Guided Self-Assembly of Refractory Metal Helices in Ultra-High Vacuum

We present a method for fabricating tungsten (W) helices via guided self-assembly of nanomembranes (NMs) in ultra-high vacuum (UHV). The process enables parallel processing of 3D structures on large-area conventional semiconductor substrates. The fabricated helices have several potential applications, including slow-wave structures in vacuum electronic devices and helical antennas for hypersonic systems, as they provide the necessary strength, thermal stability, and oxidation resistance.

The fabrication of W helices is a multi-step process beginning with creating a weak adhesion region between the refractory metal and substrate. Initially, we fabricate strips of materials that strongly adhere to W but weakly adhere to the substrate. We identify these material strips as adhesion reducers. The adhesion reducers are formed via top-down processing and include at least one stress concentrator, such as a sharp corner. Our work uses amorphous Ge adhesion reducers on bulk Si substrates. Subsequent electron-beam evaporation of W on the substrate leads to the delamination of the deposited metal/adhesion reducer combination that eventually assembles in a helical structure. We speculate that interfacial delamination begins at the stress concentrators and propagates until the majority of the W/Ge NM is released. Dry etching by a BCl₃ inductively coupled plasma removes the adhesion reducers, yielding W helices with a few tenths of micrometers diameter, as measured by scanning electron microscopy. We have performed continuum mechanical modeling to gain insight into the self-assembly of refractory metal helices in UHV. Initial results suggest that the delamination of the W/Ge bilayer is primarily driven by the buildup of residual stresses in the W films.


ACKNOWLEDGEMENT. This work was funded by the U.S. Air Force Office of Scientific Research. The work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Los Alamos National Laboratory (Contract 89233218CNA000001) and Sandia National Laboratories (Contract DE-NA-0003525).