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Description  |
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BACKGROUND OF THE INVENTION
The invention relates to an electrically conductive resist material which
can prevent electrostatic charging and the fields resulting therefrom, to
a process for its preparation and its use, in particular for the
preparation of electron beam resists.
It is known that in particular upon exposure to electron beams for
producing very fine resist structures in the submicron region on
non-conducting substrates such as glass, quartz, lithium niobate, undoped
silicon, germanium or III-V semiconductors, electric fields are generated
by means of secondary electrons, which deflect the primary beam, resulting
in unsatisfactory image reproduction (cf. Appl. Phys. Lett. 48(13), 835
(1986)). Similar resolution-limiting effects are also observed in the use
of scanning electron microscopy for inspecting and measuring integrated
circuits.
Examples of suitable compounds for avoiding charging effects are as
follows: indium oxide, tin oxide or ITO layers on glass substrates, to
which the resist is then applied (cf. Chem. Abstr. 102, 229.567n).
The use of a carbon film as conductive bottom layer requires an additional
heat treatment of up to 400.degree. C. after the deposition of the film.
However, this may have the effect that subsequent or already performed
process steps may become useless (cf. Appl. Phys. Lett. 48 (13), 835
(1986)).
By applying a solution of ammonium polystyrene sulfonate as top or bottom
layer, it is also possible to suppress charging effects (cf. New
Materials/Japan, 9 (5), 13 (1988)).
Furthermore, a resist material is known which comprises a conducting
polymer tetracyanoquinone dimethane (TCNQ)salt complex in a PMMA matrix in
order to avoid a charging effect. The disadvantage of this method is that
TCNQ complexes are, as a rule, insoluble or slightly soluble, which may
lead to phase separation when the resist material is applied. Moreover,
TCNQ complexes are difficult to handle due to their toxicity and their
decomposition at elevated temperatures(cf. Chem. Abstr. 108, 29440c).
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an
electrically conductive resist material which does not have the
above-mentioned disadvantages and which furthermore can be processed by
the methods usually used in microelectronics.
Another object of the present invention is to provide a process for
producing the above-mentioned electrically conductive resist material.
A further object of the present invention is to provide a method for
preparing electron beam resists which prevents electrostatic charging and
the production of resultant electrostatic fields.
In accomplishing the foregoing objectives, there has been provided, in
accordance with one aspect of the present invention, an electrically
conductive resist material comprising at least one polymer which is
sensitive to ionizing radiation and a soluble electrically conductive
oligomer or polymer.
In accordance with another aspect of the present invention there is
provided a process for the preparation of the foregoing resist material,
which comprises the step of admixing an electrically conductive oligomer
or polymer dissolved in a suitable solvent to at least one polymer which
is sensitive to ionizing radiation.
In accordance with a further aspect of the present invention, there is
provided a method of preventing electrostatic charging and electrostatic
fields resulting therefrom in the preparation of electron beam resists
comprising the step of applying to an insulating substrate the
above-described resist material.
Other objects, features and advantages of the present invention will become
apparent to those skilled in the art from the following detailed
description. It should be understood, however, that the detailed
description and specific examples, while indicating preferred embodiments
of the present invention, are given by way of illustration and not
limitation. Many changes and modifications within the scope of the present
invention may be made without departing from the spirit thereof, and the
invention includes all such modifications.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 represents the structure reproduced in Examples 1-4 according to the
invention.
FIG. 2 represents the structure reproduced in Comparative Example B.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
It has been found that the addition of soluble electrically conductive
oligomers or soluble electrically conductive polymers to customary resist
formulations, which are sensitive to ionizing radiation, contributes the
conductivity necessary to avoid charging effects without having to change
radically the processing steps necessary for the structuring.
In principle, all known formulations can be used for preparing the electron
beam resist. Suitable formulations are those based on novolak as binder of
both the positive-working and also the negative-working type. However,
formulations of polymers which are crosslinkable or degradable by
high-energy radiation are also suitable. Due to their low water
absorption, in particular resists of this type which have to be developed
in a solvent show particularly aggravating charging effects, which can be
prevented by the addition of soluble electrically conductive oligomers or
polymers. Examples are halogenated or nonhalogenated polyacrylates,
polymethacrylates, polyolefin sulfones, if appropriate in combination with
another binder, or chlorinated or chloromethylated polystyrenes and
copolymers thereof or epoxidized polybutadienes.
The conductive oligomers or conductive polymers used can be basically
compounds whose solubility is sufficiently high in the solvents used in
the resist formulations. These include oligo- and polythiophene
derivatives which additionally carry substituents in the 3-position.
3-Alkoxy-substituted oligo- and polythiophenes are preferably used.
Examples are 3-butylthiophene, 3-pentylthiophene,
3-hexylthiophene,3-octylthiophene, 3-dodecylthiophene,
3-(methoxyethoxyethoxymethyl)thiophene, 3,4-diethylthiophene,
3-butyl-4-methylthiophene, 3-methoxythiophene, 3-ethoxythiophene,
3-propoxythiophene, 3-(methoxyethoxy)thiophene,
3-methoxy-4-methylthiophene, 3-ethyl-4-methoxythiophene,
3-butyl-4-methoxythiophene, 3-ethoxy-4-methylthiophene,
3-ethoxy-4-ethylthiophene, 3-butoxy-4-methylthiophene,
3,4-dimethoxythiophene, 3-ethoxy-4-methoxythiophene or
3-butoxy-4-methylthiophene.
Particularly good properties (e.g. particularly high solubilities) are
achieved if the alkoxy group contains at least 6 carbon atoms, preferably
6 to 25 carbon atoms, as in the case of 3-hexyloxythiophene,
3-heptyloxythiophene, 3-octyloxythiophene, 3-nonyloxythiophene,
3-decyloxythiophene, 3-undecyloxythiophene, 3-dodecyloxythiophene,
3-tetradecyloxythiophene, 3-pentadecyloxythiophene,
3-hexadecyloxythiophene, 3-octadecyloxythiophene, 3-eicosyloxythiophene,
3-docosyloxythiophene, 3-(2'-ethylhexyloxy)thiophene,
3-(2',4',4'-trimethylpentyloxy)thiophene, 3,4-dihexyloxythiophene,
3,4-dioctyloxythiophene, 3,4-dinonyloxythiophene,
3,4-didodecyloxy-thiophene, 3-methoxy-4-pentyloxythiophene,
3-hexyloxy-4-methoxythiophene, 3-methoxy-4-nonyloxythiophene,
3-dodecyloxy-4-methoxythiophene, 3-docosyl-4-methoxythiophene,
3-ethoxy-4-pentyloxythiophene, 3-ethoxy-4-hexloxythiophene,
3-butoxy-4-dodecyloxythiophene or 3-(2'-ethylhexyloxy)-4-methoxythiophene.
Depending on the conductivity of the oligomer or polymer present in the
material and on the basic conductivity of the formulation, the addition of
about 0.01 to 10% by weight of the conductive oligomer or polymer, based
on the dry matter of the formulation has proven to be favorable, the
preferred concentrations being between 0.1 and 5% by weight, in particular
between 0.1 and 3% by weight.
The solvents used are those which dissolve not only the components of the
formulation but also the conductive oligomer or conductive polymer. The
solvents which can be used are glycol ethers, for example glycol
monomethyl ether, glycol dimethyl ether, glycol monoethyl ether or
propylene glycol monoalkyl ether, preferably propylene glycol methyl
ether, aliphatic esters, for example ethyl acetate, hydroxyethyl acetate,
alkoxyethyl acetate, n-butyl acetate, propylene glycol monoalkyl ether
acetate, preferably propylene glycol methyl ether acetate or amyl acetate,
ethers, for example dioxane or tetrahydrofuran, ketones, for example
methyl ethyl ketone, methyl isobutyl ketone, cyclopentanone and
cyclohexanone, dimethylformamide, dimethylacetamide, hexamethylphosphoric
triamide, N-methylpyrrolidone, butyrolactone, aromatics, such as toluene,
chlorobenzene, xylenes, and mixtures of the solvents mentioned, if
appropriate also in combination with nonsolvents. If appropriate safety
procedures and measures for appropriate disposal are taken, the coating
can also be carried out from other solvents, for example acetonitrile,
methoxypropionitrile, methyl cyanoacetate or mixtures thereof.
The application of the conductive resist material according to the
invention is not limited to insulating substrates. However, the particular
advantage is only displayed when applied to highly insulating substrates.
These are undoped semiconductor substrates, such as Si, Ge or GaAs having
a small surface area or volume conductivity, substrates having highly
insulating coatings of the organic or inorganic type, such as oxide
layers, quartz, lithium niobate, strontium titanate, sapphire, diamond,
but also insulating sheets of organic polymers, such as polyethylene
terephthalate or sheets of inorganic materials, such as silicon, silicon
carbide or beryllium membranes.
To improve the adhesion on the substrate, an adhesion promoter can be added
to the resist material. However, it is also possible to apply an adhesion
promoter to the substrate before the coating.
Furthermore, it is possible to add, if desired, colorants, pigments,
plasticizers, wetting agents and flow-improving agents, for example
polyglycols, cellulose ethers, preferably ethylcellulose, to the
radiation-sensitive mixtures according to the invention to improve
specific requirements such as flexibility, adhesion and gloss.
The preparation of the resist material according to the invention can be
carried out in such a manner that the electrically conductive oligomer or
electrically conductive polymer is first dissolved in a suitable solvent
and then added to a finished resist formulation, or the polymer or
oligomer forms a layer separate from the resist layer. However, it is also
possible to dissolve the conductive compound together with the individual
components of the resist formulation in a suitable solvent or solvent
mixture. This is done at a temperature between the melting and boiling
point of the solvent or solvent mixture used, preferably in the range from
about 0.degree. C. to 80.degree. C., in particular from 20.degree. C. to
60.degree. C., if necessary with stirring or other mixing techniques. The
concentration of the main components in the solution is about 0.1 to 30,
preferably 0.5 to 10% by weight.
The resist material can be processed by conventional methods, such as
spin-coating or spraycoating. The layer thickness can be adjusted within
wide limits as desired by choosing the coating parameters and solids
content of the solution.
Drying is carried out, depending on substrate and application, for example,
in an oven, on a hot plate, or, when processed in webs, in a drying
channel at elevated temperature, preferably between about 80.degree. and
130.degree. C. The drying at elevated temperature can be preceded by a
predrying step at room temperature.
The irradiation is carried out by means of ionizing radiation, for example
by means of electron radiation or X-ray radiation, preferably by means of
electron radiation.
In principle, the structuring of this type of resist can also be carried
out by means of UV light.
The development after the exposure is carried out by means of the
developers customary for the particular resist formulations. These are
predominantly developers which essentially are organic solvents. In the
case of resist formulations containing alkali-soluble or watersoluble
binders, aqueous or aqueous-alkaline developers can also be used.
The etching resistance can be increased in the following process steps by
maintaining the developed resist layers for some time, for example 5 to 40
minutes at elevated temperature,if appropriate with simultaneous exposure
to UV light.
Electron beam exposure of a resist on a sufficiently conductive substrate
yields a pattern after the development, which is shown in FIG. 1. If,
however, charging effects occur, the structures reproduced in FIG. 2 are
obtained.
The use of the resist material according to the present invention prevents
electrostatic charging and the fields resulting therefrom and thus yields
structures according to FIG. 1 also on highly insulating substrates.
The examples which follow are intended to illustrate the invention in more
detail.
COMPARATIVE EXAMPLE A
1 g of polymethyl methacrylate (PMMA) was dissolved in 100 ml of
tetrahydrofuran, and the mixture was applied to a p-doped Si wafer on a
spincoater at 1,000 rpm. Drying at 100.degree. C. for 1 minute (hot plate)
gave a layer thickness of 400 nm. It was then irradiated with 100
.mu.C/cm.sup.2 at an accelerating voltage of 25 kV. It was then developed
for 40 seconds in a developer consisting of methyl ethyl
ketone/isopropanol (ratio by volume 1:4) at 21.degree. C. On top of this
sufficiently conductive substrate, the pattern reproduced in FIG. 1 was
obtained, which corresponded to the desired pattern.
COMPARATIVE EXAMPLE B
Comparative Example A was repeated, except that an insulating substrate
(quartz) was used instead of the conductive substrate of Comparative
Example 1. However, following the same procedure, it was not possible to
transfer the desired pattern (analogously to FIG. 1) to the resist. Due to
the charging effects, the structures reproduced in FIG. 2 were observed.
EXAMPLE 1
Preparation of polydodecyloxythiophene
4.34 g of tetraethylammonium tetrafluoroborate, 5.36 g of
3-dodecyloxythiophene and 200 g of acetonitrile were placed in an
undivided electrolytic cell equipped with cooling jacket. The cathode
consisted of a stainless steel sheet 60 mm in length and 55 mm in width.
The anode used was a platinum sheet 60 mm in length and 55 mm in width. At
an electrolysis temperature of 20.degree. C. and an anodic current of 50
mA, a cell voltage of 3 to 6 volt was obtained. After a quarter of the
theoretically required amount of current had been consumed, the polymer
deposited on the anode was separated off by mechanical means, and the
anode was used again. This procedure was repeated until the theoretically
required amount of current had been consumed. The collected crude product
was comminuted mechanically, washed with water, dried, washed with pentane
and acetonitrile, and dried again. The product was taken up in
tetrahydrofuran, filtered through a sintered-glass crucible of pore size
G3, and the filtrate was evaporated to dryness on a rotary evaporator.
This gave 1.88 g of a blue-black shiny solid. Elemental analysis gave the
following values: 65.7% C, 9.0% H, 11.1% S, 5.3% F. A compressed powder
pellet of the milled product had a specific conductivity of
1.5.multidot.10.sup.-2 S/cm. A weight loss of less than 10% was observed
up to 255.degree. C. by DTG. DSC showed a maximum at 350.degree. C. (130
J/g). The average molecular weight of the undoped form determined by GPC
was about 5,400.
0.196 g of polymethyl methacrylate and 0.04 g of polydodecyloxythiophene,
prepared according to Example 1, were dissolved in 20 ml of
tetrahydrofuran and applied to p-doped silicon according to Comparative
Example A. The layer thickness was 380 nm and the specific conductivity
1.multidot.10.sup.-8 S/cm. It was also irradiated with 100 .mu.C/cm, at 50
kV. Development within 40 seconds in the developer described in
Comparative Example A gave transfer of the structure as in FIG. 1.
EXAMPLE 2
The resist material from Example 1 was applied to an insulating quartz
substrate according to Comparative Example B. Irradiation and development
as in Comparative Example A gave good structure transfer without
distortions according to FIG. 1.
EXAMPLE 3
Example 2 was repeated except that the polydodecyloxythiophene content was
increased to 5% by weight of the solid substance. The resulting specific
conductivity was 4.3.multidot.10.sup.-7 S/cm. The result was as in FIG. 1.
EXAMPLE 4
Preparation of polymethoxythiophene
4.34 g of tetraethylammonium tetrafluoroborate, 4.56 g of
3-methoxythiophene and 250 g of acetonitrile were placed in an undivided
electrolytic cell equipped with cooling jacket. The cathode consisted of
stainless steel sheets 80 mm in length and 55 mm in width. The anode used
was a carbon felt (weight per unit area about 0.3 kg/m.sup.2, specific
surface area (BET) about 1.5 m.sup.2 /g) 80 mm in length, 55 mm in width
and 3 mm in thickness (geometrical area on both sides about 90 cm.sup.2).
The anode was attached parallel to the cathode at a distance of 2 cm and
separated via a spacer made of polypropylene net. At an electrolysis
temperature of 20.degree. C. and an anodic current of 400 mA, a cell
voltage of 5 to 11 volt was obtained. After half of the theoretically
required amount of current had been consumed, the anode covered with the
oligomers was exchanged for a new one, and, after the theoretical amount
of current had been consumed, the electrolysis was stopped. The anodes
covered with the crude product were each dried immediately after the
exchange and placed in a bath containing methylene chloride and triturated
several times for an extended period of time. After repeated drying, the
carbon felts covered with the oligomers were triturated in a bath
containing acetonitrile until the oligomers had virtually entirely gone
into solution. The deep dark blue solution was evaporated to dryness in a
rotary evaporator. The crude product was comminuted mechanically, washed
with water, dried, triturated with methylene chloride for 12 hours, then
filtered off and dried. It was purified further by redissolving the
material obtained in acetonitrile and centrifuging it at 10,000 rpm for
0.5 hour, and evaporating the centrifugate to dryness in a rotary
evaporator. This gave 1.92 g of a bronze metallic shiny solid. Elemental
analysis gave the following values: 44.5% C, 3.2% H, 22.1% S, 9.4% F. A
compressed powder pellet of the milled product had a specific conductivity
of 1.8.times.10.sup.-3 S/cm. A weight loss of less than 10% was observed
up to 245.degree. C. by DTG. DSC showed a maximum at 325.degree. C. The
molecular ions of the pentamers (m/e=562) and hexamers (m/e=674) were
detected in the mass spectrum of the undoped form. GPC of the undoped form
showed that more than 80% of the product were pentamers and hexamers. In
the UV/VIS/NIR spectrum in tetrahydrofuran, the undoped pentamer had a
maximum at .lambda.=460 nm, and the undoped hexamer a maximum at
.lambda.=486 nm.
A solution of 0.02 g of polymethoxythiophene and 0.98 g of PMMA in a
solvent mixture of 7 ml of acetonitrile and 3 ml of tetrahydrofuran was
applied to an insulating substrate. After irradiation with 100
.mu.C/cm.sup.2 at 25 kV, development was carried out in methyl ethyl
ketone/isopropanol (1:4) for 60 seconds. The structures obtained were as
in FIG. 1.
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