Arm Shows Backside Power Delivery as Path to Further Moore’s Law

Delivering power to devices from the other side of the silicon will enable smaller processors

2 min read
Photo of a silicon wafer, with a lightning bolt imaged on top
Photo-illustration: iStockphoto/IEEE Spectrum

People often think that Moore’s Law is all about making smaller and smaller transistors. But these days, a lot of the difficulty is squeezing in the tangle of interconnects needed to get signals and power to them. Those smaller, more dense interconnects are more resistive, leading to a potential waste of power. At the IEEE International Electron Devices Meeting in December, Arm engineers presented a processor design that demonstrates a way to reduce the density of interconnects and deliver power to chips with less waste.

“If you supply one volt at the power regulator, most likely you don’t get one volt at the transistor,” explained Divya Prasad, a senior research engineer at Arm. A 10 percent voltage margin between the regulator and the transistors it’s meant to power is considered acceptable. However, “it’s getting harder and harder to meet this.”

Prasad and her colleagues implemented a Cortex A53 CPU design using a future 3-nanometer process developed by Belgian research firm imec. The world’s largest foundry, TSMC, is on track to reach full 5-nanometer production in 2020, so 3-nanometers is only a short way off.

They made three variations on the design. One dealt with the problem of power delivery in the same manner as is done today, where circuits access power from the network of interconnects above the transistors. Another used what imec calls buried power rails. Here, rails of ruthenium were built into the space below the silicon surface, these were energized by vertical connections to the network above and then tapped at regular intervals to provide power to circuits.

Earlier this year, imec engineers showed advantages for SRAM cells using buried power lines. SRAM, typically a 6-transistor circuit, is particularly sensitive to interconnect resistance, which can slow read and write performance. In simulations, buried ruthenium power rails led to 31 percent faster reads and 340-millivolt lower write voltages.

The final variation had buried power rails that were instead energized by micrometer-scale vias that ran vertically through the backside of the silicon.

Using the ordinary approach, the Arm engineers were unable to meet the 10 percent voltage margin, because too much power was lost in the interconnect network’s resistance, Prasad explained. With buried power rails and frontside power delivery, the design was able to hit the margin, but the engineers had to trade performance for power loss.

Buried power rails with backside delivery was the clear winner, meeting the margin and dropping only 1 percent of the voltage without harming performance. The only trade-off is the complexity of manufacturing the backside network, Prasad noted. To make it, the frontside of the wafer must be fully processed, including the construction of the buried power rails. The wafer is then flipped over, and the silicon is removed down to a mere 500 nanometers thickness. Then vertical connections less than 1 micrometer across, called micro-through-silicon vias (microTSVs), were built to contact the buried power rails. A simple network of interconnects on the backside links the microTSVs into a power delivery network.

“We’re pretty excited about backside power delivery,” said Prasad. In the best case scenario, it provided seven-fold improvement in voltage loss over the other two options.

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The First Million-Transistor Chip: the Engineers’ Story

Intel’s i860 RISC chip was a graphics powerhouse

21 min read
Twenty people crowd into a cubicle, the man in the center seated holding a silicon wafer full of chips

Intel's million-transistor chip development team

In San Francisco on Feb. 27, 1989, Intel Corp., Santa Clara, Calif., startled the world of high technology by presenting the first ever 1-million-transistor microprocessor, which was also the company’s first such chip to use a reduced instruction set.

The number of transistors alone marks a huge leap upward: Intel’s previous microprocessor, the 80386, has only 275,000 of them. But this long-deferred move into the booming market in reduced-instruction-set computing (RISC) was more of a shock, in part because it broke with Intel’s tradition of compatibility with earlier processors—and not least because after three well-guarded years in development the chip came as a complete surprise. Now designated the i860, it entered development in 1986 about the same time as the 80486, the yet-to-be-introduced successor to Intel’s highly regarded 80286 and 80386. The two chips have about the same area and use the same 1-micrometer CMOS technology then under development at the company’s systems production and manufacturing plant in Hillsboro, Ore. But with the i860, then code-named the N10, the company planned a revolution.

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