Atomic layer deposition literature review

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A good place to start: a list of companies that provide various ALD technologies, precursors etc.

This page describes selected literature available on atomic layer deposition.[edit | edit source]

Applications of atomic layer deposition to nanofabrication and emerging nanodevices[1][edit | edit source]

Abstract: Recently, with scaling down of semiconductor devices, need for nanotechnology has increased enormously. For nanoscale devices especially, each of the layers should be as thin and as perfect as possible. Thus, the application of atomic layer deposition (ALD) to nanofabrication strategies and emerging nanodevices has sparked a good deal of interest due to its inherent benefits compared to other thin film deposition techniques. Since the ALD process is intrinsically atomic in nature and results in the controlled deposition of films at the atomic scale, ALD produces layers with nanometer scale thickness control and excellent conformality. In this report, we review current research trends in ALD processes, focusing on the application of ALD to emerging nanodevices utilizing fabrication through nanotechnology.

  • Atomic layer deposition consists of four essential steps: 1)precursor exposure, 2) evacuation or purging of the precursors and any byproducts from the chamber, 3) exposure of the reactant species, typically oxidants or reagents, and 4) evacuation or purging of the reactants and byproduct molecules from the chamber .
  • A clear and distinctive feature of ALD lies in the self-limitation for precursor adsorption and alternate, sequential exposure of precursors and reactants
  • A challenge of deposition on nanoscale three-diamension surface is that while saturated adsorption is achieved on the flat surface at a given amount of precursor exposure, incomplete saturation reaction may occur deep inside of nanosize holes or vias, leading to poor overall conformality
  • The required exposure to achieve good saturation can be controlled by varying exposure time or working pressure. Higher pressure during precursor exposure was shown to enhance precursor adsorption on the inside surface of the pores, resulting in the reduction of exposure time for saturation.
  • Some materials, particularly metal ALD material, may yield islands during ALD process, which affects the growth rate, resulting in a non-linear increase in film thickness vs. growth cycles, and makes it difficult to obtain atomically smooth surfaces
  • Another important characteristic of ALD is the ability to deposit relatively high quality films at low temperature, that gives opportunities to material with low evaporation temperature, like InN.( low compared to CVD even at low temperature).
  • Another advantage is that ALD can be performed on polymer substrates with nucleation site. TMA molecules into the pores of polymers was found to facilitate the nucleation of ALD Al2O3.
  • Method: fluid bed reactor can be used for nanoparticle ALD.
  • ALD processes the advantage of biomtemplating from existing materials such as bacteria, butterfly wings, spider silk, etc. due to its high conformality and low growth temperature.
  • High aspect ratio AAO with ALD perform as templates for complex nanomorphology fabrication, particular the nanoarrays, nanotube and nanorods.

Atomic Layer Deposition: An Overview[2][edit | edit source]


  • Precise thickness control at Angstrom or monolayer level
  • Extremely smooth and conformal to the original substrate due to removal of the randomness of the precursor flux, which drives the reactions to completion during every reaction cycle and leaves no surface site behind
  • Being extendible to very large substrates and to parallel processing of multiple substrates

Al2O3 ALD as a Model ALD System:

  • Driven by extremely high enthalpy, Al2O3 formation reaction is very efficient and self-limiting
  • Higher temperature leads to decreasing of growth rate since the loss of surface species is progressive

Thermal and Plasma or Radical-Enhanced ALD: Difference and similarities between ALD and CVD:

  • Both use binary reactions, for CVD process, the reactants are present at the same time and form the product film continuously on the substrate. In ALD, the substrate is exposed to the A and B reactants individually and the product film is formed in a stepwise and very digital fashion(cycle by cycle).
  • Higher the temperature, growth rate per cycle increase as a result of CVD

Plasma or Radical-Enhanced ALD:

  • Plasma sources can be used to generate hydrogen radicals that induce the reaction.
  • For single-element ALD materials
  • Can be deposited using a binary reaction sequence
  • Limitation: not conformal in trenched samples due to attenuated radical concentration in trenched area
  • Si and Ge ALD can only be performed on pure Si and Ge surfaces since they will react with the oxygen in other kind of substrate
  • Plasma ALD has also been useful to deposit metal nitrides which generally cannot be grown with high quality using organometallic precursors.

Reactors for ALD

  • Most ALD reactors operate with an inert carrier gas in viscous flow. The optimum pressure for viscous flow reactors is ~1 Torr.
  • One ALD reactor that optimizes the residence times during reaction and purging is known as synchronously modulated flow and draw(SMFD), Which enables high-speed gas flow switching, more efficiently reactant utilization and thus shorten ALD cycle times of <1s for ALD system like Al2O3 ALD
  • “Hot wall” reactors heat the walls, gas and substrates in the reactor to the temperature of the walls, while in “cold wall” reactor, only the substrate is heated and the walls remain at room temperature or are only warmed slightly.
  • Batch reactors can coat multiple samples at the same time and can dramatically shorten the required time to coat one sample. Even though the reactant and purging time constants are longer in these reactors, the multiplex advantage can offset the time.
  • Plasma source during plasma ALD usually operated at ~100-500 mTorr.

Metal ALD Using Thermal Chemistry

  • Metal ALD based on thermal chemistry does not have the limitations caused by surface recombination that restrict radical-enhanced ALD in high aspect ratio structures.
  • Fluorosilane elimination results from the reaction of metal fluorides and silicon precursors such as SiH4 and Si2H6 with an enthalpy change around ~200kcal
  • FTIR and QCM are efficient in situ methods in surface chemistry studies.
  • Si2H6 insertion into Si-H bonds leads to Si CVD, which is more pronounced at higher temperatures and larger Si2H6 exposures.
  • In combustion chemistry process, the organic ligands of the organometallic metal precursors react with oxygen to produce CO2 and H2O as combustion products. Used for VIII group, especially Ru and Pt ALD.
  • mass gain in combustion ALD observed by QCM. Ixidation of the surface organic species initially produces a mass loss. A subsequent mass gain is produced when O2 deposits oxygen to the deposited metal surface.
  • Hydrogen reduction chemistry is used for Cu ALD and Pd ALD using Cu(thd)2 and Pd(hfac)2.
  • Alternative approaches to metal ALD have focused on depositing a metal oxide and then reducing this metal oxide with H2 or other reducing agents.

Nucleation and Growth during ALD

  • Effective nucleation for ALD requires surface chemical species that will react with the ALD precursors during the very first cycle.
  • Oxide surfaces have MOH* hydroxyl groups that are typically reactive with organometallic precursors.
  • ALD on H-Si(100) turns to be very difficult due to lack of reactive species on the surface. ALD process often takes dozens of cycles before reaching a linear growth feature. Some metal oxide even produce islands on H-Si(100) surface when employed to ALD. One approach to solving the problem is to use more active precursors like AlCH3OH.
  • Metal ALD on oxide surface, even not significant, is another ALD system that displays nucleation difficulties. Auger electron spectroscopy shows that W ALD requires approximately 8-9 cycles to nucleate on SiO2 surfaces.
  • The W ALD growth per cycle increases dramatically and reaches a maximum before reducing to a slightly smaller W ALD growth per cycle. This behavior is expected as W ALD islands grow and then grow together and coalesce to form a continuous film.
  • Al2O3 ALD on carbon nanotubes(CNTs) only yields Al2O3 nanospheres since the surface of CNTs is very smooth and has no reaction species on it and the nanospheres originates from the defects on the surface of the CNTs. However, the nucleation of Al2O3 ALD can be facilitated by functionalization of CNTs with nitroaniline or NO2.

Low Temperature ALD:

  • Al2O3 ALD can be conducted at room temperature due to the high exothermal nature. The major issue is the required purge times to avoid possible CVD process.
  • Using catalysts for gas phase deposition during ALD or CVD is very rare. However, for binary chemical reaction with very slow rate, catalysts can be used to boost the reaction rate. One example is SiO2 ALD using SiCl4 and H2O, which only grow in the presence of Lewis bases like NH3 and pyridine.

ALD on Polymers:

  • Mechanism for ALD on polymers: 1) one of the ALD precursors, such as TMA, diffuses into the near surface region of the polymer; 2) clusters of the ALD material form in the near surface region as a result of the bimolecular reaction between the two ALD precursors; 3) the clusters grow and eventually begin to coalesce; 4) a continuous film is formed that prevents the diffusion of additional precursor into the polymer; and 5) the ALD material grows linearly on the continuous ALD film.

ALD on High Aspect Ratio Structures:

  • SEM analysis has revealed that conformal ALD coating of high aspect ratio structures is dependent on the ALD exposure times.
  • Different models are presented for calculating the required exposure time for conformal ALD

ALD on particles:

  • Applications: 1) to modify the surface chemical properties of particles while retaining the bulk properties of the original particles. 2) to deposit protective and insulating coatings on particles to prevent particle oxidation or electrical conduction. 3) ALD coatings on particles can also serve to modify the optical or mechanical properties of the particles.
  • ALD on particles has been demonstrated in a fluidized particle bed where particles are suspended in the flux of reactant gases. Since the aggregation of particles is dynamic, the particles won’t glued together during ALD
  • A rotary reactor is also used to prevent particles from agglomeration.

ALD of Nanolaminates and Alloys: Polymer ALD:

  • Organic polymers can be growth by molecular layer deposition(MLD) which applies to various polyamides using acyl chlorides and amines as the reactants.
  • MLD of hybrid organic-inorganic polymers has been demonstrated using inorganic precursors from ALD with various organic precursors.
  • Low vapor pressure and thermally sensitive organic precursors are main challenge of MLD. These risks may lead to porous polymer layers, adding additional growth mechanism for MLD when reactants diffuse into the MLD films.

Additional Topics:

  • Reactions that are not self-limiting due to species decomposition or never reach completion cannot be used to ALD systems.
  • “ALD window” is the region of processing temperature in which nearly ideal ALD behavior can be observed. Temperature below the window may cause condensation or incomplete reaction while temperature higher than the region leads to decomposition or desorption or loss of surface species.
  • Spatial patterning can be achieved by conventional semiconductor processing like photoresist and photolithography methods. In addition self-assembled monolayers can also be used as a masking layer followed by removal methods.

Atomic Layer Deposition for Novel Dye-Sensitized Solar Cells[3][edit | edit source]

Abstract: Herein we present the latest fabrication and characterization techniques for atomic layer deposition of Al2O3, ZnO, SnO2, Nb2O5, HfO2, Ga2O3 and TiO2 for research on dye-sensitized solar cell. In particular, we review the fabrication of state-of-the-art 3D host-passivation-guest photoanodes and ZnO nanowires as well as characterize the deposited thin films using spectroscopic ellipsometry, X-ray diffraction, Hall effect, J-V curves and electrochemical impedance spectroscopy.

  • Author(s) proposed a completely novel 3D host-passivation-guest structure using self-assembly and atomic layer deposition techniquies that enabled an increase of more than 100mV in a liquid state DSC.
  • ALD technique can be used to directly passivate surface with random morphology with atomic scaled dense layer, which could be beneficial for DSC fabrication by reducing interfacial recombination and improving the host chemical stability.
  • SE are used to confirm the uniformity of the deposition throughout the reactor.

Coking- and Sintering-Resistant Palladium Catalysts Achieved Through Atomic Layer Deposition[4][edit | edit source]

Abstract: We showed that alumina (Al2O3) overcoating of supported metal nanoparticles (NPs) effectively reduced deactivation by coking and sintering in high-temperature applications of heterogeneous catalysts. We overcoated palladium NPs with 45 layers of alumina through an atomic layer deposition (ALD) process that alternated exposures of the catalysts to trimethylaluminum and water at 200°C. When these catalysts were used for 1 hour in oxidative dehydrogenation of ethane to ethylene at 650°C, they were found by thermogravimetric analysis to contain less than 6% of the coke formed on the uncoated catalysts. Scanning transmission electron microscopy showed no visible morphology changes after reaction at 675°C for 28 hours. The yield of ethylene was improved on all ALD Al2O3 overcoated Pd catalysts.

Atomic layer-deposited tunnel oxide stabilizes silicon photoanodes for water oxidation[5][edit | edit source]

Abstract: A leading approach for large-scale electrochemical energy production with minimal global-warming gas emission is to use a renewable source of electricity, such as solar energy, to oxidize water, providing the abundant source of electrons needed in fuel synthesis. We report corrosion-resistant, nanocomposite anodes for the oxidation of water required to produce renewable fuels. Silicon, an earth-abundant element and an efficient photovoltaic material, is protected by atomic layer deposition (ALD) of a highly uniform, 2 nm thick layer of titanium dioxide (TiO2) and then coated with an optically transmitting layer of a known catalyst (3 nm iridium). Photoelectrochemical water oxidation was observed to occur below the reversible potential whereas dark electrochemical water oxidation was found to have low-to-moderate overpotentials at all pH values, resulting in an inferred photovoltage of ~550 mV. Water oxidation is sustained at these anodes for many hours in harsh pH and oxidative environments whereas comparable silicon anodes without the TiO2 coating quickly fail. The desirable electrochemical efficiency and corrosion resistance of these anodes is made possible by the low electron-tunnelling resistance (<0.006 Ω cm2 for p+-Si) and uniform thickness of atomic-layer deposited TiO2.

  • TiO2 is an useful photoanode material but its large bandgap limits the absorption of solar spectrum to only a fraction in the ultraviolet, resulting in low efficiency as a photoanode
  • Small bandgap semiconductor such as silicon are capable of absorbing a large portion of the solar spectrum, but are not stable at the highly oxidative potentials required for water oxidation
  • TiO2 thickness of 2nm was found to prevent oxidation of the Si while being thin enough to allow facile electron tunnelling between an overlying catalyst layer(Ir) and the base substrate
  • Intrinsic decoupling of electrochemical reactions site from photovoltaic device in the nanocomposite anode provided by ALD-TiO2 layer, results in high current density
  • It is expected to have even higher current density because in this experiment the surface is smooth and thus provides little reactive sites per nominal substrate area

Mimicking the colourful wing scale structure of the Papilio blumei butterfly[6][edit | edit source]

Abstract: The brightest and most vivid colours in nature arise from the interaction of light with surfaces that exhibit periodic structure on the micro- and nanoscale. In the wings of butterflies, for example, a combination of multilayer interference, optical gratings, photonic crystals and other optical structures gives rise to complex colour mixing. Although the physics of structural colours is well understood, it remains a challenge to create artificial replicas of natural photonic structures. Here we use a combination of layer deposition techniques, including colloidal self-assembly, sputtering and atomic layer deposition, to fabricate photonic structures that mimic the colour mixing effect found on the wings of the Indonesian butterfly Papilio blumei. We also show that a conceptual variation to the natural structure leads to enhanced optical properties. Our approach offers improved efficiency, versatility and scalability compared with previous approaches.

Atomic layer deposition of transition metals[7][edit | edit source]

Abstract: Atomic layer deposition (ALD) is a process for depositing highly uniform and conformal thin films by alternating exposures of a surface to vapours of two chemical reactants. ALD processes have been successfully demonstrated for many metal compounds, but for only very few pure metals. Here we demonstrate processes for the ALD of transition metals including copper, cobalt, iron and nickel. Homoleptic N,N′-dialkylacetamidinato metal compounds and molecular hydrogen gas were used as the reactants. Their surface reactions were found to be complementary and self-limiting, thus providing highly uniform thicknesses and conformal coating of long, narrow holes. We propose that these ALD layers grow by a hydrogenation mechanism that should also operate during the ALD of many other metals. The use of water vapour in place of hydrogen gas gives highly uniform, conformal films of metal oxides, including lanthanum oxide. These processes should permit the improved production of many devices for which the ALD process has previously not been applicable.

Electrical characterization of thin Al2O3 films grown by atomic layer deposition on silicon and various metal substrates[8][edit | edit source]

Abstract: Al2O3 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 Al2O3 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 Al2O3 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 Al2O3 films. For a typical Al2O3 ALD film with a 120 Å thickness, leakage currents of <1 nA/cm2 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 Al2O3 ALD films. The measured dielectric constant decreased with decreasing Al2O3 film thickness and suggested the presence of a thin interfacial oxide layer. For Al2O3 ALD films grown on n-type Si(1 0 0), capacitance measurements were consistent with an interfacial layer with a SiO2 equivalent oxide thickness of 11 Å. Spectroscopic ellipsometry investigations also were in agreement with a SiO2 interfacial layer with a 13 Å thickness.

Improved Functionality of Lithium-Ion Batteries Enabled by Atomic Layer Deposition on the Porous Microstructure of Polymer Separators and Coating Electrodes[9][edit | edit source]

Abstract: Atomic layer deposition (ALD) of Al2O3 is applied on a polypropylene separator for lithium-ion batteries. A thin Al2O3 layer (<10 nm) is coated on every surface of the porous polymer microframework without significantly increasing the total separator thickness. The thin Al2O3 ALD coating results in significantly suppressed thermal shrinkage, which may lead to improved safety of the batteries. More importantly, the wettability of Al2O3 ALD-coated separators in an extremely polar electrolyte based on pure propylene carbonate (PC) solvent is demonstrated, without any decrease in electrochemical performances such as capacity, rate capability, and cycle life. Finally, a LiCoO2/natural graphite full cell is demonstrated under extremely severe conditions (pure PC-based electrolyte and high (4.5 V) upper cut-off potential), which is enabled by the Al2O3 ALD coating on all three components (cathode, anode, and separator).

Ab Initio Calculations of the Reaction Mechanisms for Metal−Nitride Deposition from Organo-Metallic Precursors onto Functionalized Self-Assembled Monolayers[10][edit | edit source]

Abstract: An atomistic mechanism has been derived for the initial stages of the adsorption reaction for metal?nitride atomic layer deposition (ALD) from alkylamido organometallic precursors of Ti and Zr on alkyltrichorosilane-based self-assembled monolayers (SAMs). The effect of altering the terminal functional group on the SAM (including ?OH, ?NH2, ?SH, and ?NH(CH3)) has been investigated using the density functional theory and the MP2 perturbation theory. Reactions on amine-terminated SAMs proceed through the formation of a dative-bond complex with an activation barrier of 16?20 kcal/mol. In contrast, thiol-terminated SAMs form weak hydrogen-bonded intermediates with activation barriers between 7 and 10 kcal/mol. The deposition of Ti organometallic precursors on hydroxyl-terminated SAMs proceeds through the formation of stronger hydrogen-bonded complexes with barriers of 7 kcal/mol. Zr-based precursors form dative-bonded adducts with near barrierless transitions. This variety allows us to select a kinetically favorable substrate for a chosen precursor. The predicted order of reactivity of differently terminated SAMs and the temperature dependence of the initial reaction probability have been confirmed for Ti-based precursors by recent experimental results.53 We predict that the replacement of methyl groups by trifluoromethyl groups on the SAM backbone decreases the activation barrier for amine-terminated SAMs by 5 kcal/mol. This opens a route to alter the native reactivities of a given SAM termination, in this case making amine termination energetically viable. The surface distribution of SAM molecules has a strong effect on the adsorption kinetics of Ti-based precursors. Unimolecular side decomposition reactions were found to be kinetically competitive with adsorption at 400 K.

Atomic Layer Deposition of Metal Tellurides and Selenides Using Alkylsilyl Compounds of Tellurium and Selenium[11][edit | edit source]

Abstract: Atomic layer deposition (ALD) of metal selenide and telluride thin films has been limited because of a lack of precursors that would at the same time be safe and exhibit high reactivity as required in ALD. Yet there are many important metal selenide and telluride thin film materials whose deposition by ALD might be beneficial, for example, CuInSe2 for solar cells and Ge2Sb2Te5 for phase-change random-access memories. Especially in the latter case highly conformal deposition offered by ALD is essential for high storage density. By now, ALD of germanium antimony telluride (GST) has been attempted only using plasma-assisted processes owing to the lack of appropriate tellurium precursors. In this paper we make a breakthrough in the development of new ALD precursors for tellurium and selenium. Compounds with a general formula (R3Si)2Te and (R3Si)2Se react with various metal halides forming the corresponding metal tellurides and selenides. As an example, we show that Sb2Te3, GeTe, and GST films can be deposited by ALD using (Et3Si)2Te, SbCl3, and GeCl2·C4H8O2 compounds as precursors. All three precursors exhibit a typical saturative ALD growth behavior and GST films prepared at 90 °C show excellent conformality on a high aspect-ratio trench structure.

Blocking the Lateral Film Growth at the Nanoscale in Area-Selective Atomic Layer Deposition[12][edit | edit source]

Abstract: Area-selective atomic layer deposition (ALD) allows the growth of highly uniform thin inorganic films on certain parts of the substrate while preventing the film growth on other parts. Although the selective ALD growth is working well at the micron and submicron scale, it has failed at the nanoscale, especially near the interface where there is growth on one side and no-growth on the other side. The reason is that methods so far solely rely on the chemical modification of the substrate, while neglecting the occurrence of lateral ALD growth at the nanoscale. Here we present a proof-of-concept for blocking the lateral ALD growth also at the nanoscale by combining the chemical surface modification with topographical features. We demonstrate that area-selective ALD of ZnO occurs by applying the diethylzinc/water ALD process on cicada wings that contain a dense array of nanoscopic pillars. The sizes of the features in the inorganic film are down to 25 nm which is, to the best of our knowledge, the smallest obtained by area-selective ALD. Importantly, our concept allows the synthesis of such small features even though the film is multiple times thicker.

Growth of Crystalline Gd2O3 Thin Films with a High-Quality Interface on Si(100) by Low-Temperature H2O-Assisted Atomic Layer Deposition[13][edit | edit source]

Abstract:This work documents the first example of deposition of high-quality Gd2O3 thin films in a surface-controlled, self-limiting manner by a water-based atomic layer deposition (ALD) process using the engineered homoleptic gadolinium guanidinate precursor [Gd(DPDMG)3]. The potential of this class of compound is demonstrated in terms of a true ALD process, exhibiting pronounced growth rates, a high-quality interface between the film and the substrate without the need for any additional surface treatment prior to the film deposition, and most importantly, encouraging electrical properties.

Growth of Ge Nanofilms Using Electrochemical Atomic Layer Deposition, with a “Bait and Switch” Surface-Limited Reaction[14][edit | edit source]

Abstract:Ge nanofilms were deposited from aqueous solutions using the electrochemical analog of atomic layer deposition (ALD). Direct electrodeposition of Ge from an aqueous solution is self-limited to a few monolayers, depending on the pH. This report describes an E-ALD process for the growth of Ge films from aqueous solutions. The E-ALD cycle involved inducing a Ge atomic layer to deposit on a Te atomic layer formed on Ge, via underpotential deposition (UPD). The Te atomic layer was then reductively stripped from the deposit, leaving the Ge and completing the cycle. The Te atomic layer was bait for Ge deposition, after which the Te was switched out, reduced to a soluble telluride, leaving the Ge (one ?bait and switch? cycle). Deposit thickness was a linear function of the number of cycles. Raman spectra indicated formation of an amorphous Ge film, consistent with the absence of a XRD pattern. Films were more stable and homogeneous when formed on Cu substrates, than on Au, due to a larger hydrogen overpotential, and the corresponding lower tendency to form bubbles.

Coaxial Heterogeneous Structure of TiO2 Nanotube Arrays with CdS as a Superthin Coating Synthesized via Modified Electrochemical Atomic Layer Deposition[15][edit | edit source]

Abstract:We report the fabrication and characterization of CdS/TiO2 nanotube-array coaxial heterogeneous structures. Such structures may potentially be applied in various photocatalytic fields, such as water photocatalytic decomposition and toxic pollutant photocatalytic degradation. Thin films of CdS are conformally deposited onto TiO2 nanotubes using a modified method of electrochemical atomic layer deposition. We propose that such nanostructured electrodes can overcome the poor absorption and high charge-carrier recombination observed with nanoparticulate films. The practical electrochemical deposition technique promotes the deposition of CdS onto the TiO2 tube walls while minimizing deposition at the tube entrances, thus preventing pore clogging. The coaxial heterogeneous structure prepared by the new electrochemical process significantly enhances CdS/TiO2 and CdS/electrolyte contact areas and reduces the distance that holes and electrons must travel to reach the electrolyte or underlying conducting substrate. This results in enhanced photon absorption and photocurrent generation. The detailed synthesis process and the surface morphology, structure, elemental analysis, and photoelectrochemical properties of the resulting films with the CdS/TiO2 nanotube-array coaxial heterogeneous structure are discussed. In comparison with a pure TiO2 nanotube array, a 5-fold enhancement in photoactivity was observed using the coaxial heterogeneous structure. This methodology may be useful in designing multijunction semiconductor materials for coating of highly structured substrates.

Atomic Layer Deposition of Metal Oxides on Pristine and Functionalized Graphene[16][edit | edit source]

Abstract:We investigate atomic layer deposition (ALD) of metal oxide on pristine and functionalized graphene. On pristine graphene, ALD coating can only actively grow on edges and defect sites, where dangling bonds or surface groups react with ALD precursors. This affords a simple method to decorate and probe single defect sites in graphene planes. We used perylene tetracarboxylic acid (PTCA) to functionalize the graphene surface and selectively introduced densely packed surface groups on graphene. Uniform ultrathin ALD coating on PTCA graphene was achieved over a large area. The functionalization method could be used to integrate ultrathin high-? dielectrics in future graphene electronics.

Thin film atomic layer deposition equipment for semiconductor processing[17][edit | edit source]

Abstract:Atomic layer deposition (ALD) of ultrathin high-K dielectric films has recently penetrated research and development lines of several major memory and logic manufacturers due to the promise of unprecedented control of thickness, uniformity, quality and material properties. LYNX-ALD technology from Genus, currently at beta phase, was designed around the anticipation that future ultrathin materials are likely to be binary, ternary or quaternary alloys or nanolaminate composites. A unique chemical delivery system enables synergy between traditional, production-proven low pressure chemical vapor deposition (LPCVD) technology and atomic layer deposition (ALD) controlled by sequential surface reactions. Source chemicals from gas, liquid or solid precursors are delivered to impinge on reactive surfaces where self-limiting surface reactions yield film growth with layer-by-layer control. Surfaces are made reactive by the self-limiting reactions, by surface species manipulation, or both. The substrate is exposed to one reactant at a time to suppress possible chemical vapor deposition (CVD) contribution to the film. Precisely controlled composite materials with multiple-component dielectric and metal–nitride films can be deposited by ALD techniques. The research community has demonstrated these capabilities during the past decade. Accordingly, ALD equipment for semiconductor processing is unanimously in high demand. However, mainstream device manufacturers still criticize ALD to be non-viable for Semiconductor device processing. This article presents a broad set of data proving feasibility of ALD technology for semiconductor device processing.

Chemical vapour deposition and atomic layer deposition of amorphous and nanocrystalline metallic coatings: Towards deposition of multimetallic films[18][edit | edit source]

Abstract:This paper provides a prospective insight on chemical vapour deposition (CVD) and atomic layer deposition (ALD) as dry techniques for the processing of amorphous and nanocrystalline metallic thin films. These techniques are part of major technologies in application fields such as microelectronics, energy, or protective coatings. From thermodynamic analysis, areas of investigation to generate a set of materials with the strongest propensity for amorphization as well as useful guidelines for the target phase material deposition are provided. Prospective to develop MOCVD (metalorganic chemical vapour deposition) and ALD of intermetallic films, in view of fabrication of metallic glass thin films is proposed. Examples from selected ALD and MOCVD single element metallic deposition processes will be described to illustrate the effect of deposition parameters on the physico-chemical properties of the films. This processing approach is particularly promising for metallic glass thin films.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process[19][edit | edit source]

Abstract:Atomic layer deposition (ALD), a chemical vapor deposition technique based on sequential self-terminating gas #x2013;solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD #x2014;including the question who made the first ALD experiments #x2014;and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions, reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the exp- erimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Formation of strontium template on Si(1 0 0) by atomic layer deposition[20][edit | edit source]

Abstract:The formation of ordered Sr overlayers on Si(1 0 0) by Atomic Layer Deposition (ALD) from bis(triisopropylcyclopentadienyl) Strontium (Sr(C5iPr3H2)2) and H2O has been investigated. SrO overlayers were deposited on a 1–2 nm SiO2/Si(1 0 0) substrate, followed by a deoxidation process to remove the SiO2 layer at high temperatures. Auger electron spectroscopy, Rutherford backscattering spectrometry, spectroscopic ellipsometry, and low-energy electron diffraction were used to investigate the progress of both ALD and deoxidation processes. Results show that an ordered Sr/Si(1 0 0) surface with 2 × 1 pattern can be obtained after depositing several monolayers of SrO on Si using ALD followed by an anneal at 800–850 °C. The (2 × 1) ordered Sr/Si(1 0 0) surface is known to be an excellent template for the epitaxial growth of SrTiO3 (STO) oxide. The present results demonstrate that ALD is a potential alternative to molecular beam epitaxy methods for the fabrication of epitaxial oxides on semiconductor substrates.

Atomic layer deposition of palladium films on Al2O3 surfaces[21][edit | edit source]

Abstract:We examined the atomic layer deposition (ALD) of Pd films using sequential exposures of Pd(II) hexafluoroacetylacetonate (Pd(hfac)2) and formalin and discovered that formalin enables the efficient nucleation of Pd ALD on Al2O3. In situ quartz crystal microbalance measurements revealed that the Pd nucleation is hampered by the relatively slow reaction of the adsorbed Pd(hfac)2 species, but is accelerated using larger initial Pd(hfac)2 and formalin exposures. Pd nucleation proceeds via coalescence of islands and leaves hfac contamination at the Al2O3 interface. Pd films were deposited on the thermal oxide of silicon, glass and mesoporous anodic alumina following the ALD of a thin Al2O3 seed layer and analyzed using a variety of techniques. We measured a Pd ALD growth rate of 0.2 Å/cycle following a nucleation period of slower growth. The deposited films are cubic Pd with a roughness of 4.2 nm and a resistivity of 11 μΩ cm at 42 nm thickness. Using this method, Pd deposits conformally on the inside of mesoporous anodic alumina membranes with aspect ratio ∼1500 yielding promising hydrogen sensors.

Effects of surface passivation during atomic layer deposition of Al2O3 on In0.53Ga0.47As substrates[22][edit | edit source]

Abstract:In this work we investigate the effect of different III–V surface passivation strategies during atomic layer deposition of Al2O3. X-ray photoelectron spectroscopy indicates that bare As-decapped and sulfur passivated In0.53Ga0.47As present residual oxides on the surface just before the beginning of the Al2O3 deposition while the insertion of a Ge interface passivation layer results in an almost oxide free Ge/III–V interface. The study of the initial growth regimes, by means of in situ spectroscopic ellipsometry, shows that the growth of Al2O3 on Ge leads to an enhanced initial growth accompanied by the formation of Ge–O–Al species thus affecting the final electrical properties of the stack. Alternatively, deposition on decapped and S-passivated In0.53Ga0.47As results in a more controlled growth process. The sulfur passivation leads to a better electrical response of the capacitor that can be associated to a lower oxide/semiconductor interface trap density.

Surface Chemistry for Atomic Layer Growth[23][edit | edit source]

Abstract: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.

Low-Temperature Al2O3 Atomic Layer Deposition[24][edit | edit source]

Abstract: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.

Ultrathin Coatings on Nano-LiCoO2 for Li-Ion Vehicular Applications[25][edit | edit source]

Abstract:To deploy Li-ion batteries in next-generation vehicles, it is essential to develop electrodes with durability, high energy density, and high power. Here we report a breakthrough in controlled full-electrode nanoscale coatings that enables nanosized materials to cycle with durable high energy and remarkable rate performance. The nanoparticle electrodes are coated with Al2O3 using atomic layer deposition (ALD). The coated nano-LiCoO2 electrodes with 2 ALD cycles deliver a discharge capacity of 133 mAh/g with currents of 1400 mA/g (7.8C), corresponding to a 250% improvement in reversible capacity compared to bare nanoparticles (br-nLCO), when cycled at this high rate. The simple ALD process is broadly applicable and provides new opportunities for the battery industry to design other novel nanostructured electrodes that are highly durable even while cycling at high rate.

  • The ability to fabricate nanoversions of known materials can also greatly enhance power performance by simply increasing the surface-to-volume ratio.
  • Each ALD cycle deposits a uniform Al2O3 layer of approximately 1.1-2.2 A in thickness. 6 ALD cycles yields ab 1out one nanometer thick uniform and conformal Al2O3 ALD coated LiCoO2.
  • For bulk powders, even a slight increase in impedance from surface passivation can seriouly degrade the performance of a material at high rates because of the long Li+ ion diffusion lengths.For the nanopowders, however, increased surface area resulting in a short Li+ ion diffusion length can buffer this abrupt capacity fade as long as increased surface reactions do not prove to be detrimental.
  • Increased interfacial area between LiCoO2 and electrolyte can also contribute to accelerate charge transfer reaction. Al2O3-coated electrode with 2 ALD cycles further reduces the overpotential and thus increases the capacity.
  • Thick ALD coatings(11A) results in poor Li+ conductivity and further demonstrates the need for control of ultrathin coatings at the nanoscale.
  • Al2O3 ALD can transport Li+ ions faster than the SEI formed on the uncoated nano LiCoO2 surface.
  • The Al2O3-coated nano-LiCoO2 electrode with 2 ALD cycles exhibits a discharge capacity of 133 mAh/g at 1400 mA/g, which corresponds a 250% improvement in reversible capacity compared to the bare nanoparticles.

Effective Passivation of AlGaN/GaN HEMTs by ALD-Grown AlN Thin Film[26][edit | edit source]

Abstract:An effective passivation technique for AlGaN/GaN high-electron-mobility transistors (HEMTs) is presented. This technique features an AlN thin film grown by plasma-enhanced atomic layer deposition (PEALD). With in situ remote plasma pretreatments prior to the AlN deposition, an atomically sharp interface between ALD-AlN and III-nitride has been obtained. Significant current collapse suppression and dynamic ON-resistance reduction are demonstrated in the ALD-AlN-passivated AlGaN/GaN HEMTs under high-drain-bias switching conditions.

references[edit | edit source]

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