Molecular Layer Deposition Films Research Articles

Molecular Layer Deposition for Energy Conversion and Storage

The development of nanoscale coatings with well-controlled properties is critical to the future of nanotechnology for energy applications. As an extension of atomic layer deposition (ALD), molecular layer deposition (MLD), has recently emerged as a thin-film coating technique that can enable the development of high-performance materials in energy-related applications. MLD fabrication can be classified into two categories: polymer-based organics and inorganic–organic hybrid materials. The unique properties of low growth temperature, precise control of film thickness, uniformity, flexibility, and low density make MLD films very promising for energy-related applications. In this Review, we focus on the recent developments and understanding of MLD in the application of batteries, supercapacitors, water splitting, photodegradation, solar cells, and membranes. The different types of MLD films and nanomaterials derived from MLD are discussed based on the specific application and properties. Finally, the future d...

Uniform Surface Characteristics in Sequentially Polymerized Polyurea Films

We successfully fabricated uniformly aligned polyurea thin films using molecular layer deposition (MLD) technique. By measuring the film thickness as a functional of the number of cycles, we confirmed the linear growth behavior of the polyurea MLD film. We also fabricated a spin-coated polyurea films after synthesis of the polyurea polymer in solution, and compared their roughness and surface potential with the prepared MLD film. The polyurea MLD film had a low root-mean square value and homogeneous surface potentials, whereas the spin-coated film had very rough surface and a highly variable surface potential. We believe that our results will contribute to performance improvement of polymer-based electrical devices and provide greater insight into the fabrication of uniformly aligned polymer thin films. Table S1. All FTIR peaks measured for the polyurea film along with vibration mode assignment for the in situ FTIR spectra of the (PDI/PDA)n polyurea MLD film. Figure S1. Two-dimensional (a) AFM and (b) SKPM images of the bare Si (100) wafer. (a) The surface of the bare Si (100) wafer is smooth with a RMS roughness of 0.068 nm, in agreement with the line profile. (b) At a 0 V sample bias, the average surface potential is 0.161 V and the standard deviation of the surface potential is 0.024 V. The scan area of each image is 4 μm × 4 μm. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Spatial molecular layer deposition of polyamide thin films on flexible polymer substrates using a rotating cylinder reactor

Polyamide thin films were rapidly grown using spatial molecular layer deposition (MLD) in a rotating cylinder reactor. The polyamide MLD films were coated on flexible metalized polyethylene terephthalate substrates using sequential exposures of trimesoyl chloride (TMC) and m-phenylenediamine (mPD). The rotating cylinder reactor was housed in a custom oven enclosure that maintained a constant temperature of 115 °C. One MLD cycle (TMC/mPD) was performed per rotation of the cylinder. Polyamide growth rates of 4.5 Å/cycle or 90 Å/min were observed at a slow rotation speed of 20 revolutions per minute (RPM) at a substrate speed of 0.33 m/s. Growth rates of 2.27 Å/cycle or 477 Å/min were measured at higher rotation speeds of 210 RPM at substrate speeds of 3.5 m/s. The polyamide film thicknesses were uniform over the substrate widths as large as ∼16.5 cm. The polyamide MLD surface chemistry was also self-limiting. Constant polyamide growth rate per cycle was measured at larger TMC and mPD exposures. In addition, infrared analysis and thermal annealing studies confirmed the chemical identity of the polyamide films. With four MLD cycles per rotation of the rotating cylinder, spatial MLD should be able to obtain polymer growth rates of >1900 Å/min at a rotation speed of 210 RPM. These rapid growth rates are much higher than the growth rates that can be obtained in temporal MLD reactors. The spatial MLD of polyamide films has many possible applications including the deposition of ultrathin reverse osmosis membranes for water desalination.

Orientation and Ordering of Organic and Hybrid Inorganic–Organic Polyurea Films Using Molecular Layer Deposition

The molecular orientation and ordering of films produced by a vapor-phase deposition technique known as molecular layer deposition (MLD) is investigated. Specifically, the influence of incorporating an inorganic component into MLD films is determined. Film growth properties and surface saturating conditions of a purely organic polyurea film and a mixed inorganic–organic polyurea film are investigated by in situ quartz crystal microbalance. Polarization-dependent X-ray absorption spectroscopy was employed to determine the orientation and ordering of these films. Both films demonstrate preferential orientation parallel to the substrate surface. However, the degree of ordering between the films is found to be starkly different. In-depth analysis revealed that the hybrid inorganic–organic films display a high degree of ordering compared to purely organic polyurea films. This study sheds light on the ability of MLD to finely tune oligomer packing density, growth rate, molecular orientation, and film ordering.

Correcting defects in area selective molecular layer deposition

Area selective deposition, as used with atomic layer deposition and molecular layer deposition (MLD), is a rapidly emerging field of study due to its potential applications in micro- and nanoelectronic devices. Previous studies have shown successful area selective MLD of nanoscale organic thin films using self-assembled monolayers (SAMs) as blocking layers. In this report, the authors expand upon the use of SAMs by implementing an etching process for the removal of the SAM resist once the MLD film has been deposited. This process has the benefit of removing unwanted deposits, which improves MLD selectivity on a dielectric by 380-fold over using a SAM layer alone and enables twice the amount of polymer film to be selectively deposited compared to that in previous reports. Although the etching leaves intact the polymer film in unblocked regions, the clean removal of the polymer film is limited at pattern interfaces. This effect is attributed to intermolecular interactions, such as polymer entanglement and hydrogen bonding between the chains.

Molecular layer deposition using cyclic azasilanes, maleic anhydride, trimethylaluminum, and water

Molecular layer deposition (MLD) is used to grow hybrid organic–inorganic films based on two or more self-limiting surface chemical reactions. A four-step ABCD sequence using N-(2-aminoethyl)-2,2,4-trimethyl-1-aza-2-silacyclopentane (AZ), maleic anhydride (MA), trimethylaluminum, and H2O and a three-step ABC sequence consisting of AZ, MA, and H2O are reported for growing hybrid organic–inorganic thin films. The ABCD sequence exhibited self-limiting growth behavior with a constant growth rate of 3.5 Å/cycle at 100 °C, while the growth rate of the ABC sequence increased to 90 Å/cycle after approximately 50 cycles. The growth rate using the ABC chemistry is much larger than for MLD/ALD chemistries that involve exclusively surface reactions, and displayed a strong dependence on the substrate temperature. Fourier transform infrared measurements of the as-deposited films indicated -COOH functionalities in the ABC film, and provided evidence for the reaction mechanisms. These results indicate that precursor diffusion into the ABC MLD film plays a key role in the large growth rate. The density, roughness, and dielectric constant (κ) of the films are reported in the as-deposited state and after treatment in air at temperatures up to 400 °C. The as-prepared and thermally treated ABC films with densities <1 g/cm3 and κ < 3 may be useful in applications requiring porous and low-κ films.

Fine-Tunable Absorption of Uniformly Aligned Polyurea Thin Films for Optical Filters Using Sequentially Self-Limited Molecular Layer Deposition.

Development of methods enabling the preparation of uniformly aligned polymer thin films at the molecular level is a prerequisite for realizing their optoelectronic characteristics as innovative materials; however, these methods often involve a compromise between scalability and accuracy. In this study, we have grown uniformly aligned polyurea thin films on a SiO2 substrate using molecular layer deposition (MLD) based on sequential and self-limiting surface reactions. By integrating plane-polarized Fourier-transform infrared, Raman spectroscopic tools, and density functional theory calculations, we demonstrated the uniform alignment of polyurea MLD films. Furthermore, the selective-wavelength absorption characteristics of thickness-controlled MLD films were investigated by integrating optical measurements and finite-difference time-domain simulations of reflection spectra, resulting from their thickness-dependent fine resonance with photons, which could be used as color filters in optoelectronics.

Pyrolysis of Alucone Molecular Layer Deposition Films Studied Using In Situ Transmission Fourier Transform Infrared Spectroscopy

The pyrolysis of alucone molecular layer deposition (MLD) films was studied in vacuum using in situ transmission Fourier transform infrared spectroscopy. The initial alucone MLD films were grown using trimethylaluminum (TMA) and either ethylene glycol (EG) (HO–(CH2)2–OH) or hydroquinone (HQ) (HO–C6H4–OH) at 150 °C. The alucone MLD films were then pyrolyzed in vacuum at temperatures ranging from 400 to 750 °C. The absorbance features for the C–H, C–C, and C–O stretching vibrations were observed to be lost at pyrolysis temperatures from 350 to 500 °C. For the alucone films grown using TMA and EG, the loss of these absorbance features was coupled to an increase in carboxylate (R-COO–) absorbance features. The carboxylate absorbance features reached their peak at a pyrolysis temperature of 450 °C and then decreased slowly with higher pyrolysis temperatures. The carboxylate absorbance features are consistent with an Al2O3/carbon composite material with Al3+/COO– species at the interface. In addition, the prese...

Improving area-selective molecular layer deposition by selective SAM removal.

Area selective molecular layer deposition (MLD) is a promising technique for achieving micro- or nanoscale patterned organic structures. However, this technique still faces challenges in attaining high selectivity, especially at large MLD cycle numbers. Here, we illustrate a new strategy for achieving high quality patterns in selective film deposition on patterned Cu/Si substrates. We employed the intrinsically selective adsorption of an octadecylphosphonic acid self-assembled monolayer (SAM) on Cu over Si surfaces to selectively create a resist layer only on Cu. MLD was then performed on the patterns to deposit organic films predominantly on the Si surface, with only small amounts growing on the Cu regions. A negative potential bias was subsequently applied to the pattern to selectively desorb the layer of SAMs electrochemically from the Cu surface while preserving the MLD films on Si. Selectivity could be enhanced up to 30-fold after this treatment.

Growth and properties of hafnicone and HfO(2)/hafnicone nanolaminate and alloy films using molecular layer deposition techniques.

Molecular layer deposition (MLD) of the hafnium alkoxide polymer known as "hafnicone" was grown using sequential exposures of tetrakis(dimethylamido) hafnium (TDMAH) and ethylene glycol (EG) as the reactants. In situ quartz crystal microbalance (QCM) experiments demonstrated self-limiting reactions and linear growth versus the number of TDMAH/EG reaction cycles. Ex situ X-ray reflectivity (XRR) analysis confirmed linear growth and measured the density of the hafnicone films. The hafnicone growth rates were temperature-dependent and decreased from 1.2 Å per cycle at 105 °C to 0.4 Å per cycle at 205 °C. The measured density was ∼3.0 g/cm(3) for the hafnicone films at all temperatures. Transmission electron microscopy images revealed very uniform and conformal hafnicone films. The XRR studies also showed that the hafnicone films were very stable with time. Nanoindentation measurements determined that the elastic modulus and hardness of the hafnicone films were 47 ± 2 and 2.6 ± 0.2 GPa, respectively. HfO2/hafnicone nanolaminate films also were fabricated using HfO2 atomic layer deposition (ALD) and hafnicone MLD at 145 °C. The in situ QCM measurements revealed that HfO2 ALD nucleation on the hafnicone MLD surface required at least 18 TDMAH/H2O cycles. Hafnicone alloys were also fabricated by combining HfO2 ALD and hafnicone MLD at 145 °C. The composition of the hafnicone alloy was varied by adjusting the relative number of TDMAH/H2O ALD cycles and TDMAH/EG MLD cycles in the reaction sequence. The electron density changed continuously from 8.2 × 10(23) e(-)/cm(3) for pure hafnicone MLD films to 2.4 × 10(24) e(-)/cm(3) for pure HfO2 ALD films. These hafnicone films and the HfO2/hafnicone nanolaminates and alloys may be useful for flexible thin-film devices.

Molecular Layer Deposition of Nanoscale Organic Films for Nanoelectronics Applications

The trend toward nanoelectronics motivates the development of new materials and processes. Novel organic materials are of interest for various applications, and this talk will focus on the development of nanoscale organic films for lithography and patterning. Current photoresist materials are facing many challenges introduced by advanced lithography, particularly the need for excellent compositional homogeneity and ultrathin film thickness. Traditional spin-on polymeric resists have inherent limitations in achieving a high level of control over the chemical composition, leading to interest in alternative methods for making photoresists. In this work, we demonstrate that molecular layer deposition (MLD) is a promising method for synthesizing photoresists because it allows for precise control over organic film thickness and composition. As an analogue to atomic layer deposition (ALD), MLD utilizes sequential, self-limiting reactions of organic precursors to build a thin film directly on a substrate surface. An advantage of MLD is that it grows organic films by depositing only one molecular layer at each precursor dose, which in turn allows for fine tuning of the position and concentration of various functionalities in the deposited film. In our study, we use bifunctional precursors, diamine and diisocyanate, to build polyurea resist films via urea coupling reaction. Acid-labile groups are embedded in the backbone of the diamine precursor and incorporated into the resist film with a highly uniform distribution. Two methods of incorporating photo acid generator (PAG) are employed. The first method is to soak a PAG into the resist film after deposition and the second approach is to form in-situ polymer-bound PAG. By this novel synthetic approach, both aromatic and aliphatic polyurea films were successfully deposited by MLD and characterized for both materials properties and resist response. Electron-beam lithographic tests are carried on the MLD photoresists. Among the various MLD films studied, the aliphatic film performed best as a photoresist material, with a good sensitivity of 30 µC/cm2 with 100kV e-beam exposure, and a patterned line resolution of sub-100 nm. Other uses of MLD for advanced patterning methods, as well as fundamental studies into the mechanism of MLD, will also be described.

Molecular Layer Deposition on Carbon Nanotubes

Molecular layer deposition (MLD) techniques were used to deposit conformal coatings on bulk quantities of carbon nanotubes (CNTs). Several metalcone MLD chemistries were employed, including alucone (trimethylaluminum/glycerol and trimethylaluminum/ethylene glycol), titanicone (TiCl4/glycerol), and zincone (diethyl zinc/glycerol). The metalcone MLD films grew directly on the CNTs and MLD initiation did not require atomic layer deposition (ALD) of an adhesion layer. Transmission electron microscopy revealed that MLD formed three-dimensional conformal deposits throughout a CNT scaffold. Mechanical testing was also performed on sheets of CNT networks coated by MLD. Young's Modulus values improved from an initial value of 510 MPa for uncoated CNT sheet to values that ranged from 2.2 GPa, for 10 nm of glycerol alucone (AlGL), to 8.7 GPa for a composite 5 nm AlGL + 5 nm Al2O3 coating.

Pyrolysis of Titanicone Molecular Layer Deposition Films as Precursors for Conducting TiO2/Carbon Composite Films

Titanium alkoxide films known as “titanicones” were grown using molecular layer deposition (MLD) techniques using the sequential exposure of TiCl4 and glycerol. These titanicone MLD films were then...

Cross-Linked Ultrathin Polyurea Films via Molecular Layer Deposition

Ultrathin cross-linked polymer thin films are highly desirable materials because of their important roles in many applications. However, they are difficult and challenging to fabricate. Here we report a one-step process for depositing cross-linked polyurea thin films using a vapor-phase molecular layer deposition (MLD) technique. 1,4-Diisocyanatobutane and a series of different multiamines, including diethylenetriamine, triethylenetetramine, and tris(2-aminoethyl)amine, were used to grow polyurea MLD films via urea-coupling reactions. The deposited cross-linked polyurea films exhibit characteristic MLD film growth behaviors, such as constant growth rates, infrared absorption by expected urea modes, and stoichiometric chemical compositions. More importantly, the cross-linking is shown to be capable of improving the film properties. Based on cross-linking, the thin film density can be increased by approximately 50%. In addition, the film decomposition temperature is increased by about 30 °C, suggesting an enhanced thermal stability of the cross-linked ultrathin polyurea films.

Molecular Layer Deposition of Zircone and ZrO2/Zircone Alloy Films: Growth and Properties

AbstractHybrid organic/inorganic polymer films based on zirconium are grown using molecular layer deposition (MLD) techniques. The zirconium alkoxide films, known as “zircones”, are grown using sequential exposures of zirconium tert‐butoxide (ZTB) and ethylene glycol (EG) as the reactants at temperatures from 105 to 195°C. In‐situ quartz crystal microbalance (QCM) and ex‐situ X‐ray reflectivity (XRR) experiments confirm linear growth versus the number of reaction cycles. The growth rates decrease versus temperature from 1.6 Å per cycle at 105°C to 0.3 Å per cycle at 195°C. The measured density is ∼2.17 g cm−3 for all the growth temperatures. Transmission electron microscopy (TEM) images reveal very uniform and conformal zircone films. ZrO2/zircone alloys are also fabricated by combining ZrO2 atomic layer deposition (ALD) and zircone MLD at 145°C. The composition of the ZrO2/zircone alloy is varied by adjusting the relative number of ZrO2 ALD and zircone MLD cycles in the reaction sequence. The ZrO2/zircone alloys display varying density, refractive index, elastic modulus, and hardness. The refractive index and elastic modulus change progressively from n = 1.63 and E= 27 ± 0.6 GPa for pure zircone MLD films, to n = 1.86 and E= 97 ± 5 GPa for pure ZrO2 ALD films, respectively. In capacitor structures, the zircone films display low leakage currents and a dielectric constant of ∼6.7. The zircone films are also utilized as the dielectric layer in pentacene‐based thin film transistors (TFTs), which display a high field effect mobility of 2.11 cm2 V−1 s−1 operating at −3 V with an on/off current ratio of ∼103. The zircone and ZrO2/zircone alloy films provide a new class of hybrid organic/inorganic polymer films for many functional film applications.

Molecular Layer Deposition of Aluminum Alkoxide Polymer Films Using Trimethylaluminum and Glycidol

Molecular layer deposition (MLD) of aluminum alkoxide polymer films was examined using trimethlyaluminum (TMA) and glycidol (GLY) as the reactants. Glycidol is a high vapor pressure heterobifunctional reactant with both hydroxyl and epoxy chemical functionalites. These two different functionalities help avoid "double reactions" that are common with homobifuctional reactants. A variety of techniques, including in situ Fourier transform infrared (FTIR) spectroscopy and quartz crystal microbalance (QCM) measurements, were employed to study the film growth. FTIR measurements at 100 and 125 °C observed the selective reaction of the GLY hydroxyl group with the AlCH(3) surface species during GLY exposure. Epoxy ring-opening and methyl transfer from TMA to the surface epoxy species were then monitored during TMA exposure. This epoxy ring-opening reaction is dependent on strong Lewis acid-base interactions between aluminum and oxygen. The QCM experiments observed linear growth with self-limiting surface reactions at 100-175 °C under certain growth conditions. With a sufficient purge time of 20 s after TMA and GLY exposures at 125 °C, the mass gain per cycle (MGPC) was 19.8 ng/cm(2)-cycle. The individual mass gains after the TMA and GLY exposures were also consistent with a TMA/GLY stoichiometry of 4:3 in the MLD film. This TMA/GLY stoichiometry suggests the presence of Al(2)O(2) dimeric core species. The MLD films resulting from these TMA and GLY exposures also evolved with annealing temperature to form thinner conformal porous films with increased density. Non-self-limiting growth was a problem at shorter purge times and lower temperatures. With shorter purge times of 10 s at 125 °C, the MPGC increased dramatically to 134 ng/cm(2)-cycle. The individual mass gains after the TMA and GLY exposures in the CVD regime were consistent with a TMA/GLY stoichiometry of 1:1. The MGPC decreased progressively versus purge time. This behavior was attributed to the removal of reactants that could lead to CVD and the instability of the surface species after the reactant exposures. These results reveal that the TMA and GLY reaction displays much complexity and must be carefully controlled to be a useful MLD process.

Nanocoating zinc alkoxide (zincone) hybrid polymer films on particles using a fluidized bed reactor

Ultrathin zinc alkoxide (zincone) hybrid polymer films were coated on large quantities of primary titania nanoparticles using molecular layer deposition (MLD) in a fluidized bed reactor. Diethyl zincand ethylene glycol were used as reactants at a reaction temperature of 80 or 120 °C. The composition and conformal zincone films were confirmed by x-ray photoelectron spectroscopy and TEM, respectively. The growth rate was higher at a lower reaction temperature. Primary nanoparticles were coated individually despite their strong tendency to aggregate during fluidization. Porous zinc oxide was formed by the oxidation of the hybrid zincone films in air at 400 °C. The surface area of the porous film itself was ∼40 m2/g for the MLD films after oxidation at 250 °C, and 31 m2/g for the films after oxidation at 400 °C. Methylene blue oxidation tests indicated that the zincone MLD film can reduce the photoactivity of TiO2 particles, but the passivation effect of the aged zincone films decreased due to the decomposition of zincone in the presence of water at room temperature to form a more stable composite containing zinc. The passivation effect of the oxidized zincone films further decreased due to the nonuniformity of the zinc oxide films after crystallization during the oxidation process.

Molecular Layer Deposition of Conductive Hybrid Organic-Inorganic Thin Films Using Diethylzinc and Hydroquinone

Conductive hybrid organic-inorganic thin films were fabricated by molecular layer deposition (MLD) methods using diethyl zinc (DEZ) and hydroquinone (HQ). Sequential exposures of DEZ and HQ led to linear MLD film growth. The mass gains measured by quartz crystal microbalance investigations were 52.3 ng/cm2 per MLD cycle. Scanning electron microscope images of focused ion beam milled cross-sections of the MLD film were consistent with a MLD growth of 1.6 Aå per cycle at 150oC. The DEZ/HQ MLD films were conductive based on four-point electrical probe measurements.

Importance of Trimethylaluminum Diffusion in Three-Step ABC Molecular Layer Deposition Using Trimethylaluminum, Ethanolamine, and Maleic Anhydride

Hybrid organic-inorganic films were grown by molecular layer deposition (MLD) with a three-step ABC reaction sequence using (A) trimethylaluminum (TMA), (B) ethanolamine (EA), and (C) maleic anhydride (MA) at 90 °C. Very large steady state mass gains of 1854-4220 ng/(cm(2) cycle) were measured depending on reaction conditions. These mass gains are much larger than typical mass gains for surface reactions. The quartz crystal microbalance (QCM) mass profiles during the TMA reaction were consistent with TMA diffusion into and out of the ABC films. The ABC mass gains per cycle also displayed a strong dependence on the TMA dose and purge times that was consistent with the effects of TMA diffusion. Multiple dose experiments conducted at 130 °C revealed that the ABC reactions were self-limiting for thin ABC films. For thicker ABC films, increased TMA diffusion into the ABC film led to non-self-limiting behavior. Numerical modeling assuming Fickian diffusion for TMA diffusing into and out of the ABC film could fit the QCM mass profiles. The results all indicate that TMA diffusion into the ABC MLD film plays a key role in the thin film growth. In addition, X-ray reflectivity (XRR) measurements revealed that the ABC films were exceptionally smooth.

Fabrication of Organic Thin Films for Copper Diffusion Barrier Layers Using Molecular Layer Deposition

AbstractDevice scaling predicts that copper barrier layers of under 3 nm in thickness will soon be needed in back-end processing for integrated circuits, motivating the development of new barrier layer materials. In this work, nanoscale organic thin films for use as possible copper diffusion barrier layers are deposited by molecular layer deposition (MLD) utilizing a series of self-limiting reactions of organic molecules. MLD can be used to tailor film properties to optimize desirable barrier properties, including density, copper surface adhesion, thermal stability, and low copper diffusion. Three systems are examined as copper diffusion barriers, a polyurea film deposited by the reaction of 1,4-phenylene diisocyanate (PDIC) and ethylenediamine (ED), a polyurea film with a sulfide-modified backbone, and a polythiourea films using a modified coupling chemistry. Following deposition of the MLD films, copper is sputter deposited. The copper diffusion barrier properties of the film are tested through adhesion and annealing tests, including 4-point bend testing and TEM imaging to examine the level of copper penetration. The promise and challenges of MLD-formed organic copper diffusion barriers will be discussed.