380 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 9, NO. 2, MARCH/APRIL 2003 Multichannel Ultrathin Silicon-on-Sapphire Optical Interconnects J. Jiang Liu, Senior Member, IEEE, Zaven Kalayjian, Brian Riely, Wayne Chang, Member, IEEE, George J. Simonis, Senior Member, IEEE, Alyssa Apsel, Member, IEEE, and Andreas Andreou, Associate Member, IEEE Abstract—Multichannel optical interconnects were developed using vertical-cavity surface-emitting laser (VCSEL) arrays and metal-semiconductor-metal photodetector (PD) arrays and driven by complementary metal-oxide-semiconductor circuits that were fabricated using ultrathin silicon-on-sapphire (SOS) technology. Low-threshold oxide-confined top-emitting VCSEL 8 8 arrays were designed and fabricated with off-site contact bonding pads. The arrays were flip-chip bonded to driver arrays on sapphire sub- strates and mounted on high-speed printed-circuit boards as op- tical transmitter arrays. The laser output was transmitted through the transparent sapphire substrate and coupled to MSM PD arrays and the SOS receiver. This optical interconnect system was demon- strated to operate at a data rate of 1.0 Gb/s per channel with a power consumption of 28 mW for each channel including trans- mitter and receiver. Index Terms—Optical interconnect, oxide aperture, pho- todetector (PD), sapphire substrate, silicon-on-sapphire (SOS), vertical-cavity surface-emitting laser (VCSEL). I. INTRODUCTION I N THE development of next generation board-to-board and chip-to-chip level optoelectronic (OE) interconnects, high- density two-dimensional (2-D) optical links that are composed of VCSEL and photodetector (PD) arrays integrated with high- speed drivers and receivers offer promising solutions to achieve high-bandwidth, low-power-consumption, and parallel digital data communication and switching [1]–[3]. These new optical interconnects can also significantly enhance and change the fun- damental architectures for future datalinking systems [4]. To improve the operating speed and power consumption of transmitters, VCSELs are usually made with low-resistance electrical contacts and oxide-confined apertures for lower lasing current thresholds [5]–[8]. Recently, the new evolving technology of ultrathin silicon-on-sapphire (SOS) CMOS was applied to fabricate high-bandwidth driving circuits [9]. CMOS circuits fabricated on ultrathin silicon films deposited on sapphire substrate are becoming an attractive technology for ultrafast drivers and receivers. Because of the insulating properties of the sapphire substrate, this CMOS process reduces parasitic capacitance and enables very fast circuitry (projected bandwidth limit up to 40 GHz for 0.13- m processing) [10]. Manuscript received November 8, 2002; revised February 17, 2003. J. J. Liu, Z. Kalayjian, B. Riely, W. Chang, and G. J. Simonis are with the U.S. Army Research Laboratory, Adelphi, MD 20783 USA (e-mail: jiliu@arl.army.mil). A. Apsel and A. Andreou are with the Department of Electrical and Computer Engineering, The Johns Hopkins University, Baltimore, MD 21218 USA. Digital Object Identifier 10.1109/JSTQE.2003.814182 Since these drivers and receivers are fabricated on a transparent substrate, 850-nm top-emitting VCSELs and top-receiving PDs can be flip-chip bonded onto them and transmit/receive through the plane of circuitry, without extra device processing steps for substrate removal. The thermally conductive sapphire substrate also provides a better heat sink for the electrical driving circuitries than the SiO used in other silicon-on-in- sulator (SOI) technologies. Another obvious advantage of the optical interconnects composed of transparent transmitters and receivers is that they can easily evolve to bidirectional and three-dimensional cascade links [11]. The simplicity in device processing and hybridization of these interconnects will significantly lower the manufacturing cost. In this paper, we report our fabrication of 8 8 top-emitting 850-nm VCSEL arrays and 1 12 metal-semiconductor-metal (MSM) PD arrays, their hybridization with SOS drivers and re- ceivers, and integration to produce complete optical intercon- nects. Electrical and optical properties of the devices as well as the interconnect system were investigated. These optical inter- connects were demonstrated to operate up to a bandwidth of 1.0 Gb/s per channel at a bias voltage of 3.3 V. The electrical power consumption was measured only 28 mW per channel. II. SOS TRANSMITTER Our optical transmitters are composed of oxide-aperture-con- fined 850-nm top-emitting VCSEL arrays and SOS drivers. The VCSEL structure was grown inhouse at the U.S. Army Research Laboratory on n-type GaAs substrates by molecular beam epi- taxy (MBE). Driver circuits were customer designed and fabri- cated on sapphire substrates through the MOSIS Foundry Ser- vice using 0.5- m SOS-processing technology. A. 850-nm Top-Emitting VCSEL Array Our VCSEL structure was grown on an n-type GaAs substrate by MBE. The VCSEL epitaxial structure consists of a 35-pair n-doped Al Ga As/Al Ga As bottom distributed Bragg reflector (DBR), a cavity, and a 25-pair p-doped Al Ga As/Al Ga As top DBR. The active region consists of three 70 GaAs quantum wells with 70 Al Ga As barriers. The heavy-hole exciton resonant energy of the quantum well was designed with a gain offset of 15 meV above the cavity resonant energy to account for the band-gap narrowing at high carrier concentrations. This ensures a good match between the gain spectrum and the cavity characteristics during actual device operation. Test structures for this design were repeatedly grown and characterized by 1077-260X/03$17.00 © 2003 IEEE