1. Department of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA

Correspondence to: K. J. Vahala Correspondence and requests for materials should be addressed to K.J.V. (e-mail: Email: vahala@caltech.edu).

The circulation of light within dielectric volumes enables storage of optical power near specific resonant frequencies and is important in a wide range of fields including cavity quantum electrodynamics^{1, 2}, photonics^{3, 4}, biosensing^{5, 6} and nonlinear optics^{7, 8, 9}. Optical trajectories occur near the interface of the volume with its surroundings, making their performance strongly dependent upon interface quality. With a nearly atomic-scale surface finish, surface-tension-induced microcavities such as liquid droplets or spheres^{10, 11, 12, 13} are superior to all other dielectric microresonant structures when comparing photon lifetime or, equivalently, cavity Q factor. Despite these advantageous properties, the physical characteristics of such systems are not easily controlled during fabrication. It is known that wafer-based processing¹⁴ of resonators can achieve parallel processing and control, as well as integration with other functions. However, such resonators-on-a-chip suffer from Q factors that are many orders of magnitude lower than for surface-tension-induced microcavities, making them unsuitable for ultra-high-Q experiments. Here we demonstrate a process for producing silica toroid-shaped microresonators-on-a-chip with Q factors in excess of 100 million using a combination of lithography, dry etching and a selective reflow process. Such a high Q value was previously attainable only by droplets or microspheres and represents an improvement of nearly four orders of magnitude over previous chip-based resonators.