Darren Attard

Transmission electron microscopy studies of atomic layer deposition TiO2 films grown on silicon

Transmission electron microscopy techniques have been used to characterise atomic layer deposition TiO films grown on 2 silicon substrates after RCA and HF treatment. The influence of deposition temperature (250–350 8C) and substrate type on the film microstructure have been determined. The major influence of substrate type is to control nucleation of crystallisation. HF treated silicon, which was devoid of the native oxide layer, promoted a crystalline, island growth mode. The nucleation of crystalline particles at the onset of deposition resulted in films with very fine grain sizes (f20 nm). The RCA treated silicon, which was coated with amorphous native oxide, caused the growth of an initially amorphous TiO film, which crystallised once 2 a critical film thickness had been exceeded. The major influence of temperature on the films grown on RCA treated silicon was to control nucleation of crystallisation within the amorphous layers, resulting in grain size refinement at higher deposition temperatures. Under the processing conditions used, other than the transient amorphous films formed on RCA treated silicon, anatase was the only phase formed. No evidence for preferred orientation was found. 2003 Elsevier Science B.V. All rights reserved.

Atomic layer deposition of TiO2 and Al2O3 thin films and nanolaminates

We have been developing our capability with atomic layer deposition (ALD), 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 (nanolaminates). 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 and secondary-ion mass spectrometry. 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.

Characterisation of PI3 and RF plasma nitrided austenitic stainless steels using plan and cross-sectional TEM techniques

Abstract Type 304 and 316 austenitic stainless steels treated with plasma immersion ion implantation (PI 3 ™) and RF plasma nitriding have been studied with transmission electron microscopy (TEM). A range of TEM specimen preparation techniques have been used to enable characterisation of the nature of the treated surface layers. Plan view TEM specimens, prepared by thinning from the back, highlighted the removal by sputtering of original surface features in steels during PI 3 processing, while the less energetic RF plasma nitriding retained the original surface features. All treatments resulted in the formation of an austenite phase heavily expanded by the high levels of interstitial nitrogen. Decomposition of this expanded austenite into CrN and α Fe occurred at higher treatment temperatures, and was found to nucleate at the grain boundaries preferentially. Conventional cross-sectional TEM specimens also showed the importance of the surface as a nucleation site for this transformation. The transformation process and the products of expanded austenite decomposition were the same regardless of the treatment process used. The expanded austenite layer was aligned with the underlying steel, with no orientational difference across the interface observed. Focused ion beam milling has the ability to consistently section treatment depths of several micrometers, although further work is necessary to produce through-section specimens thin enough for EELS and EFTEM analysis.

Nanostructured TiO2 membranes by atomic layer deposition

Conformal TiO2 films have been deposited on track-etched polycarbonate membranes by means of atomic layer deposition. Membranes with pore aspect ratios of 25 ∶ 1 and 50 ∶ 1 were selected for initial study. A suite of complementary characterisation techniques was used to probe the nanostructure, stoichiometry and uniformity of the TiO2 coatings. These established that both the surfaces and pores of the membranes were completely covered with an amorphous film. Further, the stoichiometry of the films was close to that of TiO2 though they were found to contain ∼6 atom% chlorine. While the internal walls of the membrane pores were covered with film over their full length, the coating thickness decreased in the direction of gas flow. This was attributed to a variation in reactant concentrations along the pores due to the process conditions. Gas conductance measurements were performed on several coated membranes. These showed that membrane conductance could be controlled by varying the coating thickness.

Atomic Layer Deposition of TiO2 / Al2O3 Films For Optical Applications

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.

The Effect of a Rapid Heating Rate, Mechanical Vibration and Surfactant Chemistry on the Structure–Property Relationships of Epoxy/Clay Nanocomposites

The role of processing conditions and intercalant chemistry in montmorillonite clays on the dispersion, morphology and mechanical properties of two epoxy/clay nanocomposite systems was investigated in this paper. This work highlights the importance of employing complementary techniques (X-ray diffraction, small angle X-ray scattering, optical microscopy and transmission electron microscopy) to correlate nanomorphology to macroscale properties. Materials were prepared using an out of autoclave manufacturing process equipped to generate rapid heating rates and mechanical vibration. The results suggested that the quaternary ammonium surfactant on C30B clay reacted with the epoxy during cure, while the primary ammonium surfactant (I.30E) catalysed the polymerisation reaction. These effects led to important differences in nanocomposite clay morphologies. The use of mechanical vibration at 4 Hz prior to matrix gelation was found to facilitate clay dispersion and to reduce the area fraction of I.30E clay agglomerates in addition to increasing flexural strength by over 40%.

Atomic layer deposition (ALD) of TiO2 and Al2O3 thin films on silicon

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.

The Effect of Radiation Damage on Zirconolite Dissolution

Polished tiles (7x7x2 mm3) of Nd-bearing zirconolite were fabricated and then some were irradiated on both large faces with 3 MeV or 2 MeV Au2+ ions (total fluence of {approx} 1 x 1015 ions/cm2) in order to render the zirconolite amorphous and so simulate displacement damage caused by alpha decay. Both the irradiated and non-irradiated tiles were then subjected to static dissolution tests in 0.01M nitric solution (pH2) at 90 C, for periods of 0-1, 1-7, 7-14 and 14-28 days. It was found that radiation damage did not affect the dissolution rate of zirconolite as indicated by the elemental leach rates of Nd, Ti, Ca and Al. The results of solution analyses are consistent with those obtained from X-ray Photoelectron Spectroscopy (XPS) in that the Ca, Nd, Ti and Al concentrations in the top surface layer (< 5 nm) all decreased with respect to that of Zr after dissolution testing, and the leached surface composition of the non-irradiated zirconolite is very similar to that of the two irradiated specimens. The implications of these results are discussed in the context of previous work.