Freeform optics have had a significant impact in airborne systems and other applications and are a key enabler for spaceborne systems. Their complex shapes allow the production of optical systems with reduced physical size and weight, together with improved performance. The sophisticated fabrication technologies that have enabled freeform optics also give rise to manufacturing challenges not encountered in the production of traditional, rotationally symmetric spherical and aspheric optics. Since manufacturing complexity has both a cost and lead time impact, it’s useful to have some means to predict how readily a specific design can be fabricated. This paper shows how an analysis of low- and mid-spatial frequency errors on various types of aspheric components leads to the use of rate of change of surface curvature as an easily derived and useful predictive metric to manufacturing complexity.
Aluminum telescopes are usually considered only for IR wavelengths, due to the inability of aluminum to be surfaced to the surface quality and roughness needed to minimize scatter at visible wavelengths. Yet Al6061T6, a material routinely specified into IR optical sensors due to its heritage, high yield strength, and fracture toughness, has been economically surfaced to equivalent mid and high spatial frequency error as glass surfaces typically specified into visible wavelength sensors. However, microroughness data can correlate poorly with scatter performance where microdefects dominate scatter. To show the performance of aluminum surfaces for visible wavelength applications, BRDF data is measured for low roughness Al6061T6 surfaces. The impact of the BRDF derived scatter for Al6061T6 is evaluated for a nominal TMA telescope.
Laser-produced plasma (LPP) sources for extreme ultraviolet lithography (EUVL) systems utilize CO2 lasers operating
with wavelength 10.6μm. Since multilayer-coated optics have high reflectivity for this infrared radiation (IR), a
significant and detrimental amount of IR is passed through the EUVL system. One method to remove the IR from the
system is to utilize a binary diffraction grating. When this grating is applied directly to the surface of the primary
collector optic of the source, the majority of the IR is diverted outside the radius of the exit aperture at the intermediate
focus (IF). This paper will report details on the performance of a full size (410mm diameter) Demonstration Collector
utilizing IR rejection (IRR) technology with the capability to produce over 125X suppression of IR, equaling the
performance of a IR spectral filter. Additional details will be reported on the technology development and use of a
glassy smoothing layer to enable high EUV performance, a weighted average multilayer reflectance of 50.9% for
unpolarized EUV radiation.
Polished 1.5m bare beryllium, off-axis aspheric mirror segments, constituting the cryogenic primary
mirror of NASA's ambitious Flagship Mission, James Webb Space Telescope (JWST), have been
successfully completed at L-3 Communications -Tinsley. Tinsley has finished the secondary, tertiary, fine
steering and spare mirrors as well. We will describe both the end results, where it was demonstrated that
visible quality mirror results can be achieved on large extremely lightweighted compliant off-axis mirrors,
and the steps taken at Tinsley to achieve these results. Over 26 square-meters of bare beryllium were
optically processed twice, first for room temperature figure, then for the cryo-null figure for the cryogenic
differences.
Latest progress on Tinsley methods are described for faster stress mirror polishing (SMP) of the Thirty Meter Telescope
(TMT) primary mirror segments as well as the European Extremely Large Telescope (EELT). Most recent data is shown
which illustrates that the SMP process is capable of producing very smooth surfaces to 1um PV, independent of the
segment type being produced.
In the preceding part I of this paper stressed mirror polishing was stated as one of the processes assumed for the polishing of
non-axisymmetric mirror blanks like those for the two ELT projects (the ESO E-ELT and the TMT). For this process it is
important to have a precise knowledge of the elastic behavior of the glass ceramic mirror substrate materials. In reality
glasses and glass ceramics do not react instantaneously to stresses at room temperature. This effect is called "delayed
elasticity".
It was shown that the delayed elasticity effect of ZERODUR® is small in size (less than approximately 1% of the applied
deformation) and fully reversible in time. A mathematical model on the relaxation of shear modulus and bulk modulus of
ZERODUR® has been introduced to predict the delayed elasticity at room temperature and different load cases. This second
paper is focusing on an updated model approach with the target to improve the model prediction accuracy. The model
results will be compared to measurements of the effect on a 1.5 m E-ELT mirror blank at L-3 Communications, Tinsley.
Latest progress on Tinsley methods are described for faster stress mirror polishing of the Thirty Meter Telescope primary mirror segments. These methods are outlined, and full scale segment data results are presented. The Tinsley SMP process complements additional processes at ITT Industries Space Systems, with the potential to effectively optically finish all TMT segments.
During 2009, Tinsley finished most of the Configuration 1 pre-cryo test Computer Controlled Optical Surfacing (CCOS)
operations on the James Webb Space Telescope primary mirror segments and in mid-2009 we began the Configuration 2
post-cryo test CCOS operations. After completing the grinding and polishing operations, including final figuring to a
cryo-null target, we delivered the finished Engineering Development Unit (EDU) to Ball Aerospace Technology
Corporation on 4 December 2009. Achieving fabrication and metrology conditions to meet the specifications for this off-axis
~1.5 m hexagonal point-to-point segmented mirror required special methods. Achieving repeatable and accurate
interferometric alignment of the off-axis aspherical mirror surface and stable thermal gradient control of the beryllium
substructure during tests required rigorous component and system-level validation. Final optical wavefront
measurements over the various spatial frequency ranges have demonstrated that all of the requirements are met. This
success has validated our processes of fabrication and metrology and allows us to proceed with the production of the 18
flight mirror segments. The first finished flight mirror, the Tertiary Mirror, was shipped to BATC on 24 February, 2010.
Performance of that mirror is reported here also.
JWST optical component in-process optical testing and cryogenic requirement compliance certification, verification &
validation is probably the most difficult metrology job of our generation in astronomical optics. But, the challenge has
been met: by the hard work of dozens of optical metrologists; the development and qualification of multiple custom test
setups; and several new inventions, including 4D PhaseCam and Leica Absolute Distance Meter. This paper summarizes
the metrology tools, test setups and processes used to characterize the JWST optical components.
Advanced shapes can now be produced for the corrective optics placed near a reimaged pupil, or even a deformable
mirror surface. These surfaces can be improved and even apodization added to improve contrast. In this paper, we
describe a special form of Narrow Ion Beam Figuring (NIBF) developed at L-3 Tinsley. In contrast to existing Ion Beam
Figuring (IBF) machining schemes, the FWHM beam width is controlled in a much narrower band while still providing
high beam currents.
The EUV optical system of the Reticle Imaging Microscope (RIM) for EUV mask inspection consists of a pinched Xeplasma source, a pupil-relayed Koehler-type illumination system and an equal-radii Cassegrain-type microscope with a 10x magnification1.
The 3D surface topologies were characterized over spatial wavelengths ranging from the clear apertures down to a few nanometers by using a portfolio of instruments including contacting profilometry, phase-shifting interferometry at 633 nm at various magnifications and Atomic Force Microscopy. Measured 3D topography maps were Fourier analyzed and Power Spectral Densities (PSDs) are computed over spatial periods ranging from the critical aperture down to a few nm. Integrated RMS surface errors over typically reported spatial period ranges were computed. For a different optical system we improved our polishing process to reduce surface errors for spatial periods below 10 mm. PSDs and integrated RMS surface errors will be shown in comparison with typical RIM surfaces.
All surfaces of the RIM optical system were coated with high-reflectivity coatings to maximize optical throughput. A description of the coatings and their performance had been published recently by Michael Kriese et al.2 The transmitted wavefront error (TWF) of the imager module was measured in a double pass configuration using a Fizeau-type Interferometer at 633 nm wavelength and a convex retrosphere. The measured TWF will be shown over the entire Numerical Aperture (NA = 0.0625) of the microscope. The integrated RMS of the TWF measured 0.79 nm.
To perform actinic inspection of patterned EUV reticles with diffraction-limited resolution at 13.5 nm wavelength aspheric optical surfaces with surface figure errors and roughnesses well below 1 nm had to be developed.
The 3D surface topologies of prototype optical components were characterized over spatial periods ranging from the clear apertures down to 25 nanometers over 6 orders of magnitude by using a portfolio of instruments.
3D topography maps were Fourier analyzed and averaged Power Spectral Densities (PSDs) computed over the entire spatial frequency range. A good fit to the PSD was achieved with a linear function on a log-log scale. RMS values were computed over several spatial period ranges.
All optical surfaces were coated with high-reflectivity coatings to maximize optical throughput at 13.5 nm for the average angle-of-incidence of each optic. The spectral reflectivity of the HR coatings, consisting of Molybdenum-Silicon bi-layers (40 periods) were measured using synchrotron instruments at the NIST/DARPA EUV Reflectometry Facility and the Center for X-Ray Optics at Lawrence Berkeley National Laboratory. Total variations (PV) of peak-position within the clear-apertures ranged from 0.005 nm to 0.020 nm, with the one exception being a highly-curved convex surface yielding a PV variation of 0.040 nm. Peak reflectivity variation was typically 0.2% to 1% PV over the clear aperture, with some of the variation being instrument precision. One optic was coated with Ruthenium only, approximately 16nm thick, with less than ±0.1 nm variation in thickness. Detailed information on the spectral reflectivity for the coatings is discussed.
The complexity of photomasks is rapidly increasing as semiconductor devices are scaled down and optical proximity correction (OPC) becomes commonplace. Raster scan architectures are well suited to the challenge of maintaining mask throughput despite these trends. Electron-beam techniques have the resolution to support OPC requirements into the foreseeable future. The MEBES® eXara mask pattern generator combines the resolution of a finely focused electron probe with the productivity and accuracy of Raster Graybeam patterning. Features below 100nm can be created, and OPC designs are produced with consistent fidelity. Write time is independent of resist sensitivity, allowing high-dose processes to be extended, and relaxing sensitivity constraints on advanced chemically amplified resists. The system is designed for the production of 100nm photomasks, and will support the development of 70nm masks.
The write time of an ALTA 3000HT mask writer has been observed to be up to 36% better than that of the ALTA 3000 system. The ALTA 3000HT system enables users to meet their performance requirements at increased production capacity with the use of new writing strategies. The ability to change between eight and four averaging passes, as well as the addition of key hardware improvements, gives users increased flexibility in meeting the throughput and print quality requirements for high-volume mask manufacturing. Observed throughput and print performance data, as well as benefits analysis and cost of ownership data, are presented.
New writing strategies have been developed to meet the demand for high-volume mask manufacturing. The ALTA® 3000HT system enables users to meet their performance requirements at increased production capacity. The write time of an ALTA 3000HT mask writer has been observed to be substantially shorter than that of the ALTA 3000 system. The ability to change between eight and four averaging passes, as well as the addition of key hardware improvements, give users increased flexibility in meeting the throughput and print quality requirements for volume production mask manufacturing. Observed throughput and print performance data are presented.
A three-aspherical mirror system for extreme ultraviolet lithography (EUVL) has been developed. The mirrors were fabricated using a computer-controlled optical surfacing (CCOS) process and a phase-shift interferometer. The figure error of the mirrors is 0.58 nm. To achieve a high reflectivity in the clear aperture, Mo/Si multilayer films with an optimized d-spacing were successfully deposited on the mirrors. These results show that we have nearly achieved the target specifications for EUVL mirrors.
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