Atomic layer deposition (ALD) of SiO2 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 TiCl4+H2O as the precursors while two different precursors, Al(CH3)3+H2O and AlCl3+H2O, 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.
We report a naturally grown stripe structure with a nanometer scale wavelength in REBa2Cu3O7-δ (RE = Sm and Eu) superconductors investigated with scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). Such a periodic array was unveiled owning to the 3 dimensionally spatial oscillation of RE and Ba around the stoichiometric ratio. The study displayed that novel nanostripes function as robust pinning sites and effectively enhance the peak effect and the irreversibility line at 77K. This illustrates an approach to fabricate high performance REBa2Cu3O7-δ superconductors for application in liquid nitrogen temperature.
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 TiO2 /Al2O3 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.
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 TiO2 and Al2O3 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.
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 in-situ. 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 TiO2 and Al2O3 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.
Ti-based shape memory alloy (SMA) thin films have the potential to become high performance actuator materials for microelectromechanical systems. The major challenge in fabricating Ti-based SMA thin films is composition control, since a small compositional deviation can result in very large changes in the phase transformation temperatures. Owing to extreme reactivity of titanium, oxygen contamination is a major problem during the sputter deposition of TiNi and TiNiPd SMA thin films as it alters the Ni/Ti ratio. Oxygen as a contaminant has deleterious effects both on shape memory properties and mechanical properties of these alloys. Not much work in this field has been focussed on identification, determination and elimination of oxygen contamination. Rutherford Backscattering Spectrometry (RBS)is a useful technique for accurate determination of stoichiometry and depth profiling of these alloy films. RBS is less
sensitive to light elements. For this reason RBS has been complemented by Heavy Ion Elastic Recoil Detection Analysis (HIERDA) for the determination of oxygen. TiNi and TiNiPd films were deposited by DC- Magnetron Sputtering on unheated Silicon substrates by using Ti1.08Ni0.92 and Ti1.08 Ni 0.74 Pd 0.18 alloy targets. RBS measurements were carried out with 2 MeV He ions whereas HIERDA used 77 MeV I10+ ions with a ToF-E detector. It was found that oxygen contamination is almost negligible in TiNiPd films compared to TiNi films deposited under similar conditions. Palladium is effective as a catalyst in removing the oxygen from the deposition system resulting in reduced oxygen pick-up. This paper presents the stoichiometric analysis and depth profiling of these films by RBS and HIERDA
This paper presents the results on single-shot laser micromachining of filtered arc deposited TiN films and compares the machining characteristics of the films deposited under partially and fully filtered conditions. Machining performance was evaluated in terms of patterning quality and the ability to perform selective removal of top TiN film with minimal interference to an underlying layer. TiN was arc-deposited onto silicon substrate with a chromium layer on the top. These films were analysed for their composition and microstructure using Rutherford Backscattering Spectroscopy (RBS) and Scanning Electron Microscopy (SEM) before and after laser machining. Under single shot conditions the effect of fluence on the machined features has been investigated. The results showed selective removal of TiN films with a single shot from the underlying Cr layer. Further, this work clearly shows a distinction between the laser machining characteristics of the films deposited under different filtering conditions and substrate temperatures.