NOVOLAK RESIN FOR POSITIVE PHOTORESIST

Abstract: This invention provides a novolak resin for a positive photoresist and which is produced by addition condensation reaction of phenol compound with formaldehyde, characterised in that the area ratio of the gel permeation chromatographic pattern (GPC pattern) [measured by the use of a UV(254 nm) detector] is as follows : the range wherein the molecular weight calculated as polystyrene is from 150 to less than 500 (not including phenol and unreacted monomer) is from 8 to 35% (hereinafter referred to as A region); the range wherein the molecular weight calculated as polystyrene is from 500 to 5000 is from 0 to 30% (hereinafter referred to as B region); and the range wherein the molecular weight calculated as polystyrene exceeds 5000 is from 35 to 92% (hereinafter referred to as C region); with the ratio of the B region to the A region being 2.50 or less.

Description: The present invention relates to a positive photoresist composition having sensitivity, heat resistance and dimensional reproducibility. A light-sensitive resin composition containing a compound having a quinone diazide group such as a naphthoquinone diazide or benzoquinone diazide group, upon irradiation with light of from 300 to 500 nm, produces a carboxyl group by the decomposition of the quinone diazide group, whereby the composition, which is alkali-insoluble in itself, turns alkali-soluble. This property is made use of to use the composition as a positive photoresist. In this case, a novolak resin is generally used in combination. This resin is important to obtain a uniform and strong resist film. This positive photoresist has a resolving power markedly superior to that of a negative photoresist. This high resolving power is made use of to use the positive photoresist as an etching protective film when a photographic etching method is applied to produce copper-clad printed circuit boards for printed circuit and integrated circuits such as IC and LSI. With the integrated circuits, miniaturization has proceeded with a rise in the integration level to result in a demand for the formation of patterns of a submicron order. Hitherto, a mask contact printing method has been used for the formation of integrated circuits, but it is said that the limit of this method is 2 µm, and a reduction projection exposure method is attracting attention as a substitute therefor. This method involves reducing and projecting the pattern of a master mask (reticule) by passing light through a lens system, and a resolving power of up to a submicron order can be obtained. However, one of the problems of this method is low through-put. That is, unlike the 1:1 contact or projection exposure methods such as the conventional mask contact printing method, this reduction projection exposure method, because of the need for divided and repeated light exposure, has the problem that the total of exposure times per piece of wafer is long. For solving this problem, raising the sensitivity of the resist used is most important, not to speak of improvement in the apparatus. If the exposure time can be shortened by raising the sensitivity, a rise in the through-put and as a result, a rise in the yield can be attained. When the positive photoresist now in practical use is viewed from this standpoint, it may not always be said that it is satisfactory in terms of the sensitivity. Generally, the positive photoresist is lower in sensitivity than the negative photoresist, an improvement in the sensitivity being desired. The simplest method to raise the sensitivity is to reduce the molecular weight of novolak resin used in the positive photoresist; this method accelerates the dissolution of the photoresist in alkaline developing solution to apparently raise the sensitivity of the photoresist. This method, however, causes the very serious disadvantage that the heat resistance of the photoresist is reduced, in addition to problems such as : film thickness loss in the unexposed area is large (reduction of the so-called film thickness retention); the shape of the pattern changes for the worse, and the so-called γ value drops because the difference in the rates at which the exposed area and unexposed area dissolve in the developing solution is small. The term "heat resistance of the positive photoresist" referred to herein means the resistance to heat of the resist pattern after development. More specifically, when heat is applied from outside to the resist pattern formed on a substrate, the resist pattern softens and flows, or changes in shape above a certain temperature, and the ambient atmospheric temperature the resist pattern can stand to retain its original sharp shape without deformation is the measure of the heat resistance of the photoresist. When the resist pattern softens and flows, or changes in shape, the dimensions of the etched substrate after etching go wrong, or the dimensions fluctuate, so that the resist cannot perform the role required for it as an etching protective film. That is, the yield drops sharply, which is undesirable. Japanese Patent Publication Kokai (Laid-open) No. 189739/1985 discloses that a photoresist composition of superior heat resistance is obtained when the content of 1 - 3 nucleus of a novolak resin is made less than 10% by weight. Three great characteristics required for the resist are sensitivity, resolution and heat resistance. If the resist lacks any one of these characteristics, it cannot be used as resist, so that all the three characteristics need to be excellent. Among these three, sensitivity and heat resistance tend to conflict with each other, and resists satisfying these two at the same time have not been on the market up to now. In view of this, we have sought to develop a positive photoresist which has one of sensitivity and heat resistance improved without impairing the other one and has excellent resolution. We have found that a positive photoresist composition having excellent heat resistance can be obtained with high sensitivity maintained, by using a novel novolak resin having a gel permeation chromatographic pattern (GPC pattern) of a predetermined shape. The present invention provides a novolak resin for use in a positive photoresist which is produced by addition condensation reaction of phenol compound with formaldehyde and characterized in that the area ratio of the gel permeation chromatographic pattern (GPC pattern) [measured by the use of a UV(254 nm) detector] is as follows the range wherein the molecular weight calculated as polystyrene is from 150 to less than 500 (not including phenol and the unreacted monomer) is from 8 to 35% (hereinafter referred to as A region); the range wherein the molecular weight calculated as polystyrene is from 500 to 5000 is from 0 to 30% (hereinafter referred to as B region); and the range wherein the molecular weight calculated as polystyrene exceeds 5000 is from 35 to 92% (hereinafter referred to as C region); the ratio of the B region to the A region being 2.50 or less. The term "the molecular weight of novolak resin" referred to herein means a value obtained by gel permeation chromatography (hereinafter referred to as GPC) with monodispersed polystyrene as a standard. The GPC chromatogram was measured by HLC-802A chromatograph (produced by Toyo Soda K.K.) with one piece each of G-4000H₈ column and G-2000H₈ column (both are produced by Toyo Soda K.K.) in series, and streaming tetrahydrofuran, a carrier solvent, at a flow rate of 1 ml/min. Chromatography was carried out using a UV(254 nm) detector. The molecular weight was obtained from a calibration curve prepared with monodispersed polystyrene. That is, the calibration curve was prepared by third order regression using monodispersed polystyrenes of which the weight average molecular weight is 300,000, 100,000, 35,000, 4,000 and 800, respectively and a styrene monomer (molecular weight, 104). In the accompanying drawings : Fig. 1 is the GPC pattern of a particular representative novolak resin defined by the present invention; the novolak resin showing such a GPC pattern not only satisfies both sensitivity and heat resistance requirements, but also sharply improves the dimensional reproducibility of mask patterns which is a guide to the resolution. Fig. 2 is a representative GPC pattern of conventionally known novolak resin; the sensitivity and heat resistance of resists with novolak resin having such a pattern tend to conflict with each other, which is undesirable. Fig. 3 represents the GPC pattern of novolak resin similar to that described in the foregoing Japanese Patent Publication Kokai (Laid-open) No. 189739/1985; a resist with such resin is improved to some degree in balance between sensitivity and heat resistance, but the balance is not yet satisfactory. Fig. 4 shows the GPC pattern of novolak resin originally prepared by fractionation, etc.; this resin is one wherein the area ratio of the A and B regions has been made small. Resists with this resin have improved heat resistance, but sharply lower sensitivity. The GPC pattern in Fig. 1 is characterized in that the area B region is small compared with A and C regions. Specific examples of phenol compound for the novolak resin of the present invention include phenol, cresol, xylenol, ethylphenol, trimethylphenol, propylphenol, butylphenol, dihydroxybenzene, naphthols, etc. These phenols may be used alone or in combination. The use of cresol is particularly preferred. As a cresol, ortho-, meta- or para-cresol may be used alone or in combination. Specifically, to use meta-cresol alone or a meta-cresol/para-cresol mixture is preferred. In the latter case, the meta-cresol/para-cresol ratio is preferably 100/0 to 20/80. Particularly, in order to attain the object of the present invention more effectively, it is most preferred that the ratio of meta-cresol and para-cresol contained in the novolak resin corresponding to the A region described above is from 10/0 to 2/8, and that of meta-cresol and para-cresol contained in the novolak resin corresponding to the C region is from 10/0 to 4/6. In the present invention, for the addition condensation reaction with the phenol, aqueous formaldehyde solution (formalin) and paraformaldehyde, which is the oligomer of formaldehyde, may be used - advantageously 37% formalin is mass-produced in industry. In the present invention, the addition condensation reaction of phenol with formaldehyde can be carried out by the usual methods. This reaction can be carried out at a temperature of from 60° to 120°C for from 2 to 30 hours. Organic acids, inorganic acids or divalent metal salts can be used as catalyst - e.g. oxalic acid, hydrochloric acid, sulfuric acid, perchloric acid, p-toluenesulfonic acid, trichloroacetic acid, phosphoric acid, formic acid, zinc acetate, magnesium acetate, etc. The reaction may be carried out in bulk or with a suitable solvent. The area ratios of the GPC pattern [measured by the use of a UV(254 nm) detector] of novolak resin obtained by the usual method are, for example, from 5 to 20% for the A region, from 32 to 45% for the B region and from 46 to 60% for the C region. By adjusting the area ratio of this GPC pattern to from 8 to 35% for the A region, from 0 to 30% for the B region and from 35 to 92% for the C region, and besides adjusting the B region/A region ratio to 2.50 or less, the particular novolak resin for resist defined by the present invention can be obtained. For regulating the GPC pattern of novolak resin as described above, fractionating the novolak resin and blending the fractions obtained is preferred. For the fractionation, there are a method of dissolving the synthesized novolak resin in a good solvent described below and precipitating the resin by adding water, and a method of adding the resin to a poor solvent (e.g. heptane, hexane, pentane, cyclohexane) and separating the resin by liquid separation. The good solvent includes for example alcohols (e.g. methanol, ethanol, propanol), ketones (e.g. acetone, methyl ethyl ketone, methyl isobutyl ketone), ethylene glycol or its ethers and ether esters (e.g. ethyl cellosolve, ethyl cellosolve acetate), propylene glycol or its ethers and ether esters (e.g. propylene glycol monomethyl ether acetate), acetic acid esters (e.g. methyl acetate, ethyl acetate), formamide, dimethylformamide, tetrahydrofuran, dioxane, dimethyl sulfoxide, acetonitrile, etc. An alternative method to regulate the GPC pattern of this novolak resin is one wherein novolak resins consisting mainly of the A, B and C regions, respectively, are separately synthesized, and the fractions obtained by fractionation of these resins are combined with one another. By this method, novolak resin having a preferred GPC pattern can also be obtained. Novolak resin thus regulated has a weight average molecular weight, as calculated as polystyrene, of e.g. 3000 to 35000, preferably from 5000 to 30000, more preferably from 7000 to 26000. In a positive photoresist composition with the novolak resin of the present invention, a quinone diazide compound is used as a light sensitive component. This quinone diazide compound is obtained by well-known methods, for example, by condensing naphthoquinonediazidesulfonyl chloride or benzoquinonediazidesulfonyl chloride with compound having a hydroxyl group in the presence of weak alkali. Examples of the compound having a hydroxyl group include hydroquinone, resorcinol, phloroglucinol, 2,4-dihydroxybenzophenone, 2,3,4-trihydroxybenzophenone, 2,3,4,4′-tetrahydroxybenzophenone, 2,2′,4,4′-tetrahydroxybenzophenone, 2,3,4,3′-tetrahydroxybenzophenone, 2,3,4,2′,3′-pentahydroxybenzophenone, 2,3,4,2′,4′-pentahydroxybenzophenone, 2,3,4,2′,5′-pentahydroxybenzophenone, 2,3,4,3′,4′-pentahydroxybenzophenone, 2,3,4,3′,5′-pentahydroxybenzophenone, 2,3,4,3′,4′,5′-hexahydroxybenzophenone, gallic acid alkyl esters, etc. Preparation of the positive photoresist is carried out by mixing and dissolving the foregoing quinone diazide compound and the novolak resin of the present invention in a solvent. The ratio of novolak resin to quinone diazide compound is preferably from 1:1 to 6:1. A preferred solvent used is such that it evaporates at a suitable drying rate to give a uniform and smooth coating film. Such solvents include ethyl cellosolve acetate, methyl cellosolve acetate, ethyl cellosolve, methyl cellosolve, propylene glycol monomethyl ether acetate, butyl acetate, methyl isobutyl ketone, xylene, etc. To the positive photoresist composition obtained by the foregoing method, small amounts of resins, dyes, etc. may be added as additive as need arises. The positive photoresist compositions prepared by combining novolak resin of the present invention and the quinone diazide compound, for example, as a light sensitive component have excellent sensitivity and markedly improved heat resistance and dimensional reproducibility. The present invention is illustrated more specifically with reference to the following Reference Examples and Examples, but it is not limited to these. Reference examples 1 to 4 To a 1000-ml three-necked flask were added 270 g of m-cresol, 2.7 g of zinc acetate dihydrate and 27 g of ion-exchanged water, and 182 g of an aqueous formalin solution (37.0%) was added dropwise over 40 minutes while heating and stirring on an oil bath at 90°C. Thereafter, reaction was carried out with heating and stirring for further 3 hours. Thereafter, 250 g of ethyl cellosolve acetate was added, and then 40 g of 5% aqueous oxalic acid solution was added dropwise over 40 minutes. Reaction was carried out with heating and stirring for a pre-determined period of time, and the reaction solution was neutralized, washed with water and dehydrated to obtain an ethyl cellosolve acetate solution of novolak resin. The weight average molecular weight and the area ratio of the GPC pattern measured by GPC chromatography were as follows : Reference examples 5 and 6 To a 1000-ml three-necked flask were added 189 g of m-cresol, 81 g of p-cresol, 2.7 g of zinc acetate dihydrate and 27 g of ion-exchanged water, and 182 g of an aqueous formalin solution (37.0%) was added dropwise over 40 minutes while heating and stirring on an oil bath at 90°C. Thereafter, reaction was carried out with heating and stirring for a further 6 hours. Thereafter, 250 g of ethyl cellosolve acetate was added, and then 40 g of 5% aqueous oxalic acid solution was added dropwise over 40 minutes. Reaction was carried out with heating and stirring for 10 hours, and the reaction solution was neutralized, washed with water and dehydrated to obtain an ethyl cellosolve acetate solution of novolak resin. The weight average molecular weight and the area ratio of the GPC pattern measured by GPC chromatography were as follows : Example 1 First fractionation : 308 Grams of the ethyl cellosolve acetate solution of novolak resin obtained in Reference example 1 (content of the novolak resin, 46.4%) was added to a 5-liter separable flask, and 646 g of ethyl cellosolve acetate and 1892 g of n-heptane were additionally added. After stirring for 30 minutes at 20°C, the resulting mixture was allowed to stand still and separated. Second fractionation : To a 5-liter separable flask was added 424 g of the lower-layer mass, and 2134 g of ethyl cellosolve acetate and 1042 g of n-heptane were added additionally, and the same procedure as in the first fractionation was repeated. To the lower-layer mass obtained by liquid separation was added the upper-layer mass obtained by the first fractionation, and heptane was removed by means of an evaporater to obtain an ethyl cellosolve acetate solution of novolak resin. The weight average molecular weight and the area ratio of the GPC pattern measured by GPC chromatography were as follows : Examples 2 to 4 Ethyl cellosolve acetate solutions of novolak resin were obtained by completely the same procedure as in Example 1 except that the amounts of the ethyl cellosolve acetate solution of novolak resin, which is the material before fractionation, ethyl cellosolve acetate and n-heptane were changed. The amounts of the material before fractionation, ethyl cellosolve acetate and n-heptane are as follows : The weight average molecular weight and the area ratio of the GPC pattern, as measured by GPC chromatography, of the novolak resin obtained by fractionation were as follows : Example 5 To a 1000-ml three-necked flask were added 270 g of m-cresol and 50 g of 5% oxalic acid, and 120 g of an aqueous formalin solution (37.0%) was added dropwise over 40 minutes while heating and stirring on an oil bath at 90°C. Reaction was then carried out with heating and stirring for a further 3 hours. Thereafter, the reaction solution was neutralized, washed with water and dehydrated to obtain an ethyl cellosolve acetate solution of novolak resin (Resin-I). The weight average molecular weight of the Resin-I was 710. 481 Grams of the ethyl cellosolve acetate solution of novolak resin obtained in Reference example 1 (content of the novolak resin, 46.4%) was added to a 10-liter separable flask, and 3927 g of ethyl cellosolve acetate and 1795 g of n-heptane were additionally added. After stirring for 30 minutes at 20°C, the resulting mixture was allowed to stand still and separated. To the lower-layer mass obtained by liquid separation was added the foregoing Resin-I so that the content of the Resin-I was 20.0% by weight Thereafter, n-heptane was removed by an evaporater to obtain an ethyl cellosolve acetate solution of novolak resin. The weight average molecular weight and the area ratio of the GPC pattern, measured by GPC chromatography, were as follows : Reference examples 7 and 8 Ethyl cellosolve acetate solutions of novolak resin were obtained by completely the same procedure as in Example 1 except that the resin of Reference example 2 was used as a material before fractionation, and that the amounts of ethyl cellosolve acetate and n-heptane were changed. The weight average molecular weight, calculated as polystyrene, of the novolak resin obtained by the fractionation and the area ratio of the GPC pattern of the same resin measured by GPC chromatography were as follows : Example 6 To a 3000-ml three-necked flask were added 625 g of a m-/p-cresol mixture (content of m-cresol, 61.5%), 185 g of p-cresol, 91 g of 5% aqueous oxalic acid and 756 g of ethyl cellosolve acetate, and 444 g of an aqueous formalin solution (37%) was added dropwise over 40 minutes while heating and stirring on an oil bath at 80°C. Reaction was then carried out with heating and stirring for 6 further hours. Thereafter, the reaction solution was neutralized, washed with water and dehydrated to obtain an ethyl cellosolve acetate solution of novolak resin. An ethyl cellosolve acetate solution of novolak resin was obtained by completely the same procedure as in Example 1 except that the novolak resin obtained above was used, and that the amounts of ethyl cellosolve acetate and n-heptane were changed. The weight average molecular weight, calculated as polystyrene, of the novolak resin obtained by fractionation and the area ratio of the GPC pattern of the same resin measured by GPC chromatography were as follows : Example 7 To a 1000-ml three-necked flask were added 270 g of m-cresol and 50 g of 5% aqueous oxalic acid, and 57 g of an aqueous formalin solution (37%) was added dropwise over 40 minutes while heating and stirring on an oil bath at 90°C. Reaction was then carried out with heating and stirring for a further 2 hours. Thereafter, the reaction solution was neutralized, diluted with ethyl cellosolve acetate, washed with water and dehydrated to obtain an ethyl cellosolve acetate solution of novolak resin (Resin-II). The weight average molecular weight of the Resin-II was 330. To a 3000-ml three-necked flask were added 724 g of a m-/p-cresol mixture (content of m-cresol, 61.5%), 86 g of p-cresol, 91 g of 5% aqueous oxalic acid and 756 g of ethyl cellosolve acetate, and 444 g of an aqueous formalin solution (37%) was added dropwise over 40 minutes while heating and stirring on an oil bath at 80°C. Reaction was then carried out with heating and stirring for a further 8 hours. Thereafter, the reaction solution was neutralized, washed with water and dehydrated to obtain an ethyl cellosolve acetate solution of novolak resin. Completely the same procedure as in Example 5 was carried out except that the novolak resin obtained above was used, and that the amounts of ethyl cellosolve acetate and n-heptane were changed. To the lower-layer mass thus obtained was added the foregoing Resin-II so that the content of the Resin-II was 25.0% by weight Thereafter, n-heptane was removed by an evaporater to obtain an ethyl cellosolve acetate solution of novolak resin. The weight average molecular weight and the area ratio of the GPC pattern, as measured by GPC chromatography, were as follows : Example 8 To a 1000-ml three-necked flask were added 270 g of m-cresol, 2.7 g of zinc acetate dihydrate and 27 g of ion-exchanged water, and 121 g of an aqueous formalin solution (37.0%) was added dropwise over 40 minutes while heating and stirring on an oil bath at 80°C. Reaction was then carried out with heating and stirring for a further 3 hours. Thereafter, 30 g of 5% aqueous oxalic acid solution was added, and reaction was carried out with heating and stirring for a pre-determined period of time. The reaction solution was then neutralized, diluted with ethyl cellosolve acetate, washed with water and dehydrated to obtain an ethyl cellosolve acetate solution of novolak resin (Resin-III). The weight average molecular weight of this resin was 710. To a 3000-ml three-necked flask were added 658 g of a m-/p-cresol mixture (content of m-cresol, 61.5%), 152 g of p-cresol and 91 g of 5% aqueous oxalic acid, and 426 g of an aqueous formalin solution (37%) was added dropwise over 40 minutes while heating and stirring on an oil bath at 80°C. Thereafter, reaction was carried out with heating and stirring for a further 3 hours. The reaction solution was then neutralized, diluted with ethyl cellosolve acetate, washed with water and dehydrated to obtain an ethyl cellosolve acetate solution of novolak resin. Completely the same procedure as in Example 5 was carried out except that the novolak resin obtained above was used, and that the amounts of ethyl cellosolve acetate and n-heptane were changed. To the lower-layer mass thus obtained was added the foregoing Resin-III so that the content of the Resin-III was 32.0% by weight Thereafter, n-heptane was removed by an evaporator to obtain an ethyl cellosolve acetate solution of novolak resin. The weight average molecular weight, calculated as polystyrene, of the novolak resin thus obtained and the area ratio of the GPC pattern of the same resin measured by GPC chromatography were as follows : Example 9 To a 1000-ml three-necked flask were added 189 g of m-cresol, 113 g of p-tert-butylphenol and 30 g of 5% aqueous oxalic acid, and 152 g of an aqueous formalin solution (37%) was added dropwise over 40 minutes while heating and stirring on an oil bath at 80°C. Reaction was then carried out with heating and stirring for 6 further hours. Thereafter, the reaction solution was neutralized, diluted with ethyl cellosolve acetate, washed with water and dehydrated to obtain an ethyl cellosolve acetate solution of novolak resin. Completely the same procedure as in Example 5 was carried out except that the novolak resin obtained above was used, and that the amounts of ethyl cellosolve acetate and n-heptane were changed. To the lower-layer mass thus obtained was added the foregoing Resin-II of Example 7 so that the content of the Resin-II was 25.0% by weight Thereafter, n-heptane was removed by an evaporater to obtain an ethyl cellosolve acetate solution of novolak resin. The weight average molecular weight, calculated as polystyrene, of the novolak resin thus obtained and the area ratio of the GPC pattern of the same resin measured by GPC chromatography were as follows : Example 10 To a 1000-ml three-necked flask were added 154 g of a m-/p-cresol mixture (content of m-cresol, 61.5%), 36 g of p-cresol, 82 g of o-cresol and 30 g of 5% aqueous oxalic acid, and 170 g of an aqueous formalin solution (37%) was added dropwise over 40 minutes while heating and stirring on an oil bath at 80°C. Reaction was then carried out with heating and stirring for a further 9 hours. Thereafter, the reaction solution was neutralized, diluted with ethyl cellosolve acetate, washed with water and dehydrated to obtain an ethyl cellosolve acetate solution of novolak resin. Completely the same procedure as in Example 5 was carried out except that the novolak resin obtained above was used, and that the amounts of ethyl cellosolve acetate and n-heptane were changed. To the lower-layer mass thus obtained was added the foregoing Resin-II of Example 7 so that the content of the Resin-II was 25.0% by weight Thereafter, n-heptane was removed by an evaporater to obtain an ethyl cellosolve acetate solution of novolak resin. The weight average molecular weight, calculated as polystyrene, of the novolak resin thus obtained and the area ratio of the GPC pattern of the same resin measured by GPC chromatography were as follows : Application examples and Comparative examples The novolak resins obtained in Examples and Reference Examples were each dissolved together with a sensitizer in an ethyl cellosolve acetate/n-butyl acetate (8:2) solvent in the proportions shown in Table 1, to prepare resist solutions. The amount of solvent was regulated so that the film thickness was 1.28 µm when the resist solution was applied under the coating conditions below. Every composition was filtered through a Teflon filter 0.2 µm thick to prepare a resist solution. The solution was then coated onto a silicon wafer, rinsed as usual, at a rate of 4000 rpm by means of a spinner. The silicon wafer was baked for 1 minute on a vacuum adsorption-type hot plate kept at 100°C, and exposed to light with the exposure time being stepwise varied at every shot by means of a reduction projection exposure apparatus with a 350 W super-high pressure mercury vapor lamp as a light source. Thereafter, the silicon wafer was developed using a developing solution SOPD (product of Sumitomo Chemical Company, Limited). After rinsing and drying, the amount of film thickness loss for each shot was plotted against the exposure time to obtain sensitivity. The film thickness retention was obtained from the film thickness on the unexposed area. Also, the silicon wafer having a resist pattern was placed for 30 minutes in a clean oven set at varying temperatures and allowed to stand in an aerial atmosphere, and the heat resistance was evaluated by observing the resist pattern by means of a scanning microscope. These results are collectively shown in Table 1. It is found from Table 1 that balance between the sensitivity and heat resistance in the Application Examples is markedly improved as compared with the Reference Examples.

Claims: A novolak resin for a positive photoresist and which is produced by addition condensation reaction of a phenolic compound with formaldehyde, characterised in that the area ratio of the gel permeation chromatographic pattern (GPC pattern) [measured by the use of a UV(254 nm) detector] is as follows : the range wherein the molecular weight calculated as polystyrene is from 150 to less than 500 (not including phenol and unreacted monomer) is from 8 to 35% (hereinafter referred to as A region); the range wherein the molecular weight calculated as polystyrene is from 500 to 5000 is from 0 to 30% (hereinafter referred to as B region); and the range wherein the molecular weight calculated as polystyrene exceeds 5000 is from 35 to 92% (hereinafter referred to as C region); with the ratio of the B region to the A region being 2.50 or less. A novolak resin according to claim 1 characterised in that the weight average molecular weight, calculated as polystyrene, of the novolak resin is from 3000 to 35000. A novolak resin according to claim 1 or 2 wherein cresol is used as a phenol compound. A novolak resin according to claim 3 wherein the cresol comprises m-cresol and p-cresol. A novolak resin according to claim 4 wherein the ratio of m-cresol to p-cresol contained in the novolak resin corresponding to the A region is from 10:0 to 2:8, and the ratio of m-cresol to p-cresol contained in the novolak resin corresponding to the C region is from 10:0 to 4:6. A positive photoresist comprising novolak resins according to any preceding claim and light sensitive component.

Inventors: FURUTA, AKIHIRO, HANABATA, MAKOTO, NINOMIYA, TAKAO, OI, FUMIO, OSAKI, HARUYOSHI, UEMURA, YUKIKAZU, UETANI, YASUNORI
Applicants: SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1987
Main classification: C08G8
IPC codes: G03C1, C08G8, G03F7