Time-of-Flight secondary ion mass spectrometry (TOF-SIMS)
What can a time-of-flight secondary ion mass spectrometer do?
The abbreviation TOF-SIMS is an acronym for the combination of secondary ion mass spectrometry (SIMS) with a time-of-flight (TOF) mass analyser.
The sample to be analysed is bombarded with a primary ion beam with a kinetic energy of a few keV. The ions collide with the surface, releasing their energy and generating a shock cascade. This leads to the detachment of individual surface atoms and molecules, or of clusters and fragments. Some of these secondary particles are present as ions and can be analysed separately according to their mass.
The time-of-flight principle is used for mass separation. Particles of different mass reach different velocities at the same acceleration voltage. The accelerated particles travel a certain distance in the analyser and reach the detector at different times, the lighter the particle the faster it is. Since times can be measured with very high accuracy, this method achieves a very high spectral mass resolution and gives a very accurate picture of the sample surface and its composition. The procedure is shown in the film in Figure 1.
With this method one achieves a spatial resolution < 100 nm, with a surface sensitivity to the first 1-2 layers of the sample. Depth profiles with a depth accuracy of < 1nm and a measurement speed of up to 10µm/h are achievable.
With the Integrated Focused Ion Beam Source (FIB) it is possible to cleanly generate cross-sectional areas of highly porous surfaces and layered structures with strongly varying ion etch rates and then to investigate these cross-sections. The creation of such a crater is shown in the film in Figure 2. The crater has a size of about 30µmx15µm with a depth of about 20µm. Duration about 6h.
At ICAN we use the TOF.SIMS 5 from Ion-TOF.
Figure 1: Illustration of the Time-of-Flight principle. Animation provided by the Ion-TOF GmbH.
Figure 2: Secondary electron images of the creation of an FIB crater in doped diamond to examine the exposed cross-sectional area. Size of the crater about 30µm x 15µm x 20µm. Duration about 6h.
Depth profiling
Figure 3 shows the mass spectrum of a europium-doped aluminium oxide layer on silicon on a linear and logarithmic scale. Since this technique measures differences in the time of flight, the conversion of this time scale into a mass scale is very important. As a result, the evaluation of the data often takes considerably more time than its generation.
The logarithmic plot shows the enormous sensitivity of the technique. It is possible to measure doping concentrations down to the ppm range.
Figure 4 shows the intensity curve of the aluminium and europium signal as a function of the sputtering time. Between the recording of 2 mass spectra, the surface was bombarded with fast oxygen molecules to expose deeper atomic layers. In this way, a depth profile of the material is generated. In this example, it can be seen that the europium concentration increases towards the depth, has a maximum at the interface to the silicon and then decreases again towards the silicon.
There are several reasons why the transition from Al2O3 to silicon is not arbitrarily sharp. In this case, the high roughness of the Al2O3 surface was largely responsible for this. The aluminium oxide layer examined was about 8 µm thick.
TOF-Sims is not a technique to directly determine total concentration with one measurement. The reason for this is that the removal rates and ionisation probability of a given element/molecule strongly depends on the matrix in which it is located.
For example, a certain amount of doping of boron in silicon would give a significantly different signal intensity than the same amount of boron in diamond. Therefore, ideally one needs a reference sample for each measurement, with the same matrix, where the concentration of the dopant is known.
Figure 3: Mass spectrum of europium-doped alumina on linear and logarithmic scales.
Figure 4: Course of the intensity of the Al+ and Eu+ signals as a function of the ion etching time (etched with oxygen). The thickness of the Al2O3 layer was approx. 8 µm.
Ihr Ansprechpartner für das TOF-SIMS:
ICAN - Interdisciplinary Center for Analytics on the Nanoscale
47057 Duisburg
Functions
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Wissenschaftliche/r Mitarbeiter/in, Center for Nanointegration Duisburg-Essen
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apl. Professor/in, Physikalische Chemie
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WiSe 2025
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The following publications are listed in the online university bibliography of the University of Duisburg-Essen. Further information may also be found on the person's personal web pages.
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