Simone Ritter

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.

Optical Materials and Their Properties

This chapter provides an extended overview on todayʼs optical materials, which are commonly used for optical components and systems. In Sect. 5.1 the underlying physical background on light–matter interaction is presented, where the phenomena of refraction (linear and nonlinear), reflection, absorption, emission and scattering are introduced. Sections 5.2–5.8 focus on the detailed properties of the most common types of optical materials, such as glass, glass ceramics, optoceramics, crystals, and plastics. In addition, special materials displaying “unusual nonlinear” or “quasi-nonreversible” optical behavior such as photorefractive or photorecording solids are described in Sect. 5.10. The reader could use this chapter as either a comprehensive introduction to the field of optical materials or as a reference text for the most relevant material information.

Development of a new process for manufacturing precision gobs out of new developed low Tg optical glasses for precise pressing of aspherical lenses

Aspherical lenses or refractive elements out of optical glass can be produced either by grinding and polishing of glass or by precise molding of glass preforms. The first process is applied for lenses with larger geometries and smaller production quantities. On the other hand, precise molding is used for volume production of lenses within a diameter range between 1 mm and around 30 mm. The addressed products can be found in the consumer markets (digital imaging, digital projection and digital storage). Different preform types can be used for precise molding: polished spherical near shape preforms, polished balls, polished discs and precision gobs. The latter are made directly from the glass melt. This paper describes a newly developed process, which results in fire-polished gobs with very low surface roughness and excellent volume accuracy. Since precision gobs are mostly made for precise molding, they must meet specific process requirements apart form their optical values, such as allowing low molding temperatures and shorter process cycles times. Therefore, this paper also describes the latest results in the development of low Tg glasses, which are designed for the volume production of precision molded optical components. Beside the important parameters like nd, nd as well as Tg, other properties like chemical durability, devitrification resistance, thermal expansion and conductivity coefficients are important for optimizing the precise molding process. The characteristics of three new low Tg glasses in the FK-, PK- as well as SK-region are presented. These glasses are environmentally friendly, since they are free of lead and arsenic.