U.S. patent application number 10/793815 was filed with the patent office on 2004-09-02 for plasma reactor coil magnet.
This patent application is currently assigned to Tokyo Electron Limited. Invention is credited to Mitrovic, Andrej S..
Application Number | 20040168771 10/793815 |
Document ID | / |
Family ID | 23239991 |
Filed Date | 2004-09-02 |
United States Patent
Application |
20040168771 |
Kind Code |
A1 |
Mitrovic, Andrej S. |
September 2, 2004 |
Plasma reactor coil magnet
Abstract
A method for processing a workpiece is carried out with a plasma
derived from a process gas in a plasma chamber of a plasma
processing apparatus during a plasma processing operation. The
apparatus includes an array of electromagnets mounted
circumferentially around the plasma chamber. The method comprises
generating a plasma from a process gas within the chamber and
causing plasma particles to strike the workpiece, selecting
distributions of current signals for the electromagnets, and
applying each selected distribution to the electromagnets to impose
more than one magnetic field topology on the plasma during the
plasma processing operation.
Inventors: |
Mitrovic, Andrej S.;
(Phoenix, AZ) |
Correspondence
Address: |
PILLSBURY WINTHROP, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
Tokyo Electron Limited
Tokyo
JP
|
Family ID: |
23239991 |
Appl. No.: |
10/793815 |
Filed: |
March 8, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10793815 |
Mar 8, 2004 |
|
|
|
PCT/US02/27978 |
Sep 4, 2002 |
|
|
|
60318890 |
Sep 14, 2001 |
|
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Current U.S.
Class: |
156/345.46 |
Current CPC
Class: |
H01J 37/3266 20130101;
H01J 37/32935 20130101; H01J 37/32623 20130101 |
Class at
Publication: |
156/345.46 |
International
Class: |
C23F 001/00 |
Claims
What is claimed is:
1. A method for processing a workpiece with a plasma derived from a
process gas in a plasma chamber of a plasma processing apparatus
during a plasma processing operation, the apparatus including an
array of electromagnets mounted circumferentially around the plasma
chamber, the method comprising: generating a plasma from a process
gas within the chamber and causing plasma particles to strike the
workpiece; selecting distributions of current signals for said
electromagnets; and applying each said selected distribution to
said electromagnets to impose more than one magnetic field topology
on the plasma during the plasma processing operation.
2. The method of claim 1, wherein at least one magnetic field
topology is a non-rotating magnetic field topology.
3. The method of claim 1, wherein at least one magnetic field
topology is a rotating magnetic field topology.
4. The method of claim 3, wherein the at least one rotating
magnetic field topology corrects a non-uniformity in plasma density
of said plasma while said at least one rotating magnetic field
topology is imposed on said plasma.
5. The method of claim 4, wherein the at least one rotating
magnetic field topology is a cross field topology.
6. The method of claim 5, wherein the magnetic field lines of the
cross field topology are non-linear.
7. The method of claim 2, wherein the at least one non-rotating
magnetic field topology is a bucket field topology.
8. The method of claim 1, wherein the more than one magnetic field
topologies include a cross field topology and a bucket field
topology.
9. The method of claim 1, wherein the applying includes supplying
the current signals such that a bucket field topology is imposed on
the plasma during a first portion of plasma processing operation
and a cross field topology is imposed on the plasma during a second
portion of plasma processing operation.
10. The method of claim 9, wherein a plurality of bucket field
topologies are imposed on the plasma to decrease plasma density at
a predetermined rate during the first portion of the plasma
processing operation.
11. The method of claim 10, wherein the magnetic field lines of the
cross field topology are nonlinear.
12. The method of claim 11, wherein the cross field topology
corrects a nonuniformity of said plasma.
13. The method of claim 1, wherein at least one magnetic field
topology changes angular orientation during processing.
14. The method of claim 13, wherein said at least one magnetic
field topology that changes angular orientation during processing
changes angular orientation by rotating.
15. A method for processing a workpiece with a plasma derived from
a process gas in a plasma chamber of a plasma processing apparatus
during a plasma processing operation, the apparatus including an
array of electromagnets mounted circumferentially around the plasma
chamber, the method comprising: generating a plasma from a process
gas within the chamber and causing plasma particles to strike the
workpiece; and supplying a distribution of current signals to said
electromagnets so that said electromagnets impose a rotating bucket
magnetic field topology on the plasma during the plasma processing
operation.
16. The method of claim 15, wherein the array of electromagnets
comprises a first system of electromagnets and a second system of
electromagnets, each electromagnet of each system being positioned
between a pair of electromagnets of the other system.
17. The method of claim 16, wherein the current signal in at least
one electromagnet system has a nonzero magnitude at each instant
during said field rotation.
18. A plasma processing apparatus for processing a workpiece, the
plasma processing apparatus comprising: a plasma chamber including
an interior region for supporting a plasma; a plasma generating
source; a vacuum system in fluidic communication with the interior
region of the plasma chamber; a gas supply system in fluidic
communication with the interior region of the plasma chamber; a
plurality of coil magnets mounted circumferentially around the
plasma chamber, each coil magnet having an axis extending radially
from an axis of the plasma chamber; a plurality of arbitrary
waveform generators, each being electrically communicated to an
associated one of the plurality of coil magnets; a control system
electrically coupled to the gas supply system, the vacuum system,
the cooling system, and the plurality of arbitrary waveform
generators, the control system being configured to operate the
arbitrary waveform generators so that the coil magnets impose a
magnetic field topology on the plasma during the plasma processing
operation.
19. A plasma processing apparatus as defined in claim 18, said
plasma generating source comprising one or more electrode assembly
mounted within the chamber and one or more RF power sources each
electrically coupled to an associated electrode assembly.
20. A plasma processing apparatus as defined in claim 19, wherein
each coil magnet is an air coil.
21. A plasma processing apparatus as defined in claim 19, wherein
each coil magnet has a core of magnetically permeable material.
22. A plasma processing apparatus as defined in claim 21, further
comprising an outer flux conducting structure mounted in
surrounding relation to the array of coil magnets, each coil magnet
and each core being in magnetic flux communication with the flux
conducting structure.
23. A plasma processing apparatus as defined in claim 22, wherein
the flux conducting structure is an annular wall structure.
24. A plasma processing apparatus as defined in claim 23, wherein
the annular wall structure is constructed of a magnetically
permeable material.
25. A plasma processing apparatus as defined in claim 24, wherein
each core is mounted on the annular wall structure.
26. A plasma processing apparatus as defined in claim 18, wherein
each arbitrary waveform generator of said plurality thereof is
electrically coupled to an associated one of plurality of coil
magnets through an associated one of a plurality of amplifiers.
Description
[0001] This is a continuation of International Application No.
PCT/US02/27978, filed on Sep. 4, 2002, which, in turn, claims the
benefit of U.S. Provisional Application No. 60/318,890, filed Sep.
14, 2001, the contents of both of which are incorporated herein in
their entirety by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to plasma processing systems
and more particularly to a method and apparatus for using a
magnetic field imposed on a plasma to control plasma
characteristics to improve plasma processing of a workpiece.
BACKGROUND OF THE INVENTION
[0003] A plasma is a collection of charged particles that may be
used to remove material from or deposit material on a workpiece.
Plasma may be used, for example, to etch (i.e., remove) material
from or to sputter (i.e., deposit) material on a semiconductor
substrate during integrated circuit (IC) fabrication. A plasma may
be formed by applying a radio frequency (RF) power signal to a
process gas contained in a plasma chamber to ionize the gas
particles. The RF source may be coupled to the plasma through a
capacitance, through an inductance, or through both a capacitance
and an inductance. Magnetic fields may be imposed on the plasma
during plasma processing of a workpiece to improve plasma
characteristics and thereby increase control over the plasma
processing of the workpiece.
[0004] Magnetic fields are sometimes used during the plasma
processing of a workpiece to contain the plasma within the chamber
or to change plasma properties during plasma processing. Magnetic
fields may be used, for example, to contain the plasma within the
chamber, thereby reducing plasma loss to the chamber walls, and to
increase plasma density. Increasing plasma density increases the
number of plasma particles striking the workpiece, which improves
the processing of the workpiece by, for example, decreasing the
processing time required to etch a workpiece. Containment of the
plasma using magnetic fields also prevents plasma particle
deposition on surfaces within the chamber such as chamber wall
surfaces and electrode surfaces.
[0005] Magnetic fields are also used to increase the uniformity of
the distribution of plasma within the chamber. Non-uniform
distribution of plasma within a plasma chamber is undesirable
because non-uniform distribution may result in non-uniform
processing of the workpiece. Non-uniformly distributed plasmas may,
in some situations, result in plasma-induced damage to the
workpiece being processed in the chamber.
[0006] Arrays of either permanent magnets or electromagnets are
sometimes used to impose a magnetic field on the plasma. An array
of permanent magnets can be arranged, for example, so that they
impose a magnetic field on the plasma within the interior of the
chamber, or, alternatively, they can be arranged and moved (by
rotation with respect to the chamber, for example) so that they
impose a rotating magnetic field on the plasma, which improves
plasma uniformity.
SUMMARY OF THE INVENTION
[0007] The present invention includes methods and apparatuses for
utilizing magnetic fields to control the processing of a workpiece
with the plasma.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic diagram of an example plasma
processing system for illustrating the present invention, the
plasma processing system showing a workpiece and plasma within a
plasma chamber of a plasma processing apparatus and showing an
outer flux conducting structure and an array of electromagnets
surrounding the processing chamber;
[0009] FIG. 2 is a schematic top plan view of a portion of the
apparatus of FIG. 1, FIG. 2 showing the processing chamber, a lower
electrode, the outer flux conducting structure and the array of
electromagnets surrounding the processing chamber and showing a
magnetic cross field topology imposed on the interior of the
chamber;
[0010] FIG. 3 is identical to FIG. 2 except showing a magnetic
bucket field topology imposed on the interior of the chamber;
[0011] FIG. 4 is a schematic representation of an example power
supply circuit for supplying an array of magnets with electrical
power;
[0012] FIG. 5 is a schematic representation of a second example
power supply circuit for supplying an array of magnets with
electrical power;
[0013] FIG. 6 is a schematic view similar to FIG. 3 except showing
a bucket field topology imposed on the processing chamber by two
systems of electromagnets; and
[0014] FIG. 7 is a graph showing current flows in four adjacent
electromagnets of the apparatus of FIG. 6.
DETAILED DESCRIPTION OF THE INVENTION
[0015] FIG. 1 shows a schematic representation of an example of a
plasma processing apparatus (or reactor) 10 of a plasma processing
system 12. The plasma processing apparatus 10 includes a plasma
chamber 14, which provides an interior region 16 for containing and
supporting a plasma. A plurality of electrodes may be mounted
within the chamber 14 in plasma generating relation to one another
and to a process gas within the chamber 14. The electrodes are
energized to generate a plasma from the process gas within the
chamber 14. To facilitate the description of the invention, only
two electrode assemblies are included in the apparatus 10.
Specifically, a first electrode assembly 18 is mounted on a first
side of the chamber 14 (in an upper portion of the interior 16 of
the chamber 14 in the example apparatus 10). A second electrode
assembly in the form of a chuck electrode assembly 20 is mounted on
a second side of the chamber 14 opposite the first side of the
chamber 14 (in a lower portion of the chamber interior 16 in the
example apparatus 10) in a position spaced from the first electrode
assembly 18.
[0016] The first electrode assembly 18 may include a plurality of
electrode segments, each segment being electrically isolated from
the other segments and each segment being independently powered by
an associated RF power source and independently supplied with a
selected process gas for transmission at a predetermined rate into
the interior of the plasma chamber. To facilitate the description
of the present invention, however, the first electrode assembly 18
is in the form of a single showerhead-type electrode. The first
electrode assembly 18 includes an inner chamber 22 (indicated
schematically by a broken line in FIG. 1) that is in pneumatic or
fluidic communication with a gas supply system 24 through a gas
supply line. A selected gas (or gasses) may be supplied to the
electrode assembly 18 to purge the chamber 14, for example, or to
serve as a process gas (or source gas) for plasma formation in the
chamber interior 16. The process gas is transmitted from the
chamber 22 into the interior 16 of the plasma chamber 14 through a
plurality of gas ports (not shown). The flow of gas through the
ports of the first electrode is indicated by a series of
directional arrows G.
[0017] The first and second electrodes 18, 20 are electrically
communicated through associated matching networks 30, 32 to
respective RF power sources 34, 36 which provide voltage signals
V.sub.B1, V.sub.B2, respectively, to the associated electrodes 18,
20. Matching networks 30, 32 may be inserted between respective RF
power sources 34, 36 in order to maximize the power transferred to
the plasma by the respective electrode assemblies 18, 20.
Alternately, the matching networks 30, 32 may be coupled to control
system 60.
[0018] Each electrode assembly 18, 20 may be independently cooled
by a fluid that circulates from a cooling system 38 through a fluid
chamber 39, 41 (indicated by a broken line) in each electrode
assembly 18, 20, respectively, and then back to the cooling system.
The plasma processing apparatus 10 further includes a vacuum system
40 in pneumatic or fluidic communication with the plasma chamber 16
through a vacuum line. The plasma processing apparatus 10
optionally includes a voltage probe 44, 46 in the form of a pair of
electrodes capacitively coupled to the transmission lines between
the associated RF power sources 34, 36, respectively, and the
associated electrode assembly 18, 20, respectively. (An example
voltage probe is described in detail in commonly assigned pending
U.S. application 60/259,862 (filed on Jan. 8, 2001), and it is
incorporated in its entirety herein by reference.) The plasma
processing apparatus 10 optionally includes an optical probe 48 for
determining plasma characteristics and conditions based on spectral
and optical properties of the plasma.
[0019] A system or array of electromagnets 51 are mounted
circumferentially around the plasma chamber 14. The electromagnets
51 are operable to impose one or more magnetic fields on a plasma
during a plasma processing operation on a workpiece. The imposition
of a magnetic field improves the condition of the plasma and
thereby improves the processing of the workpiece.
[0020] FIG. 2 shows an example of an arrangement of the plurality
of electromagnets 51 with respect to the plasma chamber 14. The
example apparatus 12 includes twelve electromagnets, designated
51A-L. Each electromagnet 51 shown is in the form of a coil magnet
that includes a coil of an electrically conductive material. Each
coil is in electrical communication with an electrical power source
53 (shown schematically in FIG. 1).
[0021] Each coil magnet 51 of a particular array may be provided by
a coil of conductive material wound on an air core (not shown) or,
alternatively, may be provided by a coil of conductive material
wound around a core 55 (partially visible in FIG. 1) of, for
example, a magnetically permeable material. Each core 55 may have a
cylindrical cross section (as shown) or, alternatively, may have an
arbitrary elongated cross section (with the longer dimension
extending in the vertical direction in the example apparatus 10).
The axis of each coil magnet 51 is radially aligned with the plasma
chamber 14. That is, the axis of each coil magnet 51 extends
radially from an imaginary axis that extends (vertically in the
example reactor 10) between the electrode assemblies 18, 20 through
the center of the plasma chamber 14. An outer flux conducting
structure 57 may be mounted in surrounding relation to the array of
coil magnets 51 as best seen in FIG. 2. Each coil magnet 51 and
each core 55 is in magnetic flux communication with the flux
conducting structure 57. An example of the flux conducting
structure 57 is an annular wall structure. Both the outer wall
structure 57 and the core 55 of each coil magnet 51 may be
constructed of a magnetically permeable material such as iron. Each
core 55 may be integrally formed on the outer ring structure 57 or
may be formed separately from the outer wall structure 57 and then
mounted on the outer ring structure 57.
[0022] It can be appreciated from FIG. 2 that each coil magnet 51
and its associated core 55 extends in a radial direction between
the outer ring structure 57 and the wall structure 59 of the plasma
chamber 14. In the example apparatus 10, the wall structure 59 is
cylindrical and comprises the side wall of the processing chamber
14. The wall structure 59 of the plasma chamber 14 may be
constructed of either a suitable dielectric material or a suitable
metallic material. If the wall structure 59 is constructed of a
metallic material, a non-magnetic metallic material is used in the
construction so that the wall structure 59 does not interfere with
a magnetic field imposed on a plasma within the plasma chamber 14
by the coil magnets 51.
[0023] The array of magnets in the example apparatus 10 is
vertically aligned with the plasma in FIG. 1, but this vertical
positioning is an example only. The array of magnets could have any
vertical position with respect to the processing chamber and the
structures (the electrodes, for example) and materials (the
workpiece or plasma, for example) contained therein. For example,
the apparatus 10 could be constructed and arranged so that the
array of magnets are vertically aligned with the top of the
workpiece, aligned with the center of the workpiece, slightly above
the workpiece, for example, or aligned with the vertical center of
the plasma, or slightly above or below the plasma, for example.
[0024] A control system 60 of the plasma processing apparatus 10 is
electrically communicated to various components of the apparatus 10
to monitor and/or control the same. The control system 60 is in
electrical communication with and may be programmed to control the
operation of the gas supply system 24, vacuum system 40, the
cooling system 38, the voltage probe 44, 46, the optical probe 48,
each RF power source 34, 36 and the power source 53. The control
system 60 may send control signals to and receive input signals
(feedback signals, for example) from the probes 44, 46, 48 and
system components 24, 34, 36, 38, 40, 53. The control system 60 may
monitor and control the plasma processing of a workpiece. By
controlling the power source 53, the control system 60 is able to
control the transfer of electrical power to each coil magnet
comprising the array of coil magnets 51, and thereby control the
properties of the magnetic field imposed on the plasma.
[0025] The control system 60 may be provided by a computer system
that includes a processor, computer memory accessible by the
processor (where the memory is suitable for storing instructions
and data and may include, for example, primary memory such as
random access memory and secondary memory such as a disk drive) and
data input and output capability communicated to the processor.
[0026] The methods of the present invention are illustrated with
reference to the example plasma processing system 12. The operation
of the plasma processing system 12 can be understood with reference
to FIG. 1. A workpiece (or substrate) 62 to be processed is placed
on a support surface provided by the chuck assembly 20. The control
system 60 activates the vacuum system 40 which initially lowers the
pressure in the interior 16 of the plasma chamber 14 to a base
pressure (typically 10.sup.-7 to 10.sup.-4 Torr) to assure vacuum
integrity and cleanliness for the chamber 14. The control system 60
then raises the chamber pressure to a level suitable for forming a
plasma and for processing the workpiece 62 with the plasma (a
suitable interior pressure may be, for example, in the range of
from about 1 mTorr to about 1000 mTorr). In order to establish a
suitable pressure in the chamber interior 16, the control system 60
activates the gas supply system 24 to supply a process gas through
the gas inlet line to the chamber interior 16 at a prescribed
process flow rate and the vacuum system 40 is throttled, if
necessary, using a gate valve (not shown). The process gas may flow
through ports in the first electrode assembly as indicated in FIG.
1 by arrows G.
[0027] The particular gas or gases included in the gas supply
system 24 depends on the particular plasma processing application.
For plasma etching applications, for example, the gas supply system
24 may supply chlorine, hydrogen-bromide, octafluorocyclobutane, or
various other gaseous fluorocarbon compounds; for chemical vapor
deposition applications, the system 24 may supply silane, ammonia,
tungsten-tetrachloride, titanium-tetrachloride, or like gases. A
plasma may also be used in chemical vapor deposition (CVD) to form
thin films of metals, semiconductors or insulators (that is,
conducting, semiconducting or insulating materials) on a
semiconductor wafer. Plasma-enhanced CVD uses the plasma to supply
the required reaction energy for deposition of the desired
materials.
[0028] The control system 60 then activates the RF power sources
34, 36 associated with the first and second electrode assemblies
18, 20. The RF power sources 34, 36 may provide voltages to the
associated electrodes 18, 20 at selected frequencies. The control
system 60 may, during a plasma processing operation, independently
control the RF power sources 34, 36 to adjust, for example, the
frequency and/or amplitude of the voltage at which each source 34,
36 drives the associated electrode assembly 18, 20.
[0029] The RF power sources 34, 36 may be operated to convert the
low-pressure process gas to a plasma. The power sources 34, 36 may
be operated, for example, to cause an alternating electric field to
be generated between the first and second electrodes 18, 20 which
induces an electron flow between the electrodes 18, 20. Electrons,
for example, are accelerated in this electric field and the flow of
heated electrons in the field ionizes individual atoms and
molecules of the process gas by transferring kinetic energy thereto
through multiple collisions between the electrons and the gas atoms
and molecules. This process creates a plasma 54 that is confined
and supported within the chamber 14.
[0030] Because each RF power source 34, 36 is independently
controllable by the control system 60, either power source may be
operated to have a relatively low frequency (i.e., a frequency
below 550 KHz), an intermediate frequency (i.e., a frequency around
13.56 MHz), or a relatively high frequency, around 60 to 150 MHz.
In an example of an etch reactor, the RF power source 34 for the
first electrode assembly 18 can be driven at a frequency of 60 MHz
and the RF power source 36 for the second electrode assembly 20 can
be driven at a frequency of 2 MHz. In order to improve the
performance of the aforementioned reactor, or, more generally, a
plasma processing device having one or more electrodes that are
driven at one or more frequencies, the control system 60 can be
programmed and operated to impose one or more magnetic fields on
the plasma during processing of the workpiece to control the
characteristics (such as, for example, magnetic field topology and
orientation, magnetic field strength, magnetic field duration and
so on) of the magnetic fields.
[0031] The invention allows a large number of possible magnetic
field topologies to be generated using a single array of magnets 51
having no moving parts. FIGS. 2 and 3 show two magnetic field
topologies that can be imposed on the plasma 54 (the plasma 54
being shown schematically in FIG. 1 only) using the magnet system.
FIG. 2 shows a cross field topology and FIG. 3 shows a magnetic
bucket field topology.
[0032] The cross field topology illustrated has nonlinear (i.e.,
arcuate) magnetic fields lines. The cross field topology may be
used to improve the uniformity of the plasma. Increasing the plasma
uniformity increases the process uniformity both for a single
substrate 62 and also increases process uniformity among a
plurality of substrates processed in succession by the apparatus
10. The array of electromagnets 51 may be operated to rotate the
cross field topology in a manner described below. A magnetic bucket
topology (FIG. 3) may be imposed on the plasma to reduce plasma
wall loss and to increase plasma density.
[0033] An example of a circuit 68 for realizing the electrical
power source 53 for powering the coil magnets 51A-L to create a
desired magnetic field topology is shown schematically in FIG. 4.
Specifically, each of a series of arbitrary waveform generators
70A-L may be electrically communicated to a respective coil magnet
51A-L (not shown in FIG. 4) of the system of electromagnets through
an associated amplifier 71A-L.
[0034] Each arbitrary waveform generator 70A-L may be electrically
communicated to the control system 60 (through electrical
connections that are not shown in FIG. 4). The control system 60
can be programmed to control each of the arbitrary waveform
generators 70A-L independently of one another to generate from each
a current waveform of arbitrary shape, magnitude and phase for
transmission to the associated coil magnet 51A-L to polarize the
same and to create the magnetic field that is imposed on the
plasma. All of the arbitrary waveform generators 70 may be phase
locked to a single low power reference signal source 72. Each
generator 70 is capable of shifting the phase of its output
relative to the reference signal from 72.
[0035] The power source arrangement of FIG. 4 enables the control
system 60 (acting through the series of arbitrary waveform
generators 70A-L) to supply each coil magnet 51 with a current
waveform having a wave shape, amplitude, phase and period that is
independent of the current waveforms generated by all other
arbitrary waveform generators in the series. Thus, the reference
signal from the reference signal source 72 is used to synchronize
the current waveforms transmitted from the system of arbitrary
waveform generators to the coil magnets 51. The control system 60
can independently program each arbitrary waveform generator 70 to
generate a different waveform with the starting phase locked to the
reference signal from source 72. This arrangement provides great
flexibility in imposing, for example, two or more magnetic field
topologies on the plasma. This arrangement allows the control
system 60 to impose, for example, two magnetic field topologies in
succession with one another during a plasma processing operation on
a particular substrate. These two topologies can be the same as one
another or can be different from one another. A topology may be
stationary or may be rotated.
[0036] For example, this arrangement (that is, using a separate
arbitrary waveform generator for each coil magnet) allows an
operator to program the control system 60 to impose on the plasma a
stationary magnetic field topology (azimuthally, for example) and a
rotating magnetic field topology, during a processing operation.
Each imposed field topology can be selected to achieve a particular
change in the plasma. For example, the rotating cross field
topology may be applied to improve plasma uniformity. As another
example, this arrangement also allows the waveforms to be generated
such that even though the imposed magnetic field is rotating, there
is a localized, field (e.g., a low- or a high-field region) imposed
at a particular location within the processing chamber. This
localized field may be used to correct for azimuthal variations of
plasma properties which result from, for example, non-axisymmetric
gas injection and the pumping of the plasma, and so on.
[0037] Another circuit 76 that can be used as a power source 53 is
shown schematically in FIG. 5. A single arbitrary waveform
generator 77 drives a series of amplifiers 71A-L that each supply
current to an associated coil magnet (not shown in FIG. 5). A phase
delay circuit 78 is coupled between the arbitrary waveform
generator and all but one of the amplifiers. Essentially the same
signal is sent to each coil magnet 51A-L, the only difference being
that the signals may be out of phase with one another because of
the presence of the phase delay circuits. Therefore, the circuit 76
can be used in instances in which the current waveforms to be
applied to the coil magnets 51 have identical wave shapes and
periods but differ from one another in phase. The power supply
circuit 76 can provide a rotating magnetic field topology or a
magnetic field topology that changes angular orientation. The
topology of the field that is produced and rotated by the power
source circuit 76 depends on several factors including the shape of
the current waveforms transmitted to the coil magnets, the number
of and the relative positions of the coil magnets in the coil
magnet system, relative field strength of each coil magnet 51 and
the phase difference between the current waveform signals. The
control system 60 can be programmed to control the arbitrary
waveform generator 70 of circuit 76 to produce, for example, a
rotating cross field topology having nonlinear (e.g., arcuate)
fields lines.
[0038] Operation
[0039] The control system 60 can produce steady currents in any (or
all) coil magnets 51 or can produce a time changing current in any
(or all) coil magnets 51A-L (using, for example, the power supply
circuitry 68). The distribution of steady and/or time-varying
currents passing through the coil magnets 51 determines the
topology of the magnetic field imposed on the plasma and determines
the change in time of the magnetic field topology. Appropriate
current waveforms can be sent to the coil magnets 51 to cause the
magnetic field imposed on the plasma to rotate, for example.
[0040] The current waveform applied to each coil magnet 51A-L
radially polarizes each coil. During radial polarization, opposite
ends of each coil magnet assume respective North and South magnetic
polarities. Generally, the magnetic fields lines extend between
opposite poles of the coil magnets 51. The direction of current
flow in each coil determines the polarity of each coil magnet. The
magnitude of the current flowing through the coil magnet determines
the strength of the magnetic field produced by each coil magnet,
and therefore the strength of the magnetic field imposed on the
plasma.
[0041] Other arrangements of the array of magnets are possible. For
example, although the axis of each coil magnet 51 extends radially
from an imaginary axis that extends between the electrode
assemblies 18, 20 in the example reactor 10, other arrangements are
possible. For example, each coil magnet 51 could be oriented so
that its axis is "tangential" to the reactor 10. Each tangentially
oriented coil could be an air coil or could be wound around a core
material. When each coil is wound around a core of material, each
core could be a separate structure or form part of a continuous
structure such as a ring or yoke.
[0042] This tangential arrangement has several disadvantages
(relative to the radial alignment in the example reactor 10). For
example, when a radially extending array of electromagnets are used
to generate magnetic fields, most of the magnetic flux lines enter
the chamber. When a tangential arrangement is used, however, most
of the magnetic flux field lines tend to flow around the exterior
of the plasma chamber 14, particularly when the coils are wound
around a yoke surrounding the chamber, and a relatively small
amount "leaks" or "fringes" out of the side of each tangentially
arranged coil and enters the plasma chamber 14. Thus, the
tangential arrangement relies on the fringing fields on one side of
each tangential coil to impose a magnetic field on the plasma in
the chamber. Because a magnet system that utilizes a tangential
arrangement of the coils relies on fringing fields to impose a
magnetic field on a plasma, more power is required to create a
particular topology having a particular field strength relative to
the use of a radial arrangement to create the same field topology
having the same strength. A radially arranged magnet system
utilizes less current than required by a comparable tangentially
arranged magnet system. Because a tangential arrangement relies on
field lines emerging from the sides of each coil, each coil emits
field lines toward the chamber and field lines out the opposite
side, for example, away from the chamber. An outer surrounding
structure is also needed to shield the area surrounding the plasma
processing apparatus from the magnetic field. When the tangentially
oriented electromagnets are wound around a yoke, for example, a
second permeable shield is needed if the area surrounding the
apparatus is to be shielded from magnetic fields. A second
permeable shield or other flux shielding structure is not required
in the example arrangement shown in FIGS. 1 and 2, for example,
because the structure 57 performs both a flux transmitting function
and a shielding function.
[0043] FIGS. 2, 3 and 6 illustrate examples of magnetic field
topologies that can be imposed on the plasma using the coil magnets
51A-L. The direction of the current flowing through each coil
magnet 51A-L is indicated by a directional arrow in FIGS. 2, 3 and
6. The relative magnitude of the current in each coil magnet is
roughly indicated by the relative size of the directional arrows in
FIGS. 2, 3 and 6. Absence of a directional arrow indicates an
instantaneous current of zero magnitude in the associated coil
magnet 51. The iron ring structure 57 closes the magnetic field
lines for each topology.
[0044] A rotating cross field topology can be imposed on the plasma
utilizing, for example, either power supply circuit 68 or 76. For
example, a complex current waveform can be fed to each coil magnet
51 that is phase shifted with respect to the previous coil in a
rotational direction that is opposite the direction of magnetic
field topology rotation. This method allows the cross field
topology to be rotated without mechanically moving any of the coil
magnets.
[0045] FIG. 2 shows a rotating cross field topology at a particular
instant in time. At this instant, the coil magnets 51A and 51B have
currents that are oppositely directed to one another and are of
relatively high magnitude, the coil magnets 51L and 51C have
currents that are oppositely directed to one another and are of
lesser magnitude than the currents in coil magnets 51A and 51B, and
the coil magnet pairs 51K and 51D, 51J and 51E, and 51I and 51F
have oppositely directed currents of successively lesser magnitude
(as indicated by the relative size of the directional arrows).
Nonlinear magnetic fields lines extend generally between the coils
of each pair of coil magnets as indicated by the arcuate arrows in
the processing chamber 14. The coil magnets 51H and 51G may have
instantaneous currents of zero magnitude (depending, for example,
on the exact field that one is trying to impose).
[0046] It can also be appreciated from FIG. 2 that magnetic field
lines extend generally from coil magnets 51A, 51L, 51K, 51J and 51I
on one side of the chamber to respective associated coil magnets
51B, 51C, 51D, 51E and 51F on an opposite side of the chamber. The
decreasing magnitude of the currents (on opposite sides of the
chamber) creates, in effect, a magnetic field gradient of
increasing strength from the approximately eleven o'clock azimuthal
position to approximately the five o'clock azimuthal position. This
gradient can help compensate for ExB drift. ExB drift can occur if
a homogeneous field crosses a plasma chamber 14 parallel to the
workpiece while an electric field perpendicular to the workpiece is
present in the chamber. The vector product of these electromagnetic
fields is parallel to the workpiece and perpendicular to both sets
of field lines. This results in having the electrons directed in
the direction of the vector product (i.e., the "preferred"
direction) which causes the plasma to be denser in one area (or
"corner") of the plasma chamber. This results in a nonuniformity of
the processing of the workpiece, which is undesirable. To correct
for this ExB drift, the magnetic field topology is rotated. If the
magnetic field topology is uniform, however, rotating the field
merely causes the "hot spot" (area of relatively high electron
density) to rotate around the periphery of the plasma. To correct
for this effect, the field lines of the magnetic field topology are
curved which causes the electrons to "fan out" sufficiently to
reduce the hot spot effect.
[0047] FIG. 3 shows a bucket-type field topology (or bucket field
topology) which forms a magnetic "bucket" around the walls of the
chamber 14. This topology produces arcuate lobes of magnetic field
lines that extend toward the center of the chamber. These lobes
tend to concentrate the plasma in the center of the chamber. This
has a number of benefits including, for example, tending to reduce
the number of plasma particles striking the chamber side wall and
other surfaces within the chamber 14 and increasing plasma density
(by confining it to a smaller volume of space). The greater the
plasma density, the faster the rate of etching or deposition, for
example. Faster processing of the workpiece increases commercial
productivity during, for example, semiconductor fabrication.
[0048] As shown in FIG. 3, the bucket field topology can be
achieved by conducting equal currents of opposite polarity (that
is, currents of opposite direction) to pairs of adjacent coil
magnets 51 of the array. The reactor 12 may also be constructed to
provide magnetic field lines having a bucket topology that rotates
or oscillates.
[0049] A schematic view of an apparatus 80 for imposing a rotating
bucket field topology on the plasma is shown in FIG. 6. The
apparatus 80 is identical to apparatus 12 except for the number of
coil magnets mounted around the chamber 14 thereof. Identical
structures between the two embodiments 12 and 80 are identified
with identical reference numbers and are not commented upon
further. The number of coil magnets mounted around the chamber 14
determines the resolution of the magnetic field produced by the
magnet system. That is, the more coil magnets that are positioned
circumferentially around a chamber, the more "finely" the bucket
field topology covers the interior of the chamber wall. To better
control the "peripheral" magnetic field (that is, the portion of
the magnetic field that is adjacent the wall), a relatively large
number of relatively smaller coils are mounted around the chamber
14 of apparatus 80. When a fine resolution field is required, the
inner ends of adjacent coil magnet 51 cores almost touch one
another as shown in FIG. 6. Because the apparatus 80 has twice the
number of coil magnets 51 mounted around its chamber compared to
apparatus 12, for example, the apparatus 80 can be operated to
achieve a bucket field topology that is finer resolution than the
bucket field topology achieved using apparatus 12. The number of
coils utilized depends on the resolution of the field that is
required. Generally, the greater the number of magnets, the finer
the field resolution.
[0050] The lobe length can be increased by operating the magnets in
pairs, in threes, and so on. That is, when the electromagnets 51
are operated in "pairs" to produce a bucket field topology, at each
instant the magnitude and direction of the current in coils 51A and
B are identical to one another. Similarly, the magnitude and
direction of the current in coils 5 IC and D are identical to one
another. Thus, coils 51A and B (and coils 51C and D and so on)
function, in effect, as a single coil. The longer the lobes of the
bucket field topology extend into the chamber, the more the plasma
is "squeezed" into the center of the plasma chamber 14, thereby
raising plasma density and reaction rate.
[0051] The apparatus 80 can also be operated (using the circuit 76
of FIG. 5, for example) to produce a "rotating" or oscillating
bucket field topology that has the same resolution as the
non-rotating bucket field topology illustrated in FIG. 3 but which
produces a series of overlapping lobe patterns which tend to more
uniformly "squeeze" the plasma (relative to the magnetic field
topology of FIG. 3). The bucket field topology produced according
to the example method described below is also advantageous because
at all times at least some location has a non-zero instantaneous
field strength. That is, the imposed field is always non-zero at
some location in the processing chamber at each point in time.
Oscillating or rotating the bucket field is advantageous because it
prevents the magnetic field lines from always striking the wall (or
walls) of the processing chamber at the same place (or places). If
the bucket magnetic field lines are not rotated and therefore
strike a wall, for example, at the same locations, this can cause
plasma particles to be directed along the field lines into the wall
that these locations which can result in degradation of the wall
material at these locations. This local degradation of wall
material from a stationary bucket magnetic field can happen, for
example, in places between the lobes where the field lines from
adjacent lobes enter the chamber wall 14 together. Thus, it can be
understood that while the imposed bucket magnetic field can be made
to be static, can be made to oscillate or can be made to rotate, it
may not be desirable to impose a static bucket (or other type)
magnetic field on the plasma for a prolonged period because this
may result in localized damage to the walls of the processing
chamber. FIG. 6 shows an instantaneous view of the electrical
currents and magnetic field in the apparatus 80 when the apparatus
80 is operated to produce a rotating bucket field topology. FIG. 7
shows a graphic representation of the magnitudes of the currents
flowing through four coil magnets 51 over time while the example
rotating bucket field topology is being produced. The rotating
bucket field topology in apparatus 80 has essentially the same
field resolution as is imposed utilizing the apparatus 12.
[0052] The coil magnets 51A-X are essentially operated as two
separate magnet systems that each provide a bucket field topology
independently of the other magnet system. The first magnet system
includes 51A, 51C, 51E, 51G, 51I, 51K, 51M, 510, 51Q, 51S, 51U, and
51W, and the second magnet system includes the remaining coil
magnets 51. The graph of FIG. 7 shows the currents through coil
magnets 51A-D. It can be appreciated that the current waveforms in
adjacent coil magnets (51A and 51B, for example) are ninety degrees
out of phase with one another. The currents in every other coil
magnet (such as coil magnets 51A and C, for example) are one
hundred and eighty degrees out of phase with one another.
[0053] FIG. 6 shows the magnetic field lines that occur at a
time=t.sub.x. The time t.sub.x is also indicated on the graph of
FIG. 7. At time t.sub.x one set of a coil magnets (the set that
includes 51B and 51D) each have maximum current and the other set
of coil magnets (the set that includes 51A and 51B) each have a
current of zero magnitude. Adjacent coils in each set (51B and 51D,
for example) have oppositely directed currents as indicated by the
oppositely directed current directional arrows in FIG. 6 and by the
graph of FIG. 7. It can be appreciated from FIG. 7 that each
current waveform is sinusoidal. It can also be appreciated from the
graph of FIG. 7 that the magnetic field produced by the rotating
(or oscillating) bucket field topology does not vanish at any point
during the plasma processing operation because the currents are
never zero in all coil magnets 51A-X, at any instant.
[0054] The structure and operation of the apparatus 80 is an
example only. It is contemplated to construct an apparatus that
includes three or more independent magnetic systems to produce, for
example, three or more rotating magnetic field topologies.
[0055] One or more magnetic field topologies can be imposed on the
plasma during the processing of a particular workpiece (such as a
semiconductor, as an example) processing quality and yield. For
example, selected magnetic field topologies can be imposed on the
plasma during an etching operation (or, alternatively, a deposition
operation) in which a pattern is etched on a surface of a wafer of
semiconductor material. Because a system of arbitrary waveform
generators and coil magnets 51 may be used to create the magnetic
fields, and because the arbitrary waveform generators can be
controlled by the control system 60, a manufacturer is able to
select an appropriate magnetic field topology (or magnetic field
topologies) for a particular semiconductor material and a
particular semiconductor etching (or deposition) application. The
determination of the optimal combination of magnetic field
topologies for a particular application may be done experimentally.
That is, particular current waveforms can be fed to selected coil
magnets of one or more magnet systems during processing of a
particular type of wafer and the results examined. The quality of
the results of the etching/deposition can be correlated with or
examined in light of the magnetic field topologies used in the
etching/deposition process. If damage to the workpiece occurs, for
example, or if the processing results are not uniform, the
distributions of current waveforms fed to the coil magnets 51 can
be changed (by reprogramming the control system 60) to, for
example, change the topology (or topologies), strength, gradient,
period, and so on of the magnetic field topologies imposed on the
plasma.
[0056] When a semiconductor is processed in a plasma chamber, the
semiconductor is susceptible to damage caused by nonuniform
concentrations (either areas of high concentration or low
concentration) of electrons in the plasma. Most of the damage that
occurs due to nonuniformities in the concentration of the plasma
occurs towards the end of a processing operation. Two or more
magnetic fields topologies can be used during the processing of a
workpiece (such as a semiconductor) to mitigate against the damage
that may occur from plasma nonuniformities. During the first
portion of a processing operation, when the workpiece is relatively
unsusceptible to damage from plasma density nonuniformities, one or
more bucket field topologies may be imposed on the plasma to
increase plasma density and thereby increase the rate of
processing. By increasing plasma density in the early part of a
processing operation, therefore, material can be etched away from
the workpiece faster, for example, and then, toward the end of the
process, when it becomes risky to run at such a high processing
rate, another magnetic field or fields can be imposed on the
processing chamber to improve plasma uniformity during the final
critical stages of the processing operation. As another example, a
bucket field topology having relatively large lobes can be imposed
on the plasma during the initial stages of processing, then a
bucket field topology having intermediate sized lobes can be
imposed on the plasma, and then a bucket field topology having
relatively small lobes can be imposed on the plasma. By decreasing
the size of the lobes (either in steps or continuously over time)
of the bucket field topology during a processing operation, the
density of the plasma can be gradually reduced as processing
occurs. During the final critical stages of plasma processing, a
rotating cross field topology having curved field lines can be
imposed on the plasma to increase plasma uniformity during the
final critical stages of processing.
[0057] Localized nonuniformities can occur in a plasma for a number
of known reasons including, for example, because of nonuniform gas
injection, nonuniform RF excitation fields being applied to the
plasma, nonuniform pumping within the plasma chamber, and so on.
Because each coil magnet can be driven by an independent arbitrary
waveform generator, the controller can be programmed to control the
distribution of currents sent to the array of magnets to compensate
for a local nonuniformity in the plasma. Thus, the controller can
be programmed to create a rotating field that provides a localized
nonuniformity in the imposed magnetic field to compensate for the
density nonuniformity occurring in the plasma.
[0058] It will be understood that while the electrodes of a plasma
chamber were described as each being driven by an associated
voltage source, this does not imply that each electrode has to be
driven by the associated voltage source. Thus, for example, it is
possible for one or the other of the pair of electrodes 18, 20 of
the system 10 to be constantly at ground level or at any other
static (i.e., unchanging) voltage level during processing.
[0059] The many features and advantages of the present invention
are apparent from the detailed specification and thus, it is
intended by the appended claims to cover all such features and
advantages of the described method which follow in the true spirit
and scope of the invention. Further, since numerous modifications
and changes will readily occur to those of ordinary skill in the
art, it is not desired to limit the invention to the exact
construction and operation illustrated and described. Moreover, the
method and apparatus of the present invention, like related
apparatus and methods used in the semiconductor arts that are
complex in nature, are often best practiced by empirically
determining the appropriate values of the operating parameters, or
by conducting computer simulations to arrive at best design for a
given application. Accordingly, all suitable modifications and
equivalents should be considered as falling within the spirit and
scope of the invention.
* * * * *