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

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.

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. Please click here to view a larger version of this figure.

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).

1. 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’s axis, the resist will not be uniform due to the membrane’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.
   1. Prebake the SiN membranes on a hot plate at 180 °C for 15 min to remove any moisture.
   2. Inserts adaptor onto spin coater and then places membranes in adapter.
   3. 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.
   4. Post-bake the samples on the hot plate at 180 °C for 3 min to harden the PMMA layer.

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. Please click here to view a larger version of this figure.

2. Electron-beam lithography (EBL)
   1. Draw the desired pattern of discs in the Graphic Database System (GDS) format and upload it to the electron-beam (e-beam) lithography system.
   2. Load the samples into the e-beam writer system, set the stage, and beam.
   3. Expose the disc area to an electron dose of 260 µC/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.
3. Chemical development
   1. 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.
   2. Wash each sample in deionized water for 30 s and blow-dry it using nitrogen while holding with a tweezer.
   3. 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.
4. 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.
   1. 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.
   2. 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 Aͦ/s. Use the acceleration voltage of 8 kV and beam a current of approximately 120 mA.
5. Lift-off
   1. Put the membranes for 1 h into a beaker with acetone (with at least 99.5% purity).
   2. Now spray the membranes with acetone while holding each with a tweezer until the excess metal is removed.
   3. 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.
   4. 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.
6. LTEM imaging
   1. Mount the sample into the TEM sample holder and insert it into the microscope.
   2. 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.
   3. Introduce the magnetic signal by defocusing the Lorentz lens.
   4. Carry on with the experiment. Tilt the sample in order to introduce the in-plane field component (e.g., the suitable angle is 30°, check the holder specification for the maximum tilt angle).
   5. 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.
   6. 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.

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. Please click here to view a larger version of this figure.

2. Fabrication of the Samples for MTXM

Note: In MTXM measurements, we can take advantage of the technique’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.

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. Please click here to view a larger version of this figure.

1. 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.
   1. Prebake the SiN samples on a hot plate at 180 °C for 15 min to remove any moisture from the samples.
   2. 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.
   3. Post-bake the samples on a hot plate at 150 °C for 1 min to harden the resist layer.
2. Electron beam lithography of the waveguide
   1. 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.
   2. Load the samples into the e-beam writer system, set the stage, and beam.
   3. Expose the disc area to an electron dose of 65 µC/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 μC/cm² at the beam energy of 20 keV and, therefore, is favorable to speed up the exposition.
3. Chemical development
   1. After the exposure, develop the samples in the developer for 1 min and follow that with a stopper (IPA) for 30 s.
   2. Wash the samples in deionized water for 30 s and blow-dry them with nitrogen while holding them with the tweezers.
4. Electron-beam evaporation
   1. Use the e-beam evaporator to deposit the Ti(3nm)/Au(100nm) bilayer for the waveguide and alignment marks.
   2. 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.
   3. Use a deposition rate for Ti of 0.5 – 0.7 Aͦ/s and for Au of ~2.5 Aͦ/s. Maintain the base pressure of the e-beam system at approximately 10^-7 mbar or better.
   4. Alternatively, use Cu instead of Au for the fabrication of the waveguide for a better transparency for the soft x-rays.
5. Lift-off
   1. After the deposition of the Ti/Au thin film, put the samples in acetone for 1 h.
   2. Now, spray the membranes with acetone while holding them with tweezers until the excess metal is removed.
   3. 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.
6. TiO₂ deposition
   1. 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.
   2. 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 Aͦ/cycle.
7. 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.
   1. Prebake the samples on the hot plate at 180 °C for 15 min to remove the moisture.
   2. Spin coat the copolymer resist at 4,000 rpm for 1 min.
      Note: The resulting film thickness is approximately 30 nm.
   3. Post-bake the samples on the hot plate at 180 °C for 3 min to harden the resist.
   4. Spin coat the PMMA 950K resist at 4,000 rpm for 1 min. The resulting film thickness is approximately 270 nm.
   5. Post-bake the samples at 180 °C for 3 min to harden the resist.
8. Electron-beam lithography of discs
   1. Create the second lithographical pattern of discs in the GDS format and upload it to the e-beam lithography system.
   2. Use the global marks to align the UV coordination system to the sample.
   3. Use the local marks to align the write-field in order to calibrate the write-field’s size, rotation, and shift to ensure a correct position of the discs on the waveguide.
   4. Expose the disc area to an electron dose of 220 µC/cm² at a beam energy of 20 keV. Use a beam current of 200 – 300 pA and a step size of 10 nm for exposing the pattern.
9. Chemical development
   1. Develop the samples in an MIBK-based developer for 1 min, and follow that with a stopper (IPA) for 30 s.
   2. Then rinse the samples in deionized water for 30 s and blow-dry them with nitrogen while holding them with tweezers.
10. Ion-beam sputter deposition
    1. Insert the samples into the ion-beam sputtering system.
    2. Tilt the sample holder by 30° 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.
    3. Deposit a 20 – 50 nm thick permalloy (Ni₈₀Fe₂₀) layer using a deposition rate of 0.5 – 0.7 Aͦ/s at a working pressure of approximately 10^-5 mbar.
       Note: The base pressure should be 10^-7 mbar or better.
11. Lift-off
    1. Put the samples in acetone for 1 h.
    2. Now, spray the membranes with acetone while holding them in a tweezer until the excess metal is removed.
    3. 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.

Figure 5: SEM image of the final structure of 30 nm thick and 2 µm wide permalloy discs on a gold waveguide with alignment marks. The samples are used further for time-resolved MTXM experiments. Please click here to view a larger version of this figure.