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Parallel fabrication of magnetic tunnel junction nanopillars by nanosphere lithography.

Wang WG, Pearse A, Li M, Hageman S, Chen AX, Zhu FQ, Chien CL - Sci Rep (2013)

Bottom Line: We present a new method for fabricating magnetic tunnel junction nanopillars that uses polystyrene nanospheres as a lithographic template.Novel voltage induced switching has been observed in these structures.This method provides a cost-effective way of rapidly fabricating a large number of tunnel junction nanopillars in parallel.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, USA. wgwang@physics.arizona.edu

ABSTRACT
We present a new method for fabricating magnetic tunnel junction nanopillars that uses polystyrene nanospheres as a lithographic template. Unlike the common approaches, which depend on electron beam lithography to sequentially fabricate each nanopillar, this method is capable of patterning a large number of nanopillars simultaneously. Both random and ordered nanosphere patterns have been explored for fabricating high quality tunneling junctions with magnetoresistance in excess of 100%, employing ferromagnetic layers with both out-of-plane and in-plane easy axis. Novel voltage induced switching has been observed in these structures. This method provides a cost-effective way of rapidly fabricating a large number of tunnel junction nanopillars in parallel.

No MeSH data available.


Schematic fabrication procedure of MTJ nanopillars using NSL.(a) Bottom mesa structures are created from the blanket MTJ film by the first photolithography and ion beam etching, then the bottom contacts are protected by the second photolithography. (b) Random nanospheres are deposited on the entire wafer. (c) MTJ nanopillars are defined by the second ion beam etching. (d) SiO2 insulating layer is deposited. (e) Nanospheres and photoresist are lifted off, exposing the contact window. (f) Complete samples with Ta/Au top electrodes.
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f1: Schematic fabrication procedure of MTJ nanopillars using NSL.(a) Bottom mesa structures are created from the blanket MTJ film by the first photolithography and ion beam etching, then the bottom contacts are protected by the second photolithography. (b) Random nanospheres are deposited on the entire wafer. (c) MTJ nanopillars are defined by the second ion beam etching. (d) SiO2 insulating layer is deposited. (e) Nanospheres and photoresist are lifted off, exposing the contact window. (f) Complete samples with Ta/Au top electrodes.

Mentions: The essential steps of this fabrication method are schematically shown in Fig. 1, involving three photolithography processes and one NSL process. First, blanket multilayer thin films of the constituent layers of MTJs are deposited on a Si wafer. The multilayer films are grown in a UHV magnetron sputtering system. More details on sample growth are presented in the supplementary section. The first photolithographic step with subsequent ion beam etching defines the base mesa structure of a 2 μm-wide line connecting two large contact pads (200 μm × 200 μm) as shown in Fig. 1a. For illustrative purpose we consider the simplest MTJ consisting of two FM electrodes separated by a thin tunneling barrier of MgO. We stop the ion beam etching at the top of the Si wafer. The second photolithographic step deposits photoresist pads with the exact size (200 μm × 200 μm) to protect the two bottom contact pads from the SiO2 deposited afterwards. We next employ NSL to place monodispersive polystyrene nanospheres of a specific diameter with a desired density on the top of the entire wafer. This is accomplished by placing an excessive amount of positive charge on the wafer through chemical treatment. After submerging the treated wafer into the nanosphere solution, the negatively charged nanospheres are then deposited with a density dictated by the concentration of the nanosphere solution and the dwell time (Fig. 1b). A uniform but random nanosphere coverage over a large area of several square inches can be readily accomplished. The nanospheres deposited on the 2-μm line serve as the hard masks to define the size of the nanopillars during the ion beam etching process. We stop the etching right at the MgO tunnel barrier layer (Fig. 1c). Next, a SiO2 insulating layer is deposited over the entire wafer, electrically isolating the top and the bottom electrodes of the MTJ pillars (Fig. 1d). Due to the good mechanical strength, the nanospheres also function as self-aligned mask for the SiO2 lift off, which exposes only the top of each nanopillar while the rest structures are covered by SiO2 (Fig. 1e). We then use the third photolithography step to pattern a series of 2-μm lines that are orthogonal to the 2-μm wire of the base mesa structure. The top contact pads are created by depositing and lifting-off a Ta/Au bilayer (Fig. 1f). The illustration in Figure 1 only shows one dumbbell-shape base mesa with six pair of top contact pads. In practice, a large quantity of such structure can be fabricated, providing thousands or even more of nanopillars on a single wafer.


Parallel fabrication of magnetic tunnel junction nanopillars by nanosphere lithography.

Wang WG, Pearse A, Li M, Hageman S, Chen AX, Zhu FQ, Chien CL - Sci Rep (2013)

Schematic fabrication procedure of MTJ nanopillars using NSL.(a) Bottom mesa structures are created from the blanket MTJ film by the first photolithography and ion beam etching, then the bottom contacts are protected by the second photolithography. (b) Random nanospheres are deposited on the entire wafer. (c) MTJ nanopillars are defined by the second ion beam etching. (d) SiO2 insulating layer is deposited. (e) Nanospheres and photoresist are lifted off, exposing the contact window. (f) Complete samples with Ta/Au top electrodes.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
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getmorefigures.php?uid=PMC3674430&req=5

f1: Schematic fabrication procedure of MTJ nanopillars using NSL.(a) Bottom mesa structures are created from the blanket MTJ film by the first photolithography and ion beam etching, then the bottom contacts are protected by the second photolithography. (b) Random nanospheres are deposited on the entire wafer. (c) MTJ nanopillars are defined by the second ion beam etching. (d) SiO2 insulating layer is deposited. (e) Nanospheres and photoresist are lifted off, exposing the contact window. (f) Complete samples with Ta/Au top electrodes.
Mentions: The essential steps of this fabrication method are schematically shown in Fig. 1, involving three photolithography processes and one NSL process. First, blanket multilayer thin films of the constituent layers of MTJs are deposited on a Si wafer. The multilayer films are grown in a UHV magnetron sputtering system. More details on sample growth are presented in the supplementary section. The first photolithographic step with subsequent ion beam etching defines the base mesa structure of a 2 μm-wide line connecting two large contact pads (200 μm × 200 μm) as shown in Fig. 1a. For illustrative purpose we consider the simplest MTJ consisting of two FM electrodes separated by a thin tunneling barrier of MgO. We stop the ion beam etching at the top of the Si wafer. The second photolithographic step deposits photoresist pads with the exact size (200 μm × 200 μm) to protect the two bottom contact pads from the SiO2 deposited afterwards. We next employ NSL to place monodispersive polystyrene nanospheres of a specific diameter with a desired density on the top of the entire wafer. This is accomplished by placing an excessive amount of positive charge on the wafer through chemical treatment. After submerging the treated wafer into the nanosphere solution, the negatively charged nanospheres are then deposited with a density dictated by the concentration of the nanosphere solution and the dwell time (Fig. 1b). A uniform but random nanosphere coverage over a large area of several square inches can be readily accomplished. The nanospheres deposited on the 2-μm line serve as the hard masks to define the size of the nanopillars during the ion beam etching process. We stop the etching right at the MgO tunnel barrier layer (Fig. 1c). Next, a SiO2 insulating layer is deposited over the entire wafer, electrically isolating the top and the bottom electrodes of the MTJ pillars (Fig. 1d). Due to the good mechanical strength, the nanospheres also function as self-aligned mask for the SiO2 lift off, which exposes only the top of each nanopillar while the rest structures are covered by SiO2 (Fig. 1e). We then use the third photolithography step to pattern a series of 2-μm lines that are orthogonal to the 2-μm wire of the base mesa structure. The top contact pads are created by depositing and lifting-off a Ta/Au bilayer (Fig. 1f). The illustration in Figure 1 only shows one dumbbell-shape base mesa with six pair of top contact pads. In practice, a large quantity of such structure can be fabricated, providing thousands or even more of nanopillars on a single wafer.

Bottom Line: We present a new method for fabricating magnetic tunnel junction nanopillars that uses polystyrene nanospheres as a lithographic template.Novel voltage induced switching has been observed in these structures.This method provides a cost-effective way of rapidly fabricating a large number of tunnel junction nanopillars in parallel.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, USA. wgwang@physics.arizona.edu

ABSTRACT
We present a new method for fabricating magnetic tunnel junction nanopillars that uses polystyrene nanospheres as a lithographic template. Unlike the common approaches, which depend on electron beam lithography to sequentially fabricate each nanopillar, this method is capable of patterning a large number of nanopillars simultaneously. Both random and ordered nanosphere patterns have been explored for fabricating high quality tunneling junctions with magnetoresistance in excess of 100%, employing ferromagnetic layers with both out-of-plane and in-plane easy axis. Novel voltage induced switching has been observed in these structures. This method provides a cost-effective way of rapidly fabricating a large number of tunnel junction nanopillars in parallel.

No MeSH data available.