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&amp;nbsp;</td><td>used for TiO2 thin film deposition by ALD&amp;nbsp;</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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Universidad de Cartagena UDEC

Research

JoVE Journal

Engineering

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

Fabricating Nanogaps by Nanoskiving

There are several methods of fabricating nanogaps with controlled spacings, but the precise control over the sub-nanometer spacing between two electrodes-and generating them in practical quantities-is still challenging. The preparation of nanogap electrodes using nanoskiving, which is a form of edge lithography, is a fast, simple and powerful technique. This method is an entirely mechanical process which does not include any photo- or electron-beam lithographic steps and does not require any special equipment or infrastructure such as clean rooms. Nanoskiving is used to fabricate electrically addressable nanogaps with control over all three dimensions; the smallest dimension of these structures is defined by the thickness of the sacrificial layer (Al or Ag) or self-assembled monolayers. These wires can be manually positioned by transporting them on drops of water and are directly electrically-addressable; no further lithography is required to connect them to an electrometer.

The fabrication of electrically addressable, high-aspect-ratio (> 1000:1) metal nanowires separated by gaps of single nanometers using either sacrificial layers of aluminum and silver or self-assembled monolayers as templates is described. These nanogap structures are fabricated without a clean room or any photo- or electron-beam lithographic processes by a form of edge lithography known as nanoskiving.

There are several methods of fabricating nanogaps with controlled spacings, but the precise control over the sub-nanometer spacing between two ...

Chemistry

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 and Operation of a Nano-Optical Conveyor Belt

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological and physical sciences over the past few decades. The progress made in this field invites further study of even smaller systems and at a larger scale, with tools that could be distributed more easily and made more widely available. Unfortunately, the fundamental laws of diffraction limit the minimum size of the focal spot of a laser beam, which makes particles smaller than a half-wavelength in diameter hard to trap and generally prevents an operator from discriminating between particles which are closer together than one half-wavelength. This precludes the optical manipulation of many closely-spaced nanoparticles and&#160;limits the&#160;resolution of optical-mechanical systems. Furthermore, manipulation using focused beams requires beam-forming or steering optics, which can be very bulky and expensive. To address these limitations in the system scalability of conventional optical trapping our lab has devised an alternative technique which utilizes near-field optics to move particles across a chip. Instead of focusing laser beams in the far-field, the optical near field of plasmonic resonators produces the necessary local optical intensity enhancement to overcome the restrictions of diffraction and manipulate particles at higher resolution. Closely-spaced resonators produce strong optical traps which can be addressed to mediate the hand-off of particles from one to the next in a conveyor-belt-like fashion. Here, we describe how to design and produce a conveyor belt using a gold surface patterned with plasmonic C-shaped resonators and how to operate it with polarized laser light to achieve super-resolution nanoparticle manipulation and transport. The nano-optical conveyor belt chip can be produced using lithography techniques and easily packaged and distributed.

The scalability and resolution of conventional optical manipulation techniques are limited by diffraction. We circumvent the diffraction limit and describe a method of optically transporting nanoparticles across a chip using a gold surface patterned with a path of closely spaced C-shaped plasmonic resonators.

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological ...

Atomically Traceable Nanostructure Fabrication

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process being used in order to maintain exquisite control over both feature size and feature density. Here, we demonstrate a method for tracking atomically resolved and controlled structures from initial template definition through final nanostructure metrology, opening up a pathway for top-down atomic control over nanofabrication. Hydrogen depassivation lithography is the first step of the nanoscale fabrication process followed by selective atomic layer deposition of up to 2.8 nm of titania to make a nanoscale etch mask. Contrast with the background is shown, indicating different mechanisms for growth on the desired patterns and on the H passivated background. The patterns are then transferred into the bulk using reactive ion etching to form 20 nm tall nanostructures with linewidths down to ~6 nm. To illustrate the limitations of this process, arrays of holes and lines are fabricated. The various nanofabrication process steps are performed at disparate locations, so process integration is discussed. Related issues are discussed including using fiducial marks for finding nanostructures on a macroscopic sample and protecting the chemically reactive patterned Si(100)-H surface against degradation due to atmospheric exposure.

We report a protocol for combining the atomic metrology of the Scanning Tunneling Microscope for surface patterning with selective Atomic Layer Deposition and Reactive Ion Etching. Using a robust process involving numerous atmospheric exposures and transport, 3D nanostructures with atomic metrology are fabricated.

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process ...

Advanced Experimental Methods for Low-temperature Magnetotransport Measurement of Novel Materials

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film synthesis) for the purpose of exploratory materials research. Certain materials pose a challenge wherein the traditional bulk Hall bar device fabrication method is insufficient to produce a measureable device for sample transport measurement, principally because the single crystal size is too small to attach wire leads to the sample in a Hall bar configuration. This can be, for example, because the first batch of a new material synthesized yields very small single crystals or because flakes of samples of one to very few monolayers are desired. In order to enable rapid characterization of materials that may be carried out in parallel with improvements to their growth methodology, a method of device fabrication for very small samples has been devised to permit the characterization of novel materials as soon as a preliminary batch has been produced. A slight variation of this methodology is applicable to producing devices using exfoliated samples of two-dimensional materials such as graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenides (TMDs), as well as multilayer heterostructures of such materials. Here we present detailed protocols for the experimental device fabrication of fragments and flakes of novel materials with micron-sized dimensions onto substrate and subsequent measurement in a commercial superconducting magnet, dry helium close-cycle cryostat magnetotransport system at temperatures down to 0.300 K and magnetic fields up to 12 T.

We describe the methodology of mechanical exfoliation and deposition of flakes of novel materials with micron-sized dimensions onto substrate, fabrication of experimental device structures for transport experimentation, and the magnetotransport measurement in a dry helium close-cycle cryostat at temperatures down to 0.300 K and magnetic fields up to 12 T.

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film ...

Scanning SQUID Study of Vortex Manipulation by Local Contact

Local, deterministic manipulation of individual vortices in type 2 superconductors is challenging. The ability to control the position of individual vortices is necessary in order to study how vortices interact with each other, with the lattice, and with other magnetic objects. Here, we present a protocol for vortex manipulation in thin superconducting films by local contact, without applying current or magnetic field. Vortices are imaged using a scanning superconducting quantum interference device (SQUID), and vertical stress is applied to the sample by pushing the tip of a silicon chip into the sample, using a piezoelectric element. Vortices are moved by tapping the sample or sweeping it with the silicon tip. Our method allows for effective manipulation of individual vortices, without damaging the film or affecting its topography. We demonstrate how vortices were relocated to distances of up to 0.8 mm. The vortices remained stable at their new location up to five days. With this method, we can control vortices and move them to form complex configurations. This technique for vortex manipulation could also be implemented in applications such as vortex based logic devices.

We present a protocol for manipulation of individual vortices in thin superconducting films, using local mechanical contact. The method does not include applying current, magnetic field or additional fabrication steps.

Local, deterministic manipulation of individual vortices in type 2 superconductors is challenging. The ability to control the position of individual ...

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and forging. Adoption of this technique is proceeding despite disagreement concerning the underlying mechanism responsible for EAD. The experimental procedure described herein enables a more explicit study compared to previous EAD research by removing thermal effects, which are responsible for disagreement in interpreting previous EAD results. Furthermore, as the procedure described here enables EAD observation in situ and in real time in a transmission electron microscope (TEM), it is superior to existing post-mortem methods that observe EAD effects post-test. Test samples consist of a single crystal copper (SCC) foil having a free-standing tensile test section of nanoscale thickness, fabricated using a combination of laser and ion beam milling. The SCC is mounted to an etched silicon base that provides mechanical support and electrical isolation while serving as a heat sink. Using this geometry, even at high current density (~3,500 A/mm2), the test section experiences a negligible temperature increase (&#60;0.02 &#176;C), thus eliminating Joule heating effects. Monitoring material deformation and identifying the corresponding changes to microstructures, e.g. dislocations, are accomplished by acquiring and analyzing a series of TEM images. Our sample preparation and in situ experiment procedures are robust and versatile as they can be readily utilized to test materials with different microstructures, e.g., single and polycrystalline copper.

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Isolating electrical and thermal effects on electrically assisted deformation (EAD) is very difficult using macroscopic samples. Metallic sample micro- and nanostructures together with a custom test procedure have been developed to evaluate the impact of applied current on the formation without joule heating and evolution of dislocations on these samples.

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and ...

Iron Nanowire Fabrication by Nano-Porous Anodized Aluminum and its Characterization

Magnetic nanowires possess unique properties that have attracted the interest of different fields of research, including basic physics, biomedicine, and data storage. We demonstrate a fabrication method for iron (Fe) nanowires&#160;via electrochemical deposition into anodic&#160;alumina oxide (AAO) templates. The templates are fabricated by anodization of aluminum (Al) discs, and the pore length and diameter are controlled by changing the anodizing conditions. Pores with an average diameter of around 120 nm are created using oxalic acid as the electrolyte. Using this method, cylindrical nanowires are synthesized, which are released by dissolving the alumina using a selective chemical etchant.

In this work, we describe a&#160;protocol to fabricate iron nanowires, including the formation of the porous alumina membrane that is used as the template, electrodeposition into templates using electrolyte solution, and the release of the nanowires into the solution.

Magnetic nanowires possess unique properties that have attracted the interest of different fields of research, including basic physics, biomedicine, and ...

Magnetic Field Mapping using Off-Axis Electron Holography in the Transmission Electron Microscope

Off-axis electron holography is a powerful technique that involves the formation of an interference pattern in a transmission electron microscope (TEM) by overlapping two parts of an electron wave, one of which has passed through a region of interest on a specimen and the other is a reference wave. The resulting off-axis electron hologram can be analyzed digitally to recover the phase difference between the two parts of the electron wave, which can then be interpreted to provide quantitative information about local variations in electrostatic potential and magnetic induction within and around the specimen. Off-axis electron holograms can be recorded while a specimen is subjected to external stimuli such as elevated or reduced temperature, voltage, or light. The protocol that is presented here describes the practical steps that are required to record, analyze, and interpret off-axis electron holograms, with a primary focus on the measurement of magnetic fields within and around nanoscale materials and devices. Presented here are the steps involved in the recording, analysis, and processing of off-axis electron holograms, as well as the reconstruction and interpretation of phase images and visualization of the results. Also discussed are the need for optimization of the specimen geometry, the electron optical configuration of the microscope, and the electron hologram acquisition parameters, as well as the need for the use of information from multiple holograms to extract the desired magnetic contributions from the recorded signal. The steps are illustrated through a study of specimens of B20-type FeGe, which contain magnetic skyrmions and were prepared with focused ion beams (FIBs). Prospects for the future development of the technique are discussed.

Guidelines are presented for recording and interpreting off-axis electron holograms to provide quantitative images of magnetic fields in nanoscale materials and devices in the transmission electron microscope.

Off-axis electron holography is a powerful technique that involves the formation of an interference pattern in a transmission electron microscope (TEM) by ...

Optimizing Magnetic Force Microscopy Resolution and Sensitivity to Visualize Nanoscale Magnetic Domains

Magnetic force microscopy (MFM) enables mapping local magnetic fields across a sample surface with nanoscale resolution. To perform MFM, an atomic force microscopy (AFM) probe whose tip has been magnetized vertically (i.e., perpendicular to the probe cantilever) is oscillated at a fixed height above the sample surface. The resultant shifts in the oscillation phase or frequency, which are proportional to the magnitude and sign of the vertical magnetic force gradient at each pixel location, are then tracked and mapped. Although the spatial resolution and sensitivity of the technique increases with decreasing lift height above the surface, this seemingly straightforward path to improved MFM images is complicated by considerations such as minimizing topographical artifacts due to shorter range van der Waals forces, increasing the oscillation amplitude to further improve sensitivity, and the presence of surface contaminants (in particular water due to humidity under ambient conditions). In addition, due to the orientation of the probe's magnetic dipole moment, MFM is intrinsically more sensitive to samples with an out-of-plane magnetization vector. Here, high-resolution topographical and magnetic phase images of single and bicomponent nanomagnet artificial spin-ice (ASI) arrays obtained in an inert (argon) atmosphere glovebox with &lt;0.1 ppm O2 and H2O are reported. Optimization of lift height and drive amplitude for high resolution and sensitivity while simultaneously avoiding the introduction of topographical artifacts is discussed, and detection of the stray magnetic fields emanating from either end of the nanoscale bar magnets (~250 nm long and &lt;100 nm wide) aligned in the plane of the ASI sample surface is shown. Likewise, using the example of a Ni-Mn-Ga magnetic shape memory alloy (MSMA), MFM is demonstrated in an inert atmosphere with magnetic phase sensitivity capable of resolving a series of adjacent magnetic domains each ~200 nm wide.

Magnetic force microscopy (MFM) employs a vertically magnetized atomic force microscopy probe to measure sample topography and local magnetic field strength with nanoscale resolution. Optimizing MFM spatial resolution and sensitivity requires balancing decreasing lift height against increasing drive (oscillation) amplitude, and benefits from operating in an inert atmosphere glovebox.

Magnetic force microscopy (MFM) enables mapping local magnetic fields across a sample surface with nanoscale resolution. To perform MFM, an atomic force ...

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

Summary

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Chapters in this video

Fabricating Nanogaps by Nanoskiving

Fabrication of Uniform Nanoscale Cavities via Silicon Direct Wafer Bonding

Fabrication and Operation of a Nano-Optical Conveyor Belt

Atomically Traceable Nanostructure Fabrication

Advanced Experimental Methods for Low-temperature Magnetotransport Measurement of Novel Materials

Scanning SQUID Study of Vortex Manipulation by Local Contact

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Iron Nanowire Fabrication by Nano-Porous Anodized Aluminum and its Characterization

Magnetic Field Mapping using Off-Axis Electron Holography in the Transmission Electron Microscope

Optimizing Magnetic Force Microscopy Resolution and Sensitivity to Visualize Nanoscale Magnetic Domains