Abstract
We present the spoked-ring microcavity, a nanophotonic building block enabling energy-efficient, active photonics in
unmodified, advanced CMOS microelectronics processes. The cavity is realized in the IBM 45nm SOI CMOS process –
the same process used to make many commercially available microprocessors including the IBM Power7 and Sony
Playstation 3 processors. In advanced SOI CMOS processes, no partial etch steps and no vertical junctions are available,
which limits the types of optical cavities that can be used for active nanophotonics. To enable efficient active devices
with no process modifications, we designed a novel spoked-ring microcavity which is fully compatible with the
constraints of the process. As a modulator, the device leverages the sub-100nm lithography resolution of the process to
create radially extending p-n junctions, providing high optical fill factor depletion-mode modulation and thereby
eliminating the need for a vertical junction. The device is made entirely in the transistor active layer, low-loss crystalline
silicon, which eliminates the need for a partial etch commonly used to create ridge cavities. In this work, we present the
full optical and electrical design of the cavity including rigorous mode solver and FDTD simulations to design the Qlimiting
electrical contacts and the coupling/excitation. We address the layout of active photonics within the mask set of
a standard advanced CMOS process and show that high-performance photonic devices can be seamlessly monolithically
integrated alongside electronics on the same chip. The present designs enable monolithically integrated optoelectronic
transceivers on a single advanced CMOS chip, without requiring any process changes, enabling the penetration of
photonics into the microprocessor.
