Nucleation and growth during atomic layer deposition

Atomic Layer deposition (ALD) is a chemical vapour deposition technique based on two binary reactions involving two precursors and a substrate. It allows controlling the thickness of the deposited films down to the atomic layer level on substrates of arbitrary complexity. Hollow areas, undercuts and high-aspects can be conformally covered. Furthermore, a wide range of inorganic materials can be deposited on a variety of materials. Although ALD`s main driver is the semiconductor industry, it is used in other application with demanding requirements, too. The manufacturing of complex 3D constructs is not possible with ALD. It requires a substrate. Two-photon polymerisation (2PP) is a rapidly developing technique used for the micro- and nanofabrication of 3D structures. Focused femtosecond laser pulses interact with a photosensitive resin in the focal point of a microscope objective and cause its curing. Along the focal point`s trace, a 3D polymeric line appears. The spatial resolution obtainable range from 65 nm to close to 1 cm. Current research in 2PP spreads from nanophotonics to micro- mechanical systems and tissue engineering. However, currently only polymeric materials can be efficiently processed. For some applications, where the 3D capabilities and resolution of 2PP is beneficial, material properties are required that cannot be obtained. The combination of these two techniques combines the advantages of both. With 2PP, parts of any arbitrary shape and complexity are created and used as templates for ALD of thin films. Such, application-relevant material properties can be introduced. However, ALD on polymer samples is challenging and not yet completely understood. To obtain conformal films on polymer substrates without compromising its preformed shape, this research proposal aims to understand the fundamentals of nucleation within the first few ALD cycles. The group of Prof. Prinz from the Nanoscale Prototyping Laboratory (NPL) at Stanford University proved that this phase is key for obtaining high performing constructs for optoelectronic applications. Using in situ characterization tools only available at the NPL, three main factors compromising thin film quality on 2PP constructs will be addressed: the temperature, the substrate composition and its geometry. First, semiconducting materials will be deposited on flat polymer substrates. The type of growth will be evaluated. Finally, deposition on complex 3D polymer templates will be done evaluating the geometric influence. Addressing these fundamental aspects will open the road towards a successful combination of both emerging technologies, which will help developing new and/or optimising existing research fields.

The combination of Two-Photon Polymerization (2PP) and Atomic Layer Deposition (ALD) allows integrating the advantages of both technologies. Leveraging from 2PPs 3D structuring capabilities, polymer parts of any arbitrary shape and complexity can be created and used as templates for the ALD of thin films to introduce application-relevant material properties. However, ALD on polymer-based samples is challenging and not completely understood.Alkaline earth materials have become an active field of research field due to their interesting electrical and optical properties. Especially Barium Titanate (BTO) has been explored for a wide range of applications including solar cells and high-k thin films for DRAM devices. The deposition of this material on 3D constructs can open the road to completely new prototyping options for optic and electronic devices. However, BTOs microfabrication is difficult and expensive.A novel process was developed allowing to deposit BTO at temperatures below 200C. Complex substrates with aspects of ~1:4 were conformally covered. The process will be commercialized in the near future and allows fabricating novel thin film capacitors, solar cells and micro-electrical and mechanical devices.Due to Nano effects, thin films differ drastically from bulk material. A newly developed analysis tool allowed exploring and understanding Nano effects during the evolution of a thin film with synchrotron radiation. The electric properties of thin film BTO was linked to its structure. A better understanding of the initial growth of an ALD film was obtained. The insight gathered allowed linking process parameters to desired properties and functionalizing organic surfaces like graphene for ALD growth.