High Frequency, High Density Interconnect Using AC Coupling
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High Frequency, High Density Interconnect Using AC Coupling Paul Franzon, Angus Kingon*, Stephen Mick, John Wilson, Lei Luo, Karthik Chandrasakhar, Jian Xu, Salvatore Bonafede+, Alan Huffman+, Chad Statler+, Richard LaBennett+ Department of Electrical and Computer Engineering, *Department of Materials Science and Engineering, + MCNC-RDI, Contact : Box 7914 NC State University, Raleigh NC 27695 [email protected]; 919.515.7351 Abstract AC Coupled Interconnection (ACCI), in conjunction with buried solder bump technology, provides a method to achieve signal I/O pitches of less than 75 µm and signaling rates greater than 5 Gbps per I/O on integrated circuits, while preserving excellent signal integrity. This paper presents a summary of approaches, status, and discusses material issues important to performance. Introduction Achieving high-density connectivity with conventional I/O schemes presents a number of difficulties. All these schemes have, at their root, the concept of using a mechanical connection for signal transmission. To achieve reliable connections, the mechanical connection must be sufficiently compliant to withstand thermal cycling. Achieving a tight pitch, while maintaining compliancy is very difficult, as it usually results in a tall, thin, and thus relatively fragile and difficultto-make mechanical structure. In addition, high density interconnect connections require greater smoothness and flatness of the surfaces being mated, often increasing their cost. Transmission of digital (and many analog) signals do not require the DC and low frequency components. Thus the signals Figure 1. Basic Concepts in AC coupled can be transmitted through Interconnect. Tight pitch signaling via aligned series capacitors and series capacitors or transformers and buried solder transformers, no mechanical bumps for DC power delivery and alignment connection is needed. Previous efforts to explore control. capacitive coupling [2,3] have
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shown success but did not address issues such as how to connect power and ground. An approach has been identified that solves this, and other, problems with AC coupling (Figure 1). Half capacitors, or spiral inductors, are fabricated on the chip and the opposing chip or package surface. The chip side is covered with a thin dieletric, to prevent accidental shorting. DC connections are provided through a dense field of conventional solder bumps. The bumps are buried either in the package or in the redistribution layer on the chip. This geometry brings the opposing half capacitors or spirals into close and controlled proximity. Single-sided, partial and full differential can all be supported, with the normal tradeoffs (Figure 2). Inductive coupling provides the interesting potential for creating a differential circuit with only one pad per I/O. This scheme has a number of advantages over the mechanical alternatives, including excellent compliance, good tolerance to temperature changes etc. It also permits high-speed signaling with excellent signal integrity.
Figure 2. Single sided a
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