FEATURE PHOTONIC INTEGRATED CIRCUITS
g
integrated thermoelectric temperature control in our research,’ Enright revealed. ‘However, we are still in the research phase with regards to this technology so, for the time being, resistive heaters are the only practical thermal tuning approach, whether that be for lasers or optical resonators.’
Less stress
Issues due to diminished performance of active components now loom larger than the mismatch in coefficients of thermal expansion (CTEs) between InP lasers and silicon wafers they’re bonded to. The stress caused by one component expanding at a different rate than its surroundings ‘can be challenging’, Enright remarked, but he’s not greatly troubled by it. ‘Advancements in this integration strategy over the last decade can now routinely produce well-functioning active devices with a thermal budget of around 300°C during processing,’ he said. Europe’s Waferscale Integration of Photonics and Electronics (WIPE) consortium is working on connecting PICs and micro-electronic integrated circuits (ICs) in this way. Led by Eindhoven University of Technology (TU/e) in the Netherlands and with six other partners, WIPE seeks to eliminate expensive and bulky wire bond connections to PICs. WIPE bonds a thin InP membrane containing a complete PIC onto a BiCmos IC using an adhesive polymer, electronically connecting the layers with wafer-scale processing. This approach allows very short, high-bandwidth, interconnections. However, cooling both layers simultaneously is difficult, which brings thermal challenges that Manuel van Rijn, a TU/e PhD student, is studying. Van Rijn looks at average temperatures
in both layers, for example, to cool and stabilise the optical layer at a temperature that keeps lasers and other components operating stably. But he has also simulated hot spots, such as those in the InP layer that would adversely affect the nearby electronics. Managing such hotspots requires a 2µm-thick gold heat spreader on the InP membrane, his simulations show. However, the adhesive layer between
the electronics and the photonics increase the thermal resistance between the heat sources and ambient air, van Rijn noted. And although InP chips themselves can also help spread heat, the thinness of WIPE’s membrane limits its ability to do this. ‘It is very important that we minimise the thermal resistance,’ van Rijn explained. ‘It’s also good to think about how we can help that horizontal spreading.’ The WIPE project is looking to create a design library of photonic/electronic standard modules using process design
28 Electro Optics March 2018
IBM has developed a compliant polymer interface to use in its silicon nanophotonics packaging to manage stresses within the package with environmental temperature cycling
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