Standard UV materials, such as ArF-grade fused silica, have impurities that lead to low transmittance, high absorption,
and fluorescence when exposed to high irradiance. Calcium fluoride (CaF<sub>2</sub>), on the other hand, is a promising material
for use as an optical diffuser for applications at 157nm, 193nm, and 248nm due to its low defect density and high
transmission in the deep UV regime. In this paper, we discuss our method for fabricating Gaussian homogenizers in
calcium fluoride using a grayscale photolithography process. Refractive microlens array homogenizers and Gaussian
homogenizers have been fabricated in CaF<sub>2</sub> and tested at 193nm for efficiency and uniformity. Using an excimer laser,
uniformity results were obtained for cylindrical lens arrays in tandem and crossed to observe the homogeneity in an
imaging configuration and for producing a square output. Efficiency, uniformity, and zero order measurements are
provided for the Gaussian homogenizers.
Many manufacturing techniques have been developed and implemented to fabricate a wide range of micro-optical
products. The challenges of the micro-optics business are diverse and tend to resist a widely accepted manufacturing process such as has been implemented for CMOS fabrication. Many of the challenges that have been addressed with various solutions include optical waveband of operation from DUV through LWIR, material systems, cost of manufacturing for the intended application space, feature sizes based on device functionality, and fabrication technology based on the manufacturing volume. Some of the technologies to be discussed include device patterning by e-beam lithography, optical lithography, direct CNC machining and micro-polishing, and plastic replication.
Micro and nano optics enable the control of light for producing intensity distributions with given profiles, propagation properties and polarization states. The higher the requirements on the optical function, the more complicated will be its realizing with a single element surface or a single element class. Combinations of refractive and diffractive, both diffractive or sub-wavelength structures with each other give the ability to link the advantages of different element classes or different element functions for realizing the optical functionality. In the paper we discuss two different examples of combinations for DUV applications. In detail we present a diffractive - diffractive beam homogenizer with NA of 0.3 that show no zero order. A binary phase grating for polarization control combined with a beam shaping element will be presented. The polarized order of this grating shows an efficiency of about 90% with a degree of polarization better than 90%. Wave optical and rigorous design strategies and simulations as well as the optical measurements will be discussed for the given examples.
As lithographers push to extend optical lithography technologies to create smaller features with higher NA, especially immersion lithography, and lower k1 values, the polarization of the light in the illumination system becomes relevant. Nowadays the fabrication with electron-beam-lithography has the ability the produce diffractive structures for the DUV spectral range. We present binary phase gratings with polarization depended optical properties for 193nm designed with the rigorous coupled wave analysis (RCWA). The element function is based on a shift of the diffraction efficiencies between the first and the zero diffraction orders for different incident polarization angles. The properties of a 193nm polarizing phase grating will be discussed in detail. For TE-polarized light a zero order signal of less than 1% was measured. The efficiency for TE-polarized light in the first order is about 80%. For TM-polarized light the binary phase gratings achieve an efficiency of nearly 90% in the first and the zero order in transmission. The deflection angle of the first diffracted order can be varied between 14° and 40°.
The rigorous modelling as well as the fabrication technology and optical measurements will be discussed.
In the optimization process of DUV-illumination systems for inspection tools and lithographic devices, more and more an exact control of angular distribution and homogeneity of the illumination will be required. On the one hand, diffractive homogenizers enable homogeneous illumination of areas with almost arbitrary shape with a high numerical aperture. On the other hand, diffractive optical elements produce a zero order or so called “hot spot”. If the optical axis is within the illuminated area, this hot spot will decrease the homogeneity of illumination. The zero order is caused by profile aberrations and its intensity can be decreased by increasing the fabrication accuracy. But the higher the numerical aperture, the larger the ratio between zero order brightness and brightness of the surrounding homogenized area. I.e., in cases of high NA the zero order of a homogenizer cannot be reduced to the brightness of the surrounding area. We present a novel approach of beam homogenization using a combination of two serially arranged diffractive optical elements that produces an intensity distribution without hot spot. Such compact two-element homogenizers have been realized for wavelengths down to 193nm. A homogenizer for 193nm producing a homogeneously illuminated rectangle with 0.3 NA will be presented.
We have investigated the propagation of TM-polarized light through periodic Chromium structures. The fabrication of periodic structures was performed with electron-beam lithography. The Chromium layer covered with Chromium Oxide is opaque. Typical slit widths of the periodic structures are varied between 100nm and 400nm. The periods are ranged between 500nm and 10μm.
To analyze these structures we have performed FDTD-modeling for wavelengths in the visible and infrared spectral range. The near field region behind these structures was modeled. The propagation of surface waves was observed. Furthermore spectral measurements on periodic structures with varying periods are carried out. We propose a method for the determination of propagation lengths of surface waves across the interfaces of the metal layer. The experimental findings were compared with FDTD modeling. Resonances of the surfaces waves were also modeled with a RCWA algorithm. A comparison between the numerical findings and the experimentally achieved results will be given.
We evaluate the optical response of binary quarter micron slits and gratings in thin opaque chromium layers for measuring critical dimensions on photomasks. At present the CD is typically inspected by time expensive SEM measurements. A main disadvantage of the SEM measurements is that it determines only the geometric parameters. Starting from the optical properties that come closer to the application of the masks we have evaluated a new approach to inspect the CD of test structures like quarter micron slits and gratings. The CD of the test structures has been varied between 100nm and 400nm. Slit widths of these structures have been characterized. Based on the combination of spectral and polarization resolved transmission and reflection measurements in a spectral range between 500 nm and 1700 nm with RCWA calculations we propose a new method for measuring the CD of test structures below the resolution limit of the classical microscopy with visible and infrared light.
The spectral response of transparent quarter micron lines in opaque Chromium layers has been studied with visible light. These lines are design elements of binary components used in advanced optical systems for microlithography. In the current fabrication process lines of 200nm line width and more than 20 μm length are fabricated by electron beam lithography. The geometric parameters of these lines, e.g. width, length and the microstructural quality are controlled by scanning electron microscopy (SEM). In this work want to report about a new possibility to control the optical properties of quarter micron lines with respect to line width and microstructure. We have performed spectral resolved measurements with a tuneable light source in a spectral range between 450nm and 1600nm. Due to the dimensions of the quarter micron lines polarization depend transmission properties are expected. By performing polarization sensitive optical measurements we are able to distinguish quarter micron lines with different widths below the resolution limit of the classical microscopy with visible light.
For future spacecrafts a lot of new UV or x-ray instruments are proposed. To enhance resolution and reduce scattering, new optical materials with optical surfaces with a roughness range of about 0.1 nm, i.e. supersmooth surfaces will be used to build the optics of the instruments. Some of the spacecrafts or instruments will be operated in the Low Earth Orbits (LEO's) of the Space Shuttle or the International Space Station. The natural and induced space environment can damage spacecraft and instrument materials. Since the LDEF-EURECA- and D2-experiments and the recovery of the Hubbel Space Telescope solar arrays it is well known that a large group fo materials degrade under space conditions. However very few data are available from inflight experiments especially on supersmooth materials. To fill the gap, the Surface Effects Sample Monitor SESAM has been developed. This space flight instrument was designed to expose test materials to the conditions of space during flight missions. Also included in the experiment is an online Atomic Oxygen (ATOX) Measurement Facility to monitor the impact of the ATOX on the exposed samples. The experiment was flown on four missions. The results show a degradation of supersmooth samples under space conditions which has to taken in account for the design of future UV or X-ray instruments.
A UHV surface analysis system combined with an optical setup was used in the present work to study the conditioning mechanism on MgF<SUB>2?</SUB>/LaF<SUB>3</SUB> HR coatings at 248 nm excimer laser wavelength. During laser irradiation of the sample the laser-induced emission of contaminants and coating material from the sample surface was recorded by a quadrupole mass spectrometer. To analyze changes in composition and chemical bonds of the coating surface XPS-measurements were performed before, during and after irradiation in dependence on sample design, number of pulses and oxygen atmosphere.
The laser damage thresholds of optical coatings lie, as a rule, markedly below those of the respective bulk materials. This is due to diverse specific realstructure properties with regard to composition, crystallography, microstructure and the physico-chemical structure of the interfaces. These properties depend in a highly complex and sensitive way on the substrate treatment, coating techniques and deposition conditions. With evaporated and sputtered oxide coatings as example, some correlations between structural thin film properties (e.g. crystallography, microstructure, anisotropy, chemical composition, defects) and the ultraviolet (248 nm) or near infrared (1064 nm) laser damage thresholds are discussed with concern to a further increase of the damage resistance. It is evident from data that an approach to the problem requires complex investigations of the technology-structure-properties relationships.
LaF<SUB>3</SUB>/MgF<SUB>2</SUB>-dieletric thin film combinations can be applied in optics for wavelengths down to 150 nm. Several such HR systems for a wavelength of 248 nm were investigated. In these coatings, the influence of laser conditioning on damage threshold and absorptivity was found to be remarkable. XPS- and TEM-investigations showed that the conditioning effect is related to structural and stoichiometric changes in the multilayers, especially in the near-surface-sublayers.
Electron-gas SNMS offers favorable properties, often useful or even necessary for the depth profiling of optical coatings. A short introduction into this method, examples of SNMS depth profile analysis in the fields of composite oxide layers, control of complex thin film components, damage of coatings and characterization of multilayers for soft X-ray optics, as well as a report of ongoing developments are given, demonstrating the performance, the limits and the future potential of the SNMS depth profile analysis.
Optical components for space optics--especially coated optical elements which represent the external surfaces of optical space instrumentation--have to work under harsh operation conditions like thermal loads, irradiation by photons, electrons and protons, as well as in atomic oxygen environments at low earth orbits. Additionally they have to withstand other cross contamination coming from the spacecraft. Therefore, the stability against these influences is a decisive factor for the application performance of optical coatings in space- borne devices. Some very recent results, based on the Surface Effects Sample Monitor flight experiment carried out aboard the ORFEUS-Shuttle Pallet Satellite, STS-51, Discovery, are presented here along with laboratory experiments in an UHV-surface analysis system. The topics include ground simulation of selective and complex particle bombardment of optical coatings analyzed by XPS as well as the verification of these results by flight experiments in combination with optical measurements (transmission, scattering).
Pulsed laser deposition is described as a technique for the synthesis of multilayers showing X- ray optical quality. The state of the art is characterized by results that demonstrate a development of the instrument basis superior to that of conventional PLD systems. Multilayers of the Ni/C, Mo/Si- and W/C-types prove the versatility of the method and the output of layer stack characterization by HREM, SPM, XD, AES, XPS, ellipsometry and image processing ensures a high quality with regard to stack regularity, layer homogeneity and interface smoothness.
Optical components for space optics - especially coated optical elements which represent the external surfaces of optical space instrumentation - have to work under harsh operation conditions like thermal loads, irradiation by photons, electrons and protons, as well as in atomic oxygen environments at low earth orbits. Additionally they have to withstand other cross contamination coming from the spacecraft. Therefore, the stability against these influences is a decisive factor for the application performance of optical coatings in space-borne devices. Some very recent results, based on the Surface Effects Sample Monitor (SESAM) flight experiment carried out aboard the ORFEUS-Shuttle Pallet Satellite (SPAS), STS-51, Discovery, are presented here along with laboratory experiments in an UHV-surface analysis system. The topics include ground simulation of selective and complex particle bombardment of optical coating analyzed by XPS as well as the verification of these results by flight experiments in combination with optical measurements (transmission, scattering).
Absorbing coatings for space-borne optical instruments must accept - besides optical requirements - unusual challenges regarding their chemical stability and their emission of gaseous and condensable materials under space conditions. Consideration of mass balances according to ANSI/ASTM E 595 do not provide enough information for proper choice of the coatings in the vicinity of sensitive optical elements. By in-situ mass spectrometric and XPS measurements we have investigated chemical changes of different black paints and inorganic coatings, their outgassing and the contamination of the surfaces near the coatings. In general, organic black paints are threatened by UV- irradiation and by thermal loads. Changes of their chemical composition under energetic UV-irradiation and the intense emission of contaminating materials were proved. The inorganic coatings investigated do not change their chemical composition and cause only comparatively small contamination on surfaces in their vicinity. For reducing the problem of contamination in optical systems it is still necessary to make available appropriate inorganic coatings with improved absorbing characteristics. However, if lower absorption can be tolerated, the use of inorganic coatings in sensible optical equipment should be considered already now.
A direct method for quantitatively controlling organic as well as inorganic contamination on optical surfaces of witness plates of only 1 cm<SUP>2</SUP> is suggested. Measurements by X-ray Photoelectron Spectroscopy (XPS) quantified by appropriate software give the quantity and chemical composition of contamination on any interesting surface with a typical detection limit of better than 10<SUP>-9</SUP> gcm<SUP>-2</SUP>. Results regarding the contamination growth on different optical materials and coatings under atmospheric conditions are presented. The contamination strongly depends on the chemical and structural properties of the surfaces. Therefore, reliable contamination control has to be done by sensitive measurements on either the surfaces of interest or on similarly treated resp. coated witness plates. To utilize the high sensitivity of the XPS-method, the witness plates have to be prepared and handled very carefully. Appropriate procedures for the contamination control of sensible optical surfaces as well as for the general control of clean rooms and fabrication facilities are described.