Coupling of Waveguides and Photodetectors for Microsystem Applications

Optical waveguides and microelectronic circuits have been integrated monolithically on one silicon chip. The integration process is based on a 0.8 ¿m SWAMI-LOCOS CMOS-process. The optoelectronic interfaces are fabricated in three different techniques: waveguides are coupled to integrated photodiodes by leaky wave coupling, butt coupling or mirror coupling. Leaky wave coupling results in a non-reproduceable coupling due to insufficient wafer surface planarity after thermal field oxidation. Coupling efficiency depends on the oxide layer thickness between the light guiding SiON-film and the active detector area. Butt coupling causes high photocurrents, but it leads to an increased leakage current in the MOS-transistors caused by parasitic channels. Only coupling by mirrors has the advantage of high efficient signal coupling from the waveguide to the photodetector in a MOS compatible technology. The coupling efficiency is defined by the depth of the mirror, which is etched into the waveguide. Because of the modular integration technique MOS processing is not affected by the waveguide deposition process and the mirror etch. This paper rates the efficiencies of the diferent coupling techniques. The reproduceabilities of the fabrication processes are discussed.