S.T. Wilkinson

The heterogeneous integration of optical interconnections into integrated microsystems

Emerging techniques for integrating optoelectronic (OE) devices, analog interface circuitry, RF circuitry, and digital logic into ultra-mixed signal systems offers approaches toward and demonstrations of integrated optical interconnections in electrical microsystems. As rising data rates dictate the use of optical interconnections and interfaces at increasingly smaller distances, optical interconnections stand at a threshold of opportunity for pervasive implementation if cost-effective integration process technology and performance can be implemented. Heterogeneous integration is one approach toward the integration of compound semiconductor OE devices, Si CMOS circuits, and organic materials. Heterogeneous integration approaches, which utilize dissimilar materials which can be independently grown and optimized, and are subsequently bonded together into an integrated system, are particularly attractive methods for creating high-performance microsystems. This paper describes a variety of optical interconnections integrated into microsystems using thin film heterogeneous integration. Thin film heterogeneous integration is attractive from the standpoint that the topography of the integrated microsystem can remain flat to within a few microns, substrates which are often optically absorbing are removed, both sides of the thin film devices can be processed (e.g., contacted, optically coated), and three-dimensionally stacked structures can be implemented. Demonstrations of interconnections using thin film heterogeneous integration technology include an integrated InGaAs/Si CMOS receiver circuit operating at 1 Gbps, an InGaAs thin film photodetector bonded onto a foundry Si CMOS microprocessor to demonstrate a single chip optically interconnected microprocessor, smart pixel emitter and detector arrays using resonant cavity enhanced P-i-N photodetectors bonded on top of per-pixel current controlled oscillators and resonant cavity enhanced light emitting diodes integrated onto digital to analog converter gray-scale per-pixel driver circuitry, and photodetectors embedded in waveguides on electrical interconnection substrates to demonstrate chip-to-chip embedded waveguide interconnections.

Thin-film multimaterial optoelectronic integrated circuits

The multimaterial integration of thin-film optoelectronic devices with host substrates ranging from silicon circuits to glass waveguides to polymer micromachines offers to the system designer the freedom to choose the optimal materials for each component to achieve performance and cost objectives. Thin-film compound semiconductor optoelectronic devices are comparable to, and, in some cases, better than, their on-wafer counterparts. Thin-film detectors have been integrated with receiver circuits and movable micromachines, thin-film emitters with drive circuitry, and both have been used to demonstrate three-dimensionally interconnected systems. Vertical electrical integration of detector arrays on top of circuits is examined for massively parallel processing of images. Vertical optical interconnections of stacked silicon circuits (which are transparent to the wavelength of light used) are explored, and are used to develop a massively parallel processing architecture based upon low memory, high throughput, and high input/output.

A three-dimensional high-throughput architecture using through-wafer optical interconnect

This paper presents a three-dimensional, highly parallel, optically interconnected system to process high-throughput stream data such as images. The vertical optical interconnections are realized using. Integrated optoelectronic devices operating at wavelengths to which silicon is transparent. These through-wafer optical signals are used to vertically optically interconnect stacked silicon circuits. The thin film optoelectronic devices are bonded directly to the stacked layers of silicon circuitry to realize self-contained vertical optical interconnections. Each integrated circuit layer contains analog interface circuitry, namely, detector amplifier and emitter driver circuitry, and digital circuitry for the network and/or processor, all of which are fabricated using a standard silicon integrated circuit foundry. These silicon circuits are post processed to integrate the thin film optoelectronics using standard, low cost, high yield microfabrication techniques. The three-dimensionally integrated architectures described herein are a network and a processor. The network has been designed to meet off-chip I/O using a new offset cube topology coupled with naming and renting schemes. The performance of this network is comparable to that of a three-dimensional mesh. The processing architecture has been defined to minimize overhead for basic parallel operations. The system goal for this research is to develop an integrated processing node for high-throughput, low-memory applications. >

Communication through stacked silicon circuitry using integrated thin film InP-based emitters and detectors

To demonstrate optical communication through stacked silicon circuitry, thin film InGaAsP-based emitters and photodetectors have been bonded directly onto silicon circuitry. These optoelectronic devices operate at a wavelength to which silicon is transparent. The thin film emitters and detectors were integrated onto a MOSIS foundry silicon CMOS integrated circuit which contained driver and amplifier circuits. Bidirectional vertical optical communication between two layers of circuitry was demonstrated by stacking the layers, exciting the emitter driver circuit on one layer with an electrical signal, and measuring the output electrical signal from the detector amplifier located on the other circuit in the vertical stack.<<ETX>>

Resonant-cavity-enhanced thin-film AlGaAs/GaAs/AlGaAs LED’s with metal mirrors

Resonant-cavity-enhanced light-emitting diodes (RCE LEDs) are of increasing interest as a low-cost alternative to lasers for short-distance applications. We report on the characteristics of thin-film AlGaAs/GaAs/AlGaAs double-heterostructure RCE LEDs with metal mirrors on both sides fabricated by means of epitaxial liftoff and bonded to silicon host substrates. The devices exhibit typical turn-on voltages of 1.3 V, operating resistances of 31 Ω, linewidths of 10.4 nm, efficiencies of 1.4%, dispersion half-angles of 23.7°, and stable output over more than 1700 h. These devices exhibit significant improvement over conventional LEDs without additional complicated processing or growth steps, resulting in a manufacturable, low-cost device.

Integration of thin film optoelectronic devices onto micromachined movable platforms

The integration of a thin film optoelectronic device onto a micromachined movable platform is reported in this letter. This micro-opto-mechanical system, consisting of a thin film AlGaAs/GaAs double heterostructure p-i-n detector integrated onto a polyimide micromachined platform on silicon, has applications which range from fiber optic coupling to sensors. Fiber optic coupling is demonstrated using a stationary fiber positioned above the thin film detector. By applying a voltage between the platform and actuation strips, the platform moves and a change in fiber to detector coupling is observed.<<ETX>>

A 155 Mbps digital transmitter using GaAs thin film LEDs bonded to silicon driver circuits

A low cost, low power digital silicon transmitter integrated with a GaAs-based surface LED has been fabricated and characterized. Operation at 155 Mbps has been demonstrated with a digital input, resulting in a transmitter which is integrable with digital Si CMOS circuitry, and is suitable for short haul optical communication.

A 100 Mbps, LED Through-Wafer Optoelectronic Link for Multicomputer Interconnection Networks

Through-wafer optoelectronic interconnect offers some architectural alternatives that are not available with wire-based interconnects. In order to compete with wire-based technologies, optoelectronic interconnects must provide reasonable performance in terms of bandwidth, bit error rate (BER), and power, using inexpensive and manufacturable devices. This paper presents a 100 Mbps link design under development as part of a scalable three-dimensional multicomputer network for a 4096 node system. Empirical and analytical data for emitters, detectors, receivers, and optical coupling is used to examine the tradeoffs between link power and bit error rate (BER). Because multicomputer networks demand extremely low BERs (10?15?10?20),hop-by-hoperror correction circuitry is incorporated to optimize BER, providing a robust channel. This approach employs a novel adaptation of the widely used wormhole routing protocol to minimize overhead and maximize compatibility with existing interconnect techniques.

Design issues for through-wafer optoelectronic multicomputer interconnects

The paper presents several design issues associated with the implementation of a three dimensional optically interconnected parallel processing system. A technique for improving bit error rate in low power multistage networks is presented. Error detection codes are transmitted along with message data to guarantee the integrity of the data during each optical hop. To realize three dimensional through silicon wafer interconnect, thin film emitters and detectors operating at a wavelength of 1.3 /spl mu/m (to which silicon is transparent) will be bonded to the silicon circuitry. A transfer diaphragm process is used to realize this integration; this process has been used to demonstrate the basic concept: a single silicon circuit has been integrated with both a thin film emitter and detector operating at 1.3 /spl mu/m wavelength (K.H. Calhoun et al., 1993). We utilize one possible integration scenario to illustrate the trade offs associated with a system of this type, which includes device design, circuit design, and issues which include manufacturability, alignment tolerance, crosstalk, and power dissipation.

An 8/spl times/8 array of resonant cavity enhanced light emitting diodes integrated onto silicon grayscale (32 level) driver circuitry

We demonstrate a fully operational 8/spl times/8 array of GaAS RCELEDs integrated directly on top of a silicon grayscale array silicon driver circuit. This integrated optoelectronic circuit allowed 32-level grayscale emission from each individually addressable pixel. A yield of 100% was achieved from the array, with full circuit functionality demonstrated.