Ute Natura

Formation of metallic colloids in CaF2 by intense ultraviolet light

Highest purity CaF2 single crystals are irreversibly modified when irradiated with millions of pulses of 193 nm light at fluences of 120 mJ/cm2. Mie theory explains the observed haze by attributing the wavelength dependent extinction and the ratio between absorption and scattering to metallic colloids with radii in the range of 20 to 30 nm and a fractional volume of up to 2.8·10-7. Non-contact scanning force microscopy (NC-AFM) measurements performed on a surface produced by in-vacuo cleavage reveals that laser irradiation additionally produces a 104 times higher volume density of colloids with a radius of 1 to 2 nm.

Mechanisms of radiation induced defect generation in fused silica

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 H2 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.

Pulse stretcher with variable pulse length for excimer laser applications

Excimer laser systems are often combined with pulse stretchers, e.g., in lithography and Raman spectroscopy to reduce pulse peak intensities. We developed a pulse stretcher which is suited especially for pulse length dependent applications, e.g., in material research. This pulse stretcher is based on imaging and provides identical beam profiles and divergences at different pulse lengths in combination with the possibility of switching between four pulse lengths. Therefore, beam splitters are mounted onto motorized translation stages. These features are important for the characterization and development of improved optical materials for deep-ultraviolet applications. Here, two 193 nm single tube excimer lasers (Lambda Physik: LPX 240i, Cymer: ELS 5600) were evaluated concerning their combinability with an attached pulse stretcher. The wave-front radii of the ELS 5600 are dependent on repetition rate or pulse energy and are changing during warming-up. Without adapted pulse shaping, this can cause destruction of stretcher optics due to moving foci. We found that the LPX 240i could be combined easily with a pulse stretcher using beam shaping with only one cylindrical lens. We could demonstrate that stretched 193 nm laser pulses retard aging of CaF(2) crystals.

Kinetics of laser induced changes of characteristic optical properties in Lithosil with 193nm excimer laser exposure

Fused silica is used as lens material in DUV microlithography systems. The kinetics of slow radiation induced defect generation in Lithosil® 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.

Excimer-laser-induced defect generation in Lithosil

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.

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.

248 nm high fluence irradiation of CaF2 crystals

Transmission, absorption and laser induced fluorescence (LIF) measurements were performed to reveal the applicability of different grade CaF2 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/cm2. 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 CaF2 promise high life expectancy at 248 nm if a standard laser polish is used and hot spots are avoided.

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.

353 nm high fluence irradiation of fused silica

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 [3]. 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.