Abstract:
An apparatus for removing selected metal ions from a plasma includes a plasma chamber and at least one silica substrate mounted inside the chamber. More specifically, the substrate is exposed in the chamber so that when metal ions from the plasma contact the substrate they diffuse into the substrate to create a liquified layer. A receptacle is also provided to receive the liquid from the layer as it flows from the substrate.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to devices, apparatus and methods for separating selected metal ions from a plasma. In particular, the present invention pertains to collectors for removing ions of a selected material from a plasma separator. More particularly, the present invention pertains to metal ion collectors for plasma separators that include silica (glass) substrates which liquify when diffused by metal ions to create a collectable liquid. 
     BACKGROUND OF THE INVENTION 
     In the operation of a plasma mass filter, as well as in the operation of other types of plasma separators, it is always necessary to somehow establish a mechanism for effectively collecting the target material that is being processed. To this end, the collection process needs to be as efficacious and as easy to implement as possible. Various techniques for collecting material from a plasma mass separator have been heretofore suggested. These suggestions include the more obvious task of physically removing and replacing collectors of the target material. Additionally, these suggestions include more subtle processes, such as introducing a medium into the plasma chamber for in situ cleaning of collectors, as disclosed in U.S. Pat. No. 6,139,681, which issued to Ohkawa for an invention entitled “Plasma Assisted Process Vessel Cleaner” and which is assigned to the same assignee as the present invention. Still, the very nature of plasmas lend themselves to other possible collection methods. 
     It is known that when oxygen and metal vapors of a plasma come into contact with the surface of a solid glass substrate close to its melting point, the nature of the substrate surface is changed. An important change results from the fact that as oxides of the metal diffuse into the substrate, a surface layer of the substrate softens and turns from a solid state into a liquid state. The viscosity of this resulting liquified surface layer will depend on both the species of the metal oxides that are involved in the process and their concentration in the surface layer. 
     Once a surface layer of the glass substrate begins to liquify, the tendency is for the surface layer to drip from the substrate. In this condition, it is possible that the diffusion rate at which metal atoms diffuse into the substrate will exceed the rate at which metal atoms are deposited onto the substrate surface. If so, there will be no solid deposit on the surface layer. On the other hand, it is also possible that the deposition rate will be higher than the diffusion rate. In this latter case, a solid deposit will form on the liquified surface layer. In either event, at some point in time, the liquified surface layer of the substrate will begin to drip from the substrate. When this happens, the drip will include the metal atoms that have diffused into the surface layer of the substrate, as well as any solid deposit that may have formed on the surface layer. 
     A mathematical expression for the motion of a liquified surface layer relative to an underlying substrate can be obtained by balancing the viscous forces in the surface layer and the gravitational forces that are acting on the layer. For this expression, we assume the substrate is vertically oriented and we obtain 
     
       
           dy/dt≈[ρg/η]d[d+w]   (eqn. 1)  
       
     
     where “y” is the vertical position of the substrate, “ρ” is the mass density of the surface layer, “g” is the gravitational acceleration, “η” is the viscosity of material in the surface layer, “d” is the thickness of the surface area, and “w” is the thickness of the solid deposit. 
     As indicated above, any movement of the liquified surface layer should account for the possibilities that for a given throughput per unit area, Γ, a solid deposit layer may, or may not, form on the surface layer. Accordingly, the throughput, Γ, will include a diffusion term (surface layer) and a deposition term (deposition layer), and can be expressed as 
     
       
         Γ= nD/d+ndw/dt   (eqn. 2)  
       
     
     where “n” is the solid density, and “D” is the diffusion coefficient of metal atoms in the substrate glass. 
     As diffusion occurs, the diffusion depth of the surface layer at any given time “t” can be expressed as 
     
       
         d˜[Dt] 1/2 .  (eqn. 3)  
       
     
     Returning for the moment to the expression for throughput, Γ (eqn. 2), it will be appreciated that the diffusion term (nD/d) dominates early in time until the thickness of the surface layer reaches d 1 , given by the expression 
     
       
         d 1 ˜nD/Γ.   (eqn. 4)  
       
     
     With the above in mind, it is possible to define a low throughput regime wherein no solid deposit is formed on the liquified surface layer, and a high throughput regime wherein there is a solid deposit. For this purpose we can define the low throughput regime by the transit time it takes for the surface layer to move across the collector substrate through a vertical height “h.” Specifically, this low throughput regime persists when the transit time “t t ” for the thickness of the surface layer “d” to reach “d 1 ” (eqn. 4) is shorter than the diffusion time “t 1 ”. In this regime, the expression given above for movement of the surface layer (eqn. 1) becomes 
     
       
           dy/dt≈[ρg/η]d   2   (eqn. 5)  
       
     
     and the transit time t 2  is given by 
     
       
           t   2 ≈[2 hη/ρgD]   1/2 .  (eqn. 6)  
       
     
     Under conditions wherein t 2 &lt;t 1 , the throughput, Γ, becomes 
     
       
         Γ&lt; n[ρgD   3 /2 hη]   1/4 .  (eqn. 7)  
       
     
     If the throughput, Γ, is larger than the critical value defined in the above expression, the movement of the surface layer can be approximated by 
     
       
           dy/dt≈[ρg/η]d   1   w.   (eqn. 8)  
       
     
     The thickness of the deposition layer “w” during the transit time can then be given by the expression 
     
       
           w≈[Γ/n][ 2 hη/Dρg]   1/2 .  (eqn. 9)  
       
     
     In light of the above it is an object of the present invention to provide an apparatus and method for removing selected metal ions from a plasma which includes a collector that can be used in the vacuum chamber of a plasma separator to continuously remove collected material from the chamber. It is another object of the present invention to provide an apparatus and method for removing selected metal ions from a plasma which incorporates a collector that allows for a periodic replenishment of the collector in the vacuum chamber of a plasma mass separator. Still another object of the present invention is to provide an apparatus and method for removing selected metal ions from a plasma which allows material having a relatively high throughput, and other material having a relatively low throughput, to be simultaneously collected on respective collectors. 
     Yet another object of the present invention is to provide an apparatus and method for removing selected metal ions from a plasma which is relatively easy to implement, is simple to use, and is comparatively cost effective. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     An apparatus for removing selected metal ions from a plasma uses a plasma filter which has a chamber (container) for processing a multi-species plasma. More specifically, the plasma filter separates the multi-species plasma into relatively light ions which have a charge to mass ratio M 1 , and relatively heavy ions which have a charge to mass ratio M 2 . The plasma filter does this by directing the ions M 1  and the ions M 2  onto separate trajectories inside the plasma chamber. Within this context, the present invention is directed to the collectors that are mounted in the chamber for the purposes of separately collecting the ions M 1  and the ions M 2 , and removing them from the chamber. 
     In accordance with the present invention, both the M 1  ion collector and the M 2  ion collector include a substrate that is mounted and exposed in the plasma chamber for contact with the selected metal ions (M 1  or M 2 ). Preferably, each substrate is made of a crystalline compound, such as silica (SiO 2 ), which is interactive with the metal ions (M 1  and M 2 ) of the multi-species plasma. Specifically, the interaction of interest for the present invention involves the diffusion of these metal ions into the respective substrates. In each case this diffusion creates a liquified layer of the substrate that will eventually flow or drip from the substrate. Also, in cases where the deposition rate of the metal ions onto the substrate exceeds the diffusion rate of the metal ions into the substrate material (silica), a solid deposit is formed that will be transported from the substrate along with the underlying liquified layer. The liquid (and solid deposit), containing the metal ions, can then be subsequently collected in a receptacle and removed from the chamber. 
     As implied above, depending on the throughput of the multi-species plasma, Γ, the substrate will be liquified and may possibly include solid deposits. Mathematically, the throughput, Γ, will include a diffusion term (liquified surface layer) and a deposition term (solid deposition layer), that can be expressed as 
     
       
         Γ= nD/d+ndw/dt    
       
     
     where “n” is the solid density, “D” is the diffusion coefficient of metal atoms in the substrate glass, “d” is the thickness of a liquified surface area of the crystalline compound, and “w” is the thickness of a solid deposit on the liquified surface. 
     For purposes of the present invention, it is envisioned that both the light ions M 1  and the heavy ions M 2  will have a substantially same diffusion term (nD/d), and a substantially same diffusion rate, with respect to the crystalline compound substrates of the collectors. It is also envisioned, however, that their deposition terms (ndw/dt) 1  and (ndw/dt) 2 , and their respective deposition rates onto the collectors will differ significantly. This is so because, in general, it is expected that the throughput of the heavy ions M 2  will be only approximately 5% of the total throughput, Γ. Further, in the case of the light ions M 1 , they will most likely have a deposition term (ndw/dt) 1  that exceeds the diffusion term (nD/d). Consequently, a solid deposit from the light ions M 1  will form on its respective collector. On the other hand, the heavy ions M 2  will most likely have a deposition term (ndw/dt) 2  that is substantially small and, thus, no cognizable solid deposit from the ions M 2  will be formed. In both cases, however, the resultant liquified layer and the possible solid deposit that may form on the collectors will drip from the collectors and be removed from the chamber. 
     An important factor in the operation of the present invention described above involves heat control. Specifically, the diffusion constant “D” and the viscosity of the liquified substrate layer on the collectors are functions of temperature. Thus, the ability to maintain substrate surface temperatures within a particular range is crucial. For the present invention, this is accomplished by controlling the plasma throughput, Γ, and selectively moving the substrates relative to the plasma column in the chamber to control the radiative cooling of the substrate surfaces. Specifically, when the plasma in the chamber provides a heat input per unit area, P, and the temperature, T, of the substrate surfaces in the chamber generally need to satisfy the expression 
     
       
         σεT 4 =P  
       
     
     where σ is the Stephan Boltzman constant, and ε is emissivity. For example, if the desired glass melt temperature of the silica is 1000° C., the power input to match the radiative cooling is 1.5×10 5  w/m 2  (ε=1). 
     Another aspect of the present invention involves replenishment of the crystalline compound (silica) on the collector plates. Importantly, as envisioned for the present invention, this can be done without compromising the vacuum chamber of the plasma filter. In detail, this is done by first evacuating the plasma from the chamber. Then, once the plasma has been evacuated, a reactant gas is introduced into the chamber. Preferably, this reactant gas includes a mixture of silane and oxygen. Next, the collector plates are heated to cause a chemical vapor deposition (CVD) of the reactant gas onto the substrate on the collector plate. In the case of silane and oxygen, the result is a build-up of the crystalline compound, silica, on the collector plate. After a desired replenishment of the compound (silica) has been achieved, the multi-species plasma can again be introduced into the chamber (container) of the plasma filter and its operation continued. 
    
    
     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 a plasma mass filter for use with the present invention with portions broken away for clarity; and 
     FIG. 2 is a schematic cross sectional view of the collectors of the present invention shown collecting metal ions from a multi-species plasma. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, a plasma mass filter for use with the present invention is shown and is generally designated  10 . For purposes of the present invention, a plasma mass filter as disclosed and claimed in U.S. Pat. No. 6,096,220, which issued to Ohkawa on Aug. 1, 2000, for an invention entitled “Plasma Mass Filter” is suitable for use as the filter  10 . Accordingly, U.S. Pat. No. 6,096,220 (hereinafter referred to as the &#39;220 patent) is incorporated herein by reference. It is to be appreciated, however, that any apparatus or device that is capable of generating a plasma may be made suitable for use with the present invention. 
     In a general overview for the operation of the plasma mass filter  10 , it is to be appreciated that a useful purpose of the filter  10  is to separate the different metal ions of a multi-species plasma  12  from each other. For purposes of disclosure, these ions of the multi-species plasma  12  will include light ions (M 1 )  14  of relatively small mass to charge ratios, and heavy ions (M 2 )  16  of relatively large mass to charge ratios. To accomplish the separation of these ions  12  and  16 , the filter  10  includes several magnetic coils  18  that are mounted on the wall  20  of the plasma chamber  22 . Specifically, the coils  18  generate a substantially uniform magnetic field B z  that is directed along the axis  24  of the chamber  22 . Also, the filter  10  includes an electrode  26  that is mounted on the chamber  22  to generate a radially directed electric field, E r , that is crossed with the magnetic field B z . The resultant crossed electric and magnetic fields (E×B) then separate the ions (M 1 )  14  from the ions (M 2 )  16  as disclosed in the &#39;220 patent. 
     As disclosed in the &#39;220 patent, the plasma mass filter  10  is effective for separating the ions (M 1 )  14  from the ions (M 2 )  16  because the ions  14 ,  16  react differently to the crossed electric and magnetic fields (E×B) in the chamber  22 . Specifically, as taught and disclosed in the &#39;220 patent, the light ions (M 1 )  14  will be confined by the crossed electric and magnetic fields (E×B) for transit through the chamber  22  along the axis  24  and generally on a trajectory  28  (See FIG.  2 ). On the other hand, the heavy ions (M 2 )  16  will be ejected into the wall  20  ( 34 ) of the chamber  22  on a trajectory  30  (See FIG.  2 ). Consequently, by positioning a collector  32  at the end of the chamber  22 , as shown in FIG. 1, the collector  32  is positioned to capture the light ions (M 1 )  14  after they have transited the chamber  22 . Likewise, by positioning a collector  34  on the wall  20  inside the chamber  22 , the collector  34  is positioned to capture the heavy ions (M 2 )  16  as they are ejected into the wall  20 . A more detailed appreciation of how the collectors  32  and  34  work in accordance with the present invention will be had by reference to FIG.  2 . 
     In FIG. 2 a cross-sectional schematic of the collectors  32  and  34  is presented for purposes of discussion. With reference to FIG. 2, it is to be understood that the actual positioning and orientation of the collectors  32  and  34  in the chamber  22  can be varied according to the desires of the operator. Further, it is also to be understood that the collectors  32  and  34  can be moved and repositioned inside the chamber. This can be done by mechanical means known in the art, to alter respective perspective of the collectors  32  and  34  relative to the column of the multi-species plasma  12  as it is introduced into the chamber  22 , and as it transits through the chamber  22 . 
     As intended for the present invention, both the collector  32  and the collector  34  comprise a crystalline compound, such as silica (SiO 2 ). Importantly, as is well known to those skilled in the pertinent art, both the light ions (M 1 )  14  and heavy ions (M 2 )  16  will diffuse into a crystalline compound, such as silica (SiO 2 ). Accordingly, as the light ions (M 1 )  14  and the heavy ions (M 2 )  16  collide with the collectors  32  and  34 , they create respective liquified layers  36  and  38  which, under the influence of gravitational forces, will begin to eventually drip from the collectors  32  and  34 . Furthermore, for a given throughput per unit area, Γ, of the multi-species plasma  12  through the chamber  22 , a solid deposit layer  40  may, or may not, form on the surface layer  38 . Accordingly, the throughput, Γ, will include a diffusion term (liquified surface layer  34 ,  38 ) and a deposition term (solid deposition layer  40 ), and can be expressed as 
     
       
         Γ= nD/d+ndw/dt    
       
     
     where “n” is the solid density, and “D” is the diffusion coefficient of metal atoms in the substrate silica glass of the collectors  32  and  34 . As shown in FIG. 2, and as disclosed above, the possibility of the formation of a solid deposit  40  is dependent on whether the deposition rate in the term (ndw/dt) of the throughput, Γ, exceeds the diffusion rate of the diffusion term (nD/d). In either case, as the liquified layers  36  and  38  on the respective collectors  32  and  34  begin to drip, they will be caught by the respective receptacles  42  and  44  for removal from the chamber  22 . 
     As disclosed above, another important aspect of the present invention involves control of the surface temperatures for the collectors  32  and  34 . In accordance with earlier disclosure this temperature control is accomplished by metering the through put, Γ, of the multi-species plasma  12 . Still another important aspect of the present invention involves the replenishment of the crystalline compound (e.g. silica) that is used to trap the separated ions (M 1  and M 2 ). In accordance with the present invention, this replenishment can be accomplished without compromising the vacuum of the chamber  22  by sequentially accomplishing several tasks. These include: evacuating the plasma  12  from the chamber  22 ; introducing a gas reactant (e.g. silane and oxygen) into the chamber  22 ; and heating the collectors  32  and  34  to cause a chemical vapor deposition (CVD) of the reactant gas onto the collectors  32  and  34 . 
     While the particular Liquid Substrate Collector 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.