A High Brightness Negative Ion Source for SIMS

Secondary ion mass spectrometry (SIMS) often utilizes a negative oxygen (O$^{-}$ or O$_{2}^{-})$ focused ion beam for trace element analysis of materials. The primary advantage of SIMS over other mass spectrometric techniques is the ability to provide spatial information in addition to high detection sensitivity. However, the scanned ion image resolution of these instruments is limited by the performance of the ion source employed to create the focused primary ion beam. To date, the duoplasmatron ion source has been the most suitable source of negatively charged oxygen ions, with an energy normalized brightness ($\beta _{r})$ for O$^{-}$ of $\sim $40 Am$^{-2}$sr$^{-1}$V$^{-1}$ and an energy spread ($\Delta $E) of $\sim $10-15eV. This produces imaging resolution in the range of 1-30$\mu $m for the ASI SHRIMP and Cameca 1280 tools and an impressive 150nm spot size for the NanoSIMS 50. Here we describe a newly developed inductively coupled plasma source with negative ion extraction. This source has $\beta _{r }$= 600Am$^{-2}$sr$^{-1}$V$^{-1}$ and $\Delta $E of only 3.5eV for O$^{-}$ ion extraction. This translates to a nominal 100x gain in current density in the focused beam, when operated for high resolution imaging.