Apparatus including ring-shaped resonators for producing microwave plasmas

A low-pressure high-density plasma is excited in a plasma chamber surrounded by a cylindrical inner wall of a ring-shaped waveguide resonator to which the microwave energy is fed by a coupling from a microwave generator. The output coupling of the microwave energy from the standing wave maintained in the waveguide resonator to the plasma chamber is effected through a multiplicity of equispaced slits whose spacing is one half or one waveguide wavelength and which extend parallel to the generatrices of the cylindrical inner wall of the ring-shaped waveguide resonator.

FIELD OF THE INVENTION 
The present invention relates to an apparatus for producing a microwave 
plasma. 
BACKGROUND OF THE INVENTION 
Chemically reactive low-pressure plasma processes have become standard in 
the coating and etching of semiconductor wafers. Apart from the use of 
such plasma for the machining of flat substrates, plasma processes can be 
used for the treatment of objects of complex shapes, for example, molded 
plastic products, fiber bundles and materials in the form of webs or 
strips. There are a number of processes and apparatuses available which 
serve for the plasma treatment of substrates of different shapes, purposes 
and materials. 
DE 31 17 257 C2, for example, describes an apparatus for plasma deposition 
of thin films in which the microwave power is outputted from a rectangular 
waveguide through a coupling window of a dielectric material into a 
cylindrical resonance chamber which simultaneously serves as a plasma 
chamber. Apparatus of this type and those which use air-core coils or 
composites of permanent magnets around the plasma chamber for the purpose 
of generating the electron-cyclotron-resonance, produce a directed plasma 
beam which can be employed, for example, for disk-shaped substrates like 
the wafers described previously. Large substrates, however, can only be 
treated by movement of them in the plasma beam or with the simultaneous 
use of a plurality of such plasma sources. 
In DE 40 38 091 A1, an apparatus is described for generating a controllable 
microwave field which enables the ignition of a plasma which is 
homogeneous over a relatively long path. This is achieved by outputting 
the microwave power through a row of preferably inductive coupling 
antennae from a rectangular or cylindrical resonator. 
On the vacuum side, the antennae are in contact with the plasma and as a 
consequence, a contamination of the substrate with the antenna material 
cannot be avoided. While with this earlier apparatus the machinery of 
large-area substrates is possible, the apparatus does not solve the 
problem of treating with plasma a large-volume substrate from all sides. 
The French patent document FR-A-2 112 733 describes an apparatus for 
uniform microwave power coupling into a cylindrical volume and in which a 
standing wave is generated in a cylinder formed with a slit antenna. The 
wave pattern can meander. The connection of a microwave generator with the 
antenna here can be realized only with a coaxial cable. 
This limits the possibility of upscaling this type of microwave excitation 
of a plasma because, with such a cable, transferable power over long 
periods of time with stability cannot exceed several hundred watts. 
In the European patent EP 0 209 469 A1, an apparatus for producing an 
electron-cyclotron-resonance plasma is described in which the inputting of 
the microwave power is effected by a multiplicity of antennas which are 
disposed on the wall of the plasma chamber. The plasma chamber is 
surrounded by permanent magnets which, in the vicinity of the antennas, 
produce electron-cyclotron-resonance regions. As a consequence of the ion 
sputtering, the antennas directly in the plasma constitute a source of 
metallic contamination. To avoid such contamination EP 0 402 282 A2 
describes an apparatus in which shielding plates are provided between the 
antennas and the plasma and upon which the substrate material can be 
captured. The disadvantageous effect of sputtering or atomization of the 
antenna materials is thereby reduced, although it is not completely 
eliminated. 
Both of these earlier systems have the common drawback with respect to the 
nature of the distribution of the microwave power to the antennas. The 
apparatus for power splitting is comprised of a waveguide and 
corresponding connecting couplers and cannot guarantee a uniform coupling 
of the microwave power to all of the antennas and thus the formation of a 
cylindrical symmetrical plasma cannot be ensured. 
A direct inputting of microwave power from a rectangular waveguide into an 
annular resonator is disclosed in EP 0 398 832 A1. The dielectric inner 
wall of the resonator here simultaneously fulfills the function of a 
plasma chamber wall through which the microwave power is outputted to the 
plasma. The flat sides of the annular resonator are provided as shunt 
slides which enable adjustment of the optimum resonance conditions. 
However, since here the plasma chamber has the dielectric walls, the 
dimensions of a substrate to be treated in the chamber is limited. 
Furthermore, a complex and expensive mechanical arrangement is required 
for tuning of the resonator. 
OBJECTS OF THE INVENTION 
It is, therefore, the principal object of the present invention to provide 
an improved apparatus for generating a low-pressure and especially, a 
high-volume plasma which is free from the drawbacks of the earlier systems 
described and thus allows treatment of large volume articles in the plasma 
chamber. 
Another object of the invention is to provide an apparatus for producing a 
large-volume low-pressure plasma which has a sufficiently high plasma 
density (up to several 10.sup.12 cm.sup.-3) for practical treatment of 
large-volume and large-area workpieces but yet can avoid contamination 
during such treatment. 
Still another object of the invention is to provide an apparatus in which a 
substantially uniform and symmetrical plasma can be generated and 
maintained for effective treatment of complex articles on all sides in the 
plasma chamber. 
SUMMARY OF THE INVENTION 
These objects and others which will become more readily apparent 
hereinafter are attained, in accordance with the invention in an apparatus 
for producing a low-pressure plasma which comprises: 
means forming a plasma chamber adapted to contain a low-pressure plasma; 
a circular ring-shaped waveguide resonator surrounding the plasma chamber 
and provided in an inner wall with a plurality of coupling slits spaced 
and oriented to effect substantially equal microwave power output coupling 
from the resonator to the chamber at all of the coupling slits; 
a microwave generator; and 
coupling means connecting the microwave generator with the waveguide 
resonator for input coupling of microwave energy from the microwave 
generator to the waveguide resonator. 
The object to be treated can be inserted into the plasma in the plasma 
chamber and can be treated uniformly on all sides of the object. 
According to a feature of the invention, the ring-shaped waveguide 
resonator is dimensioned to generate a standing wave therein and the 
coupling slits are spaced equidistantly from one another around the inner 
wall and around the standing wave. The inner wall is preferably 
cylindrical and the coupling slits can extend parallel to the generatrices 
of the inner wall. 
Advantageously, among the array of coupling slits are coupling slits which 
are spaced apart by a distance equal to characteristic wavelengths of the 
waveguide forming the resonator and still more advantageously, so that 
successive coupling slits around the inner wall are spaced apart by half 
of the wave-length of the waveguide forming the resonator. 
The standing wave is preferably a TE.sub.10 wave in the waveguide resonator 
and the coupling slits thus output couple the energy of the standing 
TE.sub.10 wave by a penetration of an azimuthal component of the magnetic 
field through the coupling slits. The coupling slits can have lengths 
equal to a half free-space wave-length. 
According to another feature of the invention, the means for coupling can 
include a tuning unit between the micro-wave generator and the waveguide 
resonator and a capacitative or inductive coupler for feeding microwave 
energy from the tuning unit into the waveguide resonator. Instead of a 
capacitative or inductive coupler, an adjustable coupling pin can be 
provided for this purpose, the coupling pin being a screw or the like 
adjustable along its axis. 
The plasma chamber can be separated from the inner wall of the ring-shaped 
waveguide resonator by a cylinder of a dielectric material selected from 
the group which consists of quartz glass and aluminum oxide ceramic. 
The plasma chamber can be composed of metal and the coupling means can 
include a vacuum-tight window closing the chamber and composed of a 
dielectric material. 
A magnetic coil can be provided on one side or with the magnetic coils 
disposed on opposite sides of the ring-shaped waveguide resonator for 
producing the electron-cyclotron-resonance zones and such zones can also 
be produced by an assembly of permanent magnets polarized perpendicular to 
a wall of the plasma chamber and disposed between the inner wall of the 
waveguide resonator and the plasma chamber wall for producing the 
electron-cyclotron-resonance zones. 
A plurality of microwave generators can be coupled to the resonator at 
respective coupling locations and, conversely, a plurality of resonators 
can be provided and coupled to a common feed resonator.

SPECIFIC DESCRIPTION OF THE INVENTION 
In FIGS. 1a and 1b, we have shown an embodiment of the invention in which a 
vacuum or plasma chamber K is comprised of a quartz cylinder 2 and has two 
connecting flanges 20 and 21 (see FIG. 1b) which serve for connection to a 
vacuum pump 22 via the fitting 18 (see FIG. 1b) so that the pressure in 
the plasma chamber can be drawn down to a residual pressure, i.e. the low 
pressure of a plasma. 
In the flange 20, a gas inlet fitting 17, as seen in FIG. 1b is provided 
for admitting the working gas which is to be transformed into the plasma. 
In the case of the treatment of wafers, that working gas can include a 
reactive component, such as oxygen. 
Between the vacuum pump and the supply of gas, a pressure between 10.sup.-5 
and 100 mbar can be generated within the chamber. 
The apparatus utilizes a magnetron vacuum tube 9 (see FIG. 1a) to generate 
the microwaves at a frequency sufficient to excite the gas within the 
plasma chamber and produce the plasma. In a typical case that frequency is 
2.45 GHz. The microwaves are coupled to the resonator via a coupling unit 
from the microwave generator 9 which can include a waveguide 10 (see FIG. 
1a). 
The coupling means also can include a circulator 8 (FIG. 1a) which is 
provided upstream from a three-pin tuner 7 and connected therewith to the 
waveguide 10 (see FIG. 1a). Downstream of the tuning unit 7, 8 is a 
rectangular waveguide 23 which may be connected by a capacitative, 
inductive or pin-type coupler 5 with a ring-shaped waveguide resonator 3 
as is clearly visible from FIGS. 1a and 1b. 
The diameter of the ring-shaped waveguide resonator 3 which has an annular 
space R (FIGS. 1b and 3) is so chosen that within this annular space a 
standing TE.sub.10 microwave is formed. 
The metallic circular annular waveguide resonator 3 has an outer 
cylindrical wall 37, an inner cylindrical wall 26 and planar annular or 
ring-shaped walls 38 and 39 (see FIG. 1b) connected to the inner and outer 
walls, all of these walls being circular. In the illustrated embodiment, 
the center of the annular chamber R, represented by the dot-dash circle 24 
in FIG. 1a has a circumferential length corresponding to five wave-lengths 
36 (see FIG. 2) in the case in which the annular waveguide resonator 3 is 
filled with air under normal pressure. 
In the inner wall 26 of the ring-shaped waveguide resonator 3 (FIG. 2), 
there are a multiplicity of coupling slits 4 which are equidistant from 
one another with a spacing of a half of the wavelength 25 of the waveguide 
(half of the characteristic waveguide wavelength), see FIG. 3, with 
respect to the circle 24 in the center of the ring-shaped waveguide 
resonator and parallel to the direction x (see FIG. 1a and FIG. 2) of the 
wavespread. 
The coupling slits 4 extend in the direction of or are parallel to the 
generatrices of the circularly cylindrical inner wall and the lengths of 
the coupling slits 4 can each amount to a half of the free space 
wavelength. 
Since the, e.g. capacitative, coupler 5 (FIG. 2) lies precisely in the 
middle between two coupling slits, the standing wave in the annular space 
R (FIG. 2) of the waveguide resonator 3, is so formed that, at the 
coupling slits 4, the azimuthal component of the high-frequency H field Hx 
has its maximums 27 and the z component of the H field H.sub.z and the 
radial component of the E field Er, their minima 28 (see FIG. 2). 
As a result, the output coupling of the microwave power is effected by the 
penetration of the azimuthal component of the H field Hx through the 
coupling slits 4 into the interior plasma space. 
The microwaves injected into the circular waveguide resonator 3 (having 
outer wall 37) as seen in FIG. 2 and by this resonator into the plasma 
space ignite the plasma to generate an externally intensive low-pressure 
plasma 1 in the space P surrounded by the inner wall 26 (FIG. 2) of the 
ring-shaped waveguide resonator 3. The space P is also circular in a cross 
sectional plane perpendicular to its axis corresponding to the dot-dash 
line z in FIG. 1b. 
At least alternate ones of the coupling slits 4 have equispacing around the 
inner wall 26 which is equal to a full characteristic wave-length of the 
waveguide resonator as represented at 36 (FIG. 2). To prevent the escape 
of microwaves from the plasma chamber outwardly, the quartz cylinder 2 is 
surrounded by an electrically conductive shield 19 (see FIG. 1b). 
In practice we can obtain an ion concentration of 10.sup.11 to 10.sup.12 
cm.sup.-3 in a pressure range of 0.1 to 10 mbar of the plasma. A dense 
plasma at low pressures (below 10.sup.-3 mbar) can also be generated. By 
ensuring an electron-cyclotron-resonance, the charge intensity can be 
significantly increased. In that case, a magnet coil 12 can be provided on 
one side of the resonator or on both sides of the resonator (FIG. 1b) or 
these zones 13 can be confined by permanent magnets 14 as shown in FIG. 3. 
The embodiment wherein the cylinder 2 is composed of quartz glass or 
aluminum oxide does not allow very large plasma chamber diameters to be 
utilized, above say about several decimeters, because then the quartz wall 
2 must be very thick. A practically unlimited upscaling can be obtained in 
the dimensions of the plasma chamber when the entire chamber is not lined 
with the dielectric but dielectric windows are provided. In that case, 
much larger plasma chambers K can be provided. 
As can be seen from FIG. 3, which illustrates a segment 29 (having an inner 
wall 26 and an outer wall 37) utilizing the latter principles, and 
providing a different output coupling of the resonator to the plasma 1, 
permanent magnets 14 (with poles N,S) are so positioned in the metallic 
plasma chamber wall 35 (e.g. of copper) that the lines of the 
high-frequency electric field 15 cross the lines of the static magnetic 
field 16. With such an arrangement, an optimal transfer of the microwave 
oscillation to the electrons accelerated in a spiral trajectory in the 
plasma is ensured. 
The windows 11 between the slits 4 and the plasma space P can be composed 
of 99.7% Al.sub.2 O.sub.3 ceramic or quartz glass and can be sealed with a 
soft metal seal 30 in a vacuum-tight manner. 
The space between the window 11 and each slit 4 is formed as a short 
rectangular horn radiator 31. The assembly formed by the coupling slits 4, 
the horn radiators 31 and the coupling windows 11 of dielectric material, 
form slit couplers 34 (FIG. 4) within the definition of the invention. 
It should be noted that other vacuum seals and systems for fastening the 
permanent magnets in place can allow the metallic wall 35 to be 
substantially thinner. 
The apparatus of the invention can thus utilize the comparatively large 
diameter of the plasma chamber to permit treatment of relatively large 
objects from all sides with the plasma. The length of the plasma 
generating region and not only the diameter can also play an important 
role in some cases. For example, when a long plasma chamber is used, an 
embodiment like that of FIG. 4 can be used with three ring-shaped 
waveguide resonators 3 (with inner walls 26, outer walls 37 and resonating 
annular spaces R) which are connected to a common feed resonator 32 
utilizing axially adjustable coupling pins 33, e.g. screws, to ensure 
equal injection of microwave energy at all three resonators. As is 
described hereabove with reference to FIG. 1 each of the resonators is 
formed with planar annular walls 38, 39 bridging respective inner and 
outer cylindrical walls 26, 37 as seen in FIG. 4. 
To achieve a coherent microwave coupling, the adjustable coupling pins 33 
are spaced from one another by a by a spacing 25 (FIG. 4) corresponding to 
a waveguide wavelength 36 (FIG.2). 
For the optimum establishment of a standing wave in the rectangular feed 
resonator 32, a short-circuit slide 6 is provided at one side of the feed 
resonator 32 (see FIG. 4).