Atomic Layer Deposition Window Research Articles

Wafer-scale growth of MoS2 thin films by atomic layer deposition.

The wafer-scale synthesis of MoS2 layers with precise thickness controllability and excellent uniformity is essential for their application in the nanoelectronics industry. Here, we demonstrate the atomic layer deposition (ALD) of MoS2 films with Mo(CO)6 and H2S as the Mo and S precursors, respectively. A self-limiting growth behavior is observed in the narrow ALD window of 155-175 °C. Long H2S feeding times are necessary to reduce the impurity contents in the films. The as-grown MoS2 films are amorphous due to the low growth temperature. Post-annealing at high temperatures under a H2S atmosphere efficiently improves the film properties including the crystallinity and chemical composition. An extremely uniform film growth is achieved even on a 4 inch SiO2/Si wafer. These results demonstrate that the current ALD process is well suited for the synthesis of MoS2 layers for application in industry.

Phase Formation Behavior in Ultrathin Iron Oxide.

Nanostructured iron oxides, and especially hematite, are interesting for a wide range of applications ranging from gas sensors to renewable solar hydrogen production. A promising method for deposition of low-dimensional films is atomic layer deposition (ALD). Although a potent technique, ALD of ultrathin films is critically sensitive to the substrate and temperature conditions where initial formation of islands and crystallites influences the properties of the films. In this work, deposition at the border of the ALD window forming a hybrid ALD/pulsed CVD (pCVD) deposition is utilized to obtain a deposition less sensitive to the substrate. A thorough analysis of iron oxide phases formation on two different substrates, Si(100) and SiO2, was performed. Films between 3 and 50 nm were deposited and analyzed with diffraction techniques, high-resolution Raman spectroscopy, and optical spectroscopy. Below 10 nm nominal film thickness, island formation and phase dependent particle crystallization impose constraints for deposition of phase pure iron oxides on non-lattice-matching substrates. Films between 10 and 20 nm thickness on SiO2 could effectively be recrystallized into hematite whereas for the corresponding films on Si(100), no recrystallization occurred. For films thicker than 20 nm, phase pure hematite can be formed directly with ALD/pCVD with very low influence of the substrate on either Si or SiO2. For more lattice matched substrates such as SnO2:F, Raman spectroscopy indicated formation of the hematite phase already for films with 3 nm nominal thickness and clearly for 6 nm films. Analysis of the optical properties corroborated the analysis and showed a quantum confined blue-shift of the absorption edge for the thinnest films.

SnO2 thin films grown by atomic layer deposition using a novel Sn precursor

SnO2 thin films were grown by atomic layer deposition (ALD) with dimethylamino-2-methyl-2-propoxy-tin(II) (Sn(dmamp)2) and O3 in a temperature range of 100–230°C. The ALD window was found to be in the range of 100–200°C. The growth per cycle of the films in the ALD window increased with temperature in the range from 0.018 to 0.042nm/cycle. Above 230°C, the self-limiting behavior which is a unique characteristic of ALD, was not observed in the growth because of the thermal decomposition of the Sn(dmamp)2 precursor. The SnO2 films were amorphous in the ALD window and exhibited quite a smooth surface. Sn ions in all films had a single binding state corresponding to Sn4+ in SnO2. The concentration of carbon and nitrogen in the all SnO2 films was below the detection limit of the auger electron spectroscopy technique and a very small amount of carbon, nitrogen, and hydrogen was detected by secondary ions mass spectroscopy only. The impurity contents decreased with increasing the growth temperature. This is consistent with the increase in the density of the SnO2 films with respect to the growth temperature. The ALD process with Sn(dmamp)2 and O3 shows excellent conformality on a hole structure with an aspect ratio of ∼9. This demonstrates that the ALD process with Sn(dmamp)2 and O3 is promising for growth of robust and highly pure SnO2 films.

Study of Y2O3 Thin Film Prepared by Plasma Enhanced Atomic Layer Deposition

Atomic layer deposition (ALD) is a promised thin film deposition technique to fabricate nanometer scale thin films. Unlike chemical vapor depositions (CVDs), one cycle of ALD process is composed of separated precursor pulsing step, purging step and reactant pulsing step. Due to separated steps in an ALD process, precise thickness control of thin films is available. Step coverage, uniformity and conformality of thin films are also superior compared with other thin film techniques, such as sputter, pulsed laser deposition (PLD).Plasma enhanced atomic layer deposition (PEALD) is energy reinforced ALD technique by inserting plasma. In case of a direct PEALD, plasma is induced during the reactant step to improve reactivity of reactant. The ALD window of PEALD is expanded especially at lower temperature due to high reactivity of plasma species. Better quality of thin films is available due to decreased contaminations within thin films and improved stoichiometry of thin films.Y2O3 has wide band gap, good thermal stability, and high dielectric constant. Due to these characteristics, Y2O3 is used for various applications, such as, replacement of SiO2 in complementary metal oxide semiconductor (CMOS) devices, protective coatings, and buffer layers for ferroelectrics and superconductors. Y2O3 is also used to fabricate Yttria-stabilized zirconia (YSZ) which is the most typical electrolyte material for solid oxide fuel cells (SOFCs).In this study, we prepared Y2O3 thin films by direct PEALD. Tris (methylcyclopentadienyl) yttrium ((MeCp3)3Y) precursor is used as a precursor of Y2O3 and high purity oxygen gas with O2 plasma is used as the reactant. The growth rate of Y2O3 thin films are measured by ellipsometer as a function of deposition temperature. Physical and chemical characteristics of Y2O3 thin films are also investigated which is deposited at different temperature.

Low-Temperature Atomic Layer Deposition of Copper Films Using Borane Dimethylamine as the Reducing Co-reagent

The atomic layer deposition (ALD) of Cu metal films was carried out by a two-step process with Cu(OCHMeCH2NMe2)2 and BH3(NHMe2) on Ru substrates and by a three-step process employing Cu(OCHMeCH2NMe2)2, formic acid, and BH3(NHMe2) on Pd and Pt substrates. The two-step process demonstrated self-limited ALD growth at 150 °C with Cu(OCHMeCH2NMe2)2 and BH3(NHMe2) pulse lengths of ≥3.0 and ≥1.0 s, respectively. An ALD window was observed between 130 and 160 °C, with a growth rate of about 0.13 A/cycle. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) revealed rough Cu films that likely originate from the Cu nanoparticle seed layer. The Cu films exhibited poor electrical conductivity because of their nanoparticulate natures. The three-step process showed self-limited ALD growth on Pd and Pt at 150 °C with Cu(OCHMeCH2NMe2)2, formic acid, and BH3(NHMe2) pulse lengths of ≥3.0, ≥ 0.3, and ≥1.0 s, respectively. ALD windows were observed between 135 and 165 °C on both Pd and Pt, with growth rates o...

Atomic layer deposition of Er2O3 thin films from Er tris-guanidinate and water: process optimization, film analysis and electrical properties

For the first time, the combination of the homoleptic erbium tris-guanidinate metalorganic complex ([Er(NMe2-Guan)3]) simply with water yielded high quality Er2O3 thin films on Si(100) substrates employing the atomic layer deposition (ALD) process. The process optimization to grow good quality Er2O3 layers was performed by varying the Er precursor pulse time, water pulse time and purge time. The high reactivity of the Er compound towards water and good thermal stability in the temperature range of 150–275 °C (ALD window) resulted in homogeneous, stoichiometric Er2O3 layers with high growth rates (1.1 A per cycle) and the as-deposited films crystallized in the cubic phase. The saturation behavior at different temperatures in the ALD window and the linear dependence of film thickness as a function of precursor pulse time confirmed the true ALD process. The potential of Er2O3 thin films as gate dielectrics was verified by performing capacitance–voltage (C–V) and current–voltage (I–V) measurements. Dielectric constants estimated from the accumulation capacitance were found to be in the range of 10–13 for layers of different thicknesses (15–30 nm).

Structural and optical properties of lanthanide oxides grown by atomic layer deposition (Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb)

Ln2O3 thin films with optically active f-electrons (Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb) have been grown on Si(100) and soda lime glass substrates by atomic layer deposition (ALD) using Ln(thd)3 (Hthd = 2,2,6,6-tetramethyl-3,5-heptanedione) and ozone as precursors. The temperature range for depositions was 200-400 °C. Growth rates were measured by spectroscopic ellipsometry and a region with a constant growth rate (ALD window) was found for Ln = Ho and Tm. All the compounds are grown as amorphous films at low temperatures, whereas crystalline films (cubic C-Ln2O3) are obtained above a certain temperature ranging from 300 to 250 °C for Nd2O3 to Yb2O3, respectively. AFM studies show that the films were smooth (rms < 1 nm) except for depositions at the highest temperatures. The refractive index was measured by spectroscopic ellipsometry and was found to depend on the deposition temperature. Optical absorption measurements show that the absorption from the f-f transitions depends strongly on the crystallinity of the material. The clear correlation between the degree of crystallinity, optical absorptions and refractive indices is discussed.

Low Temperature Epitaxial Oxide Ultrathin Films and Nanostructures by Atomic Layer Deposition

Highly epitaxial and pure (001) CeO2 ultrathin films have been prepared by atomic layer deposition (ALD) at 275 °C on Y-stabilized ZrO2 cubic fluorite single crystal substrate using cerium β-diketonate (Ce(thd)4) and ozone (O3) as precursors. Substrate temperature and precursor pulses have been optimized to set the ALD window obtaining a growth per cycle of ≈0.2 Å/cycle. This extremely low growth rate has been identified as a key parameter to ensure epitaxial growth at these low temperatures. Post-thermal treatments at 900 °C in oxygen further improve ALD-CeO2 film texture while maintaining film stoichiometry and ultrasmooth surface, rms < 0.4 nm. ALD-CeO2 thin film growth has also been tested on perovskite single crystal substrates, SrTiO3 and LaAlO3, exhibiting CeO2 epitaxial growth and thus validating ALD as an outstanding method for low temperature epitaxial growth. Furthermore, we demonstrate that by combining e-beam lithography and ALD it is feasible to obtain size-controlled CeO2 nanostructures.

Novel Heteroleptic Precursors for Atomic Layer Deposition of TiO2

Two novel heteroleptic titanium precursors for the atomic layer deposition (ALD) of TiO2 were investigated, namely, titanium (N,N′-diisopropylacetamidinate)tris(isopropoxide) (Ti(OiPr)3(NiPr-Me-amd)) and titanium bis(dimethylamide)bis(isopropoxide) (Ti(NMe2)2(OiPr)2). Water was used as the oxygen source. These two precursors are liquid at room temperature and present good volatility, thermal stability and reactivity. The self-limiting ALD-growth mode was confirmed at 325 °C for both precursors. The titanium (N,N′-diisopropylacetamidinate)tri(isopropoxide)/water process showed an ALD window at 300–350 °C, and titanium bis(dimethylamide)bis(isopropoxide) exhibited an interestingly high growth rate of 0.75 A/cycle at 325 °C. The films were crystallized to the anatase phase in the as-deposited state. X-ray photoelectron spectroscopy analysis demonstrated that the films were pure and close to the stoichiometric composition. The refractive indexes and absorption coefficient of the films were measured by spectro...

Atomic Layer Deposition of Copper Oxide using Copper(II) Acetylacetonate and Ozone

AbstractWe report on the deposition of copper oxide by atomic layer deposition (ALD) using copper acetylacetonate (Cu(acac)2) and ozone (O3) as precursors. The aim is to deposit oxides of copper as possible candidates for materials in all‐oxide photovoltaics. The present study results in ALD‐type deposition of the tenorite phase of copper(II) oxide in a temperature window of 150 °C to ca. 230 °C, with a growth rate of ∼0.038 nm per cycle. The resulting ALD characteristics, with a relatively large ALD window in deposition temperature, should be well suited for combination with additional deposition processes for the formation of complex compounds. The film thickness is studied using X‐ray reflectivity (XRR), and phase is determined by X‐ray diffraction (XRD). Surface roughness is studied using atomic force microscopy (AFM), and X‐ray photoelectron spectroscopy (XPS) is used to investigate the chemical state of the deposited films.

Atomic Layer Deposition of HfO2 Thin Films Employing a Heteroleptic Hafnium Precursor

AbstractThe application of a heteroleptic hafnium amide‐guanidinate precursor for the deposition of HfO2 thin films via a water‐assisted atomic layer deposition (ALD) process is demonstrated for the first time. HfO2 films are grown in the temperature range 100–300 °C using the compound [Hf(NMe2)2(NMe2‐Guan)2] (1). This compound shows self‐limiting ALD‐type growth characteristics with growth rates of the order of 1.0–1.2 Å per cycle in the temperature range 100–225 °C. The saturation behavior and a linear dependence on film thickness as a function of number of cycles are verified at various temperatures within the ALD window. The as‐deposited HfO2 films are characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS), X‐ray photoelectron spectroscopy (XPS), and electrical measurements. For a direct comparison of the precursor performance with that of the parent alkyl amide [Hf(NMe2)4] (2), ALD experiments are also performed employing compound 2 under similar process conditions, and in this case no typical ALD characteristics are observed.

Atomic Layer Deposition of Gd2O3 and Dy2O3: A Study of the ALD Characteristics and Structural and Electrical Properties

Gd2O3 and Dy2O3 thin films were grown by atomic layer deposition (ALD) on Si(100) substrates using the homoleptic rare earth guanidinate based precursors, namely, tris(N,N′-diisopropyl-2-dimethylamido-guanidinato)gadolinium(III) [Gd(DPDMG)3] (1) and tris(N,N′-diisopropyl-2-dimethylamido-guanidinato)dysprosium(III) [Dy(DPDMG)3] (2), respectively. Both complexes are volatile and exhibit high reactivity and good thermal stability, which are ideal characteristics of a good ALD precursor. Thin Gd2O3 and Dy2O3 layers were grown by ALD, where the precursors were used in combination with water as a reactant at reduced pressure at the substrate temperature ranging from 150 °C to 350 °C. A constant growth per cycle (GPC) of 1.1 Å was obtained at deposition temperatures between 175 and 275 °C for Gd2O3, and in the case of Dy2O3, a GPC of 1.0 Å was obtained at 200–275 °C. The self-limiting ALD growth characteristics and the saturation behavior of the precursors were confirmed at substrate temperatures of 225 and 250 °C within the ALD window for both Gd2O3 and Dy2O3. Thin films were structurally characterized by grazing incidence X-ray diffraction (GI-XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) analyses for crystallinity and morphology. The chemical composition of the layer was examined by Rutherford backscattering (RBS) analysis and Auger electron spectroscopy (AES) depth profile measurements. The electrical properties of the ALD grown layers were analyzed by capacitance–voltage (C–V) and current–voltage (I–V) measurements. Upon subjection to a forming gas treatment, the ALD grown layers show promising dielectric behavior, with no hysteresis and reduced interface trap densities, thus revealing the potential of these layers as high-k oxide for application in complementary metal oxide semiconductor based devices.

Growth of Bi2O3 Ultrathin Films by Atomic Layer Deposition

Bismuth trioxide (Bi2O3) ultrathin films were successfully synthesized on silicon substrates by means of atomic layer deposition (ALD) using Bi(thd)3 (thd: 2,2,6,6-tetramethyl-3,5-heptanedionato) and H2O as precursors. The optimum ALD window was about 270–300 °C, and an ALD-type growth mechanism via surface saturation reaction was identified; the growth rate was about 0.1 Å/cycle. The X-ray diffraction and high-resolution transmission electron microscopy investigation revealed that Bi2O3 films crystallized into a predominant alpha phase above 250 °C. The resistivity at room temperature was about 1.2 × 106 Ω·cm, which is also proof of the α-phase of as-deposited Bi2O3 films. In addition, a new method to obtain γ-Bi2O3 film was discovered. The α-Bi2O3 films (synthesized by ALD) transformed into metastable γ-Bi2O3 with preferred orientation (222) after annealing above 512 °C, and γ-phase could persist at room temperature.

Atomic Layer Deposition and Characterization of Amorphous ErxTi1-xOy Dielectric Ultra-Thin Films

Er1-xTixOy dielectric thin films were deposited on Si(100) substrates by atomic layer deposition (ALD) using tris(methylcyclopentadienyl)erbium [(CpMe)3Er] and tetrakis(diethylamino)titanium [TDEAT] as metal precursors, and O3 as oxidant. The deposition temperature dependence of both Er2O3 and TiO2 film growths showed the overlapping ALD window of 175–250°C. Compositional tunability of Er1-xTixOy films was obtained through deposition process control. Carbon and nitrogen impurities in as-deposited films were found to be below X-ray photoelectron spectroscopy detection level. Glancing incidence X-ray diffraction and phase-shifting interferometry results showed that ErxTi1-xOy films with ALD cycle ratios (Er2O3/TiO2) of 1:8 or higher have a good thermal stability and remain amorphous with unchanged surface roughness after post-deposition rapid thermal processing (RTP) at 700°C in O2. Electrical measurements showed that optimized amorphous Er1-xTixOy films after RTP exhibited a dielectric constant of ∼36, a hysteresis voltage of less than 10 mV, and a leakage current density of 10−8 A/cm2 at −1 MV/cm; such properties compare favorably with the properties of other reported amorphous titanium-based ternary dielectrics on Si. The sizable reduction of leakage current density of Er1-xTixOy found in our study suggests that amorphous Er1-xTixOy thin films are promising future dielectrics in Si integrated circuit technology.

Deposition and Characterization of Atomic Layer Deposited ZnS Thin Films on p-type GaSb(100) Using Diethylzinc Precursor and Hydrogen Sulfide

Diethylzinc precursor and hydrogen sulfide were used for the atomic layer deposition (ALD) of ZnS films on thioacetamide (TAM)-treated p-type GaSb substrates for the passivation of GaSb-based devices. The film growth rate was found to be 0.13±0.01 nm/cycle with an ALD window of 175 - 275 °C. X-ray photoelectron spectroscopy (XPS) analyses of as-deposited ZnS films showed that the resulting films were stoichiometric ZnS with no evidence of other sulfur species in the film. XPS indicated the surface of GaSb treated with TAM was a mixture of Ga-S and Sb-S without detectable oxides. After the ALD of 1 nm-thick ZnS on TAM-treated GaSb, the major component at the interface of ZnS/TAM-treated GaSb system was found to be gallium sulfide. Surface morphology and structure of as-deposited ZnS were studied with phase-shifting interferometry and grazing incidence X-ray diffraction.

Self-Limiting Film Growth of Transparent Conducting In2O3 by Atomic Layer Deposition using Trimethylindium and Water Vapor

Conformal and self-limiting film growth of transparent conducting In2O3 is demonstrated with atomic layer deposition (ALD) using trimethylindium (TMIn) and water vapor. Various parameters, including pulsing times of TMIn and H2O and deposition temperature, are varied to confirm the ALD processing window of In2O3. It is found that an extremely large Langmuir exposure of H2O (∼2 Torr s) is required to fully react the surface-adsorbed In-(CH3)* groups with H2O. Self-limiting film growth is obtainable at the deposition temperatures between 200 and 251 °C where the film growth is controlled by surface adsorption of TMIn molecules. Within the ALD window, ALD-In2O3 exhibits a film growth rate of ∼0.039 nm/cycle at 217 °C. In addition, the film exhibits an excellent step coverage of ∼97% on a high-aspect-ratio (∼11) nanostructure. The resistivity of the ALD-In2O3 film deposited at 200 °C is about 2.8 × 10–3 Ω cm, and this is attributed to the high Hall mobility of 84 cm2/(V s). Finally, the reduction of the Hall mobility of the films with respect to increases in the deposition temperature is correlated to the evolution of microstructures.

ALD of ZnO using diethylzinc as metal-precursor and oxygen as oxidizing agent

We report on ZnO atomic layer deposition (ALD) with a precursor combination of diethylzinc as metal-precursor and pure oxygen (O2) as oxidant as an alternative to H2O as oxygen precursor. The temperature region of self-limiting ALD growth (ALD window) is determined and shows an increase in growth rate of about 60% compared to water as oxygen-precursor. Finally, in situ X-ray photoelectron spectroscopy (XPS) and synchrotron-radiation photoelectron spectroscopy (SR-PES) have been used to analyze the initial growth and film properties of ALD-ZnO deposited in monolayer steps using both precursor combinations.

The Atomic Layer Deposition of SrB2O4 Films Using the Thermally Stable Precursor Bis(tris(pyrazolyl)borate)strontium

AbstractThe atomic layer deposition (ALD) of strontium borate films is carried out using bis(tris(pyrazolyl)borate)strontium (SrTp2) and water as precursors. Self‐limiting ALD growth is established at 350 °C with SrTp2 and water pulse lengths of ≥ 2.0 s and ≥ 0.3 s, respectively. An ALD window is observed from 300 to 375 °C, in which the growth rate is 0.47 Å per cycle. The thin film compositions are assessed by elastic recoil detection analysis (ERDA) and X‐ray photoelectron spectroscopy (XPS). ERDA suggests compositions of SrB2O4 at growth temperatures of &lt; 350 °C, but the boron/strontium and oxygen/strontium ratios are lower than those of SrB2O4 at 350 and 400 °C. Within the ALD window, hydrogen concentrations range from 0.37(42) to 0.87(7) at.‐%, and the carbon and nitrogen concentrations are below the detection limits. XPS analyses on representative strontium borate thin films show all expected ionizations. X‐ray diffraction (XRD) experiments reveal that the as‐deposited films are amorphous. The surface morphology is assessed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The rms surface roughness of typical 2 µm × 2 µm areas for films deposited at 325 and 350 °C are 0.3 and 0.2 nm, respectively. SEM images of these films show no cracks or pinholes.

Characteristics of ZnO Films Prepared by Atomic Layer Deposition for Transparent Electronic Devices

Recently, nanostructured zinc oxide (ZnO) for many different applications in micro- and opto-electronic devices has been studied intensively. However, its structural and electrical properties still require improvements for ZnO-based transparent electronics. In this study, we fabricated ZnO films (thicknesses from 30 to 70 nm) on glass substrates using atomic layer deposition (ALD) and investigated the film properties in relation with substrate temperatures. The processing window (thermal ALD window) of self-limiting growth was observed at 110-190 degrees C. In our thermal ALD window, the average growth rate of ZnO films was 0.26 nm/cycle, and the (002) orientation became dominant with increasing substrate temperatures. For all growth temperatures, ZnO films have shown n-type conductivity. At 170 degrees C, ZnO with good electrical properties of carrier concentration (1.3 x 10(19) cm(-3)), mobility (18 cm2/Vs), and resistivity (2.7 x 10(-2) omegacm) was successfully obtained.

Effect of Al Distribution on Carrier Generation of Atomic Layer Deposited Al-Doped ZnO Films

The effect of the Al distribution on the electrical properties of Al-doped ZnO (AZO) films deposited by atomic layer deposition (ALD) is investigated. In order to control the Al distribution, the pulsing time of trimethylaluminum (TMA) is varied from 2 (within an ALD window) to 0.1 s. As a result, the areal density of Al atoms incorporated in a single dopant layer decreases from 3.3 × 1014 to 1.2 × 1014 cm−2. Hall measurements reveal that the minimum resistivity of the ALD-AZO films is decreased from 3.2 × 10−3 to 1.7 × 10−3 Ω cm as a result of reducing the TMA pulsing time from 2 to 0.1 s. This decrease is due to the obvious increase of the carrier concentration from 1.4 × 1020 to 4.7 × 1020 cm−3. It is suggested that both the improved doping efficiency (from 13 to 58%) and the insertion of more dopant layers within the ZnO matrix are responsible for the increase of the carrier concentration.