Electric lamp and discharge devices – Cathode ray tube – Electron permeable window
Reexamination Certificate
1999-08-23
2001-05-29
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Electron permeable window
C313S421000, C250S492300, C250S400000
Reexamination Certificate
active
06239543
ABSTRACT:
TECHNICAL FIELD
The invention relates to formation of electron beam sustained high pressure gas plasmas for use in surface chemistry, including thin film deposition, grafting, cleaning, depyrogenation, and sterilization of surfaces.
BACKGROUND ART
Surface chemistry means the instigation of chemical reactions upon or within surfaces. In the former case, growth of thin films or inactivation of organic matter, such as bacteria or its byproducts known as pyrogens, is known to occur using ion or electron beams, usually at low pressure. In the latter case, certain surfaces, such as silicon, can be converted to oxides with surface reactions. An example of such conversion is the conversion of the surface of a silicon wafer to silicon dioxide by introducing a reactive oxygen species near the wafer surface, usually in vacuum.
In U.S. Pat. No. 5,508,075 A. Roulin et al. disclose formation of an oxygen barrier layer on a surface as a packaging laminate. A plasma is formed in a vacuum chamber in order to carry out plasma enhanced chemical vapor deposition (PECVD). Organic silicon compounds are combined with oxygen within the plasma such that the two compounds react and are deposited upon and chemically bonded to the surface. Silicon oxide can be formed directly on the surface. The patent indicates that preferred substrates or surfaces are flexible thermoplastic materials.
It is known that electron beams are useful for surface treatments. In U.S. Pat. No. 5,909,032 G. Wakalopulos discloses an arrangement of electron beam tubes which produce a linear or stripe-like electron beam suitable for surface treatment. The Wakalopulos patent features electron beam tubes which are sealed vacuum tubes emitting a beam through a thin window into an ambient environment, such as air, a relatively high pressure environment compared to the vacuum environment of the interior of the tube where the electron beam is generated. The construction of the Wakalopulos tube is shown in U.S. Pat. Nos. 5,637,953 and 5,414,267, both assigned to the assignee of the present invention.
The above mentioned beam tubes produce electron beams which interact with air, causing ionization of the air, creating secondary electrons which participate, together with the primary electrons of the beam, in surface treatment. One of the problems which is encountered is that the beam is evanescent, quickly become neutralized. Since the beam is continuous, the evanescent nature of the beam is not important for some applications. In other applications, a more persistent beam is needed to achieve the desired effect.
Electron beams have previously been used for sterilization. However, in many cases a residue of proteinaceous material is left because microorganism debris tenaciously adheres to a surface. The disabling of the microorganism does not necessarily disable the chemical bonds retaining the microorganism to a surface.
An object of the invention was to devise a high pressure electron beam tube apparatus with a persistent beam for surface sterilization, and a sustainer discharge for inactivation of proteinaceous material from treated surfaces.
Another object of the invention was to devise a high pressure plasma apparatus for chemical vapor deposition, grafting, and in particular thin film formation.
SUMMARY OF THE INVENTION
The above object has been achieved using a Wakalopulos electron beam tube plus an exterior field, electric or magnetic, to momentarily confine charged particles generated by the electron beam, with beam energy under 100 kV, emitted by the tube. The effect of the exterior confinement field is to sustain a gas plasma built by the electron beam interaction with air or other gases. The structure outside of the beam tube can be at ambient temperature and pressure, such as an air environment or other special purpose gases.
In the case of an electric field, an anode and cathode outside of beam tube is used. Closest to the tube is a screen anode which allows the electron beam to pass through the screen. At a slightly further distance is a damper cathode which will repel electrons back towards the screen anode, forming an electron trap. As the electron beam egresses the beam tube, electrons ionize air molecules forming a plasma which is almost neutral, except for an excess of electrons due to the electron beam, similar to a glow discharge. Secondary electrons, as well as a substantial fraction of beam electrons, impinge on objects between the screen anode and the damper cathode. The beam tube produces a stripe shaped beam which reacts in a swath on target surfaces. A belt or web carrying objects into the plasma can expose surfaces of such objects to the electrons for surface treatment.
In the case of a magnetic field, a coil outside of the beam tube is used. The coil may have a field with an axis parallel to the beam tube or at an angle to the tube axis. Electrons from the beam tube follow magnetic flux lines, so an article with surfaces to be sterilized, such as a vial, is placed in the flux path. For sterilization, the plasma disables microorganisms and interacts with the molecular residue on the surfaces, causing depyrogenation, allowing the surfaces to be free from infectious organic matter. Ash can be removed by simple washing in pure water after depyrogenation.
The screen anode and damper cathode electrodes or the coil structure tend to sustain the field making higher density and longer lasting plasmas than the evanescent plasmas usually associated with electron beams in air. Such a plasma is said to exist in a sustainer field which is energized by an independent power supply.
If the plasma is created in an inert environment in a sealed housing, special gases can be introduced into the electron beam which will interact with the beam. An example of such gases is organic silicon gases which decompose into silicon dioxide or similar film materials which can be grown merely by introducing organic silicon gases into the beam. Similarly, other decomposing gases can be introduced for plasma enhanced chemical vapor deposition (PECVD).
REFERENCES:
patent: Re. 35203 (1996-04-01), Wakalopulos
patent: 5414267 (1995-05-01), Wakalopulos
patent: 5508075 (1996-04-01), Roulin et al.
patent: 5612588 (1997-03-01), Wakalopulos
patent: 5637953 (1997-06-01), Wakalopulos
patent: 5909032 (1999-06-01), Wakalopulos
American International Technologies, Inc.
Patel Ashok
Schneck Thomas
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