Properties of Ordered Nanoporous Alumina as a Template for Pattern Transfer by MeV Ion Irradiation
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R9.30.1
Properties of Ordered Nanoporous Alumina as a Template for Pattern Transfer by MeV Ion Irradiation
Alenka Razpet 1*, Göran Possnert1, Anders Johansson1, Mohamed Abid1 and Anders Hallén2 1 Uppsala University, Ångström Laboratory, Uppsala, Sweden 2 Royal Institute of Technology, Department of Microelectronics & IT, Stockholm, Sweden *on leave from Jožef Stefan Institute, Department for Medium and Low Energy Physics, Ljubljana, Slovenia
ABSTRACT Ordered nanoporous alumina has recently been considered as a template for pattern transfer by MeV ion irradiation. Porous alumina of thickness 500 nm has already been used as a mask for low energy heavy ion implantation. However, in order to apply this material as a template for production of structures with higher aspect ratios, lighter ions with MeV energies and larger penetration depth should be used. The aim of this contribution is to examine the transmission of such ions through alumina templates with sufficient thickness to mask ions that are not passing the pores. A careful orientation of the pores in the direction of the incident beam is therefore needed since the acceptance angle for ions passing through the pores decreases with increasing thickness. Typical size of pores in the investigated alumina foils was 70 nm with a repetition frequency of 100 nm. Transmission through the foil and the energy of transmitted MeV ions were measured for aligned alumina foils by standard backscattering technique. It was shown that the sample transmission is a characteristic of each sample, but for some investigated samples the proportion of transmitted ions was the same as the relative area covered by pores. The energy spectra of transmitted ions were used to further evaluate the quality of foil templates for pattern transfer. It was shown that 2 µm thick alumina films can mask 3 MeV O+ ions, which can be used for ion lithography in polymer materials. INTRODUCTION Self ordered nanoporous alumina produced by two step anodization of high purity aluminum [1] has been successfully used as a template for chemical synthesis of nanoarrays [2]. Recently, this material has been applied as a mask for pattern transfer by heavy keV ion implantation and selective chemical etching [3]. As lighter ions with MeV energies have larger penetration depths in materials, structures with higher ratio between depth and diameter (aspect ratio) could be manufactured. However, thicker masks must also be used for this purpose, and thicker masks mean that the acceptance angle for transmission decreases. The acceptance angle for ion transmission through the pores is 2α, where the angle α is defined by tan α = d/t, and d is the pore diameter and t is the foil thickness. This angle is below 1° already for samples with pore diameters of 70 nm and thickness close to 10 µm. A careful alignment of the mask is essential.
R9.30.2
In this work, several nanoporous alumina samples of different thicknesses were characterized mainly using a 2 MeV He+ ion beam. An alignment procedure was developed based on our sta
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