Jochen Alkemper

Optimized glass-ceramic substrate materials for EUVL applications

EUV substrate materials have to meet enhanced requirements with respect to extreme low thermal expansion, high homogeneity and superior surface quality. A SCHOTT R&D program aims at the development of advanced materials covering these various aspects. The glass-ceramic Zerodur (registered trademark) of SCHOTT represents a substrate material currently used for EUV masks and optics of first generation tools due to its extremely low coefficient of thermal expansion (CTE) and its excellent homogeneity. Zerodur(registered trademark) even allows continuous shifting of the position of zero crossing of the CTE-slope to control the thermal expansion behavior according to varying customer requirements: As a result of specifically adjusted process parameters, samples of Zerodur (registered trademark) exhibit a coefficient of thermal expansion CTE < 5 ppb/K corresponding to the lowest expansion class of the SEMI standard P37 (19 to 25°C) for EUV mask blanks. By further variation of process parameters, the position of zero crossing, e.g. at 22.5°C or 30°C, can be varied, revealing an attractive attribute feature of Zerodur (registered trademark). A new dilatometer type reveals an improved reproducibility of ~ 1ppb/K in the temperature range of 0 to 50°C. A series of CTE(0;50°C) measurements with a test-cube of Zerodur (registered trademark) provides information on CTE homogeneity on a cm-scale: no CTE variation was observed within the error of measurements (1ppb/K) for a block exhibiting ± 3.5*10-6 variation in refractive index. CTE variation can cause surface deformations during changing temperature conditions. A Fizeau-Interferometer was used to record surface roughness at two different temperatures. This non- destructive metrology is regarded as a method to distinguish CTE variation < 1ppb/K. The surface deformation of Zerodur (registered trademark) due to elevated temperature was determined to be lower than the resolution. Both methods to analyze the CTE homogeneity of Zerodur (registered trademark) lead to the result of CTE variation below 1 ppb/K. Surface treatment of glass-ceramic material is a major challenge as final finishing of EUV substrates may increase roughness of super-polished surfaces significantly. Improved new glass-ceramic materials demonstrate optimization of glass-ceramic compositions to nearly meeting the specification of surface roughness after a standard finishing process. Recent achievements of material development reveal CTE-performance of this new glass-ceramic to also be adjustable to varying customer needs as already known for Zerodur (registered trademark). These results are regarded as a promising milestone to develop an optimized glass-ceramic material, because the features of the modified New-Glass Ceramic now better match the key requirements of EUVL substrate materials.

Improved materials meeting the demands for EUV substrates

The enhanced demands for substrate materials for next-generation optics and masks have initiated detailed investigations on Zerodur as a proposed EUVL substrate material with focus on thermal expansion behavior and surface roughness. As a result of specifically adjusted process parameters, the coefficient of thermal expansion (CTE) was tailored to be a minimum at 22.5°C. Laboratory samples of Zerodur exhibit a CTE < 5 ppb/K corresponding to the lowest expasnion class of the SEMI standard P37 (19 to 25°C) for EUV mask blanks. By further variation of process parameters, the position of zero crossing, e.g. at 30°C, can be varied, revealing an attractive attribute feature of Zerodur. A new dilatometer type was mounted in 2002 with first operatinoal results revealing an improved reproducibility of ~1ppb/K in the temperature range of 0 to 50°C. A series of CTE measurements with a small block of Zerodur provides information on CTE homogeneity on a cm-scale: No CTE variation was observed within the error of measurements for a block exhibiting ± 3.5*10-6 vairtion in refractinve index. CTE variation can cause surface deformations during changing temperature conditions. A first setup of Fizeau-Interferometer with a current resolution of 0.3 nm rms was used to record surface deformation of Zerodur due to elevated temperature was determined to be lower than the current resolution. Both methods to analyze the CTE homogeneity of Zerodur lead to the result of CTE variation below 1 ppb/K, still identifying todays need to improve metrology further. Final finishing of EUV substrates may increase roughness of super-polished surfaces significantly. Using appropriate processes a to surface roughness < 0.25 nm rms under production conditions can be achieved after final finishing of Zerodur. As an improved Zerodur-type material, recent achievements of material development demonstrate the optimization of glass-ceramic composition to nearly meeting the specification of surface roughness after a standard finishing process. These results are regarded as a promising milestone to develop an optimized glass-ceramic material providnig adjusted thermal expansion behavior and surface processability according to the specific demands of EUV technology.

Dynamic change of transmission of CaF2 single crystals by irradiating with ArF excimer laser light

The laser induced absorption of CaF2 caused by ArF excimer laser light has been observed at energy densities of F equals 2-30 mJ/cm2 per pulse and a repetition rate of R equals 50 Hz. The experiments show that the transmission of CaF2 samples depends on the pulse energy density. The change of the absorption coefficient with the time of irradiation can be described by an exponential model. Different experiments were performed where the energy density was increased and decreased stepwise. They prove that color centers not only are formed but also are annihilated by irradiation. Laser induced decrease of absorption was observed in all samples as soon as the energy density was decreased. Coloring and bleaching of the samples are completely reversible processes. The level of transmission depends on the energy density of the laser light and the quality of the material but not on the history of irradiation. The damage resistance of the material can be adjusted by the appropriate choice of the raw material and the process parameters. The reversibility of the laser induced absorption can be explained by a reaction equilibrium. This leads to a model where the concentration of absorbing defects depends on the current irradiation conditions. Using these equations the reversibility and the observed exponential dependence of the change of transmission with time can be explained. Assuming different dependencies of the reaction constants of coloring and bleaching on the energy density, the change of the absorption coefficient with pulse energy density can be calculated.

Thermal expansion and internal quality of a proposed EUVL mask substrate material: Zerodur

Detailed thermal expansion measurements and internal homogeneity measurements of the glass-ceramic material Zerodur were undertaken to examine its usefulness for EUVL. Repeat measurements on 100-mm long samples from three castings exhibit an expansion of approximately 12 +/- 2 ppb/K 2 (sigma) in the temperature range of interest for EUVL, corresponding to Class C of the draft SEMI 3148 standard. Internal homogeneity measurements reveal extremely small refractive index variations, suggesting comparably small compositional variations. This in turn is viewed as a necessary but not sufficient condition for high CTE uniformity, a factor required by EUVL applications.

Evaluation of fused silica for DUV laser application by short-time diagnostics

Excimer laser pulses ((lambda) equals 193 nm, (lambda) equals 248 nm) induce transient and permanent defects in highly UV transparent optical glass for microlithography. Usually laser damage of fused silica is evaluated by time consuming and expensive marathon tests characterized by about 109 pulses at repetition rates of 400-1000 Hz and fluences of 0.5-10 mJ/cm2. Alternatively, short time tests using high laser energy densities have been developed to quickly evaluate influences of changes in the production technology. The following evaluation methods are used: Laser induced absorption at 193 nm measured by laser induced deflection (LID), Laser induced fluorescence at 650 nm (LIF) excited by 193 nm or 248 nm laser irradiation, H2 content measurement by means of a pulsed Raman spectroscopy at 248 nm laser excitation. Both, the LIF signal and the H2 concentration are measured locally resolved in a non-destructive way. The applied energy densities of the above methods vary from 1 mJ/cm2 to 600 mJ/cm2. The front face technique for investigating large diameter samples, e.g. mask blanks (6 inches and 9 inches), have been established.