Optimum collimator shape and maximum emittance for submicron focusing of ion beams. Determination of the probe-forming system resolution limit

Abstract

Improvements in the spatial resolution of instrumentation using focused ion beams are connected with further reduction of the beam spot size, while the beam intensity is kept sufficiently high. In search for probe-forming systems (PFS) of new generation it is important to determine the lower theoretical resolution limit for each system. In this case the lower resolution limit can be taken to be quality-factor of the system. As a general approach permitting the resolution limit to be found the authors propose the method of the maximum beam emittance. By proper selection of the object and angular slit dimensions and their positions with respect to the axis of the PFS, one attains the maximum beam emittance for a prescribed beam spot size. Calculations involving only demagnification and aberration coefficients allow a comparison to be made between different PFS irrespective of their dimensions and focusing elements employed. Investigations into conventional magnetic quadrupole PFS show that parasitic sextupole field components present in the lenses can lead to the asymmetric position of the angular slit and thus, to asymmetric position of the beam along the optical line.