AXON Dose: A Solution for Measuring and Managing Electron Dose in the TEM
John Damiano,* Stamp Walden, Alan Franks, Kate Marusak, Ben Larson, Mike Coy, and
David Nackashi Protochips Inc., 3800 Gateway Centre Blvd, Suite 306, Morrisville NC 27560 *
john@protochips.com
Abstract: The interaction of the electron beam with materials during TEM/STEM imaging often leads to radiation damage. While a variety of low-dose techniques can help mitigate beam damage, true dose man- agement starts with knowing the precise total accumulated dose and dose rate that a sample has seen throughout an experiment. AXON Dose allows users to calibrate their instruments, track electron dose/ dose rate across a sample as a function of time and location, and quantify the impact of dose on individual samples.
Keywords: in situ electron microscopy, transmission electron microscopy, electron dose, dose rate, beam damage
Introduction Te transmission electron microscope (TEM) is a power-
ful tool for imaging and analyzing samples at the nanoscale. Te interaction of the electron beam with the sample yields a wide range of signals that are useful for determining a sample’s size, shape, composition, and morphology. Unfortunately, the same electron probe used for imaging can also damage many samples. A microscopist can unintentionally destroy a sample that required significant time and money to prepare. Worse yet, beam damage can change the sample in ways that lead to false conclusions about the data and create experimental ambiguity. Beam damage is especially challenging for in situ studies since the goal of in situ work is to characterize a reaction independent of beam damage. For example, was an observed reaction due to applied stimulus, beam damage, or both? One common approach is to use “low dose” techniques
that seek to minimize the beam/sample interactions to a level where they do not substantially impact the sample or the analy- sis results [1]. Tese efforts to limit both the cumulative electron dose (e-
/Å2 ) and the dose rate (e- /Å2 s) are well documented in
the literature both for conventional and in situ TEM, especially for techniques sensitive to radiolysis, such as liquid EM [2–4]. A more advanced approach would allow users to calibrate their TEM, quantify dose and dose rate throughout an experiment, and control either parameter within a user-defined threshold. An accurate measure of the dose imparted to a particular loca- tion on a sample as a function of time is essential so that images or other data can be compared across sites, across samples, across instruments, and across time. Until now, a robust solution for managing the electron dose
during TEM experiments—one that enables a user to track and manage beam effects—has been hampered by several key issues. First, the calibration of a TEM’s beam current can be a tedious and difficult process that requires many steps to accurately account for all microscope variables [1]. Once calibrated, the workflow requires the microscopist to either remain at a specific magnification or to work within other severe limitations to keep dose rates constant and simplify dose calculations. Finally, aſter
22 doi:10.1017/S1551929522000840
a dose threshold is determined, the operator must keep track of imaged regions within the sample and try to compensate when the limit is approached. Here, we review the features and benefits of AXON Dose, a
machine-vision solution designed to address and eliminate the challenges of calibrating, managing, and tracking a sample’s electron dose exposure throughout the course of a TEM experiment.
Calibration An improved dose workflow starts with beam current
calibration. A Faraday cup in the sample plane, connected to a picoammeter, can fully collect and accurately measure the current. By measuring the current for each aperture, spot size, and intensity, the beam current can be calibrated across all lens settings. Having characterized beam current, the corre- sponding beam area can then be measured over the same set of conditions. Te Protochips Dose holder, shown in Figure 1, is designed to support a workflow that measures both current and area. It features a Faraday cup for current collection, a through- hole to allow measurement of beam area using the camera, and fiducials that, along with soſtware control, allow the entire cali- bration process to be automated. Tis encourages regular use of the calibration workflow for the highest level of accuracy over time. With calibrated beam current and area for all microscope settings, the electron dose rate and, when tracked to specific sites on the sample over time, cumulative electron dose can be determined precisely for any pixel on any image.
Integration Freely operating the instrument as normal, through vari-
ous spot sizes or magnifications, while keeping track of the total accumulated dose and/or dose rate over the entire sample area, isn’t practical without a means to simultaneously track the beam’s position on the sample. To achieve this goal, the AXON Synchronicity system can be used to compensate for sample driſt and other sample movement through adjustments to stage, beam, and camera acquisition conditions. Synchronicity also continuously collects images and metadata produced during an experiment, automatically saving all information to the image metadata. Integration of dose information into the image metadata
enables robust analysis and visualization using heatmaps from which accurate, quantifiable information on cumulative dose and fluence can be obtained from any point on the experi- mental timeline. Figure 2 shows an example of images from the AXON Dose user interface, which allows investigators to easily measure the dose exposure at any point in the image series and to graphically visualize any or all of the collected
www.microscopy-today.com • 2022 July
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