Instrumentation Secondary ion mass spectrometry

1 instrumentation

1.1 vacuum
1.2 primary ion source
1.3 mass analyzer
1.4 detector

instrumentation

schematic of typical dynamic sims instrument. high energy (usually several kev) ions supplied ion gun (1 or 2) , focused on target sample (3), ionizes , sputters atoms off surface (4). these secondary ions collected ion lenses (5) , filtered according atomic mass (6), projected onto electron multiplier (7, top), faraday cup (7, bottom), or ccd screen (8).

a secondary ion mass spectrometer consists of (1) primary ion gun generating primary ion beam, (2) primary ion column, accelerating , focusing beam onto sample (and in devices opportunity separate primary ion species wien filter or pulse beam), (3) high vacuum sample chamber holding sample , secondary ion extraction lens, (4) mass analyser separating ions according mass-to-charge ratio, , (5) detector.

vacuum

sims requires high vacuum pressures below 10 pa (roughly 10 mbar or torr). needed ensure secondary ions not collide background gases on way detector (i.e. mean free path of gas molecules within detector must large compared size of instrument), , prevents surface contamination adsorption of background gas particles during measurement.

primary ion source

three types of ion guns employed. in one, ions of gaseous elements generated duoplasmatrons or electron ionization, instance noble gases (ar, xe), oxygen (o, o2, o2), or ionized molecules such sf5 (generated sf6) or c60 (fullerene). type of ion gun easy operate , generates focused high current ion beams. second source type, surface ionization source, generates cs primary ions. cesium atoms vaporize through porous tungsten plug , ionized during evaporation. depending on gun design, fine focus or high current can obtained. third source type, liquid metal ion gun (lmig), operates metals or metallic alloys, liquid @ room temperature or above. liquid metal covers tungsten tip , emits ions under influence of intense electric field. while gallium source able operate elemental gallium, developed sources gold, indium , bismuth use alloys lower melting points. lmig provides tightly focused ion beam (<50 nm) moderate intensity , additionally able generate short pulsed ion beams. therefore commonly used in static sims devices.

the choice of ion species , ion gun respectively depends on required current (pulsed or continuous), required beam dimensions of primary ion beam , on sample analyzed. oxygen primary ions used investigate electropositive elements due increase of generation probability of positive secondary ions, while caesium primary ions used when electronegative elements being investigated. short pulsed ion beams in static sims, lmigs deployed analysis; can combined either oxygen gun or caesium gun during elemental depth profiling, or c60 or gas cluster ion source during molecular depth profiling.

mass analyzer

depending on sims type, there 3 basic analyzers available: sector, quadrupole, , time-of-flight. sector field mass spectrometer uses combination of electrostatic analyzer , magnetic analyzer separate secondary ions mass-to-charge ratio. quadrupole mass analyzer separates masses resonant electric fields, allow selected masses pass through. time of flight mass analyzer separates ions in field-free drift path according velocity. since ions possess same kinetic energy velocity , therefore time of flight varies according mass. requires pulsed secondary ion generation using either pulsed primary ion gun or pulsed secondary ion extraction. analyzer type able detect generated secondary ions simultaneously, , standard analyzer static sims instruments.

detector

a faraday cup measures ion current hitting metal cup, , used high current secondary ion signals. electron multiplier impact of single ion starts off electron cascade, resulting in pulse of 10 electrons recorded directly. microchannel plate detector similar electron multiplier, lower amplification factor advantage of laterally-resolved detection. combined fluorescent screen, , signals recorded either ccd-camera or fluorescence detector.