Removing Mid-Spatial Frequency Errors with VIBE

The Optimax VIBE process is a full-aperture, conformal polishing process incorporating high frequency motion that rapidly removes sub-surface damage in a VIBE pre-polish step and eliminates mid-spatial frequency (MSF) errors created by deterministic polishing in a VIBE finishing step. This Phase I feasibility study will focus on the VIBE finishing step to remove undesirable MSF errors while at the same time maintaining the desired low spatial frequency form accuracy.

Non-NASA commercial applications include high energy laser systems, EUV optics (lithography), imaging systems and X-ray synchrotrons. High energy laser applications, such as Inertial Confinement Fusion National Ignition Facility (NIF) at Lawrence Livermore National Laboratory are susceptible to mid-spatial frequency errors. The MSF errors are a source of damaging intensity, specifically in the region of 120μm 33mm. In EUV lithography, flare is a significant problem. Flare is directly associated with mid-spatial frequency error. The mid-spatial frequency errors cause light to scatter into small angles and reduce image contrast. The specific mid-spatial frequency region of interest to the EUV lithography community is between 1μm 2mm. The mid-spatial frequency error scales as 1/(lambda)^2, which causes an increasingly significant problem as the lithography industry heads toward shorter and shorter wavelength systems.

VIBE technology to reduce and/or eliminate mid-spatial frequency errors has potential to be used for optics in many applications. Specifically those applications that are susceptible to small angle scatter sometimes referred to as flare. The International X-Ray Observatory (IXO), consisting of 361 grazing incidence nested, azimuthally segmented shell mirrors, is susceptible to mid-spatial frequency errors. These thin (0.4mm) mirrors are produced through a thermal slumping technique where a thin glass substrate replicates the shape of a mandrel (fused quartz or stainless steel). The current budgeted error for the IXO mandrels is 1.4nm rms over the 2 20mm spatial frequency range. In addition, exo-planet imaging systems require minimal scattering due to mid-spatial frequency errors on their primary and secondary mirrors. An example is the specification for the Jovian planet finder optical system was less than 1nm rms in the 4 50cycles/aperture range.

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