Elastic Emission Machining of Substrates for EUV, Optical, and Infrared Optics

The need for highly accurate and cost efficient deterministic polishing methods on a variety of materials is apparent. Diffraction limited, low cost, and low weight normal-incidence mirror systems are required to enabletelescopes for missions of all sizes, ranging from CubeSatsto Probe and Flagship missions. Asolution that is applicable to any size and material is crucial. Elastic Emission Machiningcan achieve these tolerances and is scalablewhile remaining highly precise with highly stable removal rates. EEM is a chemical polishing method which causes low-energy, non-damaging collisions to occur between particulates and the substrate surface. These gentle collisions do not directly remove material from the surface. Chemical reactions occur between the particulates and the surface whereby the particles become chemically bound to the surface atoms. The fluid in which the particles are immersed is accelerated to flow and produces a sheering force on the bound particle, which pulls the particle and its attached substrate atom from the substrate, thereby producing an atomic-level machining operation. This type of material removal process does not inject heat, nor does it damage the atomic lattice structure, preventing sub-surface damage. Due to the material being removed efficiently from the surface by the polishing particulate, a Beilby layer is not allowed to form and heal over the optical surface. During the Phase I effort it wassuccessfully demonstrated on Ultra Low Expansion Glass (ULE)and Zerodur. Mirror support through mitigation on light-weighted ULE and Zerodur substrates was also a goal which OptiPro wished to complete in this Phase I project. Support print through is a problem with light-weighted mirrors in which the support structures are apparent in the mirror surface whichEEM can address and solve. These factors were found to be successfully mitigated during the project and successful correction of light-weighted mirrors was accomplished. The need for highly accurate, cost efficient, deterministic polishing methods on a variety of materials is apparent. Diffraction limited and low weight normal-incidence mirror systems are required to enable and enhance telescopes for missions of all sizes, from CubeSats at sub 100mm optic sizes to Probe or Flagship missions which need mirrors and mirror segments on the order of several meters. These missions also vary in operating wavelength; from ultraviolet to far-infrared. Light-weighting of mirrors is also a key performance objective. With light-weighting, print through of the support structure onto the optical surface can occur, reducing the quality and imposing difficult to correct surface figure errors. In this proposal, we offer a means to both achieve nanometer tolerances on standard optical surfaces and capable of mitigating support print through on light-weighted mirrors. This process, Elastic Emission Machining, is a highly precise and stable chemical material removal method and will be accomplished on Zerodur and ULE, common materials used for their thermal stability. Elastic Emission Machining has been proven through the Phase I project to be capable of polishing Zerodur and ULE substrates. EEM causes low-energy non-damaging collisions to occur between particulates and the substrate surface. This type of material removal process does not inject heat into the surface, nor does it damage the atomic lattice structure, allowing for no sub-surface damage. The polishing of light-weighted mirrors was also proven to be a success, leaving smooth surfaces where other polishing methods introduce print-through errors into the mirror surface. The technical objectives of the Phase II project focus on overall improvements to the stability of the EEM polishing process, and polishing full aperture mirror substrates to the following tolerances: Mid-spatial frequency (6 - 60 cycles per aperture (CPA) errors <5nm RMS High-spatial frequency (>60 CPA) <1.5nm RMS Microroughness <0.5nm RMS The deliverables at the end of the Phase II are a large meter class EEM polishing platform capable of polishing optics up to 1 meter in diameter, improve the EEM polishing process to achieve the above mirror tolerances, full aperture 300mm x 300mm mirror substrates made to the prescriptions of James Webb and Hubble Space Telescopes, and the final report, which will include detailed results of the above goals, stating how each was attempted and ultimately met.
Benefits: With the capability to manufacture ULE and Zerodur optical components to the diffraction limit, NASA will be able to ensure that their telescopes, CubeSats, and Flagship missions have highest opportunity for success, enabling and enhancing future systems. OptiPro’s Elastic Emission Machining Platform, with the success of this proposed SBIR project, will be capable of manufacturing any light-weighted optical component, made of Silicon, ULE or Zerodur, to the diffraction limit of their operating wavelength. EEM has proven its capabilities on diffraction limited Silicon polished for use in the Department of Energy’s Synchrotron and Linear X-ray accelerators, which also use Zerodur and ULE in their beamline systems. Extreme Ultraviolet Lithography systems also use Zerodur and ULE in their systems and with the ever-growing need for microchips, more efficient and accurate optics are needed.