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Time-of-Flight Mass Spectrometer TOF mass spectrometry is based on the fact that ions with the same energy but different masses travel with different velocities. Basically, ions formed by a short ionisation event are accelerated by an electrostatic field to a common energy and travel over a drift path to the detector. The lighter ones arrive before the heavier ones and a mass spectrum is recorded. Measuring the flight time for each ion allows the determination of its mass. This cycle is repeated with a repetition rate which depends on the flight time of the highest mass to be recorded. In a more sophisticated design, the TOF analyser corrects for small differences in initial energy and angle in order to achieve high mass resolution. Combinations of linear drift paths and electrostatic sectors or ion mirrors are used and results with mass resolutions, M/dM, above 10,000 can be achieved. Major advantages of this approach over quadrupole and magnetic sector type analysers are the extremely high transmission, the parallel detection of all masses and the unlimited mass range. In TOF-SIMS, a start time of all secondary ions is defined by using a pulsed primary ion beam. Extremely short ion pulses with a duration below 1 nanosecond are applied for high mass resolution analysis. These ion pulses are formed from a continuous beam by a pulsing unit and can be compressed in time by electro-dynamic fields (Bunching). The pulsed beam can be focussed to a small spot (Ion Microprobe) to address a small area of interest and can be rastered to determine the lateral distribution of elements and molecules (Imaging SIMS). During the drift time of the secondary ions, the extraction field is switched off and low energy electrons are used to compensate for any surface charging caused by primary or secondary particles (Charge Compensation). Thus all types of bulk insulators can be analysed without any problems. The time during which the extraction field is switched off can also be used to apply low energy ion beams for sample erosion. In this case the low energy beam forms a sputter crater, the centre of which is analysed by the pulsed beam (Dual Beam Mode). For the analysis of the sputtered neutrals, pulsed laser beams can be used. The cloud of neutral particles above the surface desorbed by the pulsed primary ion beam are ionised by very intense laser radiation (Laser post-ionisation). In doing so, the high number of neutrals also being desorbed by the primary ion pulse, become accessible for mass analysis and therefore increase the sensitivity for elemental species. |
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