Planar Optical Waveguides: A Hot New Application
Peter Singer, Editor-in Chief -- Semiconductor International, 1/1/2001
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One critical application in the optical arena is dense wavelength division multiplexing (DWDM), used to separate a beam of light into "channels." Two years ago, DWDM systems achieved 16 channels with 200 GHz channel spacing; today, they offer 40 channels or more with 50 GHz channel spacing. The challenges of producing DWDM devices include the narrowness of channel spacing, cost, reliability and manufacturability. Although several approaches have been developed, including diffraction gratings and cascaded interferometers, one of the most promising is the planar array waveguide.
Beyond planar array waveguides, other types of planar lightwave circuits (PLCs) that are fabricated on silicon include add/drop components, optical attenuators, ring resonators, space switches and thermo-optic matrix switches. Analysts are expecting a 50% annual growth rate through to 2003, at which point it will be a $7B business.
Planar array waveguides and other PLCs consist of a few layers of glass deposited on a silica or silicon substrate. In an approach pioneered by IBM, for example, silicon-oxynitride (SiON) is used as the core layer (Figure). The film has an atomic fraction of 7%-8% nitrogen and results in a minimum bending radius of about 1.5 mm. The ridge-type waveguide channel is formed by reactive ion etching and has a width and height of 3.0 and 1.9 µm, respectively. IBM researchers say the as-grown SiON planar waveguides show high propagation losses caused by the vibrational overtone of the N-H bond. However, annealing the films in a nitrogen atmosphere at 1140°C reduces the H content of the film and hence the N-H induced absorption loss.
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The composition of the glass must be carefully controlled to present the correct index of refraction to the incident light. Several devices may be produced on a single wafer (i.e., four to eight devices from a 4 in. wafer). The primary challenge is to control layer thickness, composition and defect inclusions. Temperature stability is often an issue, requiring active heating to bring the devices above ambient temperature.
Trikon recently sold an Omega MORI etch system to Optical Micro Devices (OMD), which aims to be the world's first optoelectronic foundry for this type of device. OMD has $23M in funding, and is setting up a 1000 m2 cleanroom capable of producing 1000 8 in. silicon wafers per month. Customers are said to include giants Nortel and Agilent.
Thomas said the PLC devices produced today are not active devices yet, but people are already talking about putting on metalization that will enable the refractive index to be changed based on the voltage applied. "Ultimately, because these things are being made on silicon, the ultimate aim is to have circuitry built on wafer that does some of the switching as well, or least helps with the optical switching," Thomas said. •
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