  Extraordinary physical properties arise at polar interfaces in oxide
materials, including the emergence of two-dimensional electron gases,
sheet-superconductivity, and multiferroicity. A special type of polar interface
are ferroelectric domain walls, where electronic reconstruction phenomena can
be driven by bound charges. Great progress has been achieved in the
characterization of such domain walls and, over the last decade, their
potential for next-generation nanotechnology has become clear. Established
tomography techniques, however, are either destructive or offer insufficient
spatial resolution, creating a pressing demand for 3D imaging compatible with
future fabrication processes. Here, we demonstrate non-destructive tomographic
imaging of ferroelectric domain walls using secondary electrons. Utilizing
conventional scanning electron microscopy (SEM), we reconstruct the position,
orientation, and charge state of hidden domain walls at distances up to several
hundreds of nanometers away from the surface. A mathematical model is derived
that links the SEM intensity variations at the surface to the local domain wall
properties, enabling non-destructive tomography with good noise tolerance on
the timescale of seconds. Our SEM-based approach facilitates high-throughput
screening of materials with functional domain walls and domain-wall-based
devices, which is essential for monitoring during the production of device
architectures and quality control in real-time.
