Silicon-On-Insulator based guided wave optical clock distribution K. E. Moselund, P. Dainesi, and A. M. Ionescu Electronics Laboratory Swiss Federal Institute of Technology
People and funding EPFL Project leader: Prof. Adrian M. Ionescu Post. Doc: Dr. Paolo Dainesi Ph.D. student: Kirsten E. Moselund Visiting academic host: Rodica Plugaru External partner: ST Microelectronics Thomas Skotnicki, Philippe Coronel, Matthieu Bopp (Ph.D.) Funding: Integrated Systems Center, EPFL.
Overview Introduction Optical interconnect Components for Optical Signaling Silicon based light sources Waveguiding Optical Modulator Different approaches GAA optical phase modulator Simulation GAA MOSFET at EPFL Intensity modulator SiGe based light detection Conclusion
Trends in Microelectronics Increase of complexity and functionality Transistors get smaller and faster. Clock signal frequency is raised. Interconnection cross section is squeezed. Interconnection length can become even longer. Some possible proposed classical solutions Line equalization. Use of repeater amplifier (on chip only) but with power and complexity penalty. On chip asynchronous networking (AMD 64). Our approach On-chip optical interconnect
Bandwidth and delay Electrical interconnect Electrical interconnect RC time constant does not scale with the technology, but remain constant. Synchronous clock distribution is problematic. New generations will face the RC-LC behavior boundary. ITRS 2003 ITRS 2003 Performance dominated by interconnects. Optical interconnect a solution? - critical length above which optical system is faster than electrical = a few mm. P. Kapur et al. 2002
Potentials On chip guided wave optical clock distribution Inherently synchronous for wafer-scale dimensions. Design does not need to be reconsidered for a change in modulation frequency. Immune to electromagnetic interference with adjacent electronics or RF signals. WDM and TDM are possible. Limitations Difficult to implement light sources, waveguide and detectors in the same substrate? High propagation and coupling loss. Challenges with regards to layout and process development. Dispersion may become relevant for long waveguides. Power still an issue, K. especially Moselund CSI for Seminar short Oct. links 25th (receiver dominated).
Building blocks of a photonic IC http://www.intel.com/technology/silicon/sp/
Components for on-chip optical signaling Waveguiding Low propagation losses (<1dB/cm). Low bending losses and small band radii Small waveguides with high Dn. Compact and low loss coupling devices. Waveguide dimension compatible with electronics size scale. Detection Monolithically integrated. Small size and low power. High efficiency. High speed. Clock source External: Low loss, high efficiency coupling (mode size converter or grating). Internal: Monolithically or hybrid light source and GHz rate modulator. SOI photonic wire SiGe detector External laser + modulator
Light Source
Wavelength Telecom: Short haul 850nm Long Haul 1.3-1.55um On-chip signaling: Visible Si for detection, and polymers for waveguiding. IR (>1.1um) Si is ideal for waveguiding, but requires more advanced detection scheme (SiGe, III-V s). Monolithic integration on the Si-chip
Silicon Light-Emitting Diode Structure Silicon Rich Oxide (SRO) doped with rareearth ions. SRO is SiO2 enriched with Si-nanocrystals. Sandwiched between n+ poly-si and p+ Si. http://www.st.com/stonline/press/magazine/challeng/3rdedi02/chal1.htm Small size of nanocrystals (1-2nm) Larger bandgap Greater excitation efficiency of rare earth ions. Quantum efficiencies (LED) ~20%.
Si-based light sources Raman amplification: Pump photon absorbed emission of signal photon (lower energy) + a phonon. Small effect need high pump intensity + very low absorption. Two-photon absorption: Creates electron/hole pairs, which absorb both pump and signal light. Turns off laser light. Unwanted effect! J. Faist, Nature 2005.
CW Silicon Raman Laser Rong et al. Nature 2005 SOI technology Tight confinement. Optical pump intensity = 25MWcm -2. Reverse-bias PiN diode removes electrons and holes from the waveguide. Continuous operation.
Waveguiding
Waveguiding - requirements Waveguiding Low propagation losses (<1dB/cm). Since optical signaling is only advantageous for global signals. Low bending losses and small band radii. Small waveguides with high Dn. Compact and low loss coupling devices. For splitters and/or multiplexers. Low-loss input/output coupling. Waveguide dimensions compatible with electronics size scale. Compatible with CMOS processing. Good control of material and surface quality. Scattering is the most important loss mechanism for nanowires.
Free space External light source broadcasted to different photodetectors. Issues: Holographic element required. Photogeneration outside PD area. Complicated, bulky(3d) and fragile. Guided wave External or internal light sources. Issues: Propagation losses. Bend losses. Light coupling (for external source). On chip clock distribution
Guided wave approach Polymer waveguide Visible region. Si as photodetector. Small Dn low confinement Large Devices Photonic wire Rib waveguide H = W ~ 1-2mm Modest coupling efficiency with optical fibers. Moderate confinement moderately long devices IR region (>1.1um) Good confinement very compact devices and small (2-5mm) radius bends. Very Low coupling efficiency requires coupling structure. Fraction of light travels outside the physical guide surface roughness is very important.
Sub-mm SOI Waveguides Illustration Process steps SOI P-type substrate: BOX=1mm, SOI Si=340nm Hard mask 20nm SiO2 (thermal) + 50nm Si3N4 Lithography Resist: S1805, 0.5um, Laser direct writing Resolution: 0.8mm Etch hard mask Etch waveguides Oxidation (400-600nm much less due to stress) Roughness reduction. Reduction of lateral dimensions. Deep etch Define waveguide and chip dimension (end coupling). Chip Dicing
Characterization? Nanoscale (0.3x0.8um) SOI waveguides on Si-chip, size limited by lithographic resolution. Deep etch in order to approach fiber to the guide (end coupling). 1. Substrate coupling, due to large size of fibre compared to guide. 2. missing corners at taper level, the origin of which is not established. 3. Currently working on the implementation of a polymer taper by spray-coating.
Polymer taper Spraycoating of polymer: Polymer taper as the last step before dicing. Technologically simple. Allows for high aspect ratio polymer deposition spin coating is not possible. Problems Adhesion of polymer. Uneven coating close to edge. Roughness of polymer layer. IR losses in the polymer.
Suspended Nanowires Rough waveguides Taper delamination Suspended waveguides Single-crystal Si waveguides on bulk Si. Smooth sidewalls due to oxidation. Anchors required during processing.
Characteristics Cleaved waveguide, surrounded by 200nm oxide and air. Dimensions ~630nm x 380nm. Characteristics Triangular or rectangular cross section. Dimensions from 1um down to a few 100nm. Surrounded by air or oxide Taper at end ~0.4x1um. Problems Light coupling has been unsuccessful so far. Polymer taper is not readily implemented. Possible polymer deposition at input, due to deep etch process. Very fragile during CMP.
Light Modulator (CSI funded project)
Optical phase modulator previous work Functionality Modulation of refractive index by free carrier injection phase modulation of propagating light. P. Dainesi et al. Photon. Tech. Lett. 2000 M. mazza et al. Transducers. 2003 PiN - diode based High modulation efficiency VpL= 0.2 Vcm. Low speed limited by recombination -100 s MHz. Current injection heating. Schottky diode based Potentially faster than PiN, but still limited by recombination. Work in progress by R. Plugaru and M. Mazza
Optical phase modulator capacitive operation A. Liu et al. Nature 427 (6975) 2004 MOS Capacitive Operation High speed (1GHz achieved). Low efficiency VpL= 10 Vcm Negligible static power consumption. Novel GAA MOS modulator Si-core, gate oxide and Poly serves as waveguide. High efficiency VpL= 0.47 Vcm (ideal). High loss ~30dB/cm. K. Moselund et al. Transducers K. 2005 Moselund CSI Seminar Oct. 25th
Gate-All-Around Phase Modulator GAA optical modulator Operation mode: accumulation or inversion. Creation of thin charge region at both sides of the interface. No problem with corner effect turned into a positive effect.
Optimum dimensions Shrinking of core dimension The fraction of light confined to the modulated region is reduced. The average modulation in the core is increased. There exists an optimum dimension with respect to efficiency
Transient performance Switching speeds in the 10s of GHZ (device only). High Doping High speed, but also high loss. Speed proportional to source-drain distance a series connection K. Moselund of several CSI Seminar shorter Oct. 25th devices is required.
Intensity modulator Mach-Zender interferometer. Easy to implement. Large (p-phase shift). Intensity modulator Fabry-Perot resonator Resonant structures Small phase shift required (value depends on finesse) Compact. Very sensitive to optical loss difficult to design. Normalized transmission 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 R = 87.8% T = 11.9% " scat = 0.3% L cav = 80#/2n OFF state " cav = 0 (ideal) " cav = 30dB/cm " cav = 100dB/cm!" si-core = 9dB/cm 0.0 1.548 1.550 1.552 1.554 Wavelength (µm) ON state P. Dainesi et al. CLEO 2005
Micro Ring resonator R. Baets et al., PTL 16 (5), 2004 V. R. Almeida et al. Nature 431 2004 S. Pradhan et al. CLEO 2005 Micro ring resonator No need for Bragg gratings. Good control of gap required. Vertical implementation is possible. Active functionality using a PiN diode has been implemented.
Implementation based on utility Low doping NA = 10 17 ND = 10 18 Modulation depth = 91% Loss = 3 db Max. frequency = 38 GHz Modulation depth = 91% Loss = 1.5 db Max. frequency = 38 GHz Low Loss! High doping NA = 10 18 ND = 10 19 Modulation depth = 85% Loss = 6 db Max. frequency = 80 GHz High Speed! Modulation depth = 85% Loss = 4.5 db Max. frequency = 80 GHz P. Dainesi et al. CLEO 2005
Short term goals (1 year) Experimental validation of new GAA MOSFET. different architecture for light propagation, compared to previously fabricated GAA at CMI. Finalization of suspended wire fabrication technology. Surface roughness, anchor dimensions. Experimental validation of light modulation based on the suspended nanowire approach. No resonant cavity in this technology! Enhanced modeling of GAA light modulator device based on experimental data.
Light detection
SiGe Detectors Ge absorbs in the IR. SiGe detectors SiGe already used in CMOS technology (mobility enhancement). 4% lattice mismatch between Si and Ge strain, dislocations. H. Presting 1998
Conclusion Optical interconnects is a possible candidate for future on-chip global signal distribution. Capacitive modulation in a MOS structure is extremely fast and with low power consumption. Using a GAA in a resonant cavity approach it should be possible to combine both high efficiency and high speed operation. Light coupling into sub-mm structures is extremely difficult, and a solution to this problem must be found. Efficient light emission and detection are still critical issues, but not prohibitive.