edfas.org 25 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 3 WHAT IS NEEDED FOR PHOTONIC FAILURE ANALYSIS? Optical failure analysis (OFA) is in many ways different from the traditional physical or electrical FA (PFA or EFA). New tools, new synergies, and new skill sets are needed when starting in optical FA (Fig. 4). New tools: Due to the wavelength of the light, optical devices and waveguides are on the order of microns in size, not nanometers. Therefore, optical FA relies mainly on good optical microscopes and occasionally the SEM. The most important difference between OFA and EFA is in terms of fault isolation (FI). For fault iso- lation, light needs to be coupled into the PIC. Therefore, lasers in the C and L band have to be available. For coupling the light into the wave guides, OFA mostly relies on vertical couplers. These vertical couplers need to be integrated into the test vehicles when chip design and layout takes place. Finally, the FA engineer needs to be able to detect the locations where light is lost, scattered, or absorbed. Therefore, infrared microscopes with IR detectors for light and heat need to be utilized. New synergies: Although OFA needs new sets of tools to isolate failures in the optical domain, it will still be complemented by EFA and PFA. Electrical FA is needed for active devices like modulators, ring resonators, or photodetectors. Physical FA will help find the root cause for insertion and propagation loss, random scattering fail sites, and misalignments. All in all, there will be new synergies between OFA, EFA, and PFA on a silicon photonics manufacturing site. New skill sets: By far the most important ingredient needed for optical FA is the skill set of the engineer. Photonic interactions with matter are completely different from electronic interactions with matter, and often optical phenomena are even counterintuitive. In some cases, defects in the mere vicinity of the waveguide but not actually in the waveguide can severely impact the light propagation. Physical “opens” can conduct light,[9] and physical “shorts” can block light if defects are nearby. From the experience of one of the authors (FHB), it can take up to a year for an experienced PFA engineer to become a productive photonics FA engineer. Fig. 5 A nitride waveguide showing strong propagation loss (top left). STEM reveals small particles under the waveguide (top right), and EDX profiling (bottom) along the orange arrow determines the nature of the foreign material causing the loss. Fig. 4 Optical failure analysis (OFA) will be complemented by electrical and physical failure analysis.
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