GB2240549A

GB2240549A - Photoresist materials

Info

Publication number: GB2240549A
Application number: GB9102023A
Authority: GB
Inventors: Tsutomu Noguchi; Hidemi Tomita
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1990-01-31
Filing date: 1991-01-30
Publication date: 1991-08-07
Anticipated expiration: 2011-01-30
Also published as: DE4102946A1; GB9102023D0; GB2240549B

Abstract

Resist materials comprise a novolac resin prepared from cresol and formalin wherein the formalin is used in excess to introduce a branched structure in the novolac resin, and a diazo compound chemically combined with the novolac resin. The resist materials may further comprise a dihydroxybenzene derivative. The molar ratio of cresol to formalin ranges from 1:1 to 1:4.

Description

2 2 -10 -ED- -41 SE) 1 PHOTORESIST MATERIALS This invention relates to

photoresist materials which are applicable, for example, to the fabrication of semiconductor integrated 5 circuits or magnetic bubble memory elements.

Moves towards a higher degree of integration such as of semiconductor integrated circuits and magnetic bubble memories require formation of finer micropatterns, along with the use of a light source capable of emitting a shorter wavelength in photolithography. The micropatterning requires a resist pattern to be accurately transferred to a substrate, for which dry etching using oxygen, fluorine or chlorine gases have been employed instead of wet etching.

Accordingly, there is a demand for resist materials which are sensitive to far UV rays with wavelengths in the range of 200 to 300 nm, and which are resistant to dry etching.

Resist materials which have sensitivity to far UV light and which have already been proposed include those of the positive type such as polymethyl methacrylate resists or novolac resists, and those of the negative type such as chloromethylated styrene resists and polyglycidyl methacrylate resists.

However, the polymethyl methacrylate resists and the polyglycidyl methacrylate resists are poor in dry etching resistance and very low in sensitivity. In addition, the chloromethylated styrene resists require, for example, an energy of 1 to 2 jlcm2 generated from a KrF laser, thus presenting a sensitivity problem.

On the other hand, the novolac resists have a good etching resistance with respect to chlorine gases or fluorine gases and exhibit the best sensitivity among these resists, but an energy of approximately 200 to 300 mJ/cm 2 is necessary. Thus, the resists are not necessarily satisfactory. Moreover, since the novolac resists contain a large amount of components capable of absorbing far UV light, there is a tendency for the section of the resist after exposure to have a triangular form. Thus, it is difficult to obtain a fine pattern with good squareness when a light source of short wavelength is used.

According to the present invention there is provided a resist material which comprises a novolac resin which is prepared from cresol and formalin at a molar ratio of 1:1 to 1:4 to have a branched structure therein, and a diazo compound chemically combined with said 2 a novolac resin at phenolic hydroxyl groups thereof.

Intensive studies have revealed that when formalin is used in excess upon preparation of a novolac resin to introduce a branched structure into the resin, the developing speed with a developer (an alkaline solution) can be increased, resulting in an improvement of resist sensitivity and gamma characteristic, so that when a photosensitizing agent and the novolac resin are chemically combined, satisfactory sensitivity is ensured using a small amount of the photosensitizer, thus leading to an improved light transmittance, thereby improving the resist shape properties.

Moreover, when dihydroxybenzene derivatives are contained in the resin, the resist can be more clearly held in a rectangular form in section.

According to the present invention there is also provided a resist material which comprises a novolac resin which is prepared from cresol and formalin at a molar ratio of 1:1 to 1:4 to have a branched structure therein and containing a dihydroxybenzene derivative, and a diazo compound chemically combined with said novolac resin at phenolic hydroxyl groups thereof.

According to one embodiment of the invention, there is provided a resist material which comprises a novolac resin prepared from cresol and formalin at a molar ratio of 1:1 to 1A, and a diazo compound chemically combined with the novolac resin at phenolic hydroxyl groups thereof.

According to another embodiment of the invention, there is also provided a resist material which comprises a novolac resin prepared from eresol and formalin at a molar ratio of 1:1 to 1:4 and containing a dihydroxybenzene derivative, and a diazo compound chemically combined with the novolae resin at phenolic hydroxyl groups thereof.

The invention will now be further described by way of example with reference to the sole figure which is a structural formula showing a branched structure of a novolac resin according to the invention.

Embodiments of resist material according to the invention comprise a major proportion of a novolac resin prepared from cresol and formalin. For the preparation, the molar ratio between cresol and formalin is controlled such that formalin is in excess, whereby a branched structure is introduced into the resin.

Accordingly, the cresol/formalin ratio at the time of the 1 3 preparation should be 1:1 to 1A, preferably 1:1.5 to 1:2. If the molar ratio of formalin to cresol (formal in/cresol) is less than 1, the branched structure is not appreciably introduced, and an intended effect on the sensitivity and gamma characteristic cannot be expected.

When the molar ratio exceeds 4, the novolac resin is gelled and is difficult to handle.

The novolac resin is further chemically combined with a diazo compound which reacts by the action of light in the vicinity of 249 nm. The diazo compound is a compound having a diazo group and functioning as a photosensitizer and includes, for example, naphthoquinone diazide derivatives, cyclic or linear 1, 1-diketone-2-diazo compounds (Meldrum's acid-diazo compounds), and the like.

Specific examples are shown below.

0 @IC:::7_ N 2 0 j _ N 0 0 0. X 0 It will be noted that these diazo compound should preferably have functional group or groups such as, for example, a sulphonic group, - S02X, -COX wherein X is a halogen or the like, capable of chemical reaction with the phenolic hydroxyl groups of the novolac resin.

Accordingly, preferable compounds are those indicated below.

4 0 0 1 lr_Nz cl ' 01 SO2C1 0 1 Nz 01 502CL Z_ -9-9-CHn SO2C1 0 NZ 0 When the diazo compound has, for example an -S02C1 group, the chemical combination is accomplished through the sulphonic ester bond with the hydroxyl group of the novolac resin.

The amount of the diazo compound should preferably be in the range of 3 to 30 wt% of the novolac resin. If the amount of the diazo compound is smaller, satisfactory strength cannot be obtained. On the contrary, larger amounts lead to a lowering of light transmittance, resulting in a lowering of sensitivity.

In another embodiment of the invention, the novolac resin further comprises dihydroxybenzene derivatives. The dihydroxybenzene derivative is represented by the following formula R, I- 1 (0 H)z wherein R, and R2, respectively, represent hydrogen, chlorine, an alkyl group or an alkoxy group.

In particular, hydroquinone is mentioned as a typical compound.

The dihydroxybenzene derivative should preferably be copolymerized with cresol and formalin, but is not always limited to the copolymerization. For instance, the derivative may be contained by mixing with the novolac resin prepared from cresol and formalin.

The dihydroxybenzene derivative develops its effect when contained only in small amounts and its upper limit is not critical.

Too large an amount may lower the sensitivity and may impede characteristics of the novolac resin. Accordingly, the amount should preferably be in the range of 2 to 30 mole%. It will be noted that the content by mole% of the dihydroxybenzene derivative is a ratio to cresol and the content in the prepared resin is in the range of 2 to 30 wt% substantially corresponding to the molar ratio.

The novolac resin is used as a resist material after dissolution in solvent. The solvent is not critical so far as it is able to dissolve the novolac resin therein. Accordingly, the type of solvent is properly selected depending on the purpose.

The light source with which the resist material is exposed to light is preferably one which can emit light with a wavelength, for example, of not longer than 300 nm. Various far UV light sources may be used. There may be used, for instance, a KrF exima laser.

For the development, an alkaline solution is used. An alkaline solution with a lower concentration than those used for prior resist materials may be used. An alkali aqueous solution having an alkali concentration of 0.6 to 0.8 wt% is preferred.

As described hereinbefore, when the novolac resin is prepared, formalin is used in excess, by which a branched structure is introduced into the resin. This is particularly shown in the figure. The cresol of the branched moieties is likely to react with an alkali, so that the novolac resin introduced with the branched structure is very likely to be developed.

The chemical combination of a diazo compound used as a photosensitizer with the novolac resin ensures satisfactory characteristics only in small dmounts, solving the problems of a lowering of sensitivity owing to the lowering of light transmittance and a disorder in shape of the resist.

6 0 The invention will not be further described by way of examples. Example 1 Preparation of Novolac Resins g of meta-cresol, 24.3 g of formalin and 180 mg of oxalic acid were weighed and heated and agitated in ethyl cellosolve acetate for reaction at 1200C for 5 hours.

The resultant product was re-precipitated in water. to remove unreacted formalin therefrom, followed by drying under reduced pressure. The resultant. resin had a weight average molecular weight of about 2000.

In the above reaction, the molar ratio (cresol/formalin) was 1/1-3. The amounts were changed to prepare novolac resins with cresol/formalin ratios of 1/1, 1/1-1, 1/1-5, 1/2 and 1/3. These had a weight average molecular weight of about 2000 by controlling the polymerization time.

The thus prepared novolac resins were confirmed through NMR spectra with respect to the branching at about 1 per the number of bonds of 10 for the molar ratio of 1/1.3, about 1 per the number of bonds of 5 for the molar ratio of 1/1-5, and about 1 per the number of bonds of 3 for the molar ratio of 1/2. Introduction of Diazo Compound

9 g of the respective novolac resins prepared above and 0.63 g of naphthoquinonediazido-4-sulphonyl chloride (7 % by weight of the resin) were dissolved in dioxane, in which 0.33 ml of triethylamine was dropped to cause naphthoquinonediazido-4-sulphonyl chloride to ester bond with the novolac resin.

The resultant products were each re-precipitated in a hydrochloric acid aqueous solution to remove the amine salt therefrom.

Finally, the reaction product was dissolved in ethyl cellosolve acetate, followed by filtration through a filter with a pore size of 0.2, g m to provide a photoresist.

The respective resists had a gamma value, sensitivity and resin dissolution rate indicated in Table 1 below.

7 Table 1

Resin Dissolution Molar Ratio Gamma Value Sensitivity (mJ) Rate 0.96 220 810 nm/min.

0.95 170 990 nm/min.

111.3 1.09 160 980 nm/min.

111.5 1.17 98 3000 nm/min.

112 1.13 56 3750 nm/min.

1/3 1.01 95 3100 nm/min.

As will be apparent from Table 1, a higher ratio of formalin results in improved sensitivity with a higher dissolution rate.

Formation of Pattern From the photoresists prepared above, the samples having cresol/formalin = 1/1.3, 1/1.5 and 1/2 were each spin coated with a thickness of 1,LL- m and baked at 900C for 90 seconds.

Subsequently, the coating was exposed to light from a KrF exima stepper (NA = 0.42) and developed for 60 seconds with a tetraethylammonium hydroxide aqueous solution.

The section of the resultant resist pattern was observed through a scanning-type electron microscope, revealing that a pattern of 0.35 i.t-mL/S, which is nearly at the resolution limit of the exima stepper, could be obtained in good shape for all the samples.

The angle of the side walls of the resist patterns of the respective samples and the concentration of the alkaline solution (tetraethylammonium hydroxide aqueous solution) are shown in Table 2.

Table 2

Concentration of Molar Ratio Side Wall Angle Liquid Developer 1/1.3 830 0.8 wt% 1/1.5 850 0.8 wt% 1/2 830 0.7 wt% -------------------------------------------------- -------------------- As will be apparent from Table 2, the change in the molar ratio 8 0 between cresol and formalin results in an increase of the dissolution rate, with the result that the resist sensitivity is improved with improved shape properties.

For comparison, a novolac resist having high resolving power was selected from commercial resists and subjected to similar measurements, with the result that the resist sensitivity was in the range of 200 to 460 mJ/cM2, the resolving power was in the range of 0.4 to 0.5,oAkm and the side wall angle was in the range of 60 to 700. Thus, the resist sensitivity, resolving powder and the shape of the resist were found to be significantly poorer than those of the resist materials according to the invention.

Example 2 Preparation of Resin 25.8 g (0.239 moles) of meta-cresol, 1.38 g (0.0126 moles = about 5 mole%) of hydroquinone, 30.6 g of formalin and 193 mg of oxalic acid were weighed and heated and agitated in ethyl cellosolve acetate for reaction at 1200C for 3 hours.

The resultant product was re-precipitated in water to remove unreacted formalin therefrom, followed by drying under reduced pressure. The resultant resin had a weight average molecular weight of about 2300.

Similarly, a novolac resin having 20 mole% of hydroquinone was prepared.

In these novolac resins, the copolymerization of hydroquinone was confirmed through 'H-NMR. The molar ratios between cresol and formalin in the resins were approximately 1:1.5 for both cases.

Introduction of Diazo Compound

9 g of the respective novolac resins prepared above and 0.72 g of naphthoquinonediazido-4-sulphonyl chloride (8 % by weight of the resin) were dissolved in dioxane, in which 0.38 ml of triethylamine was dropped to cause naphthoquinonediazido-4-sulphonyl chloride to ester bond with the novolac resin. The resultant product was re-precipitated in a hydrochloric acid aqueous solution to remove the amine salt therefrom. 35 Finally, the reaction product was dissolved in propylene glycol methyl ether acetate, followed by filtration through a filter with a pore size of 0.2 /1 m to provide a photoresist. Formation of Pattern 9 0 Resists copolymerized with 0 mole%, 5 mole% and 20 mole% of hydroquinone were each spin coated in a thickness of 0.7 pm on a silicon wafer and baked at 900C for 90 seconds.

Subsequently, the coating was exposed to light from a KrF exima stepper (NA = 0.42) and developed with a tetraethylammonium hydroxide aqueous solution for 90 seconds.

The section of each of the resultant resist patterns was observed through a scanning-type electron microscope, revealing that a pattern of 0.35,a mL/S, which was nearly at the resolution limit of the exima.

stepper, could be obtained in good shape for all the samples.

The angle of the side walls of the resist patterns of the respective samples and the sensitivity of the resists and the transmittance after the exposure are shown in Table 3.

Table 3

Side Wall Resist Hydroquinone Angle Sensitivity Transmittance 0 mole% 820 150 n1J/CM2 10% 5 mole% 850 100 mj/cm2 15% mole% 860 70 mj/0m2 15% As will be apparent from Table 3, the copolymerization of hydroquinone and cresol can increase the transmittance of the resist and the dissolution rate in the developer, thus leading to an improvement of the resist and dissolution rate in the developer, thus leading to an improvement of the resist sensitivity and the retention of the shape.

For comparison, a novolac resit having high resolving power was selected from commercial resists and subjected to similar measurements as set forth with respect to Example 1, with the result that the resist sensitivity was 200 to 460 MjICM2, the resolving power was 0.4 to 0.5 am and the side wall angle was 60 to 700. In this case, the resist sensitivity, resolving powder and the shape of the resist were found to be significantly poorer than those of the resist material according to the invention.

Investigation of the Concentration of Developer Then, a resist copolymerized with 5 mole% of hydroquinone was 0 spin coated in a thickness of 1 k& m on a 'silicon substrate and subjected to measurement of a developing rate by immersion in a liquid developer. The developing time was 60 seconds and the concentration of the developer was changed.

The developers used were tetraethylammonium hydroxide (THAH) aqueous solutions with concentrations of 2 wt%, 1 wt%, 0.7 wt%, 0.5 wt%, 0.3 wt% and 0.1 wt%, respectively.

During the development, it was observed that the developing rates were different in the surface and in the inside of the resist. These were named as a surface sparingly soluble layer and a bulk layer. The ratio in the developing rate between the surface sparingly soluble layer and the bulk layer (the surface sparingly soluble layer/the bulk layer), an etching rate of the bulk layer and a resist side wall angle were measured. The results are shown in Table 4.

Table 4

Etching Rate Concentration of Ratio in of Bulk Layer Resist Side Developer (wt%) Developing Rate (,&m/second) Wall Angle 2 1/1 5 750 1 1/2 3 800 0.7 1/5 1 830 0.5 1/10 1 850 0.3 1/15 0.5 850 0.1 no dissolved -0.1 - From the above experiment, it was confirmed that the resist pattern form became better ata slower developing rate of the sparingly soluble layer relative to the bulk layer.

Usually, the resist thickness should be 0.7 to 1 k m and the developing time should be approximately 60 seconds. The concentration of the liquid developer should preferably be approximately 0.5 to 0.7 wt%.

As will become apparent from the foregoing, the resist materials of the invention have a branched structure by using an excess of formalin at the time of preparation of the novolac resin, so that a dissolution rate in an alkali aqueous solution can be substantially 1 11 increased. In addition, since diazo compounds having good sensitivity to far UV light are chemically combined, it is possible resolve a fine pattern of not larger than 0.5,kAm while keeping a rectangular form by exposure to an exima laser beam and development.

Accordingly, when the resist materials are applied to fabrication, for example, of semiconductor integrated circuits, the degree of fineness and the yield of the semiconductor elements can be improved.

12

Claims

1. A resist material which comprises a novolac resin which is prepared from cresol and formalin at a molar ratio of 1:1 to 1:4 to have a branched structure therein, and a diazo compound chemically combined with said novolac resin at phenolic hydroxyl groups thereof.

0

2. Resist material according to claim 1 wherein said molar ratio is in the range of from 1:1.5 to 1:2.

3. Resist material according to claim 1 wherein said diazo compound has a functional group capable of chemical combination with the phenolic hydroxyl groups of said novolac resin.

4. Resist material according to claim 3 wherein said diazo compound is of the following formula (1), (2) or (3) 0 1 i:.-- N 2 01 (51 502CL (1) 0 901 1 i:r- N2 i S02CL (2) z--- 9 - 9 - C - C H SOZCL ', 0N2 0 1 (3) X

5. Resist material according to claim 1 wherein said diazo compound is used in an amount of from 3 to 20 wt% of said novolac resin.

A 13

6. A resist material which comprises a novolac resin which Is prepared from cresol and formalin at a molar ratio of 1:1 to 1:4 to have a branched structure therein and containing a dihydroxybenzene derivative, and a diazo compound chemically combined with said novolac resin at phenolic hydroxyl groups thereof.

A

7. Resist material according to claim 6 wherein said molar ratio is in the range of from 1:13 to 1:2.

8. Resist material according to claim 6 wherein said diazo compound has a functional group capable of chemical combination with the phenolic hydroxyl groups of said novolac resin.

9. Resist material according to claim 8 wherein said diazo compound is of the following formula (1), (2) or (3).

0 1 t-l N -2 1 S 0 z c 1 (1) 0 1 o 1 SO2CL N z (2) zc -C -C -CH:i 11 11 11 50?.CL 0N z 0 (3)

10. Resist material according to claim 6 wherein said diazo compound is used in amount of from 3 to 20 wt% of said novolac resin.

14

11. Resist material according to claim 6 wherein said dihydroxybenzene derivative is contained in said novolac resin by mixing of said derivative with said novolac.

Z JP

12. Resist material according to claim 6 wherein said dihydroxybenzene derivative is contained in said novolac resin in copolymerizing said derivative with the cresol and formalin.

13. Resist material according to claim 6 wherein said 10 dihydroxybenzene derivative is of the following formula R i -- (0 H) R 2 wherein R, and R2, respectively, represent hydrogen, chlorine, an alkyl group or an alkoxy group.

14. Resist material according to claim 6 wherein said dihydroxybenzene derivative is contained in an amount of from 2 to 30 wt% based on the novolac resin.

15. A resist material substantially as hereinbefore described with 25 reference to any one of the Examples.

Published 1991 at The Patent Office. State House. 66/71 High Holborn, Iondon WCIR 47P. Further copies may be obtained from Sales Branch, Unit 6. Nine Mile Point, Cwmfelinfach, Cross Keys. Newport, NPI 7HZ. Printed by Multiplex techniques ltd, St Mary Cray, Kent.