Entering the Deep Sub-angstrom Resolution Regime in Electron Microscopy
Updated: Aug 9, 2018
Until recently, TEMs have heavily relied on charge coupling devices (CCDs) for electron imaging. These devices exhibited a number of advantages compared to their predecessors, namely the emulsion films and the imaging plates. For instance, although the emulsion films provided good spatial resolution, larger field of view and good contrast, the dynamic range was much lower, and the detector response was non-linear. These issues were improved upon in imaging plates that came later along with an added advantage of digital readout. However, the signal-to-noise ratio provided by the imaging plates was still relatively poor.
This communication is part of a series of articles contributed to the Springer's Handbook of Microscopy - G. Botton and S. Prabhudev 'Analytical Electron Microscopy' (2019)
Furthermore, both emulsion films and imaging plates were inherently offline in nature, thus any advanced imaging experiments requiring automation was limited. The CCDs on the other hand provides improved performance in areas noted above particularly with the advent of direct electron detection. However, the dynamic range is still limited by the fact that the charge created by only a few primary electrons per pixel saturates the CCD. The scintillator-coupled CCDs can extend the dynamic range delivered per frame by minimizing the average signal recorded per incident electron, but the spatial resolution is compromised owing to scattering (of electron and photons) in the scintillator material, this having an impact on the point-spread function. These limitations are overcome by the recently developed pixel area detectors (PADs) (Tate & Muller et al, 2016). The PADs are compact and have single electron sensitivity, offering high speed electron imaging, where a single frame can be read out in less than 1 ms. Importantly, the PADs provide high dynamic ranges on the order of 10^6, in other words the capability to detect between 1 to 1,000,000 electrons for every pixel. These characteristics enable them to essentially record an image of all the transmitted electrons (corresponding to ZLP to HOLZ lines) for every probe position on the sample.
One particular implementation of the PAD arrangement developed by Tate et al, (2016) and Muller et al 2016) is shown in the below (a). The detector consists of Si-based pixelated sensors (~500 μm thick) that are bump bonded pixel-by-pixel to an underlying integrated circuit, which processes the charge generated in each sensor pixel. A pixel array of 128 x 128 is used in sensing, by further dividing it into eight banks of 128 x 16 pixels each. As shown in (b), the entire detector is housed separately, sliding in and out pneumatically as and when required. Figure (c) illustrates the practical advantage of using a PAD detector, showing the diffraction pattern recorded in 1 ms for a BaFiO3 sample (along  zone-axis). Even at such small readout time, the recorded pattern clearly shows the central beam and the details of the Kikuchi bands. Figure (d) shows the accumulation of this data over 100 frames, and as can be seen, the Kikuchi band and the HOLZ lines and the central beam are much clearer.
More details on the construction and the advantages of these PAD detectors can be found elsewhere (https://doi.org/10.1017/S1431927615015664). A recent application of the EMPADs in the full-field Electron Ptychography of MoS2 now allows for an unprecedented deep sub-ansgtrom resolution down to 0.39 Angstroms, read more at https://www.nature.com/articles/s41586-018-0298-5