 |
Description  |
|
|
The invention relates to a method of manufacturing a pattern of an
electrically conductive polymer on a substrate surface, a solution from
which the conductive polymer is formed being provided on the substrate
surface as a liquid layer, after which said liquid layer is optionally
dried, exposed to patterned radiation and then heated, thereby forming the
pattern of electrically conductive polymer. The invention also relates to
the metallization of such a pattern.
In general, organic polymers are electrical insulators and are therefore
used as insulation material in electrical and electronic components. It is
known that the polymer can become electrically conductive if it comprises
a poly-conjugated bond system consisting of a chain of, for example,
double bonds, triple bonds, aromatic or hetero-aromatic rings. This
conductivity is referred to as intrinsic conductivity. Examples of such
polymers are polyacetylene, polythiophene and polypyrrole. The
conductivity of these polymers is generally low, since said polymers are
semiconductors with a relatively large band gap of 1.5-4 eV. The
conductivity can be increased by, for example, (electro)chemically
oxidizing the polymer, termed doping, resulting in the formation of p-type
conductors. Suitable flopants for obtaining p-type conductance are, for
example, I.sub.2, AsF.sub.5, SbF.sub.5, HBF.sub.4, perchlorates,
sulphonates, SO.sub.3 and FeCl.sub.3. This doping causes the formation of
positive charge carriers on the polymer chains, which charges are
compensated by negatively charged counterions.
In general, polymers are cheap and can be provided on a substrate by means
of simple techniques, such as spin coating. Consequently, conductive
polymers are attractive for use in conductive and semiconductive
structures in (integrated) electronic circuits, printed circuit boards,
anti-static coatings and electromagnetic protective layers. To realise
completely organic electronic systems, interconnects based on conductive
polymers must be available.
A method of manufacturing a pattern of an electrically conductive polymer
is known from European Patent Application EP-A-399299. In said known
method, a mixture of a conjugated polymer, such as polyaniline, a
radiation-sensitive onium salt and a solvent is provided on a substrate
surface. After heating, the substrate surface is exposed to patterned
radiation with, for example, UV light and heated again. An example of an
onium salt is triphenylsulphonium hexafluoroantimonate. As a result of
said exposure to radiation, the onium salt generates free acid which
protonates the polymer, thereby forming a conductive polymer having
negative ions as counterions. The known method is a positive process. The
exposed areas of the polymer layer become electrically conductive, whereas
the non-exposed areas remain electrically non-conductive. The unexposed
areas are removed by treating them with a suitable solvent.
A drawback of the known method is that the specific conductivity of the
conductive polymer formed is too low, so that said conductive polymer
cannot be used as an interconnect in electronic circuits. The stated
maximum conductivity values are 0.01-0.1 S/cm. This specific conductivity
is also too low to provide, if desired, by electroplating polymer layers
of a customary small layer thickness with a metal layer of, for example,
copper for use in printed circuit boards.
It is an object of the invention to provide, inter alia, a simple method of
manufacturing an electrically conductive polymer pattern, which method is
a negative process in which the specific conductivity of the conductive
parts of the polymer pattern is higher than 0.1 S/cm and the specific
conductivity of the other ("non-conductive") parts of the polymer pattern
is at least a factor of 10.sup.4 lower, and in which the conductive
polymer pattern can be galvanically re-inforced with a metal layer.
According to the invention, this object is achieved by a method as
described in the opening paragraph, which is characterized in that the
solution also comprises an oxidation agent and a base, and in that
conductive polymer is formed in the unexposed areas and non-conductive
polymer is formed in the exposed areas. Non-conductive polymer is to be
understood to mean herein, a polymer whose specific conductivity is at
least a factor of 10.sup.4 lower than the specific conductivity of the
conductive polymer. The presence of an oxidation agent causes the specific
conductivity values of the polymers formed to be higher than they would be
if no oxidation agent were used. However, solutions comprising monomers or
undoper oligomers or polymers and, in addition, typical oxidation agents
used for doping, such as Fe(III) salts, are very unstable. When these
components are mixed, instantaneously doped polymers are formed in the
solution, so that the formation of a conductive polymer pattern on a
substrate is impossible. Surprisingly, it has been found that the reaction
rate of the polymerization reaction and/or doping reaction is lowered by
adding small quantities of a base to the solution. Depending on the
concentration of the base, the reaction can be suppressed completely at
room temperature. An effective base concentration enables solutions
comprising monomers, oligomers or polymers and a suitable oxidation agent
to remain stable, at least at room temperature, for approximately 12
hours: no polymerization takes place. These stable solutions enable thin
layers to be provided on a substrate by means of, for example, spin
coating. Following the heating of the layer, an electrically conductive
polymer is formed. If the spin coated layer of monomers or oligomers is
exposed, prior to heating, to for example UV light, surprisingly, a
non-conductive polymer is formed after heating. If the spin coated layer
is partially exposed, conductive polymer is formed, after heating, in the
unexposed areas and non-conductive polymer (i.e. having a specific
conductivity which is at least a factor of 10.sup.4 lower) is formed in
the exposed areas. Patterned irradiation via, for example, a mask results,
in accordance with this method, in conductive polymer patterns in the
unexposed areas and areas of insulating polymer in the exposed areas.
Thus, the process conducted in this manner is a negative process: the
exposed areas become non-conductive. Unlike other known methods, the
manufacture of a conductive polymer pattern in accordance with the
invention does not require a separate photoresist layer. As the conductive
and non-conductive areas of the polymer layer are situated in one plane, a
separate planarization layer is not necessary to further finish, for
example, an electronic circuit.
Suitable monomers for the method in accordance with the invention are
aromatic compounds, such as aniline and heterocyclic aromatic compounds,
such as pyrrole, furan and thiophene. To increase the solubility and
improve the processability, the monomers can be substituted with, for
example, alkyl groups or alkoxy groups. An example of such a compound is
3-dodecylthiophene. Suitable oligomers are, for example, terthiophene and
.alpha., .alpha.'-coupled undecathiophene substituted with four dodecyl
side chains, referred to in literature as T.sub.12 d.sub.4 (2,5,8,11),
i.e. 12 thiophene rings (T.sub.12) with 4 dodecyl side chains (d.sub.4) on
the rings 2,5,8 and 11. Oligomers of thiophene can more easily be doped
than thiophene itself. Suitable polymers for the method in accordance with
the invention are, for example, substituted or unsubstituted polyaniline
and polythiophene.
Suitable oxidation agents for the method in accordance with the invention
are, for example, soluble Fe(III) salts, such as
tris(toluenesulphonate)Fe(III), Cu(II)-salts, such as Cu(BF.sub.4).sub.2,
Ce(IV)-salts and Cr(III)-salts. A suitable organic oxidation agent is, for
example, 2,3-dichloro-5,6-dicyanoparabenzoquinone (DDQ). The choice of the
oxidation agent depends on the redox potential of the monomer, oligomer or
polymer used. In order to be able to suppress the polymerization/doping
reaction at room temperature, the redox potentials of the oxidation agent
and the monomer, oligomer or polymer must not differ too much.
Suitable soluble bases for this method are, for example, imidazole,
dicyclohexylamine and 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU).
A suitable embodiment of the method in accordance with the invention is
characterized in that 3,4-ethylenedioxythiophene is used as the monomer. A
solution of 3,4-ethylenedioxythiophene (EDOT), a suitable base quantity
and an Fe(III)-salt as the oxidation agent is stable at room temperature.
A layer of this solution can be provided on a substrate by spin coating.
Optionally the layer is subsequently dried and exposed to patterned
radiation. The polymerization reaction is initiated by heating to
approximately 110.degree. C., resulting in the formation of a conductive
polymer having a high specific conductivity of 300 S/cm in the unexposed
areas and a non-conductive polymer (specific conductivity 10.sup.-3 S/cm)
in the areas exposed to deep UV light. Following the exposure and heating
of the layer, the layer is extracted with, for example, methanol or
1-butanol. This extraction causes the reduced oxidation agent, for example
the Fe(II)-salt formed, to be removed from the layer. The above-mentioned
conductivity and difference in conductivity between exposed and unexposed
areas of at least a factor of 10.sup.4 are sufficient for the use as
interconnect and for a possible galvanic reinforcement with a metal layer.
Layers of conductive poly-3,4-ethylenedioxythiophene are stable when they
are exposed to UV light (.lambda.>300 nm). The temperature stability of
this conducting polymer is much better than that of other known polymers.
Besides, this conductive polymer hardly absorbs visible light, so that
this polymer can suitably be used as a transparent conductor. The ethylene
group of EDOT can be substituted with C.sub.1 -C.sub.12 the
above-mentioned alkyl groups or alkoxy groups. Also oligomers of EDOT,
whether or not substituted, can be used in the method in accordance with
the invention, an example of such an oligomer being the tetramer of
EDOT-C.sub.6 H.sub.13.
The specific conductivity of the conductive polymer pattern and the
difference in conductivity between exposed and unexposed areas of at least
a factor of 10.sup.4 are such that a metal layer can be grown on this
pattern in an electroplating bath. Consequently, the method in accordance
with the invention can particularly suitably be used for the manufacture
of metal tracks on insulating substrate surfaces, such as printed circuit
boards. The metal layer to be provided may for example consist of silver,
copper, nickel or chromium. For this purpose, customary, commercially
available electroplating baths of simple composition can be used. In this
manner, the use of electroless metallization baths is circumvented.
Electroless metallization baths have the drawback that expensive noble
metals (mostly palladium) are required to activate the substrate surface
and that the metallization baths used readily become unstable. Besides,
toxic reducing agents, such as formaldehyde, are often used in said baths.
A further drawback of the standard electroless method is that the
activation is non-selective, so that the deposited uniform metal layer
must subsequently be patterned photolithographically and by means of
chemical etching, which is a very time-consuming process. The method in
accordance with the invention makes it possible to provide conductive
metal patterns of, for example, copper on an electrically insulating
substrate in a simple manner by means of electrodeposition. For the
substrate use can be made of synthetic resins, glass, quartz and ceramics.
The invention will be explained in greater detail by means of exemplary
embodiments and drawings, in which
FIG. 1 shows the structural formula of 3,4-ethylenedioxythiophene (EDOT),
FIG. 2 shows the structural formula of the tetramer of EDOT-C.sub.6
H.sub.3, and
FIG. 3 shows the formula of imidazole.
Exemplary embodiment 1.
A quantity of 0.35 mmol 3,4-ethylenedioxythiophene (EDOT, supplier Bayer
AG, formula see FIG. 1) is mixed with a solution of 0.81 mmol
tris(toluenesulphonate)Fe(III) and 0.25 mmol imidazole (formula see FIG.
3) in 1.5 g 1-butanol. The solution obtained is stable for approximately
12 hours at room temperature. After filtration through a 0.5 .mu.m filter,
the solution is spin coated onto a silicon substrate. The layer obtained
is dried at 50.degree. C. for 3 minutes. The dried layer is exposed, via a
mask, to patterned radiation with UV light (.lambda.<300 nm) by means of a
Hg lamp and, subsequently, heated to 110.degree. C. for 1 minute. After
cooling, the Fe(II)-salt formed is extracted from the layer by means of
1-butanol. After extraction, the average layer thickness is 650 .ANG.. The
unexposed parts of the layer form a track pattern having a track width of
up to 1 .mu.m and comprise electrically conductive
poly-3,4-ethylenedioxythiophene having a specific electric conductivity of
300 S/cm. The exposed parts of the layer comprise a non-conductive polymer
having a specific conductivity of only 10.sup.-2 -10.sup.-4 S/cm. Thus,
this embodiment of the method is a negative process.
Both the conductive and the non-conductive polymers are transparent to
visible light; thus, this embodiment of the invention yields transparent
conductors. The conductive polymer which is doped in situ is surrounded by
areas of non-conductive polymer, rendering further planarization steps
superfluous. The conductive patterns are stable in UV light (.lambda.>300
nm) and, besides, have a higher thermal stability than the conductive
polymers known to date. After heating for 3 hours at 100.degree. C., the
specific conductivity is 200 S/cm, while heating for the same period of
time at 150.degree. C. causes the specific conductivity to decrease only
to 50 S/cm. After heating for only 1 minute at 110.degree. C., the dopant
is completely removed from the conductive polymer of, for example, the
thiophene oligomer T.sub.12.
Exemplary embodiment 2.
A solution is prepared which consists of 0.055 mmol of the tetramer of
EDOT-C.sub.6 H.sub.13 (formula see FIG. 2), 0.037 mmol 1,4-benzoquinone,
0.163 mmol orthonitrobenzyltosylate in 1.5 g tetrahydrofuran. The solution
is spin coated onto a substrate and dried for 1 minute at 90.degree. C.
The layer is exposed to patterned radiation with UV light (.lambda.<300
nm). After heating for 2 minutes at 140.degree. C. the specific
conductivity of the unexposed areas is 10 S/cm, whereas the exposed areas
remain electrically insulating (resistance>10.sup.9 /.quadrature.;
specific conductivity 10.sup.-5 S/cm). This method enables conductive
polymer patterns having a line width of 5 .mu.m to be obtained.
Exemplary embodiment 3.
A solution is prepared which consists of 0.35 mmol
3,4-ethylenedioxythiophene, 0.25 mmol imidazole and 0.81 mmol
tris(toluenesulphonate)Fe(III) in 1.5 g 1-butanol. After filtration
through a 0.5 .mu.m filter, the solution is spin coated onto a beaded PMMA
(polymethyl methacrylate) substrate. The layer obtained has a thickness of
approximately 1 .mu.m. Parts of the layer are exposed to deep UV light
(.lambda.<300 nm). Subsequently, the layer is polymerized to
poly-3,4-ethylenedioxythiophene by heating at 110.degree. C. for 1 minute.
After cooling, the layer is extracted with 1-butanol, causing the layer
thickness to decrease to 700-1000 .ANG.. The unexposed areas of the layer
have a resistance of 0.4 k.OMEGA./.quadrature.. The exposed areas of the
layer have a conductivity which is at least a factor of 10.sup.4 lower.
The layer is then copper-plated in an aqueous copper bath containing 0.5
molar CuSO.sub.4 and 0.5 molar H.sub.2 SO.sub.4 at room temperature. The
current density is 0.1 A/cm.sup.2. Copper is deposited only on the
conductive (i.e. unexposed) parts of the polymer layer. The copper layer
bonds very well to the polymer layer.
The method in accordance with the invention enables polymer patterns to be
manufactured in a polymer layer, the specific conductivity of the
conductive parts of the polymer pattern being at least 0.1 S/cm and being
a factor of 10.sup.4 higher than the specific conductivity of the
non-conductive pans. In accordance with the invention, the conductive
polymer pattern can be galvanically provided with a metal layer, thereby
forming a corresponding metal pattern. Thus, the method in accordance with
the invention can very suitably be used for the additive manufacture of
metal patterns and, hence, is a favouable alternative to electroless
metallization.
* * * * *
|
|
 |
|
 |
Description |
|
