The Atomic Layer Deposition of HfO2and ZrO2using Advanced Metallocene Precursors and H2O as the Oxygen Source

The surface chemistry of the atomic layer deposition of metal thin films

In this perspective we discuss the progress made in the mechanistic studies of the surface chemistry associated with the atomic layer deposition (ALD) of metal films and the usefulness of that knowledge for the optimization of existing film growth processes and for the design of new ones. Our focus is on the deposition of late transition metals. We start by introducing some of the main surface-sensitive techniques and approaches used in this research. We comment on the general nature of the metallorganic complexes used as precursors for these depositions, and the uniqueness that solid surfaces and the absence of liquid solvents bring to the ALD chemistry and differentiate it from what is known from metalorganic chemistry in solution. We then delve into the adsorption and thermal chemistry of those precursors, highlighting the complex and stepwise nature of the decomposition of the organic ligands that usually ensued upon their thermal activation. We discuss the criteria relevant for the selection of co-reactants to be used on the second half of the ALD cycle, with emphasis on the redox chemistry often associated with the growth of metallic films starting from complexes with metal cations. Additional considerations include the nature of the substrate and the final structural and chemical properties of the growing films, which we indicate rarely retain the homogeneous 2D structure often aimed for. We end with some general conclusions and personal thoughts about the future of this field.

First-Principles Molecular Dynamics Simulations on Water-Solid Interface Behavior of H2O-Based Atomic Layer Deposition of Zirconium Dioxide

As an important inorganic material, zirconium dioxide (ZrO₂) has a wide range of applications in the fields of microelectronics, coating, catalysis and energy. Due to its high dielectric constant and thermodynamic stability, ZrO₂ can be used as dielectric material to replace traditional silicon dioxide. Currently, ZrO₂ dielectric films can be prepared by atomic layer deposition (ALD) using water and zirconium precursors, namely H₂O-based ALD. Through density functional theory (DFT) calculations and first-principles molecular dynamics (FPMD) simulations, the adsorption and dissociation of water molecule on the ZrO₂ surface and the water-solid interface reaction were investigated. The results showed that the ZrO₂ (111) surface has four Lewis acid active sites with different coordination environments for the adsorption and dissociation of water. The Zr atom on the surface can interacted with the O atom of the water molecule via the p orbital of the O atom and the d orbital of the Zr atom. The water molecules could be dissociated via the water-solid interface reaction of the first or second layer of water molecules with the ZrO₂ (111) surface. These insights into the adsorption and dissociation of water and the water-solid interface reaction on the ZrO₂ surface could also provide a reference for the water-solid interface behavior of metal oxides, such as H₂O-based ALD.

Reaction mechanism of atomic layer deposition of zirconium oxide using zirconium precursors bearing amino ligands and water

As a unique nanofabrication technology, atomic layer deposition (ALD) has been widely used for the preparation of various materials in the fields of microelectronics, energy and catalysis. As a high-κ gate dielectric to replace SiO₂, zirconium oxide (ZrO₂) has been prepared through the ALD method for microelectronic devices. In this work, through density functional theory calculations, the possible reaction pathways of ZrO₂ ALD using tetrakis(dimethylamino)zirconium (TDMAZ) and water as the precursors were explored. The whole ZrO₂ ALD reaction could be divided into two sequential reactions, TDMAZ and H₂O reactions. In the TDMAZ reaction on the hydroxylated surface, the dimethylamino group of TDMAZ could be directly eliminated by substitution and ligand exchange reactions with the hydroxyl group on the surface to form dimethylamine (HN(CH₃)₂). In the H₂O reaction with the aminated surface, the reaction process is much more complex than the TDMAZ reaction. These reactions mainly include ligand exchange reactions between the dimethylamino group of TDMAZ and H₂O and coupling reactions for the formation of the bridged products and the by-product of H₂O or HN(CH₃)₂. These insights into surface reaction mechanism of ZrO₂ ALD can provide theoretical guidance for the precursor design and improving ALD preparation of other oxides and zirconium compounds, which are based ALD reaction mechanism.

Group IV Transition Metal (M = Zr, Hf) Precursors for High-κ Metal Oxide Thin Films

This paper describes the synthesis of eight novel zirconium and hafnium complexes containing N-alkoxy carboxamidate-type ligands, as potential precursors for metal oxides and atomic layer deposition (ALD) for HfO₂. A series of ligands, viz., N-ethoxy-2,2-dimethylpropanamide (edpaH), N-ethoxy-2-methylpropanamide (empaH), and N-methoxy-2,2-dimethylpropanamide (mdpaH), were used to afford complexes Zr(edpa)₄ (1), Hf(edpa)₄ (2), Zr(empa)₄ (3), Hf(empa)₄ (4), Zr(mdpa)₄ (5), Hf(mdpa)₄ (6), ZrCp(edpa)₃ (7), and HfCp(edpa)₃ (8). Thermogravimetric analysis curves assessed for the evaporation characteristics of complexes 1-8 revealed single-step weight losses with low residues, except for the mdpa-containing complexes. Single-crystal X-ray diffraction studies of 1, 2, 5, and 6 revealed that all the complexes have monomeric molecular structures, with the central metal ion surrounded by eight oxygen atoms from the four bidentate alkoxyalkoxide ligands. Among the complexes prepared, 8 exhibited a low melting point (64 °C), good volatility (1 Torr at 112 °C), high thermal stability, and excellent endurance over 6 weeks at 120 °C. Therefore, an ALD process for the growth of HfO₂ was developed using HfCp(edpa)₃ (8) as a novel precursor. Furthermore, the HfO₂ film exhibited a low capacitance equivalent oxide thickness of ∼1.5 nm, with J_g as low as ∼3 × 10^-4 A/cm² at V_g -1 V in a metal-insulator-semiconductor capacitor (Au/HfO₂/p-Si).

PEALD of HfO2 Thin Films: Precursor Tuning and a New Near-Ambient-Pressure XPS Approach to in Situ Examination of Thin-Film Surfaces Exposed to Reactive Gases

A bottom-up approach starting with the development of new Hf precursors for plasma-enhanced atomic layer deposition (PEALD) processes for HfO₂ followed by in situ thin-film surface characterization of HfO₂ upon exposure to reactive gases via near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is reported. The stability of thin films under simulated operational conditions is assessed, and the successful implementation of HfO₂ dielectric layers in metal-insulator-semiconductor (MIS) capacitors is demonstrated. Among the series of newly synthesized mono-guanidinato-tris-dialkyl-amido class of Hf precursors, one of them, namely, [Hf{η²-(ⁱPrN)₂CNEtMe}(NEtMe)₃], was representatively utilized with oxygen plasma, resulting in a highly promising low-temperature PEALD process at 60 °C. The new precursors were synthesized in the multigram scale and thoroughly characterized by thermogravimetric analyses, revealing high and tunable volatility reflected by appreciable vapor pressures and accompanied by thermal stability. Typical ALD growth characteristics in terms of linearity, saturation, and a broad ALD window with constant growth of 1.06 Å cycle^-1 in the temperature range of 60-240 °C render this process very promising for fabricating high-purity smooth HfO₂ layers. For the first time, NAP-XPS surface studies on selected HfO₂ layers are reported upon exposure to reactive H₂, O₂, and H₂O atmospheres at temperatures of up to 500 °C revealing remarkable stability against degradation. This can be attributed to the absence of surface defects and vacancies. On the basis of these promising results, PEALD-grown HfO₂ films were used as dielectric layers in the MIS capacitor device fabrication exhibiting leakage current densities less than 10^-7 A cm^-2 at 2 MV cm^-1 and permittivities of up to 13.9 without postannealing.

Theoretical study on the initial reaction mechanisms of ansa-metallocene zirconium precursor on hydroxylated Si(1 0 0) surface

The initial reaction mechanisms for depositing ZrO2 thin films using ansa-metallocene zirconium (Cp2CMe2)ZrMe2 precursor were studied by density functional theory (DFT) calculations. The (Cp2CMe2)ZrMe2 precursor could be absorbed on the hydroxylated Si(1 0 0) surface via physisorption. Possible reaction pathways of (Cp2CMe2)ZrMe2 were proposed. For each reaction, the activation energies and reaction energies were compared, and stationary points along the reaction pathways were shown. In addition, the influence of dispersion effects on the reactions was evaluated by non-local dispersion corrected DFT calculations.

Low-temperature sol–gel processed AlOx gate dielectric buffer layer for improved performance in pentacene-based OFETs

An approach to achieve improved performance in pentacene-based organic field effect transistors (OFETs) using high-k AlO_x prepared by a low temperature sol–gel technique as a thin buffer layer on a SiO₂ gate dielectric was demonstrated.

Effect of bromide ions on the corrosion behavior of hafnium in anhydrous ethanol

The electrochemical behaviors of hafnium (Hf) in Et₄NBr ethanol solutions were investigated using cyclic voltammetry, potentiodynamic polarization, chronoamperometry, impedance and SEM techniques.

Low-voltage organic field-effect transistors (OFETs) with solution-processed metal-oxide as gate dielectric

In this study, low-voltage copper phthalocyanine (CuPc)-based organic field-effect transistors (OFETs) are demonstrated utilizing solution-processed bilayer high-k metal-oxide (Al(2)O(y)/TiO(x)) as gate dielectric. The high-k metal-oxide bilayer is fabricated at low temperatures (< 200 °C) by a simple spin-coating technology and can be controlled as thin as 45 nm. The bilayer system exhibits a low leakage current density of less than 10(-5) A/cm(2) under bias voltage of 2 V, a very smooth surface with RMS of about 0.22 nm and an equivalent k value of 13.3. The obtained low-voltage CuPc based OFETs show high electric performance with high hole mobility of 0.06 cm(2)/(V s), threshold voltage of -0.5 V, on/off ration of 2 × 10(3) and a very small subthreshold slope of 160 mV/dec when operated at -1.5 V. Our study demonstrates a simple and robust approach that could be used to achieve low-voltage operation with solution-processed technique.