Atomic layer deposition

1 Introduction to Atomic Layer Deposition and the Chemical Modification of Poly(Styrene) 

The deposition process is governed by the precursor volatility, stability, and reactivity which mainly depend on the substrate/reactor temperature for a given reactor pressure, as illustrated in Fig. 8.2. The ideal process window, the so-called ALD window, in which growth is saturated and insensitive to overexposure with precursor. 

Area-selective ALD is possible, since deposition is based on chemisorption which only occurs in regions of the surface where reactive sites, such as OH and NH2, are present [8]. Typically, monolayers of silane with chemically inert tails are 

The fact that ALD is based on a self-terminating gas-solid reaction yielding excellent deposition conformality aliows to coat high aspect ratio nanostructures, including colloidal arrays, anodized alumina and track etched poly(carbonate) membranes [19, 20]. 

An important point is that both of the reaction steps are self-limiting. In reaction step 1, once all available AlOH(s) surface sites have reacted with the Al(CH3)3(g) precursor, the reaction comes to a stop because the CH3 groups that now terminate the surface are unreactive toward Al(CH3)3(g). Thus, only a single atomic layer of 

FIGURE5.il Conformal deposition over high-aspect-ratio structures can be difficult to achieve by CVD due to keyhole effects and other difficulties posed by the long diffusion paths required to access the inner regions of such complex structures. 

FIGURE 5.12 Schematic illustration of the ALD reaction scheme for the deposition of AljOj. The deposition process involves two distinct reaction steps (step 1, step 2) followed by purge steps (purge 1, purge 2) that are repeatedly cycled to build up the AljOj film layer by layer. 

Question In the atomic layer deposition of alumina (AI2O3), the average deposition rate is approximately 0.5 ML (ML = monolayer) per cycle. A single cycle consists of two purge steps and two reaction steps. Even with careful attention to valve, chamber, and gas flow design, typically at least a few seconds is required to purge the chamber, introduce the precursor gas, and then ensure that all surfaces undergo complete reaction for each step. For the purposes of this question. 

Solution The problem statement indicates that 10 s is required for one complete ALD cycle, which results in the deposition of 0.5 ML of AI2O3. Thus, we can approximate the growth rate as 

W Species A adsorbs strorigly on material B O Reaction product desorbs 

The essential element in all of these options is to saturate the B surface with one and only one layer of A atoms in some carrier form foUowed by coating this A surface with one and only one layer of B. 

In ALD the deposition rate is proportional to the reaetion eyele time rather than the flux of precursor on the surfaee and the total thiekness of material deposited is exactly related to the number of deposition eyeles, making eontrol of growth straightforward at the priee, in general, of a slower overall proeess. However, for a sufficiently reactive precursor the individual steps may be very rapid. A typical 

Bulky ligands may limit the number of adsorbate molecules 

Reconstructed surfaces may only adsorb on certain sites 

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Several patents dealing with the use of volatile metal amidinate complexes in MOCVD or ALD processes have appeared in the literature.The use of volatile amidinato complexes of Al, Ga, and In in the chemical vapor deposition of the respective nitrides has been reported. For example, [PhC(NPh)2]2GaMe was prepared in 68% yield from GaMes and N,N -diphenylbenzamidine in toluene. Various samples of this and related complexes could be heated to 600 °C in N2 to give GaN. A series of homoleptic metal amidinates of the general type [MIRCfNROilnl (R = Me, Bu R = Pr, BuO has been prepared for the transition metals Ti, V, Mn, Fe, Co, Ni, Cu, Ag, and La. The types of products are summarized in Scheme 226. The new compounds were found to have properties well-suited for use as precursors for atomic layer deposition (ALD) of thin films. 

Atomic layer deposition of hafnium silicate thin films using... 

Vaidyanathan R, Cox SM, Happek U, Banga D, Mathe MK, Stickney JL (2006) Preliminary studies in the electrodeposition of PbSe/PbTe superlattice thin films via electrochemical Atomic Layer Deposition (ALD). Langmuir 22 10590-10595... 

In molecular beam epitaxy (MBE), the constituent elements of the desired film in the form of molecular beams are deposited epitaxially onto a heated crystalline substrate. These molecular beams are typically from thermally evaporated elemental sources (e.g., evaporation of elemental As produces molecules of As2, As3, and As4). A refinement of this is atomic layer epitaxy (ALE) (also known as atomic layer deposition, ALD) in which the substrate is exposed alternately to two... 

Reductive UPD is the major atomic layer deposition processes used in EC-ALE, Equation 1. Many metals can be obtained in a soluble oxidized form, from which atomic layers can be deposited at underpotentials. Control points are the reactant... 

Preiner MJ, Melosh NA (2008) Creating large area molecular electronic junctions using atomic layer deposition. Appl Phys Lett 92 213301... 

Carcia, R F. McLean, R. S. Reilly, M. H. 2006. High performance ZnO thin-film transistors on gate dielectrics grown by atomic layer deposition. Appl. Phys. Lett. 88 123509/1-123509/3. 

The electrochemical atomic layer epitaxy (ECALE) technique, also known as electrochemical atomic layer deposition (EC-ALD), is based on layer-by-layer electrodeposition. Each constituent of the thin him are deposited separately using underpotential deposition (UPD) of that element. UPD is a process wherein an atomic layer of one element is deposited on the surface of a different element at a potential under that needed to deposit the element on itself. ECALE has been used to grow mainly II-VI and III-V compounds. A thorough review of ECALE research has been published by Stickney.144 A summary of the materials deposited using ECALE are given in Table 8.4, with a more detailed discussion for a few select examples given below. 

Nistorica, C. Liu, J.-F. Gory, I. Skidmore, G. D. Mantiziba, F. M. Gnade, B. E. Kim, J. 2005. Tribological and wear studies of coatings fabricated by atomic layer deposition and by successive ionic layer adsorption and reaction for micro-electromechanical devices. J. Vacuum Sci. Technol. A. 23 836-840. 

Chemical Vapor Deposition Electrochemical Deposition Molecular Beam Epitaxy Atomic Layer Deposition Thermal Oxidation Spin Coating... 

Increased control of film composition, structure and size can be achieved by limiting the rate of reaction. This is possible using gas phase deposition where the amount of reactant is relatively low. Gas phase deposition loosely covers any hybridization strategy where at least one of the hybrid components is in the gas phase. This includes chemical vapor deposition (CVD), physical vapor deposition (PVD) and atomic layer deposition (ALD) as well as various plasma, sputtering and evaporation processes. 

Atomic layer deposition is a high vacuum process where small amounts of the precursors are leaked into the system sequentially with intermittent evacuations. The ALD enables the conformal deposition of atomically thin layers with precise thickness control at low temperatures without the typical aggregate formation in the gas-phase. 

Li, X., et al., Atomic layer deposition ofZnO on multi-walled carbon nanotubes and its use for synthesis of CNT-ZnO heterostructures. Nanoscale Research Letters, 2010. 5(11) p.1836-1840. 

Jin, S.H., et ah, Conformal coating of titanium suboxide on carbon nanotube networks by atomic layer deposition for inverted organic photovoltaic cells. Carbon, 2012. 50(12) p. 4483-4488. 

Xiangbo, M., et ah, Controllable synthesis of graphene-based titanium dioxide nanocomposites by atomic layer deposition. Nanotechnology, 2011. 22(16) p. 165602. 

Dameron, A.A., et ah, Aligned carbon nanotube array functionalization for enhanced atomic layer deposition of platinum electrocatalysts. Applied Surface Science, 2012. 258(13) ... 

Wang, X., S.M. Tabakman, and H. Dai, Atomic layer deposition of metal oxides on pristine and functionalized graphene. Journal of the American Chemical Society, 2008.130(26) p. 8152-8153. 

Alaboson, J.M.P., et ah, Seeding atomic layer deposition of high-kdielectrics on epitaxial graphene with organic self-assembled monolayers. ACS Nano, 2011. 5(6) p. 5223-5232. 

J. W. Elam, D. Routkevitch, P. P. Mardilovich, and S. M. George, Conformal coating on ultrahigh-aspect-ratio nanopores of anodic alumina by atomic layer deposition, CherrL Mater. 15, 3507 (2003). 

M. Leskela, and M. Ritala, Atomic layer deposition chemistry Recent developments and future challenges, Angew. Chem. Int. Ed. 42(45), 5548-5554 (2003). 

An alternative method for producing thin deposits consists in avoiding single-step processes by chemisorbing the precursor and then having it decomposed thermally [1]. Such a method, often named atomic layer deposition or gas phase impregnation-deposition, provides very thin deposits due to the presence of a... 

Chen R, Bent SF (2006) Chemistry for positive pattern transfer using area-selective atomic layer deposition. Adv Mater 18 1086... 

Chen R, Kim H, McIntyre PC, Bent SF (2005) Investigation of self-assembled monolayer resists for hafiiium dioxide atomic layer deposition. Chem Mater 17 536... 

Park MH, Jang YJ, Sung-Suh HM, Sung MM (2004) Selective atomic layer deposition of titanium oxide on patterned self-assembled monolayers formed by microcontact printing. Langmuir 20 2257-2260... 

Park KJ, Doub JM, Gougousi T, Parsons GN (2005) Microcontact patterning of ruthenium gate electrodes by selective area atomic layer deposition. Appl Phys Lett 86 051903... 

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