Optomechanical design of a portable metrology system for measurement of precision rotary stage errors

Abstract

The Advanced Photon Source Upgrade (APS-U) project will construct several new, best-in-class beamlines and enhancements to existing beamlines to exploit the massive increase in coherent flux enabled by the new storage ring lattice. APS-U will also enhance several existing beamlines to boost their performance. X-ray tomography is a common imaging mode for several of these beamlines, so there is demand for the highest-precision rotation of the sample. For example, the In Situ Nanoprobe (ISN, 19-ID), a next-generation hard x-ray nanoprobe, will use x-ray fluorescence tomography and ptychographic 3D imaging as key imaging modes with a spot size of 20 nm. It will require <100 nm runout and single-micro-radian wobble errors of the rotation stage to achieve full 3D resolution. Such precise requirements for a rotation stage can be achieved with air bearing rotation stages. However, this approach puts constraints on sample positioning design in terms of the sample environment (air bearing stages are generally not vacuum compatible) and the large mass of air bearing rotation stages. Mechanical bearing stages do not equal the precision runout/wobble specifications of air bearings. In order to use mechanical stages and approach air bearing level precision, the errors of the mechanical stage have to be measured precisely. We have then designed a metrology system using interferometer or capacitive sensors for the nanopositioning support lab as a diagnostic tool and to be portable for quality assurance testing of stages at the beamline.