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Dynamic Secondary Ion Spectrometry


Atoms or groups of atoms are etched by a primary ion beam (0.25÷20keV). A small fraction of these (typically 1%) is ionized and can therefore be analyzed according to the mass/charge ratio. Either a magnetic sector or a quadrupole are generally used as mass spectrometers. Typical performances of the SIMS technique are the ability to detect each isotope of the periodic table with detection limits ranging from 1ppm to 1ppb, a mass resolution M/DM up to 20000, a lateral resolution of the order of 1µm and a depth resolution of 1nm.

Two distinct approaches towards lateral resolution are used in SIMS instruments. Lateral resolution is in fact either associated with the use of a finely focused primary ion beam (ion microprobe), or with the optics dealing with secondary ions in such a way as to preserve their spatial relationships (ion microscope).

Lateral resolution is therefore limited by the primary beam diameter in the former case and by lenses aberrations in the second case.

The sequential detection of masses simultaneously generated makes the technique best suited for depth-profiling. The most important feature is in fact its ability to follow elemental depth distributions with very low detection limits and with high lateral resolution.

As a consequence, wide applications of dynamic SIMS are found primarily in microelectronics (depth profiles of dopants and impurities are the typical examples), but also in metallurgy, geology and biology.

Cameca SC ULTRA (installed July 2001)

Ion sources: Oxygen and cesium (from 250 eV up to 10 keV for O2+ and Cs+)
Beam intensity stability: < 1% over 10 min for O2+ and Cs+
Maximum beam intensity: > 400 nA for Cs+ and > 800 nA for O2+
Analyzer: The instrument can operate both as ion microprobe and as ion microscope.
Secondary extraction voltage: <= 3 kV for a primary impact energy of 250 eV
Sample surface imaging: via optical microscope
Mass resolution: M/DM< 20.000
Mass range: > 500 amu at 5 kV secondary extraction voltage
Spatial resolution: 1µm
In-depth information: intrinsically associated with the sputtering process needed for ion generation; a depth resolution of 1÷20nm is typically obtainable
Charge compensation: an electron gun is provided
Detection limit (deep profile): B, P and As < 1E1015 at/cm3
Detection limit (shallow profile): B, P and As < 1E1016 at/cm3
Detection limit (surface contamination): Metals < 5E109 at/cm²