Observation of DC field-evaporated ion species extracted from transition metal nitride thin film deposited on tungsten-tip

The ion species extracted from transition metal nitride thin films were investigated in order to understand the field evaporation mechanism of nitrogen in atom probe analysis. Nitrides of group IV transition metal, ie, titanium (Ti), zirconium (Zr), and hafnium (Hf) nitrides, were chosen for analysis, owing to their good electrical conductivities. The samples were prepared by sputtering deposition of nitride thin film on a tungsten needle. Measurements were performed at 3 different direct current voltages, and for each voltage, we observed different ion species. For TiN and ZrN, both atomic metal ions and molecular ions were detected and TiN and ZrN tended to evaporate in the form of triple-charged molecular ions. For ZrN, Zr²⁺, Zr³⁺, ZrN³⁺, and (ZrN)₂³⁺ were observed at lower direct current voltages. For a higher tip voltage, N⁺ ions were detected in addition to these ions. These results suggest that the evaporation field of nitrogen is higher than those of Zr³⁺ and (ZrN)₂³⁺. In the analysis of an HfN tip, no ions could be detected. These results can be explained in terms of the differences between evaporation fields that were roughly estimated from the work functions and the bond energies of the analyzed nitrides.