- Thin-film deposition
Thin-film deposition is any technique for depositing a
thin filmof material onto a substrate or onto previously deposited layers. "Thin" is a relative term, but most deposition techniques allow layer thickness to be controlled within a few tens of nanometers, and some ( molecular beam epitaxy) allow single layers of atoms to be deposited at a time.
It is useful in the manufacture of
optics(for reflective or anti-reflective coatings, for instance), electronics(layers of insulators, semiconductors, and conductors form integrated circuits), packaging(i.e., aluminum-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 copperby electroplating, and the deposition of siliconand enriched uraniumby a CVD-like process after gas-phase processing.
Here, a fluid 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:
Platingrelies on liquid precursors, often a solution of water with a saltof 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. It was not commonly used in semiconductor processing for many years, but has seen a resurgence with more widespread use of Chemical-mechanical polishingtechniques.
Chemical Solution Deposition(CSD) uses a liquid precursor, usually a solution of organometallicpowders dissolved in an organic solvent. This is a relatively inexpensive, simple thin filmprocess that is able to produce stoichiometrically accurate crystalline phases.
Chemical vapor deposition(CVD) generally uses a gas-phase precursor, often a halideor hydrideof the element to be deposited. In the case of MOCVD, an organometallicgas is used. Commercial techniques often use very low pressures of precursor gas.
** Plasma enhanced CVD uses an ionized vapor, or plasma, as a precursor. Unlike the soot example above, commercial PECVD relies on electromagnetic means (electric current,
microwaveexcitation), rather than a chemical reaction, to produce a plasma.
Physical deposition uses mechanical 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 depositionor PVD.
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:
* A thermal evaporator 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
scatteringagainst other gas-phase atoms in the chamber, and reduce the incorporation of impurities from the residual gas in the vacuum chamber. Obviously, only materials with a much higher vapor pressurethan the heating elementcan be deposited without contamination of the film. Molecular beam epitaxyis a particular sophisticated form of thermal evaporation.
electron beam evaporatorfires a high-energy beam from an electron gunto boil a small spot of material; since the heating is not uniform, lower vapor pressurematerials 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 nanometersper second.
Sputteringrelies 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.
Pulsed laser depositionsystems work by an ablationprocess. Pulses of focused laserlight vaporize the surface of the target material and convert it to plasma; this plasma usually reverts to a gas before it reaches the substrate.
Cathodic Arc Depositionor Arc-PVDwhich is a kind of ion beam depositionwhere an electrical arc is created that literally blasts ions from the cathode. The arc has an extremely high power densityresulting 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, ionizationand excitationcan occur during interaction with the ion fluxand a compound film will be deposited.
Other deposition processes
Some methods fall outside these two categories, relying on a mixture of chemical and physical means:
* In reactive
sputtering, a small amount of some non-noble gas such as oxygenor nitrogenis mixed with the plasma-forming gas. After the material is sputtered from the target, it reacts with this gas, so that the deposited film is a different material, i.e. an oxide or nitride of the target material.
Molecular beam epitaxy(MBE), 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 arsenideare usually deposited by repeatedly applying a layer of one element (i.e., Ga), then a layer of the other (i.e., As), so that the process is chemical, as well as physical. The beam of material can be generated by either physical means (that is, by a furnace) or by a chemical reaction ( chemical beam epitaxy).
* In Topotaxy, a specialized technique similar to
epitaxy, thin film crystal growth occurs in three dimensions due to the crystal structure similarities (either heterotopotaxyor homotopotaxy) between the substrate crystal and the growing thin film material.
* CVD, MBE, and
List of coating techniques
Thin-film thickness monitor
VINFVirtual Institute of Nanofilms
* [http://www.ornl.gov/sci/fed/Technology/tsa/tfd/tfd.html ORNL] (
Oak Ridge National Laboratory)
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