WO2007001848A2 - High refractive index fluids with low absorption for immersion lithography - Google Patents

Abstract

An immersion lithography fluid, including a solvent, and at least one additive soluble in the solvent, in which the immersion lithography fluid has a refractive index greater than the refractive index of the solvent at a selected wavelength and the immersion lithography fluid is acceptable for use in immersion lithography. In one embodiment, the immersion lithography fluid includes an ionic liquid. In one embodiment, an immersion lithography system including an optical surface, a wafer support for holding a workpiece, and the immersion lithography fluid disposed between the optical surface and the workpiece and contacting at least a portion of the optical surface.

Description

TITLE: HIGH REFRACTIVE INDEX FLUIDS WITH LOW ABSORPTION

FOR IMMERSION LITHOGRAPHY

CROSS-REFERENCE TO RELATED APPLICATIONS The present application is based on and claims priority under applicable law to U.S. Provisional Application No. 60/693,748, filed 24 June 2005, and to U.S. Provisional Application No. 60/735,912, filed 10 November 2005; the entire contents of both are hereby incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to fluids for use in immersion lithography. More specifically, the present invention relates to fluids having high refractive index and low absorption of electromagnetic radiation in a range of wavelengths useful in immersion lithography.

BACKGROUND

Optical lithography has conventionally been practiced such that the medium through which imaging occurs is air. The practical limit is defined by the effective wavelength equation:

λ_eff = λ/2 ^• n ^• NA

where λ_eff is the effective wavelength obtained, λ is the wavelength of incident or source light, NA is the numerical aperture of the projection optical system, and n is the index of refraction of the medium. In immersion lithography, by introducing an immersion lithography fluid, having a refractive index n > 1 , instead of air (refractive index ~ 1 ) as the medium between the last lens element of the projection optical system and the wafer being imaged, the refractive index of the medium is increased, thereby enabling enhanced resolution by lowering the effective wavelength, λ_eff, of the light source operating at a wavelength λ.

Lowering a light source's wavelength enables finer resolution of details. In this way, immersion lithography becomes attractive by, for example, effectively lowering a 193 nm light source to about 145 nm, depending on the refractive index of the immersion lithography fluid, thereby gaining resolution while enabling the printing of critical layers with the same photolithographic tools in use in the industry today, thereby avoiding or at least delaying replacing expensive capital equipment. Thus, the older 193 nm tools can still be used, thereby avoiding many difficulties associated with 157 nm lithography, such as the use of CaF₂, hard pellicles, a nitrogen purge to remove oxygen, etc.

Resolution is the smallest feature of a given type that can be formed or printed with acceptable quality and control. Resolution is sometimes defined as the smallest feature of a given type that meets a given depth of focus requirement. The resolution of an optical system, W, is determined by the

Rayleigh equation:

W = k., ( λ / n sin θ )

where λ is the wavelength, sin θ is the angular half aperture of the lens, n is the index of refraction of the medium, and k-, is the resolution coefficient. The R₁ parameter declines with feature size. The resolution, W, is also sometimes referred to as the critical dimension, CD. Although both the wavelength λ and the resolution coefficient K₁ have both been reduced, and sin θ has been increased, the limits of these improvements have been or will be soon reached. Thus, resolution can be increased further for a given wavelength only by increasing the index of refraction, n, of the immersion lithography fluid.

Another important aspect of lithography, the depth of focus, DOF also depends on refractive index. Depth of focus is the total range of focus that can be tolerated so that the resulting feature is within specifications such as linewidth, sidewall angle, resist loss, and exposure latitude. DOF is defined by the following equation:

DOF = k₂ ( λ / n sin² (θ/2) ) where λ is the wavelength, sin θ is the angular half aperture of the lens, n is the index of refraction of the medium, and k₂ is a constant related to the process. Depth of focus, as is known in the art, is important in obtaining resolution between fine structures. As shown by the effective wavelength equation, the effective wavelength, λ_Θff, can be reduced for a given NA and source wavelength, λ, by increasing the refractive index, n. Similarly, the DOF and resolution can both be improved by increasing the refractive index, n. Conventionally, water and perfluoroethers have been used as immersion lithography fluids to accomplish the increase in refractive index. Water has a refractive index of about 1.43-1.47 (depending on the source of information) at 193 nm. Perfluoroethers have a refractive index of about 1.4-1.5 at 193 nm. Absorptivity, a measure of the amount of the electromagnetic radiation absorbed by the immersion lithography fluid, generally increases as the source wavelength decreases, and can result in chemical changes in the immersion fluid. Pure water and perfluoroethers have an acceptable absorptivity at the wavelengths of interest for immersion lithography.

However, a need remains for immersion lithography fluids having a higher refractive index and being acceptable for use in immersion lithography, e.g., at least having a low absorptivity at the wavelengths of interest for immersion lithography.

SUMMARY

In one embodiment, the present invention relates to an immersion lithography fluid, including a solvent, and at least one additive soluble in the solvent, in which, at a selected wavelength, the immersion lithography fluid has a refractive index greater than the refractive index of the solvent and the immersion lithography fluid is acceptable for use in immersion lithography. The selected wavelength may be a wavelength useful in immersion lithography.

In another embodiment, the present invention relates to an immersion lithography system, including an optical surface, a wafer support for holding a workpiece, and an immersion lithography fluid disposed between the optical surface and the workpiece and contacting at least a portion of the optical surface, the fluid including a solvent, and at least one additive soluble in the solvent, in which, at a selected wavelength, the immersion lithography fluid has a refractive index greater than the refractive index of the solvent and the immersion lithography fluid is acceptable for use in immersion lithography. In one embodiment of the system, the fluid contacts at least a portion of the optical surface and at least a portion of the workpiece.

In one embodiment, the refractive index of the fluid, at the selected wavelength, ranges from greater than the refractive index of water to about 1.70, and in one embodiment, from about 1.5 to about 1.7, at the selected wavelength. In one embodiment, the immersion lithography fluid includes an ionic liquid. In one embodiment, the ionic liquid has a refractive index in the range from about 1.5 to about 1.7, at the selected wavelength.

The present invention thus provides an immersion lithography fluid for use in an immersion lithography system in which the fluid provides improved refractive index and acceptable for use in immersion lithography, including at least acceptable absorptivity at the selected wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic side elevational and cross-section ai view of an immersion lithography system including an embodiment of the present invention.

Fig.2 is a schematic side elevational and cross-sectional magnified view of a portion of an immersion lithography system illustrating details of a function of an embodiment of the present invention.

It should be appreciated that for simplicity and clarity of illustration, elements shown in the Figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to each other for clarity. Further, where considered appropriate, reference numerals have been repeated among the Figures to indicate corresponding elements. It should be appreciated that the process steps and structures described herein do not form a complete system or process flow for carrying out an immersion lithography process, such as would be used in manufacturing a semiconductor device or other device. The present invention can be practiced in conjunction with fabrication techniques and apparatus currently used in the art, and only so much of the commonly practiced materials, apparatus and process steps are included as are necessary for an understanding of the present invention.

DETAILED DESCRIPTION

As used herein the term "acceptable for use in immersion lithography" means that the material so described (1) has a transmissivity of at least about 95%, or an absorptivity of less than about 1 cm^"1 at the selected wavelength, and (2) is chemically stable under the conditions of use. As noted, the selected wavelength may be a wavelength useful in immersion lithography. Thus, the first criteria for acceptability is that the immersion lithography fluid should have the indicated transmissivity or absorptivity. Absorptivity of a solute is related to absorbance by Beer's Law at a particular wavelength, where A = B x C x e, where A = absorbance (unit-less), B = path length in cm., C = Concentration of the solute in moles/liter, and e = molar absorptivity or extinction coefficient, in L/mol cm. Absorptivity in the present application is A/B, measured at a given wavelength. As is known, electromagnetic radiation at the wavelengths of interest in immersion lithography, e.g., from about 190 nm to about 350 nm, is highly energetic and can break many chemical bonds. Thus, the second criteria for acceptability is that the immersion lithography fluid should be chemically stable under the conditions of use. The conditions of use include, for example, time, temperature and pressure, as well as exposure to the highly energetic electromagnetic radiation used in immersion lithography. Stable (and "stability" and cognate terms), as used herein, means that the immersion lithography fluid does not break down under the influence of the electromagnetic radiation employed to such as extent that the breakdown products would interfere in the lithography process by, for example, reacting to a deleterious extent with the photoresist or with other elements of the lithography apparatus with which it is being used. As will be recognized by the person of ordinary skill, there is likely to be at least some amount of breakdown of almost any immersion lithography fluid when exposed to such energetic electromagnetic radiation. What is needed for the fluid to be considered stable is that it meet the criteria of not interfering with the overall process, to an extent that renders the process impracticable.

In one embodiment, a further a characteristic of an acceptable immersion lithography fluid is that it should not alter or chemically attack the photoresist with which it is in contact, and that it will not alter or chemically attack the lens materials, such as quartz or CaF₂ or MgF₂. It is known that water will attack even quartz (SiO₂), resulting in the dissolution of about one monolayer per year; this or similar degrees of change is not considered to "alter or chemically attack" the lens material.

In one embodiment, a useful characteristic of an acceptable immersion lithography fluid is that it should not exhibit a change in refractive index greater than about 300 ppm, in the temperature range at which immersion lithography processes are commonly carried out. In one embodiment, the refractive index of the immersion lithography fluid should not be changed to the fourth decimal place by any variable in the immersion lithography process as commonly carried out. In one embodiment, the refractive index of the immersion lithography fluid should not be changed to the fifth decimal place by any variable in the immersion lithography process as commonly carried out.

Based on the foregoing, and on the knowledge of those of ordinary skill in the art, it is It is considered that such person of ordinary skill in the art can readily determine whether a given candidate fluid for use as an immersion lithography fluid in the present invention is "acceptable for use in immersion lithography."

In one embodiment of the present invention, addition of a dissolved salt and/or organic additive to an aqueous solution or a solvent increases the refractive index of the solution. In one embodiment, the resulting solution may be used in sub-micron patterning of photoresist by immersion lithography, for example, at a 193 nm wavelength, and thereby significantly reduce the effective wavelength. Such high refractive index solutions, matched with higher numerical aperture optics in immersion lithography can effectively reduce the wavelength of the incident radiation resulting in printing of the photoresist at higher resolution according. As described in more detail below, the immersion lithography fluid having a high refractive index is applied as a thin layer between the lens and photoresist during the irradiation/patterning process. By being in contact with the final lens in an imaging assembly and having a high refractive index, the immersion lithography fluid of the present invention reduces or avoids loss of the electromagnetic radiation due to internal reflection at the interface between the lens surface and the adjacent air that would occur in the absence of the immersion fluid. The refractive index of a solution is a function of a number of factors including electron density, dipole moment and polarizability.

In one embodiment, the present invention relates to a composition of matter, e.g., an immersion lithography fluid, in which the refractive index of aqueous or solvent solutions is greater than about 1.5 (measured at 193 nm). In one embodiment, the refractive index of the immersion lithography fluid is greater than water at the selected wavelength. In one embodiment, the refractive index of the immersion lithography fluid is in the range from about 1.5 to about 1.7 (measured at 193 nm). In one embodiment, the refractive index of the immersion lithography fluid of the present invention ranges from greaterthan the refractive index of water to about 1.70, at the selected wavelength.

In one embodiment, such increased refractive indices may be obtained by the addition of highly soluble ionic materials and/or organic additives to the aqueous or solvent solution, in the immersion lithography fluid of the present invention. In another embodiment, a high purity ionic liquid may serve as the immersion lithography fluid of the present invention. In either case, the materials used as such fluids may be ions or neutral compounds with high ionization energies that exhibit a relatively low absorption at the irradiation wavelength of interest. Any ionic liquid having suitable properties for immersion lithography may be used. Thus, the ionic liquid should not have too high a viscosity and should not absorb radiation at the operating wavelength to a degree that would interfere with the immersion lithography. Thus, it should be acceptable for use in immersion lithography, as defined herein. In one embodiment, ionic liquids comprising compounds such as tetraalkylammonium, sulfonium and phosphonium (including, e.g., tetramethylammonium,tetrakis(hydroxymethyl)phosphonium,trimethylsulfonium, (2-iodoethyl)trimethylammonium) may be used. All of these ions form highly soluble salts and include one or more relatively high atomic number atoms, and so are likely to increase the refractive index of the solvent significantly without imparting high absorptivity. Exemplary ions that meet this requirement include Cs⁺, Ba²⁺, La³⁺, Ce⁴⁺, Hf⁴⁺, Ta⁵⁺, W⁶⁺. In another embodiment, the ions include more generally, alkali, alkaline earth, lanthanide, 2nd and 3rd transition series metal ions, and Group 3, 4, 5 and 6 metals in high oxidation states, and main group elements in high oxidation states. As used herein, the term "high oxidation state" when applied to an ion refers to an ion having no electrons in its outer valence shell (in the ground state). The above exemplary and other cations may be paired with polarizable anions containing high atomic number atoms, e.g. thiosulfate, iodate, methylsulfonate, dihydrogen phosphate, methylsulfate to make a salt that would have high solubility and refractive index with optical transparency at the irradiation wavelength.

In one embodiment, optical transparency is better for methyl substituted quaternary ammonium, phosphonium and sulfonium cations as opposed to longer chain alkylammonium or phosphonium salts.

In one embodiment, a suitable ionic liquid comprises one or more of the ionic liquids disclosed in WO 2004/016570, the disclosure of which relating to ionic liquids is incorporated herein by reference.

In one embodiment, as disclosed in WO 2004/016570, the immersion lithography fluid comprises an ionic liquid comprising an anion having the general formula:

R₁-O-C(O)-CH(SO₃^₃-C(O)-O-R₂

wherein R₁ and R₂ may be substituted or unsubstituted and independently may be a C₁-C₁₂ alkyl group, and R₃ may be substituted or unsubstituted and may be a C₁-C₁₂ alkylene group, O- , N- or S-containing heteroarylene group, C₆ or C₁₀ arylene group, or C₃-C₁₄ cycloalkylene group, and in one embodiment, R₃ is ~ (CH₂Xr where n is an integer ranging from 1 to about 10.

As used herein, "cycloalkyl" includes mono-cyclic, di-cyclic and tri-cyclic rings, e.g., 2 or 3 fused cycloalkyl rings, such as perhydronaphthalene and perhydroanthracene.

As used herein, O-, N- or S-containing heteroarylene group" includes 5- and 6-membered single rings and 9- and 10-membered fused rings, containing one or more O, N or S atom in the ring; such heteroarylene groups include furan, thiophene, thiazole, dioxin, oxathiazine, benzofuran and benzoxazole, for example.

In one embodiment, R₁ and R₂ may be bonded together as members a ring, such as a 5-7 membered carbocyclic ring, e.g., cyclopentyl or cyclohexyl.

As noted, R₁, R₂ and/or R₃ may be unsubstituted or substituted with one or more substituents. With respect to the substitutents on these R₁, R₂ and/or R₃ groups, and for all other similar groups (e.g., R groups) disclosed herein, the type of the substituent is not particularly critical so long as the compound or mixture of compounds has the desired ionic liquid properties and the desired suitability for immersion lithography. Thus, in one embodiment, the substituent may include typical substituents such as C₁-C₁₂ alkyl, C₁-C₁₂ alkoxy, C_1-C₁₂ alkylthio, nitro, halo, cyano, silyl, OH, and other suitable substituents used for modifying the characteristics of organic compounds. The alkyl portions of the substituent organic group may be branched or unbranched. In one embodiment, the substituent organic (e.g., alkyl, alkylene, or cycloalkyl) group itself may be further branched and/or substituted with additional such substituents. In one embodiment, the immersion lithography fluid comprises an ionic liquid comprising a docusate variant having a general formula:

R₁-N(R₄)-C(O)-CH(SO₃-)-R₃-C(O)-N(R₅)-R₂

wherein R₁ , R₂ and R₃ have the same definitions as set forth above, and R₄ and

R₅ are independently H, C₁-C₆ alkyl or C_{1 -}C₆ alkyl-ether. Mixtures of any two or more of the foregoing anions may be suitably used. Processes for preparing the foregoing ionic liquids are disclosed in WO 2004/016570.

In one embodiment, the immersion lithography fluid comprises an ionic liquid such as disclosed in U.S. Published Application No. 2004/0007693, the disclosure of which relating to ionic liquids is incorporated herein by reference.

In one embodiment, as disclosed in US 2004/0007693, the immersion lithography fluid comprises an ionic liquid comprising an anion and a cation in which the anion is represented generally by the following structure (I):

In the above structure (I), X is a Group IHA element (Group 13 in IUPAC nomenclature), for example, boron, or a Group VA element (Group 15 in IUPAC nomenclature), for example, phosphorus or arsenic. If X is a Group IHA element then the anion has two ligands and m is two (2) whereas if X is a Group VA element then the anion has three ligands and m is three (3). In one embodiment X is either boron (B) or phosphorus (P). In one embodiment, R₁ may be substituted or unsubstituted and independently may be C_1-C₁₂ alkylene, C₁X₁₂ alkenylene, C₃-C₁₄ cycloalkylene, C₆ or C₁₀ arylene, O- , N- or S-containing heteroarylene, -C(O)-R₂-, and -C(O)-R₂-C(O)-. In one embodiment, R₂ may be substituted or unsubstituted and independently may be C₁-C₁₂ alkylene, C_1-C₁₂ alkenylene, C₃_C₁₄ cycloalkylene, C₆ or C₁₀ arylene, O- , N- or S-containing heteroarylene. In one embodiment, R₃ is independently O or S in each ligand designated by m. In one embodiment the cation is a quaternary ammonium or phosphonium cation. Since R₁ and R₂ may be independently selected, bidentate anions may have two different ligands and tridentate ligands may have three different ligands. As used herein, "cycloalkyl" includes mono- cyclic, di-cyclic and tri-cyclic rings, e.g., 2 or 3 fused cycloalkyl rings, such as perhydronaphthalene and perhydroanthracene.

It should be noted that the definitions of R₁, R₂, R₃ and R₄ in structure (I) and in the following structures (H)-(VIII) may differ from the definitions of the R₁, R₂, R₃, R4 and R₅ groups defined with respect to the sulfonate-containing compounds disclosed in WO 2004/016570 and disclosed above.

In one embodiment, in the structure (I), R₁ and R₂ may optionally be substituted with one or more substituents. The type of the substituent is not particularly critical so long as the compound or mixture of compounds is a liquid at ambient or near ambient temperatures. Thus, in one embodiment, the substituents may include typical substituents such as C^C₁₂ alkyl, C₁-C₁₂ alkoxy, C_{1 -}C₁₂ alkylthio, SO₃H₁ nitro, halo, cyano, silyl, OH, and other suitable substituents used for modifying the characteristics of organic compounds. The alkyl portions of the substituent organic group may be branched or unbranched. In one embodiment, the substituent organic (e.g., alkyl, alkylene, or cycloalkyl) group itself may be further branched and/or substituted with such substituents.

In one embodiment, in the structure (I), the substituents on R₁ and R₂, particularly when R₁ or R₂ is arylene or heteroarylene, are electron-withdrawing groups such as halo or nitro. In one embodiment, two or more adjacent substituents on an arylene or a heteroarylene group may be taken together to form a ring such as a 5-7 membered carbocyclic or heterocyclic ring. Examples of such carbocyclic rings include cyclopentyl and cyclohexyl rings while examples of such heterocyclic rings include morpholino and piperidino rings.

I n one embodiment, the anion has a structure represented by I I-VI 11 below.

π

VT

In structures II-VIII, X and m are defined as above for structure (I), and each R₄ is independently selected from H, C₁-C₁₂ alkyl, C₁-C₁₂ alkoxy, C₁-C₁₂ alkylthio, SO₃H, NO₂, halo, cyano, silyl, OH, and other suitable substituents used for modifying the characteristics of organic compounds. The alkyl portions of the organic substituents may be branched, unbranched or cyclic. In one embodiment, the substituent organic (e.g., alkyl, alkylene or cycloalkyl) group itself may be further branched and/or substituted with such substituents.

Processes for preparing the foregoing ionic liquids are disclosed in U.S. Published Application No. 2004/0007693.

In one embodiment, the cation of the foregoing ionic liquids is not particularly critical so long as the ionic liquid has properties to make it suitable for its intended use. Typical useful cations include, for example, "onium"cations, such as those described in detail below with respect to the organic additive embodiments. Onium cations include cations such as substituted or unsubstituted ammonium, phosphonium, and sulfonium cations. In one embodiment, the onium cations include, for example, substituted or unsubstituted N-alkyl or N-aryl pyridinium, pyridazinium, pyrimidinium, pyrazinium, imidazolium, pyrazolium, thiazolium, oxazolium, triazolium, imidazolinium, methylpyrrolidinium, isothiazolium, isoxazolium, oxazolium, pyrrolium, and thiophenium, to the extent the cation does not render the ionic liquid unacceptable for use in immersion lithography. In one embodiment, when the cation moiety is substituted, the substituents include one or more of the following groups: halo, alkyl, and aryl groups such as phenyl. In addition, two adjacent substituents may be joined together to form an alkylene radical thereby forming a ring structure converging on N. The alkyl, phenyl, and alkylene radicals may be further substituted. In one embodiment, the cation is an ammonium cation substituted by one or more groups such as alkyl and aryl groups such as phenyl. Many such cations and substituted cations are described in U. S. Patent Nos. 5,827,602 and 5,965,054, which are incorporated by reference in their entirety. Other suitable cations include 1 -butyl-3-methylimidazolium ("BMIM"),

1 -ethyl-3-methylimidazolium ("EMIM"), tetrabutyl ammonium, tributylethyl ammonium, tetrabutyl phosphonium, tetraethyl ammonium, N,N-dialkyl pyrrolidinium, trimethyl 2-hydroxyethyl ammonium, N,N'-dialkyl imidazolium, N-alkylpyridinium, or mixtures of two or more thereof. The cation may be an onium cation and optionally contains more than 4 carbon atoms. Mixtures of any two or more of the foregoing cations may be used.

In one embodiment, the ionic liquid includes one or more of the following: tetrabutylammonium heptadecafluorooctanesulfonate; tetrabutylphosphonium methanesulfonate; tetrabutylammonium nonafluorobutanesulfonate; tetrapentylammonium thiocyanate; trihexyltetradecylphosphonium dicyanamide; trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide; and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate.

In another embodiment organic additives such as a nitrile, an alcohol, a carbonate and/or a sulfone, with appropriately low absorbance at 193 nm, may be used as solvent, or used as an additive to water, or used neat, as the immersion lithography fluid having a high refractive index. These compounds exhibit high dipole moments and polarizability that leads to their high refractive index. Specific examples include acetonitrile, propionitrile, methoxyacetontrile, sulfolane, carbon disulfide, dimethylsulfoxide and dimethylsulfone, or a combination of two or more thereof as the solvent.

In one embodiment, the organic additive includes one or more functionalized sulfone or fluorinated functionalized sulfone. Such fluorinated functionalized sulfone may include one or more fluorinated or partially fluorinated functional group. Thus, for example, the additive may include one or more compound having the general structural formula IX:

R₆SO₂(CH₂)_XSO₃ ^" A⁺ (IX)

wherein R₆ may be C₁-C₂₀ branched or unbranched alky! or fluoroalkyl, x = 1- 20; the fluoroalkyl may have any number of fluoro- substitutions from monofluoro to perfluoro; A = H, an alkali metal, an alkaline earth metal or a metal from any of IUPAC Groups 3-12 and the Lanthanides, or any of the onium cations described hereinabove, including, for example, any ammonium, mono-, di-, tri- or quaternary-substituted ammonium, any phosphonium, mono-, di-, tri- or quaternary-substituted phosphonium or any sulfonium, mono-, di- or tri-substituted sulfonium.

In one embodiment, the organic additive includes one or more fluorinated sulfone. In one embodiment the fluorinated sulfone has the general structural formula X:

(CH_wF₃__w)(CH_xF₂__x)_a -SO₂-(CH_yF₂._y)_b(CH_zF₃._z) (X) wherein, independently, w = 0, 1 , 2 or 3; x = 0, 1 or 2, y = 0, 1 or 2, z = 0, 1 , 2 or 3, a = 0-20; b = 0-20, provided that at least one F atom is included in the sulfone having formula X. In another embodiment the fluorinated sulfone has the general structural formula Xl:

(CH^₃J(CH_xF₁J₃ -SO₂-(CHyF₁^(CH₂F₃.,) (Xl)

R₇ R₈

wherein, independently, w = 0, 1 , 2 or 3; x = 0 or 1 , y = 0 or 1 , z = 0, 1 , 2 or 3, a = 0-20; b = 0-20, R₇ and R₈ independently may be H, F or branched or unbranched C₁-C₂₀ alkyl or fluoroalkyl, and the fluoroalkyl may have any number of fluoro- substitutions from monofluoro to perfluoro, provided that at least one F atom is included in the sulfone having formula Xl. When both R₇ and R₈ = H and/or F, the resulting structure falls within the definition of both formulae X and IX.

Additional examples of suitable organic additives are described below. In one embodiment, the immersion lithography fluid in accordance with the present invention have a high refractive index, for example greater than 1.5, up to at least about 1.7, and low absorptivity, e.g, less than about 1 cm^"1, or less than about 0.5 cm^"1, or less than about 0.3 cm^"1 at the selected wavelength. In one embodiment, the immersion lithography fluid has a refractive index in the range from greater than 1.5 up to at least about 1.7 and is substantially transparent at the selected wavelength. Substantially transparent means that the absorptivity is less than about 0.3 cm^"1 at the selected wavelength.

In one embodiment, the immersion lithography fluid in accordance with the present invention is of high metallic and optical purity. High metallic purity means that the fluid contains less than about 1 part per million (ppm), and in another embodiment, less than about 0.1 ppm, and in another embodiment, less than about 0.001 ppm, of metal ions other than any specified metal ion present, e.g., in a cation or anion forming a component of the immersion lithography fluid. High optical purity means that the fluid contains substantially no impurity that results in an absorptivity of 0.3 cm^"1 or more. In one embodiment, the immersion lithography fluid in accordance with the present invention is stable towards the electromagnetic radiation to which it is to be exposed, does not alter the photoresist with which it is used and in contact, and does not attack the lens materials, such as quartz, calcium fluoride (CaF₂) or magnesium fluoride (MgF₂), with which it is used and in contact.

As described briefly above, in one embodiment, the present invention relates to an immersion lithography fluid, including a solvent; and at least one additive soluble in the solvent. The additive may be a salt or an organic compound, as described herein. The immersion lithography fluid has a refractive index greater than the refractive index of the solvent at a selected wavelength and the immersion lithography fluid is acceptable for use in immersion lithography. In one embodiment, the selected wavelength is in the range about 190 nm to about 360 nm, and in another embodiment, the selected wavelength is either about 193 nm or about 248 nm. In one embodiment, the selected wavelength is produced by a ArF excimer laser.

In one embodiment, the at least one additive has an absorptivity at the selected wavelength to provide or result in a transmission of about 90% or greater of incident electromagnetic energy therethrough when used in immersion lithography apparatus. The transmission of about 90% or greater of course depends on the thickness or pathlength of the incident electromagnetic radiation through the immersion lithography fluid, as described above.

In one embodiment, the salt or organic compound added as the at least one additive, is substantially transparent at the selected wavelength. Thus, by being substantially transparent at the selected wavelength, the salt or organic compound does not unduly absorb the electromagnetic radiation used in the immersion lithography. In one embodiment, the salt or organic compound is substantially transparent at the selected wavelength and contains a refractive index affecting functional group. As described above, for use in immersion lithography, the solvent and the at least one additive, of which the immersion lithography fluid is comprised, should be stable to electromagnetic radiation at the selected wavelength. That is, the immersion lithography fluid should not be unduly degraded by the electromagnetic radiation to which it is exposed. As is known, the electromagnetic radiation used in state of the art immersion lithography, i.e., from about 190 nm to about 360 nm, or in some specific cases, at about 193 nm or about 248 nm, is highly energetic and is capable of breaking chemical bonds in a wide variety of materials, including some of those used as solvent and/or as additive in the present invention. While it is understood that a certain degree of chemical bond breakage will occur, the degree of such breakage should not interfere with the successful conduct of the immersion lithography process itself. That is, the degradation products should not absorb a significant quantity of the electromagnetic radiation, and the degradation products should not significantly react chemically with any of the surfaces or materials to which the immersion lithography fluid is in contact during use. It is understood that a certain amount of such absorbance and/or chemical reaction may take place, but the degree of such absorbance and/or chemical reaction should not interfere with the successful conduct of the immersion lithography process itself.

As described above, absorbance is a function of the absorptivity of the material. Thus, in one embodiment, the immersion lithography fluid exhibits an absorptivity less than about 1 cm^"1 at the selected wavelength. In another embodiment, the immersion lithography fluid exhibits an absorptivity of less than about 0.5 cm^"1 at the selected wavelength, and in another, less than about 0.3 cm^"1 at the selected wavelength. In one embodiment, the fluid is substantially transparent at the selected wavelength.

As discussed, the refractive index of the immersion lithography fluid of the present invention is substantially increased with respect to currently available fluids, such as water and perfluoroethers, at the selected wavelength. As noted above, water has a refractive index of about 1.43-1.47 (depending on the source of information) at 193 nm. Perfluoroethers have a refractive index of about 1 ,4-1.5 at 193 nm. In one embodiment, the refractive index of the immersion lithography fluid of the present invention ranges from greater than the refractive index of water to about 1.70, at the selected wavelength.

The following Table 1 provides data relating to concentration, composition, refractive index, absorbance and chemical properties of some exemplary immersion lithography fluids which are suitable for use in accordance with the present invention.

Table 1

Additive Composition Rl* ABSORBANCE Chemical

Cone. at 193 nm Properties

1.54 M La(OH)₃ 1.4030 0.47 Salt/Aqueous

+ 3 CH₃SO₃H Acidic

Lanthanum

Methanesulfonate

2.0 M Ba(OH)₂ 1.3806 0.27 Salt/Aqueous

+ 2 CH₃SO₃H Acidic

Barium Methanesulfonate

Cs(OH) + CH₃SO₃H 1.3636 1.0 Salt/Aqueous

2.0 M Cesium Methanesulfonate Acidic

La(CH₃SO₃) 1.59 0.6 Salt/Aqueous

2.7 M Lanthanum (193 nm) methanesulfonate

La(OH)₃ ' 1.3737 2.4 Salt/Aqueous

1 M + 3 HCIO₄ Lanthanum Acidic

Perchlorate

Ba(H₂PO₄) ₂ in 1.3990 0.6 Salt/Aqueous/Acid

2.0 M 85% H₃PO₄ Acidic

Cs(OH) 1.3607 2.2 Salt/Aqueous/Acid

1 M + x H₃PO₄ Acidic

Cesium Dihydrogen phosphate

2 M (CHa)₄N⁺OH-+ CF₃SO₃H 1.45 0.6 Salt/Aqueous

Tetramethyl ammonium (193 nm) Acidic

Triflate

(CHg)₄N⁺OH^" 1.47 1.2 Salt/Aqueous

2 M + CH₃SO₃H (193 nm) Acidic

Tetramethyl ammonium methanesulfonate

(CH₃J₃S⁺ (CH₃OSO₃-) 1.3551 2.1 Salt/Aqueous

1 M Trimethyl sulfonium Acidic methylsulfate

(CH₂)₄SO₂ 1.49 2.2 Solvent/Water

2 M Sulfolane (193 nm)

(CH₃)₂SO₂ 1.3472 0.45 Solvent/Water

2 M Dimethyl Sulfone

CH₃CN 1.3434 0.51 Solvent

100% Acetonitrile H₂O 1.33 0.1 Water Reference

100% Water (589.3 nm)

1.437 (193 nm)

* Refractive index at 589.3 nm unless otherwise indicated

As shown in the above Table 1 , in one embodiment, the solvent comprises water. In one embodiment, the water comprises from about 0.1 wt% to about 99 wt% of the fluid. In other embodiments, the solvent comprises an organic solvent, such as acetonitrile, propionitrile, sulfolane, dimethylsulfone, carbon disulfide, dimethylsulfoxide or a combination of two or more thereof, or in combination with water, as the solvent. Thus, in one embodiment, the immersion lithography fluid comprises water, acetonitrile, propionitrile, sulfolane, dimethylsulfone, carbon disulfide, dimethylsulfoxide, or a combination of two or more thereof, as the solvent. Table 2 provides some additional, non-limiting examples of immersion lithography fluids that are within the scope of the present invention.

Table 2

N NRR_44-._1n1HH_nn((CCHH₃₃SSOO₃₃ )) M M..PP..,, S Saalltt CCooine, Density Salt Cone, Rl @ / \bsorbaι n = 0,1 ,2,3,4 ⁰C in Water g/mL in Water 589.3 nm @ 193 n R = H, Me, Et, etc (Wt %) (M) /193 nm (cm^"1)

Ammonium 190-193 61.63 1.2338 6.72 1.4045 0.25

Methanesulfonate /1.54

Methyl ammonium 94-97 81.25 1.2513 8.00 1.4291 1.64

Methanesulfonate /1.57

Dimethyl ammonium 132-134 73.87 1.1852 6.20 1.4170 0.29

Methanesulfonate /1.55

Trimethyl 200-202 69.78 1.1468 5.16 1.4138 - ammonium / -

Methanesulfonate

Tetramethyl 327 dec. 65.71 1.1247 4.37 1.4122 0.91 ammonium /1.54

Methanesulfonate Additives

In one embodiment, the additive includes at least one of a salt or an organic compound, or a combination of any two or more thereof. That is, the additive may include two or more salts in combination, two or more organic compounds in combination, or a combination of one or more salts together with one or more organic compounds.

In one embodiment, the salt includes an alkali metal ion, an alkaline earth metal ion, a lanthanide metal ion in a high oxidation state, an ion of the first, second or third transition metal series in a high oxidation state, a main group ion in a high oxidation state or a combination of two or more thereof.

In one embodiment, the salt includes Cs⁺, Ba²⁺, La³⁺, Hf⁴⁺, Ta⁵⁺, W⁶⁺, or a combination of two or more thereof. In one embodiment, the salt includes a polarizable anion containing at least one high atomic number atom. As used herein, the term "high atomic number atom" includes atoms having an atomic number greater than about 14.

In one embodiment, the salt includes at least one organic onium ion. In one embodiment, the at least one organic onium ion includes an ammonium, phosphonium or sulfonium group or a mixture or combination of any two or more thereof. Thus, for example, the salt may include a quaternary ammonium ion, a quaternary phosphonium ion, a ternary sulfonium ion or a mixture of such salts, and may also include a compound containing within a single molecule more than one or a combination of two or more of these onium ions.

Useful organic onium salts for the present invention include organic onium salts and organic onium compounds such as quaternary ammonium salts, quaternary phosphonium salts, tertiary sulfonium salts, tertiary sulfoxonium salts and imidazolium salts. As used herein, disclosure of or reference to any onium salt should be understood to include the corresponding salts, such as halides, carbonates, formates, sulfates and the like. As will be understood, such salts may be produced from the corresponding onium hydroxides, by reaction with a suitable acid, which provides the anion X in the formula (I) below. In one embodiment, the onium compounds may generally be characterized by the formula XII:

A_y(X^"y) (XII)

wherein in Formula XII, A is an onium group, X is an anion and y is a stoichiometric value balancing the positive charge on the onium group A and the negative charge (-y) on the anion X. Examples of onium groups include ammonium groups, phosphonium groups, sulfonium, sulfoxonium and imidazolium groups. In one embodiment, the onium compound should be sufficiently soluble in a solution such as water, alcohol or other organic liquid, or mixtures thereof, to provide a uniform solution. Suitable anions include, for example, halides, carbonates, formates, acetates, sulfates, sulfonates, phosphonates, phosphites, sulfites, thiocarbamates, thiocarboxylat.es, thiophosphonates, thiosulfate any other anions mentioned herein and other anions known for use with onium ions and that are suitable for use in immersion lithography.

In one embodiment, the onium compound may include an ammonium salt or a phosphonium salt, which may be characterized by the formula XIII:

R² +

R¹-A— R³ χ-y (XIII)

wherein in Formula XIII, A is a nitrogen or phosphorus atom, R¹, R², R³ and

R⁴ are each independently hydrogen, an alkyl group containing from 1 to about 20, or 1 to about 10 carbon atoms, a hydroxyalkyl or alkoxyalkyl group containing from 2 to about 20, or 2 to about 10 carbon atoms, or R¹ and R² (or any combination of R¹, R², R³ and R⁴) together with A may form a heterocyclic group provided that if the heterocyclic group contains a C=A group (i.e., a carbon double-bonded to A), R³ is the second bond, X^~ is an anion of an acid, and y is a number equal to the valence of X. Thus, in one embodiment, the onium compound may comprise a primary, secondary, tertiary or quaternary ammonium or phosphonium salt or compound.

Examples of suitable anions of acids for use as the X^~ anions for all of the onium compounds include bicarbonates, halides, nitrates, formates, acetates, sulfates, carbonates, phosphates, etc. that are suitable for use in immersion lithography. In one embodiment, the anion includes a relatively high-Z atom (e.g., relatively high atomic number) and is polarizable. Relatively high Z atoms tend to impart a higher refractive index to materials containing them. Such anions include, for example, thiosulfate, methanesulfonate, dihydrogen phosphate, dihydrogen arsenate, etc., where

S, P and As are the relatively high-Z atoms.

In formula XIII, the alkyl groups R¹ to R⁴ may be linear or branched, and specific examples of alkyl groups containing from 1 to 20 carbon atoms include methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, isooctyl, nonyl, decyl, isodecyl, dodecyl, tridecyl, isotridecyl, hexadecyl and octadecyl groups.

R¹, R², R³ and R⁴ also may be hydroxyalkyl groups containing from 2 to 5 carbon atoms such as hydroxyethyl and the various isomers of hydroxypropyl, hydroxybutyl, hydroxypentyl, etc. In one embodiment, R¹, R², R³ and R⁴ are independently alkyl and/or hydroxyalkyl groups containing 1 to about 4 or 5 carbon atoms. Specific examples of alkoxyalkyl groups include ethoxyethyl, butoxymethyl, butoxybutyl, etc. that are suitable for use in immersion lithography.

In one embodiment, the onium compound is a salt of a primary, secondary or tertiary ammonium or phosphonium group, e.g., R¹H₃N⁺, R¹R²H₂N⁺, R¹R²R³HN⁺, R¹H₃P⁺, R¹R²H₂P⁺, or R¹R²R³HP⁺. The R¹, R², R³ and

R⁴ groups may be any of those defined herein. The anion forming the salt may be any of the anions disclosed herein.

In one embodiment, the quaternary ammonium salts which can be used in accordance with the process of the present invention may be represented by Formula XIV:

wherein in Formula XIV, R¹, R², R³, R⁴, and y are as defined in Formula XIII, and X^" is an anion of an acid such as the halides, sulfates, nitrates, carbonates, etc., described herein that are suitable for use in immersion lithography. In one embodiment, R¹- R⁴ are alkyl and/or hydroxyalkyl groups containing from 1 to about 4 or 5 carbon atoms. Specific examples of ammonium ions with formula XIV include tetramethylammonium, tetraethyl- ammonium, tetrapropylammonium, tetrabutylammonium, tetra-n-octylam- monium, methyltriethylammonium, diethyldimethylammonium, methyltripropylammonium, methyltributylammonium, cetyltrimethylammonium, trimethylhydroxyethylammonium , trimethylmethoxyethylammonium, dimethyldihydroxyethylammonium, methyltrihydroxyethylammonium, dimethylpiperidinium, etc. In one embodiment, the quaternary ammonium ions used in accordance with this invention are tetramethylammonium and tetraethylammonium. The quaternary ammonium salts represented by Formula XIV may be obtained from to the corresponding quaternary ammonium hydroxides by replacing the hydroxide anion with, for example, a sulfate anion, a chloride anion, a carbonate anion, a formate anion, a phosphate ion, etc. Thus, for example, the salt may be tetramethylammonium chloride, tetramethylammonium sulfate (y=2), tetramethylammonium bromide, 1 -methyl-2-butyl imidazolium hexafluorophosphate, n-butyl pyridinium hexafluorophosphate, etc.

Examples of quaternary phosphonium salts representative of Formula XIV wherein A=P which can be employed in accordance with the present invention include salts corresponding to phosphonium ions including tetramethylphosphonium, tetraethylphosphonium, tetrapropylphosphonium, tetrabutylphosphonium, trimethylhydroxyethylphosphonium, dimethyldihydroxyethylphosphonium, methyltrihydroxyethylphosphonium, phenyltrimethylphosphonium, phenyltriethylphosphonium and benzyltrimeth- ylphosphonium, etc, and the corresponding anions such as halides, sulfates, carbonates, phosphates, etc., as described herein. In one embodiment, larger onium cations, including those with larger organic groups, provide more compatibility with photoresist materials. In one embodiment, smaller onium cations provide higher resistance to breakdown by the electromagnetic radiation of the immersion lithography process. In one embodiment, asymmetric onium cations, such as benzyltrimethylammonium, provide a good balance between photoresist compatibility and acceptable breakdown. Thus, in one embodiment, the organic onium salt comprises an asymmetric onium cation, in which one or more of the organic groups contain, on average, at least about four carbon atoms, in one embodiment, at least about six carbon atoms, and in another embodiment, at least about 8 carbon atoms.

In another embodiment, the tertiary sulfonium salts which can be employed in accordance with the present invention may be represented by the formula XV:

wherein in Formula XV, R¹, R² and R³, X^~ and y are as defined in Formula

XIII.

Examples of the tertiary sulfonium salts represented by Formula XV include sulfonium ions such as trimethylsulfonium, triethylsulfonium, tripropylsulfonium, etc, with the corresponding anions such as the halides, sulfates, nitrates, carbonates, etc. that are suitable for use in immersion lithography. In another embodiment, the tertiary sulfoxonium salts which can be employed in accordance with the present invention may be represented by the formula XVI:

wherein in Formula XVI, R¹, R² and R³, X^" and y are as defined in Formula XIII.

Examples of the tertiary sulfoxonium salts represented by Formula XVI include trimethylsulfoxonium, triethylsulfoxonium, tripropylsulfoxonium, etc, with the corresponding anions such as the halides, sulfates, nitrates, carbonates, etc. that are suitable for use in immersion lithography.

In another embodiment, the imidazolium salts which can be employed in accordance with the present invention may be represented by the formula XVII:

wherein in formula XVII, R¹ and R³ are as defined in Formula XIII, and X is an anion of an acid such as the halides, sulfates, nitrates, carbonates, etc., described herein that are suitable for use in immersion lithography.

As will be understood, in formula XVII and in the foregoing formulae XII-XVI, if X- is an anion of a dibasic acid, such as SO₄ ^"2, the stoichiometry will be adjusted accordingly, for example, for the dibasic acid anion, instead of 2 X , there would be only one X^~, and if X is an anion of a tribasic acid, such as PO₄ ^"3 a corresponding stoichiometric adjustment would be made. In one embodiment, the onium salt is an ionic liquid with a low molecular weight cation (such as an alkylammonium cation) paired with a highly refractive anion (e.g., the above-noted relatively high-Z atom containing anion) to obtain the highest possible concentration of refractive material. In one embodiment, organic compounds, such as nitrile, alcohol, carbonate, and sulfone compounds with appropriately low absorbance at 193 nm may be used as solvent, as additive to water, or as a eutectic-forming material in conjunction with the onium salt. Examples of suitable solvents include acetonitrile, propionitrile, methoxypropionitrile, dimethylsulfone and mixtures of these or other suitable materials, with or without water as a co- solvent or additive.

Onium salts may be made by reacting the appropriate amine with the acid form of the anion with which it is to be paired. For example, ammonium methanesulfonate may be synthesized by reacting equimolar amounts of ammonium hydroxide and methanesulfonic acid. The ammonium methanesulfonate product may be crystallized from water and thereafter used to generate the immersion fluid by dissolving it in the appropriate solvent.

Onium salts also may be commercially available, often as the hydroxide. As is known, onium salts, such as onium halides, carbonates, formates, sulfates and the like, can be prepared from the corresponding onium hydroxides . Various methods of conversion are described in U.S. Patents 4,917,781 (Sharifian et al) and 5,286,354 (Bard et al) which are hereby incorporated by reference. There is no particular limit as to how the onium salt is obtained or prepared. Additional information relating to onium salts and hydroxides can be found in U.S. Patent Nos. 6,787,021 , 6,508,940,

6,217,743, 6,207,039, 5,968,338, 5,910,237, 5,853,555 and 5,833,832, the disclosures of which relating to onium compounds are incorporated herein by reference.

In one embodiment, the organic onium ion comprises one or more of tetramethylarnmonium, tetraethylammonium, tetrapropylammonium, tetrabutylammonium, methyltriethanolammonium, tetrabutylphosphonium, trihexyltetradecylphosphonium, tributyltetradecylphosphonium, [(CH₃)₃NCH₂CH(OH)CH₂N(CH₃)₃]²⁺[OH^"]₂, i-butyl-3-mθthylimidazolium, trimethylsulfonium, trimethylsulfoxonium, trimethyl (2,3-dihydroxypropyl) ammonium, [(C₆H₅)CH₂N(CH₃)₂CH₂CH(OH)CH₂N(CH₃)₂. CH₂CH(OH)CH₂N(CH₃)2CH₂-CH(OH)CH₂N(CH₃)₂CH₂(C₆H₅)]⁴⁺ [OH^"]_4> and [(CH₃)₃NCH₂CH(OH)CH₂OH]⁺ [OH^"], and hexamethonium dihydroxide.

The concentration of the onium salt in the immersion lithography fluid of the present invention may range from about 1 M to about 5 M. In another embodiment, the concentration of the onium salt in the immersion lithography fluid of the present invention may range from about 1.5 M to about 2.5 M- In another embodiment, the concentration of the onium salt in the immersion lithography fluid of the present invention may range from about 2 M to about 3

M-

In one embodiment, the salt includes thiosulfate, iodate, methylsulfonate, dihydrogen phosphate, methylsulfate or a combination of two or more thereof as counterion (e.g., anion) to the onium ion or metal ion.

In one embodiment, the salt comprises MO₄ ^n" as the anion, wherein M = Mo, W, Re; or MO₃ ^π~ as the anion, wherein M = V, Nb, Sn, Si, or mixtures of any two or more thereof. In this embodiment, n is the total anion charge, equal to the metal oxidation state minus the aggregate anion charge, as appropriate to the metal in a high oxidation state as described herein.

In one embodiment, the salt comprises [MX]^π", wherein M = metal, X = a salt forming species comprising one or more of O, OH, RSO₃ ^", PO₄ ^3", PO₃ ^2", RPO₃ ^2", SO₃ ^2", HSO₃-, H₂PO₄ ^", HPO₄ ^2", B₄O₇ ²-, ROSO₃-, OCN^", CN^", or SCN and n is the total anion charge, equal to the metal oxidation state minus the aggregate anion charge, as appropriate to the metal and atoms of the X species, taking into account the number of M atoms and the number of X species.

In one embodiment, the organic compound comprises a nitrile, an alcohol, a carbonate, a sulfone, a sulfide, a sulfoxide or a combination of two or more thereof.

Suitable examples of nitriles include organic nitriles such as acetonitrile, propionitrile, methoxyacetonitrile. In general, the organic nitrile may include an organic material containing at least one nitrile (or cyano) (-CN) group. The organic nitrile material may be an aliphatic-, aromatic-, cycloaliphatic-, heterocyclic-, heteroaliphatic-nitrile. The organic material may have more than one nitrile group. Such nitriles may include propionitrile, 2-methylglutaronitrile, isobutyronitrile, dicyanocyclooctane, nitrilotriacetonitrile, iso- and terephthalonitrile, 1 ,3,5-tricyanobenzene, o-, m-, or p-tolunitrile, phthalonitrile, 1-naphthonitrile, 2-naphthonitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 1 ^-cyclohexanedicarbonitrile, 1 ,2,4,5-cyclohexanetetracarbonitrile, cycloheptanecarbonitrile, S-methylcycloheptanecarbonitrile, cyclooctanecarbonitrile, butyronitrile, valeronitrile, capronitrile, 2,2-dimethylpropanenitrile, caprylnitrile, decanenitrile, hendecanenitrile, lauronitrile, tridecanenitrile, myristonitrile, pentadecanenitrile, palmitonitrile, heptadecanenitrile, stearonitrile, phenylacetonitrile, malononitrile, succinonitrile, glutaronitrile, adiponitrile, and similar nitriles.

Suitable examples of alcohols include methanol, ethanol, n-propanol, isopropanol, and, in general, C₁-C₂₀ alcohols, both branched and unbranched, and including polyols, such as glycerol, hexanediol, etc.

Suitable examples of carbonates include organic carbonates such as alkylene carbonates. Suitable alkylene carbonates include, for example, ethylene carbonate, propylene carbonate, butylene carbonate, and glycerine carbonate. In one embodiment, the organic carbonate is represented by the general formula:

R⁵-(OC(O)-OR⁶)_n

wherein R⁵ and R⁶ are linear or branched C₁-C₂₀ alkyl radicals and n = 1-20. Thus, the organic carbonate may be a polycarbonate or a monocarbonate.

Suitable examples of sulfides include organic sulfides such as dimethyl sulfide, diethyl sulfide, dithiane, and in general sulfides substituted with C₁-

C₂₀ aliphatic or aromatic moieties, both branched and unbranched. Suitable examples of sulfides include organic sulfides, such as dimethyl sulfide, diethyl sulfide, dithiane, and in general C₁-C₂₀ aliphatic or aromatic moieties, both branched and unbranched. In one embodiment, the organic sulfide may be represented by the general structure R⁷-S-R⁷ wherein each R⁷ is independently an unsubstituted or inertly substituted alkyl group, cycloalkyl, or, together with the other R⁷, forms part of a ring structure that includes the sulfur atom of the sulfide group.

Suitable examples of sulfones include organic sulfones such as dimethyl sulfone, diethyl sulfone, dibutylsulfone, and in general sulfones substituted with C₁-C₂₀ aliphatic or aromatic moieties, both branched and unbranched. In one embodiment, the organic sulfone may be represented by the general structure R⁷-S(O)₂-R⁷ wherein each R⁷ is independently an unsubstituted or inertly substituted alkyl group, cycloalkyl, or, together with the other R⁷, forms part of a ring structure that includes the sulfur atom of the sulfone group. The sulfone may comprise one or more of the functionalized sulfones or fluorinated functionalized sulfones described above.

Suitable examples of sulfoxides include organic sulfoxides such as dimethylsulfoxide, diethylsulfoxide, tetramethylsulfoxide, and in general C₁- C₂₀ aliphatic or aromatic moieties, both branched and unbranched. In one embodiment, the organic sulfoxide may be represented by the general structure R⁷-S(O)-R⁷ wherein each R⁷ is independently an unsubstituted or inertly substituted alkyl group, cycloalkyl, or, together with the other R⁷, forms part of a ring structure that includes the sulfur atom of the sulfoxide group. In one embodiment, using ammonium as an example of the onium compound embodiment, a general formulation for an immersion lithography fluid in accordance with the present invention includes:

10-90 % NR₄._nH_n(X^~) where: n = 0-4

R = H, Me, Et, t-Butyl, etc; X = RSO₃-, H₂PO₃-, H₂PO₄-, etc. 90-10% Water

0-30% Solvent (e.g., dimethylsulfone, sulfolane, acetonitrile, etc.) As described in detail above, the nitrogen atom in this formulation may be replaced with P or S or combined in a heterocyclic ring, together with an appropriate number of R groups and anions (X^~).

In one embodiment, the immersion lithography fluid comprises lanthanum methanesulfonate, barium methanesulfonate, cesium methanesulfonate, lanthanum perchlorate, barium dihydrogen phosphate, cesium dihydrogen phosphate, tetramethylammonium triflate, tetramethylammonium methanesulfonate, sulfolane, dimethyl sulfone, acetonitrile, carbon disulfide, dimethylsulfoxide or a combination of two or more thereof as the additive.

In one embodiment, each of the at least one additive is present independently at a concentration in the range from about 0.5 M. to about 16 M- In one embodiment, each of the at least one additive is present independently at a concentration in the range from about 1 M to about 10 M- In one embodiment, each of the at least one additive is present independently at a concentration in the range from about 2 M to about 5 M.

In one embodiment, the additive includes as anion, species such as tetraborate, perchlorate, periodate, phosphate, phosphite, dihydrogen phosphate, dihydrogen phosphite, alkylsulfonate, 1 ,2-ethanedisulfonate, perfluorobutylsulfonate, triflate, alkyl phosphonate, alkylphosphinate, alkyl sulfate, or mixtures of two or more thereof, wherein alkyl comprises C₁-C₂₀, branched or unbranched. The foregoing anions may be used for any of the anions in any embodiment of the present invention.

In one embodiment, the salt or organic compound contains a refractive index affecting functional group that is polarizable, has a high atomic number, has a high dipole moment, or a combination of two or more of these characteristics.

In one embodiment, the immersion lithography fluid is free of added surfactant. That is, in this embodiment, no surfactant is purposely added to the fluid. In another embodiment, the immersion lithography fluid is substantially free of surfactant. That is, in this embodiment, no surfactant is purposely added to the fluid and none is believed to be present from any source.

In one embodiment, the immersion lithography fluid is free of added polyfluoroether. That is, in this embodiment, no polyfluoroether is purposely added to the fluid. In another embodiment, the immersion lithography fluid is substantially free of polyfluoroether. That is, in this embodiment, no polyfluoroether is purposely added to the fluid and none is believed to be present from any source.

In one embodiment, the present invention relates to an immersion lithography system, including an optical surface; a wafer support for holding a workpiece; and an immersion lithography fluid disposed between the optical surface and the workpiece and contacting at least a portion of the optical surface. The immersion lithography fluid, as described in detail above, includes a solvent and at least one additive soluble in the solvent, in which, at a selected wavelength, the immersion lithography fluid has a refractive index greater than the refractive index of the solvent and the immersion lithography fluid is acceptable for use in immersion lithography. The following briefly describes some of the basic elements of an immersion lithography system in which an immersion lithography fluid in accordance with the present invention may be employed.

Fig. 1 is a schematic side elevational and cross-sectional view of an immersion lithography system 100 including an embodiment of the present invention. The system 100 shown in Fig. 1 is a simplified example of an immersion lithography system. The system 100 contains a source 102 emitting a beam of electromagnetic energy through a lens 104. The electromagnetic energy then passes through a mask 106 and an imaging subsystem 108 having a final optical surface 108a. As shown in Fig. 1 , the system 100 further includes a stage 110, upon which is mounted a substrate 112, e.g., a semiconductor wafer. Various layers (not shown) may be positioned on the substrate over which is formed a photoresist material 114.

The photoresist material 114 is to be illuminated by the electromagnetic radiation to form a pattern corresponding to the mask 106 in the photoresist material 114 upon exposure and development.

In accordance with the present invention, an immersion lithography fluid 116 having a high refractive index fills at least a portion of the space between the final optical surface 108a and at least a portion of the surface of the photoresist material 114.

In one embodiment, the substrate 112 is a semiconductor wafer that is being fabricated as an integrated circuit. For example, the wafer can be a silicon substrate (e.g., monolithic silicon substrate or a silicon-on-insulator) in which transistors (and other components) are to be or have been formed. As is known in the art, these components may be interconnected with metal layers.

The photoresist material 114 may be a photoresist or other masking material. Suitable materials include, for example, (meth)acrylic polymers (acrylates and methacrylates). Such (meth)acrylic polymers are often used for 193-nm lithography resist design because of their excellent optical transparency and easily tailored structure. Suitable, commercially available examples include, Rohm and Haas, Epic 2000, 2100, 200, 2300, 300Oi, 320Oi; Fujifilm, GAR7307Y2, GAR8105G1 , GAR8205B15; and AZ Electronic Materials, AZ AX 112OP, AZ AX 4181 P. The present invention is not limited to these specific resist materials, but may be used with any suitable resist for use in immersion lithography at the appropriate selected wavelength.

Although Fig. 1 illustrates the liquid 116 disposed only between the optical element of the imaging subsystem 108 and the photoresist material 114, it should be understood the substrate 112 and/or the stage 110 can be immersed in the immersion lithography fluid 116. The important criterion, as will be understood, is that the immersion lithography fluid of the present invention be disposed between the final optical surface 108a and the portion of the photoresist 114 upon which the system is operating, e.g., by focusing the source radiation thereupon. The extent to which any portion of the immersion lithography system, including the imaging subsystem 108, the photoresist material 114, and other elements are immersed in or covered by the immersion lithography fluid can be suitably determined and selected by the skilled person designing the immersion lithography system as a whole. Fig. 2 is a schematic side elevational and cross-sectional magnified view of a portion of an immersion lithography system illustrating details of a function of an embodiment of the present invention. Fig. 2 schematically illustrates how an immersion lithography system is different from a non- immersion system. Fig. 2 illustrates a portion of the system 100 of Fig. 1 , including the imaging subsystem 108, the photoresist material 114, and the immersion lithography fluid 116. In addition, Fig. 2 includes the final optical surface 108a and a penultimate optical surface 108b, here illustrated as the two concave sides (108a, 108b) of a convex lens. Fig. 2 schematically illustrates two electromagnetic beam portions a, a¹ and b,b'. The beam portion a,a' represents an electromagnetic beam that would be refracted by the lens at the surfaces 108b and 108a, and that would be focused upon the photoresist 114 even without the presence of the immersion lithographic fluid

116. The path taken by the beam portion a, a' in the absence of the immersion lithography fluid 116 is shown in dotted lines. As illustrated, in the absence of the immersion lithography fluid 116, the beam portion a.a¹ would arrive at the surface of the photoresist at an angle that would result in a relatively shallow depth of focus.

The beam portion b,b' represents an electromagnetic beam that would be refracted by the lens at the surface 108b, but in the absence of the immersion lithography fluid 116, would be internally reflected at the final optical surface 108a due to the difference in refractive index between the lens and the air between the final optical surface 108a and the photoresist material

114. The reflection is schematically illustrated by the dashed-line upwardly- pointing arrows b_r and b'_r shown in Fig. 2. Thus, in the absence of the immersion lithography fluid 116, the electromagnetic radiation illustrated by the beam portion b,b' would not reach the photoresist material 114, but would instead be reflected as b_r and b'_r. As schematically illustrated in Fig. 2, with the immersion lithography fluid 116 in place, due to the similarity in the refractive indices of the lens and the fluid 116, the beam portion b,b' is transmitted to the surface of the photoresist material 114.

Thus, as illustrated in Fig. 2, the presence of the immersion lithography fluid allows light to pass at angles and/or from apertures that would be internally reflected, possibly totally, at the optic-air interface illustrated by the final optical surface 108a in the absence of the immersion lithography fluid 116.

As noted above, further improvements in resolution and depth of focus only can be attained by increasing the refractive index of the immersion lithography medium. The methods and compositions of the present invention provide a variety of materials for use in increasing the refractive index of the immersion lithography fluid.

Throughout the foregoing specification and the following claims, the numerical limits of the ranges and ratios, including concentrations, pH, wavelengths and other ranges, may be combined. That is, for example, where ranges of 1 to 10 and 2 to 5 are disclosed, although not specifically stated, this disclosure should be understood to also include the range from 2 to 10 and from 1 to 5, as well as intervening integral values as range limits. All of the compositions and processes disclosed and claimed herein can be made and executed by those of ordinary skill in the art without undue experimentation in light of the present disclosure and based upon the knowledge of such persons. While the compositions and processes of this invention have been described in terms of certain preferred embodiments, it will be apparent to those of ordinary skill in the art that variations may be applied to the compositions and/or processes and in the steps or in the sequence of steps of the processes described herein without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain agents that are chemically or physiologically related may be substituted for the agents described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.

Claims

CLAIMSWhat is claimed is:

1. An immersion lithography fluid, comprising: a solvent; and at least one additive soluble in the solvent, wherein at a selected wavelength the immersion lithography fluid has a refractive index greater than the refractive index of the solvent and the immersion lithography fluid is acceptable for use in immersion lithography.

2. The immersion lithography fluid of claim 1 wherein the at least one additive has an absorptivity at the selected wavelength to provide a transmission of about 90% or greater of incident electromagnetic energy therethrough when used in immersion lithography apparatus.

3. The immersion lithography fluid of either of claim 1 or claim 2 wherein the refractive index of the fluid, at the selected wavelength, ranges from greater than the refractive index of water to about 1.70, at the selected wavelength.

4. The immersion lithography fluid of any preceding claim wherein the selected wavelength is in the range about 190 nm to about 360 nm.

5. The immersion lithography fluid of any preceding claim wherein the selected wavelength is either about 193 nm or about 248 nm.

6. The immersion lithography fluid of any preceding claim wherein the selected wavelength is produced by a ArF excimer laser.

7. The immersion lithography fluid of any preceding claim wherein the flui dd eexxhhiibits an absorptivity less than about 1 cm^"1 at the selected wavelength.

8. The immersion lithography fluid of claim 7 wherein the absorptivity is less than about 0.5 cm^"1 at the selected wavelength.

9. The immersion lithography fluid of any preceding claim wherein the solvent comprises water.

10. The immersion lithography fluid of claim 9 wherein the water comprises from about 0.1 wt% to about 99 wt% of the fluid.

11. The immersion lithography fluid of any preceding claim wherein the additive comprises at least one of a salt or an organic compound or a combination of any two or more thereof.

12. The immersion lithography fluid of claim 11 wherein the salt comprises an alkali metal ion, an alkaline earth metal ion, a lanthanide metal ion in a high oxidation state, an ion of the first, second or third transition metal series in a high oxidation state, a main group ion in a high oxidation state or a combination of two or more thereof.

13. The immersion lithography fluid of claim 11 wherein the salt comprises Cs⁺, Ba²⁺, La³⁺, Hf⁴⁺, Ta⁵⁺, W⁶⁺, or a combination of two or more thereof.

14. The immersion lithography fluid of claim 11 wherein the salt includes a polarizable anion containing at least one high atomic number atom.

15. The immersion lithography fluid of claim 11 wherein the salt comprises at least one organic onium ion.

16. The immersion lithography fluid of claim 15 wherein the at least one organic onium ion comprises an ammonium, phosphonium or sulfonium group.

17. The immersion lithography fluid of claim 11 wherein the salt comprises thiosulfate, iodate, methylsulfonate, dihydrogen phosphate, methylsulfate or a combination of two or more thereof.

18. The immersion lithography fluid of claim 11 wherein the organic compound comprises a nitrile, an alcohol, a carbonate, a sulfone, a sulfide, a sulfoxide or a combination of two or more thereof.

19. The immersion lithography fluid of claim 11 wherein the salt or organic compound is substantially transparent at the selected wavelength and contains a refractive index affecting functional group.

20. The immersion lithography fluid of claim 19 wherein the refractive index affecting functional group is polarizable, has a high atomic number, has a high dipole moment or a combination of two or more thereof.

21. The immersion lithography fluid of claim 11 wherein the salt comprises MO₄ ^n", wherein M = Mo, W, Re; MO₃ ^n", wherein M = V, Nb, Sn, Si, or mixtures of any two or more thereof.

22. The immersion lithography fluid of claim 11 wherein the salt comprises [MX]^n~, wherein M = a metal, X = a salt forming species comprising one or more of O₁ OH, RSO₃-, PO₄ ^3", PO₃ ^2", RPO₃ ^2", SO₃ ^2", HSO₃-, H₂PO₄ ^",

HPO₄ ^2", B₄O₇ ^2', ROSO₃-, OCN-, CN-, or SCN^" and n is the total anion charge, equal to the metal oxidation state minus the aggregate anion charge, as appropriate to the metal and atoms of the X species, taking into account the number of M atoms and the number of X species.

23. The immersion lithography fluid of any preceding claim wherein the fluid comprises water, acetonitrile, propionitrile, methoxyacetontrile, sulfolane, dimethylsulfone, carbon disulfide, dimethylsulfoxide or a combination of two or more thereof as the solvent.

24. The immersion lithography fluid of any preceding claim wherein the fluid comprises lanthanum methanesulfonate, barium methanesulfonate, cesium methanesulfonate, lanthanum perchlorate, barium dihydrogen phosphate, cesium dihydrogen phosphate, tetramethylammonium triflate, tetramethylammonium methanesulfonate, sulfolane, dimethyl sulfone, acetonitrile carbon disulfide, dimethylsulfoxide or a combination of two or more thereof as the additive.

25. The immersion lithography fluid of any preceding claim wherein the at least one additive is present at a concentration in the range from about 0.5 M to about 16 M-

26. The immersion lithography fluid of any preceding claim wherein the at least one additive is present at a concentration in the range from about 1 M to about 5 M-

27. The immersion lithography fluid of any preceding claim wherein the additive comprises tetraborate, perchlorate, periodate, phosphate, phosphite, dihydrogen phosphate, dihydrogen phosphite, alkylsulfonate, 1 ,2- ethanedisulfonate, perfluorobutylsulfonate, triflate, alkyl phosphonate, alkylphosphinate, alkyl sulfate, or mixtures of two or more thereof, wherein alkyl comprises C₁-C₂₀, branched or unbranched.

28. The immersion lithography fluid of any preceding claim wherein the fluid is free of added surfactant.

29. The immersion lithography fluid of any preceding claim wherein the fluid is substantially free of surfactant.

30. The immersion lithography fluid of any preceding claim wherein the fluid is free of added polyfluoroethers.

31. The immersion lithography fluid of any preceding claim wherein the fluid is free of particulate matter.

32. An immersion lithography system, comprising: an optical surface; a wafer support for holding a workpiece; and an immersion lithography fluid disposed between the optical surface and the workpiece and contacting at least a portion of the optical surface, the fluid comprising: a solvent; and at least one additive soluble in the solvent, wherein at a selected wavelength the immersion lithography fluid has a refractive index greater than the refractive index of the solvent and the immersion lithography fluid is acceptable for use in immersion lithography.

33. The immersion lithography system of claim 32 wherein the fluid contacts both the at least a portion of the optical surface and at least a portion of the workpiece.

34. The immersion lithography system of either of claim 32 or claim 33 wherein the at least one additive has an absorptivity at the selected wavelength to provide a transmission of about 90% or greater of incident electromagnetic energy therethrough when used in immersion lithography apparatus.

35. The immersion lithography system of any of claims 32-34 wherein the refractive index of the fluid, at the selected wavelength, ranges from greater than the refractive index of water to about 1.70, at the selected wavelength.

36. The immersion lithography system of any of claims 32-35 wherein the selected wavelength is in the range about 190 nm to about 360 nm.

37. The immersion lithography system of any of claims 32-36 wherein the selected wavelength is either about 193 nm or about 248 nm.

38. The immersion lithography system of any of claims 32-37 wherein the selected wavelength is produced by a ArF excimer laser.

39. The immersion lithography system of any of claims 32-38 wherein the fluid exhibits an absorptivity less than about 1 cm^"1 at the selected wavelength.

40. The immersion lithography system of claim 39 wherein the absorptivity is less than about 0.5 cm^'1 at the selected wavelength.

41. The immersion lithography system of any of claims 32-40 wherein the solvent comprises water.

42. The immersion lithography system of claim 41 wherein the water comprises from about 0.1 wt% to about 99 wt% of the fluid.

43. The immersion lithography system of any of claims 32-42 wherein the additive comprises at least one of a salt or an organic compound.

44. The immersion lithography system of any of claims 32-43 wherein the salt comprises an alkali metal ion, an alkaline earth metal ion, a lanthanide metal ion in a high oxidation state, an ion of the first, second or third transition metal series in a high oxidation state, a main group ion in a high oxidation state or a combination of two or more thereof.

45. The immersion lithography system of claim 43 wherein the salt comprises Cs⁺, Ba²⁺, La³⁺, Hf⁴⁺, Ta⁵⁺, W⁶⁺, or a combination of two or more thereof.

46. The immersion lithography system of claim 43 wherein the salt includes a polarizable anion containing at least one high atomic number atom.

47. The immersion lithography system of claim 43 wherein the salt comprises at least one organic onium ion.

48. The immersion lithography system of claim 48 wherein the at least one organic onium ion comprises an ammonium, phosphonium or sulfonium group.

49. The immersion lithography system of claim 43 wherein the salt comprises thiosulfate, iodate, methylsulfonate, dihydrogen phosphate, methylsulfate or a combination of two or more thereof.

50. The immersion lithography system of claim 43 wherein the organic compound comprises a nitrile, an alcohol, a carbonate, a sulfone, a sulfide, a sulfoxide or a combination of two or more thereof.

51. The immersion lithography system of claim 43 wherein the salt or organic compound is substantially transparent at the selected wavelength and contains a refractive index affecting functional group.

52. The immersion lithography system of claim 51 wherein the refractive index affecting functional group is polarizable, has a high atomic number, has a high dipole moment or a combination of two or more thereof.

53. The immersion lithography system of claim 43 wherein the salt comprises MO₄ ^n", wherein M = Mo, W, Re; MO₃ ^n", wherein M = V, Nb, Sn, Si, or mixtures of any two or more thereof.

54. The immersion lithography system of claim 43 wherein the salt comprises [MX]^n", wherein M = a metal, X = a salt forming species comprising one or more of O, OH, RSO₃-, PO₄ ^3", PO₃ ^2", RPO₃ ^2", SO₃ ^2", HSO₃-, H₂PO₄ ^",

HPO₄ ^2", B₄O₇ ^2', ROSO₃-, OCN-, CN^~, or SCN^" and n is the total anion charge, equal to the metal oxidation state minus the aggregate anion charge, as appropriate to the metal and atoms of the X species, taking into account the number of M atoms and the number of X species.

55. The immersion lithography system of any of claims 32-54 wherein the fluid comprises water, acetonitrile, propionitrile, sulfolane, dimethylsulfone, carbon disulfide, dimethylsulfoxide or a combination of two or more thereof as the solvent.

56. The immersion lithography system of any of claims 32-55 wherein the fluid comprises lanthanum methanesulfonate, barium methanesulfonate, cesium methanesulfonate, lanthanum perchlorate, barium dihydrogen phosphate, cesium dihydrogen phosphate, tetramethylammonium triflate, tetramethylammonium methanesulfonate, sulfolane, dimethyl sulfone, acetonitrile carbon disulfide, dimethylsulfoxide or a combination of two or more thereof as the additive.

57. The immersion lithography system of any of claims 32-56 wherein the at least one additive is present at a concentration in the range from about 0.5 M to about 16 M-

58. The immersion lithography system of any of claims 32-57 wherein the at least one additive is present at a concentration in the range from about 1 M to about 5 M-

59. The immersion lithography system of any of claims 32-58 wherein the additive comprises tetraborate, perchlorate, periodate, phosphate, phosphite, dihydrogen phosphate, dihydrogen phosphite, alkylsulfonate, 1 ,2-ethanedisulfonate, perfluorobutylsulfonate, triflate, alkyl phosphonate, alkylphosphinate, alkyl sulfate, or mixtures of two or more thereof, wherein alkyl comprises C₁-C₂₀, branched or unbranched.

60. The immersion lithography system of any of claim 32- 59 wherein the fluid is free of added surfactant.

61. The immersion lithography system of any of claim 32- 59 wherein the fluid is substantially free of surfactant.

62. The immersion lithography system of any of claim 32- 59 wherein the fluid is free of added polyfluoroethers.

63. The immersion lithography system of any of claim 32- 59 wherein the fluid is free of particulate matter.

64. An immersion lithography fluid comprising an ionic liquid, wherein the immersion lithography fluid has a refractive index in the range from about 1.5 to about 1.7, at a selected wavelength for use in immersion lithography and the immersion lithography fluid is acceptable for use in immersion lithography.

65. The immersion lithography fluid of claim 64 wherein the immersion lithography fluid consists essentially of the ionic liquid.

66. The immersion lithography fluid of any of claims 1-31 wherein the selected wavelength is a wavelength useful in immersion lithography.

67. The immersion lithography system of any of claims 32-63 wherein the selected wavelength is a wavelength useful in immersion lithography.