Radiation-sensitive resist composition comprising a diazoketone

A radiation-sensitive resist composition for manufacturing highly resolved relief structures is characterized by the following components: PA1 a film-forming base polymer; PA1 a radiation-active component that releases an acid when irradiated; PA1 a radiation-sensitive ester-former; and PA1 a solvent.

BACKGROUND OF THE INVENTION 
The invention relates to a radiation-sensitive resist composition, as well 
as to a method for manufacturing highly resolved relief structures. 
In semiconductor production, highly resolving, radiation-sensitive resists 
are needed to produce fine patterns. In lithographic processes, a 
substrate is covered with a thin layer of this type of resist, and the 
desired pattern is transferred, initially as a latent image, into this 
layer--through projection exposure or beam control. If indicated, 
following further treatment steps, such as baking and silylation, the 
latent image is then developed, a relief pattern that serves as a mask for 
subsequent substrate etching processes being formed in the resist layer. 
The substrate can thereby consist of a semiconductor material such as 
silicon, or it can be a polymeric layer of organic material, i.e., one 
then has a so-called two-layer resist (c.f., e.g., L. F. Thompson, C. G. 
Willson, M. J. Bowden "Introduction to Microlithography", ACS Symposium 
Series 219, American Chemical Society, Washington 1983, pp. 16, 20 and 21; 
W. M. Moreau "Semiconductor Lithography", Plenum Press, New York 1988, 
page 4). 
Besides a film-forming base polymer, a radiation-active compound 
constitutes the main component of the resist or of the resist layer. The 
radiation-active component, which is generally a diazo compound or a 
so-called crivello salt, is converted into an acid through absorption of 
incident radiation, such as visible light, near ultraviolet light (NUV), 
deep ultraviolet light (DUV), X-ray radiation, electron-beam radiation and 
ion-beam radiation. In the case of positively working, wet-developable 
resists, in some instances after a baking process, this acid increases the 
solubility of the irradiated regions in a developer (c.f.: L. F. Thompson 
et al., loc. cit., pp. 88-90 and 113-115). On the other hand, in the case 
of negatively working, wet-developable resists, for example, so-called 
"image reversal" resists, an acid-catalyzed cross-linking takes place in 
the irradiated regions, through which means the solubility in the 
developer is reduced (c.f.: "Proc. SPIE", vol. 1086 (1989), pp. 117 128). 
When working with dry-developable resist systems, a silylation of the 
resist layer is carried out at the surface using suitable gaseous or 
liquid silicon-containing agents, and, in fact, when working with 
positively working resists, selectively in the unexposed regions and, when 
working with negatively working resists, selectively in the exposed 
regions. The development takes place in this case in an anisotropic, 
oxygen-containing plasma, the silylated regions serving at the surface of 
the resist layer as a resistant etching mask (c.f.: "Encycl. Polym. Sci. 
Eng.", vol. 9 (1987), p. 132). A high resolution capability is achieved 
both when working with wet-developable as well as with dryodevelopable 
resist systems through a greatest possible difference in the acid 
concentration between the exposed and the unexposed regions. 
Because of diffraction phenomena, lens defects, etc., an image of a given 
object pattern (=mask pattern) having a modulation of unexposed regions. 
Because of diffraction phenomena, lens defects, etc., an image of a given 
object pattern (=mask pattern) having a modulation of M.sub.object =1 is 
only able to be formed in the image plane with a modulation of M.sub.image 
&lt;1, i.e., the value for the optical modulation-transfer function 
(MTF.sub.optic =M.sub.image /M.sub.object) lies clearly under the maximum 
value of 1, particularly for fine patterns to be imaged (c.f.: L. F. 
Thompson et al., loc. cit., p. 36; W. M. Moreau, loc. cit., p. 357). 
The result is that with an increasingly finer structure dimension, the 
image of the mask pattern formed in the image plane (=resist plane) 
becomes less and less sharp. For this reason, the resist layer is, in 
principle, also always exposed somewhat at those sites that actually 
should remain unexposed. Therefore, with an increasingly finer structure 
dimension, the difference in the acid concentration between those regions 
of the resist layer that are to be irradiated and those that are not to be 
irradiated becomes less and less. The limit of the resist resolution is 
then reached when the difference in concentration is too small for there 
to be a difference in solubility characteristics (or also silylation 
characteristics) of these two regions, i.e., when--for a given mask 
pattern--the value for the so-called critical modulation-transfer function 
of the resist 
CMTF.sub.resist =(10.sup.1/.gamma. -1)/(10.sup.1/.gamma. +1) 
(.gamma.=contrast) exceeds the value of the optical modulation-transfer 
function (MTF.sub.optic &lt;CMTF.sub.resist). Therefore, a high contrast is 
indispensable for a high resolution capability of the resist (c.f., also: 
W. M. Moteau, loc. cit., pp. 368-371). 
Now, the problem is that when conventional resist compositions are used, 
for example on the basis of novolak polymers and diazo naphthoquinones, or 
of t-boc-protected polymers and crivello salts, one cannot prevent acid 
from forming undesirably in those regions which are supposed to remain 
unexposed per se. The resultant reduced contrast of the resist increases 
the value for the CMTF.sub.resist and, thus, limits its resolution 
capability for fine patterns. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide a radiation-sensitive resist 
composition for fabricating highly resolved relief patterns, which will 
enable the contrast, i.e., the resolution to be improved. 
DETAILED DESCRIPTION OF THE INVENTION 
This objective is achieved in accordance with the invention by means of a 
resist composition having the following components: 
a film-forming base polymer; 
a radiation-active component that releases an acid when irradiated; 
a radiation-sensitive ester-former; and 
a solvent. 
Besides the base polymer and a radiation-active compound (photo-acid 
former), as well as, if need be, other components--the resist composition 
according to the invention has a radiation-sensitive ester-former as an 
essential constituent, i.e., a compound which, together with the acid 
produced by the radiation-active component, forms an ester. This procedure 
is based on the following mechanism of action of the ester-former. 
The light modulation in the resist plane (M.sub.image) corresponds more or 
less to the concentration profile in the latent image of the acid produced 
from the radiation-active component through absorption of the radiant 
energy. The radiation-sensitive ester-former absorbs the radiation being 
used for pattern generation and thereby loses the capacity to form esters. 
What is decisive when working with the resist composition according to the 
invention is that the concentration profile of the radiation-sensitive 
ester-former in the latent image complements the concentration profile of 
the photo-acid. 
Thus, when a radiation-active component and a radiation-sensitive 
ester-former are irradiated simultaneously, regions are formed in the 
latent image where both acid as well as ester-former are present. The acid 
in these regions is esterified and converted into a neutral compound in a 
spontaneous reaction or in a reaction produced by an afterbake step. This 
results in an acid concentration profile, which shows an abrupt, i.e., a 
steep rise precisely below the line edge on the mask, so that a 
substantially sharper and, thus, higher-contrast image is produced. 
Therefore, the resist composition according to the invention makes it 
possible to resolve substantially finer patterns than had previously been 
possible. 
In general, as ester-formers, one can use those compounds which 
react--spontaneously or after baking--with the acid being produced by the 
radiation-active component due to exposure to form an ester, and which 
absorb the radiation used during exposure and are thereby broken down 
photochemically to form products that can no longer enter into any further 
esterification reaction; moreover, these products should be hydrophilic, 
i.e., be soluble in the developer. The ester-former itself--in the same 
way as the ester--should be hydrophobic, i.e., have a 
solubility-inhibiting effect, or be insoluble in the developer. In the 
resist solvent, however, the ester-former should be readily soluble 
and--with respect to a good storage stability of the resist--should not 
enter into any reaction with the base polymer, the radiation-active 
component, and other resist additives. 
As ester-formers, one preferably uses diazoketones of the structure 
##STR1## 
R.sup.1 being an aromatic or a saturated or unsaturated--aliphatic, cyclic 
or heterocyclic hydrocarbon residue, and R.sup.2 signifying hydrogen (H) 
or R.sup.1. The hydrocarbon residues can also carry substituents, such as 
halogens, as well as O-alkyl and nitro-groups. 
Suitable diazoketones are, for example, those having substituted alkyl-, 
cycloalkyl- and aryl residues, as well as those having polycyclic 
cycloalkyl- and aryl residues and, furthermore, those having 
polyfunctional residues R.sup.1 and R.sup.2. The following residues 
R.sup.1 and R.sup.2 have proven to be particularly advantageous: 
##STR2## 
In principle, the residues R.sup.1 and R.sup.2 are freely selectable. It 
must be guaranteed, however, when chemically preparing the diazoketones, 
that compounds are obtained, which are stable at room temperature and, 
moreover, do not show any significant decomposition at the temperature 
required to dry the resist on the substrate surface, which is usually 
between 50.degree. and 130.degree. C. Via the residues R.sup.1 and 
R.sup.2, the absorption of the diazoketones can also be advantageously 
adapted to the radiation used for the exposure and, in addition, by this 
means the solution properties and the thermal stability can be influenced. 
The diazoketones react with the acid (X-H) formed from the radiation-active 
component while releasing nitrogen, to form a neutral ester: 
##STR3## 
It is unimportant here which acid is formed from the radiation-active 
component during exposure. What is important, rather, is that this acid 
reacts with the ester-former to forman ester. 
The residue X can, therefore, be both an organic, as well as an inorganic 
or organometallic residue. Examples of X are: halogen-, 
##STR4## 
where R.sup.3 =saturated or unsaturated, aliphatic, cyclic or heterocyclic 
hydrocarbon residue (if indicated, halogenated), acyl, alkoxy, aryloxy, 
alkoxycarbonyl or aryloxycarbonyl; 
R.sup.4 =hydrogen, acyl, alkoxy, aryloxy, alkoxycarbonyl, aryloxycarbonyl 
or halogenated hydrocarbon residue; 
R.sup.5 =saturated or unsaturated, aliphatic, cyclic or heterocyclic 
hydrocarbon residue (if indicated, halogenated). 
The diazoketones of the afore-mentioned type being used as ester-formers 
are radiation-sensitive compounds. When irradiated, they decompose and 
form an acid photochemically--in the presence of moisture: 
##STR5## 
Compounds of this type are relatively weak acids, i.e., they are not able 
to react with the ester-former, as the acids released from the 
radiation-active component do. Therefore, it is also not possible for the 
diazo-compounds to act at the same time as ester-formers and as 
photo-active components. 
The concentration of ester-former in the resist composition is determined, 
on the one hand, by its effectiveness with respect to the acid released 
from the radiation-active component and, on the other hand, by the 
quantity of the radiation-active component applied. Generally, relative to 
the radiation-active component, 1 to 200 mole-% of radiation-sensitive 
ester-former are added to the resist; quantities of between 10 and 100 
mole-% have proven to be especially advantageous. 
Suited as radiation-active components are preferably diazodicarbonyl 
compounds, diazoquinones, crivello salts and nitrobenzyltosylates. 
Suitable diazodicarbonyl compounds are, in particular, diazoketo esters 
and diazo diesters, as well as diazomeldrum acid 
(2,2-dimethyl-5-diazo-1,3-dioxane-4,6-dione); suited as diazoquinones are, 
above all, diazo naphthoquinones. Suitable crivello salts are, for 
example, triflates, i.e., salts of the trifluoromethane sulphonic acid, 
such as triphenyl sulfoniumtriflate. The radiation-active component is 
applied in a quantity of 1 to 100% by weight, relatively to the base 
polymer. 
The polymer advantageously contains acid-labile groups, such as epoxy-, 
terto-butylester- and tert.-butoxycarbonyl groups (t-boc groups); 
therefore, it is preferably a copolymer or terpolymer with monomer units 
on the basis of tert.-butoxycarbonyl maleinimide. Other applicable 
polymers are those on the basis of novolak or cresol novolak, 
polyvinylphenol and acrylic acid or methacrylic acid. Also suited are 
styrene-maleinimide copolymers, anhydride-containing polymers and 
silicon-containing polymers. 
The following compounds, in particular, can be used as solvents: 
ethylene-glycol-monoethyl-ether acetate, propylene-glycol-monomethyl-ether 
acetate, ethyl lactate, ethyl pyruvate (pyruvic acid ethyl ester), 
methyl-3-methoxy propionate and cyclohexanone. 
The resist composition according to the invention is not restricted to 
certain resist systems. Rather, many resists systems of the most widely 
varying kind are able to benefit from the improvement in contrast, such as 
dry- and wet-developable resists, thermally developable resists, dyed 
resists, image-reversal resists, multilayer resists and photo-, X-ray, 
electron-beam, or ion-beam resists. Individual examples are: positively or 
negatively working, wet-developable single-layer resists, as well as 
corresponding dry-developable resists, and wet- or dry-developable, 
positively or negatively working two-layer resists. 
When working with so-called acid-catalyzed resists, where a strong acid is 
released during irradiation, which--during a post-exposure bake 
step--splits off protective groups, such as t-boc groups, from the base 
polymer, the resist composition according to the invention offers the 
further advantage--besides the improvement in contrast--that the acid is 
prevented from diffusing from the exposed regions of the resist layer into 
the unexposed regions in the time interval between irradiation and baking 
(=delay time). In this manner, a stability of the line widths is achieved, 
and, in fact, independent of the delay time (post exposure delay time 
stabilization). 
The radiation-sensitive resist composition according to the invention is 
used in lithographic processes to produce highly resolved relief patterns. 
To this end--to produce a radiation-sensitive resist layer--this resist 
composition is first applied to an organic or inorganic substrate and 
subsequently dried. To produce a latent image in the resist layer, it is 
first irradiated patternwise and, in fact, by means of photo-, X-ray, 
electron, or ion beams. Finally, the irradiated resist layer is treated 
with a gaseous or liquid developer to convert the latent image into a 
relief pattern, and is dried. 
To accelerate the ester formation, it is beneficial to subject the resist 
layer to a bake step following the patternwise exposure ("post exposure 
bake"). Moreover, following the patternwise exposure or following the bake 
step, the resist layer can be treated with a silylating agent ("top 
surface imaging"). Alternatively, a silylation of the resist patterns can 
take place in that, following the treatment with the developer, i.e., 
after the drying operation, the resist layer is treated with a silylating 
agent. In addition, subsequent to the bake step, for purposes of image 
reversal, the resist layer can still be subjected to a flood exposure, 
i.e., be irradiated over its entire surface. It should also be mentioned 
that during the treatment with the developer, either the exposed regions 
of the resist layer (positive resists) or the unexposed regions (negative 
resists) can be removed. 
One advantageous refinement of the method according to the invention 
provides that, after the patternwise irradiation or after the bake step, 
but before a silylation to be carried out in some instances, the resist 
layer be subjected to a flood exposure using a shorter wavelength than in 
the case of the patternwise irradiation. This procedure is called for when 
a photoresist is used and the patternwise irradiation takes place in the 
NUV (near UV), but the ester-former absorbs in DUV (deep UV). The 
ester-former is then destroyed, namely, during the flood exposure. The 
flood exposure takes place, for example, using radiation of a wavelength 
of 250 nm when the patternwise exposure (in the NIIV) is carried out at 
365 nm (i-line) or 436 nm (g-line). 
The invention will be further elucidated on the basis of exemplary 
embodiments.

EXAMPLE 1 
(Comparative Test) 
A resist solution consisting of 1.74 parts by weight of 
2.6-dinitrobenzyltosylate (as a radiation-active component), 8.66 parts by 
weight of a terpolymer of N-tert.-butoxycarbonyl maleinimide, maleic 
anhydride and styrene (for preparation, see EP Patent Application 0 492 
253) and 89.6 parts by weight of propylene glycol monomethyletheracetate 
is spin-coated on to a silicon wafer. After drying on a hot plate 
(90.degree. C./60 s), a resist layer with a thickness of 352 run is 
obtained. This layer is exposed polychromatically through a multidensity 
mask in the contact mode (device MJB 3, firm Karl Suss; Hg-Xe lamp). The 
resist layer is subsequently baked at 110.degree. C./60 s on a hot plate, 
developed for 120 s in the commercial developer NMD-W 2.38% (firm Tokyo 
Ohka Kogyo Co.), rinsed with water, and dried on a hot plate for 60 s at 
90.degree. C. The evaluation of the characteristic curve yields a contrast 
of 1.9 and a sensitivity of 22.8 mJ/cm.sup.2. 
EXAMPLE 2 
A resist solution consisting of 1.61 parts by weight of 
2.6-dinitrobenzyltosylate, 8.06 parts by weight of a terpolymer of 
N-tert.-butoxycarbonyl maleinimide, maleic anhydride and styrene, 0.81 
parts by weight of (diazomethyl-2-naphthyl)-ketone (as a 
radiation-sensitive ester-former) and 89.6 parts by weight of propylene 
glycol monomethyletheracetate is spin-coated on to a silicon wafer. After 
drying on a hot plate (90.degree. C./60 s), a resist layer with a 
thickness of 344 nm is obtained. This layer is exposed polychromatically 
through a multidensity mask in the contact mode (device MJB 3, firm Karl 
Suss; Hg-Xe lamp). The resist layer is subsequently baked at 110.degree. 
C./60 s on a hot plate, developed for 120 s in the commercial developer 
NMD-W 2.38% (firm Tokyo Ohka Kogyo Co.), rinsed with water, and dried on a 
hot plate for 60 s at 90.degree. C. The evaluation of the characteristic 
curve yields a contrast of -14.5 and a sensitivity of 31.7 mJ/cm.sup.2. 
EXAMPLE 3 
A resist solution consisting of 1.81 parts by weight of 
2.6-dinitrobenzyltosylate, 8.06 parts by weight of a terpolymer of 
N-tert.-butoxycarbonyl maleinimide, maleic anhydride and styfete, 0.61 
parts by weight of .omega.-diazoacetophenone (as a radiation-sensitive 
ester-former) and 89.6 parts by weight of propylene glycol 
monomethyletheracetate is spin-coated on to a silicon wafer. After drying 
on a hot plate (70.degree. C./60 s), a resist layer with a thickness of 
361 nm is obtained. This layer is exposed polychromatically through a 
multidensity mask in the contact mode (device MJB 3, firm Karl Suss; Hg-Xe 
lamp). The resist layer is subsequently baked at 110.degree. C./60 s on a 
hot plate, developed for 120 s in the commercial developer NMD-W 2.38% 
(firm Tokyo Ohka Kogyo Co.), rinsed with water, and dried on a hot plate 
for 60 s at 90.degree. C. The evaluation of the characteristic curve 
yields a contrast of -11.7 and a sensitivity of 28.6 mJ/cm.sup.2. 
EXAMPLE 4 
(Comparative Test) 
A resist solution consisting of 2.21 parts by weight of diphenyl 
iodoniumtriflate (as a radiation-active component), 16.3 parts by weight 
of a copolymer of N-tert.-butoxycarbonyl maleinimide and styrene 
(preparation analogous to EP Patent Application 0 492 253; see also EP 
Patent Application 0 492 254, as well as European Patent Application 0 234 
327), and 81.5 parts by weight of ethylene glycol monoethyletheracetate is 
spin-coated on to a silicon wafer. After drying on a hot plate (90.degree. 
C./60 s), a resist layer with a thickness of 1.2 .mu.m is obtained. This 
layer is exposed monochromatically, using a 250 nm filter, through a 
multidensity mask in the contact mode (device MJB 3, firm Karl Suss; Hg-Xe 
lamp). The resist layer is subsequently baked at 110.degree. C./60 s on a 
hot plate, developed for 180 s in the commercial developer NMD-W 2.38% 
(firm Tokyo Ohka Kogyo Co.), rinsed with water, and dried on a hot plate 
for 60 s at 90.degree. C. The evaluation of the characteristic curve 
yields a contrast of -1.6 and a sensitivity of 5.7 mJ/c.sup.2. 
EXAMPLE 5 
A resist solution consisting of 2.19 parts by weight of diphenyl 
iodoniumtriflate, 16.2 parts by weight of a copolymer of 
N-tert.-butoxycarbonyl maleinimide and styrene, 0.49 parts by weight of 
(diazomethyl-1-naphthyl)-ketone (as a radiation-sensitive ester-former) 
and 81.1 parts by weight of ethylene glycol monoethyletheracetate is 
spin-coated on to a silicon wafer. After drying on a hot plate (90.degree. 
C./60 s), a resist layer with a thickness of 1.2 .mu.m is obtained. This 
layer is exposed monochromatically, using a 250 nm filter, through a 
multidensity mask in the contact mode (device MJB 3, firm Karl Suss; Hg-Xe 
lamp). The resist layer is subsequently baked at 110.degree. C./60 s on a 
hot plate, developed for 180 s in the commercial developer NMD-W 2.38% 
(firm Tokyo Ohka Kogyo Co.), rinsed with water, and dried on a hot plate 
for 60 s at 90.degree. C. The evaluation of the characteristic curve 
yields a contrast of -10.3 and a sensitivity of 7.6 mJ/c.sup.2. 
In comparison to Example 4, Example 5 clearly shows that when working with 
the resist composition according to the invention, a considerable 
improvement in contrast is also to be achieved even in the case of thick 
photoresist layers. 
EXAMPLE 6 
(Comparative Test) 
A resist solution consisting of 2.21 parts by weight of diphenyl 
iodoniumtriflate, 16.3 parts by weight of a terpolymer of 
N-tert.-butoxycarbonyl maleinimide, maleic anhydride and styrene, and 81.5 
parts by weight of propylene glycol monomethyletheracetate is spin-coated 
on to a silicon wafer. After drying on a hot plate (90.degree. C./60 s), a 
resist layer with a thickness of 1.0 .mu.m is obtained. This layer is 
exposed on a KrF excimer-laser projection exposure device (.lambda.=248 
nm; numerical aperture =0.37) through a mask having structures of between 
2.0 and 0.25 .mu.m with a dose of 28.5 mJ/c.sup.2. The resist layer is 
subsequently baked at 110.degree. C./60 s on a hot plate, developed for 
180 s in the commercial developer NMD-W 2.38% (firm Tokyo Ohka Kogyo Co.), 
rinsed with water, and dried on a hot plate for 60 s at 90.degree. C. The 
evaluation of the patterns in the scanning electron microscope reveals a 
resolution of 2.0 to 0.7 .mu.m structures having sidewall angles of 
between 80.degree. (2.0 .mu.m structures) and 30.degree. (0.7 .mu.m 
structures). In the case of the 0.7 .mu.m structures, 50% of the original 
layer thickness is still present. 
EXAMPLE 7 
A resist solution consisting of 2.19 parts by weight of diphenyl 
iodoniumtriflate, 16.2 parts by weight of a terpolymer of 
N-tert.-butoxycarbonyl maleinimide, maleic anhydride and styrene, 0.49 
parts by weight of (diazomethyl-1-naphthyl)-ketone and 81.1 parts by 
weight of propylene glycol monomethyletheracetate is spin-coated on to a 
silicon wafer. After drying on a hot plate (90.degree. C./60 s), a resist 
layer with a thickness of 1.0 .mu.m is obtained. This layer is exposed on 
a KrF excimer-laser projection exposure device (.lambda.=248 nm; numerical 
aperture=0.37) through a mask having structures of between 2.0 and 0.25 
.mu.m with a dose of 45.2 mJ/cm.sup.2. The resist layer is subsequently 
baked at 110.degree. C./60 s on a hot plate, developed for 180 s in the 
commercial developer NMD-W 2.38% (firm Tokyo Ohka Kogyo Co.), rinsed with 
water, and dried on a hot plate for 60 s at 90.degree. C. The evaluation 
of the patterns in the scanning electron microscope reveals a resolution 
of 2.0 to 0.6 .mu.m structures having sidewall angles of between 
89.degree. (2.0 .mu.m structures) and 85.degree. (0.6 .mu.m structures). 
In the case of all the structures, the original layer thickness is still 
present. 
In comparison to Example 6, Example 7 clearly shows that when working with 
the resist composition according to the invention, both the resolution 
capability, as well as the structure quality can be substantially 
improved.