SIMS on FIB instruments: a powerful tool for high-resolution high-sensitivity analytics at the nano scale

Advanced Instrumentation for Ion Nano-Analytics (AINA), Luxembourg Institute of Science and Technology (LIST), 41 rue du Brill, L-4422 Belvaux, Luxembourg

FIB-based instruments become of increasing importance in materials and life sciences. They are an ideal tool for high resolution 2D and 3D imaging and for nanofabrication (nano-machining and deposition, etc.), yet their analytical capabilities are currently limited. By contrast, secondary ion mass spectrometry (SIMS) is an extremely powerful technique for surface analysis owing to its excellent sensitivity, good dynamic range, high mass resolution and its ability to differentiate between isotopes. Adding SIMS capability to FIB instruments offers not only the prospect of obtaining elemental information at much higher resolution than state-of-the-art SIMS instruments, but allows for a direct correlation of SIMS images with high resolution secondary electron images.  Past attempts of integrating mass spectrometers on FIB instruments were rather unsuccessful because of poor detection limits which were due to (i) low ionization yields of the sputtered matter, (ii) extraction optics with low secondary ion extraction efficiency, and (iii) mass spectrometers with a low duty cycles and/or transmission. In order to overcome these limitations, reactive gas flooding during FIB-SIMS has been investigated and compact high-performance magnetic sector mass spectrometers with dedicated high-efficiency extraction optics developed by our group. Secondary ion yields have been maximised by using O2 flooding for positive secondary ions and Cs flooding for negative secondary during the SIMS analysis. Compared to Ga or noble gas primary ion species without flooding, the ionisation probabilities have been increased by up to 4 orders of magnitude, which leads to detection limits varying from 10-3-10-7 for lateral resolutions between 10 nm and 100 nm. Images recorded with poorer lateral resolution but excellent detection limit (e.g. 1 ppm @ 50 nm) can be correlated with secondary electron images recorded at high lateral resolution using Ga, He, Ne or electron beams, overcoming the trade-off between detection limit and smallest feature size. The SIMS add-on has been optimised for FIB-SIMS applications. The extraction optics have maximum efficiency without distorting the primary ion beam and are coupled to a specially designed compact magnetic sector mass spectrometer with a high mass resolution (>2000), full mass range (H-U) and detection of several elements in parallel.   The results are very encouraging and the prospects of correlating SIMS with high-resolution imaging and nanofabrication instruments are very interesting. High-resolution microscopy and high-sensitivity elemental mapping are combined on a single instrument, which represents a new level of correlative microscopy.