ST makes its first PSM4 optical engine deliveries

Flavio Benetti is upbeat about the prospects of silicon photonics. “Silicon photonics as a market is at a turning point this year,” he says.

What gives Benetti confidence is the demand he is seeing for 100-gigabit transceivers in the data centre. “From my visibility today, the tipping point is 2016,” says Benetti, group vice president and general manager, digital and mixed processes ASIC division at STMicroelectronics.

Flavio Benetti

Benetti and colleagues at ST have spent the last four years working to bring to market the silicon photonics technology that the chip company licensed from Luxtera.

The company has developed a 300mm-wafer silicon photonics production line at its fabrication plant in Crolles that is now up and running. ST also has its first silicon photonics product - a mid-reach PSM4 100-gigabit optical engine - and has just started its very first deliveries.

At the OFC show in March, ST said it had already delivered samples to one unnamed 'customer partner', possibly Luxtera, and Benetti showed a slide of the PSM4 chips as part of a Lumentum transceiver.  

Another ST achievement Benetti highlights is the development of a complete supply chain for the technology. In addition to wafer production, ST has developed electro-optic wafer testing. This allows devices to be probed electrically and optically to select working designs before the wafer is diced. ST has also developed a process to 3D-bond chips.

“We have focussed on building an industrial environment, with a supply chain that can deliver hundreds of thousands and millions of devices,” says Benetti. 

PSM4 and CWDM4

ST’s first product, the components for a 4x25 gigabit PSM4 transceiver, is a two-chip design.

One chip is the silicon photonics optical engine which integrates the PSM4’s four modulators, four detectors and the grating couplers used to interface the chip to the fibres. The second chip, fabricated using ST’s 55nm BiCMOS process, houses the transceiver’s associated electronics such as the drivers, and trans-impedance amplifiers.

The two chips are combined using 3D packaging. “The 3D packaging consists of the two dies, one copper-pillar bonded to the other,” says Benetti. “It is a dramatic simplification of the mounting process of an optical module.” 

The company is also developing a 100-gigabit CWDM4 transceiver which unlike the PSM4 uses four 25-gigabit wavelengths on a single fibre.

The CWDM4 product will be developed using two designs. The first is an interim, hybrid solution that uses an external planar lightwave circuit-based multiplexer and demultiplexer,  followed by an integrated silicon photonics design. The hybrid design is being developed and is expected in late 2017; the integrated silicon photonics design is due in 2018.

With the hybrid design, it is not just a question of adding a mux-demux to the PSM4 design. “The four channels are each carrying a different wavelength so there are some changes that need to be done to the PSM4,” says Benetti, adding that ST is working with partners that will provide the mux-demux and do the integration.   

We need to have a 100-gigabit solution in high volume for the market, and the pricing pressure that is coming has convinced us that silicon photonics is the right thing to do

Opportunities 

Despite the growing demand for 100-gigabit transceivers that ST is seeing, Benetti stresses that these are not 'mobile-phone wafer volumes'. “We are much more limited in terms of wafers,” he says. Accordingly, there is probably only room for one or two large fabs for silicon photonics globally, in his opinion. 

So why is ST investing in a large production line? For Benetti, this is an obvious development for the company which has been a provider of electrical ICs for the optical module industry for years.

“ST has entered silicon photonics to provide our customers with a roadmap,” says Benetti. “We need to have a 100-gigabit solution in high volume for the market, and the pricing pressure that is coming has convinced us that silicon photonics is the right thing to do.”

It also offers chip players the possibility of increasing its revenues. “The optical engine integrates all the components that were in the old-fashioned modules so we can increase our revenues there,” he says.

ST is tracking developments for 200-gigabit and 400-gigabit links and is assessing whether there is enough of an opportunity to justify pursuing 200-gigabit interconnects.

For now though, it is seeing strong pricing pressure for 100-gigabit links for reaches of several hundred meters. “We do not think we can compete for very short reach distances,” says Benetti.  “We will leave that to VCSELs until the technology can no longer follow.” As link speeds increase, the reach of VCSEL links diminishes. “We will see more room for silicon photonics but this is not the case in the short term,” says Benetti.

Market promise

People have been waiting for years for silicon photonics to become a reality, says Benetti. “My target is to demonstrate it [silicon photonics] is possible, that we are serious in delivering parts to the market in an industrial way and in volumes that have not been delivered before.”

To convince the market, it is not just showing the technological advantages of silicon photonics but the fact that there is a great simplification in constructing the optical module along with the ability to deliver devices in volume. “This is the point,” he says. 

Benetti’s other role at ST is overseeing advanced networking ASICs. He argues that over the mid- to long-term, there needs to be a convergence between ASIC and optical connectivity.

“Look at a switch board, for example, you have a big ASIC or two in the middle and a bunch of optical modes on the side,” says Benetti. For him, the two technologies - photonics and ICs - are complementary and the industry’s challenge is to make the two live together in an efficient way.