The dominance of SiO₂ -based integrated optics is being challenged by new materials that offer higher integration density and improved sensitivity in sensing applications.

Integrated optics circuits are typically made using doped SiO₂ for both core and cladding material to attain low attenuation per centimeter (0.05 dB/cm or better). To achieve low attenuation, SiO₂ surface waveguides typically have very low contrast ( 1% difference in refractive index between core and cladding). Unfortunately, low-contrast means that the propagating optical mode is only weakly confined to the core. As a result, optical circuit features must have a minimum waveguide bending radius of tens of millimeters, and a minimum spacing of at least 100 µm between waveguides to avoid optical cross-talk.

In addition, the thickness of the layers that compose SiO₂ waveguides often exceed 20 µm. It is difficult to control doping level and residual stress (and thus, modal birefringence) in such thick films, which leads to propagation problems such as higher attenuation, polarization-dependent loss, and polarization-mode dispersion. As a result, SiO₂ integrated optic circuits generally require substantial chip area and are expensive to produce.

SiON and SiN waveguides
Alternative materials, such as silicon oxynitride (SiON) and silicon nitride (SiN) might provide solutions to many of the issues with SiO₂ . Work at the Univ. of Twente, Enschede, The Netherlands, and IBM's Zurich Research Lab, where the SiO₂ core layer has been replaced by SiON, demonstrates that low-loss, compact optical circuits can be made. Through control of the gas mixture during plasma-deposition, the refractive index of SiON can be tailored from 1.46 (SiO₂ ) to 1.98 (SiN). Therefore, SiON can be used to form waveguides with contrast that can range from 1% to as high as 35%.

Click the image to enlarge
(a) Schematic of a Mach-Zehnder Interferometer (MZI) sensor based on Si3 N4 -core waveguide. Modulation of the output signal by the zinc-oxide electro-optic modulator enables higher-sensitivity measurement. (b) Photo of a commercially- available, 5-gas, environmental optical sensor based on MZI technology (http://www.mierijmeteo.demon.nl/).

Low- to medium-contrast ( 1 to 5%) waveguides are useful for telecommunications applications. Until recently, the focus has been on developing SiON waveguides with a contrast of ~ 2.5%, which allows for a minimal bending radius of ~ 500 µm. The results have been surprisingly good. With typical attenuation levels over the telecom bands below 0.10 dB/cm, plus low modal birefringence, the technology has been adopted by foundries like NKTI, Denmark (http://www.nkti.com/), and LioniX, the Netherlands (www.lionixbv.com), which have installed dedicated production lines.

For optical sensors, higher contrast (up to 35%) waveguides are more useful, and therefore, core materials closer to SiN are attractive. Due to its high index, very tight curves can be designed in SiN. Thin layers of this material, typically 100 nm, readily enable definition of single-mode channels, which have very high surface sensitivity and very large modal birefringence, factors that make these waveguides extremely attractive for optical sensing applications. Attenuation levels are roughly 0.5dB/cm to 1dB/cm in the visible, and 0.2 to 0.5 dB/cm in the infrared.

For its high-contrast waveguides, LioniX deposits stoichiometric silicon nitride (Si₄ ) using low-pressure chemical vapor deposition (LPCVD) rather than plasma-deposited SiN. According to Hans van den Vlekkert, CEO of LioniX, "LPCVD-deposited stoichiometric material is predictable and repeatable from run-to-run. In addition, since it is a batch-process, production costs can be lower than for plasma-deposited films."

Stoichiometric films of SiO₂ and Si₄ form the basis of a new waveguide technology recently reported by LioniX. Although still in development, experiments on 1x1 µm waveguides have demonstrated an attenuation level of roughly 0.10 dB/cm. According to Rene Heideman, CTO of LioniX, "The challenge with this very high-contrast waveguide is to further reduce the attenuation level and to perfectly control modal birefringence. With our new waveguides, we have achieved both. We feel it could be a breakthrough for applications in the telecom field."

But challenges do remain for SiON and SiN waveguides, in particular, how to couple optical fibers without significant loss. For butt-coupled SiN waveguides, for example, loss can exceed 15 dB. Progress toward efficient fiber-couplers is, however, being made. According to Heideman, "Through a combination of micromachining and integrated optic technology, the propagation mode that exits the waveguide can be better matched to the optical fiber. Coupling-efficiency that exceeds 90% has been achieved in some cases."

High-sensitivity optical sensor
LioniX has leveraged their expertise in SiON and SiN waveguide technology into the development of high-sensitivity optical sensors based on an integrated Mach-Zehnder interferometer (MZI). Each arm of the MZI sensor contains a substantially identical window, on which a layer of material is deposited whose refractive index is affected by absorption of a target gas. The material on the sensor arm is exposed to ambient air, while that on the reference arm is protected. In the presence of target gas, a change in the index of the sensor chemical affects the phase of the light in the sensor arm.

Through the appropriate choice of material, sensors for any one of a wide variety of gases, compounds, or organic materials can be fabricated. Additionally, since high-contrast wave- guides enable high-density integration, multiple interferometers and therefore multiple sensors can be fabricated on a single chip. Figure b shows a five-gas atmospheric sensor available from Mierij-Meteo, De Bilt, The Netherlands (www.lionixbv.com/services/ r_mierij.html) that is temperature insensitive, low power, and immune to electro-magnetic interference.

Regime change
Although SiO₂ -based surface waveguides have proven their worth, particularly in telecommunications appli- cations, challenging manufacturing issues and large real estate requirements combine to limit their utility in many other applications. SiON and SiN-based waveguides have been demonstrated to have suitable optical properties for optical telecommunications as well as optical sensors. These new waveguide materials might provide the solutions necessary to open new application spaces and even perhaps displace SiO₂ in many existing applications due to their higher sensitivity and lower fabrication cost.

--James Walker
Founder and President of JayWalker Consulting
http://www.jaywalkerconsulting.com/

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