U.S. patent application number 10/626330 was filed with the patent office on 2004-07-01 for high peak power plasma pulsed supply with arc handling.
Invention is credited to Christie, David J..
Application Number | 20040124077 10/626330 |
Document ID | / |
Family ID | 31993275 |
Filed Date | 2004-07-01 |
United States Patent
Application |
20040124077 |
Kind Code |
A1 |
Christie, David J. |
July 1, 2004 |
High peak power plasma pulsed supply with arc handling
Abstract
A magnetron sputtering system is provided comprising a pulsed DC
power supply capable of delivering peak powers of 0.1 megawatts to
several megawatts with a peak power density greater than 1
kW/cm.sup.2. A sputtering plasma in a highly ionized state is
created without first adopting an arc discharge state. The power
supply has a pulsing circuit comprising an energy storage capacitor
and serially connected inductor with a switching means for
disconnecting the pulsing circuit from the plasma and recycling the
inductor energy back to the energy storage capacitor at the
detection of an arc condition. The energy storage capacitor and the
serially connected inductor provide an impedance match to the
plasma, limits the current rate of rise and peak magnitude in the
event of an arc, and shapes the voltage pulses to the plasma.
Inventors: |
Christie, David J.; (Fort
Collins, CO) |
Correspondence
Address: |
BENJAMIN HUDSON, JR.
1625 SHARP POINT DR.
FORT COLLINS
CO
80525
US
|
Family ID: |
31993275 |
Appl. No.: |
10/626330 |
Filed: |
July 24, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10626330 |
Jul 24, 2003 |
|
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|
10254158 |
Sep 25, 2002 |
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Current U.S.
Class: |
204/192.12 ;
204/298.08 |
Current CPC
Class: |
H01J 37/34 20130101;
H01J 37/3405 20130101; C23C 14/35 20130101; H01J 37/3444
20130101 |
Class at
Publication: |
204/192.12 ;
204/298.08 |
International
Class: |
C23C 014/32 |
Claims
I claim:
1. A method of sputter deposition, comprising: a) providing a
plasma chamber with a sputtering gas disposed therein; b) providing
a material target disposed in the plasma chamber; c) providing a
pulsed DC power supply that periodically applies a voltage pulse to
the material target, the voltage pulse ionizing the sputtering gas
to create a plasma, the plasma adopting a highly ionized state
without first adopting an arc discharge state; and d) sputtering
atoms from the target by bombarding the material target with ions
from the plasma, the atoms being then deposited on the surface of a
substrate in proximity to the plasma.
2. The method of claim 1, wherein the pulsed DC power supply
delivers power greater than 0.1 MW with a peak power density
greater than 1 kW/cm.sup.2.
3. The method of claim 1, wherein the plasma adopts the highly
ionized state without first adopting an arc discharge state by
controlling the voltage rate of rise of the voltage pulse applied
to the material target.
4. The method of claim 3, wherein the voltage rate of rise of the
voltage pulse is controlled using a circuit comprising a resistor
in series with a capacitor.
5. The method of claim 1, wherein the plasma adopts the highly
ionized state without first adopting an arc discharge state by
limiting the magnitude of the voltage pulse to a maximum level.
6. The method of claim 5, wherein the magnitude of the voltage
pulse is limited using a circuit comprising a resistor in series
with a capacitor.
7. The method of claim 5, wherein the magnitude of the voltage
pulse is limited using a circuit comprising a reverse biased diode,
a capacitor, and a clamp voltage supply.
8. A sputter deposition system, comprising: a) a plasma chamber
with a sputtering gas disposed therein; b) a material target
disposed in the plasma chamber; c) a pulsed DC power supply that
periodically applies a voltage pulse to the material target, the
voltage pulse ionizing the sputtering gas to create a highly
ionized plasma; and d) pulse shaping circuitry that shapes the
voltage pulse so as to allow the plasma to adopt a highly ionized
state without first adopting an arc discharge state.
9. The sputter deposition system of claim 8, wherein the pulse
shaping circuitry controls the voltage rate of rise of the voltage
pulse.
10. The sputter deposition system of claim 9, wherein the pulse
shaping circuitry comprises a resistor in series with a
capacitor.
11. The sputter deposition system of claim 8, wherein the pulse
shaping circuitry limits the magnitude of the voltage pulse to a
maximum level.
12. The sputter deposition system of claim 11, wherein the pulse
shaping circuitry comprises a reverse biased diode, a capacitor,
and a clamp voltage supply.
Description
RELATED APPLICATIONS
[0001] This is a continuation-in-part of U.S. patent application
Ser. No. 10/254,158, filed Sep. 25, 2002.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to apparatus and methods
for magnetron sputtering, and more particularly to magnetron
sputtering apparatus that delivers high peak powers to a sputtering
magnetron plasma load with arc handling capability.
[0004] 2. Brief Description of the Prior Art
[0005] It is desirable to coat some substrates by generating metal
ions and attracting the ions to the work piece by means of an
electrical bias. The utility of this approach includes application
of coatings to surfaces with irregularities that would prevent
uniform deposition by normal sputtering, which essentially requires
line of sight from the sputtering source to the workpiece feature.
Coating and even filling high aspect ratio trenches in
semiconductor devices is possible by biasing the wafer to attract
the ions, as reported by Monteiro in JVST B 17(3), 1999 pg. 1094
and Lu and Kushner in JVST A 19(5), 2001 pg. 2652.
[0006] Sputtering deposition may be enhanced by making use of
plasmas in a highly ionized state. A technique for generating
highly ionized, high density metal plasma by driving conventional
sputtering magnetrons with electrical pulses having high peak power
and low duty factor has been reported by Kouznetsov, et al. in
Surface and Coatings Technology 122 (1999) pg. 290. Additional
teachings can be found by Macak, et al, JVST A 18(4), 2000 pg.
1533; Gudmundsson, et al., APL, Vol. 78, No. 22, 28 May 2001, pg.
3427; and Ehiasarian, et al., Vacuum 65 (2002) p. 147.
[0007] U.S. Pat. No. 6,296,742 B1 describes a method of producing a
fully ionized plasma for use in magnetron sputtering applications.
A pulse generator delivers pulses of up to 10 MW to a sputtering
target, thereby completely ionizing a sputtering gas. In this
method, the sputtering gas is described as first adopting a glow
discharge state, then continuing to an arc discharge state, and
finally adopting a fully ionized state. As shown in FIG. 1 of that
patent, the arc discharge state is described as a break-down
condition occurring at current densities beyond those of the
abnormal glow discharge region, and is characterized by a sudden
drop in plasma impedance, as shown by an abrupt drop in plasma
voltage as the current density further increases. In practical
systems, this is usually represented by a drop in the voltage
across the plasma to at most a few tens of volts and may be
accompanied by a discharge between some part of the sputtering
target and the chamber. The '742 patent indicates that, under that
patent's teachings, the plasma, after passing through this arc
region, develops into a fully ionized state.
[0008] Part of the appeal of these techniques is the ability to
generate a large population of ionized species that can in turn be
attracted to the work piece by the application of a bias voltage.
The above references on the high peak power techniques appear to
use a simple capacitor discharge through an inductor. However, the
technique taught by these references does not disclose any arc
handling capability, and in fact suggests that it is possible, once
the fully ionized state is attained, to achieve thereafter arc-free
operation. Unavoidable imperfections in hardware, however, make the
physical realization of a completely arc free region of operation
(after the initial passing through of the arc state) essentially
impossible, even if its existence is suggested by theory. Use of
the technique, therefore, without arc handling capability, may make
commercial utilization impractical. It may also, at the least, make
processing time excessive because of the long time which may be
needed to condition the target to operate in near arc-free
conditions, and may at the very least prevent operation at the
highest power levels due to an inability to condition the target
adequately. Thus, it would be desirable to provide apparatus that
enables commercial processes using high peak power pulses to
magnetrons to produce high density, highly ionized plasmas by
minimizing arc energy that in turn keeps product and target damage
due to arcing within acceptable limits. In view of the possible
damage created by passing through the arc state at the outset of
the pulsing, it would also be very desirable to prevent the
occurrence of the arc state in the initial establishment of the
highly ionized condition.
[0009] Typically arc control and arc diverting apparatus have been
comprised of circuits that either detect the arc and disconnect the
power supply from the load or are comprised of a switching circuit
that effectively short circuits the power supply to extinguish the
arc. These types of arc handling methods are very costly because
they may result in a complete shut down of the process, wasting
expensive stock material, or require complete dissipation of all of
the energy stored in the power supply circuits. In high power
applications, short circuits to the power supply may result in
extremely high currents--even enough to cause destruction of the
power supply itself--and repetitive dissipation of stored energy in
any case requires expensive resistive elements capable of high peak
power and high average power, as well as the means for cooling
them.
[0010] It would also be desirable, then, if there were provided a
magnetron sputtering apparatus and method that could deliver peak
powers of 1 Megawatt or greater, with arc handling capability for
high yield commercial applications. It is an object of this
invention to provide a magnetron sputtering plasma system that has
the capability both to detect arcs and to take action to limit the
energy delivered to the arc. It is a further object of this
invention to provide a magnetron sputtering system that creates
sputtering plasmas in a highly ionized state without first adopting
an arc discharge state, which may cause damage to the chamber,
substrate, or target, even if only as a transient condition on each
pulse.
SUMMARY OF THE INVENTION
[0011] There is provided by this invention an apparatus and method
for producing high current pulses suited for delivering high peak
power to high-density magnetron plasmas with efficient arc handling
capability. In one embodiment of the invention, a pulsing circuit
comprised of an energy storage capacitor is repetitively charged
and then discharged through an inductor in series with the plasma.
The combination of the inductor and capacitor serve to shape the
pulse, which accomplishes three functions. First, it has been found
that it is possible to avoid the initial arc condition by properly
shaping the pulse. This is done by controlling the beginning of the
voltage pulse. In one embodiment, a network is added for the
purpose of controlling the voltage rate of rise, the unclamped peak
amplitude of the voltage pulse in the event that the plasma does
not ignite, and the frequency at which the voltage waveform rings,
particularly in the case that the plasma does not ignite. This
circuit in this embodiment amounts to a resistor in series with a
capacitor shunt connected at the output of the pulser, or its
equivalent implemented as a distributed circuit with a number of
discrete capacitors and resistors, possibly also utilizing
parasitic capacitors and resistors in devices and circuit
conductors. In addition, a circuit is provided to clamp or limit
the voltage pulse to a maximum level, implemented with a diode,
normally reverse biased in series with a capacitor held at the
clamp voltage, connected to the output of the pulser. This circuit
is activated when the amplitude of the voltage pulse exceeds a
preset adjustable value and acts to prevent the voltage from
exceeding a preset level. This has the benefit of preventing
undesirable arcs both inside and outside the vacuum chamber. All of
this makes it possible to reach a highly ionized plasma state
without first passing through the arc state.
[0012] Second, the pulsing network, or mesh, serves to provide an
impedance match to the plasma. Third, the network serves to limit
the current rate of rise and peak magnitude in the event of a later
occurrence of an arc. An arc may be detected by either the fall of
the discharge voltage below a preset voltage threshold during a
pulse, or an increase in discharge current above a preset current
threshold. Note that the arc condition represents a lowering of the
impedance of the plasma, which is represented by the ratio of the
voltage to the current, so either or both detection methods will
serve. When an arc is detected, the energy storage capacitor is
disconnected from the series inductor to stop the current rise. The
pulsing circuit is then disconnected from the plasma load and the
inductor energy is recycled to the energy storage capacitor.
[0013] For a typical sputtering plasma in a glow or abnormal glow
state, the proportion of ionized species is relatively low, on the
order of a few percent at most. Using the present invention,
sputtering plasmas in a highly ionized state may be achieved,
having ionization fractions of ten percent or more. In sputtering
systems wherein only very small ionization fractions are normally
present, such as systems for sputtering carbon, a highly ionized
plasma may be achieved using the present invention by raising the
proportion of ionized species in the plasma by a factor of five or
more.
[0014] Using the apparatus and method of this invention, a
sputtering plasma in a highly ionized state may be created without
first adopting an arc discharge state. The arc handling features of
the invention serve to mitigate and extinguish any arcs that
develop while the sputtering plasma is present.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic representation of a magnetron plasma
processing system incorporating the principles of this
invention.
[0016] FIG. 2 illustrates the waveforms for normal operation of the
magnetron plasma processing system shown in FIG. 1.
[0017] FIG. 3 illustrates the waveforms for arc handling
operation.
[0018] FIG. 4 illustrates the current, voltage, and impedance
characteristics of the sputtering plasma during operation of the
magnetron plasma processing system.
DETAILED DESCRIPTION
[0019] Referring to FIG. 1 there is shown one embodiment of a
magnetron plasma processing system 10. A DC power supply 12 is
connected to a magnetron plasma-processing chamber 14 via a pulsing
circuit 16. The magnetron plasma-processing chamber may be a
conventional magnetron chamber well known to those skilled in the
art having a magnetron cathode 18 and an anode 20. In sputtering
applications, a material target serves as the cathode 18. The
pulsed DC supply 16 may be of the type such as a MegaPulser.TM.
model manufactured by Advanced Energy Industries, Inc. which
supplies a high voltage pulse across the cathode 18 and anode 20 to
ignite a plasma 22 between the electrodes. The plasma acts upon the
material of the cathode 18 so as to result in a coating on a
substrate 26 located within the chamber. This is accomplished by
bombarding the material target or cathode 18 with ions from the
plasma 22, which results in the atoms sputtered from the target
being then deposited on the surface of the substrate 26. The
embodiment of FIG. 1 also comprises a smaller optional dc power
supply 28 that maintains a minimum voltage to the magnetron to keep
the plasma ignited between the high voltage pulses from the pulse
circuit 16. This power supply may also be used to initially ignite
the plasma before the high pulse operation begins.
[0020] For application of high voltage pulses to the magnetron
processing chamber the pulsing circuit 16 is comprised of energy
storage capacitor C.sub.1 serially connected to an inductor L.sub.1
via switch S.sub.1. The inductor L.sub.1 is connected to the
cathode of the magnetron via switch S.sub.2.
[0021] FIG. 2 illustrates the waveforms for normal operation of the
circuit. S.sub.2 is closed and S.sub.3 are open for the whole
sequence. The capacitor C, is charged to its initial voltage by the
dc power supply 12. The discharge is initiated by S.sub.1, and
capacitor C.sub.1 is discharged through inductor L.sub.1 into the
plasma load. A control circuit initiates the timing of the switches
to control the charge time of the capacitor C, and its pulse
discharge to the load. The shapes of the V.sub.C, I.sub.inductor,
V.sub.LOAD, and I.sub.load waveforms are determined solely by the
initial value of V.sub.C, the values of C.sub.1 and L.sub.1 and the
characteristics of the plasma load and the output cable.
[0022] FIG. 3 shows waveforms representative of an arc occurring
during a pulse from the pulsing circuit 16. The sequence begins as
shown in FIG. 2 and described above, but when an arc occurs the
current rises and the voltage falls until the arc is detected. Arcs
are detected by one of two means. Specific circuit techniques
required to implement the arc detection means are well known to
those skilled in the art. First, an arc may be detected as the load
current exceeding a preset threshold. This threshold can actually
be updated on a pulse by pulse basis by predicting the output
current, based on plasma load characteristics and the initial value
of V.sub.C and the values of C.sub.1 and L.sub.1 and adding a
margin to prevent false detections. Prediction of the output
current based on these parameters is well known to those skilled in
the art. Alternately, the current threshold may be based on the
average peak current, with some margin added to prevent false arc
detections. In this case, it may be desirable to leave pulses with
high arc currents out of the average calculation. Second, an arc
may be detected as the load voltage being below a preset threshold
when the load current is above a second current threshold, used
only for this second method. When the arc is detected, S.sub.1 is
opened immediately and S.sub.3 is closed after a short delay, and
then S.sub.2 is opened after another short delay. This disconnects
the load from the pulse circuit 16 and initiates the resonant
transfer of energy from inductor L.sub.1 to capacitor C.sub.1. The
result is that the energy present in inductor L.sub.1 when the arc
occurred is recycled to capacitor C.sub.1. This arc handling
sequence minimizes the energy delivered to the load in the event of
an arc. Without the arc handling provisions, the energy stored in
C.sub.1 and the energy stored in L.sub.1 would be delivered to the
arc, almost certainly causing damage to the target and the work
piece. Arc handling provisions enable commercial use of this
process.
[0023] The embodiment depicted in FIG. 1 also comprises circuitry
for shaping the voltage pulse delivered to the magnetron plasma. A
ring-up circuit 30 is provided for the purpose of controlling the
voltage rate of rise of the pulse, the magnitude of the voltage
pulse, and the frequency at which the voltage waveform rings. The
ring-up circuit 30 comprises a resistor R.sub.1 in series with a
capacitor C.sub.2 shunt connected at the power supply output. The
ring-up circuit may also be implemented as distributed networks of
discrete capacitors or resistors, and may make use of parasitic
capacitance or resistance found in other circuit elements or
components of the device. By shaping the voltage pulse delivered to
the plasma through use of the ring-up circuit, the occurrence of
arcing on initiation of the pulse may be eliminated, so that the
arc state is not entered during the initiation of the pulse.
[0024] The voltage pulse generated by the embodiment of FIG. 1 is
also shaped and controlled by a clamp circuit 32. The clamp circuit
comprises a diode D.sub.3, reverse biased in series with a
combination of a capacitor C.sub.3 in parallel with a clamp voltage
supply 34. The clamp circuit 32 acts to prevent the voltage level
of a pulse from exceeding a predetermined maximum level. By
limiting the pulse voltage, the clamp circuit prevents arcing
conditions from developing due to voltage excursions, as when for
example the pulse voltage increases due to parasitic capacitance at
the power supply output. Thus, the clamp voltage may be set to a
value that is high enough to allow the plasma to reach a highly
ionized state, but not so high as to lead to arcing conditions due
to overvoltage.
[0025] FIG. 4 demonstrates operation of the magnetron plasma
processing system to create a sputtering plasma in a highly ionized
state without first adopting an arc discharge state. Illustrated in
FIG. 4 are the voltage, current, and impedance characteristics of
the sputtering plasma as a function of time during one pulse in the
operation of the device. The pulsed DC power supply first applies a
high negative voltage, exceeding minus 1800 volts, to the material
target (cathode) of the sputtering system. As the plasma is
established, current develops through the plasma, rising to a level
approaching minus 400 amps. At this point, the plasma is in a
highly ionized state. At no time during the pulse lasting
approximately 150 microseconds does the voltage suddenly drop to
levels of a few tens of volts, which would be characteristic of an
arc discharge state. The plasma state can also be understood with
reference to the plasma impedance. As the plasma is established,
the plasma impedance is high at first, and then settles to a nearly
constant value of approximately 3.5 ohms. The impedance rises again
sharply at the end of the pulse as the current drops to zero. At no
time does the plasma impedance drop suddenly to arc levels, i.e.,
to values significantly below the steady state level of
approximately 3.5 ohms.
[0026] The present invention therefore provides a novel high peak
power plasma pulsed supply for magnetron sputtering with arc
handling that minimizes damage due to arcs. It accomplishes this by
tailoring the initiation of the pulse to prevent entering the arc
state before entering the highly ionized state, and further by
disconnecting the pulsing circuit from the plasma load and
recycling the inductor energy stored for the high peak power pulse
back to the energy storage capacitor at the detection of an arc
condition, should such a condition occur once the highly ionized
state is established.
[0027] Although there is illustrated and described specific
structure and details of operation, it is to be understood that
these descriptions are exemplary and that alternative embodiments
and equivalents may be readily made therein by those skilled in the
art without departing from the spirit and the scope of this
invention. Accordingly, the invention is intended to embrace all
such alternatives and equivalents that fall within the spirit and
scope of the appended claims.
* * * * *