Atomic Layer Deposition Precursors Research Articles

Additive Manufacturing in Atomic Layer Processing Mode.

Additive manufacturing (3D printing) has not been applicable to micro- and nanoscale engineering due to the limited resolution. Atomic layer deposition (ALD) is a technique for coating large areas with atomic thickness resolution based on tailored surface chemical reactions. Thus, combining the principles of additive manufacturing with ALD could open up a completely new field of manufacturing. Indeed, it is shown that a spatially localized delivery of ALD precursors can generate materials patterns. In this "atomic-layer additive manufacturing" (ALAM), the vertical resolution of the solid structure deposited is about 0.1 nm, whereas the lateral resolution is defined by the microfluidic gas delivery. The ALAM principle is demonstrated by generating lines and patterns of pure, crystalline TiO2 and Pt on planar substrates and conformal coatings of 3D nanostructures. The functional quality of ALAM patterns is exemplified with temperature sensors, which achieve a performance similar to the industry standard. This general method of multimaterial direct patterning is much simpler than standard multistep lithographic microfabrication. It offers process flexibility, saves processing time, investment, materials, waste, and energy. It is envisioned that together with etching, doping, and cleaning performed in a similar local manner, ALAM will create the "atomic-layer advanced manufacturing" family of techniques.

Cover Feature: Role of Anionic Backbone in NHC‐Stabilized Coinage Metal Complexes: New Precursors for Atomic Layer Deposition (Chem. Eur. J. 16/2022)

A series of new N-heterocyclic carbene (NHC) based Cu(I) and Ag(I) complexes were synthesized, analyzed, and evaluated for their potential usage in atomic layer deposition (ALD) of coinage metal thin films. Employing the same set of diketonate-type ligands for both metals did result in thermally stable Cu(I) complexes suitable for ALD, while the Ag(I) complexes exhibit an inherently unstable nature and different bonding features. More information can be found in the Research Article by Anjana Devi et al. (DOI: 10.1002/chem.202103798).

Role of Anionic Backbone in NHC-Stabilized Coinage Metal Complexes: New Precursors for Atomic Layer Deposition.

Cu and Ag precursors that are volatile, reactive, and thermally stable are currently of high interest for their application in atomic‐layer deposition (ALD) of thin metal films. In pursuit of new precursors for coinage metals, namely Cu and Ag, a series of new N‐heterocyclic carbene (NHC)‐based CuI and AgI complexes were synthesized. Modifications in the substitution pattern of diketonate‐based anionic backbones led to five monomeric Cu complexes and four closely related Ag complexes with the general formula [M( tBuNHC)(R)] (M=Cu, Ag; tBuNHC=1,3‐di‐tert‐butyl‐imidazolin‐2‐ylidene; R=diketonate). Thermal analysis indicated that most of the Cu complexes are thermally stable and volatile compared to the more fragile Ag analogs. One of the promising Cu precursors was evaluated for the ALD of nanoparticulate Cu metal deposits by using hydroquinone as the reducing agent at appreciably low deposition temperatures (145–160 °C). This study highlights the considerable impact of the employed ligand sphere on the structural and thermal properties of metal complexes that are relevant for vapor‐phase processing of thin films.

Formation of mononuclear N,O-chelate zirconium complexes by direct insertion of epoxide into tetrakis(dimethylamido)zirconium: highly promising approach for developing an ALD precursor of ZrO2 thin films.

A zirconium complex containing an N,O-chelate and alkylamide ligand has great potential for application in atomic layer deposition (ALD). However, the synthesis of this mononuclear Zr complex remains a major challenge because of the highly electrophilic nature and rich coordination of the Zr(IV) atom. Herein, a nonclassical and highly effective route for synthesizing the mononuclear N,O-chelate Zr complex was envisaged and verified using rationally designed reactions, involving the ring-opening insertion of epoxide into tetrakis(dimethylamido)zirconium(IV) (TDMAZ) at room temperature. To the best of our knowledge, very few studies in which mononuclear Zr complexes comprising alkylamide in combination with aminoalkoxide are structurally characterized. This method is extremely simple, atom economical, and easily scalable. Importantly, the produced precursor complex enables ALD of ZrO2 at a satisfactory growth rate (0.93 Å per cycle), close to that of the commercial ALD precursor CpZr(NMe2)3 (0.9 Å per cycle).

SnO deposition via water based ALD employing tin(II) formamidinate: precursor characterization and process development.

Tin monoxide (SnO) is a promising oxide semiconductor which is appealing for a wide range of applications from channel materials in p-type field effect transistors (FET) to electrode materials searched for next-generation batteries. For the controlled growth of SnO films at low temperatures, atomic layer deposition (ALD) is employed in this study, where the choice of the precursor plays a significant role. A comparative thermal evaluation of four different amidinate-based tin(II) precursors and the influence of the ligand sphere on their physicochemical properties revealed that bis(N,N'-diisopropylformamidinato tin(II) (1) possesses the required volatility, good thermal stability and sufficient reactivity towards water, to be implemented as the ALD precursor. The water-assisted ALD process resulted in crystalline SnO films on Si substrates with a growth per cycle (GPC) of 0.82 Å at temperatures as low as 140 °C. By employing complementary analytical tools, namely, X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray reflectivity (XRR), Rutherford backscattering spectrometry/nuclear reaction analysis (RBS/NRA) and X-ray photoelectron spectroscopy (XPS), the formation of tin monoxide was confirmed. Finally, the optical properties of the as-deposited films were analyzed via UV-Vis spectroscopy, exhibiting a band gap of 2.74 eV, which further confirms the formation of the targeted SnO phase.

Hybrid hierarchical nanolattices with porous platinum coating

Nanolattices, defined as lattice structures composed of nanometer-sized building blocks, exhibit a wide range of unique properties and represent a new and rapidly growing class of mechanical metamaterials. In this work, we fabricated a core-shell polymer-platinum nanolattice by combining several fabrication techniques. The CAD-designed polymer nanolattices were fabricated by a two-photon lithography technique to achieve the required architecture. The lattices were then coated with a thin film of platinum using the atomic layer deposition (ALD) method. The unique interaction of the gaseous ALD precursor with the polymer surface resulted in a porous nanogranular film, providing a structural hierarchy ranging from the micrometer-size overall dimensions down to the microstructural features of a few nanometers. The nanoporous structure of platinum coating caused its creep-like shrinkage and buckling of the polymer struts during the initial stages of platinum ALD, thus weakening the whole structure. Increasing the thickness of the platinum layer eliminated the porosity and strengthened the lattices, indicating the load-bearing capacity of the architected platinum coating. The yield stress of platinum coating estimated from the results of microcompression tests was comparable to the yield stress of annealed bulk platinum and much lower than the flow stress of solid nanocrystalline platinum nanopillars.

Investigating the Nucleation of AlOx and HfOx ALD on Polyimide: Influence of Plasma Activation

There is an increasing interest in atomic layer deposition (ALD) on polymers for the development of membranes, electronics, (3D) nanostructures and specially for the development of hermetic packaging of the new generation of flexible implantable micro-devices. This evolution demands a better understanding of the ALD nucleation process on polymers, which has not been reported in a visual way. Herein, a visual study of ALD nucleation on polymers is presented, based on the different dry etching speeds between polymers (fast) and metal oxides (slow). An etching process removes the polyimide with the nucleating ALD acting as a mask, making the nucleation features visible through secondary electron microscopy analyses. The nucleation of both Al2O3 and HfO2 on polyimide was investigated. Both materials followed an island-coalescence nucleation. First, local islands formed, progressively coalescing into filaments, which connected and formed meshes. These meshes evolved into porous layers that eventually grew to a full layer, marking the end of the nucleation. Cross-sections were analyzed, observing no sub-surface growth. This approach was used to evaluate the influence of plasma-activating polyimide on the nucleation. Plasma-induced oxygen functionalities provided additional surface reactive sites for the ALD precursors to adsorb and start the nucleation. The presented nucleation study proved to be a straightforward and simple way to evaluate ALD nucleation on polymers.

Elucidating the Surface Chemistry of Atomic Layer Deposition on Complex Metal Oxide Cathode Materials

Current research in the field of lithium-ion batteries (LIBs) is geared towards stabilizing the electrode-electrolyte interfaces to improve the LIB cycling stability and storage capacity. In particular, irreversible reactions between non-aqueous electrolytes and metal oxide cathode surfaces can corrode the active material and reduce LIB performance. Protective coatings on the cathode surface can mitigate these issues, and atomic layer deposition (ALD) is well-suited to prepare ultrathin, conformal, corrosion-resistant barrier layers on the surface of the electrodes. While there are reports that some metal oxide and metal fluoride barrier coatings on select cathodes can improve LIB performance, there is no general understanding of why certain ALD coating/cathode combinations are beneficial while others are not. This problem is complicated by the move towards complex LIB cathodes such as lithium nickel manganese cobalt oxide (NMC) materials that may have multiple phases present on the surface including hydroxides and carbonates. Understanding how the ALD precursors react on these materials to create protective coatings is challenging and requires a systematic study to elucidate the surface chemistry of the ALD precursors with each component of the complex oxide surface. Therefore, this work aims at understanding the effects of ALD precursor exposures and ALD coatings on the surface electronic structure of LIB cathode materials.Al2O3 and AlF3 were selected as prototypical metal oxide and metal fluoride surface coatings as they are widely reported in literature due to their ability to act as barrier coatings on cathodes. Additionally, the ALD precursor vapors used for these coatings, i.e., trimethyl aluminium (TMA) and hydrogen fluoride pyridine (HFpy), were adsorbed on the surfaces of these cathode materials under typical ALD conditions to observe their effect on the surface composition and electronic state. The cathode materials chosen for this study included all of the component oxides for NMC cathodes (i.e. Li, Ni, Mn, and Co) in all common oxidation states (e.g., MnO, MnO2, and Mn2O3) as well as selected ternary, quaternary, and quinary forms. In each case, the substrate surface was characterized using X-ray photoelectron spectroscopy (XPS) before and after surface treatment to elucidate changes in the surface composition and charge state induced by the individual ALD precursors and the ultrathin coating. Overall, this fundamental study will assist in the design and synthesis of novel ALD barrier coatings that demonstrate the highest sensitivity on multi-element cathode materials, ultimately leading to robust electrode-electrolyte interfaces for high-performance LIBs.

Infiltration of Atomic Layer Deposition Precursors for Modification of Polymers and Covalent Organic Networks

Incorporation of transition metal oxides in polymers and covalent organic networks could add unique physical and chemical properties to the parent materials. Understanding the diffusion and reactions of atomic layer deposition (ALD) precursors in these organic substrates are key to the rational design and precise control for synthesizing the organic/inorganic hybrid materials. Current challenges of developing new vapor phase infiltration methods include difficulties in finding proper precursor-polymer pairs with moderate reactivity and enlarging the permeation depth of the ALD precursors in the substrates.In this talk, we will first discuss vapor phase infiltration of diethyl zinc (DEZ) in a variety of polymers. DEZ is known to possess lower reactivity but larger kinetic dimeter compared with trimethylaluminum (TMA). As a result, the mass uptake of DEZ in poly(methyl methacrylate) is negligible, while the DEZ loading in polybutylene terephthalate was found one order of magnitude lower than that in cellulose acetate and poly(1,3,5-trimethylcyclotrisiloxane). We investigated the effect of glass-transition temperature, free volume and functional groups of the polymers on the vapor phase infiltration of DEZ. We will also show how AlOx priming affects the subsequent diffusion and reactions of DEZ in the polymers. Furthermore, we will present our recent study on ALD ZnO in covalent organic frameworks (COFs). Mesoporous COFs with OH functional groups were synthesized in forms of powders and thin films. Using ALD, we were able to modify the pore dimension as well as internal functionalities. Additionally, we applied ALD as a pore engineering tool for gas separation membranes and achieved enhanced H2/CO2 selectivity while maintaining high permeance of H2.

(Keynote) Status & Challenges in Today's Atomic Layer Processing Market

The global market for atomic-layer deposition (ALD) and chemical-vapor deposition (CVD) metal precursors for semiconductor manufacturing in 2020 was worth ~US$640M [1]. Driven by advanced logic, DRAM and 3D-NAND memory chip fabrication needs, demand for ALD and CVD metal precursors is forecast to see a 2020-2025 compound annual growth rate (CAGR) of ~10%. In the short-term forecast, sees the global semiconductor market as very healthy with 15-20% Year-over-Year (YoY) growth in 2021 revenues [2], and overall critical materials market growth >7% YoY. The greatest material demand increases are seen in ALD/CVD metal precursors, and in specialty wet cleans that often are integrated with metal deposition and etching. All ALD/CVD metal precursors are in healthy demand, since ALD is critical for fabs running 22-45nm nodes as well as for fabs at the leading edge at 16/14 nm and below. In particular, cobalt (Co) and hafnium (Hf) precursors saw 18-20% year-over-year (YoY) growth in 2020, and are forecasted to see strong demand through 2025. Zirconium (Zr) used for DRAM capacitors saw a lower growth but is a big segment on its ow. Ruthenium (Ru) metal is slowly replacing some of the Co and tungsten (W) interconnects on the most advanced logic chips, with anticipated precursor revenue growth to US$10M in 2025 for this highly strategic material. Metals to observe the next years include molybdenum (Mo), niobium (Nb) and lanthanum (La). The long-term revenue outlook for precursors continues to be good driven by: replacement of PVD processes to meet challenging specification for 3D and high aspect ratio device architecture, driven my lateral and stacked scalingReplacement of high themperature diffusion processes (oxidation and nitridation) with low thermal budget plasma ALD processes.Implementation of leading-edge logic nodes with a higher penetration of ALD processes in automotive and the EV transition of the global vehicle fleet.Growing need for ALD films in new semiconductor device technologies and new products: Spin-transfer torque MRAM (STT-MRAM), Resistive RAM (ReRAM), Ferroelectric FET (FeFET) and RAM (FRAM) and Cross-Point memoryImplementation of ALD in emerging new technologies: Photovoltaic, Display, MEMS, Power Electronics, LED/µLED, Optical, Lithium Battery (EVs, Mobile), Parts and powder coating, Medical / Healthcare. New precursors will be needed to meet improved performance for 3D devices and lower thermal budgets as well as reduced cost of ownership. New metals and new metal precursors, mainly for interconnects and contacts, are being continuously researched (e.g. W, Co, Ru, La, Mn).ALD materials and Original Equipment Manufacturer (OEM) tools are needed for multi-patterning lithography used with Deep-UV (DUV) Immersion and with Extreme-UV (EUV) in leading logic and memory fabs. Multi-patterning typically uses low-temperature Plasma-Enhanced ALD (PEALD), either in clusters of 8-16 single-wafer chambers, or in “Spatial” high-throughput tools. It is anticipated that fab investments in Asia will allow South Korean and Chinese OEMs to win near-term ALD orders, and they may soon compete with US, EU, and Japanese OEMs in the global tool market for standard processes that are commoditized. Recent actions taken by the new US administration has boosted logic fab investments in the US and in total USD 70 B leading edge fab investments are on the horizon the next three years, which will generate a need for both ALD equipment and a healthy precursor supply chain on all materials above going forward.

Selective Fluoride Transport in Subnanometer TiO2 Pores.

Synthesizing nanopores which mimic the functionality of ion-selective biological channels has been a challenging yet promising approach to advance technologies for precise ion-ion separations. Inspired by the facilitated fluoride (F-) permeation in the biological fluoride channel, we designed a highly fluoride-selective TiO2 film using the atomic layer deposition (ALD) technique. The subnanometer voids within the fabricated TiO2 film (4 Å < d < 12 Å, with two distinct peaks at 5.5 and 6.5 Å), created by the hindered diffusion of ALD precursors (d = 7 Å), resulted in more than eight times faster permeation of sodium fluoride compared to other sodium halides. We show that the specific Ti-F interactions compensate for the energy penalty of F- dehydration during the partitioning of F- ions into the pore and allow for an intrapore accumulation of F- ions. Concomitantly, the accumulation of F- ions on the pore walls also enhances the transport of sodium (Na+) cations due to electrostatic interactions. Molecular dynamics simulations probing the ion concentration and mobility within the TiO2 pore further support our proposed mechanisms for the selective F- transport and enhanced Na+ permeation in the TiO2 film. Overall, our work provides insights toward the design of ion-selective nanopores using the ALD technique.

Direct Atomic Layer Deposition of Ultrathin Aluminum Oxide on Monolayer MoS2 Exfoliated on Gold: The Role of the Substrate

AbstractIn this paper, the authors demonstrate the atomic layer deposition (ALD) of highly homogeneous and ultrathin (≈3.6 nm) Al2O3 films with very good insulating properties (breakdown field of ≈10–12 MV cm−1) directly onto monolayer (1L) MoS2 exfoliated on gold. Differently than in the case of 1L MoS2 supported by a common insulating substrate (Al2O3/Si), a better nucleation process of the high‐k film is observed on the 1L MoS2/Au system since the ALD early stages. Atomic force microscopy analyses show a ≈50% Al2O3 surface coverage just after 10 ALD cycles, its increase to &gt;90% (after 40 cycles), and a uniform ≈3.6 nm film (after 80 cycles). The Al2O3 density on bilayer MoS2 is found to be significantly reduced with respect to 1L MoS2/Au, suggesting a role of screened interface charges with the metal substrate on the adsorption of ALD precursors. Finally, Raman and photoluminescence spectroscopy show a p‐type doping and tensile strain of 1L MoS2 induced by the Au substrate, providing an insight on the evolution of vibrational and optical properties after the Al2O3 deposition. The direct ALD growth of Al2O3 on large‐area 1L MoS2 induced by the Au underlayer can be of wide interest for electronic applications.

Comparison of the Growth and Thermal Properties of Nonwoven Polymers after Atomic Layer Deposition and Vapor Phase Infiltration

The growth mechanism of Atomic Layer Deposition (ALD) on polymeric surfaces differs from growth on inorganic solid substrates, such as silicon wafer or glass. In this paper, we report the growth experiments of Al2O3 and ZnO on nonwoven poly-L-lactic acid (PLLA), polyethersulphone (PES) and cellulose acetate (CA) fibres. Material growth in both ALD and infiltration mode was studied. The structures were examined with a scanning electron microscope (SEM), scanning transmission electron microscope (STEM), attenuated total reflectance-fourier-transform infrared spectroscopy (ATR-FTIR) and 27Al nuclear magnetic resonance (NMR). Furthermore, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis were used to explore the effect of ALD deposition on the thermal properties of the CA polymer. According to the SEM, STEM and ATR-FTIR analysis, the growth of Al2O3 was more uniform than ZnO on each of the polymers studied. In addition, according to ATR-FTIR spectroscopy, the infiltration resulted in interactions between the polymers and the ALD precursors. Thermal analysis (TGA/DSC) revealed a slower depolymerization process and better thermal resistance upon heating both in ALD-coated and infiltrated fibres, more pronounced on the latter type of structures, as seen from smaller endothermic peaks on TA.

Atomic layer deposition of iron oxide on a porous carbon substrate via ethylferrocene and an oxygen plasma

Ethylferrocene, a novel precursor for atomic layer deposition (ALD) of iron oxide, was investigated using an oxygen plasma co-reactant. Iron oxide deposition showed a saturating growth rate of 0.1 Å/cycle in the temperature range of 150 °C to 250 °C. The iron oxide coating was subsequently deposited on a porous gas diffusion layer (GDL) for use as an air electrode in a zinc-air battery. X-ray microanalysis confirmed deep penetration of the iron oxide into the porosity of the GDL, with a lower substrate temperature providing deeper coverage. Transmission electron microscopy revealed that a uniform ~10 nm thick iron oxide coating encased the GDL particles. Electron diffraction and X-ray photoelectron spectroscopy identified the iron oxide film as α-Fe2O3. Electrochemical characterization of the ALD-coated GDL showcased promising catalytic activity towards the oxygen evolution reaction.

Ambient-Stable Two-Dimensional CrI3 via Organic-Inorganic Encapsulation.

Two-dimensional transitional metal halides have recently attracted significant attention due to their thickness-dependent and electrostatically tunable magnetic properties. However, this class of materials is highly reactive chemically, which leads to irreversible degradation and catastrophic dissolution within seconds in ambient conditions, severely limiting subsequent characterization, processing, and applications. Here, we impart long-term ambient stability to the prototypical transition metal halide CrI3 by assembling a noncovalent organic buffer layer, perylenetetracarboxylic dianhydride (PTCDA), which templates subsequent atomic layer deposition (ALD) of alumina. X-ray photoelectron spectroscopy demonstrates the necessity of the noncovalent organic buffer layer since the CrI3 undergoes deleterious surface reactions with the ALD precursors in the absence of PTCDA. This organic-inorganic encapsulation scheme preserves the long-range magnetic ordering in CrI3 down to the monolayer limit as confirmed by magneto-optical Kerr effect measurements. Furthermore, we demonstrate field-effect transistors, photodetectors, and optothermal measurements of CrI3 thermal conductivity in ambient conditions.

(Invited) Precursors for the Thermal Atomic Layer Deposition of Cobalt Metal Films and Alloys Thereof

Herein, we will describe recent progress in our laboratory on the development of chemical precursors for the thermal atomic layer deposition (ALD) of cobalt metal, titanium metal, cobalt-titanium alloy, and other metal and metal alloy films. The nanoscale dimensions of current and future microelectronics devices now require alternative, ultrathin barrier materials for copper and other interconnect metals as well as new ultrathin liner materials. Sputtered cobalt-titanium alloys have been demonstrated to function as barrier materials and liners at dimensions as small as 1 nm. Hence, these materials may replace current barrier and liner materials such as TiN, TaN, and W metal. However, the narrow and deep nanoscale features require growth of cobalt-titanium alloy films by ALD to meet the strict conformality and thickness requirements. Moreover, thermal ALD is preferred over plasma-based processes, since recombination of atomic hydrogen on feature walls can afford poor conformal coverage. Thermal ALD processes for Co and Ti metal films are poorly developed, because the negative electrochemical potentials of the ions in precursors (Co2+ ↔ Co, E° = -0.280 V, Ti2+ ↔ Ti, E° = -1.628 V) require strong reducing co-reactants, which are not available. We will report new thermal ALD processes for cobalt metal films that employ various ligands, in combination with hydrazine-based co-reactants. These processes afford high purity cobalt metal films. We will also describe our efforts to develop new precursors for the thermal ALD of titanium metal films. Initial depositions of cobalt-titanium metal films will also be described.

ZnO nucleation into trititanate nanotubes by ALD equipment techniques, a new way to functionalize layered metal oxides

In this contribution, we explore the potential of atomic layer deposition (ALD) techniques for developing new semiconductor metal oxide composites. Specifically, we investigate the functionalization of multi-wall trititanate nanotubes, H2Ti3O7 NTs (sample T1) with zinc oxide employing two different ALD approaches: vapor phase metalation (VPM) using diethylzinc (Zn(C2H5)2, DEZ) as a unique ALD precursor, and multiple pulsed vapor phase infiltration (MPI) using DEZ and water as precursors. We obtained two different types of tubular H2Ti3O7 species containing ZnO in their structures. Multi-wall trititanate nanotubes with ZnO intercalated inside the tube wall sheets were the main products from the VPM infiltration (sample T2). On the other hand, MPI (sample T3) principally leads to single-wall nanotubes with a ZnO hierarchical bi-modal functionalization, thin film coating, and surface decorated with ZnO particles. The products were mainly characterized by electron microscopy, energy dispersive X-ray, powder X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. An initial evaluation of the optical characteristics of the products demonstrated that they behaved as semiconductors. The IR study revealed the role of water, endogenous and/or exogenous, in determining the structure and properties of the products. The results confirm that ALD is a versatile tool, promising for developing tailor-made semiconductor materials.

Synthesis and Thermal Study of Hexacoordinated Aluminum(III) Triazenides for Use in Atomic Layer Deposition.

Amidinate and guanidinate ligands have been used extensively to produce volatile and thermally stable precursors for atomic layer deposition. The triazenide ligand is relatively unexplored as an alternative ligand system. Herein, we present six new Al(III) complexes bearing three sets of a 1,3-dialkyltriazenide ligand. These complexes volatilize quantitatively in a single step with onset volatilization temperatures of ∼150 °C and 1 Torr vapor pressures of ∼134 °C. Differential scanning calorimetry revealed that these Al(III) complexes exhibited exothermic events that overlapped with the temperatures of their mass loss events in thermogravimetric analysis. Using quantum chemical density functional theory computations, we found a decomposition pathway that transforms the relatively large hexacoordinated Al(III) precursor into a smaller dicoordinated complex. The pathway relies on previously unexplored interligand proton migrations. These new Al(III) triazenides provide a series of alternative precursors with unique thermal properties that could be highly advantageous for vapor deposition processes of Al containing materials.

Organoselenium Precursors for Atomic Layer Deposition.

Organoselenium compounds with perspective application as Se precursors for atomic layer deposition have been reviewed. The originally limited portfolio of available Se precursors such as H2Se and diethyl(di)selenide has recently been extended by bis(trialkylsilyl)selenides, bis(trialkylstannyl)selenides, cyclic selenides, and tetrakis(N,N-dimethyldithiocarbamate)selenium. Their structural aspects, property tuning, fundamental properties, and preparations are discussed. It turned out that symmetric four- and six-membered cyclic silyl selenides possess well-balanced reactivity/stability, facile and cost-effective synthesis starting from inexpensive and readily available chlorosilanes, improved resistance toward air and moisture, easy handling, sufficient volatility, thermal resistance, and complete gas-to-solid phase exchange reaction with MoCl5, affording MoSe2 nanostructures. These properties make them the most promising Se precursor developed for atomic layer deposition so far.

In situ ellipsometry monitoring of TiO[formula omitted] atomic layer deposition from Tetrakis(dimethylamido)titanium(IV) and H[formula omitted]O precursors on Si and In[formula omitted]Ga[formula omitted]As substrates

TiO2 Atomic Layer Deposition (ALD) is used in microelectronics due to its ability to produce conformal thin films whose thickness is controlled at the sub-nanometer scale. Tetrakis(dimethylamido)titanium(IV) and water are frequently used precursors for TiO2 ALD, although there are still some differences in growth behavior in the literature. In this parametric study, the growth of TiO2 was controlled in situ by Multi-Wavelength Ellipsometry in combination with ex situ thickness measurements by X-ray Reflectometry. The injection and purge times were optimized to reach self-saturation on the surface. We put in evidence two regions of the Growth Per Cycle (GPC) as a function of substrate temperature: GPC decreases from 0.8 Å cy−1 to 0.5 Å cy−1 as the temperature rises from 50 °C to 200 °C, and then GPC increases from 0.6 Å cy−1 to 0.9 Å cy−1 as the temperature rises from 250 °C to 350 °C. There is no evidence of an ALD window - the temperature region where the growth rate is the same. The stoichiometry of the layer grown at 200 °C, determined by X-ray Photoelectron Spectroscopy, is TiO2 (1 Ti atom per 2 O atoms). The 200 °C as-grown sample becomes crystalline (Anatase crystals) after annealing 3 h in air at 400 °C, or at 600 °C. The initial growth stages of TiO2 were studied by Multi-Wavelength Ellipsometry and Atomic Force Microscopy. Regarding TiO2 ALD on (100)Si substrate (with a native SiO2), we observed a substrate enhanced growth which turned into steady-state ALD beyond 20–30 cycles. Additionally, TiO2 ALD on (001)In0.53Ga0.47As substrate demonstrates substrate inhibited growth of type 2 (islands formation) which turned into TiO2 steady-state ALD beyond 20–30 cycles. The results of TiO2 ALD are compared with those of the existing literature.