Optical TEM Specimen Holder
side following the procedure outlined
in Torlabs’
Fiber laser spot
Connectorization Guide [9]. With single-mode fibers, we achieved a spot size of about 20 μm at a working distance of 1 mm, and with a 50 μm multimode fiber we achieved a spot size of 100 μm. Motion control. Since our size was designed
to be between 20 and 100 μm, which is much smaller than a TEM grid, we needed to be able to control the position of the laser spot on the TEM grid. To achieve this, we developed two designs: (1) the fiber is positioned via a short metal tube held by screws close to the tip of the TEM holder, and (2) the fiber is enclosed in a long metal tube that is attached to a 3-axis external micrometer stage at the rear handle of the holder (Figure 3). Te latter has the advantage of
letting
the user move the fiber in situ, whereas the former is more stable because of
the shorter
metal tube and is also more affordable. Te 3-axis external micrometer stage was designed in-house using off-the-shelf micrometer screws. In addition to the micrometer screws, we incorporated guide rods to make the stage more stable and reduce driſt and backlash. Vacuum. Te specimen an
chamber of
microscope typically operates below a pressure of 10-5
electron mbar.
Te optical feedthrough of the TEM holder needs to have effective vacuum seals, while also allowing for easy switching out of optical fibers. Terefore, we used Viton®
o-rings at all
temporary vacuum interfaces. For permanent vacuum seals (like the inside of the optical fiber coupler), we used vacuum-compatible epoxies such as TorrSeal®
and EPO-TEK® 353ND. Tese were used in small
quantities to minimize any potential contamination on the sample during imaging. For the 3-axis external micrometer stage, we used metal bellows to allow for motion while maintaining vacuum. Te bellows were custom made by Metal Flex Welded Bellows, Inc., Newport, VT.
42
Figure 3: Motion control designs; (left) using fiber positioning screws at the tip of the holder; (right) micrometer stage at the rear handle of the holder.
X-ray safety. TEMs operate between 60 and 300 keV,
which generates a significant amount of X-rays at the specimen. Te TEM holder needs to efficiently shield users from these X-rays. Stainless steel, copper alloys and titanium have X-ray penetration depths of a few mm [10] at 100 keV and are therefore suitable for shielding. Te penetration depth of X-rays through aluminum, however, is a few centimeters, making it unsuitable
www.microscopy-today.com • 2021 September
Figure 1: Process flow for constructing a TEM holder.
Figure 2: (left) Schematic side view of the optical setup design. (right) Top view of the actual setup.
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