U.S. patent number 3,860,507 [Application Number 05/310,446] was granted by the patent office on 1975-01-14 for rf sputtering apparatus and method.
This patent grant is currently assigned to RCA Corporation. Invention is credited to John Louis Vossen, Jr..
United States Patent |
3,860,507 |
Vossen, Jr. |
January 14, 1975 |
RF SPUTTERING APPARATUS AND METHOD
Abstract
An RF bias sputtering apparatus is provided having simple on-off
operation in which there is no detuning of the apparatus to an RF
power source when sputtering conditions within the ionization
chamber change. The capacitive reactances within the chamber and
without the chamber are divided into a reactance bridge network, a
capacitive reactance means being disposed across an arm of the
bridge for balancing the reactances of the bridge network. Critical
tuning requirements are reduced by the use of an RF power source
with wide bandwidth tuning capabilities.
Inventors: |
Vossen, Jr.; John Louis
(Somerville, NJ) |
Assignee: |
RCA Corporation (New York,
NY)
|
Family
ID: |
23202531 |
Appl.
No.: |
05/310,446 |
Filed: |
November 29, 1972 |
Current U.S.
Class: |
204/192.12;
204/298.08; 204/192.15 |
Current CPC
Class: |
H01J
37/34 (20130101); H01J 37/3444 (20130101) |
Current International
Class: |
H01J
37/32 (20060101); H01J 37/34 (20060101); C23c
015/00 () |
Field of
Search: |
;204/192,298 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Tufariello; T. M.
Attorney, Agent or Firm: Norton; Edward J. Squire;
William
Claims
What is claimed is:
1. In combination:
a plurality of RF power input terminals,
an electrically conductive housing arranged to form an ionization
chamber when coupled to a source of reference potential,
a first target holder electrode mounted in said chamber
electrically insulated from said housing,
a second target holder electrode mounted in said chamber spaced
from said first target holder electrode and electrically insulated
from said housing,
first and second RF power connecting means each coupled between a
separate, different one of said electrodes and a corresponding,
separate, different one of said RF power input terminals, said
first connecting means and said chamber exhibiting first and third
impedances between the corresponding one electrode and said
housing, said second connecting means and said chamber exhibiting
second and fourth impedances different from said first and third
impedances between the corresponding other electrode and said
housing, and
impedance balancing means coupled across one of said first and
second impedances for balancing the impedances between said
electrodes and said housing.
2. In combination:
a source of RF power including a self-excited oscillator,
first, second and third electrical terminals,
means for connecting said first and second terminals to said source
of RF power and said third terminal to a source of reference
potential,
an ionization chamber formed by an electrically conductive
housing,
a first electrode target holder disposed in said chamber and
electrically insulated from said housing,
a second electrode target holder disposed in said chamber spaced
from said first electrode target holder and electrically insulated
from said housing,
first connecting means electrically connecting said first electrode
to said first terminal,
second connecting means electrically connecting said second
electrode to said second terminal,
means for coupling said RF power source, said housing and said
first and second connecting means to said third terminal, the
coupling of said first connecting means to said third terminal
exhibiting a first impedance, the coupling of said second
connecting means to said third terminal exhibiting a second
impedance different from said first impedance producing an
impedance unbalance therebetween, and
impedance balancing means coupled between one of said first or
second terminals and said third terminal for balancing the coupling
impedances between said first and second connecting means and said
third terminal.
3. In a method for sputtering a material by RF stimulated glow
discharge in an apparatus in which power from a RF power source is
impressed through coupling means across a first target electrode
associated with a source of material and power from said RF power
source is impressed through said coupling means across a second
target electrode associated with a substrate workpiece holder for
depositing said material on a workpiece, said electrodes being
electrically insulated from a conductive surface in contact with
the plasma generated in the environment of said electrodes, the
steps comprising:
forming first and second impedances between said first and second
target electrodes and said conductive surface within said
environment,
forming said coupling means including said first and second target
electrodes and said conductive surface into third and fourth
impedances,
coupling said first, second, third and fourth impedances in a
bridge configuration, and
balancing said impedances.
4. The method of claim 3 wherein said third and fourth impedances
are capacitive reactances.
5. The method of claim 3 further including the steps of impressing
said RF power at the same level across said first and second target
electrodes, and then reducing the level across said second target
electrode.
6. The method of claim 3 further including the steps of placing a
first variable impedance between said RF power source and said
first target electrode and placing a second variable impedance
between said RF power source and said second target electrode.
7. The method of claim 6 wherein said third and fourth impedances
and said variable impedances are capacitive reactances.
8. In an RF sputtering apparatus for establishing a discharge
between a bias target electrode and an ionization chamber in which
said bias target electrode is located and establishng a discharge
between a deposition target electrode and said ionization chamber
in which said deposition target electrode is located, by coupling a
first source of RF power between said bias target electrode and
said ionization chamber and coupling a second source of RF power
between said deposition target electrode and said chamber through
coupling means, the improvement wherein:
said coupling means is arranged to provide balanced impedances
between (i) said bias target electrode and said ionization chamber
and (ii) said deposition target electrode and said ionization
chamber.
9. In the apparatus of claim 8 wherein there are two discharge
portions each corresponding to a separate, different electrode and
a third discharge portion common to said two discharge portions
when said chamber is grounded, said coupling means grounding said
chamber whereby the junction of said two discharge portions and
said third discharge portion is at ground potential.
10. In the apparatus of claim 8 wherein said coupling means
includes a first power connecting means for connecting said first
source of RF power to said bias target electrode, a second power
connecting means for connecting said second source of RF power to
said deposition target electrode, and means for grounding said
chamber, said first power connecting means exhibiting a first
impedance, said second power connecting means exhibiting a second
impedance different than said first impedance thereby producing an
impedance unbalance,
said first mentioned coupling means including a third impedance
which together with said first and second impedances balances said
impedance unbalance.
Description
BACKGROUND OF THE INVENTION
The sputtering of materials by radio frequency fields is generally
known and is discussed in an article by J. L. Vossen appearing in
the Journal of Vacuum Science and Technology, Vol. 8, No. 5,
Sept./Oct. 1971. Deposition of a thin film by RF bias sputtering is
achieved in a vacuum ionization chamber which is typically a
metallic housing. The material to be sputtered is placed on a
target electrode disposed within the chamber and electrically
insulated therefrom. Positioned parallel to and spaced from the
target electrode is a second target electrode which supports a
workpiece to be coated with a film that is sputtered from the
material on the first-mentioned target electrode. An RF power
source is arranged to supply an RF potential across both target
electrodes to produce a glow discharge in the region between the
two targets. The conductive surfaces of the chamber determine the
reference potential of the plasma within the chamber. The plasma is
defined as a nearly field free region in the space occupied by
positive and negative charges.
In addition to plasma, the space between the two target electrodes
also includes what is known as a "dark space." The "dark space" is
a region adjacent the electrode wherein the voltage drops from the
RF potential to a potential close to that of the metallic housing
of the chamber. With a single RF potential and a grounded second
electrode, there is but one dark space which is adjacent to the
first-mentioned electrode. Thereafter the glow discharge region is
nearly field free up to the second electrode.
In the known RF sputtering techniques, the RF power may be supplied
solely across the first-mentioned electrode with the second
electrode having the potential of the conductive housing of the
ionization chamber. In this case there is a grounded workpiece
target electrode and the material to be sputtered is removed from
the first-mentioned electrode to the second electrode by the
sputtering technique.
In a negative bias sputtering apparatus the negative bias is
applied to the second electrode. Negative bias results in some of
the material on the second electrode being sputtered therefrom and
removed. The differences in RF potential applied to the two
electrodes can be used to improve the properties of certain
materials coated on the workpiece on the second electrode, the rate
of deposition and other characteristics as known in the RF
sputtering art.
It has been found that where the material to be sputtered is a
metal, the sputtering technique introduces electrical properties in
the deposited film which are different from the electrical
properties of a bulk material. That is, there is increased
resistivity among other characteristics in the deposited film. It
has been found that these differences in characteristics can be
reduced. In some cases, the characteristics of the bulk material
may be imposed into the deposited thin film by the use of what is
known as bias sputtering. However, with present state of the art RF
sputtering techniques, such an operation is limited by the limited
voltages available or by the complexity of the equipment
itself.
Tuning in the range of 13.36 MHz .+-. 0.00678 MHz has been
generally considered required for apparatus in the field of RF
sputtering. Power sources are generally frequency sensitive and
tend to self-destruct should the apparatus detune as might occur by
slight changes of the load impedances within the ionization
chamber. Several different controls are usually provided with such
apparatus for the tuning of the power source and the various
impedances in the matching networks between the load including the
connections of the apparatus and the power source. As many as six
different tuning and adjusting controls may be present for a single
Rf sputtering operation. Additionally, because of the narrow range
of frequencies to which the system must be tuned, skilled operators
are required to maintain constant monitoring alert over the
adjustment of the various controls and even then, uncontrolled
sputtering conditions may occur. As a result, such sputtering
techniques are not readily adapted to mass production
operations.
In accordance with the present invention, high RF powers may be
utilized up to several thousand volts on both the target electrode
mounting the sputtering material and the target electrode mounting
the workpiece. In the present invention, an RF sputtering apparatus
is coupled as a bridge network and the impedances in the system are
then balanced. An operator need only turn the system on and off
after the system is balanced. Regardless of any change in
impedances within the ionization chamber the balanced bridge
arrangement as originally established within the apparatus remains,
permitting an unskilled operator to utilize the apparatus.
SUMMARY OF THE INVENTION
The aforementioned problems are overcome in an RF sputtering
apparatus including first and second target holder electrodes
disposed within an ionization chamber exhibiting a glow discharge
and RF power connecting means for coupling the electrodes to a
source of Rf power, the glow discharge exhibiting first ans second
impedances between the electrodes, the apparatus within and without
the chamber exhibiting third and fourth impedances between the
electrodes. A fifth impedance is provided having a value such that
when it is coupled across one of the third and fourth impedances a
sixth impedance is formed therewith having a value the same as the
value of the other of the third and fourth impedances.
The method comprises changing the value of one of the third and
fourth impedances to match the other of the third and fourth
impedances so that the values thereof are the same.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic circuit diagram and apparatus constructed and
operated in accordance with an embodiment of the present
invention,
FIG. 2 is a more detailed circuit diagram of the arrangement of
FIG. 1,
FIG. 3 is a graph of curves useful in explaining the present
invention, and
FIG. 4 is a cross sectional view of a preferred embodiment of a
portion of a sputtering apparatus constructed in accordance with
the present invention.
DETAILED DESCRIPTION
In FIG. 1 sputtering apparatus 10 has a low pressure ionization
chamber 12 formed by a metallic housing 14. A suitable gas, as
known in the RF sputtering art, is introduced into the chamber 12
inlet (not shown) and maintained at a low pressure by means of a
vacuum pump (not shown). Target electrodes 16 and 18 are disposed
within chamber 12 by suitable means and electically insulated from
housing 14. As shown, electrical insulation 20 serves as a
mechanical support and electrical insulation means for electrode
16, while insulation 22 serves as a mechanical support and
electrical insulation means for electrode 18.
Mounted on electode 16 is a disk 24 formed of material to be
sputtered. Mounted on electrode 18 is a disk 26 of material on
which the film of material sputtered from disk 24 is to be
deposited. Disk 26 will hereinafter be referred to as the
substrate. Surrounding electrode 16 and disk 24 is a metallic
cylindrical shield 28 which is ohmically connected to housing 14. A
similar shield 30 encloses the outer periphery of disk 18 and
substrate 26 and is ohmically connected to housing 14. Shields 28
and 30 serve to shield the electrodes and associated disks mounted
thereon from exposure to the ionization chamber at the peripheral
edges of the electrodes and disks.
Electrode 16 is coupled to an RF power source 32 by way of center
conductor 34, air dielectric 36 and outer shielding conductor 38
which is connected to housing 14. Center conductor 34, air
dielectric 36 and shield conductor 38 form a first coaxial RF
transmission line 37. This transmission line is coupled to source
32 by way of lead 40 through double pole switch 44 in one switch
position thereof by way of switch terminal 63. The other switch
position of switch 44 couples terminal 66 by way of switch terminal
64 to conductor 34. In this latter switch position a capacitane
meter (not shown) can be serially connected between conductor 34
and housing 14 as will be explained. Source 32, terminal 62 and
housing 14 are connected to a point of reference potential,
preferably ground.
In a similar manner, electrode 18 is connected to center conductor
50 surrounded by an air dielectric 52 in turn, surrounded by a
conductive shield 54, which together form a second RF coaxial
transmission line 55. Conductor 50 is electrically coupled to
housing 14 by way of serially connected double pole switch 60 and
RF power source 56. First terminal 68 of switch 60 is ohmically
coupled to RF power source 56 while a second terminal 70 is coupled
to terminal 72.
Switch 60 conductively couples either terminal 68 or 70 with
conductor 50 in accordance with the switch position of this switch.
As provided in accordance with the present invention, a variable
capacitance 74 is coupled between housing 14 and conductor 50.
Capacitance 74 is adjustd so as to balance the capacitive reactive
impedances between housing 14 and conductor 50 formed by
transmission line 55 and associated fixturing to the capacitive
reactive impedances between housing 14 and conductor 34 formed by
transmission line 37 and associated fixturing. The capacitive
reactance between either of conductors 34 and 50 and housing 14
comprises the capacitance between electrodes 16 and 18 to the
housing 14 and the capacitance between leads 34 and 50 and outer
shielding conductors 38 and 54, repectively. One source of
difference in capacitance between transmission lines 37 and 55 is,
of course, a difference in length thereof.
Adjustment of variable capacitor 74 is achieved by placing a
capacitance measuring meter (not shown) between terminals 62 and 66
and measuring capacitive reactances present between conductor 34
and housing 14 making note of same. The capacitor meter is then
placed between terminals 62 and 72 to measure the capacitive
reactance present between housing 14 and conductor 50. Of course,
the switch positions of switches 44 and 60 are positioned such as
to place terminals 66 and 72 respectively in ohmic contact with the
respective center conductors 34 and 50 of transmission lines 37 and
55. Capacitor 74 is then adjusted such that the capacitive
reactance between housing 14 and conductor 50, comprising the
reactance of capacitor 74 in addition to the capactive impedance
provided by the RF coaxial transmission line including conductor
50, air dielectric 52 and outer conductor 54, and the fixturing
within the chamber 12 is the same as the capacitive reactance
previously noted between conductor 34 and housing 14. Note that
switches 44 and 60 each disconnect the respective power sources 32
and 56 during the measurement of the capacitive reactances.
In FIG. 2 there is illustrated a circuit schematic diagram
representing the electrical equivalents of the apparatus of FIG. 1
in addition to additional components provided in accordance with a
particular practical application of the apparatus constructed in
accordance with the present invention. In FIG. 2 like numerals
refer to like components of FIG. 1. In FIG. 2, the impedance 100 of
the deposition target such as provided by disk 24 of FIG. 1 and its
connection to electrode 16 is considered to be negligible. This may
be in the order of a few ohms. Impedance 100 is coupled by way of
lead 102 to switch 44. In effect lead 102 represents the ohmic
coupling of electrode 16 to switch 44 of FIG. 1. Capacitor 104
represents the capacitive reactances between lead 102 and the
reference potential at terminal 106 manifested by housing 14 of
FIG. 1.
Coupled across capacitor 104 is RF choke 107 which is serially
connected to a D.C. volt meter 108. Across D.C. volt meter 108 is
an RF bypass capacitor 110. Terminal 62 is connected to the center
tap of RF transformer 112 whose one power output side is serially
connected to terminal 63 through variable voltage setting capacitor
114.
In a like manner a volt meter 116 is serially connected to RF choke
118 across capacitance 120 which manifests the capacitance of the
RF connections between electrode 18 and switch 60 of FIG. 1 to
reference terminal 106. Across D.C. volt meter 116 is coupled RF
bypass capacitor 122. RF choke 107 and capacitor 110 serve
respectively to reduce and bypass the RF power to D.C. volt meter
108, permitting meter 108 to measure D.C. voltages without being
destroyed by the RF power applied from transformer 112. In a like
manner RF choke 118 and capacitor 112 reduce and bypass RF power
D.C. meter 116 to protect meter 116 from destruction by the RF
power. Variable capacitance 74 is coupled between terminal 62 and
switch 60 across capacitance 120. Terminal 68 of switch 60 is
serially coupled through variable voltage setting capacitor 124 and
the RF power source manifested by transformer 112 to terminal 62 by
way of the center tap of transformer 112. Capacitors 114 and 124
each independently set the power level applied across terminals 63
and 62 and across terminals 68 and 62, respectively.
In many sputtering operations, the active agent is a glow discharge
maintained between spaced electrodes in a suitable gaseous medium.
Under the influence of the electric field established between
electrodes, ionization of the gas is produced by the collision of
free electrons with the gas molecules, producing positively charged
gas ions. These ions are attracted toward the electrodes thereby
creating what is known as an "ion sheath" around the electrode.
This "ion sheath" is the same as the "dark space" referred to
earlier. Within this region the ions are subjected to a high
potential which accelerates them toward the electrodes so that they
bomard the target disposed on the electrodes with sufficient impact
to eject particles therefrom. These ejected or "sputtered"
particles of target material will be deposited on nearby objects.
In the particular apparatus described herein the sputtering
material is deposited upon articles that are mounted on the
substrate electrode.
That portion of the glow discharge present outside the ion sheath
(dark space) surrounding the electrodes is a nearly field free
area. That is, nearly all the entire potential drop between the
electrode and ground or reference potential occurs across the dark
space. The dark space noted will appear adjacent to that electrode
at which there is applied a negative RF potential. If one of the
electrodes in a system such as illustrated in FIG. 1 does not have
an RF power source applied thereto, but is at ground potential,
then the dark space will appear at the one electrode at which the
RF power is applied. If on the other hand, a negative RF potential
is applied to the other electrode in the system, then the dark
space will appear adjacent to both electrodes.
This latter arrangement is known as RF bias sputtering. RF
potential applied to the second electrode will cause some removal
of the material (substrate) or sputtering thereof due to the RF
potential applied to this electrode. In effect with a high voltage
applied to one electrode and a low voltage to the other, the low
voltage electrode and its corresponding substrate mounted thereto
will receive a thin film of a sputtered material some of which is
resputtered therefrom.
This resputtering action or RF bias sputtering has been found with
respect to certain materials to cause the deposited materials on
the substrate to exhibit bulk properties of the sputtered material
from the high powered electrode whereby without bias sputtering the
thin deposited film will have properties which are not the same as
the bulk material being sputtered from the high powered
electrode.
It has been found that with certain sputtering material such as
refractory materials including tantalum and molybdenum, the
deposited thin films without bias sputtering do not exhibit the
same electrical resistivity properties as the bulk material.
However, to provide the deposited thin film with nearly the same
electrical properties as the bulk materials it has been found that
a negative bias voltage is required in the neighborhood of 150-200
volts. Prior art systems have proved to be extremely complex and
sensitive in providing such bias sputtering voltages.
As indicated above, in an RF bias sputtering apparatus, there is
present a dark space adjacent a deposition target and the substrate
bias target. This dark space is represented electrically as a
capacitance. Between the two dark spaces is the glow discharge
region. This glow discharge region is represented electrically as a
resistance. In prior art systems there is generally provided
impedance matching networks between both the deposition target and
the power source and the substrate bias target and the power source
in addition to tuning controls in the RF power source itself which
is critically tuned to the frequency of the matching impedance
networks. Any slight shift in the values of the disributive
capacitance impedances within the apparatus will cause a detuning
to result and possible burnout of the output tubes of the power
amplifier which ordinarily are tetrodes or pentodes. As sputtering
conditions change which very often will occur rapidly, the power
source and load may not always be in perfect matching tuning. As a
result a skilled operator has to manipulate a minimum of four
tuning controls and a power control to return the source to the
load.
It has been found that the dark space capacitance 130 associated
with electrode 16 and a portion of the resistance 132 formed by the
glow discharge in the chamber is coupled to the ground terminal 106
manifested by housing 14 through a small resistive impedance 134 of
a few ohms which is the glow discharge resistance to ground. The
second dark capacitance 136 is formed by the dark space associated
with the substrate bias target 18, and a resistive impedance 138 is
formed by the glow discharge resistance between the capacitance 136
and ground terminal 106 again through resistive impedance 134.
Capacitance 120 in combination with dark space capacitance 136 and
glow discharge resistance 138 form one-half of a reactance bridge
network. The capacitance 104 and dark space capacitance 130 and
glow discharge resistance 132 associated with the deposition target
electrode form the other half of that bridge network. By providing
an impedance bridge matching capacitor 74 in accordance with the
present invention, the capacitive reactance associated with the
substrate bias target may be matched to the capacitive reactance
assoicated with the deposition target. By so doing, any shift of
capacitive reactances within the ionization chamber due to changing
sputtering conditions will have no substantial effect on the
balanced reactance of the bridge, since the glow discharge coupling
to ground will itslef be uniformly divided with respect to the
deposition targets and substrate bias targets and ground. As long
as this division is present within the ionization chamber, then the
effects of changing sputtering conditions will not cause an
appreciable change in load conditions with respect to the RF power
source. With this in mind, the power source after the bridge
network is balanced, need only be turned on and off once the source
is tuned to the load by way of tuning capacitor 140 disposed across
transformer 112 and the power to each of the deposition and
substrate bias targets is set by variable capacitances 114 and 124,
respectively. The meters 108 and 116 permit setting of these
respective voltages. The RF power source at transformer 112 is
indicated as source 142 in FIG. 2. As a result, no matching
impedances need be inserted in the circuit nor set by a skilled
operator to match the load to the power source In this case, the
matching impedance is provided by matching the characteristic
impedance of the coaxial transmission lines 37 and 55 (FIG. 1) to
the RF power source (transformer 112).
Since the RF connections icluding the coaxial transmission lines
and fixtures are rigid and, therefore, fixed in a given apparatus,
then once capacitor 74 is adjusted such that the combination
thereof with capacitance 120 balances the capacitance of
capacitance 104, there is no need for further variation of
capacitor 74 in the operation of the apparatus of the present
invention. In a similar manner, tuning capacitor 140 need be set
once during an operation at the start thereof for matching the
source to the load in a conventional manner. It is to be understood
that switches 44 and 60 coupled to terminals 66 and 72 disconnect
capacitors 114 and 124 and transformer 112 from the remainder of
the circuit thereby enabling conventional capacitance meters, when
placed across respective terminals 66 and 72 and terminal 62 to
measure only the capacitance provided by the apparatus.
FIG. 3 illustrates the plot of sputtering sheath potential in
arbitrary units (ARB) versus frequency in MHz. Curve 150 is a plot
of the Industrial, Scientific and Medical (ISM) frequency allocated
by the Federal Communications Commission of the United States
Government which has found widespread use with RF sputtering
equipment. This ISM frequency is 13.56 MHz .+-. 0.00678 MHz. In the
use of such a narrow frequency band, a typical RF sputtering
apparatus is easily detuned and crystal controlled oscillators are
necessarily employed therewith for maintaining such a narrow range
of frequencies. The ISM frequencies have been designated in order
to prevent unlicensed operation and unlimited radiation in ranges
outside this frequency to avoid interference with other equipment
such as communication equipment and the like allocated to adjacent
frequencies in the crowded RF spectrum. In effect, the Federal
Communications Commission has indicated that spurious and harmonic
radiation frequencies other than those specified shall be
suppressed so that such radiations do not exceed a field strengh of
25 microvolts per meter at a distance of 1,000 feet or more from
the equipment causing such radiations. In sputtering equipment
ordinarily using 13.56 MHz, the eighth harmonic falls in an
aircraft navigation band which uses receivers with a sensitivity of
2 microvolts per meter. It has been found that by employing
apparatus of the present invention levels of RF power radiated by
the equipment are less than those amounts prohibited by the Federal
Communications Commission. As a result, frequencies outside the ISM
range may be utilized in this equipment without generating spurious
harmonic radiations in the prohibited frequency ranges. In view of
this, a crystal controlled oscillator need not be used as a signal
source 142 but rather a self-excited oscillator with over-critical
coupling may be provided having the characteristics of curve 152 of
FIG. 3. In effect, as provided by the present invention, tuning of
the source may be set somewhere around the 13.54 MHz range or at
any other convenient frequency, permitting deviations of the tuning
without substantially affecting the sputtering sheath potential.
Note on curve 150 the large drop off of sheath potential when the
ISM frequency of 13.56 is slightly deviated from the center
frequency as might occur with changing sputtering conditions in a
sputtering apparatus. In accordance with the present invention, an
operator need only turn on the source of RF power once the various
variable capacitances 140, 114, 124 and 74 have been initially
adjusted. Thus this apparatus is readily adapted for production
techniques utilizing unskilled labor. This provides a marked
improvement over prior art systems in that highly skilled labor is
required for those systems for the reasons discussed above.
Transformer 112 of FIG. 2 includes a primary inductance 160 and a
secondary inductance 162 and the slightly overcritical coupling may
be achieved by choosing appropriate values of the inductances 160
and 162 and the value of tuning capacitance 140 across transformer
112. Inductance 162 is concentric with inductance 160 in a
particular practical embodiment thereof and is water cooled to
prevent RF heating. This results in a loosely coupled transformer
that is primarily tuned by varying capacitance 140, when the
sputtering chamber geometry is fixed. Capacitance 140 can be tuned
once and locked into position. In one embodiment, a three kilowatt
push-pull power supply, 142, applies D.C. sheath potentials that
can be impressed across 8 inch diameter targets. The target voltage
range from -15 to -3,000 volts. This is accomplished with the
primary tuning of the source at a frequency to a little to the left
of the broad maximum tuning peak of waveform 150 of FIG. 3.
With this system, depending upon the arrangement of the internal
fixturing, a wide range of operating frequencies is possible. For
example, the operating frequency can be in the range of 3 to 8
MHz.
If bias voltages lower than -15 volts are required, the system is
first tuned to minimize voltage on the target in question, and then
the target is directly shorted to ground through a switch (not
shown). When high voltages are required on a lower target in a
particular operating system, and, simultaneously, very low voltages
are required on the upper target, it is desirable to reduce the
capacitance of capacitor 74 to its minimum to conserve power.
Ordinarily the value of capacitor 74 is fixed to balance the bridge
circuit and left at that value permanently.
In a typical sputtering process the sequence involved includes
simultaneously pre-sputtering the upper target to clean it and
sputter-etching the substrates either to clean them or to back
scatter material onto them. The voltage on the substrate target is
then reduced to the desired bias voltage level. A shutter (shield
disposed between the two electrodes in the ionization chamber) is
removed and deposition is commenced. In many different mechanical
target configurations in a practical embodiment constructed and
operated in accordance with the present invention, it has been
found possible to change the sputtering voltages from one condition
to another by simply changing the value of the series capacitor
124. To de-skill the operation for production purposes, it is
possible to use two capacitors (not shown) and a switch (not shown)
to replace capacitor 114 of FIG. 2. One of the capacitors would be
set to the target clean and sputter-etch condition, while the other
is set for the bias deposition, and the change from one condition
to the other would be by way of an RF switch (not shown). Thus the
operator would only have an on-off control, a switch and a power
control to manipulate. The system has been found to be extremely
stable. This can be shown as illustrated by the following
examples:
EXAMPLE 1
A system constructed and operated in accordance with the present
invention has been run for continuous periods of up to eight hours
each with different targets and target geometries. (See table
I).
TABLE I
__________________________________________________________________________
Target Materials, Geometry, Voltages
__________________________________________________________________________
Test V Upper V Lower Time of No. Upper Target Lower Target Target
Target Test
__________________________________________________________________________
1 8" dia., 304 Same as Upper -2000 V - 200 V 8 hrs. Stainless Steel
Target (Bonded) 2 Same as Test Same as Test -2000 V -2000 V 2 hrs.
No. 1 No. 1 3 8" dia, NiO 8" dia., 304 - 500 V - 500 V 1 hr. (Flame
Sprayed) Stainless Steel (Bonded) 4 Same as Test Same as Test -1000
V - 20 V 1 hr. No. 3 No. 3 5 Same as Test Same as Test -1000 V - 75
V 3 hrs. No. 3 No. 3 6 Same as Test Same as Test -1000 V - 150 V 3
hrs. No. 3 No. 3 7 6" dia., Mo 53/4" dia., -1000 V - 150 V 5 hrs.
(Clamped) Mo (Unclamped) 8 Co-sputtering 8" dia., 304 -1000 V - 50
V 3 hrs. 1/2 Ni Target Stainless Steel 1/2 SiO.sub.2 Target
(Bonded) (Clamped) 9 Same as Test Same as Test -1000 V - 150 V 3
hrs. No. 8 No. 8 10 Same as Test Same as Test -1000 V - 150 V 3
hrs. No. 8 No. 8
__________________________________________________________________________
In no case did the target voltages vary from those initially set up
to the limits of readibility of the voltmeters (2%). These voltages
were also stable when there were wide variations in gas pressure.
(See Table II).
TABLE II ______________________________________ Gas Pressure
Fluctuation Effects on Target Voltages; Targets, Geometry, and
Tuning Fixed ______________________________________ Argon Volts
Volts Pressure Upper Lower (Varied) (Fixed) (Fixed)
______________________________________ Initial Condition: 30
millitorr -1000 V - 200 V 40 millitorr -1000 V - 200 V 50 millitorr
-1000 V - 200 V 60 millitorr -1000 V - 200 V 70 millitorr - 980 V -
190 V 80 millitorr - 950 V - 185 V 90 millitorr - 930 V - 180 V
______________________________________
For example, the variations might occur during very severe
outgassing from very porous targets, and during the initial
sputtering of oxidized metal targets when the secondary electron
emission coefficient changes very rapidly as the oxide is sputtered
away. When either the pressure or the secondary electron yield
varies, the impedance of the discharge is changed. It is well known
that the secondary election emission yield of compounds (and
particularly, oxides) is greater than that of the constituent
elements of the compound. Thus, in a sputtering process, if a metal
target surface is oxidized, the initial sputtering will be under
conditions of high secondary electron yield. Then, as the oxide is
sputtered away, the second electron yield decreases. For most
oxides, this change can be as large as a factor of 10:1 depending
on the exact condition of the target surface.
Additional secondary electrons ejected into the glow discharge
produce more current, and so reduce the impedance of the discharge.
In contradistinction in crystal controlled systems, even small
changes in either pressure or secondary electron yield result in
gross system de-tuning.
EXAMPLE II
To provide a worst-case test reproducibility of the system, the
following experiment was performed: The upper target was tungsten
six inches in diameter, clamped. The other target was also tungsten
but was three inches in diameter and was clamped. This yielded an
asymmetric geometry. In particular a set of voltage conditions were
established as follows on the two targets and the system was run
for two hours to assure that there was no possibility that any
oxide remained on either target.
Argon Pressure: 50 millitorrs
Upper Target Voltage: -1,000 V
Lower Target Voltage: -200 V
The power and tuning controls were locked in place. Then the RF
generator was shut off and O.sub.2 was immediately bled into the
system through a pressure of 1 torr and left that way over night.
Under these conditions a relatively thick WO.sub.3 layer grows
rapidly on tungsten (W) surface. Seventeen hours later, the vacuum
chamber was pumped out and the argon sputtering gas pressure used
at the start of the test in the ionization chamber was
re-established and the generator was switched on. Exactly the same
sputtering conditions as existed at the beginning of the test were
instantly established without retuning.
For this test, capacitance 114, capacitance 124 and capacitor 74
were variable capacitances in the range of 7 to 1,000 picofarads at
5 kilovolts. The tuning capacitor 140 for the RF source was a
variable capacitor in the range of 10 to 450 picofarads at ten
kilovolts rating. Transformer coil 160 was an inductance comprising
seven turns of 3/8 inch diameter tubing, six inches in diameter and
twelve inches in length. The inductance 162 of transformer coil 112
was twenty turns of 1/4 inch diameter tubing, 3 inches in internal
diameter, 18 inches in length and was water cooled.
In FIG. 4 there is illustrated a more detailed structure of the
ionization chamber, electrodes, and connecting RF coaxial structure
described in connection with FIG. 1. In FIG. 4, like numerals refer
to like elements in FIG. 1. In FIG. 4, apparatus 200 includes
housing 14, electrodes 16 and 18, insulating mounting supports 20
and 22 and shields 28 and 30 as described previously in connection
with FIG. 1. Additionally, metal plates 21 and 23 clamp insulation
20 and 22 and the respective electrodes 16 and 18 to housing 14 as
by bolts or other suitable means, not shown. In addition, electrode
16 is shown connected to an electrode terminal 202 which is
connected to RF coaxial pipe structure center conductor 204 by way
of a braided copper conductor 206. Coolant inlet pipe 208 is
connected to a circular cavity 210 adjacent electrode 16 and
exhausted by way of exhaust pipe 212. Substrate bias electrode 18
is connected to electrode 220 which in turn is connected to
conductor 222 by way of braided conductor 224 within the hollow
pipe structure of outer conductive shield 54. Inlet water pipe 230
is connected to the circular channel 232 for cooling electrode 18.
Coolant exhaust pipe 234 is also coupled to channel 232 for
exhausting the coolant therefrom.
* * * * *