Konrad Knapp

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.

Recent results on EUV mask blank multilayers and absorbers

Mask Blanks for EUV Lithography require a lot of new properties and features compared to standard Chrome-on-Glass mask blanks. SCHOTT Lithotec has introduced all relevant technology steps to manufacture EUV mask blanks. Starting from completely new low thermal expansion substrate materials with significantly improved surface quality over multilayer coatings for EUV reflection up to new absorber layers with improved dry etching and inspection properties. New polishing and cleaning technologies, improved sputter technology and updated metrology enable us to routinely produce EUVL mask blanks meeting already many of the ITRS roadmap requirements. Further R&D is ongoing to path the way to the production of EUV mask blanks which meet all requirements An important focus of this report is to present recent results on EUVL multilayer properties such as defect density, optical properties like reflectivity and uniformity in the EUV range. In addition a new design of EUVL absorber material will be reported, including optical performance at inspection wavelength, dry etch performance and resistance to cleaning steps. Finally improvements on our metrology methods for EUVL components, such as high throughput EUV-reflectometry will be elucidated.

Absorber stack optimization toward EUV lithography mask blank pilot production

EUV Lithography requires high end quality defect free layers from the backside coating to the absorber stack. Low thermal expansion materials (LTEM) substrates with super flat surfaces are already available with low defect backside coating for E-Chuck technology. The multilayer stack is well developed from a physical point of view and major effort relies nowadays on the layer defectivity. On the other hand, absorber stack becomes one of the main challenges in terms of stress, optical behavior for ultraviolet wavelengths and dry etching behavior. Schott Lithotec is currently developing absorber stack solutions that will fulfill the requirements of next generation lithographies. There are several options for achieving the mechanical, optical and chemical specs for buffer layers and absorber coatings. Some of them are already integrated in our production processes. Buffer layers were evaluated and reach almost the physical and chemical level necessary to fit with the mask processing. TaN based absorber coatings were designed and deposited by an ion beam sputter tool optimized for low defect deposition (LDD-IBS). The chemical composition of our layer and its manufacturing process is already optimized to achieve high quality etching behavior. The current results of defect density for the absorber stack will be presented.

CaF2 for DUV lens fabrication: basic material properties and dynamic light-matter interaction

Lens fabrication for the short wavelengths of the DUV spectral range requires the replacement of glasses, by the crystalline material CaF2. We review mechanism for the interaction of CaF2 with electromagnetic radiation, especially at wavelengths of 193 nm and 157 nm. In the ideal material an absorption process can occur only via a two photon process where charges are separated and an electron--hole pair is created in the material. These excited charges can localize as charge centers or as as localized excitonic state, a bound F--H+-pair. At room temperature all charge centers should recombine within a few pico seconds and no long time change of the optical material properties should be observable. In the real material not only charge center formation but also the stabilization of these charge centers at room temperature due to impurities is identified as a key for the understanding of a radiation induced change of optical material properties.

Production challenges of making an EUV mask blank

Mask Blanks for EUV Lithography require a lot of new properties and features compared to standard COG blanks. Starting from completely new low thermal expansion substrate materials with significantly improved surface quality over multilayer coatings for EUV reflection, buffer layers, up to new absorber layers with improved dry etching and inspection properties. This papers introduces in the special features of Low Thermal Expansion Materials (LTEM), their manufacturing and the special metrology for the Coefficient ofThermal Expansion (CTE). We will look into some details ofpolishing methods for much better flatness of the substrates. The process and the metrology of low defect EUV multilayer coatings will be elucidated and some aspects of this will be explained in detail. In addition we will present new results from no-chrome alternative absorber materials.

Liquid immersion lithography: microscopic polarizabilities and the role of orientation contributions to light scattering

When investigating fluids for liquid immersion lithography scattering of light is more crucial than absorption. The reason is that pure absorption can be compensated by an increase of exposure time or light intensity, while scattered light decreases the imaging contrast on the photoresist. We therefore carefully investigate the scattering of light in a molecular liquid. Light can be scattered due to a number of mechanism. These are inelastic mechanism like Raman scattering, elastic scattering on micro- and nano bubbles but also quasielastic scattering on density fluctuations. In addition to the quasielastic scattering on density fluctuations a molecular fluid shows scattering on orientation degrees of freedom. Based on the known anisotropy of the polarizability of the water molecule, we calculate the scattering components due to orientation fluctuations. Among these the polarization ratio under 90° scattering is calculated and the relation between orientation and center-off-mass contributions for different polarization directions is evaluated. While the amount of scattering due to molecular orientations seems to be still moderate for water it is expected to be larger for most fluids, like fluorinated polymers. For three different fluor--organic molecules, which are in discussion for immersion fluids, the molecular polarizability is calculated using an abinitio method. The resulting polarizabilities are used to estimate the scattering due to orientation motion of these molecules. As a result the scattering due to orientation motions has the potential to increase the scattering level remarkably.

Strong improvement of critical parameters of CaF2 lens blanks for 193-nm and 157-nm lithography

Homogeneity residuals of the refractive index have a strong influence on the performance of lithography tools for both 193 and 157 nm application wavelengths. By systematic investigations of various defects in the real structure of CaF2 crystals, the origin of homogeneity residuals can be shown. Based on a quantitative analysis we define limiting values for the individual defects which can be either tolerated or controlled by optimized process steps, e.g. annealing. These correlations were carried out for all three relevant main crystal lattice orientations of CaF2 blanks. In conclusion we achieved a strong improvement of the critical parameters of both refractive index homogeneity and striae for large size lens blanks up to 270mm diameter.

Liquid immersion lithography using water as an immersion liquid: role of orientation contributions to light scattering at 193 nm

The orientation and center-of-mass contributions of water to light scattering are calculated based on the known anisotropy of the polarizability of the water molecule. Further, the polarization ratio under 90-deg scattering is calculated and the relation between orientational and center-off-mass contributions for different polarization directions is evaluated. An experiment is proposed that enables us to separate the different scattering contributions. While the amount of scattering due to molecular orientations seems to be still moderate for water, it is expected to be larger for most fluids, like fluorinated polymers.

Critical enabling properties of CaF2 lens blanks for state-of-the-art lithography tools

F2 lens designs considering Intrinsic birefringence imposed more severe challenges to CaF2 manufacturing technology. In order to compensate the intrinsic birefringence other crystal orientations (100) / (110) are necessary. These other crystal orientation beside (111) require individual process optimization. In this paper the achieved improvements for CaF2 lens blank material will be presented. Furthermore the conversion of stress birefringence results from 633nm to 193nm or 157nm is unclear until now. At wavelength birefringence measurement results of different orientated lens blanks will be shown and discussed.