U.S. patent application number 10/949427 was filed with the patent office on 2005-02-17 for plasma generation using multi-step ionization.
Invention is credited to Chistyakov, Roman.
Application Number | 20050034666 10/949427 |
Document ID | / |
Family ID | 32987021 |
Filed Date | 2005-02-17 |
United States Patent
Application |
20050034666 |
Kind Code |
A1 |
Chistyakov, Roman |
February 17, 2005 |
Plasma generation using multi-step ionization
Abstract
The present invention relates to a plasma generator that
generates a plasma with a multi-step ionization process. The plasma
generator includes an excited atom source that generates excited
atoms from ground state atoms supplied by a feed gas source. A
plasma chamber confines a volume of excited atoms generated by the
excited atom source. An energy source is coupled to the volume of
excited atoms confined by the plasma chamber. The energy source
raises an energy of excited atoms in the volume of excited atoms so
that at least a portion of the excited atoms in the volume of
excited atoms is ionized, thereby generating a plasma with a
multi-step ionization process.
Inventors: |
Chistyakov, Roman; (Andover,
MA) |
Correspondence
Address: |
RAUSCHENBACH PATENT LAW GROUP, LLC
P.O. BOX 387
BEDFORD
MA
01730
US
|
Family ID: |
32987021 |
Appl. No.: |
10/949427 |
Filed: |
September 24, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10949427 |
Sep 24, 2004 |
|
|
|
10249202 |
Mar 21, 2003 |
|
|
|
6805779 |
|
|
|
|
Current U.S.
Class: |
118/723E |
Current CPC
Class: |
H01J 37/32623 20130101;
H01J 37/3405 20130101; H01J 37/32321 20130101 |
Class at
Publication: |
118/723.00E |
International
Class: |
C23C 016/00 |
Claims
What is claimed is:
1. A plasma generator that generates a plasma with a multi-step
ionization process, the plasma generator comprising: a feed gas
source comprising ground state atoms; an excited atom source that
is coupled to the feed gas source, the excited atom source
generating excited atoms from the ground state atoms; a plasma
chamber that is coupled to the excited atom source, the plasma
chamber confining a volume of excited atoms generated by the
excited atom source; and an energy source that is coupled to the
volume of excited atoms confined by the plasma chamber, the energy
source raising an energy of excited atoms in the volume of excited
atoms so that at least a portion of the excited atoms in the volume
of excited atoms is ionized, thereby generating a plasma with a
multi-step ionization process.
2. The plasma generator of claim 1 wherein the feed gas source
comprises ground state atoms that are chosen from the group
comprising noble gas atoms, a mixture of different noble gas atoms,
reactive gas atoms, a mixture of different reactive gas atoms, and
a mixture of noble and reactive gas atoms.
3. The plasma generator of claim 1 wherein the feed gas source
comprises a volume of ground state argon atoms.
4. The plasma generator of claim 1 wherein the excited atom source
comprises a metastable atom source that generates metastable atoms
from the ground state atoms.
5. The plasma generator of claim 1 wherein the excited atom source
comprises a first electrode and a second electrode, the first
electrode and the second electrode generating a discharge that
excites the ground state atoms.
6. The plasma generator of claim 1 wherein the excited atom source
further comprises a magnet that generates a magnetic field that
substantially traps electrons proximate to the ground state atoms,
thereby increasing at least one of a rate at which the excited
atoms are generated from the ground state atoms and a density of
excited atoms.
7. The plasma generator of claim 1 wherein the excited atom source
comprises an electron gun that directs an electron beam into the
ground state atoms, the electron beam exciting the ground state
atoms.
8. The plasma generator of claim 1 wherein a pressure differential
exists between a pressure in the excited atom source and a pressure
in the plasma chamber, the pressure differential increasing at
least one of a rate at which the excited atoms are generated from
the ground state atoms and a density of the excited atoms.
9. The plasma generator of claim 1 wherein the excited atom source
comprises an inductively coupled discharge source that generates a
discharge that excites ground state atoms.
10. The plasma generator of claim 1 wherein the excited atom source
is positioned inside the plasma chamber.
11. The plasma generator of claim 1 wherein the excited atom source
is positioned outside the plasma chamber.
12. The plasma generator of claim 1 wherein the excited atoms
generated by the excited atom source have a lower ionization energy
compared with an ionization energy of the ground state atoms.
13. The plasma generator of claim 1 wherein the energy source is
chosen from the group comprising a DC discharge source, a radio
frequency (RF) source, an X-ray source, an electron beam source, an
ion beam source, an inductively coupled plasma (ICP) source, a
capacitively coupled plasma (CCP) source, a microwave plasma
source, an electron cyclotron resonance (ECR) plasma source, a
helicon plasma source, a magnetron source, and an AC discharge
source.
14. The plasma generator of claim 1 wherein the energy source
comprises a power supply.
15. The plasma generator of claim 14 wherein the power supply is
chosen from the group comprising a pulsed (DC) power supply, a RF
power supply, an AC power supply, and a DC power supply.
16. The plasma generator of claim 1 further comprising an
electron/ion absorber that receives the excited atoms from the
excited atom source, the electron/ion absorber trapping elections
and ions.
17. The plasma generator of claim 1 wherein the plasma that is
generated with the multi-step ionization process has a higher
plasma density than a plasma that is generated by direct ionization
of the ground state atoms.
18. A plasma generator that generates a plasma with a multi-step
ionization process, the plasma generator comprising: a feed gas
source comprising ground state atoms; a metastable atom source that
is coupled to the feed gas source, the metastable atom source
generating metastable atoms from the ground state atoms; a plasma
chamber that is coupled to the metastable atom source, the plasma
chamber confining a volume of metastable atoms generated by the
metastable atom source; and a power supply that is electrically
coupled to the volume of metastable atoms confined by the plasma
chamber, the power supply generating a power that raises an energy
of metastable atoms in the volume of metastable atoms so that at
least a portion of the metastable atoms in the volume of metastable
atoms is ionized, thereby generating a plasma with a multi-step
ionization process.
19. The plasma generator of claim 18 wherein the metastable atom
source comprises a first electrode and a second electrode, the
first electrode and the second electrode generating a discharge
that excites the ground state atoms to a metastable state.
20. The plasma generator of claim 18 wherein the metastable atom
source comprises an electron gun that directs an electron beam into
the ground state atoms, the electron beam exciting the ground state
atoms to a metastable state.
21. The plasma generator of claim 18 wherein the metastable atom
source comprises an inductively coupled discharge source that
generates a discharge that excites the ground state atoms.
22. The plasma generator of claim 18 wherein the metastable atom
source comprises a magnet that generates a magnetic field that
substantially traps electrons proximate to the ground state atoms,
thereby increasing at least one of a rate at which the metastable
atoms are generated from the ground state atoms and a density of
the metastable atoms.
23. The plasma generator of claim 18 wherein a pressure
differential exists between a pressure in the metastable atom
source and a pressure in the plasma chamber, the pressure
differential increasing at least one of a rate at which the
metastable atoms are generated from the ground state atoms and a
density of the metastable atoms.
24. The plasma generator of claim 18 wherein the metastable atom
source is positioned inside the plasma chamber.
25. The plasma generator of claim 18 wherein the metastable atom
source is positioned outside the plasma chamber.
26. The plasma generator of claim 18 wherein the metastable atoms
generated by the metastable atom source have a lower ionization
energy compared with an ionization energy of the ground state
atoms.
27. The plasma generator of claim 18 wherein the power supply is
chosen from the group comprising a pulsed (DC) power supply, a RF
power supply, an AC power supply, and a DC power supply.
28. The plasma generator of claim 18 further comprising an
electron/ion absorber that receives the metastable atoms from the
metastable atom source, the electron/ion absorber trapping
elections and ions.
29. The plasma generator of claim 18 wherein the plasma that is
generated with the multi-step ionization process has a higher
plasma density than a plasma that is generated by direct ionization
of the ground state atoms.
30. A method for generating a plasma with a multi-step ionization
process, the method comprising: generating a volume of metastable
atoms from a volume of ground state atoms; and raising an energy of
the metastable atoms so that at least a portion of the volume of
metastable atoms is ionized, thereby generating a plasma with a
multi-step ionization process.
31. The method of claim 30 wherein the volume of ground state atoms
comprises a volume of noble gas atoms.
32. The method of claim 30 wherein the generating the volume of
metastable atoms comprises generating a discharge that excites at
least a portion of the ground state atoms in the volume of ground
state atoms to a metastable state.
33. The method of claim 32 further comprising generating a magnetic
field proximate to the volume of ground state atoms, the magnetic
field substantially trapping electrons proximate to the volume of
ground state atoms, thereby increasing excitation of the portion of
the ground state atoms to a metastable state.
34. The method of claim 30 wherein the generating the volume of
metastable atoms comprises generating an electron beam that excites
at least a portion of the ground state atoms in the volume of
ground state atoms to a metastable state.
35. The method of claim 30 wherein the raising the energy of the
metastable atoms comprises exposing the metastable atoms to an
electric field.
36. The method of claim 30 wherein the raising the energy of the
metastable atoms comprises exposing the metastable atoms to X-ray
radiation.
37. The method of claim 30 wherein the raising the energy of the
metastable atoms comprises exposing the metastable atoms to a
plasma.
38. The method of claim 30 further comprising trapping electrons
and ions in the volume of metastable atoms.
39. The method of claim 30 wherein the raising the energy of the
metastable atoms comprises exposing the metastable atoms to an
electron source.
40. A method for generating a plasma with a multi-step ionization
process, the method comprising: generating a volume of metastable
molecules from a volume of ground state molecules; and raising an
energy of the metastable molecules so that at least a portion of
the volume of metastable molecules is ionized, thereby generating a
plasma with a multi-step ionization process.
41. An apparatus for generating a plasma comprising: means for
generating a volume of metastable atoms from a volume of ground
state atoms; and means for raising an energy of the metastable
atoms so that at least a portion of the volume of metastable atoms
is ionized, thereby generating a plasma with a multi-step
ionization process.
42. The apparatus of claim 41 further comprising means for trapping
elections and ions in the volume of metastable atoms.
Description
BACKGROUND OF INVENTION
[0001] Plasma is considered the fourth state of matter. A plasma is
a collection of charged particles that move in random directions. A
plasma is, on average, electrically neutral. One method of
generating a plasma is to drive a current through a low-pressure
gas between two conducting electrodes that are positioned parallel
to each other. Once certain parameters are met, the gas "breaks
down" to form the plasma. For example, a plasma can be generated by
applying a potential of several kilovolts between two parallel
conducting electrodes in an inert gas atmosphere (e.g., argon) at a
pressure that is between about 10.sup.-1 and 10.sup.-2 Torr.
[0002] Plasma processes are widely used in many industries, such as
the semiconductor manufacturing industry. For example, plasma
etching is commonly used to etch substrate material and films
deposited on substrates in the electronics industry. There are four
basic types of plasma etching processes that are used to remove
material from surfaces: sputter etching, pure chemical etching, ion
energy driven etching, and ion inhibitor etching.
[0003] Plasma sputtering is a technique that is widely used for
depositing films on substrates and other work pieces. 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 are then drawn out of the
plasma and accelerated across a cathode dark space. The target
surface has a lower potential than the region in which the plasma
is formed. Therefore, the target surface attracts positive
ions.
[0004] Positive ions move towards the target with a high velocity
and then impact the target and cause atoms to physically dislodge
or sputter from the target surface. The sputtered atoms then
propagate to a substrate or other work piece 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.
[0005] Reactive sputtering systems inject a reactive gas or mixture
of reactive gases into the sputtering system. The reactive gases
react with the target material either at the target surface or in
the gas phase, resulting in the deposition of new compounds. The
pressure of the reactive gas can be varied to control the
stoichiometry of the film. Reactive sputtering is useful for
forming some types of molecular thin films.
[0006] Magnetron sputtering systems use magnetic fields that are
shaped to trap and concentrate secondary electrons proximate to the
target surface. The magnetic fields increase the density of
electrons and, therefore, increase the plasma density in a region
that is proximate to the target surface. The increased plasma
density increases the sputter deposition rate.
BRIEF DESCRIPTION OF DRAWINGS
[0007] 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.
[0008] FIG. 1 illustrates a cross-sectional view of a known plasma
sputtering apparatus having a DC power supply.
[0009] FIG. 2 illustrates a cross-sectional view of an embodiment
of a plasma generator that generates a plasma with a multi-step
ionization process according to the present invention.
[0010] FIG. 3 illustrates a cross-sectional view of another
embodiment of a plasma generator that generates a plasma with a
multi-step ionization process according to the present
invention.
[0011] FIG. 4 illustrates a cross-sectional view of an embodiment
of a metastable atom generator that includes a metastable atom
source according to the present invention.
[0012] FIG. 5 illustrates a cross-sectional view of an embodiment
of a chamber of a metastable atom source according present
invention.
[0013] FIG. 6 illustrates a cross-sectional view of a metastable
atom source according to the invention.
[0014] FIG. 7 is a perspective view of a metastable atom source
according to one embodiment of the invention.
[0015] FIG. 7A illustrates a cross-sectional view of the metastable
atom source of FIG. 7 that illustrates the magnetic field.
[0016] FIG. 8 illustrates a cross-sectional view of another
embodiment of a metastable atom source according to the
invention.
[0017] FIG. 9 illustrates a cross-sectional view of another
metastable atom source according to the invention.
[0018] FIG. 10 illustrates a cross-sectional view of another
metastable atom source according to the invention
[0019] FIG. 11 illustrates a cross-sectional view of another
metastable atom source according to the invention.
[0020] FIG. 12A through FIG. 12C illustrate various embodiments of
an electron/ion absorber according to the invention.
[0021] FIG. 13 is a flowchart of an illustrative process of
generating a plasma with a multi-step ionization process according
to the present invention.
DETAILED DESCRIPTION
[0022] FIG. 1 illustrates a cross-sectional view of a known plasma
sputtering apparatus 100 having a DC power supply 102. The known
plasma sputtering apparatus 100 includes a vacuum chamber 104 where
a plasma 105 is generated. The vacuum chamber 104 can be coupled to
ground. The vacuum chamber 104 is positioned in fluid communication
with a vacuum pump 106 via a conduit 108 and a valve 109. 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 10.sup.-1 Torr. A feed gas 110 from a feed gas
source 111, such as an argon gas source, is introduced into the
vacuum chamber 104 through a gas inlet 112. The gas flow is
controlled by a valve 113.
[0023] The plasma sputtering apparatus 100 also includes a cathode
assembly 114. The cathode assembly 114 is generally in the shape of
a circular disk. The cathode assembly 114 can include a target 116.
The cathode assembly 114 is electrically connected to a first
terminal 118 of the DC power supply 102 with an electrical
transmission line 120. An insulator 122 isolates the electrical
transmission line 120 from a wall of the vacuum chamber 104. An
anode 124 is electrically connected to a second terminal 126 of the
DC power supply 102 with an electrical transmission line 127. An
insulator 128 isolates the electrical transmission line 127 from
the wall of the vacuum chamber 104. The anode 124 is positioned in
the vacuum chamber 104 proximate to the cathode assembly 114. An
insulator 129 isolates the anode 124 from the cathode assembly 114.
The anode 124 and the second output 126 of the DC power supply 102
are coupled to ground is some systems.
[0024] The plasma sputtering apparatus 100 illustrates a magnetron
sputtering system that includes a magnet 130 that generates a
magnetic field 132 proximate to the target 116. The magnetic field
132 is strongest at the poles of the magnet 130 and weakest in the
region 134. The magnetic field 132 is shaped td trap and
concentrate secondary electrons proximate to the target surface.
The magnetic field increase the density of electrons and,
therefore, increase the plasma density in a region that is
proximate to the target surface.
[0025] The plasma sputtering apparatus 100 also includes a
substrate support 136 that holds a substrate 138 or other work
piece. The substrate support 136 can be electrically connected to a
first terminal 140 of a RF power supply 142 with an electrical
transmission line 144. An insulator 146 isolates the RF power
supply 142 from a wall of the vacuum chamber 104. A second terminal
148 of the RF power supply 142 is coupled to ground.
[0026] In operation, the feed gas 110 from the feed gas source 111
is injected into the chamber 104. The DC power supply 102 applies a
DC voltage between the cathode assembly 114 and the anode 124 that
causes an electric field 150 to develop between the cathode
assembly 114 and the anode 124. The amplitude of the DC voltage is
chosen so that it is sufficient to cause the resulting electric
field to ionize the feed gas 110 in the vacuum chamber 104 and to
ignite the plasma 105.
[0027] The ionization process in known plasma sputtering apparatus
is generally referred to as direct ionization or atomic ionization
by electron impact and can be described by the following
equation:
Ar+e.sup.-.fwdarw.Ar.sup.++2e.sup.-
[0028] where Ar represents a neutral argon atom in the feed gas 110
and e.sup.- represents an ionizing electron generated in response
to the voltage applied between the cathode assembly 114 and the
anode 124. The collision between the neutral argon atom and the
ionizing electron results in an argon ion (Ar.sup.+) and two
electrons.
[0029] The plasma 105 is maintained, at least in part, by secondary
electron emission from the cathode assembly 114. The magnetic field
132 that is generated proximate to the cathode assembly 114
confines the secondary electrons in the region 134 and, therefore,
confines the plasma 105 approximately in the region 134. The
confinement of the plasma in the region 134 increases the plasma
density in the region 134 for a given input power.
[0030] Since the cathode assembly 114 is negatively biased, ions in
the plasma 105 bombard the target 116. The impact caused by these
ions bombarding the target 116 dislodges or sputters material from
the target 116. A portion of the sputtered material forms a thin
film of sputtered target material on the substrate 138.
[0031] Known magnetron sputtering systems have relatively poor
target utilization. The term "poor target utilization" is defined
herein to mean undesirable non-uniform erosion of target material.
Poor target utilization is caused by a relatively high
concentration of positively charged ions in the region 134 that
results in a non-uniform plasma. Similarly, magnetron etching
systems (not shown) typically have relatively non-uniform etching
characteristics.
[0032] Increasing the power applied to the plasma can increase the
uniformity and density of the plasma. However, increasing the
amount of power necessary to achieve even an incremental increase
in uniformity and plasma density can significantly increase the
probability of establishing an electrical breakdown condition
leading to an undesirable electrical discharge (an electrical arc)
in the chamber 104.
[0033] Applying pulsed direct current (DC) to the plasma can be
advantageous since the average discharge power can remain
relatively 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. An undesirable electrical discharge will corrupt the
plasma process and can cause contamination in the vacuum chamber
104. However, very large power pulses can still result in
undesirable electrical discharges regardless of their duration.
[0034] In one embodiment, an apparatus according to the present
invention generates a plasma having a higher density of ions for a
giving input power than a plasma generated by known plasma systems,
such as the plasma sputtering apparatus 100 of FIG. 1.
[0035] FIG. 2 illustrates a cross-sectional view of an embodiment
of a plasma generator 200 that generates a plasma 202 with a
multi-step ionization process according to the present invention.
In one embodiment, the plasma generator 200 includes a metastable
atom source 204 that generates metastable atoms from a feed gas
source 206. The feed gas source 206 provides a volume of ground
state atoms 208 to the metastable atom source 204. The feed gas
source 206 can provide any type of feed gas or mixture of feed
gases, such as, noble gases, reactive gases, and mixtures of noble
gases and reactive gases. In one embodiment, the feed gas source
206 comprises a source of ground state noble gas atoms. For
example, in one embodiment, the feed gas source 206 comprises a
source of ground state argon atoms.
[0036] The feed gas source 206 is coupled to the metastable atom
source 204 through a gas flow control system 210. In one
embodiment, the gas flow control system 210 includes a first gas
valve 212, a mass flow controller 214, and a second gas valve 216.
The gas flow control system 210 can include any number of gas
valves and/or mass flow controllers. The gas flow control system
210 controls the volume and the flow rate of the ground state atoms
208 flowing into the metastable atom source 204. In one embodiment,
the metastable atom source 204 includes a means of controlling the
pressure of the feed gas inside the metastable atom source.
[0037] The metastable atom source 204 receives the ground state
atoms 208 from the gas flow control system 210 at an input 217. The
metastable atom source 204 generates a volume of metastable atoms
218 from the volume of ground state atoms 208. In one embodiment,
the metastable atom source generates a volume of ions that is
relatively small compared with the volume of metastable atoms 218.
A first output terminal 220 of a power supply 222 is coupled to an
electrical input 224 of the metastable atom source 204. The type of
power supply depends upon the type of metastable atom source. For
example, the power supply 222 can be a pulsed power supply, a radio
frequency (RF) power supply, an alternating current (AC) power
supply, or a DC power supply.
[0038] The plasma generator of the present invention can use any
type of metastable atom source 204. Skilled artisans will
appreciate that there are many methods of exciting ground state
atoms 208 to a metastable state, such as electron impact
ionization, photo excitation, or thermal excitation. The operation
of specific embodiments of metastable atom sources are discussed in
more detail herein. For example, in one embodiment, the metastable
atom source 204 includes a parallel plate discharge chamber (not
shown) that receives the volume of ground state atoms 208 from the
gas flow control system 210 and that generates a discharge that
excites a portion of the volume of ground state atoms 208 to a
metastable state.
[0039] In another embodiment, the metastable atom source 204
includes an electron gun (not shown) that receives the volume of
ground state atoms 208 from the gas flow control system 210 and
that generates and accelerates an electron beam that excites a
portion of the volume of ground state atoms 208 to a metastable
state. In yet another embodiment, the metastable atom source 204
includes an inductively coupled discharge chamber that receives the
volume of ground state atoms 208 from the gas flow control system
210 and that generates a discharge that excites a portion of the
volume of ground state atoms 208 to a metastable state.
[0040] A flange 226 couples an output 227 of the metastable atom
source 204 to an input port 228 of a plasma chamber 230. The
metastable atom source 204 can be coupled to any type of process
chamber, such as the chamber 104 of FIG. 1. In fact, a plasma
generator according to the present invention can be constructed by
coupling a metastable atom source to a commercially available
plasma chamber. Thus, commercially available plasma generators can
be modified to generate a plasma using a multi-step ionization
process according to the present invention.
[0041] In one embodiment, a diameter of the input 217 of the
metastable atom source 204 is different than a diameter of the
output 227 of the metastable atom source 204. This difference in
diameters creates a pressure differential between the input 217 and
the output 227 of the metastable atom source 204. The rate of
metastable generation in the metastable atom source 204 depends
upon the pressure inside the source 204. In some embodiments, at
least one of the diameter of the input 217 and the diameter of the
output 227 of the metastable atom source 204 is chosen so that a
pressure differential is created that increases the generation rate
of the metastable atoms 218 in the metastable atom source 204.
[0042] The plasma chamber 230 confines the volume of metastable
atoms 218. In one embodiment, the output of the metastable atom
source 204 is positioned so as to direct the volume of metastable
atoms 218 towards the cathode assembly 114. In one embodiment, the
geometry of the plasma chamber 230 and the cathode assembly 114 is
chosen so that the metastable atoms reach the cathode assembly 114
at a time that is much less than an average transition time of the
metastable atoms to ground state atoms. In some embodiments, ground
state atoms from the metastable atom source 204 gain energy in the
metastable atom source, but do not actually become metastable atoms
until they reach the plasma chamber 230. Ground state atoms from
the metastable atom source 204 can become metastable atoms at any
place along the path from the metastable atom source 204 to the
cathode assembly 114.
[0043] The plasma chamber 230 is positioned in fluid communication
with the vacuum pump 106 via the conduit 108 and the vacuum valve
109. The vacuum pump 106 evacuates the plasma chamber 230 to high
vacuum. The pressure inside the plasma chamber 230 is generally
maintained at less than 10.sup.-1 Torr for plasma processing. In
one embodiment, a feed gas (not shown) from a second feed gas
source (not shown), such as an argon gas source, is introduced into
the plasma chamber 230 through a gas inlet (not shown).
[0044] The power supply 201 is a pulsed power supply that is
electrically coupled to the cathode assembly 114 with the
electrical transmission line 120. In one embodiment, the duration
of the pulse is chosen to optimize a process parameter. In other
embodiments, the power supply 201 is a RF power supply, an AC power
supply, or a DC power supply. The isolator 122 insulates the
electrical transmission line 120 from the plasma chamber 230. The
second output 126 of the power supply 102 is electrically coupled
to the anode 124 with the electrical transmission line 127. The
isolator 128 insulates the electrical transmission line 127 from
the plasma chamber 230. Another isolator 129 insulates the anode
124 from the cathode assembly 114. Numerous other cathode and anode
configurations known in the art can be used with the plasma
generator of the present invention. In one embodiment, the plasma
chamber 230 is coupled to ground potential.
[0045] The cathode assembly 114 is formed of a metallic material,
such as stainless steel or any other material that does not
chemically react with reactive gases. In one embodiment (not
shown), the cathode assembly 114 includes a sputtering target 116
that is used for sputtering materials onto a substrate or other
work piece. The sputtering target 116 can include any type of
material. For example, the sputtering target 116 can be formed of
magnetic, non-magnetic, dielectric, metals, and semiconductor
materials.
[0046] In one embodiment, a magnet (not shown) is disposed
proximate to the cathode assembly 114. The magnet generates a
magnetic field that traps electrons in the plasma proximate to the
cathode assembly 114 and, therefore, increase the plasma density in
the region proximate to the cathode assembly 114.
[0047] The substrate support 136 is disposed in the plasma chamber
230. The substrate support 136 is designed to support a substrate
138 or other work piece. In one embodiment, a temperature
controller 240 is positioned in thermal communication with the
substrate support 136. The temperature controller 240 can increase
or decrease the temperature of the substrate 138. In some
embodiments, the temperature controller 240 is used to control the
temperature of the substrate for various reasons including
enhancing a chemical reaction, increasing a growth rate, and
improving adhesion.
[0048] In one embodiment, the power supply 142 is used to apply a
bias voltage to the substrate 138. The first output 140 of the
power supply 142 is coupled to the substrate support 136 with the
transmission line 144. The isolator 146 insulates the transmission
line 144 from a wall of the plasma chamber 230. The second output
148 of the power supply 142 is coupled to ground. The power supply
142 can be any type of pulsed power supply such as a RF power
supply, an AC power supply, or a DC power supply.
[0049] The plasma generator 200 of FIG. 2 uses a multi-step or
stepwise ionization process to generate the plasma 202. The term
"multi-step ionization process" is defined herein to mean an
ionization process whereby ions are ionized in at least two
distinct steps. However, the term "multi-step ionization process"
as defined herein may or may not include exciting ground state
atoms to a metastable state. For example, one multi-step ionization
process according to the present invention includes a first step
where atoms are excited from a ground state to a metastable state
and a second step where atoms in the metastable state are ionized.
Another multi-step ionization process according to the present
invention includes a first step where atoms are excited from a
ground state to an excited state and a second step where atoms in
the excited state are ionized. The term "multi-step ionization
process" also includes ionization processes with three or more
steps.
[0050] In operation, the plasma generator 200 operates as follows.
The gas flow control system 210 supplies ground state atoms 208
from the feed gas source to the metastable atom source 204. The
power supply 222 applies a voltage to the volume of ground state
atoms 208. The voltage excites at least a portion of the volume of
the ground state atoms 208 to creates a volume of metastable atoms
218. In one embodiment, the power supply 222 applies a voltage to
the volume of ground state atoms 208. In one embodiment, the
duration of the voltage pulse is chosen to optimize a process
parameter, such as the rate of metastable atom generation or the
efficiency of metastable atom generation.
[0051] The term "metastable atoms" is defined herein to mean
excited atoms having energy levels from which dipole radiation is
theoretically forbidden. Metastable atoms have relatively long
lifetimes compared with other excited atoms. Metastable atoms are
created because, in theory, the selection rules forbid relaxation
of these excited atoms to the ground state and the emission of
dipole radiation. However, the selection rules were determined
using certain approximations. Consequently, in practice, there is a
finite probability that the metastable atoms relax to the ground
state and emit dipole radiation. The actual lifetime of metastable
atoms is on order of milliseconds to minutes. For example,
lifetimes for argon metastables are 44.9 seconds and 55.9 seconds
for metastable energies of 11.723 eV and 11.548 eV,
respectively.
[0052] All noble gases have metastable states. For example, argon
metastable atoms can be generated by a two-step ionization process.
In the first step, ionizing electrons e.sup.- are generated by
applying a sufficient voltage between the cathode assembly 114 and
the anode 124. When an ionizing electron e.sup.- collides with a
ground state argon (Ar) atom, a metastable argon atom and an
electron are generated. Argon has two metastable states, see
Fabrikant, I. I., Shpenik, O. B., Snegursky, A. V., and Zavilopulo,
A. N., Electron Impact Formation of Metastable Atoms,
North-Holland, Amsterdam. The first metastable state is represented
in jl-coupling notation as follows:
4s[3/2].sub.0.sup.0
[0053] and is represented in LS-coupling configuration as
follows:
3p.sup.5(.sup.2P.sub.3/2.sup.0)4s.sup.3P.sub.2
[0054] The energy and lifetime of the first metastable state is
11.548 eV and 55.9 seconds, respectively.
[0055] The second metastable state is represented in jl-coupling
notation as follows:
4s.sup.1[1/2].sub.0.sup.0
[0056] and is represented in LS-coupling notation as follows:
3p.sup.5(.sup.2P.sub.3/2.sup.0)4s.sup.3P.sub.0
[0057] The energy and lifetime of the second metastable state is
11.723 eV and 44.9 seconds, respectively.
[0058] Metastable atoms can be present in considerable densities in
weakly ionized discharges. In the second step, an ionizing electron
e.sup.- collides with the metastable argon atom and the metastable
argon atom is ionized and two electrons are generated, as shown
below.
Ar+e.sup.-.fwdarw.Ar*+e.sub.-
Ar*+e.sup.-.fwdarw.Ar.sup.++2e.sup.-
[0059] Plasma generation using multi-step ionization according to
the present invention is described in connection with the
generation of metastable atoms. However, the present invention is
not limited to multi-step ionization using metastable atoms. Plasma
generation using multi-step ionization according to the present
invention can be achieved by generating metastable molecules.
[0060] Electrons are formed in the metastable atom source 204 along
with the volume of metastable atoms 218. In addition, a relatively
small volume of ions are formed by direct ionization. In one
embodiment, the volume of ions and volume of electrons are removed
from the volume of metastable atoms 218 before the metastable atoms
218 are injected into the plasma chamber 230, as described herein.
The volume of metastable atoms 218 are injected into the plasma
chamber 230 adjacent to the cathode assembly 114.
[0061] In one embodiment, a pressure at the input 217 of the
metastable atom source 204 is lower than a pressure at the output
227 of the metastable atom source 204. The pressure differential
increases the efficiency at which the metastable atoms 218 are
generated in the metastable atom source 204. In addition, the
pressure differential causes the volume of metastable atoms 218 to
be rapidly injected into the plasma chamber 230. The rapid
injection generally increases the density of the metastable atoms
218 at the cathode assembly 114.
[0062] After a sufficient volume of metastable atoms 218 is present
in the plasma chamber 230, the DC power supply 102 generates an
electric field 150 proximate to the volume of metastable atoms 218
between the cathode assembly 114 and the anode 124. The electric
field 150 raises the energy of the volume of metastable atoms 218
causing collisions between neutral atoms, electrons, and metastable
atoms 218. These collisions generate the plasma 202 proximate to
the cathode assembly 114. The plasma includes ions, excited atoms
and additional metastable atoms 218. The efficiency of the
multi-step ionization process increases as the density of
metastable atoms 218 in the plasma chamber 230 increases.
[0063] In one embodiment (not shown) a magnetic field is generated
proximate to the center of the cathode assembly 114. The magnetic
field can increase the ion density of the plasma 202 by trapping
electrons in the plasma 202 and also by trapping secondary
electrons proximate to the cathode 114.
[0064] Also, in one embodiment (not shown), a feed gas from a feed
gas source (not shown) is injected directly into the plasma chamber
230. The feed gas source supplies an additional volume of ground
state atoms to the plasma 202. These ground state atoms are ionized
by direct ionization. The directly ionized ground state atoms
increases the ion density of the plasma 202.
[0065] The multi-step ionization process described herein
substantially increases the rate at which the plasma 202 is formed
and, therefore, generates a relatively dense plasma. The rate is
increased because only a relatively small amount of energy is
required to ionize the metastable atoms. For example, ground state
argon atoms require an energy of about 15.76 eV to ionize. However,
argon metastable atoms require only about 4 eV of energy to ionize.
Although energies of about 11.55 eV and 11.72 eV are necessary to
reach argon metastable states, this energy is provided by the
metastable atom source. Therefore, a volume of metastable atoms 218
will ionize at a much higher rate than a similar volume of ground
state atoms 208 for the same input energy.
[0066] Furthermore, as the density of the metastable atoms 218 in
the plasma 202 increases, the efficiency of the ionization process
rapidly increases. The increased efficiency results in an
avalanche-like process that substantially increase the density of
the plasma 202. In addition, the ions in the plasma 202 strike the
cathode 114 causing the secondary electron emission from the
cathode 114. The secondary electrons interact with ground state
atoms 208 and with the metastable atoms 218 in the plasma 202. This
interaction further increases the density of ions in the plasma 202
as additional volumes of metastable atoms 218 enter the plasma
chamber 230. Thus, for the same input energy, the density of the
plasma 202 that is generated by the multi-step ionization process
according to the present invention is significantly greater than a
plasma that is generated by direct ionization of ground state
atoms.
[0067] FIG. 2 describes an electric field 150 that raises an energy
of metastable atoms 218 in the volume of metastable atoms 218 so
that at least a portion of the metastable atoms 218 are ionized,
thereby generating the plasma 202 with a multi-step ionization
process. However, other energy sources can be used to raise the
energy of the metastable atoms 218 without departing from the scope
of the invention. For example, the energy source can be chosen from
the group comprising a planar discharge source, a radio frequency
(RF) diode source, an ultraviolet (UV) source, an X-ray source, an
electron beam source, an ion beam source, an inductively coupled
plasma (ICP) source, a capacitively coupled plasma (CCP) source, a
microwave plasma source, an electron cyclotron resonance (ECR)
source, a helicon plasma source, a magnetron source and an AC
discharge source.
[0068] Once a plasma having the desired characteristics is
generated, the plasma 202 can be used in the processing of the
workpiece 138. For example, in a plasma etch process, ions in the
plasma can be used to etch the workpiece 138 when the workpiece is
appropriately biased by the power supply 142. In a plasma
sputtering application, ions in the plasma can be used to sputter
material from the target 116. The sputtered material is deposited
on the workpiece 138 to form a thin film.
[0069] FIG. 3 illustrates a cross-sectional view of another
embodiment of a plasma generator 300 according to the present
invention that generates a plasma 302 with a multi-step ionization
process according to the present invention. The plasma generator
300 is similar to the plasma generator 200 of FIG. 2. The plasma
generator 300 includes a metastable atom source 304, a cathode 306
and an anode 308. However, the metastable atom source 304 is
positioned inside the plasma chamber 230 rather than coupled to the
plasma chamber 230 with a flange as shown in FIG. 2. The metastable
atom source 304 can be retrofitted to commercially available plasma
chambers. The plasma generator 300 also includes a magnet assembly
342.
[0070] The plasma generator 300 can use any type of metastable atom
source 304 that can be positioned inside of the plasma chamber 230.
In one embodiment, the metastable atom source 304 includes a
parallel plate discharge chamber (not shown). In another
embodiment, the metastable atom source 204 includes an electron gun
(not shown) that receives the volume of ground state atoms 208 from
the gas flow control system 210 and that generates and accelerates
an electron beam that excites a portion of the volume of ground
state atoms 208 to a metastable state. In yet another embodiment,
the metastable atom source 204 includes an inductively coupled
discharge chamber that receives the volume of ground state atoms
208 from the gas flow control system 210 and that generates a
discharge that excites a portion of the volume of ground state
atoms 208 to a metastable state.
[0071] The metastable atom source 304 is coupled to the feed gas
source 206 through a gas line 309 that is connected to the gas flow
control system 210. In one embodiment, an isolator 310 isolates the
gas line 306 from a wall of the plasma chamber 230. Additional
in-line insulating couplers (not shown) can be used to insulate the
gas line 309 and/or the gas flow control system 210 from the feed
gas source 206. The feed gas source 206 supplies the ground state
atoms 208 to an input 311 of the metastable atom source 304. The
gas flow control system 210 controls the volume and the flow rate
of the ground state atoms 208 to the metastable atom source
304.
[0072] A first output 220 of the power supply 222 is coupled to an
electrical input 312 of the metastable atom source 304. The type of
power supply depends upon the type of metastable atom source. For
example, the power supply 222 can be a pulsed power supply, a RF
power supply, an AC power supply, or a DC power supply. The
metastable atom source 304 receives the ground state atoms 208 and
generates a discharge which excites at least a portion of the
ground state atoms 208 to a metastable state. The operation of
specific embodiments of the metastable atom source 304 will be
discussed in more detail herein.
[0073] An output 314 of the metastable atom source 304 is adapted
to inject a volume of metastable atoms 218 proximate to the cathode
306. In one embodiment, the metastable atom source 304 injects the
volume of metastable atoms 218 proximate to the cathode 306 through
the use of a gas injector (not shown), such as a showerhead-type
injector. In one embodiment, a diameter of the input 311 of the
metastable atom source 304 is different from a diameter of the
output 314 of the metastable atom source 304. In this embodiment,
the difference in diameters creates a pressure differential that
increases the generation rate of the metastable atoms 218 in the
metastable atom source 304.
[0074] In one embodiment, ground state atoms 326 from a second feed
gas source 328, such as an argon gas source, are introduced into
the plasma chamber 230 through one or more gas inlets 340. In some
embodiments, the gas inlets 340 introduce the ground state atoms
320 directly into the region 324 between the anode 308 and the
cathode 306. A gas valve 341 controls the flow rate of the ground
state atoms 326 into the plasma chamber 230. The feed gas source
206 can provide any type of feed gas or mixture of feed gases, such
as, noble gases, reactive gases, and mixtures of noble gases and
reactive gases.
[0075] In one embodiment (not shown), the feed gas source 328 is
replaced with a metastable atom source, such as the metastable atom
source 204 described in connection with FIG. 2. In this embodiment,
metastable atoms 218 are injected directly between the anode 308
and the cathode 306. Direct injection of metastable atoms 218
between the anode 308 and the cathode 306 increases the density of
the plasma 302 because the metastable atoms require less energy
than ground state atoms to ionize.
[0076] A power supply 316 is electrically coupled to the volume of
metastable atoms 218. The power supply 316 can be any type of power
supply, such as a pulsed power supply, a RF power supply, an AC
power supply, or a DC power supply. A first output 318 of the power
supply 316 is coupled to the cathode 306. A second output 320 of
the power supply 316 is coupled to the anode 308. The power supply
316 generates an electric field 322 between the cathode 306 and the
anode 308 that raises the energy of the volume of metastable atoms
218 so that at least a portion of the volume of metastable atoms
218 are ionized, thereby generating the plasma 302.
[0077] In one embodiment, a magnet assembly 342 is disposed
proximate to the cathode 306. The magnet assembly 342 includes one
or more magnets 344. The one or more magnets 344 generate a
magnetic field 346 that traps electrons in a region 348 of the
plasma 302 that is proximate to the cathode 234. The trapped
electrons increases the ionization rate of the metastable atoms 218
and, therefore, increases the density of the plasma 302. In one
embodiment (not shown), a magnetic field is generated in the region
324 in order to substantially trap electrons in the area where the
plasma 302 is ignited. In this embodiment, the magnetic field in
the region 324 assists in the ignition of the plasma 302.
[0078] The magnetic field 346 also traps secondary electrons
generated at the cathode 306 by the electric field 322. The
secondary electrons move in a substantially circular motion
proximate to the cathode 306 according to crossed electric and
magnetic fields. The substantially circular motion of the electrons
generates the electron ExB drift current 350. The magnitude of the
electron ExB drift current 350 is proportional to the magnitude of
the discharge current in the plasma 302 and, in one embodiment, is
approximately in the range of about three to ten times the
magnitude of the discharge current.
[0079] In one embodiment, the electron ExB drift current 350
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 350 tends to
have a more complex shape as the interaction of the magnetic field
346 generated by the magnet assembly 342, the electric field 322
generated by the power supply 316, and the magnetic field generated
by the electron ExB drift current 350 becomes more acute. The exact
shape of the electron ExB drift current 350 can be quite elaborate
and depends on various factors. For example, in one embodiment, the
electron ExB drift current 350 has a substantially cycloidal
shape.
[0080] The electron ExB drift current 350 generates a magnetic
field that interacts with the magnetic field 346 generated by the
magnet assembly 342. The magnitude of the magnetic field generated
by the electron ExB drift current 350 increases with increased
electron ExB drift current 350. The magnetic field generated by the
electron ExB drift current 350 has a direction that is
substantially opposite to direction of the magnetic field 346
generated by the magnet assembly 342.
[0081] The interaction of the magnetic field 346 generated by the
magnet assembly 342 and the magnetic field generated by the
electron ExB drift current 350 generates magnetic field lines that
are somewhat more parallel to the surface of the cathode 306 than
the magnetic field lines generated by the magnet assembly 342. The
somewhat more parallel magnetic field lines allow the plasma 302 to
more uniformly distribute itself across the surface of the cathode
306 and, therefore, improves the uniformity of the plasma 302
proximate to the cathode 306.
[0082] The magnitude of the electron ExB drift current 350 is
relatively high because of the presence of the metastable atoms 218
in the chamber 230. The metastable atoms 218 ionize at a much
higher rate than a similar volume of ground state atoms 208.
Therefore, the current density associated with the plasma 302 is
higher than a current density associated with a plasma generated
from ground state atoms for the same input energy. The high current
density associated with the plasma 302 generates the relatively
large ExB drift current 350.
[0083] As the magnitude of the electron ExB drift current 350
increases, the magnetic field generated by the electron ExB drift
current 350 becomes stronger and eventually overpowers the magnetic
field 346 generated by the magnet assembly 342. The magnetic field
lines that are generated by the magnet assembly 342 exhibit
substantial distortion that is caused by the relatively strong
magnetic field that is generated by the relatively large electron
ExB drift current 350. Thus, a large electron ExB drift current 350
generates a magnetic field that strongly interacts with and can
dominate the magnetic field 346 that is generated by the magnet
assembly 342.
[0084] A substrate support 352 is disposed in the plasma chamber
230. The substrate support 352 is designed to support a substrate
354 or other work piece. In one embodiment, a temperature
controller 356 is positioned in thermal communication with the
substrate support 352 to regulate the temperature of the workpiece
354. The temperature controller 356 can increase or decrease the
temperature of the substrate 354. In some embodiments, the
temperature controller 356 is used to control the temperature of
the substrate for various reasons including enhancing a chemical
reaction, improve adhesion, and increasing deposition rate.
[0085] A first output 140 of the power supply 142 is coupled to the
substrate support 352. The second output 148 of the power supply
142 is coupled to ground. The power supply 142 can by any type of
power supply, such as a pulsed power supply, a RF power supply, an
AC power supply, or a DC power supply. In some embodiments, the
power supply 142 is used to apply a bias voltage to the substrate
354.
[0086] The operation of the plasma generator 300 is similar to the
operation of the plasma generator 200 of FIG. 2. The gas flow
control system 210 supplies ground state atoms 208 to the
metastable atom source 304 from the gas source 206. The metastable
atom source 304 generates a volume of metastable atoms 218 from the
volume of ground state atoms 208 supplied by the feed gas source
206.
[0087] Ground state atoms 326 from the feed gas source 328 are
injected in the region 324 between the anode 308 and the cathode
306. The metastable atoms 218 interact with the ground state atoms
326 in the region 324 between the anode 308 and the cathode 306.
The power supply 316 then generates the electric field 322 across
the mixture of metastable atoms 218 and ground state atoms 326. The
electric field 322 raises the energy of the metastable atoms 218
and ground state atoms 326 so that at least a portion of the
metastable atoms 218 and the ground state atoms 326 are ionized,
thereby generating the plasma 302 with a combination of a direct
ionization process and a multi-step ionization process. In other
embodiments, the feed gas source 328 contains a molecular gas.
[0088] In other embodiments, the energy of the metastable atoms 218
and ground state atoms 326 is raised by other means, such as a
planar discharge, a radio frequency discharge, an inductively
coupled plasma discharge, a capacitively coupled plasma (CCP)
discharge, a microwave plasma discharge, an electron cyclotron
resonance plasma discharge, a helicon plasma discharge, ultraviolet
light, X-ray radiation, electron beam radiation, or ion beam
radiation.
[0089] The one or more magnets 344 generate a magnetic field 346
that traps electrons in a region 348 of the plasma 302 that is
proximate to the cathode 234. The magnetic field 346 also traps
secondary electrons generated at the cathode 306 by the electric
field 322. The trapped electrons further increases the ionization
rate of the metastable atoms and the ground state atoms and, thus
further increases the density of the plasma 302.
[0090] As the plasma 302 is being generated, additional ground
state atoms 326 are injected in the region 324. These additional
ground state atoms 326 displace the plasma 302 in the region 324.
Meanwhile, additional metastable atoms 218 are generated by the
metastable atom source 304. These additional metastable atoms 218
commingle with the plasma 302 and with the ground state atoms 326.
The commingling of additional metastable atoms 218 further
increases the density of ions.
[0091] The resulting plasma 302 is much denser than plasmas
generated solely by direct ionization using similar plasma
generating equipment and power levels. In one embodiment, the
resulting plasma is at least twice as dense as a plasma generated
solely by direct ionization. There are many applications for such a
high density plasma. For example, the high density plasma can be
used for high-deposition rate sputtering or high-deposition rate
ion-assisted chemical vapor deposition.
[0092] FIG. 4 illustrates a cross-sectional view of an embodiment
of a metastable atom generator 400 that includes a metastable atom
source 402 according to the invention. The metastable atom
generator 400 includes the gas source 206 and the gas flow control
system 210. There are many possible configurations for the
metastable atom generator 400. In one embodiment, the gas flow
control system 210 includes the first gas valve 212, the mass flow
controller 214, and the second gas valve 216.
[0093] An output 406 of the gas source 206 is coupled to one end of
a gas line 405. The other end of the gas line 405 is coupled to an
input 408 of the first gas valve 212. An output 410 of the first
gas valve 212 is coupled to an input 412 of the mass flow
controller 214. An output 414 of the mass flow controller 214 is
coupled to an input 416 of the second gas valve 216. One end of a
gas line 418 is coupled to an output 420 of the second gas valve
216. The other end of the gas line 418 is coupled to an input 422
of the metastable atom source 402. An output 423 of the metastable
atom source 402 generates the volume of metastable atoms 218.
[0094] A power supply 404 is electrically coupled to metastable
atom source 402. The power supply 404 can be a DC, an AC, a RF, or
a pulsed power supply. A first output 428 of the power supply 404
is coupled to a first input 430 of the metastable atom source 402
with a first transmission line 432. A second output 434 of the
power supply 404 is coupled to a second input 436 of the metastable
atom source 402 with a second transmission line 438. The first
input 430 of the metastable atom source 402 is coupled to a first
electrode 440 in the metastable atom source 402. The second input
436 of the metastable atom source 402 is coupled to a second
electrode 442 in the metastable atom source 402.
[0095] In operation, ground state atoms 208 from the gas source 206
flow to the metastable atom source 402 through the gas flow control
system 210. The gas flow control system 210 controls the flow rate
of the ground state atoms 208 from the gas source 206. The ground
state atoms 208 flow between the first electrode 440 and the second
electrode 442. The first 440 and the second electrodes 442 are
energized by the power supply 404, such that a discharge is created
in a discharge region 444 between the first 440 and the second
electrodes 442. At least a portion of the ground state atoms 208
that are injected through the discharge region 444 are energized to
a metastable state. For example, Argon atoms require a 11.56 eV
energy to excite ground state Argon atoms to a metastable state.
The energy required to excite ground state Argon atoms is lower
than the 15.76 eV energy that is required to ionize ground state
Argon atoms. Therefore, a relatively large number of Argon atoms
are excited to the metastable state.
[0096] Some of the ground state atoms 208 are directly ionized,
which releases ions 424 and electrons 426 into the stream of
metastable atoms 218. Direct ionization occurs when bound electrons
in an atom are ejected from that atom. The metastable atoms 218,
the free ions 424 and electrons 426 then pass through the output
423 of the metastable atom source 402.
[0097] FIG. 5 illustrates a cross-sectional view of one embodiment
of a chamber 450 of a metastable atom source according present
invention. The chamber 450 includes an input 452 having a first
diameter 454. A gas line 456 from a gas source (not shown) is
coupled to the input 452 of the chamber 450. The chamber 450 also
includes an output 458 having a second diameter 460.
[0098] In one embodiment, the first diameter 454 of the input 452
is greater than the second diameter 460 of the output 458. The
difference in the first 454 and the second diameters 460 creates a
pressure differential between the input 452 and the output 458 of
the chamber 450. In one embodiment, the pressure differential is
chosen so that the pressure in the chamber 450 is increased. The
increase in pressure can improve the efficiency of the generation
of the metastable atoms 218 from the ground state atoms 208. In one
embodiment, the ratio of the first diameter 454 to the second
diameter 460 is chosen to optimize the excitation process in the
chamber 450. In addition, the pressure differential can increase
the velocity of the metastable atoms 218 flowing through the output
458.
[0099] FIG. 6 illustrates a cross-sectional view of an embodiment
of a metastable atom source 500 according to the invention. The
metastable atom source 500 is similar to the metastable atom source
402 of FIG. 4. The metastable atom source 500 includes a chamber
502. The metastable atom source 500 also includes first 504a, b and
second magnets 506a, b that create magnetic fields 508a, b through
the chamber 502.
[0100] A power supply 510 is coupled to the metastable atom source
500. For example, the power supply 510 can be a DC, an AC, a RF, or
a pulsed power supply. A first output 512 of the power supply 510
is coupled to a first input 514 of the metastable atom source 500
with a first transmission line 516. A second output 518 of the
power supply 510 is coupled to a second input 520 of the metastable
atom source 500 with a second transmission line 522. The first
input 514 of the metastable atom source 500 is coupled to a first
electrode 524 in the chamber 502. The second input 520 of the
metastable atom source 500 is coupled to a second electrode 526 in
the chamber 502.
[0101] A gas line 528 is coupled to an input 530 of the chamber
502. An output 532 of the chamber 502 is coupled to an input 534 of
an electron/ion absorber 536. The electron/ion absorber 536
prevents a substantial fraction of the electrons 426 and ions 424
in the chamber 502 from passing to an output 538 of the
electron/ion absorber 536. Specific embodiments of the electron/ion
absorber 536 are described herein. In one embodiment, a diameter
540 of the input 530 of the chamber 502 and a diameter 542 of the
output 532 of the chamber 502 can be varied to optimize the process
of generating the metastable atoms 218.
[0102] In operation, ground state atoms 208 from the gas source
(not shown) flow to the metastable atom source 500 through the
input 530 of the chamber 502. The ground state atoms 208 flow
between the first electrode 524 and the second electrode 526. The
first 524 and the second electrodes 526 are energized by the power
supply 510, such that an electric field is created that generates a
discharge in a discharge region 544 between the first 524 and the
second electrodes 526. The ground state atoms 208 that are injected
through the discharge region 540 are energized to a metastable
state. Some of those ground state atoms 208 are energized to the
point of ionization, which releases free ions 424 and electrons 426
into the stream of metastable atoms 218. Additionally, some ground
state atoms 208 are either not excited or are initially excited and
decay back to the ground state.
[0103] In one embodiment, ions in the chamber 502 impact the more
negatively biased electrode (either the first 524 or the second
electrode 526) and generate secondary electrons (not shown) from
the that electrode. The magnetic fields 508a, 508b confine many of
the electrons 426 and the secondary electrons in the chamber 502
and, thus improves the efficiency of the excitation process in the
chamber 502. The metastable atoms 218, ground state atoms 208, ions
424 and electrons 426 all pass through the output 532 of the
chamber 502.
[0104] The electron/ion absorber 536 receives the metastable atoms
218, ground state atoms 208, ions 424 and electrons 426 through the
input 534. The electron/ion absorber 536 traps the ions 424 and the
electrons 426 and allows the metastable atoms 218 and the ground
state atoms 208 to pass through an output 538. In one embodiment
(not shown), depending parameters, such as the flow rate of the
ground state atoms 208 from the gas source, the strength of the
magnetic fields 508a, 508b, and the strength of the electric field
generated by the power supply 510, substantially all of the ions
424 and the electrons 426 can be absorbed in the chamber 502. In
this embodiment, the electron/ion absorber 536 is not
necessary.
[0105] FIG. 7 is a perspective view of a metastable atom source 550
according to one embodiment of the invention. The metastable atom
source 550 is shown coupled to a flange 552. The flange 552 is
adapted to couple to an industry standard port (not shown) on a
commercially available plasma chamber (not shown).
[0106] The metastable atom source 550 includes a chamber 554.
Ground state atoms (not shown) enter the chamber 554 through one or
more gas inputs 555. In one embodiment, the metastable atom source
550 includes a first electrode 556. The first electrode 556 can be
a plate-type electrode, a cylindrical-shaped electrode, or a
conical-shaped electrode as shown. The first electrode 556 can be
any shaped that produces an electric field 558 that excites ground
state atoms to a metastable state.
[0107] A second electrode 560 is disposed inside the chamber 554
proximate to the first electrode 556. In one embodiment, the first
electrode 556 is a cathode and the second electrode 560 is an
anode. A first input terminal 562 couples the first electrode 556
to a power supply (not shown). A second input terminal 564 couples
the second electrode 560 to the power supply.
[0108] In one embodiment, magnets 566a-d are positioned on the top
surface 568 of the first electrode 556. In this embodiment, magnets
570a-d are also positioned in the bottom surface 572 of the second
electrode 560 opposite to the magnets 566a-d. The magnets 566a-d
trap electrons and increase the probability that electrons will
collide with ground state atoms and generate metastable atoms. In
one embodiment, the metastable atom source 550 includes at least
one mirror (not shown) that is positioned so as to reflect light
that is generated when excited and metastable atoms decay to the
ground state.
[0109] FIG. 7A illustrates a cross-sectional view of the metastable
atom source 550 of FIG. 7 illustrating the magnetic field 574. The
magnets 566a-d, 570a-d create a magnetic field 574 that
substantially traps and accelerates electrons (not shown) in the
chamber 554. The trapped electrons (not shown) collide with the
ground state atoms (not shown), thereby raising the energy of the
ground state atoms to a metastable state. The metastable atoms (not
shown) exit the chamber 554 through one or more gas outputs
576.
[0110] The operation of the metastable atom source 550 is similar
to the operation of the metastable atom source 500 of FIG. 6.
However, in this embodiment, the metastable atom source 550 does
not include an electron/ion absorber 536. Thus, a small volume of
ions and/or electrons that are not trapped by the magnetic field
574 will likely exit the chamber 554 of the metastable atom source
550 through the gas outputs 576.
[0111] FIG. 8 illustrates a cross-sectional view of another
embodiment of a metastable atom source 600 according to the
invention. The metastable atom source 600 includes a chamber 602.
The metastable atom source 600 also includes an electron gun 604
and an electron trap 606. The electron gun 604 includes a power
supply 626 that is coupled to a filament electrode 628. The power
supply 626 can be any type of power supply, such as a DC, an AC, a
RF, or a pulsed power supply. A first output 630 of the power
supply 626 is coupled to a first terminal 632 of the filament
electrode 628 with a first transmission line 634. A second output
636 of the power supply 626 is coupled to a second terminal 638 of
the filament electrode 628 with a second transmission line 640.
[0112] The electron gun 604 also includes an acceleration grid 642
that is adapted to accelerate the electrons 608 that are emitted by
the filament electrode 628. An input 642 of the acceleration grid
642 is coupled to a first output 644 of a power supply 646. In one
embodiment, the power supply 646 is a DC power supply or a pulsed
power supply. The first output 644 of the power supply 646 couples
a positive voltage to the input 642 of the acceleration grid 642.
The positive voltage accelerates the negatively charged electrons
towards the acceleration grid 642. In one embodiment, a second
output 648 of the power supply 646 is coupled to the second input
636 of the power supply 636. However, many different power supply
configurations are possible.
[0113] A gas line 610 is coupled to an input 612 of the chamber
602. An output 614 of the chamber 602 is coupled to an input 616 of
an electron/ion absorber 618. In one embodiment, a diameter 622 of
the input 612 of the chamber 602 and a diameter 624 of the output
614 of the chamber 602 is chosen to optimize the process of
generating the metastable atoms 218.
[0114] In operation, ground state atoms 208 from the gas source
(not shown) flow into the chamber 602 through the input 612. The
ground state atoms 208 flow into a region 649 proximate to the
electron gun 604. The electron gun 604 generates and accelerates
electrons 608 into the region 649. A portion of the ground state
atoms 208 that are injected through the region 649 collide with the
electrons 608 and are energized to a metastable state. Some of
those ground state atoms 208 are energized to the point of
ionization and release free ions 424 and electrons 426 into the
stream of metastable atoms 218.
[0115] The electron trap 606 traps electrons 608 that are generated
and accelerated by the electron gun 604. In one embodiment, the
electron trap 606 is negatively biased. In this embodiment, ions
424 in the chamber 602 impact the surface of the electron trap 606
and generate secondary electrons from the surface of the electron
trap 606. In another embodiment, the electron trap 606 is
positively biased. In this embodiment, electrons 608 in the chamber
602 are further accelerated and trapped by the electron trap
606.
[0116] The metastable atoms 218, the ground state atoms 208, the
ions 424 and electrons 426 then pass through the output 614 of the
chamber 602. The electron/ion absorber 618 receives the metastable
atoms 218, ground state atoms 208, ions 424 and electrons 426
through the input 616. The electron/ion absorber 618 traps the ions
424 and the electrons 426 and allows the metastable atoms 218 and
the ground state atoms 208 to pass through the output 620.
[0117] FIG. 9 illustrates a cross-sectional view of another
metastable atom source 650 according to the invention. The
metastable atom source 650 includes a chamber 652. In one
embodiment, the chamber 652 is formed of a non-conducting pipe or a
dielectric tube. The metastable atom source 650 also includes an
inductive coil 654 that surrounds the chamber 652. The inductive
coil 654 is adapted to inductively couple energy into the chamber
652.
[0118] A gas line 656 is coupled to an input 658 of the chamber
652. An output 660 of the chamber 652 is coupled to an input 662 of
a electron/ion absorber 664. The metastable atoms 218 pass through
an output 666 of the electron/ion absorber 664. In one embodiment,
a diameter 668 of the input 658 of the chamber 652 and a diameter
670 of the output 660 of the chamber 652 is chosen to optimize the
process of generating the metastable atoms 218.
[0119] The metastable atom source 650 includes a power supply 672.
Any type of power supply can be used, such as a DC, an AC, a RF, or
a pulsed power supply. A first output 674 of the power supply 672
is coupled to a first terminal 676 of the inductive coil 654 with a
first transmission line 678. A second output 680 of the power
supply 672 is coupled to a second terminal 682 of the inductive
coil 654 with a second transmission line 684.
[0120] In operation, ground state atoms 208 from the gas source
(not shown) flow into the chamber 652 through the input 658. The
ground state atoms 208 flow into a region 686 and are surrounded by
the inductive coil 654. The inductive coil 654 couples energy into
the region 686 as current generated by the power supply 672 flows
through the inductive coils. A portion of the ground state atoms
208 that are injected through the region 686 are energized to a
metastable state. A portion of the ground state atoms 208 are
ionized and release free ions 424 and electrons 426 into the stream
of metastable atoms 218. Some ground state atoms 208 are also
present in the stream of metastable atoms 218. The metastable atoms
218, the ground state atoms 208, the ions 424 and the electrons 426
then pass through the output 660 of the chamber 652.
[0121] The electron/ion absorber 664 receives the metastable atoms
218, the ground state atoms 208, the ions 424 and the electrons 426
through the input 662. The electron/ion absorber 664 traps the ions
424 and the electrons 426 and allows the metastable atoms 218 and
the ground state atoms 208 to pass through the output 666.
[0122] FIG. 10 illustrates a cross-sectional view of another
metastable atom source 700 according to the invention. This
embodiment of the metastable atom source 700 includes a cylindrical
chamber 702. In one embodiment, the cylindrical chamber 702 is
formed of a dielectric material or a non-conducting material. The
metastable atom source 700 also includes a first cylindrical
electrode 704 and a second cylindrical electrode 706. One of the
first 704 and the second cylindrical electrodes 706 is adapted to
be an anode and the other is adapted to be a cathode.
[0123] In one embodiment, the size and shape of the first 704 and
the second electrodes 706 is chosen to optimize the process for
generating metastable atoms 218 in the metastable atom source 700.
For example, the shape of the second cylindrical electrodes 706 and
the dimensions of a gap 708 can be chosen to control the pressure
in a region 710 in the cylindrical chamber 702 so as to optimize
the process of generating metastable atoms 218. For example,
increasing the pressure in the region 710 can increase the
efficiency of the excitation process and, therefore the efficiency
of generating the metastable atoms.
[0124] A power supply (not shown) is electrically connected between
the first 704 and the second cylindrical electrodes 706. In another
embodiment, one terminal of the power supply is coupled to ground.
In this embodiment, one of the first 704 and second cylindrical
electrodes 706 is also coupled to ground (not shown).
[0125] In one embodiment, the metastable atom source 700 includes
electromagnetic coils 712, 714. The electromagnetic coils 712, 714
generate a magnetic field 716 having magnetic field lines 718, 720.
The magnetic field 716 traps electrons proximate to the region 710.
The trapped electrons assist in trapping ions proximate to the
region 710. In other embodiments, the metastable atom source 700
includes magnets (not shown).
[0126] A gas line (not shown) is coupled to an input 722 of the
chamber 702. An output 724 of the chamber 702 is coupled to an
input 726 of an electron/ion absorber 728. The electron/ion
absorber 728 passes the metastable atoms 218 through an output
730.
[0127] In operation, ground state atoms 208 from the gas source
(not shown) flow into the chamber 702 through the input 722. The
ground state atoms 208 then flow into the region 710. The power
supply (not shown) generates a voltage between the first 704 and
the second cylindrical electrodes 706. The voltage creates an
electric field that raises the energy of the ground state atoms
208. A portion of the ground state atoms 208 that are injected
through the region 710 are energized to a metastable state. A
fraction of the ground state atoms 208 are ionized and release free
ions 424 and electrons 426 into the stream of metastable atoms
218.
[0128] The metastable atoms 218, the ground state atoms 208, the
ions 424 and electrons 426 then pass through the output 724 of the
chamber 702. The electron/ion absorber 728 receives the metastable
atoms 218, the ground state atoms 208, the ions 424 and the
electrons 426 through the input 726. The electron/ion absorber 728
traps the ions 424 and the electrons 426 and allows the metastable
atoms 218 and the ground state atoms 208 to pass through the output
730.
[0129] In other embodiments of the invention, the ground state
atoms 208 are energized to a metastable state by using an energy
source, such as a DC plasma source, a radio frequency (RF) plasma
source, an ultraviolet (UV) radiation source, an X-ray radiation
source, an electron beam radiation source, an ion beam radiation
source, an inductively coupled plasma (ICP) source, a capacitively
coupled plasma (CCP) source, a microwave plasma source, an electron
cyclotron resonance (ECR) plasma source, a helicon plasma source,
or a magnetron plasma discharge source.
[0130] FIG. 11 illustrates a cross-sectional view of another
metastable atom source 735 according to the invention. The
metastable atom source 735 includes a tube 736. The tube 736 is
formed of non-conducting material, such as dielectric material,
like boron nitride or quartz. A nozzle 737 is positioned at one end
of the tube 736. The tube 736 is surrounded by an enclosure 738. A
skimmer 739 having an aperture 740 is positioned adjacent to the
nozzle 737 forming a nozzle chamber 741. The skimmer 739 can be
connected to the enclosure 738. In one embodiment, the skimmer 739
is cone-shaped as shown in FIG. 11. In one embodiment, the
enclosure 738 and the skimmer 739 are electrically connected to
ground potential.
[0131] The tube 736 and the enclosure 738 defines an electrode
chamber 742 that is in fluid communication with the a gas inlet
743. A feed gas source (not shown) is coupled to the gas inlet 743
so as to allow feed gas to flow into the electrode chamber 742. An
electrode 744 is positioned inside the electrode chamber 742
adjacent to the nozzle 737 and to the skimmer 739. In one
embodiment, the electrode 744 is a needle electrode, as shown in
FIG. 11. The needle electrode will generate a relatively high
electric field at the tip of the electrode. The electrode 744 is
electrically isolated from the skimmer 739.
[0132] A power supply 745 is electrically coupled to the electrode
744 with a transmission line 746. The transmission line 746 may be
feed into the electrode chamber 742 though an insulator 747. The
power supply 745 can be any type of power supply suitable for
plasma generation, such as a DC power supply, pulsed power supply,
RF power supply, or an AC power supply. In one embodiment, the
power supply 745 generates a constant power or a constant
voltage.
[0133] In operation, feed gas flows into the electrode chamber 742
from the feed gas source. Some of the feed gas flows through the
nozzle 737 into the nozzle chamber 741. In one embodiment,
parameters such as the flow rate of the feed gas, the diameter of
the nozzle 737, and the diameter of the skimmer aperture 740 are
chosen to increase the generation of metastable atoms. The power
supply 745 applies a voltage to the electrode 744. An electric
field is developed between the electrode 744 and the skimmer 739.
The electric field raises the energy of the volume of excited atoms
thereby causing collision between neutral atoms, electrons, and
excited atoms. The collisions create excited atom and metastable
atoms.
[0134] FIG. 12A through FIG. 12C illustrate various embodiments of
an electron/ion absorber 750, 750', 750" according to the
invention. Referring to FIG. 12A, the electron/ion absorber 750
includes a first 756 and a second electrode 758 that are positioned
in a chamber 760. A first output 762 of a power supply 764 is
coupled to the first electrode 756. A second output 766 of the
power supply is coupled to ground. The second electrode 758 is also
coupled to ground. In one embodiment, the power supply 764 is a DC
power supply or a pulsed power supply.
[0135] In operation, metastable atoms 768, ground state atoms 770,
electrons 772, and ions 774, flow through the input 752 of the
electron/ion absorber 750 and enter the chamber 760. In one
embodiment, the power supply 760 applies a negative potential to
the first electrode 756 and, thus attracts and traps the ions 774
passing through the chamber 760. The second electrode 758 is
positively biased and, thus attract and traps the electrons 772.
The metastable atoms 768 and the ground state atoms 770 flow
through the output 754 of the electron/ion absorber 750.
[0136] FIG. 12B illustrates an electron/ion absorber 750' that
includes a chamber 760'. First 776 and second magnets 778 are
positioned inside the chamber 760'. The first 776 and the second
magnets 778 generate a magnetic field 780 in the chamber 760' that
traps the electrons and the ions. In operation, metastable atoms
768, ground state atoms 770, electrons 772, and ions 774 flow
through the input 752' of the electron/ion absorber 750' and enter
the chamber 760'. The electrons 772 are trapped by the magnetic
field 780. The trapped electrons 772 then trap the ions 774 in the
chamber 760'. The metastable atoms 768 and the ground state atoms
770 flow then through the output 754' of the electron/ion absorber
750'.
[0137] FIG. 12C illustrates an electron/ion absorber 750" that
includes a chamber 760". An absorber 782 is disposed in the chamber
760" in the direction of electron and ion propagation. Any type of
electron/ion absorber can be used. In operation, metastable atoms
768, ground state atoms 770, electrons 772, and ions 774, flow
through the input 752" of the electron/ion absorber 750" and enter
the chamber 760". The absorber 782 traps the electrons 772 and the
ions 774 in the chamber 760". The metastable atoms 768 and the
ground state atoms 770 flow through the output 754" of the
electron/ion absorber 750".
[0138] FIG. 13 is a flowchart of an illustrative process 800 of
generating a plasma with a multi-step ionization process according
to the present invention. The process 800 includes flowing ground
state atoms 208 from the gas source 206 (FIG. 2) into the
metastable atom source 204 (step 802). In one embodiment, the
volume of ground state atoms 208 includes a volume of noble gas
atoms.
[0139] The metastable atom source 204 then generates a volume of
metastable atoms 218 from the volume of ground state atoms 208
(step 804). In one embodiment, the volume of metastable atoms 218
is generated by generating a discharge that excites the ground
state atoms 208 to a metastable state. In another embodiment, the
volume of metastable atoms 218 is generated by generating an
electron beam that excites the ground state atoms 208 to a
metastable state.
[0140] In one embodiment, a magnetic field is generated proximate
to the ground state atoms 208. The magnetic field can be a static
or a pulsed magnetic field. The magnetic field substantially traps
electrons proximate to the ground state atoms 208 and, thus
increases the excitation rate of the ground state atoms 208 to a
metastable state.
[0141] In one embodiment, at least a portion of electrons and/or
ions are removed from the volume of metastable atoms (step 806).
Next, the energy of the metastable atoms 218 is raised so that at
least a portion of the volume of metastable atoms 218 are ionized
(step 808), thereby generating a plasma with a multi-step
ionization process. The volume of metastable atoms 218 requires
less energy to become ionized compared with a similar volume of
ground state atoms 208.
[0142] In one embodiment, raising the energy of the metastable
atoms 218 includes exposing the metastable atoms 218 to at least
one of an electric field and a magnetic field. In another
embodiment, raising the energy of the metastable atoms 218 includes
exposing the metastable atoms 218 to an electron source, an X-ray
radiation source, a plasma source.
[0143] While the invention has been particularly shown and
described with reference to specific preferred embodiments, it
should be understood by those skilled in the art that various
changes in form and detail may be made therein without departing
from the spirit and scope of the invention as defined herein.
* * * * *