ABSTRACT

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 256×256 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.

ACKNOWLEDGEMENTS

This work was funded by the EPSRC (Fast Pixel Detectors: a paradigm shift in STEM imaging, EP/M009963/1).  Financial support from the European Union under the Seventh Framework Program under a contract for an Integrated Infrastructure Initiative (Ref 312483-ESTEEM2) is gratefully acknowledged.

REFERENCES

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