Chemically amplified resist material and process for the formation of resist patterns

Chemically amplified resist material which comprises: I. an acid-sensitive terpolymer produced upon copolymerization of (i) a first monomer unit having a structure which contains an alkali-soluble group protected with an alicyclic hydrocarbon-containing protective group, (ii) a second monomer unit having a lactone structure, and (iii) a third monomer unit having a structure which contains an alkali-soluble group protected with a protective group different from that of said first monomer unit; and II. a photoacid generator capable of producing an acid upon exposure to a patterning radiation, and the process for forming resist patterns using this resist material.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a resist material and, more particularly,
 to a novel chemically amplified resist material which can exhibit a high
 resolution, high sensitivity, excellent resistance to dry etching and can
 be produced at a low cost. The present invention also relates to a process
 for the formation of resist patterns using such a novel resist material.
 The resist material of the present invention can be advantageously
 utilized in the production of semiconductor devices such as semiconductor
 integrated circuits, for example LSIs, VLSIs and ULSIs, and other devices.
 2. Description of the Related Art
 Recently, in the production of semiconductor integrated circuits, the
 degree of integration thereof has notably increased and, accordingly, LSIs
 and VLSIs have been produced on a commercial scale. The minimum line width
 of the circuit patterns in these devices approaches the order of a
 submicron or a quarter micron. In other words, in the production of these
 high performance devices, it is required to provide a fine fabrication
 technology for forming fine resist patterns.
 Using a fine fabrication technology, fine resist patterns can generally be
 produced by coating a substrate having on a surface thereof a layer or
 coating to be fabricated, such as a layer to be selectively etched, with a
 chemically amplified resist material, and exposing the resist film to
 patterning radiation to thereby form a latent image corresponding to a
 pattern of said radiation. The latent image of the resist film is then
 developed with a suitable developer. A desired resist pattern is thus
 obtained. The resist pattern can be effectively utilized as a masking
 means in the subsequent dry etching process to selectively etch the
 underlying layer. The patterning radiation generally includes ultraviolet
 radiation such as the g-line (wavelength of 436 nm) and i-line (wavelength
 of 365 nm). However, it also includes other radiation having shorter
 wavelengths such as deep ultraviolet radiation, vacuum ultraviolet
 radiation, electron beam (EB), X-ray and others as well as excimer laser
 radiation such as KrF laser radiation of the wavelength of 248 nm and ArF
 laser radiation of the wavelength of 193 nm. Note that the term
 "radiation" used herein means all of the above-mentioned radiations.
 In the formation of submicron-ordered resist patterns using as patterning
 radiation in the far ultraviolet or vacuum ultraviolet regions, it is
 necessary to use specific resist materials having an excellent
 transparency to the patterning radiation and also a high resistance to dry
 etching. The inventors of this application have studied this and found
 that said need is satisfied by a radiation-sensitive material comprising a
 polymer or copolymer of acrylic acid ester or a-substituted acrylic acid
 ester in which the ester portion contains an adamantyl skeleton (see,
 Japanese Unexamined Patent Publication (Kokai) No. 4-39665).
 Further, in Japanese Unexamined Patent Publication (Kokai) No. 9-73173, the
 inventors of this application have suggested a chemically amplified resist
 material which comprises an acid-sensitive, alkali-insoluble compound and
 a photoacid generator capable of producing an acid upon exposure of the
 resist material to a patterning radiation. The acid-sensitive,
 alkali-insoluble compound is characterized by containing a protected
 alkali-soluble group in which an alicyclic hydrocarbon group having,
 bonded to a carbon atom thereof, a lower alkyl group is contained as a
 protective group, and it is cleaved from the alkali-soluble group upon
 action of an acid so that the compound becomes alkali-soluble. Using this
 resist material in the formation of fine resist patterns, since the
 alicyclic hydrocarbon group can be cleaved and thus removed from the
 resist material, it becomes possible to inhibit peeling off of the resist
 patterns from the substrate during the development process.
 In addition, in Japanese Unexamined Patent Publication (Kokai) No. 9-90637,
 the inventors of this application have suggested another chemically
 amplified resist material which is characterized by using, in combination,
 an acid-sensitive polymer with the monomeric unit having in a side chain
 thereof a protective group-containing carboxyl group, the protective group
 being a specific lactone structure, and a photoacid generator. Using this
 resist material, it becomes possible to form fine resist patterns having a
 practically usable sensitivity and an excellent adhesion to the substrate,
 because of presence of the specific lactone structure.
 As is described in detail in each of the above-referred patent
 publications, the chemically amplified resist materials invented by the
 inventors of this application can provide many attractive effects. In
 particular, the resist materials having both of a cleavable cyclic
 hydrocarbon structure and a lactone structure can exhibit highly improved
 performances. These resist materials, however, suffer from a high
 production cost. This is because, in the production of the resist
 materials, particularly in the production of a monomer with the lactone
 structure used as a starting material, various steps of the production are
 required, and a purification of the monomer must be carried out using a
 purification column, because the conventional distillation process cannot
 be used. Further, the lactone structure, since it contains oxygen atoms in
 a high content, can deteriorate a resistance to dry etching.
 SUMMARY OF THE INVENTION
 Accordingly, one object of the present invention is to provide a novel
 chemically amplified resist material containing both of a cleavable cyclic
 hydrocarbon group and a lactone structure in an acid-sensitive polymer as
 the basic resin, in which material the content of the lactone structure in
 the polymer can be reduced without adversely affecting the various
 characteristics derived from the cyclic hydrocarbon group and the lactone
 structure, and can simultaneously satisfy the requirements for a high
 resolution, high sensitivity and excellent resistance to dry etching.
 Another object of the present invention is to provide an improved process
 for the formation of the resist patterns using the novel resist material
 of the present invention.
 The above objects and other objects of the present invention will be
 appreciated from the descriptions as set forth below with regard to the
 preferred embodiments thereof.
 According to one aspect thereof, the present invention provides a
 chemically amplified resist material which comprises:
 I. an acid-sensitive terpolymer produced upon copolymerization of
 (i) a first monomer unit having a structure which contains an
 alkali-soluble group protected with an alicyclic hydrocarbon-containing
 protective group and in which structure said alkali-soluble group is
 cleaved with an acid to thereby permit said terpolymer to exhibit a
 solubility in an alkaline solution,
 (ii) a second monomer unit having a lactone structure, and
 (iii) a third monomer unit having a structure which contains an
 alkali-soluble group protected with a protective group different from that
 of said first monomer unit and in which structure said alkali-soluble
 group is cleaved with an acid to thereby permit that said terpolymer
 exhibits a solubility in an alkaline solution, said second monomer unit
 being contained
 in an amount of 10 to 35% by mole based on a total amount of said
 terpolymer; and
 II. a photoacid generator capable of producing an acid upon exposure to a
 patterning radiation.
 Further, the present invention, according to another aspect thereof,
 provides a process for the formation of resist patterns which comprises
 the steps of:
 coating the chemically amplified resist material of the present invention
 on a substrate to be fabricated in order to form a resist film thereon;
 selectively exposing said resist film to a patterning radiation capable of
 causing generation of an acid by said photoacid generator; and
 developing a latent image formed in said resist film during said exposure
 step.
 The chemically amplified resist material according to the present
 invention, as described above, is characterized by containing, as a basic
 resin, an acid-sensitive terpolymer, i.e., a three component copolymer,
 formed upon copolymerization of three specific monomer units, and at the
 same time, containing a second monomer unit with the lactone structure in
 an amount of 10 to 35% by mole based on a total amount of the terpolymer,
 and these characteristic features were found by the inventors of this
 application as a result of the following study and research.
 To solve the problems concerning increase of costs and reduction of a dry
 etching resistance, the inventors have tried to reduce an amount of the
 lactone structure to be introduced into the acid-sensitive copolymer. That
 is, based on the recognization that in the resist material comprising a
 copolymer consisting of an alicyclic hydrocarbon structure and a lactone
 structure, the lactone structure can particularly play a role in
 increasing an adhesion of the resist material to the substrate, but can
 reduce a dry etching resistance, they have tried to ascertain a level or
 range of the lactone structure to be induced sufficient to obtain a
 satisfactory adhesion and dry etching resistance at the same time.
 First, in the copolymer consisting of the alicyclic hydrocarbon structure
 and the lactone structure, the content of the lactone structure was tried
 to be gradually reduced, that is, the content of the alicyclic structure
 was gradually increased. When the content of the alicyclic structure
 becomes above 50% by mole, it was observed that the resist performance is
 suddenly lowered due to distortion, falling down and other defects of the
 resist patterns. Then, to avoid this problem, a three component copolymer
 (terpolymer) containing a third monomeric component in addition to about
 50% by mole of the alicyclic structure and the lactone structure was
 prepared and used as the resist material, in place of the above-described
 copolymer. The third monomeric component used here was t-butyl
 methacrylate which has a cleavage capability and is expected to show a
 better dry etching resistance than the lactone structure, because it does
 not contain an oxygen atom in a moiety of the protective group. Generally,
 t-butyl methacrylate is known to show only an insufficient level of the
 adhesion, whereas it has a merit in availability at a very low cost, in
 addition to the expected increase of the dry etching resistance because of
 the above reason.
 Based on the above findings, the formation of resist patterns was repeated
 using different terpolymers consisting of the alicyclic structure, the
 lactone structure and t-butyl methacrylate, and a relationship between the
 amount of the introduced t-butyl methacrylate and the resulting adhesion
 was evaluated in the obtained resist patterns. As a result, it was found
 that a satisfactory adhesion, sufficient for practical use in the resist
 material, could be obtained if an amount of the lactone structure
 introduced into the terpolymer is at least 10% by mole, and a dry etching
 resistance would be deteriorated at the lactone amount above 35% by mole.
 In conclusion, the inventors have found that to solve the above-described
 problems, it is effective to reduce an amount of the lactone structure to
 be introduced into the terpolymer to an acceptable and lowest level, and
 at the same time, to add a third structure, which should be available at a
 low cost, to compensate for the demerits which will be caused upon
 reduction of the lactone structure, and have thus completed the present
 invention.
 DESCRIPTION OF PREFERRED EMBODIMENTS
 In the practice of the present invention, the acid-sensitive terpolymer
 used as a basic resin in the chemically amplified resist material is that
 produced upon copolymerization of the following monomer units:
 first monomer unit containing an alkali-soluble group protected with a
 protective group containing an alicyclic hydrocarbon group;
 second monomer unit having a lactone structure; and
 third monomer unit containing an alkali-soluble group protected with a
 protective group which is different from that for the alkali-soluble group
 of the first monomer unit. In such an acid-sensitive terpolymer, when an
 acid is generated from a photoacid generator (PAG), used in combination
 with the terpolymer, as a function of patterning exposure, the
 alkali-soluble group of each of the first and third monomer units which
 has been protected with the protective group is cleaved with the acid, and
 therefore the terpolymer now can exhibit a solubility in alkali. Since the
 terpolymer in an exposed area of the resist material is soluble in an
 alkaline solution, the exposed area of the resist material is dissolved
 and removed from the substrate, thereby providing the desired resist
 patterns.
 More particularly, the acid-sensitive terpolymer advantageously used in the
 practice of the present invention, as described hereinabove, contains one
 or more alkali-soluble groups in at least the first and second monomer
 units of the terpolymer. Examples of suitable alkali-soluble groups,
 although they are not restricted to those below-mentioned, include a
 carboxylic acid group, sulfonic acid group, amide group, imide group and
 phenol group. In particular, a carboxylic acid group can be preferably
 used as the alkali-soluble group.
 In the first monomer unit of the acid-sensitive terpolymer of the present
 invention, the above-described alkali-soluble carboxylic acid group or
 others have to be initially protected with a certain alicyclic
 hydrocarbon-containing protective group for the purpose of inhibiting
 dissolution of the terpolymer in an alkaline developer. The alicyclic
 hydrocarbon to be contained in the protective group includes a variety of
 alicyclic hydrocarbon groups which are well-known in the field of
 chemically amplified resists. Suitable alicyclic hydrocarbon groups
 include, although they are not restricted to, those containing the
 following compounds as the skeleton:
 (1) adamantane and derivatives thereof;
 (2) norbornane and derivatives thereof;
 (3) perhydroanthracene and derivatives thereof;
 (4) perhydronaphthalene and derivatives thereof;
 (5) tricyclo[5. 2. 1. 0.sup.2,6 ]decane and derivatives thereof;
 (6) cyclohexane, methylcyclohexane, dimethylcyclohexane, bicyclohexane and
 derivatives thereof;
 (7) spiro[4, 4]nonane and derivatives thereof; and
 (8) spiro[4, 5]decane and derivatives thereof.
 These compounds have the following structures:
 ##STR1##
 Note, in the above formulae, the formula (6) is a formula of bicyclohexane.
 Further, in this first monomer unit, the monomer unit itself may have any
 different structures which are generally known and adopted as a backbone
 structure of the resist polymers in the field of resist chemistry, and
 typical examples thereof include (meth)acrylate monomer units, i.e.,
 acrylate and methacrylate monomer units, vinyl phenol monomer units, vinyl
 benzoate monomer units, N-substituted maleimide monomer units, styrene
 monomer units and the like. Particularly, the (meth)acrylate monomer units
 can be preferably used as the backbone structure of the first monomer
 unit.
 In the acid-sensitive terpolymer of the present invention, the second
 monomer unit essentially contains a lactone structure. The lactone
 structure suitable in the practice of the present invention is represented
 by the following formula (I) or (II):
 ##STR2##
 in which
 R.sub.1 represents a hydrogen atom or a substituted or unsubstituted,
 direct chain or branched chain hydrocarbon group of 1 to 4 carbon atoms,
 preferably a lower alkyl group such as methyl, ethyl or butyl, and
 n is an integer of 1 to 4.
 Further, in this second monomer unit containing the lactone structure, the
 monomer unit itself may have any structure which is generally known and
 adopted as a backbone structure of the resist polymers in the field of
 resist chemistry, and typical examples thereof include (meth)acrylate
 monomer units, i.e., acrylate and methacrylate monomer units, vinyl phenol
 monomer units, vinyl benzoate monomer units, N-substituted maleimide
 monomer units, styrene monomer units and the like. Particularly, the
 (meth)acrylate monomer units can be preferably used as the backbone
 structure of the second monomer unit.
 Furthermore, typical examples of the second monomer unit containing the
 lactone structure, although they are not restricted to the
 below-mentioned, include the structure units represented by the following
 formulae. Note, in the formulae, that LAC represents a lactone structure,
 typical examples of which include those of the above-described formulae
 (I) and (II), R represents an optional substituent and preferably it may
 be the same or different and each represents a hydrogen atom, a halogen
 atom such as chlorine or bromine, a substituted or unsubstituted, direct
 chain or branched chain hydrocarbon group such as an alkyl group, for
 example, methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl and
 the like, a cyano group and the like, and m represents a content (mole
 number) of the second monomer unit in the terpolymer of the present
 invention.
 (meth)acrylate:
 ##STR3##
 (vinyl)phenol:
 ##STR4##
 vinyl benzoate:
 ##STR5##
 norbornene carboxylic acid:
 ##STR6##
 As described hereinabove, the second monomer unit may ensure its desired
 function and effects, if it is contained in an amount of at least 10% by
 mole in the acid-sensitive terpolymer. An upper limit of the content of
 the second monomer unit is preferably 35% by mole or less in view of
 resistance to dry etching.
 In the acid-sensitive terpolymer of the present invention, the third
 monomer unit essentially contains an alkali-soluble group and the
 alkali-soluble group is protected with a protective group which is
 different from that of the first monomer unit. The third monomer unit, as
 described above, is included in the acid-sensitive terpolymer to attain
 both of a reduction in the production cost and an improvement of the dry
 etching resistance, without relying upon the lactone structure as in the
 conventional chemically amplified resist materials, and thus it has to
 have the structure, function and the like enabling such improvement. The
 third monomer unit is not restricted to the specific one, as long as it
 contains a protective group capable of being cleaved upon the action of an
 acid, is available at a lower cost, and does not adversely affect the
 desired performances of the resist, however, in view of a good resistance
 to dry etching, it is preferred to use the third monomer unit in which the
 protective group contains no or few oxygen atoms. Accordingly, typical
 examples of the protective group to be contained in the third monomer unit
 include t-butyl group, t-amyl group and similar groups, and particularly
 t-butyl group can be advantageously used as the protective group. As the
 result of introduction of such third monomer unit in the acid-sensitive
 terpolymer, it becomes possible to improve the dry etching resistance,
 along with a reduction in the production cost.
 The alkali-soluble group to be contained in the third monomer unit is not
 restricted to the specific one, however, as in the first monomer unit
 described above, a carboxylic acid group, sulfonic acid group, amide
 group, imide group, phenol group and the like may be preferably used, more
 preferably, a carboxylic acid group.
 Further, in this third monomer unit, the monomer unit itself may have any
 different structures which are generally known and adopted as a backbone
 structure of the resist polymers in the field of resist chemistry, and
 typical examples thereof include (meth)acrylate monomer units, i.e.,
 acrylate and methacrylate monomer units, vinyl phenol monomer units, vinyl
 benzoate monomer units, N-substituted maleimide monomer units, styrene
 monomer units and the like, as in other monomer units. Particularly, the
 (meth)acrylate monomer units can be preferably used as the backbone
 structure of the third monomer unit.
 Preferably, the acid-sensitive terpolymer used as the basic resin in the
 chemically amplified resist materials of the present invention is
 represented by the following formula (III):
 ##STR7##
 in which
 R and LAC each is as defined above,
 R.sub.I represents a substituted or unsubstituted, direct chain or branched
 chain hydrocarbon group of 1 to 4 carbon atoms, preferably a lower alkyl
 group,
 Z represents atoms necessary to complete an alicyclic hydrocarbon group,
 described above, along with a carbon atom to which the --R.sub.I group is
 bonded, and
 k, l and m each is a number of the monomer unit constituting the
 acid-sensitive terpolymer, and a molar ratio of k:l:m is in the range of
 45 to 75:15 to 40:10 to 35.
 A molecular weight (weight average molecular weight, Mw) of the
 acid-sensitive terpolymer having the above-described structure is not
 specifically restricted, and thus can be widely varied. Generally, the
 molecular weight is preferred to be in the range of about 1,000 to 30,000.
 More particularly, the acid-sensitive terpolymer which can be
 advantageously used in the practice of the present invention, although it
 is not restricted, includes the following terpolymers. Note, in the
 below-mentioned formulae, that t-Bu is an abbreviation of t-butyl group,
 and k, l and m each is as defined above.
 Copolymer of 2-methyl-2-adamantyl methacrylate, t-butyl methacrylate and
 mevalonic lactone methacrylate:
 ##STR8##
 Copolymer of isobornyl methacrylate, t-butyl methacrylate and mevalonic
 lactone methacrylate:
 ##STR9##
 Copolymer of 2-methyl-2-adamantyl methacrylate, t-butyl methacrylate and
 .gamma.-butylolactone-3-yl methacrylate:
 ##STR10##
 Copolymer of 2-inethyl-2-adainantyl methacrylate, t-butylmethacrylate and
 3-inethyl-.gamma.-butylolactone-3-yl methacrylate:
 ##STR11##
 Copolymer of 2-methyl-2-adamantyl methacrylate, t-butylmethacrylate and
 .gamma.-butylolactone-2-yl methacrylate:
 ##STR12##
 If necessary, the above-described acid-sensitive terpolymer may
 additionally contain an alkali-soluble polymer or copolymer such as a
 novolak resin, a phenol resin, an imide resin, a carboxylic
 acid-containing resin, and other resins.
 In the chemically amplified resist material according to the present
 invention, a photoacid generator (PAG) capable of being decomposed upon
 exposure to a patterning radiation to thereby produce an acid capable of
 causing cleavage of a protective group of the protected alkali-soluble
 group is used in association with the above-described acid-sensitive
 terpolymer. The photoacid generator used herein may be any conventional
 agent which is well-known as a photoacid generator (PAG) in resist
 chemistry, namely, compounds capable of generating a protonic acid upon
 exposure of the resist film to a patterning radiation such as ultraviolet
 radiation, far ultraviolet radiation, vacuum ultraviolet radiation,
 electron beam, X-ray and laser light. Typical examples of the photoacid
 generator suitably used in the practice of the present invention include
 various compounds described hereinafter, although the present invention
 should not be restricted to these compounds.
 (1) diazonium salts represented by the following formula:
EQU Ar--N.sub.2.sup.+ X.sup.-
 in which
 Ar represents a substituted or unsubstituted aromatic group, for example,
 phenyl group or phenyl group substituted with halogen such as chlorine,
 bromine, iodine or fluorine, alkyl such as methyl or t-butyl, aryl or
 other substituent groups, or a substituted or unsubstituted alicyclic
 hydrocarbon group, and
 X represents halogen, BF.sub.4, BF.sub.6, PF.sub.6, AsF.sub.6, SbF.sub.6,
 CF.sub.3 SO.sub.3, ClO.sub.4 or anion of organic sulfonic acids.
 (2) iodonium salts represented by the following formula:
 ##STR13##
 in which Ar and X are as defined above.
 (3) sulfonium salts represented by the following formulae:
 ##STR14##
 in which R, Ar and X each is as defined above, and R.sup.1, R.sup.2 and
 R.sup.3 may be the same or different and each represents a hydrogen atom
 or any substituent group, for example, a halogen atom, an alkyl group, an
 aryl group and the like, and, for example, R is methyl and the like and
 R.sup.1, R.sup.2 and R.sup.3 each is phenyl and the like.
 (4) sulfonic esters represented by the following formula:
EQU Ar--COCH.sub.2 SO.sub.2 --Ar
 or
 ##STR15##
 in which Ar and R are as defined above.
 (5) oxazole derivatives represented by the following formula:
 ##STR16##
 in which X is as defined above with the proviso that at least one of the
 substituents --CX.sub.3 is a substituted or unsubstituted aryl or alkenyl.
 (6) s-triazine derivatives represented by the following formula:
 ##STR17##
 in which X is as defined above with the proviso that at least one of the
 substituents --CX.sub.3 is a substituted or unsubstituted aryl or alkenyl.
 (7) disulfone derivatives represented by the following formula:
EQU Ar--SO.sub.2 --SO.sub.2 --Ar
 in which
 Ar is as defined above.
 (8) imide compounds represented by the following formula:
 ##STR18##
 in which X is as defined above.
 (9) others such as oxime sulfonate, diazonaphtoquinone, benzoine tosylate
 and the like.
 More particularly, some of the above-described compounds can be represented
 by the following formulae:
 triphenylsulfonium triflate:
 ##STR19##
 triphenylsulfonium hexafluoroantimonate:
 ##STR20##
 triphenylsulfonium hexafluorophosphate:
 ##STR21##
 diphenyliodo hexafluorophosphate:
 ##STR22##
 benzoine tosylate:
 ##STR23##
 naphthylimidyl triflate:
 ##STR24##
 cyclohexylmethyl(2-oxocyclohexyl)-sulfonium trifluoromethane sulfonate:
 ##STR25##
 In the practice of the present invention, the chemically amplified resist
 material is prepared by using as the starting materials the
 above-described acid-sensitive terpolymer and photoacid generator (PAG).
 The preparation of such resist material can be prepared by using the
 terpolymer obtained in accordance with the conventional polymerization
 methods. For example, the selected monomers which are necessary to
 constitute the repeating units of the target terpolymer are polymerized in
 the presence of a suitable polymerization initiator, and then the obtained
 terpolymer is added with a photoacid generator, followed by dissolving the
 mixture in an organic solvent such as ethyl lactate and the like as a
 dispersant to form a resist solution.
 The polymerization conditions and polymerization initiators used herein can
 be freely selected from a wide variety of conditions and initiators which
 are conventional in polymer chemistry. For example, if the terpolymer
 should be prepared in accordance with a radical polymerization process, it
 is preferred to use as the polymerization initiator the following
 compounds.
 AIBN (N,N'-azoisobutylonitrile):
 ##STR26##
 MAIB (dimethyl-2,2-azoisobutylate):
 ##STR27##
 In the preparation of the chemically amplified resist material, an amount
 of the photoacid generator added to the acid-sensitive terpolymer may be
 widely varied depending upon its acid releasing capability, the desired
 properties of the resist material and other factors, however, it is
 generally preferred that the photoacid generator is added in an amount of
 about 1 to 30% by weight, more preferably in the range of about 1 to 15%
 by weight, based on the total weight of the acid-sensitive terpolymer.
 Further, the organic solvents used in the preparation of the resist
 solution may be widely varied depending on various factors such as the
 resist material used, coating conditions and the like, and typical
 examples of suitable organic solvents, although they are not restricted to
 the below-mentioned, include ethyl lactate, propyleneglycol
 methyletheracetate (PGMEA), ethyl pyruvate, cyclohexanone, methyl amyl
 ketone, methyl-3-methoxypropionate, ethyl-3-ethoxypropionate,
 diacetoalcohol and similar organic solvents. These organic solvents may be
 used alone or, if desired, may be used as a mixture of two or more
 solvents. Particularly, it has observed that ethyl lactate, PGMEA and
 ethyl pyruvate are effective in the production of resist patterns having a
 high contrast.
 In the preparation of the resist solution, if desired, an auxiliary solvent
 may be used in addition to the above-described organic solvent
 (hereinafter referred to as "main solvent"). The auxiliary solvent is not
 required if the resist components can be easily dissolved in the main
 solvent, however, if the resist components can hardly be dissolved in the
 main solvent with difficulty, the auxiliary solvent will assist in
 dissolving the resist components in the main solvent. Useful auxiliary
 solvents, although they are not restricted to those below-mentioned,
 include butyl acetate, .gamma.-butylolactone, propyleneglycol methylether
 and similar solvents.
 Furthermore, in order to prevent a striation of the resist solution after
 coating thereof, it is effective to add a surface active agent to the
 resist solution. Suitable surface active agents include, for example,
 "KR-341" (trade name), commercially available from Shin-etsu Kagaku Kogyo
 Co., and the like. In addition, if desired, a quenching compound for
 acids, i.e., weak basic compounds such as a substituted amine compound, a
 nitrile compound, a N-methyl pyrrolidone compound and the like may be
 added to the resist.
 In another aspect thereof, the present invention resides in an improved
 process for forming resist patterns, particularly positive-working resist
 patterns, on a substrate to be fabricated, the process comprising the
 steps of:
 coating the above-described chemically amplified resist material according
 to the present invention on the substrate to form a resist film;
 selectively exposing the resist film to a patterning radiation capable of
 causing generation of an acid from the photoacid generator in the resist
 film; and
 developing a latent image formed in the resist film upon the selective
 exposure in the above step.
 In the process for the formation of the resist patterns according to the
 present invention, it is essential to include a step of heating the
 exposed resist film at an elevated temperature between the exposure step
 and the development step. The heating step is generally referred in the
 field of the resist chemistry to as "Post Exposure Baking" or briefly
 "PEB".
 The formation of the resist patterns according to the present invention can
 be carried out in a manner considered conventional in the field of the
 chemically amplified resist materials, and preferably it can be carried
 out as follows.
 First, a solution of the resist material, prepared as described above, is
 coated on a substrate to be fabricated, to thereby form a resist film
 having a predetermined thickness. The substrate used herein may be any
 conventional substrate used in the field of the production of
 semiconductor devices and other devices, and typical examples of suitable
 substrates include semiconductor substrates such as a silicon substrate, a
 glass substrate, a SOS substrate, a non-magnetic substrate such as ceramic
 substrate and the like. If desired, the substrate may additionally contain
 one or more overlaying layers such as a polysilicon layer, an oxide layer,
 for example, a silicon oxide layer, a nitride layer, for example, a
 silicon nitride layer, a metallic layer, for example, an aluminum layer,
 an insulating interlayer, a magnetic layer and the like. Further, the
 substrate and/or the overlaying layer(s) may contain any elements such as
 wiring or circuits fabricated therein. Furthermore, in order to increase
 the adhesion strength of the resist film to the substrate, a surface of
 the substrate may be subjected to a conventional hydrophobic treatment.
 Typical examples of the chemicals advantageously used in this treatment
 are 1,1,1,3,3,3-hexamethyldisilazane (HMDS) and the like.
 Coating of the resist solution may be preferably made by using any
 conventional coating devices such as a spin coater, a dip coater, a roller
 coater and the like. Preferably, the spin coater can be used to dropwise
 add the resist solution on a surface of the substrate, thereby forming a
 thin film of the resist solution. A thickness of the resist film may be
 widely varied depending on the field of use of the resist patterns and
 other factors, and generally it is preferred that the layer thickness is
 in the range of about 0.3 to 2.0 .mu.m.
 Next, prior to selective exposure of the resist film to a patterning
 exposure, it is preferred that the resist film is prebaked at a
 temperature of about 40 to 170.degree. C., more preferably about 60 to
 120.degree. C., for about 60 to 180 seconds. Prebaking may be carried out
 by using any heating means conventionally used in the resist process.
 Suitable heating means include, for example, a hot plate, infrared (IR)
 heating oven, microwave heating oven and the like.
 In addition, if a topcoat layer or protective layer is to be applied over
 the resist film, it is contemplated to spin-coat a solution of an olefinic
 resin over the resist film, followed by baking the olefinic coating at a
 predetermined temperature such as about 100.degree. C.
 After the formation of the resist film and prebaking, the prebaked resist
 film is selectively exposed to a patterning radiation in a conventional
 exposure device or aligner. Suitable exposure devices include commercially
 available devices such as ultraviolet (far UV, deep UV or vacuum UV)
 exposure devices, X-ray exposure devices, electron beam exposure systems
 and excimer steppers, for example. The conditions of exposure can be
 varied to select the optimum condition in each process, after taking
 various factors into consideration. As a result of the selective exposure,
 the photoacid generator (PAG) in the exposed areas of the resist film is
 decomposed, and thus the exposed resist film is now ready for dissolving
 off from the substrate in the subsequent development step. Thus, a circuit
 pattern of the reticle used in the exposure step is transferred to and
 recorded in the resist film.
 Immediately after completion of the exposure, the resist film is heated on
 a heating means such as a hot plate. As mentioned above, the heating is
 called Post-Exposure Baking (PEB), and PEB is preferably carried out at a
 temperature sufficient to cause cleavage of the protective group from the
 protected alkali-soluble group in the acid-sensitive terpolymer in the
 resist film in the presence of a catalytic acid produced from the
 photoacid generator. PEB may be carried out in a manner similar to that of
 the above-described prebaking, and generally it can be carried out at a
 temperature of from about 60 to the temperature at which the basic resin
 of the resist can decompose, preferably about 90 to 150.degree. C., for
 about 30 to 240 seconds. Further, when the resist film is used in
 combination with the topcoat layer, it is necessary to separate and remove
 the topcoat layer in any suitable manner such as use of a remover, for
 example, an organic solvent, after PEB and before the development.
 As a final step of the present process, the heated resist film is developed
 with a liquid developer in accordance with any conventional method.
 Suitable apparatuses for use in this development step include the
 well-known developers such as a spin developer, dip developer and spray
 developer.
 Typical examples of the suitable liquid developer include an aqueous or
 alcoholic solution of hydroxides of metals belonging to the groups I and
 II of the periodic table such as potassium hydroxide or an aqueous or
 alcoholic solution of organic bases free from metal ions such as
 tetraalkylammonium hydroxide, typical examples of which will be described
 hereinafter with reference to the chemical formulas thereof. Further, if
 desired, the alkaline solution used as the developer may additionally
 contain any additives such as a surface active agent in order to improve
 the resulting development effect.
 More preferably, the liquid developers used in the practice of the present
 invention are those described in Japanese Unexamined Patent Publication
 (Kokai) No. 7-23053, that is, an aqueous or alcoholic solution containing
 as the developer an ammonium compound represented by the following
 formula:
 ##STR28##
 in which R.sub.2, R.sub.3, R.sub.4 and R.sub.5 may be the same or
 different, and each represents a substituted or unsubstituted alkyl group
 of 1 to 6 carbon atoms, a morpholine compound represented by the following
 formula:
 ##STR29##
 or a mixture thereof. The ammonium compound useful as the developer,
 although it is not restricted to the below-mentioned, includes:
 tetramethylammonium hydroxide (TMAH),
 tetraethylammonium hydroxide (TEAH),
 tetrapropylammonium hydroxide (TPAH),
 tetrabutylammonium hydroxide (TBAH), and the like.
 The developer is dissolved in water or an alcohol such as methanol,
 ethanol, isopropyl alcohol and the like to obtain a developing solution.
 The concentration of the developer in the developing solution may be
 widely varied, however, it is generally in the range of about 0.1 to 15%
 by weight, preferably in the range of about 0.1 to 10% by weight.
 Generally, an aqueous solution of 2.38% by weight of tetramethylammonium
 hydroxide (TMAH) is advantageously used as the developing solution. The
 developing time may also be widely varied, however, it is generally in the
 range of about 10 seconds to about 20 minutes, preferably in the range of
 about 30 seconds to about 5 minutes. As a result of development, the
 exposed areas of the resist film are dissolved and removed from a surface
 of the substrate, thereby forming the desired positive resist patterns
 corresponding to the exposure pattern. Finally, the resulting resist
 patterns are rinsed with deionized water, and dried in accordance with the
 conventional manner.
 As can be understood from the above-mentioned descriptions and the appended
 working examples, according to the present invention, since the chemically
 amplified resist material contains the specific acid-sensitive terpolymer
 constituted from three specific repeating units in the described ratio, it
 becomes possible to further increase a dry etching resistance and, at the
 same time, reduce a production cost, in addition to the advantages
 inherent in the chemically amplified resist materials, that is, high
 resolution, high sensitivity and excellent resistance to dry etching.

EXAMPLES
 The present invention will be further described with reference to the
 appended working examples. Note, however, that the examples are included
 herein for only explanation purpose and they do not restrict the present
 invention.
 Example 1
 Preparation of Terpolymers of 2-methyl-2-adamantyl methacrylate,
 t-butylmethacrylate and mevalonic lactone methacrylate (2MAdMA-tBuMA-MLMA)
 with Different Composition Ratios
 First, the 2MAdMA-tBuMA-MLMA terpolymer (terpolymer 3) included in the
 scope of the present invention was prepared in accordance with the
 following procedure.
 2-methyl-2-adamantyl methacrylate (2MAdMA), t-butylmethacrylate (tBuMA) and
 mevalonic lactone methacrylate (MLMA) in a molar ratio of 50:30:20 were
 charged in an eggplant type flask as a reaction container to make a
 1,4-dioxane solution containing 3 moles/L of the monomers. To the
 resulting 1,4-dioxane solution, a polymerization initiator, AIBN
 (N,N'-azoisobutylonitrile), in an amount of 15% by mole based on the total
 amount of the monomers, was added. The reaction container was dipped in an
 oil bath at a controlled temperature of 80.degree. C., and the reaction
 was continued for about 8 hours. After completion of the reaction, the
 temperature of the reaction system was reduced to room temperature, and
 then the reaction product was poured into a large amount of methanol to
 precipitate a polymerization product. After the precipitate was dried, the
 resulting polymerization product was dissolved in tetrahydrofuran (THF),
 and the solution was again poured into a large amount of methanol. The
 thus produced precipitate was filtered and dried, and the above procedure
 was repeated twice to obtain a resinous product at a yield of 60%. The
 1H-NMR analysis of the resinous product indicated that the product is a
 terpolymer of 2-methyl-2-adamantyl methacrylate, t-butyl methacrylate and
 mevalonic lactone methacrylate (composition ratio of 54:28:18) (terpolymer
 3). Further, the GPC analysis of the resulting terpolymer indicated that
 the terpolymer has a weight average molecular weight (Mw) of 9,000 and a
 degree of polydispersion (Mw/Mn) of 1.82.
 Further, the above procedure was repeated to prepare three different
 terpolymers of 2MAdMA-tBuMA-MLMA (terpolymers 2, 4 and 5) within the scope
 of the present invention with the proviso that the three monomers were
 charged with different rates as is described in the following Table 1. The
 properties of the terpolymers 2, 4 and 6 are also described in the Table
 1.
 Furthermore, for comparison purposes, the above procedure was repeated to
 prepare the terpolymer of 2-MAdMA-tBuMA-MLMA (terpolymer 1, not included
 in the scope of the present invention) and the copolymer of 2-MAdMA-MLMA
 (for convenience, referred to as "terpolymer 6", not included in the scope
 of the present invention). The properties of the terpolymers 1 and 6 are
 described in Table 1.
 TABLE 1
 weight
 charging average degree of
 ratio molecular poly-
 terpolymer 2MAdMA/ composition weight dispersion
 number tBuMA/MLMA ratio Mw Mw/Mn
 1 50:45:5 51:43:6 6800 1.79
 (comparison)
 2 50:40:10 54:36:10 7700 1.80
 3 50:30:20 54:28:18 9000 1.82
 4 50:20:30 54:19:27 9400 1.92
 5 50:10:40 53:12:35 10200 1.97
 6 50:0:50 49:0:51 14700 2.03
 (comparison)
 Note in the above table that there are small differences between the
 charging ratio of 2MAdMA, tBuMA and MLMA as the monomers and a composition
 ratio of the same monomers in the resulting terpolymer, however, such
 differences are negligible, because they were considered to be minor
 errors caused during the measurement.
 Example 2
 Formation and Evaluation of the Resist Patterns
 Each of the terpolymers 2 to 5 prepared in Example 1 was dissolved in
 polythyleneglycol methyletheracetate (PGMEA) to make a solution containing
 the terpolymer at a concentration of 14% by weight, and the resultant
 solution was added with triphenylsulfonium triflate (TPSSO.sub.3 CF.sub.3)
 as a photoacid generator in an amount of 2% by weight based on the weight
 of the terpolymer, and then the mixture was dissolved in ethyl lactate to
 make a resist solution containing the resinous components in an amount of
 14% by weight. The resist solution was spin-coated on a silicon substrate
 which has been treated with HMDS, and prebaked at 120.degree. C. for 60
 seconds on a hot plate to form a 0.4-.mu.m thick resist film.
 After prebaking, a polyolefinic resin was coated over the resist film on
 the substrate to form a protective coating. Then, the resist film was
 selectively exposed to a pattern of laser light having a wavelength of 193
 nm through a reticle pattern of an IC circuit on a ArF excimer laser
 exposure system (Nikon, NA=0.55). Immediately after the exposure, the
 resist film was subjected to the post-exposure baking (PEB) on a hot plate
 at 120.degree. C. for 60 seconds. After removal of the protective coating
 the postbaked resist film was developed with an aqueous solution of 2.38%
 by weight (0.27N) tetramethylammonium hydroxide (TMAH), NMD-3 (trade name)
 commercially available from Tokyo Ohka Co., for 60 seconds, and rinsed for
 30 seconds in a purified water.
 For each resist film, the positive resist patterns which correspond to the
 pattern of the laser light as an exposure source were thus obtained. In
 this example, a threshold energy Eth of the exposure dose was 5 to 50
 mJ/cm.sup.2, and the resolution of the patterns was 0.18 .mu.m L/S (line &
 space).
 Evaluation of Dry Etching Resistance
 The dry etching resistance of the resist patterns was evaluated from an
 amount of the thickness of the resist film reduced upon dry etching by
 producing a resist film having a thickness of 1.0 .mu.m in accordance with
 the above-described procedure. The substrate with the resist film was set
 in a parallel plate reactive ion etching system, and the resist film was
 etched under the dry etching conditions: CF.sub.4 as an etching gas, gas
 flow rate of 100 sccm, electric power of 200w and pressure of 0.02 Torr,
 for 5 minutes. For each resist film, an amount of the variation
 (reduction) of the thickness of the resist film before and after dry
 etching, was evaluated to be 1.1, whereas that of the resist film of the
 conventional novolak resist was 1.0 (control) under the same dry etching
 conditions. That is, this result shows that satisfactory dry etching
 resistance could be obtained by using the resist materials of the present
 invention.
 Comparative Example 1
 The procedure of Example 2 was repeated. However, in this example, for
 comparison purposes, the terpolymer 1 (composition ratio=51:43:6) prepared
 in Example 1 was used in place of the terpolymers 2 to 5. The formation of
 the resist patterns was made, however, only the minimum pattern size of
 0.30 .mu.m L/S could be obtained due to severe peeling of the resist film
 from the substrate.
 With regard to the dry etching resistance, the reduction of the thickness
 resistance of the resist film was evaluated to be 1.1, i.e., same as that
 of Example 2, whereas that of the conventional novolak resists was 1.0
 (control).
 Comparative Example 2
 The procedure of Example 2 was repeated. However, in this example, for
 comparison purposes, the terpolymer 6 (composition ratio=49:0:51) prepared
 in Example 1 was used in place of the terpolymers 2 to 5. The formation of
 the resist patterns was made with the satisfactory results, that is, good
 adhesion of the resist patterns to the substrate and the minimum pattern
 size of 0.20 .mu.m L/S.
 With regard to the dry etching resistance, the reduction of the thickness
 of the resist film was evaluated to be 1.2, whereas that of the
 conventional novolak resists was 1.0 (control). That is, this result
 evidences that the tested resist material shows a poor dry etching
 resistance compared to those of Example 2.
 Example 3
 The procedure of Example 2 was repeated with the proviso that the same
 amount of the terpolymer of isobornyl methacrylate, t-butyl methacrylate
 and mevalonic lactone methacrylate (IBMA-tBuMA-MLMA, composition
 ratio=51:28:21, prepared in accordance with the manner similar to that of
 Example 1) was used as a basic resin in place of the terpolymer of
 2-MAdMA-tBuMA-MLMA. The good resolution and dry etching resistance
 comparable to those of Example 2 could be obtained. The minimum pattern
 size was 0.19 .mu.m L/S.
 Example 4
 The procedure of Example 2 was repeated with the proviso that the same
 amount of the terpolymer of 2-methyl-2-adamantyl methacrylate, t-butyl
 methacrylate and .gamma.-butylolactone-3-yl methacrylate
 (2MAdMA-tBuMA-HGBMA, composition ratio=50:20:30, prepared in accordance
 with the manner similar to that of Example 1) was used as a basic resin in
 place of the terpolymer of 2-MAdMA-tBuMA-MLMA. The good resolution and dry
 etching resistance comparable to those of Example 2 could be obtained
 along with a good adhesion of the patterns to the substrate. The minimum
 pattern size was 0.18 .mu.m L/S, at a PEB temperature of 120.degree. C.
 Example 5
 The procedure of Example 4 was repeated with the proviso that the same
 amount of the terpolymer of 2-methyl-2-adamantyl methacrylate, t-butyl
 methacrylate and 3-methyl-.gamma.-butylolactone-3-yl methacrylate
 (2MAdMA-tBuMA-MBLMA, composition ratio=50:20:30, prepared in accordance
 with the manner similar to that of Example 1) was used as a basic resin in
 place of the terpolymer of 2-MAdMA-t-BuMA-HGBMA. Satisfactory results
 similar to those of Example 4 could be obtained
 Example 6
 The procedure of Example 4 was repeated with the proviso that the same
 amount of the terpolymer of 2-methyl-2-adamantyl methacrylate, t-butyl
 methacrylate and .gamma.-butylolactone-2-yl methacrylate
 (2MAdMA-tBuMA-GBLMA, composition ratio=50:20:30, prepared in accordance
 with the manner similar to that of Example 1) was used as a basic resin in
 place of the terpolymer of 2-MAdMA-t-BuMA-HGBMA. The resolution and dry
 etching resistance similar to those of Example 4 could be obtained along
 with a good adhesion of the patterns to the substrate. The minimum pattern
 size was 0.22 .mu.m L/S at the PEB temperature of 120.degree. C.