Silicon on Insulator is an ideal platform for largescale nanophotonic integration. We show that tight process control is needed for well-functioning filters, and discuss a number of devices based on these filters

Baets, Roel

Bogaerts, Wim

Dumon, Pieter

Selvaraja, shankar kumar

Van Thourhout, Dries

English

Ghent University Academic Bibliography

Silicon Nanophotonic Waveguide Circuits andDevicesWim Bogaerts, Pieter Dumon, Shankar Kumar Selvaraja, Dries Van Thourhout and Roel BaetsIMEC - Ghent University, Department of Information Technology, 9000 Gent, BelgiumEmail: wim.bogaerts@intec.ugent.beAbstract—Silicon on Insulator is an ideal platform for large-scale nanophotonic integration. We show that tight processcontrol is needed for well-functioning filters, and discuss anumber of devices based on these filters.I. INTRODUCTIONSilicon on Insulator (SOI) has already proven itself as avery suitable material for integrated photonics [1]. Not onlybecause of the high refractive index contrast, which allows forsubmicron waveguides and sharp bends with radii of 3µm orless [2], but also because the material can be processed withthe same tool set as used for electronics manufacturing. Thiscombination, a small footprint and an a potential industrialmanufacturing base, gives silicon photonics the scalabilityneeded for low-cost, large-volume applications.We use these photonic wires to implement a variety ofwavelength-selective components. These include ring res-onators, but also arrayed waveguide gratings scaled to verysmall footprints. However, the submicron dimensions of thephotonic wires make these filters exceptionally sensitive toprocess variations. While in the end there will always be theneed for a tuning mechanism, good process control can alreadybring the device very close to its desired specification. This, inturn, reduces the power consumption required for tuning. Weshow that this is possible with classical CMOS technology.Finally, we also discuss a number of device concepts whichcan only become viable through SOI nanophotonics, becauseof cost and volume considerations.II. PASSIVE WAVELENGTH-SELECTIVE COMPONENTSThe high contrast photonic wires in SOI allow for verysharp bends, which in turn reduces the footprint of manycommon waveguide components. Because of that, wavelengthselective elements such as ring resonators can be made verycompact, thus they can have a very large free spectral range[2]. Also, the relatively strong dispersion of photonic wires(ng 4.0) allows for shorter delay lines. This, in combinationwith acceptable propagation losses of a few dB/cm [3], [1],makes photonic wires ideal to construct wavelength selectivecomponents.A result of this is shown in Fig. 1. This 8 × 400GHzarrayed waveguide grating (AWG) has a footprint of only200×350µm2. Because of the tight process control (discussedin the next section) the path length difference in the arms iswell controlled, resulting in a low crosstalk level of −25 dB;the lowest reported in a demultiplexer device of this size with50µm-40-30-20-1001545 1550 1555 1560 1565 1570 1575 1580wavelength [nm]A WG t ra ns mi ss io n [ dB ]channel181 2 3 4 5 6 7 81.31.1Fig. 1. Compact 8-channel arrayed waveguide gratinga similar channel spacing. Also, by using a combination ofdeep and shallow etch layers [2], the insertion loss of theentire AWG is only 1.1 dB.III. PROCESS CONTROLWith waveguide dimensions of 500nm, photonic wires fallwell within the fabrication capabilities of high-end CMOSmanufacturing tools. However, the fabrication tolerances ofphotonic components are much more strict that for electronics,where margins of 5% or 10% are acceptable. For wavelength-selective filters, a deviation of waveguide width of 1nm willcorrespond to approximately 1nm wavelength shift. Whilesuch a shift can be compensated for by thermal or other tuningmechanisms, the acceptable tolerances are still of the orderof 1%. However, we have demonstrated that with reasonableprocess control, these tolerances can be met with standardCMOS fabrication tools [4].304TuU1 (Invited)1:30 PM – 2:00 PM978-1-4244-1932-6/08/$25.00 ©2008 IEEET ra ns mi ss io n [ dB ]0-10-20-30-401520 1530 1540 1550 1560 1570 1580Wavelength [nm]T ra ns mi ss io n [ dB ]0-10-20-30-401520 1530 1540 1550 1560 1570 1580Wavelength [nm]248nm193nmFig. 2. Transmission of 6 nominally identical Mach-Zehnder interferometers.Top: made with 248nm deep UV lithography. Bottom: with 193nm.One of the critical steps in the fabrication process is thelithography. While for research purposs e-beam lithographyis attractive for its flexibility and high resolution, we makeuse of standard CMOS tools, based on optical projectionlithography at deep UV wavelengths (248nm and 193nm).The resolution of these tools is typically less than that ofhigh-end e-beam lithography, and this is not only reflectedin final device geometry, but also in the uniformity andreproducibility of the fabrication process. This is illustratedin Fig. 2, where we plotted the transmission of 6 nominallyidentical Mach-Zehnder interferometers on a single chip. Thedesign was fabricated from the same mask plate both with248nm and 193nm lithography. It is clear that the highestresolution process gives the best uniformity, with deviationsof the transmission dip wavelength of approximately 1nm.IV. PHOTONIC WIRE DEVICESWavelength-selective components can be used for variousapplications. In communications, wavelength division multi-plexing is the obvious candidate, and in its simplest form, avery low-cost duplexing system based on SOI photonic waveg-uides could drive wide deployment of fiber access networks[5]. The WDM principle could also be applied for wavelengthrouting of high-speed signals between chips or boards in high-performance computing systems [6]. When used together withfiber arrays, the actual chip of the routing backplane, has asmaller footprint than the connector, allowing for an elegantpackaging approach [7].The sharp wavelength response of a ring resonator canalso be applied for optical sensors. The group index of thewaveguide can be changed modified by an outside influence,such as mechanical stress, temperature or the presence ofspecific molecules in the cladding material. Especially thelatter property is interesting. In SOI photonic wires, about20% of the optical power is located in the top cladding ofthe waveguide (either air or an overlay material). With aring resonator one can measure index changes smaller than10−4. In addition, by chemically activating the surface of thering the sensor can be made extremely sensitive to specificbiomolecules, which enables bacterial and virus detection. Insuch miniaturized label-free biosensors the light can feel thechange in the top cladding when biomolecules attach to achemically functionalized layer above the resonator [8].V. CONCLUSIONSOI Photonic wires allow for very compact wavelengthselective functions, which can be used in a variety of devices,from telecommunications to biosensing. Modern CMOS fab-rication technologies now allow sufficient process control tomeet the strict tolerances required for such applications.ACKNOWLEDGMENTPart of this work was supported by the European Unionthrough the IST-PICCO, IST-PICMOS and ICT WaDiMOSproject and the IST-ePIXnet Network of Excellence. Part ofthis work was supported by the Belgian Science Policy IAP-PHOTONnetwork and IAP-photonics@be projects, and theInstitute for the Promotion of Innovation through Scienceand Technology in Flanders (IWT-Vlaanderen) under the SBOepSOC project.REFERENCES[1] W. Bogaerts, R. Baets, P. Dumon, V. Wiaux, S. Beckx, D. Taillaert,B. Luyssaert, J. Van Campenhout, P. Bienstman, and D. Van Thourhout,“Nanophotonic waveguides in Silicon-on-insulator fabricated with CMOStechnology,” J. Lightwave Technol., vol. 23, no. 1, pp. 401–412, 2005.[2] W. Bogaerts, P. Dumon, D. Van Thourhout, D. Taillaert, P. Jaenen,J. Wouters, B. S., V. Wiaux, and R. Baets, “Compact wavelength-selectivefunctions in silicon-on-insulator photonic wires,” J. Sel. Top. QuantumElectron., vol. 12, no. 6, pp. 1394–1401, December 2006.[3] Y. A. Vlasov and S. McNab, “Losses in single-mode silicon-on-insulatorstrip waveguides and bends,” Opt. Express, vol. 12, no. 8, pp. 1622 –1631, April 2004.[4] S. Selvaraja, W. Bogaerts, D. Van Thourhout, and R. Baets, “Fabricationof uniform photonic devices using 193 nm optical lithography in silicon-on-insulator,” Proc. ECIO, p. FrB3, 2008.[5] G. Roelkens, D. Van Thourhout, and R. Baets, “Silicon-on-insulatorultacompact duplexer based on a diffractive grating structure,,” Opt.Express, vol. 15, no. 16, p. 10091, 2007.[6] P. Dumon, W. Bogaerts, D. Van Thourhout, D. Taillaert, R. Baets,J. Wouters, S. Beckx, and P. Jaenen, “Compact wavelength router basedon a silicon-on-insulator arrayed waveguide grating pigtailed to a fiberarray,” Opt. Express, vol. 14, no. 2, pp. 664–669, January 2006.[7] L. Zimmermann, H. Schroeder, W. Bogaerts, P. Dumon, and T. Tekin,“epixpack - advanced smart packaging solutions for silicon photonics,”Proc. ECIO, p. WeB2, 2008.[8] K. De Vos, I. Bartolozzi, E. Schacht, P. Bienstman, and R. Baets, “Silicon-on-insulator microring resonator for sensitive and label-free biosensing,”Opt. Express, vol. 15, no. 12, pp. 7610–7615, 2007.305

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Silicon nanophotonic waveguide circuits and devices

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