"Exact placement of atoms can drive future chip innovations"

Brick by brick. Layer by layer. From bottom to top. All these phrases surmise to an extent the process of building a house or a microchip.

“There is a subtle difference though between building a home with bricks and a computer chip with molecules,” says Adrie Mackus, associate professor at the Department of Applied Physics and Science Education and part of the recently launched TU/e Future Chips Flagship initiative.

Mackus is working on a technique known as atomic layer deposition (ALD), which has been a cornerstone technique to make microchips over the last two decades.

The how and how old

Making tiny computer chips with ALD in industry is rather involved. So, when asked to explain how ALD works, Mackus turns to the ‘house building’ analogy.

“When you’re building a house, first you put down the bricks and then the mortar. Ignoring windows or doors, in essence, there are two building materials in each brick layer. It’s the same in ALD where we alternate two chemicals to make an atomic layer.” [See highlight block above for more technical details.]

In 2024, ALD celebrates a major milestone. “This summer it’s ALD’s 50^th birthday,” says Mackus. “It took some time for the technique to be added to the microchip manufacturing toolbox, but now it’s one of the go-to tools.”

The need for ALD change

ALD can put down a certain number of atomic layers, which is a major advantage, but it’s not as simple as just placing flat layers on top of flat layers. Computer chips are made almost completely top-down, which involves first deposition, followed by patterning and etching.  That’s as if you build a house by putting down a cube of bricks and then remove all the bricks that you don’t need, instead of locally making walls and floors.

“When you zoom in on a computer chip, you’ll see different “rooms. Sometimes you want to deposit materials at the floor of the living room but not in the kitchen,” notes Mackus.

“A computer chip consists of a stack of hundreds of thin films (each could between 10 and 100 atomic layers), but the repeated deposition and etching wastes a lot of material too,” says Mackus. “Added to that, future computer chips will rely on building tiny nanoscale structures. Basically, all of this means that we need more precise placement of smaller building blocks.”

Time to be area-selective!

ALD has played a leading role in chip fabrication, but change is afoot, driven by advancements in technology and the need to be more sustainable.

As microchips embrace the nanoscale, ALD needs to move with the times to build nanoscale structures. Enter area-selective atomic layer deposition, or AS-ALD.

“AS-ALD has been around for about 20 years, and work in the field even stagnated a decade ago,” says Mackus. “It’s become popular again, and I believe the precise placement of atoms (also known as a bottom-up approach) can drive future chip innovations.”

With area-selective in the name, AS-ALD focuses on depositing atoms on specific areas only. In comparison to ALD, it’s well-suited to depositing or aligning structures at the nanoscale. To facilitate area-selective deposition, Mackus and his colleagues have led the way in innovating the technique.

“To make future chips with state-of-the-art transistors, we need to build features only a few nanometers in size, as well as complex three-dimensional structures. If you need to deposit in every crack and hole in a three-dimensional structure, you cannot use large molecules of two to three nanometers in size. We need tiny molecules to help build tiny structures, and our starting point is that of small molecule inhibitors that block off certain areas of the chip from deposition.”

Moving to the small molecule-approach of AS-ALD also lowers the threshold for industry to change to this technology as it’s possible to work with small molecules in gas-phase in vacuum systems and avoid working in solution. “AS-ALD could be huge for industry, but it’s about making the technique robust and reliable, as well as easy to implement,” adds Mackus.

Chasing the solution to the puzzle

In terms of the future, Mackus has one main goal. “We want to solve the puzzle of AS-ALD, but to solve the puzzle, we need to find the puzzle pieces.”

The puzzle pieces that Mackus is referring to here relate to how to make sure that all the inhibitor molecules behave in the same way, that the inhibitors molecules block every precursor molecule coming in, and to find inhibitor molecules that work for every material that we want to deposit.

Added to that, in his ERC starting grant-funded project, Mackus is exploring ways to deposit atoms on a surface with any orientation. “To date, the focus has been on depositing on flat surfaces, but future chips will need complex three-dimensional structures. Stacking devices on top of each other is more commonplace in memory chips, but also logic chips are going in that direction, which means that techniques that can achieve this are needed.”

And Mackus is confident that he and his colleagues’ findings will lead to great things. “We hope to change chip manufacturing by helping industry build the nanoscale landscapes of our future chips.”

Further information

To achieve his ALD plans, Mackus is working within the Plasma & Materials processing group at the Department of Applied Physics and Science Education, of which Professor Erwin Kessels is the group leader. Currently, his team is made up of three postdoctoral researchers, seven PhD researchers, and several master’s students.

Check out the website Atomic Limits for updates on the latest advancements from Mackus and his colleagues on ALD.

You can also read a series of blog entries there in relation to AS-ALD. One entry covers fully self-aligned vias, which is proposed as the ‘killer application’ for AS-ALD. The second in the series looks at metal-on-metal area-selective deposition, and the final entry explores the AS-ALD of diffusion barriers for interconnect technology.