AU672220B2 - Negative photoresist and a process therefor - Google Patents

Description

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AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name of Applicant(s): Actual Inventor(s): NATIONAL RESEARCH DEVELOPMENT

CORPORATION

SAMBASIVAN VENKAT ESWARAN Address for Service:

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CULLEN CO., Patent Trade Mark Attorneys, 240 Queen Street, Brisbane, Qld. 4000, Australia.

Invention Title: NEGATIVE PHOTORESIST PROCESS THEREFOR AND A

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The following statement is a full description of this invention, including the best method of performing it known to us: BACKGROUND OF THE INVENTION This invention relates to photoresists and to a process for the preparation thereof.

PRIOR ART Photoresists are generally employed in applications requiring micro lithography and where image resolution is brought down to micron or submicron levels. Photoresists usually *0.

consist of a polymer and a photoactive component 10 which respond to millisecond exposure to light and resist the action of etching agents. Such hotoresists have a particular application to silicon wafers which incorporate integrated o circuits.

15 Photoresists presently known in the art o9 are either the positive or negative photoresists. In the instance of positive photoresists, the photoresist exposed to light becomes soluble in the developer solution, which is normally an aqueous alkaline solution. Thus, the exposed portion of the silicon wafer having the photoresist coating thereon becomes soluble in the developer and the unexposed portion is rendered insoluble. A positive irmage of the mask 1

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Contrary to positive photoresists, negative images are formed on the silicon S 15 wafer in the instance of negative *0 photoresists. In such photoresists, the exposed areas become insoluble to the developer solution, which normally is an organic solvent, and that contrary to positive photoresists, the unexposed area is soluble to the developer.

The known negative photoresist consists a photoactive component (PAC), a polyme a solvent and other addit ives.

As reported by Turner et.a in Journal 2 0 0*

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*5S of Chemical Education, Volume 65 No.4 April 1988, the polymer is cyclized polyisoprene and the photoactive compounds are bis azides. The solvent comprises ethoxy ethyl acetate. It is generally believed that the photoactive compound, when exposed to light undergoes a photo decomposition to reactive nitrene intermediate, and that the rate of reaction increases substantially, when entrapped within a polymer matrix. Due to such a reaction, cross linking occurs with a consequential increase in molecular weight of the polymer and rendering it insoluble in commonly used developers, such as orgainic solvents. Several distinct disadvantages are associated with negative photoresists, and particularly due to the developers used therefor. One such disadvantage is that the developer is not environmental friendly. Another disadvantage is that the developer leads to swelling of the polymer, and which adversely affects the resolution.

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The positive photoresist and as reported by Turner et.al. in the aforesaid publication comprises a novoiac resin as the polymer which is soluble in an aqueous base and naphthalene diazoquinone sulfonate compound as the photoactive compound.

However, such a publication neither discloses nor suggests that the resin should be subjected to a step of purification.

A positive photoresist has been suggested by Turnet et.al. in Polymer Engineering and Science,, Mid September, 1986, Vrl. 26 No.18 and wherein the polymer normally consisting of a novolac for positive photoresists is replaced by N-(phydroxyphenyl) maleimide with various comonomers.

Yet another photoresist has been suggested by Iwayanagi et.al. and which suggests a polymer consisting of 3,3'-diazideo diphenyl sulfona; 3-(4-(p-azidophenyl)-l,3-butadienyl)-5,5 dimethyl -2-cyclohexen-l-one) studied with polyhydi xystyrene, poly(para-methoxystyrene) and polystyrene.

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s O 15 20 4 I-IL~I1~ @0 0 000 0 00 *r 0 00. 0 *000 0 @0 00 000 000 0 15 OBJECTS OF THE INVENTION In distinction to the aforesaid known photoresists, the present invention relates to a negative photoresist, but which exhibits properties of a positive photoresist during development.

Accordingly an object of this invention is to propose an improved photoresist and to a process for the preparation thereof.

Another object of this invention is to propose a negative photoresist but having the properties of a positive photoresist with respect to development.

Yet another object of this invention is to propose a negative photoresist but having the properties of a positive photoresist, and which has a high sensitivity.

Still another object of this invention is to propose a negative photoresist but having the properties of a positive photoresist which has a high resolution.

A further object of this invention is to propose ;i negative photoresist but having the properties of a positive photoresist and which uses a developer which is environmental friendly.

See 06* 5 i I -b DESCRIPTION OF THE INVENTION According to a first embodiment of the presention invention there is provided a process for the preparation of a negative photoresist which comprises purifying a novolac resin by leaching the resin in an alkaline medium to remove any alkaline insoluble material therefrom, adding at least part of the purified resin to a photoactive compound selected from the group as herein described, a solvent and known additives for increasing the adhesion and viscosity of the photoresist composition when applied as a coating.

According to a second embodiment of the present invention there is provided a negative photoresist which is able to be developed in an alkaline medium, the photoresist 6 S 15 comprising a photoactive compound selected from the gro'"p described herein, a purified polymer comprising m-cresol and/or p-chloro phenol and a solvent comprising 2-ethoxy ethyl acetate, wherein the polymer is purified by leaching in an alkaline medium so as to remove any alkaline 20 insoluble material from the polymer.

The photoresist of the present invention contains a polymer comprising a novolac resin. As described hereinabove, the use of novolec resin as a polymer in a positive photoresist is known in the art but which according to the report by Turner was not subjected to the step of purification. In distinction, the present invention envisages the step of subjecting the novolac resin to the step of purification and si'-n that only the portion soluble in an aqueous alkali .nedium is utilized for R:6.

Iga -P the preparation of the negative photoresist of the present invention. Thus,

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a -3e -~Sa~sly~U the photoresist comprising the purified polymer, photoactive component and solvent undergoos a photodecomposition, when exposed to light, to reactive nitrene intermediate.

As such a reactive nitrene intermediate is entrapped within the polymer matrix, the reaction rate increases and resulting in 0@ S' either an increase in molecular weight or a eo functional group transformation and whereby 1. the exposed portion becomes insoluble to the aqueous alkali developer.

The purification step consists in leaching out that portion of the polymer which is soluble in an. aqueous alkali medium. In 15 accordance with this invention, the polymer a is treated with an aqueous alkali and heated a to a to,rerature of 70 to 85°C so as to dissolve that portion of 'the polymer which **ee 0 dissolves in an aqueous alkali medium. The S" 20 step or heating should normally not be adopted in order to avoid a possibility of a further polymerization of the polymer. Further, in the presence of such heat, even that portion which does not undergo any further polymerization could melt, which upon subsequent cooling comprise an impurity.

7 PI ~I r~ Simultaneously, it has also been found that an absence of a heating step would require prolonged reaction periods for causing a dissolution of the polymer. Thus, to avoid the aforesaid disadvantages, the heated solution -ontaining the dissolved polymer is cooled tr room temperature to avoid further polymerization of the polymer, and as well as to prevent any alkali insoluable novolac 10 from coming down to the filterate as an impurity.

Novolac resin treated with alkali is heated, cooled and then filtered. The filterate is then acidified with dilute 15 hydrochloric acid at a pH 2 to 3 with cooling to precipitate the resin. During the step of acidification, the filterate is maintained at a temperature below room temperature so

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as to avoid decomposition. The precipitated or purified novolac resin is filtered, washed with water and then dried preferably at room temperature.

8 P1 -I L I r;L ur I-~I IQ The novolac resin employed in the present invention comprises an acid catalyzed phenol formaldehyde polymer, such as p-chloro phenol having a melting point 112 to 116 0 C and/or mcresol having a melting point of 133-138 0 C. The melting point of the polymer is important in that the photoresist does not flow during the subsequent method of processing.

In accordance with this invention, the silicon 10 wafer having the coating of the photoresist o thereon is subjected to a step of soft baking and for which a temperature of 85± 10 0 C is employed.

Further, the dispersity of the molecular 15 weight distribution of the polymer should be small as a better resolution is then achieved.

The photoactive compound employed in the photoresist of the present invention are nitrophenyl azides selected from l-azido-2,5-dichloro-3, 6-dimethyl-4-nitrobenzene, or l-azido-2,5-dibromo-3, 6-dimethyl-4-nitrobenzene.

The photoresist composition comprises to 6 parts of polymer and 10 to 12 parts of solvent to every 1 part of the photoactive compound. If less than 5 part; of the polymer is present in the composition, then a greater 9

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quantity of the photoactive compound would be required in the composition, which would then contribute and add to the end costs. If more than 6 parts of the polymer is present in the composition, a greater quantity of solvent would then be required to dissolve the polymer.

The solvent comprises 2-ethoxy ethyl acetate.

The polymer and photoactive compound is dissolved in a solvent selected from 2-ethoxy ethyl acetate, xylenes, diglyme or cyclohexanone and so as to maintain a 35-40% solid content and in the absence of light. To every 1 part of the photoactive compound 10 to 12 parts of solvent is added to the photoresist composition. The colour of the viscous solution obtained exhibited 20 yellowish brown colour or different shades depending upon concentrations of solid and could be handled under yellow light condition. Infra red spectral study of the photoresists showed the presence of the azide group characterized by the absorption at 2100 -1.

cm The different concentration of solids in the polymer azide formulation can be improved or altered by the use of viscosity improvers like butyl acetate; additives to improve the adhesion properties and stabilizers.

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EXAMPLE

A silicon substrate was washed in Benzene,trichloroethylene and acetone. HMDS (Hexamethy disilazane) was used as adhesion promoter of the photosensitive solution over the substrate. HMDS was coated and spin dried immediately before dispensing the photoresist *a e solution over the slices. The solution was a* 0 0 filtered through Whatman filter paper No.42 *0 followed by filtration through 0.5 micron 10 filtration syringe and then uniformly coated over the slices at a constant spin speed of 5000 rpm. The UV exposures were made under S* a 200 watt Hg lamp with a peak at 365 nm a in the spectrum.

15 The mask used was an opaque rectangular pattern with opaque allignment marks. The exposure time was 15 seconds and the developer was a Shipley developer 0 EXPERIMENTAL OBSERVATIONS/RESULTS: The optimized parameters for optical lithography with the photoresist solutions are tabulated below: Exposure time Developer/Concentration Sec. Shipley Developer 351 11

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*0SSe o a B. S 10 15 The observations made over the slices after performing all optical lithography steps ire discussed below.

clear pattern was seen on the substrate. The unexposed area of the layer was totally dissolved in the shipley 351 developer. The exposed layer did not dissolve but exhibited coloured fringes/striation which can be attributed to thinning of the layer.

After post bake for half an hour at 80*C, the patterned Si slice was subjected to an etching in buffered HF solution for sec. There was no smudging of the pattern observed and the sharpness was maintained indicating that the photoresist can withstand the etchantand provide selectivity to etching in buffered HF. But it did not show any resistance to HNO 3 based etchant and smudging of the pattern was observed.

The photoresist exhibited a very sharp negative image of the mask on the slice.

It, therefore, behaved as a negative photoresist, but which was developed in a positive developer developer used for positive photoresists).

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Claims (12)

1. A process for the preparation of a negative photoresist which comprises purifying a novolac resin by leaching the resin in an alkaline medium to remove any alkaline insoluble material therefrom, adding at least part of the purified resin to a photoactive compound selected from the group as herein described, a solvent and known additives for increasing the adhesion and viscosity of the photoresist composition when applied as a coating.

2. A process as claimed in claim 1 wherein to 1 part of the photoactive compound is added 5 to 6 parts of the novolac resin and 10 to 12 parts by weight of the solvent. S.

3. A process as claimed in claim 1 or claim 2 wherein the step of leaching comprises treating novolac 15 resin with an alkali solution and heating to a temperature which avoids polymerization of said resin, and cooling.

4. A process as claimed in cla'im 3 wherein the cooled resin is filtered, the filterate is acidified at a pH 2 to 3 to precipitate the resin and maintained at room 20 temperature, which is then filtered, washed and dried.

5. A process as claimed in any one of claims 1 to 4 wherein the purified novolac resin is p-chloro phenol and/or m-cresol.

6. A process as claimed in any one of claims 1 to wherein the solvent is 2-ethoxy ethyl acetate.

7. A process for the preparation of a negative photoresist, substantially as hereinbefore described with reference to the Example. s

8. A negative photoresist prepared by the process of 13 I I I_ I -II any one of claims 1 to 7.

9. A negative photoresist which is able to be developed in an alkaline medium, the photoresist comprising a photoactive compound selected from the group described herein, a purified polymer comprising m-cresol and/or p- chloro phenol and a solvent comprising 2-ethoxy ethyl acetate, wherein the polymer is purified by leaching in an alkaline medium so as to remove any alkaline insoluble material from the polymer.

10. A negative photoresist as claimed in claim 9 wherein 5 to 6 parts of polymer and 10 to 12 parts of solvent is present to every 1 part of said photoactive "compound.

:11. A negative photoresist as claimed in claim 9 15 having solid content of 35 to e*

12. A negative photoresist substantially as hereinbefore described with reference to the Example. DAT'ED this 23rd day of July 1996 SNATIONAL RESEARCH DEVELOPMENT CORPORATION 20 By their Patent torneys So CULLEN CJ. *o «o~o :14 rr pllC- 6 E ABSTRACT A negative photoresist and a process for the preparation thereof. However, the negative photoresist uised developers normally employed for positive photoresists. The photoresist comprises *U 5a photoactive compound, a novolac resin and a Ssolvent. The novolac resin is subjected to the step of purification prior to it being used in the photoresist.