IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 33, NO. 2, MAY 2010 389 Aligning Chips Face-to-Face for Dense Capacitive and Optical Communication John E. Cunningham, Ashok V. Krishnamoorthy, Ivan Shubin, Xuezhe Zheng, Senior Member, IEEE, Mehdi Asghari, Dazeng Feng, and James G. Mitchell Abstract—We report a new method that precisely self-aligns face-to-face semiconductor chips or wafers to enable commu- nication between the chips using electromagnetic waves. Our alignment mechanism takes advantage of miniaturized versions of two of nature’s idealized shapes: an inverse pyramidal shape defined by a self-terminating wet-etch process in silicon and micro-spheres with radii accurate to submicron accuracy. This approach allows chips to be packaged using passive alignment that is self locating and reaches nearly one micron level of chip misalignment tolerance. Packages for applications to capacitive and optical connections are presented. Additionally, we describe a physical architecture for a multi-chip array packages with “bridge” and “island” chips where the function of the bridge is to transfer electromagnetic signals between island chips using either capacitive or optical proximity communication. The bridge chip can provide a predetermined amount of compliance to help maintain alignment and thereby accommodate topology variants in first level package or in chip thickness when required. Experi- mental packages providing precise alignment between 1-D arrays and 2-D arrays of chips are presented. We show that our precision alignment mechanism enables high fidelity 10 Gb/s optical-prox- imity-communication with reflecting mirrors micro-machined into Silicon and co-integrated to low loss silicon-on-insulator waveg- uides for chip-to-chip communication. The alignment mechanism was also applied to a demonstration of chip-to-chip capacitive proximity communication in a linear array of six chips. Alignment measurements on a 4 4 array of chips are reported. Index Terms—Communication systems, computer performance, electromagnetic coupling, electromagnetic fields, mirrors, optical communication, optical coupling, semiconductor devices, self-or- ganizing control. I. CAPACITIVE AND OPTICAL PROXIMITY COMMUNICATION C ONDUCTIVE electrical interconnections and on-chip transceivers have long been used to provide reliable interconnections between very-large-scale integration (VLSI) electronic components, and have dominated the interconnect Manuscript received October 19, 2008; revised July 28, 2009; accepted September 24, 2009. First published February 25, 2010; current version published May 05, 2010. This work was based upon work supported, in part, by DARPA under Agreement HR0011-08-09-0001 and Agreement W911NF-07-1-0529. This work was recommended for publication by Asso- ciate Editor L. Zhang upon evaluation of the reviewers comments. J. E. Cunningham, A. V. Krishnamoorthy, I. Shubin, X. Zheng, and J. G. Mitchell are with the Microelectronics Physical Sciences Center, Sun Microsys- tems, San Diego, CA 92121 USA. M. Asghari and D. Feng are with the Kotura Inc., Monterey Park, CA 91754 USA. Color versions of one or more of the figures in this paper are available at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TADVP.2009.2037437 hierarchy for reasons of manufacturing cost, system packaging, and ease-of-use. VLSI linewidths and on-chip clock speeds have continued to scale, putting pressures on the ability of traditional wires to achieve the off-chip bandwidths necessary to fully and efficiently utilize the resources available on-chip. When optimizing chip input and output circuits that communi- cate conductively, electronic circuit and system architects must design within the constraints of VLSI packages and circuit boards by using advanced circuit techniques such as predistor- tion, equalization, multilevel coding, and digitally controlled feed-forward clock and data recover blocks—commonly referred to as serializer-deserializer (SerDes) transceivers. However, this generally increases the area and power consump- tion and limits the maximum number of input/output (I/O) circuits per chip. Current best-in-class SerDes transceivers [1] are expected to yield signaling densities between 1–5 Tb/cm . Proximity communication represents the general concept for face-to-face integrated circuits communicating by capacitive coupling [2] that we refer to as PxC. We extend this concept to optical proximity communication or Optical Proximity Com- munication (OPxC). In both cases, very high communication signal density can be achieved when compared to wire-bonding or solder-ball connections. In addition, to communicate off-chip, the circuits need drive only a small, high-impedance, capacitive pad (or optical modulator), much akin to the gate of a transistor. In the capacitive case, the electrical pad pitch may be on the order of 20 m. Each pad can drive signals at line rates of 2.5–5 Gb/s or higher [3]. This provides a potential communication density in excess of 1.25 Pb/cm . Experimental capacitive proximity communication circuits have yielded areal densities up to 43 Tb/cm to date [4]. Sun Microsystems has built and tested capacitive PxC circuits in 180 nm CMOS and is presently testing a 90 nm PxC test chip. In past prototypes we have used capacitive pad pitches from 50 m down to 24 m. Other academic research efforts have demonstrated pad pitches as low as 9 m although without full packaging solutions [5]. Another group has demonstrated capacitive coupling between a chip and the first level package using alignment based on solder reflow [6]. At 24 m, PxC pads offer about a 50 areal density improvement over traditional 150- m pitch chip I/O solder bumps. In this comparison we consider short-reach SerDes, which are links optimized for short traces within a package or a very short board route; these SerDes consume much less power compared to traditional memory or I/O links running over long PCB traces [7]. In the optical proximity case, an optical coupler can be as small as 20 m on a side. The optical coupler may communicate 1521-3323/$26.00 © 2010 IEEE