Conformal Films Research Articles

Selective Deposition of Platinum by Atomic Layer Deposition Using Terraced Oxide Surfaces

Atomic layer deposition (ALD) is widely used in science and technology, particularly in microelectronics, because it allows well-controlled production of highly conformal thin films. Technoindustri...

Nanoengineering of Crystal and Amorphous Surfaces of Pharmaceutical Particles for Biomedical Applications.

The morphology, size, and surface properties of pharmaceutical particles form an essential role in the therapeutic performance of active pharmaceutical ingredients (APIs) and excipients as constituents in various drug delivery systems and clinical applications. Recent advances in methods for surface modification, however, rely heavily on liquid-phase-based modification processes and afford limited control over the thickness and conformality of the coating. Atomic layer deposition (ALD), on the other hand, enables the formation of conformal nanoscale films on complex structures with thickness control on the molecular level, while maintaining the substrate particle size and morphology. Moreover, this enables nanoengineering of surfaces of pharmaceutical particles also in the dry state. Successful nanoengineeering of crystal and amorphous surfaces of pharmaceutical particles is demonstrated in this study whereby functional properties, such as dissolution and dispersibility, were tailored for drug delivery applications. This expands on our initial work on ALD of alumina on pharmaceutical particles within the lower micro- to higher nanosize ranges to here probe both crystalline and amorphous lactose substrate surfaces (d50 = 3.5 and 21 μm). In addition, both water and ozone coreactants were evaluated, the latter having not been evaluated previously for pharmaceutical particles. The deposition process is carried out at ambient conditions in a fluidized bed reactor for a low number of cycles (i.e., from 4 to 14). Improved dissolution and extended release were achieved by the ALD nanoengineering of both crystalline and amorphous surfaces. This novel concept opens up exciting opportunities to produce more complex materials and structures using temperature- and moisture-sensitive drugs, e.g., targeting and drug delivery opportunities, as well as delivering new functionalities for novel applications in the pharmaceutical, medical, biological, and advanced materials fields. The prospects for advancing inhaled drug delivery are exemplified by the ALD surface nanoengineering concept.

Nanoscale Surface and Interface Engineering of Solid Oxide Fuel Cells by Atomic Layer Deposition

Recently, atomic layer deposition (ALD) has drawn much attention as a suitable tool for fabrication and engineering of high performance fuel cell catalysts and electrodes. Intrinsic merits of ALD enable synthesis of conformal and uniform film even at a sub-nm scale with excellent stoichiometry control and reproducibility. Leveraging the unique characteristics, ALD has proven its promising potential as a solution to achieve two major challenges of solid oxide fuel cell research: sluggish kinetics at low operational temperatures and long-term stability. In this review, recent efforts to address the challenges by the use of ALD-based functionalization of surfaces and interfaces of cell components are discussed.

Very High Refractive Index Transition Metal Dichalcogenide Photonic Conformal Coatings by Conversion of ALD Metal Oxides

Materials for nanophotonic devices ideally combine ease of deposition, very high refractive index, and facile pattern formation through lithographic templating and/or etching. In this work, we present a scalable method for producing high refractive index WS2 layers by chemical conversion of WO3 synthesized via atomic layer deposition (ALD). These conformal nanocrystalline thin films demonstrate a surprisingly high index of refraction (n > 3.9), and structural fidelity compatible with lithographically defined features down to ~10 nm. Although this process yields highly polycrystalline films, the optical constants are in agreement with those reported for single crystal bulk WS2. Subsequently, we demonstrate three photonic structures - first, a two-dimensional hole array made possible by patterning and etching an ALD WO3 thin film before conversion, second, an analogue of the 2D hole array first patterned into fused silica before conformal coating and conversion, and third, a three-dimensional inverse opal photonic crystal made by conformal coating of a self-assembled polystyrene bead template. These results can be trivially extended to other transition metal dichalcogenides, thus opening new opportunities for photonic devices based on high refractive index materials.

Preparation and Characterization of Microsphere ZnO ALD Coating Dedicated for the Fiber-Optic Refractive Index Sensor.

We report the fabrication of a novel fiber-optic sensor device, based on the use of a microsphere conformally coated with a thin layer of zinc oxide (ZnO) by atomic layer deposition (ALD), and its use as a refractive index sensor. The microsphere was prepared on the tip of a single-mode optical fiber, on which a conformal ZnO thin film of 200 nm was deposited using an ALD process based on diethyl zinc (DEZ) and water at 100 °C. The modified fiber-optic microsphere was examined using scanning electron microscopy and Raman spectroscopy. Theoretical modeling has been carried out to assess the structure performance, and the performed experimental measurements carried out confirmed the enhanced sensing abilities when the microsphere was coated with a ZnO layer. The fabricated refractive index sensor was operating in a reflective mode of a Fabry–Pérot configuration, using a low coherent measurement system. The application of the ALD ZnO coating enabled for a better measurement of the refractive index of samples in the range of the refractive index allowed by the optical fiber. The proof-of-concept results presented in this work open prospects for the sensing community and will promote the use of fiber-optic sensing technologies.

A vapor printed electron-accepting conjugated polymer for textile optoelectronics

Reactive vapor printing has emerged as an advantageous method to create conjugated polymer films with enviable electronic properties on prewoven fabrics. However, the conjugated polymers that have been vapor printed to date are exclusively hole transporters, since only monomers that can form radical cations participate in the oxidative polymerization reaction that underpins all existing vapor printing recipes. Here, we vapor print a conjugated polymer, PDTCPD, containing electron-withdrawing cyclopentadienone (CPD) moieties that depress its electronic energy levels and qualify it to serve as an electron acceptor when interfaced with known vapor-printed polymers. A custom-built hot wall reactor is shown to enable vapor-phase oxidative polymerization of a heavy CPD derivative to create PDTCPD films. Vapor printing is particularly useful for creating films of CPD-containing polymers because these intractable materials cannot be processed using solution-based methods. Vapor printing yields uniform and conformal PDTCPD films on topographically complex fabrics with minimal evident erosion upon solvent rinsing. This work provides a route to construct wearable optoelectronic devices that include monolithically-integrated active layers.

Electrical Properties of Ultrathin Platinum Films by Plasma-Enhanced Atomic Layer Deposition

The ability to deposit thin and conformal films has become of great importance because of downscaling of devices. However, because of nucleation difficulty, depositing an electrically stable and thin conformal platinum film on an oxide nucleation layer has proven challenging. By using plasma-enhanced atomic layer deposition (PEALD) and TiO2 as a nucleation layer, we achieved electrically continuous PEALD platinum films down to a thickness of 3.7 nm. Results show that for films as thin as 5.7 nm, the Mayadas–Shatzkes (MS) model for electrical conductivity and the Tellier–Tosser model for temperature coefficient of resistance hold. Although the experimental values start to deviate from the MS model below 5.7 nm because of incomplete Pt coverage, the films still show root mean square electrical stability better than 50 ppm over time, indicating that these films are not only electrically continuous but also sufficiently reliable for use in many practical applications.

Physical and Chemical Morphology of Passively Sampled Environmental Films

This work presents spatially resolved physical and chemical surface characterization of passively sampled environmental films developed in urban, suburban, and metropolitan locations. Environmental surface films are important mediators of atmospheric particulate matter (PM) and persistent organic pollutants (POPs) fate and transport. The films are developed via dry deposition onto silicon wafer substrates for time scales ranging from 1 to 52 weeks. Spatial and chemical morphologies of the films are analyzed by bright-field, scanning electron, and atomic force microscopies. Surface feature sizes span 6 orders of magnitude, from the millimeter to nanometer regimes, indicative of super- and sub-micrometer PM as well as conformal films of nanoscale PM and semivolatile molecules. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging data are analyzed to report the films’ chemical morphology and speciation which include a range of organic and inorganic species. Increases in surface coverages (ca. 5%...

Conformal deposition of GeTe films with tunable Te composition by atomic layer deposition

Atomic layer deposition (ALD) of chalcogenide amorphous films has attracted attention, thanks to its ability to deposit highly conformal and uniform thin films on three-dimensional structures; these film properties are critical for the development of ultrahigh-density integrated electronic circuits such as nanoscale 3D cross-point memory stacks. Control over the composition of chalcogenide compounds is indispensable for fabricating functional electronic devices. In this work, the authors used trichlorogermane (HGeCl3) and bis(trimethylsilyl)telluride [(Me3Si)2Te] as ALD precursors to develop conformal and uniform germanium telluride (GeTe) films at low temperature (60 °C). The growth of GeTe resulted in moderately Ge rich films independently of the ALD precursor injection time and the use of single or discrete (Me3Si)2Te doses. To achieve control over the GeTe film composition, the authors developed a unique ALD of elemental tellurium (Te) using tellurium ethoxide [Te(OEt)4] and (Me3Si)2Te as precursors. Combining GeTe and elemental Te in an ALD supercycle fashion, they obtained GeTex films with a controllable Te content showing a uniform and conformal morphology on high aspect ratio trench structures. A detailed analysis of the variations in Ge and Te contents upon exposure of GeTex films to Te(OEt)4 revealed that an interfacial replacement of Ge atoms to form volatile Ge(OEt)2 takes place and produces a decrease in Ge and an increase in Te content. The ALD studies of GeTe and Te films described in this work provide a facile route for deposition of GeTe with controllable composition. This material system is important for the development of ovonic threshold switching selectors and phase change memories.

Electrochemically Controlled Deposition of Ultrathin Polymer Electrolyte on Complex Microbattery Electrode Architectures

Solid state microbatteries are highly sought after for emerging microsensor technologies. To overcome the problem of the dwarfing capacity resulting from the miniaturization of the battery, 3D-structured platform consisting of high surface area micropillar-shaped electrodes are used. However, applying a conformal and continuous solid polymer electrolyte films onto the intricate 3D electrodes is a crucial step toward achieving functional microbatteries. In this work, we present our approach for the development of polyethylene oxide (PEO)-acrylate based ion conducting polymer thin films which function as solid polymer electrolyte (SPE) and a separator. The SPEs were electrochemically deposited on the 3D electrodes resulting in ultrathin, continuous, conformal, and pinhole-free polymer films. The electrochemical and Li+ ions transport properties of the SPEs were characterized by EIS measurements and cyclic voltammetry. Furthermore, the homogenous composition of the SPEs at various depths were confirmed by XPS depth profiling techniques.

Conformal Deposition of RuO2 on Cu via a Galvanic Cementation Reaction

We develop a seedless wet chemical process to deposit uniform RuO2 on a Cu substrate. Our approach entails the synthesis of volatile RuO4 colloids in a pH 9 aqueous solution from the oxidation of RuCl52− by ClO−, followed by a galvanic cementation reaction in which the Cu undergoes a mild oxidation in conjunction with the reduction of RuO4 to produce a dense and conformal RuO2 thin film on the Cu surface. The nature for the chemical reactions and constituents involved are investigated by UV-Vis, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy. Both as-deposited and annealed RuO2 films are characterized by scanning electron microscope and X-ray diffractometer. The as-deposited RuO2 film, with a thickness of 199 nm, reveals an amorphous structure and after an Ar annealing, becomes crystalline in rutile phase. In between the RuO2 and Cu, there are presence of CuO from the displacement cementation reaction, and Cu2O from the parasitic Cu corrosion as expected from the Pourbaix diagram. These Cu oxides, with a thickness of 330 nm, provide sufficient adhesive bonding between RuO2 and Cu.

Plasma enhanced atomic layer deposition of cobalt nitride with cobalt amidinate

Cobalt nitride (Co3Nx) thin films were deposited via the technique of plasma enhanced atomic layer deposition (ALD) at low temperatures down to 100 °C, using bis(N,N′-di-iso-propylacetamidinato)cobalt(II) [Co(ipr2AMD)2] and NH3 plasma. Saturation curves demonstrate that the deposition processes follow the ideal self-limiting ALD fashion with a growth rate of 0.075 nm/cycle. The x in the nominal formula of Co3Nx is approximately 0.78, and the films are demonstrated polycrystalline with a hexagonal Co3N crystal structure. This process can deposit a pure, smooth, and highly conformal Co3Nx film in trenches with 20:1 aspect ratio, which can be extended to the deposition of other metal nitrides at low temperature.

Single-Atom Electroplating on Two Dimensional Materials

Catalyst doped with a single-atom noble metal displays distinctive catalytic behavior from the bulk counterparts, with tunable electronic structures and spatial versatilities, which excels in today’s heterogeneous catalysis. To deposit noble metals in a single atomic level requires a restricted chemical environment and precise thermodynamic control. Electroplating methods are commercially used to deposit uniform and conformal metal thin films on different hardware surfaces. Yet the atomic level electroplating has never been achieved. Herein we demonstrate a voltage gauged electrochemical deposition method to synthesize single-atom Pt, Au, and Pd on MoS2 and other two-dimensional (2D) materials. The surface atomic doping level for Pt, Au, and Pd can reach 1.1, 7.0, and 14%, respectively, and the doping sites are precisely positioned at Mo- and S-vacancies. The monodispersed noble atoms show enhanced hydrogen evolution activity and saturated CO tolerance, as explained by density functional theory calculatio...

Characteristics of NiO films prepared by atomic layer deposition using bis(ethylcyclopentadienyl)-Ni and O2 plasma

Plasma-enhanced atomic layer deposition (PEALD) is well-known for fabricating conformal and uniform films with a well-controlled thickness at the atomic level over any type of supporting substrate. We prepared nickel oxide (NiO) thin films via PEALD using bis(ethylcyclopentadienyl)-nickel (Ni(EtCp)2) and O2 plasma. To optimize the PEALD process, the effects of parameters such as the precursor pulsing time, purging time, O2 plasma exposure time, and power were examined. The optimal PEALD process has a wide deposition-temperature range of 100–325 °C and a growth rate of 0.037±0.002 nm per cycle. The NiO films deposited on a silicon substrate with a high aspect ratio exhibited excellent conformality and high linearity with respect to the number of PEALD cycles, without nucleation delay.

Conformal Physical Vapor Deposition Assisted by Atomic Layer Deposition and Its Application for Stretchable Conductors

AbstractPhysical vapor deposition (PVD) is a versatile thin‐film coating technique that can deposit a wide selection of inorganic materials at low cost. However, the process is based on line‐of‐sight transfer, which can lead to shadowing effects and limit film uniformity over nonplanar topographies. This work describes improving conformal PVD coating on polymer nanostructures by increasing surface energy using a thin oxide interlayer deposited by atomic layer deposition (ALD). The proposed ALD‐assisted PVD process allows conformal coating at low cost, and can be adopted for a wide variety of materials compatible with tradition PVD. Conformal gold films over nanostructures with 500 nm half‐pitch and aspect ratio up to 1.5 are demonstrated. The film uniformity is characterized using cross‐sectional electron microscopy, energy‐dispersive X‐ray spectroscopy, and electrical measurements, showing a clear improvement in coating uniformity with the oxide interlayer. This PVD process is then used to fabricate metallic nano‐accordion structures, which can be used for stretchable conductors. The demonstrated process can improve material selection and reduce process cost of conformal coating, which can find applications in integrated circuit manufacturing, stretchable electronics, and wearable sensors.

Boron Nitride as a Novel Support for Highly Stable Palladium Nanocatalysts by Atomic Layer Deposition.

The ability to prepare controllable nanocatalysts is of great interest for many chemical industries. Atomic layer deposition (ALD) is a vapor phase technique enabling the synthesis of conformal thin films and nanoparticles (NPs) on high surface area supports and has become an attractive new route to tailor supported metallic NPs. Virtually all the studies reported, focused on Pd NPs deposited on carbon and oxide surfaces. It is, however, important to focus on emerging catalyst supports such as boron nitride materials, which apart from possessing high thermal and chemical stability, also hold great promises for nanocatalysis applications. Herein, the synthesis of Pd NPs on boron nitride (BN) film substrates is demonstrated entirely by ALD for the first time. X-ray photoelectron spectroscopy indicated that stoichiometric BN formed as the main phase, with a small amount of BNxOy, and that the Pd particles synthesized were metallic. Using extensive transmission electron microscopy analysis, we study the evolution of the highly dispersed NPs as a function of the number of ALD cycles, and the thermal stability of the ALD-prepared Pd/BN catalysts up to 750 °C. The growth and coalescence mechanisms observed are discussed and compared with Pd NPs grown on other surfaces. The results show that the nanostructures of the BN/Pd NPs were relatively stable up to 500 °C. Consequent merging has been observed when annealing the samples at 750 °C, as the NPs’ average diameter increased from 8.3 ± 1.2 nm to 31 ± 4 nm. The results presented open up exciting new opportunities in the field of catalysis.

Atomic Layer Deposition of Intermetallic Co3Sn2 and Ni3Sn2 Thin Films

AbstractIntermetallics form a versatile group of materials that possess unique properties ranging from superconductivity to giant magnetoresistance. The intermetallic Co–Sn and Ni–Sn compounds are promising materials for magnetic applications as well as for anodes in lithium‐ and sodium‐ion batteries. Herein, a method is presented for the preparation of Co3Sn2 and Ni3Sn2 thin films using diamine adducts of cobalt(II) and nickel(II) chlorides, CoCl2(TMEDA) and NiCl2(TMPDA) (TMEDA = N,N,N′,N′‐tetramethylethylenediamine, TMPDA = N,N,N′,N′‐tetramethyl‐1,3‐propanediamine) combined with tributyltin hydride. The films are grown by atomic layer deposition (ALD), a technique that enables conformal film deposition with sub‐nanometer thickness control. The Co3Sn2 process fulfills the typical ALD qualifications, such as self‐limiting growth, excellent film uniformity, and conformal coverage of a trench structure. X‐ray diffraction (XRD) shows reflections characteristic to the hexagonal Co3Sn2 phase, which confirms that the films are, indeed, intermetallic instead of being mere alloys of Co and Sn. The films are extremely pure with impurity levels each below 1.0 at.%. Ni3Sn2 films similarly exhibit the expected XRD reflections for the intermetallic phase and are of high purity. The Co3Sn2 film show magnetic hysteresis with high coercivity values exceeding 500 Oe, indicating great potential in terms of applicability of the films.

Particle atomic layer deposition of alumina for sintering yttria‐stabilized cubic zirconia

AbstractThe addition of aluminum oxide (Al2O3) as a sintering aid to yttria‐stabilized zirconia (YSZ) reduces the required densification temperature. Sintering aids are incorporated using a number of processes which can lead to ambiguity when determining the effect of the sintering aid on the densification mechanism. In this study, a novel method for sintering aid addition, Particle Atomic Layer Deposition (ALD), was used to deposit an amorphous Al2O3 thin film on YSZ particles. Transmission electron microscopy confirmed the deposition of conformal Al2O3 thin films on the surface of the YSZ particles. The addition of Al2O3 to YSZ reduced the temperature at which densification began by ~75°C, and 2.2 wt% Al2O3 addition resulted in a minimum activation energy for the intermediate stage of densification. This concentration is well in excess of the solubility limit of Al2O3 in YSZ, showing that Al2O3 does not enhance the densification of YSZ solely by dissolving into the YSZ lattice and activating volume diffusion. The addition of 0.7 wt% Al2O3 with one Particle ALD cycle enhanced the ionic conductivity of YSZ by 23% after sintering at 1350°C for 2 hours, demonstrating that dense parts with high oxygen ion conductivities can be produced after sintering at reduced temperatures. One Particle ALD cycle is a fast, easily scaled‐up process that eliminates the use of solvents and has substantial cost/performance advantages over conventional processing.

Silicon nanowire pH sensors fabricated with CMOS compatible sidewall mask technology

Silicon nanowire (SiNW) field-effect transistors (FETs) have been developed as various bio/chemical sensors due to their high sensitivity as a result of the huge surface-to-volume ratios. This paper presents a CMOS compatible sidewall mask technology (SMT) for top-down fabrication of SiNWs and a highly sensitive pH sensor based on SiNW FETs. The essence of SMT is to use stand-alone vertical sidewalls of a conformal thin film, fabricated by anisotropically etching away the planar part of the thin film deposited on a thin mesa, as hard masks for SiNW etching. As the feature size of the sidewalls, depending on the film thickness rather than lithography, can be controlled readily to tens to hundreds of nanometers by tailoring the film thickness, SMT allows high output and low cost fabrication of SiNWs without need of advanced lithography facilities. Complete SMT processes have been developed for fabrication of SiNW FETs, and pH sensors using SiNW FETs have been realized. Measurement results show that the pH sensors have good static characteristics and transient performance, and the sensitivity reaches 54.5 mV/pH for a wide range of pH values from 1 to 12, approaching the theoretical Nernstian limit.

Surface passivation of InP using an organic thin film

We demonstrate the surface passivation of InP using thin layers of a perylenetetracarboxylic diimide derivative (PTCDI-C9) applied via organic vapor phase deposition (OVPD). The organic layer forms a conformal crystalline film on the InP surface, which is confirmed by atomic force microscopy and X-ray diffraction. Area-dependent photoluminescence measurements indicate that the coating reduces surface recombination. The organic thin film deposited by OVPD exhibits improved photoconductivity compared to an unpassivated InP sample, and to a layer deposited via vacuum thermal evaporation. Our results suggest that semiconductor surface passivation using organic thin films deposited by OVPD has applications to a variety of optoelectronic devices, particularly with structures requiring sidewall or conformal coatings.