10Gbit/s transceiver on silicon Jeremy Witzens a , Gianlorenzo Masini a , Subal Sahni a , Behnam Analui a , Cary Gunn a , Giovanni Capellini b a Luxtera, Inc, 2320 Camino Vida Roble, Carlsbad, CA, USA 92011; b Physics Department, University 'Roma Tre', Via Vasca Navale, 84, 00146 Roma, Italy ABSTRACT We discuss our approach to monolithic integration of Germanium photodectors with CMOS electronics for high speed optical transceivers. Integration into the CMOS process and optimization of optical coupling into the devices is described, followed by a discussion on how the devices are deployed in 4×10 Gbs receiver and transmitter subsystems. We demonstrate -19 dBm optical sensitivity for a bit error rate of 1e-12. An improvement of several dB resulted from optimizing the transimpedance amplifier relative to a design that was targeted for hybrid integration with flip-chip photodetectors, in order to take advantage of the drastically reduced capacitance of the integrated photodetectors (below 20 fF). As an example of how the versatility of on-chip waveguides and integrated photodiodes can be used, we further describe how the Germanium photodetectors are deployed to obtain a fully autonomous Mach-Zehnder interferometer subsystem with built-in monitoring and control, that can be instantiated as a single cell in an IC design. A fully differential layout is implemented for optical, electro-optic and electrical components yielding very small mismatch between components and enabling control of the interferometer with a minimum penalty. Silicon Photonics, Germanium Photodetectors, Waveguide Photodetectors, High-speed Transceivers, Integrated Optics 1. INTRODUCTION Since the early pioneering works of Soref on the electro-optical properties of Si in the near infrared [1], the promises of monolithic integration of optical components on a Silicon (Si) platform have fuelled an increasing amount of interest in the research community over the last two decades, and has led several companies to commercially pursue the development of silicon photonics in pursuit of technological advantages over hybrid approaches, such as lower production cost, compactness, channel count, scalability to high volume production and wafer scale testability. In some cases integration can also change in more fundamental ways how opto-electronic building blocks interact with electronic circuitry, by removing the parasitic capacitance associated with flip-chip or wire bonding and by enabling easy, distributed access to optoelectronic components. For example, Luxtera has implemented a distributed Mach Zehnder interferometer (MZI) driver that drastically reduces power consumption [2]. This design is facilitated by the integration of the driver and the high speed phase modulator on the same chip, since a very large number of electrical contacts between these two building blocks are needed. Several optical building blocks have been demonstrated already on Si: waveguides [3], high speed modulators [4], photodetectors [5], as well as complete transceivers. Luxtera recently introduced a 40 Gbps (4 channel, 10Gbps each) monolithic optical transceiver built on a Si CMOS platform [2]. While waveguiding and modulation can be done in Si and do not require the integration of new materials in the CMOS process, photodetection at long wavelengths (1.3 and 1.55 um) requires either Germanium or a III-V compound such as InGaAs. The former is preferred over the latter due to its better compatibility with the Si technology in terms of lattice structure and parameter and contamination concerns. In this paper we first describe our approach to integration of a Germanium (Ge) module for photodetectors in LuxG, Luxtera's optical-enabled CMOS process which is based on a commercial CMOS process. We will show how constraints linked to process thermal budget and to geometry dictate where in the CMOS process the Ge module is inserted, and why a waveguide photodetector (WPD) configuration is preferred. We will then discuss how the optical coupling from the silicon waveguides into the waveguide photodiodes was optimized. The second half is devoted to the integration of the photodiodes into transmitter (Tx) and receiver (Rx) subsystems. In particular we will demonstrate how the reduced capacitance of the integrated detectors can be exploited to yield an enhanced sensitivity after adjusting the connected transimpedance amplifier (TIA). The very small footprint of the WPD, combined with the versatile routing provided by on-chip single mode waveguides, enables the integration of complex control and monitoring functionalities on the chip at Invited Paper Silicon Photonics and Photonic Integrated Circuits, edited by Giancarlo C. Righini, Seppo K. Honkanen, Lorenzo Pavesi, Laurent Vivien, Proc. of SPIE Vol. 6996, 699610, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.786641 Proc. of SPIE Vol. 6996 699610-1 2008 SPIE Digital Library -- Subscriber Archive Copy