Corning’s ULE® is an ultra-low expansion glass used for machine tool blocks to astronomical mirrors. Its primary
alternative is a glass ceramic. In many applications, ion beam milling is used for final surface figuring. Ion milling
removes material at an atomic level and is typically a slow, expensive process. Experiments have determined the upper
limits of removal rate for ion beam milling during optical figuring. The goal was to increase the power density of the ion
beam during figuring to achieve higher removal rates with no negative effects on surface properties. Testing shows that
the removal rate on ULE® is about 50% higher than on glass ceramics under the same conditions. With an increase in
material removal rate, both ULE® and the glass ceramic show an increase in surface roughness. Average birefringence
of both materials increases slightly after milling; however the level of birefringence in the glass ceramic is seven times
larger than for ULE®. Therefore using higher ion milling power densities, the surface figuring of ULE® can be
accelerated to produce shorter processing times without adverse effects on surface properties. This can help lower the
cost for manufacture of ULE® optics.
Corning has focused its recent efforts on coefficient of thermal expansion (CTE) metrology improvements. Due to the unique environment required for EUVL technology, EUVL optics (and photomasks) require extremely uniform CTE properties, with targeted variations of less than 1ppb/K. Until now, no practical metrology technique existed that could accurately verify if a material met such requirements due to the lack of precision. Corning has previously introduced the idea of measuring CTE in ULE (registered trademark) Glass using Phase Measuring Interferometry (PMI) by discovering the correlation between refractive index and CTE in ULE (registered trademark) Glass. However, refinement of the correlation was necessary. This paper focuses on the progress made towards that end, which has resulted in the ability to non-destructively measure peak to valley CTE variations to within 57 parts per trillion per degree Kelvin (ppt/K) at possible spatial resolutions in the micron range on thick or thin samples.
A method for measuring symmetric aberrations in large departure aspheric surfaces to nearly single digit nanometer precision is demonstrated. Interferometry can accurately measure plano, spherical and small departure aspheric surfaces. However, null correction is normally required for accurate interferometric measurement of large departure aspheres. When using conventional null lenses, asymmetric aberrations are easily measured by simply rotating the surface under test to a finite number of positions and comparing them to one another. The rotationally symmetric errors are more difficult to know with certainty due to possible systematic rotationally symmetric errors with the null lens itself. The proposed system can measure aspheres on planar to f/6.0 spherical surfaces with a maximum sag of 1 mm and from 800 mm to 25 mm spherical surfaces down to f/0.55. A non-contact interferometric probe is used to measure the surface profile with the optic mounted on either a linear or rotary air bearing, depending on the base radius of curvature of the optic. Measurement results are shown for several aspheres and compared with interferometer measurements.
As optical lithographers push to extend optical lithography technologies to create smaller features with higher NA, lower k1 values and shorter wavelengths, transmitted wavefront specifications for HPFSR fused silica blanks continue to tighten. HPFSR fused silica blanks are typically certified for acceptance using an interferometer operating at a wavelength of 632.8 nm. As the market demands increasingly tighter homogeneity specifications, it has become critical to understand the sources of variation in wavefront measurements. Corning has recently initiated a study to identify those sources of variation. One glass attribute being studied is the impact of residual stress on the wavefront. It is known that residual stresses can alter the refractive index of fused silica. To obtain the residual stress measurements, birefringence measurements were obtained at 632.8 nm for comparison to wavefront measurements at 632.8 nm. The relationship between residual birefringence and transmitted wavefront measurements, at 632.8 nm on Corning HPFSR fused silica blanks, is explored in this paper.
Interferometrically measuring the index of refraction variation (index homogeneity) of glass blanks requires that the blanks be made transparent to the interferometer laser. One method for achieving this is to 'sandwich' a rough ground blank between two polished flats while adding an index matching liquid at each surface interface. This is better known as oil-on-flat (OOF) or oil-on-plate testing. Another method requires polishing both surfaces and is better known as polished homogeneity (PHOM) testing or the Schwider method. Corning Inc. historically has used OOF testing to measure the index homogeneity of disk-shaped, fused silica boules over multiple 18' diameter apertures. Recently a boule polishing and PHOM testing process was developed by Corning for measuring the homogeneity over 24' diameter apertures to support fused silica production for the National Ignition Facility (NIF). Consequently, the PHOM technique has been compared to the OOF process using a number of different methods including repeatability/reproducibility studies, data stitching, and vibration analysis. The analysis performed demonstrates PHOM's advantages over OOF testing.
The performance of various passive and active vibration control devices for equipment aboard spacecraft is analyzed. Several vibration control mechanisms, including pure friction, spring- damper, resilient friction and active friction control systems, are studied. Peak responses of equipment and their light subsystems under spacecraft g-jitter excitation are evaluated, and the results are compared to those for a payload with a fixed-base (rigidly attached) condition. For the frictional vibration control mechanisms, the nonlinear Coulomb model is used in the numerical simulation and the stick/slip condition at each time step is carefully evaluated. A tentative design for a new active friction control mechanism, by varying the normal force, is described. The device uses a velocity feedback to control the normal interface force with the aid of a magnet. By eliminating and/or minimizing the stick duration, the overall payload response reduces. Sensitivity of the performance of various vibration isolation mechanisms, to variations in the isolator properties, spacecraft excitation amplitude and equipment and subsystem frequency is also analyzed. The results show that the passive and active vibration control devices considered are generally highly effective in controlling the g-jitter vibration. In addition, these vibration control systems are insensitive to small deviations from their design conditions.
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