  Optical frequency combs, coherent light sources that connect optical
frequencies with microwave oscillations, have become the enabling tool for
precision spectroscopy, optical clockwork and attosecond physics over the past
decades. Current benchmark systems are self-referenced femtosecond mode-locked
lasers, but four-wave-mixing in high-Q resonators have emerged as alternative
platforms. Here we report the generation and full stabilization of
CMOS-compatible optical frequency combs. The spiral microcomb's two
degrees-of-freedom, one of the comb line and the native 18 GHz comb spacing,
are first simultaneously phase-locked to known optical and microwave
references. Second, with pump power control, active comb spacing stabilization
improves the long-term stability by six orders-of-magnitude, reaching an
instrument-limited 3.6 mHz/sqrt(t) residual instability. Third, referencing
thirty-three of the nitride frequency comb lines against a fiber comb, we
demonstrate the comb tooth-to-tooth frequency relative inaccuracy down to 53
mHz and 2.8x10-16, heralding unprecedented chip-scale applications in precision
spectroscopy, coherent communications, and astronomical spectrography.
