NEW TIME-OF-FLIGHT ELECTRONICS FOR ATOM-PROBE FIELD-ION MICROSCOPY

We present new time-of-flight electronics for atom-probe field-ion microscopy that is based on the LeCroy 2277/EXP2 time-to-digital converter (TDC) and is significantly superior to the commonly used LeCroy 4208 TDC. The maximum number of ions detected for each cycle is increased, the electronics dead time is decreased, and the pulse width from the Phillips Scientific 6904 discriminator is measured. The LeCroy 2277/EXP2 TDC records up to 128 ions per field evaporation pulse as opposed to a maximum of eight ions for a single LeCroy 4208 TDC. The dead time is reduced by increasing the pulse pair resolution, in the multihit mode of the TDC, from 7.3 to 3.3 ns. The LeCroy 2277/EXP2 TDC has both leading and trailing edge detection capability, allowing us to measure the pulse width generated by the Phillips Scientific 6904 discriminator. Combining the pulse width measurement capability with the updating capability of the Phillips Scientific 6904 discriminator, in which the discriminator extends the output pulse width by the time difference between two closely spaced (<2 ns) pulses, we resolve two ions arriving within 2 ns of each other. With these improvements, we find that 66% of the total number of multiply field-evaporated ions arriving with a pulse separation of up to 8 ns are incorrectly counted as a single ion. The percentage of multiply field-evaporated ions occurring with a pulse separation of 2 ns is 6%.