Laser-tip interactions in the laser-assisted atom probe tomography technique: the case of III-V semiconductors

Abstract

With the scaling down of semiconductor device dimensions to achieve improved performance, alternative materials and novel device architectures are being explored. III&V materials exhibit high electron mobilities and are being used instead of silicon as the channel material in new device architectures such*as*FinFETs*and*nanowires.*Consequently*there*is*a*growing*need*for*metrology* techniques*that*can*provide*accurate*information*at*the*sub&nm*scale*about*parameters*critical*to* device*performance*such*as*composition,*doping*levels,*layer*thicknesses,*dopant*distribution,*etc.* Atom*probe*tomography*(APT)*is*a*powerful*metrology*technique*that*can*offer*3D*information*with* near&atomic*spatial*resolution*(~0.2&0.3*nm)*and*high*chemical*sensitivity*(10*ppm).*It*is*based*on* controlled*field*evaporation*of*atoms*from*a*needle*shaped*specimen*(apex*~100*nm).*Voltage*or* laser*pulses*are*used*to*trigger*the*field*evaporation*of*atoms*which*are*then*collected*on*a*position* sensitive*detector.*Using*the*(x,y)*position*and*the*sequence*of*evaporated*ions,*we*obtain*a* reconstructed*image*of*the*specimen*wherein*the*time&of&flight*is*used*to*extract*the*chemical* identity*of*individual*ions.** In*this*project,*we*propose*to*look*experimentally*and*theoretically*at*the*interactions*of*III&V* semiconducting*tips*with*the*laser*as*used*in*laser&assisted*APT*(LA&APT),*with*a*particular*focus*on* InGaAs*and*InAlAs*illuminated*by*light*from*the*ultraviolet*to*the*near&infrared.*The*way*these*highly* absorptive*materials*(narrow*and*direct*bandgap)*interact*with*light*when*shaped*like*an*atomprobe* needle*is*yet*to*be*fully*understood.*Experimentally,*this*project*will*look*at*the*impact*of*laser* wavelength*and*power*on*the*hit&maps,*which*deliver*local*information*of*the*light*absorption*in*the* tip*apex.*We*will*also*look*at*the*shape*of*the*mass*spectra**as*these*contain*very*valuable* information*of*the*heat*transport*and*dissipation*inside*the*tip.*Finally,*also*the*impact*of*the*tip* preparation*with*focused*ion*beam*(FIB)*will*be*looked*at*as*it*has*recently*been*demonstrated*to* dramatically*change*the*optical*absorptivity*of*these*nanoscale*objects.*Theoretically,*this*project* will*make*use*of*the*Amsterdam*Discrete*Dipole*Approximation*(ADDA)*simulation*software*to* evaluate*the*light*absorption*inside*the*tips*as*a*function*of*their*shape*and*FIB*preparation*as*well* as*the*excitation*wavelength.*The*transport*of*the*generated*heat*inside*the*tip*will*be*evaluated* using*simple*to*more*complex*heat*diffusion*models*as*the*elongated*tip*geometry*as*well*as*the* nanoscale*dimensions*may*significantly*alter*the*thermal*properties.* * Besides*the*obvious*interesting*physics*this*project*should*shed*light*on,*the*better*understanding*of* the*laser*light*absorption*and*heat*diffusion*in*InGaAs*and*InAlAs*tips*during*LA&APT*is*expected*to* significantly*improve*the*capabilities*of*this*technique*for*the*analysis*of*III&V*materials.**

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