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Description  |
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BACKGROUND OF THE INVENTION
The invention relates to a magnetron cathode for a cathode sputtering
system for reactive sputtering including a target holder for holding a
target where the erosion zone is located opposite a substrate and
including a magnet system which has a pole shoe and a yoke. The magnetic
flux lines penetrate the target and in the discharge chamber, the electric
flux lines are superimposed thereon so that the electrons are concentrated
directly before the target.
A magnetron cathode for a cathode sputtering system of the aforesaid kind
which has a target holder for a target where the erosion zone is opposite
a substrate is known. The corresponding magnet system has a pole shoe and
a yoke and the magnetic flux lines penetrate the target. In the discharge
chamber, the electric flux lines are superimposed so that the electrons
are concentrated directly before the target. A reactive sputtering with
aggressive reactive gases may cause arcing problems so that, by reaction,
non-sputtering target surfaces are gradually covered by a dielectric
layer. Particularly in case of ZPT cathodes, it is possible that the
target surface is converted, in particular at those surface parts which
are opposite the floating magnet shield.
SUMMARY OF THE INVENTION
It is an object of the invention to configure the target surface at
critical points such that formation of dielectric layers is avoided.
This object has been accomplished in that the non-sputtering target
surfaces are coated with a barrier layer or a protective layer. The
non-sputtering surfaces with the protective layer are advantageously
located opposite the floating magnet shield. In this respect, it is
advantageous that the non-sputtering target surface be covered with a an
electrically conductive, in particular a metallic protective layer. If
these surfaces are covered with a metallic layer which reacts only
gradually or not at all with the reactive gas or remains largely
conductive despite the reaction, these arcing problems can be prevented or
greatly reduced. Since these target surfaces do not participate in the
sputtering, no foreign atoms will enter the layer on the substrate.
It is particularly advantageous that the protective layer applied onto the
non-sputtering target surface be one of copper oxide or zinc oxide and
that the protective layer be provided only on the outer sides of the
target. The barrier layer can either be conductive or be a non-oxidizing
layer. In both cases it fulfills its task in simple manner and prevents a
dielectric layer from being deposited on the non-sputtering target
surface. This also prevents charge-carrying particles from breaking
through the layer. The breaking through of the layer depends, inter alia,
from the electric field and the layer density. In the arrangement
described, the electric field can have approximately 10.sup.11 volt/m. It
is difficult to maintain such a field strength when the layer does not
become conductive.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is section of a round magnetron,
FIG. 2 is a diagrammatic representation of a target in a recess,
FIG. 3 is a top view of the magnetron cathode of FIG. 1,
FIG. 4 is a bottom view of the magnetron cathode of FIG. 1,
FIG. 5 is a perspective view of the magnetron of FIG. 1,
FIG. 6 shows the curve of the flux lines in a magnet arrangement.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a power supply plate 1 which is connected to a vacuum chamber
3 via a pedestal-type insulator 2. The vacuum chamber in turn is connected
to ground 4. A magnet yoke 5 is connected to the power supply plate 1 in
an insulated but tight manner. On its top side, the magnet yoke 5 has a
flat surface 6 with annularly closed support surfaces 6a and 6b for a
magnet arrangement 7. The magnet system 7 comprises a inner ring magnet 7a
and a closed outer row of bar magnets 7b all of which are axially
magnetized with respect to axis A of the system. The direction of the
polarity of the inner ring magnet 7a is opposite that of the outer bar
magnets 7b. The pole position is expressly included in FIG. 1. On the side
facing away from the magnet yoke 5, the magnet system 7 has pole surfaces
7c and 7d which are located on a common plane.
The principle and the course of the magnetic flux lines are principally
illustrated in FIG. 5.
FIG. 6 is a section across the magnet arrangement with an ideal course of
the flux lines and the direction of the electric field. The various zones
of the course of the flux lines are indicated in FIG. 6 by boxes. These
areas are of particular interest for the movement of charged particles and
especially for the movement of electrons in the arrangement. In the course
of the description, these zones are only discussed with respect to the
movement of the electrons since only they affect the ionization of the
inert gas and hence, the operation of the magnetron.
In zone 1, there applies EB. This means, the flux lines of the electric and
the magnetic fields have only components which are directed in an opposite
parallel sense. An electron that starts from the cathode toward this area,
will follow a helical line around the magnetic flux line. The effect of
the electric field can be noticed from an increased pitch in the helical
line. The magnetic flux lines start to bend at some distance away from the
cathode resulting in a horizontal component which runs perpendicular the
electric field component. The horizontal component will cause an E.times.B
drift of the electron. At the same time, due to the arcing of the magnetic
flux line, an arcing drift affects the electron.
For an electron, the directions of the arcing drifts are the same, they add
up to a total drift. For particles that start in zone 1 on the right half
of the target, the direction of drift points into the plane of the paper.
On the left half of the target in zone 1, the drift points out of the
plane of the paper. The curve described by the electron is a
superimposition of helical line and drift. In its movement, the electron
follows the arc of the magnetic flux lines. The helical line describes an
increasingly larger radius and with the horizontal component increasing,
it assumes a cycloidal shape. When the vertical component has the value
zero, the electron follows the curve of an actual cycloid; it can be found
in zone 2. Electrons which are affected from the magnetic flux lines
leading away from the magnetron will drift out of the arrangement and go
to the anode.
In zone 2, there applies E.perp.B. This means, the electric flux lines
stand perpendicular on the magnetic flux lines. The electric drift
(E.times.B) has a maximum value in this zone. Electrons starting from a
quiescent position will follow an elongated cycloid. Due to the curve of
the cycloid, there is a long residence time of the electrons in this zone
which in turn increases the probability that the atoms of the inert gas
are ionized. Due to the increased ionization, there will also be in
increased sputtering of the cathode in this area, the erosion zone.
In zone 3, there applies EB. This means, the components of the flux lines
of the electric and magnetic fields are oriented in the same direction and
parallel. In this area close to the cathode, there are only components
running perpendicular to the surface. Electrons starting in zone 3 will
describe a course that is identical to the one of electrons starting in
zone 1. Due to the fact, that the magnetic flux lines are oriented in the
same direction as the electric flux lines, and not in an opposite sense as
in zone 1, the turning direction of the helical lines will be reversed.
Between the support surfaces 6a and 6b and between the pole surfaces 7c and
7d, there is an annularly closed, trough-like recess 8 extending into the
magnetic yoke 5 and which has an essentially rectangular cross section.
This recess accommodates an annular target 9. The target surface 9a
thereof is flat and in a common plane with the pole surfaces 7c and 7d.
However, it is very advantageous to arrange the pole surfaces 7c and 7d
above the target surface 9a by correspondingly selecting the thickness of
the insulating body 11.
The recess 8 has a flat bottom 8a with a plate-like insulating body 11
resting thereon. The body essentially fills the entire width of the recess
8.
The rear side of target 9, which is made of a ferromagnetic material, is
provided with a circumferential rib 9b. Inserted therein and distributed
over the circumference are threads for several draw-in bolts 10. These
draw-in bolts allow the target 9 to be braced with respect to the magnetic
yoke 5 and the power supply plate 1.
Between target 9 and insulating body 11, there is a target cooling system
12 which can be configured in the form of a pipe wound in a bifilar manner
and located on two different radii. The pipe can have a square-like or a
rectangular cross section, and the axes of the individual windings are
located in a common plane. The two windings are connected by means of a
radial, mitred pipe segment 12a (FIG. 3). Perpendicular thereto, the
opposite ends 12b and 12c form a right angle. At sufficient insulating
distances, they pass through not-represented bores in the magnet yoke 5
and through a radial recess in the power supply plate 1.
When the draw-in bolts are tightened, the target 9 rests on the target
cooling system 12 and supports the latter on the insulating body 11 and on
the bottom 8a of the recess 8.
As seen in FIG. 1, a bore 13 runs from bottom 8a of the trough-like recess
8 to the opposite flat surface 14 of the magnetic yoke 5. The bore 13
represented in FIG. 1 represents a total of six draw-in bolts all which
are conductively connected to the target 9, on the one hand, and
conductively connected to the power supply plate 1, on the other hand.
In order to avoid a short circuit between the power supply plate 1 with the
draw-in bolts 10 and the magnet yoke 5, a sleeve-like insulating body 15
is disposed in the bore 13. Outside this bore 13 and resting on the flat
surface 14, this insulating body has a collar 15a. Due to the force of the
draw-in bolt 10, the power supply board 1 rests on this collar.
It can be seen that the power supply plate 1 with the target 9 is thus
electrically insulated with respect to both the magnet yoke 5 with the
magnet arrangement 7 and vacuum chamber 3 with ground 4. Whereas the
target 9, via the power supply plate 1, can be brought to a defined
negative potential, and whereas ground 4 also has a defined (zero
potential), the magnet arrangement 7 with the magnet yoke 5 is free and
can adjust itself to an intermediate potential prescribed by the operating
conditions. When this potential is reached, an action suppressing the
sputtering of magnet arrangement 7 will independently take effect within a
shortest possible time.
In the present case, the draw-in bolts can be referred to as target
holders. While it is possible to directly anchor the draw-in bolts 10 in
the metallic material of the target 9, non-metallic targets, e.g.
soft-metallic parts, can be sputtered such that a metallic ring is
disposed in the recess 8 onto which the soft-metallic target material is
mounted. This metallic ring then functions as a good thermally conductive
connection to the target cooling system 12. Moreover, it compensates
mechanical forces which a brittle, dielectric target could not compensate.
It can also be seen from FIG. 1 that the magnet arrangement is accommodated
in a housing 16 made of a non-ferromagnetic material. The housing 16
includes two rotationally balanced parts, one being a pot-like central
housing part 16a which covers the annular magnet 7a and is braced with the
magnet yoke by means of a screw 17 and the other being an annular outer
housing part 16b which, in a cross sectional view, surrounds the bar
magnets in an endless, i.e. closed, row. Several screws 18, of which only
one is represented, serve to secure the magnet yoke.
Further, it can be seen that, starting on the plane in which there are the
pole surfaces 7c and 7d, both parts 16a and 16b of the housing are
configured so as to extend into the recess. In the form of two cylindrical
brackets, they cover the cylindrical inner surface and the cylindrical
outer surface of the target 9 with two annular air gaps 19 and 20
remaining. These bracket-like parts of the housing 16 can be considered a
part of the air gaps 19 and 20 since they are made of a non-ferromagnetic
material.
In the area of each of their edges, the two inner housing parts 16a and 16b
have a flange-like projection 16c and 16d parallel to the target surface
9a or the plane thereof. Each of these flange-like projections covers the
plate-like insulating body 11 in the area of its inner and outer edges so
far that the insulating body is not visible through the air gaps 19 and 20
between the target 9 and the halves 16a and 16b of the housing. In case of
a back-sputtering, no conductive material can thus be deposited on the
insulating body 11 and impair the insulating properties. One must take
into consideration that the available distances are relatively small so
that the insulating properties must be largely maintained even over longer
periods of operation.
It is, however, possible that arcing problems occur during sputtering with
aggressive reactive gases so that non-sputtering target surfaces by
reaction are gradually covered with a dielectric layer. Particularly in
the case of ZPT cathode, this may lead to a conversion of the target
surface, especially at surface portions that are opposite the floating
magnet shield. For this purpose, it is particularly advantageous when the
non-sputtering target surfaces 22 are covered with a barrier layer or a
protective layer. The protective layer may be opposite the floating magnet
shield. It is, for example, possible to configure the protective layer so
as to be non-conductive or such that it does not lose its conductivity
during the sputtering. Further, it is possible to configure the protective
layer applied onto the non-sputtering target surface 22 as copper oxide or
zinc oxide. As seen in FIG. 2, the protective layer is provided only at
critical spots on the non-sputtering sides, i.e. on the outer sides of
target 9.
A particular advantage of the housing 16 resides in that its two parts 16a
and 16b provide a clean end of the connecting parts of the magnet
arrangement 7 with the magnet yoke 5, keeping the magnets 7a and 7b in
their positions even without the use of an adhesive agent. This is of
particular importance with respect to the arrangement of the outer bar
magnets 7b which, due to their annular arrangement, surround the
consequently wedge-like gaps in between.
If the surfaces 22 are covered with a metallic layer 21 which reacts only
slowly or not at all with the reactive gas or remains conductive despite a
reaction, these arcing problems can be prevented or greatly reduced. Since
these target surfaces do not participate in the sputtering, no foreign
atoms will enter the layer on the substrate.
The subject matter of the invention is by no means restricted to the
largely rotationally balanced arrangement represented in FIGS. 1 to 3.
With a particular advantage, it can also be used with elongated
rectangular cathodes not represented in the drawing as they are used for
the coating of large window panes, for example. At a width of
approximately 30 to 40 cm, cathodes of this kind can reach a length of
approximately 4 m. Any other form between a circle and a rectangle is also
possible. FIG. 5 shows a round magnetron 23.
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