Ionic Additives for Second Generation Immersion Fluid

Immersion lithography at the 193 nm exposure wavelength has become an essential technology for device fabrication at the 65 nm and 45 nm technology nodes and will likely enable patterning at the 32 nm node.  As a result, the push to develop materials for lithographic patterning at lower exposure wavelengths has been partially supplanted by the search for high refractive index (RI), low absorbance materials to increase the potential imaging capabilities of 193 nm immersion. 
Of immediate interest is a candidate for a second generation immersion fluid to replace water for imaging below 45 nm.  For imaging of 38 nm half-pitch features, a target refractive index value has been set at n = 1.65 or greater.  A third generation fluid with n = 1.9 will be needed for imaging 32 nm half-pitch.  In both cases, the fluid absorbance should be less than 0.15 cm-1, which is an order of magnitude greater than that of high purity water.[1]
The general approach to developing a second generation immersion fluid has been to study aqueous solutions with high refractive index additives.  This approach is attractive from the stand point of maintaining physical properties close to that of water, thus allowing for continued use of water handling systems currently designed for immersion exposure tools. 
Initial attempts at these fluids have involved a variety of additives including common organic materials,[2] metal salts and acids,[3] heavy metal cations stabilized with surfactants,[4] and nanoparticles.[5]  The largest fluid indices have been seen with ionic species.  Unfortunately, these increased index values are associated with absorbance values much higher than that of water.
This study seeks to understand the effects of ionic additives on the optical properties of aqueous solutions in order to design an additive which would maximize the index of the solution while minimizing the absorbance at 193 nm.  Therefore, a systematic survey of the optical properties of several series of ionic species, from simple Group I metal salts to more microelectronics-friendly quaternary ammonium salts, was performed.  Refractive index measurements were made with spectroscopic ellipsometry and absorbance measurements were taken with a vacuum UV spectrometer.
The results were compared to discern the effect of different cations and anions in the solution.  It was found that the anions had a dominate effect on the optical properties, while the cations typically showed marginal increases in index and absorbance with increasing size.  The viscosities and contact angles of several solutions were also measured to gauge the effect various ionic additives would have on the fluid handling in an exposure system.  While most of the simple metal salt solutions behaved similarly to water, salts with complex anions or quaternary ammonium cations typically had higher viscosity and lower contact angle values.
The results of this survey were used to guide the search for better performing ionic additives.  A class of salts composed of heavy metal cations and sulfonate anions were found to have high index values with moderate absorbances.  At the large concentrations needed to achieve the high index values, the solutions showed large viscosity increases over water.  Solutions of these salts were used for imaging experiments that resulted in 32 nm half-pitch features.  Efforts are being made to replace the metal cation with a quaternary ammonium cation while maintaining the optical properties of the solutions.
65nm Half-Pitch Images
32 nm Imaging with 2.82M La(O3SCH3)3, n = 1.58

References

1. X. Yan, G. Liu, M.D. Dickey, and C.G. a) French, R. H.; Sewell, H.; Yang, M. K.; Peng, S.; McCaffert, D.; Qiu, W.; Wheland, R. C.; Lemon, M. F.; Markoya, L.; Crawford, M. K. J. Microlith., Microfab., Microsyst 4(3), 031103 (2005). b) Switkes, M.; Kunz, R. R.; Sinta, R. F.; Rothschild, M. Proc. SPIE 5040, 690 (2003).
2. T.W. Holcombe, M.D. Dickey, and C.G. Conley, W. E. Proc. SPIE 5376, 16 (2004).
3. T.W. Holcombe, M.D. Dickey, and C.G. a) Smith, B. W.; Bourov, A.; Fan, Y.; Zavyalova, L.; Lafferty, N.; Cropanese, F.; Proc. SPIE 5377, 273 (2004). b) Zhou, J.; Fan, Y.; Bourov, A.; Lafferty, N.; Cropanese, F.; Zavyalova, L.; Estroff, A.; Smith, B. W. Proc. SPIE 5754, 630 (2005).
4. T.W. Holcombe, M.D. Dickey, and C.G. Lee, K.;  Kunjappua, J.; Jockuscha, S.; Turro, N. J.; Widerschpanb, T.; Zhouc, J.; Smith, B. W.; Zimmerman, P.; Conley, W. Proc. SPIE 5753, 537 (2005).
5. T.W. Holcombe, M.D. Dickey, and C.G. Chumanov, G.; Evanoff, Jr., D. D.; Luzinov, I.; Klep, V.; Zdyrko, B.; Conley W.; Zimmerman, P. Proc. SPIE 5753, 847 (2005).