It is useful in the manufacture of 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 .
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:
- 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 Reagent s 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.
- (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.
- 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.
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 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 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.
- 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.
- 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.
Some methods fall outside these two categories, relying on a mixture of chemical and physical means:
- In , 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 (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 ).
:(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)
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