Making computer chips smaller is not just about better design. It also depends on a critical step in manufacturing called patterning, where nanoscale structures are carved into materials to form the circuits inside everything from smartphones to advanced sensors.
To create these patterns, engineers use a hard mask, a thin, durable material layer that protects selected regions while the exposed areas are etched away.
"As chips get smaller, the manufacturing process becomes much more demanding," said Saptarshi Das, Penn State Ackley Professor of Engineering Science and professor of engineering science and mechanics. "The mask used to define these patterns must survive extremely harsh processing conditions. If the mask degrades, the patterns cannot be transferred reliably."
Now, Das and a team of international researchers report in a study published in Nature Materials that an atomically thin two-dimensional (2D) material, chromium oxychloride (CrOCl), dramatically outperforms conventional hard mask materials used in chip fabrication.
"The industry is really struggling to find new hard mask material," said Das, corresponding author of the study. "As chips move toward smaller dimensions and more complex 3D architectures for faster and better electronics, we need different hard mask materials to make it easier to manufacture the chips."
He noted that semiconductor manufacturers have relied largely on the same hard mask materials, such as silicon dioxide, silicon nitride, aluminum oxide, chromium, nickel, titanium nitride, etc. During manufacturing, engineers use plasma etching, a process that uses highly reactive gases to carve deep, narrow features into silicon. These harsh conditions gradually erode many traditional mask materials.
2D metal oxyhalides such as chromium oxychloride, niobium oxychloride may be the answer, according to the researchers.
Ziheng Chen, Penn State doctoral candidate in engineering science and mechanics and co-author of the study, explained that the material's layered crystal structure plays a key role in these advantageous properties.
"This 2D material is like lasagna," Chen said. "It's a layer-by-layer structure."
Instead of strong chemical bonds between layers, the sheets are loosely held together. When exposed to plasma, the material forms what Chen described as a protective surface.
"When the plasma is bombarding the surface, it will form a passivation layer," he said. "That layer becomes chemically inert and shields the material underneath from further reaction."
In thicker bulk form, these materials offer interesting magnetic and electronic properties. But their potential as ultrathin, plasma-resistant masks for chip fabrication had not previously been demonstrated, Das said. Not only did Das and his team do so, but they also discovered another advantage of 2D chromium oxychloride over traditional hard masks: It can be patterned separately and then transferred onto delicate materials such as flexible plastics or glass for use in flexible electronics or specialized sensor platforms. That flexibility could expand options for fabricating devices on unconventional materials, including flexible electronics or specialized sensor platforms.
"You make this hard mask on a rigid substrate, and you're able to transfer it onto anything else," Das said. "You remove that limitation of conventional hard masks."
The discovery that chromium oxychloride could handle exposure to plasma could work with more delicate materials was unexpected, according to Das.
"We did not really anticipate that this chromium oxychloride was going to be a hard mask material," Das said. "It was experimental serendipity."
Originally, the team tried to etch the material for an entirely different project. But unlike other 2D materials, the researchers found they could not etch it, they said.
