CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2017-145057 filed in Japan on Jul. 27, 2017, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a sulfonium salt, a polymer, a resist composition and a patterning process.

BACKGROUND ART

To meet the demand for higher integration density and operating speed of LSIs, the effort to reduce the pattern rule is in rapid progress. The wide-spreading flash memory market and the demand for increased storage capacities drive forward the miniaturization technology. As the advanced miniaturization technology, manufacturing of microelectronic devices at the 65-nm node by the ArF lithography has been implemented in a mass scale. Manufacturing of 45-nm node devices by the next generation ArF immersion lithography is approaching to the verge of high-volume application. The candidates for the next generation 32-nm node include ultra-high NA lens immersion lithography using a liquid having a higher refractive index than water in combination with a high refractive index lens and a high refractive index resist film, EUV lithography of wavelength 13.5 nm, and double patterning version of the ArF lithography, on which active research efforts have been made.

As the pattern feature size is reduced, approaching to the diffraction limit of light, light contrast lowers. In the case of positive resist a lowering of light contrast leads to reductions of resolution and focus margin of hole and trench patterns.

As the pattern feature size is reduced, the edge roughness (LWR) of line patterns and the critical dimension uniformity (CDU) of hole patterns are regarded significant. It is pointed out that these factors are affected by the segregation or agglomeration of a base polymer and acid generator and the diffusion of generated acid. There is a tendency that as the resist film becomes thinner, LWR becomes greater. A film thickness reduction to comply with the progress of size reduction causes a degradation of LWR, which becomes a serious problem.

The EUV lithogaphy resist must meet high sensitivity, high resolution and low LWR at the same time. As the acid diffusion distance is reduced, LWR is reduced, but sensitivity becomes lower. For example, as the PEB temperature is lowered, the outcome is a reduced LWR, but a lower sensitivity. As the amount of quencher added is increased, the outcome is a reduced LWR, but a lower sensitivity. It is necessary to overcome the tradeoff relation between sensitivity and LWR.

DISCLOSURE OF INVENTION

In the field of acid-catalyzed chemically amplified resist materials, it is desired to have an acid generator capable of achieving a higher sensitivity, improved LWR of line patterns, and improved CDU of hole patterns.

An object of the invention is to provide a resist composition which exhibits a high sensitivity, a reduced LWR or improved CDU, and a high resolution, independent of whether it is of positive tone or negative tone; and a pattern forming process using the same.

The inventors have found that a resist composition comprising a polymer comprising recurring units derived from a sulfonium salt of specific structure having a polymerizable group forms a resist film having a reduced LWR, improved CDU, and high resolution and is effective for precise micropatterning.

In one aspect, the invention provides a sulfonium salt comprising an anion having the formula (1a) and a sulfonium cation having the formula (1b) or (1c).


Herein R is a C1-C20straight, branched or cyclic monovalent hydrocarbon group containing at least one iodine atom, which may contain a heteroatom other than iodine, Rf1and Rf2and each independently hydrogen, fluorine or trifluoroethyl, n is an integer of 0 to 5, and L1is a single bond, or a divalent group containing an ether bond, thioether bond, ester bond, sulfonic acid ester bond, carbonate bond or carbamate bond.


Herein A is an organic group having a polymer table group, R1, R2and R3are each independently a halogen, nitro, cyano, or C1-C20straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom, where a plurality of groups R1, R2or R3are included, two adjacent groups R1, R2or R3may bond together to form a ring with the carbon atoms to which they are attached, X is a single bond, or —O—, —NH—, —S—, —SO—, —SO2—, —CO— or —CH2—, p is an integer of 0 to 4, q and r are each independently an integer of to 0 to 5, q′ and r′ are each independently an integer of 0 to 4.

In a preferred embodiment, the sulfonium cation has the formula (1b-1) or (1c-1):


wherein R1, R2, R3, X, p, q, r, q′ and r′ are as defined above, L2is a C1-C20straight, branched or cyclic divalent hydrocarbon group which may contain a heteroatom, and RAis each independently hydrogen or methyl.

In a preferred embodiment, the anion has the formula (1a-1):


wherein Rf1, Rf2, L1and n are as defined above, R4is a C1-C20straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom, s is an integer of 1 to 5. t is an integer of 0 to 4, and 1≤s÷t≤5.

In a more preferred embodiment, the anion has the formula (1a-2):


wherein R4, s and t are as defined above, L3is a C1-C20straight, branched or cyclic divalent hydrocarbon group which may contain a heteroatom, u and v are each independently 0 or 1.

In a second aspect, the invention provides a polymer comprising recurring units derived from the sulfonium salt defined above.

In a preferred embodiment, the polymer further comprises recurring units having the formula (a), (b) or (c).


Herein RBis hydrogen, fluorine, methyl or trifluoromethyl, ZAis a single bond, phenylene group, naphthylene group or (backbone)-C(═O)—O—ZB—, wherein ZBis a C1-C10slight, branched or cyclic alkylene group which may contain a hydroxyl, ether bond, ester bond or lactone ring, or a phenylene naphthylene group, XAis an acid labile group, R11is halogen, nitro, cyano, or a C1-C10straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom, w is an integer of 0 to 4, x is 1 or 2, and 1≤w+x≤5.

In a preferred embodiment, the polymer further comprises recurring units having the formula (d) or (e).


Herein RBand R11are as defined above, YAis hydrogen or a polar group having at least one structure selected from hydroxy, cyano, carbonyl, carboxy, ether bond, ester bond, sulfonic acid ester bond, carbonate bond, lactone ring, sultone ring and carboxylic anhydride, and y is an integer of 0 to 4.

In a further aspect, the invention provides a resist composition comprising a base polymer containing the polymer defined above.

The resist composition may further comprise an organic solvent, a photoacid generator free of a polymerizable group, an acid diffusion inhibitor, and/or a surfactant which is insoluble or substantially insoluble in water and soluble in alkaline developer or a surfactant which is insoluble or substantially insoluble in water and alkaline developer.

In a still further aspect, the invention provides a process for forming a pattern comprising the steps of applying the resist composition defined herein onto a substrate, prebaking to form a resist film, exposing the resist film to KrF excimer laser, ArF excimer laser, EB or EUV, baking, and developing the exposed film in a developer.

Preferably, the exposing step is by immersion lithography wherein a liquid having a refractive index of at least 1.0 is interposed between the resist film and a projection lens.

The process may further comprise the step of forming a protective film on the resist film, and in the immersion lithogaphy, the liquid is interposed between the protective film and the projection lens.

ADVANTAGEOUS EFFECTS OF INVENTION

Using a resist composition comprising a polymer comprising recurring units derived from a sulfonium salt of specific structure, a pattern of satisfactory profile having reduced LWR, improved CDU and high resolution can be formed.

