Thermal Atomic Layer Deposition Process Research Articles

Investigation of atomic layer deposition methods of Al2O3 on n-GaN

In this work, three atomic layer deposition (ALD) approaches are used to deposit an Al2O3 gate insulator on n-GaN for application in vertical GaN power switches: thermal ALD (ThALD), plasma-enhanced ALD (PEALD), and their stacked combination. The latter is a novel method to yield the most ideal insulating layer. Also, the influence of an in situ NH3 or H2 plasma pre-treatment is studied. Planar MIS capacitors are used to investigate the electrical properties and robustness of the gate insulators. In vacuo x-ray photoelectron spectroscopy (XPS) is used to study the changes in chemical composition after every surface treatment. XPS shows that all plasma pre-treatments efficiently remove all carbon contamination from the surface, but only NH3 plasma is observed to additionally remove the native oxide from the n-GaN surface. The water precursor step in the ThALD process does not completely remove the CH3 ligands of the trimethylaluminum precursor step, which might electrically be associated with a reduced forward bias robustness. The O2 plasma step in the PEALD process is associated with the removal of carbon and a tremendous increase of the O content in the GaN surface region. Electrically, this strongly correlates to an enhanced forward bias robustness and an increased forward bias hysteresis, respectively. The ThALD/PEALD stack method mitigates the shortcomings of both ALD processes while maintaining its advantages. Electrical measurements indicate that the stack method alongside NH3 plasma pretreatment provides the best characteristics in terms of hysteresis, threshold voltage, forward bias robustness, and interface trap density of states.

Crystallinity-Controlled Atomic Layer Deposition of Ti-Doped ZrO2 Thin Films

We investigated Ti-doped ZrO2 deposition using a cyclopentadienyl tris(dimethylamino) zirconium (CpZr(NMe2)3) precursor and a titanium tetraisopropoxide (TTIP) precursor using an O3 thermal atomic layer deposition process. In addition, the effect of Ti doping concentration on the chemical bonding and electrical properties of the Ti-doped ZrO2 thin films was studied. O3 was used at a high concentration of 400 g m−3. We varied the Ti doping concentration by controlling the rate of the supercycle process in the ZrO2 process window of 200 °C–300 °C. As a result, the highest dielectric constant was observed at a Ti doping concentration of 2.5% because it enhances the crystallinity of ZrO. Excessive Ti doping hinders crystal formation.

(Invited) Nanostructured Solid State Battery Architectures Enabled by Atomic Layer Deposition

Solid state batteries (SSBs) are considered the most promising next-generation energy storage technology both due to their increased safety over conventional wet Li-ion cells and their ability to fabricate novel architectures with simultaneous high energy and power density. However, the ionic conductivities of many solid electrolyte materials are orders of magnitude lower than liquid carbonate-based electrolytes. In order to offer SSBs with competitive charge/discharge rates, SSB active layers need to be thin (for fast ion diffusion) but have a large surface area (to enable large ion flux from anode to cathode). These geometric constraints establish the need for 3D electrode (and solid electrolyte) structures for a high performance battery architecture. Atomic layer deposition (ALD) is ideally suited to fabricate such devices due to its highly conformal nature and Angstrom-scale thickness control, however many challenges remain with materials fabrication and process integration.I will discuss development of both plasma-enhanced ALD (PEALD) and thermal ALD processes for the Li, Na, and K alkali polyphosphazene (A*PON) electrolytes1,2,3,4. The PEALD A*PON processes, by virtue of their 4-precursor synthesis route, allow some degree of chemical composition tuning. The thermal A*PON processes, unlike their PEALD counterparts, have a predetermined P-N ratio fixed by use of the precursors, however there is a tradeoff between the low temperature deposition enabled using plasma-based A*PON processes, and the conformality offered using the thermal A*PON processes. I will discuss and compare the reaction mechanisms and growth behavior among the entire family of A*PON solid electrolyte thin films, as well as discuss the integration challenges on the path to developing all-ALD planar and 3D SSBs. Pearse, A.; Schmitt, T.; Sahadeo, E.; Stewart, D. M.; Kozen, A.; Gerasopoulos, K.; Talin, A. A.; Lee, S. B.; Rubloff, G. W.; Gregorczyk, K. E., Three-Dimensional Solid-State Lithium-Ion Batteries Fabricated by Conformal Vapor-Phase Chemistry. ACS Nano 2018, 12 (5), 4286-4294. Pearse, A. J.; Schmitt, T. E.; Fuller, E. J.; El-Gabaly, F.; Lin, C.-F.; Gerasopoulos, K.; Kozen, A. C.; Talin, A. A.; Rubloff, G.; Gregorczyk, K. E., Nanoscale Solid State Batteries Enabled by Thermal Atomic Layer Deposition of a Lithium Polyphosphazene Solid State Electrolyte. Chemistry of Materials 2017, 29 (8), 3740-3753. Nuwayhid, R. B.; Jarry, A.; Rubloff, G. W.; Gregorczyk, K. E., Atomic Layer Deposition of Sodium Phosphorus Oxynitride: A Conformal Solid-State Sodium-Ion Conductor. ACS Applied Materials & Interfaces 2020, 12 (19), 21641-21650. Kozen, A. C.; Pearse, A. J.; Lin, C-F.; Noked, M.; Rubloff, G. W.; Atomic Layer Deposition of the Solid Electrolyte LiPON. Chemistry of Materials 2015, 27 (15), 5324-5331.

Thermal Atomic Layer Deposition of Gallium Nitride Films Using Tris(dimethylamido)Gallium and Ammonia

Gallium nitride (GaN) is a III-V semiconductor material with a wide and direct bandgap, relatively high carrier mobility, and high breakdown voltage. GaN is suitable for applications in optoelectronic devices and power devices, including light-emitting diodes (LEDs) and high electron mobility transistors (HEMTs). Typically, epitaxial GaN layers are grown by metal-organic chemical vapor deposition (MOCVD) at high deposition temperatures. However, low-temperature growth of GaN is necessary for temperature-sensitive devices and substrates. Atomic layer deposition (ALD) is a technique that offers unique advantages, such as low process temperature, excellent conformality, and atomic-level thickness control.In this study, we investigated the thermal ALD process of GaN at a low temperature of 200°C. Tris(dimethylamido)gallium (TDMAGa) was selected as the Ga precursor because tris(dimethylamido)titanium and tris(dimethylamido)aluminum were successfully adopted in the low-temperature ALD process of TiN and AlN thin films [1]. The growth and properties of ALD GaN films were investigated. ALD GaN films exhibited a self-limiting reaction with a deposition rate of 1.34 Å/cycle at 200°C, with a refractive index of 2.07. The saturation times for TDMAGa and NH3 pulses were 5 s and 30 s, respectively. Deposition at 250°C showed a deposition rate higher than 3 Å/cycle due to the thermal decomposition of TDMAGa. The ALD GaN film deposited at 200°C was amorphous and nitrogen deficient. To improve the crystallinity and composition, the as-deposited GaN films were annealed in an NH3 atmosphere in the deposition chamber without air exposure. The effects of annealing on the crystallinity and composition of the films were investigated. Acknowledgments This work was supported by Samsung Display Co., Ltd.

(Invited) Thermal Atomic Layer Deposition of Elemental Antimony and Bismuth Films

The elements antimony (Sb) and bismuth (Bi) are important components of many thin film materials. In many applications, films need to be grown in high aspect ratio features with perfect conformality and Angstrom-level control of film thicknesses. Atomic layer deposition (ALD) is a method that can afford uniform thickness and conformal films, even in narrow and deep nanoscale features. To date, there have been several ALD processes reported for elemental Sb films using SbCl3 and Sb(SiR3)3 (R = Me, Et), but no reports of Bi films grown by ALD have been published.1,2 Herein, several families of thermal ALD processes will be described for the growth of elemental Sb and Bi films. Precursors that were explored for Sb and Bi include SbCl3, BiCl3, BiPh3, and Bi(NMe2)3. Reducing precursors encompassed 2-methyl-1,4-bis(trimethylsilyl)-2,5-cyclohexadiene,3 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine,3 and nitrogen sources such as ammonia, hydrazine, alkyl hydrazines, and alkyl amines. The processes employing the nitrogen co-reactants are proposed to occur through unstable SbN and BiN, which then decompose with loss of N2 to afford elemental Sb and Bi films. Self-limited Sb and Bi film growth was established between 25 and 175 °C. High purity Sb (>98%) and Bi (>94%) films were demonstrated by X-ray photoelectron spectroscopy analysis. X-ray diffraction analyses revealed the growth of crystalline Sb and Bi films. The results of additional film characterization will be presented. Al Hareri, M.; Emslie, D. J. H. Chem. Mater. 2022, 34, 2400-2409.Pore, V.; Knapas, K.; Hatanpää, T.; Sarnet, T.; Kemell, M.; Ritala, M.; Leskelä, M.; Mizohata, K. Chem. Mater. 2011, 23, 247– 254.Klesko, J. P.; Thrush, C. M.; Winter, C. H. Chem. Mater. 2015, 27, 4918-4921.

Highly Robust Atomic Layer Deposition‐Indium Gallium Zinc Oxide Thin‐Film Transistors with Hybrid Gate Insulator Fabricated via Two‐Step Atomic Layer Process for High‐Density Integrated All‐Oxide Vertical Complementary Metal‐Oxide‐Semiconductor Applications

Highly reliable atomic layer deposition (ALD)‐derived In‐Ga‐Zn‐O thin‐film transistors with high field‐effect mobility (μFE) and hydrogen (H) resistivity are crucial for the semiconductor industry. Herein, a hybrid Al2O3 gate insulator (GI) is proposed that is designed by controlling the plasma‐enhanced ALD and thermal ALD processes in situ to demonstrate robust characteristics. A hybrid GI is applied to the top‐gate geometry of an In0.71Ga0.08Zn0.21O active layer. The optimal device exhibits exceptional electrical characteristics, including a threshold voltage of 0.37 V, μFE of 150.7 cm2 V s−1, subthreshold swing of 64.0 mV decade−1, and hysteresis of 0.02 V. It demonstrates high resistance to H annealing and reliable positive bias temperature stress, as well as changes in VTH shifts of −0.43 and 0.00 V, respectively. The excellent electrical characteristics and high robustness of the device can be attributed to the precise control of H, oxygen, and carbon species within the upper region of the hybrid GI. The achievement of robust device characteristics enables the design of a novel vertical complementary metal‐oxide‐semiconductor inverter that exhibits a voltage gain of 44.7 V V−1 and a noise margin of 87.5% at a 10 V supply voltage.

LixCoyOz thin-films deposition through thermal atomic layer deposition

3D-Li ion batteries are identified as one of the most promising technologies for improving portable and safe energy storage devices. One of the main remaining challenges to be tackled in that regard is the manufacture of efficient nanostructured electrode materials. In this paper, we report on the first realization of an electrochemically active cathodic LixCoyOz material grown via a thermal atomic layer deposition process based on the combination of Co(thd)2 and Li(thd) organometallic ligands and O3 as an oxidizing agent. Comprehensive characterizations comprising XPS, Raman, HIM (helium ion microscopy)-SIMS, and the first ever SEM images of a thermal-atomic layer deposition (ALD) deposited LixCoyOz material are shown and discussed as well and the very first electrochemical results to attest the electrochemical activity of the deposited material. Those results act as the first demonstration that lithiated materials and more precisely, LixCoyOz, can be grown via an advanced thermal ALD.

Plasma-enhanced atomic layer deposition of crystalline Ga2S3 thin films

Gallium (III) sulfide is a frontrunner for many energy storage and optoelectronic applications, which demand a deposition technique that offers a high level of control over thickness, composition, and conformality. Atomic layer deposition (ALD) is a potential technique in this regard. However, the state-of-the-art ALD processes for depositing Ga2S3 often lead to films that are amorphous and nonstoichiometric, and contain significant contaminations. Herein, we present a new plasma-enhanced atomic layer deposition (PE-ALD) process using the hexakis(dimethylamido)digallium precursor and H2S plasma coreactant to deposit high-quality Ga2S3 sulfide thin films and compare it to the thermal ALD process using the same reactants. While both cases exhibit typical ALD characteristics, substantial disparity is observed in the material properties. The PE-ALD process deposits crystalline Ga2S3 sulfide thin films at a temperature as low as 125 °C with a growth per cycle of 1.71 Å/cycle. Additionally, the PE-ALD process results in smooth and stoichiometric Ga2S3 films without any detectable carbon and oxygen contamination. Grazing incidence wide-angle x-ray scattering analysis indicates that the as-deposited Ga2S3 film crystallizes in a cubic structure with a preferred orientation along the [111] direction. The Ga2S3 film exhibits a transmittance of 70% and a bandgap of 3.2 eV with a direct transition.

ALD deposited bipolar HfOx films for silicon surface passivation

Hafnium oxide thin films consist of trap states which can degrade or enhance the device performance depending upon the application. These trap states play a crucial role in silicon surface passivation when using HfOx films. This paper investigates the dependence of crucial surface passivation parameters on different film growth components of thermal ALD process. Our study is focused on understanding different oxide charges present at the HfOx/Si interface using capacitance-voltage and constant voltage stress studies; particularly trap states introduced during thermal atomic layer deposition process. This study is performed for different film growth conditions to understand the origin of traps states. We show that the interfacial layer (IL) grows during the film deposition and therefore two interfaces are formed, i.e., HfOx/SiOx and SiOx/Si. Acting as the tunnelling oxide, this interlayer consists of trap states which are intrinsic in origin. The investigation on MOS structures fabricated using n and p type silicon substrates gives an understanding towards amphoteric nature of the trap states. Their nature (acceptor/donor) depends on the substrate doping type (n-Si/p-Si) giving it bipolar characteristic. However, the interaction of such trap states with the majority carrier band edge of the substrate strongly depends on the IL thickness. Additionally, the trap charge density increases with metal pulsing duration, suggesting that the metal precursor may be the source of these trap charges rather than the oxidant. By shedding light on the nature and behaviour of trap states in HfOx films, our study contributes to a deeper understanding of their impact on device performance. This knowledge can aid in optimizing surface passivation strategies and improving the overall performance of HfOx-based devices in various applications.

On the limitations of thermal atomic layer deposition of InN using ammonia

Chemical vapor deposition of indium nitride (InN) is severely limited by the low thermal stability of the material, and, thus, low-temperature deposition processes such as atomic layer deposition (ALD) are needed to deposit InN films. The two chemically and structurally closely related materials—aluminum nitride and gallium nitride (GaN)—can be deposited by both plasma and thermal ALD, with ammonia (NH3) as a nitrogen precursor in thermal processes. InN, however, can only be deposited using plasma ALD, indicating that there might be a limitation to thermal ALD with NH3 for InN. We use quantum-chemical density functional theory calculations to compare the adsorption process of NH3 on GaN and InN to investigate if differences in the process could account for the lack of thermal ALD of InN. Our findings show a similar reactive adsorption mechanism on both materials, in which NH3 could adsorb onto a vacant site left by a desorbing methyl group from the surfaces. The difference in energy barrier for this adsorption indicates that the process is many magnitudes slower on InN compared to GaN. Slow kinetics would hinder NH3 from reactively adsorbing onto InN in the timeframe of the ALD growth process and, thus, limit the availability of a thermal ALD process.

Reductive Thermal Atomic Layer Deposition Process for Gold

In this work, we developed an atomic layer deposition (ALD) process for gold metal thin films from chloro(triethylphosphine)gold(I) [AuCl(PEt3)] and 1,4-bis(trimethylgermyl)-1,4-dihydropyrazine [(Me3Ge)2DHP]. High purity gold films were deposited on different substrate materials at 180 °C for the first time with thermal reductive ALD. The growth rate is 1.7 Å/cycle after the film reaches full coverage. The films have a very low resistivity close to the bulk value, and a minimal amount of impurities could be detected. The reaction mechanism of the process is studied in situ with a quartz crystal microbalance and a quadrupole mass spectrometer.

Chemical and electronic properties of interfaces between RuO2 and Hf0.5Zr0.5O2 studied by x-ray photoelectron spectroscopy

Recently, the extensive study of the platinum group metal oxide electrodes for TiN replacement in Hf0.5Zr0.5O2 (HZO)-based ferroelectric memory stacks began. In this work, we use high sensitivity of x-ray photoelectron spectroscopy to the interface chemical and electronic states to investigate the chemistry and band alignment between HZO and RuO2, which is one of the most suitable noble metal oxide electrodes. We report that RuO2 is easily reduced to metallic Ru during the atomic layer deposition (ALD) of HZO because of the chemical reaction between the oxygen stored in RuO2 and metallic precursors used for HZO deposition. This process makes it impossible to use RuO2 as a bottom electrode, when HZO is grown by the conventional thermal ALD process (with H2O as a reactant). At the same time, RuO2 may be effectively conserved if one uses more active oxygen reactants for the HZO growth because of the re-oxidation of the newly formed Ru. RuO2 is also effectively conserved at the top interface, at least within the thermal budget of the growth of all functional layers. We also report the detailed investigation of the band alignment between RuO2 and HZO in both type of stacks (with reduced and conserved RuO2) and both interfaces with HZO (top and bottom), which may be useful for the understanding of the ferroelectric properties of the related stacks in the future. In addition, the potential of RuO2 utilization for the reliability improvement of HZO-based stacks is discussed.

Ionic Liquid-Mediated Route to Atomic Layer Deposition of Tin(II) Oxide via a C-C Bond Cleavage Ligand Modification Mechanism.

Atomic layer deposition (ALD) is a technologically important method to grow thin films with high conformality and excellent thickness control from vapor phase precursors. The development of new thermal ALD processes can be limited by precursor reactivity and stability: reaction temperature and precursor design are among the few variables available to achieve higher reactivity in gas-phase reactions, unlike in solution synthesis, where the use of solvent and/or a catalyst can promote a desired reaction. To bridge this synthesis gap between vapor-phase and solution-phase, we demonstrate the use of an ultrathin coating layer of a vapor phase-compatible solvent─an ionic liquid (IL)─on our growth substrate to perform ALD of SnO. Successful SnO deposition is achieved using tin acetylacetonate and water, a process that otherwise would require a stronger counter-reactant such as ozone. The presence of the layer of IL allows a solvent-mediated reaction mechanism to take place on the growth substrate surface. We report a growth per cycle of 0.67 Å/cycle at a deposition temperature of 100 °C in an IL comprising 1-ethyl-3-methylimidazolium hydrogen sulfate. Characterization of the ALD films confirms the SnO film composition, and 1H and 13C NMR are used to probe the solvent-mediated ALD reaction, suggesting a solvent-mediated addition-elimination-type mechanism which breaks a C-C bond in acetylacetonate to form acetone and acetate. Density functional theory calculations show that the IL solvent is beneficial to the proposed solvent-mediated mechanism by lowering the C-C bond cleavage energetics of acetylacetonate compared to the vapor phase. A general class of ligand modification reactions for thermal ALD is thus introduced in this work.

Atomic layer deposition of titanium phosphate onto reinforcing fibers using titanium tetrachloride, water, and tris(trimethylsilyl) phosphate as precursors

A thermal atomic layer deposition process with precursors tris(trimethylsilyl) phosphate (TTMSP), titanium tetrachloride (TiCl4), and water was used with various pulse sequences in order to deposit titanium phosphate onto bundles of carbon fibers (diameter of one filament = 7 μm, 6000 filaments per bundle) and flat silicon substrates. Pulse sequence 1, TTMSP/N2/TiCl4/N2, which comprises no water, yields no significant deposition. Pulse sequence 2, TTMSP/N2/H2O/N2/TiCl4/N2, which comprises a water pulse, yields a mixed phosphate/oxide coating and shows a self-limiting character at 200 °C with a growth per cycle of 0.22 nm cycle−1. Wet chemical analysis of the coating revealed a ratio of Ti:P between 3:1 and 2:1 in reasonable agreement with the composition Ti2.4P1O7 obtained from X-ray photoelectron spectroscopy. Thus, the deposited material can approximately be described as a mixture of Ti¾PO4 and TiO2 in a molar ratio of 1:1.5. The coating shifts the temperature of the onset of oxidation—3% weight loss in thermogravimetry—of the carbon fibers from 630 °C (uncoated C-fiber) to 750 °C (with the titanium phosphate coating).

Inhibiting Thermal and O2 Plasma Assisted ALD of SiO2 using Fluorothiol Passivation Layer on Cu

Atomic layer deposition (ALD) offers the potential for area-selective deposition of patterned structures to enable a bottom-up fabrication of semiconductor devices. Area-selective ALD can be achieved by selectively attaching a blocking molecule to a specific surface. SiO2 ALD is unique in the sense that O3 or an O2 plasma is preferable for high throughput and cleaner process: Halogenated silane such as SiCl4 and H2O/pyridine process yield pyridinium salt as byproduct. The need for O3 or O radicals for growth makes the use of organic blocking layers to inhibit growth challenging as hydrocarbons get rapidly combusted in the presence of O radicals. In this study, we explore area-selective, O2-plasma-assisted ALD of SiO2 on SiO2 while inhibiting growth on Cu with fluorinated thiols that are resistant to combustion.The surface species during the selective ALD on the growth (SiO2) and nongrowth (Cu) surfaces was monitored using in situ reflection-absorbance infrared spectroscopy (RAIRS). SiO2 ALD was done at 100 °C using di-sec-butyl-aminosilane (DSBAS) as the Si precursor and a remote O2 plasma. The nongrowth surface, CMP Cu with the native oxide (CuO x ), was functionalized with 1H,1H,2H,2H-perfluorodecanethiol (Step A) at 100 °C. The in situ infrared spectrum of the Cu surface after functionalization showed a sharp increase in absorbance in the CF2 stretching region (~1250 cm-1) indicating thiol attachment to CuO x . The functionalized Cu surface was then exposed to DSBAS (Step B) and O3 or O2 plasma (Step C). The corresponding infrared spectra showed no surface O-SiH3 species after the DSBAS cycle. In the case of thermal ALD, ozone removed ~50% of thiol while O2 plasma removed only ~15%. To maintain the inhibitor coverage on the surface, PFDT was redosed in the subsequent cycle for both thermal and O2 plasma assisted processes. This A-B-C type ALD process (A: PFDT, B: DSBAS, C: O3 or O2 plasma) (see Fig. 1(a) and (b)) was repeated for 45 cycles for thermal ALD and 35 cycles for O2 plasma assisted case which yielded ~5 nm of growth on the SiO2 surface. In the thermal ALD process, in the initial 10-15 A-B-C cycles, the amount of PFDT adsorbed onto the surface gradually increased [see Fig 1(b)]. The most likely reason for the increase in PFDT absorption is that the Cu surface is oxidized by O3, which promotes absorption of PFDT in multilayers. In contrast, in the O2 plasma assisted process, only a small fraction of the inhibitor layer was etched in the O2 plasma in a self-limiting manner. Surprisingly, during redosing, nearly the same amount of PFDT was replenished on the surface. This removal and redosing sequence could be maintained for at least 20 ALD cycles.No evidence of SiO2 deposition on the Cu surface was observed with RAIRS and x-ray photoelectron spectroscopy (XPS) for both thermal and O2 plasma assisted process after 35 A-B-C cycles, as shown in Fig. 1 (c) and (d), respectively. Characterization of surface morphology after the O2 plasma assisted selective deposition process with atomic force microscopy (AFM) revealed no evidence of damage or roughening of the Cu surface. After ALD, the inhibitor on the Cu surface was decomposed by high temperature annealing followed by cleaning of the residue with a proprietary formulation. The XPS data after cleaning indicated that Cu surface was reduced with no detectable SiO2, as shown in Fig. 1 (e) and (f). Finally, with cross section transmission electron microscopy and elemental mapping with energy-dispersive x-ray spectroscopy, we demonstrated area-selective O2-plasma-assisted ALD of 10 nm of SiO2 on SiO2 in patterned Cu/SiO2 substrates. Figure 1

Atomic Layer Deposition of Li4Ti5O12: Enabling Ultrafast Charging 3D Thin-Film Batteries

Atomic layer deposition (ALD) of lithium (Li)-compound thin-films has aroused significant interest in recent years. Promising applications are Li-ion thin-film batteries (TFB), protective particle coatings, interface model systems, and neuromorphic computing [1,2]. Here, we demonstrate the ultrafast charging with long-cycle life of Li4Ti5O12 (LTO) fabricated by ALD. The “zero-strain” anode will enable on-chip three-dimensional (3D) solid-state thin-film batteries to power upcoming autonomous sensor systems.The simultaneous increase of power and energy density of full cell 3D TFBs by coating the battery layer stack over microstructured substrates was recently demonstrated [3]. The required conformal, pinhole-free deposition and stoichiometric control of nanometer-thin films on highly structured surfaces are only accessible via ALD. The vapor-phase technique based on sequential, self-limiting surface reactions is well understood. However, the direct deposition of Li-compound anodes remains challenging [1].In this work, ALD LTO is electrochemically evaluated for the first time. The study is based on a thermal three-step ALD process for Li-containing mixed oxides on 200 mm silicon wafers [4]. This process suppresses the undesired hygroscopic nature of lithium hydroxide during deposition with water as a co-reactant. Lithium-tert-butoxide (LTB) and lithium hexamethyldisilazide (LiHMDS) are both proven as suitable precursors forming high-quality spinel LTO with low impurities after rapid thermal processing. The substrate has a strong influence on the crystallization and the film texture, determining the electrochemical behavior. Planar 30 nm films reveal capacities close to the theoretical value of 600 mAh cm-3 at 1 C. Excellent fast dis-/charging capability is achieved by optimized film texture obtaining around 50 % of the maximum capacity at extreme rates of 200 C. Outstanding cycle performance with high Coulomb efficiency is demonstrated, exhibiting only 2% capacity loss after 1000 cycles at 100 C. Excellent conformality, adhesion, and recent electrochemical results of ALD LTO in a 3D hole structure with high aspect ratios of up to 20:1 will be discussed. The superior performance of ALD LTO, compared to other deposition techniques, is a key enabler for scalable high-volume production of high-power 3D TFBs.

Demonstration of Resistive Pt Temperature Sensors Directly Printed Using Novel Atomic-Layer 3D Printer

This case study demonstrates resistive Pt temperature sensors fabricated by a novel Atomic Layer 3D-printer, a revolutionary rapid prototyping tool utilizing a combination of atomic layer deposition (ALD), microfluidics and high precision 3D printing [1], first time introduced at AVS ALD 2020 [2], and also discussed in detail in the ECS 2021 G01 symposium [3]. The printed sensors are characterized and compared to a thin film sensor made using conventional (physical vapor deposition, lithography) methods and also to a Pt100 standard.ALD works like a chemical vapor deposition (CVD) but the precursors react with the substrate one at a time, in a sequential, self-limiting manner, offering thickness control on the level of atomic layers. Combining ALD and 3D printing, the Atomic Layer 3D-printer allows for deposition of thin films with controllable thickness in the sub-nm range in a pre-programmed X-Y pattern, with no need for lithography or additional patterning, on a wide variety of substrates.The printer nozzle moves relative to the substrate in a highly controlled manner (in this case ~2 mm/s). The nozzle is a miniature spatial ALD system where the precursor (MeCpPtMe3 in this case) flows out of the center of the nozzle, surrounded by a concentric rings of vacuum and a reactant gas, in this case ozone (O3). The result is an area-selective ALD, where each nozzle pass over the substrate equals to one ALD cycle, with a typical deposition rate of ~0.9 nm/cycle.In this study, resistive Pt temperature sensors were fabricated on the SiO2/Si substrates by printing >2 mm long Pt-wires of different thicknesses (approx. range 10-40 nm thick) using 100-500 ALD cycles at 200, 225 and 250 °C, where the width of the wires (~400 µm) was defined by the used nozzle geometry (Fig. 1 shows the fabrication flow).Morphology studies of the Pt surface by scanning electron microscopy (Fig. 2) revealed that the film growth in the initial stages follows island-like nucleation, eventually becoming progressively denser with increasing number of ALD cycles. The Pt wire deposited using 200 cycles (nozzle passes) consist of interconnected network of Pt grains, while the one made using 400 cycles (~ 30 nm thick) consists of a continuous polycrystalline Pt film. This growth mechanism is typical for this thermal ALD (Pt+O3) process [4].Sensors were annealed at 600 °C in N2 to ensure temperature stability, and equipped with Au contact pads 2 mm apart (made by e-beam PVD through a shadow mask) for electrical characterization. The resistance was measured in the 25-400 °C range (selected characteristics shown in Fig. 3a), as well as in cryogenic temperatures down to 3 K (not shown here).Interestingly, higher temperature sensitivity S (Fig. 3b) was found for samples made by 200 ALD cycles, with a morphology of a network of interconnected Pt grains.The comparison of temperature coefficients of resistivity α (normalized sensitivity, Fig. 3c) clearly shown that the printed ALD Pt demonstrate better temperature sensing characteristics than the conventional 30 nm e-beam PVD Pt thin film, and is comparable also to the Pt100 standard.We will also discuss sensors printed on corrugated surfaces, i.e. black Si and Si gratings.The resistive Pt temperature sensors demonstrated in this study fabricated by a novel Atomic Layer 3D-printer are suitable for rapid printing in small-footprint areas where temperature sensing is required. Targeted, low temperature deposition with no need for lithographic masks or additional patterning paves the way for myriads of applications, e.g. on a chip, or in a battery. The characteristics of these sensors are shown to be comparable to a thin film sensor fabricated using conventional processing, as well as a Pt100 standard, thanks to high film quality delivered by the ALD process.We acknowledge the support of the H2020-EU ATOPLOT project (grant ID: 950785).

Excellent surface passivation of germanium by a-Si:H/Al2O3 stacks

Surface passivation of germanium is vital for optimal performance of Ge based optoelectronic devices especially considering their rapidly increasing surface-to-volume ratios. In this work, we have investigated the surface passivation of Ge by a stack consisting of a thin layer of hydrogenated amorphous silicon (a-Si:H) and an aluminum oxide (Al2O3) capping layer. Plasma-enhanced chemical vapor deposition was used to deposit the a-Si:H (0–10 nm), while thermal and plasma-enhanced atomic layer deposition (ALD) were employed for the Al2O3 films (0–22 nm). Transient photoconductance decay measurements revealed a recombination velocity as low as 2.7 cm s−1 for an a-Si:H layer as thin as 1.8 nm and an Al2O3 film of only ∼6 nm. In this state-of-the-art passivation scheme, the plasma-enhanced ALD process for the Al2O3 capping layer proved superior to the thermal ALD process since it resulted in an exceptionally high negative fixed charge density (Qf ∼ 1013 cm−2), which proved a key factor for the low surface recombination velocity. Transmission electron microscopy and energy x-ray dispersion revealed that a thin SiOx layer (∼1.4 nm) forms between a-Si:H and Al2O3 during the ALD process, which is thought to be the origin of this high negative fixed charge density. This passivation stack is regarded as highly interesting for applications such as solar cells, nanolasers, and nano-LEDs based on p-type Ge.

Cobalt Metal ALD: Understanding the Mechanism and Role of Zinc Alkyl Precursors as Reductants for Low-Resistivity Co Thin Films

In this work, we report a new and promising approach toward the atomic layer deposition (ALD) of metallic Co thin films. Utilizing the simple and known CoCl2(TMEDA) (TMEDA = N,N,N′,N′-tetramethylethylenediamine) precursor in combination with the intramolecularly stabilized Zn aminoalkyl compound Zn(DMP)2 (DMP = dimethylaminopropyl) as an auxiliary reducing agent, a thermal ALD process is developed that enables the deposition of Zn-free Co thin films. ALD studies demonstrate the saturation behavior of both precursors and linearity depending on the applied number of cycles as well as temperature dependency of film growth in a regime of 140–215 °C. While the process optimization is carried out on Si with native oxide, additional growth studies are conducted on Au and Pt substrates. This study is complemented by initial reactivity and suitability tests of several potential Zn alkyl-reducing agents. For the CoCl2(TMEDA)–Zn(DMP)2 combination, these findings allow us to propose a series of elemental reaction steps hypothetically leading to pure Co film formation in the ALD process whose feasibility is probed by a set of density functional theory (DFT) calculations. The DFT results show that for reactions of the precursors in the gas phase and on Co(111) substrate surfaces, a pathway involving C–C coupling and diamine formation through reductive elimination of an intermediate Co(II) alkyl species is preferred. Co thin films with an average thickness of 10–25 nm obtained from the process are subjected to thorough analysis comprising atomic force microscopy, scanning electron microscopy, and Rutherford backscattering spectrometry/nuclear reaction analysis as well as depth profiling X-ray photoemission spectroscopy (XPS). From XPS analysis, it was found that graphitic and carbidic carbon coexist in the Co metal film bulk. Despite carbon concentrations of ∼20 at. % in the Co thin film bulk, resistivity measurements for ∼22 nm thick films grown on a defined SiO2 insulator layer yield highly promising values in a range of 15–20 μΩ cm without any postgrowth treatment.

Multilayer ultraviolet reflective coating based on atomic layer deposited aluminum oxide and fluoride

Ultraviolet optical coatings employ wide bandgap dielectric materials due to their characteristic low absorption. High-reflectivity and antireflective coatings are essential for optical devices, which can be achieved by alternately depositing two dielectrics with different refractive indices. In this research, a multilayer high-reflectivity coating has been designed for middle UV wavelengths using Al2O3 and AlF3 layers on a sapphire (0001) substrate, and the initial two-layer structure has been fabricated by atomic layer deposition. The surface morphology and roughness of the coating was measured by atomic force microscopy after each deposition step. Ultraviolet spectroscopy and spectroscopic ellipsometry were used to characterize the optical performance of the single and multilayer coatings. Monochromatic x-ray photoemission spectroscopy was used to study the film composition, bonding, and impurities. A bilayer reflective coating was demonstrated, with a smooth surface (Rq < 1 nm) and peak reflectance of 25%−30% at a wavelength of 196 nm. The measured reflectance deviated from the simulations in the middle UV range, and an analysis of the AlF3 layer prepared by plasma enhanced atomic layer deposition indicated the presence of Al-rich clusters, which were associated with the UV absorption. A thermal atomic layer deposition process for AlF3 deposition showed reduced absorption, which could be more effective for shorter wavelength designs.