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Greg Blackman addresses some of the remaining issues for integrating photonics on silicon chips

ilicon photonic products are now on the market and the technology has reached a reasonably high level of maturity, but is still ‘several years

out for wide-spread applications’, according to Dr Bert Offrein, manager photonics at IBM Research, Zurich.

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The field has seen massive investment by the big semiconductor companies, largely because the electrical interconnect in microprocessors is nearing its limit in terms of bandwidth and power, explained Professor Graham Reed at the Optoelectronics Research Centre (ORC) at the University of Southampton in the UK. ‘Photonics was seen as one way forward, and now, silicon photonics is probably seen as the only way forward,’ he commented. Reed was Intel’s first consultant when the company entered the field around 10 years ago. However, there still remain some critical issues to be solved, such as how to integrate a light source on a silicon chip in a way that makes full use of CMOS production technology, and how to package that chip and align it with an optical fibre. And while the big players like IBM and Intel have the resources for investing in the field, how can smaller companies take advantage of the opportunities that silicon photonics provides? The products available commercially today

are things like active optical cables, where two points are linked optically with an optical chip at the end. Luxtera is the company best known for these devices. However, there is a drive for higher levels of integration to make full use of CMOS production capabilities. ‘The big advantage of silicon photonics is that it builds on infrastructure that is available in CMOS manufacturing. This enables high-

12 ELECTRO OPTICS l JUNE 2014 A silicon photonics chip for glucose sensing developed at Imec

volume capability and will reduce cost and simplify integration into a system,’ explained Offrein.

One of the big hurdles is integrating the light source on a silicon chip. Roel Baets, a professor at Ghent University and head of exploratory research in silicon photonics at Imec, stated: ‘You can easily integrate modulators and germanium detectors in silicon. The integration of light sources in silicon photonics is a tougher job – it is considered by many as the Holy Grail.’ The issue is that silicon is not good at emitting light, so other materials have to be used. Most of the commercial products available today are based on hybrid laser chip integration, where a complete laser chip is attached onto the silicon wafer, either in a miniature package or by flip-chip technology. The alternative is to bond III-V

semiconductors such as indium phosphide (InP) onto the silicon platform. ‘The problem with bonding technology is that by definition it cannot be a full wafer technology,’ explained Baets. Standard silicon wafers are 200mm and 300mm, whereas such wafer sizes don’t exist in the III-V world – wafer sizes in InP are typically 75mm to 100mm. ‘There is a wafer size mismatch, which means several smaller or larger III-V dies have to be bonded onto the silicon platform,’ he continued. ‘That makes it a technology which is much less directly compatible with what is available in a CMOS fab.’

Imec’s silicon photonics platform offers passive functions, but also active photonic devices including high-speed modulators, detectors, and the heterogeneous integration of III-V lasers on silicon.

The III-V wafer is bonded with its @electrooptics |


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