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CMOS-Photonics Technology Challenges

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By Ed Korczynski, Sr. Technical Editor

Fig 1

While it is very easy to talk about the potential advantages of CMOS-photonic integration, the design and manufacturing of commercially competitive products has been extraordinarily difficult. It has been well-known that the cost efficiencies of silicon wafers and CMOS fab processes could theoretically be leveraged to create low-cost photonic circuitry. However, the physics of optics is quite different from the physics of electronics, and so there have been unexpected challenges in moving R&D experiments to HVM products. During the Imec Technology Forum in Brussels held this May, Joris Van Campenhout, imec program director for Optical I/O (Fig. 1) sat down with Solid State Technology to discuss recent progress and future plans.

Data centers—also known as “The Cloud”—continue to grow along with associated power-consumptions, so there are strong motivations to find cost-effective ways to replace more of the electrical switches with lower-power optical circuits. Optical connections in modern data centers do not all have the same specifications, with a clear hierarchy based on the 3D grid-like layout of rows of rack-mounted Printed Circuit Boards (PCB). The table shows the basic differences in physical scale and switching speeds required at different levels within the hierarchy.

Data centers—also known as “The Cloud”—continue to grow along with associated power-consumptions, so there are strong motivations to find cost-effective ways to replace more of the electrical switches with lower-power optical circuits. Optical connections in modern data centers do not all have the same specifications, with a clear hierarchy based on the 3D grid-like layout of rows of rack-mounted Printed Circuit Boards (PCB). The table shows the basic differences in physical scale and switching speeds required at different levels within the hierarchy.

ESTIMATED DATA CENTER REQUIREMENTS FOR OPTICAL I/O  (Source: imec)
OPTICAL CONNECTION RACK BACKPLANE PCB CHIP
DISTANCE 5-500m 0.5-3m 5-50cm 1-50mm
RELATIVE COST $$$$ $$$ $$ $
POWER/Gbps 5mW 1mW 0.5mW 0.1mW

Rack fiberoptic lines connecting the rows of rack-mounted printed-circuit boards (PCB) in data centers represent a major portion of the total investments for capital equipment, so there is a roadmap to keep the same fibers in place while upgrading the speeds of photonic transmit and receive components over time:

40GHz was standard through 2015,

100GHz upgrades in 2016,

400GHz planned by 2019, and

1THz estimated by 2022.

Some companies have tried to develop multi-mode fiber solutions, but imec is working on single-mode. The telecommunications standard for single-mode optical fiber diameter is 9 microns, while multimode today can be up to 50 microns diameter. “Fundamentally single-mode will be the most integrate-able way to try to get that fiber on to a chip,” explained Van Campenhout. “It is difficult enough to get nine micron diameter fibers to couple to sub-micron waveguides on chip.”

Backplane is the PCB-to-PCB connection within one rack, that today uses copper connections running at up to 50 GHz. Imec sees backplane applications as a possible insertion point for CMOS-Photonics, because there are approximately 10X the number of connections compared to rack applications and because the relative cost target calls for new technologies. Imec’s approach uses 56G silicon ring-modulators to shift wavelengths by 0.1% at very low power, knowingly taking on control issues with non-linearity, and high temperature sensitivity. “We’re confident that it can be done,” stated Van Campenhout, “but the question remains if the overhead can be reduced so that the costs are competitive.” The overhead includes the possible need for on-chip thin-film heaters/coolers to be able to control the temperature.

PCB level connections are being pushed by the Consortium for On-Board Optics (COBO), an industry group working to develop a series of specifications to permit the use of board-mounted optical modules in the manufacturing of networked equipment (i.e. switches, servers, etc.). The organization plans to reference industry specifications where possible and develop specifications where required with attention to electrical interfaces, pin-outs, connectors, thermals, etc. for the development of interchangeable and interoperable optical modules that can be mounted onto motherboards and daughtercards.

Luxtera is the commercial market leader for CMOS-Photonic chips used at the Rack level today, and uses ‘active alignment’ meaning that the fiber has to be lit with the laser and then aligning to the waveguides during test and during assembly. Luxtera is fabless and uses Freescale as foundry to build the chip in an established CMOS SOI process flow originally established for high performance microprocessors. The company produces 10G chips today for advanced Ethernet connections, and through a partnership with Molex ships 40G Active Optical Cables.

Chip level optical connections require breakthrough technologies such as indium-phosphide epitaxy on silicon to be able to grow the most efficient electrically-controlled optical switches, instead of having to pick-and-place discrete components aligned with waveguides. Alignment of components is a huge issue for manufacturing and test that adds inherent costs. “The main issue is getting the coupling from the chip to the fiber with low losses, since sub-micron alignment is needed to avoid a 1 dB loss,” summarized Van Campenhout.

Figure 2 shows a simplified functional schematic of a high-capacity optical communications links employing Dense Wavelength Division Multiplexing (DWDM) to combine modulated laser beams of different colors on a single-mode fiber. Luxtera is working on DWDM for increased bandwidth as is imec.

FIGURE 2: Dense Wavelength Division Multiplexing (DWDM) scheme allows multiplication of the total single-mode fiber (SMF) bandwidth by the number of laser colors used. (Source: imec)

Difficult Design

“If you have just a 1 nm variation in the waveguide width, that device’s spectral response will be proportional as a rule of thumb,” explained Van Campenhout. “We can tune for that with a heating element, but then we lose the low-power advantage.” This results in a need for different design-for-manufacturing approaches.

“When we do photonics design we have to have round features or the light will scatter. So when we do mask making we have to use different rules, and we need to educate all of our partners that we are doing photonics,” reminded Van Campenhout. “However there are EDA companies that are becoming aware of these aspect, so things are developing nicely to create a whole ecosystem to be able to build these. We have the first version of a PDK that we use for multi-product-wafer runs, so we can deliver custom chips to partners.”

Mentor Graphics is an imec partner, and the company’s Tom Daspit, marketing manager for Pyxis Design Tools, spoke with Solid State Technology about the special challenges of EDA for photonics. “You’ve now jumped off the cliff of the orthogonal design environment. Light doesn’t bend at 45° let alone 90°. On an IC it’s all orthogonal, while if it’s photonic we have to modify the interconnect so that the final design is a nice curved one.” To produce a smooth curve the EDA tools must fracture it into a small grid for the photomask, so a seemingly simple set of curves can require gigabytes in a final GDSII file.

It was about 4 years ago that some customers began asking Mentor to modify tools to be able to support photonics, and today there are customers large and small, and some are in full volume production for communications applications. “Remember when they building the old Cray supercomputers and they had to account for all wire lengths to handle signal delays, well now with photonics we need to account for waveguide lengths,” commented Daspit.

In full volume products today are likely communications chips. Customers do not typically share product plans, so not sure of applications spaces. Everybody wants to get rid of the Cu in the backpane to eliminate power consumption, but:

“The big application is photonics for sensor integration, with universities leading the way. Medical is a huge new market,” explained Daspit. “The CMOS die could be 130- down to 65nm or maybe 28nm-nm for some digital.” So there are a wide variety of future applications for CMOS-Photonics, and despite the known manufacturing challenges there are already commercial applications in communications.

—E.K.



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