Lutz Parthier

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.

Study of haze in 193nm high dose irradiated CaF2 crystals

Crystalline calcium fluoride is one of the key materials for 193 nm lithography and is used for laser optics, beam delivery system optics and stepper/scanner optics. Laser damage occurs, when light is absorbed, creating defects in the crystal. Haze is known as a characteristic optical defect after high dose irradiation of CaF2 - an agglomeration of small scattering and absorbing centers. In order to prevent unnecessary damage of optical components, it is necessary to understand the mechanism of laser damage, the origin of haze and the factors that serve to prevent it. Stabilized M centers were described as reversible absorbing defects in CaF2, which can be annealed by lamp or laser irradiation. In this study the irreversible defects created by 193 nm laser irradiation were investigated.

A microscopic model for long-term laser damage in calcium fluoride

Single crystal calcium fluoride (CaF2) is an important lens material in deep-ultraviolet optics, where it is exposed to high radiation densities. The known rapid damage process in CaF2 upon ArF laser irradiation cannot account for irreversible damage after long irradiation times. We use density functional methods to calculate the properties of laser-induced point defects and to investigate defect stabilization mechanisms on a microscopic level. The mobility of the point defects plays a major role in the defect stabilization mechanisms. Besides stabilization by impurities, we find that the agglomeration of F-centers plays a significant role in long-term laser damage of CaF2. We present calculations on the stability of defect structures and the diffusion properties of the point defects.

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.

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.

Investigations regarding the prevention of depolarization of ArF excimer laser irradiation by CaF 2 laser optics

Crystalline calcium fluoride is one of the key materials for 193nm lithography and is used for laser optics, beam delivery system optics and stepper/scanner illumination optics. In comparison to fused silica it shows a much higher laser durability. However, even in pure calcium fluoride the irradiation by ArF excimer laser (193nm) can cause transmission loss and depolarization. Short time and long time tests of radiation induced changes of optical properties of CaF2 were carried out. Within short time tests initial and radiation induced absorption as well as the measurement of laser induced fluorescence and the measurement of laser induced depolarization are adequate methods for characterization of the material under ArF laser irradiation. Previous investigations were done by Burnett to prevent depolarization caused by spatial dispersion. Nevertheless an important challenge is the prevention of depolarization of the polarized laser beam by CaF2 laser optics caused by a temperature gradient. The dependence of depolarization on the direction of temperature gradient in comparison to the direction of the laser beam and the orientation of the CaF2 crystal was investigated. In the present work different paths to prevent or mitigate the depolarization by CaF2 due to a temperature gradient are discussed resulting in a special chance to mitigate depolarization by a laser window.

Analysis of laser durability of CaF2 for optical lithography

Photolithography is a key technolgoy for the production of semiconductor devices. It supports the continuing trend towards higher integration density of microelectronic devices. The material used in the optics of lithography tools has to be of extremely high quality to ensure the high demand of the imaging. Due to its properties CaF2 is a material of choice for the application in lithography systems. Because of the compexity of the lithography tools single lenses or lens system modules cannot be replaced. Therefore the lens material has to last the full lifetime of the tool without major degradation. According to the roadmap for next generation of optical lithography tools, like immersion lithography, the requirements of CaF2 for radiation hardness are increasing considerably. We will present a detailed analysis of the key factors influencing the laser hardness covering the complete production chain. Some aspects of the evaluation methods for testing CaF2 laser durability will be presented.

On the optical anisotropy in the cubic crystal of CaF2: scaling arguments and their relation to dispersing absorption

In this paper we present arguments for understanding the phenomenon of optical anisotropy in a perfectly cubic crystal such as CaF2. To simplify the discussion we review the basic arguments which seem to preclude any optical anisotropy in a cubic crystal. We discuss the range of validity and define clear conditions for deviations of optical isotropy in cubic crystals. Length and energy scales involved in the problem of radiation-matter interaction for the DUV wavelength range around 157 nm are discussed. These scaling arguments naturally force us to focus on the role of absorption processes at higher photon energies (i.e. smaller wavelengths). Especially the role of a strong, dispersing absorption, in the case of CaF2 caused by exciton excitation, is emphasized. Recent measurements of the anisotropy of the exciton resonance in CaF2 are described and discussed in terms of the small optical anisotropy.