The act of applying a thin film to a surface is
thin-film deposition – any technique for depositing a thin film of material onto a
substrate or onto previously deposited layers. "Thin" is a relative term, but most deposition techniques control layer thickness within a few tens of
nanometres.
Molecular beam epitaxy, the
Langmuir–Blodgett method,
atomic layer deposition and
molecular layer deposition allow a single layer of
atoms or molecules to be deposited at a time. It is useful in the manufacture of
optics (for
reflective,
anti-reflective coatings or
self-cleaning glass, for instance),
electronics (layers of
insulators,
semiconductors, and
conductors form
integrated circuits),
packaging (i.e.,
aluminium-coated PET film), and in
contemporary art (see the work of
Larry Bell). Similar processes are sometimes used where thickness is not important: for instance, the purification of copper by
electroplating, and the deposition of
silicon and enriched
uranium by a
chemical vapor deposition-like process after gas-phase processing. Deposition techniques fall into two broad categories, depending on whether the process is primarily
chemical or
physical.
Chemical deposition Here, a fluid
precursor undergoes a chemical change at a solid surface, leaving a solid layer. An everyday example is the formation of soot on a cool object when it is placed inside a flame. Since the fluid surrounds the solid object, deposition happens on every surface, with little regard to direction; thin films from chemical deposition techniques tend to be
conformal, rather than
directional. Chemical deposition is further categorized by the phase of the precursor:
Plating relies on liquid precursors, often a solution of water with a salt of the metal to be deposited. Some plating processes are driven entirely by
reagents in the solution (usually for
noble metals), but by far the most commercially important process is
electroplating. In semiconductor manufacturing, an advanced form of electroplating known as electrochemical deposition is now used to create the copper conductive wires in advanced chips, replacing the chemical and physical deposition processes used to previous chip generations for aluminum wires
Chemical solution deposition or
chemical bath deposition uses a liquid precursor, usually a solution of
organometallic powders dissolved in an organic solvent. This is a relatively inexpensive, simple thin-film process that produces stoichiometrically accurate crystalline phases. This technique is also known as the
sol-gel method because the 'sol' (or solution) gradually evolves towards the formation of a gel-like diphasic system. The
Langmuir–Blodgett method uses molecules floating on top of an aqueous subphase. The packing density of molecules is controlled, and the packed monolayer is transferred on a solid substrate by controlled withdrawal of the solid substrate from the subphase. This allows creating thin films of various molecules such as
nanoparticles, polymers and lipids with controlled particle packing density and layer thickness.
Spin coating or spin casting, uses a liquid precursor, or
sol-gel precursor deposited onto a smooth, flat substrate which is subsequently spun at a high velocity to centrifugally spread the solution over the substrate. The speed at which the solution is spun and the
viscosity of the sol determine the ultimate thickness of the deposited film. Repeated depositions can be carried out to increase the thickness of films as desired. Thermal treatment is often carried out in order to crystallize the amorphous spin coated film. Such crystalline films can exhibit certain preferred orientations after crystallization on single
crystal substrates.
Dip coating is similar to spin coating in that a liquid precursor or sol-gel precursor is deposited on a substrate, but in this case the substrate is completely submerged in the solution and then withdrawn under controlled conditions. By controlling the withdrawal speed, the evaporation conditions (principally the humidity, temperature) and the volatility/viscosity of the solvent, the film thickness, homogeneity and nanoscopic morphology are controlled. There are two evaporation regimes: the capillary zone at very low withdrawal speeds, and the draining zone at faster evaporation speeds.
Chemical vapor deposition generally uses a gas-phase precursor, often a
halide or
hydride of the element to be deposited. In the case of
metalorganic vapour-phase epitaxy, an
organometallic gas is used. Commercial techniques often use very low pressures of precursor gas.
Plasma-enhanced chemical vapor deposition uses an ionized vapor, or
plasma, as a precursor. Unlike the soot example above, this method relies on electromagnetic means (electric current,
microwave excitation), rather than a chemical-reaction, to produce a plasma.
Atomic layer deposition and its sister technique
molecular layer deposition, uses gaseous precursor to deposit
conformal thin film's one layer at a time. The process is split up into two half reactions, run in sequence and repeated for each layer, in order to ensure total layer saturation before beginning the next layer. Therefore, one reactant is deposited first, and then the second reactant is deposited, during which a chemical reaction occurs on the substrate, forming the desired composition. As a result of the stepwise, the process is slower than chemical vapor deposition; however, it can be run at low temperatures. When performed on polymeric substrates, atomic layer deposition can become
sequential infiltration synthesis, where the reactants diffuse into the polymer and interact with functional groups on the polymer chains.
Physical deposition Physical deposition uses mechanical, electromechanical or thermodynamic means to produce a thin film of solid. An everyday example is the formation of
frost. Since most engineering materials are held together by relatively high energies, and chemical reactions are not used to store these energies, commercial physical deposition systems tend to require a low-pressure vapor environment to function properly; most can be classified as
physical vapor deposition. The material to be deposited is placed in an
energetic,
entropic environment, so that particles of material escape its surface. Facing this source is a cooler surface which draws energy from these particles as they arrive, allowing them to form a solid layer. The whole system is kept in a vacuum deposition chamber, to allow the particles to travel as freely as possible. Since particles tend to follow a straight path, films deposited by physical means are commonly
directional, rather than
conformal. Examples of physical deposition include: (STM) and from the emergence of
quantum-well states characteristic of the silver film thickness in
photoemission spectroscopy (ARPES). Image size is 250nm by 250nm. A thermal
evaporator that uses an electric resistance heater to melt the material and raise its vapor pressure to a useful range. This is done in a high vacuum, both to allow the vapor to reach the substrate without reacting with or
scattering against other gas-phase atoms in the chamber, and reduce the incorporation of impurities from the residual gas in the vacuum chamber. Only materials with a much higher
vapor pressure than the
heating element can be deposited without contamination of the film.
Molecular beam epitaxy is a particularly sophisticated form of thermal evaporation. An
electron beam evaporator fires a high-energy beam from an
electron gun to boil a small spot of material; since the heating is not uniform, lower
vapor pressure materials can be deposited. The beam is usually bent through an angle of 270° in order to ensure that the gun filament is not directly exposed to the evaporant flux. Typical deposition rates for electron beam evaporation range from 1 to 10 nanometres per second. In
molecular beam epitaxy, slow streams of an element can be directed at the substrate, so that material deposits one atomic layer at a time. Compounds such as
gallium arsenide are usually deposited by repeatedly applying a layer of one element (i.e.,
gallium), then a layer of the other (i.e.,
arsenic), so that the process is chemical, as well as physical; this is known also as
atomic layer deposition. If the precursors in use are organic, then the technique is called
molecular layer deposition. The beam of material can be generated by either physical means (that is, by a
furnace) or by a chemical reaction (
chemical beam epitaxy).
Sputtering relies on a plasma (usually a
noble gas, such as
argon) to knock material from a "target" a few atoms at a time. The target can be kept at a relatively low temperature, since the process is not one of evaporation, making this one of the most flexible deposition techniques. It is especially useful for compounds or mixtures, where different components would otherwise tend to evaporate at different rates. Note, sputtering's step coverage is more or less conformal. It is also widely used in optical media. The manufacturing of all formats of CD, DVD, and BD are done with the help of this technique. It is a fast technique and also it provides a good thickness control. Presently, nitrogen and oxygen gases are also being used in sputtering.
Pulsed laser deposition systems work by an
ablation process. Pulses of focused
laser light vaporize the surface of the target material and convert it to plasma; this plasma usually reverts to a gas before it reaches the substrate.
Thermal laser epitaxy uses focused light from a continuous-wave laser to thermally evaporate sources of material. By adjusting the power density of the laser beam, the evaporation of any solid, non-radioactive element is possible. The resulting atomic vapor is then deposited upon a substrate, which is also heated via a laser beam. The vast range of substrate and deposition temperatures allows of the
epitaxial growth of various elements considered challenging by other thin-film growth techniques.
Cathodic arc deposition (arc-physical vapor deposition), which is a kind of
ion beam deposition where an electrical arc is created that blasts ions from the cathode. The arc has an extremely high
power density resulting in a high level of
ionization (30–100%), multiply charged ions, neutral particles, clusters and macro-particles (droplets). If a reactive gas is introduced during the evaporation process,
dissociation,
ionization and
excitation can occur during interaction with the
ion flux and a compound film will be deposited.
Electrohydrodynamic deposition (electrospray deposition) is a relatively new process of thin-film deposition. The liquid to be deposited, either in the form of nanoparticle solution or simply a solution, is fed to a small capillary nozzle (usually metallic) which is connected to a high voltage. The substrate on which the film has to be deposited is connected to ground. Through the influence of electric field, the liquid coming out of the
nozzle takes a conical shape (
Taylor cone) and at the apex of the cone a thin jet emanates which disintegrates into very fine and small positively charged droplets under the influence of Rayleigh charge limit. The droplets keep getting smaller and smaller and ultimately get deposited on the substrate as a uniform thin layer.
Growth modes Frank–van der Merwe growth ("layer-by-layer"). In this growth mode the adsorbate-surface and adsorbate-adsorbate interactions are balanced. This type of growth requires lattice matching, and hence considered an "ideal" growth mechanism.
Stranski–Krastanov growth ("joint islands" or "layer-plus-island"). In this growth mode the adsorbate-surface interactions are stronger than adsorbate-adsorbate interactions. Volmer–Weber ("isolated islands"). In this growth mode the adsorbate-adsorbate interactions are stronger than adsorbate-surface interactions, hence "islands" are formed right away. There are three distinct stages of stress evolution that arise during Volmer-Weber film deposition. The first stage consists of the nucleation of individual atomic islands. During this first stage, the overall observed stress is very low. The second stage commences as these individual islands coalesce and begin to impinge on each other, resulting in an increase in the overall tensile stress in the film. This increase in overall tensile stress can be attributed to the formation of grain boundaries upon island coalescence that results in interatomic forces acting over the newly formed grain boundaries. The magnitude of this generated tensile stress depends on the density of the formed grain boundaries, as well as their grain-boundary energies. During this stage, the thickness of the film is not uniform because of the random nature of the island coalescence but is measured as the average thickness. The third and final stage of the Volmer-Weber film growth begins when the morphology of the film's surface is unchanging with film thickness. During this stage, the overall stress in the film can remain tensile, or become compressive. On a stress-thickness vs. thickness plot, an overall compressive stress is represented by a negative slope, and an overall tensile stress is represented by a positive slope. The overall shape of the stress-thickness vs. thickness curve depends on various processing conditions (such as temperature, growth rate, and material). Koch states that there are three different modes of Volmer-Weber growth. Zone I behavior is characterized by low grain growth in subsequent film layers and is associated with low atomic mobility. Koch suggests that Zone I behavior can be observed at lower temperatures. The zone I mode typically has small columnar grains in the final film. The second mode of Volmer-Weber growth is classified as Zone T, where the grain size at the surface of the film deposition increases with film thickness, but the grain size in the deposited layers below the surface does not change. Zone T-type films are associated with higher atomic mobilities, higher deposition temperatures, and V-shaped final grains. The final mode of proposed Volmer-Weber growth is Zone II type growth, where the grain boundaries in the bulk of the film at the surface are mobile, resulting in large yet columnar grains. This growth mode is associated with the highest atomic mobility and deposition temperature. There is also a possibility of developing a mixed Zone T/Zone II type structure, where the grains are mostly wide and columnar, but do experience slight growth as their thickness approaches the surface of the film. Although Koch focuses mostly on temperature to suggest a potential zone mode, factors such as deposition rate can also influence the final film microstructure. == Mechanical behavior ==