Abstract:
A filter and a method for separating ions in a partially ionized plasma according to their mass includes a chamber with crossed electric and magnetic fields established therein. A feed, including metal atoms having ionization potentials in a low range, and gas atoms having an ionization potential in a high range, is introduced into the chamber. An electron temperature below the low range is generated to partially ionize the feed by dissociating the metal atoms from the gas atoms, and by ionizing the metal atoms into light and heavy ions according to their mass to charge ratio. The light and heavy ions are then influenced by the crossed electric and magnetic fields to separate the light ions from the heavy ions.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to filters and methods for separating ions of relatively low mass to charge ratios from ions of relatively high mass to charge ratios. More particularly, the present invention pertains to filters and methods for separating metal ions from a feed source material that includes metal and non-metallic atoms. The present invention is particularly, but not exclusively, useful for separating light metal ions from heavy metal ions in the presence of other gaseous components in the feed. 
     BACKGROUND OF THE INVENTION 
     It is known that the constituent elements of a multi-species plasma can be separated from each other in several different ways. One effective way to do this is to separate ions of the elements from each other according to their respective mass to charge ratios. A device and method for this purpose has recently been disclosed in U.S. Pat. No. 6,096,220, which issued to Ohkawa for an invention entitled “Plasma Mass Filter.” 
     This patent is assigned to the same assignee as the present invention, and is incorporated herein by reference for examples of a device and a method for processing a multi-species plasma to separate the ions of a heavy metal from the ions of a light metal. 
     As a practical matter, the source material for a multi-species plasma that contains both heavy metals and light metals, will contain more than just the metals. Typically, the source material (or feed) will include compounds such as oxides, hydroxides, chlorides or fluorides of the metals. Consequently, in order to fully ionize a source material, it is necessary to ionize all of the constituent elements; both metals and nonmetals. Different elements in a plasma, however, ionize at different electron temperatures. Stated differently, different elements have different ionization potentials. 
     By definition, the ionization potential of an element is the energy, expressed as electron volts (eV), that is required to detach an electron from a neutral atom. For gaseous elements (e.g. oxygen, chlorine and fluorine) the ionization potentials are relatively high and are between twelve and eighteen electron volts (12-18 eV). On the other hand, the ionization potentials for metals are relatively low and are in a range from four to eight electron volts (4-8 eV). 
     As a practical matter, the ionization of atoms in a plasma will begin to occur when electrons in the plasma have been heated to an electron temperature that is below the ionization potential of the atoms. This happens because heated electrons will evolve to a Maxwellian distribution at the electron temperature. Thus, for a given temperature, many of the electrons will have higher energy than indicated by the electron temperature. It is these energetic electrons that then do most of the ionization. 
     The efficacy of a plasma mass filter, such as the one disclosed by Ohkawa and referenced above, relies solely on the ionization of metals in the source material (feed). It does not matter whether gaseous elements in the source material have been ionized. A consequence of this is that, since only the metal elements need to be ionized, lower electron temperatures can be used. Furthermore, energy savings are significant since metal atoms will normally amount to less than half of the atoms in a typical source material and a plasma with lower electron temperature radiates less energy. Importantly, a partially ionized plasma (i.e. one wherein the metals have been ionized, but the gaseous elements of the source material have not) can still be effectively processed in a plasma mass filter for the purpose of separating metal ions from each other according to their respective mass to charge ratios. One caveat here is that the density of the gaseous elements (i.e. neutrals) in the chamber of an operational plasma mass filter may need to be controlled so as not to erode the separation quality. 
     In light of the above it is an object of the present invention to provide a device and method for separating ions in a partially ionized plasma according to mass to charge ratios that maintain separation quality during their operation. It is another object of the present invention to provide a device and method for separating ions in a partially ionized plasma according to mass to charge ratios that increases the effective throughput. Yet another object of the present invention is to provide a device and method for separating ions in a partially ionized plasma according to mass to charge ratios that reduces processing costs by reducing both the energy cost per ion and the fraction of atoms that need to be ionized. Still another object of the present invention is to provide a device and method for separating ions in a partially ionized plasma according to mass to charge ratios that is easy to use, relatively simple to manufacture and comparatively cost effective. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     In accordance with the present invention, a partially ionized plasma mass filter includes a substantially cylindrical shaped chamber that defines a longitudinal axis. Magnetic coils are mounted on the chamber to establish a magnetic field (B) in the chamber that is oriented substantially parallel to the chamber axis. Additionally, electrodes are mounted on the chamber to establish an electric field (E) in the chamber that is oriented substantially perpendicular to the chamber axis. Preferably, the electric potential is a positive along the axis and substantially zero at the wall of the chamber. Thus, crossed electric and magnetic field (E×B) are established inside the chamber. 
     An injector is provided for introducing a feed source material into the chamber along the axis. Specifically, the feed includes metal atoms and non-metallic atoms. For example, the feed may contain metallic oxides, hydroxides, chlorides or fluorides. Importantly, the present invention exploits the fact that the metals have lower ionization potentials than do oxygen or other gaseous elements (e.g. halogens such as fluorine or chlorine). Stated differently, the metals in the feed will have ionization potentials that are in a low range (e.g. 4-8 eV), and the gas atoms will have an ionization potential that is in a high range (e.g. 12-18 eV). 
     With the above in mind, an antenna is mounted on the chamber to generate an electron temperature that is slightly below the low range, but substantially below the high range. The consequence of this is that the feed source material is only partially ionized. Specifically, the metal atoms are dissociated from the gas atoms and ionized, but the non-metallic atoms are not ionized. Instead, the non-metallic atoms remain as a neutral gas. Specifically, as envisioned by the present invention the metal atoms will include both light and heavy metals. Thus, the metal atoms will ionize into light ions having a relatively low mass to charge ratio (M 1 ) and heavy ions having a relatively high mass to charge ratio (M 2 ). 
     The crossed electric and magnetic fields inside the chamber will influence the light ions (M 1 ) and the heavy ions (M 2 ) to travel on trajectories that differ according to the mass to charge ratio of the respective ions. Thus, the light ions are separated from the heavy ions. Accordingly, a first collector can be positioned in the chamber to collect the light ions (M 1 ), and a second collector can be positioned in the chamber to collect the heavy ions (M 2 ). In a preferred embodiment of the present invention, the first collector is positioned at an end of the chamber, opposite the injector, to collect the light ions after they have passed through the chamber. In this embodiment, the wall of the chamber between its two ends will serve as the second collector. 
     In addition to the structure disclosed above for the filter of the present invention, the filter may include a vacuum pump that is connected in fluid communication with the chamber. The function of this vacuum pump is to remove gas atoms (e.g. oxygen, fluorine, chlorine) as they collect near the wall of the chamber. Further, these gas atoms can then be directed back into the chamber to recombine with the light ions and the heavy ions to reform metal oxides (or hydroxides, fluorides, chlorides) which may be more easily removed from the chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
     FIG. 1 is a perspective view of the partially ionized plasma mass filter of the present invention with portions broken away for clarity; and 
     FIG. 2 is a cross sectional view of the filter as seen along the line  2 — 2  in FIG. 1 with a schematic representation of elements as they are being processed by the filter. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, a partially ionized plasma mass filter in accordance with the present invention is shown and is generally designated  10 . As shown, the filter  10  includes a plasma chamber  12  that is surrounded by a cylindrical wall  14 . For reference purposes, the wall  14  defines a longitudinal axis  16  that extends the length of the chamber  12 . As indicated in FIG. 1, the wall  14  is located at a radial distance from the axis  16 . For the purposes of the present invention, the radial distance of the wall  14  from the axis  16  is at least equal to a value “a.” 
     A plurality of magnetic coils  18 , of which the magnetic coils  18   a  and  18   b  are only exemplary, are positioned outside the wall  14 . Specifically, the coils  18  surround the chamber  12  for the purpose of generating a substantially uniform magnetic field, B z , that is axially oriented in the chamber  12 . Additionally, an electrode  20  is positioned inside the filter  10  to generate an electric field, E r , that is radially oriented relative to the axis  16  in the chamber  12 . It is an important aspect of the present invention that the electric field, E r , be characterized by a positive potential, V ctr , on the axis  16 , with a substantially zero potential at the wall  14 . Accordingly, crossed electric and magnetic fields (E r ×B z ) are established in the chamber  12  which will cause charged particles to travel on spiral trajectories  22  as they transit the chamber  12 . Insofar as the transit of charged particles through the chamber  12  is concerned, the particular trajectory  22  taken by a charged particle will depend on its mass to charge ratio. In accordance with the disclosure of U.S. Pat. No. 6,096,220 mentioned above, it is intended for the present invention that charged particles of relatively low mass/charge ratios (light ions: M 1 ) will be confined within the distance “a” as they transit the chamber  12 . Thus, the light ions M 1  will pass through the chamber  12 . On the other hand, charged particles of relatively high mass/charge ratios (heavy ions: M 2 ) will not be so confined. A so-called cut-off mass (M c ; where M 1 &lt;M c &lt;M 2 ) can be mathematically defined and determined by the expression M c =ea 2 (B z ) 2 /8V ctr . In this expression “e” is an electron charge, the wall  14  is at a distance “a” from the longitudinal axis  16 , and the magnetic field has a magnitude “B z ” in a direction along the longitudinal axis  16 . 
     Still referring to FIG. 1, it will be seen that the filter  10  of the present invention includes a pump  24  that is connected in fluid communication with the chamber  12 . Also, an antenna housing  26  is provided. For the purposes of the present invention, any device well known in the pertinent art, such as an r.f. antenna, can be positioned inside the housing  26 . In any event, the purpose here is to partially ionize a feed (source material)  28  as the feed  28  is introduced into the chamber  12  (see FIG.  2 ). 
     In FIG. 2 it will be seen that the feed  28 , as it is introduced into the chamber  12 , will include gaseous elements  30 , light metal elements  32  and heavy metal elements  34 . More specifically, the feed  28  will typically include compounds of these elements such as oxides, hydroxides, chlorides or fluorides of the metals. By way of example, such a compound for a light metal is indicated in FIG. 2 by the combined numeral  30 / 32 , while a compound for a heavy metal is indicated by  30 / 34 . 
     In the operation of the filter  10  of the present invention, the feed  28  is heated by an antenna in the housing  26  as it is being introduced into the chamber  12 . Specifically, as intended for the present invention, the feed  28  will be heated to a temperature that will accomplish two things. First, the constituent elements of the feed  28  will dissociate from each other. Second, the light metal elements  32  and the heavy metal elements  34  will both be ionized to respectively create the light ions M 1  and the heavy ions M 2 . Stated differently, the electron temperature that is used to heat the feed  28  needs to be close to the ionization potentials of both the light metal elements  32  and the heavy metal elements  34 . Typically, this means the operational energy level generated by the antenna in housing  26  will be below the low range of four to eight electron volts (4-8 eV). 
     Although light ions M 1  of the light metal elements  32  and heavy ions M 2  of the heavy metal elements  34  are introduced into the chamber  12 , it is not necessary to ionize the gaseous ions  30 . Thus, the electron temperature produced by the antenna in housing  26  can be kept well below the ionization potential of the gaseous elements  30 , which will be approximately twelve to eighteen electron volts (12-18 eV). The result is that the gaseous elements remain as neutrals in the chamber  12 . Thus, in review, the partially ionized plasma in the chamber  12  includes light ions M 1  of the light metal  32 , heavy ions M 2  of the heavy metal  34  and neutrals of the gaseous elements  30 . 
     During the processing of the partially ionized plasma in the chamber  12 , the light ions M 1  and the heavy ions M 2  will separate from each other according to the mathematical expression disclosed above for the cut-off mass M c . As indicated above, according to this expression, the light ions M 1  will transit through the chamber  12  while the heavy ions M 2  will be collected on the wall  14  of the chamber  12 . As this separation is taking place, the neutrals of the gaseous elements  30  are, of course, also in the chamber  12 . It happens that these neutrals will tend to concentrate and become more dense near the wall  14 . An adverse consequence of the presence of the neutrals in the chamber  12  is that as they become more dense they are more apt to interfere with the separation process. Thus, in order to avoid an erosion of the separation quality between M 1  and M 2 , the pump  24  is activated to draw the neutrals of the gaseous elements  30  from the chamber  12 . 
     As indicated in FIG. 2, the gaseous elements  30  that have been drawn from the chamber  12  to avoid erosion of the separation quality can be subsequently recombined with light ions M 1  of the light metal elements  32  to reform compounds  30 / 32 . As will be appreciated by the skilled artisan, the purpose here is to make it easier for the operator to collect the light metal elements  32  after they have been separated from the heavy metal elements  34 . In an alternate embodiment of the present invention, a gas source  36  can be provided for this same purpose. Also, it is envisioned that gaseous elements  30  can also be recombined with the heavy metal elements  34  to facilitate their collection. Further, if desired, some or all of the gaseous elements  30 , that are drawn from the chamber  12  by pump  24 , can be introduced into the feed  28  to help maintain the feed  28  as a source material. 
     While the particular Partially Ionized Plasma Mass Filter as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.