The idea for IDLE was to build an instrument capable of very efficient analysis of micron- and submicron-sized samples. The decision was therefore made to build a time-of-flight secondary ion mass spectrometer (TOF-SIMS) with laser post ionization for secondary neutrals. The acquisition of all secondary ions in parallel makes these instruments very efficient and keeps sample consumption to a minimum. This is essential for samples like presolar grains which are only a few microns in size or the analysis of cometary dust like the small and precious particles brought back by the Stardust mission . Further applications are depth-profiling of solar wind samples from the Genesis mission and elemental mapping of interplanetary dust and meteoritic samples.
The original instrument was equipped with a Ga primary ion gun and F2-excimer laser. Using a time-of-flight detector allows to record the whole mass spectrum for efficient sample usage and the liquid metal ion source of the Ga-gun allowed for high spatial resolution. After recent updates we now have two instruments, IDLE2 and IDLE3, the first designed especially for organic analysis and the second instrument for inorganic, general analysis.
IDLE2 (on the left in Figure 1) is equipped with a C60 primary ion gun, mainly for organic analysis and incorporates a cold stage enabling the analysis of frozen biological samples. IDLE3 (right hand side of Figure 1) is equipped with a Gold cluster liquid metal ion gun and a duoplasmatron which can be run with Argon or Oxygen. The duoplasmatron is a low-energy, high-current source especially suitable for depth-profiling and sputter-cleaning of samples whereas the Gold gun has a very high lateral resolution down to around 100 nm. It's also fitted with a 157 nm excimer laser for the post-ionization of secondary neutrals which boosts sensitivity by up to a few orders of magnitude.
Both instruments use the same analytical principles. An ion beam is rastered over the sample surface and material is sputtered at each point of the sample and then analysed in a time-of-flight mass spectrometer delivering a complete mass spectrum for each pixel of the image. The ion beam sputters the surface hence depth profiling into the sample. The four-dimensional data cube can then be post-processed for scientific analysis by defining regions of interested for which mass spectra can be extracted and quantified or distribution images can be extracted for all interesting peaks in the mass spectra. Figure 2 shows a total secondary ion image of a 1 micron chess pattern.
The laser post ionization on IDLE2 uses 157 nm photons which can ionize around 60% of all elements in a single-photon, non-resonant ionization. This boosts the detection sensitivity because typically <0.1% of the sputtered material ionizes in SIMS compared to more than one per cent ionizing in post ionization mode. Additionally, in SIMS the ionization probability for different elements can vary depending upon the material under analysis, the so-called matrix effect. The matrix effect is much lower with laser post ionization. The sketch in Figure 3 shows how a cloud of sputtered neutrals is created by the primary ion beam which is then ionized with the laser.
Figures 4 shows the inside of the IDLE2 instrument with the cold stage at the bottom and the secondary ion extractor above in the centre. The small-nosed gun is an electron flood gun for charge compensation of non-conducting samples and the wide-nosed gun is the front-end of the old Ga-primary ion gun which is now replaced by a very similar looking C60-gun.
For a more detailed description of the instrument and its applications see:
• Henkel, T. and J. Gilmour (2013). Time-of-Flight Secondary Ion Mass Spectrometry, Secondary Neutral Mass Spectrometry, and Resonance Ionization Mass Spectrometry. Treatise on Geochemistry, 2nd edition. Volume 15. K. Turekian and H. Holland, Elsevier.
• Stephan T. and Lyon I. C. (2013). Applications of ToF-SIMS in cosmochemistry. TOF-SIMS: Materials Analysis by Mass Spectrometry, 2nd edition. J. C. Vickerman and D. Briggs.
• King, A., T. Henkel, et al. (2010). "Determination of relative sensitivity factors during secondary ion sputtering of silicate glasses by Au+, Au 2+ and Au 3+ ions." Rapid Communications in Mass Spectrometry 24(1): 15-20.
• Henkel, T., D. Rost, et al. (2009). "Improvements in quantification accuracy of inorganic time-of-flight secondary ion mass spectrometric analysis of silicate materials by using C60 primary ions." Rapid Communications in Mass Spectrometry 23(21): 3355-3360.
• Henkel, T., Tizard, J., Blagburn, D. and Lyon, I. (2007) IDLE - Interstellar Dust Laser Explorer. A new instrument for elemental and isotopic analysis and imaging of interstellar and interplanetary dust. Review of Scientific Instruments 78: 055107