Isotropic grating coupler for 3D silicon photonic architectures

Continual development of optical testing methodologies for silicon photonic components and circuitry is instrumental for the overall growth of the field. A mature scheme capable of coupling light between the fiber core and single mode silicon-on-insulator (SOI) waveguides for wafer-scale testing are grating couplers. When silicon photonics are integrated into 3D architectures, the photonics layer may undergo multiple wafer flipping steps through bond/de-bond modules. Architectures such as this require optical testing on the front and backside of the wafer. Conventional grating couplers integrate reflective layers or additional layers of gratings to increase coupling efficiency. These techniques however will not work in both wafer orientations. In addition, including separate test structures for both front and backside fabrication increases device footprint and drives up manufacturing costs. We present an isotropic grating coupler that provides moderate coupling efficiency for both front and backside wafer-scale testing. The grating coupler has been designed for use in the AIM Photonics multi project wafer (MPW) platform, thus enabling non-contact optical interfacing irrespective of the wafer orientation. We have numerically demonstrated coupling efficiencies of 34% (-4.7dB) and 28% (-5.5 dB) for the wafer front and backside respectively while operating at a wavelength of 1550 nm. With proof of concept established, we foresee an eventual pathway for achieving higher efficiency isotropic grating couplers traversing into more 3D silicon photonic architectures.