The high bandwidth density and low power consumption characteristics of silicon photonics devices can provide
a high performance interconnect solution for multiprocessor systems. At the same time this technology also
poses a new set of constraints and challenges in architecting, designing, and integrating such systems.
The "macrochip" multiprocessor architecture leverages a photonically interconnected array of processor and/or
memory chips to provide a flexible platform to build heterogeneous systems. The design considerations for such
a system are influenced largely by the system architecture, the programming model and devices needed for
their implementation. This talk will first describe the macrochip platform, technology constraints and potential
interconnect solutions with the various device building blocks. Then it will present some topology choices that
range from a WDM point-to-point interconnect to more complex switched data channel networks. It will close
with a detailed analysis of these design choices and show the impact of the device constraints on performance
and power consumption along with some recent ultra-low power device implementation results.
Electroabsorption from GeSi on silicon-on-insulator (SOI) is expected to have promising
potential for optical modulation due to its low power consumption, small footprint, and more
importantly, wide spectral bandwidth for wavelength division multiplexing (WDM) applications.
Germanium, as a bulk crystal, has a sharp absorption edge with a strong coefficient at the direct
band gap close to the C-band wavelength. Unfortunately, when integrated onto Silicon, or when
alloyed with dilute Si for blueshifting to the C-band operation, this strong Franz-Keldysh (FK)
effect in bulk Ge is expected to degrade. Here, we report experimental results for GeSi epi when
grown under a variety of conditions such as different Si alloy content, under selective versus non
selective growth modes for both Silicon and SOI substrates. We compare the measured FK effect
to the bulk Ge material.
Reduced pressure CVD growth of GeSi heteroepitaxy with various Si content was studied
by different characterization tools: X-ray diffraction (XRD), atomic force microscopy (AFM),
secondary ion mass spectrometry (SIMS), Hall measurement and optical transmission/absorption
to analyze performance for 1550 nm operation. State-of-the-art GeSi epi with low defect density
and low root-mean-square (RMS) roughness were fabricated into pin diodes and tested in a
surface-normal geometry. They exhibit low dark current density of 5 mA/cm2 at 1V reverse bias
with breakdown voltages of 45 Volts. Strong electroabsorption was observed in our GeSi alloy
with 0.6% Si content having maximum absorption contrast of Δα/α ~5 at 1580 nm at 75 kV/cm.
Silicon photonics is envisioned as a promising solution to address the interconnect bottleneck
in large-scale multi-processor computing systems, owing to advantageous attributes such as wide
bandwidth, high density, and low latency. To leverage these advantages, optical proximity coupler is
one of the critical enablers. Chip-to-chip, layer-to-layer optical proximity couplers with low loss,
large bandwidth, small footprint and integration compatibility are highly desirable. In this paper, we
demonstrate chip-to-chip optical proximity coupling using grating couplers. We report the
experimental results using grating couplers fabricated in a photonically-enabled commercial 130nm
SOI CMOS process.
Ring waveguide resonating structures with high quality factors are the key components servicing silicon
photonic links. We demonstrate highly efficient spectral tunability of the microphotonic ring structures
manufactured in commercial 130 nm SOI CMOS technology. Our rings are fitted with dedicated heaters
and integrated with silicon micro-machined features. Optimized layout and structure of the devices result in
their maximized thermal impedance and increased efficiency of the thermal tuning.
We present a hybrid integration technology platform for the compact integration of best-in-breed VLSI and photonic
circuits. This hybridization solution requires fabrication of ultralow parasitic chip-to-chip interconnects on the candidate
chips and assembly of these by a highly accurate flip-chip bonding process. The former is achieved by microsolder bump
interconnects that can be fabricated by wafer-scale processes, and are shown to have average resistance <1 ohm/bump
and capacitance <25fF/bump. This suite of technologies was successfully used to hybrid integrate high speed VLSI chips
built on the 90nm bulk CMOS technology node with silicon photonic modulators and detectors built on a 130nm
CMOS-photonic platform and an SOI-photonic platform; these particular hybrids yielded Tx and Rx components with
energies as low as 320fJ/bit and 690fJ/bit, respectively. We also report on challenges and ongoing efforts to fabricate
microsolder bump interconnects on next-generation 40nm VLSI CMOS chips.
Scaling of high performance, many-core, computing systems calls for disruptive solutions to provide ultra energy
efficient and high bandwidth density interconnects at very low cost. Silicon photonics is viewed as a promising solution.
For silicon photonics to prevail and penetrate deeper into the computing system interconnection hierarchy, it requires
innovative optical devices, novel circuits, and advanced integration. We review our recent progress in key building
blocks toward sub pJ/bit optical link for inter/intra-chip applications, ultra-low power silicon photonic transceivers. In
particular, compact reverse biased silicon ring modulator was developed with high modulation bandwidth sufficient for
15Gbps modulation, very small junction capacitance of ~50fF, low voltage swing of 2V, high extinction ratio (>7dB)
and low optical loss (~2dB at on-state). Integrated with low power CMOS driver circuits using low parasitic microsolder
bump technique, we achieved record low power consumption of 320fJ/bit at 5Gbps data rate. Stable operation with biterror-
rate better than 10-13 was accomplished with simple thermal management. We further review the first hybrid
integrated silicon photonic receiver based on Ge waveguide photo detector using the same integration technique, with
which high energy efficiency of 690fJ/bit, and sensitivity of ~18.9dBm at 5Gbps data rate for bit-error-rate of 10-12 were
Ring waveguide resonating structures with high quality factors are the key components in the silicon photonics portfolio
boosting up its functionality and circuit performance. Due to a number of manufacturing reasons their peak wavelengths
are often prone to deviate from designed values. In order to keep the ring resonator operating as specified, its peak
wavelength then needs to be corrected in a reliable and power efficient way. We demonstrate the performance of the
thermally tunable mux/demux filter ring structures fabricated in the commercial 130 nm SOI CMOS line.
In this paper we present a computing system that uniquely leverages the bandwidth, density, and
latency advantages of silicon photonic interconnects to enable highly compact supercomputerscale
systems. We present the details of an optically enabled "macrochip" which is a set of
contiguous, optically-interconnected chips that deploy wavelength-division multiplexed (WDM)
enabled by silicon photonics. We describe the system architecture and the WDM point-to-point
network implementation of a "macrochip" providing bisection bandwidth of 10 TBps and discuss
system and device level challenges, constraints, and the critical technologies needed to implement
this system. We present a roadmap to lowering the energy-per-bit of a silicon photonic
interconnect and highlight recent advances in silicon photonics under the UNIC program that
facilitate implementation of a "macrochip" system made of arrayed chips.
The Ultra-performance Nanophotonic Intrachip Communication (UNIC) project aims to achieve unprecedented high-density,
low-power, large-bandwidth, and low-latency optical interconnect for highly compact supercomputer systems.
This project, which has started in 2008, sets extremely aggressive goals on power consumptions and footprints for
optical devices and the integrated VLSI circuits. In this paper we will discuss our challenges and present some of our
first-year achievements, including a 320 fJ/bit hybrid-bonded optical transmitter and a 690 fJ/bit hybrid-bonded optical
receiver. The optical transmitter was made of a Si microring modulator flip-chip bonded to a 90nm CMOS driver with
digital clocking. With only 1.6mW power consumption measured from the power supply voltages and currents, the
transmitter exhibits a wide open eye with extinction ratio >7dB at 5Gb/s. The receiver was made of a Ge waveguide
detector flip-chip bonded to a 90nm CMOS digitally clocked receiver circuit. With 3.45mW power consumption, the
integrated receiver demonstrated -18.9dBm sensitivity at 5Gb/s for a BER of 10-12. In addition, we will discuss our
Mux/Demux strategy and present our devices with small footprints and low tuning energy.
Microresonators are basic building blocks for compact photonic integrated circuits (PICs). The performance of the
microresonators depends on their intrinsic and loaded quality factors (Q). Here we demonstrate the optical
characterization of a single crystalline silicon microtoroidal resonator with integrated MEMS-actuated tunable optical
coupler. The device is realized on a two-layer silicon-on-insulator (SOI) structure. It is fabricated by combining
hydrogen annealing and wafer bonding processes. The device has been demonstrated to be operated in all three coupling
regimes: under-coupling, critical coupling, and over-coupling. At an actuation voltage of 114 V, the extinction ratio at
the resonant wavelength of 1548.18 nm reaches 22.4 dB. To characterize these quality-factor tunable microtoroidal
resonators, we have also developed a comprehensive model based on time-domain coupling theory. The intrinsic and
loaded Qs are extracted by fitting the experimental curves with the model. The intrinsic Q is 110,000. And the loaded Q
is continuously tunable from 110,000 to 5,400. This device has potential applications in variable bandwidth filters,
reconfigurable add-drop multiplexers, and optical sensors.