Direct electron detection in transmission electron microscopy
Since the first prototype of a transmission electron microscope was built in 1931 by Ernst Ruska and Max Knoll, Transmission Electron Microscopy (TEM) has proved to be an essential imaging tool for physicists, material scientists, and biologists. To record the TEM images for analysis, electron microscopists have used specialized electron micrograph film for a long time, until the new developments in TEM, such as electron tomography and cryo- electron microscopy, pushed for the needs of digital imaging. Recent years has seen the widespread use of the charge coupled device (CCD) in all fields of TEM. Although it does provide digital readout, its spatial resolution and sensitivity are fundamentally limited by the use of an indirect detection method based on scintillator screens to convert high energy electrons into photons. To realize the full imaging potential of the TEM, a new imaging detector for TEM, called the Direct Detection Device (DDD), was designed, built and characterized. The DDD is capable of directly detecting high energy electrons with a high signal to noise ratio, ensuring single electron sensitivity. Thanks to the small 5μm pixel size, the spatial resolution of the DDD exceeds any current imaging detectors in the TEM. Additionally, with the recent development of the fifth generations DDD prototype, the imaging performance of the DDD was further improved by using the DDD as a high speed 2-dimensional electron counter. The Modulation Transfer Function (MTF) of the DDD at half Nyquist frequency (50 lp/mm) reached 50% using the noiseless electron counter operation. The unique features of the DDD also enabled the possibility of specimen drift correction and fast wide-field image mosaic acquisition. With the advantages of direct electron detection, the DDD is expected to improve the image quality and resolution of all fields of TEM, especially for low dose imaging.