(Invited) Heterogeneous Photonic Integration by Direct Wafer Bonding
Authors: Davenport Michael L , Chang Lin , Huang Duanni , Volet Nicolas , Bowers John E 
Published: February 2020

The silicon photonics platform has emerged as a promising integration technique for use in many applications, particularly datacenter communications, which is expected to produce demand for large numbers of low cost photonic integrated circuits (PICs). Silicon-based PICs are almost universally based on silicon-on-insulator (SOI) wafers, which are enabled by wafer bonding technology. Silicon (Si) is useful as a waveguide material due to its high index contrast, low intrinsic absorption, and mature manufacturing infrastructure. However, it does not strongly interact with light in wavelength ranges of interest for data transmission. Heterogeneous integration of InP-based materials with Si using wafer bonding technology is able to overcome this limitation and has allowed the realization of a wide range of photonic devices on Si1. Beyond integration of InP with Si, wafer bonding enables complex vertical integration with of a wide variety of materials, such as silicon nitride (Si3N4) ultra-low loss waveguides, and magnetic and nonlinear materials.
A crystalline Si layer transferred to an ultra-low loss Si3N4 waveguide wafer by bonding allows for integration of highly confined Si photonic waveguide with low-loss passive components in Si3N4. Any device that can be realized on an SOI wafer can be integrated with the Si3N4 waveguide. Multiple die bonding of InP chips on the Si layer can add lasers, amplifiers, modulators, and photodetectors to the circuit. Lateral tapers of the waveguides allows coupling between the layers with loss below 0.5 dB. This technology was used to combine a Si3N4 arrayed waveguide grating (AWG) wavelength multiplexer with high-speed 50 Gb/s heterogeneous Si/InP photodetectors to form a 400 Gb/s wavelength division multiplexing (WDM) receiver2. A cross section of the photodetector is shown in Figure 1.
Bonding technology can allow the introduction of additional circuit functions that are unavailable using common semiconductor materials. One such device is an optical isolator, which allows light to experience loss that is dependent on the direction of optical propagation. This optical nonreciprocity can be achieved by bonding a magneto-optic material such as cerium substituted yttrium iron garnet (Ce:YIG) on Si waveguides, and applying a magnetic field through the material. Using this phenomenon, it is possible to fabricate optical isolators as well as circulators for use in optical sensors, amplifiers, and WDM systems. Our design consists of a Si microring with a bonded Ce:YIG die on top, shown schematically in Figure 2. A metallic microstrip is deposited on the backside of the bonded die following substrate removal. By applying a current through this microstrip, we generate a magnetic field, which causes a resonance split between the forward and backwards propagating light. Thus, optical isolators with up to 32 dB of isolation have been demonstrated3.
Lithium niobate (LN) is a material of paramount importance for nonlinear optics, thanks to its large nonlinear coefficient and broad transparent window (0.4-5.0 µm)4. It has attracted attention for use in PICs, because the compact scale of integrated waveguides results in high photon densities that effectively enhance nonlinear interaction. Recently, the LN-on-insulator (LNOI) technology has been successfully demonstrated by ion injection and heterogeneous bonding. This allows the implementation of LN thin films with submicron thickness on various material platforms5. We present here a quasi-phase matched, ultra-low-loss S3N4-LN platform, which reduces the loss of integrated LN waveguides down to a value comparable with best results of bulk LN waveguides 6 (Figure 3). Bonding of LN to silicon is also possible, allowing integration of LN-based devices with SOI photonics.

Wafer bonding technology allows the realization of complex and highly integrated photonic devices. The ability to combine multiple materials such as multiple-die bonding for integration of lasers, modulators, and detectors enables PICs with a wide variety of functions. Vertical integration through bonding of high-quality thin films, such as silicon nitride, lithium niobate, crystalline silicon, and thermal silicon dioxide can be used to construct integrated waveguides with useful properties and high performance.
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Figure 1