Shadow Montage and Cone-Beam Reconstruction in 4D-STEM Tomography

Diffraction images in a scanning transmission electron microscope (STEM) provide a real-space projection of the sample at sufficient probe defocus. Such shadow images may be acquired patch by patch in a 4D-STEM setup using a pixelated detector and assembled into a shadow montage. Consequently, an upscaled bright-field image is rendered efficiently in time compared to other STEM techniques. We show that this procedure achieves the result of a tilt-corrected bright-field image. Furthermore, the solution is equivalent to cone-beam reconstruction in a particular scenario of a scanning point illumination source in a plane. Contrast transfer is similar to that of conventional TEM, but like STEM, the focus is insensitive to energy loss and objective lens chromatic aberration. By adjusting the overlap between shadow patch images, the shadow montage is derived from or synchronized to specific layers in the sample, similarly to multi-slice ptychography, rendering a 3D shadow volume from a single dataset. The method is amenable to conventional tilt tomography by summing shadow volumes of all tilt views to generate a full back-projection reconstruction. This approach effectively circumvents the basic presumption of parallel-projection tomography that the depth of field must be greater than the specimen thickness.