Pushing the limits: latest developments in angle metrology for the inspection of ultra-precise synchrotron optics
The requirements on the quality of ultra-precise X-ray optical components for application in the Synchrotron Radiation (SR) community are increasing continually and strongly depend on the quality of the metrology devices available to measure such optics. To meet the upcoming accuracy goal of 50 nrad rms for slope measuring profilers, a dedicated project, SIB58 Angles, consisting of 16 worldwide partners and supported by the European Metrology Research Programme (EMRP) was started in Sep 2013. The project covers investigations on autocollimators under extremely challenging measuring conditions, ray-tracing models, 2D autocollimator calibration (for the first time worldwide), determination of error sources in angle encoders providing traceability by ‘sub-division of 2π rad’ with nrad uncertainty, angle generation by 'ratio of two lengths' in nrad level, and on the development of portable precise Small Angle Generators (SAGs) for regular in-situ checks of autocollimators’ performance. Highlights from the project will be reported in the paper and the community of metrology for X-Ray and EUV Optics will be informed about its progress and the latest work in angle metrology.
A novel instrument for generating angular increments of 1 nanoradian.
Accurate generation of small angles is of vital importance for calibrating angle-based metrology instruments used in a broad spectrum of industries including mechatronics, nano-positioning, and optic fabrication. We present a novel, piezo-driven, flexure device capable of reliably generating micro- and nanoradian angles. Unlike many such instruments, Diamond Light Source's nano-angle generator (Diamond-NANGO) does not rely on two separate actuators or rotation stages to provide coarse and fine motion. Instead, a single Physik Instrumente NEXLINE "PiezoWalk" actuator provides millimetres of travel with nanometre resolution. A cartwheel flexure efficiently converts displacement from the linear actuator into rotary motion with minimal parasitic errors. Rotation of the flexure is directly measured via a Magnescale "Laserscale" angle encoder. Closed-loop operation of the PiezoWalk actuator, using high-speed feedback from the angle encoder, ensures that the Diamond-NANGO's output drifts by only ∼0.3 nrad rms over ∼30 min. We show that the Diamond-NANGO can reliably move with unprecedented 1 nrad (∼57 ndeg) angular increments over a range of >7000 μrad. An autocollimator, interferometer, and capacitive displacement sensor are used to independently confirm the Diamond-NANGO's performance by simultaneously measuring the rotation of a reflective cube.
Sub-nm topography measurement by deflectometry: flatness standard and wafer nanotopography
Current demands of the semiconductor industry on the measurement accuracy in lithography have reached nanometer and even sub-nanometer levels. We have built a novel scanning facility based on ESAD (Extended Shear Angle Difference) deflectometry for the ultra-precise and traceable measurement of large near-flat and slightly curved optical surfaces. One primary application of the device is to establish the improved standard for straightness and flatness with sub-nanometer accuracy at PTB. Transfer standards then will allow optical devices, e.g., interferometers and wafer mappers, to be calibrated with high accuracy. The measurement principle is based on the analysis of differences between reflection angles obtained at surface points with large lateral displacements (shears). The ESAD principle minimizes error influences and in first order is independent of stage errors and whole-body movements of the specimen. ESAD scanning does not rely on external reference surfaces of matched topography and allows the accurate calibration of the angle measuring device. The measurands are directly traceable to the SI units of angle and length. We will report in detail on the new ESAD facility for the ultra-precise measurement of large (up to 500 mm in diameter) near-flat optical surfaces. We will present first measurements of a number of plane optical substrates with the device from various fields of application. The main error influences will be discussed, including the calibration of the high-resolution autocollimator used for angle measurement. Investigations into the repeatability, reproducibility and uncertainty of the topography measurement using the new facility at the sub-nanometer level will be presented.