Paul van Gerven
15 June 2021

Imec has come up with a successor to the gate-all-around transistor. At the 2021 Symposia on VLSI Technology and Circuits (VLSI 2021), the Leuven-based research institute demonstrated for the first time fully functional integrated forksheet FETs. “Our electrical characterization results confirm that the forksheet is the most promising device architecture to extend the logic and SRAM scaling roadmaps beyond 2nm, leveraging the nanosheet integration in a non-disruptive way,” commented Naoto Horiguchi, Director CMOS Device Technology at Imec.

As dimensions of transistors shrink, the proximity between the drain and the source lessens the gate electrode’s ability to control the flow of current in the channel. That’s why leading-edge chipmakers in the early 2010s moved to the FinFET architecture, in which the channel is raised to a 3D fin structure with a gate draped over it on three sides. Soon, they’ll start to envelop gates completely, hence the name gate-all-around (GAA) transistors.

Imec FinFET to forksheet
Evolution from FinFET to GAA nanosheet, and to forksheet. Credit: Imec

As the journey to higher logic density continues, a tighter spacing between nFET and pFET devices will be required. However, for both FinFET and nanosheet devices, process limitations pose a limit to how close these n and p devices can be brought together. In the forksheet transistor, a dielectric wall in between the n- and pMOS devices is introduced, which physically isolates the n-gate trench from the p-gate trench, allowing a much tighter n-to-p spacing than what’s possible with either FinFET or GAA devices.

At VLSI 2021, Imec presented the electrical characterization of its forksheet devices that were successfully integrated by using a 300 mm process flow, with gate lengths down to 22 nm. Both n- and pFETs, each with two stacked Si channels, were found to be fully functional. Their short channel control was comparable to that of vertically stacked nanosheet devices that were co-integrated on the same wafer. For the forksheet devices, dual work function metal gates were integrated as tight as 17 nm, which is about 35 percent of the spacing in state-of-the-art FinFET technology. This highlights one of the key benefits of the new device architecture.

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