Patent Number: 06326627&
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring initially to FIG. 1, a device for separating ions in accordance with the present invention is shown and generally designated 10. As shown, the device 10 includes a substantially cylindrical-shaped chamber 12 that defines a longitudinal axis 14, and has a first end 16 and a second end 18. Magnetic coils 20a and 20b are shown mounted on the chamber 12 at its first end 16, and magnetic coils 22a and 22b are shown mounted on the chamber 12 at its second end 18. Together, these magnetic coils 20a,b and 22a,b create a magnetic field (B) inside the chamber 12. The particular magnetic coils 20a,b and 22a,b that are shown in the Figures are, however, only exemplary and additional magnetic coils can be incorporated as desired. The magnetic coils 20a,b, and 22a,b are, however, shown in the Figures to illustrate that the magnetic field (B) will be strongest at the ends 16 and 18. Also, they are configured to illustrate that the coils 20a and 20b at the first end 16 are to be positioned at a greater distance from the axis 14 than are the magnetic coils 22a and 22b at the second end 18. The consequence of all this is that the magnetic field (B) will generate so-called "magnetic mirrors" at both the first end 16 and at the second end 18. Thus, in comparison with each other, there will be a full magnetic mirror across the whole cross section at the second end 18 (r&lt;b), and a generally annular-shaped magnetic mirror at the first end 16 (c&lt;r&lt;b). The exit 24 shown in FIGS. 1 and 2 is specifically positioned around the center of the annular-shaped mirror at the first end 16. Additional features of the device 10 will, perhaps, be best appreciated with reference to FIG. 2. There it will be seen that the device 10 includes a substantially rod-shaped, metallic electrode 26 that extends along the longitudinal axis 14 through the center of the chamber 12. For purposes of the present invention, this centrally located electrode 26 will preferably include two elements. One of the elements is preferably a light metal that has a mass "m.sub.1 ". As envisioned for the present invention, the second element of the central electrode 26 will be a relatively heavy impurity having a mass "m.sub.2." FIG. 2 also shows that a plurality of ring electrodes 28 are positioned in a plane around the longitudinal axis 14 at the first end 16. The electrodes 28a, 28b and 28c are only exemplary. FIG. 2 also shows that there are a plurality of ring electrodes 30 which are positioned in a plane around the longitudinal axis 14 at the second end 18. Again, the electrodes 30a, 30b, 30c, 30d and 30e are only exemplary. Together, the central electrode 26 and the ring electrodes 28 and 30 create an electric field inside the chamber 12 that will vary radially from the longitudinal axis 14 to provide a desirable radial distribution as described below. Recall, "e" is the ion charge, "m" is the mass of an ion, and "r" is a radial distance from the longitudinal axis 14. For the device 10, wherein "a" is the radius of the central electrode 26, "b" is the radius of the chamber 12, and "c" is the radius of the exit 24 (see FIG. 2), a critical potential U.sub.o can be expressed as U.sub.o =e.sup.2 B.sup.2 (b.sup.2 -a.sup.2).sup.2 /8a.sup.2 m. Desirable radial profiles 34 and 38 of the electric potential are shown in FIG. 3. For the purpose of explanation, several other profiles are also shown. For example, the radial profile 32 shown in FIG. 3 is representative of the cut-off potential for an ion of heavy mass, m.sub.2. The radial profile 34, on the other hand, is representative of the cut-off potential for an ion of light mass, m.sub.1. Stated differently, with a radial profile 32 for the electrical potential, U(r), in the chamber 12, the ions of mass m.sub.2 will be directed back toward the axis 14 for collision with the central electrode 26. The ions of light mass m.sub.1, however, will not be so directed. Further, with a radial profile 34 for the electrical potential, U(r), in the chamber 12, both the ions of mass m.sub.1 and mass m.sub.2 will be directed into collision with the central electrode 26. Thus, operationally, in order to separate the ions of mass m.sub.1 from the ions of mass m.sub.2, the device 10 is preferably operated with a radial profile 36 that is somewhere between the radial profiles 32 and 34. In some instances, as explained more fully below, it may be necessary or desirable to operate with a radial profile 38. With a radial profile 36 in the chamber 12, the heavier ions of mass m.sub.2 will generally follow a path similar to the trajectory 40 shown in FIG. 4. Thus, the heavier ions (m.sub.2) will be accelerated back into collision with the central electrode 26. The result of this is additional sputtering of the central electrode 26. At the same time, because the radial profile 36 is below the cut-off potential for the lighter ions of mass m.sub.1 (i.e. radial profile 34), the lighter ions (m.sub.1) will be confined within the chamber 12. In FIG. 4, the trajectory 42 is exemplary of a cold light ion and the trajectory 44 is exemplary of a hot light ion. In both instances, the trajectories 42 and 44 indicate that the ion does not collide with the central electrode 26. Stated differently, the ions on trajectories 42 and 44 are confined in the chamber 12. Inside the chamber 12, the sputtered particles of heavier mass m.sub.2 can either be ionized and return to the central electrode under the influence of the electric field, or, as neutrals, reach a collector 46. As seen in FIG. 2, the collector 46 is preferably a cylindrical-shaped plate that is located near the wall of the chamber 12, at a distance from the central electrode 26. The lighter ions of mass m.sub.1, which are confined within the chamber 12, will be expelled from the chamber 12 through the exit 24. This can be caused to happen by properly configuring the magnetic field (B) inside the chamber 12. In accordance with the present invention, the configuration of the magnetic field (B) inside the chamber 12 can, perhaps, be best appreciated by reference to FIG. 5. In FIG. 5, consider that the axial position Z=0 is at the first end 16 of the chamber 12, and that "z" increases along the longitudinal axis 14 in a direction from the first end 16 to the second end 18. The axial profiles 48, 50 and 52 are illustrative of magnetic field strengths for B inside the chamber 12. Recall, the device 10 incorporates respective magnetic mirrors at the first end 16 and the second end 18 of the chamber 12. Specifically, due to the configuration of the magnetic coils 20a and 20b at the first end 16 of the chamber 12 (i.e. where z=0), the field strength B will vary as shown. At the exit 24, where r&lt;c, where c is the radius of the exit 24, the magnetic field B will have the axial profile 52. At the r&gt;c, the magnetic field B will have the axial profile 52. Thus, there is a diverging magnetic field at r&lt;c which effectively creates an annular shaped magnetic mirror at the first end 16. On the other hand, due to the magnetic coils 22a and 22b at the second end 18 of the chamber 12 (i.e. where z=L), the field strength will be relatively high over the entire second end 18. The consequence here is that the magnetic mirror at the second end 18 will tend to redirect charged particles away from the second end 18 and toward the first end 16. The annular-shaped magnetic mirror at the first end 16 will, however, allow the charge particles to exit from the chamber 12 through the exit 24. In operation, the magnetic field, B, is established as described above. A vacuum of around 10.sup.-4 Torr is drawn inside the chamber 12 and a gas, such as hydrogen (H.sub.2) or Argon (Ar) is introduced into the chamber 12. The electric field, E, is then activated to initiate a plasma discharge in the chamber 12. Specifically, the electric field, E, is established with a potential that will effectively accelerate ions in the chamber 12 to an energy in the range of one to three thousand electron volts (1-3 KeV). The resultant sputtering of the central electrode 26 will then cause both light ions (M.sub.1) and heavy ions (m.sub.2) to be present in the chamber 12. With an electric field having a radial profile (e.g. radial profile 36) the heavier ions (m.sub.2) will be directed toward the central electrode 26 for further sputtering. The lighter ions (m.sub.1) will be confined inside the chamber 12 and eventually expelled through the exit 24 by the effect of the magnetic mirrors disclosed above. Heavier neutrals with mass m.sub.2 that reach the outer wall without ionization shall be collected on the collector 46. It is to be appreciated that the operation disclosed above will be effective so long as there is a sufficient amount of the heavier ions of mass m.sub.2. If the central electrode 26 contains only a minority of an impurity (i.e. the ions of mass m.sub.2 are less than 10-30% of the electrode 26), it may be necessary to adjust the electric field. Specifically, for this case, the ring electrodes 28 and 30 can be adjusted so that the radial profile 38 is established inside the chamber 12. With this potential, a fraction of the light ions that reach the plasma periphery will be directed by the electric field back to the central electrode to take part in further sputtering. Subsequently, as the proportion of heavier ions in the electrode 26 is increased, it will be possible to establish the radial profile 36 inside the chamber 12. While the particular Mass Filtering Sputtered Ion Source 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.