Apparatus for producing a beam of atoms or radicals

Radiant energy – Electrically neutral molecular or atomic beam devices and...

Reexamination Certificate

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C118S724000, C422S199000

Reexamination Certificate

active

06191416

ABSTRACT:

FIELD OF THE INVENTION
Our present invention relates to an apparatus for generating a beam or stream of atoms or radicals by thermal dissociation of a gas. More particularly, the invention relates to an apparatus for producing a stream of particles, for example atoms and radicals, produced by thermal dissociation of a gas, of the type which comprises a tube, one end of which is connected to a gas source while the stream emerges from the opposite end and a heating device is provided for heating the tube.
BACKGROUND OF THE INVENTION
The apparatus for producing a stream of atoms or radicals by thermal dissociation of a gas as it traverses a heated tube through which the gas flows axially, is known and reference may be had to the article entitled “Some Properties of Hydrogenated Amorphous Silicon Produced by Direct Reaction of Silicon and Hydrogen Atoms”, R. E. Viturro and K. Weiser in Philosophical Magazine B, 1986, Vol. 53, No. 2, pages 93-103.
For thermal dissociation of a gas at high temperatures, for example in the case of hydrogen or oxygen, the thermal dissociation temperature may have to be in the range of 1500 K to 2500 K. To be able to heat the gas passing through the tube to these high temperatures, conventional devices provide so-called electron bombardment heaters. In these, the electron source is a wire heated to glowing outside the tube and traversed by an electric current sufficient to cause the wire to emit electrons. A potential is provided between the electron source and the tube, e.g. of about 1000 volts, to accelerate the electrons toward the tube and cause them to impinge upon the tube so that a good part of the energy carried by the electrons is released on the tube and the gas flowing therethrough is thereby heated.
Electron bombardment has the advantage that a comparatively large amount of heat can be developed at the tube, i.e. that the heating power can be high, and further that the heat is concentrated at the tube rather than being dissipated. The electron bombardment technique, however, has the drawback that to generate the potential between the glow wire and the tube, the glow wire must be at close to ground potential and the tube must be brought to a high positive potential. In this case, the accelerated electrons can have in the region of the tube, sufficient energy for the impingement-type ionization of the gases which ultimately emerge from the tube as well as residual gas.
The resulting positive ions are accelerated away from the tube and thus there is an atomic beam which can cross the electron beam and dissipate energy.
In addition the beam emerging from the tube can be contaminated with ions from extrinsic sources because of the high energy of all of the atomic and subatomic particles involved in the process. Contamination of the beam of atoms or radicals may present a significant problem for the ultimate use of the beam.
It is possible to avoid contamination of the beam by ions by maintaining the tube at ground potential while providing the glow wire at a high potential. This arrangement has the drawback that it forms an “electron reflector” at the high potential which is so arranged that the electrons are accelerated only toward the tube and are not accelerated to further components also at ground potential.
The high voltage components must be sufficiently insulated and electron emission in extraneous directions must be suppressed, all of which significantly increases the fabrication cost of the apparatus and limits the use thereof in earlier systems. The need for high voltage insulation is particularly a problem since the components which must be insulated are also at high temperatures in practice. There is a significant danger that the insulator will be vaporized and the insulation effect lost.
Frequently it has also been found to be a drawback that the electrical source for electron bombardment heating comprises a high voltage unit which provides an electron emission current from about 60 mA to about 0.7 A, an electron-emission current regulator which must have positive feedback coupling between the tube temperature and the glow wire temperature to avoid unstable emission currents and other elements all of which contribute to a very high cost of the apparatus and introduce places at which failure can occur.
OBJECTS OF THE INVENTION
It is, therefore, the principal object of the present invention to provide an apparatus for generating a stream or beam of atoms or radicals by thermal dissociation of a gas whereby all of these drawbacks can be avoided.
More specifically it is the object of the invention to provide an improved apparatus for producing a particle beam of atoms or radicals by such thermal dissociation of a gas, whereby contamination of the atomic beam by ions or the like resulting from electron bombardment techniques are avoided.
It is also an object of the invention to provide a low cost and reliable apparatus for the purposes described wherein, especially, the cost factors for power supply and control and the need for electrical insulation are reduced and reliability is improved.
SUMMARY OF THE INVENTION
These objects and others which will become apparent hereinafter are attained in an apparatus for producing a stream of atomic particles in the form of free atoms or radicals which comprises:
an elongated heat-conductive and thermally conductive tube formed at one end with means for connecting the tube to a source of a gas capable of being thermally decomposed to form the atomic particles, the tube being formed at an opposite end with an outlet for the stream of atomic particles;
at last one heating wire extending along the tube and spaced from the tube over at last a major portion of a length of the wire whereby the tube is radiantly heated by the wire to a temperature sufficient to thermally decompose the gas and form the atomic particles, the wire having an electrical contact with the tube at a location proximal to the outlet and being spaced form the tube over a remainder of a length of the wire juxtaposed with the tube; and
an electrical energy source connected to the wire for passing an electric current therethrough to cause the wire to radiantly heat the tube.
In the system of the invention, therefore, the heating device comprises at least one heating wire for radiantly heating the tube and connects that heating wire electrically to the tube in the region of the gas outlet opening. Over the remainder of the length of that heating wire, it is spaced from the tube but extends along the length thereof, preferably from the end at which the gas is introduced to the point at which electrical contact is made between that heating wire and the tube.
As a consequence, the invention provides for the heating of the tube to effect thermal dissociation of the gas traversing the tube not as in the art by electron bombardment, but by thermal radiation heating. All of the drawbacks of electron bombardment heating can thus be avoided. The impact ionization of the gases at the mouth of the tube by accelerated electrons, the contamination by ions, the movement of ions or electrons across the path of the dissociated atomic particles and the drawbacks resulting from an electron reflector system are eliminated.
Indeed no high voltage is required for the resistance heating of the present invention and hence the need for a high voltage source, the special requirements for high voltage insulation, and even the need for electron emission control or regulation are all eliminated and the apparatus of the invention is greatly simplified and much less expensive by comparison with earlier apparatus.
By providing the contact of the heating wire with the tube, the heating current from the heating wire can flow through the tube to ground.
According to a feature of the invention the heating wire can be formed as a coil around the tube with at least one turn and preferably two at the end of a heating wire being wrapped tightly around the tube to hug the latter and form the electrical contact. In this configuration, the tube acts as a support for the heating

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