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.
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 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.
Combined measurements of transmission T, absorption A and total scattering TS revealed the high accuracy of all applied measurement techniques by obtaining a sum T+A+TS+R = (100±0.3)% (R denotes the Fresnel reflection). In order to investigate CaF2 at high fluences, a variety of samples from high purity excimer grade to research grade was irradiated (80 ... 150 mJ/cm2, 2*106...7*106 pulses) and characterized before and after irradiation by total scattering, laser induced fluorescence (LIF) and transmission measurements. Total scattering mappings showed negligible and
measurable scattering in excimer grade and some research samples of minor purity, respectively. For the first time to our knowledge, laser induced fluorescence measurements revealed increasing (580nm, 740 nm) as well as decreasing (313 nm, 333 nm) emissions. The small increases of the linear absorption, obtained in all samples by transmission measurements, were used to distinguish high from minor quality material. For high quality samples the linear absorption change scales with NH3 (N: number of pulses), whereas for minor quality research samples a NH2-scaling was found.
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.
The laser induced deflection technique (LID) is introduced for measuring small absorption coefficients of highly transparent DUV/VUV optical materials with high sensitivity and accuracy. The measuring principle, the calibration and the developed experimental realization are explained. At 193 nm in situ absorption and fluorescence measurements of fused silica give evidence that a commonly observed absorption decrease at the onset of laser irradiation is a bulk effect and due to a diminution of oxygen deficient centers ODC II. This decline is caused by a single photon absorption process and terminates after a dose of 4-5 kJ/cm2. Fluence dependent bulk absorption measurements of fused silica are presented which indicate the presence of a nonlinear dependence between the absorption coefficient α and the fluence H. For calcium fluoride a very good agreement between direct absorption and conventional transmission measurements is obtained. At 157 nm, a modified compact experimental setup is introduced which exhibits a significantly higher sensitivity than that applied for 193 nm experiments. First measurements of high quality calcium fluoride show that the obtained absorption is independent on the laser repetition rate. The investigation of equivalent CaF2 samples of different thickness (10 mm and 20 mm) indicates that the measured absorption coefficient is virtually free of contributions from the irradiated surfaces. Finally, a very good agreement is obtained by comparing LID data with transmission measurements of 100 mm long samples.
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.
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.
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.
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.
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.