Growth Per Cycle Research Articles

Synthesis and characteristics of Sn-doped SiO2 via plasma-enhanced atomic layer deposition for self-aligned patterning

Sn-doped SiO2 thin films as a spacer for self-aligned patterning were deposited by plasma-enhanced atomic layer deposition and their characteristics were evaluated. This doping research was conducted to improve the mechanical properties of SiO2 films, which have been conventionally used as a spacer material. Because pure SiO2 films have a low Young's modulus, the pattern is stretchable and may collapse as the patterning size decreases. The ratio of the SnO2 and SiO2 deposition cycle was varied from 15(SiO2):1(SnO2) to 3(SiO2):1(SnO2) to modify the film characteristics. X-ray reflectivity (XRR) and time-of-flight secondary ion mass spectrometer analyses revealed whether Sn was doped in SiO2 or became a nanolaminate. The x-ray photoelectron spectroscopy analysis showed that a greater amount of Sn in the SiO2 thin film resulted in a binding energy shift toward the lower binding energy Si2p and Sn3d peaks, and more Si–O–Sn chemical bonding, which increased the number of stiffer ionic bonds as the SnO2 cycle ratio was increased. Therefore, Young's modulus measured by using a nanoindenter increased from 39.9 GPa for SiO2 films to 90.9 GPa for 3(SiO2):1(SnO2) films. However, the hardness results showed a different tendency due to the not well-distributed nanolaminate film structure showing a tendency to decrease and then increase as doping increases. Moreover, the growth rate and film density were evaluated by XRR. The growth per cycle (GPC) of SiO2 was 1.45 Å/cycle and the GPC of SnO2 was 1.0 Å/cycle. The film density of SiO2 was 2.4 g/cm3 and the film density of SnO2 was 4.9 g/cm3. Also, the GPC and film density values of the Sn-doped SiO2 films were in between the values of pure SiO2 and SnO2. The dry etch rate was also measured by reactive ion etching using CF4 plasma with 150 W for 1 min.

Molecular Layer Deposition of Thermally Stable Polybenzimidazole‐Like Thin Films and Nanostructures

AbstractThe deposition of polybenzimidazole (PBI)‐like thin films by molecular layer deposition is reported here for the first time using isophthalic acid (IPA) and 3,3′‐diaminobenzidine (DAB) as monomers and trimethylaluminum (TMA) as a linker precursor. Two precursor pulsing sequences are tested, the ABCB (TMA + IPA + DAB + IPA) and ABC (TMA + IPA + DAB) type MLD processes result in different types of PBI‐like films. With the ABCB sequence thin film growth per cycle (GPC) of 6.0 Å is obtained at 225–280 °C, whereas GPC of 7.0 Å is obtained with the ABC sequence. Films are characterized in detail by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, time‐of‐flight elastic recoil detection analysis, and atomic force microscopy. The films have good thermal stability and withstand annealing at 400 °C in both air and nitrogen. PBI nanostructures are prepared by depositing PBI‐like film on electroblown polyvinylpyrrolidone fibers and removing the template fibers by annealing or dissolution into ethanol.

Atmospheric pressure spatial ALD of Al2O3 thin films for flexible PEALD IGZO TFT application

Atmospheric pressure spatial atomic layer deposition (AP S-ALD)-derived Al2O3 films were investigated on the growth temperatures (100 °C ∼ 200 °C) and demonstrated as the gate insulator for IGZO thin film transistor (TFT) applications. When the growth temperature increases to 200 °C, growth per cycle (GPC) and refractive index of the Al2O3 films were 1.33 Å/cycle and 1.63, respectively. The film density also increased from 2.55 g/cm3 to 2.79 g/cm3 on the growth temperature, which decreasing the carbon impurities of the Al2O3 film (100 °C: 3.57 at%, 150 °C: 1.73 at%, 200 °C: N/A). In addition, the impurity and low growth temperature may degrade not only film surface roughness, but also electrical characteristics. As the buffer and gate insulator Al2O3 layers, the IGZO TFT were fabricated on a polyimide substrate. The IGZO TFTs with the Al2O3 layers showed excellent device performances: 52.48 cm2/V.sec of field effect mobility, −3.09 ± 0.14 of threshold voltage, and 0.14 ± 0.01 subthreshold swing. In addition, the TFT exhibited excellent bias reliability and mechanical bending stability. This highly stability will be attributed to AP S-ALD Al2O3 acting as an excellent insulator.

Atomic layer deposition of GdF3 thin films

Gadolinium fluoride is an attractive optical material with applications in, e.g., deep-UV lithography, solar cells, and medical imaging. Despite the interest toward this material, no atomic layer deposition (ALD) process has been published. In this article, an ALD process for GdF3 using Gd(thd)3 and NH4F as precursors is presented. The deposition was studied at temperatures 275–375 °C, but 285–375 °C produce the purest films. The saturation of the growth per cycle (GPC) with respect to precursor pulses and purges was proved at 300 °C. The GPC value at this temperature is ∼0.26 Å, and the deposition temperature has very little effect on the GPC. According to x-ray diffraction, all the films consist of orthorhombic GdF3. The impurity contents, evaluated by time-of-flight elastic recoil detection analysis, is low, and the films are close to stoichiometric. The nitrogen content is less than <0.04 at. %. The antireflection properties were qualitatively evaluated by UV-vis spectrometry in a transmission mode at a 190–1100 nm range: on sapphire substrates, GdF3 serves as an antireflective coating. Dielectric properties of the films were studied, and for example, a permittivity value of 9.3 was measured for a ∼64 nm film deposited at 300 °C.

Spatial atmospheric pressure molecular layer deposition of alucone films using dimethylaluminum isopropoxide as the precursor.

Trimethylaluminum is the most used aluminum precursor in atomic and molecular layer deposition (ALD/MLD). It provides high growth-per-cycle (GPC), is highly reactive and is relatively low cost. However, in the deposition of hybrid alucone films, TMA tends to infiltrate into the films requiring very long purge steps and thereby limiting the deposition rate (nm s-1) of the process. From our previous studies, we know that dimethylaluminum isopropoxide (DMAI) could be a potential candidate to substitute TMA in alucone depositions as it does not seem to infiltrate into the films. In this study, we perform a more detailed investigation of MLD of alucone on an atmospheric pressure spatial MLD system using DMAI as the aluminum precursor. The effect of deposition temperature and reactant purge times on the overall GPC has been investigated and a decreasing GPC with increasing deposition temperature and increasing EG purge time has been observed. Furthermore, the DMAI alucone films have been compared for their chemical environment and degradation with the films prepared using TMA and EG, showing striking similarities between the two. The results demonstrate that DMAI can be used as an alternative precursor to TMA for MLD of alucone films and this work can be used as a guide for designing efficient MLD processes in the future.

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

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

Preparation of tungsten-based thin films using a F-free W precursor and tert-butyl hydrazine via 2- and 3-step atomic layer deposition process

This article reports the preparation of tungsten metal (W) and other tungsten-based thin films using a F-free W metalorganic precursor tris(3-hexyne) mono-carbonyl tungsten [W(EtCCEt)3(CO)] (Et = C2H5), THMCT], tert-butyl hydrazine (TBH), and H2 plasma via 2- and 3-step atomic layer deposition (ALD) process. Density functional theory (DFT) calculations were performed to simulate the possible mechanisms of W4(EtCCEt)4(CO)4, as the model of chemisorbed THMCT on the growing surface, with TBH. The experimental findings agree well with the DFT results that the thermal ALD of THMCT and TBH could not deposit the pure W metal films. Instead, THMCT and TBH led to form an amorphous, tungsten oxycarbide, WOxCy (with a minor nitrogen content and a negligible amount of W metal phase), resulting in a very high resistivity of ∼80,000 μΩ-cm (at 300 °C). Nevertheless, the typical characteristics of ALD, such as the self-limiting growth, and a linear dependency of the film thickness with cycle variation, were obtained with a growth-per-cycle (GPC) of ∼0.4 Å. Post-annealing of the films in the H2 atmosphere resulted in a slight increase in the metallic phase formation with a significant drop in resistivity to ∼3500 μΩ-cm. However, the annealing also facilitated the formation of the tungsten oxide phase owing to the oxygen incorporated in the as-deposited films. Introducing an additional H2 plasma pulse step at the end of each ALD cycle (3-step ALD process) significantly altered the film composition and helped reduce the as-deposited film's resistivity drastically to ∼650 μΩ-cm. Also, the process results in the formation of tungsten carbide (WCx) as the predominant phase. Finally, the additional high-temperature-post-annealing of these 3-step ALD grown films resulted in extremely low resistivity of ∼200 μΩ-cm owing to the formation of body-centered-cubic (BCC) α-W. Thus, the envisaged path in this study to achieve W thin films also provides rooms to grow oxide or carbide dominant phase of W as studied in detail with the help of X-ray photoelectron spectroscopy

Atmospheric-Pressure Plasma-Enhanced Spatial ALD of SiO2 Studied by Gas-Phase Infrared and Optical Emission Spectroscopy.

An atmospheric-pressure plasma-enhanced spatial atomic layer deposition (PE-s-ALD) process for SiO2 using bisdiethylaminosilane (BDEAS, SiH2[NEt2]2) and O2 plasma is reported along with an investigation of its underlying growth mechanism. Within the temperature range of 100–250 °C, the process demonstrates self-limiting growth with a growth per cycle (GPC) between 0.12 and 0.14 nm and SiO2 films exhibiting material properties on par with those reported for low-pressure PEALD. Gas-phase infrared spectroscopy on the reactant exhaust gases and optical emission spectroscopy (OES) on the plasma region are used to identify the species that are involved in the ALD process. Based on the identified species, we propose a reaction mechanism where BDEAS molecules adsorb on −OH surface sites through the exchange of one of the amine ligands upon desorption of diethylamine (DEA). The remaining amine ligand is removed through combustion reactions activated by the O2 plasma species leading to the release of H2O, CO2, and CO in addition to products such as N2O, NO2, and CH-containing species. These volatile species can undergo further gas-phase reactions in the plasma as indicated by the observation of OH*, CN*, and NH* excited fragments in OES. Furthermore, the infrared analysis of the precursor exhaust gas indicated the release of CO2 during precursor adsorption. Moreover, this analysis has allowed the quantification of the precursor depletion yielding values between 10 and 50% depending on the processing parameters. Besides providing insights into the chemistry of atmospheric-pressure PE-s-ALD of SiO2, our results demonstrate that infrared spectroscopy performed on exhaust gases is a valuable approach to quantify relevant process parameters, which can ultimately help evaluate and improve process performance.

Atomic Layer Deposition of Ruthenium Dioxide Thin Films Using RuO4 and Alcohols As Reactants

Ruthenium dioxide (RuO2) is the most stable oxide of Ru and has attracted tremendous attention in different applications such as supercapacitors, catalysis, and electrochemical devices. The high conductivity, good chemical stability, a work function value that is even higher than metallic Ru are few properties that make this material so demanding1. Although RuO2 is such an interesting material, atomic layer deposition (ALD) literature reports on this material are scarce. The existing ALD processes based on metalorganic Ru precursors necessitate careful tuning of the O2 partial pressure in order to deposit RuO2 films, as Ru metal is produced at lower O2 partial pressures. Furthermore, reported processes often require high temperature (>180°C) depositions and suffer from high nucleation delays, up to several hundred cycles in some cases.2 In this scenario, we present a novel ALD strategy solely for the synthesis of RuO2 films that takes advantage of the reaction between various alcohols and ruthenium tetroxide (RuO4) without any significant nucleation delay. The process, which uses methanol and RuO4 as reactants, exhibits a growth per cycle (GPC) of 1 angstrom (Å) per cycle (Figure 1a and 1b) at a substrate temperature as low as 60 °C. A constant GPC of 1Å is obtained in the temperature window of 60 °C-120 °C, while the RuO4 precursor is known to decompose above 125 °C.3 The depositions result in amorphous RuO2 films, as confirmed with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. The films can be transformed into crystalline rutile RuO2 by annealing in helium or air at around 400 °C (Figure 1c). The films are conductive, as is evident from the resistivity value of 230 µΩ.cm for a 20 nm film, and the conductivity improved slightly after the anneal.Interestingly, the reaction of other alcohol molecules such as ethanol, 1-propanol, and 2-propanol with RuO4 results in a higher GPC compared to the methanol-based process and it is concluded that the GPC can be increased by increasing the number of carbon atoms in the alcohol chain (Figure 1d). However, all these processes employing different alcohols display typical self-saturating ALD properties. The reaction mechanism of the developed ALD approach was studied in depth using in situ mass spectrometry and in vacuo XPS studies. A reaction mechanism is proposed based on these learnings, wherein during the first half-cycle, alcohol molecules are oxidized into aldehyde and H2O on a RuO2 surface. This in turn causes the partial reduction of the surface RuO2 layer into RuOx or Ru. During the next half-cycle RuO4 oxidizes the surface back to RuO2 and additional RuO2 is deposited on the surface. References Ryden, W.; Lawson, A.; Sartain, C. C., Electrical Transport Properties of Ir O 2 and Ru O 2. Physical Review B 1970, 1 (4), 1494.Austin, D. Z.; Jenkins, M. A.; Allman, D.; Hose, S.; Price, D.; Dezelah, C. L.; Conley Jr, J. F., Atomic layer deposition of ruthenium and ruthenium oxide using a zero-oxidation state precursor. Chemistry of Materials 2017, 29 (3), 1107-1115.Minjauw, M. M.; Dendooven, J.; Capon, B.; Schaekers, M.; Detavernier, C., Atomic layer deposition of ruthenium at 100° C using the RuO 4-precursor and H 2. Journal of Materials Chemistry C 2015, 3 (1), 132-137. This work is funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No.765378. Figure 1

Influence of HfO2 and SiO2 interfacial layers on the characteristics of n-GaN/HfSiOx capacitors using plasma-enhanced atomic layer deposition

We investigated the growth per cycle (GPC) for SiO2 and HfO2 on n-GaN/native oxide and p-Si/SiO2 substrates by plasma-enhanced atomic layer deposition using tris(dimethylamino)silane and tetrakis(dimethylamino)hafnium precursors, respectively, and O2 plasma gases. On the basis of the estimated GPC, we also examined the characteristics of n-GaN/Hf0.57Si0.43Ox/Pt capacitors with an inserted interfacial layer (IL) such as subnanometer-thick HfO2 and SiO2. We found that the GPC for SiO2 on n-GaN/native oxide was slightly smaller than that on p-Si/SiO2, whereas the GPC for HfO2 was the same on both substrates. The GPC for ALD-SiO2 could be reasonably plotted on the basis of the relationship between the GPC and the difference in electronegativity between the metal and oxygen in the metal-O underlayers including native oxide (Ga2O3) on GaN. On the basis of the GPC on n-GaN, Hf0.57Si0.43Ox (23 nm) capacitors were fabricated without and with a HfO2-IL (0.3 and 0.5 nm) or SiO2-IL (0.3 and 0.6 nm). These capacitors exhibited similar leakage current properties and a high breakdown electric field greater than 8.3 MV cm−1. No frequency dispersion and a flatband voltage (Vfb) hysteresis smaller than 50 mV were observed for all of the capacitors. Compared with the SiO2-IL [Si-rich HfSiOx (Si: > 0.43)] capacitors, the HfO2-IL [Hf-rich HfSiOx (Hf: > 0.57)] capacitors showed a smaller interface state density [(1.2–1.7) × 1011 cm−2 eV−1 at −0.4 eV from the conduction band] and a smaller negative Vfb shift. Therefore, the Hf-rich HfSiOx (Hf: > 0.57) grown using a HfO2-IL at the n-GaN/HfSiOx interface plays a substantial role in improving the electrical properties of n-GaN/HfSiOx capacitors.

Investigation of abnormally high growth-per-cycle in atomic layer deposition of Al2O3 using trimethylaluminum and water

Atomic layer deposition (ALD) has the excellent advantage of precisely controlling the thickness of thin films at the atomic scale owing to the self-limiting chemisorption of precursors, but its low growth-per-cycle (GPC) is a weakness in applications requiring thick films. Under the saturation process conditions for the self-limiting growth behavior in Al2O3 ALD using trimethylaluminum (TMA) and H2O, the GPC values are limited to approximately 1–1.2 Å/cycle in the temperature range of 80–250 °C. However, in the practical use of the Al2O3 ALD, abnormally high GPC often occurs, which is generally regarded as the failure to control saturation conditions such as insufficient water purge. Based on comparisons of the normal ALD processes and various modified ALD processes with abnormally high GPC, we report that the unusually high GPC in the Al2O3 ALD is not only caused by the residual H2O but also due to the dimeric nature of TMA. Dimeric aggregate fragments of TMA and dimethylaluminum hydroxide were identified by quadrupole mass spectrometry during TMA exposure and Ar purge steps. This observation supports the dimeric interaction of TMA with the residual H2O.

Comparison of thermal, plasma-enhanced and layer by layer Ar plasma treatment atomic layer deposition of Tin oxide thin films

In this work, Tin oxide (SnOx) thin films have been prepared by different atomic layer deposition (ALD) methods, namely thermal (T-ALD), plasma enhanced (PE-ALD) and subsequent layer by layer Ar plasma treatment (PE-ALD-ALT) modes. In the thermal plasma mode, ozone acts as the oxygen source and Tetrakis(dimethylamino)tin (TDMASn) provides the metal cation. In the plasma-enhanced mode, oxygen plasma was used as the oxygen source. For comparison, we added an Ar plasma treatment step after each layer deposition in the plasma enhanced ALD mode. Compared to T-ALD, the reaction process with O2 plasma as the oxygen source obtained higher growth per cycle (GPC) and crystallinity as well as higher carrier mobility. layer by layer Ar treatment further increased the film carrier concentration and decreased the film resistivity, which also inevitably caused a further increase in the roughness and residual stress in the film.

Impact of Ions on Film Conformality and Crystallinity during Plasma-Assisted Atomic Layer Deposition of TiO2.

This work demonstrates that ions have a strong impact on the growth per cycle (GPC) and material properties during plasma-assisted atomic layer deposition (ALD) of TiO2 (titanium dioxide), even under mild plasma conditions with low-energy (<20 eV) ions. Using vertical trench nanostructures and microscopic cavity structures that locally block the flux of ions, it is observed that the impact of (low-energy) ions is an important factor for the TiO2 film conformality. Specifically, it is demonstrated that the GPC in terms of film thickness can increase by 20 to >200% under the influence of ions, which is correlated with an increase in film crystallinity and an associated strong reduction in the wet etch rate (in 30:1 buffered HF). The magnitude of the influence of ions is observed to depend on multiple parameters such as the deposition temperature, plasma exposure time, and ion energy, which may all be used to minimize or exploit this effect. For example, a relatively moderate influence of ions is observed at 200 °C when using short plasma steps and a grounded substrate, providing a low ion-energy dose of ∼1 eV nm–2 cycle–1, while a high effect is obtained when using extended plasma exposures or substrate biasing (∼100 eV nm–2 cycle–1). This work on TiO2 shows that detailed insight into the role of ions during plasma ALD is essential for precisely controlling the film conformality, material properties, and process reproducibility.

Thermal Atomic Layer Deposition of Yttrium Oxide Films and Their Properties in Anticorrosion and Water Repellent Coating Applications

The thermal atomic layer deposition (ThALD) of yttrium oxide (Y2O3) was developed using the newly designed, liquid precursor, Y(EtCp)2(iPr2-amd), as the yttrium source in combination with different oxygen sources, such as ozone, water and even molecular oxygen. Saturation was observed for the growth of the Y2O3 films within an ALD window of 300 to 450 °C and a growth per cycle (GPC) up to 1.1 Å. The resulting Y2O3 films possess a smooth and crystalline structure, while avoiding any carbon and nitrogen contamination, as observed by X-ray photoelectron spectroscopy (XPS). The films showed strong resistance to fluorine-containing plasma, outperforming other resistant materials, such as silicon oxide, silicon nitride and alumina. Interestingly, the hydrophilic character exhibited by the film could be switched to hydrophobic after exposure to air, with water contact angles exceeding 90°. After annealing under N2 flow at 600 °C for 4 min, the hydrophobicity was lost, but proved recoverable after prolonged air exposure or intentional hydrocarbon exposure. The origin of these changes in hydrophobicity was examined.

Intelligent Agents for the Optimization of Atomic Layer Deposition.

Atomic layer deposition (ALD) is a highly controllable thin film synthesis approach with applications in computing, energy, and separations. The flexibility of ALD means that it can access a massive chemical catalogue; however, this chemical and process diversity results in significant challenges in determining processing parameters that result in stable and uniform film growth with minimal precursor consumption. In situ measurements of the ALD growth per cycle (GPC) can accelerate process development but it still requires expert intuition and time-consuming trial and error to identify acceptable processing parameters. This procedure is made more difficult by the presence of experimental noise in the GPC values and the complexity of ALD surface chemistries. A need exists for efficient optimization approaches capable of autonomously determining processing conditions resulting in optimal ALD film growth. In this work, we present the development of three optimization strategies and compare their performance in optimizing four simulated ALD processes. Furthermore, the effect of noise in the GPC measurements on optimization convergence is studied.

Atomic layer chemical vapor deposition of SiO2 thin films using a chlorine-free silicon precursor for 3D NAND applications

Atomic layer deposition and atomic layer chemical vapor deposition (ALD, ALCVD) of SiO2 films were investigated over a wide range of high temperatures (from 525 °C to 700 °C) using chlorine-free amino silane as the Si precursor and O3 as the oxygen reactant. The ALD window occurred at 550–600 °C. The growth per cycle (GPC) of the SiO2 films was about 1.14–1.58 Å/cycle for deposition temperatures between 525 and 700 °C. There were no chlorine impurities detected in any of the deposited ALCVD films. Above a deposition temperature of 750 °C, the SiO2 films exhibited conventional chemical vapor deposition (CVD) behavior due to precursor decomposition, as evidenced by high GPC (5.51 Å/cycle) and a significant impurity content (carbon 0.2 at% and nitrogen 0.4 at%). The SiO2 films had high film density (2.3 g/cm3), minimal roughness (rms ~ 0.16 nm). In addition, the wet etch rate (WER) of the SiO2 films decreased from 3.0 to 2.1 nm/min. The ALD SiO2 film at 600 °C exhibited excellent electrical performance, such as a leakage current density of 5.03 × 10−9 A/cm2 (at 3 MV/cm) and a breakdown field of 10.3 MV/cm.

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

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

Low-Temperature As-Grown Crystalline β-Ga2O3 Films via Plasma-Enhanced Atomic Layer Deposition.

We report on the low-temperature growth of crystalline Ga2O3 films on Si, sapphire, and glass substrates using plasma-enhanced atomic layer deposition (PEALD) featuring a hollow-cathode plasma source. Films were deposited by using triethylgallium (TEG) and Ar/O2 plasma as metal precursor and oxygen co-reactant, respectively. Growth experiments have been performed within 150-240 °C substrate temperature and 30-300 W radio-frequency (rf) plasma power ranges. Additionally, each unit AB-type ALD cycle was followed by an in situ Ar plasma annealing treatment, which consisted of an extra (50-300 W) Ar plasma exposure for 20 s ending just before the next TEG pulse. The growth per cycle (GPC) of the films without Ar plasma annealing step ranged between 0.69 and 1.31 Å/cycle, and as-grown refractive indices were between 1.67 and 1.75 within the scanned plasma power range. X-ray diffraction (XRD) measurements showed that Ga2O3 films grown without in situ Ar plasma annealing exhibited amorphous character irrespective of substrate temperature and rf power values. With the incorporation of the in situ Ar plasma annealing process, the GPC of Ga2O3 films ranged between 0.76 and 1.03 Å/cycle along with higher refractive index values of 1.75-1.79. The increased refractive index (1.79) and slightly reduced GPC (1.03 Å/cycle) at 250 W plasma annealing indicated possible densification and crystallization of the films. Indeed, X-ray measurements confirmed that in situ plasma annealed films grow in a monoclinic β-Ga2O3 crystal phase. The film crystallinity and density further enhance (from 5.11 to 5.60 g/cm3) by increasing the rf power value used during in situ Ar plasma annealing process. X-ray photoelectron spectroscopy (XPS) measurement of the β-Ga2O3 sample grown under optimal in situ plasma annealing power (250 W) revealed near-ideal film stoichiometry (O/Ga of ∼1.44) with relatively low carbon content (∼5 at. %), whereas 50 W rf power treated film was highly non-stoichiometric (O/Ga of ∼2.31) with considerably elevated carbon content. Our results demonstrate the effectiveness of in situ Ar plasma annealing process to transform amorphous wide bandgap oxide semiconductors into crystalline films without needing high-temperature post-deposition annealing treatment.

Density Functional Theory (DFT)-enhanced computational fluid dynamics modeling of substrate movement and chemical deposition process in spatial atomic layer deposition

Spatial atomic layer deposition (ALD) has been widely accepted as an effective strategy to improve the throughput in traditional ALD. This paper presents a Density Functional Theory (DFT)-enhanced Computational Fluid Dynamics (CFD) model, which includes substrate movement and a mechanistic chemical kinetics model derived from DFT in an inline spatial ALD. Substrate movement alters the steady flow field, but the disturbance does not damage the effective gas barrier built by N2 flow. Evaluation of deposition rate shows the growth per cycle (GPC) decreases as the substrate velocity increases. However, the growth per second (GPS) increases because of the considerable amount of time saved in faster substrate movements. Bottom and top purging designs are shown comparable in deposition rate. The DFT-enhanced CFD model provides a new route for accurate ALD modeling and simulation, and offers insightful information on reactor design and process optimization in spatial ALD.

Atomic layer deposited Mo2N thin films using Mo(CO)6 and NH3 plasma as a Cu diffusion barrier

Thin films of molybdenum nitride (Mo2N) are prepared using sequential exposure of molybdenum hexacarbonyl [Mo(CO)6] and NH3 plasma in a plasma-enhanced atomic layer deposition (PEALD) reactor. Several process parameters such as the deposition temperature, plasma power, and post-annealing conditions are systematically investigated to achieve the best quality films. The superior growth kinetics is evident with a significantly higher growth per cycle (GPC) value with lower incubation period for this PEALD process (~1.1 Å, ~36 cycles) when compared to thermal ALD (~0.3 Å, ~63 cycles), both carried out at 200 °C. The growth rate of the Mo2N film reveals a significant jump above 215 °C, indicating a severe decomposition of Mo(CO)6, however, polycrystalline γ-Mo2N films with face-centered-cubic structure are evident within the deposition temperature range of 200–230 °C. The sharp decrease in the resistivity of the as-grown Mo2N films is observed with increasing deposition temperature, film thickness, and plasma power. The resistivity could be further lowered by a post-annealing process and the lowest resistivity of ~395 μΩ cm is achieved for the thin film deposited with 300 watt plasma power and annealed at 700 °C. Finally, the Cu-diffusion barrier capability of an extremely thin film (~7 nm) of as-deposited Mo2N is evaluated in Cu/PEALD-Mo2N/Si structure. The X-ray diffractometry analysis confirms that such a thin layer can successfully prevent the diffusion of Cu up to 500 °C and significant Cu3Si formation was observed only at 600 °C and above. The gradual failure of the diffusion barrier upon annealing is further investigated using electrical impedance (EI) analyses in two different modes. In-plane EI measurements directly indicate the change in the Cu layer at the top, and the formation of Cu3Si can be inferred from the through-plane mode. A comparison with PEALD and thermal-ALD-grown MoNx from EI analyses further reflects the superiority of plasma-enhanced process towards fabricating diffusion barrier layer.