  The generation of dissipative Kerr solitons in optical microresonators has
provided a route to compact frequency combs of high repetition rate, which have
already been employed for optical frequency synthesizers, ultrafast ranging,
coherent telecommunication and dual-comb spectroscopy. Silicon nitride
(Si$_3$N$_4$) microresonators are promising for photonic integrated soliton
microcombs. Yet to date, soliton formation in Si$_3$N$_4$ microresonators at
electronically detectable repetition rates, typically less than 100 GHz, is
hindered by the requirement of external power amplifiers, due to the low
quality ($Q$) factors, as well as by thermal effects which necessitate the use
of frequency agile lasers to access the soliton state. These requirements
complicate future photonic integration, heterogeneous or hybrid, of soliton
microcomb devices based on Si$_3$N$_4$ microresonators with other active or
passive components. Here, using the photonic Damascene reflow process, we
demonstrate ultralow-power single soliton formation in high-Q
($Q_0>15\times10^6$) Si$_3$N$_4$ microresonators with 9.8 mW input power (6.2
mW in the waveguide) for devices of electronically detectable, 99 GHz
repetition rate. We show that solitons can be accessed via simple, slow laser
piezo tuning, in many resonances in the same sample. These power levels are
compatible with current silicon-photonics-based lasers for full photonic
integration of soliton microcombs, at repetition rates suitable for
applications such as ultrafast ranging and coherent communication. Our results
show the technological readiness of Si$_3$N$_4$ optical waveguides for future
all-on-chip soliton microcomb devices.
