Supplementary Information

Parallel fabrication of magnetic tunnel junction nanopillars

by nanosphere lithographySupplementary Information

W. G. Wang, A. Pearse, M. Li, S. Hageman, A. X. Chen, F. Q. Zhu, and C. L. Chien

Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, USA

Selection of the capping layers

In order to make use of NSL for fabricating nanopillars, it is important to have proper capping layers. The capping layer should be as thin as possible, in order to minimize damage to the nanospheres during ion beam etching. Meanwhile, the capping layer should also be thick enough to protect the MTJ stacks from the chemicals ( ACH, PEOetc) used in the deposition of nanospheres. Moreover, different metalshavedifferent wetting properties; therefore,theirattraction to the negatively charged nanospheres varies. With all these considerations, we have carried out experiments with different capping layers and found out the layer structure of Ta(8 nm)/Ru(16 nm)/Ta (5 nm) served best for our purpose. This trilayer structure offers better protection to the MTJ film than bi-layers with the same total thickness, largely due to the additional interface. Ru has a faster milling rate than Ta therefore a thick Ru provides good protection with short etching time. Making sure the last layer is Ta is also important, because Ta coated with ACH tends to give a better a nanosphere distribution when compared to Ru, Au, Ag or Al.

Switching behavior of cells containing more than one nanopillar

For samples fabricated with randomly distributed nanosphere patterns, it is important to determine the exact number of nanopillar in each 2 µm x 2 µm cell. Besides the methods described in the main text ( RA value, direct imaging and maximal resistance), in most cases the TMR curve could also directly show if a cell contains more than one nanopillar. For example, the TMR curve of a cell is plotted in the left panel of Figure S1. This is a perpendicular CoFeB/MgO/CoFeB MTJ therefore the switching is generally very sharp. However, there is an obvious step during the switching of the MTJ. By compare this curve with the TMR curve of a cell with the same MgO thickness and containing only one pillar as shown in the right panel of Figure S1, it becomes transparent that the cell in the left panel contains two nanopillars. First, both the parallel and antiparallel resistance in the left panel are half of the corresponding values in the right panel. Further, the fact that the intermediate state has the resistance of 80k kOhm, which is exactly the total resistance of two MTJ in parallel with the resistance of 240 kOhm (switched) and 120 kOhm (unswitched), tells us this cell contains two nanopillars. If the cell contains three ormore nanopillars, the P/AP resistance and the position of the intermediate state(s)would also exhibit similar behavior in the TMR curve.

Figure S1. TMR curve of a cell containing two nanopillars(left) and TMR curve of a cell containing only one nanopillar with the same MgO barrier thickness (right)

More details on ordered nanopillar array fabrication

Ordered nanosphere lithography was carried out using a modified Langmuir-Blodgett monolayer deposition process. Two types of monodisperse carboxyl-functionalized polystyrene nanospheres suspended in water were purchased from Bangs Laboratories (10.2% solids, functional group density 1.8 COOH/nm2) and Thermo Scientific (10% solids, functional group density 1.4 COOH/nm2). Both types of nanospheres had a less than 5% coefficient of variation in size. The nanosphere suspensions were diluted in a 1:1 ratio with ethanol (a spreading agent) and mixed by ultrasonication for 30 minutes.

Monolayer self-assembly took place in a 150mL Teflon Langmuir-Blodgett trough. The trough was filled with high purity DI water to the brim, and the pH was adjusted to be basic by the addition of approximately 75µL of a 0.1M NH4OH solution. Adjusting the pH allows for the control of the number of dissociated functional groups on the nanospheres.1 The MTJ film to be coated and a clean glass slide were then exposed to oxygen plasma for 30s in order to make then maximally hydrophilic. The substrate was submerged in the trough, just under the surface, and the glass slide was placed so that it could be used as a “ramp” down which nanosphere solution could be flowed onto the water surface. The nanosphere/ethanol solution (approx. 10-20 µL) was then slowly applied to the glass slide via pipette. The nanosphere solution then spread out over the surface of the water via the ethanol and began to self-assemble, and the glass slide was carefully removed.

In order to further facilitate self-assembly, polyethylene oxide (PEO) (Sigma-Aldritch MW=100000 g/mol) was added via pipette so that the resulting concentration of PEO in the trough was 3-4ppm.2 Some of the PEO diffused to the surface, forming a surfactant barrier which gently compressed the nanospheres on a time scale of minutes, while the remainder of the PEO remained in solution and screened the electrostatic repulsion between the nanospheres. PEO may also bond with the nanospheres, forming polymer “bridges” which improve the mechanical properties of the monolayer.2 Once fully assembled, the large nanospheres (350nm-420nm) exhibited bright green and blue iridescence under visible light, while the smaller nanospheres (160nm-250nm) showed no iridescence and had a flat golden color.

To transfer the monolayer to the MTJ film, the trough was slowly drained so that the water level dropped at a rate of 0.5mm/s. The coated film was then removed and left to air dry. After that the size of the nanospheres was reduced by oxygen plasma. A low power density of 0.45 W/cm2 was used to avoid overheating the nanospheres. Under this power density the size of nanospheres was reduced at 12.5 nm/min. Subsequently, ion beam etching was carried out and the dense array of nanopillarswasformed after the liftoff of nanospheres.

References

1Vogel, N., Goerres, S., Landfester, K. & Weiss, C. K. A Convenient Method to Produce Close- and Non-close-Packed Monolayers using Direct Assembly at the Air-Water Interface and Subsequent Plasma-Induced Size Reduction. Macromolecular Chemistry and Physics212, 1719-1734, doi:10.1002/macp.201100187 (2011).

2Ho, C. C., Chen, P. Y., Lin, K. H., Juan, W. T. & Lee, W. L. Fabrication of Monolayer of Polymer/Nanospheres Hybrid at a Water-Air Interface. Acs Applied Materials & Interfaces3, 204-208, doi:10.1021/am100814z (2011).

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