0° Depth of Focus at 0.35 µm 0.90 µm EXAMPLE 56 Resist Composition: Ester of Example 50 6.544 g Resin of Example 38 12.818 g PAC PAC 1.122 g Solvent 75.360 g Performance: Photospeed 68. mJ/cm 2 Resolution 0.340 µm Wall angle at 0.35 µm n/a Depth of Focus at 0.35 µm 0.90 µm EXAMPLE 57 Resist Composition: Ester of Example 48 3.452 g Resin of Example 42 6.504 g PAC 2.319 g PAC derived from benzyl-2,3,4 trihydroxyphenylmethane 0.727 g Solvent 74.238 g Performance: Photospeed 80 mJ/cm 2 Resolution 0.320 µm Wall Angle at 0.35 µm 86.0° Depth of Focus at 35 µm 1.20 µm EXAMPLE 58 Resist Composition: Ester of Example 48 5.941 g Resin of Example 42 14.050 g PAC 1.360 g PAC 1.145 g Solvent 74.814 g Performance: Photospeed 76.7 mJ/cm 2 Resolution 0.320 µm Wall Angle at 0.35 µm 89.5° Depth of Focus at 0.35 µm 0.90 µm EXAMPLE 59 Resist Composition: Ester of Example 48 12.134 g Resin of Example 44 28.786 g PAC 2.720 g PAC Derived from 2,4 dihydroxybenzophenone 2.364 g Solvent 155.934 g Performance: Photospeed 83.7 mJ/cm 2 Resolution 0.320 µm Wall Angle at 0.35 µm 88.5° Depth of Focus at 0.35 µm 1.20 µm EXAMPLE 60 Resist Composition: Ester of Example 48 5.945 g Resin of Example 42 13.818 g PAC 1.363 g PAC derived from bis(2-hydroxy-5-methylphenyl)methane 1.387 g Solvent 74.795 g Performance: Photospeed 108.3 mJ/cm 2 Resolution 0.320 µm Wall Angle at 0.35 µm 90.0° Depth of Focus at 0.35 µm 0.90 µm EXAMPLE 61 Resist Composition: Ester of Example 48 6.278 g Resin of Example 42 8.236 g Resin of Example 6 5.475 g PAC 1.361 g PAC 1.145 g Solvent 74.782 g Performance: Photospeed 73.3 mJ/cm 2 Resolution 0.335 µm Wall Angle at 0.35 µm 89.0° Depth of Focus at 0.35 µm 0.83 µm EXAMPLE 62 Resist Composition: Ester of Example 48 6.528 g Resin of Example 42 12.901 g PAC 1.047 g PAC derived from bis(4-hydroxy-2,3,5 trimethylphenyl)-(2-hydroxyphenyl)methane 2.025 g Solvent 74.804 g Performance: Photospeed 99.0 mJ/cm 2 Resolution 0.320 µm Wall Angle at 0.35 µm 86.0° Depth of Focus at 0.35 µm 1.20 µm EXAMPLE 63 Resist Composition: Ester of Example 48 6.519 g Resin of Example 44 12.946 g PAC derived from bis(2,4-dihydroxy-3,5-dimethylphenyl)methane 1.044 g PAC derived from bis(4-hydroxy-2,3,5-trimethylphenyl)-(2-hydroxyphenyl)methane 2.000 g Solvent 74.822 g Performance: Photospeed 97.7 mJ/cm 2 Resolution 0.320 µm Wall Angle at 0.35 µm 88.0° Depth of Focus at 0.35 µm 0.90 µm EXAMPLE 64 Resist Composition: Ester of Example 48 4.277 g Resin of Example 38 14.080 g PAC 1.516 g PAC derived from 2,6-bis(4-hydroxybenzyl)-p-cresol 1.127 g Solvent 73.359 g Performance: Photospeed 110. mJ/cm 2 Resolution 0.345 µm Wall Angle at 0.35 µm n/a Depth of Focus at 0.35 µm 0.9 µm EXAMPLE 65 Resist Composition: Ester of Example 48 4.342 g Resin of Example 38 14.294 g PAC 1.516 g PAC derived from bis(4-hydroxyphenyl)(benzyl)methane 1.848 g Solvent 75.358 g Performance: Photospeed 104 mJ/cm 2 Resolution 0,350 µm Wall Angle at 0.35 µm n/m Depth of Focus at 0.35 µm 0.30 µm EXAMPLE 66 Resist Composition: Ester of Example 48 4.277 g Resin of Example 38 14.080 g PAC 1.516 g PAC derived from bis(4-hydroxy-2,3,5-trimethylphenyl)methane 2.127 g Solvent 75.362 g Performance: Photospeed 124 mJ/cm 2 Resolution 0.350 µm Wall Angle at 0.35 µm n/m Depth of Focus at 0.35 µm 0. µm EXAMPLE 67 Resist Composition: Ester of Example 48 12.755 g Resin of Example 38 6.611 g PAC 0.955 g PAC derived from bis(4-hydroxy-2,3,5-trimethylphenyl)methane 1.663 g Solvent 75.369 g Performance: Photospeed 105.7 mJ/cm 2 Resolution 0.310 µm Wall Angle at 0.35 µm 89.5 Depth of Focus at 0.35 µm 1.2 µm EXAMPLE 68 Resist Composition: Ester of Example 48 7.659 g Resin of Example 38 11.879 g PAC 1.515 g PAC derived from 1,1'-bis(4-hydroxyphenyl)cyclohexane 0.946 g Solvent 75.361 g Performance: Photospeed 68.7 mJ/cm 2 Resolution 0.330 µm Wall Angle at 0.35 µm 87.5 Depth of Focus at 0.35 µm 1.05 µm EXAMPLE 69 Resist Composition: Ester of Example 53 6.422 g Resin of Example 42 13.576 g PAC 1 1.359 g PAC 1.145 g Solvent 74.811 g Performance: Photospeed 68.3 mJ/cm 2 Resolution 0.330 µm Wall Angle at 0.35 µm 88.5 Depth of Focus at 0.35 µm 1.05 µm

Claims: A light-sensitive composition comprising a mixture of esterification products, said mixture comprising a) the esterification products of an alkali-soluble novolak resin and an o-naphthoquinonediazide compound where a maximum of 20 mole percent of the phenolic hydroxyl groups are esterified with the o-naphthoquinonediazide compound, b) the esterification product of one or more low molecular weight phenols having from one to three aryl groups and from one to three hydroxyl groups with a maximum of three phenolic hydroxyl groups esterified with the o-naphthoquinonediazide compound, the molar ratio of said esterified resin to said esterified phenol varying between 1:10 and 10:1, and c) the esterification product of a high molecular weight phenol having at least four phenolic hydroxyl groups and an o-naphthoquinonediazide compound where at least 50 mole percent of the phenolic hydroxyl groups are esterified with the o-naphthoquinonediazide compound. The light sensitive-composition of claim 1 where the ratio varies between 1:5 and 5:1. The light sensitive-composition of claim 1 further including an alkali soluble resin binder. The light sensitive-composition of claim 3 where the resin binder is present in an amount up to 25 times the amount of the esterification product of the novolak resin. The light sensitive-composition of claim 4 where the amount varies from about 1 to 15 times the amount of the estrification products. The light sensitive-composition of claim 1 dissolved in a casting solvent. A light-sensitive composition comprising a mixture of esterification products, said mixture comprising the esterification product of an alkali-soluble aromatic novolak resin and an o-naphthoquinonediazide compound where a maximum of 20 mole percent of the phenolic hydroxyl groups are esterified with the o-naphthoquinonediazide compound and the esterification product of a low mw phenol having from one to three aryl groups and from one to three hydroxyl groups with a maximum of three phenolic hydroxyl groups being esterified with an o-naphthoquinonediazide compound, the molar ratio of said esterified resin to said esterified phenol varying between 1:10 and 10:1. The light sensitive-composition of claim 7 where the ratio varies between 1:5 and 5:1. The light sensitive-composition of claim 7 further including an alkali-soluble resin binder. The light sensitive-composition of claim 7 further including the esterification product of a polyhydric, polynuclear phenol having at least four phenolic hydroxyl groups and an o-naphthoquinonediazide compound where at least 50 mole percent of the phenolic hydroxyl groups are esterified with the o-naphthoquinonediazide compound. The light sensitive-composition of claim 10 where the resin binder is present in an amount of up to 25 times the amount of the esterification products. The light sensitive-composition of claim 11 where the amount varies from about 1 to 15 times the amount of the esterification products. The light sensitive-composition of claim 7 dissolved in a casting solvent. The light sensitive-composition of claim 7 where the aromatic novolak resin is the product of condensation of an aromatic aldehyde and a phenol. The light sensitive-composition of claim 14 where the aromatic aldehyde conforms to the formula: where R is a member selected from the group consisting of halogen, cyano, nitro, carboxyl, alkoxy and alkyl having from 1 to 5 carbon atoms; m is a whole integer ranging between 0 and 2, and n is a whole integer ranging between 0 and 3. The light sensitive-composition of claim 15 where the aromatic aldehyde is selected from the group consisting of a benzaldehyde, a salicylaldehyde and mixtures thereof. The light sensitive-composition of claim 7 where the aromatic novolak resin is a block copolymer of a first polymer formed by condensation of an aromatic aldehyde with a phenol, and wherein the block copolymer is formed by condensing said first polymer with an additional aldehyde. The light sensitive-composition of claim 17 where said additional aldehyde is a member selected from the group consisting of aromatic aldehydes, aliphatic aldehydes, and mixtures thereof. The light sensitive-composition of claim 7 where the aromatic novolak resin is a block copolymer of a first polymer formed by condensation of a phenol with an aliphatic aldehyde and wherein the block copolymer is formed by condensing said first polymer with an aromatic aldehyde. The light sensitive-composition of claim 7 where said aromatic novolak resin is the condensation product of a bis(hydroxymethyl)phenol and a reactive phenol. The light sensitive-composition of claim 20 where the bis(hydroxymethyl)phenol conforms to a formula selected from the group consisting of: where in each formula as applicable, o is a whole integer equal to 1 or 2, p is an integer equal to 0 or 1; R 2 is hydrogen, an alkyl, an aryl, or a heterocyclic group, R 3 is hydrogen, an alkyl or an acyl group, R 4 and R 5 are individually hydrogen, halogen, an alkyl group, an alkoxy group having 1 to 4 carbon atoms, a phenoxy group, any aryl group or an arylalkyl group; and R 5 is the same as R 4 and R 6 , and in addition a carboxylic acid group; and X is a bridging group which may be alkylene having 1 to 3 carbon atoms, -O-, -S-, -SO- or -SO 2 -. The light sensitive-composition of claim 21 where said bishydroxymethylated compound is the reaction product of formaldehyde with a phenol selected from the group consisting of o-cresol, m-cresol, p-cresol, phenol, 2-phenylphenol, 3-phenylphenol, 4-phenylphenol, 2,3-dimethylphenol, 3,4-dimethylphenol, 3,5-dimethylphenol, 3,4,5-trimethylphenol, p-ethylphenol, p-propylphenol, p-butylphenol, p-nonylphenol, t-butylphenol, bromophenols, fluorophenols, chlorophenols, trimethylsilylphenol, chloromethylphenols, acylphenols, p-hydroxybenzoic acid and p-nitro phenol. The light sensitive composition of claim 21 where said reactive phenol is selected from the group consisting of phenol, cresols, dimethylphenols, trimethylphenols, naphthols, biphenols, substituted biphenols, phenylphenols, bis(hydroxyphenyl)methane, substituted bis(hydroxyphenyl) methanes, isopropylidenebisphenols, catechols, resorcinols, thiobiphenols, of o-cresol, m-cresol, p-cresol, phenol, 2-phenylphenol, 3-phenylphenol, 4-phenylphenol, 2,3-dimethylphenol, 3,4-dimethylphenol, 3,5-dimethylphenol, 3,4,5-trimethylphenol, p-ethylphenol, p-propylphenol, p-butylphenol, p-nonylphenol, t-butylphenol, bromophenols, fluorophenols, chlorophenols, trimethylsilylphenol, chloromethylphenols, acylphenols, p-hydroxybenzoic acid and p-nitrophenol. The light sensitive-composition of claim 7 where said aromatic novolak resin is a block copolymer formed by condensing an alternating copolymer that is the condensation product of a bis(hydroxymethyl)phenol and a reactive phenol with an additional aldehyde. The light sensitive-composition of claim 24 where the additional aldehyde is an aromatic aldehyde. The light sensitive composition of claim 7 where said aromatic novolak resin is a block copolymer formed by condensing an alternating copolymer that is the condensation product of a bis(hydroxymethyl)phenol and an excess of a reactive phenol with a bis(hydroxymethyl)phenol.

Inventors: MEISTER CATHERINE C, TREFONAS PETER III, TURCI PAMELA, VIZVARY GERALD C, ZAMPINI ANTHONY, MEISTER, CATHERINE C., TREFONAS, PETER, III, TURCI, PAMELA, VIZVARY, GERALD C., ZAMPINI, ANTHONY
Applicants: SHIPLEY CO LLC, SHIPLEY COMPANY LLC
Application year: 1996
Main classification: G03F7
IPC codes: G03F7, H01L21, C08G8