# Patent

## Title
Chemically amplified negative resist composition and patterning process

## Abstract

A polymer comprising 0.5-10 mol % of recurring units having acid generating capability and 50-99.5 mol % of recurring units providing for dissolution in alkaline developer is used to formulate a chemically amplified negative resist composition. When used in a lithography process, the composition ensures an effective sensitivity, makes more uniform the distribution and diffusion of the acid generating component in a resist film, and suppresses deactivation of acid at the substrate interface. The pattern can be formed to a profile which is improved in LER and undercut.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This non-provisional application is a divisional of U.S. application Ser. No. 13/407,062, filed Feb. 28, 2012, which claims priority under 35 U.S.C. §119(a) on Patent Application No. 2011-041528 filed in Japan on Feb. 28, 2011, the entire contents of which are hereby incorporated by reference.


TECHNICAL FIELD
This invention relates to a chemically amplified negative resist composition comprising a polymer having an acid generating capability, typically for use in processing of semiconductor and photomask substrates, and a patterning process using the same.
BACKGROUND ART
In the recent drive for higher integration and operating speeds in LSI devices, it is desired to miniaturize the pattern rule. The exposure process and the resist composition are largely altered to meet such a demand. Particularly when resist patterns with a feature size of 0.2 μm or less are formed by lithography, KrF and ArF excimer laser radiation, electron beam (EB) or the like is used as the energy source for exposure, and chemically amplified resist compositions having a high sensitivity to such high-energy radiation and affording a high resolution are used as the photoresist.
Resist compositions include positive ones in which exposed areas are dissolved away and negative ones in which exposed areas are left as a pattern. A suitable composition is selected among them depending on the desired resist pattern. In general, the chemically amplified negative resist composition comprises a polymer which is normally soluble in an aqueous alkaline developer, an acid generator which is decomposed to generate an acid upon exposure to light, and a crosslinker which causes the polymer to crosslink in the presence of the acid serving as a catalyst, thus rendering the polymer insoluble in the developer (sometimes, the crosslinker is incorporated in the polymer). Typically a basic compound is added for controlling the diffusion of the acid generated upon light exposure.
A number of negative resist compositions of the type comprising a polymer which is soluble in an aqueous alkaline developer and includes phenolic units as the alkali-soluble units were developed, especially as adapted for exposure to KrF excimer laser light. These compositions have not been used in the ArF excimer laser lithography because the phenolic units are not transmissive to exposure light having a wavelength of 150 to 220 nm. Recently, these compositions are recognized attractive again as the negative resist composition for the EB and EUV lithography capable of forming finer size patterns. Exemplary compositions are described in Patent Documents 1 to 3.
In the course of development of resist compositions as mentioned above, the resist compositions are required to exhibit not only a high resolution which is the fundamental function of a resist film, but also high etch resistance. This is because the resist film must be thinned as the pattern feature size is reduced. One known means for achieving such high etch resistance is by introducing a polycyclic compound containing aromatic ring and non-aromatic ring wherein the non-aromatic ring has a carbon-carbon double bond conjugated to the aromatic ring, like indene or acenaphthylene, into a hydroxystyrene-based polymer as an auxiliary component. Examples are described in Patent Documents 1 to 3. Units having phenolic hydroxyl group are disclosed in Patent Documents 4 and 5.
On the other hand, the chemically amplified resist composition typically contains a compound which is decomposed to generate an acid upon exposure to high-energy radiation, known as acid generator, the acid serving as the catalyst for changing the solubility of the polymer in developer. A number of acid generators have been developed. In the immersion lithography, one effective technique of preventing the acid generator from being leached out into the immersion medium is by incorporating the acid generator into a polymer as one of recurring units. While this technique was applied to the positive resist composition, it was found effective for improving line edge roughness (LER) (see Patent Documents 6 to 8).










CITATION LIST


















Patent Document 1:
JP-A 2006-201532




(US 20060166133, EP1684118)



Patent Document 2:
JP-A 2006-215180



Patent Document 3:
JP-A 2008-249762




(US 2008241751, EP1975711)



Patent Document 4:
JP-A 2002-202610



Patent Document 5:
JP-A 2002-244297



Patent Document 6:
JP-A H09-325497



Patent Document 7:
JP-A 2010-116550



Patent Document 8:
JP-A 2005-084365



Patent Document 9:
JP-A 2010-164933



Patent Document 10:
JP-A 2008-102383



Patent Document 11:
JP-A 2008-304590



Patent Document 12:
JP-A H08-041150










DISCLOSURE OF INVENTION
In principle, the polymer-bound acid generator mentioned above can be applied to the negative resist composition. The recurring unit having acid generator incorporated therein is neutral and highly fat soluble prior to exposure. Upon receipt of high-energy radiation, the recurring unit is decomposed, whereby the polymer changes to be strongly acidic. When the polymer is developed with an alkaline developer, the polarity change of the polymer having acid generator incorporated therein works advantageously for positive pattern formation, but inversely for negative pattern formation. Thus, when a resist composition with higher resolution is desired, the technique of incorporating an acid generator into a polymer is not affirmatively applied to the negative resist composition.
Accordingly, an object of the invention is to provide a negative resist composition comprising a polymer having an acid generator incorporated therein and offering advantages such as high resolution and minimal LER, and a resist pattern forming process using the same.
In an attempt to apply a polymer having an acid generator incorporated therein to a negative resist composition, the inventors have found that a fine feature size pattern is formed with a likelihood of forming bridges between features and leaving scum in spaces. However, the inventors have found that when an acid generator unit of a specific structure and another recurring unit are combined in a specific range, a high resolution is obtainable despite a disadvantageous propensity of dissolution contrast and the LER reducing effect as intended is obtainable at the same time.
In one aspect, the invention provides a chemically amplified negative resist composition comprising a polymer, adapted such that the polymer may turn insoluble in alkaline developer by reacting with a crosslinker and/or a recurring unit having a crosslinkable functional group in the polymer under the catalysis of an acid generated upon exposure to high-energy radiation, to form crosslinks between polymer molecules. The polymer comprises recurring units of the general formula (1) and recurring units of at least one type selected from the general formulae (2), (3), and (4).




Herein A is a single bond or a C1-C10 alkylene group which may be separated by ethereal oxygen; Rf is each independently hydrogen, fluorine, trifluoromethyl, or pentafluoroethyl, with the proviso that not all Rf's are hydrogen; B is a C1-C10 divalent hydrocarbon group in which some or all hydrogen atoms may be replaced by fluorine and in which a methylene moiety at a position other than α- and β-positions relative to the ester oxygen atom may be replaced by oxygen; C is a single bond or a C1-C10 alkylene group which may be separated by ethereal oxygen; R1 is hydrogen, fluorine, methyl, or trifluoromethyl; R2, R3, and R4 are each independently a straight, branched or cyclic C1-C10 alkyl or alkoxy group; R5 is a substituted or unsubstituted, straight, branched or cyclic C1-C30 alkyl group which may contain an ether bond, or a substituted or unsubstituted C6-C14 aryl group; R6 is each independently a C1-C8 alkyl group; R7 is each independently a C1-C8 alkyl group; k, m and n are each independently an integer of 0 to 3, a is an integer of 0 to 4, b is an integer of 1 to 5, c and d each are an integer of 1 to 4, e is an integer of 0 to (4-c), f is an integer of 0 to (4-d), p is independently 0 or 1, and t is an integer of 0 to 2. The recurring units of formula (1) account for 0.5 to 10 mol % and the sum of recurring units of formulae (2), (3), and (4) accounts for 50 to 99.5 mol %, based on the entire recurring units of the polymer.

In a preferred embodiment, the polymer further comprises a unit having the general formula (5).




Herein D is a single bond or a C1-C10 alkylene group which may be separated by ethereal oxygen, R1 is as defined above, R8 is each independently halogen, an optionally halo-substituted C1-C8 alkyl or alkoxy group, a C6-C20 aromatic ring-containing hydrocarbon group, or an optionally halo-substituted C1-C12 acyloxy group, g is an integer of 0 to 5, q is 0 or 1, and s is an integer of 0 to 2.

In a preferred embodiment, the polymer further comprises a unit of at least one type selected from the general formulae (6) and (7).




Herein R9 is each independently halogen, an optionally halo-substituted C1-C12 acyloxy group, an optionally halo-substituted C1-C8 alkyl group, or an optionally halo-substituted C1-C8 alkoxy group, and h is an integer of 0 to 4.

In a preferred embodiment, the resist composition may comprise a crosslinker which is an alkoxymethylglycoluril or alkoxymethylmelamine.
In another preferred embodiment, the polymer further comprises recurring units of the general formula (M-1) or (M-2), and the resist composition is free of a crosslinker.




Herein R1 is hydrogen, fluorine, methyl, or trifluoromethyl, R10 is hydrogen or a straight, branched or cyclic C1-C8 monovalent hydrocarbon group, R11 is each independently a straight, branched or cyclic C1-C8 monovalent hydrocarbon group which may contain oxygen, or halogen, i is an integer of 0 to 4, and u is an integer of 0 to 2.

The resist composition may further comprise a polymer free of recurring units of formula (1) and typically a basic compound.
In another aspect, the invention provides a pattern forming process comprising the steps of applying the negative resist composition defined above onto a processable substrate to form a resist film, exposing the film patternwise to high-energy radiation, and developing the exposed film with an alkaline developer to form a resist pattern.
Typically, the high-energy radiation is EUV or EB.
In a preferred embodiment, the processable substrate comprises a chromium-containing material at the outermost surface. Typically, the processable substrate is a photomask blank.
ADVANTAGEOUS EFFECTS OF INVENTION
A polymer having a side chain with acid generating capability represented by formula (1) is used to formulate a chemically amplified negative resist composition. When used in a lithography process of forming a negative resist pattern which is required to have an ultrafine size, the composition ensures an effective sensitivity, makes more uniform the distribution and diffusion of the acid generating component in a resist film, and suppresses deactivation of acid at the substrate interface. The pattern can be formed to a profile which is improved in LER and reduced in the degree of undercut which is otherwise inherent to the negative resist composition.


DESCRIPTION OF EMBODIMENTS
As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise.
“Optional” or “optionally” means that the subsequently described event or circumstances may or may not occur, and that description includes instances where the event or circumstance occurs and instances where it does not.
The notation (Cn-Cm) means a group containing from n to m carbon atoms per group.
The abbreviations and acronyms have the following meaning.
UV: ultraviolet
DUV: deep ultraviolet
EUV: extreme ultraviolet
EB: electron beam
Mw: weight average molecular weight
Mn: number average molecular weight
Mw/Mn: molecular weight distribution or dispersity
GPC: gel permeation chromatography
PEB: post-exposure baking
PAG: photoacid generator
LER: line edge roughness
The invention is directed to a chemically amplified negative resist composition comprising a polymer, adapted to turn insoluble in alkaline developer through the mechanism that an electrophilic reaction active site originating from a crosslinker and/or a recurring unit having a crosslinkable functional group in the polymer reacts with an aromatic ring and/or hydroxyl group in the polymer under the catalysis of an acid generated upon exposure to high-energy radiation, whereby crosslinks are formed between polymer molecules. The resist composition is coated as film, exposed imagewise and developed with an alkaline developer to form a resist pattern.
As is known in the art, a resist composition comprising a polymer which is prepared by the method of incorporating an acid generator into a constituent unit of a polymer can prevent localization of the acid generating component during formation of a resist film, which is effective for the purpose of minimizing LER. However, the dissolution behavior of this polymer before and after exposure is disadvantageous for the negative tone as mentioned above. Unexpectedly, it has been found that both LER and high resolution can be met by designing the polymer as follows.
The polymer used in the resist composition is defined as comprising recurring units of the general formula (1) and recurring units of at least one type selected from the general formulae (2), (3), and (4).




Herein A is a single bond or a C1-C10 alkylene group which may be separated by ethereal oxygen. Rf is each independently hydrogen, fluorine, trifluoromethyl, or pentafluoroethyl, with the proviso that not all Rf's are hydrogen. B is a C1-C10 divalent hydrocarbon group in which some or all hydrogen atoms may be replaced by fluorine and in which a methylene moiety at a position other than α- and β-positions relative to the ester oxygen atom may be replaced by oxygen. C is a single bond or a C1-C10 alkylene group which may be separated by ethereal oxygen. R1 is hydrogen, fluorine, methyl, or trifluoromethyl. R2, R3, and R4 are each independently a straight, branched or cyclic C1-C10 alkyl or alkoxy group. R5 is a substituted or unsubstituted, straight, branched or cyclic C1-C30 alkyl group which may contain an ether bond, or a substituted or unsubstituted C6-C14 aryl group. R6 is each independently a C1-C8 alkyl group. R7 is each independently a C1-C8 alkyl group. The subscripts k, m and n are each independently an integer of 0 to 3, a is an integer of 0 to 4, b is an integer of 1 to 5, c and d each are an integer of 1 to 4, e is an integer of 0 to (4-c), f is an integer of 0 to (4-d), p is independently 0 or 1, and t is an integer of 0 to 2.
In the polymer, the recurring units of formula (1) account for 0.5 to 10 mol % and the sum of recurring units of formulae (2), (3), and (4) accounts for 50 to 99.5 mol %, based on the entire recurring units of the polymer.
If a proportion of the recurring units represented by formula (1) is less than 0.5 mol % based on the entire recurring units of the polymer, an auxiliary acid generator other than formula (1) must be used in a larger amount to acquire an effective sensitivity. When the amount of the non-polymer-bound acid generator is increased, LER is exacerbated. When the amount of weak sulfonic acid salt unit is increased, high resolution is difficultly available. If a proportion of the recurring units represented by formula (1) exceeds 10 mol %, scum and other defects may form during development. If the sum of recurring units containing a phenolic hydroxyl group represented by formulae (2), (3) and (4) is less than 50 mol %, there is a likelihood of scum formation and bridge formation between resist pattern features during development.
The recurring unit of formula (1) has a sulfonium salt attached to the polymer backbone via a linker. The linking of the sulfonium salt-containing recurring unit to the polymer backbone ensures a highly dispersed state of the sulfonium salt in the polymer. When the unit is exposed to high-energy radiation, the sulfonium cation is decomposed into an electrically neutral aromatic ring, and the counter anion is released as a sulfonic acid which serves as an acid catalyst in the chemically amplified negative resist mechanism.
In formula (1), “A” is a single bond or a C1-C10 alkylene group which may contain an ethereal oxygen atom at an intermediate of its chain. Suitable alkylene groups include methylene, ethylene, propylene, as well as isomers thereof having branched structure. Suitable alkylene groups in which one or more carbon is replaced by one or more oxygen include ethylene oxide derivatives with oxygen positioned on the phenyl side such as oxyethylene, dioxyethylene and trioxyethylene, and propylene oxide derivatives such as oxypropylene and dioxypropylene.
In formula (1), R1 is hydrogen, fluorine, methyl, or trifluoromethyl. R2, R3, and R4 are each independently a straight, branched or cyclic C1-C10 alkyl or alkoxy group. Suitable alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, iso-butyl, tert-butyl, tert-amyl, n-pentyl, n-hexyl, cyclopentyl, and cyclohexyl. Suitable alkoxy groups include methoxy, ethoxy, n-propyloxy, isopropyloxy, n-butyloxy, sec-butyloxy, iso-butyloxy, tert-butyloxy, tert-amyloxy, n-pentyloxy, n-hexyloxy, cyclopentyloxy, and cyclohexyloxy. The subscripts k, m and n are each independently an integer of 0 to 3.
In formula (1), B is a C1-C10 divalent hydrocarbon group in which some or all hydrogen atoms may be replaced by fluorine and in which one or more methylene moiety at a position other than α- and β-positions relative to the ester oxygen atom may be replaced by oxygen. Inter alia, methylene and trifluoromethylmethylene are preferred.
In formula (1), R5 is a substituted or unsubstituted, straight, branched or cyclic C1-C30 alkyl group which may contain an ether bond, or a substituted or unsubstituted C6-C14 aryl group. Suitable alkyl groups of R5 include methyl, ethyl, n-propyl, isopropyl, cyclopropyl, n-butyl, sec-butyl, iso-butyl, tert-butyl, n-pentyl, cyclopentyl, n-hexyl, cyclohexyl, n-octyl, n-decyl, n-dodecyl, 1-adamantyl, 2-adamantyl, and bicyclo[2.2.1]hepten-2-yl. Suitable aryl groups of R5 include phenyl, 4-methoxyphenyl, 4-tert-butylphenyl, 4-biphenyl, 1-naphthyl, 2-naphthyl, and 10-anthranyl. Also, R5 may stand for such an alkyl or aryl group in which one or more hydrogen is replaced by halogen atom, alkyl, aryl, hydroxyl, alkoxy, carboxyl, alkoxycarbonyl, alkoxyalkyl, acyloxy, acyloxyalkyl, or alkoxyalkoxy radical, or in which one or more —CH2— is replaced by —O—.
Preferred structures of the cation portion of the recurring unit of sulfonium salt having formula (1) are illustrated below, but not limited thereto.










Preferred structures of the anion portion of the recurring unit of sulfonium salt having formula (1) are illustrated below, but not limited thereto.



















The content of the recurring unit having formula (1) is up to 10 mol %, preferably up to 5 mol % of the polymer. The content of unit of formula (1) within this range leads to highly reliable pattern formation, that is, ensures that even when a resist pattern is formed on a substrate from which a resist pattern is otherwise strippable, typically a substrate having an uppermost layer of chromium compound, the resist pattern is substantially unstrippable. When the content of the recurring unit having formula (1) is at least 0.5 mol %, preferably at least 1.5 mol % of the polymer, an appropriate sensitivity is available along with the low diffusion effect of polymer-bound acid generator.
In the polymer used herein, recurring units of at least one type selected from the general formulae (2), (3), and (4) are also included as the unit which provides for solubility in alkaline developer and imparts a polarity to the molecule to make the polymer adhesive.




Herein, “C” is a single bond or a C1-C10 alkylene group which may be separated by ethereal oxygen. R1 is hydrogen, fluorine, methyl, or trifluoromethyl. R6 is each independently a C1-C8 alkyl group. R7 is each independently a C1-C8 alkyl group. The subscript “a” is an integer of 0 to 4, b is an integer of 1 to 5, c and d each are an integer of 1 to 4, e is an integer of 0 to (4-c), f is an integer of 0 to (4-d), p is independently 0 or 1, and t is an integer of 0 to 2.
In formula (2), “C” is a single bond or a C1-C10 alkylene group which may contain an ethereal oxygen atom (or ether bond) at an intermediate of its chain. Suitable alkylene groups include methylene, ethylene, propylene, butylene, pentylene, and hexylene, as well as structural isomers of a carbon skeleton having branched or cyclic structure. For the alkylene group containing ethereal oxygen, where p in formula (2) is 1, the ethereal oxygen atom may be incorporated at any position excluding the position between the α- and β-carbons relative to the ester oxygen. Where p is 0, the atom in “C” that bonds with the main chain becomes an ethereal oxygen atom, and a second ethereal oxygen atom may be incorporated at any position excluding the position between the α- and β-carbons relative to that ethereal oxygen atom. Alkylene groups having more than 10 carbon atoms are undesirable because of a low solubility in alkaline developer.
R6 is each independently a C1-C8 alkyl group. Preferred examples of the alkyl group include methyl, ethyl, propyl, isopropyl, butyl, pentyl, hexyl, heptyl, octyl, and structural isomers of a carbon skeleton having branched structure, cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl. Alkyl groups having more than 8 carbon atoms are undesirable because of a low solubility in alkaline developer.
The subscript “a” is an integer of 0 to 4, and b is an integer of 1 to 5. Preferably, a is an integer of 0 to 3, and b is an integer of 1 to 3 when t is 0. Also preferably, a is an integer of 0 to 4, and b is an integer of 1 to 5 when t is 1 or 2.
The subscript t is an integer of 0 to 2. The structure represents a benzene skeleton when t=0, a naphthalene skeleton when t=1, and an anthracene skeleton when t=2. In these cases, linker C may be attached to the naphthalene or anthracene ring at any desired position.
In formulae (3) and (4), R7 is each independently a C1-C8 alkyl group. Preferred examples of the alkyl group include methyl, ethyl, propyl, isopropyl, butyl, pentyl, hexyl, heptyl, octyl, and structural isomers of a carbon skeleton having branched structure, cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl. Alkyl groups having more than 8 carbon atoms are undesirable because of a low solubility in alkaline developer.
The subscript c is an integer of 1 to 4, d is an integer of 1 to 4, e is an integer of 0 to (4-c), and f is an integer of 0 to (4-d).
Since the polymer must have solubility in developer prior to exposure, the content of recurring units selected from formulae (2), (3) and (4) is 50 to 99.5 mol %, preferably 60 to 99.5 mol %, based on the entire recurring units of the polymer. The lower limit of the content of recurring units having formulae (2), (3) and (4) is preferably at least 65 mol % when at least half of the recurring units belonging to this class are not units of formula (2) wherein p is 1. The upper limit of the content of recurring units having formulae (2), (3) and (4) is high because the polymer comprising recurring units having formula (1) can be used as a blend of two or more such polymers or a blend thereof with another polymer. The content of recurring units having formulae (2), (3) and (4) relative to the overall polymer(s) will be described later.
Of the recurring units of formula (2), those recurring units wherein p is 0 and “C” is a single bond (meaning that the aromatic ring is directly bonded to the main chain of the polymer), that is, linker-free recurring units are units derived from monomers in which a 1-substituted or unsubstituted vinyl group is attached to a hydroxyl-substituted aromatic ring, as typified by hydroxystyrene units. Preferred examples include 3-hydroxystyrene, 4-hydroxystyrene, 5-hydroxy-2-vinylnaphthalene, and 6-hydroxy-2-vinylnaphthalene.
Those recurring units wherein p is 1, that is, recurring units having an ester structure as the linker are units of carbonyloxy-substituted vinyl monomers as typified by (meth)acrylates.
Preferred examples of the units of formula (2) having a linker (—CO—O—“C”—) derived from (meth)acrylates are shown below. Notably, “C” is as defined above and not carbon.







It is well known that a polymer having high etch resistance is obtained using an acenaphthylene compound (from which units of formula (3) are derived) or indene compound (from which units of formula (4) are derived) as a monomer for polymerization. The recurring units shown below are preferred in that corresponding monomers are readily available and the desired effects are achievable.




The polymer used in the resist composition preferably contains recurring units exhibiting no acidity, in order to improve pattern collapse and other performances. Preferred units which can be added to the polymer as the main constituent units include those of the general formula (5).




Herein D is a single bond or a C1-C10 alkylene group which may be separated by ethereal oxygen, R1 is as defined above, R8 is each independently halogen, an optionally halo-substituted C1-C8 alkyl or alkoxy group, a C6-C20 aromatic ring-containing hydrocarbon group, or an optionally halo-substituted C1-C12 acyloxy group, g is an integer of 0 to 5, q is 0 or 1, and s is an integer of 0 to 2.

For group R8, exemplary halogen atoms include fluorine, chlorine and bromine. When R8 is a hydrocarbon group or hydrocarbonoxy group, the hydrocarbon group or the hydrocarbon moiety of the hydrocarbonoxy group may be an alkyl group of 1 to 10 carbon atoms, preferably 1 to 8 carbon atoms. Preferred alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, pentyl, hexyl, heptyl, octyl, and structural isomers thereof, as well as cyclopentyl, cyclohexyl, and cyclooctyl. The hydrocarbon group or the hydrocarbon moiety of the hydrocarbonoxy group may also be an aromatic ring-containing group of 6 to 20 carbon atoms. Suitable aromatic ring-containing groups include optionally alkyl-substituted phenyl, naphthyl, benzyloxy, naphthyloxy, and phenethyl groups. When R8 is an optionally halo-substituted C1-C12 acyloxy group, preferred groups include methylcarbonyloxy, ethylcarbonyloxy, propylcarbonyloxy, isopropylcarbonyloxy, cyclopentylcarbonyloxy, cyclohexylcarbonyloxy, cyclooctylcarbonyloxy, phenylcarbonyloxy, and naphthylcarbonyloxy.
The recurring unit of formula (5) has a function of controlling reactivity with a crosslinker component in the polymer and for controlling solubility in alkaline developer, thus offering freedom of controlling solubility in developer. While a resist film of the resist composition of the invention is required to have an adequate dissolution rate in alkaline developer as will be described later, such a dissolution rate may be obtained, in some cases, from a blend of two or more polymers and specifically, by tailoring each of the polymers so as to have an adequate dissolution rate, or by selecting two or more polymers having different dissolution rates so that the blend thereof may have an adequate dissolution rate. In this context, a polymer comprising recurring units having formula (1) may have a higher dissolution rate than the dissolution rate of a polymer blend, and the recurring unit having a function of controlling dissolution rate is not essential. However, to ensure freedom of blending polymers, the polymer comprising recurring units of formula (1) should preferably further comprise recurring units of formula (5). The content of recurring units having formula (5) is preferably 5 to 39.5 mol %, more preferably 5 to 30 mol % based on the entire recurring units of the polymer. The recurring units having formula (5) may be of one type or a mixture of two or more types.
Further preferred units which can be added to the polymer as the main constituent units include those of the general formulae (6) and (7).




Herein R9 is each independently halogen, an optionally halo-substituted C1-C12 acyloxy group, an optionally halo-substituted C1-C8 alkyl group, or an optionally halo-substituted C1-C8 alkoxy group, and h is an integer of 0 to 4.

Examples of R9 in formulae (6) and (7) are as enumerated above in conjunction with formula (3).
While the recurring units of formulae (6) and (7) have the same dissolution rate controlling function as the unit of formula (5), the recurring units of formulae (6) and (7) are also effective for enhancing etch resistance and resistance to EB irradiation during pattern inspection because the aromatic ring has etch resistance and the cyclic structure attached to the backbone has such desirable properties.
Like the recurring units having formula (5), the content of recurring units having formulae (6) and (7) is preferably 5 to 39.5 mol %, more preferably 5 to 30 mol % based on the entire recurring units of the polymer. The recurring units having formulae (6) and (7) may be of one type or a mixture of two or more types. A combination of units of formula (5) with units of formula (6) and/or (7) is acceptable, and in this case, a polymer design is made such that the total of units of formulae (5), (6) and (7) may fall in the range.
As the recurring unit free of an acidic functional group, in another embodiment, the polymer may comprise a recurring unit having a function of forming crosslinks in the presence of an acid catalyst. The recurring unit having a function of forming crosslinks in the presence of an acid catalyst has an active structure of the reaction mechanism that undergoes dealcoholization, dehydration or ring-opening reaction in the presence of an acid catalyst to form a cation center and to form a bond to aromatic ring or hydroxyl group through electrophilic substitution reaction. The recurring unit having a function of forming crosslinks in the presence of an acid catalyst may be selected from well-known active structures as described in Patent Document 12, for example, recurring units having an oxirane structure, typically epoxy group. Besides, recurring units having a urea structure as shown below may be used as the above units and are preferred from the standpoint of shelf stability. Preferred examples of the recurring unit having such a function include those of the formulae (M-1) and (M-2):




wherein R1 is hydrogen, fluorine, methyl, or trifluoromethyl, R10 is hydrogen or a straight, branched or cyclic C1-C8 monovalent hydrocarbon group, R11 is each independently a straight, branched or cyclic C1-C8 monovalent hydrocarbon group which may contain oxygen, or halogen, e is an integer of 0 to 4, and u is an integer of 0 to 2.

In formulae (M-1) and (M-2), R10 stands for an alkyl group, examples of which include methyl, ethyl, propyl, isopropyl, butyl, pentyl, hexyl, heptyl, octyl, structural isomers thereof, cyclopentyl, cyclohexyl, and cyclooctyl. Also R10 stands for an aromatic ring-containing hydrocarbon group, examples of which include optionally alkyl-substituted phenyl, benzyl, and phenethyl groups.
For group R11, exemplary halogen atoms include fluorine, chlorine and bromine. When R11 is a hydrocarbon or hydrocarbonoxy group, the hydrocarbon group or the hydrocarbon moiety of the hydrocarbonoxy group may be alkyl. Preferred alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, pentyl, hexyl, heptyl, octyl, and structural isomers thereof, as well as cyclopentyl, cyclohexyl, and cyclooctyl. Also the hydrocarbon group or the hydrocarbon moiety of the hydrocarbonoxy group may be an aromatic ring-containing group. Preferred aromatic ring-containing groups include optionally alkyl-substituted phenyl, benzyl, and phenethyl groups. Also R11 is a C2-C8 hydrocarbon carbonyloxy group, examples of which include methylcarbonyloxy, ethylcarbonyloxy, propylcarbonyloxy, isopropylcarbonyloxy, cyclopentylcarbonyloxy, cyclohexylcarbonyloxy, and phenylcarbonyloxy.
Useful recurring units not exhibiting acidity which may be incorporated in the polymer are units of formula (M-1) or (M-2). The recurring unit of formula (M-1) or (M-2) functions such that alcohol or water is eliminated under the catalysis of the acid generated upon exposure to high-energy radiation, and the resulting moiety reacts with an aromatic ring or hydroxyl group to form crosslinks between polymer molecules. Although the incorporation of a unit having crosslinking capability into a polymer can adversely affect the shelf stability of the resist composition, the sensitivity of a resist film can be improved without sacrificing shelf stability when a crosslinker which is not a polymer to be added to the resist composition is, in part or in entirety, replaced by this recurring unit.
Where crosslinkable recurring units as represented by formula (M-1) or (M-2) are incorporated, they typically account for at least 5 mol %, based on the entire recurring units of the polymer to ensure the solubility changing capability. As long as a proportion of the crosslinkable unit is up to about 20 mol % as the upper limit, the above-mentioned polymer design is applicable without substantial changes.
For the purpose of incorporating the crosslinking unit described above, for example, a polymerizable monomer (Y-1) may be obtained through reactions according to the following scheme using 1,3-diamino-2-propanol (8) as the starting reactant. Herein, Me stands for methyl, and Et for ethyl.




Other useful examples of the recurring unit not exhibiting acidity which may be incorporated in the polymer include units of the following formula (11), (12) or (13):




wherein R1 is hydrogen, fluorine, methyl or trifluoromethyl, Y is oxygen or methylene, Z is hydrogen or hydroxyl, R′ is C1-C4 alkyl, and p is an integer of 0 to 3. These units may be used as auxiliary units which exhibit no acidity and impart adhesion-to-substrate.

The polymer comprising the recurring units defined above for use in the resist composition may be prepared in any well-known ways by selecting suitable monomers, and carrying out copolymerization, optionally with protection and deprotection reactions combined. The copolymerization reaction is preferably radical polymerization, but not limited thereto. With respect to the polymerization method, reference may be made to Patent Documents 6 to 9.
The polymer generally has a weight average molecular weight (Mw) of 1,000 to 50,000, and preferably 1,000 to 20,000, as measured by gel permeation chromatography (GPC) using polystyrene standards. A polymer with a Mw of less than 1,000 may lead to a pattern having a rounded top, reduced resolution, and degraded LER as is well known in the art. If Mw is higher than the necessity, the pattern tends to have increased LER, depending on the pattern size to be resolved. The Mw is preferably controlled to 20,000 or less particularly when a pattern having a line width of up to 100 nm is formed. It is noted that the GPC measurement generally uses tetrahydrofuran (THF) solvent. Some polymers comprising recurring units of formula (1) are not dissolvable in THF, and in this event, GPC measurement is made in dimethylformamide (DMF) solvent having up to 100 mM of lithium bromide added thereto.
The polymer preferably has a narrow dispersity as demonstrated by a molecular weight distribution Mw/Mn in the range of 1.0 to 3.0, more preferably 1.0 to 2.5. If the dispersity is broader, the pattern after development may have foreign particles deposited thereon, or the pattern profile may be degraded.
In addition to the polymer comprising recurring units having formula (1), the resist composition may further comprise another polymer free of recurring units having formula (1). For the sake of convenience, the polymer comprising recurring units having formula (1) and the other polymer free of recurring units having formula (1) are referred to as first and second polymers, respectively. The second polymer has such a function that it reacts with a crosslinker (to be described later) or a recurring unit having a crosslinking capability in the first polymer in the presence of an acid catalyst whereupon it becomes insoluble in developer. Many such polymers are known in the art. To avoid an increase of LER, the second polymer must be highly compatible with the first polymer. In this respect, the second polymer is preferably a polymer free of units of formula (1), but comprising other analogous units as major units, and specifically, a polymer comprising units of formulae (2) to (7), (M-1), (M-2) or (11) to (13) as major units. The molecular weight and dispersity of the second polymer are preferably in the same range as defined for the first polymer
When the second polymer is used in blend with the first polymer, the first polymer is preferably present in an amount of 30 to 100% by weight of the polymer blend. Less than 30% by weight of the first polymer may fail to achieve the LER improving effect.
When the second polymer comprising units of formulae (2) to (7), (M-1), (M-2) or (11) to (13) as major units, but free of units of formula (1) is used in combination with the first polymer comprising units of formula (1), the second polymer may be used as a single polymer or as a mixture of two or more second polymers. The important factors to be taken into account in the design of the second polymer are compatibility and dissolution rate.
In order to provide an optimum dissolution rate of a resist film, the first and second polymers should preferably be designed such that phenolic hydroxyl-containing units represented by formulae (2), (3) and (4) account for 50 to 95 mol %, more preferably 60 to 90 mol % based on the blend of first and second polymers. If the proportion of phenolic hydroxyl-containing units is less than 50 mol %, there is an increased likelihood of scum formation and bridge formation between pattern features during development. If the proportion of the phenolic hydroxyl-containing units exceeds 95 mol %, the pattern is prone to be undercut and on some substrates, pattern collapse may occur. The upper limit of the content of recurring units having formulae (2), (3) and (4) is preferably up to 85 mol % when at least half of the recurring units belonging to this class are units of formula (2) because the resist film is prone to be undercut at a higher content.
Among the remaining recurring units, recurring units of formulae (5) to (7) are effective for providing good compatibility and etching properties, and recurring units of formulae (M-1) and (M-2) are effective for increasing sensitivity.
Where the second polymer free of recurring units of formula (1) is used, the constitution of recurring units in the first polymer comprising recurring units of formula (1) may be determined so as to fall in the above-defined range. Given the ratio of addition of the first polymer and the second polymer, a guideline for the constitution of recurring units in the second polymer may be determined based on the constitution of recurring units in the overall polymers in the resist composition.
Like well-known chemically amplified negative resist compositions, the negative resist composition of the invention contains a crosslinker which forms crosslinks between polymer molecules and/or within polymer molecule under the catalysis of acid so that the polymer may turn insoluble in developer. Where the polymer comprises a crosslinkable recurring unit as mentioned above, the polymer itself has the crosslinker function, and a separate crosslinker need not be added. If desired, a non-polymeric crosslinker may be added for fine adjustment of properties.
The non-polymeric crosslinker may be added in a conventional manner and effectively formulated in the resist composition. The non-polymeric crosslinker reacts with the polymer to form crosslinks between polymer molecules and/or within polymer molecule under the catalysis of acid generated by the PAG, so that the polymer may turn insoluble in developer. The non-polymeric crosslinker is typically a compound having a plurality of functional groups capable of electrophilic reaction with an aromatic ring or hydroxyl group in the constitutional units of the polymer to form crosslinks. A number of suitable crosslinkers are known in the art as illustrated in Patent Documents 1 to 3.
The crosslinker used herein may be selected from well-known crosslinkers. Suitable crosslinkers include alkoxymethylglycolurils and alkoxymethylmelamines, such as tetramethoxymethylglycoluril, 1,3-bismethoxymethyl-4,5-bismethoxyethyleneurea, bismethoxymethylurea, hexamethoxymethylmelamine, and hexaethoxymethylmelamine.
In the negative resist composition, the crosslinker is preferably added in an amount of 2 to 40 parts, more preferably 5 to 20 parts by weight per 100 parts by weight of the overall polymers. The crosslinker may be used alone or in admixture of two or more.
The resist composition of the invention is characterized by controlled acid diffusion. That is, the recurring unit having formula (1) in the polymer is capable of generating an acid upon exposure to high-energy radiation. Since the acid thus generated is bound to the polymer, acid diffusion is controlled. In this sense, essentially another acid generator need not be added to the resist composition. However, when it is desired to increase sensitivity or to improve roughness or other properties, another acid generator may be added as an auxiliary component in a small amount. If the separate acid generator is added in excess, the effect assigned to the polymer-bound acid generator may be lost. Then the amount of the separate acid generator added should preferably be less than the molar equivalent based on the structure having formula (1) contained as recurring units in the polymer, and more preferably up to one half of the molar equivalent based on the structure having formula (1). Particularly when it is desired to improve roughness, an acid generator structured for more suppressed acid diffusion or a polymer comprising a recurring unit in which a sulfonate side of an acid generator is incorporated in the side chain may be used as illustrated in Patent Document 6.
Of the acid generators which can be added herein, preferred examples of the acid generator which is not incorporated in a polymer are shown below.



















Preferred examples of the acid generator which is incorporated in a polymer are shown below.













The resist composition is generally obtained by adding a solvent to the polymer(s). If desired, a basic compound, surfactant, dissolution inhibitor and the like may be added.
For the resist composition, the basic compound is, in fact, an essential component for providing high resolution. The basic compound is preferably added in an amount of 0.01 to 5 parts, more preferably 0.05 to 3 parts by weight per 100 parts by weight of the overall polymers. A number of basic compounds are known in the art, for example, from Patent Documents 1 to 5. Examples include primary, secondary, and tertiary aliphatic amines, mixed amines, aromatic amines, heterocyclic amines, nitrogen-containing compounds having carboxyl group, nitrogen-containing compounds having sulfonyl group, nitrogen-containing compounds having hydroxyl group, nitrogen-containing compounds having hydroxyphenyl group, alcoholic nitrogen-containing compounds, amide derivatives, imide derivatives, carbamate derivatives, and ammonium salts. Numerous examples of these basic compounds are described in Patent Documents 1 and 4. Generally any of these basic compounds may be used. Two or more may be selected from these basic compounds and used in admixture.
Examples of the basic compound which is preferably compounded herein include tris(2-(methoxymethoxy)ethyl)amine, tris(2-(methoxymethoxy)ethyl)amine N-oxide, morpholine derivatives, and imidazole derivatives.
An amine is effective when a resist pattern is formed on a substrate, typically a substrate having a surface layer of chromium compound, which is susceptible to a phenomenon that the resist film becomes substantially insoluble at the substrate interface during pattern formation, known as a footing phenomenon. Specifically, an amine compound or amine oxide compound having a carboxyl group, but free of hydrogen in covalent bond with nitrogen serving as basic center (exclusive of those amine and amine oxide compounds whose nitrogen atom is contained in the cyclic structure of aromatic ring) is effectively used for improving the pattern profile.
Preferred examples of the amine or amine oxide compound having a carboxyl group, but free of hydrogen in covalent bond with nitrogen serving as basic center include compounds of the general formulae (14) to (16), but are not limited thereto.




Herein R21 and R22 are each independently a straight, branched or cyclic C1-C20 alkyl group, C6-C20 aryl group, C7-C20 aralkyl group, C2-C10 hydroxyalkyl group, C2-C10 alkoxyalkyl group, C2-C10 acyloxyalkyl group, or C1-C10 alkylthioalkyl group. R21 and R22 may bond together to form a ring with the nitrogen atom to which they are attached. R23 is hydrogen, a straight, branched or cyclic C1-C20 alkyl group, C6-C20 aryl group, C7-C20 aralkyl group, C2-C10 hydroxyalkyl group, C2-C10 alkoxyalkyl group, C2-C10 acyloxyalkyl group, C1-C10 alkylthioalkyl group, or halogen. R24 is a single bond, a straight, branched or cyclic C1-C20 alkylene group, or C6-C20 arylene group. R25 is an optionally substituted, straight or branched C2-C20 alkylene group whose carbon-carbon linkage may be separated by at least one carbonyl (—CO—), ether (—O—), ester (—COO—) or sulfide (—S—) group. R26 is a straight, branched or cyclic C1-C20 alkylene group or C6-C20 arylene group.

Exemplary groups in these structural formulae are given below, but not limited thereto. Suitable C6-C20 aryl groups include phenyl, naphthyl, anthryl, phenanthryl, pyrenyl, naphthacenyl, and fluorenyl. Suitable straight, branched or cyclic C1-C20 alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, pentyl, hexyl, decyl, cyclopentyl, cyclohexyl, and decahydronaphthalenyl. Suitable C7-C20 aralkyl groups include benzyl, phenethyl, phenylpropyl, naphthylmethyl, naphthylethyl, and anthracenylmethyl. Suitable C2-C10 hydroxyalkyl groups include hydroxymethyl, hydroxyethyl, and hydroxypropyl. Suitable C2-C10 alkoxyalkyl groups include methoxymethyl, 2-methoxyethyl, ethoxymethyl, 2-ethoxyethyl, propoxymethyl, 2-propoxyethyl, butoxymethyl, 2-butoxyethyl, amyloxymethyl, 2-amyloxyethyl, cyclohexyloxymethyl, 2-cyclohexyloxyethyl, cyclopentyloxymethyl, 2-cyclopentyloxyethyl, and isomers of their alkyl moiety. Suitable C2-C10 acyloxyalkyl groups include formyloxymethyl, acetoxymethyl, propionyloxymethyl, butyryloxymethyl, pivaloyloxymethyl, cyclohexanecarbonyloxymethyl, and decanoyloxymethyl. Suitable C1-C10 alkylthioalkyl groups include methylthiomethyl, ethylthiomethyl, propylthiomethyl, isopropylthiomethyl, butylthiomethyl, isobutylthiomethyl, t-butylthiomethyl, t-amylthiomethyl, decylthiomethyl, and cyclohexylthiomethyl.
Preferred examples of the amine compound of formula (14) include, but are not limited thereto, o-dimethylaminobenzoic acid, p-dimethylaminobenzoic acid, m-dimethylaminobenzoic acid, p-diethylaminobenzoic acid, p-dipropylaminobenzoic acid, p-dibutylaminobenzoic acid, p-dipentylaminobenzoic acid, p-dihexylaminobenzoic acid, p-diethanolaminobenzoic acid, p-diisopropanolaminobenzoic acid, p-dimethanolaminobenzoic acid, 2-methyl-4-diethylaminobenzoic acid, 2-methoxy-4-diethylaminobenzoic acid, 3-dimethylamino-2-naphthalenic acid, 3-diethylamino-2-naphthalenic acid, 2-dimethylamino-5-bromobenzoic acid, 2-dimethylamino-5-chlorobenzoic acid, 2-dimethylamino-5-iodobenzoic acid, 2-dimethylamino-5-hydroxybenzoic acid, 4-dimethylaminophenylacetic acid, 4-dimethylaminophenylpropionic acid, 4-dimethylaminophenylbutyric acid, 4-dimethylaminophenylmalic acid, 4-dimethylaminophenylpyruvic acid, 4-dimethylaminophenyllactic acid, 2-(4-dimethylaminophenyl)benzoic acid, and 2-(4-(dibutylamino)-2-hydroxybenzoyl)benzoic acid.
Preferred examples of the amine oxide compound of formula (15) include oxidized forms of exemplary amine compounds of formula (14), but are not limited thereto.
Preferred examples of the amine compound of formula (16) include, but are not limited thereto, 1-piperidinepropionic acid, 1-piperidinebutyric acid, 1-piperidinemalic acid, 1-piperidinepyruvic acid, and 1-piperidinelactic acid.
The compounds having an amine oxide structure represented by formula (15) may be prepared by selecting an optimum method in accordance with a particular structure. Exemplary non-limiting methods include oxidizing reaction of a nitrogen-containing compound using an oxidizing agent and oxidizing reaction of a nitrogen-containing compound in a hydrogen peroxide water diluted solution. Reference is made to Patent Document 9.
This reaction is an oxidizing reaction of an amine using an oxidizing agent, m-chloroperbenzoic acid. The reaction may be performed using other oxidizing agents commonly employed in standard oxidizing reaction. Following the reaction, the reaction mixture may be purified by standard techniques such as distillation, chromatography and recrystallization. Reference is made to Patent Document 10.
To the resist composition, any of surfactants commonly used for improving coating characteristics may be added. A number of surfactants are well known and described in Patent Documents 1 to 5 and any suitable one may be selected therefrom. Also, fluorinated polymers as disclosed in Patent Document 11 may be added.
In the resist composition, the surfactant is preferably formulated in an amount of up to 2 parts, and more preferably up to 1 part by weight, per 100 parts by weight of the overall polymer. When used, the surfactant is preferably added in an amount of at least 0.01 part by weight.
The organic solvent used in the preparation of the resist composition may be any of organic solvents in which the polymer(s), acid generator and other additives are dissolvable. Suitable organic solvents include, but are not limited to, ketones such as cyclohexanone and methyl n-amyl ketone; alcohols such as 3-methoxybutanol, 3-methyl-3-methoxybutanol, 1-methoxy-2-propanol, and 1-ethoxy-2-propanol; ethers such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, ethylene glycol monoethyl ether, propylene glycol dimethyl ether, and diethylene glycol dimethyl ether; esters such as propylene glycol monomethyl ether acetate (PGMEA), propylene glycol monoethyl ether acetate, ethyl lactate, ethyl pyruvate, butyl acetate, methyl 3-methoxypropionate, ethyl 3-ethoxypropionate, tert-butyl acetate, tert-butyl propionate, and propylene glycol mono-tert-butyl ether acetate; and lactones such as γ-butyrolactone. These solvents may be used alone or in admixture. Of these solvents, ethyl lactate, propylene glycol monomethyl ether, PGMEA, and mixtures thereof are preferred because the acid generator is most soluble therein.
In the negative resist composition, the organic solvent is preferably used in an amount of 1,000 to 10,000 parts by weight, more preferably 2,000 to 9,700 parts by weight per 100 parts by weight of the overall polymers. When adjusted to such a concentration, the resist composition is applicable by a spin coating technique to form a resist film having a thickness of 10 to 200 nm and an improved flatness in a consistent manner.
Process
Pattern formation using the resist composition of the invention may be performed by well-known lithography processes. The process generally involves coating, prebaking, exposure to EB or EUV, PEB, and development with alkaline developer. The resist composition is first applied onto a substrate for IC fabrication (silicon wafer having a surface layer of Si, SiO2, SiN, SiON, TiN, WSi, BPSG, SOG, organic antireflective coating or the like) or a substrate for mask circuit fabrication (quartz substrate having a surface layer of Cr, CrO, CrON, MoSi or the like) by a suitable coating technique such as spin coating. The coating is prebaked on a hot plate at a temperature of 60 to 150° C. for 1 to 10 minutes, preferably 80 to 140° C. for 1 to 5 minutes to form a resist film of 0.05 to 2.0 μm thick.
Then the resist film is exposed to high-energy radiation, typically DUV, EUV, excimer laser or x-ray through a mask having a desired pattern. Alternatively, a pattern is written on the resist film directly with EB. The exposure dose is preferably 1 to 200 mJ/cm2, more preferably 10 to 100 mJ/cm2. The exposure may be performed by conventional lithography whereas the immersion lithography of holding a liquid between the mask and the resist film may be employed if desired. In this case, a protective film which is insoluble in water may be applied on the resist film. The resist film is then baked (PEB) on a hot plate at 60 to 150° C. for 1 to 5 minutes, preferably 80 to 140° C. for 1 to 3 minutes. Thereafter the resist film is developed with a developer in the form of an aqueous base solution, for example, 0.1 to 5 wt %, preferably 2 to 3 wt % aqueous solution of tetramethylammonium hydroxide (TMAH) for 0.1 to 3 minutes, preferably 0.5 to 2 minutes by conventional techniques such as dip, puddle and spray techniques. In this way, a desired resist pattern is formed on the substrate.
One advantage is that the resist film has high etch resistance. Also the resist composition is effective when it is required that the pattern experience a minimal change of line width even when the duration between exposure and PEB is prolonged. Because of these advantages, the resist composition is effective in processing a photomask blank by EB lithography. The resist composition is effectively applicable to a processable substrate, specifically a substrate having a surface layer of material to which a resist film is less adherent and which is likely to invite pattern stripping or pattern collapse, and particularly a substrate having sputter deposited thereon a surface layer material susceptible to pattern collapse, typically metallic chromium or a chromium compound containing at least one light element selected from oxygen, nitrogen and carbon. Substrates of this nature are often used in photomask blanks, and the invention is effective for pattern formation on these substrates.
EXAMPLE
Synthesis Examples, Examples, and Comparative Examples are given below by way of illustration and not by way of limitation. The average molecular weights including Mw and Mn are determined by GPC versus polystyrene standards, from which a dispersity (Mw/Mn) is computed. Me stands for methyl. The compositional ratio of a copolymer is on a molar basis.
Polymer Synthesis Example 1
In a 200-mL dropping funnel under nitrogen blanket, a solution was prepared by dissolving 57.69 g of 4-(1-ethoxyethoxy)styrene, 6.21 g of acenaphthylene, 6.51 g of 4-methylstyrene, 9.59 g of a monomer Z-1 of the structure shown below, and 7.52 g of dimethyl 2,2′-azobis(2-methylpropionate) (V601, Wako Pure Chemical Industries, Ltd.) in 90 g of methyl ethyl ketone (MEK) as a solvent. A 500-mL polymerization flask was purged with nitrogen, charged with 59 g of MEK, and heated at 80° C., after which the solution was added dropwise to the flask over 4 hours. After the completion of dropwise addition, stirring was continued for 18 hours while maintaining the polymerization temperature of 80° C. The polymerization solution was then cooled down to room temperature, combined with 60 g of methanol and 1.6 g of oxalic acid dihydrate, and stirred for 4 hours at 50° C. The solution was cooled down to room temperature and then neutralized by adding 2.1 g of triethylamine. The reaction solution was concentrated. The concentrate was dissolved in 120 g of THF again and added dropwise to 1,000 g of hexane whereupon a copolymer precipitated. The copolymer precipitate was collected by filtration and washed twice with 300 g of hexane. After washing, the copolymer was dissolved in 230 g of ethyl acetate and 90 g of water. The resulting two-layer liquid was transferred to a separatory funnel where 0.48 g of acetic acid was added and separatory operation was carried out. The lower layer was discharged, 90 g of water and 0.67 g of pyridine were added to the remaining organic layer, and separatory operation was carried out. Further separatory operation of discharging the lower layer and adding 90 g of water was repeated four times. Thereafter, the organic layer, ethyl acetate was concentrated and dissolved in 100 g of acetone. The acetone solution was added dropwise to 2 L of water whereupon a copolymer precipitated. The polymer precipitate was collected by filtration, washed twice with 1 L of water, and dried for 24 hours at 50° C., yielding 50 g of a copolymer, designated Polymer 1, having Mw=4,300 and Mw/Mn=1.42.




Polymer Synthesis Example 2
In a 200-mL dropping funnel under nitrogen blanket, a solution was prepared by dissolving 54.18 g of 4-(1-ethoxyethoxy)styrene, 4.7 g of acenaphthylene, 2.47 g of 4-methylstyrene, 9.07 g of monomer Z-1 of the above structure, 8.41 g of a monomer Y-1 of the structure shown below, and 6.93 g of dimethyl 2,2′-azobis(2-methylpropionate) (V601, Wako Pure Chemical Industries, Ltd.) in 90 g of methyl ethyl ketone (MEK) as a solvent. A 500-mL polymerization flask was purged with nitrogen, charged with 59 g of MEK, and heated at 80° C., after which the solution was added dropwise to the flask over 4 hours. After the completion of dropwise addition, stirring was continued for 18 hours while maintaining the polymerization temperature of 80° C. The polymerization solution was then cooled down to room temperature, combined with 60 g of methanol and 1.6 g of oxalic acid dihydrate, and stirred for 4 hours at 50° C. The solution was cooled down to room temperature and then neutralized by adding 2.1 g of triethylamine. The reaction solution was concentrated, dissolved in 120 g of THF again, and added dropwise to 1,000 g of hexane whereupon a copolymer precipitated. The copolymer precipitate was collected by filtration and washed twice with 300 g of hexane. After washing, the copolymer was dissolved in 230 g of ethyl acetate and 90 g of water. The resulting two-layer liquid was transferred to a separatory funnel where separatory operation was carried out. Subsequently, separatory operation of discharging the lower layer and adding 90 g of water to the remaining organic layer was repeated four times. Thereafter, the organic layer, ethyl acetate was concentrated and dissolved in 100 g of acetone. The acetone solution was added dropwise to 2 L of water whereupon a copolymer precipitated. The polymer precipitate was collected by filtration, washed twice with 1 L of water, and dried for 24 hours at 50° C., yielding 50 g of a copolymer, designated Polymer 2, having Mw=4,400 and Mw/Mn=1.45.




Polymer Synthesis Example 6
In a 200-mL dropping funnel under nitrogen blanket, a solution was prepared by dissolving 54.15 g of 4-hydroquinone monomethacrylate, 7.94 g of acenaphthylene, 7.7 g of 4-methylstyrene, 10.2 g of monomer Z-1 of the above structure, and 8.0 g of dimethyl 2,2′-azobis(2-methylpropionate) (V601, Wako Pure Chemical Industries, Ltd.) in 90 g of methyl ethyl ketone (MEK) as a solvent. A 300-mL polymerization flask was purged with nitrogen, charged with 59 g of MEK, and heated at 80° C., after which the solution was added dropwise to the flask over 4 hours. After the completion of dropwise addition, stirring was continued for 4 hours while maintaining the polymerization temperature of 80° C. Thereafter, the polymerization solution was cooled down to room temperature, and added dropwise to 1,300 g of hexane whereupon a copolymer precipitated. The copolymer precipitate was collected by filtration, washed twice with 300 g of hexane, and dissolved in 140 g of MEK. The MEK solution was passed through a nylon filter having a pore size of 0.02 μm and added dropwise to 1,300 g of hexane whereupon a copolymer precipitated. The copolymer precipitate was collected by filtration, washed twice with 300 g of hexane, and dried, yielding 75 g of a white copolymer, designated Polymer 6, having Mw=3,800 and Mw/Mn=1.55.




Polymer Synthesis Examples 3 to 5, 7 to 12
Resins (Polymers 3 to 5, 7 to 12) shown in Table 1 were synthesized by the same procedure as Polymer Synthesis Examples 1, 2 and 6 except that the type and amount of monomers were changed. The units 1 to 4 in Table 1 have the structures shown in Tables 2 to 5. In Table 1, a ratio of each unit incorporated is on a molar basis.












TABLE 1






Unit 1
Unit 2
Unit 3
Unit 4
Unit 5


Resin
(ratio)
(ratio)
(ratio)
(ratio)
(ratio)







Polymer 1 
A-1 (0.74)
B-1 (0.14)
C-2 (0.10)
Z-1 (0.03)



Polymer 2
A-1 (0.73)
B-1 (0.08) 
C-2 (0.08) 
Z-1 (0.03)
Y-1 (0.08)


Polymer 3
A-1 (0.74)
B-1 (0.14) 
C-1 (0.10) 
Z-1 (0.03)



Polymer 4 
A-1 (0.74)
B-1 (0.14) 
C-2 (0.10)
Z-2 (0.03)



Polymer 5 
A-1 (0.74)
B-1 (0.14) 
C-2 (0.10)
Z-3 (0.03)



Polymer 6 
A-2 (0.69)
B-1 (0.16) 
C-2 (0.12) 
Z-1 (0.03)



Polymer 7 
A-2 (0.68)
B-1 (0.11) 
C-2 (0.10)
Z-1 (0.03)
Y-1 (0.08)


Polymer 8 
A-3 (0.80)
B-1 (0.10) 
C-2 (0.07) 
Z-1 (0.03)



Polymer 9 
A-3 (0.80)
B-1 (0.05) 
C-2 (0.04) 
Z-1 (0.03)
Y-1 (0.08)


Polymer 10
A-1 (0.74)
B-2 (0.14)
C-2 (0.10) 
Z-1 (0.03)



Polymer 11
A-1 (0.74)
B-3 (0.14)
C-2 (0.10)
Z-1 (0.03)



Polymer 12
A-1 (0.74)
B-4 (0.14) 
C-2 (0.10) 
Z-1 (0.03)



















TABLE 2













A-1



 







A-2



 







A-3




















TABLE 3













B-1



 







B-2



 







B-3



 







B-4




















TABLE 4













C-1



 







C-2




















TABLE 5













Z-1



 







Z-2



 







Z-3










Examples 1 to 14 and Comparative Example 1
Preparation of Negative Resist Compositions
Resist compositions were prepared by using inventive polymers (Polymers 1 to 12) or comparative polymer (Polymer K), and dissolving the polymer, an acid generator (PAG-1 or 2), and a basic compound (Base-1 or 2) in an organic solvent mixture in accordance with the recipe shown in Table 6. These compositions were each filtered through a nylon or UPE filter having a pore size of 0.02 μm, thereby giving negative resist composition solutions.




The organic solvents used were propylene glycol monomethyl ether acetate (PGMEA) and ethyl lactate (EL). Each solution further contained tetramethoxymethylglycoluril (TMGU) as a crosslinker and 0.075 pbw of a surfactant PF-636 (Omnova Solutions, Inc.).













TABLE 6






Resin
PAG
Base
Additive
Solvent 1
Solvent 2



(pbw)
(pbw)
(pbw)
(pbw)
(pbw)
(pbw)







Example 1
Polymer 1

Base-1
TMGU
PGMEA
EL



(80)

(0.57)
(8.154)
(1,109)
(2,587)


Example 2
Polymer 1

Base-1
TMGU
PGMEA
EL



(40)

(0.35)
(8.154)
(1,109)
(2,587)



Polymer K








(40)







Example 3
Polymer 1

Base-1
TMGU
PGMEA
EL



(80)

(0.47)
(8.154)
(1,109)
(2,587)





Base-2








(0.1)





Example 4
Polymer 2

Base-1

PGMEA
EL



(80)

(0.57)

(1,109)
(2,587)


Example 5
Polymer 3

Base-1
TMGU
PGMEA
EL



(80)

(0.57)
(8.154)
(1,109)
(2,587)


Example 6
Polymer 4

Base-1
TMGU
PGMEA
EL



(80)

(0.57)
(8.154)
(1,109)
(2,587)


Example 7
Polymer 5

Base-1
TMGU
PGMEA
EL



(80)

(0.57)
(8.154)
(1,109)
(2,587)


Example 8
Polymer 6

Base-1
TMGU
PGMEA
EL



(80)

(0.57)
(8.154)
(1,109)
(2,587)


Example 9
Polymer 7

Base-1

PGMEA
EL



(80)

(0.57)

(1,109)
(2,587)


Example 10
Polymer 8

Base-1
TMGU
PGMEA
EL



(80)

(0.57)
(8.154)
(1,109)
(2,587)


Example 11
Polymer 9

Base-1

PGMEA
EL



(80)

(0.57)

(1,109)
(2,587)


Example 12
Polymer 10

Base-1
TMGU
PGMEA
EL



(80)

(0.57)
(8.154)
(1,109)
(2,587)


Example 13
Polymer 11

Base-1
TMGU
PGMEA
EL



(80)

(0.57)
(8.154)
(1,109)
(2,587)


Example 14
Polymer 12

Base-1
TMGU
PGMEA
EL



(80)

(0.57)
(8.154)
(1,109)
(2,587)


Comparative
Polymer K
PAG-1(8)
Base-1
TMGU
PGMEA
EL


Example 1
(80)
PAG-2(2)
(0.57)
(8.154)
(1,109)
(2,587)





*pbw: parts by weight






Evaluation of EB Image Writing

Using a coater/developer system ACT-M (Tokyo Electron Ltd.), each of the negative resist compositions (Examples 1 to 14, Comparative Example 1) was spin-coated onto a 152-mm square mask blank having a chromium oxynitride film at the outermost surface and pre-baked on a hot plate at 110° C. for 600 seconds to form a resist film of 80 nm thick. The thickness of the resist film was measured by an optical film thickness measurement system Nanospec (Nanometrics Inc.). Measurement was made at 81 points in the plane of the blank substrate excluding a peripheral band extending 10 mm inward from the blank periphery, and an average film thickness and a film thickness range were computed therefrom.
The coated mask blanks were exposed to EB using an EB writer system EBM-5000Plus (NuFlare Technology Inc., accelerating voltage 50 keV), then baked (PEB) at 120° C. for 600 seconds, and developed with a 2.38 wt % tetramethylammonium hydroxide aqueous solution, thereby yielding negative patterns.
The patterned wafer was observed under a top-down scanning electron microscope (TDSEM). The optimum exposure (Eop) was defined as the exposure dose (μC/cm2) which provided a 1:1 resolution at the top and bottom of a 200-nm 1:1 line-and-space pattern. The maximum resolution of the resist was defined as the minimum line width of a 400-nm line-and-space pattern that could be resolved and separated at the optimum exposure. The LER of a 200-nm line-and-space pattern was measured under SEM. On observation in cross section of the resist pattern under SEM, it was visually judged whether or not the pattern profile was rectangular. Table 7 tabulates the test results of the inventive and comparative resist compositions on EB image writing.













TABLE 7









Maximum






Eop
resolution
LER
Pattern




(μC/cm2)
(nm)
(nm)
profile









Example 1
20
40
4.4
rectangular



Example 2
22
45
4.4
rectangular



Example 3
21
40
4.5
rectangular



Example 4
20
40
4.3
rectangular



Example 5
21
45
4.4
rectangular



Example 6
20
40
4.4
rectangular



Example 7
20
40
4.0
rectangular



Example 8
20
40
4.6
rectangular



Example 9
20
40
4.2
rectangular



Example 10
21
40
4.1
rectangular



Example 11
21
40
4.3
rectangular



Example 12
19
40
4.5
rectangular



Example 13
23
45
4.6
rectangular



Example 14
22
40
4.4
rectangular



Comparative 
27
60
5.7
undercut



Example 1










It is evident from Table 7 that the resist compositions of Examples are improved in resolution and LER over that of Comparative Example 1 when processed by EB lithography. The chemically amplified negative resist composition of the invention is suited as ultrafine pattern-forming material for VLSI fabrication and mask pattern-forming material by EB lithography.
Japanese Patent Application No. 2011-041528 is incorporated herein by reference.
Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.

## Claims

1. A pattern forming process comprising the steps of:
applying a negative resist composition onto a processable substrate to form a resist film,
exposing the film patternwise to high-energy radiation, and
developing the exposed film with an alkaline developer to form a resist pattern;
wherein the negative resist composition is a chemically amplified negative resist composition comprising a polymer, adapted such that the polymer may turn insoluble in alkaline developer by reacting with a crosslinker and/or a recurring unit having a crosslinkable functional group in the polymer under the catalysis of an acid generated upon exposure to high-energy radiation, to form crosslinks between polymer molecules,
said polymer consisting of:
recurring units of the general formula (1),
recurring units of at least one type selected from the general formulae (2), (3), and (4),
recurring units of general formula (5),
recurring units of at least one type selected from the general formulae (6) and (7), and
recurring units of the general formula (M-1) or (M-2);





wherein A is a single bond or a C1-C10 alkylene group which may be separated by ethereal oxygen,
Rf is each independently hydrogen, fluorine, trifluoromethyl, or pentafluoroethyl, with the proviso that not all Rf's are hydrogen,
B is a C1-C10 divalent hydrocarbon group in which some or all hydrogen atoms may be replaced by fluorine and in which a methylene moiety at a position other than α- and β-positions relative to the ester oxygen atom may be replaced by oxygen,
C is a single bond or a C1-C10 alkylene group which may be separated by ethereal oxygen,
R1 is hydrogen, fluorine, methyl, or trifluoromethyl,
R2, R3, and R4 are each independently a straight, branched or cyclic C1-C10 alkyl or alkoxy group,
R5 is a substituted or unsubstituted, straight, branched or cyclic C1-C30 alkyl group which may contain an ether bond, or a substituted or unsubstituted C6-C14 aryl group,
R6 is each independently a C1-C8 alkyl group,
R7 is each independently a C1-C8 alkyl group,
k, m and n are each independently an integer of 0 to 3, a is an integer of 0 to 4, b is an integer of 1 to 5, c and d each are an integer of 1 to 4, e is an integer of 0 to (4-c), f is an integer of 0 to (4-d), p is independently 0 or 1, and t is an integer of 0,





wherein D is a single bond or a C1-C10 alkylene group which may be separated by ethereal oxygen, R1 is as defined above, R8 is each independently halogen, an optionally halo-substituted C1-C8 alkyl or alkoxy group, a C6-C20 aromatic ring-containing hydrocarbon group, or an optionally halo-substituted C1-C12 acyloxy group, g is an integer of 0 to 5, q is 0 or 1, and s is an integer of 0 to 2,




wherein R9 is independently halogen, an optionally halo-substituted C1-C12 acyloxy group, an optionally halo-substituted C1-C8 alkyl group, or an optionally halo-substituted C1-C8 alkoxy group, and h is integer of 0 to 4,




wherein R1 is hydrogen, fluorine, methyl, or trifluoromethyl, R10 is hydrogen of a straight, branched or cyclic C1-C8 monovalent hydrocarbon group, R11 is each independently a straight, branched or cyclic C1-C8 monovalent hydrocarbon group which may contain oxygen, or halogen, i is an integer of 0 to 4, and u is an integer of 0 to 2,
wherein the recurring units of formula (1) account for 0.5 to 10 mol % and the sum of recurring units of formulae (2), (3), and (4) accounts for 50 to 99.5 mol %, recurring units of formula (5) account for 39.5 mol %, recurring units of formulae (6) or (7) account for up to 39.5 mol %, and recurring units of formulae (M-1) or (M-2) account for up to 20 mol %, based on the entire recurring units of the polymer.

2. The process of claim 1 wherein the high-energy radiation is EUV or EB.

3. The process of claim 1 wherein the processable substrate comprises a chromium-containing material at the outermost surface.

4. The process of claim 1 wherein the processable substrate is a photomask blank.