On-chip optical microresonators interfaced with waveguides
Fig. 1: SEM micrograph of a microresonator disk with 50 µm diameter and a waveguide of 0.8 µm width in a distance of 1 µm: The dark circle is an ancillary pattern used to underetch the silicon oxide membrane
Fig. 2: SEM micrograph of the cross-section of an optical chip showing the facet of a ridge waveguide (marked)
Future applications of quantum technologies, including quantum-secure communication and precision measurements, call for specific and compact light sources. In this context, FBH is developing special laser modules for quantum metrology that exhibit very narrow linewidths only in the kHz range. Besides advanced distributed feedback (DFB) laser diodes, passive optical micro components are subject of intense research at FBH.
For applications at a wavelength of 1064 nm waveguides and resonators have been designed for integration on a single semiconductor chip. With respect to transparency and refractive index silicon oxide has desirable properties for the operation purpose. It can be formed on silicon by thermal oxidation and is widely used in semiconductor device fabrication. However, for use as waveguide silicon oxide has to be decoupled from the underlying silicon, i.e., the silicon underneath the waveguide structure needs to be removed by underetching to form a gap. The resulting ridge waveguides are mounted on fragile silicon oxide membranes, which challenges device and process design to take care of mechanical aspects, too. Processing steps based on optical projection lithography and electron beam lithography, wet and dry chemical etching as well as chip dicing have been developed and merged for fabrication of monolithically integrated microresonators in silicon oxide-on-silicon. Methods have been deployed to create smooth skin surfaces and facets of the optical components. The technology allows for manufacturing microresonators and waveguides in a wide range of geometry. Resonators with radii in the range of 20 - 160 µm and ridge waveguides with a width of 0.6 - 1.5 µm have been fabricated so far, examples of it are shown in Figs. 1 and 2. On-chip coupling a microresonator to a nearby waveguide could be proven, demonstrating the high quality of the arrangement of up to 106.
Part of this work was performed in the frame of Jan Schlegel’s master thesis “Preparation of waveguides and micro resonators in oxide” that was recently awarded the Jan Czochralski Award by Hochschule für Technik und Wirtschaft Berlin. The master thesis has been carried out within the Process Technology Department at FBH, combining theoretical aspects of the design with practical work for the implementation in chips.