Today’s high brilliance Laser sources cause huge thermal effects on optical components, affecting process stability.
This paper shows the holistic approach to the improvement of objective lenses to minimum thermal effects as focus shift.
A new approach to the transient simulation of thermal behavior, starting with FEM Analysis, analytical description of
surface deformation and refraction index distribution resulting in transient plot of image quality changes by optics design
simulation. Optics material selection and characterization of bulk material, surface and coating by newly developed
measurement techniques is shown. The optimum setting of opto-mechanical design, material selection, surface finish and
coating allows to produce lenses with focus shift by 0,05times the Rayleigh range @1064nm, 4kW multimode at
We report on a comparative study of a variety of fused silica materials for ArF laser applications, which differ regarding
their OH content. Laser induced deflection (LID) technique is applied to measure directly and absolutely the absorption
coefficient in fused silica materials at 193 nm as a function of the incident laser fluence in the range 1...3 mJ/cm2 before
and after applying 20 million shots at a fluence of 5 mJ/cm2. In addition the laser induced refractive index change is
detected by interferometer measurements after the prolonged irradiation for all samples.
Prior to the long term irradiation, low OH containing fused silica ([OH] < 80 wt-ppm) exhibits both, the lowest
absorption coefficient and the lowest absorption increase with fluence (dk/dH) in the range 1...3 mJ/cm2. During 20
million laser pulses at 5 mJ/cm2, however, the absorption and the dk/dH values show a strong increase for the low OH
containing fused silica. In contrast, the absorption of the medium OH containing samples ([OH] = 200...650 wt-ppm) is
highest prior to the long term irradiation but is remarkably lowered throughout the 20 million laser pulses. High OH
containing fused silica ([OH] > 900 wt-ppm) shows an intermediate absorption level, which only slightly increases or
decreases during the irradiation with 20 million laser pulses.
The ArF irradiation induced refractive index change is positive (= compaction) for all samples at the fluence 5 mJ/cm2.
For analysis, a particular material classification is taking into account. For low and medium OH containing samples,
referred to as compaction-dominated, the compaction factor increases with the OH content. For high OH containing
samples, referred to as rarefaction-dominated, the resulting compaction factor decreases with increasing H2 content.
ArF lithography technology requires minimization of optical losses due to scattering and absorption. Consequently, it is necessary to optimize the coating process of metal fluorides. The properties of metal fluoride thin films are mainly affected by the deposition methods, their parameters (temperature and deposition rate) and the vacuum conditions. A substrate temperature of more than 300°C is a condition for high density and low water content of metal fluorides.
Therefore, a substrate temperature of 150°C results in inhomogeneous films with high water content. Until now, the best results were achieved by boat evaporation. This paper will demonstrate that most of the common metal fluorides like MgF2, AlF3, and even LaF3 can be deposited by electron beam evaporation. In comparison to other deposition methods, the prepared thin films have the lowest absorption in the VUV spectral range. Furthermore, metal fluoride thin films were prepared by ion assistance. It will be demonstrated, that they have less water content, high packing density, and low absorption in the VUV spectral range. In this study, single layers of LaF3 and AlF3 and antireflection coatings were prepared by electron beam evaporation with and without
ion-assistance. The mechanical, structural, and optical properties were examined and discussed.
High reflective coatings for 193nm wavelength and 45° incidence were developed which combines the advantages of all-oxide
and all-fluorides layer stacks. Using plasma-assisted evaporation very smooth and dense Al2O3/ SiO2 multilayers showing small light scatter were deposited onto fused silica substrates. In the same coating process followed metal
fluoride stacks, which could reduce the resulting coating absorption at 193nm. The non-polarized reflectance of
combined stacks at 193nm is R>98.5% at 45° and R>98.0% for an angle range of 42°-48°. As the number of fluoride
layers could be drastically reduced compared to all-fluoride coatings any formation of micro-cracks could be avoided.
The stress of the oxide/fluoride stacks was less than 40MPa.
The absorption of ArF laser pulses in calcium fluoride, fused silica as well as in highly (HR) and partially (PR) reflecting
fluoridic coatings is directly measured using the laser induced deflection technique (LID).
For the calcium fluoride sample it is proved that the LID technique allows to separate surface and bulk absorption by
measuring only one sample with the size 20 x 20 x 10 mm3. At a laser pulse fluence Φ = 36 mJ/cm2 and a repetition rate
f = 1 kHz the bulk absorption coefficient and the surface absorption are determined to 0.0029 cm-1 and 0.00043 (two
surfaces), respectively. For the HR and PR coatings the ArF laser absorption is 0.0004 for Φ= 22 mJ/cm2 (f = 1 kHz)
and 0.0066 for Φ= 40 mJ/cm2 (f = 1 kHz), respectively. For the example of the PR coating the influence of high coating
scattering on the LID measurements is discussed and an appropriate measuring procedure is derived and applied to avoid
the scattering influence.
In addition to the established LID setup requiring rectangular substrate dimensions a modified setup is introduced
enabling the measurement of cylindrical optical elements. The principle of the new LID setup is explained and first
measurements at fused silica are presented.
Machining of aspheres represents an extra field in the manufacturing of optical components. The deviation from the sphere has a big impact on machining and testing equipment, tools and technologies, achievable specifications and costs. The production of aspheres deals with special problems such as mid-spatial frequency errors, centering tolerances and slopes, not known in that degree from manufacturing of spheres. Over the past 20 years JENOPTIK Laser, Optik, Systeme GmbH has gained a wide experience of this application area. Recent results give a review on what is required to execute the transition from standard quality to high-precision aspheres, off-axis parts and free forms.
Optical filters and windows are fundamental components of all modern infrared detectors. Their primary function consists in the transmission of the adequate portion of the electromagnetic spectrum which is to be measured by the sensor and in the rejection of undesired radiation. Therefore, the optical performance of the chosen filter has direct impact on the responsivity and on the signal-to-noise ratio of the detector. Recent developments of infrared filters optimized by JENOPTIK Laser, Optik, Systeme GmbH in accordance with several application requirements are presented.
On the other hand the optical filter also represents an integral part of the opto-mechanical system of the detector set-up. Thus, for reasons of cost efficiency and higher integration it is desirable to provide the filters with additional system functionality. At JENOPTIK Laser, Optik, Systeme GmbH processes have been developed for the production of infrared filters with such enhanced functionality. On customer demand the filters may be equipped with solderable edges, structured apertures, stray light suppressing elements or other features, all of very high precision. Depending on the application both highest-efficiency and very costeffective infrared filters and windows may be realized on industrial scale. Solution examples and design options are presented.
The performance of optical coatings for high power DUV/VUV laser applications depends amongst others on residual absorption in the thin film layers due to impurities or defects. Using pulsed F2 laser induced fluorescence measurements (LIF), characteristic non-intrinsic emissions of praseodymium, cerium and hydrocarbons are identified for several high reflecting AlF3/LaF3 based mirrors on CaF2 substrates. The separate investigations of single AlF3 and LaF3 layers on silicon wafers indicate that these emissions result from the LaF3 material. The amount of the impurities, however, varies strongly between different LaF3 material grades. The influence of different LaF3 material grades on the absorption properties of high reflecting mirrors is measured for the first time upon ArF laser irradiation using the laser induced deflection technique (LID). Low absorption values of less than 1*10-3 are obtained for all samples. The absorption, however, varies by more than a factor of 2 which is correlated to the appearance of the praseodymium and cerium emissions in the LIF spectra.
This paper discusses improvements in manufacturing of coated infrared optical components. Such components often have key functions in defence, security or space applications and must withstand severe environmental conditions. Therefore, very durable coatings are desired. As a consequence of its radioactivity the formerly used low refractive index material thorium fluoride is substituted. Examples for beam splitter cubes contacted with novel infrared transparent cement, interference filters, mirrors and AR coatings are presented.
The demand to enhance the optical resolution, to structure and observe ever smaller details, has pushed the way towards the EUV and soft X-rays. Induced mainly by the production of more powerful electronic circuits with the aid of projection lithography, optics developments in recent years can be characterized by the use of electromagnetic radiation with smaller wavelength. The good prospects of the EUV and soft X-rays for next generation lithography systems (λ = 13.5 nm), microscopy in the "water window" (λ = 2.3 - 4.4 nm), astronomy (λ = 5 - 31 nm), spectroscopy, plasma diagnostics and EUV/soft X-ray laser research have led to considerable progress in the development of different mulilayer optics. Since optical systems in the EUV/soft X-ray spectral region consist of several mirror elements a maximum reflectivity of each multilayer is essential for a high throughput. This paper covers recent results of the enhanced spectral behavior of Mo/Si, Cr/Sc and Sc/Si multilayer optics.
Multilayer mirrors with a significantly increased bandwidth in spectral and angular reflectance have been designed and deposited with a commercial magnetron sputtering system. A non-periodic multilayer design based on the thickness optimization of each layer by a stochastic method is compared to a design which consists of 3 different stacks. The EUV reflection of the samples was investigated with synchrotron radiation at the reflectometer of the PTB (Physikalisch-Technische Bundesanstalt) at BESSY II in Berlin. A reflectivity of more than 15 percent was reached in the whole wavelength range from 13 nm to 15 nm and a reflectivity of more than 30 percent was obtained for incidence angles from 0 degrees to 20 degrees with both designs. The increase in bandwidth is unavoidably connected with a decrease of peak reflectivity. Therefore, the application of such mirrors involves areas where a maximum peak reflectivity is not required, e.g. in EUV spectroscopy and for the metrology for EUV sources. Furthermore, the use of such mirrors in combination with a broadband plasma source will result in a higher integral reflectivity.
An example of a multilayer component for 248-nm excimer laser applications is explained in this contribution. By means of a polarizing beam splitter the potential of plasma- assisted deposition is shown. Such a coating shows a high reflecting and a high transmitting function for different polarization at a certain angle of incidence. High reflection requires a considerable number of layer pairs with a large total optical film thickness. Despite large film thickness, the optical absorption can be minimized and high transmission can be obtained. The laser induced damage threshold of such a coating is in the order of 2.5 J/cm2 for single shot measurements at 20 ns pulse length. The life time is greater than 5X109 shots at a fluence of 10mJ/cm2. Very stable deposition conditions must be maintained to produce these coatings within small tolerance of the optical function. It was found that plasma-assisted deposition is a more reliable technology for producing these coatings compared to classical evaporation.
The key technologies for modern production processes with enhanced spatial resolution, require high performance DUV- excimer laser optics with enhanced optical properties. Major challenges imposed onto the requested new generation of optical elements are concentrated on lowest absorption and scattering as well as stability against highest pulse number throughput. These targets are the driving force within the German Joint Research Project 'OPUS II', which is dedicated to the development of high quality optical components for the DUV spectral range. As a major contribution to these investigations, sets of reflecting stacks with four different numbers of layer pairs of LaF3/MgF2 were produced by 6 partners of the consortium and characterized in respect to their optical performance and structural properties. The characterization includes spectrophotometric measurements from the VUV up tot eh mid RI range. calorimetric absorption measurements at 193 nm, and a comparative study in total scatter behavior at 193 nm, which was performed by three laboratories within the project. Also, besides the intrinsic stress and the surface topography of the layers, the non-linear absorption behavior of selected samples have been determined. The results are presented and discussed with respect to possible applications.
Recent developments of DUV-excimer laser applications have gained in demands for radiation resistant coated components at interesting wavelengths. To meet the requirements of long term reliability and high pulse number throughput superior performance of the optical components with lowest absorption and scattering losses are necessary. In the framework of the German Joint Research Project OPUS II efforts are made to investigate the optical properties, the radiation resistance and long term stability of single layers and layer system of interest in the DUV. The evaluation of optical coatings and coating system on different substrate materials was carried out by scattering experiments, atomic force microscopy, IR spectroscopy, calorimetric absorption measurements, and determination of laser induced damage threshold. Additionally, from the spectralphotometric measurements the optical behavior of the films was examined.
Recent developments of DUV-excimer laser applications have gained in demands for radiation resistant coated components at interesting wavelengths. To meet the requirements of long term reliability and high pulse number throughput a superior performance of the optical components with lowest absorption and scattering losses is necessary. Within the framework of the German Joint Research Project "OPUS II" efforts are made to investigate the optical properties, the radiation resistance, and long term stability of single layers and layer systems of interest in the DUV. The evaluation of optical coatings and coating systems (AR and H R) on different substrate materials was carried out by scattering experiments, atomic force microscopy, infrared spectroscopy, calorimetric absorption measurements, determination of laser induced damage threshold (1-on-i, 1 000-on-i) and scratch tests. Additionally from spectrophotometric measurements the optical behaviour of the films was examined.
Dielectric mirrors are key optical components in ArF excimer laser based devices for applications in DUV photolithography as well as in material processing. In all these applications different requirements of laser radiation resistance have to be met in relation to fluence, repetition rate and pulse number lifetime. Investigations have been performed into the radiation resistance of dielectric mirrors consisting of fluorides and oxides with emphasis to the properties of bending point mirrors used in beam delivery systems of wafer steppers. Problems and limitations for the improvements of the laser-induced-damage-thresholds of the coatings are discussed.