Secondary ion mass spectrometry
|Analytes||Solid surfaces, thin films|
|Related||Fast atom bombardment
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. The mass/charge ratios of these secondary ions are measured with a mass spectrometer to determine the elemental, isotopic, or molecular composition of the surface to a depth of 1 to 2 nm. Due to the large variation in ionization probabilities among different materials, SIMS is generally considered to be a qualitative technique, although quantitation is possible with the use of standards. SIMS is the most sensitive surface analysis technique, with elemental detection limits ranging from parts per million to parts per billion.
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. In the mid-1950s Honig constructed a SIMS instrument at RCA Laboratories in Princeton, New Jersey. 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 at the University of Paris-Sud in Orsay by R. Castaing for the PhD thesis of G. Slodzian. 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. Wittmaack and C. Magee developed SIMS instruments equipped with quadrupole mass analyzers. 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. SIMS was developed in the 1960s by the French company CAMECA S.A.S and used in materials science and surface science. Instruments of this type use pulsed primary ion sources and time-of-flight mass spectrometers and were developed by Benninghoven, Niehuis and Steffens at the University of Münster, Germany and also by Charles Evans & Associates. Recent developments are focusing on novel primary ion species like C60+, ionized clusters of gold and bismuth, or large gas cluster ion beams (e.g., Ar700+). The sensitive high resolution ion microprobe (SHRIMP) is a large-diameter, double-focusing SIMS sector instrument produced by Australian Scientific Instruments in Canberra, Australia.
Typically, a secondary ion mass spectrometer consists of:
- primary ion gun generating the primary ion beam
- 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)
- high vacuum sample chamber holding the sample and the secondary ion extraction lens
- mass analyser separating the ions according to their mass to charge ratio
- ion detection unit.
SIMS requires a high vacuum with pressures below 10−4 Pa (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 (i.e. the mean free path of gas molecules within the detector must be large compared to the size of the instrument), and it also prevents surface contamination by adsorption of background gas particles during measurement.
Primary ion sources
Three basic types of ion guns are employed. In one, ions of gaseous elements are usually generated with duoplasmatrons or by electron ionization, for instance noble gases (40Ar+, Xe+), oxygen (16O-, 16O2+, 16O2-), or even ionized molecules such as SF5+ (generated from SF6) or C60+ (fullerene). This type of ion gun is easy to operate and generates roughly focused but high current ion beams. A second source type, the surface ionization source, generates 133Cs+ primary ions. Cesium atoms vaporize through a porous tungsten plug and are ionized during evaporation. Depending on the gun design, fine focus or high current can be obtained. A third source type, the liquid metal ion gun (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 (<50 nm) 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, LMIGs are most often deployed for analysis; they can be combined with either an oxygen gun or a caesium gun during elemental depth profiling, or with a C60+ or gas cluster ion source during molecular depth profiling.
Depending 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, which allow only the selected masses to pass through. The time of flight mass analyzer separates the ions in a field-free drift path according to their velocity. Since all ions possess the same kinetic energy the velocity and therefore time of flight varies according to mass. 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.
A Faraday cup measures the ion current hitting a metal cup, and is sometimes used for high current secondary ion signals. With an electron multiplier an impact of a single ion starts off an electron cascade, resulting in a pulse of 108 electrons which is recorded directly. A microchannel plate detector is similar to an electron multiplier, with lower amplification factor but with the advantage of laterally-resolved detection. Usually it is combined with a fluorescent screen, and signals are recorded either with a CCD-camera or with a fluorescence detector.
Detection limits and sample degradation
Detection limits for most trace elements are between 1012 and 1016 atoms per cubic centimetre, depending 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. 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.
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, or surface molecular 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.
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