Focusing grating couplers in unmodified 180-nm silicon-on-insulator CMOS

The design and demonstration of photonic grating couplers with improved efficiencies monolithically integrated in IBM's 7RF silicon-on-insulator (SoI) complementary metal-oxide-semiconductor (CMOS) technology offering are reported. The grating couplers are fabricated using two standard layers available in 7RF SoI CMOS, where focusing circular grating structures and a small-footprint tapered slab are fabricated on the poly-silicon and crystalline-silicon (c-Si) layers, respectively. The coupling efficiencies for 1.3- and 1.55-μm grating couplers are measured to be -2.8 and -3.8 dB, respectively. These results are at least 2× more efficient than the best previously reported grating couplers in this technology, which were fabricated using c-Si layers only. Furthermore, the devices exhibit 1-dB bandwidths of ∼40 and 52 nm for 1.3 and 1.55μm, respectively. © 2014 IEEE.