Walkie-talkies for Chips

5 April 2010—No matter how much circuitry engineers are able to cram into a semiconductor device, they can't make it work faster than the wires between such devices will allow. That's why Sony's recent development of a wireless alternative is so exciting. Today some products employ as many as 1000 pins to connect devices, and those pins take up a lot of space. More than anything, they set the limit on how large an electronic device can be.

Earlier this year, Sony Corp. unveiled the first millimeter-wave wireless technology that can serve as a short-range link among devices. A 40-nanometer complementary-metal-oxide-semiconductor (CMOS) prototype system achieved transfer speeds of 11 gigabits per second operating at 56 gigahertz over a distance of 14 millimeters. Adding a secondary antenna can increase the range to 50 mm. Sony described the technology in February at the International Solid-State Circuits Conference, held in San Francisco.

Using wireless to shift the data around instead of wires and pins, "will let us use simpler substrates and simpler IC packaging and help us produce smaller ICs," says Yoshiyuki Akiyama, senior manager in Sony's Core Device Development Group. He adds that wireless also enhances the reliability of movable and detachable parts in certain products, such as boards that have to be removed for maintenance or upgrading.

Millimeter-wave wireless operates at a frequency between 30 to 300 GHz, ideal for transferring large amounts of data. Its short wavelength—1 to 10 mm—makes it possible to miniaturize the circuitry that can be fabricated using standard CMOS technology.

Workers have described some millimeter-wave wireless systems that target interconnections between products. But because the transmission distances are much greater than those for intraconnections inside a box, they require large, complex, and power-hungry circuitry to maintain high-speed data transfers. This makes the technology impractical for short-range intraconnects.

Sony designed its system to be much more compact and to work much more effectively at low power. To this end, its system employs a free-running transmission oscillator and an injection lock system. Injection locking occurs when one oscillator causes another one to oscillate (or lock in) at the same carrier frequency, capturing it, as it were. In this case, the result is that the transmitter's signal makes the receiver's oscillator align itself to that from the transmitter, synchronizing the two carrier signals. Sony has found it to be an efficient, compact, low-power means of providing internal connections.

The differential signals of the receiver's oscillator are then applied directly to the mixer at the gates of the transistors. Bonding wire antennas are used for both the transmitter and receiver signals. The injection lock method eliminates the need for the phase-lock loop system that's generally used for such synchronization. That conventional method employs a feedback system that combines a voltage-controller and a phase comparator to track an applied frequency, and as a result, it gobbles power and occupies a lot of real estate.

"Direct connection also reduces the size of the circuits and power consumption," says Akiyama. In addition, he says, Sony has optimized the system's circuitry for use as compact low-power interconnects. The result is an overall footprint of just 0.13 square millimeters for the entire system, with a total power consumption of 70 milliwatts.

With further improvements in performance, Sony expects to first use the technology to enhance the reliability of movable and detachable parts and for high-speed connections between stacked printed circuit boards, or PCBs. Then Sony sees it being incorporated into chip packages. Eventually it will be integrated directly into system-on-chip devices employed in the company's general consumer products to replace conventional input-output circuitry.

Sony envisages the eventual disappearance of pins and wiring, resulting in clutter-free packages and PCBs. Chips needing to communicate with each other would simply be laid in close proximity, avoiding problems with crosstalk with a number of techniques, including the use of different frequency channels.

With Sony still refining the technology, it won't talk about commercialization dates, but Kenichi Kawasaki, general manager of millimeter-wave systems development at Sony, says "the potential to launch it within three years is strong. Once we are satisfied with the layout and performance, then we can start production immediately."