An injection of light into electronic circuitry

| Manufacturing

ClearCurve fibres on silicon photonics wafer

Jonathan Newell discovers how silicon photonics can increase speed and reduce power requirements in electronic systems while Andy Pye looks at other developments.

Chip designers and optical scientists dream about the possibilities of merging traditional microelectronics and photonics. Superficially, it seems easy – both fundamentally deal with electromagnetic waves, just at different frequencies (MHz and GHz for microelectronics, THz for optics).

On one side, microelectronics deals with integrated circuits and components such as transistors, copper wires, and the massively understood and employed CMOS manufacturing process. It’s the backbone of microprocessors, and at the core of conventional computing today. Conversely, photonics employs – true to its name – photons, the basic unit of light. Silicon photonics is the use of optical systems that use silicon as the primary optical medium, instead of other more expensive optical materials. Eventually, photonics has the potential to supplant microelectronics with optical analogues of traditional electrical components – but that’s decades away.

Research and development work on Silicon Photonics has been going on for at least three decades and, as a concept, the technology is nothing new. But it has never reached the level of commercial exploitation. However, according to John Ferguson of Calibre DRC, it could be the next big thing that transforms electronic circuit design.  Silicon photonics embeds lasers into chips, and carries light signals in silicon waveguides, so optical fibre can replace copper wiring.

Despite looking like an opaque piece of rock, silicon is transparent to certain wavelengths of light and so it is possible to create microscopic tracks for the light to travel along. The ratio of the dielectric coefficients of the silicon and its oxide is perfect for providing containment, so the light stays within the circuit without dispersing.

Photonics versus electronics

Electrons have mass, they require power to move them and keep them moving. Additionally, they are subject to losses due to material resistance and capacitance – these generate numerous difficulties for electronics design engineers.

In contrast, photons are less sensitive to other influences and properties of the material. So long as curves – and therefore reflections – are minimised, the light will continue travelling until it is received and its signal contents decoded. As a means of transporting signals, as witnessed by fibre-optic technology, photons are much better than electrons. However, electronics continues to be the preferred means of processing signals.

This means that silicon photonics has good power and transport properties and can be used for interconnects in electronic circuits and systems, but not for signal processing.

Reducing the power headache

Power requirements in electrical and electronic design often represent the most difficult hurdle for engineers to negotiate and so rack-mounted systems and server farm environments are ideal applications for deploying silicon photonics; here the technology can be used to reduce power requirements and save significantly on the costs of cooling.

From lab to fab

According to Ferguson, it’s unlikely in the near future that silicon photonics will be used in System-on-Chip (SoC) designs, as the technology lends itself in a far greater way to overcoming the problems of connectivity. Rather, he sees the technology being used to connect chips together in larger circuit boards or as a communication medium.

However, even in this respect, there are problems that need to be overcome. Outside bespoke low volume projects or laboratory systems, much of the industry relies very heavily on device models, modular design building blocks using industry available components and design elements, none of which cater for silicon photonics. Without this rules-based approach and modular design aids, the process of designing a complex integrated system relies very heavily on manual designs and simulation, which increase lead time and introduce difficulties in meeting international regulatory requirements.

Until the basic design building blocks have been established and the file formats for obtaining the correct precision in manufacturing (such as GDS and OASIS) have been modified to be suitable for the exact circuit curvature requirements for photonics based circuits, testing, reliability assessment and homologation of such systems are likely to be long and iterative processes.

Once these means are in place, then Ferguson’s predictions could come true and the dwindling “Moore’s law” effect could be revitalised with an injection of light.

Such computer industry giants as IBM, Google and Amazon are all investing heavily in the development of practical processes for integrating silicon photonics into electronic system designs.

And amongst those trying, Intel says its silicon photonics work, which promises to revolutionise data centres and mobile devices by combining optical signals with electronics a and integrate optical fibre into data centres, is close to fruition. Intel has been working on this field for 12 years and has developed a standard for data centre rack connectors running at up to 1.6 Terabits per second. Now, mass market silicon photonic chips are on the way, Intel says.

“Because we print these, we have the high volume capability. It’s a new era of optics. When we put this [project] together, we can finally take it to the point when optics is everywhere – in a car, in a server, in a TV,” Jeff Demain, business development director for Silicon Photonics at Intel told a London briefing.

As data centres are growing, the distances over which the hardware needs to be connected are increasing, and copper cabling simply can’t support such expansive infrastructure. Copper cables are also heavy, decrease the density of the systems and block the space that could have been otherwise used for cooling, bringing down the efficiency of the whole server farm.

Hybrid CMOS silicon photonics transceivers, transmitting and receiving data over single-mode optical fibre, are expected to play a key role in next-generation data centre connectivity. By exploiting existing CMOS manufacturing and 3-D assembly infrastructure, the hybrid CMOS silicon photonics platform enables high integration density and reduced power consumption, as well as high yield and low manufacturing cost. Combined with wavelength division multiplexing capability, highly scalable single-mode optical transceivers can be constructed, satisfying the growing need for interconnect bandwidth in next-generation cloud infrastructure.

Intel wants to see silicon photonics used both inside the servers and between the servers, to replace a number of existing connections like PCIe and Ethernet. But that’s just the first step: in time, the technology could be applied to high performance computing, telecommunications infrastructure and eventually, even consumer electronics. The company has developed silicon chips which can send and receive data through optical fibre, faster and further than copper. It then managed to put these chips on the motherboard using a simple, modular design, and even helped create the MXC cable and the connectors that will link them together.

The optical devices themselves are manufactured on silicon wafers, just like regular processors. They are tested automatically on wafer level – Intel says this alone reduces manufacturing costs by a third, in comparison with traditional fibre optic connectors.

At the February 2015 International Solid State Circuits Conference (ISSCC), nanoelectronics research centre Imec, in collaboration with Tyndall National Institute, the University of Leuven (KULeuven) and the Ghent University, demonstrated a 4x20Gb/s wavelength division multiplexing (WDM) hybrid CMOS silicon photonics transceiver, paving the way to cost-effective, high-density single-mode optical fibre links. The Tyndall National Institute is Ireland’s largest research institute and performs research in the fields of photonics, nanotechnology and microsystems.

Imec’s silicon photonics platform enables cost-effective R&D of silicon photonic ICs for high-performance optical transceivers (25Gb/s and beyond) for telecom, datacom, and optical sensing for life science applications. The integrated components include low-loss waveguides, efficient grating couplers, high-speed silicon electro-optic modulators and high-speed germanium waveguide photo-detectors.

Jonathan Newell
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