  Monolithic integration of novel materials for unprecedented device functions
without modifying the existing photonic component library is the key to
advancing heterogeneous silicon photonic integrated circuits. To achieve this,
the introduction of a silicon nitride etching stop layer at selective area,
coupled with low-loss oxide trench to waveguide surface, enables the
incorporation of various functional materials without disrupting the
reliability of foundry-verified devices. As an illustration, two distinct
chalcogenide phase change materials (PCM) with remarkable nonvolatile
modulation capabilities, namely Sb2Se3 and Ge2Sb2Se4Te1, were monolithic
back-end-of-line integrated into silicon photonics. The PCM enables compact
phase and intensity tuning units with zero-static power consumption. Taking
advantage of these building blocks, the phase error of a push-pull Mach-Zehnder
interferometer optical switch could be trimmed by a nonvolatile phase shifter
with a 48% peak power consumption reduction. Mirco-ring filters with a
rejection ratio >25dB could be applied for >5-bit wavelength selective
intensity modulation, and waveguide-based >7-bit intensity-modulation photonic
attenuators could achieve >39dB broadband attenuation. The advanced "Zero
change" back-end-of-line integration platform could not only facilitate the
integration of PCMs for integrated reconfigurable photonics but also open up
the possibilities for integrating other excellent optoelectronic materials in
the future silicon photonic process design kits.
