  Geometric dispersion in integrated microresonators plays a major role in
nonlinear optics applications, especially at short wavelengths, to compensate
the natural material normal dispersion. Tailoring of geometric confinement
allows for anomalous dispersion, which in particular enables the formation of
microcombs which can be tuned into the dissipative Kerr soliton (DKS) regime.
Due to processes like soliton-induced dispersive wave generation, broadband DKS
combs are particularly sensitive to higher-order dispersion, which in turn is
sensitive to the ring dimensions at the nanometer-level. For microrings
exhibiting a rectangular cross section, the ring width and thickness are the
two main control parameters to achieve the targeted dispersion. The former can
be easily varied through parameter variation within the lithography mask, yet
the latter is defined by the film thickness during growth of the starting
material stack, and can show a significant variation (few percent of the total
thickness) over a single wafer. In this letter, we demonstrate that controlled
dry-etching allows for fine tuning of the device layer (silicon nitride)
thickness at the wafer level, allowing multi-project wafers targeting different
wavelength bands, and post-fabrication trimming in air-clad ring devices. We
demonstrate that such dry etching does not significantly affect either the
silicon nitride surface roughness or the optical quality of the devices,
thereby enabling fine tuning of the dispersion and the spectral shape of the
resulting DKS states.
