Fabrication of add-drop filters based on frequency-matched microring resonators

• Published in: Journal of lightwave technology Vol. 24; no. 5; pp. 2207 - 2218
• Main Authors: Barwicz, T., Popovic, M.A., Watts, M.R., Rakich, P.T., Ippen, E.P., Smith, H.I.
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 01.05.2006; Institute of Electrical and Electronics Engineers; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Add-drop filters; Applied sciences; Calibration; Circuit properties; coupling-induced frequency shifts (CIFS); Detection, estimation, filtering, equalization, prediction; Dimensional changes; Electric, optical and optoelectronic circuits; electron-beam (e-beam) proximity effects; Electronics; Exact sciences and technology; fabrication; Fundamental areas of phenomenology (including applications); Information, signal and communications theory; Integrated optics. Optical fibers and wave guides; Laser optical systems: design and operation; Matched filters; Microelectronic fabrication (materials and surfaces technology); microring resonators; Optical and optoelectronic circuits; Optical device fabrication; Optical filters; Optical resonators; Optical waveguides; Optics; Optimization; Physics; Proximity effect; Resonant frequency; Resonator filters; Resonators; Resonators, cavities, amplifiers, arrays, and rings; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Signal and communications theory; Signal, noise; Silicon; Silicon nitride; Spectra; Standard deviation; Telecommunications and information theory; Spectral response; Non linear filter; Roughness; Optical frequency; Optimization; Frequency shift; Microelectronic fabrication; Dimensional control; electron-beam (e-beam) proximity effects; Standard deviation; Wafer; coupling-induced frequency shifts (CIFS); Resonator filters; Lithography; Third order; fabrication; Electron beam; Optical resonator; Calibration; Mismatching; Waveguide; Add-drop filters; Ring resonator; microring resonators; Piezoelectric resonator; High frequency; Coupled resonator
• ISSN: 0733-8724; 1558-2213
• DOI: 10.1109/JLT.2006.872298

Frequency mismatches between resonators significantly impact the spectral responses of coupled resonator filters, such as high-order microring filters. In this paper, techniques allowing fabrication of frequency-matched high-index-contrast resonators are proposed, demonstrated, and analyzed. The main approach consists of inducing small dimensional changes in the resonators through alteration of the electron-beam dose used to expose either the actual resonator on a wafer or its image on a lithographic mask to be later used in filter fabrication. Third-order microring filters fabricated in silicon-rich silicon nitride, with optical resonator frequencies matched to better than 1 GHz, are reported. To achieve this, the average ring-waveguide widths of the microrings are matched to within less than 26 pm of a desired relative width offset. Furthermore, optimization and calibration procedures allowing strict dimensional control and smooth sidewalls are presented. A 5-nm dimensional control is demonstrated, and the standard deviation of sidewall roughness is reduced to below 1.6 nm.