Coating apparatus – Gas or vapor deposition – With treating means
Reexamination Certificate
2000-02-29
2001-10-09
Mills, Gregory (Department: 1763)
Coating apparatus
Gas or vapor deposition
With treating means
C118S7230ME, C118S7230MP, C156S345420
Reexamination Certificate
active
06298806
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device for exciting a gas, of the surfaguide or surfatron-guide type, in which the gas to be excited is treated with a surface-wave plasma, in particular at atmospheric pressure.
2. Description of the Related Art
In order to excite a gas, it is passed through a dielectric tube and an electric discharge, preferably out of local thermodynamic equilibrium, is sustained in this gas by means of the electric field associated with a surface wave. This wave is excited by a field applicator which is itself supplied with microwave power from a conventional waveguide.
A first type of applicator, known by the name surfatron-guide, includes a hollow structure made of electrically conductive material, including a first part which delimits a concentration zone for the incident waves and is closed off by a moving waveguide plunger forming a short-circuit, and a second part which extends perpendicularly to the first part and in which a tube made of dielectric material, through which the gas to be excited is passed, is mounted coaxially. The second part is provided with an axially movable tuning plunger for matching the impedance of the device.
Another known type of applicator, known by the name surfaguide, also includes a hollow structure which forms a waveguide, is made of an electrically conductive material, is intended to be connected to a microwave generator and is provided with a passage intended to be passed through by a dielectric hollow tube through which the excited gas is passed. The hollow structure has a longitudinal general shape and has a wave concentration zone designed to make the microwave power delivered by the generator become concentrated towards the tube during operation of the device, with a view to producing a plasma in the said gas, preferably a surface-wave plasma.
Surfaguides do not have a tuning piston such as the one which exists in the second part of surfatron guides. It is therefore less expensive than the latter. Furthermore, the length of the plasma created by surfaguides is, for equal power, slightly higher than that of the plasma created by surfatron-guides.
However, under certain operating conditions, surfaguides are less efficient than surfatron-guides, when use is made of discharge tubes of diameter in excess of 20 mm at a frequency of 2.45 GHz.
Furthermore, for high operating powers, radiation losses occur in the environment of surfaguides, which are highly prejudicial to the energy balance of the device and furthermore raise problems of reliability and safety.
In addition, because of their construction, these two types of exciter devices have relatively limited conversion yields for the gases to be treated.
Thus, for example, for C
2
F
6
, the maximum gas flow rate which it is possible to destroy to more than 90% is of the order of 500 standard cm
3
per minute (SCCM). This flow rate is insufficient in a number of cases, for example for carrying out the treatment of effluent gases during the cleaning of thin-film deposition reactors for the fabrication of semiconductor devices.
Indeed, it is observed that an increase in the diameter of the tube for a given microwave power transmitted to the discharge tube, for a fixed flow rate, leads above a fairly low value to a decrease in the conversion yield. This can be explained, in particular, by the fact that an increase in the diameter of the discharge tube reduces the average energy of the electrons, which reduces the excitation by electron collisions.
Another important phenomenon connected with the diameter of the discharge tube is that of the radial contraction of discharges at atmospheric pressure, irrespective of whether they result from a DC, radio frequency or microwave electric field. This phenomenon becomes even more marked if the gas has a low thermal conductivity, as is the case with krypton.
If the contraction is significant, the plasma appears in filamentary form. As soon as there is contraction, the effect experienced by the gas as a result of the discharge becomes less and less as the distance from the axis of the tube increases.
In consequence, an increase in the diameter of the discharge tube is accompanied by the generation of one or more plasma filaments, between which the gas to be excited can flow without undergoing any excitation.
Furthermore, for a given gas flow rate, the power transmitted to the surface-wave discharge, to be increased with a view to increasing the conversion yield, cannot be increased, beyond a limit value, without risking the generation of an electric arc in the starting gap of the device.
The object of the invention is to overcome the drawbacks of the devices of surfaguide and surfatron-guide type in the prior art, and to provide a gas exciter device which makes it possible, on the one hand, to increase the gas flow rate for a given yield and, on the other hand, to increase the yield for a given gas flow rate.
SUMMARY OF THE INVENTION
The invention therefore relates to a device for exciting a gas, comprising a hollow structure, made of electrically conductive material, which forms a waveguide and is intended to be connected to a microwave generator, and means for passing the gas to be excited through the said structure, the said structure having a longitudinal general shape and including a zone for concentration of the radiation emitted by the said generator and is designed to produce a plasma in the gas during operation of the device, characterized in that the said means for passing the gas include a set of at least two identical hollow tubes made of dielectric material, respectively passing through the said structure in zones where the amplitude of the electric field associated with the incident wave is identical.
This provides an excitation device that can be equipped with a plurality of discharge tubes, through each of which a gas is passed with a relatively small flow rate, the total flow rate of the gas to be excited flowing through the device being increased considerably.
The excitation device according to the invention may furthermore include one or more of the following characteristics:
the said concentration zone includes a set of at least two passages which are formed in the wall of the said structure with respect to a longitudinal plane of symmetry thereof and are each intended to have one of the said hollow tubes pass through them;
the said passages are formed along the said plane of symmetry of the structure;
the said passages are formed along two longitudinal axes extending on either side of the plane of symmetry of the structure, equidistant from this plane;
the said passages are arranged in pairs along the said plane of symmetry, the said passages in each pair being arranged symmetrically on either side of the said plane;
the said passages are formed regularly with respect to the said longitudinal plane of symmetry of the structure, the distance between two passages, considered parallel to the longitudinal direction of the said structure, being equal to an integer multiple, at least equal to 1, of the half-wavelength &lgr;
g
/2 characteristic of the waveguide at the operating frequency of the device;
the said concentration zone includes a single passage which is formed in the wall of the said structure and is intended to have the said hollow tubes pass through it;
the said hollow structure which forms a waveguide has a first open end, intended to be connected to the said microwave generator, an open opposite end, intended to be equipped with impedance adjustment means forming a short-circuit, and a zone of narrowed cross-section, in which the said passage or passages are formed, which extends between the said first and second ends and delimits the said radiation concentration zone;
the said zone of narrowed cross-section has a part with constant cross-section, in which the said passage or passages are formed and which extends between two parts with cross-section increasing linearly in the direction of the said ends;
the device furthermore includes at least one ele
Etemadi Roxane
Moisan Michel
Rostaing Jean-Christophe
Zakrzewski Zenon
Burns Doane Swecker & Mathis L.L.P.
Hassanzadeh P
L'Air Liquide, Societe Anonyme pour l'Etude et l'
Mills Gregory
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