Molecular layer deposition
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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.
- 1 History
- 2 Reaction mechanism
- 3 Process considerations
- 4 Potential applications
- 5 Advantages and limitations
- 6 References
- 7 External links
molecular layer deposition is a sister technique of atomic layer deposition. While the history of atomic layer deposition dates back to the 1970's, thanks to the separate work of Valentin Borisovich Aleskovskii and Tuomo Suntola, the first MLD experiments with organic molecules didn't come out until 1991, when an article from Tetsuzo Yoshimura and co-workers was published  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 regarding molecular layer deposition has followed a steady increase over time, 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.
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/hybrid film one monoatomic 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.
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:
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.
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 occuur:
- 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.
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.
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 substate-film reaction than film-film reactions) and therefore higher GPCs in the first cycles; or a substrate inhibition (slower substate-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.
- 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 afterwards.
- Low temperatures: to increase the precursor vapour 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. This 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.
The main application for molecular scale-engineered hybrid materials relies on its synergetic properties, which surpass the individual performance of their inorganic and organic components. The main fields of application of MLD-deposited materials are
- Packaging / encapsulation: depositing ultrathin, pinhole-free and flexible coatings with improved mechanical properties (flexibility, stretchability, reduced brittleness). One example are gas-barriers on organic light emitting diodes (OLEDs).
- Electronics: Tailoring materials with special mechanical and dielectric properties, such as advanced integrated circuits that require particular insulators or flexible thin film transistors with high-k gate dielectrics. Also, the recovery of energy wasted as heat as electric power with certain thermoelectric devices.
- Batteries: electrolyte layers and buffer layers with enhanced flexibility for added performance, cycling characteristics and safety, applicable in mobile devices and wireless connections.
- Biomedical applications: to enhance either cell growth, better adhesion or the opposite, generating materials with anti-bacterial properties. These can be used in research areas like sensing, diagnostics or medicine delivery.
Combining inorganic and organic building blocks on a molecular scale has proved to be challenging, due to the different preparative conditions needed for forming inorganic and organic networks. Current routes are often based on solution chemistry, e.g. sol-gel synthesis combined with spin-coating, dipping or spraying, to which MLD is an alternative.
Advantages and limitations
The main advantage of molecular layer deposition relates to its slow, cyclical approach. While other techniques may yield thicker films in shorter times, molecular layer deposition is known for its thickness control at Angstrom level precision. In addition, its cyclical approach yields films with excelent conformality, making it suitable for the coating of surfaces with complex shapes. The growth of multilayers consisting of different materials is also possible with MLD, and the ratio of organic/inorganic hybrid films can easily be controlled and tailored to the research needs.
As well as in the previous case, the main disadvantage of molecular layer deposition is also related to it slow, cyclical approach. Since both precursors are pulsed sequentially during each cycle, and saturation needs to be achieved each time, the time required in order to obtain a film thick enough can easily be in the order of hours, if not days. In addition, before depositing the desired films it is always necessary to test and optimise all parameters for it to yield successful results.
In terms of cost, regular molecular layer deposition equipment can cost between $200,000 and $800,000. In addition, the cost of the precursors used needs to be taken into consideration.
Similar to the atomic layer deposition case, there are some rather strict chemical limitations for precursors to be suitable for molecular layer deposition.
MLD precursors must have
- Sufficient volatility
- Aggressive and complete reactions
- Thermal stability
- No etching of the film or substrate material
- Sufficient purity
In addition, it is advisable to find precursors with the following characteristics:
- Gases or highly volatile liquids
- High GPC
- Unreactive, volatile byproducts
- Easy to synthesise and handle
- Environmentally friendly
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