Letter pubs.acs.org/NanoLett
Low-Resistivity 10 nm Diameter Magnetic Sensors Mazin M. Maqableh, Xiaobo Huang, Sang-Yeob Sung, K. Sai Madhukar Reddy, Gregory Norby, R. H. Victora, and Bethanie J. H. Stadler* Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States ABSTRACT: Resistivities of 5.4 μΩ·cm were measured in 10-nmdiameter metallic wires. Low resistance is important for interconnections of the future to prevent heating, electromigration, high power consumption, and long RC time constants. To demonstrate application of these wires, Co/Cu/Co magnetic sensors were synthesized with 20−30 Ω and 19% magnetoresistance. Compared to conventional lithographically produced magnetic tunnel junction sensors, these structures offer facile fabrication and over 2 orders of magnitude lower resistances due to smooth sidewalls from in situ templated chemical growth.
KEYWORDS: Single nanowire resistivity, magnetic sensors, read sensors, magnetic recording, high density hard drives, CPP GMR
A
s device dimensions are reduced1,2 below the scale of electron mean free paths (λe), electrical resistivities at the nanoscale have increasing scientific interest and technological importance. Models of resistivity often predict degraded performance for devices of 10−40 nm dimensions (λmetals ∼ 40 nm) due to potential surface and grain boundary scattering.3−6 Experimental measurements of electron transport in metallic nanostructures have also been extensive, but challenging, due to contact design, corroded surfaces, grain boundary resistances, and scattering losses, especially for nanowires with exposed edges.7−10 For copper alone, reported resistivities have varied by several orders of magnitude depending on fabrication and measurement techniques.11−14 Here, resistivities of 10 nm diameter metallic wires were measured in densely packed, vertical arrays inside anodic alumina templates using an atomic force microscope (AFM) tip as a top contact, similar to recent measurements by Choi et al. of 20 nm wires.15 These arrays are excellent candidates for devices such as array read sensors,16,17 highly dense magnetic random-access memory,17 and self-assembled electronics.18 Therefore, the results reported here not only promise to improve near-term read sensors but also to point the way to low resistance interconnects which will quench growing concerns that the “size-effect” is threatening to halt progress along the International Technology Roadmap for Semiconductors (ITRS).19 Other high-density arrays have been proposed for hard drive media where increased recording density requirements20,21 has motivated the fabrication of magnetic nanoscale assemblies with long-range order using self-assembly,22−32 e-beam lithography,33,34 and block copolymers.35−41 The new technologies expected to be introduced at 1−1.5 TBit/in2, such as heat assisted magnetic recording (HAMR) and bit patterned © 2012 American Chemical Society
magnetic recording (BPMR), will have reduced media bit aspect ratios (BAR) with track pitches of about 30 nm, suggesting a need for 99.99%) were degreased in acetone for 10 min at room temperature and rinsed subsequently in methanol, isopropyl alcohol (IPA), and finally with deionized (DI) water. After drying in nitrogen, the native oxide layer was removed using a 1 M NaOH solution for 3 min. A DI water rinse and N2 drying were followed by electrochemical polishing to remove surface defects of the Al foil. The electrochemical polishing solution was a 1:5 mixture of perchloric acid (HClO4) and ethanol, and a constant potential of 18 V at 6 °C was applied. After this, the foils were anodized in sulfuric acid at various voltages, temperatures, and acid concentrations to explore the minimization of pore size. The resulting nanoporous oxide was etched using a mixture of 1.8 wt % chromic acid (Cr2O3) and 6 wt % phosphoric acid (H3PO4) at 60 °C for 20 min. The remaining Al foil was then anodized again under identical conditions for 3 h to produce uniform nanopores inside a new oxide layer that was 17 μm thick. For templates on Si, Ti (200 nm)/Cu (200 nm) electrodes were first evaporated onto Si, followed by evaporation of a 1 μm thick Al film which was then anodized similarly to the foils above but with different times to produce 190 nm thick AAO. This Si-integrated AAO was used for all of the read sensor studies. Next, a single electrolytic bath was used to electroplate trilayered [Co(15 nm)/Cu(5 nm)/Co(10 nm)] structures with copper leads inside the AAO nanopores. The bath composition was 155 g/L CoSO4, 1.13 g/L CuSO4·7H2O, and 50 g/L H3BO3 to maintain the pH at 3.7. At this pH, the Co crystallographic easy axis will be in-plane. Co and Cu layers were deposited using −1.00 V and −0.52 V, respectively, versus
these read sensors will be minimal because operating read sensor current densities are substantially below these values.68 The window between the aligned and antialigned states exhibited a magnetoresistance of 19%, a value that was comparable to those obtained in our typical magnetoresistance measurements, as in Figure 6. Here, the magnetizations of a
Figure 6. Magnetoresistance of 10 nm diameter Co(15 nm)/Cu(5 nm)/Co(10 nm) trilayered nanowires. Here, about six wires were contacted with low and high resistances of 17.489 Ω and 21.129 Ω, respectively.
sensor are aligned using a 6 kOe field, and then the resistance is measured as the field is swept to −6 kOe and back. At −500 Oe, the switching of the soft Co layer increases the resistance due to the GMR effect, and then at −2.5 kOe, the switching of the hard Co layer decreases the resistance back to the aligned value. Returning to +6 kOe, the resistance jumps are observed again due to the soft then the hard Co layers switching back. Other groups have also reported 10−20% MR for electrodeposited multilayered Co/Cu structures although with larger (30−100 nm) diameters.70−72 The total RA product of our samples was 0.04 Ω·μm2 with ΔRA = 0.008 Ω·μm2 including the area of the oxide matrix and the copper leads. The resistance and RA product of a single trilayer (without the nanowire leads73 or matrix) can then be calculated as 20−30 Ω and 0.002 Ω·μm2, respectively, with ΔR (= 4−5 Ω) which are nearly ideal values. Currently, commercial read sensors are individually patterned from their thin film precursors by ion milling, and then they are insulated with an aluminum oxide coating that has to be chemomechanically planarized. Therefore, unlike MRAM applications, the need to contact a single nanowire within an array is not required here because the individual sensor device will be patterned out of the array. Rather, it is a great advantage for these electrodeposited structures that they are inside a suitable insulator as-grown. Therefore, using commercial fabrication techniques, a 30 nm diameter etch will produce a 10 nm diameter device that is already surrounded by 10 nm of aluminum oxide on all sides. Even though these elements are 10× smaller in area, the properties shown above are comparable to state of the art CPP GMR structures grown by vacuum deposition and lithographic methods50 where 30 nm diameter structures have been fabricated with RA products of 0.043 Ω·μm2 and 5.5% MR. Recall that smaller diameter structures have proven exceedingly difficult via lithography. The attractive results presented here are a result of the layers being grown in situ: thus, the sidewalls never experience damaging etching conditions. Experimentally, 4106
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a Ag/AgCl reference electrode. The structures of the nanopores and nanowires were characterized by scanning electron microscopy (SEM JEOL6700). More information on the crystalline structure and magnetic properties of electrodeposited Co is given in ref 75. To measure spin torque switching, high currents are required with accurate probe pressure for controlling the number of nanowires per measurement. Therefore, a custom-made apparatus was assembled to allow an external Keithley 2400 sourcemeter to supply ±100 mA to a circuit containing an Aucoated AFM tip at the top of the nanowires, the nanowires themselves, and the growth contact at the bottom of the nanowires. The voltage across the nanowires was then measured with a National Instruments NI 9215 module. The AFM tip was controlled using a Jandel microprobe system that had adjustable pressures within the range of 30−70 g. The tip itself (Bruker NPG) had a 90 nm tip radius and was attached to the Jandel probe using In solder. The bottom film was the Ti/ Cu contact described in the growth section above, and the equipment was controlled using NI LabView. Magnetic fields of 130 Oe and 250 Oe were applied in-plane to the samples by an electromagnet. For single wire resistances, the same setup was used but with a fixed current of 100uA. Magnetoresistance was measured using a 2.5 mA current and sweeping the applied magnetic field.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge original support from NSF ECS0621868 and the GOALI program. We give special thanks to Tao Qu for dipole field calculations and Seagate Technologies, Ed Murdock and Haesok Cho for useful discussions. This work was supported primarily by the MRSEC Program of the National Science Foundation (NSF) under Award Number DMR-0819885. The work originated under NSF ECS0621868 and the GOALI program. We give special thanks to Tao Qu for dipole field calculations. Parts of this work were carried out in the University of Minnesota Nanofabrication Center and Characterization Facility which receive partial support from NSF through the NNIN program.
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REFERENCES
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