Nanofabrication of Zone Plates for Hard X-Ray Free-Electron Lasers
by
Fredrik Uhlén(Department of Applied Physics, KTH)
→
Europe/Stockholm
FA31
FA31
Description
This Thesis describes the development of hard X-ray zone plates intended for focusing
radiation at X-ray free-electron lasers (XFELs). XFELs provide unprecedented
brightness and zone plates which are put in the intense X-ray beam are at risk of
being damaged. Therefore, it is crucial to perform damage tests in order to design
zone plates which can survive the XFEL beam.
Zone plates are diffractive nanofocusing optics and are regularly used at highbrightness
synchrotron beamlines in the soft and hard X-ray regime. The resolution
of a zone plate is proportional to its outermost zonewidth and thus depends on the
smallest feature that can be fabricated. State-of-the-art nanofabrication processes
developed for zone plates are able to produce zonewidths down to 10 nm. However,
for hard X-rays, the zone plates need to be of sufficient thickness to efficiently focus
the radiation. Thus, the limit in the fabrication of hard X-ray zone plates lies
in the high aspect-ratios. This Thesis describes two processes developed for highaspect-
ratio nanostructuring. The first process uses tungsten as diffractive material.
Aspect-ratios up to 1:15 have been accomplished. Furthermore, a mounting method
of a central stop directly on the zone plate is also presented. The other fabrication
process uses diamond, in which aspect-ratios of 1:30 have been demonstrated. Both
processes rely on thin-film deposition techniques, electron-beam lithography, and
reactive ion etching. Thanks to the materials’ excellent thermal properties these
types of zone plates should be suitable for XFEL applications. Tungsten and diamond
diffractive optics have been tested at an XFEL at Stanford (LCLS), and
damage investigations were performed in order to determine the maximum fluence
that could be imposed on the optics before degradation occured. The conclusion
of these damage tests is that tungsten and diamond diffractive optics can survive
the XFEL beam and could potentially be used in beamline experiments relying on
nanofocused X-ray beams. Finally in this Thesis, characterization of two zone plates
using an interferometer is presented, where it is also shown that the interferometric
method can be used to pin-point beamline instabilities.