The making of integrated optics
A US initiative is bringing together leading companies with top academics and universities to create a manufacturing infrastructure for the widespread adoption of integrated photonics.
The US sees integrated photonics as a strategic technology and has set up the American Institute for Manufacturing Integrated Photonics - AIM Photonics - to advance the technology and make it available to a wider community of companies. AIM Photonics, with $610 million of public and private funding, is a five-year initiative ending in 2020. AIM’s long-term goal is to be self-sustaining.
Doug Coolbaugh
“Right now the infrastructure is focussed on electronics and CMOS but photonics is going to be the future,” says Doug Coolbaugh, chief operations officer at AIM Photonics. “There is no other way to do it [very high bandwidth] except using light for ultra fast communications.”
Technologies start at universities and in the labs of companies with large R&D budgets. IBM and Intel, for example, have been developing silicon photonics for over a decade and the technology is ready for deployment. However, the intellectual property developed remains with such companies.
“AIM is not only creating the manufacturing infrastructure for integrated photonics but also ideas and intellectual property that can be used by companies for new products,” says Coolbaugh.
All the elements are being addressed so that small to medium businesses and entrepreneurial ventures can use integrated photonics for their products; companies too small to develop the technology themselves. “That will accelerate the silicon photonics ecosystem and allow new products to come out much faster than it would normally take,” says Coolbaugh.
Manufacturing
Silicon photonics luminary, Lionel Kimerling, professor of materials science and engineering at MIT, and an active member of AIM Photonics, views its focus on manufacturing as an important development.
The discipline of manufacturing is something that the chip industry has mastered through designing process integration, selecting materials and all the qualification standards used to meet system requirements, he says, but is less developed in the photonics industry.
AIM is making available a chip fabrication plant to interested companies. SUNY Polytechnic Institute has been working with MIT for the last six years to develop a 300mm-wafer silicon photonics line at its Albany site. The fab offers a multi-project wafer service whereby several designs can be made on a single wafer, allowing costs to be shared among companies.
AIM is not only creating the manufacturing infrastructure for integrated photonics but also ideas and intellectual property that can be used by companies for new products
A design kit is also being developed featuring key building blocks needed to make an integrated photonics circuit. AIM is working with leading semiconductor industry design automation companies Cadence, Synopsys and Mentor Graphics to provide the software tool environment for designers to develop circuits. “This design environment is compatible with the silicon photonics process here in our fab,” says Coolbaugh.
A packaging and prototyping facility located in Rochester, New York is also being set up. “Photonics packaging is relatively new and certain aspects have not been developed that much,” says Coolbaugh.
Another issue is developing skilled engineers and technicians able to design and manufacture integrated photonics circuits. Whereas electronic chip designers typically have a first degree, photonics engineers tend to have a doctorate because of the deep understanding needed. “This is one of the things we find we are lacking significantly,” says Coolbaugh. “There are just not enough skilled people in the industry to fulfil these needs.”
Professor Kimerling says he is spending much of his time putting together educational material to help attract individuals to pursue a career in silicon photonics. Much of the technology is in place, he says, what is required is to make it accessible to people. “I don’t have 40 more years in the industry, but I could influence the next 40 years by creating these instructional materials and career paths, and getting roadmap consensus that can drive the industry,” says Kimerling.
AIM is also working with universities and companies to develop technology and intellectual property alongside the manufacturing centres. Four research areas have been chosen, covering datacom, analogue RF for telecom involving Infinera, sensors and phased arrays. These are areas where AIM sees products emerging in volume in the next five years.
Keren Bergman, whose work focusses on the intersection of photonics and computing systems, mentions how AIM Photonics has already benefited her research group through much closer interactions with companies in the area of datacom. “It has had a big impact on our work,” says Bergman, professor and director at the Lightwave Research Laboratory at Columbia University.
Each year AIM will review and add new research topics. “There are new ideas, new materials and new manufacturing processes that will be developed,” says Coolbaugh. He cites the use of silicon photonics to drive robots as an emerging application area.
Status
AIM expects the entire manufacturing infrastructure to be in place in the next couple of years.
“Right now it is only the photonics design part but we will also be putting in interposers for packaged designs," says Coolbaugh. Interposers are a key technology that allows the co-packaging of chip dice, an approach known as system-in-package or 2.5D packaging.
AIM expects to offer multi-project wafers with interposers and system-in-package by 2017, with the ability to add CMOS dice in 2018. AIM is also developing a test, assembly and packaging facility which it expects to be available by 2018. “Testing is a really critical component of this entire infrastructure,” says Coolbaugh.
The goal is to develop new ways of fast-testing photonics on wafers, while there will be the high-speed testing of circuits at Rochester. “What we design has got to work in the fab, the fab has got to test well and then what we package has to be consistent with what we deliver to the packaging house,” says Coolbaugh. “The entire flow has to integrate exactly.”
A start-up or small company wanting to make a product can already use the design kit - which continues to evolve - and benefit from AIM’s multi-project wafer service. Then there will be the Rochester packaging and prototyping site. Low volumes can be made at the Albany fab while AIM will pass higher-volume manufacturing requests to leading chip fabrication players such as GlobalFoundries.
Companies can take a concept, develop their own product and have their own business. “We provide the entire chain for the infrastructure,“ says Coolbaugh. ”Right now, this is only available to large companies.”
If all goes to plan, what impact will AIM have on integrated optics and silicon photonics in particular? “It will be a worldwide impact,” says Coolbaugh. “Just because we want to create the infrastructure in the US doesn’t mean we are limiting our customers to the US.”
Further information
For AIM Photonics presentations, click here
The text is based on an article that first appeared in Optical Connections magazine
Heterogeneous integration comes of age
Silicon photonics luminaries series
Interview 7: Professor John Bowers
August has been a notable month for John Bowers.
Juniper Networks announced its intention to acquire Aurrion, the US silicon photonics start-up that Bowers co-founded with Alexander Fang. And Intel, a company Bowers worked with on a hybrid integration laser-bonding technique, unveiled its first 100-gigabit silicon photonics transceivers.
Professor John BowersBower, a professor in the Department of Electrical and Computer Engineering at the University of California, Santa Barbara (UCSB), first started working in photonics in 1981 while at AT&T Bell Labs.
When he became interested in silicon photonics, it still lacked a good modulator and laser. "If you don't have a laser and a modulator, or a directly modulated laser, it is not a very interesting chip,” says Bowers. "So I started thinking how to do that."
Bowers contacted Mario Paniccia, who headed Intel’s silicon photonics programme at the time, and said: “What if we can integrate a laser? I think there is a good way to do it.” The resulting approach, known as heterogeneous integration, is one that both Intel and Aurrion embraced and since developed.
This is a key Bowers trait, says Aurrion co-founder, Fang: he just knows what problems to work on.
"John came up with the concept of the hybrid laser very early on," says Fang. "Recall that, at that time, silicon photonics was viewed as nothing more than people making plasma-effect phase shifters and simple passive devices. John just cut to the chase and went after combining III-V materials with silicon."
If you look at the major companies with strong photonics activities, you’ll find a leader in that group that was developed under John’s training
Fang also highlights Bowers' management skills. “John can pick players and run teams,” says Fang, who describes himself as one of those privileged to graduate out of Bowers’ research group at UCSB.
“You find yourself in an environment where John picks a team of sharp folk with complementary skills and domain expertise to solve a problem that John determines as important and has some insight on how to solve it,” says Fang. “If we look like we are going to drive off the road, he nudges with a good mix of insight, fear, and humour.”
It has resulted in some of the best trained independent thinkers and leaders in the industry, says Fang: “If you look at the major companies with strong photonics activities, you’ll find a leader in that group that was developed under John’s training”.
Silicon photonics
Bowers defines silicon photonics as photonic devices on a silicon substrate fabricated in a CMOS facility.
“Silicon photonics is not about using silicon for everything; that misses the point,” says Bowers. “The key element is using silicon as a substrate - 12-inch wafers and not 2- or 3-inch wafers - and having all the process capability a modern silicon CMOS facility brings.” These capabilities include not just wafer processing but also advanced testing and packaging.
The world is about to change and I don't think people have quite figured that out
“If you go to an advanced packaging house, they don't do 6-inch wafers and I don't know of indium phosphide and gallium arsenide wafers larger than 6 inches,” says Bowers. “The only solution is to go to silicon; that is the revolution that hasn't happened yet but it is happening now.”
Bowers adds that everything Aurrion does, there is automated test along the way. "And I think you have others; Luxtera has done a great job as well at wafer-level test and packaging," he says. "The world is about to change and I don't think people have quite figured that out."
Working with Intel was an eye-opener for Bowers, especially the process controls it applies to chip-making.
“They worry about distributions and yields, and it is clear why there are seven billion transistors on a chip and that chip will yield,” says Bowers. “When you apply that to photonics, it will take it to a whole new level.” Indeed, Bowers foresees photonics transfering to silicon.
Bowers highlights the fairly complex chips now being developed using silicon photonics.
“We have done a 2D scanner - a 32-element phased array - something one could never do in optics unless it was integrated all on one chip,” he says. The phased-array chip comprises 160 elements and is physically quite large.
This is another benefit of using 12-inch silicon wafers and fabricating the circuits in a CMOS facility. “You are not going to cost-effectively do that in indium phosphide, which I've worked on for the last 30 years,” says Bowers.
Another complex device developed at UCSB is a 2.54-terabit network-on-a-chip. “This is a larger capacity than anyone has done on any substrate,” he says.
Infinera’s latest photonic integrated circuit (PIC), for example, has a transport capacity of up to 2.4 terabit-per-second. That said, Bowers stresses that the network-on-a-chip is a research presentation while Infinera’s PIC is a commercial device.
Heterogeneous integration
Heterogeneous integration involves bonding materials such as III-V compounds onto silicon.
Bowers first worked on III-V bonding with HP to make longer wavelength - 1310nm and 1550nm - VCSELs. “We had been bonding indium phosphide and gallium arsenide to solve a fundamental problem that indium phosphide does not make good mirrors,” he says. “So I was pretty confident we could bond III-V to silicon to add gain to silicon photonics to then add all the laser capability.”
Bonding to silicon is attractive as it enables the integration of optical features that haven't been widely integrated onto any other platform, says Bowers. These include not only lasers but other active devices such as modulators and photo-detectors, as well as passive functions such as isolators and circulators.
One concern raised about heterogeneous integration and the use of III-V materials is the risk of contamination of a CMOS fabrication line.
Bowers points out that the approached used does not impact the front end of the fabrication, where silicon wafers are etched and waveguides formed. The III-V material is bonded to the wafer at the fab’s back end, the stage where metallisation occurs when making a CMOS chip.
The leading chipmakers are also experimenting with III-V materials to create faster digital devices due to their higher electron mobility. “This is part of the natural evolution of CMOS,” he says. It remains unclear if this will be adopted, but it is possible that a 5nm CMOS node will use indium phosphide.
“All the CMOS houses are doing lots of work on III-V and silicon,” says Bowers. “They have figured out how to control that contamination issue.”
New capabilities
Bowers and his team have already demonstrated the integration of new optical functions on silicon.
“Neither silicon nor indium phosphide has an isolator and one always has to use an external YIG (yttrium iron garnet) isolator to reduce the reflection sensitivity of things like widely tunable lasers,” says Bower.
His team has developed a way to bond a YIG onto silicon using the same techniques it uses for bonding III-V materials. The result is an integrated isolator device with 32dB isolation and a 2dB insertion loss, a level of performance matching those of discrete isolators.
Incorporating such functionality onto silicon creates new possibilities. “We have a paper coming out that features a 6-port circulator,” says Bowers. “It is not a tool that the community can use yet because it has never been made before but we can do that on silicon now,” he says. “That is a good new capability.”
Superior performance
Bowers stresses that heterogeneous integration can also result in optical performance superior to a III-V design alone. He cites as an example how using a silicon nitride waveguide, with its lower loss that indium phosphide or gallium arsenide, can create high-quality Q-resonators.
A Q-resonator can be viewed as a form of filter. Bowers' group have demonstrated one with a Q of 80 million. “That makes it very sensitive to a variety of things,” he says. One example is for sensors, using a Q resonator with a laser and detector to form a spectrometer.
His researchers have also integrated the Q resonator with a laser to make a widely tunable device that has a very narrow line-width: some 40kHz wide. This is a narrower than the line-width of commercially-available tunable lasers and exceeds what can be done with indium phosphide alone, he says.
Challenges
Bowers, like other silicon photonics luminaries, highlights the issues of automated packaging and automated testing, as important challenges facing silicon photonics. “Taking 10,000s of transceivers and bringing all the advanced technology - not just processing but test and packaging - that are being developed for cell phones,” he says.
Too much of photonics today is based on gold boxes and expensive transceivers. “Where Aurrion and Intel are going is getting silicon photonics to the point where photonics will be ubiquitous, cheap and high yielding,” he says. This trend is even evident with his university work. The 400-element 2.54-terabit network-on-a-chip has very high laser yields, as are its passive yields, he says.
“So, effectively, what silicon photonics can do is going up very rapidly,“ says Bowers. “If you can put it in the hands of a real CMOS player like Intel or the companies that Aurrion uses, it is going to take photonics to a whole new area that people would not have thought possible in terms of complexity.”
Yet Bowers is also pragmatic. “It still takes time,” he says. “You can demonstrate an idea, but it takes time to make it viable commercially.”
He points to the recently announced switch from Oracle that uses mid-board optics. “That is a commercial product out there now,” he says. “But is it silicon photonics? No, it is VCSEL-based; that is the battle going on now.”
VCSELs have won the initial battle in the data centre but the amount of integration the technology can support is limited. Once designers move to wavelength-division multiplexing to get to higher capacities, where planar technology is required to combine and separate the different wavelengths efficiently, that is when silicon has an advantage, he says.
The battle at 100 gigabit between VCSELs and silicon photonics is also one that Bowers believes silicon photonics will eventually win. But at 400 gigabit and one terabit, there is no way to do that using VCSELs, he says.
Status
The real win for silicon photonics is when optics moves from transceivers at the edge of the board to mid-board and eventually are integrated with a chip in the same package, he says.
Advanced chips such as switch silicon for the data centre are running into an input-output problem. There are only so many 25 gigabit-per-second signals a chip can support. Each signal, sent down a trace on a printed circuit board, typically requires equalisation circuitry at each end and that consumes power.
Most of the photonics industry has focused on telecom and datacom, and justifiably so. The next big thing will happen in the area of sensors.
A large IC packaged as a ball grid array may have as many as 5,000 bumps (balls) that are interfaced to the printed circuit board. Using photonics can boost the overall bandwidth coming on and off the chip.
“With photonics, and in particular when we integrate the laser as well as the modulator, the world doesn't see it as a photonics chip, it's an electronics chip, it just turns out that some of those bumps are optical ones and they provide much more efficient transmission of data and at much lower power,” say Bowers. A 100 terabit of even a 1000 terabit - a petabit - switch chip then becomes possible. This is not possible electrically but it is possible by integrating photonics inside the package or on the chip itself, he says.
“That is the big win eventually and that is where we help electronics extend Moore’s law,” says Bowers.
And as silicon photonics matures, other applications will emerge - More than Moore’s law - like the use of photonics for sensors.
“Most of the photonics industry has focused on telecom and datacom, and justifiably so,” says Bowers. “The next big thing will happen in the area of sensors.”
Professor Bowers was interviewed before the Juniper Networks announcement