There is an ongoing need to improve thin quality by applying suitable plasma assistance in the deposition process.  Plasmas consist of a mixtures of charged and neutral particles, and we are interested in all kinds of "activated" particle species that help in the deposition process.  For example, nitrogen in the non-activated form is a molecule, N₂.  In order to react nitrogen to form a nitride, the bond between the two nitrogen atoms has to be broken, enabling electrons to form new bonds with other atoms.  In the growth process of nitrides, such as GaN or InN, supply of activated nitrogen is highly beneficial. Activated species include ions, atoms, electronically excited atoms and molecules (that is, for nitrogen, N⁺, N₂⁺, N, N^*, and N₂^*).  These species can be produced in a variety of plasmas, and we focused develop and use a Constricted Plasma Source (CPS).

Constricted Plasma Source, here operating with oxygen and using an additional magnetic field. 

The constricted plasma source utilizes a special form of a dc glow discharge, the constricted glow discharge. It is characterized by the presence of a small orifice (or constriction) placed in the discharge path between cathode and anode. If flowing gas is used, both electric current and gas have to go trough the constriction. As a result, an electric double layers forms upstream the constriction with a voltage drop which can exceed the first ionization potential of the gas in use. Ionization processes occur in the vicinity of the constriction, and the ionized gas (plasma) is "blown" through the constriction by the pressure gradient.

The development of the CPS was honored by a R&D 100 Award (1997) of the R&D Magazine.  and has been feature on the Website of the Office of Science, US Department of Energy.

The Constricted Plasma Source has a number of advantages:

• operation in a wide range of pressure is possible
• no heated parts (such as filaments) are present, thus the source is compatible with oxygen and other reactive gases
• ion energy is low (less than 20 eV, i.e., lower than most other ion or plasma sources)
• simple, robust construction
• can be designed with different orifice shapes.

The source can be miniaturized and small modules can be arranged to form a linear plasma source. The photo shows a linear multi-cell unit of 25 cm length operating with oxygen.

Publication highlights:

• A. Anders and S. Anders, The working principle of the hollow-anode plasma source, Plasma Sources Science & Technology, vol. 4 (1995) 571-575.

• A. Anders and M. Kühn, "Characterization of a low-energy constricted-plasma source," Rev. Sci. Instrum., vol. 69, pp. 1340-1343, 1998.

• A. Anders, R. A. MacGill, and M. Rubin, "Evaluation of the plasma distribution of a quasi-linear constricted plasma source," IEEE Trans. Plasma Sci., vol. 27, pp. 82-83, 1999.
  A. E. Zhukov, R. Zhao, P. Specht, V. M. Ustinov, A. Anders, and E. R. Weber, "MBE growth of (In)GaAsN on GaAs using a constricted DC plasma source," Semiconductor Sci. Technol., vol. 16, pp. 413-419, 2001.
   

For more information on contact  André Anders.

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