Ultra-precision substrate figuring for coherency-preserving x-ray mirrors

Recent advances in synchrotron photon sources and the introduction of the free electron laser have created x- ray sources of unprecedented brightness, coherency, and energy. These sources are at the leading edge in the quest to understand the structure and interactions of matter at atomic scales, and they are indispensables tools for science and industry. The challenge now is to transport such precise short-wavelength, “diffraction-limited”, x-ray beams from the source to the experiments without distortion and dissipation. Presently the quality of the x-ray beam available to researchers is primarily limited by figure errors and surface roughness of the optics and their thin film coatings that are caused by imprecision in their fabrication (deviations from ideal surfaces). Reaching the full potential of these sources will require near perfection for the mirrors used to focus and direct the beams. Indeed, surfaces need to be smooth to the dimensions of atoms, and the required maximum allowable figure error over the entire mirror is usually less than 1 nm -- essentially a pico-scale level of precision! Nevertheless, recent advances in metrology allows such small errors to be measured and mapped, and hence these tiny errors can be corrected if a suitable tool can be developed to deterministically apply a corrective, nanoscale etch to the surface with sufficient accuracy and without introducing new errors. American Physics and Technology LLC (APT) proposes to design, develop, and demonstrate a new tool for applying precise, 3-dimensional, corrective-etches to flat and curved, meter-sized, x-ray mirrors. By iteratively measuring figure errors and correcting using our tool, we will manufacture a demonstration supermirror with the required pico-scale accuracy and smoothness. APT’s approach uses two new patented surface-modification technologies based on treatment with ionized, electrostatically-accelerated beams of “cluster ions”. These were developed and studied in the semiconductor industry for the manufacture of next generation VLSI computer chips. The first technology, Gas Cluster Ion Beams (GCIB), bombards the surface with an intense, focused beam of nanoscale, hyper-velocity “clusters” of condensed gases. These Clusters are aggregations of weakly bound “monomers” (atoms or molecules), typically about 10,000 monomers and approximately 4 nm in diameter. The surface is smoothed and etched by the many overlapping impacts of the clusters in the beam. The velocity and size of the clusters can be adjusted over a wide range to produce surface effects from gentle plastic deformation up to breaking chemical bonds and sputtering/etching away the surface material. A distinctive and attractive feature of GCIB is the shallow nature of the interaction on the surface and the minimization of sub-surface damage. The second technology, Accelerated Neutral Atom Beams (ANAB), takes the cluster beam and disrupts it into a shower of individual uncharged monomers. ANAB is often used in combination with GCIB as the final step in smoothing and cleaning surfaces. ANAB gives the ultimate in uniform smoothing and etching with the minimum of subsurface damage. APT’s Phase-I proposal will demonstrate our ability to accurately apply a variable dose of GCIB/ANAB to any location of an x-ray mirror to correct the figure with sub-nanometer vertical precision. The ultimate lateral precision will depend on the size of the GCIB/ANAB beam, the scanner-resolution, and time. Our phase-I project will demonstrate our ability to make the beams as small as 0.5 mm with the capability of varying the etch rate with sufficient accuracy and reproducibility. Advancing the state-of-the-art for x-ray optics will find wide use at the more than sixty x-ray facilities worldwide and will lead to other applications in ultra-precision fabrication.