Photoresists formed by polymerization of di-unsaturated monomers

A photoresist coating for use in microlithography comprises a polymer of a monomer of the formula ##STR1## wherein X and Y are strong electron withdrawing groups and R.sup.4 is H or, providing that X and Y are both --CN, R.sup.4 may be aliphatic hydrocarbyl, aryl or alkaryl. Preferred monomers are of the formula ##STR2## wherein R.sup.7 is a C.sub.1 -C.sub.5 alkyl or C.sub.2 -C.sub.5 alkenyl group, more particularly ethyl 2-cyanopenta-2,4-dienoate or allyl 2-cyanopenta-2,4-dienoate. Methods for applying a resist coating by vapor deposition of these monomers and exposure to radiation are described. A positive or negative tone image can be produced, depending upon the imaging method employed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to photoresists formed by polymerization of certain 
substituted butadiene monomers. The invention is especially useful in 
microlithography, particularly for producing semiconductor devices on 
silicon chips. 
2. Description of the Related Art 
The use of anionically or zwitterionically polymerisable monomers as resist 
materials for microlithography is known in the art, as discussed in U.S. 
Pat. No. 4,675,273 Woods et al and U.S. Pat. No. 4,675,270 Woods et al 
both assigned to Loctite (Ireland) Limited, the contents of which are 
incorporated herein by reference. Previous methods involved spin coating a 
solution of a cyanoacrylate polymer onto a substrate. U.S. Pat. No. 
4,675,273 describes a method for applying a polymeric resist coating to an 
etchable substrate which comprises exposing the substrate to be coated to 
the vapour of an anionically polymerizable monomer of the formula: 
EQU CH.dbd.CXY I 
where X and Y are strong electron withdrawing groups and R is H or, 
provided that X and Y are both --CN, R may be C.sub.1 -C.sub.4 alkyl, for 
sufficient time to deposit a polymerizable coating thereon. Particularly 
preferred monomers are 2-cyanoacrylate esters. The coated substrate is 
subsequently imaged using high energy radiation; the image is developed by 
conventional solvent development processes; the image is etched using a 
suitable plasma or acid etching process; and the resist coating may be 
subsequently removed by heating the coating to a temperature above the 
polymer depolymerization temperature. 
U.S. Pat. No. 4,675,270 describes an imaging method which comprises 
(a) providing a substrate having a surface reactive to activate 
polymerization of a monomer defined by the formula I as defined above; (b) 
treating the surface of the substrate with a photosensitive compound which 
releases an acid when exposed to actinic or ionizing radiation; (c) 
subsequently imagewise exposing the substrate to radiation of an energy 
effective to release said acid from said photosensitive compound; and then 
(d) exposing the substrate to vapours of one of said monomers for 
sufficient time to form a polymer coating over the substrate in the areas 
thereof not exposed to the radiation, forming a relief image. 
Cyanoacrylate polymers generally form positively imaged resists i.e. the 
relief image is in areas which have not been exposed to radiation (see 
U.S. Pat. No. 4,279,984 Matsuda et al, assigned to Matsushita Electric 
Industrial Co. Ltd.). The process of U.S. Pat. No. 4,675,270 also forms a 
positively imaged resist. 
It is also known to form negatively imaged resists i.e. the relief image is 
in areas which have been exposed to radiation. U.S. Pat. No. 4,551,418 
Hult et al describes a process for generating a negative tone resist image 
comprising the steps of: 
(1) coating a substrate with a film that contains a cationic 
photoinitiator; (2) exposing the film in an imagewise fashion to radiation 
and thereby generating cationic initiator in the exposed regions of the 
film; (3) treating the exposed film with a cationic-sensitive monomer to 
form a film of polymer resistant to plasma etching; and (4) developing the 
resist image by etching with a plasma. 
It is an object of the present invention to generate negatively-imaged 
resists using anionic or zwitterionic polymerizable monomers. 
1,1-disubstituted 1,3-butadienes are already known. U.S. Pat. No. 
3,316,227 Gerber assigned to Lord Corporation describes 1,1-disubstituted 
diunsaturated compounds having a formula selected from the groups 
consisting of 
##STR3## 
where R.sup.1 is selected from the group consisting of hydrogen, alkyl 
groups containing from 1 to 5 carbon atoms, phenyl and halogen; where 
R.sup.2 is selected from the group consisting of hydrogen and methyl, and 
where X and Y are dissimilar electron-withdrawing groups and are 
separately selected from the group consisting of cyano, carbethoxy, ethyl 
sulfone, phenyl sulfone, formyl, acetyl, benzoyl, diethyl, phosphonyl, 
amide and phenyl. These compounds are described as having utility in the 
fields of adhesives and coatings. 
U.S. Pat. No. 3,554,990 Quinn et al assigned to Eastman Kodak Company 
describes esters of 2-cyanopenta-2,4-dienoic acid having the structural 
formula: 
##STR4## 
wherein R.sup.3 is an alkenyl group of 2-10 carbon atoms or an alkoxy 
substituted alkyl group of 2-10 carbon atoms. These esters are said to be 
useful as adhesives for general and particularly for surgical uses. 
There is no suggestion in the prior art that polymers of substituted 
1,3-butadienes might find utility as photoresists. 
SUMMARY OF THE INVENTION 
The present inventors have now surprisingly found that certain substituted 
1,3-butadienes have different and/or advantageous properties, as compared 
to cyanoacrylates, in making photoresists. 
The present invention provides a polymeric resist coating comprising a 
polymer of a monomer of the formula 
##STR5## 
wherein X and Y are strong electron withdrawing groups and R.sup.4 is H 
or, providing that X and Y are both --CN, R.sup.4 may be hydrocarbyl, aryl 
or alkaryl. The term "hydrocarbyl" as used herein means "aliphatic 
hydrocarbyl" including alkyl, alkenyl and alkynyl. The polymeric resist 
coating of the present invention has an advantage over a resist coating of 
a cyanoacrylate polymer in that the coating of the present invention has 
better thermal stability. 
The polymeric resist coating may be formed by applying a solution of the 
polymer onto a substrate, for example by the known technique of spin 
coating. However it is preferred to use vapour deposition. 
In one aspect therefore the invention provides a method for applying a 
polymeric resist coating to a substrate which comprises exposing the 
substrate to the vapour of a monomer of the formula V as defined above for 
sufficient time to deposit a polymerized coating of the monomer on the 
substrate. 
In a second aspect the invention provides an imaging method comprising 
applying a polymeric resist coating as defined above to a substrate, 
imagewise exposing the coated substrate to high energy radiation, and 
developing the image by a solvent development process to form a negative 
tone image. 
Although this invention is not limited by any theory, it is believed that 
because the polymer of a dienoate monomer contains unsaturation, the 
exposure to high energy radiation leads to further crosslinking which 
reduces the solubility of the exposed areas as compared to the unexposed 
areas. Consequently the unexposed areas are dissolved more readily in the 
solvent development process. In contrast, the polymer of a cyanoacrylate 
polymer is saturated and the effect of the radiation is to degrade the 
polymer in the exposed areas, with the result that these areas are more 
readily dissolved than the unexposed areas. 
In a third aspect the present invention provides an imaging method 
comprising 
(a) providing a substrate having a surface reactive to activate 
polymerization of a monomer of the formula V as defined above; 
(b) treating the surface of the substrate with a photosensitive compound 
which releases an acid when exposed to actinic or ionizing radiation; 
(c) subsequently imagewise exposing the substrate to radiation of an energy 
effective to release said acid from said photosensitive compound; and then 
(d) exposing the substrate to vapours of one of said monomers of formula V 
for sufficient time to form a polymeric coating over the substrate in the 
areas thereof not exposed to the radiation. 
By use of the imaging method of this third aspect of the invention, a 
positive tone image is produced. Thus the monomer of the formula V has the 
advantage over a cyanoacrylate monomer that it can be used to produce 
either a negative or a positive tone image, depending upon the imaging 
method employed. 
A fourth aspect of the invention comprises an imaged article prepared by 
the foregoing inventive methods. 
In the definition of the monomers of formula V, the term "strong electron 
withdrawing groups" refers to groups which are more electron withdrawing 
than halo. Generally the electron withdrawing groups X and Y may be 
independently selected from --SO.sub.2 R.sup.6 ; --SO.sub.3 R.sup.6 ; 
--CN; --COOR.sup.5 and --COR.sup.6 wherein R.sup.5 represents a 
hydrocarbyl or substituted hydrocarbyl group such as a straight chain or 
branched chain C.sub.1 -C.sub.12 alkyl group (which may be substituted 
with a substituent such as a halogen atom or an alkoxy group), a straight 
chain or branched chain C.sub.2 -C.sub.12 alkenyl group, a straight chain 
or branched chain C.sub.2 -C.sub.12 alkynyl group, a cycloalkyl group, an 
aralkyl group or an aryl group; and R.sup.6 represents H or hydrocarbyl, 
preferably C.sub.1 -C.sub.12 hydrocarbyl. Preferably at least one of X and 
Y is --CN. 
Specific examples of the groups for R.sup.5 are a methyl group, an ethyl 
group, an n-propyl group, an isopropyl group, an n-butyl group, an 
isobutyl group, a pentyl group, a hexyl group, an allyl group, a methallyl 
group, a crotyl group, a propargyl group, a cyclohexyl group, a benzyl 
group, a phenyl group, a cresyl group, a 2-chloroethyl group, a 
3-chloropropyl group, a 2-chlorobutyl group, a trifluoroethyl group, a 
2-methoxyethyl group, a 3-methoxybutyl group and a 2-ethoxyethyl group. 
In the monomer of formula V, R.sup.4 is preferably H but provided that X 
and Y are both --CN, R.sup.4 may suitably be a C.sub.1 -C.sub.20 
hydrocarbyl group, more particularly a C.sub.1 -C.sub.20 alkyl group. 
The most preferred monomers of the formula V are those of the formula 
##STR6## 
wherein R.sup.7 is a C.sub.1 -C.sub.5 alkyl or C.sub.2 -C.sub.5 alkenyl 
group, more particularly ethyl 2-cyanopenta-2,4-dienoate or allyl 
2-cyanopenta-2,4-dienoate. 
In the case of deposition from solution, a polymer is prepared and then 
dissolved in a suitable solvent such as dichloromethane, acetone, 
nitromethane, tetrahydrofuran, acetonitrile, or chloroform. In the case of 
vapour deposition processes, the monomer vapours may be generated from the 
monomers at ambient temperatures and pressures but it is generally 
preferred to heat the monomers and/or reduce the atmospheric pressure 
above the monomer generated in the chamber in order to generate sufficient 
concentrations of vapour to accomplish the polymer deposition on the 
substrate in a reasonable time. 
Virtually any substrate upon which a polymeric image is desired may be 
utilized in the inventive processes. Most advantageously, the substrates 
will be ones which undergo subsequent acid or plasma etching during which 
the polymer coating serves as an etch resist. Suitable substrate materials 
include silicon dioxide, including SiO.sub.2 coated silicon, metallic 
oxides, and glass, all of which may be etched by plasma or acid etching 
processes. Metallic substrates which can be etched by acid processes, such 
as copper coated epoxy/glass boards used in printed circuit board 
manufacture and metal printing plates may also be utilized in the 
inventive process. Where the inventive process is used to produce an etch 
resist, the resist coating may be removed after etching by treatment with 
dilute caustic solution (e.g. NaOH) or by exposure to a suitable plasma 
(e.g. oxygen plasma). 
The preferred substrate is SiO.sub.2 coated silicon, e.g. the silicon chips 
conventionally used in preparation of semi-conductor devices. Most 
suitably, this substrate is etched by plasma etching process. 
In the case of vapour deposition processes, no surface treatment will be 
necessary if the substrate surface is inherently active for inducing 
anionic or zwitterionic polymerization of the monomer. In certain cases, 
however, where the substrate is slightly acidic or neutral it is necessary 
to activate the surface with a basic liquid or vapour which is 
substantially removed before exposing the substrate to the monomer vapour. 
Suitable activators include the known initiators for anionic or 
zwitterionic polymerization of alkyl cyanoacrylates. Especially suitable 
activators are organic amines and phosphines. 
In the imaging method of the second aspect of the invention, a conventional 
solvent development process may be used to develop the image, e.g. 
immersion in ethyl acetate, isobutyl methyl ketone, acetone or blends of 
ethyl acetate with either of isobutyl methyl ketone and acetone. Compounds 
which release acid upon irradiation for the process of the third aspect of 
the invention include any compounds which release Lewis or protonic acids 
such as those known as photoinitiators for cationically polymerizable 
resins such as epoxies or vinyl ethers. Additionally included are 
compounds which release sulfonic acids upon irradiation and are known as 
photolytically releasable latent thermal catalysts for acid curable 
stoving lacquers. 
Suitable radiation sensitive acid precursors useful in the inventive method 
include salts of complex halogenides represented by the formula 
EQU [A].sub.d.sup.+ [MX.sub.e ].sup.-(e-f) 
wherein A is a cation selected from iodonium, iodosyl, Group VIa onium, 
pyrylium, thiopyrylium, sulfonylsulfoxonium, and diazonium, M is a metal 
or metalloid, X is a halogen radical, d=e-f, f=the valence of M and is an 
integer equal to from 2 to 7 inclusive and e is greater than f and is an 
integer having a value up to 8; compounds of the formula 
EQU R.sup.8 [O.SO.sub.2 --CQ.sub.3 ].sub.n 
wherein R.sup.8 is an organic radical of valency 1 to 4 and Q is hydrogen 
or fluorine and n is an integer from 1 to 4; and compounds which release 
sulfonic acids when irradiated such as those disclosed in U.S. Pat. Nos. 
4,504,372 and 4,510,290, both incorporated herein by reference. 
The acid generating compound may be applied neat or in a solvent which is 
subsequently evaporated. If a surface activator is also to be applied to 
the substrate, both the activator and the acid generating compound may be 
applied simultaneously in a common solvent. Alternatively, the activator 
may be applied before or after application of the acid generating 
compound. 
Only trace amounts of surface activator and acid generating compound are 
necessary. Mirror finish substrates may be repolished, e.g. with a 
suitable tissue, after application of these compounds and still retain 
sufficient activator and acid generator to give sharply imaged resists 
after irradiation and exposure to monomer vapour.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention is further illustrated by the following non-limiting 
examples. 
EXAMPLE 1 
Acrolein (50 g, 0.89 moles) was added dropwise over 15 minutes to a stirred 
solution of ethyl cyanoacetate (65 g, 0.58 moles) in tetrahydrofuran (THF, 
200 mls) containing zinc chloride (50 g). After stirring for 19 hours at 
room temperature, the clear yellow solution was diluted with petroleum 
spirit b.p. 40.degree.-60.degree. C. (200 mls) and the mixture washed with 
dilute hydrochloric acid (0.1 m, 4.times.100 ml. portions) and then water 
(3.times.100 ml portions). The petroleum extract was dried (Na.sub.2 
SO.sub.4), filtered and the solvent removed under reduced pressure to 
yield an oil (66.6 g, 76%) which solidified to a waxy material after 
several hours. Spectral analysis of the product showed it to be consistent 
with the structure of ethyl 2-cyanopenta-2,4,-dienoate: 
##STR7## 
I.R. (K Br Disc); 2,220 cm.sup.-1, --CH group; 1,730 cm.sup.-1, --C.dbd.O 
group; 1,620 cm.sup.-1, H.sub.2 C.dbd.C-- group; 1,580 cm.sup.-1, 
--CH.dbd.C-- group. 
n.m.r. (CDCl.sub.3); 2.1, d, 1H, (--CH.dbd.C(CN)COOR); 3.0 m, 1H, 
(.dbd.CH--C); 3.9, m, 2H, (CH.sub.2 .dbd.C&lt;); 5.6, q, 2H, (O--CH.sub.2 
--); 8.6, t, 3H, (--CH.sub.3). 
EXAMPLE 2 
A polished silicon wafer, 3 inches (7.5 cm) in diameter, was activated by 
pouring a sufficient quantity of a solution of 10% 
N,N,N,N,-tetramethylethylene-diamine (TMED) in 
1,1,1,3,3,3,hexamethyldisilazane (HMDS) to cover the surface. The wafer 
was then spun at 4,000 rpm for 30 seconds to restore the mirror finish and 
mounted in the top of a closed cylindrical chamber 11 cm in diameter 
consisting of an aluminium base and plastic sides 2 cm in height into 
which 2.0 grams of ethyl 2-cyano-penta-2,4-dienoate (Example 1) had been 
placed. The chamber was mounted on a thermostatically controlled hot plate 
and preheated to 50.degree. C. prior to the introduction of the activated 
wafer. The wafer was mounted with the treated side 2 cms directly above 
the heated monomer liquid and exposed to its vapours for 10 minutes. 
During this period, a thin polymer film was formed on the exposed wafer 
surface. 
The coated wafer was then imagewise exposed to ultraviolet light from a 
medium pressure mercury arc lamp (operating at 80 W per cm.) through a 4 
inch (10 cm) square chrom plated quartz test mask which had alternate 
opaque and transmissive elements of varying sizes over the range 1000-1 
micrometers patterned on the surface. To ensure adequate contact between 
the mask and coated wafer a copper plate 4 inches (10 cm) square and 5/8 
inches (1.6 cm) in thickness with a 2 inch (5 cm) square centralized hole, 
was placed on the perimeter of the mask. The weight of the plate was 1 
kilogram. After 5 minutes exposure, at a distance of 20 cms. directly 
below the arc lamp, the wafer was removed and immersed in a bath of ethyl 
acetate for 60 seconds during which time a negative tone image of the mask 
had developed. The imaged wafer was rinsed in petroleum ether b.p. 
40.degree.-60.degree. C. for 30 seconds and examined microscopically. The 
minimum feature size measured using a Filer eyepiece was found to be 2.5 
micrometers. 
EXAMPLE 3 
Allyl 2-cyanopenta-2,4-dienoate was prepared by the method described in 
Example 1 by replacing ethyl cyanoacetate by an equivalent quantity of 
allyl cyanoacetate. The yield of product from this reaction was 80%. The 
structure of the product was confirmed by an infra-red spectrum. 
EXAMPLE 4 
Thin films of poly (allyl-2-cyanopenta-2,4-dienoate) were vapour deposited 
directly from the monomer (Example 3) at 40.degree. C. onto preactivated 3 
inch (7.5 cm) silicon wafers by the method described in Example 2. The 
amounts of polymeric material deposited for varying monomer exposure times 
were determined by weighing the wafers on a semi-micro balance before and 
after deposition. The results were 
______________________________________ 
Exposure time to 
Monomer Vapour (Mins) 
Coating Weight (Mgs) 
______________________________________ 
Ref A 20 1.54 
B 27 2.35 
C 40 3.95 
D 55 4.49 
______________________________________ 
EXAMPLE 5 
Resist coated wafer A (Example 4) was imagewise exposed to UV light as 
described in Example 2 for 3 minutes. After development with acetone (2 
minutes) and rinsing with ethanol a negative tone image was observed with 
a resolution of 5 micrometer sized features. 
EXAMPLE 6 
A polished silicon wafer, 3 inches (7.5 cm) in diameter, was treated with 3 
drops of photocationic catalyst UVE 1014 (a photocatalyst supplied by 
General Electric Company which is described as a 50% solution of a 
substituted triphenyl sulfonium hexafluoro-antimonate and which is known 
to produce strong acid on irradiation with UV light from a mercury arc 
lamp). The wafer was then polished with tissue paper to restore the mirror 
finish. 3 drops of amine TMED were then brushed uniformly across the 
surface of the wafer which was then polished with a paper tissue again to 
restore the mirror finish. The wafer was then imagewise exposed to UV 
light (Example 2) for 60 seconds through a 1 mm thick aluminium grip plate 
mask containing 3 mm diameter holes regularly spaced at approximately 2 mm 
intervals. After irradiation the wafer was placed in the vapour chamber 
described in Example 2 for 10 minutes. After this time, a thin polymer 
film had deposited on the unexposed regions of the wafer surface yielding 
an accurate positive tone image of the mask pattern. 
EXAMPLE 7 
A solution of photocatalyst UVE 1014 (0.3%), TMED (2%) in acetone was 
spin-coated onto a silicon wafer. The wafer was imagewise exposed to UV 
light through a test mask as described in Example 2 for 66 seconds. The 
wafer was then placed in the vapour deposition chamber (described in 
Example 2) containing allyl 2-cyanopenta-2,4-dienoate (Example 3) at 
40.degree. C. for 9.5 minutes. During this time, a positive tone image of 
the mask pattern was formed by selective polymer deposition on the 
unexposed regions of the silicon surface. While the pattern across exposed 
area of the wafer was not uniform, resolved features of 10 micrometers 
size were observed in some areas. 
EXAMPLE 8 
The experiment of Example 7 was repeated for 30 seconds UV irradiation and 
9.5 minutes monomer exposure in the vapour chamber. In this case the best 
resolved feature sizes were 2.5 micrometers. 
EXAMPLE 9 
The experiment of Example 7 was repeated for 20 seconds irradiation and 12 
minutes monomer exposure. An imaged pattern over the entire UV exposed 
area was obtained. Resolution was found to vary over the range 2.5-10.0 
micrometers. 
EXAMPLE 10 
A solution of the polymer derived from the monomer described in Example 1 
was prepared by dissolving the polymer (8 g) in dichloromethane (100 g). A 
3-inch (7.5 cm) pre-weighed silicon wafer was spin coated with an excess 
of the polymer solution for 10 seconds at 4,000 R.P.M. Residual solvent 
was removed in a nitrogen stream. The weight of coating deposited was 
found to be 6.81 mg which corresponds to a film thickness of approximately 
1.2 micrometers. 
The resist coated wafer was then imagewise exposed to ultraviolet light (as 
described in Example 2) for three minutes, cooled and immersed in an ethyl 
acetate bath for 15 seconds. A negative tone image was formed during the 
solvent development and a subsequent microscopic examination revealed 
resolved feature sizes with 15 micrometer dimensions. 
EXAMPLE 11 
Comparative Example 
A 1 cm.sup.2 polished silicon test wafer was activated by the method 
outlined in Example 2 and mounted polished side exposed on a 3 inch (7.5 
cm) support wafer by means of a small piece of two-sided adhesive tape. 
The bonded assembly was introduced into the vapour coating chamber 
containing ethyl 2-cyanopenta-2,4-dienoate (Example 1) at 50.degree. C. 
again as described in Example 2. After 5 minutes vapour exposure, the 
polymer coated wafer was withdrawn from the chamber and carefully 
separated from the support wafer such that no adhesive residue remained on 
the reverse, uncoated side of the test wafer. 
The procedure was repeated using allyl 2-cyano-penta-2,4-dienoate and the 
corresponding prior art vapour deposited monomers ethyl and allyl 
2-cyanoacrylates. 
The polymer coated wafers were placed side by side on a conventional 
ceramic coated laboratory hot plate which had been modified such that the 
surface temperature could be monitored by means of a calibrated 
thermocouple. 
The coated wafers were heated at a rate of approximately 30.degree. C. per 
minute and the phase changes which occurred were noted as a function of 
temperature. The results obtained were as follows: 
______________________________________ 
Polymer Phase Change 
Temperature .degree.C. 
______________________________________ 
Ethyl 2-cyanoacrylate 
Liquefaction 
184 
Evaporation 
193 
Ethyl 2-cyanopenta-2, 
Liquefaction 
276 
4-dienoate Evaporation 
300 
Allyl 2-cyanoacrylate 
Liquefaction 
141 
Evaporation 
146 
Allyl 2-cyanopenta-2,4-dienoate 
Liquefaction 
Not observed 
Evaporation 
235 
______________________________________ 
This result demonstrates that vapour deposited polymer layers of the 
present invention have significantly improved thermal stability over the 
prior art materials. The difference in evaporation temperature for the 
ethyl esters is 107.degree. C. and for the allyl esters 89.degree. C.