Results of a pnCCD Based Ultrafast Direct Single Electron Imaging Camera for Transmission Electron Microscopy

Abstract

Up to now in a typical transmission electron microscope (TEM) camera, the incident electrons are indirectly detected. They are converted into photons in a phosphorous layer, which are then guided via fiber optics to a CCD or CMOS imager for detection. This results in inherent disadvantages of conversion efficiency, scattering of photons, reflections at optical interfaces and absorption losses. Contrary, this can be avoided in a direct detector which directly converts the incident electrons into a spatially resolved signal. This brings an increase in resolution and sensitivity, minimizing specimen damage. Therefore, in past years various systems compromising direct detectors have been launched, however mostly CMOS imagers [1, 2]. In contrast, we present first results of a new ultrafast TEM camera based on a pnCCD detector [3].A pnCCD is a radiation hard, back-illuminated device, which is fully depleted over the whole thickness of 450µm thick, high-purity n-type silicon, which is also the sensitive region. In this version it features a 48x48µm² pixel size with 264x264 pixels. The multiparallel readout of the pnCCD enables a frame rate of up to 1000 full frames per second offering increased temporal resolution. The sensitivity is such that each incident primary electron can be easily distinguished from noise, including primary electrons that are scattered out of the detector again. With an appropriate low-dose (100-1000 incident electrons per frame), single individual primary electrons can be separately detected, enabling further analysis and processing of these electron events.The final image can be obtained by simply integrating multiple frames and their intensities, which results in a conventional intensity image (called intensity mode). Even more, by using the potential of electron event processing, the image can formed by taking the individual events with their reconstructed point of entry and count them in a subpixel grid. A simple method to determine the point of entry is to calculate the center of gravity of each event (called CoG method). Since this is done with a subpixel precision, the grid for the final image can be chosen much finer than the device pixel size of 48µm (see Figure 1). Additionally, the frames and events used for image formation can be chosen after the exposure to correct for specimen drift and selecting only those images which have been recorded before the specimen was damaged. Also temporary changes of the sample, e.g. oxidation processes, can be nicely observed with a time resolution of 1000 images per second.The pnCCD TEM camera was installed at a TEM (Titan80/300) for evaluation of the imaging capabilities at electron energies of 80, 120, 200 and 300keV. The modulation transfer function was calculated with the slanted knife edge method for the intensity mode and with event processing with no subpixel and 5x5 subpixel grids (see Figure 2). Generally, the contrast increases with lower electron energy. Looking at the higher energies of 200 and 300keV in intensity mode, the contrast fell...

Scanning transmission electron microscopy strain measurement from millisecond frames of a direct electron charge coupled device

“…A detailed technical description will be presented elsewhere. 21 A series of 160.000 CBED patterns as depicted in Fig. 1(a) have been recorded with a frame time of 1 ms while the STEM probe was scanned over a stack of four strained quantum layers buried in GaAs as shown by the darkfield STEM image in Fig.…”

Extending the dynamic range of fully depleted pnCCDs

“…The new operation mode for high charge handling capacity has already been used at various measurements performed with the pnCCD TEM camera [11] at transmission electron microscopes (TEM). Because of the radiation hardness of the pnCCD and the back illumination through a thin entrance window, pnCCDs are suitable for the direct detection of TEM electrons with energies in the range from 300 keV down to 20 keV.…”

“…Thus with a readout rate of 1000 frames per second and a TEM electron energy of 80 keV, about 18,000 TEM electrons can be collected in one pixel during 1 s acquisition time. The number of TEM electrons can be further increased by a faster readout using windowing or binning modes [11].…”

Multivariate Statistical Analysis of a Multimodal Diffraction and X-ray Spectral Series Data Set

“…Position-resolved diffraction data were acquired on a FEI Company Titan G2 80-200 operated at 200kV and equipped with three-condenser lenses and a probe spherical-aberration corrector. Diffraction patterns were acquired on a PNDetector direct electron camera [2,4]. Figure 1 shows the results of a 256 by 256 real-space pixel convergent-beam diffraction data acquired from the Arltunga meteorite where each pattern consisted of 264 by 264 reciprocal-space pixels with 20 replicate patterns at each point for a data set of 183 Gb.…”