The Ion Nanoprobe: A New Instrument for Studying the Isotopic and Elemental Composition of the Solar System and Beyond at the Few-Nanometer Scale (2009)

We are now constructing the ion nanoprobe, a new instrument designed for isotopic and chemical analysis at the few-nm scale. The new instrument will combine a recently developed high resolution liquid metal ion gun (LMIG), tunable solid state lasers for laser resonant ionization, a molecular fluorine excimer laser for UV ionization, and high transmission, high resolution timeof-flight mass spectrometer. The instrument is intended to provide a significant decrease in spot size and increase in sensitivity compared to the current state-ofthe-art SIMS instrument, the Cameca NanoSIMS-50. The ion nanoprobe is the next step in the development of microbeam resonant ionization mass spectrometers and builds on the success of two instruments of similar design, CHARISMA [1] and SARISA [2]. CHARISMA pioneered isotopic measurement of trace elements, such as Cr, Sr, Zr, Mo, Ru, and Ba, in individual presolar grains [3–8]; SARISA is now being used to measure trace element abundances in Genesis solar wind collectors by depth profiling [e.g., 2]. All three instruments share a number of characteristics, including release of atoms from samples with focused ion beams or lasers, use of laser resonant ionization to ionize specific elements, and time-of-flight mass spectrometers to mass-analyze and count ions. Ion nanoprobe components: In designing the ion nanoprobe, the availability of new technology permits analytical goals beyond the capabilities of the current instrument. We now go through the major components of the ion nanoprobe and discuss these developments. Sample stage. In CHARISMA and SARISA, the size of sample mounts is limited to 5–10 mm diameter. In order to make the ion nanoprobe more versatile, we have designed a new sample holder that will accept samples as large as 1 inch in diameter, which is the worldwide standard for polished thin sections. This has been done by making the stage a part of the ion extraction system. Fig. 1 shows the sample holder mounted on a four-axis (X, Y, Z, rotation) nanomotion stage. This stage uses piezoelectric motors with position encoders and is bakeable and UHV compatible. Releasing atoms from samples. This will be done in several ways. First, following the example of CHARISMA, a UV ablation laser can be used to efficiently desorb atoms, but the diffraction limit to spatial resolution is ~0.5 μm. The instrument will be equipped with a high current, low energy Ar ion gun for surface cleaning and depth profiling. The new instrument gets its name, however, from Ga liquid metal ion gun that can be focused to a few nm. The development of these guns has been driven by focused ion beam (FIB) sample preparation devices for electron microscopy. Ga is not widely used in SIMS, because sputtering with Ga provides a relatively low yield of ions. (TOF-SIMS instruments use Ga ion beams, but get around the sensitivity problem by presputtering, which amorphizes a few monolayers of sample and enhances ion yield.) In the ion nanoprobe, it is desirable to sputter neutral atoms and then use lasers to ionize them, so the low ionization efficiency of Ga sputtering is actually an advantage.