Atomic layer deposition : from reaction mechanisms to 3D-integrated micro-batteries

Abstract

One major difficulty in maintaining the size reduction in electronic devices is the controlled deposition of high-quality thin films with the right film properties. Besides this trend of miniaturization in information processing technology, i.e., the "More-Moore" trend, there has been another trend for non-digital technology which is designated as "More-than-Moore". "More-than-Moore" is the trend towards diversification within electronic devices, in which multiple functionalities are integrated into a single unit of the device. All-solid-state 3D-integrated micro-batteries are a good example of the More-than-Moore approach as they integrate energy storage, which is a traditionally macroscopic non-semiconductor technology, into a chip-size unit using techniques compatible with semiconductor technology. In all-solid-state 3D-integrated microbatteries, the various battery materials have to be deposited as thin films. Furthermore, to achieve a high capacity per footprint area on the chip surface, the thin films have to be deposited in high-aspect-ratio structures etched in the Si substrate. Controlled deposition of high-quality films with the right film properties is therefore also a challenge in this research field. A thin-film deposition technique which typically exhibits a high material quality, a high uniformity, precise growth control, and an excellent conformality is atomic layer deposition (ALD). ALD has the potential to be an enabling technology for a wide range of applications. To be able to develop new ALD processes and materials, detailed understanding of the reaction mechanisms and the ALD process itself are essential. Besides the conventional thermallydriven ALD processes, the usage of energy-enhanced methods is considered, i.e., plasma-assisted and ozone-based processes. Plasma-assisted ALD can for instance facilitate deposition of conductive films. Furthermore, oxygen plasma and ozone gas are well-suited to grow oxide thin films even at low substrate temperatures. The reactive reactants used can, however, recombine at surfaces which could complicate deposition in 3D structures. In this thesis new energy-enhanced ALD processes are developed and further understanding is obtaned on their reaction mechanisms. Also the ability of energy-enhanced ALD processes to conformally coat 3D structures is investigated. Moreover, the application of ALD in energy technologies is further explored by focussing on solid-state 3D-integrated batteries. The use of ALD in Li-ion battery synthesis is relatively unexplored and therefore the potential of ALD for Li-ion batteries is reviewed in this work. Not only the More-than-Moore application of all-solid-state 3D-integrated microbatteries is considered, but also larger-scale Li-ion battery concepts that can benefit from ALD as well. Nanostructuring is targeted as a solution to achieve the improvements required for implementing batteries in a wide range of applications. The potential of ALD is discussed for three battery concepts that can be distinguished, i.e., particle-based electrodes, 3D-structured electrodes, and 3D solid-state micro-batteries. It is discussed that a large range of materials can be deposited by ALD and recent demonstrations of improvements in battery technology by ALD are used to exemplify its large potential. Conformal deposition of conductive materials is needed in a variety of More-than-Moore applications, e.g., for electrodes and current collectors. TiN and TaN deposited by plasma-assisted ALD were demonstrated to serve as Li barrier and anode current collector for micro-batteries and also as Cu diffusion barrier in advanced interconnect technology for 3D-integration. Furthermore, conformal deposition of TiN films by plasma-assisted ALD was demonstrated. For some electrodes, such as the cathode current collector, a highly-chemically-stable conductive material is needed. A plasma-assisted ALD process was developed for the deposition of Pt films with excellent material properties in terms of density and resistivity. By using an additional H2-gas reduction-step, the deposition of Pt at low temperatures with good material properties was achieved, which can be of interest for deposition of Pt on plastics. Furthermore, using longer plasma exposure times, PtO2 could be deposited which is difficult to obtain by ALD. Using mass spectrometry, the reaction mechanism of plasma-assisted ALD of TaNx was investigated. For this process the reaction products released from the surface during the plasma step were found to interact with the plasma. Furthermore, the material properties of TaNx are influenced to a large extent by this interaction. Interaction of ALD reaction products with the plasma is expected to be of general significance for plasma-assisted ALD processes. The reaction products of the thermal ALD process for Pt were quantified using insitu gas-phase infrared spectroscopy and a reaction mechanism was proposed. The film growth was found to be ruled by the surface coverage of dissociatively chemisorbed oxygen with which the precursor molecules interact. The capability of plasma-assisted ALD to deposit in 3D structures was investigated using Monte Carlo simulations. It was found that deposition in 3D structures can be classified in three regimes: i.e., reaction-limited, diffusionlimited, and recombination-limited. For low values of the recombination probability, or, conformal deposition in high-aspect-ratio structures can still be achieved, as also experimentally observed for several metal oxides. For high values of the recombination probability, r, as appears to be the case for many metals, achieving a reasonable conformality becomes challenging, especially for aspect ratios >10. Sufficient conformal deposition was demonstrated for both the TiN and the Pt plasma-assisted ALD processes. For the medium aspect ratios targeted for the Li-ion micro-batteries, plasma-assisted ALD should be able to conformally deposit all materials. Similarly the loss of O3 in 3D structures was investigated where the loss of O3 on several materials was tested. To determine O3 recombination probabilities over a wide range, a method was developed using high-aspect-ratio capillaries at the inlet to a mass spectrometer. O3 typically has higher loss on materials such as MnOx and Co3O4, which can also be considered as battery materials. Several material systems were investigated and a considerable amount of fundamental understanding in ALD was generated. In particular, understanding of energy-enhanced ALD processes and understanding of their ability to coat 3D structures conformally are essential for many new application fields in which these techniques will be required