To keep up with Moore's Law — an observation made in the 1960s that the number of transistors on an integrated circuit doubles about every two years — researchers are finding ways to cram as many transistors as possible onto microchips. The newest trend is 3D transistors that stand vertically, like fins, and measure about 7 nanometers across — tens of thousands of times thinner than a human hair. Tens of billions of these transistors can fit on a single microchip, which is about the size of a fingernail.

Researchers fabricated a 3D transistor less than half the width of today's slimmest commercial models, which could help cram far more transistors onto a single computer chip. Pictured is a cross-section of one of the transistors that measures only 3 nanometers wide. (Courtesy of the researchers)

A modified chemical-etching technique, called thermal atomic level etching (thermal ALE), was used to enable precision modification of semiconductor materials at the atomic level. Using that technique, researchers fabricated 3D transistors that are as narrow as 2.5 nanometers and more efficient than their commercial counterparts. The technique repurposes a common microfabrication tool used for depositing atomic layers on materials, meaning it could be rapidly integrated. This could enable computer chips with far more transistors and greater performance.

Microfabrication involves deposition (growing film on a substrate) and etching (engraving patterns on the surface). To form transistors, the substrate surface gets exposed to light through photomasks with the shape and structure of the transistor. All material exposed to light can be etched away with chemicals, while material hidden behind the photomask remains.

The state-of-the-art techniques for microfabrication are known as atomic layer deposition (ALD) and atomic level etching (ALE). In ALD, two chemicals are deposited onto the substrate surface and react with one another in a vacuum reactor to form a film of desired thickness, one atomic layer at a time. 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.

ALE is 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. Thermal ALE has, so far, only been used to etch oxides.

Thermal ALE was modified to work on a semiconductor material, using the same reactor reserved for ALD. An alloyed semiconductor material, called indium gallium arsenide (or InGaAs), was exposed to hydrogen fluoride, the compound used for the original thermal ALE work, which forms an atomic layer of metal fluoride on the surface. Then, an organic compound called dimethylaluminum chloride (DMAC) was poured in. 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 “gate” was deposited — the metallic element that controls the transistors to switch on or off.

Using the technique, the researchers fabricated FinFETs, 3D transistors used in commercial electronic devices. FinFETs consist of a thin “fin” of silicon, standing vertically on a substrate. The gate is essentially wrapped around the fin. Because of their vertical shape, anywhere from 7 billion to 30 billion FinFETs can squeeze onto a chip. Most of the FinFETs measured under 5 nanometers in width — a desired threshold across industry — and roughly 220 nanometers in height. Moreover, the technique limits the material's exposure to oxygen-caused defects that render the transistors less efficient.

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