Pulsed-laser time-of-flight atom-probe

4 Pulsed-laser time-of-flight atom-probe 

Let us discuss some of the advantages and disadvantages of the pulsed-laser time-of-flight atom-probe as compared with the HV pulse atom-probe. Some of the advantages are as follows  

One of the most serious shortcomings of the HV pulse atom-probe is the inability to pulse field evaporate materials of very low electrical conductivity such as a high purity silicon. This is due to the difficulty of transmitting ns HV pulses across the tip. This limitation is overcome by the use of laser pulses which can be focused right to the tip apex. Thus the material applicability of the atom-probe is greatly expanded by the use of laser pulses for the pulsed-field evaporation. 

The mechanical construction of the pulsed-laser atom-probe is greatly simplified without the need of properly terminating the ns HV pulses very close to the tip. There is also no need of a flight-time focusing lens. Very short duration laser pulses can be much more easily produced 

The power of the laser unit needed for the atom-probe operation is not really well defined. It depends on the pulse width, the stability and the reproducibility of the degree of focusing of the laser beam, the cone angle of the tip, the materials to be studied, and the wavelength and reflectivity of the surface of the tip material at that wavelength, etc. A numerical calculation by Liu Tsong,72 shown in Fig. 3.20, indicates that to produce a temperature rise of a few hundred K, the energy flux should be a few 106 J cm-2 s 1. Temperature pulses of a few hundred K suffices to 

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Fig. 2.5 An ion kinetic energy distribution of field desorbed He ions taken with a pulsed-laser time-of-flight atom-probe. In pulsed-laser stimulated field desorption of field adsorbed atoms, atoms are thermally desorbed from the surface by pulsed-laser heating. When they pass through the field ionization zone, they are field ionized. Therefore the ion energy distribution is in every respect the same as those in ordinary field ionization. Beside the sharp onset, there are also secondary peaks due to a resonance tunneling effect as discussed in the text. The onset flight time is indicated by to, and resonance peak positions are indicated by arrows. Resonance peaks are pronounced only if ions are collected from a flat area of the...

Fig. 3.17 Schematic of the Penn State high resolution pulsed-laser ToF atom-probe. The flight path length of this system is now —778 cm. It uses two LeCroy 4204 TDCs of 156 ps time resolution for flight time measurement.

Fig. 3.18 To show more vividly how ions are counted one by one, we show a small time window of an oscillograph containing atom-probe signals of W3+ and HeW3+ and He2W3+ ions, taken with the Penn State pulsed-laser ToF atom-probe in its early stage of development when the flight path length was 200 cm and the laser pulse width was still 5 ns.

Fig. 3.19 Mass separated energy distributions of pulsed-laser field desorbed 4He+ and ions obtained with the Penn State pulsed-laser ToF atom-probe when the flight path length was 778 cm. Their onset flight times are separated by 34 ns, exactly that calculated from the system constants and their critical ion energy deficits at 5.5 kV.

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