edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 3 24 silicon-on insulator (SOI).[3] In this platform, silicon optics and silicon electronics are realized on the same chip at the same time, including a “socket” for the light source. That light source, a III-V laser, needs to be attached after the wafer fabrication in the packaging process.[4] On-chip light distribution is accomplished by using silicon waveguides, which are etched into the SOI and clad with oxide. The high refractive index contrast between Si and oxide provides for strong confinement of the light in the silicon and allows for tight bending of the guides. On the other hand, however, line edge roughness of the silicon wave guides’ sidewalls plays a major role in signal loss due to scattering, when compared to silica or nitride waveguides. It turns out that the superior processing capability of a modern 300 mm wafer fab with advanced lithography equipment is able to reduce the line edge roughness of the silicon wave guides considerably: Loss numbers of less than 1 dB/cm can be achieved.[5] Figure 2 shows sketches of a ridge and a rib wave guide. The halo around the mode in the core of the wave guide illustrates the evanescent field outside the waveguide. Electrical-to-optical light conversion is accomplished by using Mach-Zehnder modulators fabricated from Si wave guides. In one or both arms of the modulator, pnjunctions are diffused to obtain an optical phase shift via the plasma dispersion effect. These junction-based modulators are able to switch the light in a range of up to 50 GHz. Light detection is accomplished by integrating a Ge photodetector into the optical circuit. Because Ge has a lower band gap than Si, light in the O-band, C-band, and L-band is absorbed and converted into an electrical signal.[6] Lastly, the optical IO from the chip into the fiber and from the fiber into the chip needs to be realized with minimal insertion and return loss. Two main approaches for light IO have been developed (Fig. 3). Vertical couplers are etched into the SOI and are able to funnel light shining vertically down on the chip into the wave guides. Although less efficient than other solutions, vertical couplers are crucial elements for in-line optical test and fault isolation, because they have large alignment tolerance and there is no need for cleaving the wafer. Horizontal edge couplers can be realized by etching deep trenches (V-grooves) with exact dimensions into the silicon wafer (Fig. 3b). Single mode fibers can then be placed in the V-grooves, allowing the precise alignment of fiber core and Si waveguide.[7] Edge couplers outperform vertical couplers in insertion loss and return loss but add to fabrication complexity. (a) (b) Fig. 2 Si ridge (left) and Si rib (right) wave guide on SOI. Ridge wave guides have more confinement; rib wave guides show less propagation loss. Fig. 3 (a) A vertical grating coupler allows light insertion without breaking the wafer. (b) SEM image of a V-groove etched into the wafer. A single mode fiber can be placed into the trench and light can be coupled into the wave guide fabricated on SOI.
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