Single crystal calcium fluoride (CaF<sub>2</sub>) is an important lens material in deep-ultraviolet optics, where it is exposed
to high radiation densities. The known rapid damage process in CaF<sub>2</sub> 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 CaF<sub>2</sub>.
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 CaF<sub>2</sub> - 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 CaF<sub>2</sub>, which can be annealed by lamp or laser irradiation. In
this study the irreversible defects created by 193 nm laser irradiation were investigated.
Fused Silica is one of the key materials for 193 nm and 248 nm lithography as well as Laser Fusion experiments
(355nm windows) and is used for laser optics, beam delivery system optics and stepper/scanner optics for different
wavelengths including excimer laser wavelengths 193 nm / 248 nm / 353nm. Rising energy densities per pulse and
higher repetition rates will lead to decreasing exposure times in the future. The radiation induced defect generation of
Lithosil® at wavelength 248 nm and 193 nm is well described [1,2]. The lifetime of Fused Silica at high fluence
irradiation at 193 nm and 248 nm is limited by compaction and microchannel generation . Short time tests well
established for characterization of laser radiation induced defect generation in Lithosil® at irradiation wavelengths 193
nm and 248 nm were transferred to 353 nm laser irradiation experiments. Within these short time tests initial and
radiation induced absorption as well as the measurement of laser induced fluorescence (LIF) are adequate methods to
characterize the material under laser irradiation. Transmission and LIF measurements before and after high energy
irradiation were performed to reveal the applicability of different grades of Lithosil® for 353 nm laser applications.
We report on two approaches to strongly shorten life time testing of fused silica's absoption degradation upon 193 nm
laser irradiation. Both approaches are based on enhancing the two photon absorption (TPA) induced generation of E' and
NBOH defects centers in fused silica compared to common marathon test irradiation parameters. For the first approach
the irradiation fluence is increased from typical values H<1 mJ/cm<sup>2</sup> to H=10 mJ/cm<sup>2</sup>, therefore increasing the peak laser
power for a more efficient TPA process. To avoid microchannel formation in the samples, being a common break-down
criterion in marathon tests based on transmission measurements, a small sample of 10 mm length is irradiated and the
absorption is measured directly by the laser induced deflection (LID) technique. For comparing the experimental results
with a real marathon test at H=1.3 mJ/cm<sup>2</sup>, an experimental grade sample with very low hydrogen content, i.e. fast
absorption changes due to reduced defect annealing, is choosen. During the fluence dependent absorption measurements
after the prolonged irradiation at H=10 mJ/cm<sup>2</sup> it is found, that both experiments reveal very comparable absorption data
for H=1.3 mJ/cm<sup>2</sup>. For investigating standard material with high hydrogen content, i.e. slow absorption increase due to
effective defect annealing, a sample is cooled down to -180 °C in a special designed experimental setup and irradiated at
a laser fluence H=10 mJ/cm<sup>2</sup>. To control the increase of the defect density and to determine the end of the TPA induced
defect generation, the fluorescence at 650 nm of the generated NBOH centers is monitored. Before and after the low
temperature experiment, the absorption coefficient is measured directly by LID technique. By applying both, elevated
laser fluence and low temperature, the ArF laser induced generation of E' and NBOH centers in the investigated sample
is terminated after about 1.2*10<sup>7</sup> laser pulses. Therefore, a strong reduction of irradiation time is achieved in comparison
to about 10<sup>10</sup> pulses required in common marathon test applications.
Transmission, absorption and laser induced fluorescence (LIF) measurements were performed to reveal the applicability
of different grade CaF<sub>2</sub> for 248 nm laser applications. No emission from self-trapped excitions could be found in LIF
measurements after irradiation with 100k pulses for all grades. Therefore, three-photon excitation could be excluded up
to 1 J/cm<sup>2</sup>. Whereas emission at 420 nm and partially the double-peak at 313/333 nm could be found in LIF
measurements. UV-VIS difference spectra did not show any absorption bands after 248 nm irradiation of the samples.
Optical elements from CaF<sub>2</sub> promise high life expectancy at 248 nm if a standard laser polish is used and hot spots are
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
CaF<sub>2</sub> 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 CaF<sub>2</sub> 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 CaF<sub>2</sub>
crystal was investigated. In the present work different paths to prevent or mitigate the depolarization by CaF<sub>2</sub> due to a
temperature gradient are discussed resulting in a special chance to mitigate depolarization by a laser window.
Combined measurements of transmission <i>T</i>, absorption <i>A</i> and total scattering <i>TS</i> revealed the high accuracy of all applied measurement techniques by obtaining a sum <i>T+A+TS+R</i> = (100±0.3)% (R denotes the Fresnel reflection). In order to investigate CaF<sub>2</sub> at high fluences, a variety of samples from high purity excimer grade to research grade was irradiated (80 ... 150 mJ/cm<sup>2</sup>, 2*10<sup>6</sup>...7*10<sup>6</sup> 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 <i>NH</i><sup>3</sup> (<i>N</i>: number of pulses), whereas for minor quality research samples a <i>NH</i><sup>2</sup>-scaling was found.
Fused silica is used as lens material in DUV microlithography systems. The kinetics of slow radiation induced defect generation in Lithosil<sup>®</sup> including absorption, hydrogen consumption and changes of the refractive index is described in detail and in very good agreement with measured data in previous papers. In addition to these effects after long time irradiation fused silica is characterized by rapid damage processes (RDP) after short time irradiation. A model describing the absorption of RDP in dependence on energy density, repetition rate and time is described in this paper, the sensitivity of RDP on pre-irradiation and illumination conditions is discussed. Furthermore a method to reduce energy dependent absorption of RDP is mentioned.
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.
Excimer laser radiation changes the optical properties of fused silica. These changes include radiation induced absorption and changes of the index of refraction, which in turn determine the expected lifetime of silica lenses used in optical microlithography. A fully automated experimental setup designed for the marathon exposure of samples at low energy densities was employed. Measurements of the induced absorption, of the H<sub>2</sub> content using Raman spectroscopy as well as wavefront measurements were performed. A model to predict the aging behavior of silica in optical microlithography systems due to defect generation has been developed for both ArF laser irradiation and KrF laser irradiation. The model includes linear and nonlinear defect generation, relaxation processes and the consumption of hydrogen and describes the radiation induced changes of the index of refraction, the increase as well as the decrease. The model calculations were derived by analytical and numerical methods. A very good agreement in the range of parameters used in the experiments is observed.
Lens fabrication for the short wavelengths of the DUV spectral range
requires the replacement of glasses, by the crystalline material CaF<sub>2</sub>. We review mechanism for the interaction of CaF<sub>2</sub> 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<sup>-</sup>-H<sup>+</sup>-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.
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 H<sub>2</sub> consumption in Lithosil was proved by different experiments. The results are in very good agreement with the model calculations.
Under 193 nm excimer laser irradiation the laser induced deflection technique (LID) is applied to investigate directly the bulk absorption α of high quality fused silica and calcium fluoride. Fused silica samples are characterized by their fluence H dependent absorption α(H). Their small signal absorption coefficients α <sup>0</sup> are extrapolated by an appropriate fitting model. All investigated standard samples with high H<sub>2</sub> content fulfill the requirement for optical lithography which is determined by an α<sub>0</sub> of less than (formula available in paper). Prolonged direct absorption measurements at relatively high fluences of 10 and 20 mJ/cm<sup>2</sup> by the LID technique are compared to state of the art marathon durability tests for H<sub>2</sub> poor fused silica at a H = 1.3 mJ/cm<sup>2</sup>. The very good agreement of the results demonstrates that the measurement time for durability tests of fused silica can be reduced considerably by increasing the applied fluencs H. Calcium fluoride is investigated by both, direct bulk absorption (LID) and conventional transmission measurements. A very good agreement is found by comparing the results of both experiments. For investigations at 157 nm laser irradiation a new compact LID measurement device is introduced. Calibration measurements show that the sensitivity is significantly increased compared to the previous setup. The detection limit of the new setup is estimated to α values of (formula available in paper) for calcium fluoride and fused silica, respectively.