Polycycloalkanes as Potential Third-Generation Immersion Fluids for Photolithography at 193 nm Juan Lo ´pez-Gejo, ² Joy T. Kunjappu, ‡ J. Zhou, § B. W. Smith, § Paul Zimmerman, | Will Conley, ⊥ and Nicholas J. Turro* ,² Department of Chemistry, 3000 Broadway, Columbia UniVersity, New York, New York 10027, Chemistry Department, 2900 Bedford AVenue, Brooklyn College of CUNY, Brooklyn, New York 11210, Center for Nanolithography Research, Rochester Institute of Technology, 82 Lomb. Memorial DriVe, Rochester, New York 14623; SEMATECH, Austin, Texas 78741, and Freescale Semiconductor, Centre de Recherche, 870 rue Jean Monnet, Crolles, France ReceiVed January 18, 2007. ReVised Manuscript ReceiVed May 22, 2007 In a search for alkane candidates for 193 nm immersion fluids, several alkanes and cycloalkanes were synthesized, purified, and screened to ascertain their absorption at 193 nm, refractive index, and temperature dispersion coefficient in the context of the actual application. In general, cycloalkanes, and more specifically polycycloalkanes, possess a higher refractive index than do linear alkanes. Decalin, cyclodecane, perhydrophenanthrene (PHP), perhydrofluorene (PHF), and perhydropyrene (PHPY) are examined as potential second- and third-generation immersion fluids. The use of perhydropyrene, which possesses a high refractive index of 1.7014 at 193 nm, may be limited as an immersion fluid because of high absorption at 193 nm. Mixtures of cycloalkanes can lead to a higher enhancement of the refractive index together with a decrease of the viscosity. Exhaustive purification of the fluids is a critical step in determining the real absorption of the different fluids at 193 nm. Even very small traces of impurities possessing a high absorption coefficient at 193 nm can lead to an unacceptably high level absorption at 193 nm, previously incorrectly attributed to the alkane instead of the absorbing impurities. 1. Introduction A current holy grail in optical lithography is the develop- ment of novel materials and processes so as to push it toward dimensions < 32 nm. The minimum feature that can eventually be printed with an optical lithography system is determined by the Rayleigh expression, eq 1. 1 where hp is the 1:1 half pitch feature size, λ is the lithography wavelength (193 nm), k 1 is a measure of the lithography process capability, and NA is the numerical aperture of the stepper’s lens. NA can be defined as given by eq 2 where n IF is the immersion fluid’s index of refraction at the lithographic wavelength (193 nm) and θ is the aperture angle, which is the angle sustained by the ray of the largest spatial frequency permitted by the optical system. Therefore, half pitch (hp) can be decreased by decreasing λ or increasing the n IF . As the challenges of shorter wavelength (157 nm) became increasingly difficult, usage of immersion-based 193 nm lithography systems became more attractive. 2 Therefore, for current 193 nm photolithography systems, there is a need to develop new immersion fluids that have a higher refractive index than that of water, n ) 1.44. 3 In addition, candidates must have an acceptable transparency at 193 nm. Recently, second-generation fluids with a refractive index of ∼1.6 at 193 nm have been prepared, driven by the growing interest in this area. 4 In those studies, absorbance and viscosity have been identified as the critical and limiting issues that must be considered to obtain a valid second-generation immersion fluids. However, newer systems have been emerging with lower absorption coefficient and high refractive index values, too. In previous publications, we have suggested the combina- tion of inorganic salts (BaCl 2 ), surfactants, and crown ethers as plausible methods of increasing the refractive index of water to be used as an immersion fluid. 5 In most cases, * Corresponding author. Tel.: +1 212 854 2175. Fax: + 1 212 932 1289. E-mail njt3@columbia.edu. ² Columbia University. ‡ Brooklyn College of CUNY. § Rochester Institute of Technology. | SEMATECH. ⊥ Freescale Semiconductor. (1) Owa, S.; Nagasaka, H. J. Microlith. Microfab. Microsyst. 2004, 3, 97. (2) Mulkens, J.; Flagello, D. G.; Streefkerk, B.; Graupner, P. J. Microlith. Microfab. Microsyst. 2004,3, 104. (3) Smith, B. W.; Bourov, A.; Kang, H.; Cropanese, F.; Fan, Y.; Lafferty, N.; Zavyalova, L. J. Microlith. Microfab. Microsyst. 2004, 3, 44. (4) (a) Budhlall, B.; Parris, G.; Zhang, P.; Gao, X.; Zarkov, Z.; Ross, B., Proc. SPIE 2005, 5754, 622. (b) Zhou, J.; Fan, Y.; Bourov, A.; Lafferty, N.; Cropanese, F.; Zavyalova, L.; Estroff, A.; Smith, B. W. Proc. SPIE 2005, 5754, 630. (c) Pemg, S.; French, R. H.; Qiu, W.; Wheland, R. C.; Yang, M.; Lemon, M. F.; Crawford, M. K. Proc. SPIE 2005, 5754, 427. (5) (a) Lee, K.; Kunjappu, J. T.; Jockusch, S.; Turro, N. J.; Lopez-Gejo, J.; Widerschpan, T.; Zhou, J.; Smith, B. W.; Zimmerman, P.; Conley, W. J. Microlith. Microfab. Microsyst., submitted for publication. (b) Lopez-Gejo, J.; Kunjappu, J. T.; Turro, N. J.; Conley, W. J. Micro/ Nanolith. MEMS MOEMS 2007, 6, 013002. Microfab. Microsyst. hp ) k 1 λ NA (1) NA ) n IF sin θ max (2) 3641 Chem. Mater. 2007, 19, 3641-3647 10.1021/cm0701660 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/30/2007