edfas.org 5 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO.2 sensitive in the 0.9-1.7 μm wavelength range. Luckily for Si photonics, both the C-band (1550 nm center wavelength) and O-band (1310 nm center wavelength) fall within the sensing region of the IR microscope. IR MICROSCOPE EXPERIMENTAL SETUP Figure 2 shows a simplified overview of a near infrared diagnostic setup. A packaged module with an exposed silicon die would be placed under a microscope. The microscope can be any conventional microscope, but with a near IR sensor attached to it. The packaged module would be connected to the proper instrumentation to run the chip. In most cases the instrumentation involves using a laser source in the C or O band in conjunction with a polarizer. THE BASICS: WAVEGUIDE IMAGING UNDER LIGHT INJECTION Figure 3 demonstrates an example of an infrared image taken of a simple waveguide on a PIC under operation. The infrared camera was able to trace out the light path. In an ideal scenario, none of the light traveling through the waveguide should be able to leak out. But due to fabrication realities, there is always a little bit of line edge roughness, which leads to tiny amounts of light leaking out and illuminating the waveguide, thereby tracing the light path. Figure 3 demonstrates an example of a bend in the waveguide. FAILURE MODES CASE STUDY 1: DETECTING PACKAGING FAILURES Sometimes, the silicon itself may be operating as expected, but an error in the packaging process prevents the light from coupling into the silicon in the first place. Such errors can be detected using the infrared microscope. Figure 4a shows an example of an edge coupler where the light is properly coupled into the silicon chip. This is used as a baseline, which is used for comparing modules that are not performing as well. Figure 4b is an example of an edge coupler where the performance (a) (b) Fig. 2 Setup for infrared microscopy of photonic integrated circuit dies. Fig. 3 Example of waveguide imaging of a PIC die using IR microscope. Fig. 4 (a) Infrared image of a spot size coupler with good fiber attach process. (b) Infrared image of a spot size coupler with fiber attach excursion.
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