In the metal organic chemical vapor deposition (MOCVD) technique, reactant gases are combined at elevated temperatures in the reactor to cause a chemical interaction, resulting in the deposition of materials on the substrate. A reactor is a chamber made of a material that does not react with the chemicals being used. It must also withstand high temperatures. This chamber is composed by reactor walls, liner, a
susceptor, gas injection units, and temperature control units. Usually, the reactor walls are made from stainless steel or quartz. Ceramic or special
glasses, such as quartz, are often used as the liner in the reactor chamber between the reactor wall and the susceptor. To prevent overheating, cooling water must be flowing through the channels within the reactor walls. A substrate sits on a
susceptor which is at a controlled temperature. The susceptor is made from a material resistant to the temperature and metalorganic compounds used, often it is machined from
graphite. For growing nitrides and related materials, a special coating, typically of
silicon nitride or
tantalum carbide, on the graphite susceptor is necessary to prevent corrosion by ammonia (NH3) gas. One type of reactor used to carry out MOCVD is a cold-wall reactor. In a cold-wall reactor, the substrate is supported by a pedestal, which also acts as a susceptor. The pedestal/susceptor is the primary origin of heat energy in the reaction chamber. Only the susceptor is heated, so gases do not react before they reach the hot wafer surface. The pedestal/susceptor is made of a radiation-absorbing material such as carbon. In contrast, the walls of the reaction chamber in a cold-wall reactor are typically made of quartz which is largely transparent to the
electromagnetic radiation. The reaction chamber walls in a cold-wall reactor, however, may be indirectly heated by heat radiating from the hot pedestal/susceptor, but will remain cooler than the pedestal/susceptor and the substrate the pedestal/susceptor supports. In hot-wall CVD, the entire chamber is heated. This may be necessary for some gases to be pre-cracked before reaching the wafer surface to allow them to stick to the wafer.
Gas inlet and switching system Gas is introduced via devices known as 'bubblers'. In a bubbler a carrier gas (usually
hydrogen in arsenide & phosphide growth or
nitrogen for nitride growth) is bubbled through the metalorganic
liquid, which picks up some metalorganic vapour and transports it to the reactor. The amount of metalorganic vapour transported depends on the rate of carrier gas flow and the bubbler
temperature/
pressure, and is usually controlled automatically and most accurately by using an ultrasonic concentration measuring feedback gas control system. Allowance must be made for
saturated vapors.
Pressure maintenance system Gas exhaust and cleaning system. Toxic waste products must be converted to liquid or solid wastes for recycling (preferably) or disposal. Ideally processes will be designed to minimize the production of waste products. ==Organometallic precursors==