The influence of Na stabilized F and M centers on the DUV absorption behavior of CaF2 is comparatively studied for
nanosecond and femtosecond laser pulses by in-situ transmission and laser induced fluorescence measurements.
For 193 nm nanosecond pulses the steady state transmission of ArF laser pulses through CaF2 is measured in dependence
on the incident fluence H ≤ 10 mJ cm-2 pulse-1. The related absorption coefficients αst(H) are proportional to H and
rationalized by effective 1- and 2-photon absorption coefficients αeff and βeff, respectively. The αeff and βeff values
increase with the Na content of the CaF2 samples as identified by the fluorescence of Na related MNa centers at 740 nm.
This relation is simulated by a complex rate equation model describing the ArF laser induced MNa generation and
annealing. MNa generation starts with intrinsic 2-photon absorption in CaF2 yielding self-trapped excitons (STE). These
pairs of F and H centers can separate upon thermal activation and the F centers combine with FNa to form MNa centers.
MNa annealing occurs by its photo dissociation into a pair of F and FNa centers.
Comparative transmission measurements with DUV femtosecond pulses are done using the fourth harmonic of a Ti:Safs-
laser at 197 nm. The resulting βeff values virtually show no dependence on the MNa center concentration. Furthermore,
the absolute βeff values are lower by a factor of three compared to those obtained for nanosecond pulses. This is
explained by additional two-step absorption for nanosecond pulses after formation of self-trapped excitons (STE).
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.
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.
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.
Fluorescence techniques are known for their high sensitivity and are widely used as analytical tools and detection
methods for product and process control, material sciences, environmental and bio-technical analysis, molecular
genetics, cell biology, medical diagnostics and drug screening. According to DIN/ISO 17025 certified standards are
used for fluorescence diagnostics having the drawback of giving relative values for fluorescence intensities only.
Therefore reference materials for a quantitative characterization have to be related directly to the materials under
investigation. In order to evaluate these figures it is necessary to calculate absolute numbers like absorption/excitation
cross section and quantum yield. This can be done for different types of dopants in different materials like glass, glass
ceramics, crystals or nano crystalline material embedded in polymer matrices. Here we consider a special type of glass
ceramic with Ce doped YAG as the main crystalline phase. This material has been developed for the generation of white
light realized by a blue 460 nm semiconductor transition using a yellow phosphor or converter material respectively.
Our glass ceramic is a pure solid state solution for a yellow phosphor. For the production of such a kind of material a
well controlled thermal treatment is employed to transfer the original glass into a glass ceramic with a specific
crystalline phase. In our material Ce doped YAG crystallites of a size of several µm are embedded in a matrix of a
residual glass. We present chemical, structural and spectroscopic properties of our material. Based on this we will
discuss design options for white LED's with respect to heat management, scattering regime, reflection losses, chemical
durability and stability against blue and UV radiation, which evolve from our recently developed material. In this paper
we present first results on our approaches to evaluate quantum yield and light output. Used diagnostics are fluorescence
(steady state, decay time) and absorption (remission, absorption) spectroscopy working in different temperature regimes
(10 - 350 K) of the measured samples in order to get a microscopic view of the relevant physical processes and to prove
the correctness of the obtained data.
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.
Characterization of optical materials and components is one of the major tasks for the Radiometry Laboratory of the Physikalisch-Technische Bundesanstalt, Germany's national metrology institute, at the synchrotron radiation source BESSY II. Using spectrally dispersed synchrotron radiation, reflectometry measurements have been performed on highly pure CaF2 crystals in the VUV spectral region between 90 nm and 130 nm wavelength in the vicinity of the absorption edge. Here, the optical constants are influenced by an excitonic resonance directly correlated to the recently found anisotropy of the crystal at 157-nm wavelength. To investigate temperature-dependent effects, the reflectometer sample holder has been equipped with a heater/cooler stage, which currently enables measurements at stable temperatures in the range between -50° C and 80° C.
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.
KEYWORDS: Ions, Crystals, Absorption, Deep ultraviolet, Distortion, Electron holes, Optical properties, Chemical species, Glasses, Temperature metrology
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.
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.
Fused silica is used as lens material in DUV microlithography systems. The exposure of fused silica to high-energy excimer laser pulses over long periods of time modifies the material. Marathon experiments were conducted at different energy densities with the KrF- and ArF excimer laser to describe the material parameters under long time irradiation. A model was developed to describe the radiation induced absorption and the change of the index of refraction. The defect generation is associated with the consumption of hydrogen. The dependence of hydrogen consumption on the wavelength of irradiation, the energy density and the initial hydrogen content was investigated in detail. The saturation of H2 consumption in Lithosil was proved by different experiments. The results are in very good agreement with the model calculations.
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.
Properties of a new rare-earth doped heavy metal oxide containing silicate glass are presented. The glass has potential for fabrication of ultra-short wideband fiber and planar waveguide amplifiers. We report specific results for a fiber amplifier geometry, discussing achieved improvements in device compactness (Giles gain g* = 210 dB/m allowing up to 100 times shorter fiber) and amplification bandwidth (50% more bandwidth in C-/L-band) compared to the conventional EDFA. We also access the potential of this material for fabrication of active planar integrated waveguide devices.
Based on exact symmetry considerations one can show that a cubic system is always optically isotropic. Nevertheless even a perfectly cubic crystal such as CaF2 can show small optical anisotropy when interacting with light. Resolving this seeming contradiction leads to a phenomenon called spatial dispersion, which is an enhancement of optical anisotropy. While the initial tiny anisotropy is caused by the symmetry breaking of light, the enhancement that makes the effect observable is provided by the vicinity of a strong absorption. In semiconductors such an absorption is mainly given by the band gap but in an ionic crystal such as CaF2 the bound electron-hole pair, a deep excitonic two-particle bound state, is an additional strong absorption causing response functions to diverge as (ω−ω0)−1 in its vicinity, where ω0 is the bound state energy. We show that the exciton dispersion is able to explain in all details the optical anisotropy observed in CaF2 including the spatial-dispersion-induced birefringence, the so-called "intrinsic birefringence." As opposed to normal birefringence, the effect in CaF2 does not show up at large wavelengths and has seven optical axes instead of one.
The microscopic mechanism beyond the optical anisotropy of an ionic crystal which occurs for short wavelengths is investigated. The electron-hole, two particle propagator and its analytical behavior close to the band edge of the one particle continuum plays a major role for the mechanism of this optical anisotropy. Especially for an ionic crystal the two particle bound state, the exciton, is of special importance. In this way we argue that the so called intrinsic birefringence in CaF2 is neither intrinsic to the material nor it is birefringence. Instead it is spatial dispersion caused by the vicinity of a dispersive optical absorption given by the excitonic bound state. We propose a model which connects the bound state dispersion with the band structure and a model potential for a screened coulomb interaction. Based on these considerations we predict a wavelength dependence of the dielectric function approaching close to the bound state level (epsilon approximately ((lambda) - (lambda) 0)-1, where (lambda) 0 is the wavelength of the excitonic bound state level.
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