Abstract text (incl. figure legends and references)

Introduction

Hybrid pixel direct electron detectors are getting more popular in electron microscopy due to their excellent properties. These cameras can be affordable which makes them ideal tools for experimentation especially in combination with an SEM setup. To support this, a full detector characterisation of an Advacam Minipix detector system in the 15-30keV range was made. At low beam currents, detector layer behaviour should be dominant, which could make the findings equally applicable to Medipix and TImepix3 sensors provided the same detector layer is used.

Objectives

Determine modulation transfer function (MTF) and detective quantum efficiency (DQE) of the Advacam Minipix detector for acceleration voltages of 15 – 30 keV and different detector thresholds.

Materials & methods

Advacam Minipix (TimePix) with 300 μm thick Si sensor 256x256 pixels of 55 μm size; Tescan Mira FEG SEM; 16 μm FIB aperture.

The MTF is obtained via Fourier transform of the Point Spread Function (PSF) acquired with single spot illumination [1], mechanically scanning over individual pixels with a 16 μm FIB fabricated mask mounted on a piezo stage.

The DQE is calculated from separate homogeneous illumination measurements of DQE(0) corrected with cluster size estimation [2], Noise Power Spectrum and MTF with same conditions [3].

Results

Fig.1 and Fig.2 represent MTF and DQE obtained for beam energies of 15 – 30 keV and detector threshold settings 5 – 25 keV. It is seen that the MTF is quite close to ideal. MTF gets further from ideal with an increase of HT at constant threshold. Higher thresholds improves MTF but at the cost of reducing the number of detected electrons and thus a worse DQE.

Figure 1. MTF of TimePix detector depending on Acceleration voltage (HT) and camera threshold.

Figure 2. DQE of TimePix detector depending on Acceleration voltage (HT) and camera threshold.

Conclusion

We provide a full MTF and DQE characterisation of an Advacam Minipix detector for beam energies in the SEM range. We observe a near ideal MTF with only minor effects from clustering. In this sense the detector is very close to its theoretical limit. The DQE on the other hand remains always under 65% which we speculate to be due to a combination of backscattering and absorption in the front metal contact that is deposited on the detector layer.

References

[1] Tore Niermann et al. "A new linear transfer theory and characterization method for image detectors. Part II: Experiment", https://doi.org/10.1016/j.ultramic.2012.01.011

[2] D. Jannis et al. "Event driven 4D STEM acquisition with a Timepix3 detector: Microsecond dwell time and faster scans for high precision and low dose applications" https://doi.org/10.1016/j.ultramic.2021.113423

[3] G.McMullan et al. "Detective quantum efficiency of electron area detectors in electron microscopy", https://doi.org/10.1016/j.ultramic.2009.04.002

[4] The authors acknowledge the financial support of the Research Foundation Flanders (FWO, Belgium) project SBO S000121N.

[5] The authors are grateful to Dr. Ivan Lobato for productive discussion of methods