Mass filtering sputtered ion source

Details Mass filtering sputtered ion source Mass filtering sputtered ion source

2000-08-02

2001-12-04

Arroyo, Teresa M. (Department: 2881)

Radiant energy

Ion generation

Field ionization type

C250S3960ML, C250S492300, C250S281000, C210S695000, C204S554000, C209S127100

Reexamination Certificate

active

06326627

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains generally to devices and methods for generating ions and for separating ions of different mass charge ratios from each other. More particularly, the present invention pertains to devices and methods that are capable of effectively separating ions of different mass charge ratios after the ions have been generated by plasma sputtering. The present invention is particularly, but not exclusively, useful as a device and method for plasma sputtering a multi-metallic substrate, wherein previously-sputtered heavier ions are redirected into contact with the substrate for additional sputtering, and previously-sputtered lighter ions are prevented from doing so and, instead, are separately collected.
BACKGROUND OF THE INVENTION
For applications wherein the purpose is to separate a constituent element from a chemical compound, from a metallic alloy or from some other mixture of elements, there are several possible ways to proceed. In some instances, mechanical separation may be possible. In others, chemical separation may be more appropriate. Further, when mechanical or chemical processes are not feasible, it may happen that procedures and processes involving plasma physics may be necessary. If so, it is necessary to first generate a multi-species plasma that contains the target constituent. Then, it is necessary to separate the target constituent from the rest of the multi-species plasma.
There are many known ways in the pertinent art by which plasmas, including multi-species plasmas, can be generated. For example, the evaporation of a substrate by an electron beam or by laser ablation is often used in plasma processing applications. Another method involves sputtering. With sputtering, atoms are removed from an electrode by positive ion bombardment of a source material. Insofar as sputtering is concerned, a relatively recent development in this field is provided in an article entitled “Universal Metal Ion Source” authored by Churkin et al. of the Budker Institute of Nuclear Physics, Novosibirsk Russia, and presented in the American Institute of Physics, 1998. In particular, this article discloses an electrode that is used as a metal ion source and sputtered in a magnetic trap. As disclosed in the Churkin article, this is done with crossed electrical and magnetic fields.
As implied above, once the multi-species plasma has been generated, it is still necessary to separate the target constituent from the plasma. Again, such a separation can be accomplished in several ways known in the pertinent art. For example, plasma centrifuges and their methods of operation are well known. On the other hand, and not yet so well known, plasma filters and their methods of operation are also useful for this purposes. For example, the invention as disclosed by Ohkawa in U.S. application Ser. No. 09/192,945, filed on Nov. 16, 1998, for an invention entitled “Plasma Mass Filter” and assigned to the same assignee as the present invention is useful for separating ions of different mass charge ratios. Due to the fact that the phenomena involved with plasma filter procedures are quite different from those involved with a plasma centrifuge, it is helpful to mathematically consider these phenomena as they will apply to the situation wherein a multi-species plasma is generated using a sputtered ion source.
In a vacuum chamber, when an inwardly oriented, radial electric field (E) is crossed with an axial magnetic field (B), charged particles will have orbits that are described by the following equation:
md
2
r/dt
2
=eE+e[VB]
In the equation above, “m” is the mass of the charged particle (e.g. ion), “e” is the ion charge, and “V” is particle velocity. For a conservation of energy, it can be shown from the above equation that:
m
(
V
r
2
+V
&thgr;
2
+V
z
2
)/2+
e&phgr;+
(
r
)=&egr;
mV
&thgr;
r+eBr
2
/2=
M
where “&thgr;” is electrode potential, “&egr;” is the total energy of a particle, “M” is the angular momentum of the particle, “V
r
” is the radial component of particle velocity, “V
&thgr;
” is the angular component of particle velocity, and “V
z
” is the axial component of particle velocity.
In a cylindrical-shaped vacuum chamber, immediately after a charged particle has been ionized at a distance r
max
from the central axis, it will have a very small kinetic energy and the total energy &egr; will be:
&egr;=
e&phgr;
(
r
max
)
and its angular momentum will be:
M=eB
(
r
max
)
2
/2
Once ionized, the particle will then be influenced by the radial electric field (E) in the chamber that will accelerate it toward the axis. Acting against this acceleration of the charged particle toward the axis will be a Lorentz force that deflects the charged particle away from the axis and back to its original distance from the axis, i.e. r
max
. At the point when the charged particle (ion) is closest to the axis, i.e. at r
min
, its radial velocity will be equal to zero (V
r
=0). For this condition:
U=&phgr;
(
r
max
)−&phgr;(
r
min
)=(
eB
((
r
max
)
2
−(
r
min
)
2
/r
min
)
2
/8
m
At this point, consider that the electric field (E) is, at least in part, generated by a central electrode that is oriented along the central axis. Further, consider that the central electrode is generally rod-shaped and has a radius that is equal to “a” (i.e. r
min
=a). Thus, if r
min
is less than “a” (i.e. r
min
<a), when the charged particle is accelerated toward the electrode it will be lost to the electrode.
If, as indicated, the above-described conditions are established in a generally cylindrical shaped chamber that has a wall at a radius “b” from the central axis, there is a critical electrical potential in the chamber that can be expressed as:
U
(
r
)=
e
2
B
2
(
r
2
−a
2
)
2
/8
a
2
m=U
o
(
r
2
−a
2
)
2
/(
b
2
−a
2
)
2
  (Eq. 1)
The total voltage applied between the central electrode and the wall of the chamber can then be expressed as:
U
o
=e
2
B
2
(
b
2
−a
2
)
2
/8
a
2
m
The consequence of all this is that when U
o
is established inside the chamber with radial profile U(r), described by Eq. 1, ions with a mass greater than “m” (i.e. m
2
>m) will fall onto the central electrode. On the other hand, ions with a mass less than “m” (i.e. m
1
<m) will not fall onto the central electrode but, instead, will be confined inside the chamber for subsequent separation from the plasma.
In light of the above, it is an object of the present invention to provide a device for separating ions from each other which uses relatively heavier mass ions in a multi-species plasma to sputter a metallic electrode and, thereby, generate more of the multi-species plasma. Another object of the present invention is to provide a device for separating ions from each other that effectively confines relatively lighter mass ions to a predetermined volume in a chamber for subsequent removal therefrom. Yet another object of the present invention is to provide a device for separating ions from each other that is effective for separating metal ions from a metal alloy. Still another object of the present invention is to provide a device for separating ions from each other that is easy to use, relatively simple to manufacture and comparatively cost effective.
SUMMARY OF THE PREFERRED EMBODIMENTS
A device for separating ions of different mass charge ratios from each other includes an elongated chamber that defines a longitudinally aligned central axis and has a first end and a second end. In its configuration, the elongated chamber is preferably cylindrical shaped and has a wall that is positioned at a distance “b” from the central axis. A central electrode is positioned in the chamber and is aligned along the axis. Preferably, the electrode is rod-shaped, has a radius “a,” and is made of at least two elements. For example, one of the elements is preferably a light metal that has a mass “m
1
.” The other element is relatively heavy, such as a heavy impurity, and i

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