Memory

Paving the way to extreme scaling for logic and memory transistors

16th December 2019
Lanna Cooper

At this year’s IEEE International Electron Devices Meeting, imec, reported an in-depth study of scaled transistors with MoS2 and demonstrated best device performance to date for such materials. MoS2 is a 2D material, meaning that it can be grown in stable form with nearly atomic thickness and atomic precision. Imec synthesised the material down to monolayer (0.6nm thickness) and fabricated devices with scaled contact and channel length, as small as 13nm and 30nm respectively.

These very scaled dimensions, combined with scaled gate oxide thickness and high K dielectric, have enabled the demonstration of some of the best device performances so far. Most importantly, these transistors enable a comprehensive study of fundamental device properties and calibration of TCAD models.

The calibrated TCAD model is used to propose a realistic path for performance improvement. The results presented here confirm the potential of 2D-materials for extreme transistor scaling - benefiting both high performance logic and memory applications.

Theoretical studies recommend 2D materials as the perfect channel material for extreme transistor scaling as only little short channel effects are expected compared to the current Si-based devices. Hints of this potential have already been published with transistors built on natural flakes of 2D materials.

For the first time, imec has tested these theoretical findings through a comprehensive set of 2D-materials-based transistor data. The devices with the smallest footprint have a channel length of 30nm and less than 50nm contact pitch. ON current as high as 250µA/µm has been demonstrated with 50nm SiO2 gate dielectric. On current of ~ 100µA/µm and an excellent SSmin of 80mV/dec (for VD =50mV) have been demonstrated with 4nm HfO2 in a backgated configuration.

The device performance is not impacted by contact length scaling, confirming that carriers are injected from the edge of the contact metal directly into the channel, in line with TCAD simulations. The work confirms that TCAD models capture large parts of device physics and guide experimental validation and mapping the application space. Part of the paper thatt was presented at IEDM, is dedicated to setting the path for device optimisation for reaching Si-like performance targets.

“Although still an order of magnitude away from Si transistors, we have brought our MOSFET devices into a realm where they show promising performance for future logic and memory applications,” said Iuliana Radu, director of Exploratory and Quantum Computing imec.

“To bridge this order of magnitude, we have identified a path of systematic improvements such as a further reduction of the gate oxide thickness, the implementation of a double-gated architecture, and further reduction of channel and interface defects. We are transferring this insight to our 300mm-wafer platform for transistors with 2D materials, which was announced at last year’s IEDM.”

“As a world-leading research and innovation hub, it is imec’s role to push device scaling towards the ultimate limit. We are tackling this challenge by investigating different scaling options, providing optimal projections, and guiding the industry to adopt the proposed solutions," stated Luc Van den hove, imec CEO.

“Our partners expect us to lead the way and to support them in realising their roadmaps by demonstrating the potential of innovative concepts and novel materials. This is why I am so thrilled we have demonstrated excellent performance in ultra-scaled devices with 2D materials, and a credible path to further improvements aiming at mass production in industrial 300mm fabs.”

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