Atomic layer deposition (ALD) of SiO<sub>2</sub> onto nanoporous alumina (PA) membranes was investigated with the aim of
adjusting the pore size and transport properties. PA membranes from commercial sources with a range of pore diameters
(20 nm, 100 nm and 200 nm) were used and modified by atomic layer deposition using tris(tert-butoxy)silanol and water
as the precursor couple. By adjusting the number of deposition cycles, the thickness of the conformal silica coating was
controlled, reducing the effective pore diameter, and subsequently changing the transport properties of the PA
membrane. Silica coated PA membranes with desired pore diameters from <5 nm to 100 nm were fabricated. In addition
to the pore size, the transport properties and selectivity of fabricated silica coated PA membranes were controlled by
chemical functionalisation using a silane with hydrophobic properties. Structural and chemical properties of modified
membranes were studied by dynamic secondary ion mass spectrometry (DSIMS) and scanning electron microscopy
(SEM). Spectrophotometric methods were used to evaluate the transport properties and selectivity of silica coated
membranes by permeation studies of hydrophobic and hydrophilic organic molecules. The resultant silica/PA
membranes with specific surface chemistry and controlled pore size are applicable for molecular separation, cell culture,
bioreactors, biosensing and drug delivery.
The microstructure and optical properties of alumina and titania multilayer coatings prepared using atomic layer deposition (ALD) has been investigated. The titania layers were prepared using TiCl<sub>4</sub>+H<sub>2</sub>O as the precursors while two different precursors, Al(CH<sub>3</sub>)<sub>3</sub>+H<sub>2</sub>O and AlCl<sub>3</sub>+H<sub>2</sub>O, were used to deposit the alumina layers. The results show that ALD can be used to produce amorphous, stoichiometric alumina and titania thin films with uniform thicknesses at low temperatures (120 °C). An antireflective coating design based on 4 alternating layers of titania and alumina was prepared and the resulting reflectance compared to theoretical calculations. The results demonstrate that ALD is a suitable technique for the deposition of optical thin films at temperatures compatible with thermally sensitive substrates.
Atomic layer deposition (ALD) is an important technology for depositing functional coatings on accessible, reactive surfaces with precise control of thickness and nanostructure. Unlike conventional chemical vapour deposition, where growth rate is dependent on reactant flux, ALD employs sequential surface chemical reactions to saturate a surface with a (sub-) monolayer of reactive compounds such as metal alkoxides or covalent halides, followed by reaction with a second compound such as water to deposit coatings layer-by-layer. A judicious choice of reactants and processing conditions ensures that the reactions are self-limiting, resulting in controlled film growth with excellent conformality to the substrate.
This paper investigates the deposition and characterisation of multi-layer TiO<sub>2</sub> /Al<sub>2</sub>O<sub>3</sub> films on a range of substrates, including silicon <100>, soda glass and polycarbonate, using titanium tetrachloride/water and trimethylaluminium/water as precursor couples. Structure-property correlations were established using a suite of analytical tools, including transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS), X-ray reflectometry (XRR) and spectroscopic ellipsometry (SE). The evolution of nanostructure and composition of multi-layer high/low refractive index stacks are discussed as a function of deposition parameters.
The essential features of the ALD process involve sequentially saturating a surface with a (sub)monolayer of reactive species, such as a metal halide, then reacting it with a second species to form the required phase <i>in-situ</i>. Repetition of the reaction sequence allows the desired thickness to be deposited. The self-limiting nature of the reactions ensures excellent conformality, and sequential processing results in exquisite control over film thickness, albeit at rather slow deposition rates, typically <200nm/hr. We have been developing our capability with ALD deposition, to understand the influence of deposition parameters on the nature of TiO<sub>2</sub> and Al<sub>2</sub>O<sub>3</sub> films (high and low refractive index respectively), and multilayer stacks thereof. These stacks have potential applications as anti-reflection coatings and optical filters. This paper will explore the evolution of structure in our films as a function of deposition parameters including temperature and substrate surface chemistry. A broad range of techniques have been applied to the study of these films, including cross sectional transmission electron microscopy, spectroscopic ellipsometry, secondary ion mass spectrometry etc. These have enabled a wealth of microstructural and compositional information on the films to be acquired, such as accurate film thickness, composition, crystallization sequence and orientation with respect to the substrate. The ALD method is shown to produce single layer films and multilayer stacks with exceptional uniformity and flatness, and in the case of stacks, chemically abrupt interfaces. We are currently extending this technology to the coating of polymeric substrates.
Atomic layer deposition (ALD) is a versatile technique for producing a wide variety of thin films. It provides a method for precisely controlling film thickness and composition. In addition films produced by ALD are highly conformal and are therefore excellent for the generation of MEMS devices. In the present study, single and multi layer films of TiO<sub>2</sub> and Al<sub>2</sub>O<sub>3</sub> have been deposited on silicon substrates at 200 and 300°C. These films have been characterised by a number of surface analytical techniques including dynamic secondary ion mass spectrometry (SIMS), ion beam analysis, electron microscopy and spectroscopic ellipsometry. These methods have enabled the optical, chemical and structural properties of the films to be accurately assessed. The results obtained to date demonstrate that ALD produces highly uniform single and multi layer films with minimal impurities. These high quality films are being applied to new opportunities for the development of future MEMS devices.