U.S. patent number 3,616,405 [Application Number 04/863,601] was granted by the patent office on 1971-10-26 for continuous sputtering system.
This patent grant is currently assigned to International Plasma Corporation. Invention is credited to Harvey James Beaudry.
United States Patent |
3,616,405 |
Beaudry |
October 26, 1971 |
CONTINUOUS SPUTTERING SYSTEM
Abstract
In a system for depositing thin films of material on a workpiece
by a technique known as radiofrequency sputtering, an electrical
transformation bridge comprised of inductive and capacitive
components is electrically interfaced between an unbalanced output
of a radiofrequency generator and the excitation electrodes of a
sputtering chamber for converting the generator output signal into
opposing phase radiofrequency voltage components employed in
preferred continuous sputtering systems and for precisely matching
the impedance output of the generator with the impedance of the
excitation electrodes while the latter are under the influence of a
plasma of charged deposition particles.
Inventors: |
Beaudry; Harvey James (Fremont,
CA) |
Assignee: |
International Plasma
Corporation (N/A)
|
Family
ID: |
25341377 |
Appl.
No.: |
04/863,601 |
Filed: |
October 3, 1969 |
Current U.S.
Class: |
204/192.12;
204/298.08 |
Current CPC
Class: |
H01J
37/34 (20130101) |
Current International
Class: |
H01J
37/34 (20060101); H01J 37/32 (20060101); C23c
015/00 () |
Field of
Search: |
;204/192,298 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mack; John H.
Assistant Examiner: Kanter; Sidney S.
Claims
I claim:
1. In an RF sputtering system having an alternating current
generator and a pair of excitation electrodes for supporting
material to be sputtered the combination comprising,
an inductive-capacitive transformation bridge electrically disposed
between the generator and excitation electrodes.
2. The combination defined in claim 1, wherein said transformation
bridge comprises, a reference ground, a first inductor connected
between said ground and a first output terminal, a first capacitor
connected between said first output terminal and an input terminal
and forming with said first inductor a first network of said bridge
electrically associated with one of the electrodes, a second
inductor connected between said input and a second output terminal,
a second capacitor connected between said second output terminal
and said ground and forming with said second inductor a second
network electrically associated with the other electrode.
3. The combination as defined in claim 2, wherein said first and
second capacitors are variable for accommodating precise impedance
and phase transformations between said generator and electrodes
under sputtering conditions.
4. The combination as defined in claim 3, wherein the impedance
exhibited by each said first inductor and said second inductor and
each said first capacitor and said second capacitor is
substantially equal to the square root of a quantity equaling the
input impedance to said bridge times the real impedance of said
electrodes under deposition conditions.
5. The combination as defined in claim 2, further comprising, a
housing for said bridge having separate enclosed compartments
formed of electrically conductive walls and the inductor and
capacitor of each said network being disposed in an individual one
of said compartments for electrostatic and electromagnetic
shielding therebetween.
6. An RF sputtering system for particle deposition of a material on
a workpiece, the combination comprising,
a sputtering chamber having a pair of excitation electrodes for
supporting material to be sputtered and adapted for containing a
plasma of charged particles therein,
a generator having an unbalanced output issuing an alternating
voltage waveform, and
an inductive-capacitive transformation bridge having an input
connected to said generator output and a pair of outputs
individually connected to said chamber electrodes issuing opposed
phase components of said voltage waveform thereto and concurrently
matching the impedance of said generator output with the
dissipative impedance of said electrodes under the influence of
said plasma.
7. A system as defined in claim 6, said bridge comprising, a pair
of inductors and a pair of capacitors alternately connected end to
end and providing said bridge input across a first set electrically
nonadjacent junctions and providing said outputs at the remaining
electrically nonadjacent junctions relative to one of said first
set junctions.
8. A system as defined in claim 6, further comprising, an
unbalanced coaxial transmission line connecting said generator
output to said bridge input.
9. A system as defined in claim 8, wherein said chamber is formed
with electrically conductive wall and said bridge is mounted in an
electrically conductive housing disposed in contiguous relation
with said sputtering chamber and forming a reference ground
therewith.
10. In an RF sputtering method of depositing particles on a
workpiece by exciting a source of material with an alternating
electric field issued by electrodes mounted adjacent the source
material and energized by an alternating voltage output of a
generator to form a plasma of deposition particles the steps
comprising,
transforming the alternating voltage issued by said generator into
opposing phase voltage components by applying said generator output
to an inductive-capacitive bridge,
applying said opposing phase voltage components separately to
separate said electrodes by electrical connections to said bridge,
and
adjusting the impedance of said bridge to match the impedance of
said generator output with the dissipative impedance of said
electrodes while under the influence of said plasma.
11. The steps as defined in claim 10, wherein said bridge is formed
by alternate inductive and capacitive branches, and said steps of
adjusting comprises, simultaneously varying the capacitances of
said electrodes in the environment of said plasma.
Description
The present invention relates to systems for depositing thin films
on a workpiece by employing an alternating electric field proximate
a source of deposition material to dislodge charged particles or
ions therefrom for developing a plasma of charged deposition
particles which propagate toward and are deposited on the
workpiece. Particularly, the present invention pertains to method
and apparatus for transforming energy between the output of an
alternating current generator and the electric field required for
developing the plasma of deposition ions.
A film deposition technique having the foregoing characteristics is
commonly referred to in the art as a sputtering system. In
operation, a source of the material to be deposited, referred to as
the target, is disposed continuously with one or more electrodes
adapted to receive an alternating voltage signal, usually in the
radiofrequency range, for generating the excitation electric field.
The source (or target) and the electrodes are disposed together
with a workpiece within a chamber having a subatmospheric gas
pressure, whereby an ion plasma generated by the high-frequency
electric field dislodges charged or ion particles from the source
which are dispersed toward the workpiece for deposition of a
cohesive layer of particles thereon. Deposition processes of this
type have proved exceedingly advantageous in the manufacture of
miniaturized semiconductor circuits, such as thin film circuits,
where successive steps of deposition and masking of a workpiece are
employed to form numerous electrical circuit components on a small
substrate.
Several apparatus configurations have been employed for deposition
of thin films by sputtering. For example, in one known system, the
RF electric field is generated between a conductive work table
which is electrically grounded and a single discoidal nongrounded
electrode spaced from the worktable and to which the target or
source material is attached. The ground terminal from the RF
generator is connected to the worktable while the nongrounded
output table is connected to the discoidal electrode. A system
having this arrangement of components is sometimes referred to as a
grounded diode sputtering device and is relatively simple to
construct, particularly as to the means for interconnecting the
output of the generator with the electrode and worktable of the
sputtering chamber. For example, such interconnection may be
effected by a low cost coaxial cable connection between the
sputtering chamber and an unbalanced output of an RF generator.
However, in the operation of such units, it has been discovered
that the deposition of the source ions on the workpiece proceeds at
an undesirably slow rate due to polarity considerations and the
formation of a charge layer barrier adjacent the material source
during each half cycle of the applied alternating voltage waveform.
Deposition occurs during a positive half cycle defined by the
discoidal electrode being positive relative to the worktable,
whereas no deposition occurs during a negative half cycle in which
these respective polarities are reversed. Furthermore, during the
latter half of the positive cycle, a layer of charged particles
develops adjacent the source material and forms a barrier
preventing further deposition during the positive half cycle.
However, this layer of charge is dissipated in the negative half
cycle and the source material and surrounding ion plasma are
restored to a charged condition permitting ion deposition during
the succeeding positive half cycle. In this respect, the deposition
of the workpiece proceeds in a relatively slow pulsating rate
analogous to the conduction through a half wave rectifier or diode
from which this system derives its name.
In order to improve the deposition characteristics, i.e. uniformity
and rate, there has evolved in the art a sputtering system
employing a pair of excitation electrodes rather than the single
excitation electrode of the grounded diode apparatus in which the
pair of electrodes are energized relative to the work table with
radiofrequency voltage signals of opposing phase, i.e. 180.degree.
out of phase. By virtue of this construction, deposition proceeds
at a continuous rate, in which the source ions are carried to the
worktable and workpiece in response to the adjacent positive half
cycles alternately provided by the pair of excitation electrodes
relative to a grounded worktable. To insure uniformity of
deposition, the electrodes are shaped and arranged in a symmetrical
configuration, in this instance consisting of a central disc
electrode and an annular ring electrode disposed in radially spaced
and concentric relation. By reason of the improved deposition speed
and uniformity of the resulting deposited films, this dual
electrode sputtering system is preferentially used in the art.
However, in the construction thereof, it has heretofore been
impossible to provide an efficient yet economical means for
generating the required opposing phase RF driving signals and at
the same time efficiently conveying the electrical energy to the
pair of excitation electrodes mounted in the sputtering chamber.
For example, it has been known to derive the opposing phase RF
signals from an RF generator designed with a balanced output
network, which consists of a pair of output power tubes coupled
through a transformer to an output, wherein the secondary or output
winding of the transformer has a grounded center tap and the
opposing phase waveforms are developed relative to ground at each
end of the secondary winding. Such circuits are not only costly in
terms of materials and construction, but moreover the transformer
coupling is exceedingly lossy such that the transformation between
the power input delivered to the tubes and the signal power output
derived across the secondary winding of the transformer is highly
inefficient. In addition to this substantial insertion loss of the
transformer coupling, such output circuits require a balanced
shielded line [two spaced parallel conductors for the opposed phase
RF signals and a surrounding shield conductor for ground] to convey
the electrical energy from the generator output to the electrodes
of the sputtering chamber. Such balanced lines are inherently
large, unyielding, and expensive to construct. Also, balanced
transmission lines are not easily adapted for connection to
electrical measuring instruments, such as a serial connection with
an RF watt meter, desirable in most cases for measuring the amount
of electrical energy transmitted to the sputtering chamber.
While it has been the practice to match the impedance interface
between the transmission line and output of the RF generator, both
for the unbalanced and balanced type generator outputs, heretofore
the impedance interface between the transmission line or cable and
the electrodes of the sputtering chamber has been merely
approximated and in many cases entirely ignored. Because of this, a
portion of the electrical energy emanating from the RF generator
(which would otherwise pass to the sputtering chamber for raising
the energy level of the ion plasma developed therein), is reflected
back to the generator source at the connection of the transmission
line to the chamber electrodes. The inefficiency of the RF energy
transmission is particularly pronounced in the dual electrode
sputtering system wherein much of the theoretical advantage gained
by such systems in terms of deposition rate is lost by reason of
this impedance mismatch at the sputtering chamber. Additionally, as
the construction of the dual electrode sputtering system renders
the measurement of the RF energy applied to the sputtering chamber
impractical, it has been found difficult if not impossible to
account for, control, and reproduce the effective energy reaching
the deposition plasma. For example, one heretofore employed method
for measuring the RF sputtering energy has been to monitor the
amount of power applied at the input to the RF generator and to
estimate the amount of losses occurring between the generator and
the electrodes of the sputtering chamber. As the losses may vary,
however, this method does not adequately account for the RF energy
actually reaching the sputtering chamber during any given
deposition sequence.
Accordingly, it is an object of the present invention to provide in
a system of the type characterized, method and apparatus for
effectively and efficiently conveying alternating electrical energy
between a generator and the excitation electrodes of the sputtering
unit for maximum utilization of power developed by the
generator.
It is another object of the present invention to provide such a
method and apparatus which is conveniently adaptable for monitoring
the effective electrical energy actually reaching the excitation
electrodes of the sputtering chamber.
It is still another object of the present invention to provide such
an apparatus which lends itself to simplicity of construction for
low cost manufacture and which may be easily and rapidly assembled
and disassembled with respect to the generator and sputtering
chamber.
Other features, objects and advantages of the present invention
will become apparent from the following description to be read in
conjunction with the accompanying drawings forming a part of this
specification and illustrating the preferred embodiment of the
invention.
In the drawings:
FIG. 1 is an elevation view, partially cut away for clarity,
illustrating the transformation bridge constructed in accordance
with the present invention and as employed in a dual electrode
sputtering deposition system;
FIG. 2 is an electrical diagram partly in schematic and partly in
block form characterizing the electrical connection of the
transformation bridge between an RF generator output and a dual
electrode sputtering chamber such as shown in FIG. 1;
FIG. 3 is a top elevation view, partially cut away for clarity, of
the transformation bridge shown in FIG. 1, however, disassembled
from the sputtering chamber;
FIG. 4 is a front elevation view of the transformation bridge shown
in FIG. 3 illustrating the front instrument panel; and
FIG. 5 is a bottom elevation view of the transformation bridge as
shown in FIGS. 3 and 4.
With reference to FIGS. 1 and 2, the present invention comprises in
general a continuous sputtering system in which an L-C
[inductive-capacitive] transformation bridge 11 is connected
between an unbalanced output 12 of an RF [radiofrequency] generator
13 and a dual electrode sputtering head 14 of a sputtering chamber
16 for efficiently and effectively channeling RF energy from the
generator to the sputtering chamber for developing a plasma 17 of
deposition particles or ions therein. Briefly, the deposition
process is effected by energizing a plate 18 of source material
(sometimes called target material), by an RF electric field such
that ions are dislodged therefrom which enter a plasma 17 of
positively and negatively charged particles. The source ions
eventually propagate toward and are deposited upon a workpiece 19
to form a thin cohesive layer of the source material thereon. Here,
sputtering head 14 is formed with a pair of ungrounded electrodes
21 and 22, in this instance of discoidal and annular configuration
respectively, to which 180.degree. phase opposed RF voltage
waveforms are applied relative to a grounded electrically
conductive worktable 23 such that preferred continuous sputtering
and deposition is achieved as discussed above relative to workpiece
19.
In accordance with the present invention, the opposing phase RF
signals driving excitation electrodes 21 and 22 are derived from an
unbalanced output of generator 13, here provided by an unbalanced
RF power amplifier 26 and an unbalanced output matching network 27,
which function to issue a single phase RF voltage to an unbalanced
coaxial cable 28 connected between an output 12 of the generator
and input 29 of transformation bridge 11. Bridge 11, comprised of a
pair of inductors 31 and 32 alternately connected end to end with a
pair of variable capacitors 33 and 34, provides for the conversion
of the single phase RF signal, received through cable 28, into
opposing phase RF excitation voltages for driving electrodes 21 and
22 via leads 36 and 37. In addition, bridge 11 concurrently effects
a precise matching of the real or dissipative impedance of the
electrodes with the output of generator 13. Moreover, this
impedance match is achieved during active operation of the
sputtering system, wherein the real, i.e. dissipative impedance, of
electrodes 21 and 22 is substantially influenced by the presence
and close proximity of plasma 17 and takes on a value largely at
variance with the free space electrode impedance. A number of
advantages are achieved by this arrangement including principally,
highly efficient transmission of RF energy between generator 13 and
sputtering head 14 both in terms of low reflection losses provided
by the impedance matching capability of bridge 11 and low insertion
loss attributed to the use of an unbalanced circuit at the output
of the generator, as compared with balanced output generator
circuits which are characteristically inefficient. It is apparent
that with an increased amount of energy reaching sputtering head 14
for any given system, a greater proportion of the total power
consumed by the system is effective in generating ion plasma 17 and
thereby enhancing the uniformity and rate of the deposition
process.
Additionally, simplification in the construction of the RF
generator output is realized [unbalanced output circuits such as
that of amplifier 26 and network 27, are inherently less complex
and comprise fewer components than balanced output circuits which
have heretofore been required]. An accompanying economy and
convenience is provided by allowing employment of an unbalanced
coaxial transmission line, such as cable 28, between the generator
and sputtering unit as opposed to the heretofore used balanced
transmission lines [which are more costly and of larger, unyielding
construction]. Furthermore, as it is desirable to monitor the RF
energy passing from the generator output to the sputtering chamber,
in this instance through transformation bridge 11, this energy
transfer may be conveniently measured by serial connection of a
commercially available RF watt meter 38 with cable 28. Commercially
available RF watt meters are universally equipped for connection
with unbalanced lines, whereas the instruments must be specially
adapted by the customer should a balanced line connection be
required.
In the operation of transformation bridge 11, an input 29 extends
the single phase unbalanced RF voltage output from generator 13
across a pair of electrically nonadjacent bridge junctions 41 and
42, wherein one of these junctions, in this instance junction 42,
is extended to an earth or reference ground. By selecting the
impedance values of inductors 31 and 32 and capacitors 33 and 34 in
a manner more fully described below, separate RF voltage signals
are issued with respect to ground from the remaining pair of
electrically nonadjacent bridge junctions 43 and 44 which are
extended over leads 36 and 37 to electrodes 21 and 22 of sputtering
head 14. Thereat, the voltage signals appear as balanced,
180.degree. opposed phase RF voltage wave forms, V and V', with
respect to grounded conductive portions of chamber 16, particularly
grounded worktable 23. The real or dissipative impedance
encountered by each of electrodes 21 and 22 in the plasma
environment, appear as equivalent resistors R.sub.a and R.sub.b,
shown by dotted lines in the circuit of bridge 11, between
junctions 43 and 44 and ground, respectively. In accordance with
this equivalent circuit, the electrode impedance R.sub.a forms a
real impedance load associated with that half of the bridge
comprised of inductor 31 and capacitor 34, while resistance R.sub.b
forms an effective load associated with the remaining half of the
bridge composed of capacitor 33 and inductor 32. By selecting the
values for inductor 31 and capacitor 34 to match electrode
impedance R.sub.a and the values of capacitor 33 and inductor 32 to
match impedance R.sub.b, an effective transfer of RF energy is
achieved between input 29 and sputtering head 14. Furthermore, as
electrodes 21 and 22 are, in this instance and as a general rule,
constructed to issue equal amounts of RF energy in forming plasma
17, the dissipative impedances, R.sub.a and R.sub.b, exhibited
thereby are substantially equal. Thus, inductors 31 and 32 and
capacitors 33 and 34 are selected to provide not only a transfer
impedance between the input to the bridge across junctions 41 and
42 and the output thereof across junction terminals 43 and 44
matching the impedance between cable 28 and the sum of equivalent
resistors R.sub.a and R.sub.b [equal to the total dissipative load
impedance across the bridge output], but also to provide an equal
impedance in each branch of the bridge such that the voltage
signals appearing on leads 36 and 37 are equal in magnitude and
separated in phase by 180.degree..
To achieve the foregoing selection of values for bridge 11 in any
given system, the following formula is employed:
Z.sub.c =Z.sub.L = Z.sub.in Z.sub.out
where,
Z.sub.in = impedance of coaxial cable 28
Z.sub.out = total dissipative impedance of electrodes 21 and 22
during operation of the system
Z.sub.c = impedance of capacitors 33 and 34 at operating
frequency
Z.sub.l = impedance of inductors 31 and 32 at operating
frequency
As exemplary values for a system operating at a frequency of 13.5
megacycles, with individual electrode impedance values for R.sub.a
and R.sub.b equal to 2.5 ohms (or Z.sub.out = 10 ohms) and the
impedance of cable 28 equal to 50 ohms (or Z.sub.in = 50 ohms) the
value of inductors 31 and 32 is calculated to be 2.5 microhenrys
and the value of capacitors 33 and 34 about 800 microfarads.
With reference to FIG.(S) 3-5, the components of transformation
bridge 11 are mounted in housing 46 formed with electrically
conductive bottom and top walls 47 and 48, sidewalls 51 and 52,
back wall 53 and a front instrument panel wall 54. Furthermore,
inductor 31 and capacitor 34 associated with output junction 43 are
preferably separated from capacitor 33 and inductor 32 associated
with output junction 44 into separate shielded compartments of
housing 46 by means of an electrically conductive isolating wall 56
providing electrostatic and electromagnetic shielding therebetween.
This precludes any Faraday coupling interaction between the pairs
of associated components. Wall 47 is formed with a recess 57, in
this instance of circular shape, for mounting housing 46 on chamber
16, here of cylindrical shape, with recess 57 nested on a top mated
edge portion of the chamber as shown in FIG. 1. An electrically
conductive cylindrical wall 58 of chamber 16 thereby is in
electrical communication with bottom wall 47 of the bridge housing
and serves to extend the reference ground associated with bridge 11
to conductive work table 23 via electrical connections indicated by
dotted lines 61 and to an electrode shield 62 via connections
indicated by dotted lines 63. The recessed portion of bottom wall
47 is formed with spaced-apart openings 66 and 67 for extending
leads 36 and 37 from internally of housing 46 to connections with
stud terminals 68 and 69, upwardly extending from electrodes 21 and
22, respectively. The electrically active portions of inductors 31
and 32 and capacitors 33 and 34 are insulated from the housing
walls by means of standoff insulators such as insulator 71 for
inductor 32. Inductors 31 and 32 and each of the interconnecting
leads for bridge 11 are formed of highly conductive copper tubing
and the permanent conductor joints such as at junctions 41, 43 and
44 are silver brazed to insure positive stable electrical
connections. It is noted that housing 46 is at reference ground
such that one of the ends of both inductor 32 and capacitor 34 are
connected to housing wall 47 as shown at 42a and 42b thus forming
junction 42 as shown in FIG. 2. A coaxial female connector 72
mounted with an outer conductive portion in electrical contact with
housing wall 53 and an inner conductor insulated therefrom is
extended into electrical engagement with junction 41 to provide the
unbalanced coaxial input 29 for receiving one end of cable 28.
RF generator 13 includes an RF oscillator 76 and a preamplifier or
exciter 77 for receiving the RF frequency output of oscillator 76
and driving the grid of a power amplifying tube 78 of power
amplifier 26. In simplified form, amplifier 26 includes, in
addition to tube 78, an RF choke 79 in series with the plate supply
circuit of the tube and a DC blocking capacitor 81 through which
the amplified RF signal is fed to a junction 82 with the input of
network 27. In this instance matching network 27 is an L-C .pi.
network comprised of a pair of variable capacitors 83 and 84 and an
inductor 86 connected between junction 82 and generator output 12
and serves to match the output impedance of amplifier 26 with the
characteristic impedance of coaxial cable 28. While one form of an
unbalanced RF generator is herein disclosed by way of example, it
will be apparent to those skilled in the art that several other
types of unbalanced output networks may be employed in place of
those shown for amplifier 26 and network 27.
In preparing the sputtering system shown in FIG. 1 for deposition,
chamber 16 is initially filled with a gas having a suitable
environment for the sputtering process, such as argon, and the
pressure of such a gas is thereupon reduced substantially below
that of atmospheric pressure by drawing a vacuum on a port 88
communicating with the otherwise hermetically sealed interior of
chamber 16. Thereafter RF generator 13 is energized so as to
develop an RF electric field permeating source target 18 which in
response thereto and in conjunction with the entrapped argon gas
effects formation of plasma 17 of charged particles. Field control
magnets 97 are frequently employed for providing a preferred
direction of propagation of the deposition ions toward target 19.
During alternate half cycles of the opposed phase RF voltage
waveforms applied to electrodes 21 and 22, positive ions dislodged
from target 18 are forced to worktable 23 and workpiece 19 first by
one electrode and then by the other providing the heretofore
characterized continuous sputtering operation.
In accordance with the present invention, to insure maximum
transfer of electrical energy from generator 13 to dual electrode
sputtering head 14, capacitors 33 and 34 are simultaneously
adjusted from their central impedance values such that the power
registered by RF watt meter 38 is "peaked" or by obtaining a "null"
in case reflected power is measured by meter 38. For this purpose,
manual adjustment means are provided on the instrument panel wall
54, here taking the form of knobs 91 and 92 carried by capacitor
shafts 93 and 94. While the calculated values for capacitors 33 and
34 will in many cases serve to provide a highly efficient
transformation of the RF energy to the dual electrode sputtering
head, the adjustment thereof as above described is preferred for
reaching a precision impedance match between the generator and
electrodes.
* * * * *