The various arguments for introducing optical interconnections to silicon CMOS chips are summarized, and the challenges for optical, optoelectronic, and integration technologies are discussed. Optics could solve many physical problems of interconnects, including precise clock distribution, system synchronization (allowing larger synchronous zones, both on-chip and between chips), bandwidth and density of long interconnections, and reduction of power dissipation. Optics may relieve a broad range of design problems, such as crosstalk, voltage isolation, wave reflection, impedence matching, and pin inductance. It may allow continued scaling of existing architectures and enable novel highly interconnected or high-bandwidth architectures. No physical breakthrough is required to implement dense optical interconnects to silicon chips, though substantial technological work remains. Cost is a significant barrier to practical introduction, though revolutionary approaches exist that might achieve economies of scale. An Appendix analyzes scaling of on-chop global electrical interconnects, including line inductance and the skin effect, both of which impose significant additional constraints on future interconnects.

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Rationale and challenges for optical interconnects to electronic chips

We review the family of optoelectronic devices whose performance is enhanced by placing the active device structure inside a Fabry‐Perot resonantmicrocavity. Such resonantcavity enhanced (RCE) devices benefit from the wavelength selectivity and the large increase of the resonant optical field introduced by the cavity. The increased optical field allows RCE photodetector structures to be thinner and therefore faster, while simultaneously increasing the quantum efficiency at the resonant wavelengths. Off‐resonance wavelengths are rejected by the cavity making RCE photodetectors promising for low crosstalk wavelength division multiplexing(WDM) applications. RCE optical modulators require fewer quantum wells so are capable of reduced voltage operation. The spontaneous emission spectrum of RCE light emitting diodes(LED) is drastically altered, improving the spectral purity and directivity. RCE devices are also highly suitable for integrated detectors and emitters with applications as in optical logic and in communication networks. This review attempts an encyclopedic overview of RCE photonicdevices and systems. Considerable attention is devoted to the theoretical formulation and calculation of important RCE device parameters. Materials criteria are outlined and the suitability of common heteroepitaxial systems for RCE devices is examined. Arguments for the improved bandwidth in RCE detectors are presented intuitively, and results from advanced numerical simulations confirming the simple model are provided. An overview of experimental results on discrete RCE photodiodes, phototransistors, modulators, and LEDs is given. Work aimed at integrated RCE devices,optical logic and WDM systems is also covered. We conclude by speculating what remains to be accomplished to implement a practical RCE WDM system.

Resonant cavity enhanced photonic devices

Technologies now exist for implementing dense surface-normal optical interconnections for silicon CMOS VLSI using hybrid integration techniques. The critical factors in determining the performance of the resulting photonic chip are the yield on the transceiver device arrays, the sensitivity and power dissipation of the receiver and transmitter circuits, and the total optical power budget available. The use of GaAs-AlGaAs multiple-quantum-well p-i-n diodes for on-chip detection and modulation is one effective means of implementing the optoelectronic transceivers. We discuss a potential roadmap for the scaling of this hybrid optoelectronic VLSI technology as CMOS linewidths shrink and the characteristics of the hybrid optoelectronic transceiver technology improve. An important general conclusion is that, unlike electrical interconnects, such dense optical interconnections directly to an electronic circuit will likely be able to scale in capacity to match the improved performance of future CMOS technology.

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Scaling optoelectronic-VLSI circuits into the 21st century: a technology roadmap

Self-aligned flip-chip assembly using solder bump bonding has been applied to photonic circuits, resulting in several important advantages, including high precision, excellent ruggedness, and uncritical final assembly. Experiments with lithium niobate integrated optical chips and single-mode optical fibers have demonstrated that the optical coupling loss due to misalignment can be held to less than 1 dB in a fully self-aligned, multifiber assembly. The technique achieves approximately 1 mu m accuracy in optical-fiber placement without requiring micromanipulation of piece parts during final assembly, and simultaneously forms an essentially unlimited number of high-performance electrical bonds in an interconnect pattern distributed as required over the chip surface. The assemblies are compatible with a rugged environment and with hermetic packaging. >

Self aligned, flip chip assembly of photonic devices with electrical and optical connections

Optoelectronic-VLSI (OE-VLSI) technology represents the intimate integration of photonic devices with silicon VLSI electronics. We review the motivations and status of emerging OE-VLSI technologies and examine the performance of OE-VLSI technology versus conventional wire-bonded OE packaging. The results suggest that OE-VLSI integration offers substantial power and speed improvements even when relatively small numbers of photonic devices are driven with commodity complementary metal-oxide-semiconductor logic technologies.

Optoelectronic-VLSI: photonics integrated with VLSI circuits

The design and performance of arrays of hybrid optoelectronic detector and modulator elements for use as optical input and output pads for chip- and board-level optical interconnects are discussed. The application of these interface arrays to specific VLSI circuits is discussed, illustrating the potential improvements in performance levels. This solder bond technique is capable of very accurate component positioning at points across the entire surface of the VLSI circuit, so that precise alignment to the optical pathways can be envisaged with optical signals taken from or delivered to any position on the chip. Measurements presented indicate that submicron positioning can be achieved. In particular, a fabrication-tolerant modulator design incorporating a chirped semiconductor mirror is reported. >

Optoelectronic component arrays for optical interconnection of circuits and subsystems

Useful modulation at frequencies in excess of 1.3 GHz has been achieved in a long-wavelength InGaAs/InP multiple-quantum-well reflective modulator. Microwave frequency operation of the modulator was investigated using a free-space link, is shown schematically. The modulator was illuminated with focussed radiation from a Fabry-Perot laser emitting close to the 1610 nm resonance, use of a polarisation-selective beam splitter in conjunction with a quarter-wave retarder providing efficient resolution of power reflected from the device in the presence of stray power from other system components. RF drive to the modulator via a bias network permitted investigation of behaviour under a variety of bias conditions. Reflected modulated power was detected using a reverse biased pin photodiode, again coupled via a bias network. >

Microwave frequency operation of a low-voltage InGaAs/InP multiple-quantum-well reflective Fabry-Perot modulator

The design, fabrication and preliminary characterisation of a novel hybridised silicon/PLZT electro-optic spatial light modulator are reported. Modulation in excess of 10 dB is achieved at a drive of 30 V, with a response time less than 5 μs in a 64-element array.

8×8 element hybridised PLZT/silicon spatial light modulator array

Efficient modulation at frequencies in excess of 1.3 GHz has been achieved in long-wavelength InGaAs/InP multiple-quantum-well reflective modulators. The devices function simultaneously as efficient high-speed detectors, with demonstrated response to 2 Gbit/s.

C.J.G. Kirkby

Papers

Optoelectronic component arrays for optical interconnection of circuits and subsystems

Microwave frequency operation of a low-voltage InGaAs/InP multiple-quantum-well reflective Fabry-Perot modulator

8×8 element hybridised PLZT/silicon spatial light modulator array

Low-voltage InGaAs/InP multiple-quantum-well reflective Fabry-Perot modulators for optical communications at microwave frequencies