Advanced Low Energy Focused Ion Beam Design Based on Immersion Optics

Abstract

Form ost state-of-the-art commercial focused ion beam (FIB)s ystems an ion energy of typically 30 keVa ppears to be thew idelya ccepted standard. Nevertheless there have been severalv aluable investigations on possible applications of lowerion energies. While earlystudies carried out during thel ate1 980st o them id 1990s were mainly focusingo n techniquesf or direct device fabrication (see e.g.[1]f or an overview), more recentlyt he application of lowe nergyi on beamsf or the "gentle"( lowd amage) preparation of transmission electron microscopys amples hass parked new interest. However, most of thec urrent work is carried out using standard or onlyslightlymodified high energy instrumentation which is simply operated at its lower energy limit [2,3].In this study, an advanced lowe nergyion optical design and mode of operation is proposed which can provide comparativelysuperior resolution at beam energies of 1-3 keVw ithout compromising thesystem'sc ompatibility as an add-on to astandard transmission or scanning electron microscope [4,5].The basic systemdesign and aschematic course of theaxial (kinetic)potential/voltage distribution are shown in FIG1 . AG alliuml iquidm etal ions ource (LMIS) is used as an emitter. The beam is accelerated using ag un lens and thei nterlensd rift space is seto n high potential. Beam retardingis accomplished within theo bjectivel ens. With this designt he target is field freea nd can be kept on ground potential, whichi ndeed is very similart o theb asic operation principleo fm anym odern scanning electron microscopes [ 6,7]. As alld eployed voltage levels are comparativelyl ow, both condensera sw ella so bjectivel ensm ay be operatedi n internal acceleration mode, consequently improving theirgeneral optical performance.The systemp erformance hasb een investigated both, theoretically as well as experimentally using a newlydeveloped ion beam column [8]. Numerical simulations [9] indicate a dominating influence of Coulomb interaction effectso vert he wholec onsidered currentr ange. It will be shownt hatt he interaction limited systemperformanceisindeed critically dependent not onlyon thespecificchoice of the immersion ratio but also on the mode of operation (accel, decel) of the individual lenses.

Experimental results using an improved low-energy focussed ion beam column with booster principle and free-standing target

“…Compared to our former laboratory LEFIB system [16][17][18][19] there is now a better agreement between theory and experimental results which are very good. Due to an improved design based on a more complete simulation of the CI at a final landing energy of 3 keV our new system exhibits the same experimental resolution with an ion current at the target ten times higher than for the former system.…”

“…In Marianowski's new design the first electrode of the condenser lens is no longer electrically connected with the extraction electrode of the LMIS. This improvement was already proposed by Rauscher [19]. Another disadvantage of our former column was the post-lens scanning deflector, which is now replaced by a double-stage predeflection system, see Fig.…”

“…In our LEFIB system whose former version has already been published [16][17][18][19] the specimen is on ground potential and in a field-free region as the last electrode of the objective lens is grounded, too. Thus there is only a small penetration of the internal electrostatic lens field through the hole of the last electrode and we use the notation free-standing specimen.…”

Advanced Low Energy Focused Ion Beam Design Based on Immersion Optics

“…[1]f or an overview), more recentlyt he application of lowe nergyi on beamsf or the "gentle"( lowd amage) preparation of transmission electron microscopys amples hass parked new interest. However, most of thec urrent work is carried out using standard or onlyslightlymodified high energy instrumentation which is simply operated at its lower energy limit [2,3].…”