Michael Georgas

Single-chip microprocessor that communicates directly using light

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 overcome by using optical communications based on chip-scale electronic–photonic systems 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 chips 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 photonics, 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 microprocessors. 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.

Open foundry platform for high-performance electronic-photonic integration

This paper presents photonic devices with 3 dB/cm waveguide loss fabricated in an existing commercial electronic 45 nm SOI-CMOS foundry process. By utilizing existing front-end fabrication processes the photonic devices are monolithically integrated with electronics in the same physical device layer as transistors achieving 4 ps logic stage delay, without degradation in transistor performance. We demonstrate an 8-channel optical microring-resonator filter bank and optical modulators, both controlled by integrated digital circuits. By developing a device design methodology that requires zero process infrastructure changes, a widely available platform for high-performance photonic-electronic integrated circuits is enabled.

Addressing link-level design tradeoffs for integrated photonic interconnects

Integrated photonic interconnects have emerged recently as a potential solution for relieving on-chip and chip-to-chip bandwidth bottlenecks for next-generation many-core processors. To help bridge the gap between device and circuit/system designers, and aid in understanding of inherent photonic link tradeoffs, we present a set of link component models for performing interconnect design-space exploration connected to the underlying device and circuit technology. To compensate for process and thermal-induced ring resonator mismatches, we take advantage of device and circuit characteristics to propose an efficient ring tuning solution. Finally, we perform optimization of a wavelength-division-multiplexed link, demonstrating the link-level interactions between components in achieving the optimal degree of parallelism and energy-efficiency.

A Monolithically-Integrated Chip-to-Chip Optical Link in Bulk CMOS

Silicon-photonics is an emerging technology that can overcome the tradeoffs faced by traditional electrical I/O. Due to ballooning development costs for advanced CMOS nodes, however, widespread adoption necessitates seamless photonics integration into mainstream processes, with as few process changes as possible. In this work, we demonstrate a silicon-photonic link with optical devices and electronics integrated on the same chip in a 0.18 µm bulk CMOS memory periphery process. To enable waveguides and optics in process-native polysilicon, we introduce deep-trench isolation, placed underneath to prevent optical mode leakage into the bulk silicon substrate, and implant-amorphization to reduce polysilicon loss. A resonant defect-trap photodetector using polysilicon eliminates need for germanium integration and completes the fully polysilicon-based photonics platform. Transceiver circuits take advantage of photonic device integration, achieving 350 fJ/b transmit and 71 µA pp BER = 10 -12 receiver sensitivity at 5 Gb/s. We show high fabrication uniformity and high-Q resonators, enabling dense wavelength-division multiplexing with 9-wavelength 45 Gb/s transmit/receive data-rates per waveguide/fiber. To combat perturbations to variation- and thermally-sensitive resonant devices, we demonstrate an on-chip thermal tuning feedback loop that locks the resonance to the laser wavelength. A 5 m optical chip-to-chip link achieves 5 Gb/s while consuming 3 pJ/b and 12 pJ/bit of circuit and optical energy, respectively.

Depletion-mode carrier-plasma optical modulator in zero-change advanced CMOS.

We demonstrate the first (to the best of our knowledge) depletion-mode carrier-plasma optical modulator fabricated in a standard advanced complementary metal-oxide-semiconductor (CMOS) logic process (45 nm node SOI CMOS) with no process modifications. The zero-change CMOS photonics approach enables this device to be monolithically integrated into state-of-the-art microprocessors and advanced electronics. Because these processes support lateral p-n junctions but not efficient ridge waveguides, we accommodate these constraints with a new type of resonant modulator. It is based on a hybrid microring/disk cavity formed entirely in the sub-90 nm thick monocrystalline silicon transistor body layer. Electrical contact of both polarities is made along the inner radius of the multimode ring cavity via an array of silicon spokes. The spokes connect to p and n regions formed using transistor well implants, which form radially extending lateral junctions that provide index modulation. We show 5 Gbps data modulation at 1265 nm wavelength with 5.2 dB extinction ratio and an estimated 40 fJ/bit energy consumption. Broad thermal tuning is demonstrated across 3.2 THz (18 nm) with an efficiency of 291 GHz/mW. A single postprocessing step to remove the silicon handle wafer was necessary to support low-loss optical confinement in the device layer. This modulator is an important step toward monolithically integrated CMOS photonic interconnects.

A 45 nm CMOS-SOI Monolithic Photonics Platform With Bit-Statistics-Based Resonant Microring Thermal Tuning

The microring resonator is critical for dense wavelength division multiplexed (DWDM) chip-to-chip optical I/O, enabling modulation and channel selection at the μm-scale suitable for a VLSI chip. Microring-based links, however, require active tuning to counteract process and thermo-optic variations. Here, we present a bit-statistical tuner that decouples tracking of optical one and zero-levels to realize non-dc-balanced data transmission, an “eye-max”-locking controller, and self-heating cancellation without need for a high-speed sensing frontend. We implement the tuner on a 45 nm CMOS-SOI process with monolithically integrated photonic devices and circuits. The tuner consumes 0.74 mW in the logic while achieving a record 524 GHz (> 50 K temperature) tuning range at 3.8 μW/GHz heater efficiency. To our knowledge, this is the highest range and heater efficiency reported by an on-chip closed-loop thermal tuner to date. The tuner integrates with a 5 Gb/s 30 fJ/bit monolithic microring transmitter, achieving wavelength-lock and immunity to both tracking failures and self-heating events caused by arbitrary, nondc-balanced bitstreams. In addition, the tuner provides critical functionality for an 11-λ DWDM transmitter macro capable of 11 × 8 Gb/s bandwidth on a fiber. Together with the transmitter, a 10 Gb/s on-chip monolithic optical receiver with 10-12 BER sensitivity of 9 μA at 10 Gb/s enables a sub-pJ/bit 5 Gb/s optical chip-to-chip link, with the bit-statistical tuner providing thermally robust microring operation.

A monolithically-integrated optical transmitter and receiver in a zero-change 45nm SOI process

An optical transmitter and receiver with monolithically-integrated photonic devices and circuits are demonstrated together for the first time in a commercial 45nm SOI process, without any process changes. The transmitter features an interleaved-junction carrier-depletion ring modulator and operates at 3.5Gb/s with an 8dB extinction ratio and combined circuit and device energy cost of 70fJ/bit. The optical receiver connects to an integrated SiGe detector designed for 1180nm wavelength and performs at 2.5Gb/s with 15μA sensitivity and energy cost of 220fJ/bit.

A monolithically-integrated optical receiver in standard 45-nm SOI

A monolithically-integrated optical receiver for low-energy on-chip and off-chip communication is presented. The monolithic photodiode integration enables the energy-efficient and high-sensitivity sense-amplifier-based receiver design. The receiver is characterized in situ and shown to operate with μA-sensitivity at 3.5 Gb/s with a power consumption of 180 μW (52 fJ/bit) and area of 108 μm2. This work demonstrates that photonics and electronics can be jointly integrated in a standard 45-nm SOI process.

A 1.23pJ/b 2.5Gb/s monolithically integrated optical carrier-injection ring modulator and all-digital driver circuit in commercial 45nm SOI

Integrated photonic interconnect technology presents a disruptive alternative to electrical I/O for many VLSI applications. Superior bandwidth-density and energy-efficient operation can be realized through dense wavelength-division multiplexing (DWDM) and lower transmission losses. There are two main paths towards an integrated platform. Hybrid/heterogeneous designs [1-3] enable each component to be custom-tailored, but suffer from large packaging parasitics, increased manufacturing costs due to requisite process flows, and costly 3D integration or microbump packaging. Monolithic integration mitigates integration overheads, but has not penetrated deeply-scaled technologies due to necessary process customizations [4]. The first monolithic integration of photonic devices and electronic-photonic operation in sub-100 nm (45 nm SOI process with zero foundry changes) is demonstrated in [5]. This paper presents a monolithically integrated optical modulator with a new all-digital driver circuit in a commercial 45nm SOI process. The waveform-conditioning driver circuit enables the carrier-injection modulator to operate at 2.5Gb/s with an energy-cost of 1.23pJ/b, making it ~4× faster and more energy-efficient than the previous monolithically integrated driver/modulator presented in [5].

Integration of silicon photonics in bulk CMOS

The first monolithic process flow integrating silicon photonics on operational bulk CMOS has been developed. Features include deep-trench isolation, polysilicon waveguides, grating couplers, filters, modulators, and detectors. Fully functional on-chip CMOS enables Tx/Rx operation while minimizing interconnect parasitics. With the addition of an external 1280nm source, a fully functional optical link (5Gb/s 2.8pJ/b), capable of wavelength division multiplexing, has been demonstrated. In addition to the polysilicon resonant detector used in the link, a monolithically-integrated Silicon-Germanium selective epitaxial growth based photodetector was developed.