Unisantis and IMEC Shrink SRAM Cells

Article By : Rick Merritt

Startup working with IMEC demonstrates the smallest SRAM cells to date.

ANTWERP, Belgium — A startup led by one of the pioneers of flash memory worked with the Imec research institute to design the smallest SRAM cells to date. The 0.0205 mm² and 0.0184 mm² 6T-SRAM cells use a vertical gate-all-around transistor being developed by Unisantis as a building block for tomorrow’s leading-edge chips.

The work was one of a handful of announcements at the opening day of the Imec Technology Forum. Other news here includes work on more accurate indoor location over Bluetooth, a dense lab-on-a-chip and a camera-free approach to eye tracking, all developed solely by Imec.

A team from Unisantis and Imec is using the startup’s so-called Surrounding Gate Transistor with a 50-nm minimum pillar pitch. The design is suited to a 5nm SRAM but is less optimal for logic because it would require three of the transistors to provide the performance of a single FinFET.

The Unisantis design is similar to what’s generally known as a vertical nanowire, a candidate for years as a future transistor. They have the potential to reduce chip area significantly, one of the last remaining areas of progress in CMOS scaling.

To date, most researchers see vertical transistors as having challenges that prevent their practical use in commercial chips for many years. The Unistantis design in particular would need two to three times more performance to compete in logic with FinFETs.

FinFETs are expected to scale down to use through the 5nm node expected to hit volume production in 2020. A horizontal gate-all-around transistor, sometimes called a nanosheet, nanowire or nano-slab, is widely expected to be its successor at the 3nm node.

The CTO of Unistantis, Fujio Masuoka, was a pioneer of NAND at Toshiba in the 1980’s. His startup clearly aims to pioneer a next-generation transistor that could someday be even more fundamental to tomorrow’s semiconductors.

In February, Samsung reported on a 6T 256-Mbit SRAM with a 0.026-mm² bitcell it made using FinFETs and extreme ultraviolet lithography (EUV), the smallest device at that time. The Korean giant said it had test silicon of the design that gave it confidence in its plans to be the first to use EUV commercially.

The Unisantis design represents a significant shrink over the Samsung work, which at the time leapfrogged the work of chip giant Intel. Using EUV, the Unisantis transistor could be made in a 5nm process at a cost comparable to a FinFET SRAM, according to a press release from the Imec.

“The big three are working on gate-all-around (GAA) technology. IBM has been working on vertical nanowire GAA for years.… With Unisantis in the game, it could be the biggest legal cat-fight since Rambus took on the industry in the 90’s,” said G. Dan Hutcheson, president of VLSI Research.

However, the market watcher noted that “this is not a simple research project that can be executed in small research facilities like Imec. You need manufacturing scale to scale-up from the transistor to finished designs that can be made economically enough be saleable at anything but a loss.”

Startup Unisantis and Imec claim to have designed a smaller SRAM bitcell than Samsung, Intel and TSMC. (Images: Imec).

Bluetooth indoor location gets within 30cm

Separately, Imec announced software to deliver indoor location accuracy up to 30cm over Bluetooth using time and phase data to estimate time-of-flight information. Current techniques use signal strength to deliver 3-5 meter accuracy.

The software would let Bluetooth compete with the indoor accuracy of ultrawideband chips from companies such as DecaWave. It aims to offer similar accuracy at much lower costs with the added potential that Bluetooth is widely integrated into smartphones.

Imec demoed its software here on Atmel 802.15.4 and an NXP Bluetooth reference designs. It runs on an Arm Cortex M4F and uses less than 32 Kbits ROM and 64 Kbits RAM.

The researchers are proposing changes to Bluetooth Milan, the next version of the protocol, to enable their techniques widely. The standards process could take about a year. Meanwhile users who own both ends of a Bluetooth link could enable the software on their connections.

Separately, Imec is working with partners on a way to add encryption and authentication to the design without increasing its hardware requirements. The technology enables use of Bluetooth location as a digital lock for a variety of systems including cars, potentially supplanting NFC.

In addition, Imec created a high-end lab-on-a-chip marrying an array of 16,384 electrodes to a 16-well microfluidic array. The design aims to carry STEM-cell based human tissues to accelerate testing of new drugs.

Finally, Imec demoed a camera-free eye-tracking technology integrated on eyeglasses. It uses electro-oculography sensors to reduce weight, cost and power consumption. The glasses aim to deliver ways to control systems with eye gestures such as blinks.

Imec’s prototype eye-tracking glasses use dry electrodes from Datwyler.

— Rick Merritt, Silicon Valley Bureau Chief, EE Times