  Atomic-level imperfections play an increasingly critical role in nanophotonic
device performance. However, it remains challenging to accurately characterize
the sidewall roughness with sub-nanometer resolution and directly correlate
this roughness with device performance. We have developed a method that allows
us to measure the sidewall roughness of waveguides made of any material
(including dielectrics) using the high resolution of atomic force microscopy.
We illustrate this method by measuring state-of-the-art photonic devices made
of silicon nitride. We compare the roughness of devices fabricated using both
DUV photo-lithography and electron-beam lithography for two different etch
processes. To correlate roughness with device performance we describe what we
call a new Payne-Lacey Bending model, which adds a correction factor to the
widely used Payne-Lacey model so that losses in resonators and waveguides with
bends can be accurately predicted given the sidewall roughness, waveguide width
and bending radii. Having a better way to measure roughness and use it to
predict device performance can allow researchers and engineers to optimize
fabrication for state-of-the-art photonics using many materials.
