The primary purposes of the nano-aperture ion source is emitting ions. However, in the ionization region various other processes occur which may lead to the emission of other species. A theoretical study on such processes was conducted for argon. Besides single charged ions, multiple charged ions become constituents of the beam when using electron impact energies above the second ionization threshold. Rather surprisingly, a small amount of diatomic charged argon is predicted to become part of the beam. For a 1 keV electron beam we expect the beam to consist of about 88% single charged argon, 8% double charged argon, 2% triple charged argon, and 2% diatomic single charged argon.

When ions are extracted from the nano-aperture ion source, they inevitably interact with the neutral particles. In this chapter, a model for such interactions is presented. An important finding is that the current and the brightness tend to keep increasing with increasing particle density, despite increasing ion-neutral scattering. Another result is that the spread in energies in the beam can be reduced as a consequence of more ion-neutral scattering. The model developed forms a basis to be used in simulations that also consider coulomb interactions. Besides presenting a model for advanced calculation, simplified analytically expressions are introduced that can help to understand a particular configuration or to quickly estimate the performance.

The main goal of this chapter is demonstrating realistic simulations of the nano-aperture ion source. The optical effects due to the electric fields, the ion-neutral scattering, and Coulomb interactions are studied simultaneously using Monte-Carlo ray tracing. Two rather unusual coulomb-interactions were taken into account, namely, surface induced charge, and electron-ion scattering. Ion-to-ion coulomb repulsion is found to pose an ultimate limit to the achievable brightness. The effect is relevant inside the chip, but also in the region where the beam is accelerated up to high voltage.In a realistic configuration the simulations predict a brightness of about 3 × ^{10 6} A/m² srV in combination with an energy spread of 1 eV. Interestingly, the attainable brightness is not very sensitive to the geometry, nor was it very dependent on the noble gas species. To achieve the best performance, electric fields close to 10 kV/mm should be used, inside and outside of the gas chamber.

An essential aspect of the nano-aperture ion source is the delivery of gas towards the double aperture structure. This requires a nano-fluidic channel. In order to estimate the fluidic properties and understand the underlying transport mechanisms of such a channel, a flow model was developed. The model shows good agreement with three different sets of experimental data. The model is used to show that naive channel design will perform poorly. Nevertheless, carefully designed channels that perform well are achievable.

In this chapter the basics of focused ion beam (FIB) systems are summarized and the nano-aperture ion source (NAIS) is introduced. First a brief overview of ion-surface interactions and their applications is given. Next, the most important optical aspects of FIB systems are discussed and a comparison of different ion sources is made. After this discussion of focused ion beams in a general context, the nano-aperture ion source is introduced. A basic theoretical model from which estimates of the performance can be made is derived. In addition, an optimization for the required electron column is discussed. The basic model in combination with the electron beam optimization is used to acquire performance estimates of the NAIS.

The optics of ion emission from the nano-aperture ion source were studied in the limit of a rarefied gas. In this limit the ions do not interact with neutral particles or other ions and the emission optics are determined by the electric fields, the distribution of ionization location, and the thermal energy distribution of the particles. A first order lens effect and two types of aberrations are identified. One aberration is associated with the initial energy distribution of the ions and a second aberration is caused by the initial spatial spread in the direction of the optical axis. Simple equations are derived, which enable estimates of these optical effects. Good performance is in general achieved when the fields inside and outside the chip are equal and above 3 kV/mm.

A new type of ion source capable of delivering bright and monochromatic beams of various ionic species has been developed. The brightness of this source was measured using an ion focusing column in combination with a knife-edge ion transmission detector. The emission current was varied in the range 200 pA to 20 nA by varying the particle density and the in-chip electric field. Most data were obtained using argon ions, but helium and xenon ions were also produced. The setup was used to experimentally demonstrate a brightness of B ≈ 110 5 A/m 2 sr V. The measurements match reasonably well with ray-trace simulations.