Direct detection and electron counting ‐ A beginning of a new era for electron microscopy

Abstract

Over the last decade, great progress has been made in the instrumentation of transmission electron microscopy (TEM) through the introduction of aberration correctors, electron energy monochromators, and a wide variety of TEM specimen holders designed for in‐situ applications. With these new capabilities, a wealth of advanced experimental data can be easily generated by advanced TEM systems. However, the development of a revolutionary image capture device has been lacking; this effectively makes imaging detectors the main bottleneck when striving to achieve the full potential performance of an advanced TEM. Traditional image detectors use scintillator and optical transfer path (fiber‐coupling or lens) to convert high energy electrons to photons that are subsequently transferred to the imaging sensor to form an image. One of the disadvantages of this indirect detection approach is the loss of image resolution and sensitivity during the electron‐photon conversion and the photon transfer as additional noise sources that significantly degrade the signal‐to‐noise ratio (SNR) of the image detector. Very recently direct detection imaging devices have been successfully developed based on technology advancement made in CMOS (complementary metal oxide semiconductor) design and manufacturing, high speed data architectures, vastly increased memory densities and speed, etc. The elimination of scintillator and subsequent optical transfer path has significantly improved the detective quantum efficiency (DQE) – a critical measure of SNR for resolution and sensitivity of an imaging device. The state‐of‐the‐art direct detection imaging device further boosts the DQE and image quality under extremely low beam conditions by electron counting at high speed (e.g. 400 fps @ 4k x 4k resolution) to eliminate the sensor readout noise and minimize the electron scattering noise. Figure 1 shows the comparison of DQE measurement at 300 kV for K2 direct detection cameras operated in electron counting (solid blue) and linear (dotted blue) mode, and scintillator/fiber optical coupled cameras (purple). It is clear that electron counting has significantly restored the DQE performance of direct detection cameras for all frequencies. This new generation of direct detection imaging device has revolutionized the field of cryo‐electron microscopy (cryoEM) in structural biology and is starting to impact many applications in electron microscopy of materials science, for example, in‐situ microscopy, 4D STEM, imaging beam sensitive materials, quantitative measurement of radiation damage or quantitative electron microscopy, etc. Direct detection and electron counting are poised to advance electron microscopy into a new era.