In this paper we report quantitative measurements of the imaging performance for the current generation of hybrid pixel detector, Medipix3, used as a direct electron detector. We have measured the modulation transfer function and detective quantum efficiency at beam energies of 60 and 80keV. In single pixel mode, energy threshold values can be chosen to maximize either the modulation transfer function or the detective quantum efficiency, obtaining values near to, or exceeding those for a theoretical detector with square pixels. The Medipix3 charge summing mode delivers simultaneous, high values of both modulation transfer function and detective quantum efficiency. We have also characterized the detector response to single electron events and describe an empirical model that predicts the detector modulation transfer function and detective quantum efficiency based on energy threshold. Exemplifying our findings we demonstrate the Medipix3 imaging performance recording a fully exposed electron diffraction pattern at 24-bit depth together with images in single pixel and charge summing modes. Our findings highlight that for transmission electron microscopy performed at low energies (energies <100keV) thick hybrid pixel detectors provide an advantageous architecture for direct electron imaging.

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We report the first use of direct detection for recording electron backscatter diffraction patterns. We demonstrate the following advantages of direct detection: the resolution in the patterns is such that higher order features are visible; patterns can be recorded at beam energies below those at which conventional detectors usefully operate; high precision in cross-correlation based pattern shift measurements needed for high resolution electron backscatter diffraction strain mapping can be obtained. We also show that the physics underlying direct detection is sufficiently well understood at low primary electron energies such that simulated patterns can be generated to verify our experimental data. DOI: 10.1103/PhysRevLett.111.065506 PACS numbers: 61.05.JÀ, 07.78.+s, 68.37.Hk Electron backscatter diffraction (EBSD) is a scanning electron microscope (SEM) based method in which diffraction of low-energy-loss electrons as they exit through the topmost few tens of nanometers leads to Kikuchi diffraction. In most EBSD studies the incident electron beam is stepped across a grid of points on the sample surface and the EBSD patterns analyzed in an automated way to determine crystal phase, orientation, or lattice strain variation. The EBSD method has evolved rapidly over the last two decades [1][2][3][4][5]. Most research has been directed to the application of this versatile tool to an ever increasing array of problems in materials characterization but the analysis methods themselves have also advanced, notably in three dimensional imaging using focused ion beam (FIB)-SEM [6-9] and in strain mapping [10][11][12][13][14]. However, the detector technology used to record EBSD patterns has essentially remained unchanged for over a decade and now limits performance in several application areas, such as strain resolution and low dose mapping, and prevents the development of new areas.The earliest EBSD patterns were recorded on film either exposed directly to the electrons in the chamber [15][16][17], or indirectly imaging a phosphor screen using a camera outside the vacuum [18]. Subsequently, these were replaced by various image intensified cameras giving the convenience of a live image of the pattern at the scintillator but with degraded pattern quality compared to that recorded using film [19]. Subsequently, scintillator coupled CCDs were introduced in the early 1990s [20,21]. In a limited number of examples tapered fiber-optic bundles have been used to couple the CCD to the scintillator with good results [20] but the alternative optical lens coupling has been adopted in the vast majority (> 95%) of instruments currently in use. Departures from these detection schemes have included an investigation of microchannel plates [22] and the adoption of a retarding electrostatic field for energy filtering [23].In other fields there have been significant advances in detectors directly exposed to the imaging beam for the detection of x rays [24,25] and medium energy electrons [26][27][28][29]. The current development of TEM instr...

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Hybrid pixel sensors, originally developed for particle physics, incorporate advanced analogue processing and digital conversion circuitry at the individual pixel level. Medipix3 [1] is an example of such a sensor and we have investigated its performance as an imaging detector for transmission electron microscopy (TEM). Measurements were performed with electron beam energies in the range, 60–200 keV on a JEOLARM200cF TEM/STEM [2] utilising a 256x256 pixel Medipix3 detector with 300 µm thick Si sensor layer. In order to characterise the Modulation Transfer Function (MTF) and the Detective Quantum Efficiency (DQE) performance, 32 repeated datasets were acquired containing images of free space and a knife‐edge for known beam current conditions at each energy across the full range of relevant Medipix3 energy threshold values. Data was acquired in Single Pixel Mode (SPM) and in Charge Summing Mode (CSM) [3], where, in the latter mode, effects from charge spreading in individual electron events are corrected for on the detector. We have also measured DQE(0) using the methods described in [4]. At high lower threshold (THL) DAC values the MTF for this counting detector in single pixel mode is better than the theoretical maximum due to the reduction in the effective pixel size [4] as shown in Figure 1. However, the DQE at such high THL DAC values in single pixel mode is significantly reduced as seen in Figure 2, seeing as many real electron events are now not counted as the charge is deposited in more than one pixel and therefore falls below the threshold for detection. Consequently, there is a balance to be made between optimizing DQE and MTF, depending on the exact requirements in the given application. As is shown in Figure 3, the use of the CSM allows the achievement of a much higher MTF whilst retaining high DQE by using a lower threshold DAC value. This therefore offers additional benefits over the more conventional SPM, thus allowing very high efficiency imaging whilst preserving maximal detail in the images, which is particularly beneficial for minimizing the required electron dose to the sample required to produce interpretable data, with obvious applications in beam sensitive materials.