U.S. patent application number 11/091814 was filed with the patent office on 2005-08-18 for high deposition rate sputtering.
Invention is credited to Chistyakov, Roman.
Application Number | 20050178654 11/091814 |
Document ID | / |
Family ID | 32296386 |
Filed Date | 2005-08-18 |
United States Patent
Application |
20050178654 |
Kind Code |
A1 |
Chistyakov, Roman |
August 18, 2005 |
High deposition rate sputtering
Abstract
Methods and apparatus for high-deposition sputtering are
described. A sputtering source includes an anode and a cathode
assembly that is positioned adjacent to the anode. The cathode
assembly includes a sputtering target. An ionization source
generates a weakly-ionized plasma proximate to the anode and the
cathode assembly. A power supply produces an electric field between
the anode and the cathode assembly that creates a strongly-ionized
plasma from the weakly-ionized plasma. The strongly-ionized plasma
includes a first plurality of ions that impact the sputtering
target to generate sufficient thermal energy in the sputtering
target to cause a sputtering yield of the sputtering target to be
non-linearly related to a temperature of the sputtering target.
Inventors: |
Chistyakov, Roman; (Andover,
MA) |
Correspondence
Address: |
RAUSCHENBACH PATENT LAW GROUP, LLC
P.O. BOX 387
BEDFORD
MA
01730
US
|
Family ID: |
32296386 |
Appl. No.: |
11/091814 |
Filed: |
March 28, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11091814 |
Mar 28, 2005 |
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10065739 |
Nov 14, 2002 |
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6896773 |
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Current U.S.
Class: |
204/192.12 ;
204/298.06; 204/298.07 |
Current CPC
Class: |
H01J 37/3467 20130101;
H01J 37/3476 20130101; H01J 37/32697 20130101; H01J 37/3405
20130101; C23C 14/35 20130101; C23C 14/3485 20130101; H01J 37/3429
20130101; C23C 14/542 20130101; H01J 37/3408 20130101; H01J 37/3266
20130101; H01J 37/3455 20130101; C23C 14/228 20130101; C23C 14/3414
20130101; H01L 21/02266 20130101; C23C 14/354 20130101; C23C
14/3492 20130101 |
Class at
Publication: |
204/192.12 ;
204/298.06; 204/298.07 |
International
Class: |
C23C 014/32 |
Claims
What is claimed is:
1-30. (canceled)
31. A sputtering source comprising: a) a cathode assembly that is
positioned adjacent to an anode, the cathode assembly including a
sputtering target; b) an ionization source that generates a
weakly-ionized plasma from a feed gas proximate to the anode and
the cathode assembly; and c) a power supply that generates a
voltage pulse between the anode and the cathode assembly that
creates a strongly-ionized plasma from the weakly-ionized plasma,
at least one of an amplitude, a rise time, and a duration of the
voltage pulse being chosen to increase a density of ions in the
strongly-ionized plasma enough to generate sufficient thermal
energy from ion bombardment of a surface layer of the sputtering
target to cause a sputtering yield to be related to a temperature
of the sputtering target.
32. The sputtering source of claim 31 wherein the sputtering yield
is related to a temperature of a surface of the sputtering
target.
33. The sputtering source of claim 31 wherein the sputtering yield
is linearly related to the temperature of the sputtering
target.
34. The sputtering source of claim 31 further comprising a gas flow
controller that controls a flow of the feed gas so that the feed
gas diffuses the strongly-ionized plasma.
35. The sputtering source of claim 34 wherein the controller
controls the flow of the feed gas to allow additional power to be
absorbed by the strongly ionized plasma, thereby generating
additional thermal energy in the sputtering target.
36. The sputtering source of claim 31 wherein the thermal energy
generated in the sputtering target from ion bombardment does not
substantially increase an average temperature of the sputtering
target.
37. The sputtering source of claim 31 further comprising a magnet
that is positioned to generate a magnetic field proximate to the
weakly-ionized plasma, the magnetic field substantially trapping
electrons in the weakly-ionized plasma proximate to the sputtering
target.
38. The sputtering source of claim 31 wherein the voltage pulse
generated between the anode and the cathode assembly excites atoms
in the weakly-ionized plasma and generates secondary electrons from
the cathode assembly, the secondary electrons ionizing a portion of
the excited atoms, thereby creating the strongly-ionized
plasma.
39. The sputtering source of claim 31 wherein the power supply
generates a constant power.
40. The sputtering source of claim 31 wherein the power supply
generates a constant voltage.
41. The sputtering source of claim 31 wherein the ionization source
is chosen from the group comprising an electrode coupled to a DC
power supply, an electrode coupled to an AC power supply, a UV
source, an X-ray source, an electron beam source, an ion beam
source, an inductively coupled plasma source, a capacitively
coupled plasma source, and a microwave plasma source.
42. The sputtering source of claim 31 wherein a rise time of the
voltage pulse is chosen to increase an ionization rate of the
strongly-ionized plasma.
43. The sputtering source of claim 31 wherein a presence of
weakly-ionized plasma reduces the probability of developing an
electrical breakdown condition when the power supply generates the
voltage pulse between the anode and the cathode assembly.
44. The sputtering source of claim 31 wherein the strongly-ionized
plasma is substantially non-uniform proximate to the cathode
assembly.
45. The sputtering source of claim 31 wherein a distance between
the anode and the cathode assembly is chosen to increase an
ionization rate of strongly-ionized plasma.
46. The sputtering source of claim 31 wherein the rise time of the
voltage pulse is in the range of approximately 0.1V/.mu.sec to
100V/.mu.sec.
47. The sputtering source of claim 31 wherein the discharge voltage
for the weakly ionized plasma is in the range of approximately 100V
to 1,000V.
48. The sputtering source of claim 31 wherein the amplitude of the
voltage pulse is in the range of approximately 200V to 30,000V.
49. The sputtering source of claim 31 wherein a pulse width of the
voltage pulse is in the range of approximately 0.1 .mu.sec to 100
sec.
50. A method for high deposition rate sputtering, the method
comprising: a) ionizing a feed gas to generate a weakly-ionized
plasma proximate to a cathode assembly that comprises a sputtering
target; and b) applying a voltage pulse to the cathode assembly to
generate a strongly-ionized plasma from the weakly-ionized plasma,
at least one of an amplitude, a rise time, and a duration of the
voltage pulse being chosen so that ions in the strongly-ionized
plasma bombard a surface layer of the sputtering target to generate
sufficient thermal energy in the surface layer of the sputtering
target to cause a sputtering yield to be related to a temperature
of the sputtering target.
51. The method of claim 50 wherein the sputtering yield is linearly
related to the temperature of the sputtering target.
52. The method of claim 50 wherein the sputtering yield is
non-linearly related to the temperature of the sputtering
target.
53. The method of claim 50 wherein the rise time of the voltage
pulse is in the range of approximately 0.1V/.mu.sec to
100V/.mu.sec.
54. The method of claim 50 wherein the amplitude of the voltage
pulse is in the range of approximately 200V to 10,000V.
55. The method of claim 50 wherein a pulse width of the voltage
pulse is in the range of approximately 0.1 .mu.sec to 100 sec.
56. The method of claim 50 further comprising generating a magnetic
field proximate to the sputtering target, the magnetic field
trapping electrons proximate to the sputtering target.
57. The method of claim 50 wherein the applying the voltage pulse
to the cathode assembly generates excited atoms in the
weakly-ionized plasma and generates secondary electrons from the
sputtering target, the secondary electrons ionizing a portion of
excited atoms, thereby creating the strongly-ionized plasma.
58. The method of claim 50 further comprising diffusing the
weakly-ionized plasma with a volume of the feed gas while ionizing
the volume of the feed gas to create additional weakly-ionized
plasma.
59. The method of claim 50 further comprising exchanging a volume
of feed gas to diffuse the strongly-ionized plasma while applying
the voltage pulse to the cathode assembly to generate additional
strongly-ionized plasma from the volume of the feed gas.
60. The method of claim 50 wherein a presence of weakly-ionized
plasma reduces the probability of developing an electrical
breakdown condition when the power supply generates a voltage pulse
between cathode and anode assembly.
61. The method of claim 50 wherein the ionizing the feed gas
comprises exposing the feed gas to one of a static electric field,
an AC electric field, a quasi-static electric field, a pulsed
electric field, UV radiation, X-ray radiation, an electron beam,
and an ion beam.
62. The method of claim 50 wherein the ions in the strongly-ionized
plasma cause a surface layer of the sputtering target to evaporate
during the applied voltage pulse.
Description
BACKGROUND OF INVENTION
[0001] Sputtering is a well-known technique for depositing films on
substrates. Sputtering is the physical ejection of atoms from a
target surface and is sometimes referred to as physical vapor
deposition (PVD). Ions, such as argon ions, are generated and then
directed to a target surface where the ions physically sputter
target material atoms. The target material atoms ballistically flow
to a substrate where they deposit as a film of target material.
[0002] Diode sputtering systems include a target and an anode.
Sputtering is achieved in a diode sputtering system by establishing
an electrical discharge in a gas between two parallel-plate
electrodes inside a chamber. A potential of several kilovolts is
typically applied between planar electrodes in an inert gas
atmosphere (e.g., argon) at pressures that are between about
10.sup.-1 and 10.sup.-2 Torr. A plasma discharge is then formed.
The plasma discharge is separated from each electrode by what is
referred to as the dark space.
[0003] The plasma discharge has a relatively constant positive
potential with respect to the target. Ions are drawn out of the
plasma, and are accelerated across the cathode dark space. The
target has a lower potential than the region in which the plasma is
formed. Therefore, the target attracts positive ions. Positive ions
move towards the target with a high velocity. Positive ions then
impact the target and cause atoms to physically dislodge or sputter
from the target. The sputtered atoms then propagate to a substrate
where they deposit a film of sputtered target material. The plasma
is replenished by electron-ion pairs formed by the collision of
neutral molecules with secondary electrons generated at the target
surface.
[0004] Magnetron sputtering systems use magnetic fields that are
shaped to trap and to concentrate secondary electrons, which are
produced by ion bombardment of the target surface. The plasma
discharge generated by a magnetron sputtering system is located
proximate to the surface of the target and has a high density of
electrons. The high density of electrons causes ionization of the
sputtering gas in a region that is close to the target surface.
[0005] One type of magnetron sputtering system is a planar
magnetron sputtering system. Planar magnetron sputtering systems
are similar in configuration to diode sputtering systems. However,
the magnets (permanent or electromagnets) in planar magnetron
sputtering systems are placed behind the cathode. The magnetic
field lines generated by the magnets enter and leave the target
cathode substantially normal to the cathode surface. Electrons are
trapped in the-electric and magnetic fields. The trapped electrons
enhance the efficiency of the discharge and reduce the energy
dissipated by electrons arriving at the substrate.
[0006] Conventional magnetron sputtering systems deposit films that
have relatively low uniformity. The film uniformity can be
increased by mechanically moving the substrate and/or the
magnetron. However, such systems are relatively complex and
expensive to implement. Conventional magnetron sputtering systems
also have relatively poor target utilization. The term "target
utilization" is defied herein to be a metric of how uniform the
target material erodes during sputtering. For example, high target
utilization would indicate that the target material erodes in a
highly uniform manner.
[0007] In addition, conventional magnetron sputtering systems have
a relatively low deposition rate. The term "deposition rate" is
defined herein to mean the amount of material deposited on the
substrate per unit of time. In general, the deposition rate is
proportional to the sputtering yield. The term "sputtering yield"
is defined herein to mean the number of target atoms ejected from
the target per incident particle. Thus, increasing the sputtering
yield will increase the deposition rate.
BRIEF DESCRIPTION OF DRAWINGS
[0008] This invention is described with particularity in the
detailed description. 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.
[0009] FIG. 1 illustrates a cross-sectional view of a known
magnetron sputtering apparatus having a pulsed power source.
[0010] FIG. 2 illustrates a cross-sectional view of a prior art
cathode assembly having a cathode cooling system.
[0011] FIG. 3 illustrates a known process for sputtering material
from a target.
[0012] FIG. 4 illustrates a cross-sectional view of an embodiment
of a magnetron sputtering apparatus according to the present
invention.
[0013] FIG. 5A through FIG. 5D illustrate cross-sectional views of
the magnetron sputtering apparatus of FIG. 4.
[0014] FIG. 6 illustrates graphical representations of the applied
voltage, current, and power as a function of time for periodic
pulses applied to the plasma in the magnetron sputtering apparatus
of FIG. 4.
[0015] FIG. 7A through FIG. 7D illustrate various simulated
magnetic field distributions proximate to the cathode assembly for
various electron ExB drift currents in a magnetically enhanced
plasma sputtering apparatus according to the invention.
[0016] FIG. 8 illustrates a graphical representation of sputtering
yield as a function of temperature of the sputtering target.
[0017] FIG. 9 illustrates a process for sputtering material from a
target according one embodiment of the present invention.
[0018] FIG. 10 illustrates a cross-sectional view of a cathode
assembly according to one embodiment of the invention.
[0019] FIG. 11 is a flowchart of an illustrative process of
enhancing a sputtering yield of a sputtering target according to
the present invention.
DETAILED DESCRIPTION
[0020] The sputtering process can be quantified in terms of the
sputtering yield. The term "sputtering yield" is defined herein to
mean the number of target atoms ejected from the target per
incident particle. The sputtering yield depends on several factors,
such as the target species, bombarding species, energy of the
bombarding ions, and the angle of incidence of the bombarding ions.
In typical known sputtering processes, the sputtering yield is
generally insensitive to target temperature.
[0021] The deposition rate of a sputtering process is generally
proportional to the sputtering yield. Thus, increasing the
sputtering yield typically will increase the deposition rate. One
way to increase the sputtering yield is to increase the ion density
of the plasma so that a larger ion flux impacts the surface of the
target. The density of the plasma is generally proportional to the
number of ionizing collisions in the plasma.
[0022] Magnetic fields can be used to confine electrons in the
plasma to increase the number of ionizing collisions between
electrons and neutral atoms in the plasma. The magnetic and
electric fields in magnetron sputtering systems are concentrated in
narrow regions close to the surface of the target. These narrow
regions are located between the poles of the magnets used for
producing the magnetic field. Most of the ionization of the
sputtering gas occurs in these localized regions. The location of
the ionization regions causes non-uniform erosion or wear of the
target that results in poor target utilization.
[0023] Increasing the power applied between the target and the
anode can increase the production of ionized gas and, therefore,
increase the target utilization and the sputtering yield. However,
increasing the applied power can lead to undesirable target heating
and target damage. Furthermore, increasing the voltage applied
between the target and the anode increases the probability of
establishing an undesirable electrical discharge (an electrical
arc) in the process chamber. An undesirable electrical discharge
can corrupt the sputtering process.
[0024] Pulsing the power applied to the plasma can be advantageous
since the average discharge power can remain low while relatively
large power pulses are periodically applied. Additionally, the
duration of these large voltage pulses can be preset so as to
reduce the probability of establishing an electrical breakdown
condition leading to an undesirable electrical discharge. However,
very large power pulses can still result in undesirable electrical
discharges and undesirable target heating regardless of their
duration.
[0025] FIG. 1 illustrates a cross-sectional view of a known
magnetron sputtering apparatus 100 having a pulsed power source
102. The known magnetron sputtering apparatus 100 includes a vacuum
chamber 104 where the sputtering process is performed. The vacuum
chamber 104 is positioned in fluid communication with a vacuum pump
106 via a conduit 108. The vacuum pump 106 is adapted to evacuate
the vacuum chamber 104 to high vacuum. The pressure inside the
vacuum chamber 104 is generally less than 100 Pa during operation.
A feed gas source 109, such as an argon gas source, is coupled to
the vacuum chamber 104 by a gas inlet 110. A valve 112 controls the
gas flow from the feed gas source 109.
[0026] The magnetron sputtering apparatus 100 also includes a
cathode assembly 114 having a target 116. The cathode assembly 114
is generally in the shape of a circular disk. The cathode assembly
114 is electrically connected to a first output 118 of the pulsed
power supply 102 with an electrical transmission line 120. The
cathode assembly 114 is typically coupled to the negative potential
of the pulsed power supply 102. In order to isolate the cathode
assembly 114 from the vacuum chamber 104, an insulator 122 can be
used to pass the electrical transmission line 120 through a wall of
the vacuum chamber 104. A grounded shield 124 can be positioned
behind the cathode assembly 114 to protect a magnet 126 from
bombarding ions. The magnet 126 shown in FIG. 1 is generally shaped
in the form of a ring that has its south pole 127 on the inside of
the ring and its north pole 128 on the outside of the ring. Many
other magnet configurations can also be used.
[0027] An anode 130 is positioned in the vacuum chamber 104
proximate to the cathode assembly 114. The anode 130 is typically
coupled to ground. A second output 132 of the pulsed power supply
102 is also typically coupled to ground. A substrate 134 is
positioned in the vacuum chamber 104 on a substrate support 135 to
receive the sputtered target material from the target 116. The
substrate 134 can be electrically connected to a bias voltage power
supply 136 with a transmission line 138. In order to isolate the
bias voltage power supply 136 from the vacuum chamber 104, an
insulator 140 can be used to pass the electrical transmission line
138 through a wall of the vacuum chamber 104.
[0028] In operation, the pulsed power supply 102 applies a voltage
pulse between the cathode assembly 114 and the anode 130 that has a
sufficient amplitude to ionize the argon feed gas in the vacuum
chamber 104. The typical ionization process is referred to as
direct ionization or atomic ionization by electron impact and can
be described as follows:
Ar+e.sup.-.fwdarw.Ar.sup.++2e.sup.-
[0029] where Ar represents a neutral argon atom in the feed gas and
e.sup.- represents an ionizing electron generated in response to
the voltage pulse applied between the cathode assembly 114 and the
anode 130. The collision between the neutral argon atom and the
ionizing electron results in an argon ion (Ar.sup.+) and two
electrons.
[0030] The negatively biased cathode assembly 114 attracts
positively charged ions with sufficient acceleration so that the
ions sputter the target material from the target 116. A portion of
the sputtered target material is deposited on the substrate
134.
[0031] The electrons, which cause the ionization, are generally
confined by the magnetic fields produced by the magnet 126. The
magnetic confinement is strongest in a confinement region 142 where
there is relatively low magnetic field intensity. The confinement
region 142 is substantially in the shape of a ring that is located
proximate to the target material. Generally, a higher concentration
of positively charged ions in the plasma is present in the
confinement region 142 than elsewhere in the chamber 104.
Consequently, the target 116 is eroded more rapidly in areas
directly adjacent to the higher concentration of positively charged
ions. The rapid erosion in these areas results in undesirable
non-uniform erosion of the target 116 and, thus relatively poor
target utilization.
[0032] Dramatically increasing the power applied to the plasma can
result in more uniform erosion of the target 116 and higher
sputtering yield. However, the amount of applied power necessary to
achieve this increased uniformity can increase the probability of
generating an electrical breakdown condition that leads to an
undesirable electrical discharge between the cathode assembly 114
and the anode 130 regardless of the duration of the pulses. An
undesirable electrical discharge will corrupt the sputtering
process and cause contamination in the vacuum chamber 104.
Additionally, the increased power can overheat the target and cause
target damage.
[0033] Sputtering yields are generally determined experimentally.
The yield dependence on the bombarding ion energy approximately
exhibits a threshold that is between about 10-30 eV, followed by a
nearly linear range that extends to several hundred eV. At higher
energies, the dependence is less than linear. Sputtering processes
are generally most energy efficient when the ion energies are
within the linear range.
[0034] Sputtering systems are generally calibrated to determine the
deposition rate under certain operating conditions. The erosion
rate of the target 116 can be expressed by the following equation:
1 R = k JYM A . / min
[0035] where k is a constant, J is the ion current density in
mA/cm.sup.2, Y is the sputtering yield in atoms/ion, and M is the
atomic weight in grams, and .rho. is the density in gm/cm.sup.3 of
the target material. The deposition rate is generally proportional
to the sputtering yield Y.
[0036] FIG. 2 illustrates a cross-sectional view of a prior art
cathode assembly 114' having a cathode cooling system. The cathode
assembly 114' includes target 116'. The cathode cooling system also
includes a conduit 150 that contains a fluid 152 for conducting
heat away from the cathode assembly 114'. The fluid 152 can be a
liquid coolant or a gas, for example.
[0037] In operation, ions 154 in a plasma impact a surface 156 of
the target 116'. The impact of the ions 154 generates heat 158 at
the surface 156. Additionally, the impact of the ions 154
eventually dislodges atoms 160 from the surface 156 of the target
116' causing sputtering. The heat 158 that is generated by the ion
impact radiates through the cathode assembly 114'. The cathode
assembly 114' is in thermal communication with the conduit 150. The
fluid 152 absorbs the heat 158 and transfers it away from the
cathode assembly 114'.
[0038] FIG. 3 illustrates a known process for sputtering material
from a target 116'. An ion 154 having a mass M.sub.i and a velocity
v.sub.i impacts a target particle 162 having a mass M.sub.t which
is initially at rest on the surface 156 of the target 116'. The ion
154 impacts the surface 156 at normal incidence. The momentum from
the ion 154 transfers to the target particle 162 driving the target
particle 162 into the target 116'.
[0039] Thus, the ejection of a sputtered particle 164 from the
target 114' generally requires a sequence of collisions for a
component of the initial momentum vector to change by more than
ninety degrees. Typically, an incident ion 154 experiences a
cascade of collisions and its energy is partitioned over a region
of the target surface 156. However, the sputtering momentum
exchange occurs primarily within a region extending only about ten
angstroms below the surface 156. The incident ion 154 generally
strikes two lattice atoms 166, 168 almost simultaneously. This low
energy knock-on receives a side component of momentum and initiates
sputtering of one or more of its neighbors. The primary knock-on is
driven into the target 114', where it can be reflected and
sometimes returned to the surface 156 to produce sputtering by
impacting the rear of a surface atom 170.
[0040] A fraction of the kinetic energy of the incident ion 154 is
transferred to the target particle 162. This kinetic energy
transfer function can be expressed as follows: 2 = 4 M i M t ( M i
+ M t ) 2
[0041] The sputtering yield Y can be expressed as follows, assuming
perpendicular ion incidence onto a substantially planar surface
156: 3 Y = k .times. .times. E U .times. ( M t / M i )
[0042] where k is a constant, .epsilon. is the energy transfer
function, .alpha. is a near-linear function of the ratio of the
mass of the target atom 162 to the mass of the incident ion 154, E
is the kinetic energy of the incident ion 154, and U is the surface
binding energy for the target material. For most sputtering
materials, the mass dependence of .epsilon. .alpha. does not vary
greatly from one material to another. The primary
material-sensitive factor is the surface binding energy, and this
has only a first power dependence.
[0043] At energies above 20-30 eV, heavy particles can sputter
atoms from a surface of a target. The sputtering yield increases
rapidly with energy up to a few hundred eV, with 500-1000 eV argon
ions being typically used for physical sputtering. Above a few
hundred eV, there is a possibility that ions 154 will be implanted
in the target 116'. This can especially occur at energies over 1
keV.
[0044] FIG. 4 illustrates a cross-sectional view of an embodiment
of a magnetron sputtering apparatus 200 according to the present
invention. The magnetron sputtering apparatus 200 includes a
chamber 202, such as a vacuum chamber. The chamber 202 is coupled
in fluid communication to a vacuum system 204 through a vacuum
valve 206. In one embodiment, the chamber 202 is electrically
coupled to ground potential.
[0045] The chamber 202 is coupled to a feed gas source 208 by one
or more gas lines 207. In one embodiment, the gas lines 207 are
isolated from the chamber and other components by insulators 209.
Additionally, the gas lines 207 can be isolated from the feed gas
source using in-line insulating couplers (not shown). A gas flow
control system 210 controls the gas flow to the chamber 202. The
gas source 208 can contain any feed gas. For example, the feed gas
can be a noble gas or a mixture of noble gases. The feed gas can
also be a reactive gas, a non-reactive gas, or a mixture of both
reactive and non-reactive gases.
[0046] A substrate 211 to be sputter coated is supported in the
chamber 202 by a substrate support 212. The substrate 211 can be
any type of work piece such as a semiconductor wafer. In one
embodiment, the substrate support 212 is electrically coupled to an
output 213 of a bias voltage source 214. An insulator 215 isolates
the bias voltage source 214 from a wall of the chamber 202. In one
embodiment, the bias voltage source 214 is an alternating current
(AC) power source, such as a radio frequency (RF) power source. In
other embodiments (not shown), the substrate support 212 is coupled
to ground potential or is electrically floating.
[0047] The magnetron sputtering apparatus 200 also includes a
cathode assembly 216. In one embodiment, the cathode assembly 216
includes a cathode 218 and a sputtering target 220 composed of
target material. In one embodiment, the cathode 218 is formed of a
metal. In one embodiment, the cathode 218 is formed of a chemically
inert material, such as stainless steel. The sputtering target 220
is in physical contact with the cathode 218. In one embodiment, the
sputtering target 220 is positioned inside the cathode 218 as shown
in FIG. 4. The distance from the sputtering target 220 to the
substrate 211 can vary from a few centimeters to about one hundred
centimeters.
[0048] The target material can be any material suitable for
sputtering. For example, the target material can be a metallic
material, polymer material, superconductive material, magnetic
material including ferromagnetic material, non-magnetic material,
conductive material, non-conductive material, composite material,
reactive material, or a refractory material.
[0049] The cathode assembly 216 is coupled to an output 222 of a
matching unit 224. An insulator 226 isolates the cathode assembly
216 from a grounded wall of the chamber 202. An input 230 of the
matching unit 224 is coupled to a first output 232 of a pulsed
power supply 234. A second output 236 of the pulsed power supply
234 is coupled to an anode 238. An insulator 240 isolates the anode
238 from a grounded wall of the chamber 202. Another insulator 242
isolates the anode 238 from the cathode assembly 216.
[0050] In one embodiment (not shown), the first output 232 of the
pulsed power supply 234 is directly coupled to the cathode assembly
216. In one embodiment (not shown), the second output 236 of the
pulsed power supply 234 and the anode 238 are both coupled to
ground. In one embodiment (not shown), the first output 232 of the
pulsed power supply 234 couples a negative voltage impulse to the
cathode assembly 216. In another embodiment (not shown), the second
output 236 of the pulsed power supply 234 couples a positive
voltage impulse to the anode 238.
[0051] In one embodiment, the pulsed power supply 234 generates
peak voltage levels of between about 5 kV and about 30 kV. In one
embodiment, operating voltages are generally between about 50V and
1 kV. In one embodiment, the pulsed power supply 234 sustains
discharge current levels that are on order of about 1 A to 5,000 A
depending on the volume of the plasma. Typical operating currents
varying from less than about one hundred amperes to more than a few
thousand amperes depending on the volume of the plasma. In one
embodiment, the power pulses have a repetition rate that is below 1
kHz. In one embodiment, the pulse width of the pulses generated by
the pulsed power supply 234 is substantially between about one
microsecond and several seconds.
[0052] The anode 238 is positioned so as to form a gap 244 between
the anode 238 and the cathode assembly 216 that is sufficient to
allow current to flow through a region 245 between the anode 238
and the cathode assembly 216. In one embodiment, the gap 244 is
between approximately 0.3 cm and 10 cm. The surface area of the
cathode assembly 216 determines the volume of the region 245. The
gap 244 and the total volume of the region 245 are parameters in
the ionization process as described herein.
[0053] An anode shield 248 is positioned adjacent to the anode 238
and functions as an electric shield to electrically isolate the
anode 238 from the plasma. In one embodiment, the anode shield 248
is coupled to ground potential. An insulator 250 is positioned to
isolate the anode shield 248 from the anode 238.
[0054] The magnetron sputtering apparatus 200 also includes a
magnet assembly 252. In one embodiment, the magnet assembly 252 is
adapted to create a magnetic field 254 proximate to the cathode
assembly 216. The magnet assembly 252 can include permanent magnets
256, or alternatively, electro-magnets (not shown). The
configuration of the magnet assembly 252 can be varied depending on
the desired shape and strength of the magnetic field 254. The
magnet assembly can have either a balanced or unbalanced
configuration.
[0055] In one embodiment, the magnet assembly 252 includes
switching electro-magnets, which generate a pulsed magnetic field
proximate to the cathode assembly 216. In some embodiments,
additional magnet assemblies (not shown) can be placed at various
locations around and throughout the chamber 202 to direct different
types of sputtered target materials to the substrate 212.
[0056] In one embodiment, the magnetron sputtering apparatus 200 is
operated by generating the magnetic field 254 proximate to the
cathode assembly 216. In the embodiment shown in FIG. 2, the
permanent magnets 256 continuously generate the magnetic field 254.
In other embodiments, electro-magnets (not shown) generate the
magnetic field 254 by energizing a current source that is coupled
to the electro-magnets. In one embodiment, the strength of the
magnetic field 254 is between about fifty gauss and two thousand
gauss. After the magnetic field 254 is generated, the feed gas from
the gas source 208 is supplied to the chamber 202 by the gas flow
control system 210.
[0057] In one embodiment, the feed gas is supplied to the chamber
202 directly between the cathode assembly 216 and the anode 238.
Directly injecting the feed gas between the cathode assembly 216
and the anode 238 can increase the flow rate of the gas between the
cathode assembly 216 and the anode 238. Increasing the flow rate of
the gas allows longer duration impulses and thus, can result in the
formation higher density plasmas. The flow of the feed gas is
further discussed herein.
[0058] In one embodiment, the pulsed power supply 234 is a
component of an ionization source that generates the weakly-ionized
plasma. The pulsed power supply applies a voltage pulse between the
cathode assembly 216 and the anode 238. In one embodiment, the
pulsed power supply 234 applies a negative voltage pulse to the
cathode assembly 216. The amplitude and shape of the voltage pulse
are such that a weakly-ionized plasma is generated in the region
246 between the anode 238 and the cathode assembly 216.
[0059] The weakly-ionized plasma is also referred to as a
pre-ionized plasma. In one embodiment, the peak plasma density of
the pre-ionized plasma is between about 10.sup.6 and 10.sup.12
cm.sup.-3 for argon feed gas. In one embodiment, the pressure in
the chamber varies from about 10.sup.-3 to 10 Torr. The peak plasma
density of the pre-ionized plasma depends on the properties of the
specific plasma processing system.
[0060] In one embodiment, the pulsed power supply 234 generates a
low power pulse having an initial voltage that is between about
100V and 5 kV with a discharge current that is between about 0.1 A
and 100 A in order to generate the weakly-ionized plasma. In some
embodiments the width of the pulse can be on the order of 0.1
microseconds to one hundred seconds. Specific parameters of the
pulse are discussed herein in more detail.
[0061] In one embodiment, the pulsed power supply 234 applies a
voltage between the cathode assembly 216 and the anode 238 before
the feed gas is supplied between the cathode assembly 216 and the
anode 238. In another embodiment, the pulsed power supply 234
applies a voltage between the cathode assembly 216 and the anode
238 after the feed gas is supplied between the cathode assembly 216
and the anode 238.
[0062] In one embodiment, a direct current (DC) power supply (not
shown) is used to generate and maintain the weakly-ionized or
pre-ionized plasma. In this embodiment, the DC power supply is
adapted to generate a voltage that is large enough to ignite the
pre-ionized plasma. In one embodiment, the DC power supply
generates an initial voltage of several kilovolts between the
cathode assembly 216 and the anode 238 in order to generate and
maintain the pre-ionized plasma. The initial voltage between the
cathode assembly 216 and the anode 238 creates a plasma discharge
voltage that is on the order of 100V to 1000V with a discharge
current that is on the order of 0.1 A to 100 A.
[0063] The direct current required to generate and maintain the
pre-ionized plasma is a function of the volume of the plasma. In
addition, the current required to generate and maintain the
pre-ionized plasma is a function of the strength of the magnetic
field in the region 245. For example, in one embodiment, the DC
power supply generates a current that is on order of 1 mA to 100 A
depending on the volume of the plasma and the strength of the
magnetic field in the region 245. The DC power supply can be
adapted to generate and maintain an initial peak voltage between
the cathode assembly 216 and the anode 238 before the introduction
of the feed gas.
[0064] In another embodiment, an alternating current (AC) power
supply (not shown) is used to generate and maintain the
weakly-ionized or pre-ionized plasma. For example, the
weakly-ionized or pre-ionized plasma can be generated and
maintained using electron cyclotron resonance (ECR), capacitively
coupled plasma discharge (CCP), or inductively coupled plasma (ICP)
discharge. AC power supplies can require less power to generate and
maintain a weakly-ionized plasma than a DC power supply. In
addition, the pre-ionized or weakly-ionized plasma can be generated
by numerous other techniques, such as UV radiation techniques,
X-ray techniques, electron beam techniques, ion beam techniques, or
ionizing filament techniques. In some embodiments, the
weakly-ionized plasma is formed outside of the region 245 and then
diffuses into the region 245.
[0065] Forming a weakly-ionized or pre-ionized plasma substantially
eliminates the probability of establishing a breakdown condition in
the chamber 202 when high-power pulses are applied between the
cathode assembly 216 and the anode 238. Uniformly distributing the
weakly-ionized or pre-ionized plasma over the cathode area results
in a more uniform strongly ionized plasma when a high power pulse
is applied. The probability of establishing a breakdown condition
is substantially eliminated because the weakly-ionized plasma has a
low-level of ionization that provides electrical conductivity
through the plasma. This conductivity greatly reduces or prevents
the possibility of a breakdown condition when high power is applied
to the plasma.
[0066] Once the weakly-ionized plasma is formed, high-power pulses
are then generated between the cathode assembly 216 and the anode
238. In one embodiment, the pulsed power supply 234 generates the
high-power pulses. The desired power level of the high-power pulse
depends on several factors including the desired deposition rate,
the density of the pre-ionized plasma, and the volume of the
plasma, for example. In one embodiment, the power level of the
high-power pulse is in the range of about 1 kW to about 10 MW. In
one embodiment, the high-power pulses are rapidly applied across
the weakly-ionized plasma. In one embodiment, the high-power pulses
are substantially instantly applied across the weakly-ionized
plasma in a substantially explosive manner. This rapid application
of the high-power pulses can result in a surface layer of the
target 220 being almost instantly evaporated.
[0067] Each of the high-power pulses are maintained for a
predetermined time that, in one embodiment, is in the range of
about one microsecond to about ten seconds. In one embodiment, the
repetition frequency or repetition rate of the high-power pulses is
in the range of between about 0.1 Hz to 1 kHz. In order to minimize
undesirable substrate heating, the average power generated by the
pulsed power supply 234 can be less than one megawatt depending on
the volume of the plasma.
[0068] In one embodiment, the high-power pulse is applied so
rapidly that a surface layer of the target 220 is substantially
vaporized and only a small quantity of heat is conducted through
the cathode assembly 216. In one embodiment, the thermal energy in
at least one of the cathode assembly 216, the anode 238, and the
substrate support 212 is conducted away or dissipated by liquid or
gas cooling such as helium cooling (not shown).
[0069] The high-power pulses generate a strong electric field
between the cathode assembly 216 and the anode 238. This strong
electric field is substantially located in the region 245 across
the gap 244 between the cathode assembly 216 and the anode 238. In
one embodiment, the electric field is a pulsed electric field. In
another embodiment, the electric field is a quasi-static electric
field. The term "quasi-static electric field" is defined herein to
mean an electric field that has a characteristic time of electric
field variation that is much greater than the collision time for
electrons with neutral gas particles. Such a time of electric field
variation can be on the order of ten seconds. The strength and the
position of the strong electric field will be discussed in more
detail herein.
[0070] The high-power pulses generate a highly-ionized or a
strongly-ionized plasma from the weakly-ionized plasma. For
example, the discharge current that is formed from this
strongly-ionized plasma can be on the order of 5 kA for a pressure
that is on the order of about 100 mTorr and 10 Torr.
[0071] Since the sputtering target 220 is typically negatively
biased, the positively charged ions in the strongly-ionized plasma
accelerate at high velocity towards the sputtering target 220. The
accelerated ions impact the surface of the sputtering target 220,
causing the target material to be sputtered. The strongly-ionized
plasma of the present invention results in a very high sputtering
rate of the target material.
[0072] In addition, the strongly-ionized plasma tends to diffuse
homogenously in the region 246 and, therefore tends to create a
more homogeneous plasma volume. The homogenous diffusion results in
accelerated ions impacting the surface of the sputtering target 220
in a more uniform manner than with conventional magnetron
sputtering. Consequently, the surface of the sputtering target 220
is eroded more evenly and, thus higher target utilization is
achieved. Furthermore, since the target material is sputtered more
uniformly across the surface of the sputtering target 220, the
uniformity and homogeneity of the material deposited on the
substrate 211 is also increased without the necessity of rotating
the substrate 211 and/or the magnet assembly 252.
[0073] In one embodiment, the high-power pulsed magnetron
sputtering system 200 of the present invention generates a
relatively high electron temperature plasma and a relatively high
density plasma. One application for the high-power pulsed magnetron
sputtering system 200 of the present invention is ionized physical
vapor deposition (IPVD), which is a technique that converts neutral
sputtered atoms into positive ions in order to enhance the
sputtering process.
[0074] FIG. 5A through FIG. 5D illustrate cross-sectional views of
the sputtering apparatus 200 having the pulsed power supply 234.
For example, FIG. 5A illustrates a cross-sectional view of the
sputtering apparatus 200 having the pulsed power supply 234 at a
time before the pulsed power supply 234 is activated. FIG. 5A
illustrates the cathode assembly 216 including the sputtering
target 220. The cathode assembly 216 is coupled to the output 222
of the matching unit 224. The input 230 of the matching unit 224 is
coupled to the first output 232 of the pulsed power supply 234. The
second output 236 of the pulsed power supply 234 is coupled to the
anode 238.
[0075] The anode 238 is positioned so as to form a gap 244 between
the anode 238 and the cathode assembly 216 that is sufficient to
allow current to flow through the region 245 between the anode 238
and the cathode assembly 216. In one embodiment, the width of the
gap 244 is between approximately 0.3 cm and 10 cm. The surface area
of the cathode assembly 216 determines the volume of the region
245. The gap 244 and the total volume of the region 245 are
parameters in the ionization process as described herein. The gas
lines 207 provide feed gas 256 from the feed gas source 208 (FIG.
4) proximate to the anode 238 and the cathode assembly 216.
[0076] In operation, the feed gas 256 from the gas source 208 is
supplied by the gas flow control system 210 (FIG. 4). Preferably,
the feed gas 256 is supplied between the cathode assembly 216 and
the anode 238. Directly injecting the feed gas 256 between the
cathode assembly 216 and the anode 238 can increase the flow rate
of the feed gas 256. This causes a rapid volume exchange in the
region 245 between the cathode assembly 216 and the anode 238,
which permits a high power pulse having a longer duration to be
applied across the gap 244. The longer duration high power pulse
results in the formation of more dense plasma. This volume exchange
is described herein in more detail.
[0077] FIG. 5B illustrates the cathode assembly 216 after the feed
gas 256 is supplied between the cathode assembly 216 and the anode
238. The pulsed power supply 234 applies a voltage pulse between
the cathode assembly 216 and the anode 238. In one embodiment, the
pulsed power supply 234 applies a negative voltage pulse to the
cathode assembly 216. The characteristics of the voltage pulse are
chosen such that an electric field 260 develops between the cathode
assembly 216 and the anode 238 that creates a weakly-ionized plasma
262 in the region 245 between the anode 238 and the cathode
assembly 216. The weakly-ionized plasma 262 is also referred to as
a pre-ionized plasma.
[0078] In one embodiment, the pulsed power supply 234 generates the
weakly-ionized plasma 262 by generating a low power pulse having an
initial voltage that is in the range of 100V to 5 kV with a
discharge current that is in the range of 0.1 A to 100 A. In some
embodiments, the width of the pulse can be in the range of 0.1
microseconds to one hundred seconds. Specific parameters of the
pulse are discussed herein in more detail.
[0079] In another embodiment, an alternating current (AC) power
supply (not shown) is used to generate and maintain the
weakly-ionized or pre-ionized plasma 262. The weakly-ionized or
pre-ionized plasma 262 can be generated and maintained using
electron cyclotron resonance (ECR), capacitively coupled plasma
discharge (CCP), or inductively coupled plasma (ICP) discharge.
Generating the pre-ionized plasma using an AC power supply can be
more energy efficient than generating the pre-ionized plasma using
a DC power supply.
[0080] In addition, the pre-ionized or weakly-ionized plasma 262
can be generated by numerous other techniques, such as UV radiation
techniques, X-ray techniques, electron beam techniques, ion beam
techniques, or ionizing filament techniques. These techniques
include components used in ionization sources according to the
invention. In some embodiments, the weakly-ionized plasma is formed
outside of the region 245 and then diffuses into the region
245.
[0081] In one embodiment, as the feed gas 256 is pushed through the
region 245, the weakly-ionized plasma 262 diffuses generally in a
homogeneous manner through the region 264. The homogeneous
diffusion tends to facilitate the creation of a highly uniform
strongly-ionized plasma in the region 264. In one embodiment (not
shown), the weakly-ionized plasma 262 is trapped proximate to the
cathode assembly 216 by a magnetic field. Specifically, electrons
in the weakly-ionized plasma 262 are trapped by a magnetic field
generated proximate to the cathode assembly 216. In one embodiment,
the strength of the magnetic field is in the range of fifty to two
thousand gauss.
[0082] The pulsed power supply 234 generates high-power pulses
between the cathode assembly 216 and the anode 238 (FIG. 5C) after
the weakly-ionized plasma 262 is formed. The desired power level of
the high-power pulses depends on several factors including the
density of the weakly-ionized plasma 262 and the volume of the
plasma. In one embodiment, the power level of the high-power pulse
is in the range of about 1 kW to about 10 MW or higher.
[0083] The high-power pulses generate a strong electric field 266
between the cathode assembly 216 and the anode 238. The strong
electric field 266 is substantially located in the region 245
between the cathode assembly 216 and the anode 238. In one
embodiment, the electric field 266 is a pulsed electric field. In
another embodiment, the electric field 266 is a quasi-static
electric field. The strength and the position of the strong
electric field 266 will be discussed in more detail herein.
[0084] FIG. 5D illustrates the high-power pulses generating a
highly-ionized or a strongly-ionized plasma 268 from the
weakly-ionized plasma 262. The strongly-ionized plasma 268 is also
referred to as a high-density plasma. The discharge current that is
formed from the strongly-ionized plasma 268 can be on the order of
about 1,000 A or more with a discharge voltage in the range of 50V
to 1,000V for a pressure that is in the range of 5 mTorr and 10
Torr. In one embodiment, the strongly-ionized plasma 268 tends to
diffuse homogenously in the region 264. The homogenous diffusion
creates a more homogeneous plasma volume.
[0085] The homogenous diffusion results in accelerated ions 272 in
the strongly-ionized plasma 268 impacting the surface of the
sputtering target 220 in a more uniform manner than with
conventional magnetron sputtering. Consequently, the surface of the
sputtering target 220 is eroded more evenly and, thus higher target
utilization is achieved. Furthermore, since the target material is
sputtered more uniformly across the surface of the sputtering
target 220, the target material is deposited more uniformly on the
substrate 211 (FIG. 4) without the necessity of rotating the
substrate and/or the magnetron.
[0086] The strong electric field 266 facilitates a multi-step
ionization process of the feed gas 256 that substantially increases
the rate at which the strongly-ionized plasma 268 is formed.
Referring again to FIG. 2D, the multi-step or stepwise ionization
process is described as follows. A pre-ionizing voltage is applied
between the cathode assembly 216 and the anode 238 across the feed
gas 256, which forms the weakly-ionized plasma 262. The
weakly-ionized plasma 262 is generally formed in the region 245 and
diffuses to the region 264 as the feed gas 256 continues to
flow.
[0087] In one embodiment, a magnetic field 254 (FIG. 4) is
generated in the region 245 and extends to the center of the
cathode assembly 216. This magnetic field tends to assist in
diffusing electrons from the region 245 to the region 264. The
electrons in the weakly-ionized plasma 262 are substantially
trapped in the region 264 by the magnetic field 245. In one
embodiment, the volume of weakly-ionized plasma 262 in the region
245 is rapidly exchanged with a fresh volume of feed gas 256.
[0088] The pulsed power supply 234 applies a high-power pulse
between the cathode assembly 216 and the anode 238 after the
formation of the weakly-ionized plasma 262 (FIG. 2C). This
high-power pulse generates the strong electric field 266 in the
region 245 between the cathode assembly 216 and the anode 238. The
strong electric field 266 results in collisions occurring between
neutral atoms 270, electrons (not shown), and ions 272 in the
weakly-ionized plasma 262. These collisions generate numerous
excited atoms 274 in the weakly-ionized plasma 262.
[0089] The accumulation of excited atoms 274 in the weakly-ionized
plasma 262 alters the ionization process. In one embodiment, the
strong electric field 266 facilitates a multi-step ionization
process of an atomic feed gas that significantly increases the rate
at which the strongly-ionized plasma 268 is formed. The multi-step
ionization process has an efficiency that increases as the density
of excited atoms 274 in the weakly-ionized plasma 262 increases. In
one embodiment the strong electric field 266 enhances the formation
of ions of a molecular or atomic feed gas.
[0090] In one embodiment, the distance or gap 244 between the
cathode assembly 216 and the anode 238 is chosen so as to maximize
the rate of excitation of the atoms. The value of the electric
field 266 in the region 245 depends on the voltage level applied by
the pulsed power supply 234 and the dimensions of the gap 244
between the anode 238 and the cathode assembly 216. In some
embodiments, the strength of the electric field 266 is in range of
10V/cm to 10.sup.5V/cm depending on various system parameters and
operating conditions of the plasma system.
[0091] In some embodiments, the gap 244 is in the range of 0.30 cm
to 10 cm depending on various parameters of the desired plasma. In
one embodiment, the electric field 266 in the region 245 is rapidly
applied to the pre-ionized or weakly-ionized plasma 262. In some
embodiments, the rapidly applied electric field 266 is generated by
applying a voltage pulse having a rise time that is in the range of
0.1 microsecond to ten seconds.
[0092] In one embodiment, the dimensions of the gap 244 and the
parameters of the applied electric field 266 are varied to
determine the optimum condition for a relatively high rate of
excitation of the atoms 270 in the region 245. For example, an
argon atom requires an energy of about 11.55 eV to become excited.
Thus, as the feed gas 256 flows through the region 245, the
weakly-ionized plasma 262 is formed and the atoms 270 in the
weakly-ionized plasma 262 experience a stepwise ionization
process.
[0093] The excited atoms 274 in the weakly-ionized plasma 262 then
encounter the electrons (not shown) that are in the region 264. The
excited atoms 274 only require about 4 eV of energy to ionize while
neutral atoms 270 require about 15.76 eV of energy to ionize.
Therefore, the excited atoms 274 will ionize at a much higher rate
than the neutral atoms 270. In one embodiment, ions 272 in the
strongly-ionized plasma 268 strike the cathode assembly 216 causing
secondary electron emission. These secondary electrons interact
with neutral 270 or excited atoms 274 in the strongly-ionized
plasma 268. This process further increases the density of ions 272
in the strongly-ionized plasma 268 as the feed gas 256 is
replenished.
[0094] The multi-step ionization process corresponding to the rapid
application of the electric field 266 can be described as
follows:
Ar+e.sup.-.fwdarw.Ar*+e.sup.-
Ar.sup.*+e.sup.-.fwdarw.Ar.sup.++2e.sup.-
[0095] where Ar represents a neutral argon atom 270 in the feed gas
256 and e.sup.- represents an ionizing electron generated in
response to a pre-ionized plasma 262, when sufficient voltage is
applied between the cathode assembly 216 and the anode 238.
Additionally, Ar* represents an excited argon atom 274 in the
weakly-ionized plasma 262. The collision between the excited argon
atom 274 and the ionizing electron results in an argon ion
(Ar.sup.+) and two electrons.
[0096] The excited argon atoms 274 generally require less energy to
become ionized than neutral argon atoms 270. Thus, the excited
atoms 274 tend to more rapidly ionize near the surface of the
cathode assembly 216 than the neutral argon atoms 270. As the
density of the excited atoms 274 in the plasma increases, the
efficiency of the ionization process rapidly increases. This
increased efficiency eventually results in an avalanche-like
increase in the density of the strongly-ionized plasma 268. Under
appropriate excitation conditions, the proportion of the energy
applied to the weakly-ionized plasma 262 that is transformed to the
excited atoms 274 is very high for a pulsed discharge in the feed
gas 256.
[0097] Thus, in one aspect of the invention, high power pulses are
applied to a weakly-ionized plasma 262 across the gap 244 to
generate the strong electric field 266 between the anode 238 and
the cathode assembly 216. This strong electric field 266 generates
excited atoms 274 in the weakly-ionized plasma 262. The excited
atoms 274 are rapidly ionized by secondary electrons emitted by the
cathode assembly 216. This rapid ionization results in a
strongly-ionized plasma 268 having a large ion density being formed
in the area 264 proximate to the cathode assembly 216. The
strongly-ionized plasma 268 is also referred to as a high-density
plasma.
[0098] In one embodiment of the invention, a higher density plasma
is generated by controlling the flow of the feed gas 256 in the
region 245. In this embodiment, a first volume of feed gas 256 is
supplied to the region 245. The first volume of feed gas 256 is
then ionized to form a weakly-ionized plasma 262 in the region 245.
Next, the pulsed power supply 234 applies a high power electrical
pulse across the weakly-ionized plasma 262. The high power
electrical pulse generates a strongly-ionized plasma 268 from the
weakly-ionized plasma 262.
[0099] The level and duration of the high power electrical pulse is
limited by the level and duration of the power that the
strongly-ionized plasma 268 can absorb before the high power
discharge contracts and terminates. In one embodiment, the strength
and the duration of the high-power electrical pulse are increased
and thus the density of the strongly-ionized plasma 268 is
increased by increasing the flow rate of the feed gas 256.
[0100] In one embodiment, the strongly-ionized plasma 268 is
transported through the region 245 by a rapid volume exchange of
feed gas 256. As the feed gas 256 moves through the region 245, it
interacts with the moving strongly-ionized plasma 268 and also
becomes strongly-ionized from the applied high-power electrical
pulse. The ionization process can be a combination of direct
ionization and/or stepwise ionization as described herein.
Transporting the strongly-ionized plasma 268 through the region 245
by a rapid volume exchange of the feed gas 256 increases the level
and the duration of the power that can be applied to the
strongly-ionized plasma 268 and, thus, generates a higher density
strongly-ionized plasma in the region 264.
[0101] As previously discussed, to increase the efficiency of the
ionization process, a magnetic field (not shown) can be generated
proximate to the cathode assembly 216. The magnetic field
substantially traps electrons in the weakly-ionized plasma 262 and
secondary electrons from the cathode assembly 216 proximate to the
cathode assembly 216. The trapped electrons ionize the excited
atoms 274 generating the strongly-ionized plasma 268. In one
embodiment, the magnetic field is generated in the region 245 to
substantially trap electrons in the area where the weakly-ionized
plasma 262 is ignited.
[0102] In one embodiment, a strongly-ionized plasma 268 according
to the present invention is used to sputter magnetic materials.
Conventional magnetron sputtering generally is not suitable for
sputtering magnetic materials, since the magnetic field generated
by the magnetron can be absorbed by the magnetic target.
Traditional RF diode sputtering can be used to sputter magnetic
materials. However, this method generally results in very poor
uniformity of the sputtered film, relatively low plasma density,
and relatively low deposition rate.
[0103] According to the present invention, magnetic materials can
be sputtering by driving a target assembly including a magnetic
target material with a RF power supply (not shown). For example,
the RF power supply can provide a power of up to about 10 kW. A
substantially uniform weakly-ionized plasma can be generated by
applying RF power across a feed gas that is located proximate to
the target assembly. The strongly-ionized plasma is generated by
applying a strong electric field across the weakly-ionized plasma
as described herein. Since the RF power supply applies a negative
voltage bias to the target assembly, ions in the strongly-ionized
plasma bombard the target material causing sputtering.
[0104] In one embodiment, a strongly-ionized plasma 268 according
to the present invention is used to sputter dielectric materials.
Dielectric materials are sputtered according to the present
invention by driving a target assembly including a dielectric
target material with a RF power supply (not shown). For example,
the RF power supply can provide a power of up to about 10 kW. A
substantially uniform weakly-ionized plasma can be generated by
applying RF power across a feed gas that is located proximate to
the target assembly.
[0105] In one embodiment, a magnetic field can be generated
proximate to the target assembly in order to trap electrons in the
weakly-ionized plasma. The strongly-ionized plasma is generated by
applying a strong electric field across the weakly-ionized plasma
as described herein. Since the RF power supply applies a negative
voltage bias to the target assembly, ions in the strongly-ionized
plasma bombard the target material causing sputtering.
[0106] In one embodiment, a DC power supply (not shown) is used to
create a weakly-ionized plasma 232 for sputtering a dielectric
target material according to the present invention. In this
embodiment, the dielectric target material is positioned relative
to the cathode 218 such that an area of the cathode 218 can conduct
a direct current between the anode 238 and the cathode 218.
[0107] FIG. 6 illustrates graphical representations 320, 322, and
324 of the absolute value of applied voltage, current, and power,
respectively, as a function of time for periodic pulses applied to
the plasma in the sputtering apparatus 200 of FIG. 4. In one
embodiment, at time t.sub.0 (not shown), the feed gas 256 flows
proximate to the cathode assembly 216 before the pulsed power
supply 234 is activated. The time required for a sufficient
quantity of feed gas 256 to flow proximate to the cathode assembly
216 depends on several factors including the flow rate of the feed
gas 256 and the desired pressure in the region 245.
[0108] In the embodiment shown in FIG. 6, the power supply 234
generates a constant power. At time t.sub.1, the pulsed power
supply 234 generates a voltage 326 across the anode 238 and the
cathode assembly 216. In one embodiment, the voltage 326 is
approximately between 100V and 5 kV. The period between time
t.sub.0 and time t.sub.1 (not shown) can be on the order of several
microseconds up to several milliseconds. At time t.sub.1, the
current 328 and the power 330 have constant values.
[0109] Between time t.sub.1 and time t.sub.2, the voltage 326, the
current 328, and the power 330 remain constant as the
weakly-ionized plasma 262 (FIG. 5B) is generated. The voltage 332
at time t.sub.2 is in the range of 100V to 5 kV. The current 334 at
time t.sub.2 is in the range of 0.1 A to 100 A. The power 336
delivered at time t.sub.2 is in the range of 0.01 kW to 100 kW.
[0110] The power 336 generated by the pulsed power supply 234
partially ionizes the gas 256 that is located in the region 245
between the cathode assembly 216 and the anode 238. The partially
ionized gas is also referred to as a weakly-ionized plasma or a
pre-ionized plasma 262. As described herein, the formation of the
weakly-ionized plasma 262 substantially eliminates the possibility
of creating a breakdown condition when high-power pulses are
applied to the weakly-ionized plasma 262. The suppression of this
breakdown condition substantially eliminates the occurrence of
undesirable arcing between the anode 238 and the cathode assembly
216.
[0111] In one embodiment, the period between time t.sub.1 and time
t.sub.2 is in the range of one microsecond to one hundred seconds
to allow the pre-ionized plasma 262 to form and be maintained at a
sufficient plasma density. In one embodiment, the power 336 from
the pulsed power supply 234 is continuously applied in order to
maintain the weakly-ionized plasma 262. The pulsed power supply 234
can be designed so as to output a continuous nominal power in order
to sustain the weakly-ionized plasma 262.
[0112] Between time t.sub.2 and time t.sub.3, the pulsed power
supply 234 delivers a large voltage pulse 338 across the
weakly-ionized plasma 262. In some embodiments, the large voltage
pulse 338 has a voltage that is in the range of 200V to 30 kV. In
some embodiments, the period between time t.sub.2 and time t.sub.3
is in the range of 0.1 microsecond to ten seconds. Between time
t.sub.3 and time t.sub.4, the large voltage pulse 338 is applied
before the current across the weakly-ionized plasma 262 begins to
increase. In one embodiment, the period between time t.sub.3 and
time t.sub.4 can be between about ten nanoseconds and one
microsecond.
[0113] Between time t.sub.4 and time t.sub.5, the voltage 340 drops
as the current 342 increases. The power 344 also increases between
time t.sub.4 and time t.sub.5, until a quasi-stationary state
exists between the voltage 346 and the current 348. The period
between time t.sub.4 and time t.sub.5 is on order of several
hundreds nanoseconds.
[0114] In one embodiment, at time t.sub.5, the voltage 346 is in
the range of 50V to 30 kV, the current 348 is in the range of 10 A
to 5 kA and the power 350 is in the range of 1 kW to 10 MW. The
power 350 is continuously applied to the plasma until time t.sub.6
In one embodiment, the period between time t.sub.5 and time t.sub.6
is in the range of one microsecond to ten seconds.
[0115] The pulsed power supply 234 delivers a high power pulse
having a maximum power 350 and a pulse width that is sufficient to
transform the weakly-ionized plasma 262 to a strongly-ionized
plasma 268 (see FIG. 2D). At time t.sub.6, the maximum power 350 is
terminated. In one embodiment, the pulsed power supply 234
continues to supply a background power that is sufficient to
maintain the plasma after time t.sub.6.
[0116] In one embodiment, the power supply 234 maintains the plasma
after the delivery of the high-power pulse by continuing to apply a
power 352 to the plasma that is in the range of 0.01 kW to 100 kW.
The continuously generated power maintains the pre-ionization
condition in the plasma, while the pulsed power supply 234 prepares
to deliver the next high-power pulse.
[0117] At time t.sub.7, the pulsed power supply 234 delivers the
next high-power pulse (not shown). In one embodiment, the
repetition rate between the high-power pulses is in the range of
0.1 Hz and 1 kHz. The particular size, shape, width, and frequency
of the high-power pulses depend on various factors including
process parameters, the design of the pulsed power supply 234, the
geometry of the sputtering system 200, the volume of plasma, the
density of the strongly-ionized plasma 268, and the pressure in the
region 245. The shape and duration of the leading edge 356 and the
trailing edge 358 of the high-power pulse 354 is chosen so as to
sustain the weakly-ionized plasma 262 while controlling the rate of
ionization of the strongly-ionized plasma 268.
[0118] In another embodiment (not shown), the power supply 234
generates a constant voltage. In this embodiment, the applied
voltage 320 is continuously applied from time t.sub.2 until time
t.sub.6. The current 322 and the power 324 rise until time t.sub.6
where they maintain a constant voltage level, and then the voltage
320 is terminated. In one embodiment, the values for the current,
power and voltage are optimized for generating exited atoms.
[0119] In one embodiment of the invention, the efficiency of the
ionization process is increased by generating a magnetic field
proximate to the cathode assembly 216. The magnetic field
substantially traps electrons in the weakly-ionized plasma 262
proximate to the cathode assembly 216. The trapped electrons ionize
the excited atoms 274 thereby generating the strongly-ionized
plasma 268. In this embodiment, the magnetically enhanced plasma
has strong diamagnetic properties. The term "strong diamagnetic
properties" is defined herein to mean that the magnetically
enhanced high-density plasma discharge tends to exclude external
magnetic fields from the plasma volume.
[0120] FIG. 7A through FIG. 7D illustrate various simulated
magnetic field distributions 400, 402, 404, and 406 proximate to
the cathode assembly 216 for various electron ExB drift currents in
a magnetically enhanced plasma sputtering apparatus according to
the invention. The magnetically enhanced plasma generating
apparatus includes a magnet assembly 252 that is positioned
proximate to the cathode assembly 216. The magnet assembly 252
generates a magnetic field proximate to the cathode assembly 216.
In one embodiment, the strength of the magnetic field is in the
range of fifty to two thousand gauss. The simulated magnetic fields
distributions 400, 402, 404, and 406 indicate that high-power
plasmas having high current density tend to diffuse homogeneously
in an area 246 of the magnetically enhanced plasma sputtering
apparatus.
[0121] The high-power pulses applied between the cathode assembly
216 and the anode 238 generate secondary electrons from the cathode
assembly 216 that move in a substantially circular motion proximate
to the cathode assembly 216 according to crossed electric and
magnetic fields. The substantially circular motion of the electrons
generates an electron ExB drift current. The magnitude of the
electron ExB drift current is proportional to the magnitude of the
discharge current in the plasma and, in one embodiment, is
approximately in the range of three to ten times the magnitude of
the discharge current.
[0122] In one embodiment, the substantially circular electron ExB
drift current generates a magnetic field that interacts with the
magnetic field generated by the magnet assembly 252. In one
embodiment, the magnetic field generated by the electron ExB drift
current has a direction that is substantially opposite to the
magnetic field generated by the magnet assembly 252. The magnitude
of the magnetic field generated by the electron ExB drift current
increases with increased electron ExB drift current. The
homogeneous diffusion of the strongly-ionized plasma in the region
246 is caused, at least in part, by the interaction of the magnetic
field generated by the magnet assembly 252 and the magnetic field
generated by the electron ExB drift current.
[0123] In one embodiment, the electron ExB drift current defines a
substantially circular shape for a low current density plasma.
However, as the current density of the plasma increases, the
substantially circular electron ExB drift current tends to describe
a more complex shape as the interaction of the magnetic field
generated by the magnet assembly 252, the electric field generated
by the high-power pulse, and the magnetic field generated by the
electron ExB drift current become more acute. For example, in one
embodiment, the electron ExB drift current has a substantially
cycloidal shape. Thus, the exact shape of the electron ExB drift
current can be complex and depends on various factors.
[0124] For example, FIG. 7A illustrates the magnetic field lines
408 produced from the interaction of the magnetic field generated
by the magnet assembly 252 and the magnetic field generated by an
electron ExB drift current 410. The electron ExB drift current 410
is generated proximate to the cathode assembly 216 and is
illustrated by a substantially circularly shaped ring. In the
example shown in FIG. 7A, the electron ExB drift current 410 is
approximately 100 A.
[0125] In one embodiment of the invention, the electron ExB drift
current 410 is in the range of three to ten times as great as the
discharge current. Thus, in the example shown in FIG. 7A, the
discharge current is in the range of 10 A to 30 A. The magnetic
field lines 408 shown in FIG. 7A indicate that the magnetic field
generated by the magnet assembly 252 is substantially undisturbed
by the relatively small magnetic field that is generated by the
relatively small electron ExB drift current 410.
[0126] FIG. 7B illustrates the magnetic field lines 412 produced
from the interaction of the magnetic field generated by the magnet
assembly 252 and the magnetic field generated by an electron ExB
drift current 414. The electron ExB drift current 414 is generated
proximate to the cathode assembly 216. In the example shown in FIG.
7B, the electron ExB drift current 414 is approximately 300 A.
Since the electron ExB drift current 414 is typically between about
three and ten times as great as the discharge current, the
discharge current in this example is in the range of 30 A and 100
A.
[0127] The magnetic field lines 412 that are generated by the
magnet assembly 252 are substantially undisturbed by the relatively
small magnetic field generated by the relatively small electron ExB
drift current 414. However, the magnetic field lines 416 that are
closest to the electron ExB drift current 414 are somewhat
distorted by the magnetic field generated by the electron ExB drift
current 414. The distortion suggests that a larger electron ExB
drift current should generate a stronger magnetic field that will
interact more strongly with the magnetic field generated by the
magnet assembly 252.
[0128] FIG. 7C illustrates the magnetic field lines 418 that are
produced from the interaction of the magnetic field generated by
the magnet assembly 252 and the magnetic field generated by an
electron ExB drift current 420. The electron ExB drift current 420
is generated proximate to the cathode assembly 216. In the example
shown in FIG. 7C, the electron ExB drift current 420 is
approximately 1,000 A. Since the electron ExB drift current 420 is
typically between about three and ten times as great as the
discharge current, the discharge current in this example is
approximately between 100 A and 300 A.
[0129] The magnetic field lines 418 that are generated by the
magnet assembly 252 exhibit substantial distortion that is caused
by the relatively strong magnetic field generated by the relatively
large electron ExB drift current 420. Thus, the larger electron ExB
drift current 420 generates a stronger magnetic field that strongly
interacts with and can begin to dominate the magnetic field
generated by the magnet assembly 252.
[0130] The interaction of the magnetic field generated by the
magnet assembly 252 and the magnetic field generated by the
electron ExB drift current 420 substantially generates magnetic
field lines 422 that are somewhat more parallel to the surface of
the cathode assembly 216 than the magnetic field lines 408, 412,
and 416 in FIG. 7A and FIG. 7B. The magnetic field lines 422 allow
the strongly-ionized plasma 268 to more uniformly distribute itself
in the area 246. Thus, the strongly-ionized plasma 268 is uniformly
diffused in the area 246.
[0131] FIG. 7D illustrates the magnetic field lines 424 produced
from the interaction of the magnetic field generated by the magnet
assembly 252 and the magnetic field generated by an electron ExB
drift current 426. The electron ExB drift current 426 is generated
proximate to the cathode assembly 216. In the example shown in FIG.
7D, the electron ExB drift current 426 is approximately 5 kA. The
discharge current in this example is approximately between 500 A
and 1,700 A.
[0132] The magnetic field lines 424 generated by the magnet
assembly 252 exhibit relatively high distortion due to their
interaction with the relatively strong magnetic field generated by
the relatively large electron ExB drift current 426. Thus, in this
embodiment, the relatively large electron ExB drift current 426
generates a very strong magnetic field that is substantially
stronger than the magnetic field generated by the magnet assembly
252.
[0133] A large electron ExB drift current can enhance the rate of
ionization of the strongly-ionized plasma by trapping secondary
electrons from the cathode assembly 216. The secondary electrons
ionize neutral and excited atoms and molecules. A strongly-ionized
plasma having a high density of ions can increase the deposition
rate in a plasma sputtering process according to the invention.
[0134] The deposition rate of a sputtering process can be expressed
as follows:
R.sub.D=K.times.Y.times.I
[0135] where K is a geometrical factor, Y is the sputtering yield,
and I is the discharge current. Thus, the deposition rate is
proportional to the sputtering yield Y. The sputtering yield Y is
defined as the number of atoms sputtered per incident ion and
depends on the type of ions that bombard the target surface, the
energy of the ions, the incident angle of the bombarding ions, the
binding energy of the target material, and the target temperature.
In a typical sputtering process involving a so-called "cold
cathode," the temperature of the target is gradually heated and
that heat is dissipated using liquid cooling as described with
reference to FIG. 2. This typical sputtering process uses momentum
and energy exchange of bombarding ions to dislodge target
atoms.
[0136] To substantially increase the deposition rate, the present
invention generates a very high discharge current and a very high
sputtering yield. The high-power pulse is applied almost
instantaneously, causing an explosive reaction at the surface of
the target. The explosive reaction rapidly vaporizes a surface
layer of the target.
[0137] FIG. 8 illustrates a graphical representation 500 of
sputtering yield Y as a function of temperature T of the sputtering
target 220 of FIG. 4. A higher deposition rate can be achieved
according to the present invention by using a thermal sputtering
process. In one embodiment, the deposition rate is increased by
increasing the sputtering yield. The sputtering yield of the target
220 is increased as the temperature of the target 220 is increased.
As the temperature of the target 220 is increased, the sputtering
yield slowly increases in a substantially linear manner as shown in
region 502. In the region 502, the sputtering yield generally does
not depend on the temperature of the target 220.
[0138] When the temperature of the target reaches temperature
T.sub.0 504, the sputtering yield increases at a non-linear rate.
In one embodiment, the sputtering yield increases at an exponential
rate. The temperature T.sub.0 is approximately equal to 0.7
T.sub.m, where T.sub.m is the melting point of the target material.
In another embodiment, the temperature T.sub.0 is approximately
equal to U/40 k, where U is the binding energy for a surface atom
and k is Boltzman's constant.
[0139] The sputtering yield at or above the target temperature
T.sub.0 can be expressed as follows: 4 Y T = c T o + T M exp ( - U
T 0 + T M ) , = R 2 ( T 0 + T M U ) 2
[0140] where .DELTA.T.sub.M is the maximum difference of the target
temperature from the temperature T.sub.0, R is the initial radius
of the high temperature area on the target, .tau. is the time
period for the high temperature in the high temperature area,
.kappa. is the coefficient for the temperature conductivity, and U
is the binding energy.
[0141] When high power pulses having the appropriate power level
and duration are applied to the plasma according to the present
invention, the sputtering yield increases non-linearly. In one
embodiment, the sputtering yield increases substantially in an
exponential manner. According to one embodiment of the invention, a
high power pulse is applied to a weakly-ionized plasma 262 (FIG.
5C) for a relatively short duration. This high power pulse creates
a strongly-ionized plasma 268 (FIG. 5D) that contains a high
density of energetic ions 272.
[0142] Thus, in one embodiment, a very large quantity of explosive
energy at the target surface results in a sputtering yield that
increases exponentially. The explosive energy causes the
temperature at the target surface to increase rapidly. This rapid
increase in temperature results in a surface layer of the target
being substantially evaporated and sputtered at a very high rate
compared with known sputtering techniques. In one embodiment, the
deposition rate of a target material is greater than one micron per
minute.
[0143] FIG. 9 illustrates a process 510 for sputtering atoms 512
from a target 220 according one embodiment of the invention. In one
embodiment, the target 220 is negatively biased. The negative bias
causes the energetic ions 272 in the strongly-ionized plasma 268 to
vigorously impact the surface 514 of the target 220, thereby
causing the temperature of the target to rapidly increase. When the
temperature of the target reaches the temperature T.sub.0, the
sputtering yield increases nonlinearly. In one embodiment, the
sputtering yield increases almost exponentially. Consequently, the
deposition rate rapidly increases. The sputtering yield can depend
on the properties of the target material, such as the crystal
structure, the binding energy of the surface atoms, and/or the
melting point of the target material.
[0144] FIG. 10 illustrates a cross-sectional view of a cathode
assembly 216 according to one embodiment of the invention. When the
temperature of the target 220 reaches a certain level, the target
material is evaporated in an avalanche-like manner. In one
embodiment, the high-power pulse generates thermal energy 516 into
only a shallow depth of the target 220 so as to not substantially
increase an average temperature of the target 220. The target
material is almost instantly evaporated. However damage to the
target 220 itself is minimal because thermal energy only penetrates
into the shallow skin depth.
[0145] FIG. 11 is a flowchart 600 of an illustrative process of
enhancing a sputtering yield of a sputtering target according to
the present invention. The process is initiated (step 602) by
pumping the chamber 202 down to a specific pressure (step 604).
Next, the pressure in the chamber is evaluated (step 606). In one
embodiment, a feed gas is then pumped into the chamber (step 608).
The gas pressure is evaluated (step 610). If the gas pressure is
correct, the pressure in the chamber is again evaluated (step
612).
[0146] If the pressure in the chamber is correct, an appropriate
magnetic field 254 (FIG. 4) can be generated proximate to the feed
gas 256 (FIG. 5B) (step 614). In one embodiment, a magnet assembly
252 can include at least one permanent magnet, and thus the
magnetic field is generated constantly, even before the process is
initiated. In another embodiment, a magnetic assembly 252 includes
at least one electromagnet, and thus the magnetic field is
generated only when the electromagnet is operating. In one
embodiment, the magnetic field 254 is then evaluated (step
616).
[0147] The feed gas 256 is ionized to generate a weakly-ionized
plasma 262 (step 618). In one embodiment, the weakly-ionized plasma
262 is generated by creating a relatively low current discharge in
the gap 244 between the cathode assembly 216 and the anode 238. In
another embodiment (not shown), the weakly-ionized plasma 262 can
be generated by creating a relatively low current discharge between
an ionizing electrode and the cathode assembly 216. In yet another
embodiment (not shown), an electrode is heated to emit electrons
proximate to the cathode assembly 216. In this embodiment, a
relatively low current discharge is created between the anode 238
and the electrode in order to generate the weakly-ionized plasma
262. In the embodiment shown in FIG. 4, for example, the
weakly-ionized plasma 262 is generated by applying a potential
across the gap 244 between the cathode assembly 216 and the anode
238 after the introduction of the feed gas 256.
[0148] When the gas is weakly-ionized (step 620), A
strongly-ionized plasma 268 (FIG. 5D) is generated from the
weakly-ionized plasma 262 (step 622). In one embodiment, the
strongly-ionized plasma 268 is generated by applying a high-power
pulse between the cathode assembly 216 and the anode 238. As
described herein, the high-power pulse causes a strong electric
field 266 to be generated in the gap 244 between the anode 238 and
the cathode assembly 216.
[0149] The strong electric field 266 causes the feed gas to
experience stepwise ionization. In one embodiment, the feed gas
includes a molecular gases and the strong electric field 266
increases the formation of ions that enhance the strongly-ionized
plasma 268. In one embodiment, the strongly-ionized plasma 268 is
substantially homogeneous in the area 264 of FIG. 5D. This
homogeneity results in more uniform erosion of the sputtering
target 220 and, therefore, relatively high target utilization.
Since the cathode assembly 216 is negatively biased relative to the
anode 238, the cathode assembly 216 attracts ions from the
strongly-ionized substantially uniform plasma. This causes the ions
to bombard the cathode assembly 216, thereby resulting in
sputtering of the target material.
[0150] In one embodiment, the strongly-ionized plasma is enhanced
through the rapid exchange of the strongly-ionized plasma with a
fresh volume of feed gas 256 (step 624). This rapid exchange occurs
in the region 245. In one embodiment, the rapid exchange of the
strongly-ionized plasma occurs during the duration of the
high-power pulse.
[0151] After the strongly-ionized plasma 268 is formed (step 626),
the sputtering yield is monitored (step 628) by known monitoring
techniques. If the sputtering yield is insufficient (step 630), the
power delivered to the plasma is increased (step 632). In one
embodiment, increasing the magnitude of the high-power pulse
applied between the cathode assembly 216 and the anode 238
increases the power delivered to the plasma. In one embodiment, the
power delivered to the plasma is sufficient to vaporize a surface
layer of the target. This increases the sputtering yield in a
substantially nonlinear fashion.
[0152] The sputter yield is again evaluated (step 628). This
process continues until the sputter yield is sufficient (step 630),
and sputtering continues (step 634). Once the sputter deposition is
completed (step 636), the sputter process is terminated (step
638).
[0153] Equivalents
[0154] While the invention has been particularly shown and
described with reference to specific 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.
* * * * *