Atomic Layer Deposition Reactor Research Articles

Low‐Temperature Deposition of Hexagonal Boron Nitride via Sequential Injection of Triethylboron and N2/H2 Plasma

Hexagonal boron nitride (hBN) thin films were deposited on silicon and quartz substrates using sequential exposures of triethylboron and N2/H2 plasma in a hollow‐cathode plasma‐assisted atomic layer deposition reactor at low temperatures (≤450°C). A non‐saturating film deposition rate was observed for substrate temperatures above 250°C. BN films were characterized for their chemical composition, crystallinity, surface morphology, and optical properties. X‐ray photoelectron spectroscopy (XPS) depicted the peaks of boron, nitrogen, carbon, and oxygen at the film surface. B 1s and N 1s high‐resolution XPS spectra confirmed the presence of BN with peaks located at 190.8 and 398.3 eV, respectively. As deposited films were polycrystalline, single‐phase hBN irrespective of the deposition temperature. Absorption spectra exhibited an optical band edge at ~5.25 eV and an optical transmittance greater than 90% in the visible region of the spectrum. Refractive index of the hBN film deposited at 450°C was 1.60 at 550 nm, which increased to 1.64 after postdeposition annealing at 800°C for 30 min. These results represent the first demonstration of hBN deposition using low‐temperature hollow‐cathode plasma‐assisted sequential deposition technique.

Scalable control program for multiprecursor flow-type atomic layer deposition system

The authors report the development and implementation of a scalable control program to control flow type atomic layer deposition (ALD) reactor with multiple precursor delivery lines. The program logic is written and tested in labview environment to control ALD reactor with four precursor delivery lines to deposit up to four layers of different materials in cyclic manner. The programming logic is conceived such that to facilitate scale up for depositing more layers with multiple precursors and scale down for using single layer with any one precursor in the ALD reactor. The program takes precursor and oxidizer exposure and purging times as input and controls the sequential opening and closing of the valves to facilitate the complex ALD process in cyclic manner. The program could be used to deposit materials from any single line or in tandem with other lines in any combination and in any sequence.

Growth of Al Doped ZnO Thin Films by Atomic Layer Deposition Process

ZnO is a transparent semiconductor with a wide and direct band gape of 3.37 eV, large excitonic binding energy of ~60 meV, high mechanical and thermal stabilities, and good electrical conductivity. Therefore, ZnO based materials are extensively investigated in spintronics, gas and biological sensors, solar cells, catalysts, electronics, piezoelectric devices and high-efficiency short-wavelength optoelectronic nanodevices. In solar cell application, especially Al doped ZnO (AZO) thin films have been extensively developed and investigated due to their low cost, high thermal stability, high optical transparency and high conductivity [1, 2 , 3]. Several methods have been applied to deposit ZnO thin films in the past, such as sol gel spin coating, magnetron sputtering, pulsed laser deposition (PLD), chemical vapor deposition (CVD), and atomic layer deposition (ALD). Among these, ALD is considered as a novel technique to deposit ZnO thin films, because it possesses self-limiting surface reactions in each ALD cycle, which consequently control the layer thickness with unprecedented atomic level precision, stoichiometry, composition, uniformity of large area and maintain excellent conformity of complex nanostructures. In addition, ALD process is able to introduce separately each precursor to react on the surface of the structure. The concentration of dopant in the ZnO films can be precisely controlled by the number of ALD cycles, so that the physical properties of AZO films, such as carrier density and conductivity, can be adjusted to meet the requirements of different applications. In this study, Al doped ZnO thin films were deposited by the ALD technique with trimethylaluminium (TMA), diethylzinc (DEZ) and water vapor as aluminium, zinc and oxygen precursors, respectively. The films were grown in Savannah 100 cross-flow thermal ALD reactor. The growth temperature was maintained at 250 oC during the reaction. N2 was used as a carrier gas with a flow rate of 20 sccm. The ALD AZO films with various Al doping level were prepared by introducing TMA cycles once in every 30 pulses of DEZ and H2O injection. The structural, optical and electrical properties of the AZO films prepared by using ALD processes were investigated. The crystal structure of ZnO was analyzed by X-ray diffraction (XRD). The AZO film surface morphology was inspected using a field emission scanning electron microscopy (FE-SEM). The surface roughness was analyzed by atomic force microscopy (AFM). The sheet resistivity of AZO films was determined by using four-point probe measurements. The optical reflection was carried out by UV-VIS-NIR spectrometer.

Pulsed-bed atomic layer deposition setup for powder coating

Atomic layer deposition (ALD) provides a method for coating conformal, pinhole-free, chemically bonded, and ultra-thin films on particle surfaces. ALD is based on one or more cycles, each cycle comprising two half-reactions. As such, ALD is a process inherently discrete in time, where the coating thickness can be controlled as a function of number of cycles. A popular scheme for achieving uniform coats on powders is to combine ALD reactors with fluidization conditions. However, fluidization is not easy to attain because it is strongly dependent on particle size, density, morphology, and surface roughness. This article proposes that a pulsed-bed, instead of a continuous fluidization, is easier to achieve in most ALD reactors. Taking advantage of the discrete nature of the ALD process, with simple changes in the configurations of purge and carrier gases, the pulsed-bed mode can be completed. An adaptation made to a regular ALD reactor to work in this mode is presented. The inclusion of a capsule for powder, valve relocations, and control of times were all necessary modifications. It was found that the pulsed-bed is a very convenient alternative for research purposes, since it can coat powders of different morphological characteristics, such as carbon nanotubes, flower-like ZnO micro-arrays, and YCrO3 particles.

Scale-up analysis of continuous cross-flow atomic layer deposition reactor designs

This paper presents the development of a non-dimensional model of a continuous cross-flow atomic layer deposition (ALD) reactor with temporally separated precursor pulsing and a structured model-based methodology for scaling up the substrate dimensions. The model incorporates an ALD gas–surface reaction kinetic mechanism for the deposition of thin ZnO films from Zn(C2H5)2 and H2O precursors that was experimentally validated in our previous work (Holmqvist et al., 2012, 2013a). In order to maintain dynamic similarity, a scaling analysis was applied based on the dimensionless numbers, appearing in non-dimensionalized momentum and species mass conservation equations, that describe the convective laminar flow, mass transfer and heterogeneous reaction. The impact on these dimensionless numbers and, more importantly, the impact on the limit-cycle deposition rate and its relative uniformity was thoroughly investigated when linearly scaling up the substrate dimensions. In the scale-up procedure, the limit-cycle precursor utilization was maximized by means of dynamic optimization, while ensuring that identical deposition profiles were obtained in the scaled-up system. The results presented here demonstrated that the maximum precursor yields were promoted at higher substrate dimensions. Limit-cycle dynamic solutions to the non-dimensionalized model, computed with a collocation discretization in time, revealed that it is a combination of the degree of precursor depletion in the flow direction and the magnitude of the pressure drop across the reactor chamber that governs the extent of the deposition profile non-uniformity. A key finding of this study is the identification of optimal scaling rules for maximizing precursor utilization in the scaled-up system while maintaining fixed absolute growth rate and its relative uniformity.

Surface Selective Atomic Layer Deposition of Hafnium Oxide for Copper Diffusion Barrier Application Using Tetrakis(diethylamino)Hafnium and Ethanol

Atomic layer deposition (ALD) is a thin film deposition technique, in which substrate surface is exposed to reactants in cyclic manner separated by discrete purging steps. Thus one atomic layer of material is deposited per ALD cycle. This cycle is repeated until desired thickness is achieved. The bottom up nature of this deposition process facilitates excellent conformality, uniformity and thickness and composition tunability.Patterning of ALD films for various device applications is commonly performed by removing the specific part of the film or by selectively depositing films on specific areas of the surface. In the former method, patterning is done with the use of extra processing steps such as etching, lithography etc. The latter method involves the use of masking layers to prevent ALD on specific areas.1, 2 Maskless ALD, i.e., surface selective atomic layer deposition of thin films has recently garnered considerable interest due to its simplicity. This method of patterning ALD films is unique in that no surface mask is required. Materials nucleate on different surfaces to different extents. This difference in nucleation time is used to achieve selective atomic layer deposition (SALD). In our earlier studies on SALD of HfO2 and TiO2, we used water as the oxygen source.3, 4 In this study, ALD of HfO2 was carried out using tetrakis(diethylamino)hafnium and a primary alcohol (ethanol) in a custom built ALD reactor (Fig. 1). Unlike common ALD processes, here we used ethanol as the oxygen source. Ethanol reacts with the hydroxyl (-OH) groups on silicon surface and forms alkoxides that serve as a starting surface for the ALD film growth. Thus the film growth is obtained on the Si surface. Figure 2 shows the thickness tunability of the HfO2 ALD process on Si surface using ethanol as the oxygen source. HfO2 was found to grow at a rate of 0.05 nm/cycle, without any growth inhibition on the silicon surface. Linear dependence indicates self-limited growth of HfO2 thin film, typical to the ALD process. On the other hand, significant growth delay was observed on copper surface. The use of ethanol reduces surface copper oxide into metallic copper and reduces the number of potential ALD nucleation sites. Thus selective deposition of HfO2 was achieved on silicon surface over copper surface. Such selectivity depends on many ALD processing parameters and substrate pretreatment conditions. In this report, the effect of these parameters on the extent of selectivity will be discussed in detail along with copper diffusion barrier characteristics of such selectively deposited ultrathin (1-2 nm-thick) HfO2 layers. The growth rate on silicon side was measured using spectral ellipsometry. X-ray photoelectron spectroscopy was used to probe the presence of HfO2 on copper surface. Results of this study can be used to selectively deposit ultra-thin layers of HfO2 on silicon surface over copper surface for copper diffusion barrier applications. This work was supported in part by the National Science Foundation (CBET 1067424 and EEC 1062943). We also thank Dr. Gregory Jursich for many helpful discussions.

Atomic Layer Deposition of Environmentally Benign Tin-Titanate for Ferroelectric Application

Perovskite-based oxides that possess ferroelectric properties, such as BaTiO3 and PbTiO3, have long been a research interest because of their wide applications in devices such as actuators and sensors.1 Due to the toxicity of lead, emphasis has been put on the replacement of Pb2+ by isoelectronic, environment friendly Sn2+, in hope of maintaining similar ferroelectric performance.Theoretical studies using first principle calculations have predicted that perovskite SnTiO3 (P-SNO) would exhibit excellent ferroelectric properties,2-6 yet the actual synthesis of P-SNO by conventional methods remains unsuccessful due to the less stable Sn2+ at high temperatures.2,3 No success was achieved in depositing P-SNO thin films, either. Several studies suggest that P-SNO is metastable,2,7,8 which leads to the proposition of alternative synthetic routes that can stabilize the metastable structure,9 such as atomic layer deposition (ALD)3/chemical vapor deposition (CVD), sol-gel, etc.2 Different from the predicted perovskite structure, nonpolar, ilmentile-type thin film of SnTiO3 has been achieved on sapphire and perovskite substrates using pulsed laser deposition (PLD) techniques from ceramic SnO2 and TiO2 targets.10 This result further motivates a more thorough investigation as to whether or not thin film deposition techniques could stabilize the metastable P-SNO structure which is crucial for high ferroelectric performance.In this work, we report the result of atomic layer deposition of tin-titanate using tin(II)acetylacetonate (Sn(acac)2) and tetrakis(diethylamino)titanium (TDEAT). The ALD reaction is carried out on Si substrates using a custom-built ALD reactor (Fig. 1).11 ALD process parameters were optimized for SnOx and TiO2 deposition. Linear growth behavior was observed for both the films, indicating self-limited ALD growth. The ALD temperature windows of both ALD processes were found to overlap, which facilitates co-deposition of SnOx and TiO2 for tin-titanate deposition. The resulting film is subsequently analyzed by several characterization techniques to determine its stoichiometry, composition, impurities and structural and surface characteristics. The results of this work provide more insights into achieving lead-free, perovskite SnTiO3 structure to facilitate future research on this subject.

Dynamic parameter estimation of atomic layer deposition kinetics applied to in situ quartz crystal microbalance diagnostics

This paper presents the elaboration of an experimentally validated model of a continuous cross-flow atomic layer deposition (ALD) reactor with temporally separated precursor pulsing encoded in the Modelica language. For the experimental validation of the model, in situ quartz crystal microbalance (QCM) diagnostics was used to yield submonolayer resolution of mass deposition resulting from thin film growth of ZnO from Zn(C2H5)2 and H2O precursors. The ZnO ALD reaction intrinsic kinetic mechanism that was developed accounted for the temporal evolution of the equilibrium fractional surface concentrations of precursor adducts and their transition states for each half-reaction. This mechanism was incorporated into a rigorous model of reactor transport, which comprises isothermal compressible equations for the conservation of mass, momentum and gas-phase species. The physically based model in this way relates the local partial pressures of precursors to the dynamic composition of the growth surface, and ultimately governs the accumulated mass trajectory at the QCM sensor. Quantitative rate information can then be extracted by means of dynamic parameter estimation. The continuous operation of the reactor is described by limit-cycle dynamic solutions and numerically computed using Radau collocation schemes and solved using CasADi׳s interface to IPOPT. Model predictions of the transient mass gain per unit area of exposed surface QCM sensor, resolved at a single pulse sequence, were in good agreement with experimental data under a wide range of operating conditions. An important property of the limit-cycle solution procedure is that it enables the systematic approach to analyze the dynamic nature of the growth surface composition as a function of process operating parameters. Especially, the dependency of the film growth rate per limit-cycle on the half-cycle precursor exposure dose and the process temperature was thoroughly assessed and the difference between ALD in saturating and in non-saturating film growth conditions distinguished.

Fluidized-bed atomic layer deposition reactor for the synthesis of core-shell nanoparticles

The design of a fluidized bed atomic layer deposition (ALD) reactor is described in detail. The reactor consists of three parts that have all been placed in one protective cabinet: precursor dosing, reactor, and residual gas treatment section. In the precursor dosing section, the chemicals needed for the ALD reaction are injected into the carrier gas using different methods for different precursors. The reactor section is designed in such a way that a homogeneous fluidized bed can be obtained with a constant, actively controlled, reactor pressure. Furthermore, no filters are required inside the reactor chamber, minimizing the risk of pressure increase due to fouling. The residual gas treatment section consists of a decomposition furnace to remove residual precursor and a particle filter and is installed to protect the pump. In order to demonstrate the performance of the reactor, SiO2 particles have been coated with TiO2 using tetrakis-dimethylamino titanium (TDMAT) and H2O as precursors. Experiments with varying pulse times show that saturated growth can be obtained with TDMAT pulse times larger than 600 s. Analysis of the powder with High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy confirmed that after 50 cycles, all SiO2 particles were coated with a 1.6 nm homogenous shell of TiO2.

Vibration atomic layer deposition for conformal nanoparticle coating

A vibration atomic layer deposition reactor was developed for fabricating a conformal thin-film coating on nanosize particles. In this study, atomic layer deposition of 10–15-nm-thick Al2O3 films was conducted on a high-surface-area acetylene black powder with particle diameters of 200–250 nm. Intense vibration during the deposition resulted in the effective separation of particles, overcoming the interparticle agglomeration force and enabling effective diffusion of the precursor into the powder chunk; this phenomenon led to the formation of a conformal film coating on the nanopowder particles. It was also confirmed that the atomic layer deposition Al2O3 films initially grew on the high-surface-area acetylene black powder particles as discrete islands, presumably because chemisorption of the precursor and water occurred only on a few sites on the high-surface-area acetylene black powder surface. Relatively sluggish growth of the films during the initial atomic layer deposition cycles was identified from composition analysis.

Growth behavior and properties of atomic layer deposited tin oxide on silicon from novel tin(II)acetylacetonate precursor and ozone

In this work, a novel liquid tin(II) precursor, tin(II)acetylacetonate [Sn(acac)2], was used to deposit tin oxide films on Si(100) substrate, using a custom-built hot wall atomic layer deposition (ALD) reactor. Three different oxidizers, water, oxygen, and ozone, were tried. Resulting growth rates were studied as a function of precursor dosage, oxidizer dosage, reactor temperature, and number of ALD cycles. The film growth rate was found to be 0.1 ± 0.01 nm/cycle within the wide ALD temperature window of 175–300 °C using ozone; no film growth was observed with water or oxygen. Characterization methods were used to study the composition, interface quality, crystallinity, microstructure, refractive index, surface morphology, and resistivity of the resulting films. X-ray photoelectron spectra showed the formation of a clean SnOx–Si interface. The resistivity of the SnOx films was calculated to be 0.3 Ω cm. Results of this work demonstrate the possibility of introducing Sn(acac)2 as tin precursor to deposit conducting ALD SnOx thin films on a silicon surface, with clean interface and no formation of undesired SiO2 or other interfacial reaction products, for transparent conducting oxide applications.

The Characterization and Passivation of Fixed Oxide Charges and Interface States in the $\hbox{Al}_{2}\hbox{O}_{3}/ \hbox{InGaAs}$ MOS System

In this paper, we present a review of experimental results examining charged defect components in the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As metal-oxide-semiconductor (MOS) system. For the analysis of fixed oxide charge and interface state density, an approach is described where the flatband voltage for n- and p-type Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As MOS structures is used to separate and quantify the contributions of fixed oxide charge and interface state density. Based on an Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thickness series (10-20 nm) for the n- and p-type In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As layers, the analysis reveals a positive fixed charge density ( ~ 9 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ) distributed throughout the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and a negative sheet charge density (- 8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ) located near the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As interface. The interface state density integrated across the energy gap is ~1 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> and is a donor-type (+/0) defect. The density of the fixed oxide charge components is significantly reduced by forming gas (5 % H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> / 95% N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ambient at 350 °C for 30 minutes) annealing. The interface state distribution obtained from multi-frequency capacitance-voltage and conductance-voltage measurements on either MOS structures or MOSFETs indicates a peak density located around the In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As midgap energy, with a sharp increase in the interface state density toward the valance band and evidence of interface states aligned with the In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As conduction band. The integrated interface state density obtained from multi-frequency capacitance-voltage and conductance-voltage analysis is in good agreement with the approach of comparing the flatband voltages in n- and p -type Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As MOS structures. Finally, this paper reviews recent work based on an optimization of the In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As surface preparation using (NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S, combined with minimizing the transfer time to the atomic layer deposition reactor for Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , which indicates interface state reduction and genuine surface inversion for both n- and p -type Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As MOS structures.

Numerical modeling of carrier gas flow in atomic layer deposition vacuum reactor: A comparative study of lattice Boltzmann models

This paper characterizes the carrier gas flow in the atomic layer deposition (ALD) vacuum reactor by introducing Lattice Boltzmann Method (LBM) to the ALD simulation through a comparative study of two LBM models. Numerical models of gas flow are constructed and implemented in two-dimensional geometry based on lattice Bhatnagar–Gross–Krook (LBGK)-D2Q9 model and two-relaxation-time (TRT) model. Both incompressible and compressible scenarios are simulated and the two models are compared in the aspects of flow features, stability, and efficiency. Our simulation outcome reveals that, for our specific ALD vacuum reactor, TRT model generates better steady laminar flow features all over the domain with better stability and reliability than LBGK-D2Q9 model especially when considering the compressible effects of the gas flow. The LBM-TRT is verified indirectly by comparing the numerical result with conventional continuum-based computational fluid dynamics solvers, and it shows very good agreement with these conventional methods. The velocity field of carrier gas flow through ALD vacuum reactor was characterized by LBM-TRT model finally. The flow in ALD is in a laminar steady state with velocity concentrated at the corners and around the wafer. The effects of flow fields on precursor distributions, surface absorptions, and surface reactions are discussed in detail. Steady and evenly distributed velocity field contribute to higher precursor concentration near the wafer and relatively lower particle velocities help to achieve better surface adsorption and deposition. The ALD reactor geometry needs to be considered carefully if a steady and laminar flow field around the wafer and better surface deposition are desired.

Role of plasma enhanced atomic layer deposition reactor wall conditions on radical and ion substrate fluxes

Chamber wall conditions, such as wall temperature and film deposits, have long been known to influence plasma source performance on thin film processing equipment. Plasma physical characteristics depend on conductive/insulating properties of chamber walls. Radical fluxes depend on plasma characteristics as well as wall recombination rates, which can be wall material and temperature dependent. Variations in substrate delivery of plasma generated species (radicals, ions, etc.) impact the resulting etch or deposition process resulting in process drift. Plasma enhanced atomic layer deposition is known to depend strongly on substrate radical flux, but film properties can be influenced by other plasma generated phenomena, such as ion bombardment. In this paper, the chamber wall conditions on a plasma enhanced atomic layer deposition process are investigated. The downstream oxygen radical and ion fluxes from an inductively coupled plasma source are indirectly monitored in temperature controlled (25–190 °C) stainless steel and quartz reactors over a range of oxygen flow rates. Etch rates of a photoresist coated quartz crystal microbalance are used to study the oxygen radical flux dependence on reactor characteristics. Plasma density estimates from Langmuir probe ion saturation current measurements are used to study the ion flux dependence on reactor characteristics. Reactor temperature was not found to impact radical and ion fluxes substantially. Radical and ion fluxes were higher for quartz walls compared to stainless steel walls over all oxygen flow rates considered. The radical flux to ion flux ratio is likely to be a critical parameter for the deposition of consistent film properties. Reactor wall material, gas flow rate/pressure, and distance from the plasma source all impact the radical to ion flux ratio. These results indicate maintaining chamber wall conditions will be important for delivering consistent results from plasma enhanced atomic layer deposition systems.

Dynamic Modeling for the Design and Cyclic Operation of an Atomic Layer Deposition (ALD) Reactor

A laboratory-scale atomic layer deposition (ALD) reactor system model is derived for alumina deposition using trimethylaluminum and water as precursors. Model components describing the precursor thermophysical properties, reactor-scale gas-phase dynamics and surface reaction kinetics derived from absolute reaction rate theory are integrated to simulate the complete reactor system. Limit-cycle solutions defining continuous cyclic ALD reactor operation are computed with a fixed point algorithm based on collocation discretization in time, resulting in an unambiguous definition of film growth-per-cycle (gpc). A key finding of this study is that unintended chemical vapor deposition conditions can mask regions of operation that would otherwise correspond to ideal saturating ALD operation. The use of the simulator for assisting in process design decisions is presented.

Surface and interfacial reaction study of InAs(100)-crystalline oxide interface

A crystalline oxide film on InAs(100) is investigated with in situ monochromatic x-ray photoelectron spectroscopy and low energy electron diffraction before and after in situ deposition of Al2O3 by atomic layer deposition (ALD) as well as upon air exposure. The oxidation process leads to arsenic and indium trivalent oxidation state formation. The grown epitaxial oxide-InAs interface is stable upon ALD reactor exposure; however, trimethyl aluminum decreases oxidation states resulting in an unreconstructed surface. An increase in oxide concentration is also observed upon air exposure suggesting the crystalline oxide surface is unstable.

Direct-Write Atomic Layer Deposition of High-Quality Pt Nanostructures: Selective Growth Conditions and Seed Layer Requirements

Electron beam-induced deposition (EBID) enables the direct-write patterning of metallic structures with sub-10 nm lateral resolution without the use of resist films or etching/lift-off steps but generally leads to material of poor quality and suffers from a low throughput. These shortcomings were mitigated in recent work by combining EBID with atomic layer deposition (ALD). This direct-write ALD technique comprises the patterning of a thin seed layer by EBID followed by selective thickening of the pattern by ALD. In this work, the throughput of direct-write ALD was drastically improved based on new insights into how the ALD growth initiates on EBID material, and in addition, the conditions for selective ALD growth were identified. The required electron dose was reduced by 2 orders of magnitude to ∼11 pC/μm2 by exposing the EBID seed layers to O2 in the ALD reactor just before the ALD building step. This improvement of the technique allows for nanopatterning with a throughput comparable to electron beam li...

A model-based methodology for the analysis and design of atomic layer deposition processes—Part III: Constrained multi-objective optimization

This paper presents a structured methodology for the constrained multi-objective optimization (MO) of a continuous cross-flow atomic layer deposition (ALD) reactor model with temporal precursor pulsing. The process model has been elaborated and experimentally validated in the first two papers of this series (Holmqvist et al., 2012, 2013). A general constrained MO problem (MOP) was formulated to simultaneously optimize quasi-steady-state reactor throughput and overall precursor conversion for the controlled deposition of ZnO films from Zn(C2H5)2 and H2O, subject to a set of operational constraints. These constraints included lower bounds for the cross-substrate film thickness uniformity and post-precursor purge duration. The non-dominated Pareto optimal solutions obtained successfully revealed the relation between the incommensurable process objectives and reduced the design space of the ALD process into a feasible set of design alternatives. The results presented here show that post-precursor purge duration is essential when optimizing throughput in temporally separated ALD processes, and that this is a major drawback when considering operation at atmospheric pressure. Finally, the robustness of the process along the Pareto optimal front, i.e. the ability of the process to accommodate variations in the associated set of optimal decision variables (DVs), was assessed by Monte Carlo simulations, in which the values of the parametric uncertainties were randomly generated from a multivariate normal distribution. The uncertainty and sensitivity analysis showed that the inherent robustness of the process is progressively lost with the precursor conversion, and revealed the mechanistic dependence of all DVs on the proposed optimization specifications.

Low temperature plasma assisted atomic layer deposition in nitrogen carrier gas studied by optical emission spectroscopy

A low temperature plasma assisted atomic layer deposition process from tetrakis (dimethylamino)-titanium (TDMAT) and oxygen plasma was investigated using optical emission spectroscopy in a commercial TFS-500 atomic layer deposition reactor in industrial-like conditions with different plasma powers to optimize the plasma-assisted deposition process and to develop a tool for process control. The major emitting species recognized were the nitrogen first followed by the second positive system, carbon monoxide, nitrogen monoxide (γ)-system, atomic carbon and atomic nitrogen. The process measurements were compared to background measurements to see the process induced differences. CHx appearance in the plasma lowered the intensities of CO, NO and N2 1st+ system peaks. Also, the nitrogen atom content varied in the process measurements. By monitoring the affected species and vibrational temperature, the effect of TDMAT oxidation on the surface could be seen through the resulting changes in the plasma emission.

Electrocatalytic Activity of Pt Grown by ALD on Carbon Nanotubes for Si-Based DMFC Applications

We present an anode design for silicon-based direct methanol fuel cell (DMFC) applications. Platinum was deposited conformally by atomic layer deposition (ALD) onto vertically aligned, nitrogen-doped multi-walled carbon nanotubes (MWCNTs) grown on porous silicon. The deposition was carried out in a top-flow ALD reactor at 250{degree sign}C, using MeCpPtMe3 and O2 as precursors. The anode was tested for the methanol oxidation reaction (MOR) in a three-electrode electrochemical set-up and it showed improved catalytic activity compared to a reference sample of Pt deposited on flat Si. It is demonstrated that ALD could be a MEMS compatible deposition technique for Si-based fuel cell applications.