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High-aspect ratio zone plate fabrication for hard x-ray nanoimaging
KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.ORCID iD: 0000-0001-7569-9408
KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.ORCID iD: 0000-0002-4394-0591
2017 (English)In: Advances in X-Ray/EUV Optics and Components XII / [ed] Morawe, C Khounsary, AM Goto, S, SPIE - International Society for Optical Engineering, 2017, Vol. 10386, article id UNSP 103860SConference paper (Refereed)
Abstract [en]

We present our results in fabricating Fresnel zone plate optics for the NanoMAX beamline at the fourth-generation synchrotron radiation facility MAX IV, to be used in the energy range of 6-10 keV. The results and challenges of tungsten nanofabrication are discussed, and an alternative approach using metal-assisted chemical etching (MACE) of silicon is showcased. We successfully manufactured diffraction-limited zone plates in tungsten with 30 nm outermost zone width and an aspect ratio of 21:1. These optics were used for nanoimaging experiments at NanoMAX. However, we found it challenging to further improve resolution and diffraction efficiency using tungsten. High efficiency is desirable to fully utilize the advantage of increased coherence on the optics at MAX IV. Therefore, we started to investigate MACE of silicon for the nanofabrication of high-resolution and high-efficiency zone plates. The first type of structures we propose use the silicon directly as the phase-shifting material. We have achieved 6 mu m deep dense vertical structures with 100 nm linewidth. The second type of optics use iridium as the phase material. The structures in the silicon substrate act as a mold for iridium coating via atomic layer deposition (ALD). A semi-dense pattern is used with line-to-space ratio of 1:3 for a so-called frequency-doubled zone plate. This way, it is possible to produce smaller structures with the tradeoff of the additional ALD step. We have fabricated 45 nm-wide and 3.6 mu m-tall silicon/iridium structures.

Place, publisher, year, edition, pages
SPIE - International Society for Optical Engineering, 2017. Vol. 10386, article id UNSP 103860S
Series
Proceedings of SPIE, ISSN 0277-786X ; 10386
Keywords [en]
zone plate, high-aspect ratio, etching, RIE, MACE, tungsten, silicon, gold
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:kth:diva-220503DOI: 10.1117/12.2272695ISI: 000417334200016Scopus ID: 2-s2.0-85038958242ISBN: 978-1-5106-1230-3 ISBN: 978-1-5106-1229-7 OAI: oai:DiVA.org:kth-220503DiVA, id: diva2:1169032
Conference
Conference on Advances in XRay/ EUV Optics and Components XII held as part of the SPIE Optics + Photonics Symposium, AUG 08-09, 2017, San Diego, CA
Funder
Swedish Research Council, C0242401Knut and Alice Wallenberg Foundation
Note

QC 20171222

Available from: 2017-12-22 Created: 2017-12-22 Last updated: 2018-03-09Bibliographically approved
In thesis
1. High-Aspect Ratio Nanofabrication for Hard X-Ray Zone Plates
Open this publication in new window or tab >>High-Aspect Ratio Nanofabrication for Hard X-Ray Zone Plates
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Hard x-ray nanoimaging enables structural investigations of new materials for many applications. For high-resolution experiments, zone plate x-ray optics are commonly chosen.Two methods of zone plate nanofabrication are presented in this thesis.

Zone plates are circular diffraction gratings with radially decreasing grating period. Their optical resolution depends on the width of the smallest zone, which nowadays can be around 10 nanometers. However, the efficiency of a zone plate depends on its thickness and its material. For hard x-rays, the optimal zone plate thickness is in the order of micrometers. Therefore, high aspect ratio nanofabrication processes are needed.Two such methods are investigated in this study.

First, an existing tungsten nanofabrication process based on reactive ion etching (RIE) was extended to 22:1 aspect ratio structures at 30~nm line width. The core improvement was a resist curing step that enhanced pattern transfer during RIE. Such a zone plate with 200 micrometer diameter and 2.2% efficiency was used in the commissioning experiment of NanoMAX, the nanoimaging beamline at the Swedish synchrotron facility MAX IV. Transmission imaging with 40 nm resolution, as well as the fluorescence imaging modality were demonstrated.

Second, metal-assisted chemical etching (MACE) of silicon using gold catalyst patterns was investigated. MACE dependence on gold pattern geometry, etching solution composition, temperature, and substrate doping is described. The process is characterized in terms of etching rate, directionality, and nanostructure surface roughness.

Finally, the Ronchi test is presented as a way to quickly judge the performance of x-ray optics in terms of present aberrations and x-ray sources in terms of coherence.

Place, publisher, year, edition, pages
Stockholm, Sweden: KTH Royal Institute of Technology, 2018. p. 64
Series
TRITA-SCI-FOU ; 2018:09
Keywords
nanofabrication, x-ray optics, zone plate, tungsten, silicon, reactive ion etching, metal-assisted chemical etching, Ronchi test
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-223958 (URN)978-91-7729-700-0 (ISBN)
Public defence
2018-04-06, FB53, Albanova University Center, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20180312

Available from: 2018-03-12 Created: 2018-03-09 Last updated: 2018-03-12Bibliographically approved

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Parfeniukas, KarolisAkan, RabiaVogt, Ulrich

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