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From the bottom-up: a physico-chemical approach towards 3D nanostructures with atomic-scale control

Periodic Reporting for period 2 - BottomUp3D (From the bottom-up: a physico-chemical approach towards 3D nanostructures with atomic-scale control)

Reporting period: 2022-07-01 to 2023-12-31

The synthesis of materials from atoms as building blocks through bottom-up processing has been considered to be the ultimate ambition in the field of nanoscience. Bottom-up processing allows for new fabrication methods for nanoelectronics, but can also enable material synthesis with accurately tailored electronic, catalytic or structural properties for a broad range of applications. However, a lack of methods for high-volume synthesis limits the step towards those applications.

In the ERC Starting project BottomUp3D, we focus on the development of novel bottom-up approaches based on the thin film deposition technique of atomic layer deposition (ALD). The ability of ALD to deposit materials atomic layer by atomic layer, makes it an ideal starting point for high-volume bottom-up processing. The main task for this project is to make the growth selective to specific locations, which is referred to as area-selective deposition (ASD).

With nanoelectronics moving to increasingly more complex 3D nanostructured devices, new challenges for the processing of materials arise. For future sub-nanometer technology nodes, we reached the stage at which there is not “plenty of room at the bottom” anymore. For this post-Feynman era, we enable area-selective ALD on 3D nanostructured substrates by employing small molecules inhibitors (SMI). Furthermore, in order to develop new flavors of selective processing such as anisotropic or topographically-selective ALD, we implement plasma exposure steps involving directional ions in advanced ALD recipes. Various new opportunities emerge for the processing of 3D nanostructured substrates by bringing the surface chemistry of area-selective ALD and the physics of plasma processing together.
In the field of area-selective deposition (ASD), most groups focus on using self-assembled monolayers (SAMs) to inhibit the growth on specific areas of the surface. We pioneered the approach of using small molecule inhibitors (SMIs), motivated by the compatibility of vapor-phase dosing of SMIs with industrial process flows. Since the writing of the proposal, more research groups started to explore using SMIs for ASD, demonstrating the feasibility of our approach.

In this project we identified the main challenge for using SMIs: as a consequence of vapor-phase dosing, SMIs arrive one-by-one on the surface and adsorb at random surface sites, resulting in a relatively low surface density in saturation. To investigate this adsorption mechanism, we developed a novel simulation approach, referred to as random sequential adsorption (RSA) simulations, which gives insight into the effect of molecular size and shape on the packing of SMIs on a surface.

From an experimental point-of-view, detailed in-situ Fourier transform infrared spectroscopy (FTIR) studies were performed to assess the importance of dosing conditions for the selectivity of area-selective deposition. In previous studies, we learned that SMIs adsorb on a surface in mixture of bonding configurations, that have different stability and do not necessarily all block deposition effectively. The infrared spectroscopy results suggest that a higher coverage of the desired configuration can be obtained by choosing appropriate conditions.

The extension of the toolbox with plasma exposures allows for the development of new and innovative approaches to area-selective deposition. By combining ALD and sputter etching by ions in a process, so-called sputter yield amplification by heavy elements can exploited to achieve area-selective deposition. It was demonstrated that SiO2 can be deposited selectively on Al2O3, with respect to the heavier HfO2. This approach can be described as a physical pathway towards selectivity, in contrast to the chemical pathways that are conventionally explored.
ASD technology is maturing, meaning that some first processes are being implemented in industrial processing. With that, there is currently much interest in the field in designing processes that work on complex 3D nanostructured substrates. These developments nicely match our focus in the second part of this ERC project. Our plan is to employ ASD to enable advanced 3D nanomanufacturing, by building on recent insights into bonding configurations and dosing conditions. One of the aims is to explore whether SMIs allow for selective processing in the tight spaces of future nanoelectronics. In addition, we will put focus on the development of new flavors of selective processing, with special attention for topographically-selective deposition.
Approach for SMI screening based on DFT and random sequential adsorption simulations.