Energy-sensitive resist material and a process for device fabrication using an energy-sensitive resist material

The present invention is directed to a process for device fabrication and resist materials that are used in the process. The resist material contains a polymer that contains monomeric units that contain alicyclic formate moieties, and at least one other type of monomeric unit. The polymer may be formed by polymerization or by polymer modification of an existing polymer, and the resulting polymer either has alicyclic moieties incorporated into the polymer backbone or pendant to the polymer backbone via saturated hydrocarbon linkages. A preferred polymerization process is free radical polymerization, in which other monomers are selected for polymerization with the alicyclic moiety-containing monomer on the basis of the ability of the monomer to copolymerize by free radical polymerization. Although the polymers are contemplated as useful in resist materials that are sensitive to radiation in the ultraviolet, and x-ray wavelengths as well as sensitive to electron beam radiation, the polymers are particularly advantageous for use in process in which the exposing radiation is 193 nm, because the amount of ethylenic unsaturation in these resist materials is low.

FIELD OF THE INVENTION 
The present invention relates generally to an energy sensitive resist 
material and a process for device fabrication in which the energy 
sensitive resist material is used. 
BACKGROUND OF THE INVENTION 
Lithographic processes are typically employed in the manufacture of devices 
such as semiconductor devices, integrated optics, and photomasks. Such 
processes utilize various energy sources to create a relief image in a 
film of resist material applied onto a substrate. A positive or negative 
image of the desired device configuration is first introduced into the 
resist by exposing it to patterned radiation which induces a chemical 
change in the exposed portions of the resist. This chemical change is 
exploited to develop a pattern in the resist, which is then transferred 
into the substrate underlying the resist. 
The efficacy of a lithographic process depends at least in part on the 
resist used to transfer the pattern into the substrate. Certain types of 
resists offer particular advantages in the context of specific 
lithographic processes. For example, solution-developed resists are 
designed to have absorption characteristics appropriate for use at certain 
exposure wavelengths. It is axiomatic that, if the resist material is 
opaque to the exposing radiation, the exposing radiation will not be 
transmitted into the resist material and the desired chemical change will 
not occur. Therefore it is important to select a resist material that has 
the appropriate light transmission characteristics at the wavelength of 
the exposing radiation. Other considerations that drive the selection of 
an appropriate resist material include the etch resistance of the resist 
after it has been exposed and developed. 
In this regard, resist materials that contain polymers with ethylenic 
and/or aromatic unsaturation are typically used in lithographic processes 
for device fabrication in which the wavelength of the exposing radiation 
is in the traditional ultraviolet (UV) or deep UV range (i.e., about 240 
nm to about 370 nm). These resist materials, however, are often not 
suitable in processes in which the exposing radiation is 193 nm because 
the carbon-carbon double bond absorbs radiation at this wavelength. 
Consequently, resist materials typically used for lithographic processes 
using wavelengths of 248 nm or more are generally not useful in processes 
using a wavelength of 193 nm. Because lithographic processes for 
fabricating devices using 0.18 .mu.m, 0.13 .mu.m, and smaller design rules 
are likely to use 193 nm-wavelength light as the exposing radiation, 
resist polymers that do not contain significant amounts of ethylenic 
unsaturation are sought. 
Certain protective groups, when attached to the polymer, function as 
moieties that render the polymer relatively insoluble in alkaline (basic) 
solution. In lithographic processes, these moieties are removed upon 
irradiation and baking of the polymer film in the presence of a 
radiation-induced acid, and the polymer then becomes relatively more 
soluble in alkaline solution. After a substantial percentage of the 
moieties, for example, t-butyl carbonate, t-butyl ester, or t-butyl ether, 
have been cleaved from the exposed polymer, the polymer in the exposed 
region of the film is substantially more soluble in an aqueous alkaline 
developing solution. 
The moieties are not cleaved from polymer in the unexposed regions. 
Therefore, the resist material in those regions is not as soluble in an 
alkaline solution. If an alkaline solution is used to develop the image 
projected onto the resist, the material in the exposed regions is 
dissolved by developer solution while the material in the unexposed 
regions is not. It is by this mechanism that a positive tone image is 
developed that corresponds to the image projected into the resist 
material. 
If light is used as the energy source in a lithographic process, the 
process is referred to as photolithography. If such photolithographic 
processes utilize an exposure that occurs simultaneously over an entire 
device or a number of devices being processed on a substrate, the process 
utilizes what is considered a blanket exposure. A material, i.e., a 
resist, which is sensitive to the exposing radiation is coated onto a 
substrate, e.g., a silicon substrate, on which a plurality of devices will 
be formed. The coating material may be pre-exposure baked and is subjected 
to spatially discrete radiation, e.g., light that has been passed through 
a mask so that the light reaching the resist defines a discrete area. The 
discrete area defines a pattern that is to be transferred onto the 
underlying substrate either by negative or positive tone. The coated 
substrate is, if desired, post-exposure baked prior to image development. 
The resists used in photolithography are referred to as "photoresists." 
A blanket exposure is advantageous because it is relatively fast compared 
to other methods such as the raster scan technique that is employed when 
the energy used to expose the resist is a beam of electrons. Generally, 
however, the resolution that is achieved through a blanket exposure with 
near ultraviolet or visible light is somewhat poorer than that achieved 
with other methods such as electron beam lithography. 
Improved resolution with a blanket exposure can be achieved by using 
shorter wavelength light such as deep ultraviolet or X-ray light. One 
approach to a photoresist sensitive to shorter wavelength radiation 
employs a photoacid generator (PAG) that produces an acid moiety upon 
irradiation with deep ultraviolet light, together with a polymer that 
reacts under the influence of heat with the generated acid. Such systems 
are generally referred to as chemical amplification systems because the 
production of one molecule of acid by radiation (e.g., light) induces a 
reaction in a plurality of reactive substituents in the acid-sensitive 
polymer. Because protective groups are not cleaved from the resist polymer 
in the unexposed regions, it follows that acid is preferably not generated 
or otherwise present in the unexposed regions. 
Attempts have been made to improve the sensitivity and to reduce the fill 
shrinkage of chemically amplified resists. By improving the sensitivity of 
resists, less energy is required to create the image throughout the resist 
layer. In this regard, resist materials that have been partially 
deprotected have demonstrated enhanced sensitivity. Partial deprotection 
means that some, but not all, of the protective groups are cleaved from 
the polymer prior to use of the polymer in a lithographic process. 
Although chemically amplified resists show great promise for fine line 
resolution, these materials have demonstrated a tendency to shrink and 
crack after the exposure and post-exposure baking steps of the 
lithographic process. Shrinkage and cracking occur when the protected 
polymer is heated in the presence of acid, which releases the protecting 
groups in the form of gaseous products such as CO.sub.2 and isobutylene 
from the polymer. Such shrinkage produces a loss of image quality and, in 
part, counteracts resolution improvement available through use of deep 
ultraviolet, X-ray, or electron beam exposure. In device fabrication, 
because of particularly fine design rules, this film shrinkage can 
significantly affect the quality of the features produced in the 
lithographic process. Thus, although chemically amplified resists are 
extremely promising, some improvement is desirable. 
One type of resist material that has been suggested as suitable for 193 nm 
lithographic processes contains a derivatized acrylate or methacrylate 
copolymer. While resist materials that contain these copolymers 
demonstrate adequate sensitivity to 193 nm wavelength radiation, the 
plasma etch resistance of these copolymers does not meet current 
processing requirements. Therefore, resist materials that are compatible 
with 193 nm lithographic processes are desired. 
SUMMARY OF THE INVENTION 
The present invention is directed to materials for use in lithographic 
processes, and to the processes themselves, in which an alicyclic 
formate-containing polymer is incorporated into the resist material. The 
alicyclic formate-containing polymer has properties which make it 
particularly suited for use in a lithographic process, and has advantages 
such as faster photospeed and improved contrast over typically used 
resists. 
In addition, because the formate moieties in the resist film do not undergo 
acid catalyzed hydrolysis or acidolysis in the exposed area, there is no 
volume loss associated with the presence of these formate moieties on the 
polymer. The volume difference between exposed and non-exposed areas of 
the resist after exposure and bake is caused by moieties that undergo 
acid-catalyzed hydrolysis or acidolysis. The presence of the formate 
moieties alleviates stress which may build up in the resist film and 
adversely affect imaging. 
The polymer of the present invention is either the polymerization product 
of imaging and non-imaging monomers or analogous materials prepared by 
polymer modification. From this point on we will describe polymer 
composition in terms of the monomeric units that make up the polymer. 
Because in a polymer these monomeric units are repeated, these monomeric 
units are referred to as repeat units. Therefore, according to this 
definition, a polymer is conceived as consisting of imaging monomeric 
units and non-imaging monomeric units. It is, however, conceivable that 
analogous compositions of polymer repeat units could be prepared by 
polymer modification of an existing polymer instead of polymerizing 
distinct imaging and non-imaging monomers. 
An imaging monomer is defined as a monomer with pendant acidic moieties 
that can be deprotonated by developer (e.g., acrylic acid, norbornene 
carboxylic acid) or containing these pendant acidic moieties protected 
with a functional group capable of acidolysis or hydrolysis (e.g., 
t-butylacrylate, t-butyl norbornene carboxylate) resulting from 
photo-generated acid. A non-imaging monomer contains a moiety that has one 
or more ethylenically unsaturated bonds, and does not contain pendant 
acidic moieties. The ethylenic functionality of the monomers is either 
contained as part of a non-cyclic hydrocarbon or as part of a cyclic 
hydrocarbon or pendant to these moieties. 
The copolymer of the present invention contains about twenty-five to about 
ninety-five mole percent, preferably about fifty to ninety-five mole 
percent, of total non-imaging monomers, and about five to seventy-five 
mole percent, preferably about five to fifty mole percent, of one or more 
imaging monomers. The non-imaging monomers include alicyclic monomers and 
may include other monomers such as maleic anhydride, maleimides, and 
non-imaging acrylic or methacrylic monomers. Imaging monomers include 
monomers such as acrylic acid, methacrylic acid, norbornenecarboxylic 
acid, t-butyl acrylate, t-butyl methacrylate, or t-butyl 
nornbornenecarboxylate. 
Preferred polymers are a copolymer of norbornenyl formate and norbornene 
carboxylic acid, a copolymer of norbornenyl formate norbornenecarboxylic 
acid and t-butyl norbornenecarboxylate, a copolymer of norbornenyl 
formate, maleic anhydride and acrylic acid, a copolymer of norbornyl 
formate, maleic anhydride norbornenecarboxylic acid and t-butyl 
norbornenecarboxylate, and a copolymer of dicyclopentenyl formate and 
t-butyl norbornenecarboxylate. Copolymers containing carboxylic acid 
moiety alone are envisaged as being used along with an imageable 
dissolution inhibitor, while copolymers containing a protected carboxylic 
acid moiety may be used alone or in the presence of an imageable 
dissolution inhibitor. An imageable dissolution inhibitor is a dissolution 
inhibitor having pendent acid moieties or protected acid moieties as 
previously described. 
Additional advantages and features of the present invention will be 
apparent from the following detailed description and examples which 
illustrate preferred embodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
In the following detailed description, reference is made to the 
accompanying examples which form a part hereof, and in which is shown by 
way of illustration specific embodiments in which the invention may be 
practiced. These embodiments are described in sufficient detail to enable 
those skilled in the art to practice the invention, and it is to be 
understood that structural and chemical changes may be made without 
departing from the spirit and scope of the present invention. 
The terms "wafer" and "substrate" are to be understood as including 
silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, doped 
and undoped semiconductors, epitaxial layers of silicon supported by a 
base semiconductor foundation, and other semiconductor structures. 
Furthermore, when reference is made to a "wafer" or "substrate" in the 
following description, previous process steps may have been utilized to 
form regions or junctions in the base semiconductor structure or 
foundation. In addition, the semiconductor need not be silicon-based, but 
could be based on silicon-germanium, germanium, or gallium arsenide. The 
following detailed description is, therefore, not to be taken in a 
limiting sense, and the scope of the present invention is defined by the 
appended claims. 
The present invention is directed to a class of energy-sensitive resist 
materials that are useful in lithographic processes for device 
fabrication. Processes for device fabrication which include lithographic 
steps have been described in treatises such as S. M. Sze, VLSI Technology, 
(McGraw-Hill pub., 1983) and L. F. Thompson et al., Introduction to 
Microlithography, pp. 87-161 (American Chemical Society Symposium Series 
219, 1983) which are hereby incorporated by reference. Lithographic steps 
typically include exposing and patterning energy definable materials such 
as resist materials. An image is first introduced into the resist and 
developed to form a pattern, which is then transferred into the substrate. 
The materials are energy-sensitive; i.e., energy induces a chemical change 
in these materials. When these materials are exposed to either patterned 
radiation of the appropriate wavelength, e.g., UV light with a wavelength 
of about 190 nm to about 370 nm, x-ray radiation or particle beams such as 
electron beams in direct-write lithographic processes, the chemical change 
is induced to a significantly greater extent in the portion of the resist 
material that is directly exposed to radiation than in the portion of the 
resist material that is not directly exposed to radiation. In the context 
of the present invention, significantly greater means that the chemical 
contrast induced by the patternwise exposure is adequate to meet 
processing objectives. This chemical difference is exploited to develop 
the pattern in the energy-sensitive resist material. The developed pattern 
is then used in subsequent processing, e.g., transfer of the developed 
pattern into an underlying substrate. 
Aqueous base solutions are typically used to develop patterns in energy 
sensitive resist materials. One common example of an aqueous base solution 
is an aqueous solution of tetramethylammonium-hydroxide (TMAH) that is 
about 0.05 M to about 0.5 M, although many other solutions are well known 
to one skilled in the art. 
In the positive-tone resist materials of the present invention, the 
material that is not exposed to radiation is relatively insoluble in the 
aqueous base developer solution relative to the material that is exposed 
to radiation. This difference in aqueous base solubility is effected 
either by manipulating the aqueous base solubility of the polymer in the 
energy sensitive material, or by manipulating the aqueous base solubility 
of a dissolution inhibitor in the energy-sensitive resist material. 
The energy-sensitive resist materials of the present invention contain a 
polymer with an alicyclic moiety that is either incorporated into the 
polymer backbone or pendant to the polymer backbone via saturated 
hydrocarbon linkages. In the context of the present invention, the polymer 
is described in terms of the molecules, known as monomers, which undergo 
polymerization to form the polymer. Generally, if polymers are formed from 
monomers that all have the same chemical structure and composition, they 
are referred to as homopolymers. Polymers that are formed from two or more 
chemically distinct types of monomers, e.g., norbornenecarboxylic acid and 
maleic anhydride, are referred to herein as copolymers. 
The copolymers of the present invention contain about twenty-five to about 
ninety-five mole percent, preferably about fifty to ninety-five mole 
percent, total non-imaging monomers. The non-imaging monomers comprise at 
least a first non-imaging monomer having a moiety that contains one or 
more ethylenically unsaturated bonds. The ethylenic functionality of the 
monomers is either contained as part of a non-cyclic hydrocarbon or as 
part of a cyclic hydrocarbon or pendant to these moieties. 
The first non-imaging monomer is substituted with a formate group and will 
be referred to herein as the formate non-imaging monomer. Examples of 
suitable alicyclic monomers are formate esters of hydroxy derivatives of 
cycloolefins such as norbornene, formate esters of hydroxy derivatives of 
cyclodiolefins, such as 1,5-cyclooctadiene, 
1,5-dimethyl-1,5-cyclooctadiene, 5,6-dihydrodicyclopentadiene, and 
tetracyclododecene, as well as formate esters of hydroxy derivatives of 
cycloacetylenes. Examples of suitable non-cyclic hydrocarbons are formate 
esters of 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate. 
The polymer may also contain additional non-imaging monomer(s), which are 
chosen to further enhance the performance of the resist materials in 
lithographic processes for device fabrication. In this regard, factors 
such as aqueous base solubility, adhesion promotion, and the absorbance of 
the other monomers at the wavelength of the exposing radiation are 
considered in making the selection. Other factors such as the glass 
transition temperature (T.sub.g) of the resulting polymer are also 
considered in selecting additional monomers. The polymer should preferably 
have a T.sub.g that is higher than 30 degrees Celsius, preferably higher 
than 50 degrees Celsius. If the T.sub.g is substantially lower than the 
given limit, there is a tendency for the resist to flow during subsequent 
processing, thereby degrading image quality. 
One skilled in the art will appreciate the various factors in considering 
the monomers from which to form polymers which are suitable for use in the 
process of the present invention. Suitable additional non-imaging 
monomer(s) do not hinder the free radical polymerization of the first 
non-imaging monomer and the imaging monomer(s). The additional non-imaging 
monomer(s) may be present in an amount up to about fifty mole percent of 
the polymer, but the total amount of the non-imaging monomers, including 
the first formate non-imaging monomer and any additional non-imaging 
monomer(s), should not exceed ninety-five mole percent of the polymer. 
Examples of suitable additional non-imaging monomers include maleimide 
monomers, and maleic anhydride monomers. Other monomers such as acrylate 
monomers, fumarate monomers, and acrylonitrile monomers are also 
contemplated as suitable if the polymerization takes place in the presence 
of a Lewis acid. If the polymer is to be used in an application in which 
the exposing radiation has a wavelength of 193 nm, it is advantageous if 
the additional non-imaging monomer(s) is selected so that the resulting 
polymer contains a lithographically insignificant amount of ethylenic 
unsaturation or other functionality with an absorbance that is too high at 
the wavelength of the exposing radiation. In the context of the present 
invention, "lithographically insignificant" means that the amount is not 
sufficient to cause adverse lithographic consequences. Maleic anhydride is 
an example of a monomer that is suitable for copolymerization with the 
alicyclic moiety to provide a polymer that is useful in lithographic 
processes in which the exposure wavelength is 193 nm. 
The imaging monomer(s) is selected for its ability to copolymerize with the 
non-imaging monomer via radical polymerization, metal catalyzed 
polymerization, and other methods known by practitioners in the art. It is 
advantageous if the imaging monomer undergoes free-radical polymerization 
with the non-imaging monomer in a manner that incorporates the alicyclic 
moiety in the polymer backbone. The polymer contains about five mole 
percent to about seventy-five mole percent of the imaging monomer(s), 
preferably about five mole percent to about fifty mole percent. 
Examples of suitable imaging monomers include acrylic acid, methacrylic 
acid, acrylates, and methacrylates, or a combination thereof The imaging 
monomer(s) is chosen to provide a polymer that is useful in lithographic 
processes. For example, an acrylic acid or acrylate monomer may 
advantageously lower the glass transition temperature (T.sub.g) of the 
polymer when incorporated therein. These monomers are either substituted 
or unsubstituted, as is desired to provide the polymer with advantageous 
properties. For example, if it is desirable to decrease the solubility of 
the polymer in aqueous base solution, a monomer having acid labile 
substituents may be used. Examples of suitable pendant groups include acid 
labile groups such as acetal groups, ketal groups, beta-silicon 
substituted alkyls such as bis(trimethylsilylmethyl)methyl, 
bis(trimethylsilyl)ethyl and 1-(trimethylsilylmethyl)methyl, tert-butyl 
esters, tert-butyl esters of carboxylic acids, and tert-butyl ethers. For 
convenience, "tert" is shortened to "t" hereinafter. However, it is 
understood that a wide range of acid labile groups are operative in the 
invention. In the presence of the photoacid, these groups produce a free 
carboxylic acid and acidolysis or acid-catalyzed hydrolysis byproduct. 
More than one type of imaging monomer may be used, so long as the total 
amount of imaging monomers in the polymer does not exceed about 
seventy-five mole percent, and preferably fifty mole percent. For example, 
a preferred polymer is the polymerization product of four monomers: the 
first non-imaging (alicyclic) monomer, a second non-imaging monomer 
(maleic anhydride), and two imaging monomers - an acrylic acid or 
methacrylic acid monomer, and a substituted or unsubstituted acrylate or 
methacrylate monomer. When selecting suitable monomers for radical 
polymerization, the reactivity ratios of the monomers, and the relative 
amount of the monomers in the feed composition for the polymerization must 
be taken into account. These relationships are discussed generally in 
Polymer Handbook, chap. II.5 (Brandrup, J. et al. eds., 2nd ed., 1989). 
One example of a suitable copolymer is the polymerization product of 
norbornen-5-yl formate monomer with norbornene carboxylic acid. An example 
of the polymer is represented by the following structure: 
##STR1## 
wherein 1 is about 0.25 to about 0.95, n is about 0.05 to about 0.75, and 
1+n=1. Embodiments where 1 is about 0.50 to about 0.95 and n is about 0.05 
to about 0.50 are preferred. 
Another example of a suitable copolymer is the polymerization product of a 
norbornen-5-yl formate monomer, a maleic anhydride monomer, and acrylic 
acid. An example of the polymer is represented by the following structure: 
##STR2## 
wherein m is about 0.25 to about 0.95, n is about 0.05 to about 0.75, and 
m+n=1. Embodiments where m is about 0.50 to about 0.95 and n is about 0.05 
to about 0.50 are preferred. 
A third example of a suitable copolymer is the polymerization product of a 
norbornen-5-yl formate monomer, a maleic anhydride monomer, acrylic acid, 
and t-butyl acrylate. An example of the polymer is represented by the 
following structure: 
##STR3## 
wherein m is about 0.25 to about 0.95, n+p is about 0.05 to about 0.75, 
and m+n+p=1. Embodiments where m is about 0.50 to about 0.95 and n+p is 
about 0.05 to about 0.50 are preferred. 
Another example of a suitable copolymer is the copolymerization product of 
dicyclopentenyl formate, t-butyl norbornenecarboxylate, and norbornene 
carboxylic acid. An example of the copolymer is represented by the 
following structure: 
##STR4## 
wherein n is about 0.25 to about 0.95, m+p is about 0.05 to about 0.75, 
and n+m+p=1. Embodiments where n is about 0.50 to about 0.95 and m+p is 
about 0.05 to 0.50 are preferred. 
Resist materials are formed by combining the above-described polymers with 
other materials. In one embodiment of the present invention, the polymer, 
which is relatively insoluble in aqueous base solution due to the presence 
of the formate group of the alicyclic monomer, is combined with a 
photoacid generator (PAG) to form the resist material. In another 
embodiment, the resist material contains the polymer which is relatively 
insoluble in aqueous base solution, in combination with a dissolution 
inhibitor and a PAG. When the resist material is exposed to radiation, the 
photoacid generated by the PAG, typically in combination with a 
post-exposure bake, removes a sufficient amount of the acid cleaveable 
moieties present either on the resist polymer and/or on a dissolution 
inhibitor to render the exposed area more hydrophilic and make the formate 
moieties in this exposed area susceptible to rapid base hydrolysis in 
aqueous base solution and thus render the polymer resist soluble in 
aqueous base solution. Because the formate moieties do not undergo acid 
catalyzed hydrolysis or acidolysis in the exposed area there is less of a 
volume loss between exposed and unexposed areas after exposure and before 
development while still maintaining adequate base solubility for exposed 
resist areas. 
In the above-described embodiments, the solubility of the resist 
composition in aqueous base solution is altered when the resist material 
is exposed to radiation. Since the resist materials of the present 
invention are positive resists, the aqueous base solubility of the exposed 
resist is greater than the aqueous base solubility of the resist that is 
not exposed to radiation. If the ratio of the rate of dissolution of the 
resist before irradiation compared to that after irradiation is taken as 
1:n, n should be not be less than 2. Relative solubility ratios with 
values of n less than 2 typically produce low contrast and inferior image 
quality. 
As noted above, the PAG generates an acid (the photoacid) when the resist 
material is exposed to radiation. Generally, the resist material is about 
0.5 weight percent to about 20 weight percent (based on the weight of the 
resist material excluding solvent) PAG combined with the polymer. If the 
PAG content is above about 20 weight percent of the resist material, the 
optical density of the resist material may be too high and its presence 
above this concentration may hinder development. The amount of PAG used 
depends on the composition of the PAG and upon the wavelength of the 
exposing radiation. For example, if the resist material will be used in a 
lithographic process in which the wavelength of the exposing radiation is 
about 193 nm, and the PAG contains an aromatic moiety, the amount of PAG 
in the resist material is about 0.5 to about 4 weight percent, because the 
aromatic unsaturation in these moieties absorbs radiation at this 
wavelength. 
The photoacid cleaves the acid labile groups from the polymer or the 
dissolution inhibitor, typically during a post-exposure bake. The cleavage 
of these acid labile groups in the resist causes the exposed formate 
moieties in the exposed areas to be susceptible to rapid base hydrolysis 
during development which makes the exposed area more soluble than the 
unexposed resist area. An aqueous base developer solution is then used to 
dissolve and remove the exposed resist material. The unexposed resist 
material is then used as a patterned mask for subsequent processing of the 
underlying substrate; typically for pattern transfer into the substrate. 
Suitable PAGs include triflates (e.g. triphenysulfonium triflates, 
tris(t-butylphenyl)sulfonium triflate) and the corresponding salts of 
other perfluorinated alkyl sulfonic acids (e.g. perfluorobutanesulfonate, 
perfluorooctanesulfonate), other onium salts such as triarylsulfonium and 
diarylsulfonium hexafluoroarsenate, hexafluoroarsenate, triflate 
perfluoroalkylsulfonates and others, pyrogallol (e.g. trimesylate of 
pyrogallol), perfluoroalkylsulfonate (e.g. perfluorobutanesulfonate) and 
other sulfonate esters of hydroximides, a,a',-bis-sulfonyl diazomethanes, 
sulfonate esters of nitro-substituted benzyl alcohols and 
napthoquinone-4-diazides. Other suitable photoacid generators are 
disclosed in U.S. Pat. Nos. 5,045,432 and 5,071,730 to Allen et al., the 
disclosures of which are incorporated herein by reference. 
The dissolution inhibitor typically has acid labile substituents which mask 
its aqueous base solubility, and is therefore used in conjunction with a 
PAG as previously described. It is envisaged that the dissolution 
inhibitor in addition to its acid labile group may also possess a base 
hydrolyzable formate moiety which will act in the same fashion as 
described for the formate moieties present on the polymer backbone (e.g., 
the tris formate ester of t-butyl cholate, the bis formate ester of 
t-butyl deoxycholate). The amount of dissolution inhibitor in the 
energy-sensitive material is about 10 to about 40 weight percent. The 
contrast between the portion of the resist material that is exposed to 
radiation and the unexposed portion is enhanced because the aqueous base 
solubility of both the polymer and the dissolution inhibitor is altered by 
the acid generated by the PAG when the resist material is exposed to 
radiation and subjected to a post-exposure bake. 
The choice of a particular dissolution inhibitor for use in the process of 
the present invention depends upon the wavelength of the exposing 
radiation and the absorption characteristics of the particular dissolution 
inhibitor. In processes in which the wavelength of the exposing radiation 
is 248 nm, napthoquinone diazide dissolution inhibitors such as those 
described in Reiser, A., Photoreactive Polymers: The Science and 
Technology of Resists, chapters 5 and 6 (John Wiley & Sons, pub. 1989) and 
Dammel, R, Diazonapthoquinone-based Resists, (SPIE Optical Engineering 
Press 1993), which are hereby incorporated by reference, are contemplated 
as suitable. Diazides such as napthoquinonediazide and a pentaester of a 
hexahydroxyspirobifluorene with napthoquinone-2-diazide-5-sulfonic acid 
are examples of such dissolution inhibitors. It is advantageous if the 
energy-sensitive resist material is about 10 to about 35 weight percent 
dissolution inhibitor with a molecular weight of less than about 2000. 
The napthoquinone diazide dissolution inhibitors absorb radiation at 248 nm 
and are even more absorbent at 193 nm. In fact, these dissolution 
inhibitors absorb too strongly to be incorporated into resist materials 
for use in processes in which the wavelength of the exposing radiation is 
193 nm. Consequently, dissolution inhibitors that do not contain ethylenic 
saturation and are predominantly composed of alicyclic moieties are 
advantageous in the embodiment of the present invention in which the 
wavelength of the exposing radiation is 193 nm. Examples of suitable 
dissolution inhibitors include bile acid esters derived from cholic acid, 
deoxycholic acid, ursocholic acid, and lithocholic acid. The use of these 
dissolution inhibitors in lithographic process for device fabrication is 
described in U.S. Pat. No. 5,310,619 to Crivello et al. and U.S. Pat. No. 
5,372,912 to Allen et al., both of which are hereby incorporated by 
reference. 
Resist quality is significantly enhanced by selection of an appropriate 
optical density in the wavelength range of the exposing radiation. Too low 
an optical density results in inefficient absorption of the exposing 
radiation and in unnecessarily long exposure times. Too high an optical 
density does not permit sufficient light to reach the regions of the 
polymer film furthest removed from its ambient/resist film interface. This 
incomplete exposure tends to degrade resist image quality. In general, it 
is desirable to employ an optical density that is preferably less than 0.8 
for at least 23 percent of the exposing radiation to reach the substrate 
at the exposure wavelength. The optical density depends on the 
concentration of the absorbing species in the polymer and the PAG. Thus, 
once a suitable thickness for the resist material coating is chosen, the 
resist composition is adjusted to provide the desired optical density. 
Referring now to the drawings, an embodiment of the present invention for 
device fabrication using the energy sensitive resist material described 
above is illustrated by FIGS. 1 through 4. FIG. 1 depicts a substrate or 
wafer 20, typically a silicon wafer, being coated with a film 22 
comprising the energy sensitive resist material, which is formulated as 
described above, dissolved in a suitable solvent, such as propylene glycol 
methyl ether acetate (PGME) or cyclohexanone. The film 22 can be coated on 
the substrate using known techniques such as spin or spray coating, or 
doctor blading. The thickness of the coating depends upon a variety of 
factors such as the absorption of the resist, the quality of the film, and 
the effect of thickness on image resolution. Typically, the thickness of 
the resist is the range of about 0.2 .mu.m to about 2.0 .mu.m, preferably 
0.3 .mu.m to about 1.0 .mu.m. Thinner coatings are difficult to maintain 
pinhole-free, and thicker coatings generally have inferior resolution 
because the delineation of narrow features produces narrow columns in the 
developed pattern that tend to deform, and because greater absorption may 
occur, thereby degrading image quality. 
After coating, the resist 22 is preferably prebaked to remove any remaining 
solvent. Pre-exposure baking temperatures in the range of 70.degree. C. to 
160.degree. C. for times in the range of about 0.5 to about 60 minutes are 
desirable. The resist material is then patternwise exposed to energy 100 
such as ultraviolet radiation with a wavelength of about 190 nm to about 
370 nm, x-ray radiation, or electron beam radiation, as shown in FIG. 2. 
Typical doses in the range of 5 to 250 mJ/cm.sup.2 for 193 nm light are 
contemplated as suitable. (Corresponding doses for electron beam and x-ray 
irradiation are contemplated). Conventional exposure techniques are 
employed to delineate the pattern image in the resist material 22. 
The radiation is absorbed by the PAG to produce free acid in the exposed 
area, which then cleaves the acid labile group in the resist. It is then 
desirable to post-bake the resist film. This post bake enhances the 
reaction in the exposed area of the polymer or the dissolution inhibitor 
with the photoacid. Generally, post-bake temperatures in the range of 
70.degree. C. to about 170.degree. C. for time periods of about 20 seconds 
to about 30 minutes are effective. Heating means such as a hot plate sold 
by Brewer Sciences are contemplated as useful. 
Referring now to FIG. 3, the image in the resist 22 is then developed into 
a pattern by exploiting the difference in aqueous base solubility between 
the exposed resist material and the unexposed resist material. In the 
context of the present invention, the term "exposed resist material" 
implies the portion of the resist whose aqueous base solubility has been 
increased by exposure to the photoacid and, typically, heat. Solvents 
suitable for developing the exposed image are materials such as 
water/TMAH, water/NaOH, or mixtures of lower alkyl alcohols such as 
isopropanol, ethanol and methanol with or without water. Generally, 
immersion in the developer for time periods from 20 seconds to 5 minutes 
produces the desired delineation. During development the free acid 
moieties undergo deprotonation along with the formate moieties undergoing 
base hydrolysis. Both of these phenomena assist resist dissolution. 
After development, the pattern in the resist 22 is transferred into the 
underlying substrate 20 using conventional etching expedients well known 
to one skilled in the art, as shown in FIG. 4. For example, after the 
substrate has been exposed, circuit patterns can be formed in the exposed 
areas by coating the substrate with a conductive material such as a 
conductive metal by known techniques such as evaporation, sputtering, 
plating, chemical vapor deposition or laser induced deposition. The 
surface of the film can be milled to remove any excess conductive 
material. Dielectric materials may also be deposited by similar means 
during the process of making circuits. Inorganic ions such as boron, 
phosphorus or arsenic can be implanted in the substrate to form p or n 
doped transistors. Other means for forming circuits are well known to 
those skilled in the art. 
Application of the teachings of the present invention to a specific problem 
or environment is within the capabilities of one having ordinary skill in 
the art in light of the teachings contained herein. The following examples 
are detailed descriptions of methods of preparation and use of certain 
compositions of the present invention. The detailed preparations fall 
within the scope of, and serve to exemplify, the more generally described 
methods of preparation set forth above. The examples are presented for 
illustrative purposes only, and are not intended as a restriction on the 
scope of the invention. 
EXAMPLE 1 
Preparation of norbornen-5-yl formate 
Norbornen-5-ol (25 g) was dissolved at 55.degree. C. in 100 mL of formic 
acid (96%). Unless otherwise specified, all of the reagents specified in 
the examples herein are obtained from the Aldrich Chemical Co. After 2 
hours gas chromatography (GC) indicated that all norbornen-5-ol was 
consumed. The reaction mixture was cooled down and 100 mL of ether was 
added. Then, the reaction mixture was extracted with 10.times.100 mL of 
water. The organic layer was separated, dried and ether was removed. The 
product was distilled at 75.degree. C. (20 mm Hg). Yield: 17 g, 54% (95% 
pure by GC, no OH peak by IR). The same procedure afforded formyl ester of 
norbornen-5-methanol with 37% yield (b. p. 85.degree. C., 20 mm Hg, 96% 
pure by GC). 
EXAMPLE 2 
Preparation of 5-norbornenyl-2-yl acetate, maleic anhydride, and t-butyl 
acrylate copolymer 
A vacuum flask was charged with 5-norbornenyl-2-yl acetate (25 g, 164 
mmol), freshly distilled maleic anhydride (16.11 g, 164 mmol), acrylic 
acid (2.53 mL, 36.89 mmol), t-butyl acrylate (10.83 mL, 73.7 mmol) and 
2,2'-azobisisobutyronitrile (AIBN) (0.54 g, 3.28 mmol). A minimum amount 
of tetrahydrofuran (THF) was added to form a homogeneous mixture at 
60.degree. C. The mixture was degassed by three freeze-pump-thaw cycles 
and stirred at 65.degree. C. for 48 hours. Then, the polymerization was 
dissolved in 50-70 mL of acetone and precipitated into 500-700 mL of 
ether. The residue was collected by filtration, air dried and 
reprecipitated from acetone-ether two more times. The product, referred to 
hereinafter as polymer 1, was dried at 50.degree. C. in a vacuum oven. 
Yield: 30 g, 56%. 
EXAMPLE 3 
Preparation of 5-norbornen-2-yl formate, maleic anhydride, and t-butyl 
acrylate copolymer 
A vacuum flask was charged with 5-norbornen-2-yl formate (17.44 g, 125 
mmol), freshly distilled maleic anhydride (12.2 g, 125 mmol), t-butyl 
acrylate (8 mL, 71.42 mmol) and AIBN (0.34 g, 2.07 mmol). A minimum amount 
of THF was added to form a homogeneous mixture at 60.degree. C. The 
mixture was degassed by three freeze-pump-thaw cycles and stirred at 
65.degree. C. for 48 hours. Then, the polymerization was dissolved in 
50-70 mL of acetone and precipitated into 500-700 mL of ether. The residue 
was collected by filtration, air dried and reprecipitated from 
acetone-ether two more times. The product, referred to hereinafter as 
polymer 2, was dried at 50.degree. C. in a vacuum oven. Yield: 17.02 g, 
46%. 
EXAMPLE 4 
Analysis of the Dissolution Mechanisms of acetyloxy- and 
formyloxy-substituted Polymers in Standard Resist Developer 
A polymer containing norbornenyl acetate moiety (polymer 1), and a polymer 
containing norbornenyl formate moiety (polymer 2), were chosen for this 
work. Both polymers are formulated with 5 weight percent of thermal acid 
generator, 2-nitrobenzyl tosylate, as a 15 weight percent acetone 
solution. The formulations were poured into a beaker and the solvent was 
removed under a stream of nitrogen. Then, the beaker was placed in a 
convection oven at 150.degree. C. for 5 minutes. The resulting residue was 
ground and dissolved in a 0.262 N aqueous solution of tetramethyl ammonium 
hydroxide (TMAH). As soon as dissolution was completed (about 1 minute), 
the mixture was neutralized with acetic acid followed by dialysis against 
distilled water for two days. Water was stripped off and the polymers were 
examined by .sup.1 H and .sup.13 C NMR 
Acetate groups of polymer 1 give rise to peaks at 1.9 ppm [CH.sub.3 
C(O)O-protons] on .sup.1 H NMR spectrum or 20 ppm [CH.sub.3 C(O)O-carbons] 
and 170 ppm [CH.sub.3 C(O)O-carbons] on .sup.13 C NMR spectrum. If during 
the dissolution of polymer 1 a saponification of esters occured, peaks due 
to the acetate groups would decrease in intensity. Also, formation of 
norbornenol moiety from norbornenol ester should be accompanied by the 
shift of peaks due to the .dbd.CH--O-- group upfield from its original 
position at 4.8 ppm on .sup.1 H NMR spectrum and at 75 ppm on .sup.13 C 
NMR spectrum. None of these changes, however, were detected in .sup.1 H 
and .sup.13 C NMR spectra of polymer 1 after its dissolution 0.262 N 
aqueous TMAH, indicating that polymer 1 did not undergo saponification of 
the esters, i.e., the ester group was not removed during dissolution. 
After the deprotection, the NMR spectra of polymer 2 exhibits peaks at 8.2 
ppm (.sup.1 H NMR, HC(O)O-proton) or at 174 ppm (.sup.13 C NMR, 
HC(O)O-carbon). After the dissolution of deprotected polymer 2 in 0.262 N 
aqueous TMAH both of these peaks disappear. Moreover, peaks due to the 
norbornenol group .dbd.CH--O-- moved upfield from 74 ppm to 71 ppm on 
.sup.13 C NMR spectrum and from 5.1 ppm to 4.0 ppm on .sup.1 H NMR 
spectrum indicating removal of the ester group and formation of the free 
alcohol. 
The removal of the formate group of polymer 2 and the formation of 
tetramethyl ammonium formate can be observed directly by dissolving the 
polymer 2 in a 0.262 N solution of TMAH in D.sub.2 O and observing the 
changes by .sup.1 H NMR. Immediately after the dissolution, a sharp 
prominent peak at 8.5 ppm due to the tetramethyl ammonium formate appeared 
besides the broad peak at 8.2 ppm due to the norbornenyl formate unit. The 
reaction did not go to completion due to the small TMAH/norbornenyl 
formate ratio (about 5) which had to be employed in order to get good 
.sup.1 H NMR signal to noise ratio for polymer peaks. Five minutes after 
the beginning of the experiment, the solution was just slightly basic due 
to TMAH consumption. 
This experiment was designed to simulate actual conditions under which a 
resist containing the tested polymers would be used. The polymers were 
formulated with thermal acid generators, exposed to heat, and then 
subjected to dissolution in a standard resist developer, e.g., TMAH. If 
the ester undergoes saponification upon dissolution, then the material has 
suitable characteristics for use as a resist component. This is because 
the ester groups will be able to assist dissolution of the resist through 
the deprotonation of the free acid moieties by undergoing base hydrolysis. 
This will effectively reduce the volume loss difference observed between 
exposed and non-exposed areas of the resist after bake without 
deleteriously affecting the dissolution characteristics of the resist. The 
reduction of the stress in the resist will consequently result in better 
imaging. In contrast, if the esters do not saponify, then the polymer is 
less desirable for use because the ester moiety will inhibit dissolution 
and lead to images of poorer quality. 
It was established that during the dissolution of a polymer containing 
norbornenyl formate units (polymer 2) in 0.262 N aqueous TMAH, a 
significant saponification of the ester groups occurred. No such process 
was detected during dissolution of a polymer containing norbornenyl 
acetate units (polymer 1) in 0.262 N aqueous TMAH. Because polymer 2 
undergoes saponification of the esters upon dissolution, and polymer 1 
does not, polymer 2 is suitable for use in resist, but polymer 1 is less 
desirable. 
EXAMPLE 5 
Hydrolysis of dicyclopentenyl acetate, Preparation of dicyclopentenol 
NaOH (20 g) in 100 mL of water was added to the mixture of DDCP-acetate (60 
g) in 250 mL of methanol. The mixture was refluxed for 2 hrs and then 
stirred at 25.degree. C. overnight. After cooling, the aqueous layer was 
saturated with NaCl and extracted with 2.times.100 mL of ether. The 
combined organic layers were dried, solvents were stripped and product was 
distilled on high vacuum at 60.degree. C. Typical yield ranged between 
50-75%. 
EXAMPLE 6 
Preparation of dicyclopentenyl-formate(3) 
Dicyclopentenol (18 g) was dissolved in 50 mL of formic acid (usually it 
required a gentle warming). The mixture was stirred at 25.degree. C. for 2 
hrs. On cooling, saturated NaCl.sub.aq (50 mL) was added followed by 50 mL 
of hexanes. The organic layer was separated and washed with saturated NaCl 
(50 mL). The solvents were stripped off and the product was distilled at 
110.degree. C. (10 mmHg) yielding 19.5 g (91%) of product (95% GC pure). 
EXAMPLE 7 
Preparation of dicyclopentenyl formate, maleic anhydride, acrylic acid and 
t-butyl acrylate copolymer 
A vacuum flask was charged with dicyclopentenyl formate (5 g, 28.1 mmol), 
freshly distilled maleic anhydride (2.75 g, 28.1 mmol), acrylic acid 
(0.385 mL, 5.62 mmol), t-butyl acrylate (1.64 mL, 11.24 mmol) and AIBN 
(0.10 g, 0.61 mmol). A minimum amount of THF was added to form a 
homogeneous mixture at 60.degree. C. The mixture was degassed by three 
freeze-pump-thaw cycles and stirred at 65.degree. C. for 48 hrs. Then, the 
polymerization was dissolved in 30-50 mL of acetone and precipitated into 
300-500 mL of ether. The residue was collected by filtration, air dried 
and reprecipitated from acetone-ether two more times. The product, 
referred to hereinafter as polymer 3, was dried at 50.degree. C. in a 
vacuum oven. Yield: 3.15 g, 33% polymer 3. 
EXAMPLE 8 
Preparation of Resist Materials 
The resist solutions were prepared by dissolving 13.7 weight percent solids 
in propylene glycol methyl ether acetate (PGMEA). These solids consisted 
of polymer 3 (prepared in Example 7) (63.8 wt %), 
bis(t-butylcholate)glutarate (29.1 wt %), t-butyl cholate (4.9 wt %), 
bis(t-butylphenyl)iodonium nonaflate (BPIN) (2 wt %) and 
triphenylimidazole (0.2 wt %). The solutions were filtered through a 0.2 
.mu.m PTFE filter prior to application to silicon substrates. 
The solution was then spun at 2000-3000 RPM onto HMDS-primed silicon wafers 
with diameters of 6 inches. The coated wafers were baked at 140.degree. C. 
for 90 seconds on a hot plate. The resist films have a thickness of about 
0.51 .mu.m. The films were then patternwise exposed to radiation at a 
wavelength of 193 nm using an ISI 9300i Exposure Tool. The exposure dose 
varied systematically from about 10 to 30 mj/cm.sup.2. The resolution dose 
was determined to be about 22.55 mJ/cm.sup.2. The exposure was done 
through a mask having a standard resolution pattern containing a series of 
equal lines and spaces that varied from 0.155 microns to 0.2 microns. 
The exposed films are then baked at 155.degree. C. for two minutes. The 
patterns are then developed in an aqueous base solution (0.262 N solution 
of TMAH) for about 25 seconds. The developed resist defines features as 
small as 0.155 .mu.m. 
EXAMPLE 9 
Preparation of dicyclopentenyl formate, maleic anhydride, norbornene 
carboxylic acid and t-butyl norbornene carboxylate copolymer 
A vacuum flask was charged with dicyclopentenyl formate (5 g, 28.1 mmol), 
freshly distilled maleic anhydride (4.48 g, 45.7 mmol), norbornene 
carboxylic acid (0.775 g, 5.62 mmol), t-butyl norbornene carboxylate (2.4 
g, 12 mmol) and AIBN (0.30 g, 1.83 mmol). A minimum amount of THF was 
added to form a homogeneous mixture at 60.degree. C. The mixture was 
degassed by three freeze-pump-thaw cycles and stirred at 65.degree. C. for 
48 hrs. Then, the polymerization was dissolved in 30-50 mL of acetone and 
precipitated into 300-500 mL of ether. The residue was collected by 
filtration, air dried and reprecipitated from acetone-ether two more 
times. The product, referred to hereinafter as polymer 4, was dried at 
50.degree. C. in a vacuum oven. Yield: 5.38 g, 43% polymer 4. 
EXAMPLE 10 
Preparation of Resist Materials 
The resist solutions were prepared by dissolving 13.7 wt % solids in PGMEA. 
These solids consisted of polymer 4 (prepared in Example 9) (63.8 wt %), 
bis(t-butylcholate)glutarate (29.1 wt %), t-butyl cholate (4.9 wt %), BPIN 
(2 wt %) and triphenylimidazole (0.2 wt %). The solutions were filtered 
through a 0.2 .mu.m PTFE filter prior to application to silicon 
substrates. 
The solution was then spun at 2000-3000 RPM onto HMDS-primed silicon wafers 
with diameters of 6 inches. The coated wafers were baked at 140.degree. C. 
for 90 seconds on a hot plate. The resist films have a thickness of about 
0.51 pm. The films were then patternwise exposed to radiation at a 
wavelength of 193 nm using an ISI 9300i Exposure Tool. The exposure dose 
varied systematically from about 10 to 30 mJ/cm.sup.2. The resolution dose 
was determined to be about 26.1 mJ/cm.sup.2. The exposure was done through 
a mask having a standard resolution pattern containing a series of equal 
lines and spaces that varied from 0.145 microns to 0.2 microns. 
The exposed films were then baked at 155.degree. C. for two minutes. The 
patterns were then developed in an aqueous base solution (0.262 N solution 
of TMAH) for about 25 seconds. The developed resist defines features as 
small as 0.145 .mu.m. 
The above description, drawings and examples are only illustrative of 
preferred embodiments which achieve the objects, features and advantages 
of the present invention. It is not intended that the present invention be 
limited to the illustrated embodiments. Any modification of the present 
invention which comes within the spirit and scope of the following claims 
should be considered part of the present invention.