Gas Injection System Free Cryogenic Focused Ion Beam Sample Preparation for Complementary Cryogenic Scanning Transmission Electron Microscopy and Atom Probe Tomography

The Imperial Centre for Cryo Microscopy of Materials (I(CM)²) is a new facility at Imperial College London for the development and application of cryo microscopy to environmentally sensitive materials. The facility consists of the three state of the art microscopes (Cameca Local Electrode Atom Probe 5000 XR, FEI-Thermofisher Scientific Helios Hydra DualBeam with cryo-stage and FEI-Thermofisher Scientific Spectra 300 with cryo-holders) connected via an inert gas glovebox with cryo-vacuum transfer capability. By combining the complementary capabilities of these instruments, nanoscale compositional and structural analysis on environmentally sensitive materials of interest to a wide range of fields can be achieved.<br/>Cryogenic focused ion beam (cryo-FIB) sample preparation for site specific liftout has significant challenges compared to the standard room temperature process flow. A number of applications that require cryo-FIB have successfully bypassed the requirement for liftout such as on-grid thinning for cryo electron microscopy (cryo-EM) [1] or “satellite dish” samples for Atom Probe Tomography (APT) [2]. However, there are many number of situations in life sciences and materials science where the cryo-FIB liftout stage is essential and so reliable and reproducible processes to enable this to become routine are required. Complementary Scanning Transmission Electron Microscopy (STEM) and APT analysis on the same sample [3] is increasingly commonplace and the application of this to materials requiring cryo-FIB is an emerging field of interest. This requires samples to be fabricated and mounted in a manner that is appropriate for the specific requirements for high yield and data quality from both techniques.<br/>Room temperature FIB liftout generally uses site specific decomposition of an organometallic precursor gas supplied by a Gas Injection System (GIS) to provide protective layers and to attach liftout lamella to micromanipulators and support structures such as grids. If this gas is released into a FIB chamber with a cryogenically cooled stage then the gas immediately condenses. The thickness of this condensate is hard to control, it requires “curing” via an electron or ion beam to become conductive and using it significantly increases the difficulty of the liftout process [4]. GIS-free cryo-FIB liftout of TEM lamelle using redeposition methods has been shown to be viable [5] [6] and a cryo-FIB approach using a combined controlled cryogenic GIS deposition and redeposition has been demonstrated for APT samples [7] but has not been optimised for the extreme conditions experienced during APT analysis.<br/>In this presentation, we demonstrate a reproducible, GIS-free, process flow for cryo-FIB liftout and mounting of APT samples using selective resputtering [8] of an in-situ tungsten micro-manipulator. APT samples made using cryo-FIB liftout from single crystal silicon using this method have been successfully analysed and the data collected was found to be comparable in quality to that collected from commercial pre-sharpened silicon calibration specimens (Cameca) [9]. Progress of this approach and applications of this process to facilitate cryo-FIB sample preparation for complementary cryo-STEM and APT on the same sample will be discussed along with the numerous challenges of cryo and vacuum transfer between instruments involved in this process.<br/><b>References:</b><br/>[1] F. R. Wagner <i>et al</i>, <i>Nature Protocols</i>, 15 2041-2070 (2020).<br/>[2] A. A. El-Zoka <i>et al</i>., <i>Science Advances</i>, 6 49 (2020) .<br/>[3] M. Herbig, P. Choi, D. Raabe, <i>Ultramicroscopy</i>, 153:32-9 (2015).<br/>[4] C. D. Parmenter, Z. A. Nizamudeen, <i>Journal of Microscopy</i>, 281 (2021),p.157-174.<br/>[5] S. Klumpe <i>et al</i>., <i>Microscopy Today</i>, 30(1) (2022) p.42-47.<br/>[6] J. Kuba <i>et al</i>., <i>Journal of Microscopy</i>, 281 (2) (2021),p112-124<br/>[7] D.K. Schreiber <i>et al</i>., <i>Ultramicroscopy</i>, 194 (2018) p.89-99.<br/>[8] S. Kölling, W Vandervorst, <i>Ultramicroscopy</i>, 109(5) (2009) p.486-491.<br/>[9] J. Douglas <i>et al</i>, <i>Microscopy and Microanalysis,</i> In Review.