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Nanofabrication of Zone Plates for Hard X-Ray Free-Electron Lasers
KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics. (X-ray optics and nano-imaging)ORCID iD: 0000-0001-7554-4852
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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 high brightness 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 high aspect-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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xiii, 69 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2014:77
Keyword [en]
X-ray optics, Zone plates, Nanofabrication, X-ray free-electron lasers
National Category
Physical Sciences
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-158121ISBN: 978-91-7595-406-6 (print)OAI: oai:DiVA.org:kth-158121DiVA: diva2:774388
Public defence
2015-01-23, FA31, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (Swedish)
Opponent
Supervisors
Note

QC 20150112

Available from: 2015-01-12 Created: 2014-12-22 Last updated: 2015-01-12Bibliographically approved
List of papers
1. New diamond nanofabrication process for hard x-ray zone plates
Open this publication in new window or tab >>New diamond nanofabrication process for hard x-ray zone plates
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2011 (English)In: Journal of Vacuum Science & Technology B, ISSN 1071-1023, E-ISSN 1520-8567, Vol. 29, no 6, 06FG03-1-06FG03-4 p.Article in journal (Refereed) Published
Abstract [en]

The authors report on a new tungsten-hardmask-based diamond dry-etch process for fabricating diamond zone plate lenses with a high aspect ratio. The tungsten hardmask is structured by electron-beam lithography, together with Cl2/O2 and SF6/O2 reactive ion etching in a trilayer resist-chromium-tungsten stack. The underlying diamond is then etched in an O2 plasma. The authors demonstrate excellent-quality diamond gratings with half-pitch down to 80 nm and a height of 2.6 μm, as well as zone plates with a 75 μm diameter and 100 nm outermost zone width. The diffraction efficiency of the zone plates is measured to 14.5% at an 8 keV x-ray energy, and the imaging properties were investigated in a scanning microscope arrangement showing sub-100-nm resolution. The imaging and thermal properties of these lenses make them suitable for use with high-brightness x-ray free-electron laser sources.

Keyword
zone plate; reactive ion etching; electron beam lithography; diamond; tungsten; nanofabrication; aspect ratio; x-ray free-electron laser; x-ray optics
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-47402 (URN)10.1116/1.3656055 (DOI)000298538800086 ()2-s2.0-84255168525 (Scopus ID)
Note
QC 20111114Available from: 2011-11-08 Created: 2011-11-08 Last updated: 2017-12-08Bibliographically approved
2. Thermal stability of tungsten zone plates for focusing hard x-ray free-electron laser radiation
Open this publication in new window or tab >>Thermal stability of tungsten zone plates for focusing hard x-ray free-electron laser radiation
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2012 (English)In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 14, 043010- p.Article in journal (Refereed) Published
Abstract [en]

Diffractive Fresnel zone plates made of tungsten show great promise for focusing hard x-ray free-electron laser (XFEL) radiation to very small spot sizes. However, they have to withstand the high-intensity pulses of the beam without being damaged. This might be problematic since each XFEL pulse will create a significant temperature increase in the zone plate nanostructures and it is therefore crucial that the optics are thermally stable, even for a large number of pulses. Here we have studied the thermal stability of tungsten zone-platelike nanostructures on diamond substrates using a pulsed Nd:YAG laser which creates temperature profiles similar to those expected from XFEL pulses. We found that the structures remained intact up to a laser fluence of 100 mJ cm(-2), corresponding to a 6 keV x-ray fluence of 590 mJ cm-2, which is above typical fluence levels in an unfocused XFEL beam. We have also performed an initial damage experiment at the LCLS hard XFEL facility at SLAC National Accelerator Laboratory, where a tungsten zone plate on a diamond substrate was exposed to 105 pulses of 6 keV x-rays with a pulse fluence of 350 mJ cm-2 without any damage occurring.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-96447 (URN)10.1088/1367-2630/14/4/043010 (DOI)000303673100003 ()2-s2.0-84860151594 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20120605Available from: 2012-06-05 Created: 2012-06-04 Last updated: 2017-12-07Bibliographically approved
3. Damage investigation on tungsten and diamond diffractive optics at a hard x-ray free-electron laser
Open this publication in new window or tab >>Damage investigation on tungsten and diamond diffractive optics at a hard x-ray free-electron laser
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2013 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 21, no 7, 8051-8061 p.Article in journal (Refereed) Published
Abstract [en]

Focusing hard x-ray free-electron laser radiation with extremely high fluence sets stringent demands on the x-ray optics. Any material placed in an intense x-ray beam is at risk of being damaged. Therefore, it is crucial to find the damage thresholds for focusing optics. In this paper we report experimental results of exposing tungsten and diamond diffractive optics to a prefocused 8.2 keV free-electron laser beam in order to find damage threshold fluence levels. Tungsten nanostructures were damaged at fluence levels above 500 mJ/cm(2). The damage was of mechanical character, caused by thermal stress variations. Diamond nanostructures were affected at a fluence of 59 000 mJ/cm(2). For fluence levels above this, a significant graphitization process was initiated. Scanning Electron Microscopy (SEM) and mu-Raman analysis were used to analyze exposed nanostructures.

Keyword
X-ray optics, Zone plate, XFEL
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-122159 (URN)10.1364/OE.21.008051 (DOI)000317659300021 ()2-s2.0-84875987203 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20130514

Available from: 2013-05-13 Created: 2013-05-13 Last updated: 2017-12-06Bibliographically approved
4. Nanofabrication of tungsten zone plates with integrated platinum central stop for hard X-ray applications
Open this publication in new window or tab >>Nanofabrication of tungsten zone plates with integrated platinum central stop for hard X-ray applications
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2014 (English)In: Microelectronic Engineering, ISSN 0167-9317, E-ISSN 1873-5568, Vol. 116, 40-43 p.Article in journal (Refereed) Published
Abstract [en]

We present a nanofabrication process for producing tungsten zone plates used in hard X-ray applications including a method of integrating a high-energy absorbing central stop with the optic. Tungsten zone plates are structured with electron-beam lithography and subsequent reactive ion etching. The central stop originates from a platinum wire. It is cut to dimension by focused ion beam etching, and afterwards attached to the zone plate center using ion beam induced deposition of platinum. A zone plate with integrated central stop will simplify alignment in hard X-ray scanning microscope arrangements where the 0th order light must be eliminated. The focusing performance of the zone plate device was investigated by scanning coherent diffraction imaging (ptychography) at 8 keV photon energy. We could demonstrate a diffraction-limited focus size of 53 nm diameter full-width-at-half-maximum. Tungsten zone plates with integrated central stops show promising results for use in hard X-ray microscopes at high-brightness facilities.

Keyword
Hard X-ray microscopy, Platinum, Ptychography, Tungsten, X-ray diffractive optics, Zone plates
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-137224 (URN)10.1016/j.mee.2013.10.011 (DOI)000331161300008 ()2-s2.0-84892374286 (Scopus ID)
Funder
Swedish Foundation for Strategic Research Swedish Research CouncilEU, FP7, Seventh Framework Programme, 226716
Note

QC 20140228

Available from: 2013-12-12 Created: 2013-12-12 Last updated: 2017-12-06Bibliographically approved
5. Ronchi test for characterization of X-ray nanofocusing optics and beamlines
Open this publication in new window or tab >>Ronchi test for characterization of X-ray nanofocusing optics and beamlines
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2014 (English)In: Journal of Synchrotron Radiation, ISSN 0909-0495, E-ISSN 1600-5775, Vol. 21, 1105-1109 p.Article in journal (Refereed) Published
Abstract [en]

A Ronchi interferometer for hard X-rays is reported in order to characterize the performance of the nanofocusing optics as well as the beamline stability. Characteristic interference fringes yield qualitative data on present aberrations in the optics. Moreover, the visibility of the fringes on the detector gives information on the degree of spatial coherence in the beamline. This enables the possibility to detect sources of instabilities in the beamline like vibrations of components or temperature drift. Examples are shown for two different nanofocusing hard X-ray optics: a compound refractive lens and a zone plate.

Keyword
zone plate, compound refractive lens, Ronchi interferometer
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-153856 (URN)10.1107/S160057751401323X (DOI)000341687000025 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Foundation for Strategic Research
Note

QC 20141010

Available from: 2014-10-10 Created: 2014-10-09 Last updated: 2017-12-05Bibliographically approved

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Uhlén, Fredrik

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