The demand of high resolution diffractive optical elements (DOE) is growing. Smaller critical dimensions allow higher deflection angles and can fulfill more demanding requirements, which can only be met by using electron-beam lithography. Replication techniques are more economical, since the high cost of the master can be distributed among a larger number of replicas. The lack of a suitable mold material for precision glass molding has so far prevented an industrial use. Glassy Carbon (GC) offers a high mechanical strength and high thermal strength. No anti-adhesion coatings are required in molding processes. This is clearly an advantage for high resolution, high aspect ratio microstructures, where a coating with a thickness between 10 nm and 200 nm would cause a noticeable rounding of the features. Electron-beam lithography was used to fabricate GC molds with highest precision and feature sizes from 250 nm to 2 μm. The master stamps were used for precision glass molding of a low T<sub>g</sub> glass L-BAL42 from OHARA. The profile of the replicated glass is compared to the mold with the help of SEM images. This allows discussion of the max. aspect-ratio and min. feature size. To characterize optical performances, beamsplitting elements are fabricated and their characteristics were investigated, which are in excellent agreement to theory.
Replication techniques for diffractive optical elements (DOEs) in soft materials such as plastic injection molding are state of the art. For precision glass molding in glasses with high transition temperatures, molds with extreme thermal resistivity, low chemical reactivity and high mechanical strength are needed. Glassy Carbon can be operated up to 2000°C making it possible to mold almost all glasses including Fused Silica with a transition temperatures above 1060°C. For the structuring of Glassy Carbon wafers photolithography and a RIE process is used. We have developed a process using Si as a hard mask material. If the flow rates of the etching gases O<sub>2</sub> and SF<sub>6</sub> are chosen properly, high selectivity of GC to Si 19:1 can be achieved, which provides excellent conditions to realize high resolution elements with feature size down to 1 micron and fulfills requirements for optical applications. We fabricated several multilevel GC molds with 8 levels of structuring. Two different optical functionalities were implemented: 6x6 array beamsplitter and 1x4 linear beamsplitter. The molds were applied for precision glass molding of a low T<sub>g</sub> glass L-BAL 42 (from Ohara) with a transition temperature of 565°C. Their optical performance was measured. A more detailed analysis of the impact of mold fabrication defects on optical performance is done. Rigorous coupled wave analysis simulations are performed, where we included fabrication constrains such as duty cycle, edge depth errors, wall verticality and misalignment errors. We will compare the results with the design specifications and discuss the influence of fabrication errors introduced during the different process steps.
A consumer market for diffractive optical elements in glass can only be created if high efficient elements are available at affordable prices. In diffractive optics the efficiency and optical properties increases with the number of levels used, but in the same way the costs are multiplied by the number if fabrication steps. Replication of multilevel diffractive optical elements in glass would allow cost efficient fabrication but a suitable mold material is needed. Glassy carbon shows a high mechanical strength, thermal stability and non-sticking adhesion properties, which makes it an excellent candidate as mold material for precision compression molding of low and high glass-transition temperature materials. We introduce an 8 level micro structuring process for glassy carbon molds with standard photolithography and a Ti layer as hard mask for reactive ion etching. The molds were applied to thermal imprinting onto low and high transition temperature glass. Optical performance was tested for the molded samples with different designs for laser beamsplitters. The results show a good agreement to the design specification. Our result allow us to show limitations of our fabrication technique and we discussed the suitability of precision glass molding for cost efficient mass production with a high quality.
We demonstrate implementation and performance of microdisplay systems based on liquid-crystal technology in a
variety of applications in holographic mastering. These displays can encode 2D objects information in grey scale or
address holographic patterns in amplitude or phase.
The main advantage is here to address any content dynamically with typically 60 Hz. Furthermore they show a resolution
up to 1920×1200 pixels with a pixel size as small as 6.4 microns. Therefore they are extremely suitable for a dynamic or
multi-exposure mastering process, to incorporate image content,
phase-encode objects or any holographic features. This
technology is already being used in holographic security applications as well as in commercial and display holography.
We report about a few applications/implementations and show experimental results and performance parameters.
The Physikalisch-Technische Bundesanstalt (PTB) with its laboratory at the electron storage ring BESSY II supports the
national and European industry by carrying out high-accuracy at-wavelength measurements in the EUV spectral region,
particularly to support the development of EUV lithography, which holds the key to the next generation of computer
technology. PTB operates an EUV reflectometry facility, designed for at-wavelength metrology of full-size EUVL optics
with a maximum weight of 50 kg and a diameter of up to 550 mm and a micro-reflectometry station for reflectometry
with sub 10 μm spatial resolution. An absolute uncertainty of 0.10 % is achieved for peak reflectance, with a
reproducibility of 0.05 %. For the center wavelength an uncertainty of 2 pm is achieved with a long-term reproducibility
of 1.1 pm and a short-term repeatability below 0.06 pm. Measurements at PTB use linearly polarized radiation, whereas
EUV optics are operated with unpolarized sources and the status of polarization changes throughout the optical system.
Therefore, to transfer these high-accuracy measurements to the EUV optical components under working conditions, it is
essential to study the polarization dependence in detail. The degree of linear polarization in the EUV reflectometer is
97%. Representative polarization dependencies obtained on Mo/Si multilayer coatings over a wide range of angles of
incidence reveal that the accuracy of calculations with the IMD-code is presently limited by the optical data available.
The development of EUV lithography depends strongly on the availability of suitable metrology equipment. The Physikalisch-Technische Bundesanstalt (PTB) with its laboratory at the electron storage ring BESSY II is the European centre of EUV radiometry and supports the national and European industry by carrying out high-accuracy at-wavelength measurements in the EUV spectral region, particularly to support the development of EUV lithography, which holds the key to the next generation of computer technology. To meet the requirements of industry, PTB operates an EUV reflectometry facility, designed for at-wavelength metrology of full-size EUVL optics with a maximum weight of 50 kg and a diameter of up to 550 mm and a micro-reflectometry station for reflectometry with sub 10 μm spatial resolution. An absolute uncertainty of 0.10 % is achieved for peak reflectance, with a reproducibility of 0.05 %. For the center wavelength an uncertainty of 2 pm is achieved with a long-term reproducibility of 1.1 pm and a short-term repeatability below 0.06 pm. To transfer these high-accuracy measurements to the EUV optical components under working conditions it is essential to study the polarization dependence. Measurements at PTB use linearly polarized radiation, whereas EUV optics are operated with unpolarized sources and the status of polarization changes throughout the optical system. PTB has therefore investigated and verified the capabilities of the EUV reflectometer for measurements with variable polarization. Taking advantage of all mechanical movements for detector and sample, measurements with arbitrary orientation of the electric field vector can be carried out up to an angle of incidence of 20°. We present representative polarization dependencies obtained on Mo/Si multilayer coatings, including measurements with a 45° orientation of the polarization to the optical plane of deflection to simulate the behavior for unpolarized radiation.
CZ SMT AG produced large off-axis EUV mirrors as they are used e.g. in ASML's alpha demo tools, the predecessor for Extreme Ultraviolet Lithography (EUVL) production tools by ASML. The coating development and a large part of the actual coatings were done by the FOM-Institute. The Physikalisch-Technische Bundesanstalt (PTB) operates an EUV reflectometry facility at the electron storage ring BESSY II for at-wavelength metrology of full-size EUVL optics with a weight of up to 50 kg and a diameter of 550 mm. Critical issues for EUVL mirrors are the high reflectivity close to the theoretical limit, the matching of the period to the operating wavelength of the stepper (13.5 nm) and the imaging properties of the EUV optics. The full multilayer stack needs to be controlled laterally to such extend that the initial sub-nanometre surface figure of the substrate is preserved. The so-called added figure error should not exceed 100 pm in order to ensure faultless imaging at 13.5 nm wavelength. Here, we discuss representative results obtained at large off-axis EUV mirrors. We especially discuss the challenges of measurements at higher local angles of incidence according to the optical design and the accuracy needed in sample alignment for measurement of the coating profiles. PTB has shown excellent reproducibility for measurements of the near normal incidence reflectance of flat homogeneous mirrors over several years. For large off-axis EUV mirrors, measurements have to be done at angles significantly off normal, which dramatically increases the influence of angular alignment errors of the sample on the measured peak wavelength. Furthermore, according to the optical design, these optics have gradients of the coating thickness which require exact knowledge of the measurement position in the mirror coordinates. Extensive studies were done to estimate and validate the uncertainties connected to the sample alignment. Our results clearly show that it is possible to meet and verify the tight specifications for the lateral coating profiles of EUV multilayer mirrors. The non-correctable added figure error is significantly better than required and the overall reflectance of the coatings with a special protective capping layer is 65%.
The development of EUV lithography is critically based on the availability of suitable metrological equipment. To meet the industry's requirements, the Physikalisch-Technische Bundesanstalt (PTB) operates an EUV reflectometry facility at the electron storage ring BESSY II. It is designed for at-wavelength metrology of full-sized EUVL optics with a maximum weight of 50 kg and a diameter of up to 550 mm. A micro-reflectometry station was installed for reflectometry with high spatial resolution for, e.g., structured masks. A photon beam size of 10 μm FWHM has presently been achieved. To meet the increasing demands of metrology for future lithography production tools, the measurement uncertainty was permanently reduced. For peak reflectance, a total uncertainty of 0.10 % is achieved with a reproducibility of 0.05 %. The uncertainty of 2 pm in the center wavelength is given mainly by the uncertainty for the reference wavelength of the Kr 3d5/2-5p resonance. A long-term reproducibility of 0.8 pm has been demonstrated over a period of about 4 years. We have recently demonstrated repeatability below 0.06 pm. This good repeatability is important for the determination of the coating-thickness gradient in alpha-tool optics. We present a long-term series of measurements at a set of EUV mirrors and discuss our recent results in improving wavelength reproducibility.