ABSTRACT
The integration of bottom-up fabrication techniques and top-down methods can overcome current limits in nanofabrication. For such integration, we propose a gradient area-selective deposition using atomic layer deposition to overcome the inherent limitation of 3D nanofabrication and demonstrate the applicability of the proposed method toward large-scale production of materials. Cp(CH3)5Ti(OMe)3 is used as a molecular surface inhibitor to prevent the growth of TiO2 film in the next atomic layer deposition process. Cp(CH3)5Ti(OMe)3 adsorption was controlled gradually in a 3D nanoscale hole to achieve gradient TiO2 growth. This resulted in the formation of perfectly seamless TiO2 films with a high-aspect-ratio hole structure. The experimental results were consistent with theoretical calculations based on density functional theory, Monte Carlo simulation, and the Johnson-Mehl-Avrami-Kolmogorov model. Since the gradient area-selective deposition TiO2 film formation is based on the fundamentals of molecular chemical and physical behaviours, this approach can be applied to other material systems in atomic layer deposition.
ABSTRACT
The adsorption of metalorganic and metal halide precursors on the SiO2 surface plays an essential role in thin-film deposition processes such as atomic layer deposition (ALD). In the case of aluminum oxide (Al2O3) films, the growth characteristics are influenced by the precursor structure, which controls both chemical reactivity and the geometrical constraints during deposition. In this work, a systematic study using a series of Al(CH3)xCl3-x (x = 0, 1, 2, and 3) and Al(CyH2y+1)3 (y = 1, 2, and 3) precursors is carried out using a combination of experimental spectroscopic techniques together with density functional theory calculations and Monte Carlo simulations to analyze differences across precursor molecules. Results show that reactivity and steric hindrance mutually influence the ALD surface reaction. The increase in the number of chlorine ligands in the precursor shifts the deposition temperature higher, an effect attributed to more favorable binding of the intermediate species due to higher Lewis acidity, while differences between precursors in film growth per cycle are shown to originate from variations in adsorption activation barriers and size-dependent saturation coverage. Comparison between the theoretical and experimental results indicates that the Al(CyH2y+1)3 precursors are favored to undergo two ligand exchange reactions upon adsorption at the surface, whereas only a single Cl-ligand exchange reaction is energetically favorable upon adsorption by the AlCl3 precursor. By pursuing the first-principles design of ALD precursors combined with experimental analysis of thin-film growth, this work enables a robust understanding of the effect of precursor chemistry on ALD processes.
ABSTRACT
Atomic layer deposition (ALD) is a thin film deposition technique based on self-saturated reactions between a precursor and reactant vacuum conditions. A typical ALD reaction consists of the first half-reaction of the precursor and the second half-reaction of the counter reactant, in which the terminal groups on the surface change after each half-reaction. In this study, the effects of counter reactants on the surface termination and growth characteristics of ALD HfO2 thin films formed on Si substrates using tetrakis(dimethylamino)-hafnium (TDMAH) as a precursor were investigated. Two counter reactants, H2O and O3, were individually employed, as well as in combination with consecutive exposure by H2O-O3 and O3-H2O. The film growth behaviors and properties differed when the sequence of exposure of the substrate to the reactants was varied. Based on X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) simulation, the changes are attributed to the effects of the surface terminations formed from different counter reactant combinations. The knowledge from this work could provide insight for precisely tuning the growth and properties of ALD films.
ABSTRACT
Desflurane is a volatile anaesthetic agent with neuroprotective properties. Excitatory amino-acid carrier 1 (EAAC1) may be neuroprotective by taking up glutamate and cysteine. Therefore, the effects of desflurane on EAAC1 activity were investigated in this study. EAAC1 was expressed in Xenopus laevis oocytes. Two-electrode voltage-clamping technique was used to record membrane currents upon exposure to l-glutamate (30 µM) in the presence or absence of desflurane (0.4, 1.0, 2.0, 2.6, or 3.2mM). Currents were also measured in oocytes pre-exposed to a protein kinase C (PKC) activator (50 nM phorbol-12-myristate-13-acetate, PMA), PKC inhibitors (1 µM staurosporine or 50 µM chelerythrine), or phosphatidylinositol-3-kinase (PI3K) inhibitors (5 µM wortmannin or 10 µM LY294002). Desflurane significantly increased EAAC1 activity. The EC50 of desflurane for increasing the EAAC1 response was 0.75 mM. A kinetic study showed that desflurane significantly increased the Vmax but had no effect on the Km of the EAAC1 response for glutamate. Treatment of oocytes with desflurane plus PMA significantly increased the transporter currents compared to the control, but did not further increase the response compared to either agent alone. Staurosporine attenuated desflurane-enhanced transporter currents without decreasing the basal activity; chelerythrine did not decrease either. In addition, pretreatment of oocytes with two PI3K inhibitors (wortmannin or LY294002) significantly reduced desflurane-enhanced EAAC1 activity without decreasing basal activity. Our results suggest that desflurane increases EAAC1 activity via PKC or PI3K. This enhanced EAAC1 activity may be a mechanism for the neuroprotective effect of desflurane.
