We have fabricated arrays of Co dots of diameters 100 and 70 nm using interference lithography. Density of these arrays is 7.2x10{sup 9}/in{sup 2}. Magnetic force microscopy indicate that the Co dots are single domain with moments that can be controlled to point either in-plane or out-of-plane. Interference lithography is a process that is easily scaled to large areas and is potentially capable of high throughput. Large, uniform arrays of single-domain structures are potentially useful for high-density, low-noise data storage

Baker, S.L.

Bedrossian, P.J.

Fernandez, A.

Kania, D.R.

Vernon, S.P.

UNT Digital Library

UCRL- JC-lZ3OO3 PREPRINT Magnetic Force Microscopy of Single-Domain Cobalt Dots Patterned Using Interference Lithography DISCLAIMER This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights, Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes. I DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, m m -  mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Magnetic Force Microscopy of Single-Domain Cobalt Dots Patterned Using Interference Lithography A. Fernandez, P. I. Bedrossian, S .  L. Baker, S. P. Vernon, and D. R. Kania Lawrence Livermore National Laboratory, Livennore, CA 94550 Abstract - We have fabricated arrays of Co dots having diameters of 100 nm and 70  nm . using interference lithography. The density o f  these arrays is 7.2x10'lin2. Magnetic force microscopy measurements indicate that the C o  dots are single-domain with moments that can be controlled to point either in-plane or out-of-plane. Interference lithography is a process that is easily scaled to large areas and i s  potentially capable of high throughput. Large, uniform arrays of single-domain structures are potentially useful for high-density, low-noise data storage. INIRODUCI1ON It is anticipated that an alternate form of magnetic media will be required for very high-density data storage. ?he packing density of thin-film recording media is fundamentally limited by the jaggedness of domain walls which gives rise to noise in the readback signal [I]. In principle, a storage technology based on single magnetic domain particles can overcome this limitation and allow for much higher storage densities [2]-[4]. A single-domain particle can be magnetized in two discrete states with equal but opposite magnetic moments. An array of such particles could be used to record binary information with each particle storing one bit of information. Recently, several groups have investigated the magnetic properties of small, lithographically defined magnetic structures and considered the possibility of their use for data storage [2]-[4]. New et d. fabricated arrays of 150 nm by 200nm islands panerned from 20nm thick Co films [2], [3]. They used electron beam lithography to define the islands and a dry etch process to transfer the pattern. Magnetic force microscopy (MFM) indicates that the islands are singledomain with moments that ace in-plane 131. Using electron beam lithography and electroplating, Krauss et al. fabricated arrays of 50 nm diameter Ni piIlars with densities as high as 6 . 5 ~  1 O"/in* [4]. These particles exhibit singledomain behavior with moments that are out-of-plane. One of the main challenges in developing a storage ' technology based on single-domain particies is finding a Manuscript received March 4, 1996. A. Fernandez, e-mail Fernandez IO@lInl.gov, phone 510-424-2904, FAX 5 10-422-8761. viable and cost-effective process for fabricating high-density arrays of magnetic structures. Three key fabrication requirements must be satisfied. (1) The particles must be sufficiently small to inhibit multiple domain formation. This quires  that the paaicles be approximately 100 nm or less in diameter. (2) The size and shape of the particles must be uniform over large areas. (3) The arrays must be produced inexpensively and with a process that can be easily scaled to meet the demands of high-volume manufacturing. In this paper, we report on the use of interference lithography to fabricate high-density m y s  of single-domain Co dots. Interference lithography provides a relatively inexpensive way to generate periodic structures over large areas. It has been used in a variety of applications including the fabrication of x-ray and extreme ultra-violet gratings [SI, ultra-violet polarizers and filters is], and, more recently, field emission arrays for flat panel displays [7]-[9]. Importantly, interference lithography is capable of defining structures with dimensions of 100 nm [5]-[lo]. Here, we present MFM analysis of the fabricated Co arrays, These measurements demonstrate we can control the dimensions of the structures to permit easy-axis magnetization either in-plane or out-of-plane. elsewhere [9 ] .  Briefly, a resist coated Si wafer is exposed to the interference pattern. It is subsequently exposed a second time after being rotated 90" with respect to the first pattern. Development of the exposed layer produces a 2-dimensional array of resist hoIes or pedestals. Co dots are defined by thermally evaporating a thin layer of Co through the holes and subsequently using a lift-off process. No intermediate film is deposited between the Co and Si. The dots are centered on  a 300 nm square grid which t FABRICATION Fig. 1. MFM image of an array of type I Co dots whichare iOOomin diameter and 40 nrn tall. The dipole signature of these dots indicates the moments are in-plane. corresponds to a density of 7.2x109/in2. Their diameter is controlled by varying the exposure dose. Two types of samples are fabricated for this study. In the first type (referred to as type I), the nominal dot diameter is 100 nm and the Co thickness is 40 nm. In the second type (type 11), the nominal dot diameter is r e d u c e d  to 70 nm while the Co thickness is increased to 120 nm. A characteristic of this process is that type-I1 dots are shaped like cones. Cones are formed because the diameter of the holes decrease during deposition due to condensation of Co on the periphery of the holes. Consequently, as the deposition continues, cone shaped dots grow at the bottom of the holes. The tips of the cones are formed as the apertures close. The average height of the type11 dots is 100 nm as measured with an atomic force microscope (AFM). It follows from the above argument that type-I Co dots are shaped like truncated cones with a height of 40 nm. MAGNETlC FORCE MICROSCOPY MFM measurements are performed on a Nawscope Dimension 5000 AFh4. All MFM images are phase images acquired using Digital Instruments’ patented LiftMode [ 111 technique. Fig. 1 shows an MFM image of an array of type I Co dots. The characteristic dipole-like shape of the dots indicates that each dot is uniformly magnetized in a direction parallel to the substrate. This result is consistent with recent studies by New et af. (31 which show that patterned Co particles smaller than 200 nm are singte-domain. It is clear from Fig. 1 that the in-plane moments of these dots are not preferentially aligned. This is expeded since they are circularly symmetric in the plane and therefore do not possess a strong azimuthal shape anisotropy. Dots with an elliptical shape can be fabricated with this process by varying the relative exposure dose for the two orthogonal interference exposures. Fig. 2. MFM image of an m y  of type I1 Co dots which are 70 nm in diameter and 100 nrn tall. The symmetry of the images shows that the moments are out-of-plane. By varying the aspect ratio of the dots out of the plane, we are able to control the direction of magnetization. In Fig. 2 is an MFM image of type IJ Co dots which are taller and narrower than type I dots. The images of the dots are primarily azimuthally symmetric and either dark- indicating a negative force gradient-r light-indicating a positive force gradient. We interpret these images to show that the Co dots are single-domain and magnetized in a direction which is normal to the plane of the substrate. We find that over 95 % of the type I1 Co dots are magnetized in an out-of-plane direction. Figs. 3(a) and 3(b) show MFM images of type I1 Co dots after application of a strong magnetic field along the surface normal in a direction parallel and anti-parallel to the tip moment respectively. From these images we learn that dark dots correspond to moments which are parallel to the tip moment while light dots correspond to dots which are anti-parallel. A slightly larger percentage of dots are magnetized parallel to the tip in Fig. 3(a) compared those that are anti-parallel in Fig. 3(b). This is attributed to a weak but discernible tendency for anti-parallel moments to flip during scanning to a parallel configuration. As an example, in the lower right comer of Fig. 3(b) are two dots which flipped as the MFM tip passed o v a  them. (The image was scanned from top to bottom.) Fabricating dots with a stronger shape anisotropy should minimize this effect. CONCLUSIONS We have fabricated arrays of Co dots with diameters of 100 nm and srqaller with an areal density of 7.2x109/in2 using laser interference lithography. MFM analysis of the dots indicates that they are single-domain with moments that can be controlled to point either in-plane or out-of-plane. While a fabrication process based on lift-off has been used here, it should be possible to couple other fabrication schemes such as dry etching and electroplating Fig. 4. MFM images of an array of type I1 Co dots after application of a strong magnetic field along the surface normal in a direction (a) parallel and (b) anti-parallel to the MFM tip moment. with interference lithography to pattern arrays of small magnetic structures with a wide variety of 3-dimensional shapes. The distinct advantages of using interference lithography over other lithographic techniques are that high-density arrays of nanometer scale structures can be produced over large areas, in a relatively inexpensive way, and with potentially large throughput. ACKNOWLEDGMENT This work was performed under the auspices of the U. S .  Department of Energy by the Lawrence Livermorc National Laboratory under contract no. W-7405-Eng-48. REFEENCES [l] A. M. Barany and H. N. Bertram, “Transition noise model for longitudinal thin-film media,” IEEE Traits. Mag., vol. 23, pp. 1776-1788. March 1987. [2] R. M. H. New, R. E W. Pease. and R. L. White, “Physical and magnetic properties of submicron lithographically patterned islands”, J .  Vuc. Sci. Technof. B, vol. 13, pp. 1089-1094, May/June 131 II. M. H. New, R. E W. Pease. and R. L. White, “Effect of magnelocrystalline anisotropy in single-domain polycrystalline cobalt islands.” /EEE Truiis. Mug., vol. 31. pp. 3805-3807. Nov 1995. I 1995. ~ 141 P. R. Krauss and S. Y. Chou, “Fabrication of planar quantum magnetic disk structure using electron beam lithography. reactive ion etching, and chemical mechanical polishing,” J. Vac. Sci. Technol. B, vol. 13, pp. 2850-2852, NovlDec 1995. and H. I. Smith, “Fabrication of high energy x-ray transmission gratings for AXAF,” SPIE Proc., vol. 2280, pp. 181-190. May 1994. Schattenburg, ‘Excreme-ultraviolet polarization and filtering with gold transmission gratings,” Applied Oprics , vol. 34, pp. 648654, Feb 1995. p] CO. Bozler. C.T. Harris, S. Rabe, D.R. Rathman, M. Hollis. and H.I. Smith, ‘‘Arrays of gated fieidemitter cones having 0.32 p tip-to-tip spacing.” J. Vac. Sci. Technol. B , vol. 12, pp. 629-632, Mar/Apr 1994. [8] J. P. Spallas, A. M. Hawryluk, and D. R. Kania, “Field emitter array mask pattering using laser interference lithography,” i. VUC. Sci. TechnoL B, vol. 13, pp. 1973-1978, Sep/Oct 1995. (91 J. P. Spallas, R. D. Boyd, J. A Britten, A. Fernandez, A. M. Hawryluk, M. D. Perry, and D. R Kania, “Large scale field emitter array patterning for flat panel displays using interference lithography,” J. Vac. Sci. Technol. B., in press. [lo] S. H. Zaidi and S .  R. J. Brueck, “Multiple-exposure interference lithurgaphy,” J. VUC. Sci. Technof. B. vol. 11, pp. 658-666. May/June 1993. [ I  I J LiftMode is a trademark of Digital Instruments. LihMode, V. Elings and J. Gurley, U. S. Patent No. 5,308,974, Digital Instruments. Santa Barbara, Ca. [5] M. L. Schattenburg, RJ. Aucoin. R.C. Fleming. I. Plotnick, J. Porter, [6] E.E. Scime. E. H. Anderson, D. J. McComas. and M. L. 

Magnetic force microscopy of single-domain cobalt dots patterned using interference lithography

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Magnetic force microscopy of single-domain cobalt dots patterned using interference lithography

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