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Al2O3 films were deposited by atomic layer deposition (ALD) at temperatures as low as 33 °C in a viscous-flow reactor using alternating exposures of Al(CH3)3 (trimethylaluminum [TMA]) and H2O. Low-temperature Al2O3 ALD films have the potential to coat thermally fragile substrates such as organic, polymeric, or biological materials. The properties of low-temperature Al2O3 ALD films were investigated versus growth temperature by depositing films on Si(100) substrates and quartz crystal microbalance (QCM) sensors. Al2O3 film thicknesses, growth rates, densities, and optical properties were determined using surface profilometry, atomic force microscopy (AFM), QCM, and spectroscopic ellipsometry. Al2O3 film densities were lower at lower deposition temperatures. Al2O3 ALD film densities were 3.0 g/cm3 at 177 °C and 2.5 g/cm3 at 33 °C. AFM images showed that Al2O3 ALD films grown at low temperatures were very smooth with a root-mean-squared (RMS) roughness of only 4 ± 1 Å. Current−voltage and capacitance−voltage measurements showed good electrical properties of the low-temperature Al2O3 ALD films. Elemental analysis of the films using forward recoil spectrometry revealed hydrogen concentrations that increased with decreasing growth temperature. No other elements were observed by Rutherford backscattering spectrometry except the parent aluminum and oxygen concentrations. Low-temperature Al2O3 ALD at 58 °C was demonstrated for the first time on a poly(ethylene terephthalate) (PET) polymeric substrate. Al2O3 ALD coatings on PET bottles resulted in reduced CO2 gas permeabilities.

A chemical reactor was constructed for growing thin films using atomic layer deposition (ALD) techniques. This reactor utilizes a viscous flow of inert carrier gas to transport the reactants to the sample substrates and to sweep the unused reactants and reaction products out of the reaction zone. A gas pulse switching method is employed for introducing the reactants. An in situ quartz crystal microbalance (QCM) in the reaction zone is used for monitoring the ALD film growth. By modifying a commercially available QCM housing and using polished QCM sensors, quantitative thickness measurements of the thin films grown by ALD are obtained in real time. The QCM is employed to characterize the performance of the viscous flow reactor during Al2O3 ALD.

Polymeric films can be grown by a sequential, self-limiting surface chemistry process known as molecular layer deposition (MLD). The MLD reactants are typically bifunctional monomers for stepwise condensation polymerization and can yield completely organic films. The MLD of organic–inorganic hybrid polymers can also be accomplished using a bifunctional organic monomer and a multifunctional inorganic monomer. In this work, the growth of a poly(aluminum ethylene glycol) polymer is demonstrated using the sequential exposures of trimethylaluminum (TMA) and ethylene glycol (EG). These hybrid polymers, known as alucones, were grown over a wide range of temperatures from 85 to 175 °C. In situ quartz crystal microbalance and ex situ X-ray reflectivity experiments confirmed linear growth of the alucone film versus number of TMA/EG reaction cycles at all temperatures. The alucone growth rates decreased at higher temperatures. Growth rates varied from 4.0 Å per cycle at 85 °C to 0.4 Å per cycle at 175 °C. In situ Fourier transform infrared spectroscopy was used to monitor the surface reactions during alucone MLD. Ex situ FTIR spectroscopy, X-ray photoelectron spectroscopy, and X-ray reflectivity measurements were also employed to determine the chemical composition, thickness, and density of the alucone films. These ex situ studies revealed that the alucone films grown on Al2O3 ALD surfaces evolved under ambient conditions before reaching a stable state. Alucone films capped with rapid SiO2 ALD displayed much more stability than alucone films grown on Al2O3 ALD surfaces. These results indicated that H2O may facilitate the chemical transformation of the alucone MLD films. The alucone films represent a new class of organic–inorganic hybrid polymers. Modification of this basic alucone MLD chemistry with use of other diols or other bifunctional monomers can produce different alucone polymers with variable properties.

Anodic alumina (AA) membranes are composed of highly uniform, nanometer-scale pores arranged in a hexagonal close-packed array. Depositing conformal films inside the nanopores is extremely difficult because the nanopores have an ultrahigh aspect ratio of L/d ≈ 10 3 . Atomic layer deposition (ALD) is a thin film growth technique that can deposit highly uniform films on high-aspect-ratio substrates with monolayer thickness control. In this study, AA membranes were coated with Al 2 O 3 and ZnO ALD films and subsequently analyzed using cross-sectional scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). SEM analysis of individual nanopores revealed that the AA membranes with nanopore diameters of d ) 65 nm and lengths of L ) 50 µm could be coated conformally by Al 2 O 3 ALD using sufficient reactant exposure times. Zn concentration profiles measured by EPMA following ZnO ALD showed the progressive infiltration of the ZnO ALD into the nanopores with increasing exposure times for aspect ratios as high as L/d ∼5000. Monte Carlo simulation of the experimental results assuming Knudsen diffusion accurately reproduced the experimental Zn concentration profiles and predicted the minimum ALD reactant exposures necessary to achieve conformal films. The Monte Carlo simulation also predicted that the diffusion-limited deposition will become reaction-limited given a sufficiently low ALD reaction probability. To test this idea, Fourier transform infrared absorption measurements were performed during the coating of the AA membranes by Al 2 O 3 and SiO 2 ALD. The surface reactions during Al 2 O 3 ALD have a relatively high reaction probability of ∼10 -3 . In contrast, the surface reactions during SiO 2 ALD have a very low reaction probability of ∼10 -8 . In agreement with the predictions, diffusion-limited behavior with a t 1/2 time dependence was observed during Al 2 O 3 ALD. Reaction-limited behavior with a t 1 time dependence was observed during SiO 2 ALD.

Atomic layer controlled film growth is an important technological and scientific goal that is closely tied to many issues in surface chemistry. This article first reviews the basic concepts of atomic layer growth using molecular precursors and binary reaction sequence chemistry. Many examples are given for the various films that have been grown using this atomic layer growth technique. The paradigms for atomic layer epitaxy (ALE) and atomic layer processing (ALP) are then discussed in terms of self-limiting surface reactions. Recent investigations of the surface chemistry of SiO2 and Al2O3 ALP and GaAs ALE are examined and used to illustrate the possible mechanisms of atomic layer growth. Subsequently, the characteristics of film deposition using atomic layer growth techniques are explored using recent examples for Al2O3 ALP. The structure of the deposited films is also reviewed using results from previous Al2O3 deposition investigations. This article then concludes by discussing possible complications to studies of atomic layer controlled growth using binary reaction sequence chemistry.

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