Use of a hybrid silicon pixel (Medipix) detector as a STEM detector

Abstract

Scanning transmission electron microscopy has traditionally relied on the high angle annular dark field technique for imaging atoms [1,2], which provides a simple and easy to understand contrast, which is strongly related to atomic number. More recently, there has been a resurgence of interest in alternative imaging modes, including bright field [3,4] and annular bright field imaging [5,6]. Ultimately, however, the most flexible STEM experiment would be to record the entire back focal plane of the specimen onto a pixelated detector, and then post-process the dataset to access whichever features in the contrast are desired. This could produce all the above-mentioned signals, but many more besides, including ptychographic reconstruction of the exit wave [7,8].In order to achieve this aim, we need to have a detector that is capable of recording single electron events at typical beam energies for a scanning TEM (e.g. 100 or 200 keV) with enough pixels to allow significant flexibility for performing different imaging modes, a readout speed far quicker than that available of previous pixelated detectors such as traditional scintillator-CCD devices (typically < 30 frames per second), and synchronization to the scan system. We have installed a Medipix-3 detector onto our JEOL ARM200F STEM (probe-corrected, cold FEG version), which is a silicon based hybrid pixel detector with a CMOS readout architecture and 256 x 256 pixels. The Medipix family of detectors are true counting detectors. Each of the 55µmx55µm pixels contains amplifiers and digitisation circuitry that determines whether energy deposited by the incident radiation lies within the range set by userdefined low and high thresholds, counting digitally those that do. Medipix2 detectors have been investigated for TEM by us [9,10] and others [11]. Presently we are exploring the capabilities of the third generation Medipix-3 detector coupled to a high-speed read-out system. The MERLIN read-out system [12] is capable of frame rates of 1200fps for durations that are limited only by the ability to transfer frame data to non-volatile storage. Constant frame rates of 100fps are guaranteed but we have found no problems running continuously at 500fps. Figure 1 shows that for 200keV electrons, single electron detection is achieved but the detector settings have a strong influence on the spatial frequency response of recorded images. Utilising an exposure time of 1µs and values of 70 and 240 for the pixel low energy threshold, it can be seen that the effects of charge-sharing between pixels can be drastically reduced, but at the expense of reducing the counting efficiency from 100% to ~50%. We are currently measuring the performance of the detector with respect to Detective Quantum Efficiency (DQE) and Modulation Transfer Function (MTF). An exciting aspect of the Medipix3 is its novel architecture, which allows the detection of charge-sharing and re-attribution back to a single pixel. For high-energy electrons we expect this to significantly improve MTF values.For STEM mode d...

Fast Pixelated Detectors in Scanning Transmission Electron Microscopy. Part I: Data Acquisition, Live Processing, and Storage

“…In particular, later contributions demonstrated that the inner angle of ADF detectors had to be relatively high to exclude coherent diffraction from dominating the signal (Pennycook & Jesson, 1991;Hartel et al, 1996). Other refinements of this arrangement have been introduced over the years, including the use of split detectors for differential phase contrast (Dekkers & de Lang, 1977;Chapman, 1978;Chapman et al, 1990;McGrouther et al, 2014), multiple annular detectors (Shibata et al, 2010(Shibata et al, , 2017, and the use of bright field or annular bright field imaging (Hammel & Rose, 1995;LeBeau et al, 2009;Findlay et al, 2010;MacLaren et al, 2015). However, all these detector configurations integrate over large angular ranges of the back focal plane, resulting in the loss of most of the information contained in the diffraction pattern.…”

“…Recently, building upon technologies developed for particle physics (Turala, 2005; Wermes, 2005; Turchetta et al, 2007; Delpierre, 2014), pixelated detectors developed for X-ray imaging have been adopted for electron imaging (Clough et al, 2014; McMullan et al, 2014; McGrouther et al, 2015; Tate et al, 2016; Mir et al, 2017; Tinti et al, 2018). Compared with charge coupled device (CCD)-based detectors, these direct electron detectors (DEDs) typically offer much lower noise levels, improved detector quantum efficiency (DQE), and modulation transfer function (MTF), some degree of radiation hardness, and crucially, fast readout of the images.…”

Achieve atomic resolution in in situ S/TEM experiments to examine complex interface structures in nanomaterials

“…Just briefly, in the last decade these electron microscopes have greatly advanced by utilizing new hardware capabilities -such as correctors compensating aberrations present in electromagnetic lenses [41,51,81,96,131], monochromators compensating electron energy spread present in field emission electron guns [45,74,84,110,153,154], sensitive detectors to count even single electrons [13,50,100,101,122,133,151] -to enhance the spatial resolution in the sub-nm range, provide an energy resolution in the sub-eV range, and achieve a sensitivity to detect single atoms (see Fig. 1).…”

Towards Reproducible and Transparent Science of (Big) Electron Microscopy Data Using Version Control

“…This includes techniques such as electron energy loss spectroscopy and energy-dispersive X-ray spectroscopy, where these improvements have enabled the acquisition of large spectrum maps. A more recent development is the use of fast pixelated detectors for use in scanning transmission electron microscopy (pixelated STEM) [1], which allows for the acquisition of a large fraction of the STEM diffraction pattern for each probe position. These new developments enable new and exciting ways of characterizing materials, but also pose challenges with regards to handling large amounts of data.…”