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Reaction control of metal-assisted chemical etching for silicon-based zone plate nanostructures
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 Royal Institute of Technology.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.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Metal-assisted chemical etching (MACE) reaction parameters were investigated for the fabrication of specially designed silicon-based x-ray zone plate nanostructures using a gold catalyst pattern and etching solutions composed of HF and H2O2. Etching depth, zone verticality and zone roughness were studied as a function of etching solution composition, temperature and processing time. Homogeneous, vertical etching with increasing depth is observed at increasing H2O2 concentrations and elevated processing temperatures, implying a balance in the hole injection and silica dissolution kinetics at the gold-silicon interface. The etching depth decreases and zone roughness increases at the highest investigated H2O2 concentration and temperature. Possible reasons for these observations are discussed based on reaction chemistry and zone plate design. Optimum MACE conditions are found at HF:H2O2 concentrations of 4.7 M:0.68 M and room temperature with an etching rate of 0.7 micrometers per minute, which is about an order of magnitude higher than previous reports. Moreover, our results show that a grid catalyst design is important for successful fabrication of vertical high aspect ratio silicon nanostructures.

Keywords [en]
metal-assisted chemical etching, x-ray zone plates, silicon nanostructures
National Category
Chemical Sciences
Research subject
Chemistry; Physics
Identifiers
URN: urn:nbn:se:kth:diva-223960OAI: oai:DiVA.org:kth-223960DiVA, id: diva2:1189361
Note

QC 20180312

Available from: 2018-03-09 Created: 2018-03-09 Last updated: 2018-03-23Bibliographically 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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