U.S. patent application number 11/030254 was filed with the patent office on 2005-07-28 for photoimageable composition.
This patent application is currently assigned to SANDIA NATIONAL LABORATORIES. Invention is credited to Dentinger, Paul Michael, Krafcik, Karen L., Simison, Kelby Liv.
Application Number | 20050164125 11/030254 |
Document ID | / |
Family ID | 34134629 |
Filed Date | 2005-07-28 |
United States Patent
Application |
20050164125 |
Kind Code |
A1 |
Dentinger, Paul Michael ; et
al. |
July 28, 2005 |
Photoimageable composition
Abstract
The use of photoacid generators including an
alkoxyphenylphenyliodonium salt and/or bis(t-butylphenyl)iodonium
salt in a photoimageable composition helps improve resolution.
Suitable photoimageable compositions includes: (a) a
multifunctional polymeric epoxy resin that is dissolved in an
organic solvent wherein the epoxy resin comprises oligomers of
bisphenol A that is quantitatively protected by glycidyl ether and
wherein the oligomers have an average functionality that ranges
from about 3 to 12; and a photoacid generator comprising an
alkoxyphenylphenyliodonium salt and/or bis(t-butylphenyl)iodonium
salt. Preferred alkoxyphenylphenyliodonium salts include
4-octyloxyphenyl phenyliodonium hexafluoroantimonate and
4-methoxyphenyl phenyliodonium hexafluoroantimonate. The
photoimageable composition is particularly suited for producing
high aspect ration microstructure.
Inventors: |
Dentinger, Paul Michael;
(Sunol, CA) ; Krafcik, Karen L.; (Livermore,
CA) ; Simison, Kelby Liv; (Hattiesburg, MS) |
Correspondence
Address: |
Charles H. Jew
FLIESLER MEYER LLP
Fourth Floor
Four Embarcadero Center
San Francisco
CA
94111-4156
US
|
Assignee: |
SANDIA NATIONAL
LABORATORIES
Livermore
CA
|
Family ID: |
34134629 |
Appl. No.: |
11/030254 |
Filed: |
January 6, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11030254 |
Jan 6, 2005 |
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10125134 |
Apr 17, 2002 |
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6858378 |
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Current U.S.
Class: |
430/311 |
Current CPC
Class: |
Y10S 430/115 20130101;
G03F 7/0045 20130101; G03F 7/0046 20130101; G03F 7/40 20130101;
C07C 381/12 20130101; Y10S 430/126 20130101; G03F 7/038
20130101 |
Class at
Publication: |
430/311 |
International
Class: |
G03C 001/492 |
Goverment Interests
[0002] This invention was made with Government support under
Contract No. DE-AC04-94AL85000 awarded by the U.S. Department of
Energy to Sandia Corporation. The Government has certain rights to
the invention.
Claims
What is claimed is:
1. A method of forming a template or mold suitable for fabricating
a microstructure that comprises the steps of: (a) forming a layer
of photoimageable composition on a substrate surface wherein the
photoimageable composition comprises: (i) a multifunctional
polymeric epoxy resin that is dissolved in an organic solvent
wherein the epoxy resin comprises oligomers of bisphenol A that are
quantitively protected by glycidyl ether and wherein the oligomers
have an average functionality that ranges from about 3 to 12; and
(ii) a photoactive compound comprising an
alkoxyphenylphenyliodonium salt and/or bis(t-butylphenyl)iodonium
salt; (b) exposing the layer of photoimageable composition to a
pattern of radiation which produces a catalyst capable of changing
the photoimageable composition's susceptibility to a developer; and
(c) applying a developer to remove nonexposed portions of the
photoimageable compound which are susceptible to the developer
thereby creating a layer defining one or more open patterns therein
wherein the one or more open patterns have non-linear
dimensions.
2. The method of claim 1 wherein the photoactive compound is an
alkoxyphenylphenyliodonium salt.
3. The method of claim 1 wherein the photoactive compound is
selected from the group consisting of 4-octyloxyphenyl
phenyliodonium hexafluoroantimonate (OPI HFA), 4-methoxyphenyl
phenyliodonium hexafluoroantimonate (MPI HFA), methide salts
thereof and mixtures thereof.
4. The method of claim 1 wherein the multifunctional polymeric
epoxy resin comprises a bisphenol A novolac glycidyl ether.
5. The method of claim 1 wherein step (b) comprises exposing the
layer of photoimageable composition with a pattern of ultraviolet
radiation.
6. The method of claim 1 wherein the amount photoactive compound
present is about 1 to 12 parts by weight based on 100 parts by
weight of the resin.
7. The method of claim 1 wherein the photoactive compound is
4-octyloxyphenyl phenyliodonium hexafluoroantimonate.
8. The method of claim 1 wherein the photoactive compound is
4-methoxyphenyl phenyliodonium hexafluoroantimonate.
9. The method of claim 1 wherein the photoactive compound is
bis(t-butylphenyl)iodonium hexafluoroantimonate or methide salts
thereof.
10. The method of claim 1 wherein the oligomers have an average
functionality of about 8.
11. The method of claim 1 wherein the one or more open patterns
formed in step (c) are suitable for fabricating a microstructure
wherein at least one of its height, width, diameter or length is
less than about 100 microns.
12. A method of fabricating a metal structure, which has an aspect
ratio of from 0.1 to 70, that comprises the steps of: (a) forming a
layer of photoimageable composition on a substrate surface wherein
the photoimageable composition comprises: (i) a multifunctional
polymeric epoxy resin that is dissolved in an organic solvent
wherein the epoxy resin comprises oligomers of bisphenol A that are
quantitatively protected by glycidyl ether and wherein the
oligomers have an average functionality that ranges from about 3 to
12; and (ii) a photoacid generator comprising an
alkoxyphenylphenyliodonium salt and/or bis(t-butyphenyl)iodonium
salt; (b) exposing the layer of photoimageable composition to a
pattern of radiation which changes the photoimageable composition's
susceptibility to a developer; (c) applying a developer to remove
nonexposed portions of the photoimageable composition which are
susceptible to the developer to create a mold area within an
exposed portion of the photoimageable composition; (d) depositing a
metal into the mold area; and (e) removing the exposed
photoimageable composition to yield the metal structure.
13. The method of claim 12 wherein step (d) comprises
electroplating a metal into the mold area.
14. The method of claim 12 wherein the photoacid generator is an
alkoxyphenylphenyliodonium salt.
15. The method of claim 12 wherein the photoacid generator is
selected from the group consisting of 4-octyloxyphenyl
phenyliodonium hexafluoroantimonate (OPI HFA), 4-methoxyphenyl
phenyliodonium hexafluoroantimonate (MPI HFA), methide salts
thereof, and mixtures thereof.
16. The method of claim 12 wherein the multifunctional polymeric
epoxy resin comprises a bisphenol A novolac glycidyl ether.
17. The method of claim 12 wherein step (b) comprises exposing the
layer of photoimageable composition with a pattern of ultraviolet
radiation.
18. The method of claim 12 wherein the metal structure formed has a
nonlinear surface.
19. The method of claim 12 wherein the metal structure formed has a
curved surface.
20. The method of claim 12 wherein the amount photoacid generators
present is about 1 to 12 parts by weight based on 100 parts by
weight of the resin.
21. The method of claim 12 wherein the photoacid generator is
4-octyloxyphenyl phenyliodonium hexafluoroantimonate.
22. The method of claim 12 wherein the photoacid generator is
4-methoxyphenyl phenyliodoniu m hexafluoroantimonate.
23. The method of claim 12 wherein the photoacid generator is
bis(t-butylphenyl)iodonium hexafluoroantimonate or methide salts
thereof.
24. The method of claim 12 wherein the oligomers have an average
functionality of about 8.
25. The method of claim 12 wherein the metal structure formed in
step (e) has non-linear dimensions.
26. The method of claim 25 wherein the metal structure defines a
three-dimensional solid structure wherein at least one of its
height, width, diameter or length is less than about 100 microns.
Description
[0001] This application is a continuation application of U.S.
patent application Ser. No. 10/125,134, filed Apr. 17, 2002,
entitled PHOTOIMAGEABLE COMPOSITION by Paul Dentinger, et al.
(Attorney Docket No.: SAND-01082US0).
FIELD OF THE INVENTION
[0003] The present invention is directed to photoimageable
compositions of improved composition that exhibit superior
resolution. The invention is also directed to methods of
fabricating microstructure metal parts using the photoimageable
composition and particularly to fabricating microstructures having
non-linear features.
BACKGROUND OF THE INVENTION
[0004] There are a variety of applications for which thick-film
lithography may prove advantageous or even necessary. For instance,
thick-film lithography may be used as a plating mold to create
metal parts as in the LIGA process. Additionally, there may be
applications in thick microelectro mechanical systems or
lithograpically defined analytical systems such as, for example,
chromatography columns or mass spectrometers. Typically,
photoresist films greater than 50-100 microns thick are exposed
with a synchrotron source; the hard x-rays produced assure good
transmission within the photoresist and low diffraction from the
mask. In addition, the low run-out synchrotron sources produces
sharp side walls, but synchrotron sources are scarce and expensive.
Moreover, long exposure times on the order of hours to days are
typical.
[0005] Diazonapthoquinone/novolac (DNQ/novolac) resists that are
exposed with ultraviolet radiation are used in microcircuit
manufacturing. However, these materials suffer from several
drawbacks when used in thick (.gtoreq.50 .mu.m) films. DNQ produces
nitrogen gas upon exposure which phase separates prior to diffusing
from thick films to create bubbles. Novolac materials from highly
absorbing quinones, and the DNQ direct photolysis mechanism
typically results in photoresist formulations exhibiting low
transmittance. Careful bake steps are required to remove the
casting solvent to avoid thermally inducing reactions with the DNQ.
In addition, DNQ requires water for proper formation of the
soluble, photoproduced acid which leads to requiring long
reabsorption times after the initial post-apply bake (PAB). Finally
novolac materials have a tendency to crack, which is particularly
problematic for thick films.
[0006] A chemically-amplified negative resist, available from
MicroChem Corp., Newton, Mass., under the tradename SU-8,
circumvents many of these problems. The resist includes monomers
and oligomers of bisphenol A, which have been quantitatively
protected with glycidyl ether, and a photoacid generator (PAG). UV
exposure creates a strong acid which cationically crosslinks the
oligomers during a post-exposure bake (PEB) step to form a highly
crosslinked network. The resist exhibits high transmission, creates
no gas during exposure, and is thermally stable. UV exposures of
the resist are typically on the order of minutes, and the cured
product provides the best imaging resolution of known resists.
However, drawbacks of this resist include solvent development,
shrinkage of the cured material, and when used in very thick films,
absorption of radiation can result in degraded sidewall profiles.
These drawbacks result in poor resolution. The cured resist is also
difficult to remove.
SUMMARY OF THE INVENTION
[0007] The invention is based in part on the demonstration that the
use of alkoxyphenylphenyliodonium salt and/or
bis(t-butylphenyl)iodonium salt photoacid generators in a
photoimageable composition improves resolution.
[0008] In one aspect, the invention is directed to a photoimageable
composition that includes;
[0009] (a) a multifunctional polymeric epoxy resin that is
dissolved in an organic solvent wherein the epoxy resin comprises
oligomers of bisphenol A that are quantitatively protected by
glycidyl ether and wherein the oligomers have an average
functionality that ranges from about 3 to 12; and
[0010] (b) a photoacid generator comprising an
alkoxyphenylphenyliodonium salt and/or bis(t-butylphenyl)iodonium
salt.
[0011] Preferred photoacid generators include 4-octyloxyphenyl
phenyliodonium hexafluoroantimonate, 4-methoxyphenyl phenyliodonium
hexafluoroantimonate, bis(t-butylphenyl)iodonium
hexafluoroantimonate, and methide salts of these.
[0012] In another aspect, the invention is directed to a method of
fabricating microstructure that includes the steps of:
[0013] (a) forming a layer of photoimageable composition on a
substrate surface wherein the photoimageable composition
comprises:
[0014] (i) a multifunctional polymeric epoxy resin that is
dissolved in an organic solvent wherein the epoxy resin comprises
oligomers of bisphenol A that are quantitatively protected by
glycidyl ether and wherein the oligomers have an average
functionality that ranges from about 3 to 12; and
[0015] (ii) a photoactive compound comprising an
alkoxyphenylphenyliodoniu- m salt;
[0016] (b) exposing the layer of photoimageable composition to a
pattern of radiation which produces a catalyst capable of changing
the photoimageable composition's susceptibility to a developer;
and
[0017] (c) applying a developer to remove non-exposed portions of
the photoimageable compound which are susceptible to the
developer.
[0018] In a further aspect, the invention is directed to a method
of fabricating a metal structure that includes the steps of:
[0019] (a) forming a layer of photoimageable composition on a
substrate surface wherein the photoimageable composition
comprises:
[0020] (i) a multifunctional polymeric epoxy resin that is
dissolved in an organic solvent wherein the epoxy resin comprises
oligomers of bisphenol A that is quantitatively protected by
glycidyl ether and wherein the oligomers have an average
functionality that ranges from about 3 to 12; and
[0021] (ii) a photoacid generator comprising an
alkoxyphenylphenyliodonium salt and/or bis(t-butylphenyl)iodonium
salt.
[0022] (b) exposing the layer of photoimageable composition to a
pattern of radiation which changes the photoimageable composition's
susceptibility to a developer;
[0023] (c) applying a developer to remove non-exposed portions of
the photoimageable composition which are susceptible to the
developer to create a mold area within exposed portions of the
photoimageable composition;
[0024] (d) depositing a metal into the mold area; and
[0025] (e) removing the exposed photoimageable composition to yield
the metal structure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIGS. 1A, 1B, 1C, and 1D show the structure of 23
photoinitiators that were tested.
[0027] FIG. 2 is a SEM photograph of a trench that is formed in
resist material.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] Photoimageable compositions of the present invention
generally comprise (a) multifunctional polymeric epoxy resin that
is dissolved in an organic solvent wherein the epoxy resin
comprises oligomers of bisphenol A that are quantitatively
protected by glycidyl ether and wherein the oligomers have an
average functionality that ranges from about 3 to 12 and preferably
about 8; and (b) a photoacid generator comprising an
alkoxyphenylphenyliodonium salt and/or bis(t-butylphenyl)iodonium
salt.
[0029] Preferably, the multifunctional polymeric epoxy resin
comprises a bisphenol A novolac glycidyl ether having the following
structure (I): 1
[0030] The multifunctional polymeric epoxy resin typically
comprises about 80% to 99% and preferably about 93% to 99% by
weight of the photoimageable composition.
[0031] A photoactive compound (PAC) is a compound that undergoes a
photochemical transformation upon absorption of photon. Photoacid
generators (PAG) and photobase generators generate upon absorption
of a photon an acid or base, respectively. A PAG has a chromophore
and an acid salt. The salt can be any suitable anion, but
preferably is an anion selected from tetrafluoroborate,
hexafluorophosphate, hexafluoroarsenate, hexafluoroantimonate,
trifluoromethane sulfonate, perfluorotetraphenylbor- ate or
fluorinated methides.
[0032] Photoimageable compositions of the present invention employ
one or more photoacid generators wherein the chromophore comprises
and alkoxyphenylphenyl iodonium and/or bis(t-butylphenyl)iodonium
cation. The PAC typically comprises about 1% to 10% and preferably
about 1 % to 6% by weight based on the weight of the
multifunctional polymeric epoxy resin.
[0033] Prior to discussing this invention in further detail, the
following terms will first be defined.
[0034] The term "alkoxy" refers to the groups alkyl-O--,
alkenyl-O--, cycloalkyl-O--, cycloalkenyl-O--, and alkynyl-O--,
where alkyl, alkenyl, cycloalkyl, cycloalkenyl, and alkynyl ,
cycloalkyl, cycloalkenyl, and alkynyl are as defined herein.
Preferred alkoxy groups are alkyl-O-- and include, by way of
example, methoxy, ethoxy, n-propoxy, iso-propoxy, n-butoxy,
tert-butoxy sec-butoxy, n-pentoxy, n-hexoxy, 1,2-dimethylbutoxy,
and the like.
[0035] The term "alkyl" refers to monovalent alkyl groups
preferably having from 1 to 20 carbon atoms and more preferably 1
to 12 carbon atoms. This term is exemplified by group such a
methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, n-hexyl,
and the like.
[0036] The term "alkenyl" refers to alkenyl groups preferably
having from 1 to 20 carbon atoms and more preferably 1 to 12 carbon
atoms and having at least 1 and preferably from 1-2 sites of
alkenyl unsaturation. Preferred alkenyl groups include ethenyl
(--CH.dbd.CH.sub.2), n-propenyl (--CH.sub.2CH.dbd.CH.sub.2),
iso-propenyl (--C(CH.sub.3).dbd.CH.sub.2), and the like.
[0037] The term "alkynyl" refers to alkynyl groups preferably
having from 1 to 20 carbon atoms and more preferably 1 to 12 carbon
atoms and having at least 1 and preferably from 1-2 sites of
alkynyl unsaturation. Preferred alkynyl groups include ethynyl
(--C.dbd.CH.sub.2), propargyl (--CH.sub.2C.dbd.CH) and the
like.
[0038] The term "cycloalkyl" refers to cyclic alkyl groups of from
3 to 20 carbon atoms having a single cyclic ring or multiple
condensed rings. Such cycloalkyl groups include, by way of example,
single ring structures such as cyclopropyl, cyclobutyl,
cyclopentyl, cyclohexyl, cyclooctyl, and the like, or multiple ring
structures such as adamantanyl, and the like.
[0039] The term "cycloalkenyl" refers to cyclic alkenyl groups of
from 4 to 8 carbon atoms having a single cyclic ring and at least
one point of internal unsaturation. Examples of suitable
cycloalkenyl groups include, for instance, cyclobut-2-enyl,
cyclopent-3-enyl, cyclooct-3-enyl and the like.
[0040] The term "methide" refers to (R.sub.fSO.sub.2).sub.3C--
wherein each R.sub.f is independently selected from the group
consisting of highly fluorinated or perfluorinated alkyl or
fluorinated aryl radicals. The methides may also be cyclic, when a
combination of any two R.sub.f groups are linked to form a bridge.
The R.sub.f alkyl chains may contain from 1-20 carbon atoms, with
1-4 carbon atoms preferred. The R.sub.f alkyl chains may be
straight, branched, or cyclic and preferably are straight. When
R.sub.f is or contains a cyclic structure, such structure
preferably has 5 or 6 ring members, 1 or 2 which can be
heteroatoms. The alkyl radical R.sub.f is also free of ethylenic or
other carbon-carbon unsaturation: e.g., it is a saturated
aliphatic, cycloaliphatic or heterocyclic radical.
[0041] By "highly fluorinated" is meant that the degree of
fluorination on the chain is sufficient to provide the chain with
properties similar to those of a perfluorinated chain. More
particularly, a highly fluorinated alkyl group will have more than
half the total number of hydrogen atoms on the chain replaced with
fluorine atoms. Although hydrogen atoms may remain on the chain, it
is preferred that all hydrogen atoms be replaced with fluorine to
form a perfluoroalkyl group, and that any hydrogen atoms beyond at
least half replaced with fluorine that are not replaced with
fluorine be replaced with bromine and or chlorine. It is more
preferred that at least two out of three hydrogen on the alkyl
group be replace with fluorine, still more preferred that at least
three of four hydrogen atoms be replaced with fluorine and most
preferred that all hydrogen atoms be replaced with fluorine to form
a perfluorinated alkyl group.
[0042] The fluorinated aryl radicals of the methide structure may
contain from 6 to 22 ring carbon atoms, preferably 6 ring carbon
atoms, where at least one, and preferably at least two, ring carbon
atoms of each aryl radical is substituted with a fluorine atom or a
highly fluorinated or perfluorinated alkyl radical as defined
above, e.g., CF.sub.3. Suitable methide anions are further
described in U.S. Pat. No. 5,554,664 which is incorporated
herein.
[0043] Preferably, the alkoxyphenylphenyliodonium salt has the
following structure: 2
[0044] wherein each of R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5,
R.sup.6, R.sup.8, R.sup.9 and R.sup.10 is independently selected
from the group consisting of --H, alkyl-O--, alkenyl-O--,
cycloalkyl-O--, cycloalkenyl-O--, and alkynyl-O--, with the proviso
that not all are --H; and A is a suitable anion.
[0045] In a preferred embodiment, R.sup.3 and/or R.sup.8 are
alky-O-- and the remaining substituents are hydrogen. A preferred
alkyl-O-- is n-octyl-O--. In another preferred embodiment, R.sup.3
and/or R.sup.8 are t-butyl and remaining substituents are hydrogen.
Preferred anions include, for example, tetrafluoroborate,
hexafluorophosphate, hexafluoroarsenate, hexafluoroantimonate,
trifluoromethane sulfonate, perfluorotetraphenyl borate or
fluorinated methides.
[0046] Optionally the photoimageable composition can also include
an amine that is selected from the group consisting of
triisobutylamine (TIBA), 1,8-bis(dimethylamino)naphthalene
1,8-bis(dimethylamino)naphthalene (also know as PROTON SPONGE.TM.)
(PS), 2,2'-diazabicyclo [2.2.2] octane (DABCO) and mixtures
thereof. The amount of amine used is about 0.1 mol % to 25 mol %
and preferably about 1 mol % to 15 mol % based on the moles of PAC
present. The presence of the amine reduces process bias.
[0047] The organic solvent comprises any suitable solvent for the
components set forth above. Typical solvents include, for example,
propylene glycol methyl ether acetate (PGMEA),
.gamma.-butyrolactone and cyclopentanone. The solvent typically
comprises about 10% to 80% and preferably about 25% to 50% by
weight of the photoimageable composition.
[0048] The photoimageable composition of the present invention can
be formulated by adding the PAC to a polymer mixture. The
photoimageable composition of the present invention can be employed
in the same manner as conventional negative resist compositions but
with improved results. For example, the photoimageable composition
can be applied (e.g. spun) onto a substrate surface; the thickness
of the film is determined by the composition's viscosity. A
post-apply bake (PAB) step removes solvent from the film after
coating. The photoimaging mechanism is initiated when radiation
(e.g. ultraviolet radiation) is directed to regions of film to
cause the production of photoacids or photobases which act as a
catalyst in the subsequent crosslinking reaction that takes place
during the post-exposure bake (PEB) step. The photoimageable
composition is referred to as a negative photoimageable composition
in that the exposed portions are not susceptible to removal by the
developer while the unexposed composition portions are.
[0049] Photoimageable compositions of the present invention are
particularly suited for making metal parts that have non-linear
dimensions and/or high aspect ratio of from 0.1 to 70. In
particular, the compositions can be employed to manufacture
microstructures or microcomponents which refer to three-dimensional
solid structures whose critical features, height, width (or
diameter) or length is less than about 100 microns, i.e., at least
one dimension of the three-dimensional structure is less than about
100 microns. It has been demonstrated that the inventive
photoimageable composition significantly improves the resolution
associated with prior art resists.
[0050] Microstructures can be fabricated using LIGA processes.
"LIGA" is a German acronym for a process involving X-ray
lithography, electroplating, and plastic molding. Conventional
sources of radiation including X-ray and ultraviolet radiation can
be used with the inventive photoimageable composition. A typical
LIGA process involves applying a layer of photoimageable
composition onto a suitable substrate. The thickness of the layer
is typically equal to or greater than the desired height of the
microstructure. The photoimageable composition is then positioned
behind a patterned mask and exposed to a collimated ultraviolet
radiation. After exposure, a developer dissolves the non-irradiated
areas. The resulting template or mold is then used to electroplate
microstructures on the electroplating based. When electroplating is
completed, the wafer is planarized, and the remaining polymer is
removed to produce the microstructure.
[0051] One embodiment of photoimageable composition of the present
invention can be formulated by adding appropriate photoacid
generator(s) to a polymer formulation that is commercially
available as SU8 R50 from MicroChem Corp. This commercial
composition contains a resist mixture of monomers and oligomers of
bisphenol A that are quantitatively protected by glycidyl ether and
has an average functionality of 8 represented by structure (I)
shown above. The solids content of the resist mixture is about 69%
by weight. The composition SU8 R50 does not contain a photoacid
generator. While the invention will be illustrated being SU8 R50 it
is understood that the photoacid generators of the present
invention will also improve resolution in other photoimageable
compositions containing the requisite components as defined
above.
[0052] Experimental
[0053] Various photoimageable compositions containing different
photoacid generators (PAGs) were prepared and tested to demonstrate
the superior properties (i.e., resolution) attained by using the
alkoxyphenylphenyliodonium salt and/or bis(t-butylphenyl)iodonium
salt. The photoimageable compositions with the PAGs were formulated
with SU8 R50 as the nominal composition. The structures and
acronyms of the PAGs tested are shown in FIGS. 1A, 1B, 1C, and 1D.
As noted in the figures, the PAGs were obtained form commercial
sources including: Midori Kagaku Ltd. (MK) (Tokyo, Japan), Toyo
Gosei (TGK) (Tokyo, Japan), GE Silicones Division (GE), UCB
Chemicals, 3M, Rhodia Silicones (RS), and Aldrich via Ciba Geigy
Specialty Chemicals (CG). Typically, a mixture of 2% PAG to SU8 R50
solids was rolled overnight on a hematology mixer. Samples which
did not dissolve easily were rolled overnight at about 65.degree.
C.
[0054] Imaging experiments were done with a standard resolution
test pattern. The pattern contains both tones of elbows, isolated
square posts, isolated supported and unsupported lines, and
"bullseye" patterns. The features range in size from 80, 50, 40,
35, 30, 25, 20, 17.5, 15, 12.5, 10, 8, 6, 5, 4, 3, and 2 .mu.m.
Formulations were spun at 1500 rpm for 15 sec., baked at 80.degree.
C. for 5 min. and then subject to an edge bead removal with
streaming acetone at approximately 900 rpm. The post-apply bake
(PAB) step was from room temperature (RT) to 95.degree. at
2.degree. C./min., hold for 7 min., and then cool to RT at
1.degree. C./min. Exposure was done with broadband radiation (about
330-450 nm) in hard or soft contact on a Karl Suss MA/BA 6 aligner
with reported doses measured at 365 nm. Four sites per wafer were
exposed. Post-exposure bake (PEB) was done from 65.degree. to
85.degree. C. at 2.degree. C./min., hold for 10 min., and then
decrease at 1.degree. C./min. to RT. Development was immersion in a
crystallization dish with very gentle to no agitation at 3 times
the clearing time for the bulk fields.
[0055] After processing, the wafers were inspected with an optical
microscope for resolution and other detrimental effects such as
cracking, peeling, etc. If appropriate , another wafer was
typically exposed to get a more accurate dose. Particularly
promising PAGs were formulated with several different
concentrations and conditions for additional testing.
[0056] The following table 1 shows the compiled results for the PAG
experiments.
1 TABLE 1 Dose Number (mJ/cm.sup.2) Imaging Comments 1 600 Looked
very good. High resolution, some cracking 2 >>6000 No latent
image even at 6000. 3 10,600 Showed latent image at 6000, where 2
did not, images poor. Only 100 .mu.m OK. Some dewetting in two
layer experiment. 3.7 wt. % 4 >9000 Horrible images, exceedingly
slow 5 6000 Slow, lots of cracking, but resolution very good. 6
1600 Images quite good, but not as sharp as 5, much faster though.
7 Moderate doses, but lines sloped, pyramidal. 8 Lots of cracking,
dose pretty good, images pretty good, but clearly inferior to 1, 9
9 1700 Excellent images. Good adhesion, low cracking. Follow up
experiments consistently better imaging by several people. 10 1500
UV 9380 C Images are quite nice, but there are bubbles and cracks
everywhere. This formulation has ITX and a reactive diluent. It is
quite impure. 11 DNE UV93850 C Orange peeled and dewetted after
PAB. Supposedly the same as UV 9380C but without the ITX. 12 7200
Decent resolution, but resist cracked and peeled badly. 13
>>4000 Barely image at 4000, should be much higher dose than
12, 14 1500 Good images, but lots of cracking. Follow up experiment
at higher PAG concentration had peeling, images not as good as
commercial material. 15 1100 Reasonable resolution, but dewetting
was quite bad. 16 >>5000 17 >>5000 Same as 16 18
>12,000 Very slow even with fast acting acid. Did not dissolve
in SU- 8/solvent solution. 20 >>1600 Expect that the acid is
not strong enough. 21 Phase separation in solid. 22 Did not
dissolve in SU- 8/solvent solution. 23 >>3400 SU-8 >800
Commercial formulation shows good resolution. Other formulations
qualitatively compared to this formulation.
[0057] There are three major conclusions from Table 1. The first is
that only certain acids appear to be of sufficient "strength" to
catalyze the epoxy reaction for imaging SU-8. While certain acids
may catalyze epoxy reactions in general, for imaging, the resultant
crosslinked film must withstand the development step without
swelling, cracking, delaminating, etc. PAGs 1-4 are a direct
comparison of the affect of the acid generated if we assume that
the chromophore alone determines acid generation efficiency. Also,
5, vs. 6, 12 vs. 16, and PAGs 7 and 8 yield information on the type
of acid required. It is clear from the homologues 1-4 that the
hexafluoroantimonate is a particularly active acid, that the
perfluorobutane sulfonate is somewhat active but slow and results
in poor imaging and that the others are not active enough to
reasonably crosslink the material at these conditions. Comparing
PAGs 5 and 6 shows that the methide PAG appears to be significantly
more active than even the hexafluoroantimonate. Comparing 12 and 16
confirm that the hexafluoroantimonate is considerably more active
than the perfluorobutane sulfonate. PAGs 7 shows that the
perfluorinated tetraphenyl borate is sufficiently strong as well.
However, since the material is a formulated commercial product, it
cannot be ruled out that other photoactive sensitizers are present.
Finally, the hexafluorophosphate acid generated from PAG 8 is also
sufficiently strong to catalyze the reaction. This is similar to
the known work that the hexafluorophosphate analogue to the PAG
used in the commercial SU-8 formulation (a hexafluoroantimonic acid
generated from a mixture of triphenylsulfonium salts) is also
capable of catalyzing the reaction sufficient for imaging though
with some loss of resolution. The failure of formulations made from
PAGs 20 and 23 could be entirely due to their weak generated acids,
as both generate acids that are considerably weaker than the
perfluoroalkane sulfonate counterparts.
[0058] It is clear that only some types of acids can effectively
crosslink the SU-8 sufficient to withstand the solvent development
step. It is not clear exactly why the hexafluoroantimonate acid
creates such high resolution patterns, though its reactivity must
be considered important. However, it is also clear that the methide
acid is considerably faster but did no seem to provide as good a
resolution at least with the bis t-butylphenyl iodonium
chromophore.
[0059] Another significant conclusion from the data can be obtained
by considering only the chromophore. PAGs 1, 5, 8. 9-15 all
generate hexafluoroantimonic acid. The series provides insight into
the mechanism of acid generation in SU-8 polymer. All of the
iodonium chromophores shown have particularly low absorbance in the
wavelength range of interest. However, it is known that the
iodonium compounds are more susceptible to radiationless energy
transfer from the matrix to the PAG, so they appear to be far more
sensitive than the sulfonium counterparts. In particular, the
alkoxyphenylphenyliodonium chromophore produces particularly fast
photoresists in this matrix and both PAG 1 and 9 show excellent
images. While the alkoxyphenylphenyliodonium chromophore shows red
shifted absorbance relative to the bis(t-butyl) phenyl analogue,
the alkoxyphenylphenyliodonium chromophores themselves do not
absorb heavily in the wavelengths of interest (330 nm and longer
wavelengths are the only ones that transmit through the optics and
mask). However, it has been determined experimentally that the
alkoxyphenylphenyliodonium chromophores are more likely to receive
energy from the matrix than their alkyl phenyl counterparts. One
very significant advantage of the alkoxyphenyl phenyl iodonium
chromophores, then, is the ability to print extremely thick
structures. This arises from the absorbance of the entire
formulation being low, while maintaining adequate photospeed for
throughput considerations. PAGs 7 and 10, however, show that the
alkylphenylphenyliodonium salts are capable of reasonably high
sensitivity, though these commercial formulations are likely
sensitized with compound such as isopropyl-9H-thioxanthen-9-one
(ITX). The sensitizers may be useful at boosting photospeed but
will contribute to the overall absorbance of the film, and hence
could limit the total thickness of the parts.
[0060] PAGs 12, 14, and 15 show that it is likely that the
sulfonium counterparts require direct absorption for acid to be
released. Both PAGs 14 and 15 have appreciable absorption in the
wavelengths of interest and are reasonably fast. However, PAGs 12
and 13 are not highly absorbing and result in particularly slow
resists. That sulfonium PAGs are more difficult to sensitize by
their matrices has been shown elsewhere.
[0061] A third conclusion is that no particular structure is
obviously affecting the other imaging requirements such as
dewetting, adhesion, cracking of the film, etc. It is clear that
there are several desirable features of the chromophore such as
high transmission with good sensitivity afforded by the
alkoxyphenylphenyliodonium chromophores. It is also clear that
there are certain desirable acids such as the hexafluoroantimonate,
methide and potentially the perflourinated borates, but
unfortunately, the structure of the total PAG is not clear
correlated with improved imaging performance.
[0062] After initial experiments shown in Table 1, PAGs 1, 5, 6,
and 9 were attempted several more times. PAGs 1 and 9 consistently
produced very nice images without noticeable dewetting or adhesion
problems, and had particularly cleanly developed patterns. PAG 5
has been used successfully on several occasions, but has shown with
increasing PAG concentration to be somewhat susceptible to adhesion
issues. Because PAG 9 is a cost-effective solution and shows less
cracking than PAG 1 and commercial SU-8, it was selected for a
variety of experiments. After consistently failing to produce
acceptable shuttle patterns with the commercial SU-8, a resist with
3% OPI-HFA PAG (9) was prepared and shown to produce the patterns
shown in FIG. 2. The 6.5 micron trench is approximately 105 microns
thick. As shown, the resist has vertical sidewalls and clean
trenches. The trenches as shown in FIG. 2 could not be cleared in
regular SU-8 and there was also a bit of difficulty with the SU-8
with triisobutyl amine. The OPI-HFA has been used in films up to 1
mm thickness with results not obtained using commercially available
to SU-8.
[0063] Finally, the OPI-HFA PAG has been combined with TBA to
create an optimal formulation which has been used repeatedly in Ni,
and Ni-alloy plating baths to create parts from the LIGA
process.
[0064] Although only preferred embodiments of the invention are
specifically disclosed and described above, it will be appreciated
that many modifications and variations of the present invention are
possible in light of the above teachings and within the purview of
the appended claims without departing from the spirit and intended
scope of the invention.
* * * * *