Note: Descriptions are shown in the official language in which they were submitted.
3fi~
1 Brief Su~nary of the Invention
2 In accordance with this invention a positive exposure
3 resist process uses as the resist material a class of
4 polymers whose instability has heretofore made them of
limite~ practical utility. These po]ymers are polysulfones
6 which can be prepared by reacting olefins with sulfur
7 dioxide.
8 A film of olefin~sulfone polymer is exposed to radiation
9 in a patternwise manner wi~h sufficient energy to cause the
exposed portions of the polymer film to degrade so as ~o form
11 a positive resist image. In one aspect of the invention the
12 image is developed using a solvent which preferentially
13 dissolves the exposed portions of the film. In another aspect
14 of the invention $he exposed portions of the film are
volatili~ed during the exposure so that the image is
16 simultaneously exposed and developed.
17 The polymers require as little as one-eighth ~he energy
18 dosage nèeded to expose, for example, polymethyl methaorylate
19 and yet are effective to act ~s an etch resistant mask to
protect the substra$e in the unexposed areas.
21 Detailed Description
22 Polyme~s suitable for use in the process of the invention
23 are polysulfones having the repeating units of 1:1 olefin
24 monomer to SO2 and 2:1 olefin monomer to SO2 as follows:
~[monomer]n - SO2~ where n = 1 or 2. Preferred, for
26 example, are polymers of the type
27 ~[C(Rl~-C(R2H~]n5O2t where Rl = hydrogen, alkyl, aryl or
FI 9-72-026 -2-
.
., .
~ 3~i~3~)~
1 with R2 forn~ a cycloaliphatic ring and where R2 = hydrogen,
2 aryl, alkyl or with Rl forms a cycloaliphatic ring. An example
3 where n=l is poly (butene-l~sulfone); an example where
4 n=2 is poly(styrene sulfone).
The polymers can be formed by reacting olefins with sulfur
6 dioxide. The polymers are described ~Eor example in The
7 Encyclopaedia of Polymer ~echnology "Olefin Sulfur Dioxide
8 Polymers", page 460-485 and in Advances in Macromolecular
9 Chemistry Vol. 1 Polysulphones: Organic and Physical Chemistry
by Ivin and Rose pages 335-406. The polymers derive their
11 usefulness in the process of the invention from the relative
12 instability of the sulfone bond and the volatile nature of
13 the degradation-products. This permits degradation of the
14 polymers at low amounts of energy so that the resist image
forming process can be carried out rapidly and with less
16 energy requirement. Suitable sulfone polymers are those which
17 are stable above room temperature and whose solubility
18 characteristics permit ~he coating of films.
19 The sulfone polymers preferred for use in the invention
are derived from olefins having the general formula
21 RlHC=CHR2 where Rl = Hydrogen, alkyl, aryl or wi~h R~ forms a
22 cycloaliphatic~ring and where R2 = Hydrogen, aryl, alkyl or
23 with Rl forms a cycloaliphatic ring. Examples of such olefins
24 include butene-l, butene-2, 4-bromobutene-1, pentene-l,
cyclopentene~ hexene-l, and styrene. Polyisobutylene sulfone,
26 although having~the requisite instability is not sol~ble in
27 organic solvents.
FI 9-72-026 -3-
.
36~3~
1 The polymers can be prepared by free radical polymerizations
with sulfur dioxide and have molecular weights in the film forming
range of from about 5000 to about 5,000,000 (weight average) with
molecular weights above about 50,000 preferred.
Films of the polymers are cast from solvent solutions of the
polymers containing, for example, from about 1 to about 20~ by
weight of solution of the polymer. Suitable solvents should have
boiling points which are below the decomposition point of the polymer
to permit removal of the solvent from the cast film by heating. Ex-
amples of suitable solvents are organic liquids such as, for example,
toluene, cyclohexanone, benzene, chlorobenzene, butyl acetate, chloro-
form, acetone, dioxane, xylene, methyl ethyl ketone, tetrahydrofuran,
cellosolve* acetate, cyclohexanone, dimethyl sulfoxide, and n-butyl
acetate.
The films can be cast in various thicknesses of from about 50
angstroms to about 10 microns as is conventional in the art depending
upon the intended use of the resist image. For example, about 0.5
to about 2.0 microns thick for an etch process or from about 1.5 to
about 3 microns thick for a lift off metallurgy process. The casting
process is conventional such as by spinning or dip coat.
It is preferred to prebake the resist film in air or vacuum at
a temperature usually above the glass transition temperature of the poly-
mer but below the thermal decomposition temperature. The prebake re-
moves traces of solvent and anneals out any strains in the film. Suit-
able baking temperatures range from about 25C to a few degrees below
the polymer decomposition temperature.
The resist is exposed patternwise to radiation such as, for example,
ultraviolet, electron beam, x-ray~ and gamma radiation which acts to
rapidly degrade the polymer. The sensitivity of the polymers makes
them particularly useful in processes employing low energy focussed
scanning electron beam radiation of from about 10 to about 30 KeV with
charge densities of from about lxlO 6 coul/cm2 to about 1 coul/cm2 as
is known in
*Trade Mark - ~ -
a
.
.
~ 2 ~Ç~
1 the art. The required dosage can be reduced by heating the polymer
during exposure.
The ultimate products of the decomposition of the
polysulfone polymers are the monomers, sulfur dioxide7 and various polymer
fractions so that, in one aspect of -the invention, by using exposure
charge densities of above about 1 x 10~5 coul/cnl2 the image can be
simultaneously exposed and developed. In another aspect of the
invention the image is solvent developed using solvents which
preferentia11y dissolve the lower molecular weight degraded polymer
in the exposed portions of the film. Suitable solvents include,
for example, toluene, methyl isobutyl ketone, butyl acetate,
xylene, cyclohexanone, cellosolve acetate, benzene, chlorobenzene,
ethanol, methanol, butanol, cyclohexanol. The development rate
can be adjusted by heading or cooling the solvent.
The solvent development is carried out preferably in
~! j
- , . . . .
t3~)~
1 ~ temperature range of from about 10C to about 50C.
2 Three types ~ solvent development processes can be used.
3 In the first a good solvent for both the exposed and
4 unexposed polymer is used to gain speed. The resist
thickness is adjusted so that even though there i5 some
6 loss of unexposed resist, ~he remaini.ng unexposed resist
7 film is thick enough to protect the substrate during the
8 subsequent treatment. Alternately, al solvent for the
9 exposed areas only is employed. In the third type of
1~ development, a mixture of a solvent for both the exposed
11 and unexposed polymer and a solvent for the exposed polymer
12 only is used. The optimu~ development time is determined
13 for each case hy the factors of exposure dosa~e, film thickness,
14 solvent system and solvent temperature as known by one skilled
in the art.
16 The patt-erned resist image requires no postbake and has
17 high resolutions of less~than the fil~ thickness. For example,
la .5Q micron images line and space in a fil~ thi~kness o~ 1.0
:19 micron. The polysulfone resist is imperYious to the acids
which are used, for example, in the ~abrication of integrated
21 circuits. The resist films, however, have poor etch resistance
:22 to strong alkali (pH 14.0).
23 The resist films can be solvent stripped from the substrate
24 following the etch process. Suitable stripping solvents are
solvents such as, for example, aliphatic and aromatic
26 hydrocarbons, ketones~ and acetates which are heated from
27 about 21C to 109C. One solvent can eerve in the proces~ as
FI 9-72-026 -6-
~: , . ' '
1 th~ casting solve~t, the developer, and the stripper by
2 adjusting the processing temperature. ~or example, toluene
3 can be used at room temperature to apply and develop the
4 resist and toluene heated at 50C can be used to strip the
resist.
6 The process of the invention is further illustrated by,
7 but is not intended to be limited to the following examples
B wherein parts are part~ by weight unless otherwise indicated.
g Example 1
A butene-l-sulfur dioxide copolymer is pxepaxed by the
11 following procedure.
12 Butene-l, 56.1 grams (1.0 mole) and sulfur dioxide,
13 192 grams (3.0 mole~ are condensed into a 2 litre stainless
14 steel Parr stirring autoclave at about -78C. The catalyst,
0.5 grams ~3xlO 3 mole) of azobisisobutyronitrile is then
16 added. The reactor is sealed and heated for 18 hours at about
17 45C. The reactor i5 caoled to room temperature, opened, and
18 the viscous contents are dissolved in chloroform. The
19 chloroform solution is filtered and t~en added dropwise to
cold petroleum ether to precipitate a white polymer characterized
21 by elemental analysis, infrared and nuclear magnetic resonance
22 as poly(butene-l-sulfone). -
23 - The procedure of Example 1 was repeated to prepare the
24 following sulfone polymers . Poly (cis-butene-2~sulfone);
poly(pentene-l-sulfone~; poly(cyclopentene sulfone);
26 poly(hexene-l-sulfone); and poly(styrene sulfone)~
FI 9-72-026 -7-
~ . , . -
369~.~
1 Example 2
2 Poly-(4-bromobutene-1-sulfone) is prep~red by the
3 followiny procedure. The monomer, 4-bromo~utene-1, 135
4 grams (1.O mole) and sulfur dioxide 192 grams (3.0 mole) were
condensed into a 2 litre stainless steel Parr stirring auto-
6 clave at a temperature of -78UC and the catalyst 0.8 grams
7 (4.8 x 10 3mole) a~obisisobutyronitrile was added~ The
8 autoclave was sealed and then heated at 45C for 18 hours.
9 The viscous reaction mixture waS thlen poured into 2.5 litres ~ .
of methyl alcohol to precipitata a ~white solid product. The
11 polymer was purified by repeated dissolution and precipitation
12 from chloroform/methanol and then dried at 30C under vacuum
13 to give 8 grams of white polymer. The glass transition
14 temperature was 36C. An elemental analysis was made with
the following result calculated for tC~H7BrO2S)n. C, 24.13;
16 H, 3.54; Br, 40.14; S, 16.11; 0, 16.08. Found: C, 24.59,
17 H, 3.71; Br, 39.88; S, 14.37 0, 17.65.
18 The above polymers were further characterized as to their ~ ::
19 glass transltion temperatures, molecular weight, decomposition
temperature ~thermo gravimetric analysis using a Du Pont
21 thermobalance) with the results being shown in Table I below:
22 Table I ~
23 Glass Transitio~Onset of Thermal
24 Polymer ~ Temp. (T~)C Decomp. C Mol Wt.
Polybutene- 64 125 MW 3,080k
26 1 sulfone) Mn 1,320k
27 Poly(cis- 47 110 MW 1,180k
28 butene-2 Mn 148k
29 sulfone)
~I 9-72-026 ~ -8
. . .
. . .
:
~ 2 ~3~
1 Glass Transition Onset of Thermal
Polymer Temp. (Tg~C _ Decomp. C Mol. Wt.
Poly~4-bromo- 36 110 --
butene-l
sulfone)
Poly(pentene- 68 130 --
1 sulfone)
Poly(cyclo- 142 129 --
pentene
sulfone)
Poly(hexene-l 60 115 Mw 611,500sulfone) 122
10Poly(styrene 78 130 Mw 366,000
sulfone)
Example 3
i
Poly-l-butene sulfone (Mol. Wt. Mw 3x106) was spun cast at
3000 r.p.m. to 1.25u (12~500A) thickness on 5000 A thermal oxide of silicon.
The film was prebaked in air at 95C for 1.0 hr. The film was exposed in
vacuum with a 0.5u diameter beam to a dose of 3.0x10-6 coul/cm2 at 15 keV.
The film was developed in me~hyl isobutyl ketone for 90 seconds at 21C.
The exposed region was removed by the solvent to yield a positive image.
The net film thickness remaining in the unexposed region was 6200A. The
; sample was rinsed in`~methanol-~and dried at 95C for l~O minutes. The~5000A
oxide layer was etched with buf~ered HF at 21C to yield 0.5u images for
diffusion dop;ng. The film was stripped in several minutes with acetone
at 40C followed by aicohol and water rinse. A similar procedure was
followed to etch images in silicon oxide using the polymers prepared by
the procedures of Examples 1 and 2 as summarized in Table II below:
.
~ ,
:
g
P,
o o ~ t) u
U o o U~ ~
o 0~ 0 n
O O ~ C r
r~
N N N V ~ U Gl
O 0 -1 0 r-lO -1 Q) ~1
~ ~ U ~
, . . .
Hl a~
l ~Ij n O ~ X X X x. ,~
.
o ~ ; O , ~''
e~ . '; ~ ; '` ~
.
' v, ~, ~ U ~ ~ ~ 'h
-`:' ' ~ ' ~ .
, ' ' ` , ' ' ' . . '
'~ Wllere ~lltrathin film~ are involved, i.e. 5~.3 microns,
loss of unexposed resist d~ring liquid developm~nt must be
3 avoided. One embodiment of the process of the invention
4 takes advantage of the fact that the olefin-sulfur dioxide
polymers can be vapori~ed in the exposéd areas at dosage
6 levels of about 1 x 10 5coul/cm2 in vacuum. This is a means
7 of eliminating liquid development beccluse the degraded
8 monomer components of the polymer are removed simultaneously
9 with the exposure.
Although the invention has been particularly shown and
11 described with reference to preferred embodiments thereof, it
12 will be understood by those skilled in the art that the
13 foregoing and other changes in form and details may be made
14 therein without departing from the ~pirit and scvpe of the
invention. ~
16 Wbat is clai~ed is:
' .
, .
,
, ~ .
.
~:
'
FI 9-72-026 -11-
DMB:kmg
10/2~/72
:
:
~ .
.
-, . ~ . : .
.
