Secondary ion mass spectrometry

CAMECA IMS3f Magnetic secondary ion mass spectrometer.

Mass spectrometry, Secondary-Ion Mass Spectrometry (SIMS ) is a method for producing ions from low-volatile, polar and thermally unstable compounds in mass spectrometry .

Initially used to determine the elemental composition of low-volatile substances, but later it was used as a desorption method for soft ionization of organic substances. Used to analyze the composition of solid surfaces and thin films. MESI is the most sensitive surface analysis technique, able to detect the presence of an element in the range of 1 part per billion.

Content

The essence of the method

The sample is irradiated with a focused beam of primary ions (for example ${{\ce {Xe+}}}$ ${\ displaystyle {{\ ce {Xe +}}}}$ , ${{\ce {Cs+}}}$ ${\ displaystyle {{\ ce {Cs +}}}}$ , ${{\ce {Ga+}}}$ ${\ displaystyle {{\ ce {Ga +}}}}$ ) with energies from 100 eV to several keV (more energy is used in the FAB method). The resulting secondary ion beam is analyzed using a mass analyzer to determine the elemental, isotopic or molecular composition of the surface.

The yield of secondary ions is 0.1-0.01%.

History

Vacuum

The MSRI method requires the creation of high vacuum conditions with pressures below 10 ⁻⁴ Pa (approximately 10 ⁻⁶ m bar or mm Hg ). This is necessary to ensure that secondary ions do not collide with the surrounding gas molecules on their way to the sensor ( mean free path ), as well as to prevent surface contamination by adsorption of particles of the surrounding gas during the measurement.

Measuring instrument

The classical analyzer on the basis of MISV includes:

primary ion gun producing a primary ion beam;
primary ion collimator, accelerating and focusing the beam on the sample (in some devices with the ability to separate the primary ions with a special filter or create a pulsation of the beam);
a high-vacuum chamber containing the sample and an ionic lens to extract secondary ions;
a mass analyzer that separates ions according to their charge-to-mass ratio;
ion detection devices.

Varieties

There are static and dynamic modes of the MISS.

Static mode

A small ion flux per surface unit (<5 nA / cm²) is used. Thus, the test surface remains almost intact.

Used for the study of organic samples.

Dynamic mode

A typical diagram of the dynamic device MSCI. High-energy ions are created using an ion gun (1 or 2) and focus on the sample (3), which ionizes and knocks out a number of atoms from the surface. These secondary ions are then collected by ion lenses (5) and filtered in accordance with their atomic masses (6), then projected onto an electron multiplier (7, above), Faraday cylinder (7, below) or CCD screen (8).

The flow of primary ions is large (of the order of μA / cm²), the surface is examined sequentially at a speed of about 100 angstroms per minute.

The mode is destructive, and therefore more suitable for elemental analysis.

Erosion of the sample allows to obtain the profile of the distribution of substances in depth.

Literature

J. Maan, V. Spicer, A. Liebsch, et al. Electron and ion spectroscopy of solids / L. Firmens, J. Vannick, V. Decacer. - M .: Mir, 1981. - 467 p.

Afanasyev V.P. Electron and ion spectroscopy of solids // Soros Educational Journal, 1999, №2, p. 110-116.

Links

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