High-voltage pulse

An FIM may be modified so that the imaged atom chosen for analysis can be positioned over a small aperture in the phosphor-coated screen. If the electric field is raised to a sufficiently high value, material may be removed from the surface by field evaporation. The specimen is subjected to a high-voltage pulse, which causes a number of atoms on the specimen surface to field evaporate as positive ions. Only the atom that was imaged over the aperture (or probe hole ) passes into a time-of-flight mass spectrometer, all the other atoms being blocked off by the screen. The applied... 

Voltage-pulsed AP analysis of resistive materials is difficult, since they do not transmit high-voltage pulses effectively. In the case of semiconducting materials there is the further problem of their brittleness, so that the mechanical shock due to the voltage pulse often causes specimen fracture. The pulsed laser atom probe (PLAP) is more effective for these materials. 

Slr9a9m9 says that his relays are powered by the High Voltage pulses from the distributor (the coil is in the cap of the distributor), but he is unsure how the relay coil gets grounded (if it s grounded at all). 

Cluster ions are often observed in field evaporation of compound semiconductors such as GaAs and GaP etc. In high voltage pulse field evaporation of GaAs and GaP, only a few per cent of cluster ions are... 

Fig. 3.10 Linear type high voltage pulse time-of-flight atom-probe with an...

It is now well recognized that chemical identification of even those materials composed of the most commonly used elements requires a much better mass resolution, since most elements have more than one isotope, and isotope overlapping is often a serious problem for a reliable compositional analysis of samples. This is the reason that the linear type high voltage pulse ToF atom-probe is now gradually being replaced by other types. 

---