Dosunmu, O. I.

Emsley, M. K.

Leblebici, Y.

Muller, P.

Ünlü, M. S.

Infoscience - École polytechnique fédérale de Lausanne

MG4 230pm - 2:45pm SILICON RESONANT CAVITY ENHANCED 12x1 PHOTODETECTOR ARRAYS FOR OPTICAL INTERCONNECT RECEIVER INTEGRATION Malthew K. Emsley, Olufemi I. Dosunmu. ' Paul Muller, ' Yusuf Leblebid, M. Selim onlo Boston University College of Engineering, 8 Saint Mary's St. Boston MA 02215 USA, memsley@bu.edu ' Swiss Federal Institute of Technology. Lausanne Swikedand High bandwidth shon distance communications standards are being developed based on parallel optical interconnect fiber arrays to meet the needs of increasing data rates of inter-chip communication in modem computer architecture. To ensure that this standard becomes an anractive option for computer systems, low wst components must be implemented on both the t"itting and receiving end of the fibers. To meet this low cost r e q b e n t  all-silicon based receiver circuits are actively being pursued [I], however. manufacturing high speed, high efficiency silicon photodetectors presents a technical challenge. Resonant cavity enhanced (RCE) photcdetgtors have been shown to provide the required bandwidth-efficiency product but have lemained a challenge to reproduce thmugh commercially available fabrication techniques. We have previously developed a method to produce siliwn wafers with high reflectance buried dishiiutd Brag reflectors (DBR) [Z]. The rubskates wnsist of a hvo- period, 90% reflecting, DBR fabricated using a double siticon-on-insulator (SOI) process. Discrete RCE Si photodetectors have previously been fabricated with 40% quanhun efficiency at 860 nm, a FWHM of 25 ps. and a 3dB bandwidth in excess of IO GHz [3]. In this paper we present Si RCE 12x1 photodetector arrays that have been fabricated and packaged with silicon based amplifiers to demonstrate the feasibility of a low cost monolithic silicon photoreceiver array. The 12x1 photodetector arrays were designed for the PAROL1 p&ct line specification by lniineon 141. The specification calls for 12 photodetectors with bemeen 30 and 80 ~rm photodetector active area in a linear asray on a 250 pm pitch. The fabricated 12x1 photcdetector " a y  can Fig. 1 12x1 photodetector a m y  The photodetectors consist of vertical Si RCE p i - n  photodiodes, a cross section is shown in Fig. 2, fabricated &ugh standard Si processing techniques on double SO1 reflecting wafers. The photodetectors have a 42pm detection area. Several samples were prepared for packaging with Helix -312 3.6GHz 12 Channel Receiver arrays. The samples were sent to Helix in Zurich, Switzerland and were bonded to the Helix receiver amay using 25" Gold wire wedge bonding. There were limitations in using this device but it was the only option for this testing. The first and perhaps most important limitation is the mismatch input impedance of the Helix amplitier. The Helix amplifier is designed to have a matched capacitive load of 450 E, the typical capacitance of Optorpeed lnGaAs photodetectors which is the standard photodetector used in the optical interconned standard. Our photodetectors have a 70fF capacitance. One other draw back is that men though the specification mentioned earlier called for 30 to 80 pm diameter photodetcdoH the testing was done with S O p m  wre multi-mode fiber (hfMF). This means thaf the 42 pm photodetector was only caphtring 70% of the incident light at best. P- Fig. 2 Cras section scbemtsic of Si RCE pi-n photodiode The photodetectors were tested with the Helix receiver to measure the bit m r  rate BER and the eybdiagram The photoreceiver was tested using an HF'8664A Synthesized Signal Generator-connaed to a HP70841B Pattem Generator which was connected to a laser driver and a New Focus Model-1780 85Dnm ICG Hz Lam Diode, which was coupled to MMF and sent through an EXFO FVA-310OC Variable Attenuator. The output fib of the Variable Attmuator was buttcoupled with the photodetector by line of sight and by maximizing the received photocurrent. The receiver is then connected to a HP70842B Ermr Detector to measure BER and a Agilest infinium DCA Wide Bandwidth OsciUoswpe with HP54743A TDR Module to measure the eyediagram. During the test the photodetector is biased at 1.8V reverse bias. Typically eye diagrams are measured on a photodetector connected to a kansimpedance amplifier with a fixed gain. Therefore at higher &quencies a lowering of the output magnihde will occur due to 3- roll off. The Helix HMu312 has a rail-t+d limiting 0-7803-7888-1/03/$17.00@2003 IEEE 67 amplifier built in which means the magnitude of the output is constant for changing input magnitudes. Therefore the usefuloess of the eye diagrams is questionable in this study as it can only reasonably measure the lateral eye opeoing due to diffusing carriers. Figure 3 shows the eyediagmn for the same randomly selected detector in a 12x1 array used in the BER results. The eye shows good opening at 3 GHz and -9.5 dBm optical power. The plot does not show any chsractaistic signs of long dif€usion tails. r.L.~ws5*e?3q...* ...... i 3G?LzBitRateti2-9.SdBm ’ : . .  : I  I ! . . ~  ..;..-- ... ............... - ....... . . ~ . ,  ; ................... i ..... ~ .... .~: ....... ~ ... .i .... ~ . *  .... ~ ~ . >  ....... : . I  i . .  :5Oprldiv. ; : ; i ; ; ; 1 i Fig. 3 Eye diagram for single device in 12x1 photodetector m y  a t  36&2 The HF’70841B Pattern Generator was setup to send a PRBS 223-1 sequence to lasa diode with a clock bquency of 1 GHz, l.SGHz, 2GHz and 3GHz on four wparate runs. BER was measured using the HP70842B Prior Detktor which had the same pseudo mdom bit squencc (PRBS) 223-1 pT0gra”Sd for comparison. The BER was measured as a funetion of power hy controlling the attenuation factor on the calibrated EXFO FVA-3IMx: Variable Attenuator. Figure 4 shows the m e d  BER f” a randomly selected detector in a 12x1 array. Ideal “rrrorfrel?.traau2uss . i o n i s w n s i d d t o b c I p a r t i n 1  trillinn M a BER of 10-12. At 3GHz the Si RCE photodctsaor was able to achieve a BER of 10-12 at - 6.5 dBm an order of magnitude worse than Optospeed‘s JnGaAs photcdeteaors which require -16.5dBm at the same BER The other thing to note in Fig. 4 is the 5.5 dBm shift in minimum receival power for 1 GHz transmission to 3 GHz transmission. The designers at Helix felt that this shift in mini” received power is due to the mismatched load of the receiver on the photoaetector caused by the lower capacitance. The other cause of an increase in minimum received power, at all frequencies, is the problem of not collecting all the light f” the MMF as well as the reduced nsponsivity at 8 5 0 m  because of misaligned This work has presented Si RCE p i - n  photcdeteclor arrays designed for Use in parallel optical interconnect systems. Photodetector arrays were wire bonded with existing Si based receiver circuits to make an all-Si photoreceiver operating at 3 GHz, showing performance that could scan compete with existing compound semiconductor photodetecton for a tiaction of the cost. The wafers could also be wed to fabricate a host of Si basal integrated optoelectronics owing to the availability s p e d  pealr. of Si processing and the availability of large wafer sues. These wafers are well suited for large scale integration and are compatible with standard CMOS processing making them ideal for fabricating photodetectors monolithicalJy with receiver circuits. Fip. 4 BER measnrmcnt for single device In 1211 photodetector array Acknowledgements The auihors wish to thank MiLe Ameen and Mark Harris at Axcelis Technologies for providing ion- implantation services for this pjcxt. We would also like to thank Philippe Fliickiger and the entire slatr at the Center of Microtechnology at EPFL for help in the fabrication of these detectors. Additionally we would like to thank B m  Gbyselen of SOlTEC SA for fabrication of the double SO1 wafers. The authors also thank Martin Bossard and J6rg Wieland of Helix AG for packaging our detectors with their receiver circuits as well as for doing the BER and Eye-diagam testing. This =search was spollJored by the Army Research Laboratory (ARL) and was accomplished under the ARL Cwpaative Agreement Number DAALU7-99-2-0070. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the Labomtoy or the U.S. Government. References [I] B. Yang, 1. D. Schaub, S. M. CsuraL, D. L. Rogers, and 1. C. Campbell, “IOGWs All-Silicon Optical Receiver,” IEEE Photonics Technology Letters, Vol. 15, No. 5, May 2003, pp 745-747. [2] M. K. Emsley, 0.1. Dosrmmu, and M. S. ifnlq “Silicon Subshates With Buried Disbibuted Bra&! Reflectors for Resonant Cavity-Enhanced O p t o e l ~ ~ c s , ”  BEE Joumal of Selected Topics in Quantum Electronics, Vol. 8, No. 4, JulylAugust 2002, pp. 948-955. [3] M. K. Emsley, 0.1. Dosunmu, and M. S. OnlQ “High- Speed Resonant-Cavity-Enhanced Silicon Photodetectors on Reflecting Silicon-on-Insulator Substrates,” IEEE Photonics Technology Letters, Vol. 14, No. 4, April 2002, pp. 519-521. [4] M i e o n  Tecnologies, “Parallel Optical Links (PAROLI),” www.Xmeon.com/pamli/, July 2003. 68 

Silicon Resonant Cavity Enhanced 12x1 Photodetector Arrays for Optical Interconnect Receiver Integration

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Infoscience - École polytechnique fédérale de Lausanne