Long-range ordered nanoaperture array with uniform diameter and interpore spacing

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Long-range ordered nanoaperture array with uniform diameter and interpore spacing

Ming-Nung Lin

Department of Physics, National Taiwan University, Taipei 106, Taiwan Minn-Tsong Lin^a兲

Department of Physics, National Taiwan University, Taipei 106, Taiwan and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan

C. Y. Liu and M. Y. Lai

Institute of Atomic and Molecular Sciences, Academia Sinica, P. O. Box 23-166, Taipei 106, Taiwan N. W. Liu

Institute of Atomic and Molecular Sciences, Academia Sinica, P. O. Box 23-166, Taipei 106, Taiwan and Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan C. Y. Peng, H. H. Wang, and Y. L. Wang^b兲

Institute of Atomic and Molecular Sciences, Academia Sinica, P. O. Box 23-166, Taipei 106, Taiwan and Department of Physics, National Taiwan University, Taipei 106, Taiwan

共Received 27 May 2005; accepted 20 September 2005; published online 20 October 2005兲 In application of focused-ion-beam lithography and grazing Ar⁺milling on the U-shape barrier layer of anodic alumina nanochannels, we fabricated a hexagonally symmetry aperture array with nominal diameter of 12± 2 nm and interspacing of 100± 2 nm. Besides long-range spatial ordering, the focused-ion-beam guided-grown process has also significantly improved uniformity of both the interpore spacing and the aperture size. This aperture array membrane can be applied to the fabrication of nanostructures, such as a lithographic contact mask for ordered quantum-dot array.

on the nanoscale have attracted more and more interest.^1–3 One of the important nanostructures is the self-organized hexagonally anodic alumina ordered array of nanochannels.

With its highly uniform and straight nanopore structure, and its extremely high-density advantages, an anodic alumina ox- ide 共AAO兲 nanochannel is extensively applied to advanced nanodevices, such as carbon nanotube arrays, for flat panel displays,⁴ magnetic storage media,⁵ and single electronics.⁶ By a two-step anodization, AAO nanochannels self-organize into hexagonally close packed共hcp兲 domains with an average size of a few micrometers.⁷ Even though the nanochannels of such self-organized arrays also exhibit improved size uniformity, the presence of irregular channels along the domain boundaries sets a severe limitation on its further improvement. To overcome this drawback, five approaches have been developed to fabricate an ideally ordered nanochannel array on AAO film: Nanoimprint lithography,^8–10 electron-beam lithography,¹¹ direct focused- ion-beam 共FIB兲 lithography,^12,13 holographic lithography,¹⁴ and resist-assisted FIB lithography.¹⁵ These lithographic methods first create an ordered array of concaves on the Al surface and then use the concaves as “pinning points” to align the orientation of the nanochannel domains and therefore completely eliminate the presence of domain boundaries. Especially for nanoimprint and holographic lithography, the monodomain area can reach to ⬃cm².^10,14Besides the long-range ordered structure, many research groups^16–18 are actively seeking the way to fabricate the AAO with a smaller pore size; however, it is rather difficult to ideally

fabricate ordered AAO with a pore size less than 20 nm for the natural limit caused by the lithography.^16–18On the other hand, the pore diameter of nanochannels is 40% of the interspacing, which severely limits the application possibilities when both a large interspacing and a small pore diameter are required. In this letter, we use grazing Ar⁺ milling to shrink down and control the pore or aperture diameter of an ideally ordered nanochannel array. By this way, we fabricated a hcp nanoaperture array with a nominal diameter of 12± 2 nm and interspacing of 100± 2 nm.

Figure 1 represents the process of the sample prepara- tion. High-purity Al共99.99%兲 is electropolished in a mixture solution of HClO₄ and C₂H₅OH 共ratio 1:5兲 at 5 °C under constant stirring. A commercial 50 keV Ga FIB with a beam diameter⬃10 nm and a beam current ⬃1.1 pA is utilized to create an array of hcp concaves on a polished Al surface.

These concaves are employed to guide the anodization growth process of nanochannels. We use 0.3 M oxalic acid

a兲Electronic mail: mtlin@phys.ntu.edu.tw

b兲Electronic mail: ylwang@pub.iams.sinica.edu.tw

FIG. 1. Schematic diagram showing the process for fabrication of the nano- pattern apertures.

APPLIED PHYSICS LETTERS 87, 173116共2005兲

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solution to create nanochannels with 100 nm interspacing at 4 °C and 40 V anodization voltage. The remaining Al sub- strate is removed by a CuCl₂solution共3%兲 in order to expo- sure the naked barrier. An Ar⁺ beam miller machine MPS- 3000 PBN is set up to sputter the U-shaped barrier layer in order to create the nanoapertures. The Ar⁺beam current density is approximately 1.2 mA cm⁻² with a beam energy 500 eV. The incident Ar⁺ beam was tilted by an angle around 75°to control the aperture size. Finally, the images of nanoapertures are taken by transmission electron microscope 共TEM兲. By taking the top view of the thin 共⬃150 nm兲 alumina film, the electrons directly passing through the apertures without any interceptions will project these apertures to form the shining spots image on the screen.

Figure 2 represents the TEM image with a boundary between the two areas of both growth modes with共left-hand side兲 and without 共right-hand side兲 FIB-guided prepatterning.

The array comprised by FIB-guided channels has increased not only the degree of order of the nanoapertures but also their interpore spacing and aperture size. In contrast, the array comprised of a self-organized channel loses the long- range order and reveals a dispersive distribution in its interpore spacing and aperture size. Fourier transform was made for each area in order to further clarify the differences between two areas. In the self-organized area, the emerging peaks of the Fourier transform form an isotropic ring pattern, indicating the absence of a long-range ordering in the FIB- guided area; the hexagonally arranged peaks are sharp. This can be connected to the improvement of the degree of order in the orientation arrangement of the nanochannels.

Figures 3共a兲 and 3共b兲 show the TEM images of the nanoapertures in a self-organized area on different scales. It

can be obviously seen that the size distribution of these apertures 关Fig. 3兴 is relatively broad. Besides, the shapes of these apertures are irregular and the interpore spacing between each neighboring apertures are diverse. Figures 3共c兲 and 3共d兲 show the TEM images of the nanoapertures in a FIB-guided area on different scales. These apertures are mostly round shaped, and reveal a uniform size distribution with a diameter of 12 nm. Moreover, the interpore spacing between these neighboring apertures is 100 nm.

Two distinct characteristics in these TEM images should be discussed in more details. First, the electrons passing through these apertures were indeed exposed to form the shining spots in the TEM images. The contrast is good enough to clarify the differences between apertures and nearby alumina areas. Second, the shining spot in the TEM image is the projection of the transmitted electron beam through the whole channel; therefore, there are two important factors that affect the certification of the aperture size. Both the roughness of the channel wall and the inclined angle of the pore will modify the projection image of the electron beam. Thus, the nominal diameter of the apertures obtained from the TEM image also reflects the overall effect of geometrical factors mentioned above on electron transmission.

Finally, a statistical analysis in the geometrical distribution was made in order to have a clear comparison between the different nanoapertures from two growth modes. We randomly chose 100 apertures to measure diameter size of aperture. The size distribution of nanoapertures is shown in Fig. 4. The mean value of the FIB-guided aperture size is 12 nm with dispersion ±2 nm. More than 80% of the aperture sizes are distributed between 11–13 nm. The mean value of the interpore spacing is 100 nm with a dispersion of ±2 nm.

On the other hand, the average size of the apertures made from the self-organized AAO is 13 nm, however, with a much larger dispersion of ±6 nm. Less than 35% of the aperture sizes are spread between 12–14 nm. The mean value of interpore spacing is 102 nm with a dispersion ±25 nm.

These distinct differences can be mainly ascribed to two rea- sons. First, the aperture size depends on the pore size and curvature of the barrier layer with the same inclined grazing Ar⁺milling angle. The size distribution and the interspacing

FIG. 2. TEM image with a boundary between the two areas of different growth modes. The insets are the Fourier transform for the FIB-guided area 共right corner兲, and self-organized area 共left corner兲, respectively.

FIG. 3. TEM images of the randomly selected area in different scales, the inset is the profile of the selected aperture共the middle one in the bottom line兲.

173116-2 Lin et al. Appl. Phys. Lett. 87, 173116共2005兲

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between neighboring channels were demonstrated to be ex- cellently uniform in the FIB-guided sample.⁶In contrast, the self-organized sample cannot achieve such uniformity. Sec- ond, the relative height around neighboring channels also affects the aperture size. The lower channel 共compared to nearby channels兲 will be shielded by its neighbors under the inclined grazing Ar⁺beam and then this results in the smaller aperture. Compared to the self-organized mode, the growth rate of the FIB-guided channels is more identical and this leads to same height of each channel.

In conclusion, we demonstrated a combined process of focused-ion-beam-guided prepatterning technique and grazing Ar⁺milling to fabricate long-range ordered nanoaperture arrays. The nominal diameter of the created apertures is 12± 2 nm and the interspacing is 100± 2 nm. Compared to the self-organized process, the focused-ion-beam-guided process has increased not only long-range order but also uniformity of the aperture size as well as the interpore spacing.

This technique could be extensively applied to the manufac- turing of advanced nanodevices with requirement of high nanoscale ordering in a large area.^4,5,19,20

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FIG. 4. The statistical analysis for two growth modes:共a兲 Aperture size distribution in FIB-guided area;共b兲 interspacing distribution in FIB-guided area; 共c兲 aperture size distribution in self-organized area; and共d兲 interspacing distribution in self-organized area.

173116-3 Lin et al. Appl. Phys. Lett. 87, 173116共2005兲