Self-limiting Surface Reactions Research Articles

Low temperature (002)-oriented zinc oxide films prepared using ozone-based spatial atomic layer deposition

In this study, ZnO films are prepared at a low temperature of 150 °C by using spatial atomic layer deposition (sALD) with diethylzinc and ozone as precursor and oxidant. The ozone flow rate is varied to systematically investigate its effect on optical, structural and electrical properties. The experimental results show that the self-limiting surface reactions can occur at the low ozone flow rate of 200 sccm, confirming ALD growth mode. All the ozone flow rates lead to ZnO (002) crystalline orientation, which is difficult to be obtained for conventional water-based ALD ZnO films at the low temperature. The increased ozone flow rate results in an increased amount of oxygen vacancies, enhanced carrier concentration and a reduced stress. The resistivity and carrier concentration can be tuned in the range of 4.75–0.08 Ω-cm and (1.1–5.5) × 1018 cm−3. Finally, the sALD ZnO film on polyethylene terephthalate reveals a high stability in the film property against bending. This study is beneficial for the utilization of ZnO films in optoelectronic devices that demand the (002) preferred orientation, especially under low-temperature conditions.

Area-selective atomic layer deposition (AS-ALD) of low temperature (300 °C) cobalt thin film using octadecyltrichlorosilane (ODTS) self-assembled monolayers (SAMs)

As the semiconductor industry advances, manufacturing technologies encounter challenges in achieving precise alignment and thickness control with smaller and more intricate 3D structures. Addressing this, Area-selective atomic layer deposition (AS-ALD) emerges as a promising “bottom-up” approach, offering an alternative to current nano-patterning techniques. To achieve AS-ALD, self-assembled monolayers (SAMs) are employed as inhibitor agents. In this study, we employed plasma-enhanced atomic layer deposition (PEALD) to promote self-limiting surface reactions and enable low-temperature processing, thereby facilitating the implementation of high purity cobalt (Co) film through AS-ALD using octadecyltrichlorosilane (ODTS) SAM as an inhibitor. The selective deposition is achieved by employing SAMs as an inhibitor, belonging to the deactivation process, a common method for realizing a bottom-up approach. SAMs can form a thin organic film on a solid surface, allowing for surface modifications to become hydrophobic or hydrophilic. Through the selective deposition of Co in the PEALD low-temperature process (300 °C), we successfully addressed the degradation issue of ODTS SAM and verified the successful achievement of a high-purity selectively deposited 13.4 nm Co film. Our findings highlight the potential of selective Co deposition for realizing complex 3D structures and narrow interconnection layers.

Solution ALD Processing of Halide Perovskite Thin Films Yields Superior Functional Quality and Stability Than Classical Processing

The range of materials accessible by classical ALD is limited by the volatility requirement applied to the precursors. However, solution ALD (sALD) transfers the self-limiting surface reaction principle of ALD into the solution processing realm and allows for the same level of control in the deposition of ionic or molecular solids, such as polymers, metal-organic frameworks, and halide perovskites, which cannot be delivered from the gas phase.We have developed an sALD procedure for the direct deposition of the prototypical halide perovskite compound, methylammonium triiodoplumbate(IV), (H3CNH3)(PbI3), also referred to as methylammonium lead iodide or MAPI. The process saturates upon dosage variation to a self-limiting growth rate. The material is obtained in crystalline and highly pure, stoichiometric form at room temperature. All analytical techniques confer to indicate the absence of any lead iodide trace. The MAPI layers generated by sALD can be integrated into planar heterojunction stacks and yield functional solar cells.The direct comparison of films of several thickness values prepared in pairs by sALD and by a state-of-the-art spin-coating method shows that the sALD films always overperform their spin-coated counterparts both in terms of charge carrier lifetimes and in terms of stability towards decomposition under thermal, radiative or vacuum stress.Thus, experimental atomic-level control of solution-processed semiconductors is accessible for the first time. It can now be applied to the accurate investigation of halide perovskite photophysics and photochemistry fundamentals as they pertain to transport and interface phenomena.

Multiscale computational fluid dynamics modelling of spatial ALD on porous li-ion battery electrodes

The self-limiting surface reaction characteristic of atomic layer deposition (ALD) makes it ideal for the surface modification of electrode materials for lithium-ion batteries (LIBs). Spatial ALD shows promise as a scalable method for the coating on pre-fabricated electrode sheets. As a strong-coupled multiscale process, various process conditions and microstructure parameters have great influences on the macroscale fluid dynamics and the pore-scale diffusion–reaction process, thus affecting the coating efficiency. This study presents a multiscale numerical model that combines computational fluid dynamics (CFD) with multilevel pore-scale diffusion–reaction kinetics to explore the spatial ALD process on porous LIB electrodes. The dynamic mesh method is utilized to simulate electrode movement. The considerable active surface-to-volume ratio of the porous structure limits the precursor infiltration depth due to the low diffusion rate and inadequate precursor supply. As the electrode velocity increases, an asymmetric distribution of precursor concentration under the injector is observed with a rapid decrease. Elevating both the precursor concentration and inlet gas velocity augments the coating depth by enhancing the supply of the precursor. The experimental data aligns well with our numerical results, verifying the accuracy of the multiscale CFD model. Our observations reveal that a relatively lower operating pressure, around 0.1 atm, compared to 0.01 atm and 1 atm, optimizes the deposition rate along the electrode depth during the half-ALD cycle, especially when the pore size is larger. Electrode porosity of about 0.4 notably improves coating uniformity by elevating the precursor diffusion rate. Predictions show that with a substrate velocity of 0.2 m/s, the coating depth on an electrode having higher porosity at the top compared to the bottom via atmospheric spatial ALD could reach a depth of 38 μm with a precursor utilization of 78 %.

Multi-objective optimization for inherently selective atomic layer deposition of zirconia on silicon and not on copper

Atomic layer deposition (ALD) deposits uniform, pinhole free metal or metal oxide films via a self-limiting surface reaction. Area-selective ALD allows selective deposition of these films on targeted areas. Other than using masking to prevent deposition on undesired areas of the substrate, the process parameters can be altered to achieve selectivity. However, determination of whether an ALD process is selective under the chosen reaction conditions requires rigorous experiments to be performed. ALD is a slow process and requires expensive organometallic complexes known as metal precursors. Simulation models built on the experimental data can eliminate the need for performing multiple experiments by predicting reaction conditions that promote selective growth thus saving time and cost. A little or no information was found in literature on models that can predict selectivity window of an ALD process by looking at process parameters. Our inhouse experimental data obtained from the inherently selective atomic layer deposition (ISALD) of zirconia on silicon and not on copper were used to build a simulation model that predicts selectivity of the ISALD process over a wide range of temperature. Multi-objective optimization algorithm was developed to find optimal process condition that can maximize the deposition rate on silicon while minimizing the same on copper. The theoretical framework is based on the equilibrium reached at the substrate surface after chemisorption of the precursor molecules. The chemisorption process is influenced by the interactions of the groups present in the precursor molecules and on the substrate surface. Universal Quasi-chemical Functional-group Activity Coefficient method for group contribution was used to quantify the interactions between different groups. Further, we show that the same group interaction parameters can be used with a different zirconium precursor having same functional groups to predict growth characteristics of zirconia on silicon under similar reaction conditions.

Molecular Layer Deposition of Organic–Inorganic Hafnium Oxynitride Hybrid Films for Electrochemical Applications

The molecular layer deposition (MLD) method can be used to deposit hybrid organic–inorganic films with precisely defined composition, flexible properties, and conformality on different substrates. In this study, hafnium-based organic–inorganic hybrid polymer films were studied as potential coatings for silicon nanoparticles (SiNPs) used in composite lithium-ion battery (LIB) anodes, an application which requires the film to be both flexible and stable under electrochemical conditions. Hf-hybrid films were successfully deposited by MLD using sequential exposure of the homoleptic tetrakis(dimethylamido) hafnium complex and ethanolamine as the reactants. The self-limiting surface reactions lead to a constant growth per cycle (GPC) of ∼2.0 Å/cycle at 120 °C. Temperature-dependent growth was observed, with the GPC decreasing from ∼2.5 to ∼1.1 Å/per cycle as the temperature was increased from 65 to 145 °C. Scanning transmission electron microscopy and electron energy loss spectroscopy mapping confirm that a thin, dense, and conformal Hf-based MLD layer is deposited on the SiNPs. The presence of expected C–N, C–O, and −CH2 moieties in the MLD films was confirmed by Fourier transform infrared spectroscopy. Hafnium nitride and hafnium oxide bonds within the hybrid thin films were identified by X-ray photoelectron spectroscopy. Characterization results indicated that the deposited hafnium-based organic–inorganic hybrid films contain both metal oxynitride bonds and organic bonds, including C–C, C–O, and C–N. This Hf-based MLD thin film was tested on LIB SiNP composite anodes as an artificial solid–electrolyte interphase, with results showing that the capacity retention increased by about 35% after 110 cycles in a LIB application.

Oxidative Molecular Layer Deposition of Conjugated Amine Polymer Thin Films

Conjugated amine polymers such as polypyrrole (pPy), and polyaniline (pAni) exhibit high electrochemical capacities, making them appealing as electrode materials for energy storage, electrochemical desalination, and chemical sensing. Nanoscale thin films of these polymers are attractive to provide lower weight, faster charging, and higher sensitivity in these applications. This report describes the first demonstration of molecular layer-by-layer growth of conjugated amine polymers using alternating gas-phase exposures of amine monomers and MoCl5 oxidant in a process termed oxidative molecular layer deposition (oMLD). Pyrrole (Py) and MoCl5 undergo self-limiting surface reactions by oMLD to form conformal pPy thin films, but oMLD using aniline (Ani) and p-phenylenediamine (PDA) monomers yield unexpected azo functionality. This is attributed to a MoCl5-amine surface adduct that directs polymerization near amine groups, producing azo groups for primary amines. oMLD pPy exhibits record 282 mAh/g capacity in aqueous electrolyte, and PDA/MoCl5 oMLD yields azo-polymers of interest as anode materials for alkali-ion batteries. Alternating between Py and PDA monomers during oMLD produces molecularly mixed copolymers with qualitatively different electrochemical responses from the isolated monomer structures. This work lays the foundation for the growth of conformal thin films of conjugated amine polymers with molecular-level control of composition and thickness.

(Invited) Investigating Surface Reaction Thermodynamics: In Situ Calorimetry for Atomic Layer Deposition

Atomic layer deposition (ALD) is comprised of sequential self-limiting surface reactions that are inherently sensitive to the surface chemistry of the substrate and the reactivity of the chosen precursors. In order to better understand ALD processes and direct their future development we must investigate the fundamental mechanisms of ALD reactions. This is commonly done using a combination of computational modeling and experimental investigations using techniques like spectroscopic ellipsometry and IR spectroscopy. We present pyroelectric calorimetry for ALD as an in situ technique to measure the time resolved heat generation from ALD reactions. This provides a new dimension of thermodynamic and kinetic reaction data.As an experimental technique, pyroelectric calorimetry measures ALD reactions on realistic surfaces as opposed to idealized models. We have designed and constructed custom calorimeters that are compatible with typical ALD process parameters. These calorimeters have been calibrated with a thermal resolution down to 0.1 μJ/cm2 and a temporal resolution of 50ns. To put this in perspective for an ALD reaction, this corresponds to about 0.1% of the heat evolved in the trimethylaluminum (TMA) and water ALD process and about 104 faster than our measurement of the TMA half reaction. The temporal resolution of our calorimeters is orders of magnitude faster than most complementary in situ analysis techniques including ellipsometry, IR spectroscopy, quartz crystal microgravimetry, and mass spectrometry. This time resolution provides information on precursor flow dynamics and reaction kinetics.Pyroelectric calorimetry was used to investigate the TMA and water half reactions for ALD Al2O3 along with in situ spectroscopic ellipsometry thickness measurements and Rutherford backscattering spectrometry (RBS) composition analysis. This enabled comparisons of the change in thickness with the amount of heat generated as each half reaction saturates. Atomic growth rates calculated form RBS were used to calculate the heat generated on a per-atom basis. These results were then compared with proposed reaction mechanisms and energy changes from density functional theory (DFT) computational modeling.Measurements of reaction and adsorption heats can also be used to compare and evaluate different precursors and inhibitor molecules for ALD growth inhibition in area-selective ALD. We investigated how the thermodynamics of adsorption can guide our choice of precursor-inhibitor pairs when using small molecule inhibitors that rely on competitive adsorption and chemical passivation to prevent ALD growth. Pyroelectric calorimetry offers new insight into what drives and limits ALD reactions and provides many opportunities for future investigations.

(Invited) Energy Enhanced Atomic Layer Deposition (EEALD)

Atomic layer deposition (ALD) is a layer by layer chemical vapor deposition (CVD) technique based on alternating purge-separated self-limiting surface reactions. ALD offers inherent atomic scale controlled growth of high quality conformal thin films at relatively low temperatures. A low thermal budget is often critical in microelectronics processing, such as for back-end-of-line processing, 3D integration, deposition on glass or flexible substrates for large area electronics, avoiding unwanted diffusion, and maintaining stable flat band voltages, effective work functions, threshold voltages, and effective oxide thickness in metal/insulator/metal (MIM) and metal/oxide/semiconductor (MOS) device structures. However, low deposition temperature can lead to incorporation of excess -OH groups or other residual impurities from unreacted ligands and result in poor stoichiometry of ALD films. Poor stoichiometry may in turn lead to sub-optimal physical, optical, and electrical properties. A number of approaches may be used to reduce impurities, increase density, improve properties, and achieve the desired morphology of ALD films. An obvious approach is to increase deposition temperature, however this may move a process out of the ALD window into the CVD regime, negating many of the benefits of ALD. The most common approach is post-deposition annealing (PDA) at elevated temperatures. The PDA temperatures that are typically required however, can exceed the maximum temperature limitations of the substrate or previously formed electronics. To maintain the low thermal budget of ALD while maximizing film properties, performing annealing during, rather than after, deposition can be beneficial. An alternate approach to help drive reactions and reduce impurity / ligand incorporation is thus to add extra energy as part of each (or every few) ALD supercycles. Methods to date include in-situ rapid thermal (MTA, DADA, etc.) annealing, flash lamp annealing, plasma exposure, and UV exposure. I will collectedly refer to these techniques as energy enhanced ALD (EEALD). [1-13] (Note that these methods are distinct from plasma enhanced ALD (PEALD), an excellent review of which can be found in [14].) Documented benefits of the various forms of EEALD include higher GPC, denser films, lower temperature, improved dielectric constant and refractive index, lower leakage, lower residual impurities. A potential downside of energy enhancement is the additional time required to implement these steps into the ALD super-cycle, particularly cool down time when in-situ annealing is incorporated. This invited talk will describe, compare, and contrast these various techniques focusing on mechanisms (thermal vs. chemical), placement in ALD supercycle, benefits and drawbacks, and challenges to be addressed in finding the ideal EE-ALD technique. Finally, I will introduce an entirely new method of EE-ALD. J.F. Conley, Jr., Y. Ono, D.J. Tweet, Appl. Phys. Lett. 84(11), 1913-1915 (2004).J.F. Conley, Jr., D.J. Tweet, Y. Ono, and G. Stecker, in High-k Insulators and Ferroelectrics for Advanced Microelectronic Devices, R.M. Wallace, D. Landheer, M. Houssa, and J. Morais, eds., MRS Proc. Vol. 811, 5 (2004).J.F. Conley, Jr., Y. Ono, D. Tweet, G. Stecker, R. Solanki, and W.W. Zhuang, in Physics and Technology of High-k Gate Diectrics II, S. Kar, R. Singh, D. Misra, H. Iwai, M. Houssa, J. Morais, and D. Landheer, eds., ECS Proc. Vol. 2003-22, 11 pgs.K.H. Holden, S.M. Witsel, P.C. Lemaire, and J.F. Conley, Jr. in preparation (2022).Henke et al., ECS J. Sol. Sta. Sci. Tech. 4(7), 277 (2015).R.D. Clark et al., ECS Transactions, 41(2), 79 (2011).Miikkulainen et al., ECS Tran. 80(3), 49 (2017).Chalker et al., ECS Tran. 69(7), 139 (2015).Kwak, Y.-H. Lee, and B.-H. Choi, Appl. Surf. Sci. 230, 249 (2004).Kim et al., Electrochemical and Solid-State Lett. 14(4), H146 (2011)No et al., J. ECS 153(6), F87 (2006).Shih et al., Sci. Rep. 7(1), 39717 (2017).Österlund et al. Vac. Sci. Tech. A 39, 032403 (2021).Ueda et al., Appl. Surf. Sci. 554, 149656 (2021).Profijt, et al., J. Vac. Sci. Technol. A 29(5), 050801 (2011).

Atomic Layer Deposition of Highly Stable Manganese-Iron Oxide Bifunctional Catalysts for Zinc-Air Batteries

Incorporating intermittent renewable energy sources into the power grid will require large amounts of grid-scale energy storage. Electrochemical batteries are a versatile and scalable energy storage option and, hence, Li-ion batteries have been widely adopted to store excess wind and solar energy [1]. Li-ion batteries, however, have a relatively low energy density and serious safety concerns. An alternative electrochemical battery option lies with zinc-air batteries. This technology uses lower cost materials and is overall much safer. Furthermore, zinc-air batteries have a much larger theoretical energy density than Li-ion batteries [2]. The major impediment to wide-scale adoption of zinc-air batteries is the low energy efficiency because of the poor reaction kinetics at the air electrode. Both the charge and discharge reactions at the air electrode are sluggish and require the use of catalysts to obtain practicable performance. However, many catalysts active towards the charge reaction are not active towards the discharge reaction, and vice versa. The development of a catalyst active towards both the charge and discharge reactions, known as a bifunctional catalyst, is therefore a high priority [3]. Furthermore, catalysts employed in zinc-air batteries often show instability, with performance degradation evident after a few cycles. Ultimately, a highly stable bifunctional zinc-air battery catalyst is of the utmost importance.The aim of this work is to develop highly stable bifunctional catalysts for zinc-air batteries using atomic layer deposition (ALD). With ALD, extremely conformal catalyst coatings can be deposited directly on the air electrode of a zinc-air battery. The self-limiting surface reactions of ALD ensure that electrode porosity is maintained while maximizing the total coating surface area [4]. Since ALD operates in the gas phase, catalytic coatings can be deposited deep within the pores of the air electrode. This will help maintain the three-phase boundary necessary for the discharge reaction and ultimately improve the stability of a zinc-air battery [5]. To create a bifunctional catalyst, two ALD processes, one for manganese oxide and another for iron oxide, is combined into one ALD supercycle, depositing a mixed manganese-iron oxide. Since manganese oxide is a well-established discharge catalyst [6], and iron oxide demonstrates activity towards the charge reaction [7], this mixed manganese-iron oxide exhibits bifunctional activity in a zinc-air battery. An optimized supercycle process will be discussed and full-cell battery test results showcased. Specifically, the bifunctional efficiency of a zinc-air battery can be improved by more than 10% by using the mixed manganese-iron oxide catalyst. In addition, the high stability of the manganese-iron oxide catalyst is demonstrated, where bifunctional efficiency can be maintained at over 95% of the initial value over 200 cycles. Materials characterization of the mixed manganese-iron oxide, deposited through ALD, is also included.[1] L. Trahey, F. R. Brushett, N. P. Balsara, G. Ceder, L. Cheng, Y. M. Chiang, N. T. Hahn, B. J. Ingram, S. D. Minteer, J. S. Moore, K. T. Mueller, L. F. Nazar, K. A. Persson, D. J. Siegel, K. Xu, K. R. Zavadil, V. Srinivasan, and G. W. Crabtree, “Energy Storage Emerging: A Perspective from the Joint Center for Energy Storage Research,” Proc. Natl. Acad. Sci. U. S. A., vol. 117, no. 23, pp. 12550–12557, 2020.[2] J. Fu, R. Liang, G. Liu, A. Yu, Z. Bai, L. Yang, and Z. Chen, “Recent Progress in Electrically Rechargeable Zinc – Air Batteries,” Adv. Mater., vol. 31, no. 31, p. 1805230, 2019.[3] E. Davari and D. G. Ivey, “Bifunctional electrocatalysts for Zn – air batteries,” Sustain. Energy Fuels, vol. 2, no. 1, pp. 39–67, 2018.[4] C. Detavernier, J. Dendooven, S. Pulinthanathu Sree, K. F. Ludwig, and J. A. Martens, “Tailoring nanoporous materials by atomic layer deposition,” Chem. Soc. Rev., vol. 40, no. 11, pp. 5242–5253, 2011.[5] M. P. Clark, M. Xiong, K. Cadien, and D. G. Ivey, “High Performance Oxygen Reduction/Evolution Electrodes for Zinc − Air Batteries Prepared by Atomic Layer Deposition of MnOx,” ACS Appl. Energy Mater., vol. 3, no. 1, pp. 603–313, 2020.[6] M. P. Clark, T. Muneshwar, M. Xiong, K. Cadien, and D. G. Ivey, “Saturation Behavior of Atomic Layer Deposition MnOx from Bis(Ethylcyclopentadienyl) Manganese and Water: Saturation Effect on Coverage of Porous Oxygen Reduction Electrodes for Metal-Air Batteries,” ACS Appl. Nano Mater., vol. 2, no. 1, pp. 267–277, 2019.[7] M. Labbe, M. P. Clark, Z. Abedi, A. He, K. Cadien, and D. G. Ivey, “Atomic layer deposition of iron oxide on a porous carbon substrate via ethylferrocene and an oxygen plasma,” Surf. Coatings Technol., vol. 421, p. 127390, 2021.

Oxidative Molecular Layer Deposition of Amine-Containing Conjugated Polymer Thin Films

Conjugated polymers such as polyethylenedioxythiophene (pEDOT), polypyrrole (pPy), and polyaniline (pAni) exhibit high electrochemical capacities, making them appealing as electrode materials for energy storage, electrochemical desalination, and chemical sensing. Recent work has established the growth of thin films of pEDOT using alternating gas-phase exposures of the EDOT monomer and a metal chloride (e.g., MoCl5) oxidant in a process termed oxidative molecular layer deposition (oMLD). Here, we describe the first demonstration of oMLD of amine-containing conjugated polymers. We find that pyrrole (Py) and MoCl5 undergo self-limiting surface reactions during oMLD exposures to form conformal pPy thin films, but oMLD using aniline (Ani) and p-phenylenediamine (PDA) monomers yields unexpected azo functionality. The formation of azo groups is attributed to an MoCl5-amine surface adduct that spatially constrains polymerization reactions near the amine group and produces azo groups when coupling two primary amines. pPy grown by oMLD exhibits a record-breaking 282 mAh/g capacity in an aqueous electrolyte, and PDA/MoCl5 oMLD yields azo polymers of interest as anode materials for alkali-ion batteries. Alternating between Py and PDA monomers during oMLD produces molecularly assembled copolymers with qualitatively different electrochemical responses from the isolated monomer structures. This work lays the foundation for the growth of conformal thin films of conjugated amine polymers with molecular-level control of composition and thickness.

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.

Thermal atomic layer etching of amorphous and crystalline Al2O3 films

Thermal atomic layer etching (ALE) can be achieved with sequential, self-limiting surface reactions. One mechanism for thermal ALE is based on fluorination and ligand-exchange reactions. For metal oxide ALE, fluorination converts the metal oxide to a metal fluoride. The ligand-exchange reaction then removes the metal fluoride by forming volatile products. Previous studies have demonstrated the thermal ALE of amorphous Al2O3 films. However, no previous investigations have explored the differences between the thermal ALE of amorphous and crystalline Al2O3 films. This study explored the thermal ALE of amorphous and crystalline Al2O3 films. HF, SF4, or XeF2 were used as the fluorination reactants. Trimethylaluminum (TMA) or dimethylaluminum chloride (DMAC) were used as the metal precursors for ligand-exchange. Spectroscopic ellipsometry measurements revealed that the amorphous Al2O3 films had much higher etch rates than the crystalline Al2O3 films. When using HF and TMA at 300 °C, the amorphous Al2O3 film was removed at an etch rate of 0.78 Å/cycle. For the crystalline Al2O3 film, an etch rate of 0.06 Å/cycle was initially observed prior to the stoppage of etching after removing about 10 Å of the film. Thermal ALE with HF and DMAC resulted in similar results. Etch rates of 0.60 and 0.03 Å/cycle were measured for amorphous and crystalline Al2O3 films at 300 °C, respectively. Other fluorination agents, such as SF4 or XeF2, were also used together with TMA or DMAC for Al2O3 ALE. These reactants for fluorination and ligand-exchange were able to etch amorphous Al2O3 films at 300 °C. However, they were unable to etch crystalline Al2O3 film at 300 °C beyond the initial 10–20 Å surface layer. The investigations also examined the effect of annealing temperature on the etch rate per cycle using HF and TMA as the reactants at 300 °C. Amorphous Al2O3 films were etched at approximately the same etch rate of 0.78 Å/cycle until the crystallization of amorphous Al2O3 films at ≥ 880 °C. The differences between amorphous and crystalline Al2O3 thermal ALE could be used to obtain selective thermal ALE of amorphous Al2O3 in the presence of crystalline Al2O3.

Surface modification of PS polymer by oxygen-atom treatment from remote plasma: Initial kinetics of functional groups formation

Self-limiting surface reactions are promising for atomic layer etching of materials. Classical treatments with plasmas do not allow for self-limiting functionalization but cause the formation of several functional groups and degradation. To avoid this, the polystyrene was oxidized in the plasma afterglow with the O-atom density 7 × 1017 m−3 and dissociation fraction 7%. X-ray Photoelectron Spectroscopy was used to study the formation of various oxygen groups versus the O-atom dose. The surface was saturated with the OH groups already after the dose of 1 × 1019 m−2, indicating a large initial sticking. The functionalization remained almost intact over an order of magnitude larger doses. Measurable quantities of the CO groups appeared simultaneously with the benzene ring's degradation at a dose of 1 × 1020 m−2. The ring degradation was accomplished with a significant increase in the OH concentration what was explained by the formation of a thicker oxygen-rich film. OCO and O(CO)O groups appeared after the dose of 1 × 1021 m−2, i.e., a 100-times larger dose than required for the initial saturation with the OH. The observations are sound with the predictions from the literature obtained through the density functional theory of oxygen adsorption on polystyrene.

Conversion reactions in atomic layer processing with emphasis on ZnO conversion to Al2O3 by trimethylaluminum

Atomic layer processing such as atomic layer deposition (ALD) and thermal atomic layer etching (ALE) is usually described in terms of sequential, self-limiting surface reactions. This picture for ALD and thermal ALE leaves out the possibility that the metal precursor in ALD and thermal ALE can also convert the surface material to another new material. This perspective introduces the previous evidence for conversion reactions in atomic layer processing based on a variety of studies, including Al2O3 ALD on ZnO, growth of Zn(O,S) alloys, “self-cleaning” of III-V semiconductor surfaces, and thermal ALE of ZnO and SiO2. The paper then focuses on the reaction of Al(CH3)3 [trimethylaluminum (TMA)] on ZnO as a model conversion system. A variety of techniques are utilized to monitor ZnO conversion to Al2O3 using TMA at 150 °C. These techniques include FTIR spectroscopy, quadrupole mass spectrometry (QMS), x-ray reflectivity (XRR), gravimetric analysis, x-ray photoelectron spectroscopy (XPS), and quartz crystal microbalance (QCM) measurements. The various studies focus on ZnO conversion to Al2O3 for both hydroxyl-terminated and ethyl-terminated ZnO substrates. FTIR studies observed the conversion of ZnO to Al2O3 and provided evidence that the conversion is self-limiting at higher TMA exposures. QMS studies identified the volatile reaction products during the TMA reaction with ZnO as CH4, C2H4, C2H6, and Zn(CH3)2. The CH4 reaction product preceded the appearance of the Zn(CH3)2 reaction product. XRR investigations determined that the thickness of the Al2O3 conversion layer on ZnO limits at ∼1.0 nm at 150 °C after larger TMA exposures. A gravimetric analysis of the conversion reaction on ZnO nanoparticles with a diameter of 10 nm displayed a percent mass loss of ∼49%. This mass loss is consistent with an Al2O3 shell of ∼1 nm on a ZnO core with a diameter of ∼6 nm. XPS studies revealed that ZnO ALD films with a thickness of 2 nm were almost completely converted to Al2O3 by large TMA exposures at 150 °C. QCM investigations then measured the mass changes for lower TMA exposures on hydroxyl-terminated and ethyl-terminated ZnO films. More mass loss was observed on ethyl-terminated ZnO films compared with hydroxyl-terminated films, because TMA does not have the possibility of reacting with hydroxyl groups on ethyl-terminated ZnO films. The mass losses also increased progressively with temperatures ranging from 100 to 225 °C on both hydroxyl-terminated and ethyl-terminated ZnO films. The perspective concludes with a discussion of the generality of conversion reactions in atomic layer processing.

Bridging the Synthesis Gap: Ionic Liquids Enable Solvent-Mediated Reaction in Vapor-Phase Deposition.

Molecular layer deposition (MLD) is an attractive, vapor-phase deposition method for applications requiring ultrathin organic materials, such as photolithography, lithium batteries, and microelectronics. By using sequential self-limiting surface reactions, MLD offers excellent control over thickness and conformality, but there are also challenges such as a limited range of possible film compositions and long deposition times. In this study, we introduce a modified technique, termed ionic liquid assisted MLD (IL-MLD), that can overcome these barriers. By performing the surface reactions inside of an ultrathin layer of a compatible ionic liquid (IL), solvent effects are replicated inside a vacuum system, broadening the possible reactions to a much wider suite of chemistries. Using this strategy, the MLD of polyetherketoneketone, an industrially and research-relevant, high-performance thermoplastic, is reported. With this proof-of-concept, we demonstrate that IL-MLD can enable the synthesis of polymers via solvent- or catalyst-mediated reactions and establish an approach that may allow solution chemistries to be accessed in other vapor deposition techniques as well.

Surface modification and functionalization of powder materials by atomic layer deposition: a review.

Powder materials are a class of industrial materials with many important applications. In some circumstances, surface modification and functionalization of these materials are essential for achieving or enhancing their expected performances. However, effective and precise surface modification of powder materials remains a challenge due to a series of problems such as high surface area, diffusion limitation, and particle agglomeration. Atomic layer deposition (ALD) is a cutting-edge thin film coating technology traditionally used in the semiconductor industry. ALD enables layer by layer thin film growth by alternating saturated surface reactions between the gaseous precursors and the substrate. The self-limiting nature of ALD surface reaction offers angstrom level thickness control as well as exceptional film conformality on complex structures. With these advantages, ALD has become a powerful tool to effectively fabricate powder materials for applications in many areas other than microelectronics. This review focuses on the unique capability of ALD in surface engineering of powder materials, including recent advances in the design of ALD reactors for powder fabrication, and applications of ALD in areas such as stabilization of particles, catalysts, energetic materials, batteries, wave absorbing materials and medicine. We intend to show the versatility and efficacy of ALD in fabricating various kinds of powder materials, and help the readers gain insights into the principles, methods, and unique effects of powder fabrication by ALD.

(Invited) Atomic Layer Deposition of MOF from Dissolved Precursors ― ‘Solution ALD’ (sALD)

Metal organic frameworks (MOFs) are crystalline three-dimensional porous solids featuring a high surface area, which have attracted considerable attention in a wide range of applications e.g. in catalysis, hydrogen storage, gas separation, thermoelectric applications or as chemical sensors. Beyond solvothermal synthesis of bulk MOF crystals, growing MOFs thin films has been limited so far to rather conventional solution-based techniques. However, traditional growth methods from solution such as spin-coating, spray-coating or dip-coating generate MOFs thin films with a rather low film quality and an alarming lack of a precise film thickness control under 1 nm. This renders them unsuitable for most industrial applications and severely limits their future application potential.Atomic layer deposition (ALD) provides this capability, but is limited by the necessity to use gaseous precursors in vacuum conditions at temperatures typically above 100°C. This is incompatible with the typically high temperature sensitivity of many hybrid metal-organic compounds such as the MOFs.In this perspective, we have transferred the principles of ALD to precursors dissolved in liquid solvents. The ‘solution ALD’ (sALD) method can be implemented in a variety of microfluidic or slot-die processing devices. We have demonstrated that the principles of classical, gas-based ALD —self-limiting surface reactions— are reproduced in sALD. sALD shares the fundamental properties of standard ‘gas ALD’ (gALD), in particular the self-limiting surface chemistry and the ability to coat deep pores in a conformal manner Furthermore, sALD allows the experimentalist to design new, advantageous, reactions of known semiconductors (oxides, heavier chalcogenides), or to deposit materials otherwise inaccessible to ALD altogether, such as polymers, ionic solids, and metal-organic frameworks. They may be obtained in highly pure, crystalline form at room temperature, either as planar films or as conformal coatings of porous substrates. Based on these advances, we have been able to make the first sALD-derived films of a MOF, Cu-BDC. At any chosen thickness, sALD MOF films feature significantly lower roughness and better adhesion than those generated by the best spray-coating and dip-coating methods available currently.

Study of Novel Hafnium-Based Hybrid Thin Films By Molecular Layer Deposition

Molecular layer deposition (MLD) is a gas-phase reaction technique which relies on self-limiting surface reactions for growth of surface-bound polymers. Hybrid organic-inorganic materials usually possess properties distinct from both inorganic and organic components. MLD techniques can be used to deposit hybrid films with precisely defined composition and tunable properties that can be conformally deposited on high aspect ratio substrate. In this study, novel hybrid organic-inorganic hafnium based polymer films were successfully deposited using sequential exposures of a homoleptic tetrakis(dimethylamido)hafnium (TDMAH) complex and ethanolamine (EA) as the reactants by MLD. Hafnium nitrides and oxides have received much attention in the fields of microelectronics and lithium ion batteries, and thus are of interest as hybrid materials that may impart beneficial mechanical and chemical properties. To grow the Hf hybrid films, TDMAH and EA pulses were alternated with 120s of N2 purge. The processes were investigated as a function of temperature, precursor pulsing time and number of cycles. The results show that the self-limiting surface reactions possess a constant growth rate of ∼1.9 Å per cycle at 120 °C. Temperature-dependent growth characterization was performed for these hybrid materials. By increasing the temperature from 65 °C to 145 °C the growth rate decreases from ∼2.5 Å per cycle to ∼1.1 Å per cycle. The presence of expected C-N, C-O and -CH2 moieties in the MLD films were confirmed by Fourier transform infrared (FTIR) spectroscopy. Hafnium nitride and oxide bonds within hybrid thin films were identified by applying X-ray photoelectron spectroscopy (XPS). The results show that the deposited hafnium based organic-inorganic hybrid films contain both Hf – O and Hf – N bonds which enables flexibility, and thus have potential applications for both battery and microelectronics technologies.

(Invited) Investigations into Molecular Layer Deposition of Conjugated Amine Polymers

Conjugated amine polymers (CAPs) such as polyaniline and polypyrrole have high electrical conductivities (>500 S/cm), high specific capacities (>500 F/g), and are one of few known material classes containing redox-active sites for binding negatively-charged anions. These properties make CAPs appealing for a range of electrochemical applications including semiconductor devices, energy storage devices, sensors, and electrochemical separation processes. Unfortunately, existing synthesis routes for CAPs rely on rapid chemical or electrochemical oxidation reactions that provide limited control over the molecular structure and thickness of CAPs. This restricts the ability to use CAPs to address technological needs. In this report, we describe ongoing research in our group to deposit conformal CAP films with molecular-scale control of thickness and composition using sequential and self-limiting gas-phase surface reactions. Our approach is in line with established molecular layer deposition (MLD) techniques, and uses an oxidant as one of the MLD precursors in so-called oxidative molecular layer deposition (oMLD). Using select examples, we reveal new insights into oMLD growth behavior, characterize the molecular structures of the oMLD CAP films, and connect structural differences under varying deposition conditions with electrical and electrochemical properties of the films. The insights we report enable the deposition of electrically conductive and redox-active CAP films conformally onto high-surface area samples, opening new avenues to enable advanced electrochemical technologies based on ultrathin and conformal CAP films.