Plasma Device with an External RF Hollow Cathode for Plasma Cleaning of High Vacuum Systems

Abstract

A compact cylindrical vacuum chamber made from a dielectric ceramic or glass wrapped with a cylindrical electrode connected to an RF source make a hollow cathode RF plasma source. The dielectric cylinder is used as the vacuum container with the conductive electrode outside the vacuum region to excite plasma inside. A gas is supplied by a gas source at low flow on one end of the cylinder and after being excited exhausts into a connected vacuum chamber carrying excited metastables and radicals. RF power is applied to the electrode to excite the plasma via the hollow cathode effect. This remote RF plasma source can be used to create ions, electrons, excited metastables, and atomic radicals for use downstream depending on choices of gas, pressure, flow rates, RF power and frequency, and extraction electrodes.

1 . A device for the production of plasma using a radio-frequency hollow cathode discharge for delivery of plasma to a vacuum chamber, the device comprising:
 a) a hollow cylinder formed of a dielectric material, the cylinder having an exterior, and an interior, and an upstream end in communication with a source of gas through a gas flow control device, wherein the interior is devoid of objects other than the gas; and   b) a single cylindrical electrode formed of conducting material and surrounding and generally coextensive with the exterior of the cylinder, the electrode in communication with a source of radio-frequency electrical power, wherein, upon energizing the electrode with radio-frequency electric power and passage of gas through the cylinder under vacuum, a plasma is formed by radio-frequency, hollow cathode effect coupling inside the dielectric cylinder to generate reactive neutral species: and   c) a vacuum chamber in communication with a downstream end of the hollow cylinder.   
     
     
         2 . The device of  claim 1 , wherein the cylinder is formed of a material comprising about 55% fluorophlogopite mica and 45% borosilicate glass. 
     
     
         3 . The device of  claim 1 , wherein the vacuum chamber is in a charged-particle-beam instrument and the reactive neutral species are used to clean the vacuum chamber. 
     
     
         4 . The device of  claim 3 , wherein the gas is air and the vacuum chamber is cleaned by Oxygen radicals that are the neutral specie that remove carbon compounds by oxidation from the vacuum chamber, but do not oxidize the conducting material of the electrode. 
     
     
         5 . The device of  claim 1 , wherein the gas is hydrogen. 
     
     
         6 . The device of  claim 1 , wherein the upstream end of the cylinder is closed by an end wall formed of the dielectric material and the gas enters through an aperture in the end wall. 
     
     
         7 . The device of  claim 1 , wherein a virtual anode is formed by a hollow cathode effect along a central axis of the cylinder in the plasma and an electrical ground is defined by the vacuum chamber walls. 
     
     
         8 . The device of  claim 1 , wherein the cylindrical electrode is a brass cylinder extending around the exterior of the hollow cylinder. 
     
     
         9 . The device of  claim 1 , wherein the gas flow control device includes an aperture through which gas flows into the cylinder under vacuum. 
     
     
         10 . The device of  claim 1 , wherein the gas flow control device is a variable valve. 
     
     
         11 . The device of  claim 1 , wherein the cylinder is formed of a material comprising fluorophlogopite mica and borosilicate glass. 
     
     
         12 . A device for the production of plasma using electrical energy for delivery of plasma to a vacuum chamber, the device comprising:
 a) a hollow cylinder formed of a material including fluorophlogopite mica and borosilicate glass, the cylinder having an exterior and an interior;   b) an electrode formed of conducting material and at least partially surrounding the exterior of the cylinder, the electrode in communication with a source of electrical power, wherein, upon energizing the electrode with electric power and passage of gas through the cylinder under vacuum, a plasma is formed to generate reactive neutral species: and   c) a vacuum chamber in communication with a downstream end of the hollow cylinder.   
     
     
         13 . The device according to  claim 12 , wherein the electrode further comprises:
 a single cylindrical electrode formed of conducting material and surrounding and generally coextensive with the exterior of the cylinder, the electrode in communication with a source of radio-frequency electrical power, wherein, upon energizing the electrode with radio-frequency electric power and passage of gas through the cylinder under vacuum, a plasma is formed by radio-frequency, hollow cathode effect coupling inside the dielectric cylinder to generate reactive neutral species.   
     
     
         14 . The device according to  claim 13 , wherein the cylindrical electrode is a brass cylinder extending around the exterior of the hollow cylinder. 
     
     
         15 . The device of  claim 12 , wherein an upstream end of the cylinder is closed by an end wall formed of the dielectric material and the gas enters through an aperture in the end wall. 
     
     
         16 . A method of generating a plasma having increased numbers of secondary electrons and density, the method comprising the steps of:
 evacuating a vacuum chamber to a selected pressure;   flowing a gas through a cylinder in fluid communication with the vacuum chamber, the cylinder being formed of a material including fluorophlogopite mica and borosilicate glass; and   energizing an electrode surrounding the cylinder with RF electrical energy, wherein a plasma is formed in the cylinder from the gas, and the plasma density is enhanced by the electron and ion bombard of the mica/borosilicate glass that increases the secondary electron production.