edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 3 26 EXAMPLES OF PHOTONICS FA This section describes three examples of failure analysis of photonic devices. These devices are a nitride Fig. 6 Metallic nodules (red arrows) in the spacer of a vertical grating coupler element. EDX of the same area reveals traces of metal left behind from the CMOS processing part of the PIC. Fig. 7 Electron tomography of the transition point in a spot size converter between single mode fiber and Si waveguide. The spot size converter facilitates the insertion of light with very low loss values. Sketch is not to scale. waveguide, a vertical coupler, and a spot size converter, and the FA is mostly in the form of PFA. Applications of EFA in the photonic world can be found in the literature, see for example, Reference 8. The first example shows a multimode silicon nitride waveguide, which exhibited unusual propagation loss. Examination with TEM and STEM detected small metallic particles near the bottom surface of the guide, which were confirmed by an energy dispersive x-ray analysis (EDX) scan (Fig. 5). Because light in waveguides has an evanescent field outside the waveguide, even defects tens of nanometers away can pose a problem in photonic circuits. The second example deals with a vertical grating coupler showing very low coupling efficiency. In a fully monolithic PIC fabrication process, the vertical couplers are built at the same time as the gates in the CMOS part of the chip. Therefore, the optical elements on the PIC are exposed to the same process steps as the electrical devices. Careful examination in the TEM and subsequent EDX mapping of the vertical coupler reveals small metallic nodules in the sidewall spacers of the grating (Fig. 6). Thus, extremely careful cleaning after certain CMOS processes is of paramount importance for both optical and electrical performance. The third example of failure analysis in a silicon photonic circuit deals with an optical spot size converter. When the light from the single mode fiber is coupled to a silicon wave guide, the optical mode has to be “funneled” from a diameter of 9 mm to a diameter of less than half a micron. This is accomplished by building a spot size converter,
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