  State-of-the-art nanomechanical resonators are heralded as a central
component for next-generation clocks, filters, resonant sensors and quantum
technologies. To practically build these technologies will require monolithic
integration of microchips, resonators and readout systems. While it is widely
seen that mounting microchip substrates into a system can greatly impact the
performance of high-Q resonators, a systematic study has remained elusive,
owing to the variety of physical processes and factors that influence the
dissipation. Here, we analytically analyze a mechanism by which substrates
couple to resonators manufactured on them, and experimentally demonstrate that
this coupling can increase the mechanical dissipation of nanomechanical
resonators when the resonance frequencies of resonator and substrate coincide.
More generally, we then show a similar coupling mechanism can exist between two
adjacent resonators. Since the substrate-mode coupling mechanism strongly
depends on both the resonator position on the substrate and the mounting of the
substrate, this work provides key design guidelines for high-precision
nanomechanical technologies.
