Ion beam lithography using single ions

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Abstract

Presented here is a study to determine the conditions whereby holes etched along single ion tracks can be produced. Using standard tools of ion beam analysis a strategy has been developed to count single ions incident on a PMMA film spun onto a Si photodiode that functions as a detector. We investigate the sensitivity of PMMA to single ions as a function of the incident ion energy, mass and the PMMA development parameters. Non-contact atomic force microscopy (AFM) has been used to image the openings of holes etched along single ion tracks confirming the PMMA is sensitive to the passage of specific single ions. A high aspect ratio Si nanowhisker cantilever has been utilised to perform a quantitative analysis of the holes which are up to 45 nm in diameter for 71 MeV Cu ions.

Introduction

A beam of high energy MeV ions offers the potential to generate very high aspect ratio nanostructures. The latent damage track in a material following the passage of a single ion is cylindrical with a diameter between 10 and 100 nm and length of the order 104–105 nm. There is an extensive literature on track formation in solids and the formation of nanostructures, e.g. [1], use of ions for micromachining, e.g. [2] and many examples in the literature of studies of the tracks formed by high-energy ions [3], [4], [5] in polymers. There are also examples of using a focused beam of ions to write structures into a resist [6]. The emphasis in the present work is to use a specific number of well-oriented tracks, followed by development, to create structures at the ultimate resolution limit of a single ion. Using a precision positioning system, production of nanostructures with dimensions below the wavelength of light becomes possible. With irradiation at different tilt angles, even complex 3D nanostructures could be developed with useful applications [7], [8], [9], [10], [11].

There are four problems to overcome in order to achieve single ion resolution: (1) positioning a single ion impact in the right place, (2) detecting the passage of a single ion, (3) developing the latent damage along the length of the track to produce a hole and (4) imaging the high aspect ratio ion track. Problem (1) requires precision ion beam resolution and positioning. State of the art proton beam writing systems demonstrate a resolution of <100 nm in production of lithographic structures [12]. We propose nanometre precise positioning by use of a nanoaperture based on the method of Lüthi et al. [13], which is also being developed by Schenkel et al. [14] for highly charged keV ions. The spatial resolution limits imposed by ion scattering after passage through a nanoaperture is under investigation by Taylor et al. [15]. We overcome problem (2) by counting individual ion tracks using a substrate that acts as a detector onto which the resist film is spun. Problem (3) is addressed by studying the resist/developer combination in conjunction with different energetic ions. As a starting point the resist poly(methyl-methacrylate) PMMA was chosen. Finally problem (4) remains to be fully solved but can be partially addressed by the use of high aspect ratio scanned probes as we demonstrate here.

Section snippets

Film manufacture

PMMA films were prepared on various substrates using MicroChem 950 PMMA A2 and a spin coater with a spin speed of 5000 rpm. The resulting film thickness was approximately 60 nm after baking for 10 min at 180 °C. To expose the PMMA films to a precise number of ion impacts a method has been devised whereby a PMMA film is spun onto the face of a Hamamatsu S1223 Si PIN photodiode. After coating with PMMA the photodiodes were found to be fully functional and capable of producing signals from single MeV

Developer study

Regions of latent damage are developed to form a structure making use of the contrast in etching rates between the irradiated and unirradiated regions of resist. In lithography the emphasis is usually focused on obtaining high contrast because irradiation sources typically have a penumbra that, if etched, will worsen the resolution of the process. Since we aim to use the passage of a single ion then there is no penumbra and contrast is not so important. However, if the latent damage from a

Sensitivity determination

Holes with a diameter of a few tens of nm are difficult to image, especially if one is uncertain whether the holes have actually been formed owing to lack of resist sensitivity. A macroscopic method has been devised to measure the sensitivity of PMMA under different developing conditions. A PMMA film is exposed to a 3 MeV proton beam (LET = 11.2 eV/nm) in a 250 × 26 pixel array creating a rectangular exposed area of approximately 30 × 3 μm2. The low LET proton beam is used to achieve uniform damage

Identifying single ion tracks

The radii of ion damage tracks in PMMA are not well known. The passage of a 1 MeV/u ion through a material can produce up to 2 keV electrons. Using the range of a 1 keV electron in PMMA as a guide, we would expect to produce tracks of the order of 10 nm. For H and He with LET values <300 eV/nm, it is not certain that the damage deposited along a single track reaches the sensitivity threshold. In order to assure supra-threshold energy densities, heavier ions have been employed. We used 71 MeV Cu with

Hole analysis

Quantitative analysis of the holes has been performed with non-contact AFM utilising a Si nanowhisker type cantilever (NT-MDT, NSC05). Standard cantilevers were inadequate for this task. The high aspect ratio whisker at the cantilever tip enables imaging of the small openings to the etched ion tracks. The image of an area containing single ion impacts is shown in Fig. 3(a). To separate holes in the PMMA from the surface roughness a height discriminator of −4 nm is chosen based on the pixel

Conclusion

We have achieved counting of single MeV ion impacts into PMMA for low fluence exposures. Areas with homogeneous and exactly known fluence allow us to determine the sensitivity of a resist with respect to the developing conditions. We have performed the first step for single ion beam lithography by etching a counted number of single ion impacts.

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