CA1293084C - Copolymers having o-nitrocarbinol ester groups and preparation thereof - Google Patents

Abstract

Abstract of the Disclosure: Copolymers containing from 5 to 50 mol % of monomers having o-nitrocarbinol ester groups, from 95 to 50 mol % of 0-substituted p-hydroxy-styrene and from 0 to 30 mol X of other copolymerizable monomers are useful for producing positive-working photo-resists and light-sensitive coating materials.

Description

1;~ ! &L~
0.Z. 0050/38843 Copolymers having o-nitrocarbinol ester groups and preparation thereof The present invention relates to a copolymer hav-ing o-nitrocarbinol ester groups and the preparation thereof and to the production of a light-sensitive coat-ing material for photoresists for the production of semi-conductor elements, planographic printing plates and etch resist, in particular to a copolymer which contains, as copolymerized units, 0-substituted P-hydroxystyrene units and to a positive-uorking photoresist produced therefrom ~hich, after exposure to shortwave UV light, for example within the wavelength region from 300 nm to 190 nm, can be washed out with an alkaline solvent, is sufficiently stable to plasma etching and is usable in particular for photolithography.
In lithographic processes, for example to produce semiconductor elements, numerous light-sensitive resist materials are used. On exposure, these materials undergo a change in solubility in the exposed areas which results in so~vent discriDination bet~een exposed and unexposed areas. In posit`ive-working resists, the exPosed areas show better solubility in a suitable solvent than the unex-posed areas. Conventional positive-working photoresists are predominantly systems comprising a photo-insensitive, alkali-soluble matrix based on novolaks and a photosen-sitive component which acts as a solubility inhibitor and is converted by exposure into alkali-soluble products, so that the exposed areas in their entirety become soluble in the alkaline developer. It is true that these systems have proven very useful, but exposure to short-wave U~ light presents difficulties since the novolak matrix is strongly absorbent in the wavelength region belo~ 300 nm, making exposure in the relevant film thickness range greater than û.S ~m impractical.
Photosensitive polymers for use as resists for the shortwave UV region are for example poly(methyl meth-acrylate), copolymers of methyl methacrylate and indenone J8~

- 2 - O.Z. 0050/38843 (J. Vac. Sci. Techn. 19 (1981), 1333) and copolymers of methyl methacrylate and 3-oximino-2-butanone as photo-active component (for example US 4,382,120 and US 4,343, 889).
S There are also existing ~wo-component systems comprising a poly(methyl methacrylate-co-methacrylic acid) matrix and o-nitrobenzyl esters, for example esters of choLic acid, for use as solubility inh;bitors (cf.
for example DE-A-3,415,791).
One-component resists from copolymers which con-tain o-nitrobenzylcarbinol ester groups are described for example in DE-A-2,150,691 and DE-A-2,922,746.
~h;le the resists for the shortwave UV region are sufficiently stable to wet etch processing, their stab-ility to fluorine- and chlorine-containing plasmas, which may additionally contain oxygen, as used for etching in sem;conductor element fabrication, is limited and in need of improvement.
It is an objec~ of the present invention to de-scribe new copolymers and the use thereof, in particularpositive-working resists produced therefrom, which 0ake it possible in particular to produce resist patterns, have high thermal stability and resistance to fluorine- and chlorine-containing plasmas, and, after exposure to short-wave UV light, can be washed out with alkaline solvents.
~ e have found that this object is achieved, sur-pr;singly, vith a copolymer of a certain composition which contains, as copolymerized units, not only o-nitro-ben~ylcarbinol ester groups but also O-substituted p-hy-droxystyrene units.
The present invention accordingly provides a co-polymer which contains, as copolymeri2ed units, (a) from 5 to 50 mol ~ of one or more compounds of the general formula (I) 1=0 1 (I) X--C--H
,C C--N02 -l.'~tt~

- 3 - O.Z. 0050~38843 where A is an aromatic or heteroaromatic, substituted or unsubstituted ring system of S to 14 ring members, X is hydrogen, alkyl of 1 to 8 carbon atoms, or sub-stituted or unsubstituted aryl or aralkyl, and Y is an ethylenically unsaturated radical of 2 to 6 carbon atoms, tb) from 95 to 50 mol ~ of one or more compounds of the general formula (II) R~
CHz=C
(Il) ~R2 where R1 is hydrogen or methyl, R2 is hydrogen, halogen or alkyl of 1 to 6 carbon atoms, R3 is hy-drogen, alkyl of 1 to 6 carbon atoms, acetyl, ben-zoyl~ C1-C6-alkylsilyl, C1-C6-alkoxysilyl or tert.-butoxycarbonyl, and (c) from 0 to 30 mol % of one or more further olefinic-ally unsaturated, copolymerizable organic compounds, other than (a) and ~b), whose homopolymers are trans-parent ~ithin the ~avelength region from 250 to 400 nm, with the proviso that the percentages mentioned under (a) to (c) add up to 100.
Preference is given to those copolymers ~here radical Y in the general formula (I) is one of the radicals --CH=CH2 , --C=CH2 , --CH=CH--COOH, --CCl=CCl--COOH, --C=CH2 -CH=CHCH3 , -C-CH2-COOH , -C-CH3 , -CH=IC-CH2-COOH
CH2 CH-COOH COOH or --IC--CH z--C ~ 2--COOH

According to the invention, these copolymers are prepared by polymerizing the compounds mentioned under (a) to 1~3~:! 8~

- 4 - O.Z. 0050/38843 (c) in the presence of free radical initiators, with the prov;so that R3 of component (b) is aLkyl of 1 to 6 `carbon atoms, acetyl, benzoyl, C1-C6-alkylsilyl, C1-C6-alkoxysilyl or tert.-butoxycarbonyl, and the copoly-mer obtained is if appropriate converted into the co-polymer where R3 is H.
The present invention also prov;des in particular a photoresist ~hich contains a copolymer according t~ the invention and can be washed out with aqueous alkaline solvents, and to a process for producing a semiconductor element on the basis of this copolymer. The present invention further concerns the use of the polymer accord-ing to the invention for producing light-sensitive coating materials, dry resists and printing plates.
The light-sensitive copolymer according to the invention containing o-nitrocarbinol ester groups and 0-substituted p-hydroxystyrene units can be washed out with an a~ueous alkaline solvent after image~ise exposure and complete or partial conversion of the group R3 into R3 =
H. To obtain high thermal stability, it is advantageous to select the mononers in such a way that the glass trans-ition temperature of the copolymer is above 100C. The polymer according to the invention is highly suitable for use as a photoresist.
The hydroxy function (R3 = H) of the 0-substitu-ted p-hydroxystyrene units is blocked during the poly-meri~ation by a protective group. aefore development, this protective group is necessarily wholly or partly split off, which can be done before coating, during pre-bake or in a postbake.
In what follows, the components of which the co-polymer according to the invention is composed will be discussed in detail.
(a) Suitable o-nitrocarbinol ester monomers of the general formula tI) 1 ~3~
- S - O.Z. 0050/38843 y =0 ( I ) X--l--H
C Ç--N2 ~ , where A is an aromat;c or heteroaramatic, substituted or unsubstituted ring system of 5 to 14 ring members, X is hydrogen, aLkyl of 1 to 8 carbon atoms, or sub-stituted or unsubstituted aryl or aralkyl, and Y is an ethylenicalLy unsaturated radical of 2 to 6 carbon atoms, are for example those where the radical Y in the general formula (I) is one of the radicals -CH=CH2 , -1C=CH2 . -CH=CH-COOH , -CCl=CCl-COOH , -C=CH2 .

-CH=CHCH3 , -C-CH2-COOH , -~C-CH3 , -C~=f-CH2-COOH
CH2 CH-COOH COOh or In this definition~ an aromatic ring system A com-prises in particular benzene or a substituted benzene. The benzene ring c~n be monosubstituted or polysubstituted, for example by C1-Cg-alkyl, in particular methyl, by C1-C6-al-koxy, in particular methoxy, by halogen, such as chlorine, by n;tro or amino, and by sulfo. Also possible are substi-tuted and unsubstituted polynuclear benzene derivatives, such as naphthalene, anthracene, anthraquinone and phenanthrene.
A heteroaromatic ring system A is in particular pyridine.
Particularly suitable aromatic and heteroaromatic o-nitrocarbinols from which to derive the o-nitrocarbinol ester groups have proven to be for example: o-nitrobenzyl, 6-nitroveratryl, 2-nitro-4-aminobenzyl, 2-nitro-4-dimethyl-1.?~3~
- 6 - O.Z. 005~/38843 amino-benzyl, 2-nitro-4-methylaminobenzyl, 2-nitro-5-dimethylaminobenzyl, 2-nitro-5-aminobenzyl, 2-nitro 4,6-dimethoxybenzyl, 2,4-dinitrobenzyl, 3-methyl-2,4-dinitro-benzyL, 2-nitro-4-methylbenzyl or 2,4,6-trinitrobenzyl alcohol, and also 2-nitrobenzhydrol, 2,2 -dinitrobenz-hydrol, 2,4-dinitrobenzhydrol and 2,2 ,4,4 -tetranitro-benzhydrol. Similarly su;table are for example 2-nitro-3-hydroxymethyl-naphthalene, 1-nitro-2-hydroxymethyl-naphthalene and 1-nitro-2-hydroxymethylanthraquir,one.
The copolymer according to the inYention is based as stated above on an o-nitrocarbinol ester of an ole-finically unsaturated carboxyLic acid as monomer, the ethylenically unsaturated mono- or dicarboxylic acid being for example acrylic acid, methacrylic acid, maleic acid, dichloromaleic acid, fumaric acid, crotonic acid, itaconic acid or methyleneglutaric acid.
The o-nitrocarbinol esters of olefinically unsat-urated carboxylic acids can be prepared by kno~n methods of organic chemistry, for example by reacting the acid chlorides ~ith o-nitrocarbinols or by direce acid-cata-lyzed esterification.
Particularly preferred o-nitrocarbinol ester monomers are o-nitrobenzyl acrylate, o-nitrobenzyl meth-acrylate, o-nitro-~-methylbenzyl (meth)acrylate and 2-nitro-6-chloro--methylbenzyl (meth)acrylate.
The copolymer according to the invention contains o-nitrocarbinol ester monomers (a) in an amount from 5 to 50, preferably from 10 to 30, mol % as copolymerized units.
(b) Comonomers (b) present in the copoly-er according to the invention as copolymerized units comprise compounds of the general formula (II) CH2=1 ( I I ) ~}R 2 0~

- 7 - O.Z. 0050/38843 where R1 ;s hydrogen or methyl, R is hydrogen, halogen, for example chlorine, or alkyl of 1 to 6 carbon atoms, and R3 is hydrogen, alkyl of 1 to 6 carbon atoms, acetyl, benzoyl, C1-C6-alkylsilyl, C1-C6-alkoxysilyl or tert.-butoxycarbonyl.
Particularly preferred 0-substituted p-hydroxy-styrene derivatives are those where the phenolic hydroxy~
group is etherified or ester;f;ed, eg. p-methoxystyrene, p-acetoxystyrene or p-tert.-butoxycarbonyloxystyrene.
Component (b) is present as copolymer;zed units in an amount from 95 to S0, preferably from 90 to 70, mol % in the copolymer according to the ;nvent;on.
(c) To improve the solub;lity in aqueous alkaline sol-vents or to obtain other properties, such as increased thermal stabiLity, the copolymer according to the ;n-vention may also contain as copolymerized units one or more olef;nically unsaturated copoLymer;zable or-ganic compounds, other than (a) and (b), whose homo-polymers are transparent in the ~aveLength reg;on from 250 to 400 nm.
Examples of suitable comonomers of th;s type are ethylenically unsaturated carboxylic acids, such as (meth)acrylic acid, maleic acid, fumaric acid, crotonic acid, itaconic acid and/or methylene glutar;c acid, otner ethylenically unsaturated compounds, such as ethylene, aromatic vinyl compounds, such as styrene, dienes, such as isoprene, butadiene, esters of the abovement;oned ole-finically unsaturated carboxylic acids, such as esters of acrylic and in particular methacrylic acid with C1-Cg-alcohols, preferably methyl methacrylate, and also other derivatives of acrylic acid and/or methacrylic acid.
The monomers (c) can be present as copolymeri~ed units in the copolymer according to the invention ;n amounts from 0 to 30, preferably from 0 to 20, mol %.
The mole percentages of ~a) to (c) in the copolymer according to the invention add up to 100.
Examples of particularly preferred copolymers 3r~
- 8 - O.Z. 0050/3a843 according to the invention are those of o-nitro-~-methyl-benzyl methacrylate and p-tert.-butoxycarbonyloxystyrene, o-nitro-a- methylbenzyl acrylate, p-tert.-butoxycarbonyl-oxystyrene and p-vinylanisole, o-ntiro-~-methylbenzyl S (meth)acrylate, p-tert.-butoxycarbonyLoxystyrene and methacrylic acid, o-nitro--methylbenzyl (meth)acrylate, p-tert.-butoxycarbonyloxystyrene and acrylic acid, and o-nitro-~-methylbenzyl(meth) acrylate, p-tert.-butoxy-carbonyloxystyrene and p-methoxysilyloxystyrene.
for developability with aqueous alkaline solvents, it is necessary that the substituted styrene der;vatives be wholly or partly converted into p-hydroxystyrene deriva-tives before development of the imagewise exposed areas.
Neither the undeblocked copolymers nor copolymers of o-ni-trobenzyl acrylates with styrene prepared in a similar manner, as described in DE-A-Z,t50,691, are properly developable with aqueous alkaline solutions even after very long exposure times.
The copolymer according to the invention is pre-parable from the particular monomers by a conventionalmethod of polymerization, for ~hich the monomers are pre-ferably chosen in such a way that the glass transition temperature of the resulting copolymer is above 100C. To this end, the monom~rs can for example be heated for sev-- 25 eral hours at 60-120C in a suitable solvent, such as ethyl acetate, toluene or acetone, together with a poly-mer;zation initiator, such as benzoyl peroxide or azobis-isobutyronitrile. This reaction is advantageously con-ducted in such a way that the conversion is not substan-tially more than 50~, in order to obtain a relativelylow molecular weight.
The molecular weight can be determined by the polymerization conditions, such as reaction temperature, type of initiator and initiator concentration, and also type of solvent and monomer concentration. To obtain a molecular weight within the range from 1,000 to 500,000 which is favorable for photoresist applications, preferred 3(~ 8~
- 9 - O.Z. ooso/38843 ranges are from 60 to 120C for the polymerization tem-perature, from 0.1 to S mol % for the initiator concen-tration and from 3 to 50X for the monomer concentration.
The molecular weight and the molecular weight distri-bution have a substantial influence on the solubility ina particular developer~ The lower the molecular weight and the narrower the molecular weight distribution, the better and the more uniform ~he developability. Prefer-ence is therefore given to molecular weights within the range from 1,000 to 500,000 g~mol, particularly preferably from 5,000 to 100,000 g/mol, and molecular weight distri-butions < 3.
The use of the copolymer according to the inven-tion is not restricted to application as a photoresist for fabricating semiconductor components; it can also be used in light-sensitive coating materials such as dry resists and printing plates. For these applications, other ingredients such as dyes, pigments, sensitizers and other additives may be added to the light-sensitive materials.
As mentioned, the resist according to the invention is very highly suitable for photolithography for the structuring of semiconductors. In such a process, the polymer is dissolved in a suitable solvent, for example diethylene glycol dimethyl ether, cyclohexanone or methyl-cellosolve acetate, in a solids content in general from 5X by weight to 30X by weight. The solution can advan-tageously be f;ltered through a polytetrafluoroethylene filter tpore diameter about 0.2 ~) and for example be applied by spin coating at 1,000 - 10,000 rpm to sili-con wafers to for~ resist layers from 0.2 ~m to 2.0 ~m in thickness. After baking, the film is exposed through a quartz mask by the contact or projection technique. The bake conditions depend in each case on the copolymer used.
Copolymers having t-butoxycarbonyl protective groups are preferably treated in such a way that the protective group is filtered off thermally, at from 160 to 200C, before 1.~ 3~
- 10 - O.Z. 0050/38843 or after exposure. Other protective groups are preferably split off in solution before the resist is applied, and the bake then takes place above the glass transition tem-perature of the particular polymer. Exposure is continued until the imagewise exposed areas are completeLy deve-loped. Suitable light sources are alternatively Hg high pressure lamps, Cd-Xe lamps or Excimer lasers in the wave-length region below 350 nm, preferably 248 nm (KrF).
After exposure, the resist is developed with an aqueous alkaline developer, such as 2~ strength tetramethyl-ammonium hydroxide solution, 1g strength NaOH, 10% strength NaHC03 or a commercial positive developer, eg. Developer S tfrom Kodak) or P4 (from Merck). The development time is in general from 30 seconds to 2 minutes. It should be chosen in such a way that stripping in the unexposed areas amounts to not more than 5~, bearing in mind that the degree of stripping in the unexposed areas in a particular developer is also affected in particular by the presence of p-hydroxystyrene or other ~ater- or alkali-soluble components in the polymer.
To transfer the resist pattern thus produced to the substrate underneath, plasma processes are very fre-quently used in industry. A description of plasma etching technique can be found inter alia in Introduct;on to Microlithography, ACS Symp. Ser. 219, L.F. Thomson, C.G.
~;llson, M.J. ~owden, Eds., American Chemical Soc., Washington DC (1983) in chapter 5.
Silicon and silicon dioxide substrates are gen-erally etched with fluorine-containing plasmas. The plasma reactors used are barrel and paraLlel plated reactors.
Typical conditions for etching ~ith fluorine-containing gases, such as SF6, CF4 or CF4/6~ 2~ in a parallel plate reactor are: gas pressure from 10 mtorr to 1 torr, power from 0.11 ~/cm2 to 0.66 ~/cm2. The polymer accord-ing to the invention shows excellent etching stabiLity tofluorine-containing plasmas at strip rates from 200 ~/
minute to 700 ~/m;nute, depending on the conditions ``` 1 ~3~ 8~
1 1 - O. Z . ooso/3ss43 employed.
In the Examples which follow, parts and percen-tages are by weight, unless other~ise stated.

Preparing Light-sensitive polymers a) Synthesis of copolymers 10 parts of p-t-butoxycarbonyloxystyrene (A), prepared according to Polymer 24 (1983), 999, and 1.2 parts of o-nitro-~-methylbenzyl methacrylate (3) are dis-solved in 10 parts by volume of ethyl acetate. After0.082 part of azobisisobutyronitrile has been added, the mixture is heated at 70-80C under nitrogen for 16 hours.
The viscous solution is diluted with ethyl acetate and pre-cipitated in naphtha. After washing with naphtha and methanol the polymer is dried under reduced pressure. The yield is 6.1 parts. The IR spectrum shows carbonyL bands at 1760 cm 1 (carbonate) and 1720 cm 1 (ester) and also the nitro band at 1530 cm 1 Further polymers of different compositions are prepared analogously to the above method:
Monomer composition Initiator (a) con- Reaction Yield (A) (~) (parts) mol %, tent (~) time (%) based on ;n poly- (hours) monomers mer (A) ~
14.4 1 62.0 16 24 7.0 1 45.3 16 32 2.3 1 26.6 16 59 1.2 1 21.8 16 54 30 10 0.6 1 12.0 24 72 b) Synthesis of terpoly~ers 10 parts of p-t-butoxycarbonyloxystyrene (A), 9.4 parts of o-nitro-~-methylbenzyl methacrylate and 1.3 parts of methacrylic acid are dissolved in 50 parts by volume of ethyl acetate and polymerized by means of 0.130 part of azobis;sobutyronitri~e at 70-80C in the course of 8 3~4 - 12 - O.Z. OOSO/38843 hours. Precipitating in naphtha and drying gives 7.0 parts of polymer. T;tration w;th 0.01 N KOH indicates a methacrylic acid content of 4%.
Removal of protect;ve group S c) In substance 1.0 9 of the copolymer prepared in Example 1a) is heated at 200C in a drying cabinet for 2û minutes. The polymer ueight loss is 0.35 9. NMR and IR spectra indicate that the protective group has been completeLy removed.
d) In solution 1.0 9 of the copolymer prepared in Example 1a) is dissolved in 10 ml of dichloromethane. 0.5 ml of tri-fluoroacetic acid is added, and gas immediately begins to evolve. After gas evolution has ceased, the m;xture is st;rred at room temperature for 15 minutes and then precip-;tated in 100 ml of naphtha. NMR and IR spectra of the precipitated polymer show that the protective group has been completely removed and a phenolic Polymer has formed.

Preparation of resist solutions, exposure and development Poly~o-nitro-~-methylbenzyl methacrylate-co-p-t-butoxycarbonyloxystyrene) (Mn: 14,000 g/mol), prepared as described in Example 1a), is dissolved in diethylene glycol dimethyl ether to give a solution having a solids content of 30X. The solution is filtered through a 0.2 ~m Teflon*filter and spuncoated at 2,45û r.p.m. onto a sili-con wafer to produce a layer 1 ~m in th;ckness. The wafer is then baked at 180C for 2 minutes and exposed through a structured chromium-coated quart~ mask by the contact technique. The light source used is an Excimer laser from Lambda Physics, ~hich emits monochromatic light of wavelength 248 nm when the gas medium is krypton and fluorine. After exposure at 400 mJ/cm2 the imagewise exposed areas are developed with 2~ strength aqueous tetra-methylam~onium hydroxide solution for 60 seconds to leave positive resist structures of good quality.
* Trade mark ,,. ~7, '~' ?~
- 13 - O.Z. 0050/38843 A copolymer prepared as described in Example 1a) is dissolved, the solution is f;ltered, and the filtrate is spuncoated onto a ~afer, all three steps being car-S ried out as described in Example 2. The wafer is thenbaked at 130C for 2 minutes and exposed Directly after image~ise exPosure the exposed film is not developable ~ith 2% strength aqueous tetramethyLammonium hydroxide solution. If, however, exposure is immediately follo~ed by a bake at 180C for 2 minutes, the resist can be developed as in Example 2.

The polymer prepared as described in Example 1b) is d;ssolved in diethylene glycol dimethyl ether to give a 30% strength solution, and a û.8 ~m layer ;s applied to a wafer. The ~afer is baked at 180C for 2 minutes, ex-posed to the Excimer laser at 248 nm (140 mJ/cm2) for 4 seconds and then developed for 60 seconds with a com-mercial developer (Developer S from Kodak). The un-exposed areas are stripped of 3%. The exposed areas arecompletely stripped. Resolution of structures in the sub-micron range is good.

The poly(o-nitro-~-methylbenzyLmethacrylate-co-p-hydroxystyrene) prepared as described in Example 1d) is disso~ved in diethylene glycol dimethyl ether, and the solution is filtered. The filtrate is spuncoated onto a uafer in a 1 ~m thick polymer film..
The uafer is baked at 130C for 2 minutes and exposed and developed, the last two steps being carried out as described in Example 2. Again the result comprises resist tracks of high resolution.

Determination of plasma etch stability The etch experiments are carried out in a parallel plate reactor (from Plasma Technology). The substrates to be etched are positioned on the lo~er, cooled electrode.

`'` 1.~3~
- 14 - O.Z. 0050/38843 To produce the plasma, the upper electrode is connected to a 13.56 MH~ high fre~uency generator. The etching gas used is CF4/6X 2 under a gas pressure of 50 mtorr.
The energy setting is 0.22 W/cm2. The etch stability S of the resist structures produced as described in Example 2 is compared in the same process with commercial resists (novolak or polymethyl methacrylate positive resist). To this end, 1 ~m thick layers of Polymer are applied in each case to silicon wafers, and the wafers are baked before etching at 200C for 2 minutes. 5 wafers at a time were positioned on the lower electrode, the chamber was evac-uated, etching gas was introduced and, after the pressure had ctab;Lized, the plasma was ignited. The layer thick-ness before and after each etching cycle is measured with an ~-step profilometer. The following strip rates are found:
Polymethyl methacrylate resist 960 A/min Novolak resist 640 X/min Example 2 650 ~/min (structures as in Example 2)

Claims (11)

1. A copolymer which contains as copolymerized units (a) from 5 to 50 mol % of one or more compounds of the formula (I) (I) where A is an aromatic or heteroaromatic, substituted or unsubstituted ring system of 5 to 14 ring members, X is hydrogen, alkyl of 1 to 8 carbon atoms, or sub-stituted or unsubstituted aryl or aralkyl, and Y is an ethylenically unsaturated radical of 2 to 6 carbon atoms, (b) from 95 to 50 mol % of one or more compounds of the formula (II) (II) where R1 is hydrogen or methyl, R2 is hydrogen, halogen or alkyl of 1 to 6 carbon atoms, R3 is hy-drogen, alkyl of 1 to 6 carbon atoms, acetyl, ben-zoyl, C1-C6-alkylsilyl, C1-C6-alkoxysilyl or tert.-butoxycarbonyl, and (c) from 0 to 30 mol % of one or more further olefinic-ally unsaturated, copolymerizable organic compounds, other than (a) and (b), whose homopolymers are trans-parent within the wavelength region from 250 to 400 nm, with the proviso that the percentages mentioned under (a) to (c) add up to 100.

2. A copolymer as claimed in claim 1, wherein the radical Y in the formula (I) is one of the radicals - 16 - O.Z. 0050/38843

3. A copolymer as claimed in claim 2, wherein in the formula (I) the radical A is phenyl, the radical X
is methyl and the radical Y is a group.

4. A copolymer as claimed in any one of claims 1 to 3, wherein in the formula (II) R3 is hydrogen, methyl or tert.-butoxycarbonyl.

5. A process for preparing a copolymer as claimed in claim 1, which comprises polymerizing the compounds men-tioned under (a) to (c) in the presence of a free radical initiator, with the proviso that R3 of component (b) is alkyl of 1 to 6 carbon atoms, acetyl, benzoyl, C1-C6-alkylsilyl, C1-C6-alkoxysilyl or tert.-butoxycarbonyl, and if appropriate converting the copolymer obtained into the copolymer where R3 is H.

6. A photoresist which can be washed out with an aqueous alkaline solution, containing a copolymer as claimed in claim 1.

7. A process for producing semiconductor elements, which comprises using a copolymer as claimed in claim 1 as a light-sensitive material.

8. A process for producing a light-sensitive coating material, which comprises using a copolymer as claimed in claim 1.

9. A process for producing dry resist, which com-prises using a copolymer as claimed in claim 1.

- 17 - O.Z. 0050/38843

10. A process for producing planigraphic printing plates, which comprises using a copolymer as claimed in claim 1.

11. A process for producing an etch resist, which comprises using a copolymer as claimed claim 1.