Patent Publication Number: US-5025194-A

Title: Vapor and ion source

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a vapor and ion source, and in particular to a metal vapor and ion source. 
     In the art, the metal vapor and ion sources conventionally use the bombardment of a target by an electron beam. Complex and fragile prior art apparatuses operate under a high vacuum (in the range of 10 -3  pascals) and have a limited lifetime. 
     Accordingly, an object of the invention is to provide for another type of vapor and ion source having a very simple structure and a long lifetime. 
     SUMMARY OF THE INVENTION 
     For attaining this object, the invention provides for a vapor and ion source comprising, in a low pressure chamber, an anode (ground), a cathode and means for applying a magnetic field. The cathode constitutes an equipotential cavity provided with an aperture in a front plate. A magnetic field is applied orthogonally to the plane of said front plate. The material to be ionized is arranged inside the cathode cavity. 
     For operating this ion source, a low pressure carrier gas is initially introduced into the chamber and in particular in the cathode cavity, and the cathode is connected to a negative potential in the range of some hundreds volts with respect to the anode (the ground). Such operation causes ionization of the carrier gas and heading of the cathode. Then, the material to be ionized, arranged in the cathode, is vaporized and ionized. The vapors and the ions of the material are then projected towards a target at a negative potential with respect to the plasma potential. Once the phenomenon has started, a pumping can be provided to reduce the quantity of carrier gas which is then unnecessary. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and others, objects, features and advantages of the invention will be explained in greater detail in the following description of a preferred embodiment in connection with the attached FIGURE. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in the FIGURE, a source according to the invention comprises a cathode 10 provided with an internal cavity and an output aperture 11 arranged in a front plate of the cavity wall. This cathode is enclosed in a chamber, for example a conductive chamber M connected to the ground that will be called hereinafter either the ground or the anode as this chamber constitutes an anode with respect to the cathode 10. This chamber is provided with pumping means 13 and means 14 for introducing a gas, for example helium, argon, etc. 
     A magnet A or another means for applying a magnetic field applies a magnetic field B at the aperture 11 of the cathode orthogonally to the plane of the front plate thereof. In the cavity 10 is arranged a material to be ionized, for example a metal 15 having the form of a powder, rods, chips, etc. 
     Initially, the chamber M is evacuated and a carrier gas having a pressure in the range of some tenths to some tens pascals is introduced. When the cathode is at a negative potential in the range of some hundreds volts, an electric field appears and, due to the existence of the cavity, electric field lines 16 curved towards the inside of the cavity appear at the level of the aperture in the front plate, as shown in the FIGURE. Thus, there are areas wherein the electric field E, due to the curvature of the electric field lines, is perpendicular to the applied magnetic field B. Accordingly, a plasma is created at very low gas pressures, lower than in case such crossed electric and magnetic fields do not exit. In fact, it is the edge effect (curvature of the electric field due to the presence of the aperture in the front plate of the cavity) associated with the crossed magnetic field at some places that are at the origin of the plasma creation. Thus, at pressures of the carrier gas higher than some tenths of pascals, a plasma region appears substantially as shown by dotted lines in the FIGURE, at the neighborhood of aperture 11. 
     This plasma causes heating of the cavity walls 10 and therefore of the material 15 which is vaporized (directly or through a liquefaction). Thus, vapors of the material are created. Those vapors exist through the aperture 11 of the cavity and a substantial proportion thereof is ionized by the plasma electrons so ions of the material 15 are created and can be collected by a plate 20 arranged in front of the cavity aperture, at a negative potential with respect to the ground. This plate therefore receives ions and vapors of the material while the electrons are pushed back by the negative potential towards the ground. 
     It will be appreciated that the plasma potential is not the potential of the cathode 10, but that, if the potential of the cathode is for example -500 volts, the plasma potential will be of some tens volts. Thus, according to the impact energy that is to be given to the ions on the plate 20, this one will be at a potential varying between about one hundred and one thousand volts. 
     If the vapor pressure of the material in the cavity increases to a pressure of about one pascal, the introduction of the gas 14 which causes the creation of the plasma can be stopped while continuing pumping by means of the pump 13. The discharge is maintained because, in those conditions, it will be the vapors of material 15 that are substituted for the function of the starting gas. Therefore, a self-sustained plasma exists inside the cavity. 
     As explained, a self-sustained vapor and ion source is obtained due to the presence of material 15, this source operating under a vacuum of some tenth to some tens pascals. 
     Material 15 will be for example a metal. In a case in which the cathode is at a potential of -500 volts and plate 20 at a potential of about -1,200 volts, copper was used at about 3 amperes of Cu +  ions accelerated under 1,200 volts on a surface of 50 cm 2 . The deposition speed of the copper atoms on plate 20 were at a rate of 20 micrometers/minute. About 25% of the evaporated copper atoms were ionized. 
     This possibility of obtaining a self-sustained source due to the presence of a metal, and while being under a vacuum (in the absence of carrier gas) exists for about 60% of the known metals (the ones of which the saturated vapor pressure is higher than some pascals at temperatures lower than 1,600°-1,700° C. which are the temperatures normally occurring inside the cathode cavity). 
     The method according to the invention presents a large number of advantages over the state of the art, including for example, 
     a high deposition speed in comparison to vacuum sputtering, 
     a high ionization ratio of the metal vapors (up to 25%) while conventional ion deposition methods permit ionization ratios up to 1 to 2%, and 
     the possibility of operating without a carrier gas for 60% of the metals, which reduces the pollution ratio. 
     In addition the high ion number than can be implanted on a target 20 results in a high adherence of the deposit. 
     Due to its great simplicity, the device according to the invention can have a long lifetime. Additionally, there is no practical limitation to the size of the device and it is possible to provide bombardment of target 20 on a large length if aperture 12 is an elongated slot. 
     The invention is capable of numerous variations. The metal constituting the cathode and the ground chamber is chosen from elements that do not react at the operating temperatures, for example tungsten. The chamber can have various shapes and can be associated with a ground grid arranged in front of aperture 11 for improving the starting of the plasma. Various grids and aperture systems at various potentials can be provided in order to accelerate or focus the ions towards the target.