EP1220289A2

EP1220289A2 - Plasma mass selector

Info

Publication number: EP1220289A2
Application number: EP01202936A
Authority: EP
Inventors: Tihiro Ohkawa
Original assignee: Archimedes Technology Group Inc
Current assignee: Archimedes Technology Group Inc
Priority date: 2000-08-08
Filing date: 2001-08-02
Publication date: 2002-07-03
Also published as: JP2002150994A; EP1220289A3

Abstract

A plasma mass selector for processing a partially ionized multi-species plasma includes magnetic coils that are mounted on a cylindrical chamber to establish an axially oriented uniform magnetic field in the chamber. A segmented quadrupole electrode creates a non-uniform cyclotron electric field in the chamber that is oriented perpendicular to the chamber axis to cross with the magnetic field. With the plasma in the chamber, a controller is activated to operate the non-uniform cyclotron electric field at a predetermined cyclotron frequency (ω). The electric field thus resonates with selected charged particles having the predetermined cyclotron frequency (ω) to restrain the selected particles in orbits around the central axis during their transit through the chamber. On the other hand, non-selected particles are allowed to drift radially outward from the central axis for collision with the wall and subsequent removal therefrom.

Description

FIELD OF THE INVENTION

The present invention pertains generally to particle filters. More particularly, the present invention pertains to devices and methods for removing selected particles from a partially ionized multi-species plasma. The present invention is particularly, but not exclusively, useful as a device and method for establishing resonance with selected particles in a plasma, to control the movement of these selected particles in a manner that will cause them to be removed or separated from other non-selected particles in the plasma.

BACKGROUND OF THE INVENTION

In a partially ionized multi-species plasma there will be neutral particles (e.g. a neutral gas such as Argon) as well as charged particles. Further, in a multi-species plasma the charged particles will also have different mass-charge ratios. Thus, for applications wherein it is desirable to selectively remove specific particles having a predetermined mass-charge ratio from the plasma, it is important to appreciate how all of the various particles in the plasma will react to particular conditions, and how they will react to each other under these conditions. For instance, considering a single charged particle, it is well known that when the charged particle moves perpendicular to a magnetic field it will move on a circular path. The number of revolutions around this path per second is known as the particle's cyclotron frequency (ω). Also, it is known that cyclotron resonance can be achieved by coupling electromagnetic power into a system of charged particles that are undergoing orbital movement in a uniform magnetic field. In the context of a partially ionized multi-species plasma, these concepts still pertain. Additionally, however, in a partially ionized multi-species plasma the effect of collisions between ions and neutrals is also of importance.
It is known that in a partially ionized plasma, the effects of ion-neutral collisions become important because the drag on the azimuthal rotation that is caused by these ion-neutral collisions induces the guiding centers of the ion orbits to drift radially outward from a central axis. On the other hand, it is also known that when another force is applied to counter the collisional drag force, the direction of the radial drift can be reversed. It happens that such a countering force can be applied by using resonant electromagnetic power at the cyclotron frequency of specific particles. Importantly, this resonant power is mass dependent. Accordingly, selected particles in a partially ionized plasma can be excited at their cyclotron frequency and thereby be forced to drift or spiral inwardly toward a central axis. At the same time, non-selected particles in the plasma will be allowed to continue to drift radially outward from the central axis.
In light of the above it is an object of the present invention to provide a device and method for processing a partially ionized multi-species plasma which is capable of targeting selected particles in the plasma for separation from all other particles in the plasma. Still another object of the present invention is to provide a device and method for processing a partially ionized multi-species plasma that will effectively differentiate between particles of different mass-charge ratio in order to remove particles of a selected mass-charge ratio from the plasma. Yet another object of the present invention is to provide a device and method for processing a partially ionized multi-species plasma that is easy to use, is relatively simple to employ, and is comparatively cost effective.

SUMMARY OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, a plasma mass selector for processing a partially ionized multi-species plasma includes a generally cylindrical chamber that has a wall and defines a central axis. An injector is provided for introducing the partially ionized plasma into the chamber. As intended for the present invention the partially ionized multi-species plasma will have at least three identifiable constituents. These are: 1) neutrals, such as are present in an inert gas (e.g. Argon); 2) particles that have a relatively low mass-charge ratio (M₁) and a cyclotron frequency (ω₁); and 3) particles that have a relatively high mass-charge ratio (M₂) and a cyclotron frequency (ω₂).
The selector also includes a plurality of magnetic coils that are mounted on the chamber to establish a substantially uniform magnetic field inside the chamber. More specifically, the magnetic field is oriented so as to be substantially parallel to the central axis.
A quadrupole electrode is positioned to create a non-uniform cyclotron electric field (NCEF) inside the chamber. Specifically, the NCEF electric field is oriented substantially perpendicular to the central axis in order to establish crossed electric and magnetic fields inside the chamber. Preferably, the quadrupole electrode comprises a plurality of concentric coplanar rings that are divided into quadrant segments. With this configuration, the quadrupole electrode is selectively operable at either the cyclotron frequency ω₁ or ω₂ to respectively resonate with the particles M₁ or M₂. For example, when the quadrupole electrode is operated at ω₁ the effect is to restrain the particles M₁ in orbits around the central axis for transit through the chamber, while allowing the particles M₂ to drift radially outward from the central axis for collision with the wall. On the other hand, when the quadrupole electrode is operated at ω₂ the effect is to restrain the particles M₂ in orbits around the central axis for transit through the chamber, while allowing the particles M₁ to drift radially outward from the central axis for collision with the wall.

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 plasma mass selector in accordance with the present invention;
Fig. 2A is a representative geometric illustration of a charged particle orbit under the influence of a drag force, D;
Fig. 2B shows the motion variables associated with the charged particle orbit shown in Fig. 2A;
Fig. 3A is a representative geometric illustration of a charged particle orbit under the influence of a drag force, D, and an accelerating force, P; and
Fig. 3B shows the motion variables associated with the charged particle orbit shown in Fig. 3A.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring initially to Fig. 1, a plasma mass selector in accordance with the present invention is shown and is generally designated 10. As shown, the selector 10 includes a chamber 12 surrounded by a wall 14 that is located at a distance from a central axis 16. Preferably, the chamber 12 is cylindrically shaped, and the central axis 16 is substantially the longitudinal axis of the cylinder. As also shown in Fig. 1, the selector 10 includes an injector 18 for introducing a partially ionized multi-species plasma into the chamber 12. As intended for the present invention this partially ionized plasma contains neutrals, such as are present in an inert gas (e.g. Argon, Ar). Additionally, the partially ionized plasma will contain particles of relatively low mass-charge ratio (M₁) and particles of relatively high mass-charge ratio (M₂). These particles M₁ and M₂ will have respective cyclotron frequencies ω₁ and ω₂. It will be appreciated that the partially ionized plasma that is to be processed by the selector 10 in accordance with the present invention may also contain additional charged particles having mass-charge ratios that are different from M₁ and M₂.
Still referring to Fig. 1, it will be seen that the selector 10 includes a plurality of magnetic coils 20, of which the magnetic coils 20a and 20b are only exemplary. Specifically, the magnetic coils 20a and 20b are mounted externally on the chamber 12 and can be activated in a manner well known in the pertinent art to generate a generally uniform magnetic field, B, inside the chamber 12. Preferably, this magnetic field B will be oriented substantially parallel to the central axis 16 inside the chamber 12.
Fig. 1 also shows that the selector 10 includes a quadrupole electrode which is made up of a plurality of coplanar electrode rings 22. As shown, the electrode rings 22 collectively lie in a plane that is oriented substantially perpendicular to the central axis 16. The electrode rings 22a and 22b shown in Fig. 1 are only exemplary. In the orientation disclosed, the electrode rings 22 generate a radially oriented electric field, E, that is generally perpendicular to the central axis 16. Accordingly, there are crossed electric and magnetic fields (E x B) inside the chamber 12.
It is an important aspect of the present invention that the configuration for the electrode rings 22 is such that quadrant electrode segments 24 are created. As indicated in Fig. 1, these quadrant electrode segments 24 are identified as 24a, 24b, 24c and 24d. In a manner well known in the pertinent art, the segments 24a-d can be selectively activated such that electric fields are generated between adjacent segments by applying voltage to diametrically opposed pairs. For example, the quadrant segments 24a and 24c can be activated, while quadrant segments 24b and 24d are idle This results in a voltage between 24a and respectively 24b, 24c and 24d. This can then be changed so that the quadrant segments 24b and 24d are activated to generate electric fields while the quadrants 24a and 24c are idle. Further, this change over can be accomplished at a predetermined selectable frequency, ω.
For the purposes of the present invention, the mathematical disclosure for the operation of the selector 10 of the present invention starts with the understanding that an electrostatic potential is established inside the chamber 12 that is given by: Φ0 = -α0 r2 Bz / 2 and Φ2 = -[α2 r2 Bz/2] cos 2 cos ω t
The equations of motion for the charged particles in the multi-species plasma are given by M d vr / d t = M v 2 / r + e v Bz + e Er-M* ν vr M d r v / d t = r {- e vr Bz +e E - M* ν v } where ν is the ion-neutral collision frequency and M* is the reduced mass.
We separate the slow drift motion, v₁, and the rapid cyclotron motion, v₂, by putting r = r1 + r2,  = 1 + 2 and v = v1 + v2
We obtain d vr2 / d t ≈ Ω v2 - ν* vr2 + Ω α2 r1 cos 21 cos ω t d v2 / d t ≈ - Ω vr2 - ν* v2 - Ω α2 r1 sin 21 cos ω t where Ω = e B_z / M and ν^* = ν M^* / M
The solutions are given by vr2 + i v2 = [r1α2 Ω / 2] exp [-2 i  ] x[ { exp[iωt] -exp[-iΩt-ν*t]} {iΩ+iων*}-1 + { exp[ -iωt]-exp[-iΩt-ν*t]} {iΩ-iω+ν*}-1]
At the resonance ω = Ω, we obtain vr2 + iv2 ≈ [r1 α2 Ω / 2 ν*] exp [-2 i 1 -i ω t ] and r2 ≈ [r1 α2 / 2 ν*] sin [21 + ω t ] r1 2 ≈ [r1 α2 / 2 ν*] cos [21 + ω t ]
We calculate the electric field <E>_,t averaged over time and azimuthal angel on the ion orbits < Er2 >,t ≈ 0 < E2 >,t ≈ - [3/8] [r1 Bz α2 2 / ν* ]
The radial drift velocity of the guiding center is given by vr1 = < E2 >,t / Bz + ν* r1 α0 / Ω = [ν* r1 α0 / Ω] {1 - [3 α2 2 Ω / 8 ν*2 α0]}
The condition that the radial drift of the resonant ions is reversed is given by α2 / ν* > [8 α0 / 3 Ω]1/2
In the steady state operation, the non-resonant ions must be lost radially before they exit axially. L / vz > [Ω / ν* α0 ]
The axial velocity limited by the collisional drag is given by vz = [vs 2 / ν*L ] where L is the length of the device and v_s is the sound velocity. The above condition becomes ν*L / vs > [Ω / α0]1/2
The drift reversal condition for the resonant ions can be written using the above condition α2 > [vs / L][8/3]1/2
It shows that the required quadrupole r-f field is modest.
The practical aspects of the motion of the charged particles M inside the chamber 12 will perhaps be best appreciated by cross referencing the Figs. 2A, 2B, 3A, and 3B. When doing so, it is important to realize that although the Figs. 2A and 2B illustrate an orbit for the charged particle M_2, they could as well illustrate an orbit for the charged particle M₁. Similarly, although the Figs. 3A and 3B illustrate an orbit for the charged particle M₁ they could as well illustrate an orbit for the charged particle M₂. The point here is that the particular orbit trajectory does not depend on the mass-charge ratio, M, of a particle. Instead, the trajectory of the particular orbit depends on whether the particle is being resonantly accelerated in the partially ionized multi species plasma.
Figs. 2A and 2B specifically illustrate the trajectory for an orbit 26 that will be followed around the axis 16 by a particle of mass-charge ratio M₂ when the particle M₂ begins at a start point 28. Specifically, the orbit 26 will result when the particle M₂ is not resonantly accelerated as it traverses through the chamber 12. As mentioned above, due to collisions with neutrals in the chamber 12, a drag force, D, will be imposed on the particle M₂. Further, in accordance with the mathematics disclosed above, the drag force D will cause the particle M₂ to spiral outwardly from the central axis 16 and follow the orbit 26, as shown in Figs. 2A and 2B. On the other hand, Figs. 3A and 3B specifically illustrate an orbit 30 that will be followed by a particle of mass-charge ration M₁ when the particle M₁ begins at a start point 32 and is resonantly accelerated as it traverses through the chamber 12.
As shown in Figs. 3A and 3B, in order for a particle (e.g. the particle M₁) to be resonantly accelerated along the orbit 30, it is necessary to exert a force P on the particle that will act opposite to the drag force D. In accordance with the present invention the generation of this propelling force, P, is accomplished by selectively activating the electrode quadrant segments 24a-d. Specifically, the force P is generated on a particle M₁ by operating the electrode quadrant segments 24 at the cyclotron frequency of the particle M₁ (i.e. ω₁). Thus, a resonant condition is established with the particle M₁ that does not affect the particle M₂. On the other hand, when the electrode quadrant segments 24a-d are operated at the cyclotron frequency of the particle M₂ (i.e. ω₂) the particles M₂, instead of the particles M₁, will be resonantly accelerated to follow trajectories having a configuration similar to the orbit 30. In this manner, any particle M that is resonantly accelerated at its cyclotron frequency will spiral inwardly toward the central axis 16 (i.e. orbit 30). Consequently, resonantly accelerated particles will be allowed to travel through the chamber 12. The particles that are not resonantly accelerated, however, will spiral outwardly away from the central axis 16 (i.e. orbit 26) and will collide with the wall 14 before they complete a transit through the chamber 12.
While the particular Plasma Mass Selector 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.

Claims

A plasma mass selector which comprises:

a chamber having a wall and defining a central axis;

an injector for introducing a partially ionized plasma into said chamber, said partially ionized plasma containing neutrals, particles of mass-charge ratio (M₁) having a cyclotron frequency (ω₁), and particles of mass-charge ratio (M₂) having a cyclotron frequency (ω₂);

a magnetic means for establishing a substantially uniform magnetic field in said chamber, said magnetic field being oriented substantially parallel to said central axis; and

an electrical means for creating a non-uniform cyclotron electric field in said chamber, said electric field being oriented substantially perpendicular to said axis to cross with said magnetic field, said electric field being operable at the cyclotron frequency ω₁ to resonate with the particles M₁ to restrain the particles M₁ in orbits around said central axis for transit through said chamber while allowing the particles M₂ to drift radially outward from said central axis for collision with said wall.
A selector as recited in claim 1 wherein said magnetic means is a plurality of magnetic coils mounted on said wall of said chamber.
A selector as recited in claim 1 wherein said electrical means is a quadrupole electrode.
A selector as recited in claim 3 wherein said electrode is a plurality of concentric electrode rings, and said rings are separated to establish quadrant segments.
A selector as recited in claim 1 wherein said electrical means is operable at ω₂ to resonate with the particles M₂ to restrain the particles M₂ in orbits around said central axis for transit through said chamber while allowing the particles M₁ to drift radially outward from said central axis for collision with said wall.
A selector as recited in claim 1 wherein said particles M₁ are introduced from said selector into a plasma processor for subsequent processing.
A plasma mass selector which comprises:

a magnetic means for establishing a substantially uniform magnetic field in a chamber, said chamber having a wall and defining a central axis with said magnetic field being oriented substantially parallel to said central axis;

an electrical means for creating a non-uniform cyclotron electric field in said chamber, said electric field being oriented substantially perpendicular to said axis to cross with said magnetic field;

a means for introducing a partially ionized multi-species plasma into said chamber; and

a controller means connected with said electrical means for operating said non-uniform cyclotron electric field at a predetermined cyclotron frequency (ω) to resonate with selected charged particles in the multi-species plasma to restrain the selected particles in orbits around said central axis for transit through said chamber while allowing non-selected particles to drift radially outward from said central axis for collision with said wall.
A selector as recited in claim 7 wherein the partially ionized plasma contains neutrals, particles of mass-charge ratio (M₁) having a cyclotron frequency (ω₁), and particles of mass-charge ratio (M₂) having a cyclotron frequency (ω₂), and further wherein said controller means is operable at ω₁ to resonate with the particles M₁ to restrain the particles M₁ in orbits around said central axis for transit through said chamber while allowing the particles M₂ to drift radially outward from said central axis for collision with said wall.
A selector as recited in claim 7 wherein said magnetic means is a plurality of magnetic coils mounted on said wall of said chamber.
A selector as recited in claim 7 wherein said electrical means is a quadrupole electrode.
A selector as recited in claim 10 wherein said electrode is a plurality of concentric electrode rings, and said rings are separated to establish quadrant segments.
A selector as recited in claim 7 wherein the partially ionized plasma contains neutrals, particles of mass-charge ratio (M₁) having a cyclotron frequency (ω₁), and particles of mass-charge ratio (M₂) having a cyclotron frequency (ω₂), and further wherein said controller means is operable at ω₂ to resonate with the particles M₂ to restrain the particles M₂ in orbits around said central axis for transit through said chamber while allowing the particles M₁ to drift radially outward from said central axis for collision with said wall.
A selector as recited in claim 7 wherein said particles M₁ are introduced from said selector into a plasma processor for subsequent processing.
A method for selecting particles from a non-uniform partially ionized multi-species plasma which comprises the steps of:

establishing a substantially uniform magnetic field in a chamber, said chamber having a wall and defining a central axis with said magnetic field being oriented substantially parallel to said central axis;

creating a non-uniform cyclotron electric field in said chamber, said electric field being oriented substantially perpendicular to said axis to cross with said magnetic field;

introducing a partially ionized multi-species plasma into said chamber; and

operating said non-uniform cyclotron electric field at a predetermined cyclotron frequency (ω) to resonate with selected charged particles in the multi-species plasma to restrain the selected particles in orbits around said central axis for transit through said chamber while allowing non-selected particles to drift radially outward from said central axis for collision with said wall.
A method as recited in claim 14 wherein the partially ionized plasma contains neutrals, particles of mass-charge ratio (M₁) having a cyclotron frequency (ω₁), and particles of mass-charge ratio (M₂) having a cyclotron frequency (ω₂), and further wherein said operating step is accomplished at the cyclotron frequency ω₁ to resonate with the particles M₁ to restrain the particles M₁ in orbits around said central axis for transit through said chamber while allowing the particles M₂ to drift radially outward from said central axis for collision with said wall.
A method as recited in claim 14 wherein the partially ionized plasma contains neutrals, particles of mass-charge ratio (M₁) having a cyclotron frequency (ω₁), and particles of mass-charge ratio (M₂) having a cyclotron frequency (ω₂), and further wherein said controlling step is accomplished at the cyclotron frequency ω₂ to resonate with the particles M₂ to restrain the particles M₂ in orbits around said central axis for transit through said chamber while allowing the particles M₁ to drift radially outward from said central axis for collision with said wall.
A method as recited in claim 14 wherein said establishing step is accomplished with a plurality of magnetic coils mounted on said wall of said chamber.
A method as recited in claim 14 wherein creating step is accomplished with a quadrupole electrode.
A method as recited in claim 18 wherein said electrode is a plurality of concentric electrode rings, and said rings are separated to establish quadrant segments.
A method as recited in claim 14 further comprising the step of transferring the said particles M₁ from said chamber into a plasma processor for subsequent processing.