Electron Diffraction Based Tilt Angle Measurements in Electron Tomography

Electron tomography can provide three dimensional (3D) visualization of nanoscale objects in a transmission electron microscope (TEM) from a tilt series of projected images. The accuracy of the 3D reconstruction depends on quality of input parameters, including accurate alignment in real space and accuracy of tilt and azimuth angle measurements. Here we present an electron diffraction-based in-situ method that allows accurate measurement of the relative tilt () and azimuth () angles. Experimental data were collected in a Hitachi HF 3300 TEM / STEM instrument with a cold field emission gun operated at 300 kV. The rod-shaped samples that allow collection of full ±90° tilt range were prepared in a Hitachi NB 5000 dual beam instrument [1]. The sample used for the experiment comprised a few-layer thick ordered array of silver nanoparticles with sub-5 nm diameter deposited on Si <001>. On amorphous carbon deposited on the top of the sample, some fiducial markers were fabricated by electron beam induced deposition for accurate alignment [2]. The accurate measurement of α and β relies on recording a diffraction pattern (DP), in addition to an image, at each tilt α, which is easily achieved using a computer instrument control known as Maestro [3]. The data were collected in 3° increments. Figure 1 schematically shows the experimental set up. Individual DPs, collected at each α, were aligned over the entire tilt range so that diffraction pattern features, such as Kikuchi lines, are continuous throughout the tilt range, as shown in Figure 2. The relative shift between subsequent DPs is equivalent to change in relative angle between the incident beam and the sample. To facilitate measurement of the each shift, the individual DPs were converted to sinogram by Radon transform. The presence of Kikuchi lines in DPs results in bright peaks in sinograms. The angle of the Kikuchi lines and their distance from center of the DPs appear as the x and y value respectively of the corresponding peak in the sinogram. Using the angle and distance of Kikuchi lines, the shift among individual DPs was calculated and the DPs over the entire tilt series can be stitched together as shown in Figure 2. Low-index zone axis, for example [010] and [0-11], were used as in-situ calibration of the tilt per pixel. The known 90° tilt between [010] and [0-11] poles on stitched DP divided by their distance in pixels gives the °/pix calibration. In the …