Defect inspection system helps yield better phase masks

By Mark A. DeSorbo

FARUM, Denmark—After a “run-in” period, Ibsen Photonics A/S says the manufacturing process of its holographic phase mask line has been successfully relocated from an ISO Class 5 to a newly built 14,000 square-foot ISO Class 4 cleanroom facility in this Danish community, located 13 miles from Copenhagen.

Phase masks play a vital role in the telecommunications and sensing markets, namely in the manufacturing of fiber Bragg gratings (FBGs), says Kristian Buchwald, product manager.

Any defects can weaken transmissions, so cleanliness of masks, he adds, is vital since the units are essentially the power points of FBGs and optical components, where light—whether it is through optical fiber or from a laser—is harnessed, filtered and multiplexed to perform such feats as cramming 30,000 telephone calls into one signal.

“We have always produced our phase masks in our own cleanroom, but we can clearly see improvement in defect counts on the phase masks over the last months caused by the relocation of the manufacturing line to our new [ISO Class 4] cleanroom from an [ISO Class 5] cleanroom previously,” Buchwald adds.

Specifically, phase masks are used in two primary applications, the first of which involves inserting them directly into telecommunication network signal streams carrying voice and data traffic, says Dirk Jessen, Ibsen's vice president of sales and marketing. The second application, he says, is writing gratings on wafers used in telecom, sensing or biomedical applications.

Contamination, Jessen adds, adversely affects transmission properties even in a Class 4 cleanroom, especially with the presence of people. “Process contamination also plays a part as lithographic processes can contaminate with photoresist, chemicals, etc,” he says. “Signal quality is of utmost importance. The FBG should add as little noise as possible to the signal. Any defect on the phase mask will transfer to the FBG, and so the jump from Class 5 to Class 4 provides a significant quality improvement.”

During the three-month run-in period, Ibsen claims it has experienced improvements of more than 50 percent in the cleanliness of its phase masks using its in-house-developed defect inspection system.

The system, Jessen says, is coupled with a cleanroom environmental monitoring system, and evaluates and monitors individual process cleanliness as well as the overall improvements stemming from the ISO Class 4 upgrade. The system also locates defects on grating structures—a complex task because grating structures are sub-micron, which is the same order of magnitude as the flaws.

“In connection with the move of our phase mask production to the new cleanroom, we tightened general procedures and retrained our employees,” Jessen says. “As we are using our defect inspection system…to analyze various manufacturing steps, we have improved on some of the handling processes in critical areas.”

Officials at Ibsen Photonics A/S say the move from an ISO Class 5 to an ISO Class 4 cleanroom has done wonders for its phase mask line.
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