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-
$\mu{\rm m}$
grating couplers are measured to be
${-}{\rm 2.8}$
and
${-}{\rm 3.8}~{\rm dB}$
, respectively. These results are at least
$2\times$
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
${\sim}{40}$
and 52 nm for 1.3 and
${\rm 1.55}~\mu{\rm m}$
, respectively.