Direct Magneto-Optical Compression of an Effusive Atomic Beam for Application in a High-Resolution Focused Ion Beam

Abstract

An atomic rubidium beam formed in a 70-mm-long two-dimensional magneto-optical trap (2D MOT), directly loaded from a collimated Knudsen source, is analyzed using laser-induced fluorescence. The longitudinal velocity distribution, the transverse temperature, and the flux of the atomic beam are reported. The equivalent transverse reduced brightness of an ion beam with properties similar to the atomic beam is calculated because the beam is developed to be photoionized and applied in a focused ion beam. In a single two-dimensional magneto-optical trapping step, an equivalent transverse reduced brightness of ð1.0 þ0.8 −0.4 Þ × 10 6 A=ðm 2 sr eVÞ is achieved with a beam flux equivalent to ð0.6 þ0.3 −0.2 Þ nA. The temperature of the beam is further reduced with an optical molasses after the 2D MOT. This optical molasses increases the equivalent brightness to ð6 þ5 −2 Þ × 10 6 A=ðm 2 sr eVÞ. For currents below 10 pA, for which disorder-induced heating can be suppressed, this number is also a good estimate of the ion-beam brightness that can be expected. Such an ion-beam brightness would be a 6× improvement over the liquid-metal ion source and could improve the resolution in focused ion-beam nanofabrication.

Measurements of the energy distribution of a high brightness rubidium ion beam

“…The ion beam analyzed in this research is created by two-step photoionization of a laser-cooled and compressed beam of 85 Rb atoms. The details of the creation and analysis of this atomic beam were described earlier [11]. There, also a measurement of the longitudinal velocity distribution of the atoms is shown.…”

“…The ultracold Rb ion beam is formed in the atomic beam laser cooled ion source (ABLIS). In this source * e.j.d.vredenbregt@tue.nl an ultracold beam of 85 Rb atoms is created by means of magneto-optical compression [11]. After selection of the desired atomic flux, this beam is photoionized in the crossover of a tightly focused excitation laser beam and an ionization laser beam whose intensity is enhanced in a build-up cavity [12].…”

Cavity-enhanced photoionization of an ultracold rubidium beam for application in focused ion beams

“…Atomic equivalent reduced brightnesses of the order of 10 7 A/(m 2 sr eV) have been reported [2,3], which are roughly an order of magnitude higher than the industry standard liquid metal ion source (LMIS). Furthermore, energy spreads of less than 1 eV full width at half maximum (FWHM) are expected [2,3] as compared to 4.5 eV for the LMIS. Together with the aberrations of the focusing column these two beam parameters determine the resolution of a FIB instrument for a given current.…”

“…Photoionized ultracold atomic beams are promising to be used as source for focused ion beam (FIB) instruments for nanofabrication purposes [1]. Atomic equivalent reduced brightnesses of the order of 10 7 A/(m 2 sr eV) have been reported [2,3], which are roughly an order of magnitude higher than the industry standard liquid metal ion source (LMIS). Furthermore, energy spreads of less than 1 eV full width at half maximum (FWHM) are expected [2,3] as compared to 4.5 eV for the LMIS.…”

A high repetition deterministic single ion source

“…Due to this large energy spread, ion beam applications typically require beam energies in the 10 keV range in order to ensure good focusing. Recently, a third type of ion source based on photoionization of laser-cooled atoms is gaining increasing attention [22][23][24]. Due to the low temperature, such a source can feature a high brightness comparable to a LMIS, however, at a much lower energy spread of less than 1 eV, which allows operation also at low beam energies of a few hundred electronvolts [17].…”