The care and feeding of an optoelectronics cleanroom

by Hank Hogan

Faced with the need to improve and expand their cleanrooms, four optoelectronics startups reveal their challenges and the steps they took to overcome the hurdles

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Photons may not have any mass, but in modern technology they carry a lot of weight. Much of the world's voice, video and data travels as beams of light down strands of fiber. Music and computer data is extracted every day from CDs and DVDs using red-hued, solid-state lasers, and in many cases, optoelectronics play a crucial role in that seemingly ever-present technical magic.

Increasingly, the devices that make the magic possible are being manufactured in cleanrooms. That's long been true for solid-state lasers, but now the new integrated optoelectronic and photonic devices are being built in clean areas—spaces that aren't like those used in the creation of leading-edge integrated circuits, with their sub-micron feature size and two to three centimeter chip size.

A finished 6″ wafer full of integrated variable optical attenuator devices and ready for dicing.
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“The width of a waveguide is ten microns, but the length of it to perform certain functions—say a switch, attenuators or a coupler—is on the order of several millimeters,” says Louay Eldada, co-founder, chief technology officer and vice chairman of the optical components company Telephotonics Inc. (Wilmington, MA).

Behind these dimensions are two constraints. There's the size of the core of an optical fiber, and the need to match that drives the ten-micron width figure. Then there's the need to not lose precious photons in the waveguide. That requirement for high-efficiency throughput prevents rapid or sharp changes in beam direction, and that lies behind the length of the device. Both of these factors, in turn, compel certain cleanroom performance and contamination-control strategies. For instance, the minimum size of a particle to be concerned about is often measured in microns, not fractions of a micron.

Companies operating in this new arena include Telephotonics, Optical Micro Devices, Ltd. (Swinden, United Kingdom), Codeon Corp. (Colombia, MD), and AXSUN Technologies Inc. (Billerica, MA), among other recent startups. Many have recently constructed or upgraded cleanroom facilities, and the experience has shown that the new technology represents a learning curve for many.

“A lot of construction companies don't know the difference between an integrated circuit cleanroom, a biotech cleanroom and an optical cleanroom,” says John Bradunas, chief operating officer for Codeon.

All of these cleanrooms use some kind of air filtration, often HEPA based, with pressurization and flow arranged to suppress contamination. In the case of the new optoelectronic cleanrooms, some of the manufacturing equipment and techniques are borrowed from semiconductor fabrication.

There are, however, critical differences between the new breed of integrated optical devices and the more familiar electronics gear that impact cleanroom design, construction and operation. There's the relatively large features and forgiving nature of the optoelectronic devices, for example.

However, Bradunas points to differences in physical space requirements. At present, some of the new miniaturized optical products require a manual manufacturing approach versus the highly automated machinery found in semiconductor and other plants.

“When it comes to automation, the optical market is now where the integrated circuit market was back in the 1960s or early 1970s,” says Bradunas.

Thus, in some optoelectronic facilities the manual workflow pattern has to be taken into account in the layout of the space; whereas in an integrated circuit plant the need is to find a place for big machines. As a result of this degree of automation, power and chemical requirements in an optoelectronic facility more closely resemble those found in a pharmaceutical or a biotech cleanroom.

Because optoelectronics covers devices ranging from solar cells to waveguides, there's no one cleanroom lesson that applies to all. However, a look at these four different companies reveals the contamination challenges each faces, and the steps undertaken to overcome those hurdles.

A light fab
On an island half a world away from the largest semiconductor foundries, Optical Micro Devices (OMD) wants to be the world's photonic foundry.

Like its semiconductor analogs in Taiwan, OMD will build components for anyone. This is being done in a 10,000-square-foot, ISO Class 5, raised-floor cleanroom running 200-mm wafers. According to facilities manager Ray Miller, the company's cleanroom is not anything unusual.

“We do not have any special contamination control on our kit. The equipment is not SMIF and only has standard handling features. The fab is a pretty standard recirculation system to Class 100 [ISO Class 5],” he says.

The factory was qualified in January, and the process has been running since last June. This is being done primarily with refurbished 200-mm wafer equipment such as furnaces, steppers, sputtering gear and etching equipment. The company makes both planar lightwave circuits (PLCs) and silicon microbenches for fiber alignment.

The automated tool for fabricating the waveguides in the Class-10 minienvironment.
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PLCs are passive optoelectronic subcomponents that transport optical signals between active components within a substrate, much like a conducting wire transmits electricity between active electrical components. OMD manufactures PLCs by controlled deposition and etching of silica onto silicon wafers. The PLCs consist of a silica core encased in a silica cladding of a different refractive index. A substance's refractive index is the ratio of the speed of light in a vacuum to the speed of light in the substance. It also is a measure of the amount a ray of light bends when passing from one medium to another.

Because of the refractive index gradient and reflection, electromagnetic waves are confined to the waveguide, and light can therefore be rerouted and otherwise altered. PLC devices include optical beamsplitters and wavelength multiplexers and demultiplexers. Through the use of heat, OMD can create dynamic PLCs that are used for optical attenuators and optical switches.

Most of the processing at OMD is similar to that found in semiconductor manufacturing. One difference, and a potential contamination concern, is the use of thick silicon films. OMD's chemical vapor deposition and reactive ion-etching equipment have to deposit and etch more than is typically done in integrated circuit manufacturing. As a result, the equipment runs in a different regime, and that could be a source of trouble. But to date, company officials report no contamination problems. OMD is currently shipping product prototypes and small orders.

Optical printed circuit boards
Facing OMD, in more ways than one, is Telephotonics. Like its English counterpart, Telephotonics builds waveguides using its optical amplification-specific integrated circuits. This approach is likened to that of building an optical printed circuit board. Telephotonics' primary market is high-bandwidth, all-optical, metropolitan communications networks.

Unlike OMD, Telephotonics makes and markets standard products. This is done in a series of cleanrooms, ranging from ISO Class 7 to ISO Class 6. There are, however, minienvironments in the manufacturing area that reach ISO Class 4 for the most critical manufacturing steps.

“When you build the core of the waveguide, which is really where most of the light will be traveling, that's where you have to be the most careful that you don't get dust particles,” explains Eldada of Telephotonics.

Once the cladding is on top of the core, the device becomes relatively immune to particles, and the cleanroom class can soar without impacting yield. According to Eldada, this high-cleanliness area is only about 10 percent of the total 5,000 square feet of ISO Class 6 cleanroom area. An additional 7,000 square feet are ISO Class 7.

One important contamination-control point is that ions have no affect optically on these waveguides. For that reason, there's little to no impact from airborne molecular contamination, and that affects airflow control and filtering. Another consequence is that there's no need to use ultra-pure water. The only requirement is for water that is clean of physical particles, and that impacts incoming water treatment.

This general tolerance for more contamination and particles also shows up in the waveguides themselves. A key metric is the rate of loss of photonic signal. In the core, a particle can disrupt the transmission of light and cause a loss of perhaps as much as a decibel (dB), or a decrease in signal strength of about 20 percent. That is roughly what the loss should be for the entire optical component. Given that the industry fights over losses of tenths of a dB, such a large decrease can't be tolerated. Outside of the core, however, this requirement relaxes considerably.

For instance, Eldada notes that the refractive index can be altered by local heating, and this effect can be used to modify the optical properties of the waveguide. Such heaters are not tiny devices.

“Effects happen over several millimeters, so if your heater width is ten microns and there is some very small or slow variation, the device essentially will still work,” comments Eldada.

With a German accent
Startup AXSUN Technologies plans to use a synchrotron-derived beam and some German-originated technology to miniaturize photonic devices, thereby cutting the size and cost of instruments. Company officials, for instance, indicate that AXSUN's channel monitor is essentially an optical spectrum analyzer. Thanks to advanced technology, AXSUN's version occupies only a few square inches of board space, yet it has very close to the performance of a 30-pound, $35,000 instrument.

The company has two manufacturing facilities in the US, one on the East Coast and the other on the West. Both have cleanrooms for microelectromechanical systems (MEMS) manufacturing. The West Coast facility is close to the Lawrence Livermore and Sandia National Laboratories and sports an ISO Class 5 cleanroom.

The proprietary manufacturing flow at AXSUN involves the use of a robotic vision pick-and-place to position optical components on a small substrate. Some of these components are fairly standard, but some are built using LIGA, a German acronym meaning lithography electroplating and molding. Developed in the 1980s and 1990s, LIGA enables the creation of tiny components and alignment structures. Because the parts are molded, they can be very tall, with much greater aspect ratios than is possible with other micromachining techniques.

A critical process step in LIGA is exposure to high-energy x-rays. For volume production, AXSUN reached an agreement with the Advanced Light Source at Lawrence Berkeley Labs. The company is operating its own private beam line of high energy x-rays. With LIGA and its own custom-developed technology, AXSUN will take photonic components and chips, place them onto a substrate, and pack them densely. Part of this manufacturing will take place in an ISO Class 5 cleanroom.

AXSUN has developed a number of manufacturing techniques and technologies and has built some of its own tools. These include the machine-vision recognition and robotic technology used to align components on the substrate, which is accomplished by the robot trimming the location of the LIGA structures.

According to company officials, AXSUN is at a critical juncture due to the current economic climate and the company's developmental stage. AXSUN is now concentrating its efforts on completing successful product introduction and deployment.

Changing focus
Codeon came to life in 1999 with a single focus: To make high-speed 40-GHz optical modulators for network communications and switching.

That has since changed in response to market conditions to include a 10-GHz family and other specialized products for particular customers. The company does this manufacturing in two adjacent buildings, with a mix of ISO Class 7 and ISO Class 6 cleanrooms. Like the company's focus, Codeon's Bradunas notes that these specifications can be modified.

“We have built-in robustness,” he says. “With more air available and more filters installed, we can change the rooms to make them Class 100s [ISO Class 5] or better.”

Such a relatively easy upgrade is possible because the buildings are constructed with the standard double ceiling. There's a HEPA filter on top and a diffuser down below. The return is through the floor. Testing has shown that the current cleanrooms actually are better than rated, with the best meeting ISO Class 5 standards.

The inside of the cleanroom at OMD in the United Kingdom.
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Given that the critical particle size is on the order of microns, it's doubtful that the process will need to go to higher levels of cleanliness. The lower airflow associated with the current setup offers some advantages. During the manufacturing process, fiber-optic strands have to be attached in the modulators. This is done semi-manually using a microscope. The work is delicate, and the strands could be disturbed by air currents. So, a lower and gentler airflow actually helps.

As for the future of Codeon's cleanrooms, Bradunas isn't sure if there will ever be a need for the kind of stringent specifications found in semiconductor fabs. Part of that has to do with some basic differences between devices that manipulate light and those that manipulate electrons.

“The ideal thing in the integrated circuit world is having no distance between transistors. Here we can't do that because, as things get smaller, the requirements to drive, power or manage them get bigger and bigger,” says Bradunas.

As for the future, there is no optoelectronic equivalent yet of Moore's law. Therefore, there is no drive toward greater cleanliness. That could change, however. On the horizon are very high levels of integration from the use of photonic crystals. Elements that today are millimeters in length will shrink to dimensions measured in microns. Then optoelectronics will face many of the same contamination-control challenges confronting semiconductors today.

It's not a problem that's likely to crop up soon, according to Eldada of Telephotonics. He observes, “If I had to put a number on it, I would say 15 to 20 years before something like that becomes commercially viable.”

Hank Hogan is a special correspondent to CleanRooms. He lives in Austin, Texas.


Some light in the market gloom
The optoelectronics market is suffering from an excess of success. A recent fiber-optic building spree tore up city streets and added considerable capacity to the world's telecommunications network.

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On a smaller scale, the same explosion happened to wireless systems. The resulting capacity surplus has lead to large losses and outright bankruptcies of telecommunications vendors and equipment makers. For the optoelectronics that power voice, video and data traffic, those market realities have made vendor expansion dreams turn into contraction nightmares.

Estimates are that the total worldwide optical component market shrunk 20 to 40 percent between 2000 and 2001. However, that decline hasn't been shared equally among all market segments.

“The market downturn in components and modules has been more pronounced than in systems, or end-user equipment, because systems vendors had been stockpiling components in 2000,” notes Peter Middleton, an optical components analyst with San Jose-based Gartner Dataquest.

The finished device with fibers attached (pigtailed) and the lid open for viewing.
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The outlook, on the other hand, is brighter—especially for companies offering advanced manufacturing techniques.

The Semiconductor Industry Association (SIA) of San Jose recently stated that future growth in the optical storage market will be driven by a rapid shift to writable CD drives and the increasing adoption of DVD technology. After a 22 percent decline in 2001, the SIA predicts modest growth in 2002, followed by 15 percent growth in 2003 and 20 percent growth in 2004.

A 2002 rebound is also forecast by Gartner Dataquest, says Middleton. However, he notes that vendors are interested in automated manufacturing technologies. He expects implementation in these new integrated optical components and improved manufacturing techniques will have a lasting impact on the industry.

“When growth returns to the market in the latter half of 2002, manufacturing operations will be less labor intensive than they were,” predicts Middleton. —Hank Hogan


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