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Optics on-board

Microphotonics expands the limits of silicon IC technology

David Bak, Editor-in-Chief, Global Design News -- Design News, July 16, 2001

7:16 am, the #9007 TGV pulls out from Paris Gare du Nord headed for London. Picking up velocity, it hurtles through the French countryside, barely slowing as it plunges beneath the English Channel. Different gauge rail on the U.K. side, however, dictates a sharp reduction in speed for the remaining distance to Waterloo Station.

So it is with telecommunications today. Passing through optical fibers on pulses of light, data traverses the world at billions of bits per second. But as they near their destinations, those bits are routed electronically, creating traffic jams and delays. That's why channeling photons themselves through the interconnecting circuits, for all-optical data routing, will revolutionize the 21^st century much as semiconductor technology changed the 20^th century.

Ironically, it is proven semiconductor manufacturing technology–i.e., standard wafer fabrication techniques used for making integrated silicon circuits—that will enable development of those components necessary for optical manipulation. A leading example of this technology, called "microphotonics," integrates silicon-based optical waveguides with MEMS (micro-electromechanical systems) actuated devices to replace present-day optical-electrical-optical interconnects. Advantages are five-fold:

• Scalable manufacturing. Well-established processes such as flame hydrolysis or plasma-enhanced chemical vapor deposition can be used to create waveguide structures on silicon substrates.
  

• High densities. Components fabricated on a silicon wafer occupy far less space than their electromechanical counterparts, resulting in much smaller devices.
  

• Smaller size. A single device can provide the same functionality as many "formerly discrete" components.
  

• Fewer fibers. Fiber management is simplified since integration reduces the number of fibers required to interconnect components.
  

• Lower cost. Semiconductor fab-like manufacturing reduces labor.

One chip, many devices. Combining splitters, switches, taps, filters, and waveguides onto a single chip is the first step to eliminating today's electro-optical-electro bottlenecks. Nanovation Technologies' (Evanston, IL) hybrid silica/MEMS Nanoshutter™ technology, designed to route light between MEMS-actuated optical devices, illustrates this approach.

Nanoshutter fabrication begins with a standard silicon wafer as the optical chip substrate. A 15-micron layer of pure silica, grown directly on top of the silicon wafer, serves as the bottom-buffer layer for the waveguide structure. A six- to eight-micron layer of germanium (Ge) doped silica, which functions as the core waveguide layer, follows.

Nanovation offers an extensive library of optimized Nanoblock optical design elements, such as switches, splitters, bends, taps and couplers. Integration onto a sngle chip eliminates fiber splices, reducing cross talk and optical insertion loss.

Masking and etching through the core layer of Ge doped silica using conventional lithography techniques creates the chip's square cross-section channel waveguides, while a 15-20 micron thick top-buffer layer of boron phosphorus silica seals the waveguide layer. Adjusting the relative concentration of dopants matches its index of refraction to that of the bottom layer to confine light in the waveguides.

Further etching cuts through the top, core, and bottom layers into the silicon substrate itself. This produces trenches measuring tens of microns wide and more than 30 microns deep. MEMS-actuated optical components, placed in these trenches via flip chip mounting, can then block, reflect, or filter the beams. Moving in and out of the beam path in less than 10 msec, these components enable equipment manufacturers to customize such chips to their applications. The result, says Roydn Jones, senior vice president and executive director of the company's Silica Business Unit, is a series of optical elements, based on function, that can be used as building blocks to construct a "photonic integrated circuit."

Similarly, Digital Optics Corp., headquartered in Charlotte, NC, has developed a hybrid wafer-level integration platform called a Photonic Chip™ Optical Sub-Assembly (OSA). Photonic Chip OSAs are based on wafer-fabricated micro-optics. Passive optical functions such as collimation, focusing, splitting, and reflection are fabricated with photolithographic techniques. Standard die bonding equipment then attaches active components like lasers and detectors. A key benefit of the Photonic Chip OSA integration platform is the ability to accomplish as many of the critical alignment steps, and as many assembly steps at the wafer level as possible. This, in turn, enables high-volume capacity, tight alignment tolerance, and lower-cost optical sub-assemblies.

Coming improvements. While silica-on-silicon is emerging as one of the dominant platforms for microphotonics, other technologies are contenders. In November 2000, for example, Scotland-based Kymata announced a strategic partnership with IBM to commercialize an optical waveguide technology based on siliconoxynitride (SiON). Developed at the IBM Zurich Research Laboratory in Switzerland, the SiON waveguide core is sandwiched between SiO₂ layers deposited on silicon wafers using conventional chip manufacturing tools. The advantage over silica-on-silicon is a higher refractive index contrast between the waveguide core and cladding for tighter confinement in the waveguides.

Silica-on-silicon waveguides route light between MEMs-actuated optical devices with Nanovation's Nanoshutter technology. As a result, optical waveguides and othe rpassive products built with silica can be designed, scaled, and produced with custom features to enable equipment manufacturers to customize their products.

Bending radii, as a result, are far tighter than silica-on-silicon. This feature, says Vivek Tandon, vice president of strategic development at Kymata, will enable even more compact passive devices, further increasing scalability and cost benefits. In addition, heaters formed in the device's metalization layer allow tunability via thermo-optic control of the waveguide's refractive index for customer-specific applications.

Tandon notes, however, that there are significant challenges to overcome before SiON waveguide technology produces high manufacturing yields.

Ultimately, industry may bypass both silica-on-silicon and SiON for a material like indium phosphide (InP). Researchers are presently experimenting with InP-based wafer growth technologies for building active as well as passive components simultaneously, eliminating secondary procedures such as die bonding and flip chip mounting. Though said to be years away, such technology could represent the true integration of optics and electronics.

Benefits to the OEM. The telecom industry may be driving development of on-chip optics, but it will be the large consumer markets such as automotive that will drive big-volume optical chip production. For example, Standard MEMS Inc. (a design, fabrication, and packaging house for MEMS devices located in Burlington, MA and partnered with Germany's Fraunhofer Institute) presently supplies Ford Motor Co. with electronic chips and integrated sensing elements that monitor oil pressure, manifold absolute pressure, and exhaust gas pressure. "Integration with optics is being discussed," reports company President and CEO Nicholas Ortyl, "to reduce cost and weight of the wire harness."

Multiplexing off an optical signal, Ortyl explains, would not only reduce potential failure points in the wire harness, but would eliminate computer ports and reduce connector size. "Just think of the volumes," he notes. "Hundreds of millions of connectors will press down prices to make on-chip optics feasible for these applications." Other high-volume markets for optical chips are consumer electronics and computing. Micro fresnel lenses, built up using MEMS technology and coupled to passive optical components, could, for instance, improve DVD player performance by allowing blue lasers. Blue penetrates deeper than conventional red, permitting higher data densities.

"One of the roadblocks to using blue lasers," Ortyl says, "has been that traditional lenses cannot take the heat. MEMS technology allows a higher power laser; silicon, with applied focusing films, can focus the blue and dissipate the heat." Finally, computer CPUs themselves may benefit from the speed and bandwidth of microphotonics in the not-so-distant future. With communication between components no longer bogged down by electronic transmission, clock rates could soar 100 times faster than present.

For product design engineers, the coming message is clear: "Don't let your train slow down at the station—get on-board with optics."

Ever higher bandwidth

Ever higher bandwidth

To deliver a variety of services, from broadcast TV to video-on-demand and telephony, long-haul communications systems expand transmission capacity by what is referred to in the telecom industry as Dense Wavelength Division Multiplexing, or DWDM. This technique accommodates multiple wavelengths in a single optical fiber. Chip-level optical components, such as Kymata's Arrayed Waveguide Grating, promise to solve present day interconnect problems between fiber- optic transmission and copper access circuits. Consequently, network managers will be able to optically manipulate signals (add or drop wavelengths on demand, or switch them from one destination to another) without slowing signal transmission.

Array Waveguide Grating Theory 1. Multiple wavelengths enter the AWG. 2. Spread out into the lens area. 3. Undergo delays in each of the AWG arms. 4. Phase tilt at input to the second lens depends on the wavelength. 5. Phase tilt affects how light is recombined through constructive interface. 6. Different wavelengths are directed to the different output waveguides.

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• Another great article by design news!
  - 2006-10-10 11:41:53
• I concur. keep up the great work guys.
  - 2006-09-28 13:44:05
• agreed. great article!
  - 2006-09-28 13:10:40

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