Radiation-sensitive mixture and production of relief images

A radiation-sensitive mixture, essentially consisting of PA0 a) a binder or binder mixture which is insoluble in water but soluble in aqueous alkaline solutions, PA0 b) a compound which forms a strong acid on exposure to radiation and PA0 c) an organic compound which contains an acid-cleavable carbamate unit, is suitable for the production of relief structures.

The present invention relates to positive-working radiation-sensitive 
mixtures which contain a binder which is insoluble in water but soluble in 
aqueous alkaline solutions, a compound which forms an acid on exposure to 
radiation and carbamate groups, and a process for the production of relief 
images. The novel mixtures are sensitive to UV radiation, electron beams 
and X-rays and are particularly suitable as resist material. 
Positive-working radiation-sensitive mixtures are known. Positive-working 
resist materials which contain o-quinonediazides in binders, for example 
novolaks or poly-(p-vinylphenols), which are soluble in aqueous alkaline 
media are used commercially. However, the sensitivity of these systems to 
radiation, in particular short-wavelength radiation, is unsatisfactory in 
some cases. 
Increases in sensitivity in radiation-sensitive systems which, in the 
primary photoreaction, produce a species which then initiates a catalytic 
secondary reaction independently of the radiation have likewise been 
described. Thus, U.S. Pat. No. 3,915,706 describes, for example, 
photoinitiators which produce a strong acid which then cleaves acid-labile 
groups, such as polyaldehyde groups, in a secondary reaction. 
Radiation-sensitive mixtures which are based on acid-cleavable compounds 
and contain, as a binder, a polymer which is soluble in aqueous alkaline 
media, and a compound which forms a strong acid in a photochemical 
reaction and a further compound having acid-cleavable bonds, which 
increase their solubility in an alkaline developer as a result of the 
action of the acid, are also known (cf. DE-A-3 406 927). Compounds which 
form a strong acid in a photochemical reaction are diazonium, phosphonium, 
sulfonium, iodonium and halogen compounds. The use of these onium salts as 
photochemical acid donors in resist materials is also disclosed in, for 
example, U.S. Pat. No. 4,491,628. Crivello or Org. Coatings and Appl. 
Polym. Sci. 48 (1985), 65-69 gives an overview of the use of onium salts 
in resist materials. 
Radiation-sensitive mixtures of polymers having acid-labile side groups and 
photochemical acid donors are disclosed in, for example, U.S. Pat. No. 
4,491,628, FR-A-2 570 844 and Polymeric Material Science and Engineering, 
A.C.S., 61 (1989), 417-421. However, these polymeric binders are 
hydrophobic and become alkali-soluble only after exposure. 
Copolymers having phenolic and acid-labile groups, for example 
poly-(p-hydroxystyrene-co-tertbutoxycarbonyloxystyrene) are disclosed in 
J. Polym. Sci., Part A, Polym. Chem. Ed. 24 (1986), 2971-1980. However, if 
copolymers of this group which are also alkali-soluble are used in 
conjunction with commercial sulfonium salts, such as triphenylphosphonium 
hexafluoroarsenate, as also described in U.S. Pat. No. 4,491,628, these 
mixtures have the disadvantage that a very large amount of material is 
removed from the unexposed parts, since these sulfonium salts do not 
contribute sufficiently toward inhibition of solubility. 
DE-A-37 21 741 describes radiation-sensitive mixtures which contain a 
polymeric binder soluble in aqueous alkaline solutions and an organic 
compound whose solubility in an aqueous alkaline developer is increased by 
the action of an acid and which contains one or more acid-cleavable 
groups, this organic compound producing a strong acid on exposure to 
radiation. 
It is an object of the present invention to provide novel positive-working 
highly active radiation-sensitive systems for the production of relief 
structures, which systems can be developed with aqueous alkaline solutions 
and permit the production of layers which are sensitive to 
short-wavelength UV light. 
We have found that this object is achieved and that, surprisingly, high 
active radiation-sensitive systems for the production of relief structures 
in short-wavelength UV light can be produced by produced by incorporating 
carbamate groups, the said systems exhibiting very good reproducibility 
and high resolution. 
The present invention relates to a radiation-sensitive mixture, essentially 
consisting of 
(a) a binder or binder mixture which is insoluble in water but soluble in 
aqueous alkaline solutions and 
(b) a compound which forms a strong acid on exposure to radiation, 
wherein the mixture additionally contains an organic compound (c) which has 
an acid-cleavable carbamate unit 
##STR1## 
or component (a) or component (b) contains an acid-cleavable carbamate 
unit 
##STR2## 
Examples of suitable binders (a) are phenolic resins, in particular 
novolaks having a mean molecular weight M.sub.w of from 300 to 30,000, and 
preferably poly-(p-hydroxystyrene), poly-(p-hydroxy-.alpha.-methylstryene) 
or copolymers of p-hydroxystyrene whose mean molecular weights M.sub.w are 
from 200 to 200,000. 
In particular, sulfonium salts of the general formula (I) 
##STR3## 
where R.sup.1, R.sup.2 and R.sup.3 are identical or different and are each 
alkyl of 1 to 3 carbon atoms, aryl, substituted aryl or aralkyl and 
X.sup..crclbar. is ClO.sub.4.sup..crclbar., AsF.sub.6.sup..crclbar., 
SbF.sub.6.sup..crclbar., PF.sub.6.sup..crclbar., BF.sub.4.sup..crclbar. 
and/or CF.sub.3 SO.sub.3.sup..crclbar. are used as compounds (b) which 
form a strong acid on exposure to radiation; preferred sulfonium salts of 
the general formula (I) are those in which one or more of the radicals 
R.sup.1, R.sup.2 and R.sup.3 are a radical 
##STR4## 
where R.sup.4 and R.sup.5 are identical or different and are each H, OH or 
a radical 
##STR5## 
where R.sup.6 and R.sup.7 are identical or different and are each H or 
alkyl of 1 to 5 carbon atoms, or R.sup.6 is bonded to R.sup.7 to form a 
five-membered or six-membered, unsubstituted or monosubstituted or 
polysubstituted, heterocyclic ring. 
The novel radiation-sensitive mixtures contain the compound (b) which forms 
a strong acid on exposure to radiation in general in an amount of from 1 
to 30% by weight based on the total amount of the substances (a)+(b). 
Preferably used additional organic compounds (c) which have an 
acid-cleavable carbamate unit 
##STR6## 
are compounds of the general formula (II) 
##STR7## 
where R.sup.6 and R.sup.7 are identical or different and are each H or 
alkyl of 1 to 5 carbon atoms, or R.sup.6 is bonded to R.sup.7 to from a 
five-membered or six-membered, unsubstituted or monosubstituted or 
polysubstituted, heterocyclic ring, and R.sup.8 is straight-chain or 
branched alkyl of 1 to 10 carbon atoms, halogen-containing straight-chain 
or branched alkyl of 1 to 10 carbon atoms, phenyl, alkyl, alkoxy or 
halophenyl, or phenylalkyl where the alkyl group is of 1 to 3 carbon 
atoms, halo- or alkoxyphenylalkyl where the alkyl groups are of 1 to 3 
carbon atoms or a methyl or ethyl radical which is substituted by a 
heterocyclic radical. 
The novel mixtures may additionally contain, as a sensitizer, a polycyclic 
aromatic compound which absorbs radiation and transfers it to the compound 
(b) which forms a strong acid on exposure to radiation. 
The present invention furthermore relates to a process for the production 
of relief structure or relief images by application of a photoresist 
solution in a layer thickness of from 0.1 to 5 .mu.m to a substrate 
pretreated in a conventional manner, drying, imagewise exposure, heating 
to 50.degree.-150.degree. C. and development with an aqueous alkaline 
solution, wherein the photoresist solution used contains a novel 
radiation-sensitive mixture. 
Regarding the components of the novel radiation-sensitive mixture, the 
following may furthermore be stated specifically. 
(a) Because of the desired stability to plasma etching, a particular 
phenolic resins, for example novolaks, as described in, for example, 
Novolak Resins used in Positive Resist Systems (T. Pampalone in Solid 
State Technology, June 1984, pages 115-120), are used as binders (a) which 
are insoluble in water but soluble or dispersible in aqueous alkaline 
solutions, novolaks based on p-cresol/formaldehyde being preferred for 
exposure in the short-wavelength UV range (.lambda..ltoreq.300 nm). 
Poly-(p-hydroxystyrenes), poly-(p-hydroxy-.alpha.-methylstyrenes) and 
copolymers of p-hydroxystyrene are particularly preferred. 
If carbamate groups are present in the binder (a), they may be represented 
by the following formula 
##STR8## 
where 
m.gtoreq.n, 
X is a phenolic monomer unit, e.g. p-hydroxystryene or 
p-hydroxy-.alpha.-methylstyrene, and 
Y is a monomer unit having a 
##STR9## 
group for example N-alkoxycarbonylmaleimide of the formula 
##STR10## 
or carbamate-modified p-hydroxystyrene units of the formulae 
##STR11## 
where R.sup.6, R.sup.7 and R.sup.8 may have the abovementioned meanings. 
Component (a) is present in the novel radiation-sensitive mixture in 
general in amounts of from 70 to 99, preferably from 75 to 95, % by 
weight, based on the total amount of the mixture of (a)+(b)+(c). 
(b) In principle, all compounds which form a strong acid on exposure to 
radiation can be used as the acid donor. However, sulfonium salts of the 
general formula (I) 
##STR12## 
where R.sup.1, R.sup.2 and R.sup.3 are identical or different and are each 
alkyl of 1 to 3 carbon atoms, e.g. methyl or ethyl, aryl, e.g. phenyl, or 
aralkyl, e.g. benzyl, and X.sup..crclbar. may be the anions 
ClO.sub.4.sup..crclbar., AsF.sub.6.sup..crclbar., SbF.sub.6.sup..crclbar., 
PF.sub.6.sup..crclbar., BF.sub.4.sup..crclbar. and/or CF.sub.3 
SO.sub.3.sup..crclbar., are preferred for exposure to short-wavelength UV 
radiation. 
Particularly preferred sulfonium salts are those of the general formula (I) 
in which one or more of the radicals R.sup.1, R.sup.2 and R.sup.3 are a 
radical 
##STR13## 
where R.sup.4 and R.sup.5 are identical or different and are each H, OH or 
a radical 
##STR14## 
where R.sup.6 and R.sup.7 are identical or different and are each 
hydrogen, straight-chain alkyl of 1 to 10, preferably 1 to 5, carbon 
atoms, e.g. methyl or ethyl, branched alkyl of 3 to 10, preferably 3 to 5, 
carbon atoms, in which the alkyl groups may furthermore be substituted by 
halogen, e.g. chlorine or bromine, preferably chlorine, OH, SH, alkoxy of 
1 to 4 carbon atoms, e.g. methoxy or ethoxy, O-aryl, O-aralkyl, alkylthio 
of 1 to 4 carbon atoms, S-aryl, such as S-phenyl, or S-aralkyl, such as 
S-benzyl, aryl, e.g. phenyl, halogen-substituted aryl, e.g. chlorophenyl 
or bromophenyl, alkyl-substituted aryl, e.g. tolyl, alkoxy-substituted 
aryl, e.g. methoxyphenyl, or aralkyl, e.g. benzyl or phenylethyl, or 
R.sup.6 is bonded to R.sup.7 to form a nitrogen-containing heterocyclic 
radical having 5 to 8, preferably 5 or 6, ring members, resulting, with 
the inclusion of the amide nitrogen of the carbamate group, in, for 
example, rings such as 
##STR15## 
R.sup.6 and/or R.sup.7 may furthermore be acyl, resulting, with inclusion 
of the N of the carbamate group, in, for example, the following radicals: 
##STR16## 
Examples of suitable counter-ions X.sup..crclbar. are 
ClO.sub.4.sup..crclbar., AsF.sub.6.sup..crclbar., SbF.sub.6.sup..crclbar., 
CF.sub.3 SO.sub.3.sup..crclbar. (=triflate), PF.sub.6.sup..crclbar. and 
BF.sub.4.sup..crclbar.. 
Preferred sulfonium salts are, in particular, triphenylsulfonium 
hexafluoroarsenate, dimethyl-4-hydroxyphenylsulfonium triflate and 
dimethyl-4-hydroxy-3,5-dimethylphenylsulfonium hexafluoroarsenate. 
Examples of further acid donors (b) are iodonium salts of the general 
formula 
##STR17## 
where R.sup.1, R.sup.2 and X.sup..crclbar. have the same meanings as in 
the general formula (I). 
The compounds which form a strong acid on exposure to radiation, among 
which the abovementioned sulfonium salts are preferred, are present in the 
novel radiation-sensitive mixture in general in an amount of from 1 to 30, 
preferably from 3 to 10, % by weight, based on the total amount of 
(a)+(b)+(c). 
(c) Suitable organic compounds (c) are those which contain one or more 
acid-cleavable carbamate units 
##STR18## 
Compounds of the general formula (II) 
##STR19## 
where R.sup.6 and R.sup.7 are identical or different and are each 
hydrogen, straight-chain alkyl of 1 to 10, preferably 1 to 5, carbon 
atoms, e.g. methyl, ethyl, propyl, n-butyl or n-pentyl, branched alkyl of 
3 to 10, preferably 3 to 5, carbon atoms, e.g. isopropyl or isobutyl, a 
corresponding alkyl radicals which is substituted by halogen, such as 
chlorine or bromine, OH, SH, alkoxy or thioalkyl of 1 to 5 carbon atoms, 
aryl, e.g. phenyl, aralkyl, e.g. benzyl or phenylethyl, an aryl or aralkyl 
radical which is monosubstituted or polysubstituted by alkyl, alkoxy or 
halogen, e.g. tolyl, methoxyphenyl, 4-methoxybenzyl or 4-chlorobenzyl, or 
R.sup.6 and R.sup.7 are bonded to one another to form a 
nitrogen-containing heterocyclic radical having 5 to 8, preferably 5 or 6, 
ring members, resulting, with inclusion of the amide nitrogen of the 
carbamate group in, for example, rings such as 
##STR20## 
R.sup.6 and R.sup.7 may furthermore be acyl, resulting, with inclusion of 
the N of the carbamate group, in, for example, the radicals 
##STR21## 
R.sup.6 and R.sup.7 may furthermore be a polymer chain, for example the 
units 
##STR22## 
R.sup.8 may be straight-chain alkyl of 1 to 10, preferably 1 to 5, carbon 
atoms, e.g. methyl, ethyl, propyl, n-butyl or n-pentyl, branched alkyl of 
3 to 10, preferably 3 to 5, carbon atoms, e.g. isopropyl or isobutyl, 
tertiary alkyl of 4 to 10 carbon atoms, e.g. tert-butyl, where the 
abovementioned alkyl radicals may furthermore be halogen-substituted, i.e. 
monosubstituted or polysubstituted by halogen, such as chlorine or 
bromine, preferably chlorine, e.g. b-chloroethyl, b-bromoethyl, 
b-trichloroethyl, b-chloroisopropyl or b-trichloroisopropyl, aryl, such as 
phenyl or biphenyl, halogen-substituted aryl, e.g. chlorophenyl, aralkyl, 
such as benzyl or phenyl-ethyl or halogen-substituted aralkyl, for example 
benzyl which is halogen-substituted, for example chlorine-substituted, or 
alkoxy-substituted in the benzene ring. 
The amount of component (c) present in the novel radiation-sensitive 
mixture depends on whether, and how many, carbamate groups are present in 
components (a) and/or (b). Where neither component (a) nor component (b) 
contains carbamate groups, the amount of component (c) in the novel 
radiation-sensitive mixture is from 5 to 40, preferably from 15 to 30, % 
by weight, based on the total amount of components (a)+(b)+(c). 
The carbamate group 
##STR23## 
can be obtained by one of the methods stated below: 
##STR24## 
The novel mixtures are preferably dissolved in an organic solvent, the 
solids content generally being from 5 to 40% by weight. Preferred solvents 
are aliphatic ketones, ethers and mixtures thereof, in particular 
ethylcellosolve, butylglycol, methylcellosolve and 1-methoxy-2-propanol, 
alkylene glycol alkyl ether esters, for example methylcellosolve acetate, 
ethylcellosolve acetate, methylpropylene glycol acetate and ethylpropylene 
glycol acetate, ketones, for example cyclohexanone, cyclopentanone and 
methyl ethyl ketone, and acetates, such as butyl acetate, and aromatics, 
such as toluene and xylene. The choice of the corresponding solvents and 
mixtures thereof depends on the choice of the particular phenolic polymer, 
novolak and photosensitive component. 
Other additives, such as adhesion promoters, wetting agents, dyes and 
plasticizers, may also be added. 
Small amounts of sensitizers may, if required, also be added in order to 
sensitize the compounds in the relatively long-wavelength UV range to the 
visible range. Polycyclic aromatics, such as pyrene and perylene, are 
preferred for this purpose, but other dyes which act as sensitizers may 
also be used. 
The solution of the radiation-sensitive mixture can be filtered through a 
filter having a pore diameter of 0.2 .mu.m. By applying the resist 
solution by spin coating at speeds of from 1,000 to 10,000 rpm, a resist 
film having a layer thickness of from 0.1 to 5 .mu.m is produced on a 
wafer (for example a silicon wafer oxidized on the surface). The wafer is 
heated at from 1 to 5 minutes at 90.degree. C. or 80.degree. C. 
In the novel process for the production of relief images, a 
radiation-sensitive recording layer, which essentially consists of the 
novel radiation-sensitive mixture, is exposed imagewise to a dose such 
that the solubility of the exposed parts in aqueous alkaline solvents 
increases after heating at from 60.degree. to 120.degree. C., and these 
parts can be removed selectively with the alkaline developer. 
The resist films are generally exposed to UV light from a mercury lamp, to 
excimer laser light, electron beams or X-rays through a chromium-plated 
structured quartz mask. The imagewise exposed films are heated for 5 
seconds to 2 minutes at from 60.degree. to 110.degree. C. and then 
developed with aqueous alkaline developer, the exposed parts being 
selectively detached while little removal of material takes place in the 
unexposed parts.

In the Examples which follow, parts and percentages are by weight, unless 
otherwise stated. 
EXAMPLE 1 
A photoresist solution is prepared from 80 parts of poly(p-hydroxystyrene) 
(commercial product from Polysciences, M.sub.w =from 1500 to 7000 g/mol), 
5 parts by triphenylsulfonium hexafluoroarsenate (commercial product from 
Alfa), 15 parts of N-ethoxycarbonylmorpholine and 280 parts of 
methylglycol acetate. The solution is then filtered through a filter 
having a pore diameter of 0.2 .mu.m. 
The resist solution is applied by spin coating to an oxidized silicon wafer 
in a layer thickness of 1 .mu.m. The wafer is dried for one minute at 
90.degree. C. and then exposed for 7 seconds to excimer laser light of 
wavelength .lambda.=248 nm using a test mask by the contact process. 
Thereafter, heating is carried out for 30 seconds at 90.degree. C. and 
development is effected with a developer of pH from 12.0 to 13.5 for about 
20 seconds. 
EXAMPLE 2 
A photoresist solution is prepared from 80 parts of poly(p-hydroxystyrene), 
5 parts of triphenylsulfonium hexafluoroarsenate, 15 parts of 
N-2-trichloroethoxycarbonylpyrrolidone and 280 parts of methylglycol 
acetate. The procedure described in Example 1 is followed. The sensitivity 
for 1 .mu.m layer thickness is 175 mJ/cm.sup.2. 
EXAMPLE 3 
A photoresist solution is prepared from 80 parts of poly(p-hydroxystyrene), 
5 parts of triphenylsulfonium hexafluoroarsenate, 15 parts of tert-butyl 
N,N-diethylcarbamate and 280 parts of methylglycol acetate. The procedure 
described in Example 1 is followed. The sensitivity for 1 .mu.m layer 
thickness is 80 mJ/cm.sup.2. 
EXAMPLE 4 
A photoresist solution is prepared from 95 parts of 
poly(p-hydroxystyrene-co-20 mol % N-chloroethoxycarbonylmaleimide), 5 
parts of triphenylsulfonium hexafluoroarsenate and 300 parts of 
ethylglycol acetate. The procedure described in Example 1 is followed. The 
sensitivity for 1 .mu.m layer thickness is 150 mJ/cm.sup.2.