Characterization of detector modulation-transfer function with noise, edge, and holographic methods

Sample thickness affects contrast and measured shape in TEM images and in electron tomograms

We investigated the effect of nanoparticle (NP) image broadening and its contrast change dependence on a support matrix thickness in a transmission electron microscope (TEM). We measured the effect of NP size and atomic number on its image broadening. Based on the experimental TEM images we generated tomograms of NPs on four types of support matrix. The measured shape aspect ratio of the NPs in such tomograms depends on the geometry of the support matrix. For example, the aspect ratio of 6 nm NP placed on a thin film with window-frame support is 1.14, while the aspect ratio of 6 nm NP on a rod-shaped support with 910 nm diameter is 1.67 in a tomogram.

Performance of a silicon-on-insulator direct electron detector in a low-voltage transmission electron microscope

The performance of a direct electron detector using silicon-on-insulator (SOI) technology in a low-voltage transmission electron microscope (LVTEM) is evaluated. The modulation transfer function and detective quantum efficiency of the detector are measured under backside illumination. The SOI-type detector is demonstrated to have high sensitivity and high efficiency for the direct detection of low-energy electrons. The detector is thus considered suitable for low-dose imaging in an LVTEM.

Accurate measurement of relative tilt and azimuth angles in electron tomography: a comparison of fiducial marker method with electron diffraction

Electron tomography is a method whereby a three-dimensional reconstruction of a nanoscale object is obtained from a series of projected images measured in a transmission electron microscope. We developed an electron-diffraction method to measure the tilt and azimuth angles, with Kikuchi lines used to align a series of diffraction patterns obtained with each image of the tilt series. Since it is based on electron diffraction, the method is not affected by sample drift and is not sensitive to sample thickness, whereas tilt angle measurement and alignment using fiducial-marker methods are affected by both sample drift and thickness. The accuracy of the diffraction method benefits reconstructions with a large number of voxels, where both high spatial resolution and a large field of view are desired. The diffraction method allows both the tilt and azimuth angle to be measured, while fiducial marker methods typically treat the tilt and azimuth angle as an unknown parameter. The diffraction method can be also used to estimate the accuracy of the fiducial marker method, and the sample-stage accuracy. A nano-dot fiducial marker measurement differs from a diffraction measurement by no more than ±1°.

Determination of localized visibility in off-axis electron holography

Off-axis electron holography is a wavefront-split interference method for the transmission electron microscope that allows the phase shift and amplitude of the electron wavefront to be separated and quantitatively measured. An additional, third component of the holographic signal is the coherence of the electron wavefront. Historically, wavefront coherence has been evaluated by measurement of the holographic fringe visibility (or contrast) based on the minimum and maximum intensity values. We present a method based on statistical moments is presented that allows allow the visibility to be measured in a deterministic and reproducible fashion suitable for quantitative analysis. We also present an algorithm, based on the Fourier-ratio method, which allows the visibility to be resolved in two-dimensions, which we term the local visibility. The local visibility may be used to evaluate the loss of coherence due to electron scattering within a specimen, or as an aid in image analysis and segmentation. The relationship between amplitude and visibility may be used to evaluate the composition and mass thickness of a specimen by means of a 2-D histogram. Results for a selection of elements (C, Al, Si, Ti, Cr, Cu, Ge, and Au) are provided. All presented visibility metrics are biased at low-dose conditions by the presence of shot-noise, for which we provide methods for empirical normalization to achieve linear response.

Quantitative characterization of electron detectors for transmission electron microscopy

A new generation of direct electron detectors for transmission electron microscopy (TEM) promises significant improvement over previous detectors in terms of their modulation transfer function (MTF) and detective quantum efficiency (DQE). However, the performance of these new detectors needs to be carefully monitored in order to optimize imaging conditions and check for degradation over time. We have developed an easy-to-use software tool, FindDQE, to measure MTF and DQE of electron detectors using images of a microscope's built-in beam stop. Using this software, we have determined the DQE curves of four direct electron detectors currently available: the Gatan K2 Summit, the FEI Falcon I and II, and the Direct Electron DE-12, under a variety of total dose and dose rate conditions. We have additionally measured the curves for the Gatan US4000 and TVIPS TemCam-F416 scintillator-based cameras. We compare the results from our new method with published curves.