Large grain Ge growth on amorphous substrates for CMOS back-end-of-line integration of active optoelectronic devices

Abstract

The electronic-photonic integrated circuit (EPIC) has emerged as a leading technology to surpass the interconnect bottlenecks that threaten to limit the progress of Moore's Law in microprocessors. Compared to conventional metal interconnects, photonic interconnects have the potential to increase bandwidth density while simultaneously reducing power consumption. However, photonic devices are orders of magnitude larger than electronic devices and therefore consume valuable substrate real estate. The ideal solution, in order to take advantage of optical interconnects without decreasing transistor counts, is to monolithically implement dense threedimensional integration of electronics and photonics. This involves moving the photonic devices off the substrate, and into the metal interconnect stack. Moving photonic devices into the interconnect stack imposes two fabrication limitations. First, the available thermal budget allowed for photonic device processing is limited to 450 °C. Second, the metal interconnects are embedded within amorphous dielectrics and therefore there is no crystalline seed to initiate epitaxial growth. This thesis addresses two major barriers for integration of photonics in the back end: (1) how to fabricate high quality Ge for active regions of optoelectronic devices while adhering to back-end processing constraints, and (2) how to couple optical power to these devices. First, an approach was developed to fabricate the active region of Ge-based optoelectronic devices. A new technique, known as two-dimensional geometrically confined lateral growth (2D GCLG), has demonstrated single crystalline Ge on an amorphous substrate. This thesis presents the first application of the 2D GCLG technique to fill a lithographically defined Si0 2 trench with large grain Ge, while adhering to back-end processing constraints. A modified design is then proposed to increases the yield of 2D GCLG structures. This trench filling technique is an integral step towards fabricating Ge-based optoelectronic devices that are capable of being integrated into the back-end of a microprocessor. Once it was established that high quality Ge trenches could be fabricated in the back-end, optical coupling to devices was addressed. For dense three-dimensional integration of photonic devices, vertical coupling between photonic planes is necessary. Therefore, this thesis begins with the design and simulation of vertical couplers. These couplers utilize evanescent coupling between two overlapping inversely tapered waveguides, which ensure efficient coupling due to optical impedance matching. These couplers are designed to exhibit coupling efficiencies in excess of 98.4%, equivalent to a 0.07 dB coupling loss. The technique of evanescent coupling between overlapping inverse tapers is then applied to electro-absorption modulators (EAMs). A design for low-loss evanescent coupling from a waveguide to a Ge EAM is modeled and optimized. The design implements lateral evanescent coupling from overlapping inverse taper structures. Simulation results show that the coupling efficiency into and out of the modulator can be as high as 99%, equivalent to a 0.04 dB coupling loss.