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Time Of Flight Secondary Ion Mass Spectroscopy (TOF SIMS)

For SIMS (Secondary Ion Mass Spectrometry) analysis, a solid surface is bombarded by primary ions of some keV energy. The primary ion energy is transferred to target atoms via atomic collisions and a so-called collision cascade is generated. Part of the energy is transported back to the surface allowing surface atoms and molecular compounds to overcome the surface binding energy. The interaction of the collision cascade with surface molecules is soft enough to allow even large and non-volatile molecules with masses up to 10,000 u to escape without or with little fragmentation.
Most of the emitted particles are neutral in charge, but a small proportion is also positively or negatively charged. The subsequent mass analysis of the emitted ions provides detailed information on the elemental and molecular composition of the surface.

SIMS

SIMS is a very surface sensitive technique because the emitted particles originate from the uppermost one or two monolayers. The dimensions of the collision cascade are rather small and the particles are emitted within an area of a few nm in diameter. Hence, SIMS can be used for micro analysis with very high lateral resolution, provided that such finely focused primary ion beams can be formed.

SIMS is destructive in nature because particles are removed from the surface. This can be used to erode the solid in a controlled manner to obtain information on the in-depth distribution of elements. This dynamic SIMS mode is widely applied to analyse thin films, layer structures and dopant profiles. In order to receive chemical information on the original undamaged surface, the primary ion dose density must be kept low enough (< 1013 cm-2) to prevent a surface area from being hit more than once. This so-called static SIMS mode is widely used for the characterisation of molecular surfaces.

Most of the emitted particles are neutral in charge. Post-Ionisation of these particles by electrons, plasma or photons allows mass analysis of these particles. This technique is called Secondary Neutral Mass Spectrometry, SNMS. One of the most efficient ways to ionise the emitted neutrals is Laser Post-Ionisation (Laser-SNMS). This technique is becoming very attractive for the quantitative analysis of extremely small volumes.
 

Time of Flight

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.

Time of Flight

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.

Surface Imaging
By rastering a fine-focussed ion beam over the surface, like an electron beam in an electron microprobe, mass resolved secondary ion images (chemical maps) can be obtained simultaneously.
 
  • High lateral resolution (<60 nm)
  • Fast image acquisition (up to 50 kHz pixel frequency)
  • Field of view from µm2 to cm2

Surface Imaging

Depth Profiling
For Depth Profiling two ion beams operate in the Dual Beam Mode. While the first beam is sputtering a crater, the second beam is progressively analysing the crater bottom.
 
  • Depth resolution better than 1 nm
  • High mass resolution
  • Sputter speed of up to 10 µm/h
  • Ideally suited for insulators

Depth Profile

corresponding links: http://www.iontof.com/
russian distributor: http://eng.technoinfo.ru/


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