Viscous Flow Reactor Research Articles

Atomic layer deposition of chromium oxide—An interplay between deposition and etching

Atomic layer deposition (ALD) of chromium oxide (Cr2O3) thin films is investigated in a custom built hot wall viscous flow reactor configuration at 300 °C. Chromium(III) 2,4-pentanedionate [Cr(acac)3] and ozone (O3) are employed as the metal and the oxygen sources, respectively. In situ quartz crystal microbalance (QCM) and ex situ x-ray reflectivity studies are utilized as the two complementary techniques to monitor the growth mechanism and self-limiting deposition chemistry during Cr2O3 ALD. In situ QCM studies reveal a negligible nucleation period on the previously grown Al-OH* terminated surface before revealing the perfectly linear growth mechanism at 300 °C. The saturated growth rate is found to be ca. 0.28 Å/cycle. In addition, excessive O3 exposure also reveals an alternative, controlled, and spontaneous etching pathway of the growing film as a result of the partial surface oxidation of Cr2O3. The as-deposited thin films are found to exhibit a polycrystalline rhombohedral structure without any preferential orientation. X-ray photoelectron spectroscopy studies reveal uniform distribution of Cr and O throughout the stack of ca. 40 nm film with minimum C impurities. High resolution scans of Cr 2p core level also confirm the presence of Cr in the +3 oxidation state with the corresponding multiplet spectrum.

High-capacity rotary drum for atomic layer deposition onto powders and small mechanical parts in a hot-walled viscous flow reactor

Atomic layer deposition (ALD) is uniquely capable of providing uniform thin-film coatings on powder substrates, but powder processing has historically required an ALD reactor designed specifically for powders—e.g., a fluidized bed. Tubular hot-walled viscous-flow reactors commonly employed in laboratory-scale ALD research for coating planar substrates such as silicon have been employed previously to coat gram quantities of powder spread out in a thin layer on a tray, but larger quantities of powder introduce long diffusion pathways where reactants are unable to percolate to the bottom of the powder bed to provide uniform coating in reasonable time periods. In this work, we report a rotary drum with a capacity of 100s of grams that is compatible with conventional tubular hot-walled ALD reactors and provides uniform coatings on powders in viscous flow operation. We benchmark this system using Al2O3 ALD with trimethylaluminum and water. We examine the effect of rotation speed and powder quantity on saturation time via in situ quadrupole mass spectroscopy measurements and gravimetric analysis. The rotary drum we report is able to provide homogenous coating of up to 75 g of silica gel powder with a total surface area of ∼1500 m2 in viscous flow operation with precursor utilization as high as 70%.

Atomic layer deposition of amorphous antimony sulfide (a-Sb2S3) as semiconductor sensitizer in extremely thin absorber solar cell

Atomic layer deposition of amorphous antimony sulfide (a-Sb2S3) is demonstrated with an alternating exposure of tris(dimethylamino)antimony (TDMASb) and hydrogen sulfide (H2S) at 150 °C in a custom-built viscous flow reactor. Growth mechanism and deposition chemistry are investigated by in situ quartz crystal microbalance and in situ Fourier Transform Infrared spectroscopy. Reaction hypothesis facilitating the binary reaction is established by quantum mechanical density functional theory calculations that essentially support the experimental findings. The developed material is used as a photon harvester in solar cells under extremely thin absorber configuration, with TiO2 and Spiro-OMeTAD as electron and hole transporting layers, respectively. Investigation of charge injection properties with surface photovoltage spectroscopy reveals low but non-negligible density of interfacial (sensitizer/TiO2) electronic defects. The conventional viscous flow reactor configuration is modified to showerhead-type reactor configuration to achieve better uniformity and conformality of a-Sb2S3 on highly porous TiO2 scaffolds. a-Sb2S3 device performance is optimized to achieve the highest power conversion efficiencies of 0.5% while annealed crystalline c-Sb2S3 device reaches power conversion efficiencies of 1.9% under 1 sun illumination.

Porous Silicon Electrochemical Capacitors Using Double Sided Porous Silicon and ALD Coatings

With the advent of the internet of things (IoT), interest in integrating energy storage into silicon with energy harvesting and sensors has significantly increased. Silicon is already the materials of choice for the integrated circuits found in every IoT device; however, the efforts to integrate electrochemical (EC) capacitors on a silicon die have been limited. EC capacitors have cycle lifetimes of many thousands which is especially important for off-power-grid IoT devices where difficulty associated with regularly replacing the batteries of billions of devices is prohibitive. Porous silicon (P-Si) was previously viewed as unusable for energy storage because of its tendency to easily oxidize and difficulty in passivating [1, 2]. Bare porous-Si is not stable over a long period of time due to unwanted reactions with the electrolyte. In our work, we have developed integrated energy storage based on P-Si nanostructures with highly branched pores coated with a variety of atomic layer deposited (ALD) films including films that have the potential of pseudocapacitive properties. ALD of films in ultrahigh aspect-ratio features (AR > 100:1) presents unique challenges. To obtain uniform coatings, efficient surface reactions are needed between high volatility, low molecular weight, small molecular diameter precursors without chemical vapor deposition side reactions. ALD titanium nitride (TiN) using TiCl4 + NH3 met these criteria and resulted in passivating conductive films [3, 4]. To further increase the energy density, materials that may have pseudo capacitive properties including VN, Nb2O5, VTiN, and TiAlN were coated using stop-flow ALD processing. Whereas the majority of ALD is in a continuous viscous flow reactor, a pseudo stop-flow reactor was used to obtain uniform coatings. The substrate was allowed higher-pressure extended precursor soak times with intervening pump-purge cycles to remove excess precursor and reaction byproducts. The large open central regions needed in the pore structure for ion transport also aids in the coating process by enhancing precursor diffusion down the pores. Cross-sectional SEM images in backscatter mode and TEM images were used to examined the penetration depth and show that TiN can penetrate deep into the pores (see Fig. 2), but TiAlN tends to seal the pores and the penetration depth of Nb2O5 is limited. VN however was successfully coated into porous silicon to depths of 10 µm with a double dose process at 325 °C. Measurements from coated P-Si capacitors reveal that high areal capacitance of 3 to 6 mF/cm2 can be achieved using 2 um deep pores and 28 mF/cm2with 12 um deep pores, about two orders of magnitude higher than previously P-Si studies [1, 2]. In addition, to increase the total stored energy and operating voltage, samples were prepared with silicon that had pores etched into both sides of the substrate (see Fig. 1) using a double cell HF tank system with electrolytical backside contact. The substrates were then stacked resulting in electrochemical capacitors in series thereby doubling the maximum voltage to 5 V and also doubling the total energy stored. These devices have the potential to provide integrated on-chip energy storage. [1] S. E. Rowlands, R. J. Latham, and W. S. Schlindwein, Ionics 5, 144–149 (1999). [2] S. Desplobain, G. Gautier, J. Semai, L. Ventura and M. Roy, Phys. Stat. Sol. (c), 4, 2180, 2007. [3] D. S. Gardner, C. W. Holzwarth, Y. Liu, S. B. Clendenning, W. Jin, B. K. Moon, et al., "Integrated On-Chip Energy Storage Using Porous-Silicon Electrochemical Capacitors," presented at the Int'l. Electron Devices Meeting (IEDM), 2014. [4] D. S. Gardner, C. W. Holzwarth III, Y. Liu, S. B. Clendenning, W. Jin, B. K. Moon, C. Pint, Z. Chen, E. Hannah, C. Chen, C. P. Wang, E. Mäkilä, R. Chen, T. Aldridge, and J. L. Gustafson, “Integrated On-Chip Energy Storage Using Passivated Nanoporous-Silicon Electrochemical Capacitors”, Nano Energy (2016), doi: 10.1016/j.nanoen.2016.04.016 Figure 1

Reactor scale simulation of an atomic layer deposition process

To simulate an atomic layer deposition (ALD) process in a reactor scale, three-dimensional deposition of Al2O3 from trimethylaluminum and ozone inside a viscous flow reactor is investigated. The chemistry mechanism used includes both gas-phase and surface reactions. The simulations are performed for a fixed operating pressure of 10torr (1330Pa) and two substrate temperatures at 250°C and 300°C. The Navier–Stokes, energy, and species transport equations are discretized through the finite volume method to simulate transient, laminar and multi-component reacting flows. It is found that the larger surface reaction rate constant, and the greater concentrations of gaseous reactants at the substrate result in higher deposition rates on the substrate at 300°C. At a fixed substrate temperature, the deposition rate distributions are the same among all the cycles that indicate a constant growth rate at each cycle. As a result, Al2O3 growth rates of 3.78 angstrom/cycle and 4.52 angstrom/cycle are obtained for the substrate temperatures of 250°C and 300°C, respectively.

Alucone Alloys with Tunable Properties Using Alucone Molecular Layer Deposition and Al2O3 Atomic Layer Deposition

Ultrathin and conformal hybrid organic–inorganic thin films can be deposited by molecular layer deposition (MLD) techniques. By combining the hybrid organic–inorganic MLD process with an inorganic atomic layer deposition (ALD) process, ALD/MLD alloy films can be deposited that have an adjustable organic–inorganic composition. These alloys have tunable properties that may be useful for designing various functional films. In this study, alucone MLD using trimethylaluminum (TMA) and ethylene glycol (EG) was employed together with Al2O3 ALD using TMA and H2O to deposit alucone alloys at 135 °C. The surface species and film growth were examined using in situ Fourier transform infrared spectroscopy. The mass gain for each reactant exposure during film growth was also monitored using an in situ quartz crystal microbalance (QCM) in a viscous flow reactor. The composition of the alucone alloy was varied by adjusting the relative number of ALD and MLD cycles in the reaction sequence. Alucone alloys were grown using relative numbers of ALD and MLD cycles varying from 1:3 to 6:1 (TMA/H2O:TMA/EG). These alucone alloys displayed varying density, refractive index, elastic modulus, and hardness. The density and refractive index changed from 1.6 g/cm3 and n = 1.45 for pure alucone to 3.0 g/cm3 and n = 1.64 for pure Al2O3, respectively. The elastic modulus and hardness varied from 21 ± 8 GPa and 1.0 ± 0.1 GPa for pure alucone to 198 ± 8 GPa and 13.0 ± 0.2 GPa for pure Al2O3, respectively. These results demonstrate the potential of ALD/MLD alloy films to provide tunable properties for many functional film applications.

Fiber Containment for Improved Laboratory Handling and Uniform Nanocoating of Milligram Quantities of Carbon Nanotubes by Atomic Layer Deposition

The presence of nanostructured materials in the workplace is bringing attention to the importance of safe practices for nanomaterial handling. We explored novel fiber containment methods to improve the handling of carbon nanotube (CNT) powders in the laboratory while simultaneously allowing highly uniform and controlled atomic layer deposition (ALD) coatings on the nanotubes, down to less than 4 nm on some CNT materials. Moreover, the procedure yields uniform coatings on milligram quantities of nanotubes using a conventional viscous flow reactor system, circumventing the need for specialized fluidized bed or rotary ALD reactors for laboratory-scale studies. We explored both fiber bundles and fiber baskets as possible containment methods and conclude that the baskets are more suitable for coating studies. An extended precursor and reactant dose and soak periods allowed the gases to diffuse through the fiber containment, and the ALD coating thickness scaled linearly with the number of ALD cycles. The extended dose period produced thicker coatings compared to typical doses on CNT controls not encased in the fibers, suggesting some effects due to the extended reactant dose. Film growth was compared on a range of single-walled NTs, double-walled NTs, and acid-functionalized multiwalled NTs, and we found that ultrathin coatings were most readily controlled on the multiwalled NTs.

Molybdenum Atomic Layer Deposition Using MoF6 and Si2H6 as the Reactants

Mo ALD has been demonstrated by fluorosilane elimination chemistry using MoF6 and Si2H6 as the reactants. The nucleation and growth characteristics of Mo ALD were investigated using a variety of in situ and ex situ techniques in both high vacuum and viscous flow reactors. Quartz crystal microbalance (QCM) and X-ray reflectivity (XRR) investigations showed that Mo ALD has significant growth rate of 500−600 ng/cm2 per cycle or 6−7 Å per cycle for temperatures between 90 and 150 °C. The large growth rates could result from extra Mo deposition by MoF6 → Mo + 3F2 that may be facilitated by the very exothermic reaction of MoF6 with silicon-containing surface species. The QCM studies revealed that the Mo ALD surface chemistry is self-limiting. The QCM and Auger electron spectroscopy (AES) studies indicated that Mo ALD nucleates very rapidly on Al2O3 ALD surfaces and reaches the linear growth regime after only 4−5 ALD cycles. Oscillatory behavior for the total mass gain and individual mass gains was observed versus ALD cycle number during the nucleation region. The AES studies revealed that Mo films grown in a high vacuum reactor do not contain silicon impurities. In contrast to the AES results, Rutherford backscattering spectroscopy (RBS) analysis showed that Mo ALD films grown in a viscous flow reactor contain ∼16 at % Si impurities. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of silicon and showed that varying temperature, precursor dose and purge parameters did not lower the Si impurities significantly. Glancing incidence X-ray diffraction (GIXRD) studies indicated that Mo ALD films were nanocrystalline. The Si impurities may exist at grain boundaries or amorphous Mo silicides as a result of Si2H6 decomposition during the highly exothermic fluorosilane elimination reaction. Fourier transform infrared (FTIR) analysis revealed that MoFx surface species are reduced to metallic Mo during the Si2H6 exposure. Because of its rapid nucleation rate, Mo ALD films could serve as ultrathin continuous conducting films or as adhesion layers for other metal ALD systems on oxide surfaces.

Atomic Layer Deposition of Hafnium Dioxide on TiN and Self-Assembled Monolayer Molecular Film

Hafnium dioxide thin films with thicknesses ranging from 20 to 100 Å have been grown in a viscous flow reactor using atomic layer deposition (ALD) with tetrakis(dimethylamido)hafnium(IV) and water as the reactants. ALD films are deposited successfully at on different molecular monolayers. X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy analyses show a continuous growth of the layer on the self-assembled monolayer molecules. Electrical properties characterized by current–voltage measurements suggest that three parameters play important roles in determining the physical structure of , namely, the precursor pulse time, the sample exposure time, and the time for purging out an unreacted precursor. ALD conditions for different substrates have been optimized so that excellent electrical properties can be obtained for films on molecules. Transmission electron microscopy of 30 and 60 Å ALD on TiN and molecular monolayers shows a continuous deposition of . This process enables the development of a class of molecular electronic devices, solid-state molecular memory devices.

Conformal ZnO coatings on high surface area silica gel using atomic layer deposition

Silica gel is a mesoporous form of silica with a high specific surface area that is widely used as a support material for heterogeneous catalysis. In this study, we use atomic layer deposition (ALD) methods to deposit ZnO layers onto silica gel supports. ZnO ALD was performed using alternating exposures of diethyl zinc (DEZ) and water to coat 1 g quantities of silica gel in a conventional viscous flow reactor. The coated materials were analyzed using weight gain, X-ray fluorescence, X-ray diffraction, energy dispersive X-ray analysis, and scanning and transmission electron microscopy. These measurements revealed that the silica gel support could be conformally coated using reactant exposure times of ∼ 90 s. The ALD ZnO was amorphous for films deposited using < 5 ALD cycles, but became hexagonal, nanocrystalline ZnO for films deposited using > 5 ALD cycles. In addition to the ZnO films, we also discovered that metallic Zn was deposited in the silica gel using very large DEZ exposures and deposition temperatures > 150 °C. The metallic Zn phase begins to form as soon as the ZnO ALD surface reactions have saturated, indicating that the Zn growth is strongly dependent on the availability of excess DEZ precursor.

Growth, structure, and tribological behavior of atomic layer-deposited tungsten disulphide solid lubricant coatings with applications to MEMS

This paper describes the synthesis, structure, and tribological behavior of nanocomposite tungsten disulphide (WS 2) solid lubricant films grown by atomic layer deposition. A new catalytic route, incorporating a diethyl zinc catalyst, was established to promote the adsorption and growth of WS 2. The films were grown down to 8 nm in thickness by sequential exposures of WF 6 and H 2S gases in a viscous flow reactor on Si, SiO 2, stainless steel, and polycrystalline Si and electroplated Ni microelectromechanical systems structures. Films were studied by cross-sectional transmission electron microscopy (XTEM) with Automated eXpert Spectral Image Analysis (AXSIA) software for X-ray spectral images and X-ray diffraction to determine the coating conformality and crystallinity. The coatings exhibited a hexagonal layered structure with predominant preferentially orientated (0 0 2) basal planes. Regardless of orientation to the substrate surface, these basal planes when sheared imparted low friction with a steady-state friction coefficient as low as 0.008 to 50,000 cycles in a dry nitrogen environment. The formation of smooth transfer films during wear provided low interfacial shear stresses during sliding thus achieving low friction and wear. The XTEM combined with AXSIA of the wear tracks identified this mechanism and the effects of vapor phase reaction by-product etching on insulating and native polycrystalline Si and Ni surfaces.

SiO 2 film growth at low temperatures by catalyzed atomic layer deposition in a viscous flow reactor

SiO 2 thin films were grown by catalyzed atomic layer deposition (ALD) at low temperatures in a viscous flow reactor using sequential SiCl 4 and H 2O exposures. Pyridine (C 5H 5N) was used as a catalyst for both reactant exposures. Micropulsing was employed to avoid possible film contamination by pyridinium chloride. Quartz crystal microbalance experiments measured a SiO 2 film growth rate of 1.35 Å per SiCl 4/H 2O AB cycle at 305 K with SiCl 4, H 2O and pyridine exposures of 10 3–10 4 Langmuir (1 Langmuir = 10 − 6 Torr s). This SiO 2 ALD growth rate was verified by ex situ X-ray reflectivity, spectroscopic ellipsometry and surface profilometry measurements. The SiO 2 ALD film growth rate decreased dramatically at higher temperatures. Transmission Fourier transform infrared spectroscopy studies revealed that the hydrogen-bonded pyridine coverage was correlated with the catalyzed SiO 2 ALD growth rates from 305 to 360 K. Larger pyridine pressures were needed for catalyzed SiO 2 ALD at higher temperatures to offset the larger pyridine desorption rates. The SiO 2 ALD films contained negligible C, N, or Cl impurities as determined by X-ray photoelectron spectrometry. The deposited SiO 2 films were also extremely smooth with a surface roughness identical to the initial Si(100) substrate.

Low-Temperature Al2O3 Atomic Layer Deposition

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 A. Current−voltage and capacitance−voltage...

Atomic-layer deposition of wear-resistant coatings for microelectromechanical devices

Friction and wear are major concerns in the performance and reliability of microelectromechanical systems (MEMS) devices employing sliding contacts. While many tribological coating materials are available, most traditional surface coating processes are unable to apply conformal coatings to the high aspect ratio (height/width) structures typical of MEMS devices. We demonstrate that thin, conformal, wear-resistant coatings can be applied to Si surface-micromachined structures by atomic-layer deposition (ALD). For this demonstration, we apply 10-nm-thick films of Al2O3 using a binary reaction sequence with precursors of trimethyl aluminum and water. Deposition is carried out in a viscous flow reactor at 1 Torr and 168 °C, with N2 as a carrier gas. Cross-section transmission electron microscopy analysis shows that films are uniform to within 5% on MEMS device structures with aspect ratio ranging from 0 to &amp;gt;100. Films are stoichiometric Al2O3, with no evidence of contamination from other species, and are amorphous. Preliminary friction and wear data show that ALD films have promising properties for application to MEMS devices.

Growth of ZnO/Al2O3 Alloy Films Using Atomic Layer Deposition Techniques

Atomic layer deposition (ALD) is an ideal technique for fabricating composite thin films. The thickness and stoichiometry of composite thin films prepared using ALD is dependent on the underlying surface chemistry during ALD film growth. A set of ZnO/Al2O3 alloy films was prepared by ALD in a viscous flow reactor using alternating Zn(CH2CH3)2/H2O exposures for ZnO ALD and alternating Al(CH3)3/H2O exposures for Al2O3 ALD. The ZnO reaction cycle percentage was varied from 0 to 100%. The composite film thicknesses were measured using ex situ stylus profilometry and ellipsometry. The atomic composition of the composite films was established by atomic emission spectroscopy. Large deviations were found when the measured thicknesses and compositions were compared with “rule of mixtures” predictions. Many of the ZnO/Al2O3 alloy films showed lower than expected Zn film content and were thinner than predicted by the ALD growth rates of the pure ZnO and Al2O3 films. To understand these discrepancies, in situ quartz ...

Viscous flow reactor with quartz crystal microbalance for thin film growth by atomic layer deposition

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.

Electrical characterization of thin Al 2O 3 films grown by atomic layer deposition on silicon and various metal substrates

Al 2O 3 films with thicknesses ranging from 30 to 3540 Å were grown in a viscous flow reactor using atomic layer deposition (ALD) with trimethylaluminum and water as the reactants. Growth temperatures ranged from 125 to 425 °C. The Al 2O 3 ALD films were deposited successfully on a variety of substrates including Au, Co, Cr, Cu, Mo, Ni, NiFe, NiMn, Pt, PtMn, Si, stainless steel, W, and ZnO. Electrical properties were characterized by current–voltage and capacitance–voltage measurements using a mercury probe. These measurements focused mainly on Al 2O 3 ALD films deposited on n-type Si(1 0 0) and on Mo-coated Si(1 0 0) substrates. Excellent insulating properties were observed for nearly all of the Al 2O 3 films. For a typical Al 2O 3 ALD film with a 120 Å thickness, leakage currents of <1 nA/cm 2 were observed at an applied electric field of 2 MV/cm. Fowler–Nordheim tunneling was observed at high electric fields and dielectric breakdown occurred only at ⩾5 MV/cm. Dielectric constants of k∼7.6 were measured for thick Al 2O 3 ALD films. The measured dielectric constant decreased with decreasing Al 2O 3 film thickness and suggested the presence of a thin interfacial oxide layer. For Al 2O 3 ALD films grown on n-type Si(1 0 0), capacitance measurements were consistent with an interfacial layer with a SiO 2 equivalent oxide thickness of 11 Å. Spectroscopic ellipsometry investigations also were in agreement with a SiO 2 interfacial layer with a 13 Å thickness.