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Photonics on CMOS is a candidate technology for applications where functional ... interconnect in advanced multiprocessor systems-on-chip. ... 45-, and 32-nm CMOS technology nodes, and extracted typical performance metrics, such as.

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Heterogeneous Integration of III-V Lasers on Silicon for Photonic/Electronic Convergence Fabien Mandorlo, Corrado Sciancalepore, Ian O’Connor, Pedro Rojo Romeo, Christian Seassal, Pierre Viktorovitch, Xavier Letartre Université de Lyon, Institut des Nanotechnologies de Lyon (INL), UMR CNRS 5270 Ecole Centrale de Lyon, 36 avenue Guy de Collongue, F 69134 Ecully Cedex, France Contact [email protected] Badhise Ben Bakir, Damien Bordel, Nicolas Olivier, Jean-Marc Fedeli CEA LETI, Minatec, 17 rue des Martyrs, F-38054 Grenoble cedex 9, France

Heterogeneous integration of III-V microlasers on silicon is discussed as a promising scheme for photonic-electronic convergence. The typical performances of an optical interconnect circuit are evaluated and two types of microlasers are proposed and compared. Photonics on CMOS is a candidate technology for applications where functional integration is needed for improving system performance while reducing size and cost. It can be apply to several applications such as information processing and optical sensing. The ITRS has identified Integrated optical interconnect as a potential solution to overcome limitations of metallic interconnect in advanced multiprocessor systems-on-chip. However, at the system level, the multiphysics nature of the design problem have contributed to severe difficulties in assessing its suitability. At the device level, one of the major issues lies in the difficulty to get efficient light sources which are compatible with CMOS (silicon) technology. At present, the more promising solution is provided by heterogeneous integration of III-V based lasers onto silicon through bonding technology [1,2]. To explore the impact of microsource characteristics on integrated optical interconnect performances, we developed a systematic, fully automated synthesis method capable of optimally sizing the interface circuits based on system specifications, CMOS technology data, and optical device characteristics. We applied this according to the specifications for 65-, 45-, and 32-nm CMOS technology nodes, and extracted typical performance metrics, such as data rate, dynamic and static power. During this study, we observed that the total interconnect power decreases with a rising number of interconnected processor cores. This is due to the fact that, while there are more links, each link is shorter, and that in optical interconnect, power increases exponentially with length and is dominated by the source power. For a 10x10 matrix of processor cores for the 32-nm printed gate length technology node, this analysis gives a total power of 33.3 W for source threshold currents of 1.5 mA. Compared to the figure given by the ITRS for maximum power at this technology node (167 W), this works out as equal to 20% of total power. Since source threshold current has a significant impact on static power, we also explored the impact of threshold current on interconnect power (see figure below).

Average static power (mW) for varying interconnect length and source threshold current for a 45 nm CMOS technology node.

The integration of complex photonic circuits with CMOS systems will require devices with small footprint such as microcavity based lasers. In this paper we will discuss two approaches. The first one, based on microdisk resonators, has given rise to numerous demonstrations including low threshold electrically driven microlasers efficiently coupled with a SOI microguide [2]

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In optical circuit based on resonant wavelength devices, the possibility to control and/or tune their resonant optical frequency is mandatory not only to match the components together but also to get reconfigurable functions. We have recently proposed [3] to exploit the coupling between the III-V microlaser and the silicon waveguide in order to promote a specific resonant mode for lasing operation. An outer loop is designed which aims at creating an additional feedback to modulate the coupling loss rate of the different whispering gallery modes (WGM). We have shown that, by heating the loop but keeping the laser temperature constant, we can tune the laser wavelength by mode hoping.

Power in the heater (mW) 3D schematic view of a microdisk laser with its electrical contacts and a feddback loop (top left); SEM top view of the device (center); output power for 2 lasing modes as a function of the electrical power injected in the heater (right)

The second approach we proposed follows a generic technological/conceptual scheme for 3D microphotonics on CMOS based on III-V/Silicon heterogeneous integration. The devices consist, in general, in the vertical and high optical index contrast pile up of semiconducting membranes, which may include a 1D or 2D photonic crystal (PC), along a so called 2.5D approach [4]. The operation of the devices is based on the exploitation of hybrid optical mode resonators which possess both wave-guided components in the PC membranes and radiated components across the membranes. This conceptual approach lends itself to the production of devices which are able to both operate in the wave-guided regime and provide communication between the different membrane guiding layers and with free space. It allows also for a vertical separation of the active material (III-V) and passive (silicon) membrane layer levels. From a theoretical point of view, we will show that a strong lateral and vertical optical confinement can be achieved and that an efficient coupling with a silicon waveguide can be designed. Optical characterization of 2.5D microlasers structures will be presented. x y z

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Schematic view of a 2.5D microlaser: 3D (left), top view (center), electric field distribution (Ey) of the laser mode (right)

This work has been supported by the European projects FP7-ICT WADIMOS and HELIOS REFERENCES [1] A.W. Fang; H. Park, O. Cohen, R. Jones; M.J. Paniccia; J.E. Bowers, “Electrically pumped hybrid AlGaInAs-silicon evanescent laser”, Optics Express 14, 9203 (2006), [2] J. Van Campenhout, P. Rojo Romeo, P. Regreny, C. Seassal, D. Van Thourhout, S. Verstuyft, L. Di Cioccio, J. Fedeli, C. Lagahe, and R. Baets, “Electrically pumped InP-based microdisk lasers integrated with a nanophotonic silicon-on-insulator waveguide circuit,” Opt. Express 15, 6744 (2007). [3] F. Mandorlo, P. Rojo-Romeo, X. Letartre, R. Orobtchouk and P. Viktorovitch, “Compact modulated and tunable microdisk laser using vertical coupling and a feedback loop”, Opt. Express 18, 19612 (2010) [4] P. Viktorovitch, B. Ben Bakir, S. Boutami, J.L. Leclercq, X. Letartre, P. Rojo-Romeo, C. Seassal, M. Zussy, L. Di Cioccio and Jean-Marc Fedeli, “3D harnessing of light with 2.5D photonic crystals”, Laser & Photon. Rev. 4, 401 (2010)

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