We have developed a number of second-generation high-index candidate immersion fluids that exceed the 1.6 refractive index requirement for immersion lithography at 193 nm to replace the water used in first-generation immersion systems. To understand the behavior and performance of different fluid classes, we use spectral index measurements, based on the prism minimum deviation method, to characterize the index dispersion. In addition to fluid absorbance and index requirements, the temperature coefficient of the refractive index is a key parameter. We have used a laser-based Hilger-Chance refractometer system to determine the thermo-optic coefficient (dn/dT) by measuring the index change versus temperature at two different laser wavelengths, 632.8 and 193.4 nm. Also, we determined the batch-to-batch (within a 6-month period), before and after irradiation (at 193.4 nm), before and after air exposure, and before and after resist exposure (image printing test) variations of index and n/. The optical properties of these second-generation immersion fluids mostly compare favorably to water; the ratio of index of refraction at 193.4 nm is 1.644/1.437, the dispersion from d-line (n193-d) is 0.160 versus 0.103 and dn/dT at 193.4 is −550×10−6/K vs. −93×10−6/K, respectively.
The optical properties of three grades of Teflon® AF—AF1300, AF1601, and AF2400—were investigated using a J.A. Woollam VUV-VASE spectroscopic ellipsometry system. The refractive indices for each grade were obtained from multiple measurements with different film thicknesses on Si substrates. The absorbances of Teflon® AF films were determined by measuring the transmission intensity of Teflon® AF films on CaF2 substrates. In addition to the refractive index and absorbance per cm (base 10), the extinction coefficient (k), and absorption coefficient () per cm (base e), Urbach parameters of absorption edge position and edge width, and two-pole Sellmeier parameters were determined for the three grades of Teflon® AF. We found that the optical properties of the three grades of Teflon® AF varied systematically with the AF TFE/PDD composition. The indices of refraction, extinction coefficient (k), absorption coefficient (), and absorbance (A) increased, as did the TFE content, while the PDD content decreased. In addition, the Urbach edge position moved to a longer wavelength, and the Urbach edge width became wider.
R. French, V. Liberman, H. Tran, J. Feldman, D. Adelman, R. Wheland, W. Qiu, S. McLain, O. Nagao, M. Kaku, M. Mocella, M. Yang, M. Lemon, L. Brubaker, A. Shoe, B. Fones, B. Fischel, K. Krohn, D. Hardy, C. Chen
To identify the most practical and cost-effective technology after water immersion
lithography (Gen1) for sub-45 nm half pitches, the semiconductor industry continues to
debate the relative merits of water double patterning (feasible, but high cost of
ownership), EUV (difficulties with timing and infrastructure issues) and high index
immersion lithography (single-exposure optical lithography, needing a suitable high
index last lens element [HILLE]). With good progress on the HILLE, high index
immersion with numerical apertures of 1.55 or above now seems possible. We continue
our work on delivering a commercially-viable high index immersion fluid (Gen2). We
have optimized several fluids to meet the required refractive index and absorbance
specifications at 193 nm. We are also continuing to examine other property/process
requirements relevant to commercial use, such as fluid radiation durability, last lens
element contamination and cleaning, resist interactions and profile effects, and particle
contamination and prevention. These studies show that both fluid handling issues, as well
as active fluid recycling, must be well understood and carefully managed to maintain
optimum fluid properties. Low-absorbing third generation immersion fluids, with
refractive indices above 1.7 (Gen3), would further expand the resolution of singleexposure
193 nm lithography to below 32 nm half pitch.
Our studies of second generation immersion fluid candidates are moving beyond the discovery phase, and into addressing issues for their commercial application. Thus, we continue work to examine and fundamentally understand fluid transparency and refractive index, to fully optimize these properties. At the same time, we are now examining other process concerns, including index variation with temperature, new imaging performance studies, fluid handling considerations, and fluid property maintenance with active recycle during lithographic exposure. The systems and procedures we have developed in these areas continue to show our fluids' promise for sub-45nm immersion lithography applications.
Water-based immersion lithography using ArF illumination is able to provide optical solutions as far as the 45-nm node, but is not able to achieve the 38- or 32-nm nodes as currently defined. Achieving these lithographic nodes will require new, higher refractive index fluids to replace the water used in first-generation immersion systems. We have developed a number of such second-generation high-index fluids for immersion lithography at 193 nm. These highly transparent fluids have 193-nm indices up to 1.664. To understand the behavior and performance of different fluid classes, we use spectral index measurements to characterize the index dispersion, coupled with Urbach absorption edge analysis and Lorentz Oscillator modeling. Interference imaging printers have long been available, and they now have a new use: a rapid, cost-effective way to develop immersion lithography, particularly at extremely high resolutions. Although interference printers will never replace classical lens-based lithography systems for semiconductor device production, they do offer a way to develop resist and fluid technology at a relatively low cost. Their simple image-forming format offers easy access to the basic physics of advanced imaging. Issues such as polarization of the image-forming light rays, fluid/resist interaction during exposure, topcoat film performance, and resist line edge roughness (LER) at extremely high resolutions, can all be readily studied. 32-nm 1:1 line/space (L/S) imaging is demonstrated using two of the second-generation fluids. These resolutions are well beyond current lens-based system capabilities. Results on the performance of various resists and topcoats are also reported for 32-nm L/S features.
Water is the first generation immersion fluid for 193 nm immersion lithography. With a fluid refractive index of 1.436 and an optical absorbance of 0.01/cm at 193 nm, water immersion technology can enable optical lithography for the ITRS’ 65 nm half-pitch node. However, to achieve numerical apertures above 1.35 and to go beyond the 45 nm node, low absorbance fluids with indices higher than 1.6 are needed for the second generation of immersion lithography.
We have developed a number of Gen. 2 high index fluids for immersion lithography at 193 nm. These highly transparent fluids have 193 nm indices up to 1.67. 32 nm 1:1 line space imaging has been demonstrated using two of our Gen 2 candidate fluids, representing major advance in optical lithography. To understand the behavior and performance of different fluid classes, we use spectral index measurements to characterize the index dispersion, coupled with Urbach absorption edge analysis, and Lorentz oscillator modeling.
Immersion fluids for 157 nm and 193 nm immersion lithography have been measured over the spectral range from 156 nm to 1700 nm in a nitrogen purged environment. The refractive index n and k of several candidate fluids have been measured using the prism minimum deviation technique implemented on a commercial Variable Angle Spectroscopic Ellipsometer (VASE) system. For measurement the liquids were contained in a triangular prism cell made with fused silica windows. The refractive index of high-purity water at 21.5° C measured over the spectral range 185 nm to 500 nm. was checked against values measured on high accuracy prism minimum deviation equipment by NIST and agreement with NIST has been found to be good. The refractive index at a nominal temperature of 32°C for four fluorinated fluids in the range of n=1.308 to 1.325 at 157 nm are also reported. It was found to be extremely important to correct for temperature differences among different instruments using the thermo-optic coefficient of each liquid. The 157 nm results on fluorinated fluids are compared with measurements at NIST using a VUV Hilger-Chance Refractometer, which measured both the refractive index and the thermo-optic coefficient. In all cases results agree well.
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