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BACKGROUND OF THE INVENTION
This invention relates to plasma generating apparatus and to processes for
operating such apparatus.
Plasma discharges are extensively used in various manufacturing processes
such as:
Fabrication of semiconductor devices.
Etching of plastic films for ink adhesion
Deposition of layers on plastic roll goods to reduce oxygen and water vapor
diffusion for flexible packaging
Etching and coating fibers and tows (bundle of fibers) to improve adhesion
in composite materials or materials such as tire cords.
Etching and deposition on the inside of plastic bottles for increased
hermeticity
Etching and deposition on parts to reduce corrosion
For example, a semiconductor substrate is disposed within an evacuated
chamber coupled to a source of plasma. By appropriately biasing the
substrate, the plasma is coupled to the substrate for ion bombarding the
substrate, for etching patterns in the substrate, or depositing ions on
the substrate to grow layers of selected materials thereon.
The present invention has particular utility with (although not restricted
to) apparatus and processes described in U.S. Pat. No. 4,918,031, issued
to me (Johnson), Flamm and Ibbotson on Apr. 17, 1990, the subject matter
of such patent being incorporated herein by reference.
This patent discloses a plasma generating apparatus comprising an outside,
cylindrical enclosure of metal, an internal helical coil, and an internal,
open-ended tube of insulating material within the coil and concentric
therewith. A low pressure gas is passed through the tube and ionized by
high intensity electric fields generated within the tube by the coil. The
ions, radicals, atoms, plasma fragments, or gas phase combined species of
the plasma are used to process workpieces, e.g., semiconductor substrates
disposed adjacent to an exit end of the tube or within the tube itself.
The patent also discloses the use of a longitudinally split, metallic
shield disposed internally of the coil and surrounding the internal tube,
and explains that the shield "is useful to adjust plasma species
concentrations by application of a bias or to shield the plasma region
from radial electric fields." However, except for schematically
illustrating the split metallic shield and describing its function in the
above quoted statement, the patent provides no significant disclosure
concerning its details or use.
I have discovered, however, that the use of internal metallic shields
provides important advantages in plasma apparatus of the type described in
the patent, and the present invention provides novel shielding
arrangements and novel processes for operating apparatus, including such
shielding arrangements.
DESCRIPTION OF THE DRAWINGS
The figures are schematic views of various aspects of the inventive
apparatus.
FIG. 1 is a perspective view, partially broken away, of one embodiment of
the inventive apparatus;
FIG. 2 is a fragmentary view showing a portion of an R.F. coil mounted on a
supporting structure and illustrating a capacitive coupling between one of
the coil turns and the inside of the plasma-containing inner tube;
FIG. 3 is a vertical section showing a modification of the shield shown in
FIG. 1;
FIGS. 4, 5 and 6 are perspective views of the different two-piece shields
according to this invention;
FIG. 7 is a fragmentary view in perspective showing a planar coil and
shield combination;
FIG. 8 is a perspective view showing a coil and shield each including both
planar and cylindrical elements; and
FIG. 9 shows an example of a high voltage insulator used to support the
coil.
SUMMARY OF THE INVENTION
In one embodiment of the invention, a plasma generating apparatus includes
a longitudinally split, metallic shield disposed within a helical coil and
disposed around an internal plasma region. The shield, in contrast to the
shield shown in the patent, allows a significant capacitive coupling
between the coil and the plasma region. Such coupling can be obtained by
the use of multiple slits through the shield or by the use of shields
having a shorter axial length than the coil. In a novel use of such
apparatus, the degree of capacitive coupling allowed through the shield is
varied for tuning the characteristics of the plasma. Such variable
capacitive coupling is obtained by varying the area of exposure of the
coils through the shield to the plasma region, e.g., by varying the
dimensions and/or the configuration of the slits through the shield.
In another embodiment of the invention, the coil comprises a planar spiral,
and a multi-slotted, metallic, disc-like shield grounded at the outer
diameter is disposed between the planar coil and the plasma region.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
With reference to FIG. 1, one embodiment of a plasma generating apparatus
10 according to the invention is illustrated. This embodiment is quite
similar in appearance to the apparatus shown in FIG. 2 of the
afore-described U.S. Pat. No. 4,918,031 and can be used in much the same
manner as are the various plasma generating apparatuses disclosed in the
patent. Significant differences, however, in the use of the inventive
apparatus are described hereinafter.
The apparatus 10 shown in FIG. 1 includes an external enclosure 12 of
metal, an internal helical coil 14 of metal, a slotted shield 16 of a
nonmagnetic metal disposed concentrically within the coil 14, and an
internal tube 18 of a low loss dielectric insulating material, e.g., of
quartz. The enclosure 12, the coil 14 and the shield 16 can be, for
example, of copper or silver plated copper.
The dimensions of the apparatus 10, and the means for energizing it,
including applying an R.F. voltage to the coil 14, can be determined in
accordance with known principles. Examples of apparatus designs and means
for energizing them are provided in the afore-described patent.
The shield 16 can be mounted on the internal tube 18 in spaced apart
relation therewith, as by insulating material shims, not shown, and the
lower end 22 of the coil is electrically connected to the shield 16 by
means of a screw 24. Alternatively, the shield can be deposited directly
onto the internal tube 18. The coil 14 is sufficiently rigid to be
self-supporting, although additional means, e.g., a rigid tube of
insulating material can be used about which the coil 14 is wound and
supported. An example of such a coil supporting tube 15 is indicated in
FIG. 2.
FIG. 9 shows an example of a high voltage insulator used to support the
coil. It is important to realize that the shielded plasma operation
requires a high voltage and high current on the coil to operate. In turn
for the device to operate with reasonable input powers, the losses both
dielectric and conductive (due to resistive losses in the coil and losses
in its surface due to eddy currents), must be minimized. In other words,
the coil system must operate at high Q. Except for the lower connection
between the coil end 22 and the shield 16, the turns of the coil are
spaced from the shield 16.
For example, with a coil 14 having an inner diameter of 350 mm, the shield
16 can have an outer diameter of 260 mm, and an inner diameter of 258 mm,
and the tube 18 can have an outer diameter of 250 mm. These dimensions are
not critical.
An electrical connection 20 is provided between one of the turns of the
coil 14 and an R.F. power supply 22 that best matches the impedance of the
supply.
The shield 16 is provided with a number of longitudinally extending and
circumferentially spaced slits 30 therethrough. The parameters of the
slits are a function of the particular apparatus and the function of the
apparatus. This is discussed hereinafter. Of present significance,
however, is that more than two slits 30 are provided. (The patent shows
but a single "slit" for the purpose of avoiding the generation of low
impedance, current carrying loops around the circumference of the shield,
such loops being wasteful of energy and causing undesirable heating of the
apparatus). The multiple slits 30 through the shield 16 further reduce the
lengths of the current loops in the shield and improve efficiency. The
electrical coupling between the coil 14 and the plasma region is now
discussed.
An R.F. coil produces a combination of electric fields. The fields can be
capacitively and/or inductively coupled to the plasma.
In the capacitive coupling mode, as illustrated in FIG. 2, the coil turns
14a are capacitively coupled through the dielectric tube 18, and the
inside wall 34 of the tube becomes a capacitively coupled electrode. At
any instant, the R.F. voltage on the coil varies along the length thereof,
hence the voltage on the capacitive electrode on the inside wall 34 of the
tube also varies along the length of the quartz tube. The difference in
voltage between the various portions of the tube inner wall electrode
creates an electric field within the tube 18 which is effective to ionize
the molecules of the gas 36 passing through the tube.
In the inductively coupled mode, as illustrated in FIG. 1, the current in
the coil 14, which is in the order of amperes, generates an intense R.F.
magnetic field, B, extending axially of the tube 18. This magnetic field,
in turn, generates a circular electric field, E, within the tube. The
intensity of the electrical field rises and falls with the magnetic field
intensity, and alternates in direction every half cycle.
In the absence of an electrically conductive shield, both capacitive and
inductive couplings are present, but the capacitive coupling predominates.
It has been discovered, however, that such predominantly capacitively
coupled fields couple so efficiently with the plasma as to produce large
and generally uncontrollable R.F. plasma potentials. These potentials lead
to sputtering of the tube walls 34 and to excessively high energies of the
plasma particulates. The wall sputtering causes contamination of the
system, and the excessively high energy particles can damage the articles
being processed by the plasma.
Also, the particles of the plasma are energized to a relatively wide range
of energies, which is undesirable as introducing uncontrolled variations
in the processing of articles by the plasma.
Attempts were made to overcome these problems by the use of metal shields
of the type shown in the aforementioned patent. These shields of
non-magnetic material allow penetration of the magnetic field produced by
the R.F. coil into the plasma region while blocking capacitive coupling
between the coil and the plasma region. Thus, with such shields in place,
the coupling between the R.F. coil and the plasma region is totally
inductive. Such inductive coupling is less efficient than capacitive
coupling, and the large, uncontrollable R.F. plasma potentials, and the
problems caused thereby are avoided.
A problem with such totally inductive couplings, however, is that owing to
the relatively poor plasma coupling efficiency thereof, it is somewhat
more difficult to initiate a plasma within the plasma tube and, in
general, far more R.F. power is required in the plasma generating process.
The need for greater R.F. power raises numerous problems with respect to
the design of the R.F. coils and the supports thereafter capable of
handling large R.F. powers and voltages.
I have discovered, however, that the problems of the prior art apparatus
solved by the use of capacitive shields and the problems caused by the
shields themselves can all be solved by the use of shields which provide
only a partial, and preferably variable, capacitive shielding between the
R.F. coil and the plasma region. Shields providing such partial shielding
are described hereinafter.
It is first noted that the shield shown in the aforementioned patent is not
perfect and does allow some capacitive coupling therethrough. The current,
generated in a capacitance measurement however, is so low as not to be
directly measurable by means available to me. I do know, however, that in
an actual experimental shield of the type shown in FIG. 2 of the patent,
the area of the single vertical slit through the shield is somewhat less
than 0.2% of the total external area of the shield. Because the
capacitance of a capacitor is a direct function of the area of the
electrodes of the capacitor, it is assumed that the capacitive coupling
allowed through the single slit shield of the patent is less than 0.2% of
the capacitive coupling that would exist in the absence of the shield.
Thus, for the purpose of distinguishing the present invention from the
prior art, one definition is that the inventive shield allows a capacitive
coupling between the R.F. coil and the plasma region which is at least in
excess of 0.2% of what such capacitive coupling would be in the absence of
the shield, all other parameters being the same. I have also discovered
that allowing significant amounts of capacitive coupling can produce
process effects comparable to separately biasing the articles being
processed by the plasma with an R.F. voltage. The capacitive coupling of
the coil to the plasma in a plasma chamber having dielectric walls allows
the plasma potential with regard to the substrate to have an R.F. voltage
imposed. This voltage produces a bias in the the same manner as the R.F.
voltage applied to the substrate. Measurements of the plasma on shields
typically provide R.F. plasma voltages with regard to a grounded substrate
of several hundred volts. The same plasma configuration but with a shield
has R.F. voltage of less than 1 volt R.F. By opening the shield any value
between unshielded and totally shielded can be obtained.
As previously noted, the shield 16 includes 20 circumferentially spaced
slits 30. In this embodiment, each slit has a length of 350 mm and a width
of 1 mm. The shield 16 has an outside diameter of 260 mm and a length of
750 mm. With these dimensions, the 20 slits provide a ratio of open area
through the shield (exposing the coil 14 to the tube 18) to total external
shield area of around 1.14%. In experiments with shields of the type shown
in FIG. 1, the following results were obtained in comparison with the use
of single slit shields of the same type shown in the patent:
1.0 Increased power coupling 50% vs 85%
2.0 Increased operating range for the pressures of gases flowed through the
plasma chamber from between 10E-4 to 25 torr to 10E-5 to 50 torr
As shown in FIG. 1, the shield 16 is coextensive in length with the coil
14. One means for further increasing the capacitive coupling allowed
through the shield 16 is, as shown in FIG. 3, to make it somewhat shorter
than the axial length of the coil. For example, with a coil length of 240
mm, the shield can have a length of only 150 mm. The difference in length
between the shield and coil is 90 mm, and, with a shield having the same
parameters as the shield 16 shown in FIG. 1., with the sole exception of
the length, the ratio of open area to total shield area is increased to
37%. That is, the length difference is considered as an opening through
the shield.
I have also discovered that the conditions necessary for effective use of
the plasma in various processes often differs from those used for
initiating the plasma. Thus, by means of a variable capacitive coupling
shield, a first, and relatively high amount of capacitive coupling can be
provided and then, after the plasma has been established, the amount of
capacitive coupling can be reduced for maintaining the plasma. This
technique is particularly useful in connection with the use of gases with
which it is difficult to initiate a plasma, e.g., Chlorine, at pressures
below 10E-4 torr or above 50 torr. Thus, once the plasma has been
initiated, making use of the high coupling efficiency provided by the
capacitive coupling, the degree of capacitive coupling is reduced for
avoiding the aforementioned problems associated with the use of capacitive
couplings.
Electron bombardment of photoresist by capacitively coupled electric fields
heats the photoresist and reduces its useful exposure time.
A quite simple means for varying the capacitive coupling through the
capacitive shield is to provide mean for varying the shapes and dimensions
of the slits through the shield and/or varying the axial position of the
shields relative to the coil for uncovering or exposing more or less of
the coil surface.
One example of the former means is shown in FIG. 4. Here, a shield 40
comprises 2 concentric shield portions 42 and 44 identical to one another
except that the inner shield portion 42 has a slightly smaller diameter so
as to fit snugly within the outer shield portion 44. As illustrated, the
two shield portions are held together by screws 46, the screws 46 passing
through holes in the outer portion 5 and pressing against the outer wall
of the inner portion 42 to hold it in place. By loosening the screws, the
inner portion can be rotated relative to the outer portion for displacing
the slits in the two portions more or less relative to each other. Thus,
the effective width of the shield slits can be varied for varying the
capacitive coupling provided through the shield.
More easily adjusted shields can be provided. For example, the lower edge
of the inner shield portion 42 can ride on an inwardly projecting,
circumferential ridge through the lower wall of the outer shield portion
44 to allow free relative rotation between the two shield portions. Such
rotation can be performed by means of a radially extending lever attached
to the upper end of the inner shield portion. The lever can be moved
manually, or automatically, by a process monitoring and controlling
mechanism.
Variations of the shield shown in FIG. 4 are shown FIGS. 5 and 6.
In FIG. 5, the slits 30 are of varying width from top to bottom. In
addition to providing a variable capacitive coupling between different
settings of the shield portion 42 and 44, the longitudinally varied slit
widths provide a capacitive coupling that varies along the length of the
coil for any given setting of the two portions 42 and 44. An advantage of
this shield is that a lower voltage portion of the coil is capacitively
coupled to the plasma. Essentially, the shape of the slit in the shield
determines the impedance of the bias supplied to the plasma.
In this shield shown in FIG. 6, each slit includes a single, enlarged
circular opening 50. The opening controls the coupling from a single turn
of the coil. An advantage of this is a specific impedance of coupling to
the plasma.
In FIG. 7, the upper end of the tube 18 is closed (gases being admitted
into the tube by means of a pipe 18a) and the coil 14a comprises a planar
spiral that is mounted on the tube 18 (e.g., via shims, not shown). A
slotted, disc-like shield 16a is disposed between the tube end and the
coil 14.
In FIG. 8, the spiral coil 14a is an extension of the helical coil 14, and
the shield 16a is an integral top cap of the shield 16. The slits through
the cylindrical wall of the shield continue into the top cap 16a and
become chevrons in the top cap which provide low loss due to eddy
currents.
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