Over the past few years there have been an increasing number of applications calling for ultra-low roughness (ULR)
surfaces. A critical demand has been driven by EUV optics, EUV photomasks, X-Ray, and high energy laser applications. Achieving ULR results on complex shapes like aspheres and X-Ray mirrors is extremely challenging with conventional polishing techniques. To achieve both tight figure and roughness specifications, substrates typically undergo iterative global and local polishing processes. Typically the local polishing process corrects the figure or
flatness but cannot achieve the required surface roughness, whereas the global polishing process produces the required
roughness but degrades the figure. Magnetorheological Finishing (MRF) is a local polishing technique based on a
magnetically-sensitive fluid that removes material through a shearing mechanism with minimal normal load, thus removing sub-surface damage. The lowest surface roughness produced by current MRF is close to 3 Å RMS. A new
ULR MR fluid uses a nano-based cerium as the abrasive in a proprietary aqueous solution, the combination of which
reliably produces under 1.5Å RMS roughness on Fused Silica as measured by atomic force microscopy. In addition to the highly convergent figure correction achieved with MRF, we show results of our novel MR fluid achieving <1.5Å
RMS roughness on fused silica and other materials.
EUV mask substrates, made of titania-doped fused silica, ideally require sub-Angstrom surface roughness, sub-30 nm
flatness, and no bumps/pits larger than 1 nm in height/depth. To achieve the above specifications, substrates must
undergo iterative global and local polishing processes. Magnetorheological finishing (MRF) is a local polishing
technique which can accurately and deterministically correct substrate figure, but typically results in a higher surface
roughness than the current requirements for EUV substrates. We describe a new super-fine MRF® polishing fluid
whichis able to meet both flatness and roughness specifications for EUV mask blanks. This eases the burden on the
subsequent global polishing process by decreasing the polishing time, and hence the defectivity and extent of figure
distortion.
Aspheric surfaces can provide significant benefits to optical systems, but manufacturing high-precision aspheric surfaces
is often limited by the availability of surface metrology. The lack of 3D surface data required to drive aspheric
manufacturing equipment can create risk and unwanted variation in the manufacturing process. One typical approach to
gathering this 3D data is using dedicated null correction optics in addition to the interferometer itself. However, the
cost, lead time, inflexibility, and calibration difficulty of such null optics makes interferometric aspheric testing a far less
attractive solution than the relatively simple spherical test. Subaperture stitching interferometry was originally developed
to allow for the full-aperture 3D measurement of large-aperture spheres and flats using commercially available
interferometers and transmission elements1, 2, 3 The method was then extended to the measurement of mild aspheric
surfaces, by exploiting the local best-fitting and magnification of the high density fringe patterns associated with nonnull
interferometry.4 Subaperture stitching interferometry was then extended by an order of magnitude through the use
of a Variable Optical Null (VON) that allowed the measurement of high-departure aspheres. The automated VON has an
optical system with a range of motion control that generates an optical wavefront that closely matches the surface of the
asphere for each subaperture. The residual wavefront error is measured with a standard interferometer, and the fullaperture
surface profile of the asphere is reconstructed using advanced stitching algorithms. This method allows for the
accurate measurement of aspheres with more than 1000 waves of departure from best-fit sphere, without the use of
dedicated null lenses.
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