EP1586221B1 - Ion accelerator arrangement - Google Patents

Description

The invention relates to an ion accelerator arrangement specified in the preamble of claim 1. Art.

Ion accelerator arrangements are, for example, in use for surface treatment, in particular in semiconductor technology, or as a propulsion for spacecraft. Ions are typically generated and accelerated from a neutral working gas for propulsion purposes, particularly a noble gas. For the generation and acceleration of ions, in particular two construction principles have prevailed.

In the case of the lattice accelerators, the positively charged ions are transformed from a plasma by means of a lattice arrangement in which a first lattice adjacent to the plasma chamber lies at an anode potential and a second lattice displaced in the beam exit direction lies at a more negative cathode potential. Such an arrangement is for example from the US 3613370 known. Space charge effects limit the ion current density of such an accelerator arrangement to low values.

Another design provides for a plasma chamber which is interspersed on the one hand by an electric field for the acceleration of positively charged ions in the direction of a jet exit opening and on the other hand by a magnetic field for guiding electrons which serve to ionize a neutral working gas. In particular, accelerator arrangements with an annular plasma chamber in which the magnetic field proceeds predominantly radially and electrons under the influence of the electric and magnetic fields are reflected on closed drift paths have been in use for some time and magnetic fields move on closed drift paths. Such an accelerator arrangement is for example from the US 5,847,493 known.

In a new type of ion accelerator arrangement with electric and magnetic fields in a plasma chamber, the magnetic field shows a special structure with a longitudinal direction parallel to the longitudinal field in longitudinal sections of the second kind and predominantly to the longitudinal direction perpendicular, in particular radial course in longitudinal sections of the first kind, which in particular one show as cusp course of the magnetic field. The arrangement is preferably constructed in multiple stages with alternating successive longitudinal sections of the first and second type. Such ion accelerator arrangements are known, for example DE 100 14 033 A1 or DE 198 28 704 A1 , At one of the DE 101 30 464 A1 known plasma accelerator arrangement are provided on the inner wall radially inwardly projecting electrodes.

In JP 61 066 868 A For example, an RF ion generator is shown with excitation coil disposed on the sidewalls of a plasma chamber. A permanent magnet assembly generates a magnetic field with field lines curved around the coil turns to keep plasma away from the coil turns. The US Pat. No. 6,060,836 A describes a plasma generator with an axially protruding into a plasma chamber waveguide, which RF power of a magnetron is fed and carries its inner conductor at an end projecting into the chamber, a permanent magnet assembly.

The present invention has for its object to further improve the efficiency of an ion accelerator arrangement.

The invention is described in claim 1. The dependent claims contain advantageous refinements and developments of the invention.

The invention is based on the itself from the DE 100 14 033 A1 known magnetic field structure, which in the ionization (or plasma) chamber in the longitudinal direction of the arrangement in a section of the second type has a field direction parallel to the longitudinal direction and in a section of the first kind a stronger, in particular predominant field component perpendicular to the longitudinal direction. The magnetic field continuously and monotonically transitions from a first-type portion to a second-type adjacent portion and vice versa, the adjacent first and second-type portions being longitudinally spaced apart or immediately adjacent to one another. The longitudinal direction of an ion accelerator arrangement coincides substantially with the mean direction of movement of the accelerated ions or an axis of symmetry of the ionization chamber.

By reducing the distance between mutually perpendicular to the longitudinal direction of opposing wall surfaces of the ionization chamber limiting walls in the longitudinal section of the second type, the working gas in this section available volume is reduced compared to a design with constant wall distance and at the same time the working gas in the middle between the opposite Concentrated wall surfaces.

It turns out, surprisingly, that in this way the overall efficiency of the arrangement, in which in particular the ionization efficiency and the electrical efficiency are received, increases significantly.

Preferably, the distance of opposing wall surfaces in the section of the second type not only to each other but also with respect to a particular to the longitudinal direction parallel center line or center area reduced relative to the wall distance in an adjacent longitudinal section of the first kind.

The minimum wall distance in a section of the second type is advantageously at least 15%, preferably at least 20%, in particular at least 25% less than the maximum wall distance in a neighboring section of the first kind. Advantageously, at least one, preferably both, of the opposing wall surfaces in FIG a section of the second type to the ionization chamber out, in particular in the form of a curvature with a continuously extending in the longitudinal direction, preferably monotonically curved wall surface.

The opposing wall surfaces may be made of insulating material insulating or metallic or partially metallic, in particular in the way that in the section or sections of the second type is a metallic wall surface which forms an intermediate electrode at a fixed or sliding potential and in the longitudinal direction by insulating Wall sections is limited, and the wall surfaces in the first-type sections are electrically insulating.

Advantageously, the ion accelerator arrangement in the longitudinal course of the plasma chamber is constructed in a plurality of stages such that several sections of the first type alternate with sections of the second type, wherein preferably the longitudinal components are alternately opposite in sections of the second type separated sections of the second type, the longitudinal component the magnetic field thus reverses when passing through a section of the first kind. Such a multi-stage magnetic field structure is known per se from the prior art. The invention essential reduction of the wall distance can then in only one, several or all sections be given second kind. In this case, if the wall spacing is reduced in several or all sections of the second type relative to adjacent sections of the first type, the quantitative extent of the relative reduction can also vary from section to section. Preferably, a reduction of the wall distance is present at least in the section of the second type which follows in the longitudinal direction of the anode, and / or if the quantity is quantitatively varied over several sections, the reduction in this section is greatest.

The anode is preferably arranged on the end of the ionization chamber opposite the longitudinal direction of the ion outlet opening. The cathode is advantageously designed as a primary electron source from which primary electrons are conducted through the ion exit opening into the plasma chamber and / or which electrons serve to neutralize an ion or plasma jet emerging from the ionization chamber, and laterally offset outside the ionization chamber and against the outlet opening arranged.

The ion accelerator arrangement according to the invention can serve both for delivering a positively charged ion beam and, in particular in the preferred application in the propulsion of a spacecraft, for emitting a neutral plasma jet. In another application, the accelerated ions can be used in particular for the treatment of solid surfaces and near-surface layers.

Fig. 1

einen Magnetfeldverlauf in einer Ionisationskammer,

Fig. 2

eine mehrstufige Anordnung.

The invention is illustrated below with reference to preferred embodiments with reference to the figures still in detail. Showing:

Fig. 1

a magnetic field course in an ionization chamber,

Fig. 2

a multi-level arrangement.

At the in Fig. 1 sketched arrangement of the prerequisite for the present invention magnetic field profile is schematically sketched in an ionization chamber IK. The ionization chamber is assumed to be rotationally symmetrical about a central longitudinal axis SA, which lies in the longitudinal direction LR of the arrangement. A with respect to the ionization chamber radially inner magnet assembly MGi and a radially outer magnet assembly MGe generate a magnetic field having at least a longitudinal portion MA1 _N first type and at least one adjacent thereto in the longitudinal direction of the longitudinal section MA2 _N of two species in the ionization chamber IK. Preferably, the magnetic field in the ionization chamber in the longitudinal direction alternately successively comprises a plurality of longitudinal sections of the first and second types as in the in Fig. 2 sketched example and as in Fig. 1 indicated by a further longitudinal section MA2 _{N + 1} .

In the longitudinal section of the second type MA2 _N , the magnetic field shows a field direction parallel to the longitudinal axis SA, whereas in the longitudinal section MA1 _{N of the} first type the magnetic field has a larger radial component, that is to say perpendicular to the longitudinal axis. The longitudinal section MA1 _N first type is chosen in the example so that the radial field component significantly outweighs. Longitudinal sections of the first and second types can be defined directly adjacent to each other, but in the sketched example for clear delimitation with predominant longitudinal component in the section MA2 _N and predominant radial component in the longitudinal section MA1 _N but spaced by an unspecified transition section. In the longitudinal section MA2 _{N of the} second type, the magnitude of the magnetic flux decreases from the lateral chamber walls toward the center, as well as in the longitudinal section of the first type magnetic flux at the chamber walls is greater than in the middle between opposite wall surfaces. The magnetic field structure described so far is in itself, for. B. off DE 10014033 A1 As well as magnet arrangements for generating such a magnetic field structure. The field distribution of the magnetic field in Fig. 1 is merely schematic and not quantitative.

Essential to the present invention, that in the region of the longitudinal section MA2 _N second type is the radial distance of the mutually perpendicular opposing to the longitudinal axis SA wall surfaces WF2i _N, WF2e _N is less than the radial distance from the wall of panels WF1i _N, WF1e _N is now in the longitudinal section MA1 _N The clear radial width of the ionization chamber is thus reduced in the longitudinal section MA2 _{N of the} second type with respect to the longitudinal section MA1 _{N of the} first type. In the section MA2 _N, both opposing wall surfaces WF2i _N , WF2e _{N are} preferably displaced radially with respect to the longitudinally adjacent wall surfaces WF1i _N, WF1e _N radially toward the center of the ionization chamber. In contrast to a chamber geometry with sections of the first and second type having the same radial wall spacing, a concentration of the working gas, in particular also of the non-ionized atoms in the radially inner region, is enforced in section MA2 _N , where a higher electron density and therefore higher ionization probability are present due to lower magnetic flux.

The course of the wall surfaces in the longitudinal direction can in each case be parallel to the longitudinal axis SA with a step or ramp as a transition. However, at least in the longitudinal section MA2 _{N of the} second type, a course which is not parallel to the longitudinal axis SA is preferred, which is closer to the field line course of the magnetic field in this longitudinal section than a wall course parallel to SA. The wall surface WF2i _N and / or WF2e _N may be curved towards the radial center of the ionization chamber with a minimum wall distance D2L, which increases in the longitudinal direction of the adjacent portion MA1 _N first type. The course of the wall surface WF2i _N and / or WF2e _N can be continuously monotonically curved or such a shape, for. B. be approximated with several straight part progressions.

In a corresponding manner, the wall surfaces WF1i _N and / or WF1e _{N may have} a straight or curved course in the longitudinal direction, wherein these surfaces for the sake of simplified production typically a parallel straight line to the longitudinal axis is usually favorable.

The radial wall distance in the longitudinal section MA2 _N second type or not parallel to SA wall of the local minimum radial distance D2L is preferably at least 15%, preferably at least 20%, in particular at least 25% less than the wall distance in the adjacent longitudinal section of the first kind or for non-SA parallel course of the local maximum wall distance D1M, ie D2L ≤ 0.85 D1M or 0.80 D1M or 0.75 D1M.

The wall surfaces of the chamber wall may consist of electrically insulating material or of electrically conductive material or else partially of electrically conductive material, in particular non-magnetizable metal. In a preferred embodiment, the wall surfaces WF2i _N , WF2e _N metallic and the wall surfaces WF1 _N , WF1e _N insulating. The metallic wall surfaces can then advantageously form, as parts of the electrode arrangement, intermediate electrodes at intermediate electrical potentials between the potentials of an anode and a cathode, wherein the intermediate potentials can be predeterminable or, in the case of isolated, non-contacted intermediate electrodes, adjust slidably during operation. In metallic wall surfaces WF2i _N , WF2e _N can also be provided in particular that metallic Are on a substantially cylindrical insulating chamber shell or inserted and fixed electrodes and facing away from the chamber shell, the ionization chamber and the opposite wall surface facing surfaces form the wall surfaces WF2i _N or WF2e _N.

In Fig. 2 is a longitudinally multi-level arrangement sketched, in which in itself, z. B. off DE 100 14 033 A1 known manner in the longitudinal direction a plurality of longitudinal sections of the first and second type alternately follow one another, wherein two to an intermediate portion of the first kind (MA1 _N in Fig. 1 ) adjacent sections of the second kind (MA2 _N , MA2 _{N + 1} in Fig. 1 ) show opposite longitudinal components of the magnetic field. While in Fig. 1 an annular chamber geometry is provided about a central longitudinal center axis SA and an inner and an outer magnet arrangement Mgi, Mge, is shown in the drawing Fig. 2 a preferred chamber geometry with simply contiguous cross-sectional area of the central longitudinal axis SAZ containing ionization chamber IKZ, which may in particular be substantially rotationally symmetrical about the longitudinal axis parallel to the longitudinal axis SAZ, based. The magnet arrangement consists in this case again in a known manner only of a surrounding the chamber envelope outer magnet assembly MG. Both wall surfaces facing one another then belong to the same chamber wall which is closed around the central longitudinal axis SAZ and laterally surrounding the ionization chamber. The ionization chamber shows a jet outlet opening, from which a usually slightly divergent ion or plasma jet PB exits with an average ion movement in the longitudinal direction LR. Outside the ionization chamber at the outlet opening AU and laterally offset therefrom, a cathode KA, which is at cathode potential and emits electrons, is arranged as part of the electrode arrangement. A part IE of these electrons is passed through the electric field of the electrode assembly in the ionization chamber and serves there in a known Way for the ionization of the working gas and in particular also the generation of secondary electrons. Another part NE of the electrons emitted by the cathode can serve to neutralize a positively charged particle flow PB.

In another advantageous embodiment, no external electron source is provided for generating primary electrons for gas ionization and / or for neutralizing a plasma jet having excess positive charge. The cathode can then be given in particular by a housing part which surrounds the outlet opening of the ionization chamber and is located at the cathode potential.

An anode A0 as part of the electrode arrangement is arranged at the end of the ionization chamber opposite the outlet opening AU in the longitudinal direction LR and is at anode potential. A neutral working gas, for driving purposes preferably a heavy noble gas such as xenon (Xe) can be introduced into the ionization chamber, for which purpose an anode-side central supply line is entered in the sketch. A typical distribution of a plasma consisting of electrons and positive gas ions is shown in crossed hatching in the ionization chamber.

The magnet arrangement forms a magnetic field in the ionization chamber IKZ, which in the longitudinal direction alternately has successively longitudinal sections MA11, MA12 of the first type and longitudinal sections MA21, MA22, MA23 of the second type. Let it be assumed that, as outlined, the distance of opposite wall surfaces which is equal in this case to the diameter of the ionization chamber in all longitudinal sections of the first type and in optionally present transition sections is constant DZ.

In the sketched example, which for the sake of clarity combines several design variants for the longitudinal sections MA21, MA22, MA23 of the second kind, the ionization chamber in the longitudinal section MA21 is narrowed to a minimum diameter D21 L by a concave curvature surrounding the central longitudinal axis with a wall surface WF21. The wall surface WF21 is assumed to be electrically insulating. In the longitudinal section MA22, the diameter of the ionization chamber is reduced to a value D22L, whereby larger dimensioning of D22L compared to D21 L may allow for any expansion of the plasma in the second compared to the first stage, and wall losses affecting the electrical efficiency are kept low can. The wall surface WF22 or the total diameter constriction at this distance is metallic and forms a first intermediate electrode A1 at a fixed intermediate potential. In the section MA23, finally, an electrode A2 of small radial thickness is provided, which does not reduce the diameter D23L in this section, or does not appreciably reduce it to DZ, and which uncontactively adopts an intermediate potential during operation. The electrode arrangement may also differ in the subdivision in the longitudinal direction from the division of the magnetic field into longitudinal sections of the first and second types.

The features indicated above and in the claims, as well as the features which can be seen in the figures, can be implemented advantageously both individually and in various combinations. The invention is not limited to the exemplary embodiments described, but can be modified in many ways within the scope of expert knowledge. In particular, the wall surfaces in the sections of the second type can be shaped in various other ways and in this case be insulating, electrically conductive or else only partially conductive in an electrically conductive manner. The dimensions of the individual longitudinal sections and / or the intermediate electrodes may vary from stage to stage. characteristics known ion accelerator arrangements can be combined with the features essential to the invention. The cross section of the ionization chamber can also deviate from the rotationally symmetrical shape and assume an elongated shape.

Claims (9)

1. Ion accelerator arrangement with an ionization chamber (IK), an electrode arrangement and a magnet arrangement (MG),

   - the ionization chamber having an ion outlet opening in a longitudinal direction (LR) and being bounded by at least one side wall transverse to the longitudinal direction, and it being possible to introduce working gas into the ionization chamber via a feed opening at a spacing from the outlet opening,

   - the electrode arrangement including at least one cathode (KA) and one anode (A0), and an electric field for accelerating positively charged working gas ions in the direction of the outlet opening being generated in the ionization chamber,

   - the magnet arrangement generating a magnetic field in the ionization chamber, which has in the longitudinal direction at least one longitudinal portion of a second kind (MA2_N) with a magnetic field direction substantially parallel to the longitudinal direction, and a longitudinal portion, adjacent thereto, of a first kind (MA1_N) with, contrary thereto, a higher proportion of the field component perpendicular to the longitudinal direction, and

   - the wall spacing between opposite wall surfaces being less in the longitudinal portion of a second kind (MA2_N) than in the longitudinal portion of a first kind (MA1_N),

   characterized in that in the longitudinal portion of the second kind the extent of the wall in the longitudinal direction has a monotonic curvature towards the ionization chamber.
2. Arrangement according to Claim 1, characterized in that the minimum wall spacing in the longitudinal portion of a second kind is less by at least 15%, in particular by at least 25%, than the maximum wall spacing in the longitudinal portion of a first kind.
3. Arrangement according to Claim 1 or 2, characterized in that the longitudinal portions of a first and second kind follow one another in alternation.
4. Arrangement according to one of Claims 1 to 3, characterized in that a reversal of direction of the longitudinal component of the magnetic field occurs in a longitudinal portion of a first kind.
5. Arrangement according to one of Claims 1 to 4, characterized in that the chamber wall in a longitudinal portion of a second kind is formed at least partially by an intermediate electrode.
6. Arrangement according to one of Claims 1 to 5, characterized in that the anode is arranged at the end of the ionization chamber opposite the outlet opening in the longitudinal direction.
7. Arrangement according to one of Claims 1 to 6, characterized in that the cathode is designed as a primary electron source and is arranged outside the ionization chamber in a fashion offset laterally from the outlet opening.
8. Arrangement according to one of Claims 1 to 7, characterized in that the cathode is designed as a primary electron source and is arranged outside the ionization chamber in a fashion offset laterally from the outlet opening.
9. Arrangement according to one of Claims 1 to 7, characterized in that no external electron source is provided as neutralizer or primary electron source.

Images