Non-volatile reconfigurable integrated photonics enabled by phase change materials.

Silicon Integrated Photonics is today an effervescent field which, during the past 20 years, has enabled the implementation of increasingly complex on-chip optical circuits. Among the vast range of optical devices recently analyzed, the development of reconfigurable components has attracted larger and larger interest: the possibility to finely tune their properties enables an efficient and fully integrated manipulation of the circuit, providing many different additional features within the same area and computational cost. In particular, in recent years great effort has been concentrated on developing reconfigurable optics able to retain their properties unaltered after the actuation, without the need to maintain it ON. This quality is called non-volatility, fundamental in low-power applications. One of the most promising methods to obtain non-volatile components is the integration of phase change materials (PCM), semiconductor alloys with extremely different optical properties in crystalline or amorphous solid states. Their state transition can be achieved with nanosecond-duration heat pulses. In this project, the integration of two commonly employed PCMs, GeTe and Ge2Sb2Te5, has been analyzed both from the fabrication and from the design and simulation point of view. First, a fully integrated process based on DUV lithography has been developed, while today most of the studies employ E-beam to pattern these materials. This choice allows a faster processing of the wafer, enabling a large-scale production. After the preliminary characterization of the involved PCMs, smaller loop experiments have been performed on two alternative techniques selected to pattern them, i.e. lift-off and etching, optimizing their process parameters and performing a conclusive comparison. In parallel, the design of different optical components has been carried out through a combination of Comsol Multiphysics®, for their physical simulation, and MATLAB®, for the successive data analysis, in particular for the geometrical optimization of Si-PCM hybrid adiabatic couplers, and hybrid phase shifters. Finally, both these elements have been employed to generate the layout of different optical devices, respectively a switch matrix and a Mach-Zehnder Interferometer (MZI). The latter will be later included in a dedicated photolithography reticle, used to perform the complete process flow previously created.