Molecular layer deposition (MLD) is a vapour phase thin film deposition technique based on self-limiting surface reactions carried out in a sequential manner. Essentially, MLD resembles the well established technique of atomic layer deposition (ALD) but, whereas ALD is limited to exclusively inorganic coatings, the precursor chemistry in MLD can use small, bifunctional organic molecules as well. This enables, as well as the growth of organic layers in a process similar to polymerization, the linking of both types of building blocks together in a controlled way to build up organic-inorganic hybrid materials.
Even though MLD is a known technique in the thin film deposition sector, due to its relative youth it is not as explored as its inorganic counterpart, ALD, and a wide sector development is expected in the upcoming years.
History Molecular layer deposition is a sister technique of atomic layer deposition. While the history of atomic layer deposition dates back to the 1970s, thanks to the independent work of Valentin Borisovich Aleskovskii. and Tuomo Suntola, the first MLD experiments with organic molecules were not published until 1991, when an article from Tetsuzo Yoshimura and co-workers appeared regarding the synthesis of polyimides using amines and anhydrides as reactants. After some work on organic compounds along the 1990s, the first papers related to hybrid materials emerged, after combining both ALD and MLD techniques. Since then, the number of articles submitted per year on molecular layer deposition has increased steadily, and a more diverse range of deposited layers have been observed, including polyamides, polyimines, polyurea, polythiourea and some copolymers, with special interest in the deposition of hybrid films.
Reaction mechanism In similar fashion to an atomic layer deposition process, during an MLD process the reactants are pulsed on a sequential, cyclical manner, and all gas-solid reactions are self-limiting on the sample substrate. Each of these cycles are called MLD cycles and layer growth is measured as Growth Per Cycle (GPC), usually expressed in nm/cycle or Å/cycle. During a model, two precursor experiment, an MLD cycle proceeds as follows:
First, precursor 1 is pulsed in the reactor, where it reacts and chemisorbs to the surface species on the sample surface. Once all adsorption sites have been covered and saturation has been reached, no more precursor will attach, and excess precursor molecules and generated byproducts are withdrawn from the reactor, either by purging with inert gas or by pumping the reactor chamber down. Only when the chamber has been properly purged with inert gas/pumped down to base pressure (~ 10−6 mbar range) and all unwanted molecules from the previous step have been removed, can precursor 2 be introduced. Otherwise, the process runs the risk of CVD-type growth, where the two precursors react in the gaseous phase before attaching to the sample surface, which would result in a coating with different characteristics.
Next, precursor 2 is pulsed, which reacts with the previous precursor 1 molecules anchored to the surface. This surface reaction is again self-limiting and, followed again by purging/pumping to base pressure the reactor, leaves behind a layer terminated with surface groups that can again react with precursor 1 in the next cycle. In the ideal case, the repetition of the MLD cycle will build up an organic/inorganic film one monatomic layer at a time, enabling highly conformal coatings with precise thickness control and film purity
If ALD and MLD are combined, more precursors in a wider range can be used, both inorganic and organic. In addition, other reactions can be included in the ALD/MLD cycles as well, such as plasma or radical exposures. This way, an experiment can be freely customised according to the research needs by tuning the number of ALD and MLD cycles and the steps contained within the cycles.
Process chemistry and surface reactions Precursor chemistry plays a key role in MLD. The chemical properties of the precursor molecules drive the composition, structure and stability of the deposited hybrid material. To reach the saturation stage in a short time and ensure a reasonable deposition rate, precursors must chemisorb on the surface, react rapidly with the surface active groups and react with each other. The desired MLD reactions should have a large negative ∆G value.
Organic compounds are employed as precursors for MLD. For their effective use, the precursor should have sufficient vapor pressure and thermal stability to be transported in the gas phase to the reaction zone without decomposing. Volatility is influenced by the molecular weight and intermolecular interactions. One of the challenges in MLD is to find an organic precursor that has sufficient vapor pressure, reactivity and thermal stability. Most organic precursors have low volatility, and heating is necessary to ensure the sufficient supply of vapor reaching the substrate. The backbone of the organic precursors can be flexible i.e., aliphatic, or rigid i.e., aromatics employed with the functional groups. The organic precursors usually are homo or heterobifunctional molecules with -OH, -COOH, -NH2, -CONH2, -CHO, -COCl, -SH, -CNO, -CN, alkenes, etc. functional groups. The bifunctional nature of the precursors is essential for continuous film growth as one group is expected to react with the surface and the other one is accessible to react with the next pulse of the co-reactant. The attached functional groups play a vital role in the reactivity and binding modes of the precursor and they should be able to react with the functional groups present at the surface. A flexible backbone may hinder the growth of a continuous and dense film by back coordination, blocking the reactive sites and thus lowering the film growth rate. Thus, finding a MLD precursor with all the above-mentioned requirements fulfilled is not straightforward process.
Surface groups play a crucial role as reaction intermediates. The substrate is usually hydroxylated or hydrogen terminated and hydroxyls serve as reactive linkers for condensation reactions with metals. The inorganic precursor reacts with surface reactive groups via the corresponding linking chemistry that leads to the formation of new O-Metal bonds. The metal precursor step changes the surface termination, leaving the surface with new reactive sites ready to react with the organic precursor. The organic precursor reacts at the resulting surface by bonding covalently with the metal sites, releasing metal ligands and leaves another reactive molecular layer ready for the next pulse. Byproducts are released after each adsorption step and the reactions are summarised below.
Process considerations When performing an MLD process, as a variant of ALD, certain aspects need to be taken into account in order to obtain the desired layer with adequate purity and growth rate:
Saturation Before starting an experiment, the researcher must know whether the process designed will yield saturated or unsaturated conditions. If this information is unknown, it is a priority to get to know it in order to have accurate results. If not long enough precursor pulsing times are allowed, the surface reactive sites of the sample will not have sufficient time to react with the gaseous molecules and form a monolayer, which will be translated in a lower growth per cycle (GPC). To solve this issue, a saturation experiment can be performed, where the film growth is monitored in-situ at different precursor pulsing times, whose GPCs will then be plotted against pulsing time to find the saturation conditions.
Additionally, too short purging times will result in remaining precursor molecules in the reactor chamber, which will be reactive in the gaseous phase towards the new precursor molecules introduced during the next step, obtaining an undesired CVD-grown layer instead.
MLD window Film growth usually depends on the temperature of deposition, on what is called MLD window, a temperature range in which, ideally, film growth will remain constant. When working outside of the MLD window, a number of problems can occur:
When working at lower temperatures: limited growth, due to insufficient reactivity; or condensation, which will appear like a higher GPC than expected. When working at higher temperatures: precursor decomposition, which originates non-saturating uncontrolled growth; or desorption that will lower deposition rates.
In addition, even when working within the MLD window, GPCs can still vary with temperature sometimes, due to the effect of other temperature-dependent factors, such as film diffusion, number of reactive sites or reaction mechanism.
Non-idealities
Non-monolayer growth When carrying out an MLD process, the ideal case of one monolayer per cycle is not usually applicable. In the real world, many parameters affect the actual growth rate of the film, which in turn produce non idealities like sub-monolayer growth (deposition of less than a full layer per cycle), island growth and coalescence of islands.
Substrate effects During an MLD process, film growth will usually achieve a constant value (GPC). However, during the first cycles, incoming precursor molecules will not interact with a surface of the grown material but rather with the bare substrate, and thus will undergo different chemical reactions with different reaction rates. As a consequence of this, growth rates can experience a substrate enhancement (faster substrate-film reaction than film-film reactions) and therefore higher GPCs in the first cycles; or a substrate inhibition (slower substrate-film reaction than film-film reactions), accompanied by a GPC decrease at the beginning. In any case, process growth rates can be very similar in both cases in some depositions.
Lower than anticipated growth In MLD, it is not strange to observe that, often, experiments yield lower than anticipated growth rates. The reason for this relies on several factors, such as:
Molecule tilting: organic molecules with long chains are prone to not remaining completely perpendicular to the surface, lowering the number of surface sites. Bidentate ligands: when a reacting molecule has two functional groups, it may bend and react with two surface sites instead of remaining straight on the surface. This has been shown, for instance, for titanicones grown with ethylene glycol and glycerol. Because glycerol has an additional hydroxyl group compared to ethylene glycol and is able to provide an extra reactive hydroxyl group in the case of a double reaction of the terminal hydroxyl groups with the surface. Steric hindrance: organic precursors are often bulky, and can cover several surface groups when attached to the surface. Long pulsing times: organic precursors can have very low vapour pressures, and very long pulsing times may be necessary in order to achieve saturation. In addition, long purging times are usually needed to remove all unreacted molecules from the chamber afterward. Low temperatures: to increase the precursor vapor pressure, one might think of increasing its temperature. Nevertheless, organic precursors are usually very thermally fragile, and a temperature increase may induce decomposition. Gas-phase: many organic reactions are normally carried out in the liquid phase, and are therefore dependent of acid-base interactions or solvation effects. These effects are not present in the gaseous phase and, as a consequence, many processes will yield lower reaction rates or directly won't be possible.
This phenomenon can be avoided as much as possible by using organic precursors with stiff backbones or with more than two functional groups, using a three step reaction sequence, or using precursors in which ring-opening reactions occur.
Physical state of precursors Liquid precursors