Secondary Ion Mass Spectrometry (SIMS) is an extremely powerful technique for analyzing surfaces owing in particular to its excellent sensitivity, high dynamic range, very high mass resolution and ability to differentiate between isotopes. During the past several years, we have been performing a multidisciplinary R&D effort combining fundamental research on particle matter interactions, instrument development and application development to develop the HIM-SIMS, which is a unique tool for high-resolution high-sensitivity correlative microscopy.
Considering that the highest-resolution ion beam (sub nm spot sizes!) is obtained by the so-called Gas Field Ion Source (GFIS) technology, and taking into account that He+ and Ne+ are the ions most efficiently created by the GFIS process, the team investigated in detail the interaction between energetic He+ and Ne+ ions and surfaces (both soft and hard matter). This newly gained fundamental knowledge on e.g. sputtering and ionization mechanisms allowed the team to develop strategies to optimize the ion emission from samples exposed to GFIS beams. Encouraged by these new findings, we developed in a second step a compact but high-performance SIMS spectrometer consisting of high-efficiency secondary ion extraction optics, post-accelerating transfer optics and a double focussing magnetic sector mass spectrometer allowing parallel detection of all elements and isotopes. This SIMS system has been successfully installed and tested on the Zeiss ORION Helium Ion Microscope (HIM), which uses the GFIS as key enabler and which has emerged during the last few years as an ideal tool for nano-imaging and nano-fabrication.
This development has led to excellent results, ranging from a world record resolution in SIMS of 10 nm to applications in various R&D areas (e.g. toxicological studies on nanoparticles, photovoltaics, soil, etc.), so that the HIM-SIMS has become a commercial product in 2018. Several of these instruments are located in different laboratories and companies worldwide.
Contact: Tom WIRTZ & Jean-Nicolas AUDINOT
We have equipped different types of Focused Ion Beam (FIB) platforms with magnetic sector based SIMS technology:
- Thermo Fisher SCIOS and HELIOS: FIB-SEM platforms using different types of FIB columns (Ga+, Xe+, O+, Ar+).
- SIMS:ZERO: FIB platform equipped with a zeroK Cs+ Low Temperature Ion Source (LoTIS) providing reactive Cs+ primary ions for nano-scale analytics (a spot size of 2 nm with 1 pA can be reached).
- Raith VELION: FIB platform equipped with a Liquid Metal Alloy Ion Sources (LMAIS) FIB column providing a number of different primary ion species, i.e. Ga+, Li+, Bi+ and Bi3+, with nanoscale resolution This instrument is equipped with an extremely high precision laser interferometer sample stage allowing extremely precise beam positioning onto nanosized objects / device features, opening the pathway towards a more extended use of the FIB-SIMS technology within the semiconductor field.
Contact: Olivier DE CASTRO & Tom WIRTZ & Hung Quang HOANG
Transmission Electron Microscopy (TEM) is a well-known technique for high-resolution structural imaging down to atomic scale. However, the conventional analytical techniques associated with TEM such as Energy-Dispersive X-Ray Spectroscopy (EDX) or Electron Energy-Loss Spectroscopy is inadequate for the analyses of trace elements ( < 0.1 at. %) or isotopes. Analysis of light elements is particularly challenging by EDX. SIMS has the advantage of high-sensitivity down to the ppm level and all the elements in the periodic table can be analyzed, including isotopes. However, the lateral resolution in SIMS imaging is fundamentally limited to ~10 nm, which is more than two orders-of-magnitude poorer than TEM. To combine the benefits of TEM and SIMS in a single instrument, we developed an in-situ TEM-SIMS instrument. The concept was realized by modifying the octagon of a FEI Tecnai F20 TEM (80-200 kV) to accommodate a FIB and a SIMS around the TEM objective lens pole-pieces area. The FIB is a commercial FEI Magnum with Ga+ source operating at 30 kV. A special high-voltage TEM sample holder which can be biased to ±4.5 kV was developed for enhancing secondary ion collection efficiency. The secondary ion extraction optics and a compact mass spectrometer was completely designed and developed in house. The performance of the in-situ TEM-SIMS instrument and the benefits of correlative microscopy have been demonstrated. Ongoing research focuses on the development of analytical strategies and optimization methods for the characterization of various types of samples taken from a range of applications in materials science (e.g. battery research, energy materials, photovoltaics, tracking of hydrogen in steel, etc.), life sciences and beyond.
Contact: Santhana ESWARA
Based on the GFIS technology, a dedicated instrument platform for use in nanotoxicology and life sciences was developed in the EU’s H2020 research and innovation program funded project npSCOPE (2017 – 2021; www.npscope.eu). 9 partners (Luxembourg, Germany, Netherlands, Belgium, Switzerland and France) worked together in order to design, manufacture, assemble and test an integrated instrument featuring topographic imaging, in depth transmitted beam information and a SIMS detector able to generate a full mass spectrum per pixel scanned in the area of interest (see focal plane detector technology below).
While most material science samples have rather simple preparation and acquisition needs, beam-sensitive materials and especially life science samples request special attention. For example, to guarantee vacuum compatibility, introducing biological samples in frozen-hydrated condition is beneficial to circumvent the artefacts linked to the classical way of room temperature (RT) sample preparation (such as chemical fixation, dehydration and resin embedding). Also beam damage on sensitive (materials science) samples can be reduced under cold conditions. These beneficial aspects of cryo-EM were passed on to ion beam induced techniques during this project. Another problem in HIM-SIMS is that secondary electron imaging does not yield proper contrast on thin-sectioned biological specimen as they are typically used in (analytical)electron microscopy of life science samples.
The npSCOPE instrument is based on the extraordinarily high resolution of the helium ion microscope’s GFIS technology and incorporates the following complementary detection modalities:
- Ion induced secondary electron (SE) imaging: He+ ion beam → sub-nm lateral resolution for topographic imaging.
- Magnetic sector SIMS with a novel continuous focal plane detector (featuring a full mass spectrum per scanned pixel; FPD-SIMS, see also next paragraph below): Ne+ ion beam → sub-15 nm laterally resolved chemical information.
- Scanning transmission helium ion microscopy (2D position sensitive STIM detector): He+ ion beam → volume information with sub-6 nm resolution suitable for thin sample sections.
Moreover, the instrument incorporates cryo-cooling capabilities for sample investigation under -139 °C allowing the investigation of frozen-hydrated (vitrified) specimen.
- Actively cooled cryo shield and high precision piezo driven 5-axis cryo-sample stage.
- Cryo-compatible airlock for sample transfer.
- Actively cooled in vacuum cryo transfer suitcase connected to a cryo glove box (humidity < 1 ppm) for sample loading.
cryoTEM image (left) courtesy Petr Chlanda, University of Heidelberg
Contact: Tom WIRTZ & Antje BIESEMEIER & Olivier DE CASTRO