U.S. patent application number 17/107029 was filed with the patent office on 2021-04-15 for method and apparatus of achieving high input impedance without using ferrite materials for rf filter applications in plasma.
The applicant listed for this patent is Applied Materials, Inc.. Invention is credited to Amit Kumar Bansal, Abdul Aziz Khaja, Kwangduk Douglas Lee, Xing Lin, Juan Carlos Rocha-Alvarez, Addepalli Sai Susmita, Zheng John Ye, Jianhua Zhou.
Application Number | 20210111000 17/107029 |
Document ID | / |
Family ID | 1000005291780 |
Filed Date | 2021-04-15 |
United States Patent
Application |
20210111000 |
Kind Code |
A1 |
Ye; Zheng John ; et
al. |
April 15, 2021 |
METHOD AND APPARATUS OF ACHIEVING HIGH INPUT IMPEDANCE WITHOUT
USING FERRITE MATERIALS FOR RF FILTER APPLICATIONS IN PLASMA
Abstract
Implementations of the present disclosure generally relate to
methods and apparatus for generating and controlling plasma, for
example RF filters, used with plasma chambers. In one
implementation, a plasma processing apparatus is provided. The
plasma processing apparatus comprises a chamber body, a powered gas
distribution manifold enclosing a processing volume and a radio
frequency (RF) filter. A pedestal having a substrate-supporting
surface is disposed in the processing volume. A heating assembly
comprising one or more heating elements is disposed within the
pedestal for controlling a temperature profile of the
substrate-supporting surface. A tuning assembly comprising a tuning
electrode is disposed within the pedestal between the one or more
heating elements and the substrate-supporting surface. The RF
filter comprises an air core inductor, wherein at least one of the
heating elements, the tuning electrode, and the gas distribution
manifold is electrically coupled to the RF filter.
Inventors: |
Ye; Zheng John; (Santa
Clara, CA) ; Khaja; Abdul Aziz; (San Jose, CA)
; Bansal; Amit Kumar; (Milpitas, CA) ; Lee;
Kwangduk Douglas; (Redwood City, CA) ; Lin; Xing;
(San Jose, CA) ; Zhou; Jianhua; (Campbell, CA)
; Susmita; Addepalli Sai; (Bangalore, IN) ;
Rocha-Alvarez; Juan Carlos; (San Carlos, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Applied Materials, Inc. |
Santa Clara |
CA |
US |
|
|
Family ID: |
1000005291780 |
Appl. No.: |
17/107029 |
Filed: |
November 30, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15214063 |
Jul 19, 2016 |
10879041 |
|
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17107029 |
|
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|
62214738 |
Sep 4, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/6831 20130101;
H01L 21/67103 20130101; H01J 2237/334 20130101; C23C 16/50
20130101; H01J 37/32715 20130101; H01J 37/32082 20130101; C23C
16/5096 20130101; H01L 21/67069 20130101 |
International
Class: |
H01J 37/32 20060101
H01J037/32; C23C 16/50 20060101 C23C016/50; C23C 16/509 20060101
C23C016/509; H01L 21/67 20060101 H01L021/67; H01L 21/683 20060101
H01L021/683 |
Claims
1. An air core inductor, comprising: a first helical coil section
having a number of coil turns; and a second helical coil section
having a number of coil turns and positioned to overlap the first
helical coil section both axially and, wherein the first helical
coil section and the second helical coil section share a common
longitudinal axis and the wire of each helical coil section has a
rectangular cross-section.
2. The air core inductor of claim 1, further comprising: a third
helical coil section positioned to overlap the second helical coil
section both axially and radially, wherein the wire of the third
helical coil section has a rectangular cross-section.
3. The air core inductor claim 1, wherein the second helical coil
section has an inner diameter slightly smaller than an outer
diameter of the first helical coil section.
4. The air core inductor of claim 3, wherein insulating material
extends radially between the first helical coil section and the
second helical coil section.
5. The air core inductor of claim 1, wherein the first helical coil
section and the second helical coil section are electrically and
mechanically connected at one end of the sections.
6. The air core inductor of claim 1, wherein the number of coil
turns of the first helical coil section and the number of turns of
the second helical coil section are the same.
7. The air core inductor of claim 1, wherein the number of coil
turns of the first helical coil section is greater than the number
of coil turns of the second helical coil section.
8. The air core inductor of claim 1, wherein the coil turns of the
first helical coil section and the coil turns of the second helical
coil section are wound in the same direction.
9. The air core inductor of claim 1, further comprising an
elongated non-magnetic core assembly, wherein the first helical
coil section and the second helical coil section encircle the
elongated non-magnetic core assembly.
10. The air core inductor of claim 1, wherein the air core inductor
has an air gap and the first helical coil section and the second
helical coil section encircle the air gap.
11. A substrate support assembly, comprising: a substrate support
pedestal having a substrate-supporting surface; a heating assembly
comprising one or more heating elements disposed within the
substrate support pedestal for controlling a temperature profile of
the substrate-supporting surface; a tuning assembly comprising a
tuning electrode that is disposed within the substrate support
pedestal; and a radio frequency (RF) filter comprising an air core
inductor, wherein at least one of the heating elements and the
tuning electrode is electrically coupled to the RF filter, the air
core inductor comprising: a first helical coil section having a
number of coil turns; and a second helical coil section having a
number of coil turns and positioned to overlap the first helical
coil section both axially and radially, wherein the first helical
coil section and the second helical coil section share a common
longitudinal axis and the wire of each helical coil section has a
rectangular cross-section.
12. The substrate support assembly of claim 11, further comprising:
a third helical coil section positioned to overlap the second
helical coil section both axially and radially, wherein the wire of
the third helical coil section has a rectangular cross-section.
13. The substrate support assembly of claim 11, wherein the second
helical coil section has an inner diameter slightly smaller than
the outer diameter of the first helical coil section.
14. The substrate support assembly of claim 13, wherein insulating
material extends radially between the first helical coil section
and the second helical coil section.
15. The substrate support assembly of claim 11, wherein the first
helical coil section and the second helical coil section are
electrically and mechanically connected at one end of the
sections.
16. The substrate support assembly of claim 11, wherein the air
core inductor further comprises an elongated non-magnetic core
assembly, wherein the first helical coil section and the second
helical coil section encircle the elongated non-magnetic core
assembly.
17. The substrate support assembly of claim 11, wherein the air
core inductor has an air gap.
18. The substrate support assembly of claim 17, wherein the first
helical coil section and the second helical coil section encircle
the air gap.
19. A substrate support assembly, comprising: a substrate support
pedestal having a substrate-supporting surface; a heating assembly
comprising one or more heating elements disposed within the
substrate support pedestal for controlling a temperature profile of
the substrate-supporting surface; a tuning assembly comprising a
tuning electrode that is disposed within the substrate support
pedestal; and a radio frequency (RF) filter comprising an air core
inductor, wherein the air core inductor comprises: a first helical
coil section having a number of coil turns; a second helical coil
section having a number of coil turns and positioned to overlap the
first helical coil section both axially and radially; and a third
helical coil section positioned to overlap the second helical coil
section both axially and radially, wherein the wire of each of the
first helical coil section, the section helical coil section, and
the third helical coil section has a rectangular cross-section,
wherein the first helical coil section and the second helical coil
section share a common longitudinal axis, wherein at least one of
the heating elements and the tuning electrode is electrically
coupled to the RF filter, and wherein the RF filter comprises at
least two of the air core inductors connected in series.
20. The substrate support assembly of claim 19, wherein the air
core inductor further comprises an elongated non-magnetic core
assembly, wherein the first helical coil section, the second
helical coil section, and the third helical coil section encircle
the elongated non-magnetic core assembly.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 15/214,063, filed Jul. 19, 2016, which claims
benefit of U.S. provisional patent application Ser. No. 62/214,738,
filed Sep. 4, 2015. Both aforementioned related patent applications
are herein incorporated by reference in their entirety.
BACKGROUND
Field
[0002] Implementations described herein generally relate to an
apparatus and method for processing substrates. More particularly,
the present disclosure relates to methods and apparatus for
generating and controlling plasma, for example RF filters, used
with plasma chambers. The methods and apparatus can be applied to
semiconductor processes, for example, plasma deposition and etch
processes and other plasma processes used to form integrated
circuits.
Description of the Related Art
[0003] In the manufacture of integrated circuits and other
electronic devices, plasma processes are often used for deposition
or etching of various material layers. Plasma processing offers
many advantages over thermal processing. For example, plasma
enhanced chemical vapor deposition (PECVD) allows deposition
processes to be performed at lower temperatures and at higher
deposition rates than achievable in analogous thermal processes.
Thus, PECVD is advantageous for integrated circuit fabrication with
stringent thermal budgets, such as for very large scale or
ultra-large scale integrated circuit (VLSI or ULSI) device
fabrication.
[0004] One problem encountered with plasma processing in integrated
circuit fabrication is that devices may become damaged because of
exposure to non-uniform plasma conditions, such as electric field
gradients. The susceptibility or degree of device damage depends on
the stage of device fabrication and the specific device design.
Devices containing an insulating or dielectric layer deposited on a
substrate are susceptible to damage due to charges and/or potential
gradients accumulating on the surface of the dielectric layer.
[0005] Therefore, there is a need for an improved methods and
apparatus for plasma processing.
SUMMARY
[0006] Implementations described herein generally relate to an
apparatus and method for processing substrates. More particularly,
the present disclosure relates to methods and apparatus for
generating and controlling plasma, for example RF filters, used
with plasma chambers. The methods and apparatus can be applied to
semiconductor processes, for example, plasma deposition and etch
processes and other plasma processes used to form integrated
circuits. In one implementation, a plasma processing apparatus is
provided. The plasma processing apparatus comprises a chamber body,
a powered gas distribution manifold enclosing a processing volume
and a radio frequency (RF) filter. A pedestal having a
substrate-supporting surface is disposed in the processing volume.
A heating assembly comprising one or more heating elements is
disposed within the pedestal for controlling a temperature profile
of the substrate-supporting surface and/or substrate if present. A
tuning assembly comprising a tuning electrode is disposed within
the pedestal between the one or more heating elements and the
substrate-supporting surface. The RF filter comprises an air core
inductor, wherein at least one of the heating elements, the tuning
electrode, and the gas distribution manifold is electrically
coupled to the RF filter.
[0007] In another implementation, a substrate support assembly is
provided. The substrate support assembly comprises a substrate
support pedestal having a substrate-supporting surface, a heating
assembly comprising one or more heating elements disposed within
the pedestal for controlling a temperature profile of the
substrate-supporting surface and substrate if present, a tuning
assembly comprising a tuning electrode that is disposed within the
pedestal and a radio frequency (RF) filter. The RF filter comprises
and an air core inductor, wherein at least one of the heating
elements and the tuning electrode is electrically coupled to the RF
filter.
[0008] In yet another implementation, a substrate support assembly
is provided.
[0009] The substrate support assembly comprises a substrate support
pedestal having a substrate-supporting surface, a heating assembly
comprising one or more heating elements disposed within the
substrate support pedestal for controlling a temperature profile of
the substrate-supporting surface and substrate if present, a tuning
assembly comprising a tuning electrode that is disposed within the
substrate support pedestal, and a radio frequency (RF) filter
comprising an air core inductor. The air core inductor comprises a
first helical coil section having a predetermined number of coil
turns and a second helical coil section having a predetermined
number of coil turns and positioned to overlap the first helical
coil section both axially and radially so that the air core
inductor is resonant at a predetermined frequency. The first
helical coil section and the second helical coil section share a
common longitudinal axis. At least one of the heating elements and
the tuning electrode is electrically coupled to the RF filter. The
RF filter comprises at least two of the air core inductors
connected in series.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] So that the manner in which the above-recited features of
the present disclosure can be understood in detail, a more
particular description of the implementations, briefly summarized
above, may be had by reference to implementations, some of which
are illustrated in the appended drawings. It is to be noted,
however, that the appended drawings illustrate only typical
implementations of this disclosure and are therefore not to be
considered limiting of its scope, for the disclosure may admit to
other equally effective implementations.
[0011] FIG. 1 is a schematic view of a plasma processing apparatus,
incorporating an RF filter according to one implementation
described herein;
[0012] FIG. 2 is a schematic view of a substrate support pedestal,
incorporating an RF filter according to one implementation
described herein;
[0013] FIG. 3 is a perspective view of an air core inductor
according to one implementation described herein;
[0014] FIG. 4 is a top view of the air core inductor of FIG. 3
according to one implementation described herein;
[0015] FIG. 5 is a cross-section view of the air core inductor of
FIG. 4 taken along line A-A according to one implementation
described herein;
[0016] FIG. 6 is a schematic cross section view of a portion of the
air core inductor of FIG. 3;
[0017] FIG. 7 is a schematic cross section view of another air core
inductor according to another implementation described herein;
[0018] FIG. 8 is a schematic cross section view of another air core
inductor according to another implementation described herein;
and
[0019] FIG. 9 is a graph illustrating the response of an RF filter
containing an air core inductor designed according to
implementations described herein.
[0020] To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements
that are common to the figures. It is contemplated that elements
and features of one implementation may be beneficially incorporated
in other implementations without further recitation. It is to be
noted, however, that the appended drawings illustrate only
exemplary implementations of this disclosure and are therefore not
to be considered limiting of its scope, for the disclosure may
admit to other equally effective implementations.
DETAILED DESCRIPTION
[0021] The following disclosure describes methods and apparatus for
generating and controlling plasma, for example RF filters, used
with plasma chambers. Certain details are set forth in the
following description and in FIGS. 1-9 to provide a thorough
understanding of various implementations of the disclosure. Other
details describing well-known structures and systems often
associated with RF filters and plasma chambers are not set forth in
the following disclosure to avoid unnecessarily obscuring the
description of the various implementations.
[0022] Many of the details, dimensions, angles and other features
shown in the figures are merely illustrative of particular
implementations. Accordingly, other implementations can have other
details, components, dimensions, angles and features without
departing from the spirit or scope of the present disclosure. In
addition, further implementations of the disclosure can be
practiced without several of the details described below.
[0023] Implementations described herein will be described below in
reference to a PECVD process that can be carried out using any
suitable thin film deposition or etching system. Examples of
suitable systems include the CENTURA.RTM. systems which may use a
DxZ.TM. processing chamber, PRECISION 5000.RTM. systems,
PRODUCER.TM. systems, PRODUCER GT.TM. and the PRODUCER SE.TM.
processing chambers which are commercially available from Applied
Materials, Inc., of Santa Clara, Calif. Other tools capable of
performing PECVD or etch processes may also be adapted to benefit
from the implementations described herein. In addition, any system
enabling the PECVD or etch processes described herein can be used
to advantage. The apparatus description described herein is
illustrative and should not be construed or interpreted as limiting
the scope of the implementations described herein.
[0024] As used herein, the term "air core inductor coil" describes
an inductor that does not depend upon a ferromagnetic material to
achieve its specified inductance. This covers the case where there
is just air inside as well as windings upon a different insulator
such as, for example, Bakelite, glass, polyether ether ketone
(PEEK), or polytetrafluoroethylene(PTFE).
[0025] Implementations described herein include methods and
apparatus to accommodate the RF filtering requirements typically
seen in plasma enhanced chemical vapor deposition chambers and etch
chambers where RF power is used to produce and sustain plasmas with
substantially uniform characteristics including plasma density and
temperature. More particularly, implementations of the present
disclosure address the electrical noise of the RF interferences
originating from the plasma which may couple to a variety of
chamber and system parts adversely effecting chamber operations.
One example of RF interference is the RF noise coupled through a
heater to an electrostatic chucking power supply which could
adversely affect wafer chucking quality. Another example is the RF
power coupling through the heater elements into the AC heating
circuitry at the ground potential. These and other types of RF
coupling typically come in an uncontrolled and unexpected manner
leading to degradation of the tool performance, or in extreme cases
equipment and system damage. For example, an off-the-shelf DC power
supply is usually not designed to handle a load other than the DC,
and an AC line power supporting the heater elements is not designed
to handle a load other than the 60 Hz frequency. Once these loads
receive reflected power at RF frequencies such as those at 350 KHz,
13.56 MHz, and others, a standing wave will establish on the loads
resulting in damaging high voltage build-up. RF filters are used to
block these reflected signals while allowing for the designed DC or
60 Hz signal to operate normally.
[0026] RF filters are usually characterized by their impedance at
one particular, or multiple RF frequencies simultaneously, which
need to meet specifications for adequate blocking at the chosen
operating frequencies. An RF filter having a low or insufficiently
high RF impedance would allow the RF current to pass through to
reach the devices the RF filter is designed to protect. Another
characteristic of an RF filter is its mechanical dimension and
energy consumption. Previous RF filter designs employed lumped
circuit elements such as inductors alone, capacitors alone, and
certain combinations of the inductors and capacitors, either in
series or in parallel to form certain resonant circuitry in order
to reach the designed electrical and mechanical specifications.
These previous RF filter designs typically result in physically
large devices when high impedance is desired, or present as a low
ratio of the impedance over physical dimension. This is due to the
capacitors involved in the designs that are large in dimension, or
inductors that are large when the operating frequency is low.
[0027] Ferrite materials are often used as the core material for
inductor coils in order to achieve sufficiently high impedance
values. However, ferrite materials are sources of dielectric loss
depending upon the operating frequency, and their electrical
properties may be temperature dependent which is affected by the RF
loss itself. Thus, avoiding the use of ferrite materials in RF
filters operating at high RF powers and multiple RF frequencies is
desirable.
[0028] Implementations of the present disclosure provide compact RF
filters and methods of constructing compact RF filters at a single
operating RF frequency or multiple operating frequencies without
the use of lumped element capacitors and ferrite materials. The RF
filter devices of the present disclosure exhibit a high ratio of
impedance over physical dimensions at a single operating RF
frequency or multiple RF frequencies which have not been achieved
by prior art devices. The results achieved by the RF devices
described herein are particularly desirable in applications wherein
there is a space limitation, a weight limitation, or both.
Exemplary applications include semiconductor manufacturing
equipment where there is high demand for packaging and integration
where there are space and weight limitations. The cost reduction
achieved due to the superior impedance and physical dimension ratio
provides additional competitive advantages.
[0029] Implementations of the present disclosure, address the
deficiencies of lumped element filters associated with capacitors,
inductors, or both capacitors and inductors in constructing an RF
filter, by taking advantage of the self resonant nature of an air
core inductor structure design. Implementations described herein
achieve high input impedance at desired operating frequencies
ranging as low as 50 to 60 Hz, to radio frequency, very high
frequency, and microwave frequency, and in low physical form
factor.
[0030] Implementations of the present disclosure include methods of
constructing RF filters using insulated or enameled wires to form
helix like inductor coils on a non-magnetic dielectric core
material. The RF filters described herein exhibit desirable
characteristics and specifications including but not limited to
their frequency responses and input impedance at one or multiple
frequencies that are used to initiate and sustain plasmas under RF
power in plasma deposition and etching tools. The frequency
response of the RF filters described herein may comprise of for
example, low-pass, high-pass, and band-pass characteristics; and
their input impedance may be high, low, or at any specific
magnitude demanded by the system requirements. The RF filters
described herein may operate in one frequency or multiple
frequencies in the range from 50 Hz to 2450 MHz, under a desired
power level. The RF filters described herein may exhibit extremely
high input impedance at one or multiple operating frequencies where
extremely high impedance presents to the system components as an
open circuit in a practical sense from which no, or very low
current is drawn, and the devices which the RF filters are designed
to protect may appear invisible electrically to the main system
components.
[0031] In one implementation, the RF filters described herein are
constructed with helix-like coil windings. The desired frequency
response and input impedance are achieved by controlling at least
one of the following parameters during the coil forming process,
these parameters include but are not limited to: the coil diameter
in implementations where a wire having a circular cross-section is
used, edge length in implementations where a wire having a
rectangular cross-section is used, and in case of arbitrary
cross-section its shape and area, its length, conductivity, the
number of the stack layers, the number of turns of each layer, the
pitch both along the length and along the radial direction of the
helix coil formation, the core material and geometry where the core
material may be that of the air, or materials having relative
magnetic permeability of one, that is, non-ferrite materials. The
RF filters described herein consist of a plural number of layers
(e.g., two or more layers) forming a stack of helix coils such that
mutual electromagnetic field coupling between the neighboring coils
are enhanced resulting in desired mutual capacitance and inductance
distributed per turn and among all turns, and thereof desired
resonant frequency, or frequencies, without capacitors of lumped
elements that are typically used.
[0032] FIG. 1 is a schematic side cross-sectional view of a plasma
processing apparatus including an RF filter, according to one
implementation described herein. The apparatus includes a chamber
100 in which one or more films may be deposited on or etched from a
substrate 110 disposed on a substrate support pedestal 108. The
substrate support pedestal 108 has a substrate-supporting surface
111 for supporting the substrate 111. The chamber 100 includes a
chamber body 102 and a gas distribution assembly 104, which
distributes gases uniformly into a process volume 106. The
substrate support pedestal 108, hereafter pedestal 108, is disposed
within the process volume 106 and supports the substrate 110. The
pedestal 108 includes a heating element 150. The pedestal 108 is
movably disposed in the process volume 106 by a stem 114 that
extends through the chamber body 102, where the stem 114 is
connected to a drive system 103 and bellows to allow the pedestal
108 to be raised, lowered, and/or rotated.
[0033] The gas distribution assembly 104 includes a gas inlet
passage 116, which delivers gas from a gas flow controller 120 into
a gas distribution manifold 118. The gas distribution manifold 118
includes a plurality of holes or nozzles (not shown) through which
gaseous mixtures are injected into the process volume 106 during
processing.
[0034] A high frequency RF power source 126 and a low frequency RF
power source 127 provide electromagnetic energy through a match
circuit 129 to power the gas distribution manifold 118, which acts
as an RF powered electrode, to facilitate generation of a plasma
within the process volume 106 between the gas distribution manifold
118 and the pedestal 108. An RF filter 121 having an air core
inductor as described herein may be positioned between the gas
distribution assembly 104 and the high frequency RF power source
126 and the low frequency RF power source 127. The RF filter 121 is
generally configured to block RF energy from reaching the high
frequency RF power source 126 and the low frequency RF power source
127.
[0035] The pedestal 108 includes a tuning electrode 112, which is
electrically grounded through an RF rod 122, such that an electric
field is generated in the chamber 100 between the powered gas
distribution manifold 118 and the tuning electrode 112. In one
implementation, the tuning electrode 112 comprises a conductive
mesh, such as a tungsten or molybdenum containing mesh that is
disposed within the dielectric material that is used to form the
pedestal 108. In one configuration, the pedestal 108 includes a
ceramic material, such as aluminum nitride (AlN), silicon nitride
(SiN), silicon carbide (SiC) or the like.
[0036] A ceramic ring 123 is positioned below the gas distribution
manifold 118. Optionally, a tuning ring 124 is disposed between the
ceramic ring 123 and an isolator ring 125, which electrically
isolates the tuning ring 124 from the chamber body 102. The tuning
ring 124 is typically made from a conductive material, such as
aluminum, titanium, or copper. As depicted in FIG. 1, the optional
tuning ring 124 is positioned concentrically about the pedestal 108
and substrate 110 during processing of the substrate 110. The
tuning ring 124 is electrically coupled to an RF tuner 135, which
includes a variable capacitor 128, such as a variable vacuum
capacitor, that is terminated to ground through an inductor L1. RF
tuner 135 also includes a second inductor L2 that is electrically
coupled in parallel to the variable capacitor 128 to provide a path
for low frequency RF to ground. RF tuner 135 also includes a sensor
130, such as a voltage/current (V/I) sensor, that is positioned
between the tuning ring 124 and the variable capacitor 128 for use
in controlling the current flow through the tuning ring 124 and the
variable capacitor 128.
[0037] The tuning electrode 112 formed in the pedestal 108 is
electrically coupled through RF rod 122 to an RF tuner 145, which
includes a variable capacitor 139, such as a variable vacuum
capacitor, that is terminated to ground through an inductor L3. RF
tuner 145 also includes a second inductor L4 and a capacitor 146
that is electrically coupled in parallel to the variable capacitor
139 to provide a path for low frequency RF to ground. RF tuner 145
also includes a sensor 140, such as a V/I sensor, that is
positioned between the tuning electrode 112 and the variable
capacitor 139 for use in controlling the current flow through the
tuning electrode 112 and the variable capacitor 139.
[0038] The RF rod 122 couples the tuning electrode 112 to a power
supply 149 via RF tuner 145 and RF filter 148. The RF filter 148
may contain an air core inductor as described herein. The RF filter
148 is generally configured to block RF energy from reaching the
power supply 149. The power supply 149 may include a direct current
(DC) power source, an alternating current (AC) power source, or a
combination of both. In one embodiment, the power supply 149 is an
alternating current (AC) power source to provide AC signal to the
tuning electrode 112. The configuration of RF filter 148 will be
further discussed in detail below.
[0039] One or more heating elements 150 are disposed within the
pedestal 108 and are used to control a temperature profile across
the substrate-supporting surface 111 and across the substrate 110
if present. As depicted, the heating elements 150 may be disposed
beneath the tuning electrode 112; in other words, the tuning
electrode 112 is disposed closer to the substrate than the heating
elements 150. The heating elements 150 generally provide resistive
heating to the substrate 110. The heating elements 150 may be
comprised of any feasible material, such as a conductive metal wire
(e.g., refractory metal wire), patterned metal layer (e.g.,
molybdenum, tungsten or other refractory metal layer) or other
similar conductive structure. The heating elements 150 are
connected to one or more conductive rods 155, which may extend
along the length of the stem 114 of the pedestal 108. In one
implementation, the conductive rods 155 are positioned parallel to
or substantially parallel to the RF rod 122. The conductive rods
155 couple the heating elements 150 to a heating power source 165,
through one or more RF filters 160. RF filters 160 may contain an
air core inductor as described herein. The RF rod 122 and
conductive rod 155 are generally solid conductive elements (e.g.,
moderate diameter solid wire, non-stranded wire) that are formed
from a conductive material, such as copper, nickel, gold, coated
aluminum, and a refractory metal or other useful material. The RF
filters 160 are generally configured to block RF energy from
reaching the heating power source 165. The configuration of the RF
filters 160 will be discussed in further detail below. In one
implementation, the heating power source 165 provides a non-RF,
alternating current (AC) power to the heating elements 150. For
example, the heating power source 165 may provide three-phase AC
power at a frequency of approximately 60 Hertz.
[0040] A system controller 134 controls the functions of the
various components, such as the RF power sources 126 and 127, the
drive system 103, the variable capacitors 128 and 139, power supply
149, and heating power source 165. The system controller 134
executes system control software stored in a memory 138. The system
controller 134 comprises parts of or all of one or more integrated
circuits (ICs) and/or other circuitry components. The system
controller 134 may in some cases include a central processing unit
(CPU) (not shown), memory (not shown), and support circuits (or
I/O) (not shown). The CPU may be one of any form of computer
processor that is used for controlling various system functions and
support hardware and monitoring the processes being controlled by
and within the chamber 100. The memory is coupled to the CPU, and
may be one or more of a readily available memory, such as random
access memory (RAM), read only memory (ROM), floppy disk, hard
disk, or any other form of digital storage, local or remote.
Software instructions (or computer instructions) and data may be
coded and stored within the memory for instructing the CPU. The
software instructions may include a program that determines which
tasks are to be performed at any instant in time. The support
circuits are also connected to the CPU for supporting the processor
in a conventional manner. The support circuits may include cache,
power supplies, clock circuits, timing circuits, input/output
circuitry, subsystems, and the like.
[0041] In the plasma processing apparatus, chamber 100, an RF path
is established between the powered gas distribution manifold 118
and the tuning electrode 112 via plasma. Further, by changing the
capacitance of the variable capacitor 139, the impedance for the RF
path through the tuning electrode 112 changes, in turn, causing a
change in the RF field coupled to the tuning electrode 112 and a
change in the RF return current through the tuning electrode 112
and the RF rod 122. Therefore, the plasma in the process volume 106
may be modulated across the surface of the substrate 110 during
plasma processing. Controlling the RF field and modulating the RF
return current may thus result in higher processing rate in
depositing films onto or etching material from the substrate 110
with improved center-to-edge deposition thickness uniformity or
etch removal uniformity.
[0042] Further, an additional RF path is established between the
powered gas distribution manifold 118 and the tuning ring 124.
Additionally, by changing the capacitance of the variable capacitor
128, the impedance for the RF path through the tuning ring 124
changes, in turn, causing a change in the RF field coupled to the
tuning ring 124. For example, a maximum current and corresponding
minimum impedance of the tuning ring 124 can be achieved by varying
the total capacitance of the variable capacitor 128. Therefore, the
plasma in the process volume 106 may also be modulated across the
surface of the substrate 110 using this additional RF path.
[0043] During operation of the plasma processing apparatus, RF
energy is normally delivered to a top, powered electrode (i.e., the
powered gas distribution manifold 118), coupled through the plasma
formed in the process volume 106 and the substrate 110 and mainly
returned through the wall of the chamber body 102 and/or tuning
electrode 112 to ground. Since the heating elements 150 may be
embedded beneath the tuning electrode 112, RF energy can
capacitively couple through the ceramic materials to the heating
elements (i.e., RF leakage). The RF leakage to these undesired
paths, such as heating elements and AC lines, not only affects the
substrate processing results (i.e., deposition rate and uniformity
on the substrate) but also cause electromagnetic interference (EMI)
on or damage to the heating element AC power sources.
[0044] By adjusting the RF tuner 145 (and especially variable
capacitor 139) to compensate for the net reactance caused by other
tuning assembly components (e.g., tuning electrode 112, RF rod 122)
at an operating frequency of the powered gas distribution manifold,
a minimum impedance path through the tuning assembly may be
provided to ground. Thus, a greater proportion of RF energy will be
coupled through this path, which includes the tuning electrode, RF
rod 122 and RF tuner 145, with less RF leakage into other
components of the plasma processing apparatus. The minimum
impedance path provides greater efficiency as well as greater
control of the application of RF energy for depositing films onto
or etching material from the substrate. However, typically due to
the need to use a mesh type of tuning electrode 112 within the
pedestal 108 for manufacturing reasons, and the need to position
the heating elements 150 near the substrate-supporting surface 111
of the pedestal 108, unavoidably an amount of the RF energy
provided to the process volume 106 leaks to the heating elements
150.
[0045] Conversely, the RF filters 160 may be included in the
heating assembly to provide a relatively greater impedance path to
ground to minimize the amount of RF leakage to the heating elements
150. The RF filters 160 may be inserted in between the heating
elements 150 and the corresponding AC source(s) to attenuate RF
energy and to suppress RF leakage current. In some configurations,
the impedance of the tuning electrode 112 to ground is
substantially less than the impedance of the heating elements 150
to ground.
[0046] FIG. 2 is a schematic view of the pedestal 108,
incorporating the RF filters 148, 160 according to one
implementation described herein. The pedestal 108 may generally be
used in a plasma processing apparatus, such as the apparatus
described herein with respect to FIG. 1.
[0047] Pedestal 108 includes a tuning assembly comprising tuning
electrode 112, RF rod 122, RF tuner 145, RF filter 148 and power
supply 149. Pedestal 108 also includes a heating assembly
comprising a plurality of heating elements 150. The heating
elements may be distributed among a plurality of heating zones that
are used to adjust the temperature profile across the substrate
during processing. The plurality of heating zones may include an
inner heating zone and an outer heating zone. The heating elements
150 are coupled through conductive rods 155a-d (collectively 155)
to one or more RF filters 160. In one implementation, each of the
conductive rods 155a-d is coupled to a respective RF filter 160,
which may have the same or different properties. Groups of rods may
correspond to different heating zones; for example, conductive rods
155b and 155c correspond to the heating elements of the inner
heating zone, while conductive rods 155a and 155d correspond to
heating elements of an outer heating zone. In one implementation,
two conductive rods 155 correspond to a particular zone. For
example, the heating power source 165 delivers AC power through RF
filters 160 into a first rod of the two rods; the AC power travels
through the first rod and heating elements 150 before returning
through the second rod and RF filter 160 to ground.
[0048] Components of the heating assembly will generally have
non-zero impedance that reflects both the intrinsic electrical
properties of the components and the proximity to other components
in the pedestal or plasma processing apparatus. For example,
heating elements 150 and conductive rods 155 may couple RF energy
from tuning assembly components during operation. The resistive
portion of components' impedance is typically a non-zero value that
is not affected by changes in operating frequency and is not able
to be compensated for during processing. Therefore, for ease of
description, resistive components are not depicted in FIG. 2.
[0049] To provide greater efficiency and greater control of the
application of RF energy for depositing films onto or removing
material from a substrate, ideally a maximum amount of the RF
energy delivered by the powered gas distribution manifold will be
coupled through the wall of the chamber body 102 and tuning
assembly to ground, with no charge coupled into the heating
assembly or into other components (i.e., RF leakage). Therefore, it
is advantageous to reduce the impedance of the tuning assembly to a
minimum value, and to increase the impedance of the heating
assembly (e.g., heating elements 150 and one or more conductive
rods). As discussed above, the components may all include some
real, non-zero impedance (i.e., resistance) that cannot be
compensated for during processing. However, the reactance of
different components may be controlled by adjusting capacitive or
inductive elements within the tuning assembly and the heating
assembly.
[0050] To reduce the impedance of the tuning assembly to a minimum
value and couple more of the delivered RF energy into the tuning
assembly, components of RF tuner 145 may be adjusted to compensate
for the reactance of the tuning assembly components at the
operating frequency. For example, the capacitance of the variable
capacitor 139 may be tuned when the tuning assembly has a positive
reactance value for a particular frequency. The negative reactance
provided by the variable capacitor 139 may thus compensate for the
positive reactance.
[0051] The RF filter 148 is configured to block RF energy from
reaching the power supply 149. In one implementation, the RF filter
148 includes at least one air core inductor L5 as described
herein.
[0052] To increase the impedance of the heating assembly, one or
more RF filters 160 are coupled to the conductive rods 155a-d. The
RF filter 160 is configured to block RF energy from reaching the
heating power source 165.
[0053] The RF filters 160 may be coupled to conductive rods 155
though conductive connections 220 near the pedestal base 210. In
one implementation, the conductive connections 220 may include a
short rigid lead for each rod, so that each RF filter 160 is
directly coupled to each respective rod or disposed proximate to
the pedestal base 210 and to the respective rods. Further, each
conductive connection 220 may be maximally spaced apart to minimize
capacitive coupling between connections. It is believed that
connections that include a flexible and/or shielded multi-conductor
cable disposed between the rods and the filter may introduce
additional impedance and may provide a shunt path for RF or other
signals, as each conductor will have resistive properties and may
have coupling with the other conductors and with the grounded
shield. Therefore, in some implementations, it is desirable to
position the RF filter as close to the rods as mechanically
feasible, and in some cases directly coupling the RF filter to each
of the rods.
[0054] In one implementation, the RF filter 160 may have its
electrical components selected to provide a desired resonance
frequency for the RF filter 160. In one implementation, the RF
filter 160 includes at least one air core inductor L6, L7, L8, L9
as described herein. The air core inductors L6, L7, L8, L9 may be
designed and positioned to provide a desired resonance frequency
for the RF filter 160. The air core inductors L6, L7, L8, L9 may be
inserted in series on the conductive connections 220. The air core
conductors L6, L7, L8, L9 may be positioned within a grounded metal
enclosure, which, in some cases, surrounds the various RF filter
160 circuit elements. The air core inductors L6, L7, L8, L9 may be
electrically coupled with the power supply 165a, 165b via
transformers 166a, 166b.
[0055] Any two individual resonant filters disclosed herein can be
connected in series to achieve a dual-frequency resonant filter. It
is also conceivable that multiple resonant filters can be built on
difference sections of the same dielectric core. Multiple resonant
frequencies may be generated using various combinatorial techniques
to accommodate multiple operating frequencies.
[0056] FIG. 3 is a perspective view of an air core inductor 300
according to one implementation described herein. FIG. 4 is a top
view of the air core inductor 300 of FIG. 3 according to one
implementation described herein. FIG. 5 is a cross-section view of
the air core inductor 300 taken along line A-A of FIG. 4 according
to one implementation described herein. The air core inductor 300
comprises a first helical coil section 310 and a second helical
coil section 320 wound upon an elongated core member 330. The
second helical coil section 320 surrounds and extends along in a
spaced relation to the first helical coil section 310. Thus the
first helical coil section 310 and the second helical coil section
320 extend along the same longitudinal axis 360 with the first
helical coil section 310 being within the second helical coil
section 320.
[0057] The air core inductor 300 is a multi-layer structure. The
air core inductor 300 may contain any suitable number of layers. As
depicted in FIG. 6, the first helical coil section 310 forms a
first layer and the second helical coil section 320 forms a second
layer.
[0058] In one implementation, the first helical coil section 310
and the second helical coil section 320 are each formed of a wire
312 of an electrically conductive metal coated with an insulating
material 314. In one implementation, the first helical coil section
310 and the second helical coil section 320 are electrically and
mechanically connected at one end of each of the helical coil
sections 310, 320. In one implementation, the first helical coil
section 310 and the second helical coil section 320 have terminals
340, 350 extending from their other ends. In one implementation,
the terminals 340, 350 are spaced apart and extend substantially
perpendicular to the longitudinal axis 360.
[0059] In one implementation, the first helical coil section 310
and the second helical coil section 320 are formed of a single,
continuous length of a wire of an electrically conductive metal
with terminals 340, 350 extending from each end of the wire.
[0060] Each of the first helical coil section 310 and the second
helical coil section 320 includes the wire 312 coated with an
insulating material 314. The wire 312 is of a suitable gauge having
a predetermined thickness. Any suitable electrically conductive
material may be used. Exemplary conductive materials include copper
and the like. The insulating material 314 has a predetermined
thickness. Any suitable insulating material having a predetermined
thickness may be used. Exemplary insulating materials include
polyimide, aromatic polyimide, liquid crystal polymer,
polytetrafluoroethylene (PTFE), polyether ether ketone (PEEK),
ETFE, Parylene, tantalum oxides, any nano-dielectric coating, PFA,
FEP, Polyurethane, polyurethane with self-bonding overcoat,
polyamide, polyvinyl acetal, polyvinyl acetal overcoated with
polyamide, polyurethane overcoated with polyamide, epoxy, polyester
(amide) (imide) overcoated with polyamide, polyester (amide)
(imide), silicone-treated glass fiber, polyamide-imide,
thermoplastic compounds, polyvinylchloride (PVC), polylefin class:
{LDPE, HDPE, TPO, TPR, polyolefin alloys}, LDPE low density, HDPE
high density, polypropylene (PP), thermoplastic fluoropolymers,
TEFLON FEP, Tefzel ETFE, Kynar PVDF, TEFLON PFA, Halar ECTFE, PTFE
Teflon, PTFE Teflon film, XLPE & XLPVC, silicone rubber,
Polyimide Kapton film, Polyester Mylar film, Kaladex PEN film, and
a crosslinked polyalkenepoly.
[0061] The first helical coil section 310 has a predetermined
number of coil turns. The second helical coil section 320 has a
predetermined number of coil turns. In one implementation, the
predetermined number of coil turns of the first helical coil
section 310 and the predetermined number of coil turns of the
second helical coil section 320 are the same. In one
implementation, the predetermined number of coil turns of the first
helical coil section 310 is greater than the predetermined number
of coil turns of the second helical coil section 320. In one
implementation, the predetermined number of coil turns of the first
helical coil section 310 is less than the predetermined number of
coil turns of the second helical coil section 320.
[0062] The second helical coil section 320 is positioned to overlap
the first helical coil section 310 both axially and radially so
that the air core inductor is resonant at a predetermined
frequency. The second helical coil section 320 has an inner
diameter slightly smaller than the outer diameter of the first
helical coil section 310. The insulating material 314 extends
radially between the first helical coil section 310 and the second
helical coil section 320. In one implementation, the coil turns of
the first helical coil section 310 and the coil turns of the second
helical coil section 320 are wound in the same direction. In
another implementation, the coil turns of the first helical coil
section 310 and the coil turns of the second helical coil section
320 are wound in opposite directions.
[0063] The elongated core member 330 may be any suitable shape. In
one implementation, the elongated core member 330 is generally
cylindrically shaped. The elongated core member 330 is composed of
a non-magnetic material. In the implementation of FIG. 3, the
elongated core member 330 has a one-piece structure. It should be
understood that the elongated core member 330 may include multiple
pieces, for example, a two-piece structure. The elongated core
member 330 may be constructed of any suitable non-magnetic
material. Exemplary non-magnetic materials include polyimide,
aromatic polyimide, liquid crystal polymer, polytetrafluoroethylene
(PTFE), polyether ether ketone (PEEK), ETFE, Parylene, tantalum
oxides, any nano-dielectric coating, PFA, FEP, Polyurethane,
polyurethane with self-bonding overcoat, polyamide, polyvinyl
acetal, polyvinyl acetal overcoated with polyamide, polyurethane
overcoated with polyamide, epoxy, polyester (amide) (imide)
overcoated with polyamide, polyester (amide) (imide),
silicone-treated glass fiber, polyamide-imide, thermoplastic
compounds, polyvinylchloride (PVC), polylefin class: {LDPE, HDPE,
TPO, TPR, polyolefin alloys}, LDPE low density, HDPE high density,
polypropylene (PP), thermoplastic fluoropolymers, TEFLON FEP,
Tefzel ETFE, Kynar PVDF, TEFLON PFA, Halar ECTFE, PTFE Teflon, PTFE
Teflon film, XLPE & XLPVC, silicone rubber, Polyimide Kapton
film, Polyester Mylar film, Kaladex PEN film, and a crosslinked
polyalkenepoly. It should be understood that although the
implementation of FIG. 3 depicts the air core inductor 300 as
having an elongated core, the elongated core may be replaced by an
air gap.
[0064] FIG. 6 is a schematic cross section view of a portion of the
air core inductor 300 of FIG. 3. In FIG. 6, D1 represents the
diameter of the wire 312. P1, P2, P3 and P4 represent the pitch
between any two neighboring wires that collectively produce the
desirable mutual conductance and capacitance to achieve the target
resonant frequency. The pitches between neighboring wires may or
may not be the same. One of ordinary skill in the art may develop
equivalent winding schemes that achieve a similar resonant
frequency and the desired quality factor using the same type of
wires or different wires.
[0065] The frequency to which the coil sections 310 and 320 are
responsive in assembly with each other is determined by the usual
parameters for such coils. Some of the usual parameters for
determination of the induction are the material of which the wires
are made, the diameter of the wire, the number of turns of the
wire, and the proximity and orientation of the coils with respect
to each other. In the case of the coils assembled as shown in FIGS.
3-6, some of the parameters for determination of capacitance are
the distance the coils are overlapping each other, the composition
and thickness of the insulating material on the wire, and the
distance between the wire portions of the coils.
[0066] In one specific implementation, each coil section is formed
from 24 AWG enameled copper wire. The copper wire may have a
diameter between about 0.020 inches and about 0.025 inches. The
first coil section may have between 20 and 25 turns. The second
coil section may have between 18 and 23 turns. The elongated core
may comprise PEEK material. The elongated core may have a diameter
between 0.30 inches and 0.35 inches. In assembly, the coils may be
adjusted to provide a combined inductance and distributed
capacitance at an operating frequency of about 12.7 MHz and the
quality factor is approximately 100.
[0067] The minimum inductance and range of inductance variation of
the air core inductor 300 can be set by adjusting the physical
parameters of the air core inductor 300, including wire length l,
wire diameter D, insulation type, wire gauge and construction,
helix diameter, and twist per inch T. This air core inductor 300
may be scaled to any frequency of operation and inductance as would
be appreciated by those skilled in the art.
[0068] FIG. 7 is a schematic cross section view of another air core
inductor 700 according to another implementation described herein.
The air core inductor 700 is similar to the air core inductor 300
except that the air core inductor 700 is a tri-layer structure that
includes a third helical coil section 710 that is positioned to
overlap the second helical coil section 320 both axially and
radially so that the air core inductor 700 is resonant at a
predetermined frequency. In FIG. 7, D1 represents the diameter of
the wire 312. Also, P1, P2, P3, P4, P5 and P6 represent the pitch
between any two neighboring wires that collectively produce the
desirable mutual conductance and capacitance to achieve the target
resonant frequency. The pitches between neighboring wires may or
may not be the same.
[0069] FIG. 8 is a schematic cross section view of another air core
inductor 800 according to another implementation described herein.
The air core inductor 800 is similar to the air core inductor 700
except that the wire of each helical coil section 810, 820, 830 has
a rectangular cross-section so that the air core inductor 800 is
resonant at a predetermined frequency. It is believed that the
cross-sectional shape of the wire also has a strong influence
toward mutual coupling of the electric and magnetic field and
therefore is another factor that may be used to achieve the desired
target resonance frequency. For example, the mutual coupling would
increase if a wire of a circular cross-section and of diameter D1
as shown in FIG. 7 was replaced with a wire having a rectangular
cross-section and all other parameters and winding process being
the same, resulting in a shift of the resonant frequency and a
change of the quality factor as well.
[0070] FIG. 9 is a graph 900 illustrating the response of an RF
filter containing an air core inductor designed according to
implementations described herein. In this implementation, the
resonance frequency of the of the air core inductor is 13.1
megahertz (MHz) and the operating frequency is approximately 13.1
MHz. Line 910 represents the self-resonant frequency of the air
core inductor. Line 920 represents the operating frequency.
[0071] Based on a given (i.e., known) operating frequency, the
resonance frequency for the RF filter may be selected in order to
provide a desired impedance value of the RF filter at the operating
frequency.
[0072] Based on the selected resonance frequency, one or more
components of the RF filters described above may be selected. For
example, and as described above, the RF filter 160 may include an
air core inductor. To create an RF filter having the selected
resonance frequency, parameters of the air core inductor may then
be selected (such as a number of turns, a radius, a length, and so
forth). Selecting an air core inductor for use in the RF filter may
also include considering other electrical properties of the
component at RF frequencies. For example, an air core inductor may
also exhibit resistive and/or capacitive properties at RF
frequencies. The air core inductors of the RF filter may thus be
designed to account for these other properties in order to achieve
a desired reactance value at the operating frequency. Of course,
the RF filter may include other types of components (resistors,
capacitors, etc.) or different combinations or configurations of
these components.
[0073] As the air core inductors of the RF filter are installed,
for example installed into a heating assembly prior to operation of
the plasma processing apparatus, the air core inductors of the RF
filter may be positioned relative to each other, and/or relative to
other components of the heating assembly, to achieve a desired
reactance value for the RF filter circuit. Based on their positions
and orientations, the air core inductors may have capacitive and/or
inductive couplings with each other that may impact the frequency
response of the RF filter. Additionally, the air core inductors may
be installed into a protective enclosure, or into an assembly, that
may have electrical characteristics that cause further couplings
impacting the RF filter's frequency response. Therefore, the
individual air core inductors of the RF filter may each be
positioned so that the various couplings can be accounted for in
the resonance frequency. Of course, this may be an iterative
process, as moving one air core inductors may involve adjustment to
the properties of the same air core inductors (e.g., selecting a
different number of turns or length) or to other components.
[0074] As described above, to achieve a desired impedance of the RF
filter for a particular operating frequency, the air core inductors
of the RF filter may be selected (and appropriately positioned)
that result in the selected filter resonance frequency. In most
cases, the resonance frequency will differ from the operating
frequency. However, to ensure that the impedance value is
sufficiently large to prevent RF energy from coupling into the
heating assembly, the filter resonance frequency should be selected
relatively close to the operating frequency. In one implementation,
the filter resonance frequency may be selected so that the
operating frequency falls within a desired range of the resonance
frequency. For example, the resonance frequency may be selected so
that the operating frequency falls within .+-.5% (or .+-.10%) of
the resonance frequency. Of course, the ranges could be specified
as frequency differences (e.g., within 1 MHz) instead of
percentages. In some cases, the resonance frequency may be selected
so that the impedance of the RF filter circuit at the operating
frequency is a certain desired percentage of the peak impedance. In
one example, the resonance frequency may be selected so that the
impedance of the RF filter at the operating frequency is between
about 10% and 100% of the peak impedance, such as an impedance of
between about 15% and 60% of the peak impedance. In one
configuration, the operating frequency is within a predetermined
range that is positioned about the resonant frequency of the RF
filter. In one example, the operating frequency is within a
predetermined range that is defined as being twice the difference
of the resonant frequency and a first frequency at which the
measured reactance is within 10% of the peak reactance.
[0075] Beyond keeping the operating frequency within a
predetermined range of the resonance frequency, the resonance
frequency may also be advantageously selected so that the operating
frequency is at least a minimum range from the resonance frequency.
Therefore, it may be advantageous to select a RF filter resonance
frequency so that the operating frequency would fall outside some
minimum range of the resonance frequency. For example, the
resonance frequency may be selected so that the operating frequency
is at least .+-.2.5% (or .+-.3.5%) of the resonance frequency. In
another example, the resonance frequency may be selected so that
the operating frequency is at least .+-.2.5% (or .+-.3.5%) of the
resonance frequency, but less than a frequency at which the
impedance of the RF filter drops to a level of about 10% of the
peak impedance.
[0076] When introducing elements of the present disclosure or
exemplary aspects or implementation(s) thereof, the articles "a,"
"an," "the" and "said" are intended to mean that there are one or
more of the elements.
[0077] The terms "comprising," "including" and "having" are
intended to be inclusive and mean that there may be additional
elements other than the listed elements.
[0078] While the foregoing is directed to implementations of the
present disclosure, other and further implementations of the
disclosure may be devised without departing from the basic scope
thereof, and the scope thereof is determined by the claims that
follow.
* * * * *