Atomic layer deposition of MnO using Bis(ethylcyclopentadienyl)manganese and H2O

Manganese oxide (MnO) atomic layer deposition (ALD) was accomplished using sequential exposures of bis(ethylcyclopentadienyl)manganese (Mn(CpEt)₂) and H₂O. Rutherford backscattering analysis revealed a nearly 1:1 atomic ratio for Mn:O in the MnO ALD films. X-ray diffraction determined that the films were crystalline and consistent with the cubic phase of MnO. Quartz crystal microbalance (QCM) measurements monitored the mass deposition rate during MnO ALD and verified self-limiting reactions for each reactant. Extremely efficient reactions were observed that required reactant exposures of only 3 × 10⁴ L (1 L = 1.33 × 10^− 4 Pa s). X-ray reflectivity (XRR) studies were used to confirm the QCM measurements and determine the film density and film thicknesses. The MnO ALD film density was 5.23 g/cm³. The growth per cycle was investigated from 100-300 °C. The largest MnO ALD growth per cycle was 1.2 Å/cycle at 100 °C and the growth per cycle decreased at higher temperatures. Transmission electron microscopy images observed the conformality of MnO films on ZrO₂ nanoparticles and confirmed the growth per cycle observed by the XRR studies. Fourier transform infrared spectroscopy was used to study the -CpEt? and -OH? surface species during MnO ALD and also monitored the bulk vibrational modes of the growing MnO films. The results allowed a growth mechanism to be established for MnO ALD using Mn(CpEt)₂ and H₂O. Only 54% of the Mn sites are observed to retain the -CpEt? surface species after the Mn(CpEt)₂ exposure. Efficient MnO ALD using Mn(CpEt)₂ and H₂O should be useful for a variety of applications where metal oxides are required that can easily change their oxidation states.     

Fluorescent tags to visualize defects in Al2O3 thin films grown using atomic layer deposition

Defects and cracks in thin film barriers that are coated on polymers allow the leakage of reactive species through the polymer substrate. Fluorescent tags have been developed to visualize defects and cracks in thin film barriers and to inspect rapidly the barrier quality with minimal sample preparation. For Al₂O₃ films with a thickness of 25 nm deposited on polyethylene naphthalate polymer substrates using atomic layer deposition techniques, the fluorescent tags have identified cracks ~ 20 nm in width after applied strain and have observed individual defects as small as ~ 200 nm in diameter.     

Thin films of In2O3 by atomic layer deposition using In(acac)3

Thin films of indium oxide have been deposited using the atomic layer deposition (ALD) technique using In(acac)₃ (acac = acetylacetonate, pentane-2,4-dione) and either H₂O or O₃ as precursors. Successful growth using In(acac)₃ is contradictory to what has been reported previously in the literature [J.W. Elam, A.B.F. Martinson, M.J. Pellin, J.T. Hupp, Chem. Mater. 18 (2006) 3571.]. Investigation of the dependence of temperature on the deposition shows windows where the growth rates are relatively unaffected by temperature in the ranges 165–200 °C for In(acac)₃ and H₂O, 165–225 °C for In(acac)₃ and O₃. The growth rates obtained are of the order 20 pm/cycle for In(acac)₃ and H₂O, 12 pm/cycle for In(acac)₃.     

Atomic layer deposition of Ru films from bis(2,5-dimethylpyrrolyl)ruthenium and oxygen

Ru thin films were grown on hydrogen terminated Si, SiO₂, Al₂O₃, HfO₂, and TiO₂ surfaces by atomic layer deposition from bis(2,5-dimethylpyrrolyl)ruthenium precursor and oxygen. The 4-20 nm thick films on these surfaces consisted of nanocrystalline hexagonal metallic ruthenium, regardless of the deposition temperature. At the lowest temperatures examined, 250-255 °C, the growth of the Ru films was favored on silicon, compared to the growth on Al₂O₃, TiO₂ and HfO₂. At higher temperatures the nucleation and growth of Ru became enhanced in particular on HfO₂, compared to the process on silicon. At 320-325 °C, no growth occurred on Si-H and SiO₂-covered silicon. Resistivity values down to 18 μΩ·cm were obtained for ca. 10 nm thick Ru films.     

The uniformity of Al distribution in aluminum-doped zinc oxide films grown by atomic layer deposition

We investigated the aluminum distribution in aluminum-doped zinc oxide films grown by atomic layer deposition. Surface morphology, structure, composition and electrical properties of obtained films were studied. For the aluminum content less than 2 at.%, a periodicity of Al distribution along the layer depth was observed. This periodicity diminished significantly after annealing the samples in nitrogen atmosphere at 300 °C. For the Al content higher than 2 at.%, its distribution in ZnO:Al films was uniform within the depth measurement accuracy of ∼5-10 nm.     

Characteristics and applications of plasma enhanced-atomic layer deposition

Atomic layer deposition (ALD) is expected to play an important role in future device fabrication due to various benefits, such as atomic level thickness control and excellent conformality. Plasma enhanced ALD (PE-ALD) allows deposition at significantly lower temperatures with better film properties compared to that of conventional thermal ALD. In addition, since ALD is a surface-sensitive deposition technique, surface modification through plasma exposure can be used to alter nucleation and adhesion. In this paper, characteristics of PE-ALD for various applications in semiconductor fabrication are presented through comparison to thermal ALD. The results indicate that the PE-ALD processes are versatile methods to enable nanoscale manufacturing in emerging applications.     

Thin films of iron oxide by low pressure MOCVD using a novel precursor: tris(t-butyl-3-oxo-butanoato)iron(III)

Deposition of thin films of iron oxide on glass has been carried out using a novel precursor, tris(t-butyl-3-oxo-butanoato)iron(III), in a low-pressure metalorganic chemical vapor deposition (MOCVD) system. The new precursor was characterized for its thermal properties by thermogravimetry and differential thermal analysis. The films were characterized by X-ray diffraction (XRD), transmission electron microscopy, scanning electron microscopy, and by optical measurements. XRD studies reveal that films grown at substrate temperatures below ∼550 °C and at low oxygen flow rates comprise only the phase Fe₃O₄ (magnetite). At higher temperatures and at higher oxygen flow rates, an increasing proportion of α-Fe₂O₃ is formed along with Fe₃O₄. Films of magnetite grown under different reactive ambients—oxygen and nitrous oxide—have very different morphologies, as revealed by scanning electron microscopic studies.     

X-ray mirrors on flexible polymer substrates fabricated by atomic layer deposition

Atomic layer deposition (ALD) techniques were used to fabricate W/Al₂O₃ superlattices with high X-ray reflectivity on flexible Kapton® polyimide and polyethylene naphthalate (PEN) polymer substrates. Reflectivities of 78% and 74% at λ = 1.54 Å were measured for 6-bilayer W/Al₂O₃ superlattices on Kapton® polyimide and PEN, respectively. These excellent X-ray reflectivities are attributed to precise bilayer thicknesses and ultrasmooth interfaces obtained by ALD and smoothing of the initial polymer surface by an Al₂O₃ ALD layer. The conformal ALD film growth also produces correlated roughness that enhances the reflectivity. These W/Al₂O₃ superlattices on flexible polymers should be useful for ultralight and adjustable radius of curvature X-ray mirrors.     

Magneto-optical Kerr-effect studies on copper oxide thin films produced by atomic layer deposition on SiO2

This work demonstrates the sensitivity of magneto-optical Kerr-effect (MOKE) spectroscopy to ultra-thin nonmagnetic films using the example of copper oxide. The films with an effective thickness between 0.6 nm and 6 nm are produced by atomic layer deposition (ALD) on silicon oxide substrates based on the Cu(I) β-diketonate precursor [(ⁿBu₃P)₂Cu(acac)] (acac = acetylacetonate) at a process temperature of 120 °C. The copper oxide films exhibit magneto-optical activity in the spectral ranges around 2.6 eV and above 4 eV. The evolution of the spectral features as a function of the number of ALD cycles is simulated numerically using the dielectric function and the Voigt constant of Cu₂O as input parameters. The comparison between experimental and simulated MOKE spectra strengthens the conclusion drawn from spectroscopic ellipsometry studies that the thin film optical constants differ markedly from the bulk ones.     

Conformal nanocoating of zirconia nanoparticles by atomic layer deposition in a fluidized bed reactor

Primary zirconia nanoparticles were conformally coated with alumina ultrathin films using atomic layer deposition (ALD) in a fluidized bed reactor. Alternating doses of trimethylaluminium and water vapour were performed to deposit Al(2)O(3) nanolayers on the surface of 26?nm zirconia nanoparticles. Transmission Fourier transform infrared spectroscopy was performed ex situ. Bulk Al(2)O(3) vibrational modes were observed for coated particles after 50 and 70?cycles. Coated nanoparticles were also examined with transmission electron microscopy, high-resolution field emission scanning electron microscopy and energy dispersive spectroscopy. Analysis revealed highly conformal and uniform alumina nanofilms throughout the surface of zirconia nanoparticles. The particle size distribution and surface area of the nanoparticles are not affected by the coating process. Primary nanoparticles are coated individually despite their high aggregation tendency during fluidization. The dynamic aggregation behaviour of zirconia nanoparticles in the fluidized bed plays a key role in the individual coating of nanoparticles.     

Atomic layer deposition grown composite dielectric oxides and ZnO for transparent electronic applications

In this paper, we report on transparent transistor obtained using laminar structure of two high-k dielectric oxides (hafnium dioxide, HfO₂ and aluminum oxide, Al₂O₃) and zinc oxide (ZnO) layer grown at low temperature (60 °C-100 °C) using Atomic Layer Deposition (ALD) technology. Our research was focused on the optimization of technological parameters for composite layers Al₂O₃/HfO₂/Al₂O₃ for thin film transistor structures with ZnO as a channel and a gate layer. We elaborate on the ALD growth of these oxides, finding that the 100 nm thick layers of HfO₂ and Al₂O₃ exhibit fine surface flatness and required amorphous microstructure. Growth parameters are optimized for the monolayer growth mode and maximum smoothness required for gating.     

Atomic layer deposition and characterization of zirconium oxide-erbium oxide nanolaminates

ZrO₂ and Er₂O₃ thin films and nanolaminates were grown by atomic layer deposition from tris(2,2,6,6-tetramethyl-3,5-heptanedionato)erbium, bis(methylcyclopentadienyl)methoxymethylzirconium and ozone as precursors at 350 °C. Nanolaminates consisted of 3-8 nm thick ZrO₂ and Er₂O₃ layers alternately deposited on planar substrates and on three-dimensional substrates with aspect ratio 1:20. The erbium content was 5-15 at.%. ZrO₂-Er₂O₃ films crystallized already in as-deposited states. Upon annealing at 650 °C, the films were stabilized in the form of cubic or tetragonal ZrO₂ polymorph and cubic Er₂O₃. Dielectric properties of the nanolaminates were comparable to those of the constituent oxides.     

Atomic layer deposition of palladium films on Al2O3 surfaces

We examined the atomic layer deposition (ALD) of Pd films using sequential exposures of Pd(II) hexafluoroacetylacetonate (Pd(hfac)₂) and formalin and discovered that formalin enables the efficient nucleation of Pd ALD on Al₂O₃. In situ quartz crystal microbalance measurements revealed that the Pd nucleation is hampered by the relatively slow reaction of the adsorbed Pd(hfac)₂ species, but is accelerated using larger initial Pd(hfac)₂ and formalin exposures. Pd nucleation proceeds via coalescence of islands and leaves hfac contamination at the Al₂O₃ interface. Pd films were deposited on the thermal oxide of silicon, glass and mesoporous anodic alumina following the ALD of a thin Al₂O₃ seed layer and analyzed using a variety of techniques. We measured a Pd ALD growth rate of 0.2 Å/cycle following a nucleation period of slower growth. The deposited films are cubic Pd with a roughness of 4.2 nm and a resistivity of 11 μΩ cm at 42 nm thickness. Using this method, Pd deposits conformally on the inside of mesoporous anodic alumina membranes with aspect ratio ∼1500 yielding promising hydrogen sensors.     

Scalable production of ultra small TiO2 nano crystal/activated carbon composites by atomic layer deposition for efficient removal of organic pollutants

A high surface area photo-catalytic composite material is synthesized by depositing thin films of titanium dioxide (TiO₂) on activated carbon (AC) particles using atomic layer deposition (ALD). A rotary ALD reactor is developed for scalable fabrication of powder and grams of the catalyst is prepared in each batch. The processes of TiO₂ ALD are monitored by mass spectrometry. Saturated ALD surface reactions are confirmed so that the entire surface of the AC support is covered by conformal coatings of TiO₂. For composites fabricated by 3 or more ALD cycles of TiO₂, the amorphous oxide layers can be converted to crystalline films by high temperature annealing. The as-prepared TiO₂/AC composites are highly reactive in photo-catalyzed degradation of methyl orange. The excellent catalytic performance is attributed to the abundant and uniformly dispersed active phase, formation of very active ultra small (<5 nm) TiO₂ crystals, and easy accessibility of the active sites.     

Review of tailoring ZnO for optoelectronics through atomic layer deposition experimental variables

Atomic layer deposition (ALD) is an increasingly popular thin film deposition technique which offers unique large area capability combined with excellent conformality, thus ALD will likely be important in the development of next generation optoelectronic devices. Such device platforms include solar cells, thin film transistors and light emitting diodes, and in all of these technologies one material is frequently used – zinc oxide (ZnO) – owing to its excellent electrical and optical properties combined with earth abundance. The approaches and achievements in tailoring the properties of ALD ZnO are discussed. Key process variables include deposition temperature and purge times as well dopant incorporation, with particular attention paid to tuning band alignment and carrier concentrations (focusing on lower carrier concentration applications).

This review was submitted as part of the 2016 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.     

Influence of thickness and growth temperature on the properties of zirconium oxide films grown by atomic layer deposition on silicon

ZrO₂ films of thicknesses varied in the range of 3–30 nm were atomic layer deposited from ZrI₄ and H₂O–H₂O₂ on p-Si(100) substrates. The effects of film thickness and deposition temperature on the structure and dielectric properties of ZrO₂ were investigated. At 272 and 325 °C, the growth of ZrO₂ started with the formation of the cubic polymorph and continued with the formation of the tetragonal polymorph. The ratio between the lattice parameters increased with the film thickness and growth temperature. The effective permittivity, determined from the accumulation capacitance of Hg/ZrO₂/Si capacitors, increased with the film thickness, reaching 15–17 in 25-nm-thick films. The permittivity decreased with the increasing growth temperature. The hysteresis of the capacitance–voltage curves was the narrowest for the films deposited at 325 °C, and increased towards both lower and higher deposition temperatures.     

Atomic layer deposition on polymer based flexible packaging materials: Growth characteristics and diffusion barrier properties

One of the most promising areas for the industrial application of atomic layer deposition (ALD) is for gas barrier layers on polymers. In this work, a packaging material system with improved diffusion barrier properties has been developed and studied by applying ALD on flexible polymer based packaging materials. Nanometer scale metal oxide films have been applied to polymer-coated papers and their diffusion barrier properties have been studied by means of water vapor and oxygen transmission rates. The materials for the study were constructed in two stages: the paper was firstly extrusion coated with polymer film, which was then followed by the ALD deposition of oxide layer. The polymers used as extrusion coatings were polypropylene, low and high density polyethylene, polylactide and polyethylene terephthalate. Water vapor transmission rates (WVTRs) were measured according to method SCAN-P 22:68 and oxygen transmission rates (O₂TRs) according to a standard ASTM D 3985. According to the results a 10 nm oxide layer already decreased the oxygen transmission by a factor of 10 compared to uncoated material. WVTR with 40 nm ALD layer was better than the level currently required for most common dry flexible packaging applications. When the oxide layer thickness was increased to 100 nm and above, the measured WVTRs were limited by the measurement set up. Using an ALD layer allowed the polymer thickness on flexible packaging materials to be reduced. Once the ALD layer was 40 nm thick, WVTRs and O₂TRs were no longer dependent on polymer layer thickness. Thus, nanometer scale ALD oxide layers have shown their feasibility as high quality diffusion barriers on flexible packaging materials.     

Conformity and structure of titanium oxide films grown by atomic layer deposition on silicon substrates

Conformity and phase structure of atomic layer deposited TiO₂ thin films grown on silicon substrates were studied. The films were grown using TiCl₄ and Ti(OC₂H₅)₄ as titanium precursors in the temperature range from 125 to 500 °C. In all cases perfect conformal growth was achieved on patterned substrates with elliptical holes of 7.5 μm depth and aspect ratio of about 1:40. Conformal growth was achieved with process parameters similar to those optimized for the growth on planar wafers. The dominant crystalline phase in the as-grown films was anatase, with some contribution from rutile at relatively higher temperatures. Annealing in the oxygen ambient resulted in (re)crystallization whereas the effect of annealing depended markedly on the precursors used in the deposition process. Compared to films grown from TiCl₄, the films grown from Ti(OC₂H₅)₄ were transformed into rutile in somewhat greater extent, whereas in terms of step coverage the films grown from Ti(OC₂H₅)₄ remained somewhat inferior compared to the films grown from TiCl₄.     

The influence of the parameters of the zinc oxide layer deposition process using the atomic layer deposition method on the physical and mechanical properties of 316LVM steel

The analysis of most of the work on the use of surgical implants shows that the physical, chemical and mechanical properties of the implant surface determine the effectiveness of their use. Therefore, in the currently conducted research, the greatest attention is focused on the development of coating conditions on the surface of implants, limiting, among others, biofilm formation. In addition to the improvement of electrochemical properties, an important issue related to the production of layers is also an appropriate set of physical and mechanical properties. Therefore, the paper attempts to assess the physical and mechanical properties of the applied zinc oxide layers on the LVM 316 steel substrate using the atomic layer deposition (ALD) method. As part of the assessment of mechanical properties of the resulting surface layers, the tests of adhesion to the metal substrate (scratch test) were carried out. In turn, as part of the physical properties assessment, surface roughness tests (atomic force microscopy – AFM) and its wettability were carried out. Such a comprehensive approach will allow the development of an antibacterial layer with optimal physical and mechanical properties for applications with metal implants.     

Electroplating to visualize defects in Al2O3 thin films grown using atomic layer deposition

Thin films grown using atomic layer deposition (ALD) are known for being continuous and nearly pinhole-free. These characteristics enable ALD films to be important in many applications such as gas or copper diffusion barriers, gate dielectrics, surface modification and functionalization layers. Few methods have been demonstrated to characterize defects in ALD films. In this study, a method to render the defects visible in Al₂O₃ ALD thin films on conductive substrates has been developed by growing copper bumps locally at the defect sites using electroplating. The electroplated copper can be easily observed or inspected using conventional optical- or electron-microscopy. Using this approach, the defect density in Al₂O₃ ALD thin films grown on nickel substrates has been shown to be as low as 38 /cm².