This research has been financially supported by the Grant Agency of the Czech Republic (Project No. 15-34632L) and by the CEITEC Nano+ project, ID CZ.02.1.01/0.0/0.0/16 013/0001728. The sample fabrication and the LTEM measurement were carried out in the CEITEC Nano Research Infrastructure (ID LM2015041, MEYS CR, 2016-2019). Meena Dhankhar was supported by a Brno Ph.D. talent scholarship.

We demonstrate a method for the fabrication of samples for TEM and MTXM. The permalloy nanodiscs with diameters ranging from 250 - 4000 nm and thicknesses between 20 - 100 nm are fabricated on 30 nm thick SiN membranes for TEM and 200 nm thick SiN membranes for MTXM. Photographs of the SiN membranes are shown in Figure 1.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57817/57817fig1.jpg" /> 
Figure 1: Photograph of SiN membranes used as a substrate for MTXM (left) and TEM (right) samples. The image shows size comparison to a ruler. The MTXM frame is a 5 x 5 mm rectangle with a window thickness of 200 nm and the TEM frame fits a 3 mm circle in diameter with a window thickness of 30 nm. <a href="/files/ftp_upload/57817/57817fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
1. Fabrication of the Samples for TEM
Note: In this section, we describe the fabrication of the samples for TEM which is used for the observation of the nucleation process of magnetic vortices17. The membranes are chosen as the substrates because they offer a solid support for the lithographical fabrication of magnetic structures. An important parameter is the membrane window thickness. A higher accelerating voltage allows penetrating thicker samples, but any unnecessary thickness will cause a loss of signal19. For that reason, we use the thinnest membranes available from our supplier (30 nm).
<ol>
	<li>Substrate preparation and spin coating 
	Note: Spin coating is a widely used process to get a desired resist film on the substrate. A small amount of resist is dropped on the substrate which is then rotated at a very high speed to get the desired coating thickness. The spin coating of TEM membranes is rather peculiar because of the following reasons: (i) if the membrane is spun on the spin coater&#8217;s axis, the resist will not be uniform due to the membrane&#8217;s small diameter and (ii) vacuum holders cannot be used because they can break the membrane. For this purpose, we designed 3D printed adapters (see Figure 2) that hold the membrane off-axis and do not require a vacuum to hold the sample.
	<ol>
		<li>Prebake the SiN membranes on a hot plate at 180 &#176;C for 15 min to remove any moisture.</li>
		<li>Inserts adaptor onto spin coater and then places membranes in adapter.</li>
		<li>Spin coat 950 K PMMA (poly-methyl-methacrylate) resist at 3,000 rpm for 1 min to produce a film thickness of approximately 200 nm.</li>
		<li>Post-bake the samples on the hot plate at 180 &#176;C for 3 min to harden the PMMA layer.</li>
	</ol>
	</li>
</ol>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57817/57817fig2.jpg" /> 
Figure 2: Photograph of 3D printed adapter, used to hold the TEM membrane off-axis during the spin coating. Multiple membranes can be coated at the same time. <a href="/files/ftp_upload/57817/57817fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<ol start="2">
	<li>Electron-beam lithography (EBL)
	<ol>
		<li>Draw the desired pattern of discs in the Graphic Database System (GDS) format and upload it to the electron-beam (e-beam) lithography system.</li>
		<li>Load the samples into the e-beam writer system, set the stage, and beam.</li>
		<li>Expose the disc area to an electron dose of 260 &#181;C/cm2 at the beam energy of 20 keV. 
		Note: The suitable parameters of the exposition process are a beam current of 250 pA and a step size of 10 nm. This dose is approximately 30% higher when compared to bulk substrates as the backscattering is highly reduced on the membranes.</li>
	</ol>
	</li>
	<li>Chemical development
	<ol>
		<li>After the exposure, develop the samples in a methyl isobutyl ketone (MIBK)-based developer for 2 min. Stop the development by using isopropyl alcohol (IPA) for 30 s.</li>
		<li>Wash each sample in deionized water for 30 s and blow-dry it using nitrogen while holding with a tweezer.</li>
		<li>Check the development of the samples using an optical microscope at a low magnification first (using a 5X objective) and then at a high magnification (using a 100X objective); the optical microscope image of a developed sample is shown in Figure 3.</li>
	</ol>
	</li>
	<li>Electron-beam evaporation 
	Note: Electron-beam evaporation20 is a form of physical vapor deposition in which a target anode is bombarded with a high energy electron-beam produced by a charged tungsten filament under a high vacuum. The electron-beam causes atoms from the target to transform into a gaseous phase. These atoms precipitate into a solid form and coat everything in the vacuum chamber with a thin layer of the target material. It is better to use an e-beam evaporation system for a lift-off purpose as it gives a nice edge to the disc without depositing any extra material on the disc boundary.
	<ol>
		<li>Tape the membranes carefully using poly-oxydiphenylene-pyromellitimide (e.g., Kapton) onto the holder and transfer it into the deposition chamber of the e-beam evaporator via load lock.</li>
		<li>Use the electron-beam evaporation system to deposit a thin layer of permalloy (Ni80Fe20) with thicknesses ranging from 20 to 100 nm at the deposition rate of approximately 1 A&#870;/s. Use the acceleration voltage of 8 kV and beam a current of approximately 120 mA.</li>
	</ol>
	</li>
	<li>Lift-off 
	<ol>
		<li>Put the membranes for 1 h into a beaker with acetone (with at least 99.5% purity).</li>
		<li>Now spray the membranes with acetone while holding each with a tweezer until the excess metal is removed.</li>
		<li>If the excess metal remains on the sample, place the membranes back into the beaker and repeat the procedure. 
		Note: Optionally, a megasonic bath could be used to assist the lift-off procedure. Note that it is not possible to use a classical ultrasonic bath as it breaks the membranes.</li>
		<li>Image the final array of the magnetic discs with a scanning electron microscope (SEM) at the accelerating voltage of 5 kV and a beam current of 100 pA for the final inspection. An image at the magnification of 100,000X is shown in Figure 3b.</li>
	</ol>
	</li>
	<li>LTEM imaging
	<ol>
		<li>Mount the sample into the TEM sample holder and insert it into the microscope.</li>
		<li>Correct the sample height and align the microscope in the Lorentz mode at the desired accelerating voltage (in our case 300 kV) using the standard procedures of the microscope.</li>
		<li>Introduce the magnetic signal by defocusing the Lorentz lens.</li>
		<li>Carry on with the experiment. Tilt the sample in order to introduce the in-plane field component (e.g., the suitable angle is 30&#176;, check the holder specification for the maximum tilt angle).</li>
		<li>Apply the magnetic field by exciting the objective lens (normally turned off in the Lorentz mode). 
		Note: The field calibration curve should be provided by the TEM manufacturer.</li>
		<li>Saturate the sample, gradually decrease the magnetic field by deexciting the objective lens and capture the images on camera. Example results are shown in Figure 3c.</li>
	</ol>
	</li>
</ol>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57817/57817fig3.jpg" /> 
Figure 3: Final sample imaged by optical and electron microscopies. (a) This panel shows the silicon nitride membrane window with the arrays of discs in the resist after the electron-beam exposure and resist development. (b) This panel shows the final array of the magnetic discs imaged by the SEM. (c) This panel shows the LTEM image of the magnetic vortices nucleation states in an array of the magnetic nanodiscs. <a href="/files/ftp_upload/57817/57817fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
2. Fabrication of the Samples for MTXM
Note: In MTXM measurements, we can take advantage of the technique&#8217;s time resolution. In order to introduce a high-frequency excitation of the magnetic vortices, fabricate a gold waveguide in the first step and then place magnetic discs on top of the waveguide in the second lithography step. The whole structure is fabricated on a 200 nm thick SiN membrane which is transparent enough for soft x-rays21. The detailed steps are described in the following text and the schematic of the process is shown in Figure 4. The process of the MTXM sample fabrication goes through all the steps described above for the TEM samples fabrication but an additional lithography step is required to fabricate the waveguide.
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/57817/57817fig4.jpg" /> 
Figure 4: Schematics of preparation steps of a sample with discs and a waveguide on a silicon nitride membrane for MTXM time-resolved experiments. It involves a two-step lithography to obtain the final structure. <a href="/files/ftp_upload/57817/57817fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<ol>
	<li>Sample preparation and spin coating 
	Note: The membrane for MTXM is a 5 x 5 mm2 frame with a 3 x 3 mm2 wide and 200 nm thick central window. The membrane cannot be put on the spin coater vacuum chuck as it would break the membrane. In this case, we glued the membrane to a 10 x 10 mm2 silicon wafer to make it easy to work with.
	<ol>
		<li>Prebake the SiN samples on a hot plate at 180 &#176;C for 15 min to remove any moisture from the samples.</li>
		<li>Spin coat the positive resist CSAR at 3,000 rpm for 1 min; the resulting film thickness is approximately 500 nm. 
		Note: This type of resist was chosen for its higher sensitivity resulting in faster writing times. The thickness-spin speed dependence can be found on the resist data sheet.</li>
		<li>Post-bake the samples on a hot plate at 150 &#176;C for 1 min to harden the resist layer.</li>
	</ol>
	</li>
	<li>Electron beam lithography of the waveguide
	<ol>
		<li>Create the desired pattern of the waveguide and alignment marks (for the second lithography steps) in GDS format and upload it to the e-beam lithography system.</li>
		<li>Load the samples into the e-beam writer system, set the stage, and beam.</li>
		<li>Expose the disc area to an electron dose of 65 &#181;C/cm2 at the beam energy of 20 keV, a beam current of 10 nA, and a step size of 200 nm. 
		Note: We used CSAR positive resist for exposing the waveguide and alignment marks. This resist has a higher sensitivity than PMMA with the electron dose of 65 &#956;C/cm2 at the beam energy of 20 keV and, therefore, is favorable to speed up the exposition.</li>
	</ol>
	</li>
	<li>Chemical development
	<ol>
		<li>After the exposure, develop the samples in the developer for 1 min and follow that with a stopper (IPA) for 30 s.</li>
		<li>Wash the samples in deionized water for 30 s and blow-dry them with nitrogen while holding them with the tweezers.</li>
	</ol>
	</li>
	<li>Electron-beam evaporation
	<ol>
		<li>Use the e-beam evaporator to deposit the Ti(3nm)/Au(100nm) bilayer for the waveguide and alignment marks.</li>
		<li>Rotate the samples continuously at a rate of 10 rpm to increase the layer homogeneity. 
		Note: The titanium layer of 3 - 5 nm makes an adhesive contact between the SiN sample and the Au layer. The thickness of the Au layer is typically between 80 - 120 nm. This range of thickness is suitable for the wire bonding of the sample to custom-made printed circuit boards, which we used to inject current pulses into the waveguide.</li>
		<li>Use a deposition rate for Ti of 0.5 - 0.7 A&#870;/s and for Au of ~2.5 A&#870;/s. Maintain the base pressure of the e-beam system at approximately 10-7 mbar or better.</li>
		<li>Alternatively, use Cu instead of Au for the fabrication of the waveguide for a better transparency for the soft x-rays.</li>
	</ol>
	</li>
	<li>Lift-off
	<ol>
		<li>After the deposition of the Ti/Au thin film, put the samples in acetone for 1 h.</li>
		<li>Now, spray the membranes with acetone while holding them with tweezers until the excess metal is removed.</li>
		<li>If the excess metal remains on the samples, place them back to the beaker with acetone and repeat the procedure. 
		Note: Optionally, a megasonic bath could be used to support the lift-off procedure. The samples with the Ti/Au waveguide structure and with alignment marks go through the same lithography steps again for the fabrication of the magnetic discs on the waveguide.</li>
	</ol>
	</li>
	<li>TiO2 deposition
	<ol>
		<li>Insert the samples with the waveguide and alignment marks into the atomic layer deposition system and deposit 20 nm of the TiO2 layer to make the insulating layer between the waveguide and the discs.</li>
		<li>Use the Ti precursor Tetrakis(dimethylamido)titanium (TDMAT) and H2O for the deposition of TiO2 by oxygen plasma and grow it at a rate of 0.51 A&#870;/cycle.</li>
	</ol>
	</li>
	<li>Spin coating of samples 
	Note: We used a double layer of resist to increase the edge quality of the discs. During the e-beam exposure, the bottom resist is overdosed, and after the development, it provides a refined undercut.
	<ol>
		<li>Prebake the samples on the hot plate at 180 &#176;C for 15 min to remove the moisture.</li>
		<li>Spin coat the copolymer resist at 4,000 rpm for 1 min. 
		Note: The resulting film thickness is approximately 30 nm.</li>
		<li>Post-bake the samples on the hot plate at 180 &#176;C for 3 min to harden the resist.</li>
		<li>Spin coat the PMMA 950K resist at 4,000 rpm for 1 min. The resulting film thickness is approximately 270 nm.</li>
		<li>Post-bake the samples at 180 &#176;C for 3 min to harden the resist.</li>
	</ol>
	</li>
	<li>Electron-beam lithography of discs
	<ol>
		<li>Create the second lithographical pattern of discs in the GDS format and upload it to the e-beam lithography system.</li>
		<li>Use the global marks to align the UV coordination system to the sample.</li>
		<li>Use the local marks to align the write-field in order to calibrate the write-field&#8217;s size, rotation, and shift to ensure a correct position of the discs on the waveguide.</li>
		<li>Expose the disc area to an electron dose of 220 &#181;C/cm2 at a beam energy of 20&#160;keV. Use a beam current of 200 - 300 pA and a step size of 10 nm for exposing the pattern.</li>
	</ol>
	</li>
	<li>Chemical development
	<ol>
		<li>Develop the samples in an MIBK-based developer for 1 min, and follow that with a stopper (IPA) for 30 s.</li>
		<li>Then rinse the samples in deionized water for 30 s and blow-dry them with nitrogen while holding them with tweezers.</li>
	</ol>
	</li>
	<li>Ion-beam sputter deposition
	<ol>
		<li>Insert the samples into the ion-beam sputtering system.</li>
		<li>Tilt the sample holder by 30&#176; with respect to the direction of the sputtered material in order to taper the discs by the shadowing effect. 
		Note: The tapering is used to control the switching of the vortex circulation11.</li>
		<li>Deposit a 20 - 50 nm thick permalloy (Ni80Fe20) layer using a deposition rate of 0.5 - 0.7 A&#870;/s at a working pressure of approximately 10-5 mbar. 
		Note: The base pressure should be 10-7 mbar or better.</li>
	</ol>
	</li>
	<li>Lift-off
	<ol>
		<li>Put the samples in acetone for 1 h.</li>
		<li>Now, spray the membranes with acetone while holding them in a tweezer until the excess metal is removed.</li>
		<li>If the excess metal remains on the sample, place the sample back to the beaker with acetone and repeat the procedure. 
		Note: Optionally, a megasonic bath could be used to support the lift-off procedure. We got the final structure of the permalloy discs over a Ti/Au waveguide on a SiN membrane as shown in Figure 5.</li>
	</ol>
	</li>
</ol>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/57817/57817fig5.jpg" /> 
Figure 5: SEM image of the final structure of 30 nm thick and 2 &#181;m wide permalloy discs on a gold waveguide with alignment marks. The samples are used further for time-resolved MTXM experiments. <a href="/files/ftp_upload/57817/57817fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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The authors have nothing to disclose.

We have demonstrated the fabrication of samples for LTEM and MTXM magnetic microscopies. These samples need to be fabricated on thin SiN membranes so that the electrons, in the case of the LTEM, and the soft x-rays, in the case of the MTXM, can penetrate through the samples. These samples can be fabricated either by 1) a positive resist lithography or by 2) a negative resist lithography.
We used the positive resist lithography technique because it requires less sample preparation and fewer fabrication steps and allows easier processing. It also allows the researcher to use the shadowing effect, which we used for the precise disc shape control (a tapering of one side of the disc). This shape was used to control the circulation of the magnetic vortices during nucleation10,11.
The disadvantage of this technique is the complicated lift-off procedure because the thin film material is sometimes deposited on the resist edge and then cannot be removed by a lift-off. We solved this problem by using a double resist layer. This slightly limits the resolution (approximately 20 nm) of the lithographical process but remains sufficient for the purposes of magnetic imaging.
The negative resist lithography technique offers a higher resolution as structures with a resolution down to 7 nm can be written into the resist. The material is then etched away either by wet etching or by ion beam etching. The problem with this approach is that the resist is difficult to remove after the etching. Commonly used oxygen plasma resist stripping is not possible in the case of thin permalloy structures, as they oxidize very easily. This fact, together with the need to use the shadowing technique, favors the positive lithography process which was used throughout this work.
We used the samples prepared by the methods described in this paper for the observation of the dynamics of magnetic vortices during a circulation switching by an MTXM10,11 and for the observation of various nucleation states17. This can be extended to more types of experiments requiring lithographically prepared structures on the membranes.

The magnetism of nanostructures was intensively studied in the last two decades following technological trends towards miniaturization. As the lateral dimensions of the structures become smaller and smaller, the magnetic properties of ferromagnetic structures start to be governed by the structure geometry in addition to the properties of the magnetic material. The behavior of different magnetic elements from bulk materials to microstructures has been reviewed in detail (e.g., by Hubert and Sch&#228;fer)1. One of the most known examples of non-trivial magnetization ground state is magnetic vortices-curling magnetization structures occurring in micron- and submicron-sized thin magnetic discs and polygons. The magnetization here is curling in-plane around an out-of-plane vortex core2,3. The magnetization reversal of magnetic vortices has been extensively studied in both static4,5,6 and dynamic7,8,9,10 regimes. The possible applications of magnetic vortices are, e.g., multi-bit memory cells11, logic circuits12, radio-frequency devices13, or spin-wave emitters14.
To image a magnetic vortex and especially the vortex core, the spatial resolution of the microscopy technique should be as close as possible to fundamental magnetic length scales (below 10 nm). Lorentz transmission electron microscopy15 (LTEM) and magnetic transmission x-ray microscopy16 (MTXM) are ideal candidates for the imaging of magnetic vortices as they offer a high spatial resolution and MTXM also offers a high temporal resolution for magnetization dynamics studies. The disadvantage of these techniques is the complicated sample preparation, which is the subject of the presented paper.
The processes presented here explain the fabrication of samples used for imaging magnetic vortices by TEM17 and MTXM10,11. Both techniques are of transmission character, and because of that, it is necessary to fabricate the structures on thin membranes. The membranes are typically made from silicon nitride and their thickness ranges from tens of nanometers to a few hundreds of nanometers. Each of these two methods requires a different support frame geometry. In the case of MTXM, the frame is 5 x 5 mm2 and the window is large, 2 x 2 mm2. In the case of TEM, the frame geometry is a circle of 3 mm in diameter with the window size dependent on the experiment, typically 250 x 250 &#181;m2. The membranes bring additional challenges of more difficult sample handling with the risk of breaking the windows during all the lithography processes.
The fabrication of samples can be done by both positive and negative resist lithography techniques18. The positive resist lithography process uses a positive resist; the chemical structure of the resist changes upon irradiation and the exposed part will become soluble in the chemical developer. The exposed area will wash away while the unexposed area will remain on the substrate. In the case of a negative resist lithography process, the irradiation hardens the resist and the exposed area will remain on the substrate while the unexposed area will wash away in the chemical developer. Both techniques can be used for the fabrication of the samples, but we prefer positive resist lithography because it requires fewer fabrication steps when compared to the negative resist lithography technique. It is also easier to handle, faster, and often provides better results.

<table>
	<tbody>
		<tr>
			<td>SiN Membrane - TEM</td>
			<td>Silson</td>
			<td>SiRN-TEM-200-0.25-500</td>
			<td>TEM membrane</td>
		</tr>
		<tr>
			<td>SiN Membrane - MTXM</td>
			<td>Silson</td>
			<td>SiRN-5.0-200-3.0-200</td>
			<td>MTXM membrane</td>
		</tr>
		<tr>
			<td>3D adapter for spin coating</td>
			<td></td>
			<td></td>
			<td>The model of the adapter for 3D printing can be downloaded at: https://www.thingiverse.com/thing:2808368</td>
		</tr>
		<tr>
			<td>PMMA 950k electron beam resist</td>
			<td>Allresist</td>
			<td>AR-P 679.04</td>
			<td>used for TEM sample</td>
		</tr>
		<tr>
			<td>Electron beam resist developer</td>
			<td>Allresist</td>
			<td>AR 600-56</td>
			<td>used for TEM sample</td>
		</tr>
		<tr>
			<td>High-contrast electron beam resist</td>
			<td>Allresist</td>
			<td>AR-P 6200.13</td>
			<td>used for the waveguide on the MTXM sample</td>
		</tr>
		<tr>
			<td>High-contrast electron beam resist developer</td>
			<td>Allresist</td>
			<td>AR-600-546</td>
			<td>used for the waveguide on the MTXM sample</td>
		</tr>
		<tr>
			<td>Tetrakis(dimethylamido)titanium(IV)</td>
			<td>Sigma Aldrich</td>
			<td>669008 Aldrich&#160;</td>
			<td>used for TiO2 thin film deposition by ALD&#160;</td>
		</tr>
		<tr>
			<td>Electron beam resist for nanometer lithography</td>
			<td>Allresist</td>
			<td>AR-P 617.02</td>
			<td>used as the bottom layer of bilayer resist for easier lift-off procedure</td>
		</tr>
		<tr>
			<td>PMMA 950k electron beam resist</td>
			<td>Allresist</td>
			<td>AR-P 679.04</td>
			<td>used as the top layer of bilayer resist for easier lift-off procedure</td>
		</tr>
		<tr>
			<td>Electron beam resist developer</td>
			<td>Allresist</td>
			<td>AR 600-56</td>
			<td>used for development of the disks on waveguide</td>
		</tr>
		<tr>
			<td>Permalloy pellets</td>
			<td>Kurt J Lesker</td>
			<td>EVMPERMQXQ-D</td>
			<td>used for the deposition of the magnetic layers</td>
		</tr>
		<tr>
			<td>Titanium pellets</td>
			<td>Kurt J Lesker</td>
			<td>EVMTI45QXQD</td>
			<td>used as adhesive layer for the gold waveguide</td>
		</tr>
		<tr>
			<td>Gold pellets</td>
			<td>Kurt J Lesker</td>
			<td>EVMAUXX40G</td>
			<td>used for the deposition of the waveguide</td>
		</tr>
	</tbody>
</table>

fabrication of magnetic nanostructures on silicon nitride membranes for magnetic vortex studies using transmission microscopy techniques

Electron and x-ray magnetic microscopies allow for high-resolution magnetic imaging down to tens of nanometers. However, the samples need to be prepared on transparent membranes which are very fragile and difficult to manipulate. We present processes for the fabrication of samples with magnetic micro- and nanostructures with spin configurations forming magnetic vortices suitable for Lorentz transmission electron microscopy and magnetic transmission x-ray microscopy studies. The samples are prepared on silicon nitride membranes and the fabrication consists of a spin coating, UV and electron-beam lithography, the chemical development of the resist, and the evaporation of the magnetic material followed by a lift-off process forming the final magnetic structures. The samples for the Lorentz transmission electron microscopy consist of magnetic nanodiscs prepared in a single lithography step. The samples for the magnetic x-ray transmission microscopy are used for time-resolved magnetization dynamic experiments, and magnetic nanodiscs are placed on a waveguide which is used for the generation of repeatable magnetic field pulses by passing an electric current through the waveguide. The waveguide is created in an extra lithography step.

Figure 1 shows the photographs of the SiN frames and membranes used for the MTXM and LTEM microscopies. Figure 2 shows the design of the 3D-printed adapters to hold the TEM membrane off-axis during the spin coating process. Figure 3 shows the various steps of the LTEM sample preparation (after the resist development and after the lift-off procedure) and the final image observed by the LTEM. Figure 4 shows the schematics of the preparation steps for the fabrication of the discs and the waveguide on the SiN membrane for the MTXM time-resolved experiments. Figure 5 shows the final MTXM sample containing the discs placed on a waveguide.

Watch this Scientific Journal Video about Fabrication of Magnetic Nanostructures on Silicon Nitride Membranes for Magnetic Vortex Studies Using Transmission Microscopy Techniques at JoVE.com

Fabrication of Magnetic Nanostructures on Silicon Nitride Membranes for Magnetic Vortex Studies Using Transmission Microscopy Techniques

A protocol for the fabrication of magnetic micro- and nanostructures with spin configurations forming magnetic vortices suitable for transmission electron microscopy (TEM) and magnetic transmission x-ray microscopy (MTXM) studies is presented.

Electron and x-ray magnetic microscopies allow for high-resolution magnetic imaging down to tens of nanometers. However, the samples need to be prepared ...

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Research

JoVE Journal

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8.0K Views.  Brno University of Technology. A protocol for the fabrication of magnetic micro- and nanostructures with spin configurations forming magnetic vortices suitable for transmission electron microscopy (TEM) and magnetic transmission x-ray microscopy (MTXM) studies is presented.

Video: Fabrication of Magnetic Nanostructures on Silicon Nitride Membranes for Magnetic Vortex Studies Using Transmission Microscopy Techniques

Fabrication of Uniform Nanoscale Cavities via Silicon Direct Wafer Bonding

Measurements of the heat capacity and superfluid fraction of confined 4He have been performed near the lambda transition using lithographically patterned and bonded silicon wafers. Unlike confinements in porous materials often used for these types of experiments3, bonded wafers provide predesigned uniform spaces for confinement. The geometry of each cell is well known, which removes a large source of ambiguity in the interpretation of data.
Exceptionally flat, 5 cm diameter, 375 &#181;m thick Si wafers with about 1 &#181;m variation over the entire wafer can be obtained commercially (from Semiconductor Processing Company, for example). Thermal oxide is grown on the wafers to define the confinement dimension in the z-direction. A pattern is then etched in the oxide using lithographic techniques so as to create a desired enclosure upon bonding. A hole is drilled in one of the wafers (the top) to allow for the introduction of the liquid to be measured. The wafers are cleaned2 in RCA solutions and then put in a microclean chamber where they are rinsed with deionized water4. The wafers are bonded at RT and then annealed at ~1,100 &#176;C. This forms a strong and permanent bond. This process can be used to make uniform enclosures for measuring thermal and hydrodynamic properties of confined liquids from the nanometer to the micrometer scale.

A method for permanently bonding two silicon wafers so as to realize a uniform enclosure is described. This includes wafer preparation, cleaning, RT bonding, and annealing processes. The resulting bonded wafers (cells) have uniformity of enclosure ~1%1,2. The resulting geometry allows for measurements of confined liquids and gasses.

Measurements of the heat capacity and superfluid fraction of confined 4He have been performed near the lambda transition using lithographically patterned ...

Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees of freedom to control electronic devices. In particular, the behavior of graphene&#8217;s charge carriers in response to potentials from charged Coulomb impurities is predicted to differ significantly from that of most materials. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) can provide detailed information on both the spatial and energy dependence of graphene&#39;s electronic structure in the presence of a charged impurity.&#160;The design of a hybrid impurity-graphene device, fabricated using controlled deposition of impurities onto a back-gated graphene surface, has enabled several novel methods for controllably tuning graphene&#8217;s electronic properties.1-8 Electrostatic gating enables control of the charge carrier density in graphene and the ability to reversibly tune the charge2 and/or molecular5 states of an impurity. This paper outlines the process of fabricating a gate-tunable graphene device decorated with individual Coulomb impurities for combined STM/STS studies.2-5 These studies provide valuable insights into the underlying physics, as well as signposts for designing hybrid graphene devices.

This paper details the fabrication process of a gate-tunable graphene device, decorated with Coulomb impurities for scanning tunneling microscopy studies. Mapping the spatially dependent electronic structure of graphene in the presence of charged impurities unveils the unique behavior of its relativistic charge carriers in response to a local Coulomb potential.

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees ...

Phase Diagram Characterization Using Magnetic Beads as Liquid Carriers

Magnetic beads with ~1.9 &#181;m average diameter were used to transport microliter volumes of liquids between contiguous liquid segments with a tube for the purpose of investigating phase change of those liquid segments. The magnetic beads were externally controlled using a magnet, allowing for the beads to bridge the air valve between the adjacent liquid segments. A hydrophobic coating was applied to the inner surface of the tube to enhance the separation between two liquid segments. The applied magnetic field formed an aggregate cluster of magnetic beads, capturing a certain liquid amount within the cluster that is referred to as carry-over volume. A fluorescent dye was added to one liquid segment, followed by a series of liquid transfers, which then changed the fluorescence intensity in the neighboring liquid segment. Based on the numerical analysis of the measured fluorescence intensity change, the carry-over volume per mass of magnetic beads has been found to be ~2 to 3 &#181;l/mg. This small amount of liquid allowed for the use of comparatively small liquid segments of a couple hundred microliters, enhancing the feasibility of the device for a lab-in-tube approach. This technique of applying small compositional variation in a liquid volume was applied to analyzing the binary phase diagram between water and the surfactant C12E5 (pentaethylene glycol monododecyl ether), leading to quicker analysis with smaller sample volumes than conventional methods.

Here, we present a protocol to investigate multi-component phase diagrams using externally controlled magnetic beads as liquid carriers in a lab-in-tube approach. This approach can aid in applications that seek to gather further information on phase change in complex liquid systems.

Magnetic beads with ~1.9 &#181;m average diameter were used to transport microliter volumes of liquids between contiguous liquid segments with a tube for ...

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe2) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal&#8211;semiconductor contact of the FIB-fabricated MoSe2 devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe2-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 &#937;&#8722;1 cm&#8722;1, with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe2 multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe2 are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS2 and WS2.

We describe the approaches for the device fabrication and electrical characterization of molybdenum diselenide (MoSe2) layer semiconductor nanostructures with different thicknesses. In addition, the fabrication of ohmic contacts for MoSe2-layer nanocrystals by the focused-ion beam deposition method using platinum (Pt) as a contact metal is described.

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Use of a Multi-compartment Dynamic Single Enzyme Phantom for Studies of Hyperpolarized Magnetic Resonance Agents

Imaging of hyperpolarized substrates by magnetic resonance shows great clinical promise for assessment of critical biochemical processes in real time. Due to fundamental constraints imposed by the hyperpolarized state, exotic imaging and reconstruction techniques are commonly used. A practical system for characterization of dynamic, multi-spectral imaging methods is critically needed. Such a system must reproducibly recapitulate the relevant chemical dynamics of normal and pathological tissues. The most widely utilized substrate to date is hyperpolarized [1-13C]-pyruvate for assessment of cancer metabolism. We describe an enzyme-based phantom system that mediates the conversion of pyruvate to lactate. The reaction is initiated by injection of the hyperpolarized agent into multiple chambers within the phantom, each of which contains varying concentrations of reagents that control the reaction rate. Multiple compartments are necessary to ensure that imaging sequences faithfully capture the spatial and metabolic heterogeneity of tissue. This system will aid the development and validation of advanced imaging strategies by providing chemical dynamics that are not available from conventional phantoms, as well as control and reproducibility that is not possible in vivo.

A multi-compartment dynamic phantom is used to simulate some biology of interest for metabolic studies using hyperpolarized magnet resonance agents.

Imaging of hyperpolarized substrates by magnetic resonance shows great clinical promise for assessment of critical biochemical processes in real time. Due ...

Frequency Mixing Magnetic Detection Scanner for Imaging Magnetic Particles in Planar Samples

The setup of a planar Frequency Mixing Magnetic Detection (p-FMMD) scanner for performing Magnetic Particles Imaging (MPI) of flat samples is presented. It consists of two magnetic measurement heads on both sides of the sample mounted on the legs of a u-shaped support. The sample is locally exposed to a magnetic excitation field consisting of two distinct frequencies, a stronger component at about 77 kHz and a weaker field at 61 Hz. The nonlinear magnetization characteristics of superparamagnetic particles give rise to the generation of intermodulation products. A selected sum-frequency component of the high and low frequency magnetic field incident on the magnetically nonlinear particles is recorded by a demodulation electronics. In contrast to a conventional MPI scanner, p-FMMD does not require the application of a strong magnetic field to the whole sample because mixing of the two frequencies occurs locally. Thus, the lateral dimensions of the sample are just limited by the scanning range and the supports. However, the sample height determines the spatial resolution. In the current setup it is limited to 2 mm. As examples, we present two 20 mm &#215; 25 mm p-FMMD images acquired from samples with 1 &#181;m diameter maghemite particles in silanol matrix and with 50 nm magnetite particles in aminosilane matrix. The results show that the novel MPI scanner can be applied for analysis of thin biological samples and for medical diagnostic purposes.

A scanner for imaging magnetic particles in planar samples was developed using the planar frequency mixing magnetic detection technique. The magnetic intermodulation product response from the nonlinear nonhysteretic magnetization of the particles is recorded upon a two-frequency excitation. It can be used to take 2D images of thin biological samples.

The setup of a planar Frequency Mixing Magnetic Detection (p-FMMD) scanner for performing Magnetic Particles Imaging (MPI) of flat samples is presented. ...

Studying Dynamic Processes of Nano-sized Objects in Liquid using Scanning Transmission Electron Microscopy

Samples fully embedded in liquid can be studied at a nanoscale spatial resolution with Scanning Transmission Electron Microscopy (STEM) using a microfluidic chamber assembled in the specimen holder for Transmission Electron Microscopy (TEM) and STEM. The microfluidic system consists of two silicon microchips supporting thin Silicon Nitride (SiN) membrane windows. This article describes the basic steps of sample loading and data acquisition. Most important of all is to ensure that the liquid compartment is correctly assembled, thus providing a thin liquid layer and a vacuum seal. This protocol also includes a number of tests necessary to perform during sample loading in order to ensure correct assembly. Once the sample is loaded in the electron microscope, the liquid thickness needs to be measured. Incorrect assembly may result in a too-thick liquid, while a too-thin liquid may indicate the absence of liquid, such as when a bubble is formed. Finally, the protocol explains how images are taken and how dynamic processes can be studied. A sample containing AuNPs is imaged both in pure water and in saline.

This protocol describes the operation of a liquid flow specimen holder for scanning transmission electron microscopy of AuNPs in water, as used for the observation of nanoscale dynamic processes.

Samples fully embedded in liquid can be studied at a nanoscale spatial resolution with Scanning Transmission Electron Microscopy (STEM) using a ...

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...

Characterization of Ultra-fine Grained and Nanocrystalline Materials Using Transmission Kikuchi Diffraction

One of the challenges in microstructure analysis nowadays resides in the reliable and accurate characterization of ultra-fine grained (UFG) and nanocrystalline materials. The traditional techniques associated with scanning electron microscopy (SEM), such as electron backscatter diffraction (EBSD), do not possess the required spatial resolution due to the large interaction volume between the electrons from the beam and the atoms of the material. Transmission electron microscopy (TEM) has the required spatial resolution. However, due to a lack of automation in the analysis system, the rate of data acquisition is slow which limits the area of the specimen that can be characterized. This paper presents a new characterization technique, Transmission Kikuchi Diffraction (TKD), which enables the analysis of the microstructure of UFG and nanocrystalline materials using an SEM equipped with a standard EBSD system. The spatial resolution of this technique can reach 2 nm. This technique can be applied to a large range of materials that would be difficult to analyze using traditional EBSD. After presenting the experimental set up and describing the different steps necessary to realize a TKD analysis, examples of its use on metal alloys and minerals are shown to illustrate the resolution of the technique and its flexibility in term of material to be characterized.

This paper provides a detailed method to characterize the microstructure of ultra-fine grained and nanocrystalline materials using a scanning electron microscope equipped with a standard electron backscatter diffraction system. Metal alloys and minerals presenting refined microstructures are analyzed using this technique, showing the diversity of its possible applications.