In a joint project across three universities and MAX IV laboratory, researchers have developed a revolutionary experimental setup for atomic layer deposition. The new instrument was designed specifically for MAX IV and will allow for observations previously impossible.
SPECIES, one of the soft X-ray beamlines in MAX IV 1.5 GeV storage ring, has added to its portfolio a new cutting-edge instrument. The new experimental setup has been specially developed to use Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) for the study of atomic layer deposition (ALD), a process where thin films of material are grown depositing one layer at a time.
This experimental setup is composed of a custom cell where the ALD process is performed and observed using APXPS. The instrument is the result of an extensive collaboration between the University of Helsinki, world-leading in ALD studies, University of Oulu, Lund University, and MAX IV Laboratory, and funded by the University of Helsinki through the FiMAX consortium.
In February, the team from the University of Helsinki led by professor Mikko Ritala, and from the University of Oulu came to MAX IV for the final experiments and refinement activities on the experimental setup. We talked with the scientists to understand how the cell they have developed allows for unprecedented observations.
One layer at a time
“Atomic layer deposition is a method used to produce thin films”, explains Matti Putkonen, associate professor at University of Helsinki and expert in atomic layer deposition and etching. “It is known since the 1970s, and nowadays it is a mainstream technology in many industrial processes such as for the production of microelectronics and microprocessors.”
In ALD, different precursors interact in a controlled environment and the product of the chemical reaction is deposited on a surface, progressively forming a new atomic layer. “The knowledge of the surface chemistry is still limited. With this project, we want to better understand how the precursors react at the surface when forming the thin film.”
Up to now, scientists have only been able to analyse the surface of the material after the deposition process. Now, thanks to this next-generation experimental setup developed at SPECIES, they can finally observe the formation of the atomic layer in real-time. “There’s no other technology that can monitor what happens during the pulse, an approach known as in-operando”, continues professor Putkonen. “Furthermore, thanks to APXPS, we can now perform experiments at a pressure range that more closely resembles the true ALD growth conditions used in the industry.”
To test the capabilities of the new cell during this latest beamtime, the team from Helsinki has focused on noble metals, which are particularly hard to deposit by ALD. This new instrument opens unprecedented possibilities for understanding atomic layer deposition, and the team is already planning to send more people and start new research projects.
Seeing what was previously unseen
Samuli Urpelainen is a senior researcher at the University of Oulu. He has participated in the design and development of this new instrument since the very beginning. Samuli is an old acquaintance of MAX Lab and MAX IV, where worked at the beamlines I411 and then at SPECIES until 2019.
The idea of this project arose in 2016 from a series of conversations between Samuli, Simo Huotari from University of Helsinki, and Joachim Schnadt from Lund University. They decided to join forces to create an experimental cell for ALD studies with an entirely new concept.
Samuli explains their achievements with the clarity typical of someone who knows every bolt and pipe of such a complex machine. “Combining the high intensity of synchrotron radiation, APXPS, and modern technologies available at MAX IV, for the first time we can measure in real-time while depositing the precursors, and at pressure ranges relevant for this type of chemical reactions.”
With this new setup, researchers are getting closer to time resolutions in the order of milliseconds, which start to approach the time resolution of the chemical reaction. “In this cell, we inject the gasses in pulses, something that is not available in normal setups. It means sending fast pulses of gas into the cell to achieve that time resolution we are after. Now we can see things that people haven’t seen before while doing this ALD growth”.
Another challenge was to control the interaction of the precursors. “In normal APXPS systems, everything is injected in the cell from the same line, and there is, therefore, a risk of cross-contamination. In this cell, we have separated the precursor inlets. We have also added heating to other parts of the cell, which is typical of an ALD reactor, and this as well is something unusual in APXPS instruments.”
The ALD experimental setup is now ready and will remain at MAX IV. “Although funded by the University of Helsinki, the cell will now belong to MAX IV, available to all users as part of SPECIES equipment portfolio”. The setup is currently compatible only with the SPECIES beamline, “but there are plans to develop the instrument further so to enable similar experiments at other beamlines at MAX IV”.
Esko Kokkonen is a beamline scientist at SPECIES. He joined MAX IV in 2018 specifically for the ALD project and has been leading the design and construction of the cell. “When I joined, it was mostly just Samuli and me. We consulted a lot with the ALD experts in Helsinki, especially on how to mimic the gas flow of commercial ALD reactors.”
One of the trickiest challenges Esko encountered in building the setup for SPECIES was to incorporate all the ALD requirements in an APXPS cell design. “Ambient pressure cells are, by design, quite small and complex devices. One must think very carefully how to place each element, so to enable the proper gas flow without hindering the measurements. The last thing you want after a long design process is that a tube you have carefully placed ends up being in the way of the synchrotron beam!”
“This setup allows for in-situ and operando experiments, which helps in understanding the details of the layer growths. We get to see how individual chemical reactions, such as oxidation or reduction, happen on the sample surface”, continues Esko. Such results are possible thanks to synchrotron light: “we can already now do measurements with a sub-second resolution, and hopefully in the future, we will further improve on that.”
Esko is proud of the result of the project and is looking forward to welcoming users at SPECIES that will take full advantage of this new setup. “The cell is now available to users. However, the work is not stopping here. I like how during a project meeting someone mentioned that this could be considered as version 1.0 of the cell. We have improvements and changes we are planning on doing in the future to enhance the setup”.