Year: 2023
Type: article
Abstract: The development and improvement of new technologies is often associated with the investigation of new functional materials. In order to properly design new functionalities, thorough characterization of the spatial distribution of elements in newly prepared materials and devices is required. Changes of the local chemical composition while a device is running due to e.g. degradation processes or dendrites formations, amongst other, may impact the foreseen functionality. Such chemical changes may be of relevance even at nanometer scales (too small to be imaged by EDS). A powerful way to identify and characterize such compositional changes is to employ FIB-SIMS — a mass spectrometry technique, capable of chemical imaging with high spatial resolution and sensitivity [1, 2]. TOFWERK produces a time-of-flight (TOF) analyzer add-on for FIB-SEM instruments, the fibTOF [3]. The addition of a mass spectrometric detector to a FIB-SEM (focused ion beam-scanning electron microscope) brings SIMS (secondary ion mass spectrometry) imaging capabilities. This allows the observation of the local composition with a lateral resolution of a few tens of nanometers [2, 4]. The time-of-flight mass spectrometry principle is based on measuring the time ions with a given energy need to traverse a distance in a field free space. The heavier the ion, the more time is needed for ions with identical kinetic energy to pass the set distance. The advantage of using TOF technology is the fast and simultaneous measurements of all isotopes with a high sensitivity. The combination of an orthogonal time-of-flight mass spectrometer with a FIB-SEM microscope enables mass spectrometric measurements with no need to pulse the FIB beam. Synchronization of the SIMS signal with the scanning pattern of the FIB allows for a three dimensional data set to be built up as the FIB beam scans repeatedly over the sample surface. A mass spectrum (with a typical mass range of 0–500 Th) for each voxel (dwell time/pixel of >10 µs/voxel) is recorded. The fibTOF allows for fast measurements with a complete mass spectrum per scanned voxel, reflecting the elemental distribution at the nanometer scale. The working principle of the fibTOF is presented in figure 1. Tofwerk's implementation (fibTOF) has been shown to be compatible with FIB-SEM microscopes of the major FIB-SEM manufacturers (Thermo Fischer Scientific, Tescan, Zeiss). Here we demonstrate the capability of the fibTOF to image the distribution of alkali metal ions for industrial applications such as rechargeable batteries and zero-gap electrolyzer for CO2 conversion to reusable fuels [5, 6]. Whereas analytical techniques based on X-ray emission (e.g. EDS) struggle with the detection of lithium, the element is readily detected by FIB-SIMS with either liquid metal or common gas phase ion sources. The high ionization probability of sputtered particles allows for the detection of lithium down to the ppm level [7]. For many popular rechargeable battery technologies, lithium plays an essential role as charge carrying ion (e.g. Li+) incorporated into the electrode material and moving from the cathode to the anode during charging and vis versa during discharge [8]. fibTOF allows rapid visualization of the spatial distribution of the element of interest and gives insight into possible mass transportation processes, and degradation pathways. The formation of alkali metal precipitates on gas diffusion electrodes (GDE) during electrolysis is known to increase the cell resistance thus reducing the stability of the electrolyzer. The detection and imaging of these precipitates is important to better understand the degradation pathways and to improve the electrolysis process. To complement the measurement capabilities of the fibTOF, further measurements highlighting the detection of heavier elements, Al and Ti, playing a key role for failure analysis of semiconductor devices, are presented [9]. The presented measurements were performed with a monoisotopic 69Ga+ FIB. The 30 keV acceleration voltage of the ion beam with an optimal focus point allowed for high spatial resolution. The ion current between 1.5 pA -500 pA was adapted according to the sample and the field of view. A pixel dwell time of 10 µs was fixed. This contribution is intended to introduce fibTOF (FIB-SIMS) measurement capabilities and to show to what extent FIB-SIMS can complement state of the art analytical techniques for FIB-SEM microscopes. Working principle of fibTOF mass spectrometry.
Source: Microscopy and Microanalysis
Institutions: Tofwerk (Switzerland), University of Bern
Language: English
Subfield: Surfaces, Coatings and Films
Field: Materials Science
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W4385072136