Atomic Layer Deposition Temperature Window Research Articles

Atomic Layer Deposition Growth and Characterization of Al2O3 Layers on Cu-Supported CVD Graphene

The deposition of thin uniform dielectric layers on graphene is important for its successful integration into electronic devices. We report on the atomic layer deposition (ALD) of Al2O3 nanofilms onto graphene grown by chemical vapor deposition onto copper foil. A pretreatment with deionized water (DI H2O) for graphene functionalization was carried out, and, subsequently, trimethylaluminum and DI H2O were used as precursors for the Al2O3 deposition process. The proper temperature regime for this process was adjusted by means of the ALD temperature window for Al2O3 deposition onto a Si substrate. The obtained Al2O3/graphene heterostructures were characterized by Raman and X-ray photoelectron spectroscopy, ellipsometry and atomic force and scanning electron microscopy. Samples of these heterostructures were transferred onto glass substrates by standard methods, with the Al2O3 coating serving as a protective layer during the transfer. Raman monitoring at every stage of the sample preparation and after the transfer enabled us to characterize the influence of the Al2O3 coating on the graphene film.

Interplay of Precursor and Plasma for The Deposition of HfO2 via PEALD: Film Growth and Dielectric Properties

AbstractHfO2 thin films are appealing for microelectronic applications such as high‐κ dielectric layers, memristors, and ferroelectric memory devices. To fulfill the different requirements of each application, the properties of the deposited material need to be tuned accordingly. In this context, plasma‐enhanced atomic layer deposition (PEALD) is a powerful processing route to tailor the properties of HfO2 thin films, especially at low temperatures. Herein, a comprehensive bottom‐up approach is presented, ranging from the synthesis of molecularly engineered Hf precursors to the development of a HfO2 PEALD process and a detailed evaluation where plasma can be exploited to tune the dielectric properties. With the example of the newly synthesized bis‐(dialkylamido)‐bis‐(formamidinato) Hf(IV) precursor, [Hf{η2‐(iPrN)2CH}2(NMe2)2] which is reactive, thermally robust and volatile, successful implementation in a PEALD process for HfO2 at low temperatures is demonstrated. The typical atomic layer deposition (ALD) characteristics of precursor saturation, linearity, and ALD temperature window are demonstrated with constant growth of 0.7 Å per cycle from 125 to 200 °C, yielding high‐purity layers. The effect of plasma pulse duration on the chemical composition alongside structural, topographical, as well as dielectric properties of the films is investigated. For the latter, the films are incorporated in metal‐insulator semiconductor (MIS) structures.

Growth mechanism study of boron nitride atomic layer deposition by experiment and density functional theory

We investigated the fundamental mechanisms determining the limit of the atomic layer deposition (ALD) temperature window by density functional theory (DFT) simulations. Boron nitride (BN) ALD experiments were performed using BCl3 or tris-dimethylaminoborane (TDMAB) and NH3 on Si(100) substrates to check whether the ALD temperature window exists or not. With increasing the growth temperature, an increase in growth per cycle (GPC) between 700 and 900 °C was relatively small in the BCl3 and NH3 system, while the GPC exponentially increased in the TDMAB and NH3 system. Based on the experimental results, we performed DFT simulations to investigate the reaction mechanisms determining the lower ALD temperature window in the BCl3 and NH3 systems. The two initial reactions and two cyclic reactions were numerically analyzed: (a) BCl3 reactions on OH-terminated Si(100) surface, (b) NH3 reactions on Cl-terminated Si(100) surface, (c) BCl3 reactions on H-terminated BN surface, and (d) NH3 reactions on Cl-terminated BN surface. We found activation energies for reaction (c) is 30.0 kcal/mol, determining the lower limit of the ALD temperature window. The activation temperature of reaction (c) is obtained 789 °C which is just in the ALD temperature window by our experiments.

Fabrication of PtIrPd Noble Metal Medium Entropy Alloy Thin Film by Atomic Layer Deposition

Noble metal medium entropy alloy (MEA) thin films have recently attracted enormous research interests recently because of their great potential in the catalytic application. However, conventional bottom‐up fabrication methods for MEA thin film such as magnetron sputtering face enormous challenges including precise control over film thickness and consumption of costly high‐purity noble metal targets. Herein, a facile and tunable approach is developed coupling sequential atomic layer deposition (ALD) of noble metal layers with electric Joule heating (EJH) alloying process to grow platinum‐iridium‐palladium (PtIrPd) MEA thin films. The PtIrPd MEA thin film with a thickness of ≈20 nm and an atomic ratio of 35:35:30 is successfully fabricated. The effect of EJH processing temperature on the film morphology and structure evolution is investigated. ALD provides flexibility in the metal deposition sequence with meticulous thickness control and atomic composition precision, while the ultrafast EJH ramping/cooling rates promote homogeneous alloying of MEA combinations inhibiting phase separation. Furthermore, the integrated method circumvents a major obstacle of finding a common ALD temperature window for different noble metal precursors as well as opens new pathways for the controllable synthesis of multicomponent metal alloy thin films with potential catalytic, magnetic, and optical properties.

Surface Acidity-Induced Inherently Selective Atomic Layer Deposition of Tantalum Oxide on Dielectrics

Selective deposition shows a great perspective for the downscaling of nanoelectronics. In this work, inherently selective atomic layer deposition (ALD) of tantalum oxide was studied on a series of oxide substrates. The Ta2O5 films linearly grow on acidic oxides of MnO2, SiO2, and Ta2O5, while there are long nucleation delays on basic oxides such as Al2O3 and HfO2. The inherent selectivity is induced through acidity differences which influence the reaction path, and the H-transfer reaction is a key factor. The Ta(OEt)5 precursor with alkaline ligands tends to chemically adsorb on acidic SiO2 surfaces through H-transfer reaction between surface hydroxyls and precursor molecules. Through H-transfer reaction, the ligands of the Ta(OEt)5 precursor are dissociated into ethanol molecules and the remaining intermediates are strongly chemisorbed on the surface, initiating the following nucleation. However, it is hard to nucleate on basic Al2O3 and HfO2 substrates because the H-transfer reaction is blocked. Moreover, robust selectivity only exists within the ALD temperature window, which highlights that tuning deposition temperature is also an effective way to amplify selectivity. The obstruction of H-transfer reaction on basic oxides provides a new strategy for inherently selective ALD, which will expand the selective toolbox of nanofabrication for next-generation nanoelectronic applications.

Atomic Layer Deposition of MoS2 Decorated TiO2 Nanotubes for Photoelectrochemical Water Splitting

AbstractA thermal atomic layer deposition (ALD) process to fabricate MoS2 thin films is successfully demonstrated by using cycloheptatriene molybdenum tricarbonyl (C7H8Mo(CO)3) and H2S as precursors at an ALD temperature below 300 °C. The process is systematically investigated, showing a typical self‐limiting characteristic within an ALD temperature window of 225–285 °C and a high growth‐per‐cycle of 0.11 nm. The as‐deposited films are amorphous while they can be crystallized in situ by sulfurization with H2S at a low temperature of 300 °C. A prototypical application of the developed ALD process is demonstrated by constructing a MoS2/TiO2 heterostructure through depositing MoS2 onto anodized TiO2 nanotubes for photoelectrochemical water splitting. The MoS2/TiO2 heterostructures exhibit approximately three times superior photoelectrochemical performance than the pristine TiO2 nanotubes. This is attributed to an enhanced visible light‐harvesting ability of MoS2 and an improved separation of the photo‐generated charge carriers at the heterostructure interface, which is affirmed by a staggering gap (type II) between MoS2 and TiO2 as probed by ultraviolet photoelectron spectroscopy.

Optimization of atomic layer deposition of Al2O3 films as possible template for graphene transfer

Nanofilms of aluminium oxide Al2O3 (ALO) were grown on Si, Cu and graphene/Cu substrates by atomic layer deposition (ALD). Trimethylaluminium (TMA) and deionized (DI) water were used as precursors for the ALD process. In order to obtain the ALD temperature window for the above-mentioned precursors, ALO nanofilms were deposited on Si substrates at temperatures ranging from 50 °C to 300 °C. Based on ellipsometry measurements of the films thickness, the growth rate of ALO films obtained was between 1.15 Å/cycle and 1.25 Å/cycle. Ellipsometry and XPS analyses showed that the optimal growth temperature for ALO films is between 150 °C and 200 °C. A graphene layer was grown on a Cu foil by chemical vapor deposition (CVD) and coated with an ALD ALO nanofilm. Raman spectroscopy was used to characterize the influence of ALO films deposition on the graphene layer. A Raman intensity enhancement for all graphene bands in the Al2O3/graphene/Cu sample was observed.

Microstructures of HfOx Films Prepared via Atomic Layer Deposition Using La(NO3)3·6H2O Oxidants.

Hafnium oxide (HfOx) films have a wide range of applications in solid-state devices, including metal–oxide–semiconductor field-effect transistors (MOSFETs). The growth of HfOx films from the metal precursor tetrakis(ethylmethylamino) hafnium with La(NO3)3·6H2O solution (LNS) as an oxidant was investigated. The atomic layer deposition (ALD) conditions were optimized, and the chemical state, surface morphology, and microstructure of the prepared films were characterized. Furthermore, to better understand the effects of LNS on the deposition process, HfOx films deposited using a conventional oxidant (H2O) were also prepared. The ALD process using LNS was observed to be self-limiting, with an ALD temperature window of 200–350 °C and a growth rate of 1.6 Å per cycle, two times faster than that with H2O. HfOx films deposited using the LNS oxidant had smaller crystallites than those deposited using H2O, as well as more suboxides or defects because of the higher number of grain boundaries. In addition, there was a difference in the preferred orientations of the HfOx films deposited using LNS and H2O, and consequently, a difference in surface energy. Finally, a film growth model based on the surface energy difference was proposed to explain the observed growth rate and crystallite size trends.

In situ ellipsometry aided rapid ALD process development and parameter space visualization of cerium oxide nanofilms

Process development in atomic layer deposition (ALD) is often time-consuming, requiring optimization of saturation curves and temperature windows for controlled deposition rates. Any ALD process should be self-limiting in nature, exhibiting a temperature window of nominal deposition and a linear deposition rate. Meeting these criteria usually requires several ALD experiments, followed by film characterization, which are generally time, cost, and labor-intensive. Against this backdrop, we report a methodology using in situ ellipsometry to rapidly develop the ALD process for cerium oxide using Ce(iPrCp)2(N-iPr-amd) and water. The entire optimized process was realized in ten experiments of sequential pulsing as a function of temperature, requiring less than a day. In the traditional approach, tens of experiments and ex situ characterization may be required. The approach reported here generated a contour visualization of the time-temperature-thickness parameter space delineating the optimal deposition conditions. The cerium oxide deposition rate deposited in the ALD temperature window was ∼0.15 nm/cycle; the deposited film was further characterized using x-ray photoelectron spectroscopy, x-ray diffraction, and atomic force microscopy to probe the film composition and quality further.

In-Situ Ellipsometry Aided Rapid ALD Process Development and Parameter Space Visualization

Process development in atomic layer deposition (ALD) is a time-consuming process requiring optimization of pulse/purge times and temperature for controlling growth rates. For example, a new precursor for ceria ALD process using Ce(iPrCp)2(N-iPr-amd) takes tens of experiments for optimization [Golalikhani et al. J. Vac. Sci. Technol 36.5 (2018): 051502]. Against this backdrop, we report a powerful methodology of sequential pulsing aided by in situ ellipsometry, which is used to arrive at an optimal ceria ALD process in 10 experimental runs, requiring less than a day. We have performed saturation pulse experiments of precursor and oxidizer on a silicon surface monitored using in situ ellipsometry. This approach generates a time-temperature-thickness 3D map and makes it convenient to visualize the optimal pulse/purge times and a temperature 'valley' for the process. In situ ellipsometry data from saturation pulse experiments were correlated with adsorption isotherms. The film's growth rate deposited in the ALD temperature window was ~ 2 Å/cycle; the deposited film was further characterized using XPS, XRD, UV-Vis, and AFM. The growth rate is augmented by AI/ML algorithms that can introduce explainability between process parameters and film properties.

Low temperature Topographically Selective Deposition by Plasma Enhanced Atomic Layer Deposition with ion bombardment assistance

Area selective deposition via atomic layer deposition (ALD) has proven its utility in elementary nanopatterning processes. In the case of complex 3D patterned substrates, selective deposition processes lead to vertical sidewall coverage only, or top and bottom horizontal surface coverage only, to enable advanced nanopatterning and further miniaturization of microelectronic devices. While many fabrication strategies for vertical only Topographically Selective Deposition (TSD) have already been developed, the horizontal TSD case needs further attention. In this work, we propose a versatile route for the TSD on 3D top and bottom horizontal surfaces along with a proof-of-concept for such selective Ta2O5 thin film deposition. The strategy at stake relies on a plasma enhanced atomic layer deposition process assisted by energetic ion bombardment during the plasma step and followed by a postgrowth wet etching step. The effectiveness of this strategy is based on a careful adjustment of processing temperatures purposely set at low temperature, most probably below the ALD temperature window. Anisotropic ion bombardment via substrate biasing during the plasma step provides an extra amount of thermal energy only to exposed horizontal surfaces, which in turn enables a selective densification of the thin film under growth. The difference in thin film density on horizontal and vertical surfaces enables the property-selective etching of vertical surfaces, generating horizontal TSD. A proof-of-concept for such low temperature TSD is shown in the case of 3D trenched substrates with an aspect ratio of 14.

Toolbox for atomic layer deposition process development on high surface area powders.

Atomic layer deposition (ALD) is an industrially applied technique for thin film deposition. The vast majority of processes target flat substrates rather than powders. For ALD on powders, new processes are needed, as different reaction conditions are required. Here, two setups are described in detail, which enhance the ALD process development for powders. The first setup described is capable of directly measuring the vapor pressure of a given precursor by a capacitance diaphragm gauge. Promising precursors can be pre-selected, and suitable precursor saturation temperatures can be determined. The second setup consists of four parallel reactors with individual temperature zones to screen the optimal ALD temperature window in a time efficient way. Identifying the precursor saturation temperature beforehand and subsequently performing the first ALD half cycle in the parallel setup at four different reactor temperatures simultaneously will drastically reduce process development times. Validation of both setups is shown for the well-known ALD precursors, trimethylaluminum to deposit aluminum oxide and diethyl zinc to deposit zinc oxide, both on amorphous silica powder.

Titania-Supported Vanadium Oxide Synthesis by Atomic Layer Deposition and Its Application for Low-Temperature Oxidative Dehydrogenation of Propane

Vanadium oxide supported on TiO2(P25) catalysts were synthesized via atomic layer deposition (ALD) of vanadyl acetylacetonate (VO(acac)2). Comprehensive investigations were performed to evaluate the ALD temperature window and the saturation deposition period of VO(acac)2, which were found to be 140–210 °C and 120 min, respectively. The synthesized catalysts were compared to their corresponding impregnation ones for the oxidative dehydrogenation of propane reaction (ODH-P). The ODH-P results indicated the superior performance of the ALD catalysts for the low temperature ODH-P (< 350 °C) given its better dispersion of vanadium oxide(vanadia) in the ALD catalysts. Several characterization techniques including HR-TEM, FE-SEM, XRD, BET, XPS, O2-chemisorption, ICP-OES, and H2-TPR were utilized to evaluate the influence of the ALD and impregnation methods of synthesis on different features of the catalysts. It was observed that the ALD method could offer far better dispersion of the active phase on the support of the catalyst, which is beneficial for the catalytic performance. Variations of Hacac, acetone, and CO2 FTIR absorbance peaks’ areas versus the support temperature

Atomic layer deposition of high-quality Pt thin film as an alternative interconnect replacing Cu

High-quality Pt thin films are prepared by atomic layer deposition (ALD) using metal-organic precursors dimethyl-(N,N-dimethyl-3-butene-1-amine-N) platinum (C8H19NPt) and with diluted molecular oxygen (O2) as a reactant. The films are grown at a relatively low temperature of 225 °C on a thermally grown SiO2 substrate, and the process shows all the necessary qualities of an ideal ALD such as self-limiting growth characteristics and a well-defined ALD temperature window between 200 and 250 °C. Noticeably, the current ALD-Pt process shows a very high growth per cycle of 0.167 nm without an incubation period at 225 °C, and perfect conformality is obtained at a dual trench structure (top and bottom width: 40 and 15 nm) with an aspect ratio of around 6.3. The resistivity of the ALD-Pt film at ∼39 nm in thickness deposited at 225 °C is almost the same (∼10.8 μΩ cm) as its bulk resistivity (10.6 μΩ cm), and it is as low as ∼12 μΩ cm at 25 nm in thickness. Comprehensive analyses using x-ray photoelectron spectroscopy, x-ray diffractometry, transmission electron microscopy (TEM), and x-ray reflectance indicate that the extremely low resistivity of ALD-Pt is due to the formation of highly pure and polycrystalline films with high density (∼21.04 g/cm3) and large grain size (∼48 nm for 25 nm thick film). For comparison, ALD-Ru is deposited at the same equipment and deposition temperature, 225 °C, using (ethylbenzene)(1,3-butadiene)Ru(0) (C12H16Ru) and diluted O2 as the reactant. The higher resistivity of ∼20 μΩ cm at a similar thickness (∼23.5 nm) with ALD-Pt is obtained, which is much higher than its bulk value (7.6 μΩ cm). TEM analysis suggests that the formation of relatively smaller-sized grains of ALD-Ru is the main reason for it.

Low-Temperature Atomic Layer Deposition of Highly Conformal Tin Nitride Thin Films for Energy Storage Devices.

We present an atomic layer deposition (ALD) process for the synthesis of tin nitride (SnNx) thin films using tetrakis(dimethylamino) tin (TDMASn, Sn(NMe2)4) and ammonia (NH3) as the precursors at low deposition temperatures (70-200 °C). This newly developed ALD scheme exhibits ideal ALD features such as self-limited film growth at 150 °C. The growth per cycle (GPC) was found to be ∼0.21 nm/cycle at 70 °C, which decreased with increasing deposition temperature. Interestingly, when the deposition temperature was between 125 and 180 °C, the GPC remained almost constant at ∼0.10 nm/cycle, which suggests an ALD temperature window, whereas upon further increasing the temperature to 200 °C, the GPC considerably decreased to ∼0.04 nm/cycle. Thermodynamic analysis via density functional theory calculations showed that the self-saturation of TDMASn would occur on an NH2-terminated surface. Moreover, it also suggests that the condensation of a molecular precursor and the desorption of surface *NH2 moieties would occur at lower and higher temperatures outside the ALD window, respectively. Thanks to the characteristics of ALD, this process could be used to conformally and uniformly deposit SnNx onto an ultranarrow dual-trench Si structure (minimum width: 15 nm; aspect ratio: ∼6.3) with ∼100% step coverage. Several analysis tools such as transmission electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and secondary-ion mass spectrometry were used to characterize the film properties under different deposition conditions. XRD showed that a hexagonal SnN phase was obtained at a relatively low deposition temperature (100-150 °C), whereas cubic Sn3N4 was formed at a higher deposition temperature (175-200 °C). The stoichiometry of these thermally grown ALD-SnNx films (Sn-to-N ratio) deposited at 150 °C was determined to be ∼1:0.93 with negligible impurities. The optoelectronic properties of the SnNx films, such as the band gap, wavelength-dependent refractive index, extinction coefficient, carrier concentration, and mobility, were further evaluated via spectroscopic ellipsometry analysis. Finally, ALD-SnNx-coated Ni-foam (NF) and hollow carbon nanofibers were successfully used as free-standing electrodes in electrochemical supercapacitors and in Li-ion batteries, which showed a higher charge-storage time (about eight times greater than that of the uncoated NF) and a specific capacity of ∼520 mAh/g after 100 cycles at 0.1 A/g, respectively. This enhanced performance might be due to the uniform coverage of these substrates by ALD-SnNx, which ensures good electric contact and mechanical stability during electrochemical reactions.

Biocompatible organic-inorganic hybrid materials based on nucleobases and titanium developed by molecular layer deposition.

We have constructed thin films of organic–inorganic hybrid character by combining titanium tetra-isopropoxide (TTIP) and the nucleobases thymine, uracil or adenine using the molecular layer deposition (MLD) approach. Such materials have potential as bioactive coatings, and the bioactivity of these films is described in our recent work [Momtazi, L.; Dartt, D. A.; Nilsen, O.; Eidet, J. R. J. Biomed. Mater. Res., Part A 2018, 106, 3090–3098. doi:10.1002/jbm.a.36499]. The growth was followed by in situ quartz crystal microbalance (QCM) measurements and all systems exhibited atomic layer deposition (ALD) type of growth. The adenine system has an ALD temperature window between 250 and 300 °C, while an overall reduction in growth rate with increasing temperature was observed for the uracil and thymine systems. The bonding modes of the films have been further characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction, confirming the hybrid nature of the as-deposited films with an amorphous structure where partial inclusion of the TTIP molecule occurs during growth. The films are highly hydrophilic, while the nucleobases do leach in water providing an amorphous structure mainly of TiO2 with reduced density and index of refraction.

Thermal atomic layer deposition of Sn metal using SnCl4 and a vapor phase silyl dihydropyrazine reducing agent

This work explores a novel, thermal atomic layer deposition (ALD) process to deposit tin metal at a low temperature. The authors employ 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (DHP) to reduce SnCl4 on silicon substrates. The authors explored a range of temperatures between 130 and 210 °C to determine the ALD window, which was found to be 170–210 °C. The authors show that this process yields a growth rate of ∼0.3 Å per cycle at 190 °C. Furthermore, X-ray photoelectron spectroscopy results showed that the film impurities are reduced for depositions within the ALD window. The reaction mechanism was explored using in situ mass spectrometry and in situ quartz crystal microbalance (QCM). Within the ALD temperature window, the QCM results showed a saturated mass gain during the SnCl4 exposure and a net mass loss during the DHP dose. Consistent with the QCM results, in situ mass spectroscopy data indicate that the DHP exposure step removes surface Cl via formation of volatile trimethylsilyl chloride and pyrazine by-products, effectively reducing the oxidation state of surface-bound Sn. This work is the first thermal Sn metal ALD process to be reported in literature and the oxidation/reduction chemistry presented here may be applied to other metal precursors, increasing the applicability of metal ALD use in industry.

Atomic Layer Deposition of p-Type Bi2S3

Atomic layer deposition (ALD) of bismuth sulfide (Bi2S3) is demonstrated by the sequential exposure of bismuth(III) bis(2,2,6,6-tetramethylheptane-3,5-dionate) [Bi(thd)3] and hydrogen sulfide (H2S) at 200 °C. A saturated growth rate of 0.34–0.37 A/cycle was observed by in situ quartz crystal microbalance (QCM) and verified by ex situ X-ray reflectivity (XRR) measurements throughout the ALD temperature window. As-deposited Bi2S3 films were found to be polycrystalline in nature without any preferential orientation. In addition to the direct band gap at ca. 1.56 eV normally seen for Bi2S3, we also found evidence for an indirect band gap at ca. 1.03 eV. Ultraviolet photoelectron spectroscopy (UPS) and Seebeck measurements strongly support degenerate p-type conductivity of the as-grown thin films, in contrast to the n-type nature normally found in the literature. Temperature-dependent (70–300 K) electrical resistivity measurements showed that, in the temperature range of 70–100 K, variable-range hopping (VRH) ...

Atomic Layer Deposition of Ruthenium and Ruthenium Oxide Using a Zero-Oxidation State Precursor

Atomic layer deposition (ALD) processes are reported for ruthenium (Ru) and ruthenium oxide (RuO2) using a zero-oxidation state liquid precursor, η4-2,3-dimethylbutadiene ruthenium tricarbonyl [Ru(DMBD)(CO)3]. Both ALD Ru and RuO2 films were deposited using alternating N2 -purge-separated pulses of Ru(DMBD)(CO)3 and O2. ALD Ru metal films were deposited via short (2 s) pulses of O2. Ru films have an ALD temperature window from 290 to 320 °C with a GPC of 0.067 nm/cycle and a negligible nucleation delay on SiO2. Ru films show a strong hexagonal crystal structure with low resistivity of approximately 14 μΩ cm at 320 °C. RuO2 films were deposited using longer (20 s) pulses of either molecular O2 or O2 plasma. RuO2 films deposited via thermal ALD using molecular O2 have a temperature window from 220 to 240 °C with a GPC and nucleation delay on SiO2 of 0.065 nm/cycle and 35 cycles, respectively. Thermal ALD RuO2 films show a distinct rutile phase microstructure with a resistivity of approximately 62 μΩ cm. In ...

Comparison of the Atomic Layer Deposition of Tantalum Oxide Thin Films Using Ta(NtBu)(NEt2)3, Ta(NtBu)(NEt2)2Cp, and H2O.

The growth characteristics of Ta2O5 thin films by atomic layer deposition (ALD) were examined using Ta(NtBu)(NEt2)3 (TBTDET) and Ta(NtBu)(NEt2)2Cp (TBDETCp) as Ta-precursors, where tBu, Et, and Cp represent tert-butyl, ethyl, and cyclopentadienyl groups, respectively, along with water vapor as oxygen source. The grown Ta2O5 films were amorphous with very smooth surface morphology for both the Ta-precursors. The saturated ALD growth rates of Ta2O5 films were 0.77 Å cycle-1 at 250 °C and 0.67 Å cycle-1 at 300 °C using TBTDET and TBDETCp precursors, respectively. The thermal decomposition of the amido ligand (NEt2) limited the ALD process temperature below 275 °C for TBTDET precursor. However, the ALD temperature window could be extended up to 325 °C due to a strong Ta-Cp bond for the TBDETCp precursor. Because of the improved thermal stability of TBDETCp precursor, excellent nonuniformity of ∼2% in 200 mm wafer could be achieved with a step coverage of ∼90% in a deep hole structure (aspect ratio 5:1) which is promising for 3-dimensional architecture to form high density memories. Nonetheless, a rather high concentration (∼7 at. %) of carbon impurities was incorporated into the Ta2O5 film using TBDETCp, which was possibly due to readsorption of dissociated ligands as small organic molecules in the growth of Ta2O5 film by ALD. Despite the presence of high carbon concentration which might be an origin of large leakage current under electric fields, the Ta2O5 film using TBDETCp showed a promising resistive switching performance with an endurance cycle as high as ∼17 500 for resistance switching random access memory application. The optical refractive index of the deposited Ta2O5 films was 2.1-2.2 at 632.8 nm using both the Ta-precursors, and indirect optical band gap was estimated to be ∼4.1 eV for both the cases.