GB2180842A

GB2180842A - Positive-working photoresist composition

Info

Publication number: GB2180842A
Application number: GB08619368A
Authority: GB
Inventors: Akira Yokota; Hisashi Nakane; Hidekatsu Kohara; Hatsuyuki Tanaka; Masanori Miyabe; Yoshiaki Arai; Shingo Asaumi; Toshimasa Nakayama
Original assignee: Tokyo Ohka Kogyo Co Ltd
Current assignee: Tokyo Ohka Kogyo Co Ltd
Priority date: 1985-08-09
Filing date: 1986-08-08
Publication date: 1987-04-08
Also published as: GB2180842B; DE3626582C2; GB8619368D0; JPS6235349A; JPH0650396B2; DE3626582A1

Abstract

A positive-working photoresist composition suitable for fine patterning in the manufacture of semiconductor devices, e.g. VLSIs, with high fidelity comprises 100 parts by weight of a cresol novolac resin and 25-60 parts by weight of a naphthoquinone diazide sulfonic acid ester as the photosensitive component while the cresol novolac resin component is prepared from an isomeric mixture of cresol composed of 10-45% of n-cresol and 90-55% of p-cresol with substantial absence of o-cresol or composed of 10-45% of n-cresol, 90-55% of p-cresol and 5% or less of o-cresol.

Description

SPECIFICATION A novel positive-working photoresist composition The present invention relates to a positive-working photoresist composition or, more particularly, to a positive-working photoresist composition suitable for use in fine patterning of a photoresist layer in the manufacturing process of semiconductor-based devices such as LSls, VLSls and the like.

Recent progress in semiconductor technology has aroused a rapidly increasing demand for computerized instruments, including computers for industrial use, instruments for automatization of offices, personal computers and the like, and, similarly, semiconductor devices such as intergrated circuits display an overwhelming trend towards increased density or degree of integration. For example, VLSls having a density of 1 megabit or higher are now being produced and VLSls having a density 256 kilobits are being superceded. Such a high density of integration in VLSls naturally requires extremely fine patterning on semiconductor wafers in the so-called submicron range.For example, the minimum line width to be reproduced with high fidelity in the photoresist layer is about 2 Am in semiconductor ICs for 256 kilobits DRAMs, about 1.0 to 1.3 m in devices for 1 megabit DRAMs and about 0.7 to 0.8 ,um in devices for 4 megabits DRAMs so that the technology of patterning must permit an extremely high precision.

As is known, the patterning on semiconductor wafers for the manufacture of integrated circuits is performed by photolithography using a photoresist composition. Of the two types of photoresist compositions (positive-working and negative-working), the positive-working photoresist compositions are widely preferred in the working of fine patterning in which high-fidelity reproduction of a line pattern having a width of 1 to 2 Clm is essential.

The principal ingredients in most of the conventional positive-working photoresist compositions are an alkali-soluble novolac resin as the film-forming constituent and a quinone diazide compound or photosensitive constituent in the form of a mixture. Typical photosenitizer compounds of the quinone diazide type include sulfonic acid esters formed between a naphthoquinone diazide sulfonic acid and a compound having one or more of phonolic hydroxy groups disclosed in U.S. Patent No. 3,402,044 and other esters disclosed in U.S. Patents No. 3,046,118, No.

3,106,465 and No. 3,148,983.

In respect of the film-forming constituent of the photoresist compositions which is typically an alkali-soluble novolac resin, on the other hand, various types of novolac resins have been proposed including phenol-formaldehyde novolac resins disclosed in U.S. Patent No. 3,402,044 and cresol novolac resins disclosed in Electro-chemistry and Industrial Physical Chemistry, volume 48, page 584 (1980). Further, Japanese Patent Kokai 59-17112 teaches that the sensitivity of a positive-working photoresist composition comprising a cresol novolac resin as the film-forming constituent can be improved by suitably selecting the proportion of the cresol isomers in the cresol used in the preparation of the cresol novolac resin.

Turning now to the problems in the process of exposure of the photoresist layer to light, the exposure is carried out either by exposure by direct contacting or by exposure by minifying projection. In the former method of contacting exposure, the photoresist layer formed on the surface of a semiconductor wafer is exposed to light through a patterned photomask in direct contact with the photoresist layer. This method is advantageous in respect of the contrast of the patterned image so that a patterned photoresist layer of considerably high contrast can be obtained by this method even when the photoresist composition used is inherently inferior in respect of the contrast and fidelity of the pattern reproduction. However, this method has some disadvantages and problems.For example, the photomask is sometimes damaged mechanically as a natural consequence of direct contact with. the photoresist layer in each time of exposure so that extreme care is required in handling the photomask and the photomasks can be maintained in good condition only at more than negligible expense. Moreover, needless to say, the pattern on the photomask must be full-size relative to the pattern to be reproduced so that a patterned photomask having such a high precision is unavoidably very expensive, especially, when the line width of the pattern is in the submicron range.

In the method of exposure by minifying projection, on the other hand, the dimension of the pattern on a patterned photomask can be as much as 5 to 10 times that of the photoresist pattern to be reproduced so that even a photomask of high precision for patterning in the submicron range can be obtained at a relatively low cost. On the contrary to this advantage, this method is disadvantageous in respect of the contrast of light between the areas to be exposed and not to be exposed in comparison with the exposure by direct contacting of the photomasks.

Therefore, this method of exposure by minifying projection is applicable to the reproduction of a pattern of high precision only when the photoresist composition is inherently highly sensitive in exposure to light with relative low contrast.

In the manufacture of semiconductor devices such as VLSls, furthermore, the pattern to be formed in the photoresist layer is not composed of lines having one and the same line width but includes lines having varied line widths combined in a complicated manner. This fact causes a difficult problem affecting the quality of pattern reproduction since the minimum exposure dose by which the photoresist layer on the exposed areas can be removed away by development considerably depends on the line width. Taking the minimum exposure dose to a pattern of 2.0 ,um line width as unity, for example, the optimum exposure doses to patterns of 1.5 m and 1.0 um line widths are 1.2 to 1.3 and 1.5 to 1.7, respectively.Therefore, an exposure dose which may be optimum for a line of certain line width may be too small or too large for lines having a smaller or larger line width, respectively, in the same pattern and will cause insufficient reproduction of finer lines or excessive removal of the photoresist layer of the coarser lines so that the fidelity of pattern reproduction cannot be highest over the whole area of the pattern. Moreover, the surface of a semiconductor device during processing is not completely flat but usually has a stepwise height difference of 0.5 to 1.0 ,um from portion to portion so that the thickness of a photoresist layer formed on such a stepwise surface cannot be uniform but is smaller in the upper area of the step and larger in the lower area of the step.When such a photoresist layer is exposed to light and developed, therefore, it is usual that the line width of the pattern reproduced in the photoresist layer is smaller in the area where the photoresist layer has a smaller thickness than in the area where the thickness is larger affecting the fidelity of pattern reproduction.

In connection with the process of etching on the surface of the semiconductor wafer on which a patterned photoresist layer of submicron fineness is formed, the undesirable phenomenon of side etching is unavoidable in a wet process so that the process of etching is sometimes performed by a dry process, free from side etching, by use of plasma. In this dry etching method, however, the patterned photoresist layer as the etching mask is attacked by the plasma to cause gradual reduction in the film thickness. Accordingly, it is a desirable condition that the patterned line of the photoresist layer has a cross section in which the width of the line is not affected even when the film thickness is reduced by the attack of the plasma in the process of dry etching.

The above described problems each concern the poor reproducibility or fidelity between the original of the pattern on the photomask and the patterned image reproduced in the photoresist layer. The reasons therefore include, as is mentioned above, the decrease in the contrast in the exposure by minifying projection between the exposed and unexposed areas, difference in the optimum exposure doses between line patterns having different line widths, difference in the film thickness of the photoresist layer between the areas on both sides of a step on the wafer surface having stepwise height differences, and so on.

These problems can be solved as a whole only by the use of a photoresist composition having high fidelity in pattern reproduction and free from the influence of the exposure dose on the dimensions of the reproduced pattern. The photoresist composition free from the influence of the exposure dose on the dimensions of the reproduced pattern here implied should have following characteristics. The reproduced line pattern should have a line width which is an accurate reproduction of the line in the original pattern of the photomask without expansion or diminishment irrespective of the exposure dose or the extent of development.The patterned line of photoresist layer should have a rectangular cross section standing on the substrate surface with definitely angled shoulders while undesirable cross sectional configurations include those having trailing skirts on the substrate surface even with definitely angled shoulders because the photoresist layer may disappear in the thin skirt portions by the attack of the etching plasma to cause a change in the line width of the photoresist pattern.

It is therefore desirable to provide a positive-working photoresist composition free from the above described problems capable of giving a patterned photoresist layer which is an accurate reproduction of the orginal pattern with high fidelity and is unaffected by the influence of the exposure dose on the width of the patterned lines.

The composition for patterning semiconductor wafers by positive-working photolithography provided by the invention comprises: (A) 100 parts by weight of a cresol novolac resin as a film-forming constituent; and (B) from 25 to 60 parts by weight of a naphthoquinone diazide sulfonic acid ester as a photosensitive constituent, the cresol moiety in the cresol novolac resin being derived from an isomeric mixture of cresol composed of 10 to 45% of m-cresol and 90 to 55% of p-cresol with substantial absence of o-cresol.

As is understood from the above description, the most characteristic feature of the inventive photoresist composition consists of the specific cresol novolac resin as the film-forming constituent which is a novolac resin prepared from a mixture of cresol isomers composed of m- and pcresol with substantial absence of o-cresol. Although a mixed cresol containing o-cresol in a small proportion can be used in the invention, the content of o-cresol preferably should not exceed 5% and should be as small as possible.

The naphthoquinone diazide sulfonic acid ester, i.e. component (B), as the principal ingredient of the photosensitive ingredient in the inventive photoresist composition, may be a reaction product obtained by the esterification reaction of a naphthoquinone diazide sulfonic acid and a phenolic compound such as, for example, polyhydroxyy benzophenones, alkyl gallates, and the reaction of esterification may be performed readily according to a conventional procedure.

The phenolic compound, as one of the reactants in the esterfication reaction is exemplified, in addition to the above mentioned polyhydroxy benzophenones, e.g. tetrahydroxy benzophenone, and alkyl gallates, by trihydroxybenzenes, trihydroxybenzene monoethers, 2,2' ,4,4'-tetrahydroxy diphenyl methane, 4,4'-dihydroxy diphenyl propane, 4,4'-dihydroxy diphenyl sulfone, 2,2'-dihy droxy-l,l '-dinaphthyl methane, 2-hydroxyfluorene, 2-hydroxyphenanthrene, polyhyrdoxy anthraquinones, purpurogallin and derivatives thereof, phenyl 2,4,6-trihydroxybenzoate. Further, aromatic amine compounds can be used in place of these phonolic compounds.

The cresol novolac resin, i.e. component (A), as the film-forming constituent in the inventive photoresist composition is, as is mentioned above, a novolac resin prepared from a mixture of cresol isomers composed of 10 to 45% of m-cresol and 90 to 55% of p-cresol with substantial absence of o-cresol. When the requirement relative to the isomeric proportion of the cresol isomers in the starting cresol used in the preparation of the cresol novolac resin is not satisfied, the desired improvement in the photoresist composition tends to be unrealised.

The inventive photoresist composition should comprise from 25 to 60 parts by weight of the naphthoquinone diazide sulfonic acid ester as the photosensitive constituent to each 100 parts by weight of the cresol novolac resin as the film-forming constituent. When the amount of the photosensitive constituent is in excess of 60 parts by weight, the photoresist composition would have remarkably decreased sensitivity while, when the amount thereof is too small, an adverse effect is caused on the cross sectional configuration of the line in the patterned photoresist layer.

The inventive positive-working photoresist composition is used usually in the form of a solution prepared by dissolving the above described cresol novolac resin and the naphthoquinone diazide sulfonic acid ester in a suitable organic solvent. Exemplary of the organic solvent used in this case are ketones, e.g. acetone, methyl ethyl ketone, cyclohexanone and isoamyl ketone, polyhydric alcohols and derivatives thereof, e.g. ethylene glycol, ethylene glycol monoacetate, diethylene glycol and monomethyl, monoethyl, monopropyl and monophenyl ethers of diethylene glycol monoacetate; cyclic ethers, e.g. dioxane; and esters, e.g. methyl acetate, ethyl acetate and butyl acetate. These organic solvents can be used either singly or as a mixture of two kinds or more according to need.

The inventive positive-working photoresist composition may be admixed optionally with various kinds of known additives having compatibility with the essential ingredients and conventionally used in photoresist compositions such a auxiliary resins, plasticizers, stabilizers, colouring agents to serve for further increasing the visibility of the patterned image after development and so on.

The procedure for forming a patterned photoresist layer using the inventive photoresist composition may be conventional. For example, the surface of a substrate body such as a semiconductor silicon wafer is coated with the inventive photoresist composition in the form of an organic solution by use of a suitable coating machine such as a spinner followed by drying to form a uniform photoresist layer thereon, which is then exposed to light on a minifying projector or a suitable apparatus for exposure through a photomask bearing a desired pattern followed by development using a developing solution such as an aqueous solution of an organic base, e.g.

tetramethyl ammonium hydroxide, in a concentration of 2 to 5% by weight so that the photoresist layer is selectively dissolved away on the areas where the photoresist composition has been imparted with increased solubility in the developing solution as a result of exposure to light to give a high-fidelity minified reproduction of the pattern on the photomask. Advantageously, the thus reproduced pattern is a very accurate reproduction of the photomask pattern to the utmost fineness having a line width in the submicron range with dimensional accuracy and the dimensional accuracy is not affected even on a substrate surface having stepwise height differences in the exposure by minifying projection which may in general give poor contrast.Accordingly, the inventive positive-working photoresist composition can be used advantageously in the manufacture of high-precision semiconductor devices such as VLSls.

The following examples of the positive-working photoresist composition of the invention is illustrate the invention in more detail.

Figs. 1, 2 and 3 each schematically illustrate a cross section of a patterned line of the photoresist layer produced in the examples and comparative examples.

Example 1 A cresol novolac resin was prepared according to a conventional procedure by the condensation reaction in a reaction mixture composed of 40:60 by weight mixture of m- and p-cresols and formalin in the presence of oxalic acid as the catalyst. A positive-working photoresist composition in the form of a solution was prepared by dissolving 100 parts by weight of the above prepared cresol novolac resin and 30 parts by weight of 2,3,4-trihydroxy benzophenone ester of naphthoquinone-1,2-diazido-5-sulfonic acid in 390 parts by weight of ethylene glycol monoethyl ether acetate followed by filtration through a membrane filter having pores of 0.2 jim diameter.

A silicon wafer of 4 inch diameter was uniformly coated with the photoresist solution in a coating thickness of 1.3 jim as dried using a resist coater (Model TR-4000, manufactured by Tazmo Co.) followed by drying and pre-baking for 90 seconds on a hot plate kept at 1 100C to give a photoresist layer on the wafer. The silicon wafer provided with the photoresist coating layer was then exposed for 880 milliseconds to ultraviolet light on a minifying projector (Wafer Stepper Model DSW-4800, manufactured by GCA Co.) through a test chart photomask (manufactured by Dai-Nippon Printing Co.) and developed for 30 seconds at 230C using a 2.38% by weight aqueous solution of tetramethyl ammonium hydroxide as the developing solution.

The patterned lines of the photoresist layer thus developed has an ideally rectangular cross section as is schematically illustrated in Fig. 1 of the accompanying drawing showing the line 2 with perpendicularly upright side surfaces standing on the substrate 1.

Examples 2 to 9 and Comparative Examples 1 to 5.

The experimental procedure in each of these experiments was substantially the same as in Example 1 except that the weight ratio of the m- and p-cresols in the mixed cresol used in the preparation of the cresol novolac resin was varied as indicated in Table 1 below and that the photosensitive compound, which was one of the compounds I to Vl shown below, was used in varied amounts as indicated in the table in parts by weight per 100 parts by weight of the cresol novolac resin.

Photosensitizer compound l: an esterification product of 1 mole of 2,3,4-trihydroxy benzophenone and 1.6 moles of naphthoquinone-1 ,2-diazido-5-sulfonyl chloride II: an esterification product of 1 mole of 2,4,6-trihydroxy benzophenone and 1.8 moles of naphthoquinone- 1 ,2-diazido-5-sulfonyl chloride III: an esterification product of 1 mole of 2,3,4-trihydroxy benzophenone and 2.0 moles of naphthoquinone- 1 ,2-diazido-5-sulfonyl chloride IV: an esterification product of 1 mole of 2,3,4-trihydroxy-4'-hydroxy benzophenone and 2.2 moles of naphthoquinone-1 ,2-diazido-5-sulfonyl chloride V: an esterification procduct of 1 mole of 1,2,3-trihydroxy benzene and 1.6 moles of naphtho quinone- 1 ,2-diazido-5-sulfonyl chloride Vl: an esterification product of 1 mole of phenyl 2,4,6-trihydroxy benzoate and 1.6 moles of naphthoquinone- 1 ,2-diazido-5-sulfonyl chloride Table 1 shows the exposure time in milliseconds and the cross sectional form of the patterned line of the photoresist layer as indicated by the numerals 1, 2 and 3 which correspond to the cross section of the patterned line 2 of the photoresist layer formed on the substrate 1 by development shown in Figs. 1, 2 and 3, respectively, in the accompanying drawing. The cross sectional form may be rectangular as is illustrated in Fig. 1, plateau-like with definitely angled shoulders but with trailing skirts as is illustrated in Fig. 2 or broad with rounded shoulders and trailing skirts as is illustrated in Fig. 3.

T a b l e 1

Example Comparative Example 2 3 4 5 6 7 8 9 1 2 3 4 5 m-Cresol : p-cresol 45:55 30:70 10:90 40:60 45:55 40:60 40:60 45:55 60:40 50:50 47:53 60:40 70::30 weight ratio Compound I I II IV I I V VI I I I III VI Photosensitizer Parts by weight 30 25 25 30 50 40 30 25 30 30 30 30 40 Exposure time, 450 750 700 410 800 700 750 700 370 390 450 450 570 milliseconds Cross section of lines 1 1 1 1 1 1 1 1 3 2 2 3 2 Example 10.

The experimental procedure was substantially the same as in Example 1 except that the test chart photomask was replaced with a test chart reticle having line-and-space patterns of 1.25 jim and 2.0 jim widths. The results were that the pattern of the photoresist layer produced on the silicon wafer was an accurate, high fidelity reproduction of the pattern on the test chart reticle.

Comparative Example 6.

The experimental procedure was substantially the same as in Comparative Examp!e 1 exept that the test chart photomask was replaced with the same test chart reticle as used in Example 10 and the length of the exposure time was 450 milliseconds. The results were that the lineand-space pattern of 1.25 jim width could be reproduced on the silicon wafer while the line-andspace pattern of 2.0 jim width was reproduced incompletely as composed of a line pattern of 1.6 jim width and a space pattern of 2.4 jim width.

Claims

1. A composition for patterning semiconductor wafers by positive-working photolithography which comprises: (a) 100 parts by weight of a cresol novolac resin as a film-forming constituent; and (b) form 25 to 60 parts by weight of a naphthoquinone diazide sulfonic acid ester as a photosensitive constituent, the cresol moiety in the cresol novolac resin being derived from an isomeric mixture of cresol composed of 10 to 45% of m-cresol and 90 to 55% of p-cresol with substantial absence of o-cresol.

2. A composition as claimed in claim 1 wherein the isomeric mixture of cresol is composed of 10 to 45% of m-cresol, 90 to 55% or less of o-cresol.

3. A composition as claimed in claim 1 or claim 2 wherein the naphthoquinone diazide sulfonic acid ester is an esterification product of naphthoquinone- 1, 2-diazido-5-sulfonic acid and a polyhydroxy benzophenone.

4. A composition as claimed in claim 1 or claim 2 wherein the naphthoquinone diazide sulfonic acid ester is an esterification product of naphthoquinone-1, 2-diazido-5-sulfonic acid and an alkyl gallate.

5. A positive working photoresist composition as claimed in claim 3 wherein the polyhyroxy benzophenone is 2,3,4-trihydroxy benzophenone.

6. A positive working photoresist composition as claimed in claim 3 wherein the polyhydroxy benzophenone is 2,4,6-trihydroxy benzophenone.

7. A positive working photoresist composition as claimed in claim 3 wherein the polyhydroxy benzophenone is 2,3,4,-trihydroxy-4'-hydroxy benzophenone.

8. A composition as claimed in claim 1 or claim 2 wherein the naphthoquinone diazide sulfonic acid ester is an esterification product of naphthoquinone-1,2-diazido sulphonic acid and a trihydroxybenzene.

9. A composition as claimed in claim 1 substantially as hereinbefore described with reference to the examples.

10. A solution of a composition as claimed in any one of the preceding claims in an organic solvent.

11. A solution as claimed in claim 10 wherein the organic solvent is ethylene glycol monoethyl ether acetate.

12. A silicon wafer produced using the compositions as claimed in any one of claims 1 to 9, or the solution as claimed in claim 10 or claim 11.

13. The use of a composition as claimed in any one of claims 1 to 9 or the use of a solution as claimed in claim 10 or claim 11 for patterning semiconductor wafers by positive working photolithography.