Secondary Ion Mass Spectrometry

Characterisation Installation 4
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SIMS uses a focused, energetic primary ion beam to bombard the surface of the sample of interest. Sample atoms are sputtered and those that are ionized (secondary ions) are accelerated towards a mass spectrometer where they are mass separated before being detected. Either positive or negative secondary ions are detected. The use of a reactive primary ion beam can increase the secondary ion yield. Oxygen is used to increase positive secondary ion yield and cesium is used to increase negative secondary ion yield. There are three main types of spectrometer that are used to mass-separate the secondary ions, magnetic sector, time-of-flight, and quadrupole spectrometers. Magnetic SIMS provides the best detection limits whilst time-of-flight (TOF)-SIMS provides the highest surface sensitivity when used with a pulsed analysis gun. SIMS can provide surface mass spectra, images with lateral resolution in the 0.1 to 10 ┬Ám range and depth profiles with depth resolution in the 1 to 10 nm range. SIMS can be quantitative when reference samples are used and can achieve ppm or even ppb sensitivity. Profile depths can range from a few tens of nanometers to several tens of microns and any vacuum compatible sample can be analyzed. Some samples may require specific sample preparation (notably for biological samples) and analysis conditions. For example organic samples may be damaged by mono-atomic ion beams and the use of cluster ion beams (such as Argon clusters) can reduce this damage, thus enabling large molecular ions, characteristic of the sample to be detected.

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