Sulfonium Salts of Alicyclic Group Functionalized ... - ACS Publications

Reyes Sierra-Alvarez, ζ and Christopher K. Ober*,†. †Department of ... Chemical and Environmental Engineering,. University of Arizona, Tucson, Ar...
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Chem. Mater. 2009, 21, 4037–4046 4037 DOI:10.1021/cm901366r

Sulfonium Salts of Alicyclic Group Functionalized Semifluorinated Alkyl Ether Sulfonates As Photoacid Generators Yi Yi,† Ramakrishnan Ayothi,† Yueh Wang,‡ Mingqi Li,§ George Barclay,§ Reyes Sierra-Alvarez,ζ and Christopher K. Ober*,† †

Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, Intel Corporation, Hillsborough, Oregon 97124, §Dow Advanced Materials, the Dow Chemical Company, Marlborough, Massachusetts 01752, and ζDepartment of Chemical and Environmental Engineering, University of Arizona, Tucson, Arizona 85721

‡

Received May 18, 2009. Revised Manuscript Received July 19, 2009

We describe new sulfonium salts incorporating semifluorinated sulfonate anions and their successful application as photoacid generators (PAGs) intended to solve the environmental problems caused by perfluorooctyl sulfonate (PFOS). By utilizing simple and unique chemistries based on 5-iodooctafluoro3-oxapentanesulfonyl fluoride, a series of alicyclic-group functionalized octafluoro-3-oxapentanesulfonate anions were prepared in high purity and high yield. Triphenylsulfonium (TPS) salts of norbornyl and γ-butyrolactone groups functionalized octafluoro-3-oxapentanesulfonates were extensively studied. They have excellent thermal stability and good solubility in various polar solvents. Their optical and thermal properties were thoroughly investigated. Angle resolved X-ray photoelectron spectroscopy (XPS) analysis confirmed that the new TPS salts are uniformly distributed in polymer films, whereas TPS PFOS is heavily segregated to a polymer film surface. Lithographic performance of the new salts together with TPS PFOS and triphenylsulfonium perfluorobutyl sulfonate (TPS PFBS) in model positive tone resists was evaluated and compared at 193 nm (both dry and immersion) and extreme ultraviolet (EUV) wavelengths. Resist compositions containing the new salts are capable of resolving sub-100 nm dense lines and spaces at both tested wavelengths. Especially, a pattern of 42 nm dense lines was successfully demonstrated in 193 nm immersion exposures. A specific TPS salt of monosaccharide-functionalized octafluoro-3-oxapentanesulfonate, a “sweet” PAG, was also synthesized and studied. This PAG is capable of resolving 90 nm lines in 193 nm lithography. These new PAGs show comparable performance with TPS PFBS and better performance than TPS PFOS, which makes them very promising environmentally friendly candidates for high resolution lithography applications. Introduction Photoacid generators (PAGs) are photosensitive molecules capable of releasing protons upon exposure to irradiation.1 Because the in situ generation of acid catalysts is fast and well-controlled, PAGs have been widely used in the fields of coatings, adhesives, and photoresists.2-4 They are also of great interest for a variety of applications including photodirected oligonucleotide synthesis,5 two-photon three-dimensional microfabrication,6 organic electronics patterning,7 and mesoporous silica film patterning.8 *Corresponding author. Mailing address: 310 Bard Hall, Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853. Phone: (607) 255 8417. Fax: (607) 255 2365. E-mail: [email protected].

(1) (2) (3) (4) (5)

Shirai, M.; Tsunooka, M. Prog. Polym. Sci. 1996, 21, 1. Crivello, J. V. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 4241. Ito, H. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 3863. Ito, H. Adv. Polym. Sci. 2005, 172, 37. Serafinowski, P. J.; Garland, P. B. J. Am. Chem. Soc. 2003, 125, 962. (6) Zhou, W.; Kuebler, S. M.; Braun, K. L.; Yu, T.; Cammack, J. K.; Ober, C. K.; Perry, J. W.; Marder, S. R. Science 2002, 296, 1106. (7) Menard, E.; Meitl, M. A.; Sun, Y.; Park, J.-U.; Shir, D. J.-L.; Nam, Y.-S.; Jeon, S.; Rogers, J. A. Chem. Rev. 2007, 107, 1117. (8) Doshi, D. A.; Huesing, I. K.; Lu, M.; Fan, H.; Lu, Y.; SimmonsPotter, K.; Potter, B. G.; Hurd, A. J.; Brinker, J. Science 2000, 290, 107.

r 2009 American Chemical Society

The most important application of PAGs is in photoresist patterning which enables intrinsic imaging processes in integrated circuit fabrication. A photoresist is a delicate mixture of a PAG and a polymer. In a typical imaging process, a site-specific photogenerated strong acid initiates numerous chemical reactions which lead to solubility changes in the exposed areas of a photoresist film. After the photoresist film is treated with a developer, micro- or nanoscale features are precisely transferred into the photoresist layer. Therefore, the properties of a PAG have a great influence on photoresist performance. Several classes of PAGs including onium salts, sulfonate esters, and organohalides have been previously investigated. Their structures and photochemistry have been well-reviewed in the literature.1,4 Among these compounds, onium salts containing perfluoroalkyl sulfonate (PFAS) as anions are particularly attractive due to their high acid-generation efficiency and excellent thermal stability. Among them, perfluorooctyl sulfonate (PFOS) salts are the most commonly used because the generated PFOS acid is strong, stable, and nonvolatile. However, many recent studies reveal that PFOS and PFOS-related materials are potentially environmentally

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hazardous. Their worldwide distribution, environmental persistence, and bioaccumulation potential have been reported.9-11 Due to the increasing concerns about the environmental and public health impact of PFOS compounds, their use is being regulated and restricted for many applications. Moreover, PFOS-based PAGs have undesirable properties, for example, inhomogeneous distribution in polymer films due to self-aggregation of PFOS units, which result in nonuniform acid distribution after exposure and therefore poor patterning performance. Even though perfluorobutyl sulfonate (PFBS) based PAGs seem to be good alternatives to PFOS based PAGs, they need improved performance, for example, through control of acid diffusion to obtain precise pattern profiles. Therefore, it is necessary to look for alternatives with environmental compatibility and excellent performance. The nature of the photogenerated acid of an onium salt is directly related to its anion. To address environmental and performance issues, two strategies have been proposed to optimize the chemical structures of the anions of onium PAGs. The first strategy is based on the concept that it is possible to produce an acid and thus a PAG with similar activity to PFOS compounds with far fewer CF2 groups adjacent to the acid group. Several PAGs having methide or amide anions12 or containing nonfluorinated functional groups in the PAG anions have been prepared.13,14 The other approach is to bind PAG functionalities to polymer chains, which minimizes their self-aggregation and enhances their miscibility with the polymer matrix.15-17 Recently, we have prepared iodonium and sulfonium salts containing an environmentally compatible 2-phenoxytetrafluoroethanesulfonate anion.14 These salts demonstrated great potential as PAGs for extreme ultraviolet (EUV) lithography. To design and prepare environmentally friendly PFOS-free salts as PAGs, aromatic and alicyclic groups can substitute for CF2 units in PFOS. Compared to aromatic groups, alicyclic groups, such as norbornyl, γ-butyrolactyl, and tetra-O-acetyl-β-D-glucopyranosyl, may be considered more environmentally friendly. Furthermore, these moieties have low absorption at a range of UV wavelengths, which makes the PAGs (9) Kannan, K.; Koistinen, J.; Beckmen, K.; Evans, T.; Gorzelany, J. F.; Hansen, K. J.; Jones, O. P. D.; Helle, E.; Nyman, M.; Giesy, J. P. Environ. Sci. Technol. 2001, 35, 1593. (10) Kannan, K.; Newsted, J.; Halbrook, R. S.; Giesy, J. P. Environ. Sci. Technol. 2002, 36, 2566. (11) Kannan, K.; Newsted, J.; Halbrook, R. S.; Giesy, J. P. Environ. Sci. Technol. 2004, 38, 1264. (12) Lamanna, W. M.; Kessel, C. R.; Savu, P. M.; Cheburkov, Y.; Brinduse, S.; Kestner, T. A.; Lillquist, G. J.; Parent, M. J.; Moorhouse, K. S.; Zhang, Y.; Birznieks, G.; Kruger, T.; Pallazzotto, M. C. Proc. SPIE 2002, 4690, 817. (13) Asakura, T.; Yamato, H.; Matsumoto, A.; Murer, P.; Ohwa, M. J. Photopolym. Sci. Technol. 2003, 16, 335. (14) Ayothi, R.; Yi, Y.; Cao, H. B.; Wang, Y.; Putna, S.; Ober, C. K. Chem. Mater. 2007, 19, 1434. (15) Wu, H.; Gonsalves, K. E. Adv. Funcnl. Mater. 2001, 11, 271. (16) Wang, M.; Jarnagin, N. D.; Lee, C.-T.; Henderson, C. L.; Wang, Y.; Roberts, J. M.; Gonsalves, K. E. J. Mater. Chem. 2006, 16, 3701. (17) Wang, M.; Gonsalves, K. E.; Rabinovich, M.; Wang, Y.; Roberts, J. M. J. Mater. Chem. 2007, 17, 1699.

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incorporating these groups very attractive for 193 nm lithography. Here we report our efforts to develop a series of environmentally friendly sulfonium salts with perfluorinated anions functionalized with fluorine-free alicyclic moieties. These unfluorinated units are expected to reduce the overall fluorine content and improve PAG distribution in polymer films. In this paper, their design and synthesis are fully discussed. To demonstrate their capabilities to effectively generate strong acids for photoresist applications, triphenylsulfonium salts functionalized with norbornyl and γ-butyrolactone groups were specifically evaluated and compared at 193 nm (both dry and immersion) and the EUV wavelength. Experimental Section Materials. 5-Iodooctafluoro-3-oxapentanesulfonyl fluoride was purchased from Synquest Laboratories, Inc. Unless stated, all the other chemicals and solvents were received from SigmaAldrich and used directly. Triphenylsulfonium bromide (TPSBr) was prepared according to the literature.18 PFOScontaining PAG (TPS PFOS) was prepared by metathesis reaction of potassium perfluorooctyl sulfonate (Synquest Laboratories, Inc.) and TPSBr. Poly(4-hydroxystyrene-co-styreneco-tert-butylacrylate) (25 mol % tBA) (poly(HSt-co-St-co-tBA)) was purchased from Dupont Electronic Materials. Poly(γ-butyrolactone methacrylate-co-methyladamantyl methacrylate) (poly(GBLMA-co-MAdMA)) (56.5 mol % GBLMA) was kindly provided by Mitsubishi Rayon. Silicon wafers were received from Montco Silicon Technologies, Inc. Methods. 1H NMR and 19F NMR spectra were recorded on a Varian Mercury 300 spectrometer by dissolving samples in the corresponding deuterated solvents. Electrospray ionization/ mass spectrometry (ESI/MS) analysis was carried with a 3D ion trap Esquire (Bruker) in both positive and negative modes. Molar absorption coefficients (ε) at 193 and 248 nm were measured from the absorption of PAG in acetonitrile solutions with a Shimadzu UV-3101PC UV/vis/near-IR spectrophotometer. Thermal properties of PAGs were evaluated by differential scanning calorimetry (DSC) with a TA Q1000 and thermogravimetry analysis (TGA) with a Q500 under a nitrogen atmosphere at a heating rate of 10 °C/min. The decomposition temperature was defined as 5% weight loss of sample. The acid sizes were estimated by the ChemSketch program integrated in ACD/Laboratories 11.0 software. The boiling points of acids were estimated by MpBpWin 1.42 program integrated in EPI Suite software. The acid strength was estimated with pKalc/ Pallas 3.5.1.4 software written by CompuDrug Chemistry Ltd. Angle resolved X-ray photoelectron spectroscopy (XPS) analysis was performed with a SSX-100 utilizing monochromated aluminum KR X-rays (1486.6 eV). The takeoff angles are 20°, 60°, and 90°. The PAG load is 0.02 mmol/0.18 g polymer in the XPS study. The thickness of the sample thin films is 100 nm. Synthesis of PAGs. 5-(2-(Bicyclo[2.2.1]heptan-3-iodo-2-yl)octafluoro-3-oxapentanesulfonyl Fluoride (a). ICF2CF2OCF2CF2SO2F (8.5 g, 20 mmoml) and norbornene (4.7 g, 50 mmol) were dissolved in 50 mL anhydrous DMF. The solution was bubbled with nitrogen for 30 min. Then, sodium p-toluene sulfinate (0.9 g, 5 mmol) was added. After being stirred for (18) Imazeki, S.; Sumino, M.; Fukasawa, K.; Ishihara, M.; Akiyama, T. Synthesis 2004, 10, 1648.

Article 2 days at room temperature, the suspension was poured into 150 mL ether and 150 mL water. The ether layer was separated, washed with water, and dried over anhydrous MgSO4. Finally, 7 g product was obtained after vacuum distillation. Yield=67%. 1 H NMR (CDCl3) δ 1.28 (m, 2H), 1.66 (m, 3H), 1.88 (m, 1H), 2.30 (m, 1H), 2.41 (s, 1H), 2.49 (s, 1H), 4.26 ppm (m, 1H). 19F NMR (CDCl3) δ -82.31 (s, -OCF2CF2SO2F, 2F), -84.95 (t, -OCF2CF2CH-, 2F), -112.41 (s, -CF2SO2F, 2F), -119.91 to -122.13 ppm (m, -CF2CH-, 2F). 5-(2-(Bicyclo[2.2.1]heptan-2-yl)octafluoro-3-oxapentanesulfonyl Fluoride (b). Into 15 mL ether solution of compound a (5.3 g, 10 mmol), tributyltin hydride (3 mL, 11 mmol) was slowly dropped under nitrogen. After the addition was over, the solution was reflux for 6 h. Then, iodine was added to quench the excess tributyltin hydride, followed by adding 10 mL saturated aqueous solution of KF. The mixture was kept stirring for several hours. After the white precipitates were filtered, the ether solution was dried over anhydrous MgSO4. The final compound was purified by vacuum distillation. Yield = 2 g (51%). 1H NMR (CDCl3) δ 1.18 (m, 3H), 1.50 (m, 4H), 1.58 (m, 1H), 2.07 (m, 1H), 2.34 (s, 1H), 2.57 ppm (s, 1H). 19F NMR (CDCl3) δ -82.43 (d, -OCF2CF2SO2F, 2F), -85.27 (t, -OCF2CF2CH-, 2F), -112.46 (s, -CF2SO2F, 2F), -117.85 to -122.34 ppm (m, -CF2CH-, 2F). 5-(2-(Bicyclo[2.2.1]heptan-2-yl)octafluoro-3-oxapentanesulfonate (c). Compound b (2 g, 5.1 mmol) and NaOH (0.4 g, 10.2 mmol) were added into 3 mL water. The mixture was refluxed at 90 °C overnight. After the water was evaporated, 10 mL anhydrous ethanol was added to precipitate NaF. The product was obtained by evaporating ethanol after NaF was filtered. Yield=2 g (95%). 1 H NMR (DMSO) δ 1.15 (m, 3H), 1.50 (m, 4H), 2.25 (m, 2H), and 2.49 ppm (s, 1H). 19F NMR (DMSO) δ -82.91 (t, -OCF2CF2SO3-, 2F), -85.90 (m, -OCF2CF2CH-, 2F), -118.82 (s, -CF2SO3-, 2F), -117.93 to -121.38 ppm (m, -CF2CH-, 2F). Triphenylsulfonium 5-(2-(Bicyclo[2.2.1]heptan-2-yl)octafluoro3-oxapentanesulfonate (TPS NB). Compound c (0.5 g, 1.21 mmol) and TPSBr (0.41 g, 1.19 mmol) were dissolved in 20 mL water, respectively. Then, TPSBr solution was slowly dropped into the aqueous solution of c during stirring. After the addition was complete, the mixture was kept stirring overnight and extracted with chloroform. The chloroform solution was separated and dried over anhydrous MgSO4. After the evaporation of chloroform, the pure compound was obtained as a transparent viscous oil, which will slowly crystallize at room temperature. Yield=0.7 g (90%). 1H NMR (CDCl3) δ 1.18 (m, 3H), 1.50 (m, 4H), 1.64 (m, 1H), 2.25 (m, 2H), 2.57 ppm (s, 1H) and 7.67-7.80 ppm (m, -(C6H5)3Sþ, 13 H). 19F NMR (CDCl3) δ -82.86 (t, -OCF2CF2SO3-, 2F), -86.24 (m, -OCF2CF2CH-, 2F), -118.19 (s, -CF2SO3-, 2F), -118.20 to -122.60 ppm (m, -CF2CH-, 2F). ESI-MS calcd: C9H7F8O6S-, 391.03; C18H15Sþ, 263.09. Found: 391.1, 263.1. Elemental anal calcd: C, 53.21; H, 4.00. Found: C, 53.22; H, 3.96. Sodium 5-Iodooctafluoro-3-oxapentanesulfonate (d). ICF2CF2OCF2CF2SO2F (15.8 g, 37.1 mmol) was added into 20 mL aqueous solution of NaOH (2.97 g, 74.1 mmol). The mixture was stirred and refluxed at 95 °C overnight. After the water was evaporated, 20 mL anhydrous ethanol was added to precipitate NaF. The product was obtained by evaporating ethanol after NaF was filtered. Yield=16.4 g (99%). 19F NMR (D2O) δ -69.2 (s, -CF2I, 2F), -83.5 (t, -CF2CF2SO3-, 2F), -86.7 (s, -CF2CF2I, 2F), -118.4 ppm (s, -CF2SO3, 2F). Sodium 6-(5-Oxo-tetrahydrofuran-2-yl)-1,1,2,2,4,4,5,5-octafluoro3-oxahexanesulfonate (e). 4-Pentenoic acid (0.55 g, 5.5 mmol) was

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dissolved in 5 mL aqueous solution of NaOH (0.33 g, 8.25 mmol). Then, d (2.23 g, 5 mmol) and 15 mL acetonitrile were added. The mixture was bubbled with nitrogen gas for 30 min. Then, NaHCO3 (0.85 g, 10 mmol) and Na2S2O4 (1.1 g, 6.3 mmol) were added. The mixture was stirred at room temperature for 4 h. After the reaction, the mixture was poured into 20 mL water. The aqueous solution was then extracted with ethyl acetate. The ethyl acetate solution was washed with brine and dried over anhydrous MgSO4. After the evaporation of ethyl acetate, the product was obtained as a white crystal. Yield=1.4 g (67%). 1H NMR (D2O) δ 2.05 (m, -CF2CH2CHCHH-, 1H), 2.55 (m, -CF2CH2CHCHH-,1H), 2.65 (m, -CF2CH2CHCH2CH2COO-, 4H), and 5.05 ppm (m, -CF2CH2CH-, 1H). 19F NMR (D2O) δ -83.57 (t, -OCF2CF2SO3-, 2F), -89.26 (m, -OCF2CF2CH2-, 2F), -115.61 to -119.09 (m, -CH2CF2-, 2F), -119.17 ppm (s, -CF2SO3-, 2F). Triphenylsulfonium 6-(5-Oxo-tetrahydrofuran-2-yl)-1,1,2,2,4, 4,5,5-octafluoro-3-oxahexanesulfonate (TPS GB). Compound e (0.7 g, 1.67 mmol) and TPSBr (0.57 g, 1.66 mmol) were dissolved in 30 mL water, respectively. Then, TPSBr solution was slowly dropped into the sodium sulfonate solution during stirring. After the addition was complete, the mixture was kept stirring overnight and then extracted with chloroform. The chloroform solution was separated and dried over anhydrous MgSO4. After the evaporation of chloroform, the pure compound was obtained as transparent viscous oil. Yield=0.98 g (90%). 1H NMR (CDCl3) δ 1.96 (m, -CF2CH2CHCHH-, 1H), 2.46 (m, -CF2CH2CHCHH-,1H), 2.52 (m, -CF2CH2CHCH2CH2COO-, 4H), 4.85 ppm (m, -CF2CH2CH-, 1H), and 7.67-7.80 ppm (m, -(C6H5)3Sþ, 14H). 19F NMR (CDCl3) δ -83.55 (t, -OCF2CF2SO3-, 2F), -89.65 (m, -OCF2CF2CH2-, 2F), -114. 86 to -118.15 (m, -CH2-CF2-, 2F), -118.82 ppm (s, -CF2SO3-, 2F). ESI-MS calcd: C9H7F8O6S-, 394.98; C18H15Sþ, 263.09. Found: 395.1, 263.1. Elemental anal calcd: C, 49.24; H, 3.37. Found: C, 49.26; H, 3.26. Tetra-O-acetyl-β-D-glucopyranosyl-1,1,2,2-tetrafluoro-2-(1,1,2,2tetrafluoro-4-iodopentyloxy)-ethanesulfonyl Fluoride (f). Allyl-tetraO-acetyl-β-D-glucopyranoside (1.8 g, 4.6 mmol) and ICF2CF2OCF2CF2SO2F (4.8 g, 11.3 mmoml) were dissolved in the mixture of 3 mL water and 6 mL acetonitrile. The mixture was then bubbled with nitrogen for 30 min. Then, NaHCO3 (1.2 g, 14.3 mmol) and Na2S2O4 (1.6 g, 9.2 mmol) were added. The mixture was stirred at room temperature overnight. After the reaction, the mixture was poured into 20 mL water. The aqueous solution was then extracted with ethyl ether. The ethyl ether solution was washed with brine and dried over anhydrous MgSO4. After the evaporation, the crude product was obtained as sticky oil. Yield=3.2 g (85%). The crude product was used without purification for the next reaction. Tetra-O-acetyl-β-D-glucopyranosyl-1,1,2,2-tetrafluoro-2-(1,1,2, 2-tetrafluoro-pentyloxy)-ethanesulfonyl Fluoride (g). Compound f (3.2 g, 3.9 mmol) was dissovled in 10 mL ethyl ether. Tributyltin hydride (1.4 mL, 36.7 mmol) was slowly dropped under nitrogen. After the addition was over, the solution was reflux overnight. Then, iodine was added to quench the excessive tributyltin hydride, followed by adding a 10 mL saturated aqueous solution of KF. The mixture was kept stirring for several hours. After the white precipitates were filtered, the ethyl ether solution was separated and dried over anhydrous MgSO4. The yield of this reaction is 80% (2.2 g). Tetra-O-acetyl-β-D-glucopyranosyl-1,1,2,2-tetrafluoro-2-(1,1,2, 2-tetrafluoro-pentyloxy)-ethanesulfonate (h). Compound g (3.4 g, 5 mmol) and NaOH (0.4 g, 10 mmol) were added into 20 mL water. The mixture was refluxed at 90 °C overnight. After the water was evaporated, 10 mL anhydrous ethanol was added to

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precipitate NaF. The product was obtained by evaporating ethanol after NaF was filtered. A 3.3 g portion of crude product was obtained. Yield=93%. Triphenylsulfonium Tetra-O-acetyl-β-D-glucopyranosyl-1,1,2, 2-tetrafluoro-2-(1,1,2,2-tetrafluoro-pentyloxy)-ethanesulfonate (the “Sweet” PAG). Compound h (2.3 g, 3.25 mmol) and TPSBr (1.1 g, 3.2 mmol) were mixed together in 20 mL water. The mixture was kept stirring overnight and extracted with chloroform. The chloroform solution was separated and dried over anhydrous MgSO4. After the evaporation of chloroform, 2.0 g compound was obtained as a yellowish solid. Yield=97%. Its 1 H and 19F NMR spectrum (CDCl3) is in the Supporting Information. ESI-MS calcd: C21H25F8O14S-, 685.08; C18H15Sþ, 263.09. Found: 685.4, 263.3. Elemental anal calcd: C, 49.37; H, 4.25. Found: C, 50.47; H, 4.35. Evaluation of the PAGs Using Microlithography. A typical resist solution was prepared by dissolving the polymer (0.18 g) and PAG (5 wt % relative to polymer) in propylene glycol methyl ether acetate (PGMEA) (2.82 g). The PAG loading in different resist compositions was adjusted to the same molar concentration. A small amount of trioctylamine (TOA, 0.05 wt % relative to polymer) was added to the mixture as a quencher. After the polymer and PAG dissolved, the clear solution was filtered through a 0.45 μm PTFE filter (Whatman). Resist films with thicknesses around 110 ( 10 nm were prepared by spin-coating the solution onto 1,1,3,3,3-hexamethyldisilazane (HMDS) vapor primed silicon wafers. The spin-coated wafers were prebaked at 120 °C for 60 s for poly(HST-co-St-co-tBA) and 115 °C for 60 s for poly(GBLMAco-MAdMA). Resist-coated wafers were then exposed to either 193 nm or EUV irradiation. The exposed films were then baked at 120 °C for 60 s. The films were developed in aqueous tetramethylammonium hydroxide solution (TMAH, 0.26 N) for 10-30 s, then rinsed in distilled water, and dried with a stream of nitrogen. After development, the patterns were examined with a highresolution Zeiss Supra scanning electron microscope. Line-edge roughness (LER) was calculated by a SuMMIT image analysis software (EUV technology). Also, 193 nm lithography was performed on an ASML 1100 system with 0.75 NA. In addition, 193 nm immersion exposures with water were performed on an ASML 1900i tool with 1.35 NA. Water is the immersion media. EUV lithography was performed at the Advanced Light Source of LBNL, California. Outgassing experiments were performed at the Center of Nanotechnology at the University of Wisconsin. Toluene was used as the internal standard. The detailed procedures of the experiments and quantification methods are available in the literature.19

Results and Discussion Design and Synthesis. We designed PAG anions, each carrying either γ-butyrolactone or norbornyl units. Both of them have a ring structure, which enables excellent thermal stability, a high boiling point of the corresponding acid, and better control of acid diffusion. Compared with PFOS based PAGs, the new PAGs have fewer fluorine atoms which minimizes self-aggregation in polymer resists. Moreover, as γ-butyrolactone and norbornyl groups are widely present in 193 nm polymer resists in the form of side groups or part of the polymer backbone, there are improved noncovalent bonding interactions (19) Wang, Y.; Cao, H. B.; Thirumala, V.; Choi, H. Proc. SPIE 2005, 5753, 765.

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between PAG molecules and polymer chains, which help to improve the PAG distribution in polymer matrixes. The new PAG anions were synthesized in three steps in the form of sodium sulfonates from the starting compound 5-iodooctafluoro-3-oxapentanesulfonyl fluoride as indicated in Scheme 1. The NB anion was prepared by addition of a perfluoroalkyl iodide to norbornene by electron transfer.20 The GB anion was prepared by sulfinatodehalogenation induced in situ intramolecular ring formation.21,22 The PAG cation, triphenylsulfonium (TPS), was synthesized in the form of TPSBr by using a Grignard reagent and chlorotrimethylsilane as an activator.18 After room temperature metathesis reactions between the sodium sulfonates and TPSBr,23 the TPS salts were obtained in high yield and high purity. Their chemical structures were confirmed by 1H NMR and 19F NMR (Figures 1 and 2). It is interesting to point out that both of the PAGs are a mixture of stereoisomers due to the ring structures of the anions. The norbornyl PAG (TPS NB) slowly crystallizes at room temperature, while the γ-butyrolactone PAG (TPS GB) is a transparent viscous liquid, which is like room temperature ionic liquids. In the following discussion, our study will focus on the characterization and comparison of TPS GB and TPS NB. It should be noted that the mild and convenient sulfinatodehalogenation reaction of 5-iodooctafluoro-3-oxapentylsulfonyl fluoride in the presence of sodium dithionite and sodium bicarbonate provides a facile way to introduce different functionalities to PAG anions. For example, we have prepared a sulfonium salt functionalized with a monosaccharide unit. Saccharides are one of four basic biomolecules from which life is composed. They play many essential roles in life activities.24 Monosaccharides are the simplest form of saccharides which are abundant, inexpensive, and environmentally friendly. Compared to PFOS, the PAG anion containing a monosaccharide moiety more easily degrades to produce less hazardous short fluorine-containing molecules. These new PAGs are expected to be nonbioaccumalitive and environmentally friendly so as to lessen any impact on the environment and living organisms. We term it a sweet PAG, the structure and synthetic scheme of which are shown in Scheme 1. By strictly using a 1:1 ratio of sodium hydroxide, the sodium sulfonate was prepared without deprotection of the monosaccharide functionality, which was confirmed by negative mode ESI-MS (see the Supporting Information). The chemical structure of the sweet PAG was confirmed by 1H and 19F NMR (see the Supporting Information). The sweet acid has low fluorine content and high acid strength (Table 1). We believe that they open a very exciting opportunity for the development of a new family of “green” photoresist materials (20) Feiring, A. E. J. Org. Chem. 1985, 50, 3269. (21) Zou, X.; Wu, F.; Shen, Y.; Xu, S.; Huang, W. Tetrahedron 2003, 59, 2555. (22) Huang, W.-Y.; Wu, F.-H. Israel J. Chem. 1999, 39, 167. (23) Crivello, J. V.; Lam, J. H. W. Macromolecules 1977, 10, 1307. (24) Sinnott M. L. Carbohydrate Chemistry and Biochemistry: Structure and Mechanism; Royal Society of Chemistry: Cambridge, 2007.

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Figure 1. 1H NMR spectra of TPS NB (a) and TPS GB (b) in CDCl3.

Figure 2.

19

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F NMR spectra of TPS NB (a) and TPS GB (b) in CDCl3.

Scheme 1. Synthesis of the New Sulfonate Anions and Their Corresponding Triphenylsulfonium Salts

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Yi et al. Table 1. Sulfonic Acid Properties boiling point (°C)

Acid C8F17SO3H C4F9SO3H CF3SO3H GB acid NB acid the sweet Acid

a

3

b

F content (wt %)

molecular weight

size ((3 cm /mol)

acid strength

estimatedc

reported

64.58 56.98 37.98 38.46 38.85 22.14

500.13 300.10 150.08 396.21 392.26 686.47

272 162 80 232 240 419

3.13 1.15 1.37 2.61 2.61 2.61

229.3 214.4 202.9 340.4 388.7 561.4

NA 275 162 NA NA NA

a Calculated by the ChemSketch program integrated in ACD/Laboratories 11.0 software. b Estimated with pKalc/Pallas 3.5.1.4 software written by CompuDrug Chemistry Ltd. c Estimated by the MpBpWin 1.42 program integrated in EPI Suite software.

which have wide applications in nanobiotechnology, for example, to pattern biologically active objects. Table 1 compares the fluorine content and the estimated sizes of the corresponding acids. The fluorine content of the sweet acid is about 22%, which is the lowest among the acids. The fluorine contents of the GB and NB acids are below 40 wt %, close to that of triflic acid and quite less than the other two perfluorinated acids. Of the acids, the sweet acid size is the largest due to the glucopyranoside ring. The sizes of GB acid and NB acid are smaller than the size of the PFOS acid but larger than the size of the PFBS acid. As effective acid diffusion is mainly determined by the counterion due to the strong long-range Coulombic interactions,25 the size of the PAG anion greatly affects acid diffusion. Generally, acids having larger anions exhibit less diffusion.26 Therefore, the corresponding acids of the new PAGs have a smaller diffusion length compared with PFBS acid, which offers better control of the desired pattern dimensions. The volatility of the acid is also very important. For acids having low boiling points such as triflic acid, acid loss will happen during high temperature baking of resist films.27 We estimated the boiling points of the acids (Table 1). Although there are large errors in the estimated values, which are evident from the reported values of PFBS acid and triflic acid, the increase of evaporation temperature according to the sequence of triflic acid, PFBS acid, and PFOS acid is correct. Therefore, the boiling points of GB acid, NB acid, and sweet acid should be much higher than 162 °C. The environmental fate of the acids was also estimated (see the Supporting Information). The new acids show much better environmental properties than PFOS acid and better or similar properties to PFBS acid. The toxicity and microbial degradation experiments are under way. PAG Properties. The solubility, thermal, and optical properties of TPS NB and TPS GB were analyzed. Both of them are readily soluble in polar solvents such as PGMEA, γ-butyrolactone, and cyclohexanone. The sweet PAG has good solubility in γ-butyrolactone and cyclohexanone. In microlithography processes, high temperature baking is required to remove most of the solvent before exposure and activate chemical reactions after exposure. Therefore, PAG should be thermally stable at

high temperatures.28 Table 2 summarizes the melting and degradation temperatures of the new PAGs. In this study, TPS PFBS and TPS PFOS are used as reference materials. Thermogravimetric analysis indicated that TPS NB and TPS GB are stable until about 350 °C. The sweet PAG begins to lose mass when the temperature is more than 230 °C, which is due to elimination of acetyl groups on the monosaccharide ring. Furthermore, no melting behavior was observed until degradation in DSC analysis of the sweet PAG. TPS PFBS is a crystalline solid with a melting temperature of 84.8 °C. TPS PFOS is instead a transparent waxy solid. As stated in the previous text, both TPS GB and TPS NB are a mixture of stereoisomers. It is very interesting that while TPS NB can slowly crystallize at room temperature, TPS GB cannot crystallize at all even after 6 months of storage and remains a sticky transparent oil. Hirayama et al. found similar viscous liquid characteristics for triphenylsulfonium n-octylsulfonate, but no thermal properties of the compound have been reported.29 To fully understand the thermal behavior of TPS GB, we carried out DSC analysis. Only a thermal transition below 0 °C was detected (Figure 3), which resembles the glass transition of room temperature ionic liquids. To the best of our knowledge, no PAGs with such unique thermal properties have been reported. The optical absorption characteristic of a PAG is one of the important factors in its ability to function. While insufficient absorption by the PAG leads to low acid generation efficiency, high absorption by the PAG will limit penetration of the light through a film, which results in a sloped resist wall profile. The new PAGs’ molar absorption coefficients at 248 and 193 nm are larger than those of TPS PFOS and TPS PFBS (Table 2). But, the difference is not very much because these PAGs’ absorption is mainly determined by TPS cation.

(25) Shi, X. J. Vac. Sci. Technol. B 1999, 17, 350. (26) Stewart, M. D.; Tran, H. V.; Schmid, G. M.; Stachowiak, T. B.; Becker, D. J.; Willson, C. G. J. Vac. Sci. Technol. B 2002, 20, 2946. (27) Cronin, M. F.; Adams, T.; Fedynyshyn, T.; Georger, J.; Mori, J. M.; Sinta, R.; Thackeray, J. W. Proc. SPIE 1994, 2195, 214.

(28) Barclay, G. G.; Medeiros, D. R.; Sinta, R. F. Chem. Mater. 1995, 7, 1315. (29) Hirayama, T.; Shiono, D.; Matsumaru, S.; Ogata, T.; Hada, H.; Onodera, J.; Arai, T.; Sakamizu, T.; Yamaguchi, A.; Shiraishi, H.; Fukuda, H.; Ueda, M. Jpn. J. Appl. Phys. 2005, 44, 5484.

Table 2. Thermal and Optical Properties of the new PAGs and TPS PFOS and TPS PFBS PAG TPS PFOS TPS PFBS TPS GB TPS NB the sweet PAG

Td (°C) Tm (°C) ε248nm (M-1 cm-1) ε193nm (M-1 cm-1) 370.4 376.6 356.4 353.2 232.5

84.8 56.7

14375 14000 15700 16340 NA

68330 70050 70665 73050 NA

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Figure 3. DSC thermograms of TPS NB and TPS GB.

PAG Distribution in Polymers. To ensure photoacid concentration is the same throughout polymer thin films after exposure, PAG molecules must be well distributed in polymer matrixes. Inhomogeneous distribution of a PAG and its acid in a polymer matrix, e.g. either enrichment or deficiency on the surface, has negative effects on resist patterns. Angle resolved XPS (ARXPS) has proved to be a highly sensitive tool to profile PAG distribution near the top surface (∼10 nm) and was used to measure the fluorine concentration on the resist film surface.30 Compared with other depth-profiling tools such as Rutherford back scattering (RBS)31 and time of flight-secondary ion mass spectrometry (TOF-SIMS),32 ARXPS has advantages because it is not necessary to label the PAG with heavy atoms such as I and Sb or to damage the resist film. In this study, we employed this tool to quantify concentrations of TPS NB and TPS GB near resist film surfaces and in bulk. To increase the signal-to-noise ratio, the concentration of PAGs used in the ARXPS study was 0.02 mmol/0.18 g polymer. PAGs were mixed separately with two different types of high activation energy copolymers, poly(HSt-co-Stco-tBA) and poly(GBLMA-co-MAdMA), in the ARXPS studies. While poly(HS-co-S-co-tBA)33 is a standard polymer for 248 nm lithography, poly(GBLMA-coMAdMA)34 is a model polymer working at 193 nm. Both of them are experimental polymers for EUV lithography. Figure 4 shows a typical XPS spectrum of a spin-coated thin film of poly(HS-co-S-co-tBS) and TPS PFBS. In the XPS spectrum, C1s, O1s, and F1s peaks are clearly resolved and distinguishable. The peak at around 975 eV is due to the Auger electrons from O. The fluorine concentrations of a polymer thin film at different takeoff angles (TOA) were compared in Figure 5. A TOA of 90° corresponds to a resist thin film depth around 10 nm, and a TOA of 20°, to that around 2 nm. It is evident that the (30) Krautter, H. W.; Houlihan, F. M.; Hutton, R. S.; Rushkin, I. L.; Opila, R. L. Proc. SPIE 2000, 3999, 1070. (31) Sundararajan, N.; Keimel, C. F.; Bhargava, N.; Ober, C. K.; Opitz, J.; Allen, R. D.; Barclay, G.; Xu, G. J. Photopolym. Sci. Technol. 1999, 12, 457. (32) Hirayama, T.; Shiono, D.; Onodera, J.; Yamaguchi, A.; Fukuda, H. Polym. Adv. Technol. 2006, 17, 116. (33) Ito, H.; Breyta, G.; Hofer, D.; Sooriyakumaran, R.; Petrillo, K.; Seeger, D. J. Photopolym. Sci. Technol. 1994, 7, 433. (34) Nozaki, K.; Yano, E. J. Photopolym. Sci. Technol. 1997, 10, 545.

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Figure 4. Typical XPS spectrum of a resist film containing poly(HStco-St-co-tBA) and TPS GB.

fluorine weight concentration of a TPS PFOS-containing thin film increases as the sampling depth decreases, which suggests that TPS PFOS molecules segregate to the surface. The enormous difference between the calculated bulk concentration (3.7 wt %) and surface concentration of fluorine indicates that TPS PFOS migrates to the polymer film surface. For the other PAGs, the fluorine surface concentration detected is very close to the calculated bulk concentration of 2 wt %. Therefore, TPS GB, TPS NB, and TPS PFBS are more uniformly distributed in polymer matrixes than TPS PFOS. 193 nm Lithography Imaging. TPS GB, TPS NB, TPS PFBS, and TPS PFOS were separately blended with poly(GBLMA-co-MAdMA). The four resists were initially evaluated at 193 nm. Figure 6 compares their sensitivities for 90 nm (1:1) dense lines and spaces. The resist compositions containing TPS GB and TPS NB have very close sensitivity, 28.2 mJ/cm2 for TPS GB formulation and 27.1 mJ/cm2 for TPS NB formulation. They are more sensitive than TPS PFOS resist (31.2 mJ/cm2) and less sensitive than TPS PFBS loaded resists (24.1 mJ/cm2). The LER values of 90 nm (1:1) dense lines were also calculated. The PFOS resist has the largest value of 9.8 nm, while the resists containing TPS PFBS, TPS GB, and TPS NB have values of 8.6, 8.5, and 8.4 nm, respectively. The smaller LER of TPS GB and TPS NB resists is due to the more homogeneous distribution of the new PAGs, which has been confirmed by ARXPS. As the corresponding acids of TPS GB and TPS NB have a larger size, they can provide better control of acid diffusion than TPS PFBS. The performance of the new PAGs can be further improved through optimization of resist process conditions. For example, in Figure 7, the resist compositions with TPS NB and the sweet PAG demonstrate fine 90 nm patterns with significantly reduced LER. The excellent properties of the new PAGs prompted an investigation of their performance in 193 nm immersion lithography, which is considered as the most promising next generation technology targeting feature sizes of 45 nm and below. By using a water immersion system, we successfully demonstrated 42 nm dense lines (Figure 8) for poly(GBLMA-co-MAdMA) blended with TPS NB.

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Figure 5. Surface fluorine concentration of resist thin films containing TPS PAGs (GB and NB) as a function of takeoff angle: (a) poly(HSt-co-St-co-tBA), (b) poly(GBLMA-co-MAdMA).

Figure 6. Top-down SEM images of 90 nm dense lines (1:1 line/space) of resist films of poly(GBLMA-co-MAdMA) blended separately with TPS PFOS (a), TPS PFBS (b), TPS GB (c), and TPS NB (d) patterned by 193 nm lithography. Esize (mJ/cm2): a, 31.2; b, 24.1; c, 28.2; d, 27.1. LER (nm): a, 9.8; b, 8.6; c, 8.5; d, 8.4.

Figure 7. Top-down and cross-sectional SEM images of 90 nm dense lines (1:1 line/space) of resist films of poly(GBLMA-co-MAdMA) blended separately with TPS NB (a and c) and the sweet PAG (b and d) patterned by 193 nm lithography. Esize (mJ/cm2): a, 23.8; b, 27.3. LER (nm): a, 5.8; b, 6.5.

Figure 8. Top-down and cross-sectional SEM images of 42 nm dense lines (1:1 line/space) of a resist film of poly(GBLMA-co-MAdMA) blended with TPS NB patterned by 193 nm immersion lithography. Esize = 28.2 mJ/cm2. LER = 3.06 nm.

EUV Lithography Imaging. As EUV imaging optics operate under vacuum, one concern for resist materials is outgassing products that are released during exposure. Such outgassing products adsorb on the optical surfaces and thus can contaminate the imaging optics and lower the tool’s performance and lifetime. Therefore, outgassing fragments should be minimized to protect the imaging optics. According to previous studies, the major outgassing contaminants are deprotected groups from polymers and

decomposition products from PAGs.35 An outgassing limit of 6.5 1013 molecules/cm2 is suggested by Intel for microexposure tools, while a limit of 5  1013 molecules/cm2 is required by ITRS 2005. We evaluated the outgassing properties of TPS GB and TPS NB prior to EUV lithography. The environmentally stable chemically amplified photoresist (35) Dean, K. R.; Gonsalves, K. E.; Thiyagarajan, M. Proc. SPIE 2006, 6153, 61531E.

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Figure 9. Top-down SEM images of resist films of poly(HSt-co-St-co-tBA) blended separately with TPS GB (left column), TPS NB (middle column), and TPS PFBS (right column) patterned by EUV lithography.

Figure 10. Top-down SEM images of resist films of poly(GBLMA-co-MAdMA) blended separately with TPS GB (left column), TPS NB (middle column), and TPS PFBS (right column) patterned by EUV lithography.

(ESCAP) type polymer was selected as the polymer matrix because ESCAP type polymers have shown lower outgassing compared to the other types of polymers.36 The sample preparation and outgassing fractions are listed in the Supporting Information. The major fraction from TPS GB and TPS NB is benzene, which is the main decomposition product of the TPS cation. No significant fragments from the GB and NB anions were detected. The total measured outgassing concentration was 3.2  1013 molecules/cm2, which is below the limit suggested by ITRS 2005. Poly(HSt-co-St-co-tBA) and poly(GBLMA-co-MAdMA) were used in our EUV lithography studies. Although they are typical polymers working at deep UV wavelengths, they have potential for EUV lithography application. TPS NB and TPS GB were separately blended with poly(HSt-co-St-co-tBA) and poly(GBLMA-co-MAdMA). The other two resist compositions containing TPS PFBS and the polymers were also prepared as controls. The resist thin films were spin-coated onto silicon wafers and, then, exposed to EUV irradiation. The range (36) Chauhan, M. M.; Nealey, P. J. J. Vac. Sci. Technol. B 2000, 18, 3402.

of dose is between 3.2 and 5.8 mJ/cm2. After the exposed thin films were developed, the patterns were examined, and their images were obtained by a Zeiss Supra SEM. Important resist parameters including the resist resolution, sensitivity, and the LER of 100 nm (1:1 line/spacing) dense lines were then measured. All of the resist compositions studied have sensitivity values (Esize for 100 nm (1:1 line/spacing) dense lines) of no more than 5 mJ/cm2, which indicates that the new PAGs are as highly sensitive as the control PAG. The SEM images of the smallest features are shown in Figures 9 and 10. It is evident that all resist compositions studied can reach a resolution of 60 nm dense lines. For elbow features, the resist compositions of TPS GB blended with poly(HSt-co-St-co-tBA) and poly(GBLMA-co-MAdMA) demonstrated a higher resolution of 60 nm, while the resist compositions of TPS NB and TPS PFOS achieved 70 nm for poly(HSt-co-Stco-tBA) and 80 nm for poly(GBLMA-co-MAdMA). As the objective of this study is to compare the performance of new PAGs with that of the standard PAG of TPS PFBS and the beam time was limited, we did not optimize the process conditions. On the basis of their excellent performance, we believe the new PAGs are capable of achieving

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higher resolution with low LER after processing conditions are optimized.

new PAGs are promising candidates for high resolution lithography.

Conclusions

Acknowledgment. C.K.O. gratefully acknowledges the funding support from Intel Corporation. Part of this work was performed at the Cornell Nanoscale Facility (CNF), a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation. The LER program was provided by Patrick Naulleau. The authors thank Brian Hoef and Paul Denham for the help with EUV exposures.

We have synthesized a series of TPS salts with functionalized octafluoro-3-oxapentanesulfonate anions by efficient chemistries. Compared with conventional TPS PFOS, the new PAGs having reduced CF2 content are environmentally friendly. Moreover, XPS depth profiling demonstrated that they are distributed much better than TPS PFOS through the model polymer thin films Their capability to achieve high resolution patterning was confirmed by excellent performance at 193 nm (both dry and immersion) and at EUV wavelength even under unoptimized process conditions. Therefore, the

Supporting Information Available: 1H and 19F NMR and negative and positive ESI-MS spectra of the sweet PAG, environmental fate estimation of the sulfonic acids, and EUV outgassing results of TPS GB and TPS NB. This material is available free of charge via the Internet at http://pubs.acs.org/.