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Sun Working to Obsolete Motherboards
perl_camel_jockey writes "Sun is developing a new technology that promises to increase computing power by eliminating the need for physical, soldered chip-to-chip connections on the motherboard. Called 'proximity communications', it portends the ability for chips to talk to one another wirelessly just by being next to each other. Potential applications in computer design abound. Apparently this is part of Sun's Hero program, recipient of a $50 million grant from DARPA's High Productivity Computing Systems program to rejuvenate supercomputing in the US and regain the lead lost to Japan, in particular to NEC's Earth Simulator, ranked as the most powerful supercomputer in the world."
Here is some technical background on the subject:
Abstract
This paper reports results from wireless chip to chip communication experiments. Sixteen bit words pass from one chip to another in parallel without detectable error at 1.35 billion data items per second for a total data rate of 21.6 Gigabits per second. The experiment transmits pseudo random patterns between chips built in 350nm CMOS technology. Chips touch face-to-face to communicate. The same pseudo random data pattern is loaded onto both chips so that the receiving chip can check the accuracy of every bit communicated. Each communication channel consumes a static power of 3.6 milliWatts, and a dynamic power of 3.9 picoJoules per bit communicated. The channels lie on 50 micron centers. Because the capacitive communication works through covering oxide, ESD protection is unnecessary. Vernier position measuring circuits built into the chips indicate the relative position of transmitting and receiving arrays to assist mechanical alignment. The test chip includes a Vernier circuit that provides inter-chip position measurements with a resolution of 1.4 microns.
Background
On-chip performance has been increasing much more rapidly than off-chip communication bandwidth because both on-chip transistor density and clock frequency are increasing faster than off-chip input/output (I/O) density and frequency [2]. This difference occurs because off-chip bonding and wiring are about two orders of magnitude larger than on-chip wiring. On-chip wiring pitch is on the order of 1 micron, while off-chip wiring and ball-bond pitches are on the order of 100 microns. The performance gap between on-chip and off-chip bandwidth makes off-chip bandwidth a performance bottleneck.
Fig. 1 shows a comparison of the density of I/O pads for proximity communication versus area ball bonding [2]. Because proximity communication structures use the lithographic pitch of on-chip wires, proximity communication can achieve a density about 60 times greater than area ball bonds.
Proximity Communication
Proximity communication is based on the observation that faster, lower-cost communication is possible over shorter distances. We name this communication method "Proximity" because chips are placed in a face-to-face arrangement with the transmitter and receiver circuits aligned with only microns of distance between them.
Fig. 2 illustrates two chips positioned for proximity communication. Because the transmitter and receiver are close to each other and communicate by capacitive coupling rather than through large off-chip wires, the transmitter and receiver circuits are small. Moreover, the transmitter and receiver pads are protected from exposure to electrostatic discharge (ESD) events by the top layer dielectric and passivation. Thus the transmitter and receiver omit ESD protection, further reducing the parasitic capacitance and power consumption.
We chose to couple the transmitter pads directly to the receiver pads [1][5], in contrast to [3][4] in which the transmitter couples to a multi-chip module (MCM) substrate trace which couples in turn to a receiver pad. Coupling the transmitter pad directly to the receiver pad provides a stronger signal, improving signal integrity and reducing power.
Alignment
Alignment is a critical issue. Misalignment reduces received signal strength by reducing coupling area and increasing distance. Cross-talk also increases with misalignment because the voltage swings on adjacent transmitter and receiver pads couple into the receiver pad. In order to couple the transmitter and receiver pads best, the pads must be placed in correct alignment. Fig. 3 shows the six alignment dimensions, three translational and three rotational.
We use an on-chip Vernier measurement system to measure the relative positions of two chips in the x, y, and dimensions. Verniers are common in mechanical systems, and a micro electro-mechanical system (MEMS) Vernier strain gauge has demonstrated a resolution of 10 nm [6].
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