ST makes its first PSM4 optical engine deliveries
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.
Mario Paniccia: We are just at the beginning
During that time, his Intel team had six silicon photonics papers published in the science journals, Nature and Nature Photonics, and held several world records - for the fastest modulator, first at 1 gigabit, then 10 gigabit and finally 40 gigabit, the first pulsed and continuous-wave Raman silicon laser, the first hybrid silicon laser working with The University of California, Santa Barbara, and the fastest silicon germanium photo-detector operating at 40 gigabit.It got to the stage where Intel’s press relations department would come and ask what the team would be announcing in the coming months. “ 'Hey guys,' I said, 'it doesn't work that way ' ”.
Since leaving Intel last year, Paniccia has been working as a consultant and strategic advisor. He is now exploring opportunities for silicon photonics but in segments other than telecom and datacom.
“I didn't want to go into developing transceivers for other big companies and compete with my team's decade-plus of development; I spent 20 years at Intel,” he says.
Decade of development
Intel’s silicon photonics work originated in the testing of its microprocessors using a technique known as laser voltage probing. Infra-red light is applied to the back side of the silicon to make real-time measurements of the chip’s switching transistors.
For Paniccia, it raised the question: if it is possible to read transistor switching using light, can communications between silicon devices also be done optically? And can it be done in parallel to the silicon rather than using the back side of silicon?
In early 2000 Intel started working with academic Graham Reed, then at the University of Surrey, and described by Paniccia as one of the world leaders in silicon photonics devices. “We started with simple waveguides and it just progressed from there,” he says.
The Intel team set the target of developing a silicon modulator working at 1 gigahertz (GHz); at the time, the fastest silicon modulator operated at 10 megahertz. “Sometimes leadership is about pushing things out and putting a stake in the ground,” he says.
It was Intel’s achievement of a working 1GHz silicon modulator that led to the first paper in Nature. And by the time the paper was published, Intel had the modulator working at 2GHz. The work then progressed to developing a 10 gigabit-per-second (Gbps) modulator and then broadened to include developing other silicon photonics building-block devices that would be needed alongside the modulator – the hybrid silicon laser, the photo-detector and other passive devices needed for an integrated transmitter.
There is a difference between proving the technology works and making a business out of it
Once 10Gbps was achieved, the next milestone was 20Gbps and then 40Gbps. Once the building block devices achieved operation in excess of 40Gbps, Intel’s work turned to using these optical building blocks in integrated designs. This was the focus of the work between 2010 to 2012. Intel chose to develop a four-channel 40Gbps (4x10 gigabit) transceiver using four-wavelength coarse WDM which ended up working at 50Gbps (4x12.5 gigabit) and then, most recently, a 100Gbps transceiver.
He says the same Intel team is no longer talking about 50Gbps or 100Gbps but how to get multiple terabits coming out of a chip.
Status
Paniccia points out that in little more than a decade, the industry has gone from not knowing whether silicon could be used to make basic optical functions such as modulators and photo-detectors, to getting them to work at speeds in excess of 40Gbps. “I’d argue that today the performance is close to what you can get in III-V [compound semiconductors],” he says.
He believes silicon photonics is the technology of the future, it is just a question of when and where it is going to be applied: “There is a difference between proving the technology works and making a business out of it”.
In his mind, these are the challenges facing the industry: proving silicon photonics can be a viable commercial technology and determining the right places to apply it.
For Paniccia, the 100-gigabit market is a key market for silicon photonics. “I do think that 100 gigabit is where the intercept starts, and then silicon photonics becomes more prevalent as you go to 200 gigabit, 400 gigabit and 1 terabit,” he says.
So has silicon photonics achieved its tipping point?
Paniccia defines the tipping point for silicon photonics as when people start believing the technology is viable and are willing to invest. He cites the American Institute for Manufacturing Integrated Photonics (AIM Photonics) venture, the $610 million public and private funded initiative set up in 2015 to advance silicon photonics-based manufacturing. Other examples include the silicon photonics prototyping service coordinated by nano-electronics research institute imec in Belgium, and global chip-maker STMicroelectronics becoming a silicon photonics player having developed a 12-inch wafer manufacturing line.
Instead of one autonomous LIDAR system in a car, you could have 20 or 50 or 100 sprinkled throughout your vehicle
“All these are places where people not only see silicon photonics as viable but are investing significant funds to commercialise the technology,” says Paniccia. “There are numerous companies now selling commercialised silicon photonics, so I think the tipping point has passed.”
Another indicator that the tipping point has happened, he argues, is that people are not spending their effort and their money solely on developing the technology but are using CMOS processes to develop integrated products.
“Now people can say, I can take this process and build integrated devices,” he says. “And when I put it next to a DSP, or an FPGA, or control electronics or a switching chip, I can do things that you couldn't do next to bulky electronics or bulky photonics.”
It is this combination of silicon photonics with electronics that promises greater computing power, performance and lower power consumption, he says, a view shared by another silicon photonics luminary, Rockley Photonics CEO, Andrew Rickman.
Moreover, the opportunities for integrated photonics are not confined to telecom and datacom. “Optical testing systems for spectroscopy today is a big table of stuff - lasers, detectors modulators and filters,” says Paniccia. Now all these functions can be integrated on a chip for such applications as gas sensing, and the integrated photonics device can then be coupled with a wireless chip for Internet of Things applications.
The story is similar with autonomous vehicle systems that use light detection and ranging (LIDAR) technology. “These systems are huge, complicated, have a high power consumption, and have lots of lasers that are spinning around,” he says. “Now you can integrate that on a chip with no moving parts, and instead of one autonomous LIDAR system in a car, you could have 20 or 50 or 100 sprinkled throughout your vehicle”
Disruptive technology
Paniccia is uncomfortable referring to silicon photonics as a disruptive technology. He believes disruption is a term that is used too often.
Silicon photonics is a technology that opens up a lot of new possibilities, he says, as well as a new cost structure and the ability to produce components in large volume. But it doesn’t solve every problem.
The focus of the optical vendors is very much on cost. For markets such as the large-scale data centre, it is all about achieving the required performance at the right cost for the right application. Packaging and testing still account for a significant part of the device's overall cost and that cannot be forgotten, he says.
Paniccia thus expects silicon photonics to co-exist with the established technologies of indium phosphide and VCSELs in the near term.
“It is all about practical decisions based on price, performance and good-enough solutions,” he says, adding that silicon photonics has the opportunity to be the mass market solution and change the way one thinks about where photonics can be applied.
“Remember we are just at the beginning and it will be very exciting to see what the future holds.”
First silicon photonics devices from STMicro in 2014
STMicroelectronics expects to have first silicon photonics products by mid-2014. The chip company announced the licensing of silicon photonics technology from Luxtera in March 2012. Since then STMicro has been developing its 300mm (12-inch) CMOS wafer manufacturing line for silicon photonics at its fab at Crolles, France.
Flavio Benetti, STMicroelectronics
"We think we are the only ones doing the processing in a 12-inch line," says Flavio Benetti, general manager of mixed processes division at STMicroelectronics.
The company has a manufacturing agreement with Luxtera and the two continue to collaborate. "We have all the seeds to have a long-term collaboration," says Benetti.
"We also have the freedom to develop our own products." STMicro has long supplied CMOS and BiCMOS ICs to optical module makers, and will make the ICs and its photonic circuits separately.
The company's interest in silicon photonics is due to the growth in data rates and the need of its customers to have more advanced solutions at 100 Gig and 400 Gig in future.
"It is evident that traditional electronics circuits for that are showing their limits in terms of speed, reach and power consumption," says Benetti. "So we have been doing our due diligence in the market, and silicon photonics is one of the possible solutions."
It is evident that traditional electronics circuits for that are showing their limits in terms of speed, reach and power consumption
The chip company will need to fill its 300mm production line and is eyeing short-reach interconnect used in the data centre. STMicro is open to the idea of offering a foundry service to other companies in future but this is not its current strategy, says Benetti: "A foundry model is not excluded in the long term - business is business - but we are not going to release the technology to the open market as a wafer foundry."
The photonic circuits will be made using a 65nm lithography line, chosen as it offers a good tradeoff between manufacturing cost and device feature precision. Test wafers have already been run through the manufacturing line. "Being the first time we put an optical process in a CMOS line, we are very satisfied with the progress," says Benetti.
One challenge with silicon photonics is the ability to get the light in and out of the circuit. "There you have some elements like the gratings couplers - the shape of the grating couplers and the degree of precision are fundamental for the efficiency of the light coupling," says Benetti. "If you use a 90nm CMOS process, it may cost less but 65nm is a good compromise between cost and technical performance." The resulting photonic device and the electronics IC are bonded in a 3D structure and are interfaced using copper pillars.
A foundry model is not excluded in the long term - business is business - but we are not going to release the technology to the open market as a wafer foundry
Making the electronics and photonic chips separately has performance benefits and is more economical: the dedicated photonic circuit is optimised for photonics and there are fewer masks or extra processing layers compared to making an electro-optic, monolithic chip. The customer also has more freedom in the choice of the companion chip - whether to use a CMOS or BiCMOS process. Also some STMicro customers already have a electronic IC that they can reuse. Lastly, says Benetti, customers can upgrade the electronics IC without touching the photonic circuit.
Benetti is already seeing interest from equipment makers to use such silicon photonics designs directly, bypassing the optical module makers. Will such a development simplify the traditional optical supply chain? "There is truth in that; we see that," says Benetti. But he is wary of predicting disruptive change to the traditional supply chain. "System vendors understand the issue of the supply chain with the added margins [at each production stage] but to simplify that, I'm not so sure it is an easy job," he says.
Benetti also highlights the progress being made with silicon photonics circuit design tools.
STMicro's test circuits currently in the fab have been developed using electronic design automation (EDA) tools. "Already the first generation design kit is rather complete - not only the physical design tools for the optics and electronics but also the ability to simulate the system [the two together] with the EDA tools," says Benetti.
But challenges remain.
One is the ability to get light in and out of the chip in an industrial way. "Coupling the light in the fibre attachment - these are processes that still have a high degree of improvement," says Benetti. "The process of the fibre attachment and the packaging is something we are working a lot on. We have today at a very good stage of speed and precision in the placement of the fibres but there is still much we can do."
Luxtera's interconnect strategy
Part 1: Optical interconnect
Luxtera demonstrated a 100 Gigabit QSFP optical module at the OFC/NFOEC 2013 exhibition.
"We're in discussions with a lot of memory vendors, switch vendors and different ASIC providers"
Chris Bergey, Luxtera
The silicon photonics-based QSFP pluggable transceiver was part of the Optical Internetworking Forum's (OIF) multi-vendor demonstration of the 4x25 Gigabit chip-to-module interface, defined by the CEI-28G-VSR Implementation Agreement.
The OIF demonstration involved several optical module and chip companies and included CFP2 modules running the 100GBASE-LR4 10km standard alongside Luxtera's 4x28 Gigabit-per-second (Gbps) silicon photonics-based QSFP28.
Kotura also previewed a 100Gbps QSFP at OFC/NFOEC but its silicon photonics design uses two chips and wavelength-division multiplexing (WDM).
The Luxtera QSFP28 is being aimed at data centre applications and has a 500m reach although Luxtera says up to 2km is possible. The QSFP28 is sampling to initial customers and will be in production next year.
100 Gigabit modules
Current 100GBASE-LR4 client-side interfaces are available in the CFP form factor. OFC/NFOEC 2013 saw the announcement of two smaller pluggable form factors at 100Gbps: the CFP2, the next pluggable on the CFP MSA roadmap, and Cisco Systems' in-house CPAK.
Now silicon photonics player Luxtera is coming to market with a QSFP-based 100 Gigabit interface, more compact than the CFP2 and CPAK.
The QSFP is already available as a 40Gbps interface. The 40Gbps QSFP also supports four independent 10Gbps interfaces. The QSFP form factor, along with the SFP+, are widely used on the front panels of data centre switches.
"The QSFP is an inside-the-data-centre connector while the CFP/CFP2 is an edge of the data centre, and for telecom, an edge router connector," says Chris Bergey, vice president of marketing at Luxtera. "These are different markets in terms of their power consumption and cost."
Bergey says the big 'Web 2.0' data centre operators like the reach and density offered by the 100Gbps QSFP as their data centres are physically large and use flatter, less tiered switch architectures.
"If you are a big systems company and you are betting on your flagship chip, you better have multiple sources"
The content service providers also buy transceivers in large volumes and like that the Luxtera QSFP works over single-mode fibre which is cheaper than multi-mode fibre. "All these factors lead to where we think silicon photonics plays in a big way," says Bergey.
The 100Gbps QSFP must deliver a lower cost-per-bit compared to the 40Gbps QSFP if it is to be adopted widely. Luxtera estimates that the QSFP28 will cost less than US $1,000 and could be as low as $250.
Optical interconnect
Luxtera says its focus is on low-cost, high-density interconnect rather than optical transceivers. "We want to be a chip company," says Bergey.
The company defines optical interconnect as covering active optical cable and transceivers, optical engines used as board-mounted optics placed next to chips, and ASICs with optical SerDes (serialiser/ deserialisers) rather than copper ones.
Optical interconnect, it argues, will have a three-stage evolution: starting with face-plate transceivers, moving to mid-board optics and then ASICS with optical interfaces. Such optical interconnect developments promise lower cost high-speed designs and new ways to architect systems.
Currently optics are largely confined to transceivers on a system׳s front panel. The exceptions are high-end supercomputer systems and emerging novel designs such as Compass-EOS's IP core router.
"The problem with the front panel is the density you can achieve is somewhat limited," says Bergey. Leading switch IC suppliers using a 40nm CMOS process are capable of a Terabit of switching. "That matches really well if you put a ton of QSFPs on the front panel," says Bergey.
But once switch IC vendors use the next CMOS process node, the switching capacity will rise to several Terabits. This becomes far more challenging to meet using front panel optics and will be more costly compared to putting board-mounted optics alongside the chip.
"When we build [silicon photonics] chips, we can package them in QSFPs for the front panel, or we can package them for mid-board optics," says Bergey.
"If it [silicon photonics] is viewed as exotic, it is never going to hit the volumes we aspire to."
The use of mid-board optics by system vendors is the second stage in the evolution of optical interconnect. "It [mid-board optics] is an intermediate step between how you move from copper I/O [input/output] to optical I/O," says Bergey.
The use of mid-board optics requires less power, especially when using 25Gbps signals, says Bergey: “You dont need as many [signal] retimers.” It also saves power consumed by the SerDes - from 2W for each SerDes to 1W, since the mid-board optics are closer and signals need not be driven all the way to the front panel. "You are saving 2W per 100 Gig and if you are doing several Terabits, that adds up," says Bergey.
The end game is optical I/O. This will be required wherever there are dense I/O requirements and where a lot of traffic is aggregated.
Luxtera, as a silicon photonics player, is pursuing an approach to integrate optics with VLSI devices. "We're in discussions with a lot of memory vendors, switch vendors and different ASIC providers," says Bergey.
Silicon photonics fab
Last year STMicroelectronics (ST) and Luxtera announced they would create a 300mm wafer silicon photonics process at ST's facility in Crolles, France.
Luxtera expects that line to be qualified, ramped and in production in 2014. Before then, devices need to be built, qualified and tested for their reliability.
"If you are a big systems company and you are betting on your flagship chip, you better have multiple sources," says Bergey. "That is what we are doing with ST: it drastically expands the total available market of silicon photonics and it is something that ST and Luxtera can benefit from.”
Having multiple sources is important, says Bergey: "If it [silicon photonics] is viewed as exotic, it is never going to hit the volumes we aspire to."
Part 2: Bell Labs on silicon photonics click here
Part 3: Is silicon photonics an industry game-changer? click here