Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
NEW CLASS OF E-~EAM RESISTS BASED ON DONOR
POLYMER-DOPED HALOCARBON ACCEPTOR
TRANSFER COMPLEXES
BACKGRO~ND OF THE INVENTION
Field of the Invention
The invention lies in the field of E-beam resist
compositions and the~production of patterned thin
film layers therefrom~.
Prior Art
The prior art is replete with radiation sensitive
materials as resists and with their use in
pattern formation Ln the fabrication of micro-
electronic~devices. In the prior art, pattern
formation in these materials is dependent upon
di~ferential solubility between irradiated and
unirradiated~regions. These solubility changes
ar~e~produced by either bon~breaking, (chain
scissioD) or bond formatlon (chain crosslinking)
in polymeric sy~tems.~ This occurs in the presence
; 20 ~ of actinic~radLation,~ E-beam radiation or X-ray
radiati;on.
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1 Several prior art resists have included in them halogen
containing organic compounds or halocarbons. These
halocarbons are generally present to enhance the sensi-
tivity of the resist. Several such resists are dis-
cussed and reviewed in U.S. Patents 3,752,669; 3,769,023;
3,820,993; 3,895,954; 3,988,152 and 3,916,036. The
resists disclosed in the above references are sensitive
to either actinic or electron beam radiation. The prime
need for the halocarbon in these resists are for the
generation of free radicals to initiate polymerization.
More recently there nas been developed a new class of
E-beam resist materials based on donor-charge transfer
salts. These resist materials are described in Canadian
patent application 354,062, filed June 16, 1980, by E.M.
Engler et al and assigned to the assignee of the present
application and is entitled "Class Of E-Beam ~esists
Based On Conducting Organic Charge Transfer Salts".
These materials are different and distinct from the pre-
sent compositions in that they are crystalline saltsthat are coated onto a substrate by evaporation or sub-
limination, wherein the present compositions are amor-
phous polymeric materials which are cast from a solu-
tion. The present resist compositions are two compo-
nent systems in which differential solubility is gener-
ated via salt formation as opposed to the one compo-
nent system of the above-mentioned application in
which a neutral substance is produced. Additionally,
the materials used in the present invention are in-
sulating while the aforementioned are conductive.
The prior art materials have several drawbacks amongwhich is the difficulty of obtaining sharp images of
high resolution, particularly in negative resists.
This is due to the swelling of the polymeric material
during solvent development.
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SUM~RY OF THE INVENTION
What has been dlscovered here are novel E-beam negative
resists which can be broadly classified as donor
polymer-doped halocarbon charge transfer complexes. More
specifically, the materials comprise a polymer backbone or
skeleton having bonded thereto electroactive molecules.
These electroactive polymer species are doped with a
halocarbon.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows graphically the relationship between the
exposure dose and the normalized thickness of a TTF resist
exposed on a silicon substrate.
FIG. 2A and 2B show E-beam exposed patterns via scanning
electron microscopy.
DESCRIPTION OF THE INVENTION
The present invention teaches novel E-beam resist materials
which can be used to provide a negative resist image.
Principally, the materials are comprised of electroactive
polymers and a halocarbon. The resist includes a donor
polymer-doped halocarbon charge transfer complex.
Preferably, the electroactive polymer consists of a polymer
backbone and an electroactive molecule bonded thereto.
Several of the electroactive polymers of the type
anticipated for use in this invention are described in U.S.
Patent No. 4,142,783 and in the IBM* Techn cal Disclosure
Bulletin, Vol. 20, #7, December 1977.
The polymeric backbone can be selected from several known
homopolymer and copolymer compositions having skeletal
functional groups or side chains having functional groups
capable of reacting wîth the functional groups of donor
molecules. Polymers which can be used, include polystyrene,
a copolymer of polystyrene and chloromethylated styrene,
e.g~,
*Registered Trade Mark Of Internat.ional Business Machines
Corporation
r~
..
6~1
~ ~CH2CR
[(cH2-cH)x (CH2-CH)~-x ]n
where the value of X is varied (O<x~l), so that the
number of donor molecules per chain and their dis~ance
apart can be varied. The desired lithographic
properties are thus varied as a function of X;
Typically, other polymer backbones can be selected
from the following:
polyglutamic acid
1I H
~C-CH-N~n
(C IH2)2 ~,
COOH
., ,
polyvinyl chloride
[-CH2 - C~ H ~n
. C~
.
~ polyepichlorohydrin
,
~ ~ :
.
[~--C H 2--C H--O--]
;CH2
I
C~
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poly(ahalo phosphazenes)
X R
_ ~N-p _
n
poly(acrylic chloride)
~CH -CH ~
COOCQ
and the like.
The donor molecules that can be used in this
invention are those which can be characterized as
having the following specific molecular properties:
ta) Those that are capable of electron oxidation to
a cation.
tb) Have an oxidation potential of from about 0.lV
to about lV measured against a standard calomel
electrode:
(c) those that photoionize in the presence of a
halocar~o~; and
(d) which have a functional group which when reacted
with a polymer support will be bonded thereto.
Functional groups contemplated by the present
invention include hydroxyl, phenoxy, carboxyl, amino
groups and the like.
A wide variety of ~ donor molecules are expected
to be active in polymeric form as negative resist
materials~
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This invention may ~e effected by usinq donors
of the empirical formula C6H~X4R4 and having the
structural formula
X ~
where X=~, ~, Se and Te or any coMbination thereof.
The R groups may be of any organic substituent
including alkyls, such as methyl and ethyl, phenyls,
substituted phenyls, -SCH3, -CO2ME, halogen, fused
cyclics in which the substituent effectively
connects Rl with R2 and R3 with R4, e.g. Some
specific fulvalene compositions include
tetrathiafulvalene (TTF), its derivatives and Se
analogs (TSeF) and its derivatives. For example,
tetrathiafulvalenecarboxylic acid (TTFCO2H),
tetraselenafulvalenecarboxylic, (hydroxymethyl)-
tetrathiafulvalene (TTFCH2OH), hydroxymethyl-
tetraselenafulvalene (TSeFCH2OH), (p-hydroxyphenyl)-
tetrathiafulvalene (TTFC6H4OH), (p-hydroxyphenyl)-
tetraselenafulvalene ~TSeFC6H4OH), (p-aminophenyl)-
tetrathiafulvalene (TTFC6H4NH2), tp-carboxyphenyl)-
tetrathiafulvalene`(TTFC6~.4CO~-i), pheno::y (TTF~.
C~ <xx)
.
The fol}owing fused rings, such as cyclopentene,
cyclohexene, benzene, furan, thiophene, dihydrofuran
25~ and dihydrothiophene, and derivatives thereof can
be used. In addition, tetrathiatetracene compounds,
.
e.g.
S--S
:~ S S
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and their derivatives are also sui.table ~or the
purpose of this lnvention. In general, organic
~-electron donors having low ionization potentials
(<7.5eV)2 can be used.
Additionally, the following compositions are
contemplated by this invention;
~mines
R-NH2, R=Alkyl, Ar~l
Pyrazolines
~5~
~3
Pyrazolines of part.icular importance include
1,3-di-(p-methoxyphenyl)-5-(p-hydroxyphenyl)_~2_
pyrazoline,
1, 5-di-(p-methoxyphenyl)-3-(p-hydroxyphenyl)_~2_
pyrazoline,
3, 5-di-(p-methoxyphenyl)-l-(p-hydroxyphenyl)-~2-
pyrazoline,
l, 3-di-(p-methoxyphenyl)-5-(p-carboxyphenyl)_~2_
pyrazoline,
l, 5-di-(p-methoxyphenyl)-3-~p-carboxyphenyl)-~2-
pyrazoline,3, 5-di-(p-methoxyphenyl)-l-(p-carboxyphenyl)-~ -
pyrazoline,
l, 3-di-~p-methoxyphenyl)-5-(p-aminophenyl)-~2-
pyrazoline,
l, 5-di(phenyl)-3-(p-aminophenyl)-A2-pyrazoline,
l-(p-hydroxyphenyl)-3-(p-methoxystyryl)-5-
(p-methoxyphenyl)-~2-pyrazoline,
l-(p-hydroxyphenyl)-3~(p-diethylaminostyryl)-5-
(p-diethylaminophenyl)-Q -pyrazoline,
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1 Ferrocene
~1 . .
m
- m = Fe, etc.
Phenothiazine
~ ~3
[(n -C5H5)Fe(CO)]4
Dithiadiselenafulvalene and its derivatives may also be used
as donor molecules.
The acceptor molecules that can be used in this invention
are those which can be characterized as having the following
specific molecular properties:
(a) Contains one or more halogen a~oms.
(b) Has high electron affinity to accept electron from
donor species (0-2eV).
:
(c) Forms anionic species.
Typical halocarbon acceptors which can be used are selected
rom CC14, CBr4~ CI4~ C2 C16' C2 C12 4' 3 4 4
2 2 4~ ~H2C14, C2Br6, C3C18, Cl C C13, CHBr CHCl
CH2C12 and the like.
The halocarbon agent may be present in amounts ranging from
0.01 to lO times the concentration of the donor moiety.
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1~63671
There are two basic kinds of synthetic procedures
for covalently attaching the donor molecules to
the polymer resin. In equation (l),
~ CH2x~ D-y ~- ~ D
where~ is a polymer, x is a halogen, D is a donor
molecule, and y is a functional group capable of
coupling D to the benzene ring. In this procedure,
preformed and appropriately functionalized donor
molecules (D-Y) are reacted in single-step
coupling procedures with the polymer resin. In
this approach, the groups -x and -y are chosen
so as to lead to coupled products. Bonding to the
polymer matrix is accomplished in one step. In
an alternate method, (i.e. reactions 2-4), the desired
electroactive molecule is synthesized from
polymer precursors directly on the resin.
Thus, functionalized electroactive species
are not required; however, multiple polymer
reactions become necessary.
O
C~S~ ~ CH2--C~S~!
~ CH2O C ~ S ~ ¢ S ~ E~3N ~ CH2O-C-TTF
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1 The specific steps of the synthesis of the contemplated
compositions can be found in aforementioned U.S. Patent
No. 4,142,783.
The solvents which can be used for film coating are
toluene, chloroform, methylene chloride, cyclopentanone,
tetrahydrofuran, methyl ethyl ketone etc.
The present inventive resist compositions are exposed
to E-beam radiation.
Exposures were performed in a vacuum of about 10 6 torr
on a scanning E-beam system at 20 KV beam voltage. The
charge density is in the range of about lxlO 6 C/cm2 to
about 50xlO 6 C/cm2.
More specifically, this invention concerns new compositions
of matter which function in a novel resist process when
irradiated by E-beams. For example, films of a polymeric
TTF materials can be spin cast from a solution contain-
ing a halocarbon acceptor, such as CBr4. Under these
conditions the resulting polymer film contains CBr4 and
becomes sensitive to radiation. When X-rays are used
to irradiate these films through appropriate masks, only
the unexposed areas can be removed from the underlying
substrate by washing with a non-polar solvent. This nega-
tive resist process is a novel one and unrelated to
those suggested earlier for polymers whose solubility
decreases upon exposure to radiation because of radia-
tion induced crosslinking reactions.
YO9-79-054
llW671
11
It is suggested that these resists operate by means
of the radiation-induced formation of a salt,
according to Reaction 1, where the
Poly (TTF) + CBr4 ~ poly (TTF.) (Br ) (1)
The neutral polymer is soluble in non-polar
solvents while the salt produced is insoluble. This
reaction is well known to occur in monomeric TTF,
in solution U. S. Patent No. 4,036,648,
where the halide salt produced is insoluble in
non-polar solvents. This new mechanism for
lithographic action has been established in the
following ways.
Using visible-near ir spectrophotometry, the TTF
ions postulated in the above reaction have been
detected. By irradiating poly(TTF) doped
halocarbon films that were spun onto transparent
substrates. It has been observed that the films'
spectrum changes during irradiation to give new
absorptions at 600nm and at 800nm, previously
identified as characteristic of TTF ion and
aggregates thereof. (see the publication to
J. B. Torrance et al entitled "Optical Properties of
the Radical Cation Tetrathiafulvalenium in its
Mixed-Valence and Monovalence Halide Salts",
Phys. Rev. B, 19, 730 (1979). Additional
predictions of the suggested mechanism have also
been observed. For instance, it would be expected
that polymer films containing no halocarbon present
would be much less sensitive to incident radiation.
Confirming this point it has been observed that
irradiation of the undoped TTF polymer films gave
no lithographic images when subjected to the same
,
~ incident radiation as in the case of doped films.
~: .
~ ~ .
YO919-054
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Another facet of this mechanism is that reversing
of the charge transfer in Reaction 1, see
Reaction 2 below, could be expected to lead to a
change in the solubility properties, whereby, the
irradiation
poly(TTF )(X ) ~ poly TTF (2)
Drocess would be nullified and removal of the
polymer (now in its neutral state) could take place.
This effect has been demonstrated in the following
way. An exposed film was found to be insoluble
in the organic solvent dimethylformamide (DMF).
However, when the chemical reducing agent hydrazine
is added to the DMF solution it was observed that
the exposed polymer film was readily removed
leaving a clean Si substrate. In this process
hydrazine reduces the oxidized TTF back to its
neutral form TTF and the neutral polymer, thus
formed can be readily dissolved in the organic
solvent.
The third ramification of the postulated mechanism
is that other ~-donor polymers should be
lithographically active in the presence of
halocarbon dopants. As alluded to previously,
~-donor sensitivity to radiation with halocarbons
2S present has been observed for a large number of
donors in fluid solution. If the postulated
mechanism is correct it would be expected that
the same variety of donor halocarbon ionization
found in solution would also be observed in the
polymeric solid state. Similar lithographic
differential solubility with donors such as
pyrazoline, dimethylphenylenediamine, and ferrocene .
bound to polymeric backbones and prepared as
halocarbon doped films, has in fact been observed.
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A prime disadvantage of previous negative resist
materials ar~ the inherently low resolution of
the patterns obtainable. As discussed earlier,
negative resists typically operate by means of a
radiation-induced crosslinking process. Although
the crosslinked polymer that remains cannot be
dissolved in a developer solvent, penetration of the
solvent into the polymer causes swelling of the
~olymer chains since many of the development
sol~ents are thermodynamically good solvents for
the unexposed non-crosslinked polymer. The
swelling process grossly distorts the lithographic
pattern and attempts to alleviate this problem
by use of mixed solvents or thermal treatments
have largely been unsuccessful.
For the present resist process, however, the
lithographic patterns obtained showed no evidence
for solvent induced distortions or loss of
resolution. It is suggested that solvent
penetration of the polymer followed by swelling
is prevented in the system described herein, by
the presence of ions which act to repel the
non-polar solvent from the polymer matrix. Thus
solvent cannot penetrate into the exposed
polymer mass and cause swelling.
EXAMPLES
The following examples are given solely for
purposes of illustration and are not to be
construed as limitations on the inventions,
many variations of which are possible without
departing from the spirit or scope thereof.
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14
E~IPLE 1
Poly (vinylcarboxytetrathiaf~llvalene)
Poly(vinylbenzylchloride), prepared from the
monomer vinylbenzylchloride, 0.275g, and the cesium
salt of tetrathiafulvalene carboxylic acid, 0.750mg,
is added to 75ml of a DMF solvent and stirred at
75C for 24 hours. The solution is concentrated and
the resultant polymer is isolated by precipitation
into a rapidly stirred HzO solution~ Repeated
precipitations from THF/H2O gave a dark brown
solid. Anal- calc d for Cls 1 Hl1.653.5 l.7 C10.13
9 9t 7S4H3O2~o~87(cl)o 131 C=53.81, S=33.0g,
Cl=0.14. Found C,53.74; S,32.96; Cl,0.23.
Polymer films are prepared by mixing 4.7mg of the
polymer with 1 mg of C2~r2C14 which is added to
20~1 of cyclopentanone. The films are spin
coated at 2S00 RPM on a photoresist
spinner. No baking is required in order to obtain
good images. Solvents which can be used as
developers include tetrahydrofuran cyclopentanone,
diglyme, methylene chloride, chloroform and
mixtures thereof.
Several poly (TTF) films are prepared in this manner.
To determine the sensitivity of this resist, these
films were exposed to E-beams with doses from
2-40xlO 6 C/cm2. The films were developed in
1:1 THF: Cyclopentanone and then the thickness
of the remaining resist was determined.
From the resultin~
plot of normalized thickness remaining vs.
dose rate, see FIG. 1, it can be seen that the
sensitvity of the material for 50~ thickness
remaining is ~5 Coul/cm2. From data on other resist
materials ~Table I), it can be seen that the present
: ~ :
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1163671
material is one of the most sensitive resist known.
Determination of Resolution of Resist:
An E-beam exposed pattern was studied via scanning
electron microscopy ~see FIG. 2). These photographs
show no evidence for the classical negative resist
swelling behavior. All of the patterns are extremely
well-formed, with parallel, vertical walls and showing
no signs of pattern distortion. From the indicated
scale, it is estimated that the present resist has a
resolution of better than 20002.
EXAMPLE 2
Poly (vinylcarboxYferrocene)
Poly (vinylbenzylchloride), (15 Omg) is reacted with
the 360mg of cesium salt of ferrocene carboxylic
acid, (as prepared below) in 65ml DMF solvent. The
solution is heated to 75C, and stirred for 24
hours. The volume of the solution is reduced and the
polymer is precipitated into H20. Repeated
precipitations (THF/H20) gave a light yellow solid.
Anal. calc'd for CgHg (C7 5H5 401~4 FeO 7) (Clo 3)
C,69.9; Fe,13.4; Cl,4. Found C,67.62; Fe 12.11;
Cl,4.61.
.
Cesium salt of carboxyferrocene. ~lonocarboxyferrocene,
230 mg, is dissolved in ethanol. To this solution
is added 5ml of H20 which contained 250mg CsHC03. ;
Using slight heating, and high vacuum, the
solution was taken to dryness to produce the
desired salt.
About 5.2mg of the so prepared polymer and about
lmg of C2Br2C14, the acceptor compound are added to
Y0979-054
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16
26~1 of THF solvent and films are spun at 2500 RPM
on a Headway Photoresist Spinner. The films are
exposed to E-beams having doses in the range
2 40 x 10 6 coul/cm2. Patterns are developed by
one of the following solvents or mixtures thereof:
toluene, chloroform, methylene chloride,
cyclopentanone, THF.
No pre or post exposure baking is required to obtain
~ood images.
EXAMPLE 3
Poly (vinylphenoxy-1,3-~p methoxyphenyl)-5-
(p-hydroxyphenyl~-~2-pyrazoline):
The potassium salt of the pyrazoline is prepared
by adding 374mg pyrazoline to 40 mg KH in dry THF.
After stirring for 30 minutes, 152mg of
polyvinylbenzylchoride is added to this solution.
After refluxing for 4 days, the solvent volume
was reduced and the polymer isolated following
multiple reprecipitations from 50:5Q MeOH/H2O.
About 4.6mg of the polymer (4.6mg) and lmg of the
C2Br2C14 acceptor, are added to 32ul of THF. Films
were spun.'at 2500 RPM on a Headway Photoresist
Spinner. After E-beam exposures as in the above
examples, good images are obtained by developing
in toluene: THF mixtures. No pre-or post exposure
baking was required to obtain good images.
.
EXAMPLE 4
Poly[p-N,N-dimethylamino)-N-y-D-glutamanilide]
Poly(D-glutamic acid) (Miles-Yeda Ltd. mol wt 12400),
0.5g is dissolved in 50 mL of dry DMF and 2g
of freshly,distilled N, N-dimethyl-p-phenylenediamine
. is then added. The solution is cooled to 0C and
' yo979-054
~ ~ 17 11~6 ~
lg of DCC is added with stirring. Stirring is
continued at 0C for 1 h and at room te~perature
for an additional 24 h. One milliter of dry
methanol is added and stirring is continued for an
additional h. The precipitate is filtered off and
tlle filtrate is evaporated to dryness at 35C
(0.01 mm) (bath temperature). The residue is
dissolved in THF and filtered in a drybox under
nitrogen. Diethyl ether is added to the solution
and the precipitate is filtered and collected
under nitrogen. The collected solid is further
purified by precipitation from THF with diethyl
ether. Anal. Calcd for (Cl3Hl7N3O2)n C 62-90;
H,6.85; ~,16.93. Found: C,62.69; H,7.64; N,14.54.
About 15mg of the above prepared polymer (5.Omg)
and the C2Br2C14 acceptor, lmg, are added to 29ul
THF and films are spun (2500 RPM) on a
Photoresist Spinner. After exposure to E-beams
as in the above examples, good images are
obtained by use of methyl ethyl ketone developer
solvent. No baking was required.
EXAMPLE 5
PolYphenoxytetrathiafulvalene
About 15 mg of polyphenoxytetrathiafulvalene and
1.0 mg of C2Br2C14 are added to 29ul THF and films
are spun (2500 RPM) on a photoresist spinner.
After exposure to E-beams as in the above examples,
good images are obtained by use of methyl ethyl
ketone developer solvent. No baking was re~uired.
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18
TABLE I
Resist E-Beam (20KV) 2 Resolution
Dose 1~ Coul7cm
P~MA* 80 1002
FBM* < 1 ~m
PBS* 2 <.S um
P(GMA-co-EA)* .3 1 ilm
PS* 40 2000~~
~CA* 8 1 ~m
* PMMA - Poly(methyl methacrylate)
FBM - Poly (fluoro methacrylate~
PBS - Poly (butyl sulfone)
P(GMA-co-EA) - Poly (qlycidyl methacrylate ethyl
acrylate copolymer)
PS - Polystyrene
PCA - Polychloroacrylate
After E-beams exposure the polymer films are readily
removed by washing with a DMF solution which
contained a few drops of the reducing agent hydrazine.
Because this works by reducing the oxidized films,
it is likely that other reducing agents ~and
solvents)can similarly be used.
Other functionalized polymers, e.g.,
poly(epichlorohydrin, poly(halophosphazenes),
poly(acrylic chloride) and copolymers of the same
were reacted with donors listed above. The
resultant electroactive polymers were treated as in
Examples l-S above and provided good imases when
expo~ed to E-beam radiation.
:
: :
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