Optical Metrology for High Performancec X-Ray Mirrors

An often heard saying in optical fabrication shops is that when it comes to optics, “if you can measure it you can make it”. The converse is equally true, and poor or inadequate surface metrology more often than not leads to poorly figured optical surfaces. As a germane example, fabrication issues stemming from inadequately measured X-ray beam-line mirrors currently limit the focal quality of the resultant X- ray spot. Indeed, to obtain nanometer-scale spot sizes the peak-to-valley surface error of such 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, OptiPro has conceptualized a novel metrology system that appears capable of reaching the 100 pm target. The heart of the system is our new chromatic interferometric probe, constructed under a recently-completed Phase I SBIR project at NASA, which, when adapted for use for X-ray mirror metrology is anticipated to have a base displacement-measuring performance of 19 pm. Combining the probe with a unique referencing and spatial localization metrology sub-system results in a sub-aperture surface topography measurement system having 40 pm performance. Finally, when this sub-system is utilized with an optimized stitching algorithm, simulations have shown that a long meter-class X-ray mirror can be measured to 100 picometers over the entire length of the mirror. In Phase I we propose to construct a testbed that substantially de-risks the proposed metrology technology and lays the groundwork for a Phase II project. Specifically, the testbed includes a scaled down metrology system in which a 2” cylindrical test object having 30m of surface sag is measured. Work tasks include activities directed at proving the 19 pm base performance of the probe, the 40 pm sub-aperture performance, as well evaluations of the stitching algorithm that will indicate whether 100 pm metrology over a meter-scale optic is feasible. The performance data and metrics obtained in Phase I, due to budget and time constraints, are limited to repeatability, which is also a good proxy for the ultimate accuracy of a well-referenced well-engineered metrology system. Accordingly, accuracy considerations are deferred to Phase II. The proposed surface-measuring system, when successful, 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 will benefit most, including those operating in the short wavelength (e.g., UV and X-ray) regimes, or have large apertures such as telescope mirror segments or projection optics used in semiconductor fab equipment (i.e., “steppers”). The proposed metrology testbed will be designed, constructed, and tested at OptiPro facilities in Ontario, NY, just outside of Rochester. The project is expected to last 9.5 months.