UC San Diego shows thin-film devices actively couple to substrates, aiding 3D chips

UC San Diego physicists and collaborators report that substrates under oxide thin films are not electrically inert, challenging a long-standing materials assumption and pointing toward denser three-dimensional chips for energy-efficient neuromorphic computing, phys.org reported of a study in Science. Associate Professor Alex Frañó's lab at the Department of Energy's Q-MEEN-C center studied vanadium dioxide thin-film devices in which applied voltage forms an electric filament that drives neuron-like electrical spiking. Using dark-field X-ray microscopy developed by then-graduate student Elliot Kisiel, largely at Argonne National Laboratory, the team unexpectedly saw structural change in the substrate as well as the film, showing the two were energetically coupled. Thin films in the work were around 100 nanometers thick while substrates can be about 10,000 times thicker, yet the film could push and pull on the substrate and the substrate acted back on the film. The group spent four years reproducing the result across sample changes and instruments at Argonne and Brookhaven National Laboratory. Frañó said the assumption that substrates are inert needs to be rethought. One forward path is using the substrate as a medium that couples materials on both sides, enabling three-dimensional interconnects beyond two-dimensional thin-film layouts. The Science paper's DOI is 10.1126/science.adt9347.