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Atomic layer etching yields 2.5nm wide FinFETs

Atomic layer etching yields 2.5nm wide FinFETs

Technology News |
By eeNews Europe



Presenting their results in a paper titled “First Transistor Demonstration of Thermal Atomic Layer Etching: InGaAs FinFETs with sub-5 nm Fin-width Featuring in-situ ALE-ALD” at IEEE’s International Electron Devices Meeting last week, the researchers used an improved thermal ALE process to fabricate InGaAs-based III-V heterostructures. They reported that this was not only the first time thermal ALE was used to fabricate transistors, they were also able to integrate this process with atomic layer deposition in a single vacuum chamber.

Combining the two processes in-situ, they designed self-aligned In0.53Ga0.47As n-channel FinFETs with fins as narrow as 2.5nm, a gate length of 60nm and characterized with a transconductance gm of 0.85mS/um at Vds of 0.5V. For FinFETs designed with larger fins up to 18nm and the same gate length, they achieved a transconductance gm of 1.9mS/um at Vds of 0.5V.

In all, the authors claim the new transistors exhibited an average 60% gm improvement over devices fabricated through conventional techniques, suggesting that a very high-quality MOS interface could be obtained by in-situ ALE-ALD.

Traditional ALE techniques use plasma with highly energetic ions that strip away individual atoms on the material’s surface. But these cause surface damage. These methods also expose material to air, where oxidization causes additional defects that hinder performance.

In 2016, the University of Colorado team invented thermal ALE, a technique that closely resembles ALD and relies on a chemical reaction called “ligand exchange.” In this process, an ion in one compound called a ligand — which binds to metal atoms — gets replaced by a ligand in a different compound. When the chemicals are purged away, the reaction causes the replacement ligands to strip away individual atoms from the surface.

In this new work, the researchers modified thermal ALE to work on a semiconductor material, using the same reactor reserved for ALD. They used an alloyed semiconductor material, called indium gallium arsenide (or InGaAs), which is seen as faster, more efficient alternative to silicon.


The researchers exposed the material to hydrogen fluoride, the compound used for the original thermal ALE work, which forms an atomic layer of metal fluoride on the surface. Then, they poured in an organic compound called dimethylaluminum chloride (DMAC). The ligand-exchange process occurs on the metal fluoride layer. When the DMAC is purged, individual atoms follow.

The technique is repeated over hundreds of cycles. In a separate reactor, the researchers then deposited the “gate,” the metallic element that controls the transistors to switch on or off.

In experiments, the researchers removed just .02 nanometers from the material’s surface at a time.

 “You’re kind of peeling an onion, layer by layer,” explained first author Wenjie Lu, a graduate student in MIT’s Microsystems Technology Laboratories (MTL). “In each cycle, we can etch away just 2 percent of a nanometer of a material. That gives us super high accuracy and careful control of the process.”

Because the new atomic-level etching process merely repurposes atomic-level deposition microfabrication tools, it could be rapidly integrated, only requiring a small redesign of the deposition tool to handle new gases to do deposition immediately after etching.

MIT – www.mit.edu

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