NANO LETTERS
Direct Thermal Patterning of Self-Assembled Nanoparticles
2003 Vol. 3, No. 12 1643-1645
Hendrik F. Hamann,* S. I. Woods,† and Shouheng Sun IBM T. J. Watson Research Center, Yorktown Heights, New York 10598 Received August 26, 2003; Revised Manuscript Received October 8, 2003
ABSTRACT A general methodology for direct thermal patterning of self-assembled nanoparticle films is presented. Specifically, using laser heating we have fabricated ferromagnetic FePt nanoparticle array patterns with 0.8 µm pitch and an edge resolution of ∼100 nm. The nanoparticle arrays have been investigated by scanning SQUID microscopy and MFM. Finally, we have measured the patterning rate as a function of temperature by monitoring the change in optical reflectance.
One of the most exciting prospects of nanoparticles is their implementation as building blocks for novel electronic, photonic, and magnetic devices.1-3 While major progress has been made in synthesizing nanoparticles and in controlling the self-assembly of thin films, a general method to pattern and modify these films is not available. Previous techniques to form patterned arrays relied on conventional photo or e-beam/ion beam lithography,4-7 which can be a complicated process. Recently, a simpler patterning technique using direct e-beam exposure has been demonstrated.8,9 While this direct patterning method has successfully reduced the number of process steps (i.e., from five to three), the required vacuum for the e-beam exposure is still costly and could potentially limit the throughput of such a technology. In this paper we present an even simpler patterning methodology, which utilizes localized heating. Our technique is generally applicable; it does not require vacuum conditions and is compatible with emerging contact printing techniques,10-12 micro/nanosphere lithography,13,14 and copolymer lithography.15,16 Depending on the choice for the heat source, the potential resolution may be similar to e-beam lithography. Specifically, using laser heating we demonstrate nanoparticle array patterns with 0.8 µm pitch and an edge resolution of ∼100 nm. After further thermal annealing, the patterned FePt nanoparticle arrays are investigated by a SSM (scanning SQUID microscope) and a MFM (magnetic force microscope), demonstrating the ferromagnetism of the patterned islands. Finally, we investigate the rate of the patterning as a function of temperature by monitoring the change in optical reflectance. The patterning process is illustrated in Figure 1. As a first step nanoparticles with attached stabilizers are deposited on a substrate as described previously.3 The particle film * Corresponding author. E-mail:
[email protected] † Present address: Neocera, 10000 Virgina Manor Road, Suite 300, Beltsville, MD 20705-4215. 10.1021/nl034706d CCC: $25.00 Published on Web 10/29/2003
© 2003 American Chemical Society
Figure 1. Process flow of the direct thermal patterning method.
thickness can be tuned readily using self-assembly deposition techniques. In this work we have patterned successfully on glass as well as on Si/SiO2 (∼50 nm) substrates. In a second step we expose regions of the film to a heat source, which carbonizes the surfactants.3 In a third and final step unexposed regions of the film are removed by an appropriate solvent such as hexane. In our experiments the heat treatment is realized with a pulsed and focused laser beam (@532 nm; NA ∼0.8; pulse width ∼10 s; absorbed laser power ∼4 mW), which is scanned with respect to the sample. The size of the islands can be tuned by either adjusting the laser pulse width or controlling the laser power. Larger patterns can be made by scanning the laser beam slowly with respect to the sample. The temperatures are estimated to be ∼550 °C. The as-
Figure 3. SSM (A) and MFM (B) image of patterned FePt nanoparticle arrays.
Figure 2. Atomic force microscope (AFM) (A) and corresponding line scan (B) of patterned arrays of about four layers of 6 nm FePt nanoparticles, and high-resolution AFM images (C) showing the particle ordering within a patterned island.
synthesized and patterned particles exhibit a chemically disordered face-centered cubic (fcc) phase and are superparamagnetic. Post-deposition annealing converts the particles to a ferromagnetic, ordered face-centered tetragonal (fct) phase, which has a high magnetic anisotropy (Ku ∼ 6 × 106 J/m3) at room temperature.3 Figure 2 shows respectively an atomic force microscope (AFM) image (A) and a corresponding line scan (B) of a pattern of 6 nm FePt nanoparticles. It demonstrates that the pattern has been developed cleanly and that the background (unexposed regions) is free of particles. The nanoparticle islands have a diameter of ∼0.8 µm with fairly sharp edges (
