Secondary ion mass spectrometry

Infobox chemical analysis
name = Secondary ion mass spectrometry


caption =CAMECA IMS3f Magnetic SIMS Instrument
acronym = SIMS
classification =Mass spectrometry
analytes = Solid surfaces, thin films
related = Fast atom bombardment
Microprobe

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 [cite journal| title = Rays of positive electricity | author = Thomson, J. J |journal= Phil. Mag. 20(1910), 752–767.] . Improved vacuum pump technology in the 1940s enabled the first prototype experiments on SIMS by Herzog and Viehböck [cite journal| title = Ion source for mass spectrography | author =Herzog, R. F. K., Viehboeck, F |journal = Phys. Rev. 76(1949), 855–856] 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, [cite journal| title =Ion microprobe mass analyzer | author =Liebl, H. J |journal = J. Appl. Phys. 38(1967), 5277–5280] the other was a French project initiated at the University of Orsay by R. Castaing for the PhD thesis of G. Slodzian [cite journal| title =Optique corpusculaire—premiers essais de microanalyse par emission ionique secondaire | author =Castaing, R. & Slodzian, G. J |journal =Microscopie 1(1962), 395–399.] . Both instruments were later manufactured, the American one by "GCA Corp" and the French one by "Cameca" respectively. Cameca is located in the Paris area and is still involved in SIMS instrumentation ( [http://www.cameca.fr 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 [cite journal| author =Wittmaack, K. | title =Pre-equilibrium variation of secondary ion yield |journal =Int. J. Mass Spectrom. Ion Phys. 17(1975), 39–50] [ cite journal| author =Magee, C. W. et. al | title =Secondary ion quadrupole mass spectrometer for depth profiling design and performance evaluation |journal =Rev. Scient. Instrum. 49(1978), 477–485] . Around 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 [cite journal| title = Analysis of sub-monolayers on silver by secondary ion emission | author =Benninghoven, A |journal = Physica Status Solidi 34(1969), K169–171] . Instruments of this type 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 ( [http://www.eaglabs.com/ www.eaglabs.com] ). Recent developments are focusing on novel primary ion species like C₆₀ or cluster ions of gold and bismuth. [cite journal| title =Sputter-depth profiling for thin-film analysis| author =S.Hofmann|journal =Phil. Trans. R. Soc. Lond. A (2004) 362, 55–75]

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 with pressures below 10^-4 Pascal (roughly 10^-6 mbar or torr). This is needed to ensure that secondary ions do not collide with background gases on their way to the detector (mean free path), and is also prevents surface contamination 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. This type is easy to operate and generates roughly focused but high current ion beams. A second source type, the [http://www.eaglabs.com/training/tutorials/sims_instrumentation_tutorial/sources.php 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, recently developed sources for gold, indium and bismuth use alloys which lower their melting points. The LMIG provides a tightly 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 be analyzed. 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 being investigated. For short pulsed ion beams in static SIMS, only LMIGs are deployable, but they are 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 in a field-free drift path according to their kinetic energy. It requires pulsed secondary ion generation using either a pulsed primary ion gun or a pulsed secondary ion extraction. It is the only analyzer type able to detect all generated secondary ions simultaneously and is the standard analyzer for static SIMS instruments.

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, [cite web | title=SIMS Detection Limits of Selected Elements in Si and SIO2 Under Normal Depth Profiling Conditions | date=May 4, 2007 | publisher=Evans Analytical Group | url =http://www.eaglabs.com/files/appnotes/AN339.pdf | accessdate = 2007-11-22 ] 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.cite journal | author = Kaufman, A.J. | coauthors = Xiao, S. | year = 2003 | title = High CO 2 levels in the Proterozoic atmosphere estimated from analyses of individual microfossils | journal = Nature | volume = 425 | pages = 279–282 | doi = 10.1038/nature01902 ] 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 Bi₃^2-).

tatic 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. [cite journal| title= Chemometric evaluation of time-of-flight secondary ion mass spectrometry data of minerals in the frame of future in situ analyses of cometary’s material by COSIMA onboard ROSETTA|author=C. Engrand, J. Kissel, F. R. Krueger, P. Martin, J. Silén, L. Thirkell, R. Thomas, K. Varmuza|journal= Rapid Communications in Mass Spectrometry|volume= 20|pages=1361–1368|doi=10.1002/rcm.2448| year= 2006]

See also

* SHRIMP

References

Bibliography

* Benninghoven, A., et al., "Secondary Ion Mass Spectrometry: Basic Concepts, Instrumental Aspects, Applications, and Trends", Wiley, New York, 1987 ISBN: 0471519456
* Bubert, H., Jenett, H., "Surface and Thin Film Analysis; A compenium of Principles, Instrumentation, and Applications", p. 86-121, Wiley-VCH, Weinheim, Germany 2002 ISBN: 3-527-30458-4

External links

* Excellent tutorial pages for SIMS [http://www.eaglabs.com/training/tutorials/sims_theory_tutorial/index.php theory] and [http://www.eaglabs.com/training/tutorials/sims_instrumentation_tutorial/ instrumentation] , where the book "Secondary Ion Mass Spectrometry: Basic Concepts, Instrumental Aspects, Applications, and Trends", by A. Benninghoven, F. G. Rüdenauer, and H. W. Werner, Wiley, New York, 1987 (1227 pages), is cited as "the best SIMS reference". For a presentation describing Time of Flight SIMS see [http://www.eaglabs.com/en-US/presentations/TOFSIMS/Presentation_Files/index.html TOF SIMS presentation] .
* [http://www.simsworkshop.org/ SIMS workshop] , site includes a literature database [http://www.simsworkshop.org/literature.htm]