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

This method can help answer key questions in the fabrication of micro and nano structures, for observation by transmission electron and x-ray microscopies. The main advantage of this technique is that it allows sample fabrication on nanometer thin, silicon nitride membranes, which is the requirement for observation by transmission microscopies. Demonstrating the procedure will be Meena Dhankhar, and Marek Vanatka, grad students from my laboratory.

To begin, place the 30 nanometer silicon nitrade membranes on a hot plate at 180 degrees Celsius, and bake them for 15 minutes to remove any moisture. Put the adaptor onto the spin coater, and then place the membranes into the adaptor. Spin coat 950k pmma resist at 3, 000 rpm for one minute to produce a film thickness of approximately 200 nanometers.

Then, post-bake the samples on the hot plate at 180 degrees Celsius for three minutes to harden the pmma layer. To carryout electron beam lithography, draw the desired pattern of discs in the graphic database system, or GDS format, and upload it to the electron beam lithography system. Load the samples into the e-beam writer system.

Then set the stage and beam. Expose the disc area to an electron dose of 260 microcoulombs per square centimeter at a beam energy of 20 kilo electron volts. Following the exposure, immerse the samples in a methyl isobutyl ketone developer for two minutes.

Then transfer the samples to isopropyl alcohol for 30 seconds to stop the development. Use deionized water to wash each sample for 30 seconds. Then, while holding each sample with tweezers, use nitrogen to blow it dry.

Next, using an optical microscope, begin at low magnification to check the development of the samples, then switch to high magnification. To perform electron beam evaporation, use kapton to carefully tape the membranes onto the holder. And transfer the samples into the deposition chamber of the e-beam evaporator via load lock.

Use the electron beam evaporation system at an acceleration voltage of eight kilovolts and a beam current of approximately 120 milliamperes to deposit a thin layer of permalloy at a thickness ranging from 20 to 100 nanometers, and a deposition rate of approximately one angstrom per second. To lift off the excess metal, place the membranes in a beaker with acetone for one hour. Then, while using tweezers to hold them, use acetone to spray the membranes until the excess metal is removed.

If excess metal remains on the sample, place the membrane back in the acetone for another soak, before spraying with acetone again. To image a sample, mount it into the TEM sample holder and insert it into the microscope. Using the standard procedures of the microscope, correct the sample height and align the microscope in the lorentz mode at the desired accelerating voltage.

For example, 300 kilovolts in this case. Introduce the magnetic signal by defocusing the lorentz lens. Then tilt the sample to introduce the in plain field component.

For example, a suitable angle is 30 degrees. Apply the magnetic field by exciting the objective lens, which is normally turned off in the lorentz mode. Finally, saturate the sample, gradually decrease the magnetic field by deexciting the objective lens, and capture the images on camera.

Seen here are the silicon nitrade frames and membranes used for the MTXM and LTEM microscopie. The MTXM frame is a five by five millimeter square with a window thickness of 200 nanometers. And the TEM frame fits a three millimeter diameter circle with a window thickness of 30 nanometers.

This panel contains a silicon nitrade membrane window with the arrays of discs in the resist after the electron beam exposure and resist development. The final array of the magnetic discs imaged by the SEM is shown here. This LTEM image illustrates the magnetic vortices nucleation states in an array of magnetic nanodiscs.

Finally, this SEM image represents the final structure of 30 nanometer thick and two micron wide permalloyed discs on a gold wave guide with alignment marks. Once mastered, this technique can be done within one day. While attempting this procedure, it's important to remember to use proper parameters for electron beam exposure of the pattern.

After watching this video, you should have a good understanding of how to fabricate and characterize the structures step by step, using the presented method. Don't forget that working with chemicals is hazardous, and precautions such as wearing gloves and protective glasses while manipulating the material in a fume hood should be always be taken while performing the procedure.