U.S. patent application number 16/847967 was filed with the patent office on 2020-09-10 for transparent pressure barrier for microwave plasma processing.
This patent application is currently assigned to Lyten, Inc.. The applicant listed for this patent is Lyten, Inc.. Invention is credited to Michael W. Stowell, Peter Todd Williams.
Application Number | 20200287258 16/847967 |
Document ID | / |
Family ID | 1000004842989 |
Filed Date | 2020-09-10 |
![](/patent/app/20200287258/US20200287258A1-20200910-D00000.png)
![](/patent/app/20200287258/US20200287258A1-20200910-D00001.png)
![](/patent/app/20200287258/US20200287258A1-20200910-D00002.png)
![](/patent/app/20200287258/US20200287258A1-20200910-D00003.png)
![](/patent/app/20200287258/US20200287258A1-20200910-D00004.png)
![](/patent/app/20200287258/US20200287258A1-20200910-D00005.png)
![](/patent/app/20200287258/US20200287258A1-20200910-D00006.png)
![](/patent/app/20200287258/US20200287258A1-20200910-D00007.png)
![](/patent/app/20200287258/US20200287258A1-20200910-D00008.png)
![](/patent/app/20200287258/US20200287258A1-20200910-D00009.png)
![](/patent/app/20200287258/US20200287258A1-20200910-D00010.png)
View All Diagrams
United States Patent
Application |
20200287258 |
Kind Code |
A1 |
Stowell; Michael W. ; et
al. |
September 10, 2020 |
TRANSPARENT PRESSURE BARRIER FOR MICROWAVE PLASMA PROCESSING
Abstract
This disclosure relates to a reactor including an energy source
configured to generate a microwave in a direction of propagation. A
reaction chamber is coupled to the energy source. A waveguide
disposed in the reactor includes a window positioned at an angle,
such as being misaligned, relative to the direction of propagation
of the microwave such that an average dielectric constant
experienced by the microwave increases over a region where the
window occupies a greater fraction of a cross-sectional area of the
waveguide. The window includes one or more dielectric materials
that have respective dielectric constants that increase along the
propagation direction in a first region that is adjacent to a first
face of the window, and decrease in a second region that is
adjacent to a second face of the window. The dielectric materials
have a mass density that varies based on one or more pores formed
therein.
Inventors: |
Stowell; Michael W.;
(Sunnyvale, CA) ; Williams; Peter Todd; (San
Carlos, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lyten, Inc. |
Synnyvale |
CA |
US |
|
|
Assignee: |
Lyten, Inc.
Sunnyvale
CA
|
Family ID: |
1000004842989 |
Appl. No.: |
16/847967 |
Filed: |
April 14, 2020 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
16247002 |
Jan 14, 2019 |
10644368 |
|
|
16847967 |
|
|
|
|
62617605 |
Jan 16, 2018 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01P 3/12 20130101; H01J
37/32238 20130101; H01P 1/08 20130101; H01J 37/32229 20130101 |
International
Class: |
H01P 1/08 20060101
H01P001/08; H01J 37/32 20060101 H01J037/32; H01P 3/12 20060101
H01P003/12 |
Claims
1. A reactor comprising: an energy source configured to propagate
microwave energy in a direction; a reaction chamber coupled to the
energy source; and a waveguide including a window positioned at an
angle relative to the direction of propagation of the microwave
energy such that an average dielectric constant of the microwave
energy increases over a region in which the window incrementally
occupies additional cross-sectional area of the waveguide.
2. The reactor of claim 1, wherein the reaction chamber is
configured to receive the microwave energy through the window.
3. The reactor of claim 1, wherein the window comprises a material
configured to influence propagation of the microwave energy.
4. The reactor of claim 1, wherein the window comprises a first
face and a second face that is positioned substantially opposite to
the first face.
5. The reactor of claim 4, wherein the window comprises one or more
dielectric materials contained between the first face and the
second face.
6. The reactor of claim 5, wherein the window is disposed within
the waveguide, and each dielectric material of the one or more
dielectric materials has a dielectric constant configured to:
increase along the direction of propagation in a first region of
the waveguide that is adjacent to the first face of the window; and
decrease along the direction of propagation in a second region of
the waveguide that is adjacent to the second face of the
window.
7. The reactor of claim 5, wherein the dielectric materials include
one or more pores.
8. The reactor of claim 7, wherein the dielectric materials have a
mass density that varies based on the one or more pores.
9. The reactor of claim 6, wherein the first region of the
waveguide is configured to compress the microwave energy.
10. The reactor of claim 6, wherein the second region of the
waveguide is configured to decompress the microwave energy.
11. The reactor of claim 6, wherein the waveguide comprises a third
region positioned between the first region and the second
region.
12. The reactor of claim 11, wherein the third region comprises the
dielectric materials with respective dielectric constants that are
configured to remain uniform throughout the third region in the
direction of propagation of the microwave.
13. The reactor of claim 1, wherein the window is disposed within a
pressure barrier.
14. The reactor of claim 13, wherein the window is substantially
shaped as a parallelogram.
15. The reactor of claim 14, wherein the waveguide includes a
plurality of bends.
16. The reactor of claim 15, wherein the bends are shaped as one or
more of an "E" or an "H," and wherein the bends include twists.
17. The reactor of claim 15, wherein the bends are configured to
influence the direction of the propagation of the microwave
energy.
18. The reactor of claim 1, further comprising one or more
additional windows.
19. The reactor of claim 18, wherein the one or more additional
windows are configured to: form at least one protection volume
disposed between adjacent windows of the one or more additional
windows; and form at least one protection volume disposed between
the one or more additional windows and the window.
20. The reactor of claim 1, wherein the average dielectric constant
ranges from 2 to 10.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. patent
application Ser. No. 16/247,002, filed Jan. 14, 2019, and entitled
"Pressure Barrier Comprising a Transparent Microwave Window
Providing a Pressure Difference on Opposite Sides of the Window",
and U.S. Provisional Patent Application No. 62/617,605, filed on
Jan. 16, 2018, and entitled "Microwave Transparent Pressure
Barrier"; all of which are hereby incorporated by reference,
respectively, for all purposes.
TECHNICAL FIELD
[0002] This disclosure relates generally to materials processing
and, more specifically, to microwave plasma reactors.
DESCRIPTION OF RELATED ART
[0003] Microwave plasmas are useful for processing various
materials, such as chemical processing of gases or materials
deposition systems. For example, industrial chemical processing of
gases using microwaves can be accomplished by flowing the gases
through an elongated vessel while microwave energy is coupled into
the vessel to generate a plasma. The plasma cracks the gas
molecules into component species, such as the conversion of methane
into hydrogen and carbon particulates, or the conversion of carbon
dioxide into oxygen and carbon. Typical systems for microwave
processing (e.g., chemical gas processing) include a quartz
reaction chamber through which process materials flow, and a
microwave magnetron source coupled to the reaction chamber through
a waveguide. Systems are designed to control the effective coupling
of the microwave energy into the reaction chamber and the gas flow
within the reaction chamber to improve energy absorption by the
flowing gas.
[0004] Microwave plasma reactors typically include a pressure
barrier through which microwave energy can penetrate and create a
plasma for processing materials in the reactor. Pressure barriers
may also serve as a mechanical safety barrier to prevent a plasma
created in a processing chamber from backflowing into the microwave
energy source.
SUMMARY
[0005] In some embodiments, a pressure barrier comprises a window
with a first side and a second side, a main section comprising a
length, a first end, and a second end opposite the first end, a
first gradient compression section adjacent to the first end of the
main section, and a second gradient decompression section adjacent
to the second end of the main section. A pressure difference can be
formed between the first and second side of the window. In some
embodiments, the gradient compression section comprises a first
portion of the window, the main section comprises a second portion
of the window, and the gradient decompression section comprises a
third portion of the window. The window can comprise a dielectric
material, where an average dielectric constant of the gradient
compression section increases toward the main section, and an
average dielectric constant of the gradient decompression section
decreases away from the main section. A microwave propagating in a
propagation direction can enter the pressure barrier at the
gradient compression section, travels along the length of the main
section, and exit the pressure barrier through the gradient
decompression section.
[0006] In some embodiments, a pressure barrier, comprises a first
window and a second window separated by a distance L, wherein the
first and second windows each comprise a first and a second side.
The pressure barrier can also comprise a first waveguide adjacent
to the first side of the first widow, and a second waveguide
adjacent to the second side of the second widow. The pressure
barrier can also comprise a sealed protective volume between the
windows adjacent to the second side of the first window and the
first side of the second window. A pressure difference can be
formed between the first waveguide adjacent to the first side of
the first widow and the second waveguide adjacent to the second
side of the second widow. Microwave energy can propagate along the
first waveguide, enter the protective volume through the first
window, and enter the second waveguide through the second window.
The first and second windows can also be angled with respect to the
propagation direction of the microwave energy through the first
waveguide. The distance L can be n.lamda..sub.g/2, where n is an
integer greater than or equal to 1, and .lamda..sub.g is the guided
wavelength of the microwave energy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 shows a conventional pressure barrier.
[0008] FIG. 2 shows an example of a pressure barrier with a
waveguide and a window, in accordance with some embodiments.
[0009] FIGS. 2A-2C show pressure barriers side views, with
waveguides and windows, in accordance with some embodiments.
[0010] FIG. 2D shows a plot of the average dielectric constant of
the pressure barrier in FIG. 2C, in accordance with some
embodiments.
[0011] FIG. 3A shows a conventional pressure barrier in a waveguide
with a conventional quartz window used as a barrier.
[0012] FIG. 3B shows S-parameter S21 for the window in FIG. 3A with
varying dielectric constants.
[0013] FIG. 4A shows an example of a pressure barrier with a
waveguide and a window, in accordance with some embodiments.
[0014] FIG. 4B shows S-parameter S21 for the window in FIG. 4A with
varying dielectric constants and tilt angle, in accordance with
some embodiments.
[0015] FIG. 5A shows S-parameter S21 for the window in FIG. 4A with
varying dielectric constants, in accordance with some
embodiments.
[0016] FIG. 5B shows S-parameter S11 for the window in FIG. 4A with
varying dielectric constants, in accordance with some
embodiments.
[0017] FIG. 6A shows S-parameter S21 for the window in FIG. 4A with
varying tilt angles, in accordance with some embodiments.
[0018] FIG. 6B shows S-parameter S11 for the window in FIG. 4A with
varying tilt angles, in accordance with some embodiments.
[0019] FIGS. 7 and 8 show examples of pressure barrier window
shapes and/or materials, in accordance with some embodiments.
[0020] FIG. 9 shows a perspective view of an assembly containing a
pressure barrier and an E-bend, in accordance with some
embodiments.
[0021] FIGS. 10A-10E show perspective views of a pressure barrier
with two windows, in accordance with some embodiments.
[0022] FIG. 10F shows a side view of the pressure barrier in FIGS.
10A-10F.
[0023] FIG. 10G shows S-parameters S21 and S11 for a window similar
to those shown in FIGS. 10A-10F, in accordance with some
embodiments.
DETAILED DESCRIPTION
[0024] The present disclosure describes pressure barriers for
microwave reactors. The present pressure barriers can be used in
microwave reactors with high power levels in the range of tens to
hundreds of kilowatts at pressure differences (i.e., pressure
differences across the barrier) greater than 50 psi, or greater
than 100 psi, while transmitting sufficient microwave power (e.g.,
greater than 90%, or greater than 95%), across the pressure
barrier. In some embodiments, the present pressure barriers are
capable of operating at these high pressure differences and
transmit as much or more of the microwave power incident on the
pressure barrier, compared with conventional pressure barriers that
are not capable of operating at these high pressure
differences.
[0025] Conventional microwave reactors typically operate at low
pressures, such as in the range of vacuum to approximately 30 psi
(i.e., pressure difference across the barrier approximately 30 psi,
or less). For example, a conventional system may have a microwave
source operating at close to atmospheric pressure on one side of a
pressure barrier and a process chamber on the other side of the
pressure barrier may be at low vacuum, such that the pressure
difference across the barrier is approximately 15 psi. Since
conventional microwave plasma reactors operate at total and
differential pressures below 30 psi, there is no need for windows
within conventional systems to withstand substantially higher
pressures while minimizing power loss. As such, conventional
pressure barriers for waveguides and antennas are not effective
barriers for high pressure differences (e.g., greater than 50 psi),
and typically have operating limits of 30 psi. Furthermore,
conventional pressure barriers for 50 psi can only handle low power
levels, such as approximately 3 kilowatts (kW), and as a result,
conventional pressure barriers for these systems cannot tolerate
high microwave power levels (e.g., greater than 10 kW, or greater
than 100 kW).
[0026] The ability of a pressure barrier component, such as a
window, to withstand higher pressures typically involves increasing
the thickness of a material of the component. However, an increased
thickness in microwave applications usually results in decreased
power transmission, due to the microwave energy being absorbed by
the barrier material. In short, design features, such as found in
the prior art, that enable a window to operate at higher pressure
differentials and/or higher total pressure generally tend to reduce
the capacity of the window to operate at high microwave power, and
conversely, design modifications that allow a window to operate at
higher microwave power generally tend to reduce the capacity of the
window to handle higher total and/or differential pressures.
[0027] In contrast to conventional pressure barriers, the pressure
barriers of the present embodiments can operate at high pressure
differences (e.g., above 50 psi, or above 100 psi, or from 10 psi
to 200 psi) while transmitting sufficient microwave power (e.g.,
greater than 90% or greater than 95%) through the use of wave
compression and decompression sections. In some embodiments, the
pressure barriers will not fail at pressure differences below 300
psi, or 600 psi (e.g., to provide a safety margin for operation at
pressure differences such as 100 psi or 200 psi). In some cases,
the pressure barrier is located between a microwave source (e.g., a
magnetron) and a load (e.g., a microwave plasma) and the pressure
on the side of the pressure barrier adjacent to the microwave
source is about 7 psi, or about 15 psi, or about 35 psi, or about
50 psi, or about 100 psi, or about 150 psi, or about 300 psi, or
about 600 psi, and the pressure on the side of the pressure barrier
adjacent to the load is about 7 psi, or about 15 psi, or about 35
psi, or about 50 psi, or about 100 psi, or about 150 psi, or about
300 psi, or about 600 psi.
[0028] In some embodiments, the waves are input into a compression
section over which the average dielectric constant increases as
they are input into the pressure barrier, travel through a main
section where the average dielectric constant is approximately
constant, and are output through a decompression section over which
the average dielectric constant decreases as they exit the pressure
barrier. For example, the compression and decompression sections
can be made from wedges of dielectric material, and the angle of
those wedges can be tuned to mitigate the losses as the window main
section is made thicker. In some cases, the angle or taper of the
wedges can be changed to accommodate the main window thickness and
the other design parameters of the system (e.g., the wavelength of
the incident energy).
[0029] The term "average dielectric constant" as used herein refers
to the effective dielectric constant experienced by a propagating
wave at a particular position along the propagation direction
averaged over the cross-sectional area (normal to the propagation
direction) that is occupied by the wave. For example, the average
dielectric constant at a position along the wave propagation
direction within a waveguide is the cross-section weighted average
of the dielectric constants of the materials within the waveguide
at that position (where the cross-section is normal to the
microwave power propagation direction). In another example, the
average dielectric constant at a position along the wave
propagation direction within a waveguide is approximately the
volume weighted average of the dielectric constants of the
materials within a thin slice of the waveguide (at that position,
and with surfaces normal to the microwave power propagation
direction) of the materials within the waveguide.
[0030] The "average intrinsic impedance" can be defined similar to
the average dielectric constant described above. This is because
the intrinsic impedance of a material is directly related to (i.e.,
is inversely proportional to the square root of) the average
dielectric constant. Therefore, the compression and decompression
regions of the present windows also have gradients in the average
intrinsic impedance, which in turn lead to gradients in the
waveguide wave impedance.
[0031] The gradient of the average dielectric constant in the
compression section enable a controlled compression of the wave as
the wave slows down in the higher (compared to the gas within a
waveguide) dielectric constant of the materials. Similarly, the
gradient of the average dielectric constant in the decompression
section enables a controlled decompression as the wave exits the
dielectric material of the pressure barrier. In contrast,
conventional windows do not have gradients in the dielectric
constant, and as a result, a propagating wave experiences an abrupt
change in dielectric constant when encountering a surface of the
window. The graded compression and decompression of the wave
reduces the losses within the window, thereby enabling the use of
thicker (i.e., longer length in the direction of the microwave
propagation) materials that can withstand higher pressures while
preserving the energy of the microwave.
[0032] In some embodiments, the present pressure barriers
containing windows with compression and decompression regions can
be located between a microwave source and a plasma reaction zone in
a microwave plasma reactor. In such systems, there is an impedance
matching device (e.g., a 3-stub tuner) that seeks to maximize the
amount of power transferred from the microwave source through the
window to the plasma in the reaction zone of the reactor. When the
losses within the window are too great, the impedance matching
device can sense the window as a load in addition to (or rather
than) the plasma, resulting in power disadvantageously transferred
to the window (e.g., causing temperature of the window to rise),
which also reduces the amount of power transferred from the
microwave source to the plasma. Such a situation can occur when a
thick window is used (e.g., operable at high pressure differences)
that absorbs too much of the microwave energy. In some cases, the
losses within the window are large when the window has a relatively
well-defined distance from the source (rather than being
distributed over a range of distances from the source of roughly
the order of a half of a guided wavelength or more). In some cases,
the losses within a thick window can be reduced if the window has a
thickness that is a substantial fraction of a half loaded guided
wavelength, as is discussed further below. Higher microwave source
power causes higher temperature rises within the window, because
more power will be transferred to the window. In some cases,
thermal runaway is possible due to temperature dependent loss
properties of the dielectric window. In contrast, windows with
compression and decompression regions reduce the ability of the
matching device to couple power into the window, thereby reducing
loss within the windows and enabling thick windows to absorb less
microwave energy within such systems as described above. In some
embodiments, the present pressure barriers (containing windows with
compression and decompression regions that can operate at high
pressure differences) are located between a microwave source and a
plasma reaction zone in a microwave plasma reactor, and the
impedance matching device between the source and the reactor does
not register the window as a significant load compared to that of
the plasma. Not to be limited by theory, the compression and
decompression regions distribute the localized impedance of the
window over a range of distances from the source, and as a result
the effective lumped impedance of the window more closely matches
the impedance of the waveguide, which enables the improved power
transfer through the window. High microwave power can be
challenging for pressure barriers in microwave plasma processing
systems in particular because at high powers the microwave plasma
can densify (i.e., the plasma has a well-defined position and a
high electron number density). When the microwave plasma is dense,
the load impedance of the plasma is fixed over a narrow range of
values. However, at lower power levels, the plasma will be diffuse
(i.e., will not be dense) and the load impedance of the plasma will
be able to vary over a wider range, which in turn can help the
impedance matching device tune into the plasma more efficiently
(i.e., more easily tune the impedance of the source to transfer
more power to the load).
[0033] In some embodiments, the pressure barriers described herein
are used in microwave plasma reactors and methods, such as any
appropriate microwave reactor and/or method described in U.S.
patent application Ser. No. 15/351,858, now issued as U.S. Pat. No.
9,812,295, and entitled "Microwave Chemical Processing," or in U.S.
patent application Ser. No. 15/428,474, now issued as U.S. Pat. No.
9,767,992, and entitled "Microwave Chemical Processing Reactor,"
which are assigned to the same assignee as the present application,
and are incorporated herein by reference as if fully set forth
herein for all purposes.
[0034] Although the above example describes features of the current
pressure barriers in the context of microwave plasma reactors, the
present pressure barriers can be used in other types of processing
equipment utilizing microwave (or other wavelength) energy sources
and windows between the source and a process zone where the
microwave (or other wavelength) energy is utilized. The present
pressure barriers can provide additional benefits in systems
utilizing pulsed (rather than continuous wave (CW)) microwave (or
other wavelength) energy, since impedance matching is more
challenging when the source is pulsed.
[0035] In some embodiments, the present pressure barriers are used
in a processing system, and a differential pressure environment is
created between the two sides of the barrier. For example, the
pressure barrier can be used to isolate downstream volatile gases
from upstream inert or atmospheric gas. For example, in microwave
plasma processing systems, the pressure barrier can separate the
source from a process zone. The atmosphere on the side of the
pressure barrier adjacent to the source can be an inert gas,
nitrogen, argon, or a high breakdown potential gas such as
SF.sub.6. The atmosphere on the side of the pressure barrier
adjacent to the source can be a reactive gas such as hydrogen, a
hydrocarbon such as methane, or a dispersion such as silica
particles suspended in methane gas. Many other gas combinations are
possible, including some with toxic and hazardous gases. When toxic
and/or hazardous gases are present, the pressure barriers described
herein can also serve as safety containment systems.
[0036] Even though the present pressure barrier windows have
reduced absorption compared to conventional windows, in some
embodiments, a constant absorption of microwave energy in the
window causes a rise in the temperature of the window. The
temperature rise may be negligible, however, in some embodiments,
the temperature rise may be significant. In such cases, the
pressure barrier window may be cooled to prevent or mitigate
temperature increases. For example, the pressure barrier can have
fluid cooling channels in a frame surrounding the window. In some
cases, a gas flow may be directed at the window to assist in
cooling the window. For example, an inert gas may be directed
towards one side of a window that is outside of a process region,
and/or a precursor gas may be directed towards a second opposite
side of the window that is outside of the process region.
[0037] In a non-limiting example, the pressure barrier can contain
a window within a waveguide, and the faces of the window can be
"angled" with respect to the incident microwaves (i.e., a direction
normal to the window surface is misaligned with the microwave
propagation direction). In such a case, as the microwave propagates
down the waveguide into the window, the average dielectric constant
experienced by the microwave increases over the region where the
window occupies a greater fraction of the cross-sectional area of
the waveguide (i.e., over the compression section).
[0038] In a second non-limiting example, the pressure barrier can
contain a window within a waveguide, and the dielectric constant of
the materials within the window can increase along the propagation
direction in a region adjacent to a first face of the window (i.e.,
within a compression region), and decrease in a region adjacent to
a second face of the window (i.e., within a decompression region).
For example, such windows can include a dielectric material with
varying mass density (e.g., varying porosity) in the compression
and decompression regions. In another example, such windows can
include more than one dielectric material in an alloy or mixture
whose composition changes within the compression and decompression
regions. In another example, such windows can include a dielectric
material with varying point defect (e.g., vacancies, substitutional
defects, and/or interstitials) concentrations within the
compression and decompression regions. A method by which the above
windows can be fabricated is using 3-D (additive) printing, for
example using equipment that is capable of printing more than one
type of dielectric material (e.g., polymeric materials, such as
those described herein) in various mass-fractions, shapes and
patterns. Alternatively, the above windows can be fabricated by
assembling small units (e.g., particles, wires, spheres, rods,
etc.) of different dielectric constant materials (e.g., polymeric
materials, such as those described herein) and then annealing (and
optionally further shaping and/or polishing) to form a window with
a graded dielectric constant. For example, a first mixture of
particles with a low average dielectric constant can be poured into
a mold to form a first layer. Then, a second layer with a higher
average dielectric constant can be formed on top of the first layer
by pouring a second mixture of particles with a higher average
dielectric constant into the mold. This process can be repeated
until the desired thickness and dielectric constant profile is
established, and then the mold can be annealed and optionally
further shaped and/or polished to form the window. Alternatively,
lasers can be used to locally modify the properties of a window
material (e.g., crystallinity, defect concentration, etc.) in 3-D
to form dielectric constant gradients in materials. Other methods
can also be used to fabricate the present windows with graded
dielectric constants, and the present windows are not limited to
any particular fabrication technique.
[0039] The gradients of the average dielectric constant in the
compression and decompression sections can be formed using many
different combinations of geometries as well as materials. For
example, the present windows can contain silica, fused silica,
quartz, fused quartz, polystyrene, polypropylene, polyethylene,
Kapton (i.e., polyimide), Teflon (i.e., polytetrafluoroethylene,
PTFE), and combinations thereof.
[0040] In some embodiments, a dielectric window within the present
pressure barriers interacts with a propagating microwave where some
energy is reflected, some energy is absorbed, and some energy is
transmitted. In some embodiments, the present pressure barriers are
used in a waveguide, and the pressure barrier window is capable of
transmitting a high fraction of microwave power through the
pressure barrier in the waveguide, such as greater than 80%, or
greater than 85%, or greater than 90%, or greater than 95%, or
greater than 98%, or greater than 99%, or greater than 99.9%.
[0041] In some embodiments, the window within the pressure barrier
is angled or tapered to reduce the microwave losses due to the
window and enable the impedance matching device to more efficiently
deliver power through the window in the waveguide structure over a
narrow but non-negligible range of microwave frequencies.
Conventional low-cost microwave sources, such as magnetrons in
particular, may produce microwave power at a range of frequencies
rather than at a single frequency that is for practical purposes
arbitrarily precise. In some embodiments, a secondary window can be
added to the pressure barrier for additional safety and to provide
a protection volume whereby process gases leaking from the
processing side could be vented safely and diluted thru the central
region between two or more windows. In some embodiments, pressure
sensing elements within a volume between the two windows can detect
the loss of pressure integrity to shut down the power input through
electrical signaling means. Additionally, a high breakdown strength
gas such as SF.sub.6 can be inserted into this section to reduce
the chances of breakdown and plasma discharge.
[0042] In some embodiments, it is advantageous to increase the
pressure across the window (e.g., have higher pressure in a
reaction region compared to a microwave source region to improve
reaction rates during microwave plasma materials processing). As
the pressure difference across the window increases, the window
needs to be thicker to accommodate the higher pressure differences,
which results in increased losses (e.g., in absorption and/or
reflection). As described above, the present pressure barriers
solve this problem by introducing gradient compression and
decompression sections surrounding a main section of the
window.
[0043] FIG. 1 shows a conventional pressure barrier 100, which has
typical specifications of 3 kW, 10% loss of microwave power
minimum, 30 psi pressure difference (i.e., across the pressure
barrier), and is water cooled. The barrier has a window 110 which
is usually quartz.
[0044] FIG. 2 shows an example of a pressure barrier 200, with a
waveguide 201 and a window 210 of the present disclosure placed
inside, in accordance with some embodiments. Directions X, Y and Z
are indicated in FIG. 2. The microwave power propagates through the
waveguide 201 in the direction shown by arrow 205. The pressure
barrier 200 and the window 210 each have a main section (i.e.,
region) 211, an input section 212 adjacent to a first end of the
main section 211, and an output section 213 adjacent to a second
end of the main section 211. The input section 212 and the output
section 213 taper away from the main section; i.e., forming wedge
shapes (or right triangular prisms) and the main section forms a
right rectangular prism. The angled input section 212 compresses an
incoming wave, such as a microwave provided by an energy source,
and the angled output section 213 decompresses the wave after
exiting the pressure barrier (e.g., so that the wave can propagate
into a reaction zone of the microwave reactor). The pressure
barrier 200 can withstand approximately 100 kW of incident
microwave power, 1-2% loss of microwave power through the window
maximum, greater than 50 psi pressure difference (i.e., across the
pressure barrier), and is optionally gas or water cooled.
[0045] More generally, the present pressure barriers can withstand
pressure differences greater than 10 psi, greater than 50 psi,
greater than 100 psi, or from 10 psi to 200 psi, and incident power
levels (e.g., of microwave power) greater than 10 kW, or greater
than 20 kW, or greater than 50 kW, or greater than 100 kW, or
greater than 500 kW. The loss of power when transmitting microwaves
(or, waves with other wavelengths) through the present pressure
barriers is less than 10%, or less than 5%, or less than 1%, or is
from 0.1% to 10%, or is from 0.1% to 2%. The lengths of the
compression, decompression and main sections of the windows in the
present pressure barriers can be from 1'' (1 inch) to 10'', or from
1'' to 5'', or from 1'' to 2'', or less than 1'', in different
embodiments with different specification (e.g., window material
dielectric constant, window geometries, and required pressure
difference).
[0046] FIGS. 2A and 2B show non-limiting embodiments of pressure
barriers 200a and 200b, respectively, in side views, with
waveguides and windows 210a and 210b. The windows 210a (FIG. 2A)
and 210b (FIG. 2B) of these embodiments are parallelograms in side
view. The angles 220a (FIG. 2A) & 220b (FIG. 2B) and 225a (FIG.
2A) & 225b (FIG. 2B) of the parallelogram windows in side view
are also shown for the non-limiting embodiments in these figures.
For example, angles--220a & 220b and 225a & 225b can be
from 20.degree. to 70.degree., or from 30.degree. to 70.degree., or
from 30.degree. to 80.degree., or from 30.degree. to 60.degree., or
from 40.degree. to 60.degree., or from 50.degree. to 60.degree., or
less than 80.degree., or less than 70.degree., or about 50.degree.
or about 55.degree.. The window 210a in FIG. 2A has a similar shape
as the window 210 in FIG. 2, while the window 210b in FIG. 2B is
thinner, and since the angles 220b and 225b are similar to 220a and
225a, window 210b has a more oblique parallelogram shaped
cross-section than window 210a. The microwave 206 propagates down
the waveguide in the direction 205 in FIGS. 2A and 2B, and
encounters compression section 212a (FIG. 2A) and 212b (FIG. 2B)
first, then propagates through the main section 211a (FIG. 2A) and
211b (FIG. 2B), and then encounters decompression section 213a
(FIG. 2A) and 213b (FIG. 2B) before reentering waveguide 201 (FIGS.
2A and 2B). (Note that microwave 206 is a visual aid that
represents the magnitude and direction of the microwave electric
field (transverse to the propagation direction) as a function of
axial position along the waveguide as shown at a single instant in
time, and the wavelength and the shape of the microwave 206 in
FIGS. 2A and 2B is not drawn to scale.) The lengths of the
compression section, main section and decompression section in FIG.
2A are 212l, 211l, and 213l, respectively. The window 210a and 210b
contains one or more dielectric materials having one or more
dielectric constants (e.g., from 2.2-2.6, or from about 2 to about
3.5, or from about 2 to about 6, all at 2.45 GHz) and loss tangents
(e.g., from less than 0.0001 to 0.0005, or from less than 0.0001 to
0.001, or greater than 0.001, all at 2.45 GHz), and the waveguide
contains gases (or gas-solid dispersions, or gas-liquid
dispersions) on each side of the barrier with a lower dielectric
constant (e.g., approximately 1) than those of the window
materials. As the microwave propagates through the compression
section 212a & 212b (from left to right in FIGS. 2A-2B) the
average dielectric constant increases, and there is a corresponding
decrease in the (real part of the) intrinsic impedance. The average
dielectric constant increases in this example because the window
(i.e., a higher dielectric constant material than the surrounding
gas or dispersions) occupies a larger fraction of the
cross-sectional area of the waveguide when moving from left to
right in the compression region. Correspondingly, the average
dielectric constant decreases, and there is a corresponding
increase in the (real part of the) intrinsic impedance through the
decompression section 213a (FIG. 2A) and 213b (FIG. 2B). Note that
even though the main section of the window 210b in FIG. 2B does not
span the width of the waveguide 201 (FIGS. 2A and 2B), the average
dielectric constant throughout the main section is constant, just
as in the thicker window embodiment shown in FIG. 2A.
[0047] FIG. 2C shows another example of a window 210c for a
pressure barrier 200c that has a compression region 212c, a main
region 211c, and a decompression region 213c. The microwave 206
propagates down the waveguide in the direction 205 in FIG. 2C. In
this example, the window 210c is a rectangular prism shape
(rectangular in cross-section shown in FIG. 2C), and the
compression region 212c and the decompression region 213c are
formed using gradients in the average dielectric constant on the
materials in the window, in accordance with some embodiments. FIG.
2D shows a plot 230 of the average dielectric constant versus
position experienced by a wave propagating through the waveguide
201 and the window 210c in FIG. 2C. The x-axis in FIG. 2D is the
position along the axial direction of the waveguide 201 (FIG. 2C)
for pressure barrier 200c (FIG. 2C). The average dielectric
constant increases along length 212l (FIG. 2C), is constant over
length 211l (FIG. 2C), and decreases in length 213l (FIG. 2C),
corresponding to the compression, main and decompression regions,
respectively. In some embodiments, the gradient can be achieved by
using layers of materials with different dielectric constants. The
gradient of the dielectric constant can be linear (as shown in FIG.
2D), non-linear, stepwise, or other relationship.
[0048] In a non-limiting example, the gradients in the average
dielectric constant, or intrinsic impedance, of the present windows
(e.g., window 210c shown in FIGS. 2C and 2D) may be substantiated
by creating a window that fills the body of the waveguide (e.g., in
the shape of a rectangular prism), in which the window is created
out of two dielectric materials with substantially different
dielectric constants, and the two materials are deposited in small
"voxels" by a 3D-printer and optionally sintered, in such a manner
that gradients in average dielectric constant are formed. For
example, referring to FIG. 2D, materials A and B can have different
bulk dielectric constants, and at one axial position "z1" along the
direction of propagation of waves in the waveguide, a certain
fraction of voxels is of material "A" and the remaining fraction is
of material "B" producing an average dielectric constant
".epsilon.1" at position z1, and at another axial position "z2",
the fraction of voxels of material "A" and of material "B" differ
from their corresponding values at position "z1" producing an
average dielectric constant "c2" at position z2.
[0049] FIGS. 3A and 3B provide background information on
conventional pressure barriers, which are typically made of quartz.
In general, electromagnetic waves propagate through dielectric
media within waveguides and antennas more slowly than through a
vacuum by a factor .epsilon., which is the dielectric constant of
the material. The dielectric constant of a material, particularly
when normalized to the permittivity of free space, is also known as
its relative permittivity, or ".epsilon.r" which may also be
referred to in this disclosure as ".epsilon.." The dielectric
constant of air is .epsilon..sub.r=1.0, while for quartz
.epsilon..sub.r is approximately 3.75. The dielectric constants
and/or loss tangents of high strength materials such as quartz slow
down a wave in a waveguide, where the slowing down of the wave
results in relatively large power transfer losses. FIG. 3A shows a
conventional pressure barrier 300 in a waveguide 301 with a
conventional quartz window 310 used as a barrier. Microwave energy
propagates down the z-axis of the waveguide (i.e., in the z
direction), and the window 310 has a surface normal to the z-axis
of the waveguide (i.e. the wave propagation direction). The
conventional quartz window 310 has a rectangular prism shape (i.e.,
has rectangular cross-sections in the x-y, x-z, and y-z planes),
with a thickness (in the z direction) equal to 0.375 inches. A
thickness of 0.375 inches is thicker than typical pressure barrier
windows and enables the pressure barrier 300 to operate at pressure
differences of greater than about 100 psi across the barrier.
[0050] The microwave power transfer through pressure barrier 300 in
FIG. 3A was simulated by establishing a microwave source at a first
port 305 of the waveguide 301, and placing a load (e.g., a
microwave plasma) at a second port 306 of the waveguide 301. FIG.
3B shows S-parameter S21 for four different window dielectric
constants. S-parameter S21 describes the power transmitted from the
source (i.e., port 1) to the load (i.e., port 2) through the
window. The total power transferred from the source to the load is
S21 plus any power reflected from port 2 back to port 2 (i.e.,
S22), however, since S22 is very small in these simulations, S21 is
approximately equal to the power transferred from the source to the
load. Curve 321 simulates the transmitted power (i.e., S21) for the
pressure barrier 300 with a window 310 having a dielectric constant
equal to 1.75, while curves 322, 323, and 324 simulate windows with
dielectric constants equal to 2.75, 3.75 and 4.75 respectively. The
loss tangent for the window 310 was kept constant at 0.0002 for all
simulated curves 321-324. The x-axis of the plot in FIG. 3B is the
frequency (in GHz) of the guided microwave through pressure barrier
300. The curves 321-324 show that a higher dielectric constant
causes less power to transfer from the source to the load through
window 310. The lost power (i.e., the power transferred from the
microwave source to the quartz window 310) is absorbed by the
window 310, which causes the window 310 to become heated. For a
conventional 0.375 inch thick window 310 (i.e., that is capable of
withstanding 100 psi pressure differences) made from quartz with a
dielectric constant of 3.75, curve 323 shows that the power
transferred through the window is approximately 75% to 80% at 2.45
GHz. In other words, greater than 20%, or from 20% to 25% of the
power will be lost.
[0051] FIGS. 4A and 4B are similar to FIGS. 3A and 3B, except FIGS.
4A and 4B show an example schematic and associated S-parameter
curves of a present pressure barrier 400 capable of operating at
higher pressure differences (e.g., greater than 50 psi, or
approximately 100 psi), in accordance with some embodiments. The
window 410 of the pressure barrier within waveguide 401 is tilted
(or angled) with respect to the z-axis (i.e., the wave propagation
direction) of the waveguide 410, and has a similar geometry as the
pressure barrier shown in FIG. 2B with a parallelogram shaped
cross-section (within the waveguide 401) in the x-z plane. The
window 410 in this example is 0.375 inches thick (i.e., in the
direction normal to the major surface of the window). In this
example, the waveguide 410 is wider in the y direction than the x
direction, and the window is tilted by rotating the window around
the y-axis. In other embodiments, a waveguide that is wider in the
y direction than the x direction, can use a window that is tilted
by rotating the window around the x-axis. The angle 415, by which
the window 410 is tilted, is shown for one example in FIG. 4A, and
various angles 415 were simulated in FIG. 4B.
[0052] The microwave power transfer through pressure barrier 400 in
FIG. 4A was simulated by establishing a microwave source at a first
port 405 of the waveguide 401, and placing a load (e.g., a
microwave plasma) at a second port 406 of the waveguide 401. FIG.
4B shows S-parameter S21 for different window tilt angles 415 and
dielectric constants. S-parameter S21 describes the power
transmitted from the source (i.e., port 1) to the load (i.e., port
2) through the window, similar to the simulations as described for
FIGS. 3A and 3B. The x-axis of the plot in FIG. 4B is the frequency
(in GHz) of the guided microwave in pressure barrier 400. Curve 421
(with circle symbol markers) shows the example where the window has
a tilt angle equal to 0.degree. and a dielectric constant of 3.75,
which corresponds to curve 323 in FIG. 3B. A set of curves 420
(shown as lines without symbol markers) simulate the transmitted
power (i.e., S21) for the pressure barrier 400 with a window 410
having a dielectric constant equal to 3.75, while the tilt angle
415 is increased from 5.degree. to 50.degree. in 5.degree.
increments. The arrow shown for the set of curves 420 shows the
trend to higher transferred power with a higher tilt angle.
[0053] Curves 422, 423 and 424 (with symbol markers that are
inverted triangles, upright triangles, and squares, respectively)
in FIG. 4B simulate windows with a tilt angle equal to 53.degree.
and dielectric constants equal to 1.75, 3.75 and 5.75,
respectively. The loss tangent for the window 410 was kept constant
at 0.0002 for all simulated curves in the set of curves 420, and in
curves 421-424. The curves 422-424 show that a dielectric constant
greater than 3.75 causes less power to transfer from the source to
the load through window 410, even at a tilt angle 415 of
53.degree.. For the 0.375 inch thick window 410 in this example
that is tilted at an angle of 53.degree. and made from quartz with
a dielectric constant of 3.75, curve 423 shows that the power
transferred through the window is approximately 98% to 100% (or
about 99%) at a frequency of 2.45 GHz. In other words, only 1% to
2% of the power will be absorbed by the window 410 with the
parameters simulated in curve 423. Surprisingly, these results are
an improvement over a thick (0.375 inches thick) window with faces
normal to the wave propagation direction (i.e., with roughly 80%
loss), as well as over a thin window only capable of low pressure
difference operation (e.g., less than 30 psi pressure differences,
and with roughly 10% loss). The simulations also show that there is
virtually no increase in loss when changing the dielectric constant
of the window from 1.75 to 3.75, as both curves 422 and 423
virtually overlap at 2.45 GHz. The preservation of power for the
present window 410 is achieved by the compression region and the
decompression region formed by tilting the window with respect to
the wave propagation direction. The lost power (i.e., the power
transferred from the microwave source to the quartz window 410) is
absorbed by the window 410, which causes the window 410 to become
heated, however, the lower loss (e.g., roughly 10 to 20 times lower
loss) of the present window 410 compared to conventional quartz
window 310 means that the window 410 will heat up significantly
less than window 310.
[0054] FIG. 5A shows simulations of S-parameter S21 for the
pressure barrier 400 in FIG. 4A, where the window 410 has a tilt
angle of 53.degree. and various dielectric constants, ranging from
1.75 to 5.75. The x-axis is again the frequency (in GHz) of the
guided microwave through the pressure barrier 400. A set of curves
520 shows that S21 increases as dielectric constant of window 410
decreases, as shown by the arrow. The dielectric constant for
windows simulated by curves, 521, 522, 523, 524, 525, 526 and 527
are 3.75, 4.25, 4.5, 4.75, 5.0, 5.5 and 5.75, respectively.
[0055] FIG. 5B shows simulations of S-parameter S11 (in dB) for the
pressure barrier 400 in FIG. 4A, for the same set of simulations
depicted in FIG. 5A. The x-axis is again the frequency (in GHz) of
the guided microwave through the pressure barrier in GHz. The
S-parameter S11 in this example is the amount of power input into
source (i.e., port 1) 405 that is reflected back to the source
(i.e., port 1) 405. In practice, impedance matching devices will
more easily be able to maximize the power transfer from the source
to the load when the S11 is lower. The curves in the plot in FIG.
5B show that S11 has a resonant minimum that shifts frequency as
the dielectric constant of window 410 changes. The resonance in the
pressure barrier 400 that causes these resonant features is a
cavity formed within the pressure barrier over the distance
occupied by the window (e.g., as shown in regions 212b, 211b and
213b in FIG. 2B, which has a similar geometry as the window 410 in
FIG. 4A) The dielectric constant for windows simulated by curves,
531, 532, 533, 534, 535, 536, and 537 are 3.75, 4.25, 4.5, 4.75,
5.0, 5.5, and 5.75, respectively. The plot in FIG. 5B also shows
curves 538 and 539, which are simulations of windows with
dielectric constants of 2.75 and 1.75, respectively. FIG. 5B shows
that there exists a beneficial resonance condition at approximately
2.45 GHz when the window has a dielectric constant of 4.75 and a
tilt angle of 53.degree.. Therefore, a system using 2.45 GHz
frequency microwaves could reduce the reflection from the window
410 by using a window material with a dielectric constant of 4.75.
However, quartz, with a non-optimal dielectric constant of 3.75 has
a high power transfer coefficient S21 (as shown in FIG. 5A) and has
other beneficial properties (e.g., low loss tangent, high
mechanical strength, and is commercially available). This is an
example that shows that various factors can be taken into account
when designing a pressure barrier and window, including dielectric
constant, loss tangent, mechanical properties, and commercial
availability.
[0056] FIG. 6A shows simulations of S-parameter S21 for the
pressure barrier 400 in FIG. 4A, where the window 410 has a
dielectric constant of 4.75 and various tilt angles ranging from
55.degree. to 56.degree.. The x-axis is again the frequency of the
guided microwave through the pressure barrier 400 in GHz. The
curves in the plot in FIG. 6A show that S21 further improved by
optimizing the tilt angle of window 410. The tilt angles for
windows simulated by curves, 611, 612, 613, 614, 615, and 616 are
55.0, 55.25, 55.5, 55.6, 55.75, and 56.0, respectively. All curves
611-616 show an S21 greater than about 0.99 over the frequency (in
GHz) range from 2.3 GHz to 2.6 GHz. This shows that the present
windows have high transmission (i.e., power can be effectively
transferred from a source to a load through the window) over a
relatively wide bandwidth (e.g., greater than +/-150 MHz around a
nominal 2.45 GHz). Such a wide bandwidth is beneficial for use in
practical systems employing microwave energy sources with some
variation in wavelength (e.g., around a nominal 2.45 GHz). Not to
be limited by theory, the tilt angles of the present windows (or
the dielectric constant gradients, in other embodiments) serve to
broaden (i.e., smear out, or soften) the resonance effects by
creating less well defined cavities (compared to a conventional
rectangular prism window with no tilt) within which the waves can
resonate. In other words, tilted mirrors create cavities with lower
Q factors, which have characteristically broader resonance
features.
[0057] FIG. 6B shows simulations of S-parameter S11 (in dB) for the
pressure barrier 400 in FIG. 4A, for the same set of simulations
depicted in FIG. 6A. The x-axis is again the frequency (in GHz) of
the microwave through the pressure barrier 400 in GHz. The
S-parameter S11 in this example is the amount of power input into
source (i.e., port 1) 405 that is reflected back to the source
(i.e., port 1) 405. The curves in the plot in FIG. 6B show that S11
has a resonant minimum that shifts frequency as the tilt angle of
window 410 changes. The tilt angles for windows simulated by
curves, 621, 622, 623, 624, 625, and 626 are 55.0, 55.25, 55.5,
55.6, 55.75, and 56.0, respectively. FIG. 6B shows that the
beneficial resonance minimum of S11 is 2.45 GHz when the window has
a dielectric constant of 4.75 and a tilt angle of 55.6.degree..
Therefore, a system using 2.45 GHz frequency microwaves could
reduce the reflection from the window 410 by using a window
material with a dielectric constant of 4.75 and tilting the window
to an angle 415 of 55.6.degree.. Advantageously, all of the curves
shown in FIG. 6B have S11 less than -30 dB at 2.45 GHz, which is in
part due to the wide bandwidth of the S11 resonance dips (i.e.,
inverted peaks) for the present windows.
[0058] The simulation results shown in FIGS. 3A, 3B, 4A, 4B, 5A,
5B, 6A and 6B, comparing the present barriers and conventional
pressure barriers, illustrate that a thick window (e.g., 0.375
inches) able to operate at high pressure differences can be made to
have virtually no transmission loss if the window dielectric
material and tilt angle of the window are adjusted accordingly.
When a wave within a waveguide moves from the open portion of the
waveguide into a dielectric material, the wave slows down. In a
conventional pressure barrier, when an incident wave reaches the
window it experiences an abrupt change in dielectric constant,
which results in large power transfer losses (e.g., greater than
20%, or greater than 30%, as shown in curves 323 and 324 in FIG.
3B). The present embodiments have windows with major surfaces
tilted with respect to the wave propagation direction, enabling an
impedance matching system to deliver nearly 100% (e.g., greater
than 80%, or greater than 90%, or greater than 95%, or greater than
99%, as shown in FIGS. 4B, 5A, 5B, 6A and 6B) of the power through
the window (e.g., to a plasma in a processing zone of a reactor).
Not to be limited by theory, the tilting of the window uniquely
transitions the waveform into and out of a window of a pressure
barrier in a graded fashion (i.e., creates compression and
decompression regions with gradients in average dielectric
constants), which enables the high power transfer characteristics
of the windows described above.
[0059] Windows with compression and decompression regions (e.g.,
those that are have angled faces as shown in FIGS. 2, 2A-2B, and
4A, or those with dielectric constant gradients as shown in FIGS.
2C-2D) allow for atmospheric to high pressure differences across
the dielectric and efficient the transfer of high power levels
through the window. The dielectric constant(s) of the material(s)
in the window, as well as the geometry of the compression, main,
and decompression regions of the window, can be tuned to minimize
the lost power and maximize the power transmitted through window
(e.g., using parametric simulations similar to those shown in FIGS.
4A, 4B, 5A, 5B, 6A and 6B).
[0060] In addition to the dielectric constant, the loss tangent of
the dielectric material in a window will also affect the amount of
power transmitted through the window in the present pressure
barriers. For example, the S-parameters of a window with similar
geometry as that shown in FIGS. 2 and 2A were simulated. A first
simulation was done with a higher loss tangent ethylene
tetrafluoroethylene (ETFE, .epsilon..sub.r=2.3, loss tangent about
0.01 at 2.45 GHz) window, and a second simulation was done with a
lower loss tangent high density polyethylene (HDPE,
.epsilon..sub.r=2.3, loss tangent 0.0003 at 2.45 GHz) window.
Similar to the simulations in FIGS. 3A, 3B, 4A, 4B, 5A, 5B, 6A and
6B, the simulated structure had a waveguide with a source and a
load, where port 1 of the waveguide was the source and port 2 of
the waveguide was the load, and the window was placed in the
waveguide between the two ports. The simulated S-parameters of the
ETFE window were: S11=0.005, S21=95.3, S12=95.3, S22=0.005. The
simulated S-parameters of the HDPE window were: S11=0.001,
S21=99.7, S12=99.7, S22=0.001. These simulations show that the loss
tangent impacts the power transfer through a pressure barrier
window, and that a high amount of power can be transferred (e.g.,
greater than 99%, or greater than 99.5%, or about 99.7%) using low
loss tangent (e.g., less than 0.0005) materials in the present
windows. These simulations also illustrate that a high amount of
power can be transferred (e.g., greater than 90%, or about 95%, or
greater than 95%) using the present windows with a dielectric
material with a relatively high loss tangent (e.g., greater than
0.001).
[0061] In some embodiments, the one or more dielectric constants of
the pressure barrier window material are from 2.2 to 2.6, or from
about 2 to about 3.5, or from about 2 to about 6 (all at 2.45 GHz),
and the loss tangents are from less than 0.0001 to 0.0005, or from
less than 0.0001 to 0.001, or are greater than 0.001 (all at 2.45
GHz). The dielectric constant is chosen to be low enough to allow
manageable size of the pressure barrier for the particular
application.
[0062] FIGS. 2, 7 and 8 show some examples of shapes of the present
pressure barrier windows, where the dielectric windows are shown
within a waveguide, in accordance with some embodiments. Directions
X, Y and Z are indicated in FIG. 7. FIG. 7 is a diamond shape (as
viewed along a longitudinal cross-section, i.e., the Y-Z plane
where Z is along the path of microwave propagation in the
waveguide), while FIG. 2 is a parallelogram shape in the same
relative plane. The window in FIG. 7 has a compression section "C,"
a decompression section "D," and a main section "M" with length
"L." The length L of the main section for any geometry may be, for
example, 0.3 inches, 0.4 inches, 0.5 inches, 1 inch, 3 inches, or 6
inches, or greater than 0.3 inches, or greater than 0.5 inches, or
greater than 1 inch, or greater than 3 inches, or greater than 6
inches, or from 0.3 inches to 10 inches, or from 0.3 inches to 6
inches, or from 0.3 inches to 3 inches, or from 0.3 inches to 1
inch, in various embodiments. FIG. 8 shows a window with a geometry
similar to that of the window in FIG. 2 and provides some
non-limiting example materials for the pressure barrier, including
a barrier containing a "TEFZEL polymeric material" (i.e., ETFE
(TEFZEL.RTM.)) and a waveguide containing an "aluminum body" (with
"WR340 flanges" and an "aluminum pressure cap", in this
non-limiting example). For a microwave frequency of 2.45 GHz, the
dielectric constant of the window material can be 2.3, and the main
section can have a total length from 0.3 inches to 10 inches (e.g.,
to withstand high pressure differences). In general, the length of
the main section and the compression/decompression sections varies
with the dielectric materials used. In some embodiments, the shapes
of the compression and decompression sections are not symmetrical.
Additionally, the dielectric materials of the central (main)
section do not need to be the same as the materials in the
compression and/or decompression sections. Other dimensions and
materials may be chosen depending on the specific application, and
the concepts described herein apply also to other microwave
frequencies. Some non-limiting examples of shapes for the
compression and decompression sections of the present windows are
right triangular prism shapes and pyramidal shapes. In some
embodiments, a waveguide that is wider in the y direction than the
x direction, can use a window that is tilted by rotating the window
around either the y-axis, or the x-axis, or both the x-axis and the
y-axis (even though most examples show the window tilted around the
y-axis).
[0063] In the examples of FIGS. 2, 7 and 8, the shape--that is, the
geometrical dimensions--of the dielectric materials provide a
gradient compression of the wave within the waveguide, compressing
and decompressing over the length of the barrier rather than
introducing an abrupt change in dielectric constant within the
waveguide, thereby reducing energy loss compared to conventional
windows. Similar to the angles for the parallelogram cross-section
windows shown in FIGS. 2A and 2B (angles 220a and 220b, and 225a
and 225b) and FIG. 4A (angle 415), other window shapes can also be
defined by similar angles. For example in FIG. 8, the compression
section C and decompression section D may be pyramids with a
rectangular bases adjacent to the main section, where a first angle
at the apex of the pyramid in the compression section defines the
gradient of the dielectric constant increase within the compression
section, and a second angle at the apex of the pyramid in the
decompression section defines the gradient of the dielectric
constant decrease within the decompression section. The gradient
can be achieved in various ways, such as but not limited to grading
the geometry continuously, in steps, linearly or non-linearly.
[0064] The above embodiments demonstrate that windows with square,
rectangular, parallelogram, and diamond shaped cross-sections (in
the X-Z plane) can all be used if configured correctly to form
suitable compression and decompression regions. Factors to be
considered in the design of a specific microwave transparent high
pressure difference barriers include the microwave frequency, the
dielectric constant of the window material, the loss tangent of the
window material, the pressure specification, the waveguide
dimensions, and the type of gas and/or gas mixtures that could
contact the window (e.g., during normal operation, during abnormal
operation, or during a failure). The wave compression and
decompression regions preserve the microwave power as the wave
traverses the pressure barrier, which also results in minimal
heating in the dielectric material. This reduces the cooling
requirements for the pressure barrier, thus enabling gas or water
cooling. Cooling of the pressure barrier could be achieved by, for
example, providing cooling channels in a frame surrounding the
dielectric material or flowing a cooling substance (e.g., a gas) at
one or more faces of the pressure barrier window.
[0065] FIG. 9 provides a perspective view of an assembly 900
demonstrating that other features can be added to deliver better
performance of the system. In this example, the system is a
pressure barrier with a window 910 having a parallelogram
cross-section, combined with an E-bend 920 in the waveguide 901.
E-bends provide a smooth change in the electric field, to
correspond to a change in the direction of the waveguide. Some
non-limiting examples of other features that can be combined with
the present pressure barriers are waveguide E-bends, H-bends, sharp
E-bends, sharp H-bends, or twists. These other features in
combination with the present pressure barriers allow for microwave
energy to propagate along any desired path of a microwave waveguide
and/or reactor through different pressure zones (e.g., with
different gas species in each zone) while preserving the microwave
energy. Additionally, in some embodiments the present pressure
barriers can be beneficial in waveguides with circular or
elliptical cross-sections (even though all of the examples are
shown in the context of rectangular cross-section waveguides). For
example, a window can be tilted within waveguides with circular or
elliptical cross-sections, or a window can have conical compression
and/or decompression regions.
[0066] In some embodiments, there may be a pressure seal between
interfaces of the pressure barrier window and the walls of the
waveguide. Different mechanisms can be used to seal the pressure
barrier and window and distribute the pressure around the perimeter
of the window. For example, the sealing can be achieved by a wedge
shape of the main section of the pressure barrier adjacent to the
walls of the waveguide, where the pressure is distributed along the
sides. In other embodiments, sealing can involve an O-ring or flat
elastomeric seal, or materials that are to be coated onto the
pressure barrier. In other adaptations, the dielectric material
could be melted into the desired shape of the barrier within the
structure of the waveguide itself to innately provide a seal
between the pressure barrier and the waveguide. Other methods of
ensuring a seal around the pressure barrier are also possible.
[0067] FIGS. 10A-10E show a non-limiting example of a pressure
barrier 1000 with two quartz windows 1010a (FIGS. 10B, 10C and 10E)
and 1010b (FIGS. 10B and 10E), conductive frames 1015a (FIGS. 10B,
10D and 10E) and 1015b (FIGS. 10B and 10E), end caps 1020a and
1020b (FIG. 10A), a housing with two halves 1030a and 1030b (FIG.
10A) and O-rings 1040a, 1040b, 1050a and 1050b (FIG. 10B). The
pressure barrier in this example was designed for use with 30 kW of
2.45 GHz microwaves (either CW or pulsed), and pressure differences
during operation of about 200 psi (with a 3.times. safety margin
between the rated pressure difference during operation of about 200
psi and the maximum pressure difference tolerated before failing of
about 600 psi). FIG. 10A shows the pressure barrier with both
halves of the housing assembled, and FIG. 10B shows the pressure
barrier with one half of the housing removed. FIG. 10C shows a
cross-section view down a center plane (i.e., the x-z plane) of the
pressure barrier with only one quartz window and conductive frame
assembled. FIG. 10D shows a partially transparent view of FIG. 10C,
where the frame 1015a of the window 1010a is shown as not
transparent. FIG. 10E shows a cross-section view down a center
plane of the pressure barrier with both quartz windows and
conductive frames assembled. FIGS. 10D and 10E illustrate that the
frames 1015a and 1015b (FIG. 10E) are designed such that they form
a continuous plane of metal on the upper and lower surfaces (i.e.,
in y-z planes (FIG. 10C)) of the waveguide 1001 (FIG. 10A), which
is advantageous to avoid field concentrations and arcing around the
pressure barrier and windows.
[0068] In this example, a secondary window 1010b is added to the
pressure barrier for additional safety and to provide a protection
volume 1060 between the two windows. This pressure barrier has two
quartz windows 1010a and 1010b surrounded by conductive frames
1015a and 1015b. The quartz windows and frames are hermetically
sealed (using end caps 1020a and 1020b) to a housing with two
halves 1030a and 1030b that bolt together. The two halves of the
housing of the pressure barrier are sealed together using O-rings
1040a and 1040b, and the pressure barrier is coupled and sealed to
the waveguide using O-rings 1050a and 1050b. In some embodiments,
the protection barrier can also contain one or more dielectric
materials (e.g., be filled with a polymeric material).
[0069] The design in this example includes ports 1062 for gases
(e.g., a gas with a high dielectric breakdown such as SF.sub.6) to
be input and output from the protection volume, pressure release
valves, and ports 1062 for sensors (e.g., pressure transducers) to
detect the environment within the protection volume 1060 (e.g., the
pressure and/or types of gas species present). For example, in the
case of a failure of the window adjacent to a processing zone in a
microwave plasma reactor, volatile process gases can leak into the
protection volume 1060. The sensors in the protection volume 1060
can sense such a leak (either due to a detected pressure change, or
volatile gas, or both), and then trigger a shutdown including
venting the protection volume 1060 in a safe manner (e.g., through
a volatile gas filter or scrubber). In the case of a catastrophic
window failure the pressure release valve within the protection
volume 1060 can ensure safe equipment operation, as well as protect
the zone of the reactor adjacent to the intact window.
[0070] FIG. 10F shows a non-limiting example of a pressure barrier
1000f, showing the distance L between the windows 1010a and 1010b.
The waveguide 1001 and microwave propagation direction 1005 are
also shown in FIG. 10F. In addition to the compression section,
main section and decompression section (e.g., similar to that shown
in FIG. 2B) of each window in the pressure barrier 1000f, in some
embodiments, the distance L between the present windows is
substantially close to n.lamda..sub.g/2 (e.g., within 1%, within
5%, or within 10% of n.lamda..sub.g/2), where n is an integer
greater than or equal to 1, and .lamda..sub.g is the guided
wavelength of the microwave energy (or, in cases where a dielectric
material is arranged between the two windows, then .lamda..sub.g is
the correspondingly loaded guided wavelength of the microwave
energy). Not to be limited by theory, when the distance L follows
this relationship the two windows in the pressure barrier will form
a resonant cavity, which will further suppress the loss resulting
from a microwave passing through the present pressure barriers.
[0071] FIG. 10G shows simulations of S-parameters S21 and S11 for a
pressure barrier similar to that shown in FIGS. 10A-10F, containing
two tilted windows with a length L between the windows. The x-axis
the frequency of the guided microwave in the pressure barrier in
GHz. The S21 parameter is very close to 100% in this example. The
S11 parameter in this example shows resonant behavior with more
resonance features (i.e., multiple overlapping inverted peaks) than
were observed in the single tilted window simulations (e.g., the
single dips shown in FIGS. 5B and 6B). A source of an additional
resonance feature in this example is the resonance between the two
windows. The windows in this example were spaced such that the
distance L between them was a multiple of n.lamda..sub.g/2 (as
described above).
[0072] In some embodiments, resonance features causing the spectral
dips in S11 can come from multiple sources including, but not
limited to, the resonance within a pressure barrier region
containing a window (e.g., a single thick or tilted window), the
resonance between two windows, or a resonance between any
reflecting surface in communication with a surface of a window in
the pressure barrier. For example, a resonant cavity can be
established within a thick window when the thickness is adjusted
accordingly (e.g., the thickness is a multiple of a half wavelength
of the guided loaded wavelength within the dielectric material of
the window). In some embodiments, the resonance within a window can
also be tailored (e.g., by changing the thickness of the window) to
further increase the total power transmitted from a source to a
load through the window(s) of the pressure barrier.
[0073] Reference has been made to embodiments of the disclosed
invention. Each example has been provided by way of explanation of
the present technology, not as a limitation of the present
technology. In fact, while the specification has been described in
detail with respect to specific embodiments of the invention, it
will be appreciated that those skilled in the art, upon attaining
an understanding of the foregoing, may readily conceive of
alterations to, variations of, and equivalents to these
embodiments. For instance, features illustrated or described as
part of one embodiment may be used with another embodiment to yield
a still further embodiment. Thus, it is intended that the present
subject matter covers all such modifications and variations within
the scope of the appended claims and their equivalents. These and
other modifications and variations to the present invention may be
practiced by those of ordinary skill in the art, without departing
from the scope of the present invention, which is more particularly
set forth in the appended claims. Furthermore, those of ordinary
skill in the art will appreciate that the foregoing description is
by way of example only and is not intended to limit the
invention.
* * * * *