  On-chip optical resonators have the promise of revolutionizing numerous
fields including metrology and sensing; however, their optical losses have
always lagged behind their larger discrete resonator counterparts based on
crystalline materials and flowable glass. Silicon nitride (Si3N4) ring
resonators open up capabilities for optical routing, frequency comb generation,
optical clocks and high precision sensing on an integrated platform. However,
simultaneously achieving high quality factor and high confinement in Si3N4
(critical for nonlinear processes for example) remains a challenge. Here, we
show that addressing surface roughness enables us to overcome the loss
limitations and achieve high-confinement, on-chip ring resonators with a
quality factor (Q) of 37 million for a ring with 2.5 {\mu}m width and 67
million for a ring with 10 {\mu}m width. We show a clear systematic path for
achieving these high quality factors. Furthermore, we extract the loss limited
by the material absorption in our films to be 0.13 dB/m, which corresponds to
an absorption limited Q of at least 170 million by comparing two resonators
with different degrees of confinement. Our work provides a chip-scale platform
for applications such as ultra-low power frequency comb generation, high
precision sensing, laser stabilization and sideband resolved optomechanics.

Optics