Tomographic Atom Probe (3D-AP)
The three-dimensional (tomographic) atom probe (3D-AP) combines time-of-flight type of mass spectrometry with a multi-anode detector. A schematic sketch of the 3DAP is shown below. This instrument permits the elemental reconstruction of a small volume of metallic material of the specimen with near atomic resolution. This is achieved by determining the position and mass-to-charge ratio of the atoms detected in that volume. Specimens in the form of a fine needle with a tip radius of about 50 nm are used for investigation. Every tip prepared for the atom probe has to be examined by TEM in order to check the form of the tip and to control the position of microstructural features relative to the tip. The surface atoms are field evaporated from the tip by applying a series of high voltage pulses with nanosecond rise time. The mass-to charge ration of the ions, and their position at the specimen surface, are deduced by the time-of-flight and from the coordinates of the ion impact on the multianode detector, respectively. The data are stored sequentially and independently for each type of chemical species. This enables a three-dimensional reconstruction of the analysed volume in real space.
For a typical analysis about 106 atoms have to be counted. Metallurgical information is obtained from the reconstructed 3DAP data by application of statistical methods. Recently a refined method, the wavelength dependent filtering (WDF) was developed, which allows a sensitive analysis of the amplitude and wavelength of concentration fluctuations in nearly homogeneous alloys.
Topics of investigations
Some examples of 3DAP analyses:
Prof. J. Rösler, IfW, Technische Universität Braunschweig
Prof. Clemens, Technische Universität Leoben, Österreich