U.S. patent application number 14/800198 was filed with the patent office on 2015-11-05 for apparatus and method for sputtering hard coatings.
This patent application is currently assigned to Zond, LLC. The applicant listed for this patent is Bassam Hanna Abraham, Roman Chistyakov. Invention is credited to Bassam Hanna Abraham, Roman Chistyakov.
Application Number | 20150315697 14/800198 |
Document ID | / |
Family ID | 44081346 |
Filed Date | 2015-11-05 |
United States Patent
Application |
20150315697 |
Kind Code |
A1 |
Chistyakov; Roman ; et
al. |
November 5, 2015 |
APPARATUS AND METHOD FOR SPUTTERING HARD COATINGS
Abstract
A plasma generator includes a chamber for confining a feed gas.
An anode is positioned inside the chamber. A cathode assembly is
positioned adjacent to the anode inside the chamber. A pulsed power
supply comprising at least two solid state switches and having an
output that is electrically connected between the anode and the
cathode assembly generates voltage micropulses. A pulse width and a
duty cycle of the voltage micropulses are generated using a voltage
waveform comprising voltage oscillation having amplitudes and
frequencies that generate a strongly ionized plasma.
Inventors: |
Chistyakov; Roman; (Andover,
MA) ; Abraham; Bassam Hanna; (Millis, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Chistyakov; Roman
Abraham; Bassam Hanna |
Andover
Millis |
MA
MA |
US
US |
|
|
Assignee: |
Zond, LLC
Mansfield
MA
|
Family ID: |
44081346 |
Appl. No.: |
14/800198 |
Filed: |
July 15, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13010762 |
Jan 20, 2011 |
9123508 |
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14800198 |
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12651368 |
Dec 31, 2009 |
7898183 |
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13010762 |
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11738491 |
Apr 22, 2007 |
7663319 |
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12651368 |
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60745398 |
Apr 22, 2006 |
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Current U.S.
Class: |
204/192.15 ;
204/192.12 |
Current CPC
Class: |
H01J 2237/0206 20130101;
C23C 14/564 20130101; H01J 37/3408 20130101; C23C 14/3414 20130101;
C23C 14/35 20130101; H01J 37/32018 20130101; C23C 14/3485 20130101;
H01J 37/32009 20130101; H01J 37/3444 20130101 |
International
Class: |
C23C 14/34 20060101
C23C014/34; C23C 14/35 20060101 C23C014/35 |
Claims
1-13. (canceled)
14. A method of sputtering a thin films, the method comprising: a.
supplying a feed gas proximate to an anode and a cathode assembly
comprising a magnet assembly and a sputtering target; and b.
generating a pulsed voltage waveform between the anode and the
cathode assembly, the pulsed voltage waveform comprising voltage
oscillations having an amplitude, a frequency, and a duration that
generates a magnetron discharge with oscillating discharge current,
wherein the amplitude and the duration of the voltage oscillations
are chosen to generate an arc free magnetron discharge during a
pulsed voltage waveform, a frequency of the voltage oscillations
being chosen to increase an ionization rate of sputtered target
material atoms; and c. applying a negative bias voltage to a
substrate holder supporting a substrate, the negative bias voltage
attracting positive target material ions to the substrate, wherein
the negative voltage and the frequency of the voltage oscillations
are chosen to achieve a desired hardness of the sputtering
film.
15. The method of claim 14, further comprising moving the magnetic
field.
16. The method of claim 14, wherein the desired hardness of the
sputtered film is in a range of 20-60 GPa.
17. The method of claim 16, wherein the substrate comprises a
cutting edge of a razor blade.
18. The method of claim 14, wherein amplitude of the oscillating
discharge current is in a range of 100 A-5000 A.
19. The method of claim 14, wherein the oscillating discharge
current generates a current density on the target in the range of
0.1 A/cm.sup.2-10 A/cm.sup.2
20. The method of claim 14, wherein the feed gas comprises a noble
gas.
21. The method of claim 14, wherein the sputtering target comprises
at least one of Al, Cu, Ti, C, Ta, Mo, Ni, V, Si, B, In, Sn, W, Cr,
Fe, B, Zr, Au, Ag, Pt, Re, Co, or Zn.
22. The method of claim 14, wherein the target material comprises
carbon.
23. The method of claim 14, wherein the voltage oscillations begin
from positive value.
24. The method of claim 22, wherein the desired hardness of the
sputtered carbon coating is in a range of 20-60 GPa.
25. The method of claim 14, wherein the sputtering target comprises
at least one of Al, Cu, Ti, C, Ta, Mo, Ni, V, Si, B, In, Sn, W, Cr,
Fe, B, Zr, Au, Ag, Pt, Re, Co, or Zn.
26. The method of claim 14, wherein the frequency of the voltage
oscillations is in a range of 1-1,000 kHz.
27. The method of claim 14, wherein an absolute value of at least
one peak voltage of the voltage oscillation in the voltage pulse is
in a range of 500 V and 3000 V.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This patent application is a continuation-in-part of U.S.
patent application Ser. No. 12/651,368, filed Dec. 31, 2009,
entitled "Methods and Apparatus for Generating Strongly-Ionized
Plasmas with Ionizational Instabilites," which is a continuation of
U.S. patent application Ser. No. 11/738,491, filed Apr. 22, 2007,
entitled "Methods and Apparatus for Generating Strongly-Ionized
Plasmas with Ionizational Instabilites," now U.S. Pat. No.
7,663,319 which claims priority to U.S. Provisional Patent
Application 60/745,398, filed Apr. 22, 2006, and which is a
continuation-in-part of U.S. patent application Ser. No.
11/376,036, filed Mar. 15, 2006, entitled "Methods and Apparatus
for Generating Strongly-Ionized Plasmas with Ionizational
Instabilites," now U.S. Pat. No. 7,345,429, which is a continuation
of U.S. patent application Ser. No. 10/708,281 filed Feb. 22, 2004,
entitled "Methods and Apparatus for Generating Strongly-Ionized
Plasmas with Ionizational Instabilites," now U.S. Pat. No.
7,095,179. This application also claims priority to U.S.
Provisional Application No. 61/297,263 filed on Jan. 21, 2010,
entitled "Methods of Generating an Arc-Free Discharge for Reactive
Sputtering." The entire disclosure of these patent applications and
patents are incorporated herein by reference. In addition, U.S.
patent application Ser. No. 11/162,824, filed Sep. 23, 2005,
entitled "Apparatus for Generating High Current Electrical
Discharges;" U.S. patent application Ser. No. 12/819,914, filed
Jun. 21, 2010, entitled "High Power Pulse Magnetron Sputtering for
High Aspect-Ratio Features, Vias, and Trenches;" and U.S. patent
application Ser. No. 11/608,833, filed Dec. 10, 2006, "entitled
High Power Pulsed Magnetron Sputtering," which are all assigned to
the present assignee are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] A plasma can be created in a chamber by igniting a direct
current (DC) electrical discharge between two electrodes in the
presence of a feed gas. The electrical discharge generates
electrons in the feed gas that ionize atoms thereby creating the
plasma. The electrons in the plasma provide a path for an electric
current to pass through the plasma. The energy supplied to the
plasma must be relatively high for applications, such as magnetron
plasma sputtering. Applying high electrical currents through a
plasma can result in overheating the electrodes as well as
overheating the work piece in the chamber. Complex cooling
mechanisms can be used to cool the electrodes and the work piece.
However, the cooling can cause temperature gradients in the
chamber. These temperature gradients can cause non-uniformities in
the plasma density which can cause non-uniform plasma process.
[0003] Temperature gradients can be reduced by pulsing DC power to
the electrodes. Pulsing the DC power can allow the use of lower
average power. This results in a lower temperature plasma process.
However, pulsed DC power systems are prone to arcing at plasma
ignition and plasma termination, especially when working with
high-power pulses. Arcing can result in the release of undesirable
particles in the chamber that can contaminate the work piece.
[0004] Plasma density in known plasma systems is typically
increased by increasing the electrode voltage. The increased
electrode voltage increases the discharge current and thus the
plasma density. However, the electrode voltage is limited in many
applications because high electrode voltages can effect the
properties of films being deposited or etched. In addition, high
electrode voltages can also cause arcing which can damage the
electrode and contaminate the work piece.
BRIEF DESCRIPTION OF DRAWINGS
[0005] This invention is described with particularity in the
detailed description and claims. The above and further advantages
of this invention may be better understood by referring to the
following description in conjunction with the accompanying
drawings, in which like numerals indicate like structural elements
and features in various figures. The drawings are not necessarily
to scale, emphasis instead being placed upon illustrating the
principles of the invention.
[0006] FIG. 1 illustrates a cross-sectional view of a plasma
sputtering apparatus having a pulsed direct current (DC) power
supply according to one embodiment of the invention.
[0007] FIG. 2 is measured data of discharge voltage as a function
of discharge current for a prior art low-current plasma and a
high-current plasma according to the present invention.
[0008] FIG. 3 is measured data of a particular voltage pulse
generated by the pulsed power supply of FIG. 1 operating in a
low-power voltage mode.
[0009] FIG. 4 is measured data of a multi-stage voltage pulse that
is generated by the pulsed power supply of FIG. 1 that creates a
strongly-ionized plasma according to the present invention.
[0010] FIG. 5A-FIG. 5C are measured data of other illustrative
multi-stage voltage pulses generated by the pulsed power supply of
FIG. 1.
[0011] FIG. 6A and FIG. 6B are measured data of multi-stage voltage
pulses generated by the pulsed power supply of FIG. 1 that
illustrate the effect of pulse duration in the transient stage of
the pulse on the plasma discharge current.
[0012] FIG. 7A and FIG. 7B are measured data of multi-stage voltage
pulses generated by the pulsed power supply of FIG. 1 that show the
effect of the pulsed power supply operating mode on the plasma
discharge current.
[0013] FIG. 8 is measured data for an exemplary single-stage
voltage pulse generated by the pulsed power supply of FIG. 1 that
produces a high-density plasma according to the invention that is
useful for high-deposition rate sputtering.
[0014] FIG. 9 illustrates a cross-sectional view of a plasma
sputtering apparatus having a pulsed direct current (DC) power
supply according to another embodiment of the invention.
[0015] FIG. 10A illustrates a schematic diagram of a pulsed power
supply that can generate multi-step voltage pulses according to the
present invention.
[0016] FIG. 10B shows a multi-step output voltage waveform and the
corresponding micropulse voltage waveforms that are generated by
switches and controlled by the drivers and the controller.
[0017] FIG. 11 illustrates a schematic diagram of a pulsed power
supply having a magnetic compression network for supplying
high-power pulses.
[0018] FIG. 12 illustrates a schematic diagram of a pulsed power
supply having a Blumlein generator for supplying high-power
pulses.
[0019] FIG. 13 illustrates a schematic diagram of a pulsed power
supply having a pulse cascade generator for supplying high-power
pulses.
DETAILED DESCRIPTION
[0020] FIG. 1 illustrates a cross-sectional view of a plasma
sputtering apparatus 100 having a pulsed direct current (DC) power
supply 102 according to one embodiment of the invention. The plasma
sputtering apparatus 100 includes a vacuum chamber 104 for
containing a plasma. The vacuum chamber 104 can be coupled to
ground 105. The vacuum chamber 104 is positioned in fluid
communication with a vacuum pump 106 that is used to evacuate the
vacuum chamber 104 to high vacuum. The pressure inside the vacuum
chamber 104 is generally less than 10.sup.-1 Torr for most plasma
operating conditions. A process or feed gas 108 is introduced into
the vacuum chamber 104 through a gas inlet 112 from a feed gas
source 110, such as an argon gas source. The flow of the feed gas
is controlled by a valve 114. In some embodiments, the gas source
is an excited atom or metastable atom source.
[0021] The plasma sputtering apparatus 100 also includes a cathode
assembly 116. The cathode assembly 116 shown in FIG. 1 is formed in
the shape of a circular disk, but can be formed in other shapes. In
some embodiments, the cathode assembly 116 includes a target 118
for sputtering. The cathode assembly 116 is electrically connected
to a first terminal 120 of the pulsed power supply 102 with an
electrical transmission line 122.
[0022] A ring-shaped anode 124 is positioned in the vacuum chamber
104 proximate to the cathode assembly 116. The anode 124 is
electrically connected to ground 105. A second terminal 125 of the
pulsed power supply 102 is also electrically connected to ground
105. In other embodiments, the anode 124 is electrically connected
to the second terminal 125 of the pulsed power supply 102 which is
not at ground potential.
[0023] A housing 126 surrounds the cathode assembly 116. The anode
124 can be integrated with or electrically connected to the housing
126. The outer edge 127 of the cathode assembly 116 is electrically
isolated from the housing 126 with insulators 128. The gap 129
between the outer edge 127 of the cathode assembly 116 and the
housing 126 can be an air gap or can include a dielectric
material.
[0024] In some embodiments, the plasma sputtering apparatus 100
includes a magnet assembly 130 that generates a magnetic field 132
proximate to the target 118. The magnetic field 132 is less
parallel to the surface of the cathode assembly 116 at the poles of
the magnets in the magnet assembly 130 and more parallel to the
surface of the cathode assembly 116 in the region 134 between the
poles of the magnets in the magnetic assembly 130. The magnetic
field 132 is shaped to trap and concentrate secondary electrons
emitted from the target 118 that are proximate to the target
surface 133. The magnet assembly can consist of rotating
magnets.
[0025] The magnetic field 132 increases the density of electrons
and therefore, increases the plasma density in the region 134 that
is proximate to the target surface 133. The magnetic field 132 can
also induce an electron Hall current 135 that is formed by the
crossed electric and magnetic fields. The strength of the electron
Hall current 135 depends, at least in part, on the density of the
plasma and the strength of the crossed electric and magnetic
fields.
[0026] The plasma sputtering apparatus 100 also includes a
substrate support 136 that holds a substrate 138 or other work
piece for plasma processing. In some embodiments, the substrate
support 136 is biased with a RF field. In these embodiments, the
substrate support 136 is electrically connected to an output 140 of
a RF power supply 142 with an electrical transmission line 144. A
matching network (not shown) may be used to coupled the RF power
supply 142 to the substrate support 136. In some embodiments, a
temperature controller 148 is thermally coupled to the substrate
support 136. The temperature controller 148 regulates the
temperature of the substrate 138.
[0027] In some embodiments, the plasma sputtering apparatus 100
includes an energy storage device 147 that provides a source of
energy that can be controllably released into the plasma. The
energy storage device 147 is electrically coupled to the cathode
assembly 116. In one embodiment, the energy storage device 147
includes a capacitor bank.
[0028] In some embodiments, the plasma sputtering apparatus 100
includes an arc control circuit 151 that is used to prevent
undesirable arc discharges. The arc control circuit 151 includes a
detection means that detects the onset of an arc discharge and then
sends a signal to a control device that deactivates the output of
the power supply 102 for some period of time. The probability that
a magnetron discharge will transfer to an arc discharge is high
under some processing conditions. For example, the probability that
a magnetron discharge will transfer to an arc discharge is high for
some reactive sputtering processes which use feed gases containing
at least one a reactive gas. Arc discharges are generally
undesirable because they can create particles that can damage the
sputtered film.
[0029] In operation, the vacuum pump 106 evacuates the chamber 104
to the desired operating pressure. The feed gas source 110 injects
feed gas 108 into the chamber 104 through the gas inlet 112. The
pulsed power supply 102 applies voltage pulses to the cathode
assembly 116 that cause an electric field 149 to develop between
the target 118 and the anode 124. The magnitude, duration and rise
time of the initial voltage pulse are chosen such that the
resulting electric field 149 ionizes the feed gas 108, thus
igniting the plasma in the chamber 104.
[0030] In one embodiment, ignition of the plasma is enhanced by one
or more methods described in co-pending U.S. patent application
Ser. No. 10/065,277, entitled High-Power Pulsed Magnetron
Sputtering, and co-pending U.S. patent application Ser. No.
10/065,629, entitled Methods and Apparatus for Generating
High-Density Plasma which are assigned to the present assignee. The
entire disclosures of U.S. patent application Ser. No. 10/065,277
and U.S. patent application Ser. No. 10/065,629 are incorporated
herein by reference. U.S. patent application Ser. No. 10/065,629
describes a method of accelerating the ignition of the plasma by
increasing the feed gas pressure for a short period of time and/or
flowing feed gas directly through a gap between an anode and a
cathode assembly. In addition, U.S. patent application Ser. No.
10/065,277 describes a method of using pre-ionization electrodes to
accelerate the ignition of the plasma.
[0031] The characteristics of the voltage pulses generated by the
pulsed power supply 102 and the resulting plasmas are discussed in
connection with the following figures. The pulsed power supply 102
can include circuitry that minimizes or eliminates the probability
of arcing in the chamber 104. Arcing is generally undesirable
because it can damage the anode 124 and cathode assembly 116 and
can contaminate the wafer or work piece being processed. In one
embodiment, the circuitry of the pulse supply 102 limits the plasma
discharge current up to a certain level, and if this limit is
exceeded, the voltage generated by the power supply 102 drops for a
certain period of time.
[0032] The plasma is maintained by electrons generated by the
electric field 149 and also by secondary electron emission from the
target 118. In embodiments including the magnet assembly 130, the
magnetic field 132 is generated proximate to the target surface
133. The magnetic field 132 confines the primary and secondary
electrons in a region 134 thereby concentrating the plasma in the
region 134. The magnetic field 132 also induces the electron Hall
current 135 proximate to the target surface 133 that further
confines the plasma in the region 134.
[0033] In one embodiment, the magnet assembly 130 includes an
electromagnet in addition to a permanent magnet. A magnet power
supply (not shown) is electrically connected to the magnetic
assembly 130. The magnet power supply can generate a constant
current that generates a constant magnetic filed. Alternatively,
the magnet power supply can generate a pulse that produces a pulsed
magnetic field that creates an increase in electron Hall current
135 proximate to the target surface 133 that further confines the
plasma in the region 134. In one embodiment, the pulsing of the
magnetic field is synchronized with the pulsing the electric field
in the plasma discharge in order to increase the density of the
plasma. The sudden increase in the electron Hall current 135 may
create a transient non-steady state plasma.
[0034] Ions in the plasma bombard the target surface 133 because
the target 118 is negatively biased. The impact caused by the ions
bombarding the target surface 133 dislodges or sputters material
from the target 118. The sputtering rate generally increases as the
density of the plasma increases.
[0035] The RF power supply 142 can apply a negative RF bias voltage
to the substrate 138 that attracts positively ionized sputtered
material to the substrate 138. The sputtered material forms a film
of target material on the substrate 138. The magnitude of the RF
bias voltage on the substrate 138 can be chosen to optimize
parameters, such as sputtering rate and adhesion of the sputtered
film to the substrate 138. The magnitude of the RF bias voltage on
the substrate 138 can also be chosen to minimize damage to the
substrate 138. In embodiments including the temperature controller
148, the temperature of the substrate 138 can be regulated by the
temperature controller 148 in order to avoid overheating the
substrate 138.
[0036] Although FIG. 1 illustrates a cross-sectional view of a
plasma sputtering apparatus 100, it will be clear to skilled
artisans that the principles of the present invention can be used
in many other systems, such as plasma etching systems, hollow
cathode magnetrons, ion beam generators, plasma-enhanced chemical
vapor deposition (CVD) systems, plasma accelerators, plasma rocket
thrusters, plasma traps, and any plasma system that uses crossed
electric and magnetic fields.
[0037] FIG. 2 is measured data 150 of discharge voltage as a
function of discharge current for a prior art low-current plasma
and a high-current plasma according to the present invention.
Current-voltage characteristic 152 represents measured data for
discharge voltage as a function of discharge current for a plasma
generated in a typical commercial magnetron plasma system with a
commercially available DC power supply. The actual magnetron plasma
system used to obtain the current-voltage characteristics 152 was a
standard magnetron with a 10 cm diameter copper sputtering target.
Similar results have been observed for a NiV sputtering target.
Argon was used as the feed gas and the operating pressure was about
1 mTorr. The current-voltage characteristic 152 illustrates that
discharge current increases with voltage.
[0038] The current-voltage characteristic 152 for the same
magnetron plasma system generates a relatively low or moderate
plasma density (less than 10.sup.12-10.sup.13 cm.sup.-3, measured
close to the cathode/target surface) in a low-current regime. The
plasma density in the low-current regime is relatively low because
the plasma is mainly generated by direct ionization of ground state
atoms in the feed gas. The term "low-current regime" is defined
herein to mean the range of plasma discharge current densities that
are less than about 0.5 A/cm.sup.2 for typical sputtering voltages
of between about -300V to -1000V. The power density is less than
about 250 W/cm.sup.2 for plasmas in the low-current regime.
Sputtering with discharge voltages greater than -800V can be
undesirable because such high voltages can increase the probability
of arcing and can tend to create sputtered films having relatively
poor film quality.
[0039] The current-voltage characteristic 154 represents actual
data for a plasma generated by the pulsed power supply 102 in the
plasma sputtering system 100 of FIG. 1. The current-voltage
characteristic 154 illustrates that the discharge current is about
140 A (.about.1.8 A/cm.sup.2) at a voltage of about -500V. The
discharge current is about 220 A (.about.2.7 A/cm.sup.2) when the
voltage is about -575V. The data depends on various parameters,
such as the magnitude and geometry of the magnetic field, chamber
pressure, gas flow rate, pumping speed, and the design of the
pulsed power supply 102. For certain operating conditions, the
discharge current can exceed 375 A with a discharge voltage of only
-500V.
[0040] The voltage-current characteristic 154 is in a high-current
regime. The current-voltage characteristic 154 generates a
relatively high plasma density (greater than 10.sup.12-10.sup.13
cm.sup.-3) in the high-current regime. The term "high-current
regime" is defined herein to mean the range of plasma discharge
currents that are greater than about 0.5 A/cm.sup.2 for typical
sputtering voltages of between about -300V to -1000V. The power
density is greater than about 250 W/cm.sup.2 for plasmas in the
high-current regime. The voltage-current characteristic 154
generates high-density plasmas that can be used for high-deposition
rate magnetron sputtering.
[0041] Some known magnetron systems operate within the high-current
regime for very short periods of time. However, these known
magnetron systems cannot sustain and control operation in the
high-current regime for long enough periods of time to perform any
useful plasma processing. The pulsed power supply 102 of the
present invention is designed to generate waveforms that create and
sustain the high-density plasma with current-voltage
characteristics in the high-current regime.
[0042] FIG. 3 is measured data 200 of a particular voltage pulse
202 generated by the pulsed power supply 102 of FIG. 1 operating in
a low-power voltage mode. The pulsed power supply 102 produces a
weakly-ionized plasma having a low or moderate plasma density (less
than 10.sup.1210.sup.13 cm.sup.-3) that is typical of known plasma
processing systems. The pulsed power supply 102 is operating in a
low-power mode throughout the duration of the voltage pulse 202.
The pulsed power supply 102 supplies energy to the plasma at a
relatively slow rate in the low-power mode. The energy supplied by
the pulsed power supply 102 in the low-power mode generates a
weakly-ionized plasma by direct ionization of the ground state
atoms in the feed gas. The weakly-ionized plasma corresponds to a
plasma generated by a conventional DC magnetron.
[0043] The pulsed power supply 102 can be programmed to generate
voltage pulses having various shapes. The desired voltage pulse of
FIG. 3 is a square wave voltage pulse as shown by the dotted line
203. However, the actual voltage pulse 202 generated by the pulsed
power supply 102 is not perfectly square, but instead includes low
frequency oscillations that are inherent to the power supply 102.
Some of these low frequency oscillations can be on the order of 50V
or more. In addition, the voltage pulse 202 has an initial value
204 of about -115V that is caused by the charge accumulation on the
cathode assembly 116 for a particular repetition rate.
[0044] The voltage pulse 202 includes an ignition stage 205 that is
characterized by a voltage 206 having a magnitude and a rise time
that is sufficient to ignite a plasma from a feed gas. The
magnitude of the voltage pulse 202 rises to about 550V in the
ignition stage 205. However, the voltage of the first pulse that
initially ignites the plasma can be as high as -1500V. The ignition
of the plasma is depicted as a rise in a discharge current 208
through the plasma. The duration of the ignition stage 205 is
generally less than about 150 .mu.sec. After the ignition stage
205, the discharge current 208 continues to rise even as the
voltage 210 decreases.
[0045] The rise in the discharge current 208 is caused at least in
part by the interaction of the pulsed power supply 102 with the
developing plasma. The impedance of the plasma decreases as the
current density in the plasma increases. The pulsed power supply
102 attempts to maintain a constant voltage, but the voltage
decreases due to the changing plasma resistive load. The peak
discharge current 212 is less than about 50 A with a voltage 214
that is about -450V. The power 216 that is present at the peak
discharge current 212, which corresponds to a momentary peak
density of the plasma, is about 23 kW.
[0046] As the voltage 218 continues to decrease, the discharge
current 220 and the plasma density also decrease. As the density of
the plasma decreases, the impedance of the plasma increases. The
voltage level 222 corresponds to a quasi-static discharge current
224 that is substantially constant throughout the duration of the
voltage pulse 202. This region of quasi-static discharge current
224 is caused by the plasma having a substantially constant
resistive load. The term "substantially constant" when applied to
discharge current is defined herein to mean a discharge current
with less than a 10% variation.
[0047] After about 200 .mu.sec the oscillations dampen as the
voltage 226 fluctuates between about -525V and -575V, the discharge
current 228 remains constant with a value of about 25 A and the
power 230 is between about 10-15 kW. These conditions correspond to
a weakly-ionized or low-density plasma that is typical of most
plasma processing systems, such as the conditions represented by
the current-voltage characteristic 152 described in connection with
FIG. 2. The plasma density is in the range of about
10.sup.8-10.sup.13 cm.sup.-3.
[0048] The total duration of the voltage pulse 202 is about 1.0
msec. The next voltage pulse (not shown) will typically include an
ignition stage 205 in order to re-ignite the plasma. However,
electrons generated from the first pulse can still be present so
the required ignition voltage will typically be much less than the
first pulse (on the order of about -600V) and the ignition will
typically be much faster (on the order of less than about 200
.mu.sec).
[0049] FIG. 4 is measured data 250 of a multi-stage voltage pulse
252 that is generated by the pulsed power supply of FIG. 1 that
creates a strongly-ionized plasma according to the present
invention. The measured data 250 is from a magnetron sputtering
system that includes a 10 cm diameter NiV target with an argon feed
gas at a pressure of about 10.sup.-3 Torr. The multi-stage voltage
pulse 252 generates a weakly-ionized plasma in the low-current
regime (FIG. 2) initially, and then eventually generates a
strongly-ionized or high-density plasma in the high-current regime
according to the present invention. Weakly-ionized plasmas are
generally plasmas having plasma densities that are less than about
10.sup.12-10.sup.13 cm.sup.-3 and strongly-ionized plasmas are
generally plasmas having plasma densities that are greater than
about 10.sup.12-10.sup.13 cm.sup.-3. The multi-stage voltage pulse
252 is presented to illustrate the present invention. One skilled
in the art will appreciate that there are numerous variations of
the exact shape of the multi-stage pulse according to the present
invention.
[0050] The multi-stage voltage pulse 252 is a single voltage pulse
having multiple stages as illustrated by the dotted line 253. An
ignition stage 254 of the voltage pulse 252 corresponds to a
voltage 256 having a magnitude (on the order of about -600V) and a
rise time (on the order of about 4V/.mu.sec) that is sufficient to
ignite an initial plasma from a feed gas. The initial plasma is
typically ignited in less than 200 .mu.sec.
[0051] A first low-power stage 258 of the voltage pulse 252 has a
peak voltage 260 that corresponds to a discharge current 261 in the
developing initial plasma. In some embodiments, the ignition stage
254 is integrated into the first low-power stage 258 such that the
plasma is ignited during the first low-power stage 258. The peak
voltage 260 is about -600V and can range from -300V to -1000V, the
corresponding discharge current 261 is about 20 A, and the
corresponding power is about 12 kW. In the first low-power stage
258, the pulsed power supply 102 (FIG. 1) is operating in the
low-power mode. In the low power mode, the pulsed power supply 102
supplies energy to the initial plasma at a relatively slow rate.
The slow rate of energy supplied to the initial plasma in the
low-power mode maintains the plasma in a weakly-ionized
condition.
[0052] The weakly-ionized or pre-ionized condition corresponds to
an initial plasma having a relatively low (typically less than
10.sup.12-10.sup.13 cm.sup.-3) plasma density. As the density of
the initial plasma grows, the voltage 262 decreases by about 50V as
the current 261 continues to rise to about 30 A before remaining
substantially constant for about 200 .mu.sec. The discharge current
261 rises as the voltage 262 decreases because of the changing
impedance of the plasma. As the plasma density changes, the
impedance of the plasma and thus the load seen by the pulsed power
supply 102 also changes. In addition, the initial plasma can draw
energy from the pulsed power supply 102 at a rate that is faster
than the response time of the pulsed power supply 102 thereby
causing the voltage 262 to decrease.
[0053] The impedance of the plasma decreases when the number of
ions and electrons in the plasma increases as the current density
in the initial plasma increases. The increase in the number of ions
and electrons decreases the value of the plasma load. The pulsed
power supply 102 attempts to maintain a constant voltage. However,
the voltage 262 continues to decrease, at least in part, because of
the changing plasma load. The substantially constant discharge
current corresponds to a conventional DC magnetron discharge
current as discussed in connection with current-voltage
characteristic 152 of FIG. 2. The initial plasma can correspond to
a plasma that is in a steady state or a quasi-steady state
condition.
[0054] The peak plasma density can be controlled by controlling the
slope of the rise time of the voltage pulse 252. In a first
transient stage 264 of the voltage pulse 252, the voltage increase
is characterized by a relatively slow rise time (on the order of
about 2.8V/.mu.sec) that is sufficient to only moderately increase
the plasma density. The plasma density increases moderately because
the magnitude and the rise time of the voltage 266 in the first
transient stage 264 is not sufficient to energize the electrons in
the plasma to significantly increase an electron energy
distribution in the plasma. An increase in the electron energy
distribution in the plasma can generate ionizational instabilities
that rapidly increase the ionization rate of the plasma. The
electron energy distribution and the ionizational instabilities are
discussed in more detail with respect to generating a
strongly-ionized plasma according to the invention.
[0055] The moderate increase in the plasma density will result in a
current-voltage characteristic that is similar to the
current-voltage characteristic 152 of a conventional DC magnetron
that was described in connection with FIG. 2. The voltage 266
increases by about 50V to a voltage peak 268 of about -650V. The
discharge current 270 increases by about 20 A to about 50 A and the
power increases to about 30 kW. The pulsed power supply 102 is
still operating in the low-power mode during the first transient
stage 264.
[0056] In a second low-power stage 272 of the voltage pulse 252,
the voltage 274 increases slowly by about 40V. The slow voltage
increase is characterized by a discharge current 276 that remains
substantially constant for about 350 .mu.sec. The plasma can be
substantially in a steady state or a quasi-steady state condition
corresponding to the current-voltage characteristic 152 of FIG. 2
during the second low-power stage 272. The plasma density in the
second low-power stage 272 is greater than the plasma density in
the first low-power stage 258, but is still only weakly-ionized.
The pulsed power supply 102 is operating in the low-power mode.
[0057] In a second transient stage 278 of the voltage pulse 252,
the pulsed power supply 102 operates in the high-power mode. In
this second transient stage 278, the voltage 280 increases sharply
compared with the first transient stage 264. The rise time of the
voltage 280 is greater than about 0.5V/.mu.sec. The voltage
increase is about 60V to the peak voltage. The relatively fast rise
time (on the order of about 5V/.mu.sec) of the voltage 280 and the
corresponding energy supplied by the pulsed power supply 102 shifts
the electron energy distribution in the weakly-ionized plasma to
higher energies. The higher energy electrons rapidly ionize the
atoms in the plasma and create ionizational instability in the
plasma that drives the weakly-ionized plasma to a non-steady state
condition or a transient state. In a non-steady state, the
Boltzman, Maxwell, and Saha distributions can be modified. The
rapid increase in ionization of the atoms in the plasma results in
a rapid increase in electron density and a formation of the
strongly-ionized plasma that is characterized by a significant rise
in the discharge current 282. The discharge current 282 rises to
about 250 A at a non-linear rate for about 250 .mu.sec.
[0058] One mechanism that contributes to a sharp increase in the
electron energy distribution is known as diocotron instability.
Diocotron instability is a wave phenomena that relates to the
behavior of electron density gradients in the presence of electric
and magnetic fields. Electron electrostatic waves can propagate
along and across (parallel to and perpendicular to) field lines
with different frequencies. These electron electrostatic waves can
create electron drifts in the presence of a perpendicular electric
field that are perpendicular to magnetic field lines.
[0059] Such electron drifts are inherently unstable, since any
departure from charge neutrality in the form of charge bunching and
separation (over distances on the order of the characteristic
length scale in a plasma, the Debye length) create electric fields
which cause second order ExB drifts that can exacerbate the
perturbation. These instabilities are referred to as gradient-drift
and neutral-drag instabilities. A charge perturbation associated
with an electron Hall current developed by crossed magnetic and
electric fields can produce radial electron drift waves. Drifts
driven by the two density gradients (perpendicular and parallel)
associated with a maximum in the radial electron density
distribution can interact to cause the diocotron instability.
Diocotron instability is described in "Magnetron Sputtering: Basic
Physics and Application to Cylindrical Magnetrons" by John A.
Thorton., J. Voc. Sci. Technol. 15(2), March/April p. 171-177,
1978.
[0060] A high-power stage 283 includes voltage oscillations 284
that have peak-to-peak amplitudes that are on the order of about
50V. These "saw tooth" voltage oscillations 284 may be caused by
the electron density forming a soliton waveform or having another
non-linear mechanism, such as diocotron instability discussed
above, that increases the electron density as indicated by the
increasing discharge current 286. The soliton waveform or other
non-linear mechanism may also help to sustain the high-density
plasma throughout the duration of the voltage pulse 252. Soliton
waveforms, in particular, have relatively long lifetimes.
[0061] The discharge current 286 increases non-linearly through the
high-power stage 283 until a condition corresponding to the
voltage-current characteristic 154 of FIG. 2 is reached. This
condition corresponds to the point in which the pulsed power supply
102 is supplying an adequate amount of continuous power to sustain
the strongly-ionized plasma at a constant rate as illustrated by a
substantially constant discharge current 287. The peak discharge
current 288 in the high-power stage 283 is about 250 A at a voltage
290 of about -750V. The corresponding peak power 292 is about 190
kW.
[0062] The voltage pulse 252 is terminated at about 1.24 msec. The
cathode assembly 116 remains negatively biased at about -300V after
the termination of the voltage pulse 252. The plasma then rapidly
decays as indicated by the rapidly decreasing discharge current
294.
[0063] The high-power stage 283 of the voltage pulse is sufficient
to drive the plasma from a non-steady state in the second transient
stage 278 to a strongly-ionized state corresponding to the
voltage-current characteristic 154 of FIG. 2. The pulsed power
supply 102 must supply a sufficient amount of uninterrupted power
to continuously drive the initial plasma in the weakly-ionized
state (in the second low-power stage 272) through the transient
non-steady state (in the second transient stage 278) to the
strongly-ionized state (in the high-power stage 283). The rise time
of the voltage 280 in the second transient stage 278 is chosen to
be sharp enough to shift the electron energy distribution of the
initial plasma to higher energy levels to generate ionizational
instabilities that creates many excited and ionized atoms. The rise
time of the voltage 280 is greater than about 0.5V/.mu.sec.
[0064] The magnitude of the voltage 280 in the second transient
stage 278 is chosen to generate a strong enough electric field
between the target 118 and the anode 124 (FIG. 1) to shift the
electron energy distribution to high energies. The higher electron
energies create excitation, ionization, and recombination processes
that transition the state of the weakly-ionized plasma to the
strongly-ionized state. The transient non-steady state plasma state
exists for a time period during the second transient stage 278. The
transient state results from plasma instabilities that occur
because of mechanisms, such as increasing electron temperature
caused by ExB Hall currents. Some of these plasma instabilities are
discussed herein.
[0065] The strong electric field generated by the voltage 280
between the target 118 and the anode 124 (FIG. 1) causes several
ionization processes. The strong electric field causes some direct
ionization of ground state atoms in the weakly-ionized plasma.
There are many ground state atoms in the weakly-ionized plasma
because of its relatively low level of ionization. In addition, the
strong electric field heats electrons initiating several other
different type of ionization process, such as electron impact,
Penning ionization, and associative ionization. Plasma radiation
can also assist in the formation and maintenance of the high
current discharge. The direct and other ionization processes of the
ground state atoms in the weakly-ionized plasma significantly
increase the rate at which a strongly-ionized plasma is formed.
[0066] In one embodiment, the ionization process is a multi-stage
ionization process. The multi-stage voltage pulse 252 initially
raises the energy of the ground state atoms in the weakly-ionized
plasma to a level where the atoms are excited. For example, argon
atoms require an energy of about 11.55 eV to become excited. The
magnitude and rise time of the voltage 280 is then chosen to create
a strong electric field that ionizes the exited atoms. Excited
atoms ionize at a much higher rate than neutral atoms. For example,
Argon excited atoms only require about 4 eV of energy to ionize
while neutral atoms require about 15.76 eV of energy to ionize. The
multi-step ionization process is described in co-pending U.S.
patent application Ser. No. 10/249,844, entitled High-Density
Plasma Source using Excited Atoms which is assigned to the present
assignee. The entire disclosure of U.S. patent application Ser. No.
10/249,844 is incorporated herein by reference.
[0067] The multi-step ionization process can be described as
follows:
Ar+e.sup.-.fwdarw.Ar.sup.*+e.sup.-
Ar.sup.*+e.sup.-.fwdarw.Ar.sup.++2e.sup.-
where Ar represents a neutral argon atom in the initial plasma,
e.sup.- represents an ionizing electron generated in response to an
electric field, and Ar.sup.* represents an excited argon atom in
the initial plasma. The collision between the excited argon atom
and the ionizing electron results in the formation of an argon ion
(Ar.sup.+) and two electrons.
[0068] In one embodiment, ions in the developing plasma strike the
target 118 causing secondary electron emission. These secondary
electrons interact with neutral or excited atoms in the developing
plasma. The interaction of the secondary electrons with the neutral
or excited atoms further increases the density of ions in the
developing plasma as the feed gas 108 is replenished. Thus, the
excited atoms tend to more rapidly ionize near the target surface
133 (FIG. 1) than the neutral argon atoms. As the density of the
excited atoms in the plasma increases, the efficiency of the
ionization process rapidly increases. The increased efficiency can
result in an avalanche-like increase in the density of the plasma
thereby creating a strongly-ionized plasma.
[0069] In one embodiment, the magnet assembly 130 generates a
magnetic field 132 proximate to the target 118 that is sufficient
to generate an electron ExB Hall current 135 (FIG. 1) which causes
the electron density in the plasma to form a soliton or other
non-linear waveform that increases at least one of the density and
lifetime of the plasma as previously discussed. In some
embodiments, the strength of the magnetic field 132 required to
cause the electron density in the plasma to form such a soliton or
non-linear waveform is in the range of fifty to ten thousand
gauss.
[0070] An electron ExB Hall current 135 is generated when the
voltage pulse 252 applied between the target 118 and the anode 124
generates primary electrons and secondary electrons that move in a
substantially circular motion proximate to the target 118 according
to crossed electric and magnetic fields. The magnitude of the
electron ExB Hall current 135 is proportional to the magnitude of
the discharge current in the plasma. In some embodiments, the
electron ExB Hall current 135 is approximately in the range of
three to ten times the magnitude of the discharge current.
[0071] The electron ExB Hall current 135 defines a substantially
circular shape when the plasma density is relatively low. The
substantially circular electron ExB Hall current 135 tends to form
a more complex shape as the current density of the plasma
increases. The shape is more complex because of the electron ExB
Hall current 135 generates its own magnetic field that interacts
with the magnetic field generated by the magnet assembly 130 and
the electric field generated by the voltage pulse 252. In some
embodiments, the electron ExB Hall current 135 becomes cycloidal
shape as the current density of the plasma increases.
[0072] The electron density in the plasma can form a soliton or
other non-linear waveforms when the small voltage oscillations 284
create a pulsing electric field that interacts with the electron
ExB Hall current 135. The small voltage oscillations 284 tend to
create oscillations in the plasma density that increase the density
and lifetime of the plasma. The increase in plasma density shown in
FIG. 4 in the time period between about 900 .mu.sec and 1.2 msec
can be the result of the electron density forming a soliton or
other non-linear waveform. In this time period, the voltage is only
slightly increasing with time, but the discharge current 286
increases at a much more rapid rate.
[0073] In one embodiment, the electron density increases in an
avalanche-like manner because of electron overheating instability.
Electron overheating instabilities can occur when heat is exchanged
between the electrons in the plasma, the feed gas, and the walls of
the chamber. For example, electron overheating instabilities can be
caused when electrons in a weakly-ionized plasma are heated by an
external field and then lose energy in elastic collisions with
atoms in the feed gas. The elastic collisions with the atoms in the
feed gas raise the temperature and lower the density of the feed
gas. The decrease in the density of the gas results in an increase
in the electron temperature because the frequency of elastic
collisions in the feed gas decreases. The increase in the electron
temperature again enhances the heating of the gas. The electron
heating effect develops in an avalanche-like manner and can drive
the weakly-ionized plasma into the transient non-steady state.
[0074] FIG. 5A-FIG. 5C are measured data 300, 300'', and 300''' of
other illustrative multi-stage voltage pulses 302, 302', and 302''
generated by the pulsed power supply 102 of FIG. 1. The desired
pulse shapes requested from the pulsed power supply 102 are
superimposed in dotted lines 304, 304', and 304'' onto each of the
respective multi-stage voltage pulses 302, 302', and 302''. The
voltage pulses 302, 302', and 302'' are generated for a magnetron
sputtering source having a 10 cm diameter copper target and
operating with argon feed gas at a chamber pressure of
approximately 10.sup.-6 Torr. The repetition rate of the voltage
pulses is 40 Hz.
[0075] The voltage pulse 302 illustrated in FIG. 5A is a two-stage
voltage pulse 302 having a transient region included in both the
low-power stage and the high-power stage of the pulse. A low-power
stage 306 of the voltage pulse 302 including the first transient
region is sufficient to ignite an initial plasma and eventually
sustain a weakly-ionized plasma. The duration of the low-power
stage 306 of the voltage pulse 302 is about 1.0 msec.
[0076] The relatively fast rise time (on the order of about
6.25V/.mu.sec) of the voltage during the first transient region in
the low-power stage 306 is sufficient to shift the electron energy
distribution of the initial plasma to higher energies to generate
ionizational instability that drives the initial plasma into a
transient non-steady state condition. The rise time of the voltage
should be greater than about 0.5V/.mu.sec as previously discussed.
However, since the pulsed power supply 102 is operating in a
low-power mode during the low-power stage 306 of the voltage pulse
302, it does not supply a sufficient amount of uninterrupted power
to continuously drive the initial plasma from the transient
non-steady state to a strongly-ionized state corresponding to the
current-voltage characteristic 154 of FIG. 2. Since there is
insufficient energy stored in the pulsed power supply 102 in the
low-power mode to create conditions that can sustain a
strongly-ionized plasma, the plasma density oscillates and
eventually the transient non-steady state of the plasma becomes
weakly-ionized corresponding to the current-voltage characteristic
152 of FIG. 2.
[0077] The low-power stage 306 of the voltage pulse 302 includes
relatively large voltage oscillations 308. The voltage oscillations
308 dampen when the initial plasma reaches the weakly-ionized
condition corresponding to the current-voltage characteristic 152
of FIG. 2. The weakly-ionized plasma is characterized by the
substantially constant discharge current 312. The voltage
oscillations 308 occur because the pulsed power supply 102 does not
supply enough energy in the low-power mode to drive the transient
plasma into the strongly-ionized state that corresponds to the
high-current regime illustrated by the current-voltage
characteristic 154 of FIG. 2. Consequently, the discharge current
310 oscillates as the plasma rapidly expands and contracts. The
rapidly expanding and contracting plasma causes the output voltage
308 to oscillate in response to the changing plasma load. The
rapidly expanding and contracting plasma also prevents the electron
density in the plasma from forming a soliton or other non-linear
waveform that can increase the plasma density.
[0078] The average power 314 during the generation of the initial
plasma is less than about 50 kW. The voltage 316 and the discharge
current 318 are substantially constant after about 500 .mu.sec,
which corresponds to a plasma in a weakly-ionized condition.
[0079] A high-power stage 320 of the voltage pulse 302 includes a
second transient region 321. The voltage increases by about 30V in
the second transient region 321. The pulsed power supply 102
generates the high-power stage 320 of the voltage pulse 304 at
about 1.1 msec. The voltage in the second transient region 321 has
a magnitude and a rise time (on the order of about 5V/.mu.sec) that
is sufficient to drive the weakly-ionized plasma into a transient
non-steady state. The rise time of the voltage is greater than
about 0.5V/.mu.sec. In the high-power stage 320, the pulsed power
supply 102 is operating in the high-power mode and supplies a
sufficient amount of uninterrupted power to drive the
weakly-ionized plasma from the transient non-steady state to a
strongly-ionized state corresponding to the current-voltage
characteristic 154 of FIG. 2.
[0080] Voltage oscillations 322 occur for about 300 .mu.sec. The
voltage oscillations 322 create current oscillations 324 in the
transient plasma. The voltage oscillations 322 are caused, at least
in part, by the changing resistive load in the plasma. The pulsed
power supply 102 attempts to maintain a constant voltage and a
constant discharge current, but the transient plasma exhibits a
rapidly changing resistive load.
[0081] The voltage oscillations 322 can also be caused by
ionizational instabilities in the plasma as previously discussed.
Ionizational instabilities can occur when the degree of ionization
in the plasma changes because of varying magnitudes of the crossed
electric and magnetic fields. The degree of ionization can grow
exponentially as the ionizational instability develops. The
exponential growth in ionization may be a consequence of electron
gas overheating as a result of developing electron Hall currents.
The exponential growth in ionization dramatically increases the
discharge current.
[0082] The voltage oscillations 322 are minimized after about 1.5
msec. The minimum voltage oscillations 323 can create a pulsing
electric field that interacts with the electron ExB Hall current
135 (FIG. 1) to generate oscillations in the plasma density that
increase the density and lifetime of the plasma. The plasma is in
the high-current regime corresponding to the current-voltage
characteristic 154 of FIG. 2 in which the pulsed power supply 102
supplies an adequate amount of energy to increase the density of
the plasma non-linearly to the strongly-ionized state. The average
voltage 326 is substantially constant while the current 328
increases nonlinearly with insignificant oscillations.
[0083] After the voltage oscillations 322, the average voltage 326
remains lower than the voltage 316 present during the low-power
stage 306 of the voltage pulse 304. The discharge current 324 rises
to a peak current 330. After about 2.0 msec the average voltage 326
is about -500V, the discharge current 330 is almost 300 A and the
power 332 is about 150 kW. These conditions correspond to a
strongly-ionized plasma in the high-current regime.
[0084] The pulsed power supply 102 supplies power to the transient
plasma during the high-power stage 320 at a relatively slow rate.
This relatively slow rate corresponds to a relatively slow rate of
increase in the discharge current 328 over a time period of about
1.0 msec. In one embodiment of the invention, the pulsed power
supply 102 supplies high-power to the plasma relatively quickly
thereby increasing the density of the plasma more rapidly. The
density of the plasma can also be increased by increasing the
pressure inside the plasma chamber.
[0085] FIG. 5A illustrates that in order to sustain a
strongly-ionized plasma in the high-current regime corresponding to
the current-voltage characteristic 154 of FIG. 2 at least two
conditions must be satisfied. The first condition is that the rise
time of a voltage in a transient region must be sufficient to shift
the electron energy distribution of the initial plasma to higher
energies to generate ionizational instability that drives the
plasma into a transient non-steady state condition. The second
condition is that the pulsed power supply must supply a sufficient
amount of uninterrupted power to drive the plasma from the
transient non-steady state to a strongly-ionized state
corresponding to the current-voltage characteristic 154 of FIG.
2.
[0086] In the low-power stage 306, the voltage in the first
transient region has a sufficient rise time to shift the electron
energy distribution of the initial plasma to higher energies as
shown by current oscillations 310. However, the pulsed power supply
102 is in the low-power mode and does not supply a sufficient
amount of uninterrupted power to drive the initial plasma from the
transient non-steady state to a strongly-ionized state. In the
high-power stage 320, the voltage in the second transient region
321 has a sufficient rise time to shift the electron energy
distribution of the initial plasma to higher energies as shown by
current oscillations 324. Also, the pulsed power supply 102 (in the
high-power mode) supplies a sufficient amount of uninterrupted
power to drive the weakly-ionized plasma from the transient
non-steady state to a strongly-ionized state.
[0087] FIG. 5B is measured data 300' of another illustrative
multi-stage voltage pulse 302' generated by the pulsed power supply
102 of FIG. 1. The voltage pulse 302' is a three-stage voltage
pulse 302'. The low-power stage 306' of the voltage pulse 302'
including a first transient region has a rise time and magnitude
that ignites an initial plasma. The low-power stage 306'
corresponds to a low-power mode of the pulsed power supply 102 and
is similar to the low-power stage 306 of the voltage pulse 302 that
was described in connection with FIG. 5A.
[0088] A transient stage 340 of the three-stage voltage pulse 302'
is a transition stage where the pulsed power supply 102 transitions
from the low-power mode to the high-power mode. The duration of the
transient stage 340 is about 40 .mu.sec, but can have a duration
that is in the range of about 10 .mu.sec to 5,000 .mu.sec. The
discharge voltage 342 and discharge current 344 both increase
sharply in the transient stage 340 as previously discussed.
[0089] The transient stage 340 of the voltage pulse 302' has a rise
time that shifts the electron energy distribution in the
weakly-ionized plasma to higher energies thereby causing a rapid
increase in the ionization rate by driving the weakly-ionized
plasma into a transient non-steady state. Plasmas can be driven
into transient non-steady states by creating plasma instabilities
from the application of a strong electric field.
[0090] A high-power stage 350 of the three-stage voltage pulse 302'
is similar to the high-power stage 320 of the two-stage voltage
pulse 302 that was described in connection with FIG. 5A. However,
the discharge current 352 increases at a much faster rate than the
discharge current 328 that was described in connection with FIG.
5A. The discharge current 328 increases more rapidly because the
transient stage 340 of the voltage pulse 302' supplies high power
to the weakly-ionized initial plasma at a rate and duration that is
sufficient to more rapidly create a strongly-ionized plasma having
a discharge current 352 that increases non-linearly.
[0091] Voltage oscillations 354 in the high-power stage 350 are
sustained for about 100 .mu.sec. The voltage oscillations can are
caused by the ionizational instabilities in the plasma as described
herein, such as diocotron oscillations. The voltage oscillations
354 cause current oscillations 356. The maximum power 358 in the
third stage 350 is approaching 200 kW, which corresponds to a
maximum discharge current 360 that is almost 350 A. The third stage
350 of the voltage pulse 302' is terminated after about 1.0
msec.
[0092] FIG. 5C is measured data 300'' of another illustrative
multi-stage voltage pulse 302'' generated by the pulsed power
supply 102 of FIG. 1. The voltage pulse 302'' is a three-stage
voltage pulse 302''. The low-power stage 306'' of the voltage pulse
302'' including a first transient region has a rise time and
magnitude that ignites an initial plasma. The low-power stage 306''
corresponds to a low-power mode of the pulsed power supply 102 and
is similar to the low-power stage 306 of the voltage pulse 302 that
was described in connection with FIG. 5A and the low-power stage
306' of the voltage pulse 302' that was described in connection
with FIG. 5B.
[0093] A transient stage 370 of the three-stage voltage pulse 302''
is a transition stage where the pulsed power supply 102 transitions
from the low-power mode to the high-power mode. The duration of the
transient stage 370 is about 60 .mu.sec, which is about 1.5 times
longer than the duration of the transient stage 340 of the voltage
pulse 302' that was described in connection with FIG. 5B. The
peak-to-peak magnitude of the voltage 376 (-100V) is greater than
the peak-to-peak magnitude of the voltage 346 (-70V) of FIG. 5B.
The discharge voltage 372 and discharge current 374 both increase
sharply in the transient stage 370 because of the high value of the
peak-to-peak magnitude of the voltage 376.
[0094] The magnitude and rise time of the transient stage 370 is
sufficient to drive the initial plasma into a non-steady state
condition. The discharge voltage 372 and the discharge current 374
increase sharply. The peak discharge voltage 376 is about -650V,
which corresponds to a discharge current 377 that is greater than
about 200 A. The discharge voltage 378 then decreases as the
discharge current 374 continues to increase.
[0095] The discharge current 374 in the transient stage 370
increases at a much faster rate than the discharge current 352 that
was described in connection with FIG. 5B because the peak-to-peak
magnitude of the voltage 376 is higher and the duration of the
transient stage 370 is longer than in the transient stage 340 of
FIG. 5B. The duration of the transient stage 370 is long enough to
supply enough uninterrupted energy to the weakly-ionized plasma to
rapidly increase the rate of ionization of the transient
plasma.
[0096] A high-power stage 380 of the three-stage voltage pulse
302'' is similar to the high-power stage 350 of the three-stage
voltage pulse 302' that was described in connection with FIG. 5B.
However, the voltage pulse 302'' does not include the large voltage
oscillations that were described in connection with FIGS. 5A and
5B. The large voltage oscillations are not present in the voltage
pulse 302'' because the transient plasma is already substantially
strongly-ionized as a result of the energy supplied in the
transient stage 370. Consequently, the initial plasma transitions
in a relatively short period of time from a weakly-ionized
condition to a strongly-ionized condition.
[0097] Small voltage oscillations 384 in the voltage pulse 302''
may be caused by the electron density forming a soliton waveform or
having another non-linear mechanism that increases the electron
density as indicated by the increasing discharge current 286. The
soliton waveform or other non-linear mechanism may also help to
sustain the high-density plasma throughout the duration of the
voltage pulse 302'.
[0098] The discharge current 382 in the third stage 380 is greater
than about 300 A. The maximum power 386 in the third stage 380
approaches 200 kW. The third stage 380 of the voltage pulse 304''
is terminated after about 1.0 msec.
[0099] FIG. 6A and FIG. 6B are measured data of multi-stage voltage
pulses 400, 400' generated by the pulsed power supply 102 of FIG. 1
that illustrate the effect of pulse duration in the transient stage
of the pulse on the plasma discharge current. The multi-stage
voltage pulses 400, 400' were applied to a standard magnetron with
a 15 cm diameter copper target. The feed gas was argon and the
chamber pressure was about 3 mTorr.
[0100] The multi-stage voltage pulse 400 shown in FIG. 6A is a
three-stage voltage pulse 402 as indicated by the dotted line 404.
A low-power stage 406 of the voltage pulse 402 has a magnitude and
a rise time that is sufficient to ignite a feed gas and generate an
initial plasma. The pulsed power supply 102 is operating in the
low-power mode during the low-power stage 406. The maximum voltage
in the low-power stage 406 is about -550V. The initial plasma
develops into a weakly-ionized plasma having a relatively low-level
of ionization corresponding to the current-voltage characteristic
152 of FIG. 2. The weakly-ionized plasma can be in a steady state
corresponding to a substantially constant discharge current 408
that is less than about 50 A.
[0101] The pulsed power supply 102 is in the high-power mode during
a transient stage 410. In the transient stage 410, the voltage
increases by about 100V. The rise time of the voltage increase is
sufficient to create a strong electric field through the
weakly-ionized plasma that promotes excitation, ionization, and
recombination processes. The excitation, ionization, and
recombination processes create plasma instabilities, such as
ionizational instabilities, that result in voltage oscillations
412. The duration of the transient stage 410 of the voltage pulse
402 is, however, insufficient to shift the electron energy
distribution in the plasma to higher energies because the energy
supplied by the pulsed power supply 102 in the transient stage 410
is terminated abruptly as illustrated by the dampening discharge
current 414. Consequently, the transient plasma exhibits
ionizational relaxation and eventually decays to a weakly-ionized
plasma state corresponding to a substantially constant discharge
current 416.
[0102] A high-power stage 418 of the voltage pulse 402 has a lower
magnitude than the transient stage 410 of the voltage pulse, but a
higher magnitude than the low-power stage 406. The high-power stage
418 is sufficient to maintain the weakly-ionized plasma, but cannot
drive the plasma from the weakly-ionized condition to the
strongly-ionized condition corresponding to the current-voltage
characteristic 154 of FIG. 2. This is because the transient stage
410 did not provide the conditions necessary to sufficiently shift
the electron energy distribution in the weakly-ionized plasma to
high enough energies to create ionizational instabilities in the
plasma. The voltage pulse 402 is terminated after about 2.25
msec.
[0103] The multi-stage voltage pulse 400' illustrated in FIG. 6B is
a three-stage voltage pulse 402' as indicated by the dotted line
404'. A low-power stage 406' of the voltage pulse 402' is similar
to the low-power stage 406 of the voltage pulse 402 that was
described in connection with FIG. 6A. The low-power stage 406' has
a magnitude and a rise time that is sufficient to ignite a feed gas
and to generate an initial plasma. The pulsed power supply 102 is
operating in the low-power mode as described herein during the
low-power stage 406'. In one embodiment, the maximum voltage in the
low-power stage 406' is also about -550V. The initial plasma
develops into a weakly-ionized plasma having a relatively low-level
of ionization corresponding to the current-voltage characteristic
152 of FIG. 2. The weakly-ionized plasma can be in a steady state
corresponding to a substantially constant discharge current 408'
that is less than about 50 A.
[0104] The transient stage 410' of the voltage pulse 402' creates a
strong electric field through the weakly-ionized plasma that
promotes excitation, ionization, and recombination processes. The
excitation, ionization, and recombination processes create plasma
instabilities, such as ionizational instabilities, that result in
voltage oscillations 412'. The rise time of the peaks in the
oscillating voltage 412' create instabilities in the weakly-ionized
plasma that rapidly increase the ionization rate of the
weakly-ionized plasma as illustrated by the rapidly increasing
discharge current 414'.
[0105] The duration of the transient stage 410' of the voltage
pulse 402' is sufficient to shift the electron energy distribution
in the plasma to higher energies that rapidly increase the
ionization rate. The duration of the transient stage 410' of FIG.
6B is five times more than the duration of the transient stage 410
of FIG. 6A. The discharge current 420 increases nonlinearly as the
average discharge voltage 422 decreases. The magnitude of the
discharge current can be controlled by varying the magnitude and
the duration of the transient stage 410' of the voltage pulse
402'.
[0106] The high-power stage 418' of the voltage pulse 402' has a
lower magnitude than the transient stage 410'. The pulsed power
supply 102 provides a sufficient amount of energy during the
high-power stage 418' to maintain the plasma in a strongly-ionized
condition corresponding to the current-voltage characteristic 154
of FIG. 2. The maximum discharge current 416' for the plasma in the
strongly-ionized state is about 350 A. The voltage pulse 402' is
terminated after about 2.25 msec.
[0107] FIG. 7A and FIG. 7B are measured data of multi-stage voltage
pulses 430, 430' generated by the pulsed power supply 102 of FIG. 1
that show the effect of the pulsed power supply operating mode on
the plasma discharge current. The multi-stage voltage pulses 430,
430' were applied to a standard magnetron with a 15 cm diameter
copper target. The feed gas was argon and the chamber pressure was
about 3 mTorr.
[0108] The multi-stage voltage pulse 430 shown in FIG. 7A is a
three-stage voltage pulse 432 as indicated by the dotted line 434.
The pulsed power supply 102 generates a low-power stage 436 of the
voltage pulse 432 that has a magnitude and a rise time that is
sufficient to ignite a feed gas to generate an initial plasma. The
maximum voltage in the ignition stage is about -550V. The pulsed
power supply 102 is operating in the low-power mode. The initial
plasma develops into a weakly-ionized plasma having a relatively
low-level of ionization corresponding to the current-voltage
characteristic 152 of FIG. 2. The weakly-ionized plasma can be in a
steady state corresponding to a substantially constant discharge
current 408' that is less than about 50 A.
[0109] The pulsed power supply 102 generates a transient stage 440
of the voltage pulse 432 that increases the voltage by about 150V.
The rise time, amplitude and duration of the voltage in the
transient stage 440 of the voltage pulse 432 is sufficient to
promote enough excitation, ionization, and recombination processes
for the weakly-ionized plasma to experience a high rate of
ionization as illustrated by the rapidly increasing discharge
current 442. The pulsed power supply 102 is operating in a
high-power mode during the transient stage 440.
[0110] The high-power stage 444 of the voltage pulse 432 has a
lower magnitude than the transient stage 440 but has a sufficient
magnitude to maintain the strongly-ionized plasma in the
high-current regime corresponding to the current-voltage
characteristic 154 of FIG. 2. The discharge current 446 for the
strongly-ionized plasma is about 350 A. The pulsed power supply 102
operates in the high-power mode during the high-power stage 444 and
generates enough uninterrupted energy to sustain the
strongly-ionized plasma. The voltage pulse 432 is terminated after
about 2.25 msec.
[0111] The multi-stage voltage pulse 430' of FIG. 7B is a
three-stage voltage pulse 432' as indicated by the dotted line
434'. The pulsed power supply generates a low-power stage 436' of
the voltage pulse 432' that is similar to the low-power stage 436
of the voltage pulse 432 of FIG. 7A. The low-power stage 436' of
the voltage pulse 432' has a magnitude and a rise time that is
sufficient to ignite a feed gas to generate an initial plasma. The
pulsed power supply 102 is operating in the low-power mode. The
maximum voltage in the ignition stage is about -550V. The initial
plasma develops into a weakly-ionized plasma having a relatively
low-level of ionization. The weakly-ionized plasma can be in a
steady state that corresponds to a substantially constant discharge
current 438' that is less than about 50 A.
[0112] The pulsed power supply 102 generates a transient stage 440'
of the voltage pulse 432' that increases the voltage by about 150V.
The transient stage 440' is similar to the transient stage 440 of
FIG. 7A. The amplitude and duration of the transient stage 440' of
the voltage pulse 432' is sufficient to promote enough excitation,
ionization, and recombination processes to rapidly increase the
ionization rate of the weakly-ionized plasma as illustrated by the
rapidly increasing discharge current 442'. The pulsed power supply
102 is operating in a high-power mode during the transient stage
440'.
[0113] The pulsed power supply 102 generates a high-power stage
444' that includes a voltage having a lower magnitude than the
voltage in the second stage 440'. The voltage in the high-power
stage 444' decreases to below -500V which is insufficient to
sustain a strongly-ionized plasma. Thus, the strongly-ionized
plasma exhibits ionizational relaxation and eventually decays to a
weakly-ionized plasma state corresponding to a quasi-stationary
discharge current 449. The voltage pulse 432' is terminated after
about 2.25 msec.
[0114] FIG. 8 is measured data 450 for an exemplary single-stage
voltage pulse 452 generated by the pulsed power supply 102 of FIG.
1 that produces a high-density plasma according to the invention
that is useful for high-deposition rate sputtering. The voltage
pulse 452 is a single-stage voltage pulse as indicated by the
dotted line 453. The pulsed power supply 102 operates in a
high-power mode throughout the duration of the voltage pulse
452.
[0115] The voltage pulse 452 includes an ignition region 454 that
has a magnitude and a rise time that is sufficient to ignite a feed
gas to generate an initial plasma. The discharge current 458
increases after the initial plasma is ignited. The initial plasma
is ignited in about 100 .mu.sec.
[0116] After ignition, the discharge current 460 and the voltage
456 both increase. The initial peak voltage 462 is about -900V. The
voltage then begins to decrease. The discharge current 460 reaches
an initial peak current 464 corresponding to a voltage 466. The
initial peak discharge current 464 is about 150 A at a discharge
voltage 466 of about The peak discharge current 464 and
corresponding discharge voltage 466 corresponds to a power 468 that
is about 120 kW. The time period from the ignition of the plasma to
the initial peak discharge current 464 is about 50 .mu.sec. The
initial plasma does not reach a steady state condition but instead
remains in a transient state.
[0117] The voltage pulse 452 also includes a transient region 454'
having voltage oscillations 467 that include rise times which are
sufficient to shift the electron energy distribution in the initial
plasma to higher energies that create ionizational instabilities
that cause a rapid increase in the ionization rate as described
herein. The initial plasma remains in a transient state.
[0118] The voltage pulse 452 also includes a high-power region
454''. The voltage in the high-power region 454'' has a magnitude
that is sufficient to sustain a strongly-ionized plasma. Small
voltage oscillations 469 in the voltage pulse 452 may be caused by
the electron density forming a soliton waveform or having another
non-linear mechanism that increases the electron density as
indicated by the increasing discharge current 470. The soliton
waveform or other non-linear mechanism may also help to sustain the
strongly-ionized plasma throughout the duration of the voltage
pulse 452.
[0119] The single-stage voltage pulse 452 includes a voltage 456
that is sufficient to ignite an initial plasma, voltage
oscillations 467 that are sufficient to create ionizational
instabilities in the initial plasma, and a voltage 472 that is
sufficient to sustain the strongly-ionized plasma. The pulsed power
supply 102 operates in the high-power mode throughout the duration
of the single-stage voltage pulse 452. The peak discharge current
470 in the high-density plasma is greater than about 250 A for a
discharge voltage 472 of about -500V. The power 474 is about 125
kW. The pulse width of the voltage pulse 452 is about 1.0 msec.
[0120] FIG. 9 illustrates a cross-sectional view of a plasma
sputtering apparatus 500 having a pulsed direct current (DC) power
supply 501 according to another embodiment of the invention. The
plasma sputtering apparatus 500 includes a vacuum chamber 104 for
containing a plasma. The vacuum chamber 104 can be coupled to
ground 105. The vacuum chamber 104 is positioned in fluid
communication with a vacuum pump 106 that is used to evacuate the
vacuum chamber 104 to high vacuum. The pressure inside the vacuum
chamber 104 is generally less than 10.sup.-1 Torr for most plasma
operating conditions.
[0121] The plasma sputtering apparatus 500 also includes a cathode
assembly 502. The cathode assembly 502 is generally in the shape of
a circular ring. The cathode assembly 502 includes a target 504.
The target 504 is generally in the shape of a disk and is secured
to the cathode assembly 502 through a locking mechanism, such as a
clamp 506. The cathode assembly 502 is electrically connected to a
first terminal 508 of the pulsed power supply 501 with an
electrical transmission line 510.
[0122] In some embodiments, the plasma sputtering apparatus 500
includes an energy storage device 503 that provides a source of
energy that can be controllably released into the plasma. The
energy storage device 503 is electrically coupled to the cathode
assembly 502. In one embodiment, the energy storage device 503
includes a capacitor bank.
[0123] A ring-shaped anode 512 is positioned in the vacuum chamber
104 proximate to the cathode assembly 502 so as to form a gap 514
between the anode 512 and the cathode assembly 502. The gap 514 can
be between about 1.0 cm and 12.0 cm wide. The gap 514 can reduce
the probability that an electrical breakdown condition (i.e.,
arcing) will develop in the chamber 104. The gap 514 can also
promote increased homogeneity of the plasma by controlling a gas
flow through the gap. The anode 512 can include a plurality of feed
gas injectors 516 that inject feed gas into the gap 514. In the
embodiment shown, the feed gas injectors 516 are positioned within
the anode 512. The feed gas injectors 516 are coupled to one or
more feed gas sources 518. The feed gas sources can include atomic
feed gases, reactive gases, or a mixture of atomic and reactive
gases. Additionally, excited atom sources (not shown) or metastable
atom sources (not shown) can be coupled to the feed gas injectors
516 to supply excited atoms or metastable atoms to the chamber
104.
[0124] The anode 512 is electrically connected to ground 105. A
second terminal 520 of the pulsed power supply 501 is also
electrically connected to ground 105. In other embodiments, the
anode 512 is electrically connected to the second terminal 520 of
the pulsed power supply 501.
[0125] The anode 512 can be integrated with or connected to a
housing 521 that surrounds the cathode assembly 502. An outer edge
522 of the cathode 502 is isolated from the housing 521 with
insulators 523. The space 524 between the outer edge 522 of the
cathode assembly 502 and the housing 521 can be filled with a
dielectric.
[0126] The plasma sputtering apparatus 500 can include a magnet
assembly 525 that generates a magnetic field 526 proximate to the
target 504. The magnetic field 526 is less parallel to the surface
of the cathode assembly 502 at the poles of the magnets in the
magnet assembly 525 and more parallel to the surface of the cathode
assembly 502 in the region 527 between the poles of the magnets in
the magnetic assembly 525.
[0127] The magnetic field 526 is shaped to trap and concentrate
secondary electrons emitted from the target 504 that are proximate
to the target surface 528. The magnetic field 526 increases the
density of electrons and therefore, increases the plasma density in
the region 527. The magnetic field 526 can also induce an electron
Hall current that is generated by the crossed electric and magnetic
fields. The strength of the electron Hall current depends, at least
in part, on the density of the plasma and the strength of the
crossed electric and magnetic fields. Crossed electric and magnetic
fields generated in the gap 514 can enhance the ionizational
instability effect on the plasma as discussed herein.
[0128] The plasma sputtering apparatus 500 also includes a
substrate support 530 that holds a substrate 532 or other work
piece. The substrate support 530 can be electrically connected to a
first terminal 534 of a RF power supply 536 with an electrical
transmission line 538. A second terminal 540 of the RF power supply
536 is coupled to ground 105. The RF power supply 536 can be
connected to the substrate support 530 through a matching unit (not
shown). In one embodiment a temperature controller 542 is thermally
coupled to the substrate support 530. The temperature controller
542 regulates the temperature of the substrate 532.
[0129] The plasma sputtering apparatus 500 can also include a
cooling system 544 to cool the target 504 and the cathode assembly
502. The cooling system 544 can be any one of numerous types of
liquid or gas cooling system that are known in the art.
[0130] In operation, the vacuum pump 106 evacuates the chamber 104
to the desired operating pressure. The feed gas is injected into
the chamber 104 from the feed gas source 518 through the gas inlet
516. The pulsed power supply 501 applies negative voltage pulses to
the cathode 502 (or positive voltage pulses to the anode 512) that
generate an electric field 546 in the gap 514 between the cathode
assembly 502 and the anode 512. The magnitude and rise time of the
voltage pulse are chosen such that the resulting electric field 546
ionizes the feed gas in the gap 514, thereby igniting an initial
plasma in the gap 514.
[0131] The geometry of the gap 514 can be chosen to minimize the
probability of arcing and to facilitate the generation of a very
strong electric field 546 with electric field lines that are
perpendicular to the surface 528 of the target 504 and the cathode
502. This strong electric field 546 can enhance the ionizational
instability in the plasma by increasing the volume of excited atoms
including metastable atoms that are generated from ground state
atoms in the initial plasma. The increased volume of exited atoms
can increase the density of the plasma in a non-linear manner as
previously discussed.
[0132] The plasma is maintained, in part, by secondary electron
emission from the target 504. In embodiments including the magnet
assembly 525, the magnetic field 526 confines the secondary
electrons proximate to the region 527 and, therefore, concentrates
the plasma proximate to the target surface 528. The magnetic field
526 also induces an electron Hall current proximate to the target
surface 528, which further confines the plasma and can cause the
electron density to form a soliton waveform or other non-linear
waveform.
[0133] Ions in the plasma bombard the target surface 528 since the
target 504 is negatively biased. The impact caused by the ions
bombarding the target 504 dislodges or sputters material from the
target 504. The sputtering rate generally increases as the density
of the plasma increases.
[0134] The RF power supply 536 generates a negative RF bias voltage
on the substrate 532 that attracts positively ionized sputtered
material to the substrate 532. The sputtered material forms a thin
film of target material on the substrate 532. The magnitude of the
RF bias voltage on the substrate 532 can be chosen to optimize
parameters, such as sputtering rate and adhesion of the sputtered
firm to the substrate 532, and to minimize damage to the substrate
532. The temperature controller 542 can regulate the temperature of
the substrate 532 to avoid overheating the substrate 532.
[0135] Although FIG. 9 illustrates a magnetron sputtering system,
skilled artisans will appreciate that many other plasma systems can
utilize methods for generating high-density plasmas using
ionizational instability according to the invention. For example,
the methods for generating high-density plasmas using ionizational
instability according to the invention can be used to construct a
plasma thruster. The method of generating a high-density plasma for
a thruster is substantially the same as the method described in
connection with FIG. 9 except that the plasma is accelerated
through an exhaust by external fields.
[0136] FIG. 10A illustrates a schematic diagram 550 of a pulsed
power supply 552 that can generate multi-step voltage pulses
according to the present invention. The pulsed power supply 552
includes an input voltage 554 that charges a bank of capacitors
556. In one embodiment, the input voltage 554 is in the range of
100V to 5000V. A parallel bank of high-power solid state switches
558, such as insulated gate bipolar transistors (IGBTs), are
coupled to a primary coil 560 of a pulse transformer 562. The solid
state switches 558 are controlled by driver 557 that send signals
to the solid state switches 558 that activate or deactivate the
switches 558. When the solid state switches 558 are activated by
the drivers 557 they release energy stored in the capacitors 556 to
the primary coil 560 of the pulse transformer 562 in the form of
voltage micropulses. In some embodiments, the duration of the
voltage micropulses is in the range of two microseconds to one
hundred microseconds.
[0137] The pulse transformer 562 also includes a secondary coil
564. The voltage gain from the pulse transformer 562 is
proportional to the number of secondary turns in the secondary coil
564. A first end 566 and a second end 570 of the secondary coil 564
are coupled to an output driving circuit 568. In many embodiments,
the output driving circuit 568 includes diodes, inductors, and
capacitors. The output driving circuit 568 provides voltage pulses
across a first output 574 and a second output 576. The first output
574 can be coupled to a cathode and the second output 576 can be
coupled to an anode, for example. The pulsed power supply 552 can
provide pulse power up to about 10 MW with a relatively fast rise
time and duration up to 100 milliseconds.
[0138] The pulsed power supply 552 can include a controller or
processor 578 which determines the output waveform generated by the
pulsed power supply 552. In some embodiments, a separate controller
or processor, such as a computer, is electrically connected to the
drivers 557 of the solid state switches 558 so as to control the
operation of the solid state switches 558. In other embodiments,
the processor 578 is integrated directly into the pulsed power
supply 552 as shown in FIG. 10A. The processor 578 and drivers 557
can be used to determine parameters, such as the pulse width of the
micropulses, and the repetition rate and/or duty cycle of the
micropulses that is generated by solid state switches 558 in order
to control of output pulse trains generated by the pulsed power
supply 552.
[0139] In some embodiments, the pulsed power supply 552 is used in
conjunction with the arc control circuit 151 that was described in
connection with FIG. 1. The arc control circuit 151 includes a
detection means that detects the onset of an arc discharge and then
sends a signal to a control device in the pulsed power supply 552
that deactivates the drivers 557 for the high-power solid state
switches 558 for some period of time. The deactivation of the
drivers 557 for the high-power solid state switches 558 reduces the
voltage between the anode and the cathode assembly to levels that
can not support an arc discharge.
[0140] Energy stored in cables that connect the pulse power supply
552, magnetron, and the output driving circuit 568 still can be
released after the drivers 557 for the solid state switches 558 are
deactivated and, under some circumstances, can sustain an arc
discharge for a short period of time. In order to minimize these
undesirable arc discharges, the control circuit 151 should be
positioned close to the magnetron and the length of cables
connecting the control circuit 151 and the magnetron should also be
minimized. For example, the length of cables connecting the control
circuit 151 and the magnetron should be less than about 100 cm.
[0141] In some embodiments, the power supply 552 generates a single
step voltage pulse. For example, the processor 578 can instruct the
drivers 557 for the high-power solid state switches 558 to generate
micropulses with a ten microseconds pulse width and a fifty
microsecond period (i.e. a forty microseconds off time). These
micropulses generate output voltage pulses having a duration that
is one millisecond with an amplitude that is equal to -400 V. The
resulting voltage waveform has a 20% duty cycle. In this example,
the power supply 552 generates twenty pulses. Thus, the total
duration of the voltage waveform generated by the power supply 552
is one millisecond (50 microsecond pulse width.times.20 periods).
The resulting magnetron discharge had a current of 10 A with a
-400V voltage.
[0142] In other embodiments, the power supply 552 generates
multi-step voltage pulses by varying the duty cycle of the signals
generated by the drivers 557 for the high-power solid state
switches 558 for predetermined times. In various embodiments, a two
stage voltage pulse is used to generate plasmas having particular
properties, such as plasmas that are formed initially with a weakly
ionized plasma and then with a strongly ionized plasma as described
herein. For example, a two stage voltage pulse with a first stage
having an amplitude that is -500 V and a second stage having an
amplitude that is -600V with a total pulse width of two
milliseconds can be generated by pulsed power supply 552.
[0143] During the first stage, the processor 578 instructs the
drivers 557 for the high-power solid state switches 558 to generate
-500V pulses with a fifteen microseconds pulse width and a fifty
microsecond period (i.e. a thirty-five microseconds off time). The
resulting voltage waveform had a 30% duty cycle. Twenty pulses were
generated. The total duration of the first stage waveform was one
millisecond. During the first stage, the magnetron discharge
voltage was -500V and the magnetron discharge current was 15 A. The
first stage waveform generated a weakly ionized plasma as described
herein. The voltage rise time between the first stage waveform and
the second stage waveform was 20 V/microsecond.
[0144] During the second stage, the processor 578 instructs the
drivers 557 for the high-power solid state switches 558 to generate
-600V pulses with a 16 microseconds duration and a forty
microsecond period (i.e. a twenty-four microseconds off time). This
resulting voltage waveform had a 40% duty cycle. Twenty five pulses
were generated. The total duration of the second stage waveform was
one millisecond. The total duration of the two-step waveform was
two milliseconds. During the second stage voltage waveform, the
magnetron discharge voltage was -600V and magnetron discharge
current was -300 A. The second stage waveform generated a strongly
ionized plasma as described herein.
[0145] In various embodiments, the voltage rise time between the
first stage waveform and the second stage waveform of the two stage
voltage pulse waveform is selected to generate plasmas with
particular properties as described herein. The rise time between
the first stage waveform and the second stage waveform can be
varied by changing the width of the micropulses or the duty cycle
of the micropulses at the end of the first stage waveform and the
width of the micropulse or the duty cycle of the micropulses in the
beginning of second stage waveform. It should be understood that
multi-stage pulses with any number of stages can be generated with
the methods and apparatus of the present invention.
[0146] In one embodiment, the plasma generator described in
connection with FIG. 10 A generates a voltage pulse shape
comprising voltage oscillations with large amplitude. For example,
the amplitude of the voltage oscillations can be in the range of
500 V to 3,000 V. These voltage oscillations generate a magnetron
discharge with an oscillating discharge current. In some
embodiments, the amplitude of the oscillating discharge current is
in the range of 100 A to 5,000 A, which corresponds to a current
density in the target that is in the range of 0.1 to 10 A/cm2. In
one embodiment, the amplitude of the voltage oscillations inside
the voltage pulse begins in the range from about 0 V to negative
200 volts. In another embodiment, the amplitude of the voltage
oscillations inside the voltage pulse begins in the range from
about positive 100 V to negative 300 volts. These ranges of voltage
amplitudes can be controlled by adjusting the values of capacitors
and inductors in the output driving circuit 568 as shown in FIG. 10
A.
[0147] The discharge produced by voltage oscillations is a pure
pulse discharge because both the voltage and the current are
changing during the oscillation. In the beginning of the voltage
oscillation, electrons gain high energy. When the electrons produce
ionization, the discharge current will reach a maximum value. This
discharge is not a DC (direct current) or a quasi stationary
discharge. Instead, the discharge is formed by the voltage
oscillation rise time and the voltage oscillation fall time. The
phase shift between the discharge current and the discharge voltage
can be in the range of about 0 to 100 .mu.sec.
[0148] In one embodiment, the peak voltage amplitude of the voltage
oscillations corresponded to the current peak of the current
oscillations. In another embodiment, the current peak of the
current oscillations is shifted relative to the voltage peak of the
voltage oscillations by a time interval that in the range of about
0.1 to 10 .mu.sec. This shift can be controlled by adjusting the
values of capacitors and inductors in the output driving circuit
568 as shown in FIG. 10 A.
[0149] In one embodiment, the pulsed voltage waveform with the
voltage oscillations is used to generate a magnetron discharge with
a carbon target. The frequency, amplitude and durations of the
voltage oscillations were adjusted in order to achieve a hard
carbon coating with a hardness that is in the range of 20 and 60
GPa. One application of the present teaching is to sputter this
hard carbon coating on the sharp cutting edge of a razor blade in
order to improve wear resistance and other properties of the razor
blade. Razor blades for personal hygiene that include such carbon
coating are highly desirable. Voltage oscillations with a frequency
that is in the range of about 1 to 1,000 KHz, with an amplitude
that is in the range of 1,000 to 3,000 V, and with a discharge
current density on the carbon target that is in the range of 0.2 to
10 A/cm.sup.2 have been shown in achieve a desirable hard carbon
coating for razor blade applications.
[0150] In one embodiment, the voltage waveform with the
oscillations was used to generate a magnetron discharge in the
presence of a mixture of a noble gas and a reactive gas, such as
O.sub.2 or N.sub.2 or C.sub.2H.sub.2. The voltage oscillations
amplitude and frequency can be adjusted in order to achieve an arc
free or a nearly arc free magnetron discharge. In one embodiment,
the magnetron target material comprises one or more of Al, Cu, Ti,
C, Ta, Mo, Ni, V, Si, B, In, Sn, W, Cr, Fe, B, V, Zr, Y, Au, Ag,
Pt, Re, Zn, and Co.
[0151] FIG. 10B shows a multi-step output voltage waveform 590 and
the corresponding micropulse voltage waveforms 592 that are
generated by switches 558 and controlled by the drivers 557 and the
controller 578. The micropulse voltage waveforms 592 that generate
the multi-step output voltage waveform 590 illustrates how a
multi-step voltage waveform can be formed by varying the pulse
widths and the duty cycle of the micropulses generated by the
switches 558. In addition, the micropulse voltage waveforms 592
that generate the multi-step output voltage waveform 590
illustrates how the rise times of the multi-step voltage waveform
can be varied by varying the pulse widths and the duty cycle of the
micropulses generated by the drivers 557. In one embodiment, the
micropulse voltage waveform 592 generates negative voltage
oscillations with amplitudes in the range of about 500 to 3,000 V
that begins from about 0 V to negative 200 V. In another
embodiment, the micropulse voltage waveform 592 generates negative
voltage oscillations with amplitudes in the range of about 500 to
3,000 V that begins from about positive 100 V to negative 300
V.
[0152] FIG. 11 illustrates a schematic diagram 600 of a pulsed
power supply 602 having a magnetic compression network 604 for
supplying high-power pulses. The pulsed power supply 602 generates
a long pulse with a switch and applies the pulse to an input stage
of a multi-stage magnetic compression network 604. Each stage of
magnetic compression reduces the time duration of the pulse,
thereby increasing the power of the pulse.
[0153] The pulsed power supply 602 includes a DC supply 606, a
capacitor 608, and a power-MOS solid switch 610 for providing power
to the magnetic compression network 604. The magnetic compression
network 604 includes four non-linear magnetic inductors 612, 614,
616, 618 and four capacitors 620, 622, 624, 626. The non-linear
magnetic inductors 612, 614, 616, 618 behave as switches that are
off when they are unsaturated and on when they are saturated. The
magnetic compression network 604 also includes a transformer
628.
[0154] When the solid switch 610 is activated, the capacitor 620
begins to charge and the voltage V1 increases. At a predetermined
value of the voltage V1, the magnetic core of the non-linear
magnetic inductor 612 saturates and the inductance of the
non-linear magnetic inductor 612 becomes low causing the non-linear
magnetic inductor 612 to turn on. This results in charge
transferring from the capacitor 608 to the capacitor 620. The
electric charge stored in the capacitor 620 is then transferred
through the transformer 628 to the capacitor 622 and so on. The
charge that is transferred to the capacitor 626 is eventually
discharged through a load 630. The magnetic compression network 604
can generate high-power pulses up to a terawatt in tens of
nanoseconds with a relatively high repetition rate.
[0155] FIG. 12 illustrates a schematic diagram 650 of a pulsed
power supply 652 having a Blumlein generator 654 for supplying
high-power pulses. The pulsed power supply 652 having the Blumlein
generator 654 can deliver short duration high voltage pulses with a
fast rise time and a relatively flat top. The pulsed power supply
652 includes a high voltage DC supply 656. A first terminal 658 of
the high voltage DC supply 656 is coupled through a
current-limiting inductor 660 to a dielectric material 662 that is
located between an inner conductor 664 and an outer conductor 666
of a coaxial cable 668. The inner conductor 664 is coupled to
ground 670 through an inductance 672. The outer conductor 666 is
also coupled to ground 670. The Blumlein generator 654 operates as
follows. The high voltage power supply 656 slowly charges the
Blumlein generator 654. A very fast high-power switch 674
discharges the charge through a load 676, such as a plasma
load.
[0156] FIG. 13 illustrates a schematic diagram 700 of a pulsed
power supply 702 having a pulse cascade generator 704 for supplying
high-power pulses. A high frequency power supply 706 is coupled to
a transformer 708. The transformer 708 is coupled to a cascade of
"low voltage" (1 kV to 3 kV) pulse generators 710 that are
connected in series. The pulse cascade generator 704 operates as
follows. The high frequency power supply 706 charges capacitors 712
in each of the pulse generators 710. Switches 714 in each of the
pulse generators 710 close at predetermined times thereby
discharging energy in the capacitors 712. When the required output
voltage appears between the terminal 716 and ground 718, the stored
energy discharges through a load 720, such as a plasma load.
EQUIVALENTS
[0157] While the invention has been particularly shown and
described with reference to specific preferred embodiments, it
should be understood by those skilled in the art that various
changes in form and detail may be made therein without departing
from the spirit and scope of the invention as defined herein.
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