Low-energy ion sources from ultra-cold atoms
Focused Ion Beams (FIBs) are an extremely powerful and versatile tool for analyzing and modifying surfaces at the sub-micrometer scale. Due to its relevance, FIB technologies have attracted a great deal of R&D efforts in the last decade. Such efforts enabled designing and realizing state-of-the-art machines, while, at the same time, showing several technical and fundamental limitations in the performance. For instance, they include the achievement of truly nanometer spatial resolution, as well as the application of FIBs to a large class of emerging materials (bio-tissues, soft-matter, layered materials, etc.).
The present research aims at developing a radically new FIB source, based on the ionization of laser-cooled atom beams. The excellent abilities in controlling particle dynamics offered by laser manipulation is, hence, transferred to the production of ion beams with superior dynamical properties compared to conventional sources, where the field ionization of liquid metals is used. In particular, controlling the kinetic energy of the produced ions allows overcoming the limitations related to chromatic aberrations and focusing ions to the nanometer scale. Moreover, ion creation and their extraction/acceleration can be effectively decoupled each other in order to keep excellent dynamical properties even in mildly accelerated beams, e.g., at voltages below 5 kV, as required in the applications with brittle material surfaces.
The basic idea of the research has been developed within the frame of a European Project (MC-IAPP 251391 “Coldbeams”), coordinated by Université Paris-Sud, involving personnel and associated members of CNR-INO at Dipartimento di Fisica, Universitŕ di Pisa and a company owning a world leading position in the fabrication of FIB columns, Orsay Physics – TESCAN, Fuveau, France. Two distinct ion production strategies have been identified: direct photoionization of an atom beam produced out of a Magneto-Optical Trap and field-ionization of Rydberg atoms. The latter led to the realization of a new cesium ion microscope at Orsay Physics – TESCAN. The prototype shows an excellent potential, demonstrating by sub-10 nm resolution imaging with ions accelerated at a few kV.
Presently, the technology is under development in view of its industrialization. Nonetheless, there are many research activities to explore ranging, e.g., through the deterministic production of few ion bunches to the investigation of the ion/surface processes, largely unexplored, occurring under low-dose, low-energy bombardment of technological surfaces by cesium ions an including writing, implantation, structural/morphological modifications at the local scale.
Sketch of the few-ion production source: a pyramidal Cs MOT produces a beam of slow atoms which, after a transverse cooling phase, are laser ionized and detected.
Example of writing on a stainless steel substrate by a Cs Focused Ion Beam using an laser-cooled atom based ion source.