Top-down nanofabrication
Top-down nanofabrication builds complex micro and nanostructures by progressively patterning and removing material from a bulk substrate. In our work, the starting material is a silicon-on-insulator (SOI) wafer, which consists of a thin single-crystal silicon device layer on top of a sacrificial silicon dioxide (SiO₂) buried oxide, supported by a thick silicon handle wafer. The device layer — typically 100–500 nm thick — becomes the nanostructure itself, while the buried oxide serves as a sacrificial layer that can later be selectively removed to release free-standing suspended structures. This three-layer stack is the foundation for all the nanostructures used in our thermal and acoustic experiments, and the process flow for a typical sample is shown in the figure below.
Fabrication begins with electron-beam (EB) lithography, which uses a focused beam of electrons to write a pattern in a thin layer of electron-sensitive resist coated on the wafer surface. The first lithography step defines the position and shape of metal contact pads. After exposure and development of the resist, a thin aluminum (Al) film is deposited by physical vapour deposition (PVD), and the unwanted metal is removed by lift-off — leaving precisely placed pads on the silicon surface. In TDTR samples, these pads act simultaneously as heaters and thermometers for thermal measurements. A second EB lithography step then defines the geometry of the nanostructure itself — the pattern of holes, slits, pillars, or wire outlines — with feature sizes down to tens of nanometers.
With the pattern defined in resist, reactive ion etching (RIE) transfers it into the silicon device layer. In RIE, a plasma of reactive gases removes exposed silicon in a highly anisotropic manner, producing vertical sidewalls and faithful reproduction of the lithographic pattern across the entire structure. Once the silicon nanostructure is shaped, the final step is to release it from the substrate by removing the SiO₂ buried oxide underneath using vapour-phase hydrofluoric acid (VHF) etching. VHF selectively attacks SiO₂ without damaging silicon or aluminum, leaving a fully suspended nanostructure — a free-standing membrane, nanowire, or nanobeam — thermally isolated from the substrate, which is essential for accurate thermal transport measurements.
The scanning electron microscopy images below show examples of the suspended nanostructures fabricated using this process. Straight and serpentine silicon nanowires are used to probe ballistic phonon transport at different length scales. Silicon nanobeams decorated with periodic arrays of aluminum nanopillars combine the suspended platform with local resonators for phonon engineering. Two-dimensional phononic crystals — membranes patterned with periodic arrays of holes — enable studies of coherent, wave-based heat conduction. More complex geometries, such as the thermal lens shown in the bottom-right panel, use asymmetric hole patterns to focus or redirect heat fluxes based on the ray-phononics concept.
