Chalcogenide glasses are amorphous, glassy, semiconducting materials containing S, Se, or Te as the primary components, with network modifiers such as Ge, As, Sb and Bi. They are of interest in photonics because of their low-loss transmission in the infrared, their large third order susceptibility, low two photon absorption at telecommunications wavelengths, and variety of bond rearrangement effects such as photoexpansion, photodarkening, electron beam induced deformation, and electron beam induced second harmonic generation. The large Kerr nonlinearities and large refractive indices of chalcogenide glasses enable compact integrated optical circuits capable of all-optical switching.
In this thesis, electron beam induced deformations are explored as a way of fabricating optical waveguides in Ge0.2Se0.8 chalcogenide glasses because this is an etchless technique that potentially allows writing waveguides whose surface roughness is limited only by the roughness of the as-deposited film, allowing for a minimum of surface roughness scattering loss.
Thin films (100 nm to several microns thick) of Ge0.2Se0.8 glass are deposited using pulsed laser deposition (PLD). Another option for depositing films, which has the advantage of being lower cost, is explored: spin coating. A process to deposit thin amorphous Ge-Se films using spin coating from amine-based solutions is developed. Mixing in an alcohol such as methanol in relatively small proportions (< 10 mol %) allows for the production of thicker films while maintaining stoichiometry. Films thicker than 1 μm and with surface roughness less than 1 nm RMS can be obtained in a single layer.
Exposure to focused electron beams (generated in an electron beam lithography system) is investigated as a way to directly write waveguides into thin PLD Ge0.2Se0.8 films. These waveguides are mound-shaped and similar to rib waveguides. Electron beam parameter sweeps are run demonstrating a transition from mound formation to trench formation depending on the speed of the beam (corresponding to the electron beam charge dose) across the material. The height of the deformations tends to increase with film thickness, electron beam current and number of exposures (i.e. passes over the same pattern). Strategies are developed to smooth the mound deformations and make them uniform transversely and longitudinally. Example device structures, written completely with this direct-write electron beam technique, such as directional couplers, ring resonators and waveguide tapers are demonstrated and characterized with atomic force microscopy.
Another faster and less expensive method to fabricate waveguides is explored. A photolithography process is developed to produce shallow rib waveguides in the spin coated films. This process involves using a stepper with i-line (365 nm) radiation to pattern SPR950-0.8 photoresist which is used as an etch mask with CHF3 plasma in an inductively-coupled plasma reactive ion etcher (ICP). These waveguides have losses of about 40 dB/cm most likely due to the scattering from the voids.
A new strategy is developed to simultaneously exploit the flexibility of direct electron beam writing of waveguides and the speed of photolithography: write bus waveguides with photolithography and then write electron beam induced structures onto them or coupled to them.