In situ studies of the atomic layer deposition of thin HfO2 dielectrics by ultra high vacuum atomic force microscope

We studied in situ the initial stages of atomic layer deposition (ALD) of HfO₂ by an ultra high vacuum atomic force microscope working in frequency-modulation mode. The ALD cycles, made by using tetrakis-di-methyl-amido-Hf and water as precursors, were performed on the Si(001)/SiO₂ substrate maintained at 230 °C. After each ALD cycle we studied the influence of the HfO₂ growth on the surface height histogram, the root mean square roughness, the surface fractal dimension and the autocorrelation function. This detailed analysis of the surface topography allowed us to confirm the completion of the first HfO₂ layer after four ALD cycles.     

Formation of Hf- and Ta-aluminates by reactive ion beam mixing of X/Al interfaces (X = Hf or Ta)

Hf- and Ta-aluminates have been grown by 3 keV O₂⁺ reactive ion beam mixing (IBM) of X/Al interfaces (X = Hf or Ta). The kinetics of growth, composition and electronic structure of the films formed have been studied using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and factor analysis. A reactive IBM kinetics of two stages has been found by means of factor analysis. In a first stage, HfO₂ or a mixture of Ta suboxides and Ta₂O₅ species are formed for the Hf/Al or Ta/Al interface, respectively. Ta suboxide species are subsequently transformed into Ta₂O₅ species at the beginning of the second stage. In a second stage, HfO₂ or Ta₂O₅ species are transformed progressively into Hf-O-Al or Ta-O-Al species, respectively, leading to the synthesis of custom designed Hf- and Ta-aluminates. The evolution of the Auger parameters and UPS valence band spectra shows that this transformation is accompanied by changes in the electronic structure of the oxide films formed.     

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.     

Properties of atomic layer deposited HfO2 thin films

The growth, composition and morphology of HfO₂ films that have been deposited by atomic layer deposition (ALD) are examined in this article. The films are deposited using two different ALD chemistries: i) tetrakis ethylmethyl amino hafnium and H₂O at 250° and ii) tetrakis dimethyl amino hafnium and H₂O at 275 °C. The growth rates are 1.2 Å/cycle and 1.0 Å/cycle respectively. The main impurities detected both by X-ray Photoelectron Spectroscopy and Fourier transform infrared spectroscopy (FTIR) are bonded carbon (~ 3 at.%) and both bulk and terminal OH species that are partially desorbed after high temperature inert anneals up to 900 °C. Atomic Force Microscopy reveals increasing surface roughness as a function of increasing film thickness. X-ray diffraction shows that the morphology of the as-deposited films is thickness dependent; films with thickness around 30 nm for both processes are amorphous while ~ 70 nm films show the existence of crystallites. These results are correlated with FTIR measurements in the far IR region where the HfO₂ peaks are found to provide an easy and reliable technique for the determination of the crystallinity of relatively thick HfO₂ films. The index of refraction for all films is very close to that for bulk crystalline HfO₂.     

Compositional, structural and electronic characteristics of HfO2 and HfSiO dielectrics prepared by radio frequency magnetron sputtering

HfO₂ and HfSiO films were prepared on Si substrates by using radio frequency magnetron sputtering (RFMS). Compositional, structural and electronic properties of the two films were investigated completely. X-ray photoelectron spectroscopy (XPS) spectra showed that the atom ratio of Hf to O was about 1:2 in the HfO₂ film and the chemical composition of the HfSiO film was Hf₃₇Si₇O₅₆. Grazing incidence X-ray diffraction (GI-XRD) patterns indicated crystallization in the HfO₂ film after 400 °C annealing, but there is no detectable crystallization in the HfSiO film after 800 °C annealing. C-V measurements indicated that the dielectric constants for the HfO₂ and HfSiO film were 20.3 and 17.3, respectively. The fixed charge densities were found to be 6.0 × 10¹² cm⁻² for the HfO₂ film and 3.7 × 10¹² cm⁻² for the HfSiO film. I-V characteristics showed that the average leakage current densities were 2.4 μA/cm² for the HfO₂ film and 0.2 μA/cm² for the HfSiO film at the gate bias of 1 V.     

The effects of O2/Ar ratio on the structure and properties of hafnium dioxide (HfO2) films

HfO₂ films at various O₂/Ar flow ratios were prepared by reactive dc magnetron sputtering. The effects of O₂/Ar ratio on the structure and properties of HfO₂ films were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and UV-Visible spectroscopy. The results showed that the HfO₂ films were amorphous at different O₂/Ar ratios, and the atomic ratio of O/Hf in the HfO₂ films at high O₂/Ar ratio was nearly to 2:1. The peaks of Hf4f and O1s shifted to higher binding energy with increasing the oxygen flow proportion. The HfO₂ films at high O₂/Ar ratio had high transmissivity at the range of 400-1100 nm.     

Textured strontium titanate layers on platinum by atomic layer deposition

Formation of textured strontium titanate (STO) layers with large lateral grain size (0.2-1 μm) and low X-ray reflectivity roughness (~ 1.36 nm) on Pt electrodes by industry proven atomic layer deposition (ALD) method is demonstrated. Sr(t-Bu₃Cp)₂, Ti(OMe)₄ and O₃ precursors at 250 °C were used to deposit Sr rich STO on Pt/Ti/SiO₂/Si ∅200 mm substrates. After crystallization post deposition annealing at 600 °C in air, most of the STO grains showed a preferential orientation of the {001} plane parallel to the substrate surface, although other orientations were also present. Cross sectional and plan view transmission electron microscopy and electron diffraction analysis revealed more than an order of magnitude larger lateral grain sizes for the STO compared to the underlying multicrystalline {111} oriented platinum electrode. The combination of platinum bottom electrodes with ALD STO(O₃) shows a promising path towards the formation of single oriented STO film.     

Characteristics of high-k gate oxide prepared by oxidation of multi-layered Hf/Zr/Hf/Zr/Hf metal films

We investigated the physical and electrical properties of Hf-Zr mixed high-k oxide films obtained by the oxidation and annealing of multi-layered metal films (i.e., Hf/Zr/Hf/Zr/Hf, ∼ 5 nm). We demonstrated that the oxidation of multi-layered metal films results in two distinctive amorphous layers: That is, Hf-Zr mixed oxide film was formed on the top of silicate film due to inter-diffusion between Hf and Zr layer. This film shows the improved dielectric constant (k) and the raised crystallization temperature. Compared with HfO₂ and ZrO₂ gate dielectric, the crystallization temperature of Hf-Zr mixed oxides was raised by more than 200 °C. Using AES and XPS, we observed that Zr oxide has more fully oxidized stoichiometry than Hf oxide, irrespective of annealing temperatures. We also found that the thickness of an interfacial layer located between Hf-Zr mixed oxide and Si substrate also increases as annealing temperature increases. Especially, the thin SiO_x interfacial layer starts to form if annealing temperature increases over 700 °C, deteriorating the equivalent oxide thickness.     

Effects of physical growth conditions on the structural and optical properties of sputtered grown thin HfO2 films

HfO₂ thin films were prepared by reactive DC magnetron sputtering technique on (100) p-Si substrate. The effects of O₂/Ar ratio, substrate temperature, sputtering power on the structural properties of HfO₂ grown films were studied by Spectroscopic Ellipsometer (SE), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrum, and X-ray photoelectron spectroscopy (XPS) depth profiling techniques. The results show that the formation of a SiO_x suboxide layer at the HfO₂/Si interface is unavoidable. The HfO₂ thickness and suboxide formation are highly affected by the growth parameters such as sputtering power, O₂/Ar gas ratio during sputtering, and substrate temperature. XRD spectra show that the deposited films have (111) monoclinic phase of HfO₂, which is also supported by FTIR spectra. XPS depth profiling spectra shows that highly reactive sputtered Hf atoms consume some of the oxygen atoms from the underlying SiO₂ to form HfO₂, leaving Si-Si bonds behind.     

HfO2 gate insulator formed by atomic layer deposition for thin-film-transistors

We have investigated the effects of annealing temperature on the physical and electrical properties of the HfO₂ film deposited by an atomic layer deposition (ALD) method for high-k gate oxides in thin-film-transistors (TFTs). The ALD deposition of HfO₂ directly on the Si substrate at 300 °C results in the formation of thin HfSi_xO_y interfacial layer between Si and HfO₂. The subsequent low temperature N₂-annealing of HfO₂ films (i.e., 300 °C) using a rapid thermal processor (RTP) improves the overall electrical characteristics of HfSi_xO_y-HfO₂ films. Based on the current work, we suggest that HfO₂ film deposited by the ALD method is suitable for high-k gate oxides in TFTs, which have to be fabricated at low temperature.     

Influence of underlayers on the c-axis distribution in Co80Pt20 thin films

A series of approximately 40 nm thick Co₈₀Pt₂₀ thin films have been sputter-deposited onto a combination of Ta, Pt and Ru underlayers grown at different layer thicknesses. The addition of a Ta seed layer to the Pt and Ru underlayers caused the {0002} hexagonal close packed (hcp) Co₈₀Pt₂₀'s c-axis dispersion's full width at half maximum to narrow from approximately 12° to approximately 2°. In-situ stress measurements taken during deposition showed that the Ta seed layer reduced the growth stresses for the Pt and an initial 1 nm of growth for the Ru underlayers. The Ru layer thickness controlled the c/a ratio of the hcp Co₈₀Pt₂₀ film which regulated the degree of magnetic easy-axis alignment in the Co₈₀Pt₂₀ film. The optimal underlayer material stack for Co₈₀Pt₂₀ with a narrow c-axis dispersion and a high degree of magnetic easy-axis alignment was 5 nm Ta/10 nm Pt/5 nm Ru.     

Growth and characterization of epitaxial anatase TiO2(001) on SrTiO3-buffered Si(001) using atomic layer deposition

Epitaxial anatase titanium dioxide (TiO₂) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer grown by molecular beam epitaxy (MBE) to serve as a surface template. The growth of TiO₂ was achieved using titanium isopropoxide and water as the co-reactants at a substrate temperature of 225-250 °C. To preserve the quality of the MBE-grown STO, the samples were transferred in-situ from the MBE chamber to the ALD chamber. After ALD growth, the samples were annealed in-situ at 600 °C in vacuum (10^− 7 Pa) for 1-2 h. Reflection high-energy electron diffraction was performed during the MBE growth of STO on Si(001), as well as after deposition of TiO₂ by ALD. The ALD films were shown to be highly ordered with the substrate. At least four unit cells of STO must be present to create a stable template on the Si(001) substrate for epitaxial anatase TiO₂ growth. X-ray diffraction revealed that the TiO₂ films were anatase with only the (004) reflection present at 2θ = 38.2°, indicating that the c-axis is slightly reduced from that of anatase powder (2θ = 37.9°). Anatase TiO₂ films up to 100 nm thick have been grown that remain highly ordered in the (001) direction on STO-buffered Si(001) substrates.     

Effect of the composition on the bandgap width of high-κ MexTiyOz (Me = Hf, Ta, Sr) layers

To explore the possibility of bandgap engineering in Ti-oxide based insulators, we investigated the effect of added cations of another kind (Hf, Ta, Sr) on the optical absorption and photoconductivity of thin titanate films. A bandgap of 3.1-3.4 eV, typical for pure polycrystalline TiO₂, was found in crystallized Sr_xTi_yO_z of different composition as well as in amorphous Ta₂Ti₃O_z. By contrast, the gap width of Hf titanates increases starting from 3.5 eV for 30% Hf/(Hf + Ti) to 4.2 eV for 84% Hf/(Hf + Ti). We suggest that this gap widening is associated with reduced interaction between electron states of neighboring Ti cations as influenced by a wide-gap (E_g = 5.6 eV) HfO₂ sub-network.     

Grid emission suppression characteristics of molybdenum grids coated with Hf and Pt films

Hf and Pt were deposited onto molybdenum grids by ion beam-assisted deposition (IBAD). Electron emission characteristics from these coated molybdenum grids contaminated by active electron emission substances (Ba, BaO) of the cathode were measured using the analogous diode method. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were performed on the surface of molybdenum grids coated with Hf and Pt films. When emission substances were deposited onto the grid surface, the reaction between BaO and Hf formed a BaHfO₃ compound that markedly reduced the accumulation of BaO and accordingly decreased grid emission. The decomposition of BaO on the surface of Pt under high temperature was believed to lead to the formation of Ba and its re-evaporation from the surface, reducing active electron emission substances with a consequent significant reduction in grid emission. According to the comparison of their grid emission suppression mechanisms, it could be concluded that grid emission suppression was mainly the reduction of active electron emission substances on the grid surface, in particular BaO. This could be a worthwhile approach for the development of new grid coating materials.     

Direct observation of crystallization of HfO2 promoted on silicon surfaces in gate dielectric stacks

High-angle annular dark-field scanning transmission electron microscopy was used to investigate the crystallization mechanism of amorphous hafnium dioxide (HfO₂) layers in gate stacks (polysilicon/HfO₂/SiON/Si substrate). A 0.9-nm-thick HfO₂ layer remained amorphous with a uniform thickness on annealing at 1050 °C. In contrast, crystalline islands with a cubic structure formed when a 1.8-nm-thick HfO₂ layer was annealed. These islands had commensurate interfaces with both the silicon substrate and the polysilicon film. These results suggest that crystallization is promoted on a silicon surface.     

Adsorption of methanol on mesoporous SBA-15

The adsorption of methanol on mesoporous SBA-15 has been studied by using Brunauer-Emmett-Teller (BET) surface area analysis, scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). The BET surface area analysis shows decreases of the surface area from 387 to 383 m²/g, pore volume from 0.88 to 0.81 cm³/g and pore diameter from 9.07 to 8.4 nm after methanol adsorption. The appearance of strong IR bands at 2862 and 2964 cm^− 1 due to methyl (-CH₃) symmetric and asymmetric stretching demonstrate the presence of methanol and evidence of successful methanol adsorption. XPS results show increase of carbon and oxygen content on the surface of SBA-15. Thermogravimetric analysis shows that the methanol adsorbed on SBA-15 is stable up to a temperature of 265 °C and that the methanol adlayers decompose between 265 and 588 °C.     

Atomic layer deposition of an HfO2 thin film using Hf(O-iPr)4

A thin film of hafnium dioxide (HfO₂) was formed on the surface of Si(100) by atomic layer deposition (ALD) using Hf(O-iPr)₄ (Hf(OCH(CH₃)₂)₄, hafnium tetrakis-iso-propoxide) as an Hf source and O₂ as an oxidant. The temperature window of the process was 250–350 °C, which is about 100 °C lower than that of a process using Hf(O-tBu)₄ as a source. This result was in accordance with the decomposition characteristics of the Hf precursor, as investigated by the temperature-programmed decomposition of the compound in an ultra-high vacuum and by thermogravimetric analysis in air. The thickness of a film deposited under the above conditions increased in proportion to the ALD cycles, indicating that the film-growth rate per cycle remained nearly constant during the process. The deposited film consisted of a monoclinic crystal phase included in an amorphous matrix, which was confirmed by X-ray diffraction. The film showed an equivalent-oxide thickness (EOT) of 2.1 nm and a leakage current density of 8.9 × 10^? 6 A/cm² at ? 1 V. The leakage current was three orders of magnitude lower than that of SiO₂ with the same EOT.     

Atomic layer deposition of SiO2 from Tris(dimethylamino)silane and ozone by using temperature-controlled water vapor treatment

Atomic layer deposition of SiO₂ from tris(dimethylamino)silane (TDMAS) and ozone as precursors on Si(100) surfaces at near-room temperatures was studied by infrared absorption spectroscopy with a multiple internal reflection geometry. TDMAS can be adsorbed at OH sites on hydroxylated Si surfaces at room temperature. Ozone oxidation of the TDMAS-treated Si surface is effective in removing hydroaminocarbon adsorbates introduced during TDMAS adsorption at room temperature. After oxidation by ozone, treatment with H₂O vapor at a substrate temperature of around 160 °C causes regeneration of OH sites for TDMAS adsorption. Cycles involving TDMAS adsorption and ozonization at room temperature followed by H₂O treatment at 160 °C permit the buildup of layers of SiO₂. The amount of residual hydroaminocarbon at the interface between the growing SiO₂ film and the substrate can be reduced with the ozone treated Si surface as a starting surface.     

Electrical characterisation of HfYO MIM-structures deposited by ALD

By an ALD process with the solid precursors HfCl₄ and (CpCH₃)₃Y and the oxidant water Yttrium doped HfO₂ was deposited on TiN layer on highly doped silicon. The films were analysed by ellipsometry, XRR, RBS and XRD. For the electrical characterisation, capacitance and I-V measurement on MIM structure were used. By doping the HfO₂ with 6.2 at.% Yttrium and annealing the film at 500 °C in N₂ the k-value increased by 60% for a 9.5 nm thick film, the leakage current also increased. The deposited amorphous film crystallises at 450 °C into the cubic phase.     

Effects of annealing temperature on the characteristics of ALD-deposited HfO2 in MIM capacitors

We have investigated the annealing effects of HfO₂ films deposited by an atomic layer deposition (ALD) method on the electrical and physical properties in the Si/SiO₂/Pt/ALD-HfO₂/Pd metal-insulator-metal (MIM) capacitors. If the annealing temperature for HfO₂ films was restricted below 500 °C, an annealing step using a rapid thermal processor (RTP) improves the electrical properties such as the dissipation factor and the dielectric constant. On the other hand, annealing at 700 °C degrades the electrical characteristics in general; the dissipation factor increases over the frequency range of 1∼4 MHz, and the leakage current increases up to 2 orders at the low electric field regions. We found that the degradation of electrical properties is due to the grain growth in the HfO₂ film (i.e., poly-crystallization of the film) by the high temperature annealing processing. We suggested that the annealing temperature must be restricted below 500 °C to obtain the high quality high-k film for the MIM capacitors.