Ghent University - IMEC, Department of Information TechnologySint-Pietersnieuwstraat 41, 9000 Gent, BELGIUM

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Silicon Photonics

What is Silicon Photonics?Silicon photonics is the science and engineering of optical (photonic) functions on the surface ofa chip using silicon as a base material. This includes passive functionality, such as routing andwavelength filtering, but also active functions such as switching, signal modulation, as well as lightdetection and ultimately generation. By extension, silicon photonics is also referred to as group IVphotonics, with the inclusion of (silicon-)germanium, and recently also carbon and tin.

imecAs an associated laboratory of imec, the Photonics Research Group has access to the industrialCMOS processes in imec’s 200mm clean room, including high-resolution optical lithography. In ajoint effort between the Photonics Research Group and imec, new of enhanced processes forphotonics are being developed.

Originally, only processes for passive waveguide circuits were developed, which requireslithography and etch, but since 2009 additional development is started on active components,employing a wider variety of imec’s process portfolio, such as dopant implantation, metallization,surface treatments, silicidation, …

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Design toolsSilicon photonics enables larger-scale integration than hitherto possiblewith other planar lightwave circuit technologies. This implies thattraditional photonic design tools are often no longer sufficient for siliconphotonics.

The Photonics Research Group is developing a set of tools whichintegrate with electronic design toolkits (e.g. Cadence) to apply thedesign methodology of electronic circuits to photonics. This softwareframework also integrates existing photonics tools to enable physicalmodelling where needed.

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WaveguidesThe key component in a photonic integrated circuit is the opticalwaveguide, which is used to route light over the chip. For most pruposes,we make use of deep-etched strip waveguides, so-called photonicwires. Their high lateral contrast allows for very tight bends (a fewmicrometers). For large-scale circuitry, the propagation loss in thewaveguide should be negligible. Using optimized dry etching as well assurface treatments, we managed to reduce losses to less than 2dB/cm.

Fabrication processIn imec we develop processes for silicon photonics based on industrialCMOS tools. In contrast with many research groups which rely on e-beamlithography, we use 193nm optical projection lithography and dryetching.

Process control is extremely important: nanometer variations inwaveguide geometry can result in a similar wavelength shift in spectralfilters. Therefore, we optimized the fabrication process to obtain control ofthe critical dimensions to the level of nanometers.

Filter ComponentsOne of the key functions of many photonic integrated circuits is spectralfiltering and multiplexing. A wavelength filter is the essential buildingblock for WDM communication systems, but also in many optical sensingand spectroscopy systems.

We have developed different concepts for wavelength filters in silicon.Compact ring resonators allow the filtering of a comb of wavelengths.Simple Mach-Zehnder interferometers can be cascaded into complexfilters. For all-out demultiplexers we have demonstrated ultra-compactarrayed waveguide gratings (AWG) and planar concave echellegratings (PCG).

Wavelength selective functions typically rely on a given phase delay at aspecific wavelength, ans this requires extremely good control in thefabrication. We have demonstrated nanometer-level control of spectralfilters, which is enough to allow for low-power thermal tuning.

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A flexible platformSilicon photonics is a generic technology, which can be used for a largenumber of applications. With this in mind, imec is developing a general-purpose photonics platform, consisting of pluggable process modules thatcan be added or removed from a flow on demand.

Based on this flow, further optimization can be considered when specificusers required additional (or enhanced) functionality.

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Coupling light into nanophotonic waveguides, with their submicron crosssection, is far from trivial, as the mode mismatch with a 10µm fiber moderesults in 30dB coupling loss when directly coupled.

Our solution is to introduce a surface diffraction grating coupler. Whenusing a single etch process, coupling efficiency of 30% is achieved. Usinga further optimized process with a overlay cladding and additional etchsteps, the coupling efficiency is boosted to almost 70% (-1.6dB), but thiscomes at a cost of additional processing steps.

Grating fiber couplers are flexible, can be positioned anywhere on thewaveguide, and can be used for wafer-scale testing.

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Silicon photonics allows a dense integration of photonic functions on asingle chip. However, many of these photonic building blocks needelectrical actuation, either for tuning, detection or signal modulation. Forthis, close integration with electronics is indispensable.

Imec is a pioneer in 3-D integration which allows the stacking ofelectronic chips with thousands of electrical interconnects using through-silicon vias. This technology can be adapted for photonics, where theTSVs are ‘drilled’ in the photonics wafer, which can then be bonded ontoan electronics carrier. The TSVs are fabricated after the photonicsprocess, and before the metallization process, which can be identical tothe Cu-damascene metallization of electronics.

The nature of the industrial tools used for silicon photonics makes itexpensive to use for research purposes. On the other hand, mask-basedlithography and wafer-scale processes allow the fabrication of a verylarge number of chips with a large number of structures on each. Thismakes the technology exceptionally attractive for cost-sharing .

The ePIXfab initiative provides a multi-project-wafer-like service ofshuttle runs in both the imec and the CEA-LETI pilot lines, fabricatingsilicon chips for many users over the entire world using stable fabricationprocesses, at only a fraction of the cost of a dedicated silicon run.

In addition, ePIXfab also provides design support and validation, as wellas training for new users. ePIXfab is supported by the EU-PhotonFABsupport action.

For electrical signal modulation, the Photonics Research Group andimec are developing carrier dispersion modulators. By implanting a p-n orp-i-n junction in the waveguide core, an electrical bias can modulate thecarrier density in the waveguide, which changes the optical loss as wellas the refractive index. This can be used for phase modulators, or incombination with an interferometer, for intensity modulation.

On top of that, we are collaborating in the SOFI project to develop electro-optic silicon-organic hybrid modulators where compact silicon waveguidesare combined with strong electro-optic polymers to obtain efficient phasemodulation.

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Why Silicon Photonics?There are three key argument why Silicon is a good material system for photonic integrated circuits:

Silicon is transparent at the most widely used telecom wavelengths, and could therefore bedeployed in existing telecom infrastructure.

Silicon and its native oxide form a material pair with a high refractive index contrast. Thisenables tight confinement of light in submicron waveguides with sharp bends, effectively reducingthe footprint of an integrated circuit with several orders of magnitude

Silicon is the main material used for electronics. Therefore, the infrastructure and processes forelectronics manufacturing can be leveraged to fabricate silicon photonics.

Silicon Photonics

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