There is growing interest at synchrotron light and X-ray free electron laser facilities to explore and improve the dynamic performance of piezoelectric bimorph deformable X-ray mirrors. Many beamlines, especially those dedicated to Macromolecular Crystallography, need to measure hundreds of samples per day. Shorter acquisition time requires rapid changes in the focus of the X-ray beam to condense the maximum photon density onto the sample. This is necessary to match the X-ray beam to the dimensions of the sample, or to probe variable sized regions of larger samples. Fine control of the X-ray beam becomes crucial for ensuring the highest quality of scientific data and increased throughput. Previous work at Diamond Light Source successfully changed the X-ray beam focus and stabilised it in under 10 seconds using piezoelectric bimorph deformable mirrors. Further updates to the controls software of the programmable HV-ADAPTOS high-voltage power supply (from CAEN / S.RI. Tech) now make it possible to control individual electrodes at 1 Hz using custom voltage profiles. This allows localized compensation of piezo creep, thus improving X-ray beam shape, significantly reducing stabilisation time, and eliminating curvature drift. For ex-situ validation, dynamic changes in the surface of the bimorph mirror need to monitored in real-time with sufficient spatial sensitivity. In this paper, we show that the active optical surface of a bimorph mirror (from Thales-SESO) can be accurately changed with sub-nanometre height sensitivity by dynamically monitoring the mirror’s surface using an array of high-speed (up to 200 kHz) Zygo ZPS™ absolute interferometric displacement sensors mounted in an independent metrology frame.
The need for smaller focal spot sizes to meet the demands for higher resolution imaging is driving the adoption of adaptive optics in x-ray beamlines. Closed-loop control of the mirror shape, position, and orientation can greatly enhance the performance of these optics by allowing for rejection of perturbations. We demonstrate the performance of an array of interferometric absolute position sensors as a means of providing real-time feedback on shape changes of the reflecting surface of a bimorph mirror by comparison to a Fizeau interferometer.
The evaluation of the measurement uncertainty of a robust all-fiber-based low-coherence interferometer for the measurement of absolute thickness of transparent artifacts is described. The performance of the instrument is evaluated by measuring the length of air-gaps in specially constructed artifacts and the observed measurement errors are discussed in the context of the uncertainty associated with them. A description of the construction of the artifacts is presented, accompanied by an uncertainty analysis to estimate the uncertainty associated with the artifacts. This analysis takes into account the dimensional uncertainty of the artifacts (including wringing effects), thermal effects, and effects of the environment on refractive index. The 'out-of-the-box' performance of the instrument is first evaluated. A maximum error of 350 nm for an air-gap of 10.1 mm is observed. A linear trend between the measured length and the error is also observed. The relative magnitude of the errors and the uncertainty associated with the error suggests that this trend is real and that a performance enhancement can be expected by mapping the error. Measurements of the artifacts are used to develop an error map of the instrument. The uncertainty associated with the predicted error is determined based on the uncertainty associated with the error. This analysis suggests that the uncertainty in the predicted error at the 2σ level may be conservatively estimated to be (2.9L+37.5) nm, where L is in units of mm.
Microlithographic systems rely on precision alignment and a high-level of dimensional stability to achieve required performance. In critical applications, immunity to thermally induced dimensional changes is achieved by the use of low linear coefficient of thermal expansion (hereafter referred to as CTE and denoted by a) materials such as ULE in components such as reflective optics and machine structures. ULE has a CTE that is typically in the 0 + 30 ppb K-1 range and it may be engineered to achieve a specific value. A high-accuracy determination of the CTE is essential for both process control and for providing an essential input to the design of such systems for error budgeting purposes. Currently, there is a need for CTE determination with an expanded uncertainty U(a)(k=2) < 1 ppb K-1 in the 273-373 K temperature range. A survey of the state-of-the-art of high-accuracy absolute measurement of CTE is presented along with a discussion of the significant error sources in each of the current techniques. The metrology techniques, sample design and instrumentation are described along with uncertainty estimates for representative instruments. The design philosophy and prospects for a new instrument that satisfies the above mentioned need are described.