Single-chip microprocessor that communicates directly using light

TL;DR: This demonstration could represent the beginning of an era of chip-scale electronic–photonic systems with the potential to transform computing system architectures, enabling more powerful computers, from network infrastructure to data centres and supercomputers.

Abstract: An electronic–photonic microprocessor chip manufactured using a conventional microelectronics foundry process is demonstrated; the chip contains 70 million transistors and 850 photonic components and directly uses light to communicate to other chips. The rapid transfer of data between chips in computer systems and data centres has become one of the bottlenecks in modern information processing. One way of increasing speeds is to use optical connections rather than electrical wires and the past decade has seen significant efforts to develop silicon-based nanophotonic approaches to integrate such links within silicon chips, but incompatibility between the manufacturing processes used in electronics and photonics has proved a hindrance. Now Chen Sun et al. describe a 'system on a chip' microprocessor that successfully integrates electronics and photonics yet is produced using standard microelectronic chip fabrication techniques. The resulting microprocessor combines 70 million transistors and 850 photonic components and can communicate optically with the outside world. This result promises a way forward for new fast, low-power computing systems architectures. Data transport across short electrical wires is limited by both bandwidth and power density, which creates a performance bottleneck for semiconductor microchips in modern computer systems—from mobile phones to large-scale data centres. These limitations can be overcome1,2,3 by using optical communications based on chip-scale electronic–photonic systems4,5,6,7 enabled by silicon-based nanophotonic devices8. However, combining electronics and photonics on the same chip has proved challenging, owing to microchip manufacturing conflicts between electronics and photonics. Consequently, current electronic–photonic chips9,10,11 are limited to niche manufacturing processes and include only a few optical devices alongside simple circuits. Here we report an electronic–photonic system on a single chip integrating over 70 million transistors and 850 photonic components that work together to provide logic, memory, and interconnect functions. This system is a realization of a microprocessor that uses on-chip photonic devices to directly communicate with other chips using light. To integrate electronics and photonics at the scale of a microprocessor chip, we adopt a ‘zero-change’ approach to the integration of photonics. Instead of developing a custom process to enable the fabrication of photonics12, which would complicate or eliminate the possibility of integration with state-of-the-art transistors at large scale and at high yield, we design optical devices using a standard microelectronics foundry process that is used for modern microprocessors13,14,15,16. This demonstration could represent the beginning of an era of chip-scale electronic–photonic systems with the potential to transform computing system architectures, enabling more powerful computers, from network infrastructure to data centres and supercomputers.

Summary

Methods

• The key chip characteristics are summarized in Extended Data Table 1 .
• Afterwards, the chips are placed in a chamber which supplies XeF 2 gas to isotropically etch the silicon substrate, removing it as the volatile product SiF 4 .
• The authors use lensed fibers available from Oz Optics with a spot size of 5 µm and a working distance of 26 µm to couple light into the vertical grating couplers through the chip backside (after substrate removal).
• Programs are compiled from C source code using a gcc-based C compiler targeted for the RISC-V ISA.

Full text

UC Berkeley
UC Berkeley Previously Published Works
Title
Single-chip microprocessor that communicates directly using light.
Permalink
https://escholarship.org/uc/item/4dh1v4px
Journal
Nature, 528(7583)
ISSN
0028-0836
Authors
Sun, Chen
Wade, Mark T
Lee, Yunsup
et al.
Publication Date
2015-12-01
DOI
10.1038/nature16454
Copyright Information
This work is made available under the terms of a Creative Commons Attribution-
NonCommercial-NoDerivatives License, availalbe at
https://creativecommons.org/licenses/by-nc-nd/4.0/
Peer reviewed
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University of California

1
Single-Chip Microprocessor with Integrated Photonic I/O
Chen Sun
*1,2
, Mark T. Wade
*3
, Yunsup Lee
*1
, Jason S. Orcutt
*2,4
, Luca Alloatti
2
, Michael S.
Georgas
2
, Andrew S. Waterman
1
, Jeffrey M. Shainline
3,5
, Rimas R. Avizienis
1
, Sen Lin
1
, Benjamin
R. Moss
2
, Rajesh Kumar
3
, Fabio Pavanello
3
, Amir H. Atabaki
2
, Henry M. Cook
1
, Albert J. Ou
1
,
Jonathan C. Leu
2
, Yu-Hsin Chen
2
, Krste Asanović
1
, Rajeev J. Ram
2
, Miloš A. Popović
3
, Vladimir
M. Stojanović
1
*
Contributed equally to this work
1
University of California, Berkeley, Berkeley, CA
2
Massachusetts Institute of Technology, Cambridge, MA
3
University of Colorado, Boulder, Boulder, CO
4
Now at IBM T.J. Watson Research Center, Yorktown Heights, NY
5
Now at National Institute of Science and Technology, Boulder, CO
Data transport across short-reach electrical wires is both bandwidth-density and power-
density-limited, creating a performance bottleneck for semiconductor microchips in modern
computer systems, from mobile phones to large-scale datacenters. Optical communications
based on chip-scale electronic-photonic systems
1–4
enabled by silicon-based nanophotonic
devices
5
can overcome these limitations
6–8
. However, combining electronics and photonics on
the same chip has proved challenging due to microchip manufacturing conflicts between
electronics and photonics. Consequently, current electronic-photonic chips
9–11
are limited to
niche manufacturing processes and integrate only a few optical devices alongside simple

2
circuits. Here we report an electronic-photonic system-on-chip (SoC) integrating over 70
million transistors and 850 photonic components that work in concert to provide logic,
memory, and interconnect functions, realizing a microprocessor chip that can optically
communicate directly to the outside world for the first time. To integrate electronics and
photonics at this scale, we adopt a zero-change approach to the integration of photonics.
Instead of developing a custom process to enable the fabrication of photonics
12
, which
complicates or eliminates the possibility of integration with state-of-the-art transistors at
large scale and at high yield, we design optical devices directly within a standard
microelectronics foundry process used for modern microprocessors
13
(Cell
14
, BlueGene/Q
15
,
Power7
16
, etc.). We expect that this demonstration signals the beginning of an era of
electronic-photonic SoCs with potential for transformative impact on computing system
architecture, enabling a leap to new kinds of more powerful computers, from network
infrastructure to datacenters and supercomputers.
The electro-optic SoC (Figure 1) contains a dual-core RISC-V instruction set architecture
17
(ISA)
microprocessor and an independent 1 MB bank of static random access memory used for memory.
The on-chip electro-optic transceivers for data input/output (I/O) enable both the microprocessor
and the memory to communicate directly to off-chip components using light, without the need for
separate chips or components to host the optical devices. The chip was fabricated in a commercial
high-performance 45 nm complementary metal-oxide semiconductor (CMOS) silicon-on-insulator
(SOI) process
18
. No changes to the foundry process were necessary to accommodate photonics and
all optical devices were designed to comply with the native process manufacturing rules. This
zero-change integration enables high-performance transistors on the same chip as optics, reuse of
all existing designs in the process, compatibility with electronics design tools, and manufacturing

3
in an existing high-volume foundry.
The process includes a crystalline Si (c-Si) layer which is patterned to form both the body of
electronic transistors and the core of optical waveguides. A thin buried oxide (BOX) layer
separates the c-Si layer from the silicon handle wafer (Extended Data Figure 1). As the BOX is
<200 nm thick, light propagating in c-Si waveguides will evanescently leak into the silicon handle
wafer, resulting in high waveguide loss. To resolve this, we perform selective substrate removal on
the chips after electrical packaging to etch away the silicon handle under regions with optical
devices (Extended Data Figure 2). We leave the silicon handle intact under the microprocessor and
memory (which dissipate the most power) to allow a heat sink to be contacted, if necessary.
Substrate removal has a negligible impact on the electronics
13
and the processor is completely
functional even with a fully-removed substrate.
Silicon-germanium (SiGe) is present, though in low germanium mole fractions, in advanced
CMOS processes for enhancing hole mobility and transistor performance via compressive strain
engineering of p-channel transistors
18
. Selecting an 1180 nm wavelength band for the optical
channel enables use of photodetectors (PDs) built using this SiGe
19
. Silicon is transparent at 1180
nm and no adverse effects are observed. At these wavelengths, the optical propagation loss in
silicon strip waveguides is 4.3 dB/cm (losses at industry-standard wavelengths of 1300 nm and
1550 nm are 3.7 dB/cm and 4.6 dB/cm, respectively
13
). The receiver circuit
20
resolves photocurrent
produced by the illuminated PD into digital ones and zeros. The receiver sensitivity in optical
modulation amplitude (OMA) is −5 dBm for a better than 10
−12
bit-error-rate.
The electro-optic transmitter consists of an electro-optic modulator and its electronic driver. The

4
modulator is a 10 µm diameter silicon microring resonator, coupled to a waveguide. We dope the
structure with the n-well and p-well implants used for transistors to form radially extending p-n
junctions, interleaved along the azimuthal dimension
21, 22
, taking the form of a “spoked ring”. The
ring exhibits a sharp, notched filter optical transmission response, with a stop-band at the ring’s
resonant wavelength (λ
0
). Applying a negative voltage across the junctions depletes the ring of free
carriers (electron and hole concentrations), while a small positive voltage refills the carriers. A
change in carrier concentration influences the ring waveguide’s index of refraction through the
carrier plasma dispersion effect
23
which, in turn, shifts λ
0
. Electro-optic modulation (on-off keying)
is achieved by changing the voltage applied across the junction to move the λ
0
stop-band in and out
of the laser wavelength (λ
L
). The modulator has a loaded quality factor of approximately 10,000,
and a voltage swing of only 1 V
pp
across the modulator achieves 6 dB on-to-off ratios at a 3 dB
insertion loss for non-return-to-zero (NRZ) binary data. The low voltage, near-zero quiescent
current, and low capacitance (15 fF, including wiring capacitance) result in an energy-efficient
modulator driven by a standard CMOS logic inverter at gigabit datarates using the same 1 V
nominal supply that powers digital electronics.
As a resonant device, the modulator is highly sensitive to c-Si layer thickness variations within and
across SOI wafers
24
as well as spatially and rapidly temporally varying thermal environments
created by the electrical components on the chip
25, 26
. Both effects cause λ
0
to deviate from the
design value, necessitating tuning circuitry. We embedded a 400 Ω resistive microheater inside the
ring to efficiently tune λ
0
and added a monitoring PD weakly coupled to the modulator drop port.
When light resonates in the modulator ring, a small fraction of the light couples to and illuminates
the PD. This generates photocurrent proportional to the amount of resonating light, which is