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Nanofabrication of Diffractive Soft X-ray Optics
KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics. (Biomedicinsk Fysik och Röntgenfysik)
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis summarizes the present status of the nanofabrication of diffractive optics, i.e. zone plates, and test objects for soft x-ray microscopy at KTH. The emphasis is on new and improved fabrication processes for nickel and germanium zone plates. A new concept in which nickel and germanium are combined in a zone plate is also presented. The main techniques used in the fabrication are electron beam lithography for the patterning, followed by plasma etching and electroplating for the structuring of the optical materials. The process for fabricating nickel zone plates has been significantly improved. The reproducibility of the electroplating step has been increased by the implementation of an in-situ rate measurement and an end-point detection method. We have also shown that pulse plating can be used to obtain zone plates with a uniform height profile. New plating mold materials have been introduced and electron-beam curing of the molds has been investigated and implemented to increase their mechanical stability so that pattern collapse in the electroplating step can be avoided. The introduction of cold development has improved the achievable resolution of the process. This has enabled the fabrication of zone plates with outermost zone widths down to 16 nm. The nickel process has also recently been adapted to fabrication of gold structures intended for test objects and hard x-ray zone plates. For the fabrication of germanium zone plates we developed a highly anisotropic plasma-etch process using Cl2 feed and sidewall passivation. Germanium zone plates have been fabricated with zone widths down to 30 nm. The diffraction efficiency is comparable to that of nickel zone plates, but the process does not involve electroplating and thus has for potential for highyield fabrication. The combination of nickel and germanium is a new fabrication concept that provides a means to achieve high diffraction efficiency even for thin nickel. The idea is to fabricate a nickel zone plate on a germanium film. The nickel zone plate itself is then used as etch mask for a highly selective CHF3- plasma etch into the germanium layer. Proof of principle experiments showed an efficiency increase of about a factor of two for nickel zone plates with a 50- nm nickel thickness.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2009. , vii, 71 p.
Series
Trita-FYS, ISSN 0280-316X ; 2008:57
Keyword [en]
Nanofabrication, Zone plate, x-ray, diffractive x-ray optics, x-ray microscopy
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-9800ISBN: 978-91-7415-205-0 (print)OAI: oai:DiVA.org:kth-9800DiVA: diva2:133159
Public defence
2009-02-06, FR4, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100728Available from: 2009-01-13 Created: 2009-01-07 Last updated: 2011-10-20Bibliographically approved
List of papers
1. Controlled electroplating for high-aspect-ratio zone plate fabrication
Open this publication in new window or tab >>Controlled electroplating for high-aspect-ratio zone plate fabrication
2006 (English)In: Journal of Vacuum Science & Technology B, ISSN 1071-1023, E-ISSN 1520-8567, Vol. 24, no 6, 2592-2596 p.Article in journal (Refereed) Published
Abstract [en]

The authors report a method for monitoring, control, and end-point detection of electroplating in nanostructures. The method is demonstrated on nickel plating into polymer molds, which is an important process in the fabrication of soft x-ray zone-plate diffractive optics. The lack of reproducibility presently limits the achievable nickel aspect ratio and, thus, reduces the zone-plate diffraction efficiency. The reported method provides reproducible plating via real-time control of the plating rate. It combines in situ light transmission measurements with current measurements to determine the thickness of the growing layer. The accuracy of the thickness prediction was better than ±4% (1) for 100–300  nm nickel layers. Furthermore, a slight change in the light transmission signal indicates when a gratinglike zone-plate structure is slightly overplated and the plating should be stopped. This end-point detection provides the optimal filling of high-aspect-ratio molds for improved diffraction efficiency.

Keyword
X-RAY MICROSCOPY; HIGH-EFFICIENCY; CONDENSER; OPTICS
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-5987 (URN)10.1116/1.2362761 (DOI)000243324400017 ()2-s2.0-33845257285 (Scopus ID)
Note
QC 20100616Available from: 2006-06-08 Created: 2006-06-08 Last updated: 2017-12-14Bibliographically approved
2. Pulse reverse electroplating for uniform nickel height in zone plates
Open this publication in new window or tab >>Pulse reverse electroplating for uniform nickel height in zone plates
2006 (English)In: Journal of Vacuum Science & Technology B, ISSN 1071-1023, E-ISSN 1520-8567, Vol. 24, 2848- p.Article in journal (Refereed) Published
Abstract [en]

Nickelsoft x-ray zone plates are fabricated by through-mask electroplating. Theauthors report on how a uniform nickel thickness can beobtained over the entire zone plate using pulse and pulsereverse plating. If the plating is carried out at aconstant current the nickel thickness has been observed to decreasewith radius. This results in lower outer zones and reduceddiffraction efficiency in the outer parts of the zone plates.Here they show that the height profile can be controlledby adjusting the current density of the pulses. A highcurrent density is found to primarily affect the edges whilea low current density was observed to affect the centralparts of the structures. This is true for both cathodicand anodic currents, which means that local plating and dissolutionrates can be adjusted to obtain a uniform mass distribution.

Keyword
x-ray microscopy; high-efficiency; electrodeposition; fabrication; deposition; resolution; copper; holes
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-9804 (URN)10.1116/1.2395953 (DOI)000243324400063 ()2-s2.0-33845274933 (Scopus ID)
Note
QC 20100728 QC 20110929. Conference: 50th International Conference on Electron, Ion, and Photon Beam Technology and Nanofavrication. Baltimore, MD. MAY 30-JUN 02, 2006 Available from: 2009-01-07 Created: 2009-01-07 Last updated: 2017-12-14Bibliographically approved
3. SU8 plating mold for high aspect-ratio nickel zone plates
Open this publication in new window or tab >>SU8 plating mold for high aspect-ratio nickel zone plates
2007 (English)In: Microelectronic Engineering, ISSN 0167-9317, E-ISSN 1873-5568, Vol. 84, 1136- p.Article in journal (Refereed) Published
Abstract [en]

Nickel zone plates are fabricated by electrodeposition into a mold with high aspect ratio and narrow line width. This process requires high-mechanical stability of the mold to avoid pattern collapse in the plating bath. In the present paper we demonstrate how SU-8 can be used as plating mold material in a tri-layer resist to fabricate 35-nm half-pitch nickel gratings with an aspect ratio exceeding 11:1. To attain sufficient stability of the mold the SU-8 was cured by e-beam exposure with a dose of 25 mC/cm2 at 5-keV electron energy.

Keyword
SU-8; zone plate; aspect ratio; electroplating
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-9805 (URN)10.1016/j.mee.2007.01.109 (DOI)000247182500105 ()2-s2.0-34247645044 (Scopus ID)
Note
QC 20100728Available from: 2009-01-07 Created: 2009-01-07 Last updated: 2017-12-14Bibliographically approved
4. High-aspect-ratio germanium zone plates fabricated by ractive ion etching in chlorine
Open this publication in new window or tab >>High-aspect-ratio germanium zone plates fabricated by ractive ion etching in chlorine
Show others...
2009 (English)In: Journal of Vacuum Science & Technology B, ISSN 1071-1023, E-ISSN 1520-8567, Vol. 27, no 2, L1-L3 p.Article in journal (Refereed) Published
Abstract [en]

This article describes the fabrication of soft x-ray germanium zone plates with a process based on reactive ion etching (RIE) in Cl-2. A high degree of anisotropy is achieved by sidewall passivation through cyclic exposure to air. This enables structuring of higher aspect ratios than with earlier reported fabrication processes for germanium zone plates. The results include a zone plate with a 30 nm outermost zone width and a germanium thickness of 310 tun having a first-order diffraction efficiency of 70% of the theoretical value. 25 nm half-pitch gratings were also etched into 310 nut of germanium. Compared to the electroplating process for the commonly used nickel zone plates, the RIE process with Cl-2, for germanium is a major improvement in terms of process reproducibility.

Keyword
x-ray optics; nickel; ti
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-9806 (URN)10.1116/1.3089371 (DOI)000265839400001 ()2-s2.0-64549134292 (Scopus ID)
Note
QC 20100728Available from: 2009-01-07 Created: 2009-01-07 Last updated: 2017-12-14Bibliographically approved
5. Nickel-germanium soft x-ray zone plates
Open this publication in new window or tab >>Nickel-germanium soft x-ray zone plates
Show others...
2009 (English)In: Journal of Vacuum Science & Technology B, ISSN 1071-1023, E-ISSN 1520-8567, Vol. 27, no 3, L5-L7 p.Article in journal (Refereed) Published
Abstract [en]

This article presents a fabrication process for soft x-ray zone plates in which nickel and germanium are combined to achieve high diffraction efficiency. A nickel zone plate is first fabricated on a germanium film and then used as a hardmask for a CHF3-plasma etch into the germanium. Zone plates with 50-60 nm nickel and 110-150 nm of germanium are presented. The measured diffraction efficiencies were 10%-11% at lambda=2.88 nm, which shows that high efficiency is possible even with thin nickel. Thus, the method has a potential for improving the efficiency of high-resolution zone plates for which the high-aspect-ratio structuring of nickel is difficult.

Keyword
elemental semiconductors; germanium; light diffraction; nanofabrication; nickel; semiconductor thin films; X-ray microscopy; X-ray optics; zone plates
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-9807 (URN)10.1116/1.3117256 (DOI)000266500300001 ()2-s2.0-70449726046 (Scopus ID)
Note
Tidigare titel: High-efficency nickel-germanium soft x-ray zone plates QC 20100728Available from: 2009-01-07 Created: 2009-01-07 Last updated: 2017-12-14Bibliographically approved
6. High-resolution differential-interference-contrast x-ray zone plates: Design and Fabrication
Open this publication in new window or tab >>High-resolution differential-interference-contrast x-ray zone plates: Design and Fabrication
Show others...
2007 (English)In: Spectrochimica Acta Part B - Atomic Spectroscopy, ISSN 0584-8547, E-ISSN 1873-3565, Vol. 62, no 6-7, 539-543 p.Article in journal (Refereed) Published
Abstract [en]

Differential interference contrast is a potentially powerful technique for contrast enhancement in soft X-ray microscopy. We describe the design and fabrication of single-element diffractive optical elements suitable as objectives for high-resolution differential interference contrast microscopy in the water-window spectral range. A one-dimensional pattern calculation followed by an extension to two dimensions results in a pattern resolution of 1 nm, which is well below fabrication accuracy. The same fabrication process as for normal zone plates is applicable, but special care must be taken when converting the calculated pattern to a code for e-beam lithography.

Keyword
Differential interference contrast, Diffractive optics, X-ray microscopy
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-5986 (URN)10.1016/j.sab.2007.03.005 (DOI)000249213700005 ()2-s2.0-34547643613 (Scopus ID)
Note

QC 20100616

Available from: 2006-06-08 Created: 2006-06-08 Last updated: 2017-12-14Bibliographically approved
7. Single-optical-element soft-x-ray interferometry with a laser-plasma x-ray source
Open this publication in new window or tab >>Single-optical-element soft-x-ray interferometry with a laser-plasma x-ray source
Show others...
2005 (English)In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 30, no 15, 2167- p.Article in journal (Refereed) Published
Abstract [en]

We report on a compact interferometer for the water-window soft-x-ray range that is suitable for operation with laser-plasma sources. The interferometer consists of a single diffractive optical element that focuses impinging x rays to two focal spots. The light from these two secondary sources forms the interference pattern. The interferometer was operated with a liquid-nitrogen jet laser-plasma source at lambda = 2.88 nm. Scalar wave-field propagation was used to simulate the interference pattern, showing good correspondence between theoretical and experimental results. The diffractive optical element can simultaneously be used as an imaging optic, and we demonstrate soft-x-ray microscopy with interferometric contrast enhancement of a phase object.

Keyword
Diffraction; Imaging techniques; Interferometers; Light propagation; Plasmas; X rays; Compact interferometer; Focal spots; Laser-plasma source; Liquid nitrogen; Interferometry
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-7017 (URN)10.1364/OL.30.002167 (DOI)000231072700039 ()2-s2.0-24344492433 (Scopus ID)
Note
QC 20100616Available from: 2007-04-20 Created: 2007-04-20 Last updated: 2017-12-14Bibliographically approved
8. High-resolution compact x-ray microscopy
Open this publication in new window or tab >>High-resolution compact x-ray microscopy
Show others...
2007 (English)In: Journal of Microscopy, ISSN 0022-2720, E-ISSN 1365-2818, Vol. 226, no 2, 175-181 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate compact full-field soft X-ray transmission microscopy with sub 60-nm resolution operating at λ= 2.48 nm. The microscope is based on a 100-Hz regenerative liquid-nitrogen-jet laser-plasma source in combination with a condenser zone plate and a micro-zone plate objective for high-resolution imaging onto a 2048 × 2048 pixel CCD detector. The sample holder is mounted in a helium atmosphere and allows imaging of both dry and wet specimens. The microscope design enables fast sample switching and the sample can be pre-aligned using a visible-light microscope. High-quality images can be acquired with exposure times of less than 5 min. We demonstrate the performance of the microscope using both dry and wet samples.

Keyword
Compact; Laser plasma; Liquid-jet; Soft x-ray microscopy; Water window; Zone plate optics
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-9810 (URN)10.1111/j.1365-2818.2007.01765.x (DOI)000245744400008 ()17444946 (PubMedID)2-s2.0-34247246221 (Scopus ID)
Note
QC 20100728Available from: 2009-01-07 Created: 2009-01-07 Last updated: 2017-12-14Bibliographically approved
9. Compact high-resolution differential interference contrast soft x-ray microscopy
Open this publication in new window or tab >>Compact high-resolution differential interference contrast soft x-ray microscopy
Show others...
2008 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 92, no 064104Article in journal (Refereed) Published
Abstract [en]

We demonstrate high-resolution x-ray differential interference contrast (DIC) in a compact soft x-ray microscope. Phase contrast imaging is enabled by the use of a diffractive optical element objective which is matched to the coherence conditions in the microscope setup. The performance of the diffractive optical element objective is evaluated in comparison with a normal zone plate by imaging of a nickel siemens star pattern and linear grating test objects. Images obtained with the DIC optic exhibit typical DIC enhancement in addition to the normal absorption contrast. Contrast transfer functions based on modulation measurements in the obtained images show that the DIC optic gives a significant increase in contrast without reducing the spatial resolution. The phase contrast operation mode now available for our compact soft x-ray microscope will be a useful tool for future studies of samples with low absorption contrast.

Keyword
diffractive optical-elements, zone plates, phase-contrast, fabrication, condenser
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-9811 (URN)10.1063/1.2842422 (DOI)000253237900123 ()2-s2.0-39349084101 (Scopus ID)
Note
QC 20100728Available from: 2009-01-07 Created: 2009-01-07 Last updated: 2017-12-14Bibliographically approved
10. Compact Zernike phase contrast x-ray microscopy using a single-element optic
Open this publication in new window or tab >>Compact Zernike phase contrast x-ray microscopy using a single-element optic
Show others...
2008 (English)In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 33, no 9, 932-934 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate Zernike phase contrast in a compact soft x-ray microscope using a single-element optic. The optic is a combined imaging zone plate and a Zernike phase plate and does not require any additional alignment or components. Contrast is increased and inversed in an image of a test object using the Zernike zone plate. This type of optic may be implemented into any existing x-ray microscope where phase contrast is of interest.

Keyword
spatial-resolution, zone plates
National Category
Biomedical Laboratory Science/Technology
Identifiers
urn:nbn:se:kth:diva-17561 (URN)000256166900014 ()2-s2.0-44949260817 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved

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  • apa
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