Secondary ion mass spectrometry

Secondary ion mass spectrometry (SIMS) is a technique used to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions. While only charged secondary ions emitted from the material surface through the sputtering process are used to analyze the chemical composition of the material, these represent a small fraction of the particles emitted from the sample. These secondary ions are measured with a mass spectrometer to determine the elemental, isotopic, or molecular composition of the surface. SIMS is the most sensitive surface analysis technique, being able to detect elements present in the parts per billion range.

History
In 1910 British physicist J. J. Thomson observed a release of positive ions and neutral atoms from a solid surface induced by ion bombardment. Improved vacuum pump technology in the 1940s enabled the first prototype experiments on SIMS by Herzog and Viehböck in 1949, at the University of Vienna, Austria. Then in the early 1960s two SIMS instruments were developed independently. One was an American project, led by Liebel and Herzog, which was sponsored by NASA at GCA Corp, Massachusetts, for analyzing moon rocks, the other was a French project initiated at the University of Orsay by R. Castaing for the PhD thesis of G. Slodzian. Both instruments were later manufactured, the American one by GCA Corp and the French one by Cameca respectively, latter is located in the Paris area and is still involved in SIMS instrumentation (www.cameca.fr). These first instruments were based on a magnetic double focusing sector field mass spectrometer and used argon for the primary beam ions. In the 1970s, K.Wittmack and C. Magee respectively developed SIMS instruments equipped with quadrupole mass analyzers. At the same time A. Benninghoven introduced the method of static SIMS, where the primary ion current density is so small that only a negligible fraction (typically 1%) of the first surface layer is necessary for surface analysis. Instruments of this type are use pulsed primary ion sources and 'time-of-flight mass spectrometers and were developed by Benninghoven, Niehus and Steffens at the University of Munster, Germany and also by Charles Evans & Associates (Redwood City, CA, USA) respectively (www.eaglabs.com). Recent developments are focusing on novel primary ion species like C60 or cluster ions of gold and bismuth. 

Instrumentation
A classical SIMS device consists of 1) primary ion gun generating the primary ion beam, 2) a primary ion column, accelerating and focusing the beam onto the sample (and in some devices an opportunity to separate the primary ion species by wien filter or to pulse the beam), 3) high vacuum sample chamber holding the sample and the secondary ion extraction lens, 4) mass analyser separating the ions according to their mass to charge ratio, 5) ion detection unit.

Vacuum
SIMS requires a high vacuum of at least 10-6 mbar to ensure secondary ions to move undisturbed to the detector (mean free path) and to prevent surface recovery by adsorption of background gas particles during measurement.

Primary ion sources
There are three basic types of ion guns. In one, ions of gaseous elements are usually generated with Duoplasmatrons or by electron ionization - for instance noble gases (Ar+, Xe+), oxygen (O-, O2+), or even SF5+ ionized molecules (generated from SF6) and C60+ respectively. It is easy to operate and generates roughly focused but high current ion beams. A second source type, the surface ionization source, is singularly used to generate Cs+ primary ions. Caesium atoms vaporize through a porous tungsten plug and get ionized during evaporation. It is applicable both in fine focus or high current mode, respectably - depending on the gun design. A third, the liquid metal ion source (LMIG), operates with metals or metallic alloys, which are liquid at room temperature or slightly above. The liquid metal covers a tungsten tip and emits ions under influence of an intense electric field. While a gallium source is able to operate with elemental gallium, recent developed sources for gold, indium and bismuth use alloys lowering their melting points. The LMIG provides a fine focused ion beam (<50nm) with moderate intensity and is additionally able to generate short pulsed ion beams. It is therefore commonly used in static SIMS devices.

The choice of the ion species and ion gun respectively depends on the required current (pulsed or continuous), the required beam dimensions of the primary ion beam and on the sample which is to investigate. Oxygen primary ions are often used to investigate electropositive elements due to an increase of the generation probability of positive secondary ions - while caesium primary ions often are used when electronegative elements are to investigate. For short pulsed ion beams used in static SIMS, only LMIGs are deployable, but often combined with either an oxygen gun or a caesium gun for sample depletion.

Mass analyzers
Dependent on the SIMS type, there are three basic analyzers available: sector, quadrupole and time-of-flight. A sector field mass spectrometer uses a combination of an electrostatic analyzer and a magnetic analyzer to separate the secondary ions by their mass to charge ratio. A quadrupole mass analyzer separates the masses by resonant electric fields, where only masses of choice are able to pass.The time of flight mass analyzer separates the ions at a field free drift path according to their kinetic energy. It needs a pulsed secondary ion generation generated with a pulsed primary ion gun or a pulsed secondary ion extraction. It is the only analyzer type able to detect all generated secondary ions together and is standard analyzer for static SIMS devices.

Detectors
A faraday cup measures the ion current hitting a metal cup, sometimes used for high current secondary ion signals. With an electron multiplier an impact of a single ion releases an electron cascade finally generating a pulse of 108 electrons which is recorded directly. A microchannel plate detector is similar to an electron multiplier but with lower amplification factor but the advantage of a lateral resolved detection. Usually it is combined with a fluorescent screen and signals are recorded either with a CCD-camera or with a fluorescent detector.

Detection limits
Detection limits for most trace elements are between 1012 and 1016 atoms per cubic centimeter, although this is dependent on the type of instrumentation used, the primary ion beam used and the analytical area, and other factors. Samples as small as individual pollen grains and microfossils can yield results by this technique. The amount of surface cratering created by the process depends on the current (pulsed or continuous) and dimensions of the primary ion beam (often Cs+, O2-, Ga+ or Bi clusters like Bi32-).

Static and dynamic modes
In the field of surface analysis, it is usual to distinguish static SIMS and dynamic SIMS. Static SIMS is the process involved in surface atomic monolayer analysis, usually with a pulsed ion beam and a time of flight mass spectrometer, while dynamic SIMS is the process involved in bulk analysis, closely related to the sputtering process, using a DC primary ion beam and a magnetic sector or quadrupole mass spectrometer.

Applications
The COSIMA instrument on board the Rosetta was the first instrument to determine the composition of cometary’s dust with secondary ion mass spectrometry.