Abstract:
A fluorine generator includes a vacuum chamber filled with a working gas. An r-f antenna is positioned outside the chamber across a dielectric window from a potassium fluoride (KF) source located in the chamber. The r-f antenna radiates through the window to heat the working gas and sublime the PK source to create a plasma. Crossed electric and magnetic fields in the chamber drive the heavier potassium ions in the plasma toward a collector in the chamber while confining the lighter fluorine and working gas ions for evacuation from the chamber.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention pertains generally to fluorine radical generators. More particularly, the present invention pertains to ion filters that are able to separate fluorine ions from a plasma. The present invention is particularly, but not exclusively, useful as a system for creating a plasma by sublimating a potassium fluoride source to create a plasma from which fluorine ions can be evacuated.  
         BACKGROUND OF THE INVENTION  
         [0002]    Fluorine (F) is a pale greenish yellow gas. It is the most electronegative (non-metallic) of the elements and is the first of the halogens, having only one electron vacancy in its outer energy level. Chemically, fluorine is highly corrosive and is a very reactive gas. Thus, it is never found free and, accordingly, its storage and delivery are problematic and raise serious safety implications. Moreover, fluorine gas is expensive.  
           [0003]    Despite the difficulties that are confronted during the handling and use of fluorine, fluorine radicals have been found to be very useful in many different applications. For instance, NF 3  has been used extensively in semiconductor applications. More specifically, NF 3  has been used as a feed for etching semiconductors and, more recently, for the cleanup of vacuum chambers following an etching process. Many other examples, including other types of cleanup operations, can be cited wherein fluorine gas, or a fluorine radical, is useful.  
           [0004]    It happens that potassium (K) reacts with fluorine (F) to create a potassium fluoride salt (KF). Unlike a fluorine radical, however, potassium fluoride salt (KF) is relatively inexpensive, and is more easily handled. Further, it also happens that a potassium fluoride salt (KF) is volatized into its constituents, K and F, when heated by a plasma. The resultant fluorine can then be used for the applications referred to above.  
           [0005]    In light of the above, it is an object of the present invention to provide a fluorine generator that can be used as an “on-demand” source of fluorine radicals. Another object of the present invention is to provide a fluorine generator that circumvents the need for implementing the safety procedures that are required for the long-term storage of fluorine. Still another object of the present invention is to provide a convenient source of relatively clean, uncontaminated fluorine. Yet another object of the present invention is to provide a fluorine generator that is simple to use, is relatively easy to manufacture, and is comparatively cost effective.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with the present invention, a fluorine generator includes a generally cylindrical shaped vacuum chamber having a first end and a second end. The chamber defines a longitudinal axis and it is filled with a working gas. For purposes of the present invention this working gas is preferably either Neon (Ne) or Nitrogen (N 2 ).  
           [0007]    A dielectric window is located at the first end of the chamber, and a potassium fluoride (KF) source is positioned in the chamber adjacent the window. Preferably, the KF source will be a flat, generally annular shaped disk made of a potassium fluoride salt. An r-f (radio frequency) antenna is positioned outside the chamber with the dielectric window located between the KF source and the r-f antenna. Further, the generator includes a generally annular shaped collector that is positioned inside the chamber between said first and second ends, and at a distance “a” from the axis.  
           [0008]    In further detail, the generator includes a plurality of concentric electrodes that are mounted inside the chamber. Specifically, these electrodes are centered on the axis between the KF source and the second end of the chamber, and they are used to create a radially oriented electric field, E r , in the chamber. Importantly, the electrodes are biased to create a parabolic profile of electric potential for the electric field, E r , that can be defined as φ¢(r)=U(1−r 2 /a 2 ). In this case, “U” is the voltage on the axis (which will preferably be below 200 volts) and “r” is a radial distance from the axis. Additionally, the generator includes magnetic coils that are mounted on the outside of the chamber to create an axially oriented and substantially uniform magnetic field, B, inside the chamber. Thus, the electric and magnetic fields are crossed (E r ×B).  
           [0009]    In the operation of the generator, the working gas is first introduced into the chamber. The r-f antenna is then activated to heat the working gas. This heating ionizes the working gas and, in turn, causes the KF source to sublimate. The result is the creation of a plasma in the chamber that includes potassium ions, fluorine atoms, fluorine ions and working gas ions. It is an important aspect of the present invention that the crossed electric and magnetic fields (E r ×B) are controlled to drive the potassium ions on unconfined orbits toward the collector inside the chamber. Thus, the potassium ions, which have a mass weight of 39 and are therefore heavier than the ions of either the working gas or of fluorine, are separated from the plasma. Specifically, by being on unconfined orbits, the potassium ions do not exit the chamber and are, instead, collected on the collector in the chamber. On the other hand, contrary to its effect on the potassium ions, the controlled (E r ×B) is established to place the fluorine ions and the working gas ions on confined orbits around the axis. Thus, these ions are directed toward the second end of the chamber. These ions are then evacuated from the chamber through its second end for the collection of the fluorine.  
           [0010]    An important consideration for the operation of the generator of the present invention is the maintenance of a pressure of approximately 1-10 mTorr in the chamber. This is done by concerted control over the introduction of the working gas into the chamber and the evacuation of fluorine and working gas ions from the chamber. A consequence here is that the potassium ions, fluorine ions and working gas ions have respective densities in the plasma, wherein the densities of the potassium and fluorine ions are in a range of 5-20% of the density of the working gas ions.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    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:  
         [0012]    [0012]FIG. 1 is a perspective view of a fluorine generator with portions broken away for clarity; and  
         [0013]    [0013]FIG. 2 is a schematic diagram of the component parts of a fluorine generator according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    Referring initially to FIG. 1, a fluorine generator in accordance with the present invention is shown and generally designated  10 . As shown the generator  10  includes a substantially cylindrical shaped vacuum chamber  12 , and a dielectric window  14  that is positioned to enclose the chamber  12  at one end. Further, generator  10  is shown to include a potassium fluoride (KF) source  16  that is located inside the chamber  12 . Preferably, the KF source  16  is a flat, annular shaped disk of a potassium fluoride salt and is positioned inside the chamber  12  and near the dielectric window  14  substantially as shown. Additionally, the generator  10  is shown to include a radio frequency (r-f) antenna  18  that is positioned outside the chamber  12  and across the dielectric window  14  from the KF source  16 .  
         [0015]    Still referring to FIG. 1, it will be seen that the generator  10  also includes a plurality of magnetic coils  20 , of which the coils  20   a  and  20   b  are exemplary. Also, it will be appreciated by the skilled artisan that permanent magnets could be used for the purposes of the present invention in lieu of the coils  20 . In either case, the purpose of the coils  20  (or permanent magnets) is to generate a constant uniform magnetic field, B, inside the chamber  12  that is substantially oriented parallel to the longitudinal axis  22  of the chamber  12  (see FIG. 2).  
         [0016]    [0016]FIG. 1 further shows that the generator  10  includes a plurality of concentric electrodes  24 , of which the electrodes  24   a - c  are exemplary. As shown, the electrodes  24  are centered on the axis  22  and are located at a distance “L” from the window  14 . Importantly, the KF source  16  is located between the electrodes  24  and the window  14 . The purpose of the electrodes  24  is to generate a radially oriented electric field, E r , inside the chamber  12 . Preferably this electric field E r  will have a positive potential on the axis  22  that is equal to of less than approximately 200 volts, and it will have a zero potential at a radial distance “a” from the axis  22 . Also, it is envisioned for the present invention that there will be a parabolic profile of electric potential for the electric field E r , wherein the profile is defined as φ(r)=U(1−r 2 /a 2 ), with “U” being the voltage on the axis  22  (e.g. 200 volts) and “r” being a radial distance from the axis  22 . In any event, the consequence here is to create crossed electric and magnetic fields (E r ×B) inside the chamber  12 .  
         [0017]    By cross referencing FIG. 1 with FIG. 2, it will be seen that a generally annular shaped collector  26  is positioned around the chamber  12  at a radial distance “a” from the axis  22 . As so positioned, the collector  26  is in a plane that is substantially perpendicular to the axis  22 . Further, pumping ports  28   a  and  28   b  are connected in fluid communication with the chamber  12 . With specific reference to FIG. 2, it will be seen that the present invention also incorporates a working gas source  30  that is used to introduce a working gas into the chamber  12 . Preferably, the working gas that is used with the generator  10  will be either Neon (Ne) or Nitrogen (N 2 ). Additionally, FIG. 2 indicates that a controller  32  is electronically connected to both the electrodes  24  and to the magnetic coils  20 . Further, FIG. 2 shows that a flow control unit  34  is provided to evacuate gases and plasmas from the chamber  12 . More specifically, the flow control unit  34  is positioned at the end of chamber  12  opposite the dielectric window  14 . In particular the flow control unit  34  is located so that both the KF source  16  and the electrodes  24  are between the dielectric window  14  and the flow control unit  34 .  
       OPERATION  
       [0018]    In the operation of the generator  10  of the present invention, a working gas  36  is introduced into the chamber  12  from the working gas source  30 . The r-f antenna  18  is then activated to heat and vaporize the working gas  36 . In turn, heat from the vapors of working gas  36  will cause the KF source  16  to sublimate and create potassium ions  38 , fluorine ions  40  and fluorine atoms. The result is a multi-species plasma  42  that is held in the chamber  12 .  
         [0019]    With the plasma  42  in the chamber  12 , the controller  32  is set to establish crossed electric and magnetic fields (E r ×B) in the chamber  12  that will effect a predetermined cut-off mass (M c ). For purposes of the present invention, the cut-off mass (M c ) can be calculated in accordance with the disclosure of U.S. Pat. No. 6,096,220 which issued to Ohkawa for an invention entitled “Plasma Mass Filter” and which is assigned to the same assignee as the present invention. Preferably, M c =0.12B 2 a 2 /U for the present invention, with M c  set to be less than, or approximately equal to, 39 (the mass for potassium). The result of this being, because the cut-off mass (M c ) can be set above the mass of the working gas  36  (M g ) and above the mass of fluorine, but below the mass for potassium (M K ) (i.e. M g &lt;M c &lt;M K ) the effect of the crossed electric and magnetic fields (E r ×B) is predictable. Specifically, this condition will place the heavier mass potassium ions on unconfined orbits  44 . Specifically, as shown in FIG. 2, the unconfined orbits  44  cause the potassium ions  38  to become collected on the collector  26 . Meanwhile, the crossed electric and magnetic fields (E r ×B) place the working gas ions  36  and the fluorine ions  40  on confined orbits  46  around the axis  22  in chamber  12  between the KF source  16  and the electrodes  24 . As indicated in FIG. 2, these confined orbits  46  cause the working gas ions  36 , and the fluorine ions  40 , to proceed along the axis  22  for evacuation from the chamber  12 . Between the electrodes  24  and the flow control unit  34 , the fluorine ions  40  become neutrals as they continue for evacuation from the generator  10  with the assistance of the flow control unit  34 .  
         [0020]    As the generator  10  is being operated, it is important that a pressure of approximately 1-10 mTorr be sustained in the chamber  12 . This is done by concerted control over the working gas source  30  and the flow control unit  34 . Specifically, this control requires that the introduction of the working gas  36  into the chamber  12  be balanced with the evacuation of fluorine ions  40  and working gas ions  36  from the chamber  12 . A consequence here is that the potassium ions  38 , fluorine ions  40 , fluorine atoms and working gas ions  36  have respective densities in the plasma, wherein the densities of the potassium ions  38 , fluorine ions  40 , and fluorine atoms are in a range of 5-20% of the density of the working gas ions  36 .  
         [0021]    While the particular Fluorine Generator 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.