Ion Tracks and Microtechnology

The fabrication of microelectronic circuits depends decisively on the use of lithography. The main steps of this technique are the irradiation and the development of a thin radiation sensitive film — the resist — deposited onto a silicon substrate. The irradiation by visible light, uv, x rays or electrons changes the solubility of the resist film in the developing medium and enables in this way the transfer of information from the mask onto the resist film. The structured resist film itself serves in turn as a stencil for transferring the resist film pattern onto the silicon substrate.

The generation of nuclear tracks requires the availability of a source of heavy ions of sufficiently high nuclear charge and energy. There exist in principle four different ways to create artificial ion tracks in solids: Nuclear reactors, radioactive sources, ion accelerators, and scanning ion microbeams (Figure 1–1).

On the way from the ion irradiation to the observation of the developed track three distinct fields exist, associated with different degrees of complexity (Figure 2–1):

As result of the electronic and atomic collision-cascades close to the ion path a cloud of interstitial atoms and vacancies is formed. At larger distances, the electronic collision-cascade leads to excited atoms and molecules prone to chemical reactions. Their local distribution can be obtained from computer simulations [1] and defines the starting condition for diffusion processes and long-term secondary reactions. Ultimately the atomic defects reorganize in the form of a track core, a heavily disturbed zone of about 0.01 μm diameter along the ion path. It involves the diffusion of many particles in a highly disturbed solid and can be roughly described by semi-empirical models. The electronic defects lead to chemically activated sites (radicals) up to distances of about 1 μm.

During track development the damaged zone of the latent track is either transformed chemically or removed by an etchant, leading to the observable track, the end-product of all the preceding steps and ultimate goal of a structuring tool.

The use of ion tracks as a structuring tool depends very much on the availability of suitable observation techniques characterized according to the observed effects and recovered information:

The ion track etch technique enables the generation of concrete shapes which can either be used individually in the form of single-ion tracks, or collectively in the form of track arrays composed of many tracks. In the following, a short overview on the possible shapes is given. The used techniques comprise mainly (1) variation of the track recording material, (2) variation of the etchant, and (3) partial or complete annealing.

This chapter describes two examples where single-ion tracks are used as critical apertures ruling the flow between two adjacent reservoirs of a fluid. First, the classical flow of an aqueous suspension of microparticles. Second, the quantum mechanical flow of a superfluid.

Solid state properties depend on size and shape as soon as these are only becoming small enough:

“When the dimensions of artificially created structures approach or become smaller than certain characteristic distances — such as grain size, domain size, wavelength, mean free path, coherence length — it becomes possible to access new phenomena or manipulate materials in new ways” [1].

While single tracks lead to property changes that are extremely localized, ensembles of many tracks, distributed over a given volume, induce globally distributed property changes which gradually determine the gross behavior of the solid with increasing density. Thus the integrated effect of many distributed ion tracks is capable to change existing properties and to induce new properties on a global scale. Simultaneously the direction of the track array defines the axis of a new, track-induced anisotropy in the solid.

At high track densities, the ion tracks can be utilized for imprinting structures onto solids in the same way as conventional lithographies [1], [2], This technique has several advantages in comparison with conventional techniques (Table 10-1).

The sweeping progress of ion source and accelerator technology is a challenge for engineers and scientists to promote as well their practical uses.