Thin-film deposition
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Thin-film deposition is 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 allow layer thickness to be controlled within a few tens of nanometers, and some (molecular beam epitaxy) allow one layer 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), and packaging (i.e., aluminum-coated PET film. 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 CVD-like process after gas-phase processing.
Deposition techniques fall into two broad categories, based on whether they are understood in terms of chemistry, or of physics.
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. It was not commonly used in semiconductor processing for many years, but has seen a resurgence with more widespread use of Chemical-mechanical polishing techniques.
- Chemical vapor deposition (CVD) generally uses a gas-phase precursor, often a halide or hydride of the element to be deposited. In the case of MOCVD, an organometallic gas 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, microwave excitation), rather than a chemical reaction, to produce a plasma.
Physical deposition
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 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.
Some examples of physical deposition are given below:
- 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 scattering against 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 pressure than the heating element can be deposited without contamination of the film. Molecular beam epitaxy is a particular 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 nanometers per second.
- 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.
- 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.
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 oxygen or nitrogen is 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.
- In 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 arsenide are 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 heterotopotaxy or homotopotaxy) between the substrate crystal and the growing thin film material.
See also
External links
- (A Google search on "thin film deposition" leads to 25,000 links, mostly for related equipment, textbooks, and university courses; very little of a tutorial or explanatory nature)
- [Angstrom Engineering Inc. - Supplier of Thin Film Research Equipment]
- [University of Wisconsin: Laboratory for Thin Film Deposition]
- [Cambridge University Facilities]
- [Thin Film Deposition Processes and Techniques] (A textbook summary and order form)
- [ORNL] (Oak Ridge National Laboratory)]
- [Thin Film Science] The European White Book of Materials, Chap.6
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