Organometallic chemical vapor deposition

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The metal organic chemical vapor deposition ( English metal-organic chemical vapor deposition or metallo-organic chemical vapor deposition , MOCVD) is a coating method from the group of chemical vapor deposition ( chemical vapor deposition , CVD), in which the deposition of a solid layer from the chemical vapor phase of an organometallic precursor (predecessor molecule ) takes place.

functionality

Surface processes during layer growth

MOCVD corresponds to a normal CVD process in which a gaseous precursor is fed into a reaction chamber and reacts there with other substances or the substrate to be coated and forms a solid layer. The process taking place in the reaction chamber can be divided into the following sub-processes:

  1. Transport of the precursor molecules into the reaction chamber
  2. Convection and diffusion
  3. Adsorption of the precursor molecules on the substrate through physi- (weak bond) or chemisorption (with charge transfer, strong binding forces )
  4. thermal decomposition of the precursor metal complex on the heated substrate (150–1200 ° C)
  5. possibly further surface diffusion of the deposited atoms towards nuclei or energetically better positions
  6. Installation of the atoms on the surface or in the respective lattice sites (important for epitaxy )
  7. Desorption of gaseous reaction products
  8. Removal of the reaction products and unreacted precursor molecules, which are then pumped out

The disadvantage of MOCVD or CVD in general is that, with multicomponent reaction gases, a chemical reaction of the components can take place in the gas space. This leads to nucleation, which can manifest itself in the form of powder deposition and poor layer properties. This can be minimized by a good reactor design and suitable deposition parameters (including low pressure and high carrier gas flows).

Basic system structure

The structure of MOCVD systems essentially corresponds to that of conventional CVD systems. These are usually vacuum coating systems. A fundamental distinction must be made between systems in which the source substances are conducted in a horizontal gas stream over the substrates to be coated and those in which the source gas is fed vertically onto the substrates. In the horizontal system, under normal flow conditions, one can expect a laminar flow, in which the concentration of the source substances determining the growth rates decreases along the convection direction . With the vertical variant, it is extremely important to determine the flow rate and the height of the reactor chamber, since otherwise recirculation and "backwater" can occur.

The precursors generally have a boiling temperature higher than 100 ° C. There are differences in how the precursor is fed. On the one hand they concern the evaporation of the precursors and on the other hand the structure of the supply lines. There are three established methods for precursor evaporation:

  1. the sublimation of a solid precursor,
  2. Liquid delivery systems (LDS) and
  3. Bubbler systems for liquid and melted precursors.

In the case of provision by sublimation, the precursor, which is initially in solid form, is heated and thus transferred directly (without the liquid phase) into the gas phase. The advantage of this method is that auxiliary materials such as solvents or carrier gases can be dispensed with. However, it is disadvantageous that the precursor is constantly subjected to thermal stress and the solid precursor also gradually decomposes, that is, its quality deteriorates.

In bubbler systems, an inert carrier gas - often argon, nitrogen or hydrogen - is usually introduced into a container (similar in structure to gas washing bottles ) with a liquid precursor. Rising gas bubbles of the carrier gas lead to an ideally saturated vapor above the liquid. The gas mixture is then passed to the reaction chamber. In principle, the bubbler system is suitable for precursors that are in solid and liquid form at room temperature and have an essential vapor pressure. However, solid precursors should be in the form of finely divided powders and with high carrier gas flows usually lead to undersaturated gas mixtures or flow-dependent fluctuations in the precursor content in the gas mixture. To avoid this, batches of bubblers connected in series are often used to make saturation easier. With all liquid or solid precursors, the vapor pressure, and thus the proportion in the gas mixture, is highly temperature-dependent, so that these precursors are ideally kept in temperature-controlled baths. A disadvantage of excessive heating above ambient temperature is that the feed lines to the reactor must also be heated, since there is a risk of condensation in the feed line system.

variants

A variant or subgroup of the MOCVD is the metal-organic gas phase epitaxy (English metal organic chemical vapor phase epitaxy , MOVPE). It is similar to MOCVD in many respects, but aims to deposit epitaxial layers . Despite this difference, both terms are often used synonymously due to the strong similarity of the procedures. Furthermore, numerous processes of atomic layer deposition can also be viewed as modified CVD processes with organometallic precursors as a subgroup of MOCVD.

Applications

MOCVD processes are widely used in many areas of semiconductor and microsystem technology , for example metals or semiconductors with (semi) metallic components are deposited on wafers . Above all, precursors are used, the organic part of which splits off easily under process conditions and is highly volatile. A well-known example is the production of gallium arsenide (GaAs) using trimethylgallium (TMGa) and arsine (arsine); Strictly speaking, this is a MOVPE process.

List of organometallic precursors (selection)

literature

  • Hugh O. Pierson (Ed.): Handbook of chemical vapor deposition (CVD): principles, technology, and applications . William Andrew, 1999, ISBN 978-0-8155-1432-9 , pp. 84-107 (section: Metallo-Organic CVD (MOCVD) ).

Individual evidence

  1. ^ Anthony C. Jones, Michael L. Hitchman: Chemical Vapor Deposition: Precursors, Processes and Applications . Royal Society of Chemistry, 2009, ISBN 978-0-85404-465-8 , pp. 18 .