  Despite tremendous progress in the research on self-assembled
nanotechnological building blocks such as macromolecules, nanowires, and
two-dimensional materials, synthetic self-assembly methods bridging nanoscopic
to macroscopic dimensions remain unscalable and inferior to biological
self-assembly. In contrast, planar semiconductor technology has had an immense
technological impact owing to its inherent scalability, yet it appears unable
to reach the atomic dimensions enabled by self-assembly. Here we use surface
forces including Casimir-van der Waals interactions to deterministically
self-assemble and self-align suspended silicon nanostructures with void
features well below the length scales possible with conventional lithography
and etching, despite using nothing more than conventional lithography and
etching. The method is remarkably robust and the threshold for self-assembly
depends monotonically on all governing parameters across thousands of measured
devices. We illustrate the potential of these concepts by fabricating
nanostructures, which are impossible to make with any other known method:
Waveguide-coupled high-Q silicon photonic cavities that confine telecom photons
to 2 nm air gaps with an aspect ratio of 100, corresponding to mode volumes
more than 100 times below the diffraction limit. Scanning transmission electron
microscopy measurements confirm the ability to build devices even with
subnanometer dimensions. Our work constitutes the first steps towards a new
generation of fabrication technology that combines the atomic dimensions
enabled by self-assembly with the scalability of planar semiconductors.
