In the conventional atom probe, atoms on tip apecs were field evaporated by the application of ns high voltage pulsed on DC standing volatege. This requires specimens to be electrical conductive. Because of this principle limitation, the major application areas for the atom probe technique were metals. Some investigations reported the analysis of high doped Si of 10^-2 Ohm・cm or thin oxide film layers. However, it was thought that the analysis of the materials with the higher electrical resistivity was impossible. Moreover, the most difficulty of the atom probe analysis comes from the frequent specimen rupture which is cause by the repetitive filed stress on the tip apex. Even metals, some materials like martensitic steels were extremely difficult to analyze due to frequent specimen rupture. In general, the field stress applied to tip apex is higher than yield stress of standard materials. This causes frequent specimen rupture using voltage pulses.

In oder to overcome such principle limitations of the atom probe technique, laser assisted fiedl evaporation has been proposed. Attempts of laser assisted field evaporation were made in 1970's by various goups, but due to the limited performance of the laser systems in those days, laser assisted atom probe has been obsolete till recently. The Rouen group has revived the laser assisted atom probe in 2004 by employing femoto second pulsed laser to their tomographic atom probe [IFES2004]. Thereafter, several groups impremented either pico or femoto second laters to 3D atom probe, and it is now established that laser assisted field evaporation widens the applicaiton areas of atom probe tomography in various types of materials including metals, semiconductors and their devices. Several attempts have been made to analysing insulator materials like MgO in tunneling junctions of some other metallic oxides, but all of the successful reports of insulator anlysis were on thin films.

As shown in Fig. 3, the laser assisted 3D atom probe is the same as the standard 3D atom probe except that atoms are field evaporated by the irradiation of laser pulses. Since voltage pulses are not applied to the tip apex, the field stress is substantially alleviated, so the chance of tip rupture can be substantially decreased. Since energy deficit is not expected by the laser triggered field evaporation, the mass resolution of the time-of-flight mass spectroscopy is expected to be substantially improved. However, currently available experimental results suggest that time delay occurs by the laser triggered field evaporation due to the time lag of the termal conductance in poor conductive materials. However, our recent studeis have shown that the mass resolution can be substantiall improved by emplying short wave length (UV: 310 nm) femoto second (400 fs) laser pulses. This indicates that the field evaporation mechanism is not a simple thermal mechanism when short wave length laser is employed. Since the mass resolution can be improved substantially, it is possible to keep the flight distance short (~15 cm) so that a large acceptance angle (~0.3 sr) can be kept. Accordingly, a large angle of view can be achived in laser assisted 3DAP compared to the conventional 3D atom probe (both straight and energy compensated atom probe). We have also found that the emplyment of short wave length laser (310 nm) makes the field evaporation of insulator ceramis possible, thereby even bulk insulator ceramics can be analyzed with the laser assisted 3D atom probe.

Fig. 2 Schematic illustration of the laser assisted wide angle 3D atom probe at NIMS