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Langmuir 2004, 20, 11208-11212
Fabrication of Large-Area Ferromagnetic Arrays Using Etched Nanosphere Lithography Shemaiah M. Weekes,* Feodor Y. Ogrin, and William A. Murray School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, United Kingdom Received May 27, 2004. In Final Form: September 14, 2004 Nanosphere lithography (NSL) is a simple, cost-effective, and powerful technique capable of producing large-area arrays of ferromagnetic nanostructures with dimensions below 100 nm. These properties make NSL an attractive process for the fabrication of arrays of magnetic elements with applications in magnetic data storage. The main disadvantage with conventional NSL is that the monolayer of spheres always contains imperfections that are transferred to the resulting nanostructures. This can significantly affect the structural and magnetic properties of the fabricated array. In this paper we present a novel adaptation of NSL that reduces the effect of such defects on the resulting nanostructures. The technique also offers excellent control over the diameter, aspect ratio, and pitch of the fabricated elements. These properties are demonstrated through the fabrication of arrays of Ni elements of 210 nm diameter and arrays of Co elements with diameters between 200 and 320 nm.
1. Introduction Self-assembled and patterned magnetic nanostructures are currently the subject of much interest due to their enormous potential in future nanotechnology. Highdensity magnetic recording is one area where a move toward the use of patterned media promises to increase storage densities by several orders of magnitude compared to that currently available in magnetic disk drives. In these systems nanofabrication or self-assembly techniques are used to produce artificially isolated elements of magnetic material, typically of dimensions below 100 nm. Each artificially fabricated magnetic element in the pattern has the properties of a single domain particle with only two possible magnetic states, “up” or “down”, corresponding to those of a recorded bit, “1” or “0”. Given the low dimensions and high anisotropy, the overall density of information which can be achieved is estimated to be well above 1 Tbit/in.2.1 Although the advantages of using patterned recording media are evident, the mass production of these systems for both commercial and research purposes is complicated due to the cost and time scales involved in fabrication. For instance, a typical fabrication process of a patterned array would involve electron-beam lithography (EBL). EBL can achieve very high resolution, down to 5 nm/ feature. However, the area of the pattern is unlikely to exceed an area of 1 mm2, which is insufficient for application as a recording medium. Even with the possibility of replicating the pattern over a larger area,2 this method is not yet sufficiently fast and reliable. The limited area of patterns has also presented problems with regard to the application of magnetic and structural characterization techniques. The limitation in the overall volume of magnetic material directly relates to the maximum amplitude, which can be obtained from measurements. With the samples available from EBL, many standard magnetization techniques are simply not sensitive enough to resolve the signals. In this sense the main problems in * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (2) Cui, B.; Wu, W.; Kong, L. S.; Sun, X. Y.; Chou, S. Y. J. Appl. Phys. 1999, 85, 5534.
the fabrication of patterned recording media still remain: finding a fast and cost-effective process, capable of producing patterns over a large area and with high resolution. Nanosphere lithography (NSL) is a simple and very effective technique, which fulfils the above criteria3 and is thus valuable for producing prototype patterned media samples. In a typical procedure, fabrication of samples is achieved by depositing a single layer of polystyrene (PS) spheres from a colloidal suspension. Under correct conditions, evaporation of the solvent causes the spheres to form an ordered monolayer at the sample’s surface. The monolayer can then be used as a matrix for metal deposition. After metal deposition and removal of the spheres, a pattern of sub-micrometer-size particles is achieved.4 Commercial availability of nanospheres with diameters from 50 nm to greater than 1 µm makes it possible to vary the size and pitch of the magnetic elements. However, one of the main problems associated with this method is that the monolayer of spheres always contains dislocations resulting in agglomerations of particles after metal evaporation. In this paper, we suggest a method which not only avoids agglomerations but also improves control over the dimensions of the fabricated elements. This method follows the approach described previously by Haginoya et al.,5 where a monolayer of PS spheres is used to create an array of holes. We have extended this idea by developing a technique that allows electrodeposition of ferromagnetic material into the array of holes and offers control over the size and aspect ratio of the resulting elements. Below it is demonstrated that using this fabrication process, one can produce patterned, magnetic arrays, over an area greater than 1 cm2, with the diameter of a single element in the range of 100 nm to 1 µm. Although this technique is not suggested as a commercial method for the production of patterned media, it provides an ideal basis for studies (3) Burmeister, F.; Scha¨fle, C.; Matthes, T.; Bo¨hmisch, M.; Boneberg, J.; Leiderer, P. Langmuir 1997, 13, 2983. (4) Rybczynski, J.; Ebels, U.; Giersig, M. Colloids Surf. A 2003, 219, 1. (5) Haginoya, C.; Ishibashi, M.; Koike, K. Appl. Phys. Lett. 1997, 71, 2934.
10.1021/la048695v CCC: $27.50 © 2004 American Chemical Society Published on Web 11/03/2004
Ferromagnetic Arrays Using Nanosphere Lithography
Langmuir, Vol. 20, No. 25, 2004 11209
Figure 2. Array of 210 nm diameter holes in MgF2 formed by NSL. Figure 1. Schematic diagram of the fabrication process: (a) Deposition of a monolayer of PS spheres; (b) reduction of the sphere diameter using RIE and subsequent evaporation of MgF2; (c) removal of the spheres and RIE to leave an array of holes; (d) electrodeposition of ferromagnetic material to form an array of columns.
of the local magnetic properties of nanostructures in relation to their geometrical dimensions. 2. Experimental Section The fabrication procedure involved the following steps. A 100 nm metallic layer of Ti or Cu was evaporated onto a glass substrate of thickness 0.25 mm to create a conductive layer for the final-stage electrodepositon. A further 200 nm of SiO2 was then deposited on top of the metallic layer using RF sputtering. A sputtering time of 30 min and an Ar ion pressure of 5 mTorr were used to obtain the required thickness. The predeposition base pressure was 10-6 mTorr. Following the SiO2 growth, the samples were placed in an ultrasonic bath, first in alcohol and then in Milli-Q water before being thoroughly dried. It should be noted that using conventional methods, it is necessary to carry out surface treatments to make the substrate hydrophilic before depositing the PS spheres.6,7 This ensures even coverage and plays an important role in the self-assembly of the close-packed monolayer.8 It was found, however, that SiO2 prepared in the way detailed above is already hydrophilic and requires no special surface treatments. It is likely that this is a result of exposure to the Ar ion plasma during the sputtering process.9 The deposition of spheres was performed using the method described by Jensen et al.10 A mixture of solution containing PS spheres and Milli-Q water was dispensed onto the substrate using an Eppendorf pipet (Figure 1a). A ratio of 4.5 µL of solution to 65 µL of water achieved best results, although the optimum ratio can depend on a number of factors, including the age of the solution, drying conditions, and surface quality. The PS carboxyl spheres were obtained from “Duke Scientific”. The solution contained 4 wt % spheres with diameter 390 nm and a quoted size distribution of
