On the use of silicon photonics — Part

35 0278-6648/09/$25.00 © 2009 IEEE s today’s complementary metal oxide semiconductor (CMOS) technologies are scaled down to below 65 nm, conventional metal lines carrying signals in an integrated circuit face increasing challenges. As a short-term solution, the semiconductor industry adopted copper instead of aluminum for interconnects due to its low resistive behavior and good electromigration property. However, issues such as delay degradation, interference, and power dissipation will remain for metal interconnects as we further scale down feature sizes. The International Technology Roadmap for Semiconductors (ITRS) suggested optical interconnects as one of the viable solutions to continue to improve on device performance. Optical interconnects usually require less power to operate for distances involved in off-chip communication and are also naturally resistant to electromagnetic interference. The industry has been accustomed to using direct band gap emitters for fiber optic communication for many years. Most optical interconnect applications prefer to use light emitters based on III–V or other direct band-gap materials due to their higher quantum efficiency. Researchers have mostly considered multi quantum well (MQW) optical modulators and vertical cavity surface emitting lasers (VCSELs) for inter-chip data communication. MQW modulators are formed by a series of quantum wells contained within the intrinsic region of a reverse biased PIN (p-type, intrinsic, ntype) junction. These modulators require that an external laser beam be brought onto the modulator, which can be considered a disadvantage. Application of an electric field across the wells changes the absorption coefficient of the well material according to the quantum confined Stark effect resulting in modulation of the external beam incident on the device. The light beam is reflected by a mirror at one end of the PIN stack and routed using optics. However, bringing in the external beam and rerouting the reflected light complicates the opto-mechanical packaging of modulators. VCSELs emit light in a cylindrical beam vertically from the surface of a fabricated wafer and offer significant advantages compared to the edge-emitting lasers used in fiber optic communications devices since they can be placed on top of a chip in large numbers (i.e., thousands). Both MQW modulators and VCSELs can provide a high number of connections between chips for inter-chip data communication and offer large data rates. These emitters need to be integrated to silicon CMOS as it is the base very large scale integration (VLSI) technology due to its low cost and widespread use. Monolithic integration of VCSEL and MQWs on Si CMOS has been problematic due to lattice mismatch. Hence, these optoelectronic devices need to be fabricated separately and hybrid bonded to silicon chips or wafers using flip-chip technology to