“Quilted” Circuits

The semiconductor industry road map, which corresponds with Moore’s Law and states that the density of chips doubles approximately every 18 months, is facing a bit of a roadblock. The experts agree: If all else remains the same, the gains in cost and performance of monolithically integrated chips are going to diminish substantially in the near future, which means that the processing speed of computers will also suffer.

A seamless interface, tailor-made by Notre Dame faculty, may be just the thing to solve the interchip interconnect problem. In fact, a research team led Gary H. Bernstein, professor of electrical engineering, has demonstrated a new paradigm for interchip communication called Quilt Packaging (QP).

QP involves the fabrication of contact nodules that protrude from the edge of a chip. The chips needed to form a system are placed side-by-side, with the nodules allowing a direct electrical interconnection. Linking together like the blocks of a card-trick pattern quilt, the various integrated circuits can, in fact, be of heterogeneous materials. In this way, for example, silicon processors could be combined efficiently and inexpensively with optical processors, microwave devices, or memory.

This type of interchip contact offers high-speed signal paths for the high-fidelity transmission of signals between chips at very high frequencies, into the hundreds of GHz. Signals could also be transferred between chips far more faithfully than the conventional approach of going from one chip to another through packages and printed circuit boards. The net result, as demonstrated by the team — Bernstein, Patrick J. Fay, associate professor of electrical engineering; Gregory L. Snider, professor of electrical engineering; and Qing Liu, a graduate student in electrical engineering — is a more efficient use of the die area and better performance in an overall smaller system with the need for fewer chip packages. In short, a better system is achieved at a lower cost.

The team presented experimental results at the Second International Work-shop on SOP, SIP, SOC (3S) Electronics Technologies in September 2006, where they unveiled world-record transmission efficiencies at frequencies up to 40GHz. “We used the equipment in the Notre Dame Nanofabrication Facility to build and test the system,” says Bernstein, “and we are very excited because this milestone shows that by using Quilt Packaging, the cost of integrated systems, energy use, size, and weight decreases, and performance can be improved. Additional studies may impact portable devices such as laptops and cell phones but could also improve the performance of high-speed systems, such as radar and microwave communication systems.”

Self-aligning keys and slots bridge the gap between neighboring chips, enabling direct chip-to-chip contact. The multi-chip “quilted” system these links create offers high signal bandwidth, reduced power dissipation, reduced production costs, and the option to use heterogeneous materials. Nodules as small as 10 microns have been demonstrated by the Notre Dame team in this National Science Foundation funded project.