Objetivos Económicos y Financieros perseguidos a través de la Emisión

Atomic layer deposition (ALD) is a sub-category of CVD, where thin film deposition occurs via self-limiting reactions on the substrate.202,207,208 _{This can be explained by considering a typical}

reaction cycle, outlined in Figure 4.4. A first precursor (precursor A) is pulsed into a vacuum chamber, where it chemisorbs to the substrate. This is the first half -reaction. The reaction is self- limiting, meaning only a monolayer of chemisorbed material can form, so long as the precursor pulse is long enough for the substrate to become saturated. A purge step is then used to remove any reaction by-products and unreacted precursor, before the second precursor (precursor B) is pulsed into the chamber. Precursor B reacts with the chemisorbed layer of precursor A in a second half-reaction, forming the first layer of a thin film. Again, this reaction is self-limiting, as it is limited by the reaction sites of activated precursor A available.207 _{Another purge step removes by-}

products and excess precursor B, and the cycle is repeated until the desired film thickness is achieved.202 _{Whilst formation of binary films is most common, ternary and elemental films can}

be deposited using two precursors or a plasma in place of precursor B respectively.209–211

ALD is typically used for the growth of metal oxides, sulphides and nitrides, thus one precursor is usually a metal source, such as an organometallic or metal halide, and the other is a non-metal source (e.g. H2O, O3, NH3, H2S). Due to the ligands of the metal precursor, growth

rates in practice are limited to less than one monolayer per cycle as a result of steric effects.201,202

Despite this, the film thickness vs. number of cycles (Figure 4.5)212 _{remains linear for ALD}

processes resulting in sub-nanometre control of thickness, with typical growth per cycle (GPC) values of < 0.1 nm/cycle.207 _{This lends to applications requiring very thin films such as optical}

coatings.208,213 _{Additionally, due to the slow reaction times and reliance on self-limiting reactions,}

conformal coverage of high aspect ratio substrates is achieved,207,214 _{important for applications}

such as trench capacitors in memory applications208,214 _{and barrier materials for integrated circuit}

components.214 _{This is otherwise unachievable with physical vapour deposition methods such as}

evaporation and sputtering,207 _{and whilst CVD can conformally coat rough surfaces it struggles}

with such high aspect ratio substrates due to the fast reaction times.207 _{Finally, the separation of}

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Figure 4.4 Schematic of an ALD cycle. Precursor A is introduced into a reaction vessel where it

reacts with the substrate to form a monolayer. Vacuum and inert gas purge steps are used to remove excess precursor and by-products before precursor B is introduced into the chamber. Precursor B reacts with precursor A to form a monolayer. This gives a single layer of the desired compound AB. Further vacuum and inert gas purges to remove waste follow. The cycle is repeated to deposit films of the desired thickness.

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Figure 4.5 Schematic of a typical growth curve for an ALD reaction. Thickness vs number of

cycles is linear due to the self-limiting nature of ALD reactions.

It should be noted that a temperature regime exists at which the GPC is independent of temperature and reactions are fully self-limiting. This is termed the ‘ALD window’ (Figure 4.6). Within this window any fluctuations in temperature will not affect surface uniformity; an advantage over CVD, particularly when working in the ‘kinetic regime’ (Figure 4.3). At either end of this window the GPC can drop, due to desorption of reactants or insufficient energy for reactions to go to completion, at higher and lower temperatures respectively. Otherwise condensation at temperatures below the ALD window or decomposition processes at higher temperatures result in an increased GPC. In the decomposition case reactions are described as CVD-like. To limit condensation, precursors with high vapour pressures, such as diethyl zinc (2.7 kPa at 25 ºC), are used.215 _{Precursors must also have high stability to decomposition to extend the}

ALD window to as high temperature as possible,216 _{whilst being sufficiently reactive to deposit}

films at reasonable rates below the decomposition temperature (ALD processes usually occur below 350 ºC). As a result, they tend to be pyrophoric and must be handled under inert environments.

However, despite the use of highly reactive precursors, the long purge steps and multi- step nature of ALD renders very slow growth rates. These are typically 0.03 – 0.08 nm/s (100 – 300 nm/hour).207,216 _{As metal oxide films in photovoltaic devices are on the order of tens of}

nanometres, deposition rates are too slow to be viable for high-throughput manufacture.212 _Over

the last few decades spatial atomic layer deposition reactors, which maintain the advantages of ALD whilst demonstrating improved reaction times, have been researched. The theory and operation of these will be discussed in the following section.

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Figure 4.6 Typical temperature profile of an ALD process. At temperatures within the ALD

window, reactions are fully self-limiting as the substrate becomes fully saturated with precursor. Outside of this temperature window, growth per cycle may be governed by condensation of precursor, incomplete surface saturation, desorption or decomposition of precursors.