What is TOF-SIMS?
Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) uses a pulsed primary ion beam to desorb and ionize species from a sample surface. The resulting secondary ions are accelerated into a mass spectrometer, where they are mass analyzed by measuring their time-of-flight from the sample surface to the detector. There are three different modes of analysis in TOF-SIMS; 1) mass spectra are acquired to determine the elemental and molecular species on a surface; 2) images are acquired to visualize the distribution of individual species on the surface; and 3) depth profiles are used to determine the distribution of different chemical species as a function of depth from the surface.
An image is generated by rastering a finely focused beam across the sample surface. Due to the parallel detection nature of TOF-SIMS, the entire mass spectrum is acquired from every pixel in the image. The mass spectrum and the secondary ion images are then used to determine the composition and distribution of sample surface constituents.
TOF-SIMS provides spectroscopy for characterization of chemical composition, imaging for determining the distribution of chemical species, and depth profiling for thin film characterization.
In the spectroscopy and imaging modes, only the outermost (1-2) atomic layers of the sample are analyzed. The actual desorption of material from the surface is caused by a "collision cascade" which is initiated by the primary ion impacting the sample surface. The emitted secondary ions are extracted into the TOF analyzer by applying a high voltage potential between the sample surface and the mass analyzer. TOF-SIMS spectra are generated using a pulsed primary ion source (very short pulses of <1 ns). Secondary ions travel through the TOF analyzer with different velocities, depending on their mass to charge ratio (ke=½mv 2). For each primary ion pulse, a full mass spectrum is obtained by measuring the arrival times of the secondary ions at the detector and performing a simple time to mass conversion.
Chemical images are generated by collecting a mass spectrum at every pixel as the primary ion beam is rastered across the sample surface. The images below show an example of elemental and molecular imaging from a cross-section of a sheet of coated paper. The total ion image is the sum of all peaks in the spectrum. The C3H7O+ peak is an organic fragment of cellulose. The sodium (Na+) image is from the clay coating on the paper. The iron (Fe+) image is contamination from the razor blade used to cut the paper and illustrates the high sensitivity of the TOF-SIMS technique.
TOF-SIMS Depth Profiling
TOF-SIMS is capable of shallow sputter depth profiling. An ion gun is operated in the DC mode during the sputtering phase in order to remove material, and the same ion gun or a second ion gun is operated in the pulsed mode for acquisition phase. Depth profiling by TOF-SIMS allows monitoring of all species of interest simultaneously, and with high mass resolution. The figure below shows a TOF-SIMS depth profile of a thin MoSi2 layer on a silicon substrate. By monitoring the entire mass spectrum at each depth any unexpected contamination at the interface, such as fluorine in this case, can be detected.