Digital direct electron imaging of energy-filtered electron backscatter diffraction patterns

Electron backscatter diffraction is a scanning electron microscopy technique used to obtain crystallographic information on materials. It allows the nondestructive mapping of crystal structure, texture, and strain with a lateral and depth resolution on the order of tens of nanometers. Electron backscatter diffraction patterns (EBSPs) are presently acquired using a detector comprising a scintillator coupled to a digital camera, and the crystallographic information obtainable is limited by the conversion of electrons to photons and then back to electrons again. In this article we will report the direct acquisition of energy-filtered EBSPs using a digital complementary metal-oxide-semiconductor hybrid pixel detector, Timepix. We show results from a range of samples with different mass and density, namely diamond, silicon, and GaN. Direct electron detection allows the acquisition of EBSPs at lower $(\le 5$ keV) electron beam energies. This results in a reduction in the depth and lateral extension of the volume of the specimen contributing to the pattern and will lead to a significant improvement in lateral and depth resolution. Direct electron detection together with energy filtering (electrons having energy below a specific value are excluded) also leads to an improvement in spatial resolution but in addition provides an unprecedented increase in the detail in the acquired EBSPs. An increase in contrast and higher-order diffraction features are observed. In addition, excess-deficiency effects appear to be suppressed on energy filtering. This allows the fundamental physics of pattern formation to be interrogated and will enable a step change in the use of electron backscatter diffraction (EBSD) for crystal phase identification and the mapping of strain. The enhancement in the contrast in high-pass energy-filtered EBSD patterns is found to be stronger for lighter, less dense materials. The improved contrast for such materials will enable the application of the EBSD technique to be expanded to materials for which conventional EBSD analysis is not presently practicable.

Figure 1

(a) Illustration of the EBSD detection geometry and a conventional EBSD detector. (b) An EBSP from a GaN thin film acquired at an energy of 5 keV, a probe current of $\approx 1.5\mathrm{nA}$, a detector-to-specimen distance of $\approx 10\mathrm{mm}$ (capture angle $\approx {60}^{\circ })$, and a 100 s exposure time using the Timepix digital direct electron imaging detector. The red lines outline a pair of Kikuchi bands; the large yellow circles indicate HOLZ rings and the small green circles highlight RHEED spots.

Figure 2

(a) Timepix detector, (b) simplified schematic of a detector's pixel: sensor (A), bias voltage (B), solder bumps (C), preamplifier (D), threshold (E), discriminator (F), threshold adjustment (G), and counter (H).

Figure 3

As-acquired EBSPs from GaN for an incident beam energy of 20 keV with a threshold energy of 19.4 keV, a probe current of $\approx 20\mathrm{nA}$, and a detector-to-specimen distance of $\approx 5\mathrm{mm}$ providing a capture angle of 100°, with an acquisition time of (a) 0.5 ms, (b) 1 ms, and (c) 5 ms.

Figure 4

(a–c) Experimental EBSPs from diamond, Si, and GaN for an incident beam energy of 20 keV, a probe current of $\approx 10\mathrm{nA}$, and a threshold energy of 4.6 keV. The EBSPs from diamond, Si, and GaN had acquisition times of 0.8, 50, and 10 s, respectively. The insets are two-dimensional (2D) fast Fourier transforms of each image. (d–f) Experimental EBSPs from diamond, Si, and GaN for an incident beam energy of 20 keV, a probe current of $\approx 10\mathrm{nA}$, and a threshold energy of 19.4 keV. The EBSPs from diamond, Si, and GaN had acquisition times of 100, 1482, and 60 s, respectively. The insets are 2D fast Fourier transforms of each image. (g–i) 19.5 keV dynamical simulations of EBSPs from diamond, Si, and GaN. The detector-to-specimen distances and capture angles for the EBSPs were estimated by comparison with the simulated EBSPs. For diamond the detector-to-specimen distance was $\approx 8\mathrm{mm}$ with a capture angle $\approx {80}^{\circ }$, for Si the detector-to-specimen distance was $\approx 15\mathrm{mm}$ with a capture angle $\approx {50}^{\circ }$, and for GaN the detector-to-specimen distance was $\approx 10\mathrm{mm}$ with a capture angle $\approx {70}^{\circ }$.

Figure 5

Threshold calibration using backscattered electrons showing the differentiation of the particle count as a function of the digital threshold value (THL). The inset at the top left shows the energy calibration line derived by extrapolating the intercept to zero of the curves plotted in the main graph.

Figure 6

Monte Carlo simulations of the backscattered electron energy spectra for Si and Au, respectively. The intensities have been scaled so that the backscattered intensity from 10 to 20 keV is taken as 1.

Figure 7

Zoomed-in regions from EBSPs from Si acquired with (a) low threshold and (b) high threshold energies; (c, d) show the extracted $\left(220\right)$ and higher-order Kikuchi bands from the EBSPs shown in (a, b), respectively.

Figure 8

EBSPs from silicon for an incident beam energy of 30 keV, a probe current of $\approx 2.5\mathrm{nA}$, an acquisition time of $\approx 100\mathrm{s}$, and a detector-to-specimen distance of $\approx 30\mathrm{mm}$ providing a capture angle of $\approx {30}^{\circ }$. (a) Threshold energy of 4.6 keV, (b) threshold energy of 28.6 keV, (c) difference between (a) and (b), (d) mean Kikuchi band profile for band A in (a), (e) mean Kikuchi band profile for band B in (a), where the energy-dependent effect is much smaller than for band A.