May_EDFA_Digital

edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 2 24 process and was expected by the manufacturer. For thermal management and reliability reasons, it was important to evaluate the total dimensions of this manu- facturing signature. X-ray conventional tomography at 240 KeV did not allow visibility through the heavy metal thermal dissipators. To remedy this problem,microtomog- raphy was performed with a synchrotron x-ray generator supplying x-rays with an energy up to 400 KeV, allowing part of the photons to go through the component. The computed slice images of the chip/heat dissipator and heat dissipator/chip solders revealed the 3D structure of the process signature (Fig. 4). In the top image of Fig. 4, the very low porosity level at the most critical interface is visible, appearing between the heat dissipator and the chip, on the epitaxial side. Previously observed aligned porosities at the chip/heat dissipator interface represent a limited ratio of the total solder volume and are therefore considered a noncritical risk from a thermal dissipation and reliability standpoint. Moreover, the construction analysis process flow did not allow the encapsulation of all the studied compo- nents before sawing. One component was hardly cut without resin encapsulation. The wire saw generated significant mechanical stress in the stack and led to the crack of several AsGa chips (Fig. 5). This preparation artefact confirms the robustness of the solders, which were not damaged by mechanical stress out of the mission profile. After assessing the techno- logical robustness of the assembly/package system, the following analysis of the intrinsic defect mode of AsGa laser diodes, namely the catastrophic optical damage (COD), was compiled. QUALIFICATION HAZARDS Qualification tests are carried out in a laboratory to simulate the specific operat- ing conditions of the space mission, using dedicated components taken from the pro- duction batch of the flight model units. Overheating of a cable inside one of the climate chambers occurred during one step of the qualification process, which led to a strong pulverization and redeposition of contaminatingmaterial over all surfaces of the chamber. Samples in the chamber exhibited an abnormal COD count, which provided an opportunity to study the component’s resilience to CODs. CATASTROPHIC OPTICAL DAMAGE Catastrophic optical damage (COD) is classified as a sudden degradation mode because there is no time dependency in this degradationmechanism, as the device can be heavily affected without any previous signature. This is therefore the most hazardous degradation mode of all, and the most complicated to screen. The COD is specifically related to themirror facet where it takes place. COD is known to arise froma thermal runaway process at the facet, which takes place in the following sequence: 1. The surface defect appears at the optical (output) facet, which acts as an optical absorption site for the radiation emitted from the cavity. 2. Optical absorption induces a sudden increase in the facet temperature. 3. The temperature gradient between the surface and the bulk results in shrinkage of the bandgap at the surface. Fig. 4 X-ray tomography slice view of the heat dissipator/chip interface (top right) and of the chip/heat dissipator interface (bottomright). Courtesy of Insidix. Fig. 5 Aligned porosities in the heat dissipator/chip solder and cracks in the AsGa chips due to hard sawing preparation.