Transmission Electron Microscope Using a Linear Accelerator

High-voltage transmission electron microscopes (HVTEMs), which can visualize internal structures of micron thick samples, intrinsically have large instrument sizes because of the static voltage isolation. In this Letter, we develop a compact HVTEM, employing a linear accelerator, a subpicosecond beam chopper, and a linear decelerator. 100 kV electrons initially accelerated by a static field are accelerated at radio frequency (rf) up to 500 kV, transmitting through the sample and finally rf decelerated down to 200 kV to be imaged through a 200 kV energy filter. 500 kV imaging, as well as subnanometer resolution at 200 kV, have been demonstrated.

Figure 1

Overview of the Linac-TEM. (a) Illustration of the whole system. CL and IL stand for condenser lens and intermediate lens, respectively. (b) Illustration and photograph of the rf deflector cavity. (c) Illustration and photograph of the accelerator cavity. (d) Schematic graph to show the acceleration condition. (e) Photograph of the whole instrumentation showing the TEM column (left) and rf waveguide circuit block (right). Choke flanges (inset) are used for the rf input of the acceleration and deceleration cavities in order to isolate the mechanical vibration.

Figure 2

Acceleration in the linac TEM. (a) Diffraction patterns of a (001)-oriented single-crystal Au sample at different acceleration voltages. (b) Acceleration plotted as a function of the input power in the acceleration cavity. The solid line shows the theoretically calculated acceleration voltage.

Figure 3

Electron beam chopping. (a) Schematic illustration of the electron optics of the chopper system consisting of pairs of identical transfer lenses and beam deflecting rf cavities. (b) Observed electron beam traces on the slit plane at different relative phases of the two deflector cavities with the same input power. The slit-chopped image is shown in the rightmost image. (c)–(e) The spectral beam images on the energy plane using the energy filter (c) without chopping, (d) with chopping at 4 ps in the in phase condition, and (e) with chopping in the off phase condition by 1 mrad. The horizontal arrow direction corresponds to the energy loss. The spectra are obtained through a polycrystalline gold film. The images of (b)–(e) are recorded without rf acceleration.

Figure 4

Linac TEM Images. (a)–(c) Images of a Au film on a microgrid with acceleration and deceleration. (a) Image at 200 kV acceleration from the original 100 kV beam without deceleration (rf accelerated by 100 kV). (b) Image with a 300 kV electron beam at the sample position by 200 kV rf acceleration and detected at 200 kV by 100 kV deceleration. (c) Image with a 500 kV electron beam at the sample position by 400 kV rf acceleration and detected at 200 kV by 300 kV deceleration. All the images were acquired through the energy filter operated for 200 kV. (d),(e) Lattice imaging of a crocidolite sample obtained at 200 kV acceleration without deceleration. (d) Raw TEM Image. (Inset) Fourier transformed pattern showing 0.9 nm lattice fringe. (e) Fourier-filtered image using the 0.9 nm lattice spot. The lattice position at the right bottom is more clearly visible. The intensity profile integrated along the lattice direction of the raw data in (d) is shown in the inset. The profile is plotted along the red arrow direction in (d).