Strain Mapping Using HR-EBSD Versus 4D STEM on a Direct Electron Detector

Strain in materials is important for the performance of electronic devices by affecting the mobility of charge carriers [1]. This can hinder performance and be used to tune electronic properties. Strain can also be used to stabilize certain phases and is critical in the development of certain high-performance alloys [2]. At the nanoscale, 4-dimensional scanning transmission electron microscopy (4D STEM) and scanning precession electron diffraction (SPED) are commonly used to measure strain with spatial accuracies of 1-10 nm and precision of +/- 0.05-0.1% [3]. The precision of these strain measurements is largely dependent on sample preparation, and fundamentally limited by dynamical diffraction within the diffraction disks. High resolution electron backscatter diffraction (HR-EBSD) provides an attractive alternative for measuring strain with much higher precision of greater than 0.01% although with much lower spatial resolution of 20-80 nm [4]. In this study we explore the usage of a Monolithic Active Pixel Sensor (MAPS) for HR-EBSD to offer an alternative to strain mapping via 4D STEM. A 4k x 4k MAPS detector designed for low kV electrons, with 6.5 μm pixels (DE-SEMCam, Direct Electron LP) was used to collect HR-EBSD patterns across an indented silicon wafer at 10kV, 13 nA current, 125 nm step size (Fig. 1(B)). The number of electrons was directly measured and controlled to limit shot noise in the data. Patterns were acquired with 4k x 4k resolution at 92 frames per second (fps) and using 2x binning at 281 fps. Frames were summed until a desired number of electrons per pixel were collected, with total collection times of 0.25 -1 second per pattern. While these acquisition parameters are similar to HR-EBSD using a scintillator-coupled detector, MAPS detectors offer advantages in quantum efficiency and point spread function, resulting in higher quality patterns (Fig. 2). The pattern in Fig. 1(B) has higher contrast, sharper bands and contains additional features compared to similar datasets acquired using a scintillator-coupled detector. The higher quality pattern allowed the rotation of the silicon crystal around the indent to be mapped very precisely to within < 0.05 degrees (Fig. 3). Elastic strain maps were calculated with a precision of 0.01%, detecting strain present at the very edges of the scanned area. The high quantum efficiency of the MAPS detector at low kV offers the possibility of reducing interaction volumes and pushing HR-EBSD spatial resolution closer to that of 4DSTEM [5]. (A) Initial kernel average misorientation map obtained from the HR-EBSD scan of the indent. (B) A representative 10kV HR-EBSD pattern from the silicon wafer. A fast Fourier transform of Fig. 1(B), and EBSD back transform, showing the quality of the pattern. High resolution kernel average map showing the silicon crystal rotation and cracking near the indent.