Source of fast neutral molecules

Details Source of fast neutral molecules Source of fast neutral molecules

1998-09-24

2001-09-04

Berman, Jack (Department: 2881)

Radiant energy

Electrically neutral molecular or atomic beam devices and...

Reexamination Certificate

active

06285025

ABSTRACT:

FIELD OF THE INVENTION
The field of this invention is vacuum-plasma equipment and namely—broad cross-section beam sources of fast neutral molecules intended for cleaning and heating of tools or other products in a working vacuum chamber before coating deposition aimed at improving of adhesion and quality of the coatings.
PRIOR ART
Broad cross-section beam sources of ions and/or fast neutral gas molecules are widely known which use thermionic cathodes to generate the plasma ion emitter of the source. They allow to clean and to heat products in vacuum before coating deposition. Products treatment rate with fast neutral molecules is equal to the treatment rate with corresponding ions of the same kinetic energy. But in comparison with widely used ion sources the sources of fast neutral molecules are remarkable for easier transportation in vacuum of neutral accelerated particle beam and for independence of kinetic energy and of flow density distribution of particles bombarding a product on the product surface potential.
Disadvantages of known fast neutral molecule sources are: complex constructions and high cost prices, comparatively short useful life of the sources and impossibility of products etching with fast neutral molecules of reactive gases because of the use of thermionic cathodes which cannot work in reactive gases as well as damage of the products with arc spot traces as a result of electrical break-downs between the products under treatment and the plasma ion emitter of the source.
Another known accelerated particle source comprises a gas discharge chamber with a cold cathode, with an anode and with a gas discharge power supply, a case enveloping the gas discharge chamber with a flange for hermetic junction and electrical connection with a working vacuum chamber, an accelerating grid positioned between the gas discharge chamber and the part of the case adjoining the flange, an accelerating voltage power supply its positive pole being connected to one of the electrodes of the gas discharge chamber, exactly—to the cold cathode, and its negative pole being connected to the flange of the case as well as a means to admit an ion-producing gas to fill the gas discharge chamber (A. S. Metel. Broad cross-section accelerated particle beam sources on the basis of hollow cathode glow discharge.—In: Plazmennaya emissionnaya elektronika, tez. dokl., Ulan-Ude: Buriatsky institut yestestvennykh nauk SO AN USSR, p. 77-81, FIG.
2
).
Up to 30-40 per cent of all ions produced in the gas discharge chamber are being accelerated with the potential drop between the plasma ion emitter established in the gas discharge chamber and a 35 cm long, 8 cm wide and 70-90 per cent transparent accelerating grid and are entering through the holes of the grid the adjoining the flange part of the case and are further entering the working vacuum chamber.
The adjoining the flange part of the case and the working vacuum chamber are in fact being used as a charge transfer chamber because in the ion-producing gas pressure range of about 0.1 Pa the charge transfer length L being equal to a distance from the accelerating grid at which 63.2 per cent (1-1/e) of all accelerated ions turn into fast neutral molecules as a result of charge transfer collisions with gas molecules and being defined with the following equation:
L=1
&sgr;, where
n is gas molecule density in the chamber, and
&sgr; is charge transfer collision cross-section,
is about several dozens cm for ions with 0.3-3 keV energy. For this reason practically all the accelerated ions turn into fast neutral molecules as a result of charge transfer collisions in a working vacuum chamber that is typically 0.5-1 m wide. Kinetic energy and velocity direction of the accelerated particles change negligibly after the charge transfer collisions and electric current in the working vacuum chamber circuit is mainly carried with slow secondary ions produced as a result of the charge transfer collisions and coming to the walls of the chamber. The ions come also to the walls of the adjoining the flange part of the case, to the products being treated with accelerated particles inside the working vacuum chamber as well as to the accelerating grid.
However only a very small part of slow secondary ions produced as a result of charge transfer collisions come to the accelerating grid because the charge transfer length exceeds the grid width and charge transfer collisions occur mainly at distances from the grid exceeding its width. For this reason the current in the working vacuum chamber circuit is practically equal to the current of accelerated ions entering the working vacuum chamber through the accelerating grid. When this current is less than the minimum current of a stable vacuum arc which may result from an electrical break-down between the working vacuum chamber and the plasma ion emitter of the gas discharge chamber, ignition of stationary arc with arc spots appearing on the walls of the working vacuum chamber and on the surfaces of the products under treatment can be eliminated by means, for instance, of the positive pole of the accelerating voltage power supply connection to the electrode of the gas discharge chamber through a current-limiting resistor.
When this current exceeds the minimum current of a stable vacuum arc, it is impossible to prevent the arc ignition by means of a current-limiting resistor. In this case the accelerating voltage power supply unit should include an arc extinguishing circuit which react, for instance, upon an abrupt drop of accelerating voltage between the electrode of the gas discharge chamber and the working vacuum chamber down to the value of arc discharge voltage (less than 100 V) and switches off the accelerating voltage for a time interval long enough for expiring of arc spots and then switches on the accelerating voltage again. The damage extent of the products under treatment with the arc spot traces depends on the arc current and on the arc duration, i. e. on the quickness of the arc extinguishing circuit response. But the damage of the products is inevitable because in any case the arc extinguishing needs at first the arc ignition.
Positive space charge of accelerated ions and of slow secondary ions in the working vacuum chamber is neutralised with electrons appearing in the chamber as a result of ion-electron emission from the inner walls of the chamber. As a result the vacuum chamber and the adjoining the flange part of the case are filled with quasi-neutral synthesised plasma with a positive relative to the chamber and the case walls potential. To limit electron flow from the synthesised plasma through the accelerating grid into the gas discharge chamber in order to prevent overheating of the gas discharge chamber with accelerated electrons a negative to the case potential U
g
(about 100 V) is put to the accelerating grid from an additional power supply unit.
In order to prevent contamination of the products under treatment with the material of the accelerating grid being sputtered with secondary ions from the synthesised plasma the negative potential of the grid is to be regulated in such a way that it would not exceed the minimum electron cut-off potential which is decreasing with the accelerated ion current decrease.
While using a gas discharge chamber with a cold cathode, the electron current should not be cut off completely as bombardment of the gas discharge chamber cold cathode with accelerated electrons results in a decrease of the discharge voltage and of energetic cost of ions produced.
For this reason the grid potential U
g
is to be regulated in such a way that the current of accelerated electrons, mainly being less than 5-10 per cent of the accelerated ion beam current, would flow in the gas discharge chamber circuit.
Disadvantages of the device are low quality of treated with fast molecules products resulting from their surfaces damage with arc spot traces during electrical brake-downs and complexity of the device control by regulating the negative potential of the accelerating grid by every change of be

Affiliated with

Also associated with

Rating

Source of fast neutral molecules does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Source of fast neutral molecules, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Source of fast neutral molecules will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2456011