Optical Metrology of High Performance X-Ray Mirrors

The performance of the X-ray beam-lines at the Department of Energyĺs synchrotron light sources is limited by the quality of the mirrors in those beam-lines, wherein the quality of the mirrors in turn is limited during their manufacture by the capabilities of the metrology equipment used to measure the mirror surfaces. In the proposed Phase IIB project a novel metrology system will be constructed and tested in order to prove that the underlying technology can obtain X-ray mirror surface measurement accuracies at or better than 100 picometers. This new metrology system, when incorporated into the mirror fabrication process, is thought to be sufficient to completely resolve the beam-line limitations caused by the mirror. Researchers using X-ray beam-lines know that to obtain diffraction limited nanometer-scale focal spot sizes the peak-to-valley surface error of the beam-line mirrors must be approximately a nanometer, and the metrology error, which should consume less than 10% of the surface error budget, should have performance on the order of 100 picometers. Unfortunately, state of the art surface metrology is currently no better than several nanometers. To address this metrology shortcoming, a metrology system has been conceptualized that utilizes a new confocal interferometric probe. Scanning the probe across multiple sub-apertures of the surface of the mirror, combining the probe with a unique referencing and spatial localization sub-system, and further augmenting the system with sub-aperture stitching methods, allows for the surface of an X-ray mirror to be measured to 100 picometers accuracy over its entire length. In Phase IIB we propose to demonstrate that the metrology technology can indeed perform at the 100pm accuracy level over the length of an X-ray mirror. Work tasks include integrating the sub-systems developed in Phase II, namely the measurement reference frame, machine model, refractive index sensor, and sub-aperture stitching algorithm into a testbed, and then evolving the engineering testbed into a system that can be used by an optician on the shop floor during the deterministic polishing of an X-ray mirror. The proposed surface-measuring system will significantly advance the state of the art of large-aperture optical surface metrology. While the system can measure the surface of nearly any optical element (e.g., lenses, mirrors, etc.), those devices having extraordinarily stringent surface tolerances over large surface areas will benefit most, such as X-ray beamline mirrors, space telescope mirror segments, and projection optics used in semiconductor fab equipment (i.e., ôsteppersö). The proposed Phase IIB project will be performed at OptiPro facilities in Ontario, NY, just outside of Rochester. The project is expected to last 24 months.