Note: Descriptions are shown in the official language in which they were submitted.
3'7~
I ~151 --1--
~IIG~I RUSS LI'rHOGP~APlllC Rosetta
AND METHOD
This invention relates to resist compositions for
high resolution lithography. More particularly, it is
concerned with resist compositions based upon cat ionic
polymers.
Photolithograph has been employed for some time in
-the electronics industry for the production of circuit
patterns. In Nina processes, a layer of resist material
it applied to the substrate and patterned by exposure to
light through a mask which defines the desired pattern.
Upon exposure to light, the photo resist changes volubility,
becoming either more soluble (positive working resist ) or
less soluble (negative working resist) in the developer
solvent.
Most known positive working resists are polymeric
materials which undergo a degradative reaction upon ox-
posture to yield products which are more soluble in the
developer solvent. Because most positive working resists
function by this mechanism, they tend to be less sensitive
than negative working resists. Negative resists generically
function by a mechanism involving a radiation-induced in-
crease in molecular weight, usually as a result of cross-
linking, to produce a change in volubility. Negative
working resists are generally preferred because of their
greater sensitivity, but often exhibit undesirable Swahili-
in upon development due to cross-linking and entrapment
owe solvent in the cross-linked polymer net. Swelling can
be a serious problem in applications which require high
resolution.
In the production of integrated electronic circuit
devices by micro lithography, design trends are toward
increasing the scale of device complexity, and hence the
density of circuit patterns, to reduce fabrication costs
~.2~}3~S
23451 ON -2-
and increase performance. This goal imposes a number of
requirements upon lithographic resist materials employed
in the manufacture of such devices, notably resolution,
sensitivity and etch resistance.
Electron beam lithography recently has follnd in-
creased use in the production of microcircuit devices but
there are only a limited number of photo resist materials
which can be used for electron beam lithography. More-
over, since many known electron beam resists are positive
working and do not have the sensitivity or etch nests-
lance of negative working resists, the latter are
preferred.
According to one aspect of the invention, there is
provided
1. A medium for electron beam lithographic record-
.
no comprlslng:a substrate; and
a film consisting essentially of a negative working
cat ionic vinyl polymer resist consisting of the repeating
unit t
-(~C~12-~CH-)-
Arks
where An is an alkyd substituted aromatic quaternized
nitrogen-containinq~rpen~ant group and X is selected from
the group consisting of F , Of , By , I , and PF6 , said
phylum being deposited on said substrate.
According to another aspect of the invention, there
is provided a method of micro lithographic recording come
prosing the steps of pa) applying a thin film of cat ionic
polymer resist to a substrate, wherein said cat ionic
polymer consists essentially of a vinyl polymer having
aromatic quaternized nltrogen-containing heterocyclic
pendant groups; (b) exposing said thin film of cat ionic
polymer to actinic radiation to convert said polymer in
the exposed regions thereof from a charged form soluble in
, .
, ",*
f
ZAP
23,~51 -3-
solvents of hicJh dipole momerlt to on uncharged for
insoluble ion solvents of high dole moment; (c) washing
the exposed coated substrate in a solvent of high dipole
moment to remove therefrom a portion of said thin film
of cat ionic polymer resist from the unexposed regions
thereof; (d) patterning the substrate; and (3) removing
any remaining cat ionic polymer film from said substrate.
Some embodiments of thy invention will now be
described, by way of example, with reference to the
acco-npanyiny drown in which:
The sole drawing FIGURE is a graph showing the
sensitivity to electron beam exposure of several cat ionic
polymer resist materials in accordance with -the invention.
For a better understar-dilly of the invention, together
with other end further objects, advantages, and capably-
ties thereof, reference is made to the following disclosure
and ap~erlded claims in collection with the above-described
drawing.
Cat ionic polymers useful as microlithocJraphic record-
ivy resists in the present invention are polymers having
a polyvinyl chain with cat ionic pendant groups. Pendant
groups having aromatic rink systems have been found to be
particularly stable to attack by enchants. Preferred
cat ionic polymers owe this invention are vinyl polymers
having nitrogen-containing aromatic heterocyclic rink
pendant groups. As used throughout this specification
and appended claims, the term "aromatic" is meant to
denote any cyclic system ox atoms having a delocalized pi-
electron structure satisfying Heckle'-; Rule and having
exceptional stability resulting from such electron demo-
calizat.ion (cf. James B. Hendrickson et at., Organic
Chemistry, 3rd Edition, Mc~ra~-Hill, New York, 1970,
pp. ~.67-170).
Examples owe quaterllized aromatic nitrogen-containing
3~7S
23,~51 -I-
heterocyc.lic systems contemplated as falling within the
scope of this invention and useful as pendant groups in-
elude, but are not necessarily limited to, monocyclic
systems such as:
NUN I+ N
R R R
pyridinium l,2-diazinium 1,3-diazinium l,~-diazinium
and condensed polycyclic systems such as:
N
Jo No N N J No
R R
quinolinium isoquinolinium naphtll~ridinium phthalazinium
No ON
N J N J No
R R
quinoxalinium quinazolinium acridinium
N
phenanthradinium phenazium.
;'3>7~
51 -5-
While shown as unsubstituted rink systems above,
pendant groups havillg small alkyd group rincJ substituents
such as methyl, ethyl, propel, isopropyl and bottle groups
or the like attached to carbon atoms of the ring sustains
are also contemplated as falling within the scope of the
present invention.
Cat ionic polymer resists in aeeordanee with this in-
mention havinc3 pendant croups of the types mentioned above
are soluble in solvents of high dipole moment such as
water or low molecular weight alcohols because of the
electrical err associated with the qua-ternized vitro-
gun atom. Solvents such as water, methanol, ethanol, pro-
panel, isopropanol and the like thus serve as effective
developer solvents. There is, however, a decrease in such
volubility as the e~rbon content of the pendant croup in-
creases from mainsail to twirl systems, or when such
ring systems are substituted with alkyd side chains. Thus,
for better volubility of the polymeric resist material in
its charcJed quaternized form, unsubstituted monocle
pendant croups such as pyridinium or diazinium are pro-
furred. Because of superior etch resistance and ease of
formulation, eationie polymer resist compositions Hanukkah
pendant pyridinium groups are most preferred.
In the formulas indicated above, R is hydrogen or an
alkyd group containincJ from 1 to 12 carbon atoms, inlay-
size. For reasons already stated, there is a decrease in
volubility of the eationie polymer resist materials of
this invention as -the carbon content of the alkyd group
attached to the quaterniz~d nitrogen atom increases.
Smaller R groups such as hydroc3en or alkyd groups contain-
inch up to about 5 carbon atoms are thus preferred.
In cohesion polymer resists Go the present invention,
the positive charge of the c~uaternized nitrocJen atom is
balanced by a necJatively charged counter ion, preferably
a halide ion or halogen-eontainincJ ion such as F , Of ,
By , I or PF6 . Cat ionic polymer resist materials in
s
23,451 -6-
accordance with the present invention are conveniently
prepared by polymerizcltion of the appropriate vinyl
monomer followed by reaction of the resulting molecular
polymer with a halogen-containing acid or an allele halide
to produce the ~uaternized nitrogen cat ionic polymer.
Details of the preparation of several posy (vinyl-N-
alkylpyridinium salt) cat ionic polymers are given as
illustrative examples below.
Cat ionic polymer resist materials of this invention
undergo a substantial change i-n volubility in solvents of
high dipole moment, such as water or low molecular weight
alcohols, upon exposure to actinic radiation such as
ultraviolet light, x-ray, or electron beam. Although the
exact mechanism of the change is not well understood and
lo no theory is adhered to at the exclusion of others, it is
believed that the change is attributed to a change in the
polar character of the polymer from a charged form to an
uncharged or neutral form. In this manner, the resist
compositions of the present invention differ from prior
JO art negative working resist materials which function by a
change in molecular weight, usually due to cross-linking,
Fox a change in volubility. Resist compositions of the
present invention thus undergo a transformation in kind,
i.e. iron ionic to non-ionic, upon exposure rather than a
change in degree, i.e. a change in molecular weight, as is
characteristic of most prior art negative-working resist
compositions. As a result, the resist compositions of
this invention exhibit good contrast and differential
volubility upon exposure.
The average molecular weight of cat ionic polymers
useful as resist compositions herein ranges between about
5000 to about 100,000. Because of the charge associated
with each monomer unit of the cat ionic polymers, there
seems to be little effect upon the volubility in polar
solvents as the molecular weight of the polymer varies.
For particular applications, the molecular weight range
S
~3,~51 -7-
is tailored to obtain desirable film-forming qualities of
the polymers.
To prepare micro lithographic recording media of the
invention, a solution of the polymer in water or a low
molecular weight alcohol such as methanol is cast or spume
on a substrate to obtain a thin uniform film. After de-
positing the solution on the substrate, which may be any
conventional lithographic support such as glass, plastic,
silicon substrates and the like, the solution is dried to
yield a thin film of the polymer. Film thicknesses are
varied to suit the particular application, but films of
up to about lam in thickness are effective as microlitho-
graphic recording media. Solutions of about 2 weight
percent to about 10 weight percent cat ionic polymer in
water or alcohol are effective in producing dried films
of adequate thickness
Following the casting and drying of the polymer film
on the substrate, the film is exposed to actinic radiation.
The method of exposure may be by flooding the cast film
with the radiation through a patterned mast to replicate
the pattern in the exposed film, or by tracing out the
desired pattern by exposure to an electron beam control-
ably deflected to produce the pattern in the film.
Exposure may be by ultraviolet light, x-ray, or electron
beam in the cases of flood exposure through a pattern
mask, or by deflected tight electron beam in the case
where the pattern is traced. Examples XII - XVII below
illustrate the effectiveness of several posy (2--vinyl-N-
al~ylpyrid.inium iodide) resists in accordance with the
invention when exposed by ultraviolet light and electron
beam.
In a preferred embodilllent of the invention, cat ionic
polymer resists are exposed by electron beam in a vector
scanning electron microscope. This method yields the
requisite resolution and density of pattern to produce
very large scale integrated electronic circuit patterns on
substrates.
I
23,~51 -8-
s shown by Examples XIII - XVIII and illustrated in
Figure 2, exposure o F several posy (vinyl-N-alkylpyridinium
iodide) resist materials of this invention to a 20 TV elect
iron beam with current densities ranginc3 up to about 50
ucoulombs/cm were sufficient to insolubilize better than
70% of the resist film applied initially to the substrate
in each case. In particular cases, exposures at much
lower levels were elective.
Following exposure of the polymer film, the unexposed
portion of cat ionic polymer film is removed by washing the
substrate and film in a developer solvent of high dipole
moment. Water, methanol, or other low molecular weight
alcohols or organic solvents having a dipole moment
greater than about 1.5 D are effective in removincJ the
unexposed cat ionic polymer film while leaving the major
amount of exposed film on the substrate.
RecJions of the substrate not covered by the resist
film remaining on the substrate are next processed to
produce a pattern in the substrate. In the particular
application where etched microelectronic circuit patterns
are produced in a metallic substrate, the etching step may
be carried out by plasma etching. The cat ionic polymer
resists ox this invention are particularly stable to
attack by plasma arc as illustrated by Example XVIII
where the etch resistance of posy (2-vinyl-N-methylpyrid-
ilium iodide) in accordance with the invention is compared
to that of a prior art electron beam resist material noted
for its etch resistance.
One surprisincJ and beneficial aspect of cat ionic
polymer resist materials of this invention is the apparent
lack of any need to deposit an electrically conductive
layer either over or under the polymer film when it is
deposited on a substrate end employed as an electron beam
resist. It is a common practice in electron beam lithe-
cJraphy to deposit a thin film of electrically conductive material such as a metal film or a layer of tin oxide or
~3,~51 I
indium oxide either over or under the electron beam resist
to dissipate the charge which otherwise tends to build up
in the resist film during electron beam exposure. If this
charge is not removed, the possibility arises of electron
static deflection or broadening of the electron beam at the point of impact on the film and substrate resulting in
diminished resolution. Cat ionic polymer resist films of
the present invention appear to have sufficient inherent
electrical conductivity -to carry away such charge during
exposure, eliminating the need for the deposition of a
metal or metal oxide film on the substrate. This desire
able property of the resists of the invention simplifies
the production of microelectronic circuit devices by
eliminating several steps thereby in the overall process.
Employing cat ionic polymer resist compositions of the
present invention and electron beam lithographic methods,
microelectronic circuit patterns have been obtained with
sub-micron line widths and resolution.
In order to enable one skilled in the art to better
practice the present invention, the following Examples are
provided. It is to be understood, however, that the
Examples are merely illustrative of the invention and are
not to be viewed as limiting the scope thereof.
Examples I - XI illustrate methods of preparing
cat ionic polymer resist compositions of the invention.
In general, -the methods include polymerization of the
appropriately substituted vinyl monomer to produce a
molecular polymer product. The molecular polymer is
treated with a haloyen-containing acid or on alkyd halide
to quatrains the nitrogell atom of the aromatic nitrogen-
containing heterocyclic pendant group to obtain the de-
sired cat ionic polymer resist material.
This Example illustrates the general method employed
to produce molecular polymers used as intermediates in
the formulation of cat ionic polymer resist compositions.
23,~51 -10-
Thirty ml of freshly distilled vinyl porn were
mixed with 0.2 g of bouncily peroxide. The mixture was
out-gassed by repeated freezing and thawing under vacuum
in a liquid nitrogen bath. After the mixture had been
thoroughly out-gassed, it was allowed to germ to room
temperature and stand for 67 hours at 40C to polymerize.
At the end of this time the black residual mass was
purified by repeated washing with tetrahydrofuran from
which the polymer was precipitated each time with hexane
or Hutton. The final product had an off-white color.
Example II
Five grams of poly(2-vinylpyridine) were dissolved
in 50 ml of ethylene chloride. To this mixture were
added drops with stirring a solution of 11.39 g ~0.08
mole) of methyl iodide dissolved in 10 ml of ethylene
chloride. The resulting mixture was stirred overnight at
ambient temperature. At the end of this time the super-
Nate was decanted from the rubbery mass of polyvinyl-
N-methylpyridinium iodide) which had precipitated from
solution.
The product was dried in vacuum overnight. Different
trial scanning calorimetry of the product indicated an
endothermic melting transition point of 205C and a deco-
position temperature of about ~0C.
Example III
Poly(2-vinylpyridinium hydrochloride) was prepared
by the general method detailed in Example II by reacting
.8 g of poly(2-vinylpyridine) with 3.8 ml ox 12 molar Hal.
Example IV
Poly(2-vinylpyridinium hydrobromide) was prepared by
the general method of example II by reacting 5.0 g of
poly(2-vinylpyridine) with 7.9 ml of I weight percent
hydrobromic acid.
Pi
~3,451 -11-
Example V
Poly(2-vinylpyridinium hydrofluoride) was prepared
by the general method detailed in Example II by reacting
1.1 g of poly(2-vinylpyridine) with 2 ml of ablate 48
weight percent hydrofluoric acid.
Example VI
Poly(2-vinylpyridinium hexafluorophosphate) was
prepared by the general method of Example II by reacting
1.1~ g of poly(2-vinylpyridine) with 1.6 g of hexafluoro-
phosphoric acid.
Example VII
Poly(2-vinyl-N-ethylpyridinium iodide) was prepared
by the general method of Example II by reacting 3.28 g of
poly(2-vinylpyridine) with ~.88 g of ethyl iodide. It
was found necessary -to heat the ethylene chloride soul-
lion under rollicks for about 24 hours to insure reaction
between the reagents.
Example VIII
Poly(2-vinyl-N-propylpyridini~n iodide) was prepared
by reacting 5.2 g of poly(2-vinylpyridine) with 8.3~ g of
n-propyl iodide in ethylene chloride. The cat ionic
polymer product was precipitated from solution by addition
of hexane.
ox pie IX
Poly(2-vinyl-N-butylpyridinium bromide) was prepared
by reacting 5.1~ g of poly(2-vinylpyridine) with 6.70 g
of l-bromobutane in ethylene chloride solution. The
yellow cat ionic polymer product was precipitated from
solution by the addition of cyclohexane.
Example X
Poly(2-vinyl-N~heptylpyridinium iodide) was prepared
by reacting 1.2 g of poly(vinylpyridine) with 2.65 g of
l-iodoheptane in ethylene chloride solution. The methyl
tone chloride solution was stirred at ambient temperatures
overnight after which time it was heated under reflex for
2 hours. The polymer was separated by pouring the mixture
23,451 -12-
Pinto hexane. The resulting precipitate was dried under vacuum overnight.
Example XI
Poly(2~vinyl-N dodecylpyridinium iodide) was prepared
by reacting I g of poly(2-vinylpyridine) with 13.8 y of
dodecyl iodide in ethylene chloride. The mixture was
stirred at ambient temperatures for 4 days. After this
time, the mixture was poured into hexane and the powdered
polymeric product separated. The precipitated polymer0 was filtered and dried under vacuum overnight.
example XII
This example illustrates the use of cat ionic polymer
resists of the present invention in processes employing
ultraviolet light exposure.
A film of poly(2-vinyl-N-methylpyridinium iodide)
approximately 8000 Angstrom units in thickness was cast
by conventional spinning techniques on a silicon wafer
substrate. The solvent employed for film casting was
distilled water. The coated wafer was dried and then
exposed to ultraviolet Lotte of 266 no wavelength from
a Nd:YAG pulsed laser. Following exposure, the coated,
exposed silicon wafer was washed in distilled water.
Measurement of the remaining film thickness indicated
that most of the film originally cast on the wafer no-
mined. reflectance infrared spectrum of the exposed
film remaining on the silicon substrate indicated that
absorption bands at 1470 cm 1 and 1435 cm 1 were intent
silted relative to the band at 1512 cm 1 compared with
the preexposed film.
In Examples ZOO - XVII following, the sensitivities
to exposure by electron beam of several resist materials
of this invention were evaluated. In each Example, a thin
film of the appropriate resist material was cast on a 2
inch silicon wafer substrate which previously had been
oxidized to produce a thin surface layer of Sue. The
films were cast in each Example by applying a few drops
'US
23,~51 -13-
of a met~lanolic solution of the resist and spinning at
2000 rum to spread the film evenly over the substrate.
Each cast film was dried by baking at 120C for about
1/2 hour.
Each coated silicon wafer was exposed to a 20 TV
electron beam in a vector scan electron beam instrument
which was computer controlled. Exposure produced a 9 x 9
array of rectangles each having a different exposure by
varying -the dwell time of the beam on each rectangle.
The beam current was measure by means of a Faraday cup
located in the instrument and a Keithley electrometer.
The approximate minimum exposure required -to fix at
least kiwi of the resist material to the substrate appears
in the Table. As -the data indicate, exposure of the no-
sits of this invention to an electron beam with current
densities of up to about 50 ~coulombs/cm2 is sufficient to
fix at least 70% of the resist.
Example XVIII
In this Example, the resistance to plasma etching of
a resist material in accordance with the invention is come
pared to that of a prior art resist material employed for
microelectronic circuit fabrication, known for its good
etch resistance.
Two 2 inch silicon wafers were coated with electron
beam resist materials. In one case, the resist was posy
(2-vinyl-N-methylpyridinium iodide), and in the other case
a prior art resist formulation. Both coated silicon sub-
striates were exposed by methods detailed in Examples ~III-
XVII above, and then developed to produce a pattern of
resist malarial remaining on the substrate.
The two patterned substrates were then simultaneously
etched in a plasma etcher for five minutes at 200 watts,
6~CC14 in ale at 430 mTorr; five minutes at 150 watts,
4% CC14 in He at 420 mTorr; and 7 minutes at 50 watts,
10'~ 2 in ale at 480 mTorr.
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23, 45L -14-
day
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23,~,51 -15-
Under these conditions, the prior art resist material
was eroded at an average rate of about 120 Amman and the
resist of -this invention at an average rate of about 60
A/min.
While there have been shown and described what are
at present considered the preferred embodiments of the
invention, it will be obvious to those skilled in the art
that various changes and modifications may be made there-
in without departing from the scope of the invention as
defined by the appended claims.
.. . . . . . . . . ..
