An interesting paper was brought to my attention recently, on a new technology for ion beams based on magneto-optic trapping. My only experience with ion beams was with the transport of radioactive ions at TRIUMF (it’s far easier to shuttle your particles around macroscopic distances as ions), but ions beams are also used for etching and implantation in nanofabrication, among other things. There are other ion etching techniques — I’ve used reactive ion etching, but that’s a bulk etch which involves a mask to expose areas to be etched and hide areas to be protected. Using a beam allows you to focus the ions down to a small area and do precision work.

Drawbacks of ion-beam sources available today (see how I avoided the “currently available” pun?) include liquid metal ion source, (LMIS), gas-phase sources and plasma sources, each having a drawback in one of the important areas of energy spread, brightness (ion beam current per unit solid angle, limited by either or both variables) and available elements. A small energy spread is desirable, because different energies will focus differently, leading to the equivalent of chromatic aberration and limiting spot size. Brightness tells you how many ions you can deliver to your target, and limits on the different species of atom you can deliver restricts what kind of structures you can build through implantation.

A Magneto-Optic Trap gives one the advantage of a very cold source of atoms which limits the energy spread of the ion source and reduces the divergence of the beam, which improves the brightness. And there are a large number of species that can be trapped, so this addresses the main shortcomings of ion beams.

The idea is that the atoms are trapped and cooled, and then ionized and extracted, giving one a Magneto-Optical Trap-based Ion Source. The ionization takes place by tuning a laser to the minimum energy that will ionize only excited-state atoms, so that ground-state atoms in the background vapor are not ionized, and so there is no excess photon energy that would become kinetic energy of the ion and electron after the interaction. The ion is extracted by the presence of an accelerating potential. Perturbations from the trapping fields are small, and could be reduced if one were to operate in a pulsed mode, intermittently turning the trap field off while the ionization laser is on.

MOTIS website. Publications linked therein.