US2007074811A1PendingUtilityA1
Method and apparatus for measuring plasma density in processing reactors using a long dielectric tube
Est. expirySep 30, 2025(expired)· nominal 20-yr term from priority
H05H 1/0081H01J 37/32935
36
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Claims
Abstract
An apparatus for measuring plasma density of a plasma processing reactor, comprises a probe having a dielectric tube with a coaxial cable inserted therein. The coaxial cable has an open antenna tip, distance constancy is kept between the antenna tip and the dielectric tube despite varying thermal conditions. The probe can be utilized to determine resonant plasma frequency near its tip location and the corresponding plasma density.
Claims
exact text as granted — not AI-modified1 . An apparatus for measuring plasma density in a plasma-processing reactor, comprising:
a probe comprising a coaxial cable inserted into a closed dielectric tube and having an open metal antenna tip; a coaxial cable connected to the probe; a network analyzer connected to the probe through the coaxial cable, supplying a high-frequency signal to the probe and measuring the intensity of the reflected signal; and a high-pass filter located between the coaxial cable or the probe and the network analyzer to cut off strong low frequency signals; wherein distance constancy is kept between the antenna tip and the dielectric tube; and plasma density can be measured in a chemically active environment.
2 . The apparatus of claim 1 , wherein the high frequency signal is in the range of 0.5-5 GHz.
3 . The apparatus of claim 1 , wherein the antenna tip is a straight naked metal wire at least a few millimeters long representing the center electrode of the coaxial cable stripped of isolation and metal screening.
4 . The apparatus of claim 3 , wherein an end of the antenna tip does not touch an inner end of the dielectric tube, and there is a space of at least a few millimeters between them.
5 . The apparatus of claim 4 , wherein a space between the antenna tip end and the inner end of the dielectric tube is maintained constant.
6 . The apparatus of claim 5 , wherein space is maintained constant, in spite of possible thermal expansion of the coaxial cable.
7 . The apparatus of claim 6 , further comprising a dielectric spacer disposed inside the main dielectric tube and placed around the antenna tip so that the space is maintained constant.
8 . The apparatus of claim 7 , wherein the dielectric spacer includes a dielectric tube with an inner radius approximately equal to the radius of the antenna tip to increase constancy of the antenna tip shape under varying thermal conditions.
9 . The apparatus of claim 1 , wherein various resonances in the reflected signal are interpreted based on surface wave modes.
10 . The apparatus of claim 9 , wherein resonances are mapped with corresponding plasma density values.
11 . The apparatus of claim 10 , wherein resonance modes are selected from the measured absorption resonances, wherein the selected resonance modes are those modes that are the strongest and also provide information about local values of the plasma density around the antenna tip.
12 . The apparatus of claim 1 , wherein the dielectric tube is made of material selected to have a dielectric property used in correspondence with an expected plasma density range to produce a resonance in a frequency range of the network analyzer.
13 . The apparatus of claim 12 , wherein a material with higher dielectric permittivity is chosen for the dielectric tube for measurements in a higher plasma density range to maintain the resonant frequency below about 5 GHz.
14 . The apparatus of claim 1 , wherein the coaxial cable has a smaller radius than the inner radius of the dielectric tube, and a spacer ring is provided at the end of the coaxial cable between the radius of the coaxial cable and the inner radius of the dielectric tube, the ring providing more sharply emphasized boundary conditions for the surface wave reflection, making absorption resonances more pronounced.
15 . The apparatus of claim 1 , wherein the coaxial cable has a smaller radius than the inner radius of the dielectric tube, and there is at least one dielectric ring or short dielectric tube along the coaxial cable, surrounding the coaxial cable, and located inside the dielectric tube, diminishing the amplitude of parasitic surface waves running along the dielectric tube, which otherwise might interfere with the main absorption resonances used for measurements.
16 . The apparatus of claim 7 , wherein the dielectric spacer is metal.
17 . An apparatus for determining plasma density of plasma in a plasma processing reactor, comprising:
a probe comprising a long dielectric tube and a coaxial cable inserted in the dielectric tube, the coaxial cable having an open antenna tip; wherein distance constancy is kept between the antenna tip and the dielectric tube despite varying thermal conditions.
18 . The apparatus of claim 17 , wherein distance constancy is also kept between the coaxial cable and the dielectric tube despite varying thermal conditions.
19 . The apparatus of claim 17 , wherein plasma density is measured in a chemically active environment.
20 . The apparatus of claim 17 , further comprising a spacer proximate the antenna tip to keep the distance constancy.
21 . The apparatus of claim 18 , further comprising:
a spacer proximate the antenna tip to keep the distance constancy between the antenna tip and the dielectric tube; and a spacer proximate the coaxial cable to keep the distance constancy between the coaxial cable and the dielectric tube.
22 . The apparatus of claim 17 , further comprising a dielectric tube spacer, an inner radius of the dielectric tube spacer being equal to or about the radius of the antenna tip, the tube spacer extending from a portion of the coaxial cable from which the antenna tip extends to the dielectric tube in the direction of the antenna tip to keep the distance constancy.
23 . The apparatus of claim 18 , further comprising:
a dielectric spacer; a metal spacer; or any combination thereof, disposed between the coaxial cable and the dielectric tube, to keep the distance constancy between the coaxial cable and the dielectric tube.
24 . The apparatus of claim 18 , further comprising a ring spacer disposed between the coaxial cable and the dielectric tube to keep the distance constancy between the coaxial cable and the dielectric tube.
25 . The apparatus of claim 17 , wherein the antenna tip is a naked metal wire representing a center electrode of the coaxial cable without isolation and/or metal screening.
26 . The apparatus of claim 17 , wherein the antenna tip is straight.
27 . The apparatus of claim 17 , wherein the antenna tip is not straight.
28 . The apparatus of claim 27 , wherein the antenna tip bent in one direction.
29 . The apparatus of claim 17 , wherein the antenna tip is bent in the shape of a partial loop.
30 . The apparatus of claim 17 , wherein the shape of the antenna tip stays constant under varying thermal conditions.
31 . The apparatus of claim 17 , wherein a material for the dielectric tube corresponds to an expected plasma resonant frequency.
32 . The apparatus of claim 17 , wherein the dielectric tube is made of a material with a high dielectric permittivity when a high plasma density is expected to keep the resonant frequency under 3 GHz.
33 . The apparatus of claim 32 , wherein the dielectric permittivity is selected so a plasma resonant frequency falls in a pre-determined range of values.
34 . The apparatus of claim 17 , wherein the distance constancy kept between the antenna tip and the dielectric tube provides more sharply emphasized boundary conditions for surface wave reflection, making absorption resonances more pronounced.
35 . The apparatus of claim 17 , farther comprising a base coupled to the dielectric tube.
36 . The apparatus of claim 35 , wherein the base is made of dielectric material.
37 . The apparatus of claim 17 , further comprising a network analyzer coupled to the probe through the coaxial cable.
38 . The apparatus of claim 37 , wherein the network analyzer supplies a high-frequency signal to the probe and measures intensity of a reflected signal.
39 . The apparatus of claim 38 , further comprising a high-pass filter located between and the probe and the network analyzer, the high-pass filter reducing low frequency signals.
40 . The apparatus of claim 17 , wherein the distance constancy is selected so as to diminish the amplitudes of parasitic surface waves running along the dielectric tube, which otherwise might interfere with main absorption resonances used for measurements.
41 . The apparatus of claim 17 , wherein the plasma density is determined around at least one probe, and the plasma density at other locations in the plasma processing reactor is determined based on the determined plasma density around the at least one probe and a model of relative plasma densities in the plasma processing reactor.
42 . A method for determining plasma density of plasma in a plasma processing reactor, comprising:
utilizing a probe configured and arranged so that resonancy of a chosen mode is maximized, the probe comprising a dielectric tube and a coaxial cable inserted therein, the coaxial cable having an open antenna tip, wherein distance constancy is kept between the antenna tip and the dielectric tube despite varying thermal conditions; determining a resonant frequency or wavelength of the plasma in the plasma processing unit in the chosen mode; and determining the dielectric permittivity of the plasma using the resonant frequency or wavelength; and determining the density of the plasma using the resonant frequency or wavelength.
43 . The method of claim 42 , wherein determining the resonant frequency or wavelength of the plasma in the plasma processing unit comprises:
providing radio frequency signals to the probe antenna tip; receiving back reflected radio frequency signals which carry a plasma wave resonance signature; reducing strong low frequency signals; determining the resonant frequency or wavelength of the plasma waves.
44 . The method of claim 42 , wherein various resonances in the reflected signal are interpreted based on surface wave modes.
45 . The method of claim 44 , wherein resonances are mapped with corresponding plasma density values.
46 . The method of claim 45 , wherein resonance modes are selected from the measured absorption resonances, wherein the selected resonance modes are those modes that are the strongest and also provide information about local values of the plasma density around the antenna tip.
47 . The method of claim 45 , wherein the dielectric permittivity of the plasma is determined as described in the body of this patent using the dispersion relation:
D
(
ω
,
k
z
,
m
)
=
ɛ
p
-
ɛ
d
·
K
m
(
k
z
a
)
K
m
′
(
k
z
a
)
·
α
I
m
′
(
k
z
a
)
+
β
K
m
′
(
k
z
a
)
α
I
m
(
k
z
a
)
+
β
K
m
(
k
z
a
)
=
0
where:
ω=2πƒ (where ƒ is a wave frequency)
k z =2π/λ (where k z is a longitudinal wave vector; λ is a longitudinal wavelength)
m=azimuthal mode number ε d =dielectric permittivity of the dielectric tube 115
a=external radius of dielectric tube 115
b=internal radius of dielectric tube 115
I m =modified Bessel function of first kind of order m
K m =modified Bessel function of second kind of order m
I′ m and K′ m are derivatives, respectively, for I m and K m .
and
α
=
1
ɛ
d
·
sK
m
(
k
z
b
)
-
p
ɛ
d
K
m
′
(
k
z
b
)
K
m
(
k
z
b
)
I
m
′
(
k
z
b
)
-
I
m
(
k
z
b
)
K
m
′
(
k
z
b
)
β
=
1
ɛ
d
·
p
ɛ
d
I
m
′
(
k
z
b
)
-
sI
m
(
k
z
b
)
K
m
(
k
z
b
)
I
m
′
(
k
z
b
)
-
I
m
(
k
z
b
)
K
m
′
(
k
z
b
)
where r a =radius of antenna tip
48 . The method of claim 42 , wherein various resonances in the reflected signal are interpreted based on surface wave modes.
49 . The method of claim 42 , wherein resonances are mapped with corresponding plasma density values.
50 . The method of claim 42 , wherein resonance modes are selected from measured absorption resonances, wherein the selected resonance modes are those modes that are the strongest and also provide information about local values of the plasma density around the antenna tip.
51 . The method of claim 42 , wherein the dielectric tube is made of material selected to have a dielectric property used in correspondence with an expected plasma density range to produce a resonance in a desired frequency range.
52 . The method of claim 42 , wherein a material with higher dielectric permittivity is chosen for measurements in a higher plasma density range.
53 . The method of claim 42 , wherein the coaxial cable has a smaller radius than the inner radius of the dielectric tube, and a spacer ring is provided at the end of the coaxial cable between the radius of the coaxial cable and the inner radius of the dielectric tube, the ring providing more sharply emphasized boundary conditions for the surface wave reflection, making absorption resonances more pronounced.
54 . The method of claim 42 , wherein the coaxial cable has a smaller radius than the inner radius of the dielectric tube, and there is at least one dielectric ring or short dielectric tube along the coaxial cable, surrounding the coaxial cable, and located inside the dielectric tube, diminishing the amplitude of parasitic surface waves running along the dielectric tube, which otherwise might interfere with the main absorption resonances used for measurements.
55 . The method of claim 54 , wherein the dielectric spacer is metal.
56 . The method of claim 44 , wherein resonances are mapped with corresponding plasma density values.
57 . The method of claim 44 , wherein resonance modes are selected from the measured absorption resonances, wherein the selected resonance modes are those modes that are the strongest and also provide information about local values of the plasma density around the antenna tip.
58 . The apparatus of claim 17 , wherein the probe is slidable through a wall of the plasma processing reactor.
59 . The apparatus of claim 17 , further comprising a spring coupled to the coaxial cable to bias the coaxial cable into the dielectric tube to maintain the relative positions of the antenna tip and the dielectric tip.
60 . The method of claim 42 , further comprising sliding the probe through a wall in the plasma processing reactor so that the antenna tip is positioned at a desired position within the plasma processing reactor.
61 . The method of claim 42 , further comprising biasing the coaxial cable into the dielectric tube to maintain the relative positions of the antenna tip and the dielectric tip.
62 . The apparatus of claim 1 , wherein the probe is slidable through a wall of the plasma processing reactor.
63 . The apparatus of claim 1 , further comprising a spring coupled to the coaxial cable to bias the coaxial cable into the dielectric tube to maintain the relative positions of the antenna tip and the dielectric tip.
64 . The apparatus of claim 1 , wherein the dielectric tube is of a shape which limits cable expansion and helps provide a relatively constant distance between the antenna tip and the dielectric tube.
65 . The apparatus of claim 17 , wherein the dielectric tube is of a shape which limits cable expansion and helps provide a relatively constant distance between the antenna tip and the dielectric tube.
66 . The method of claim 42 , wherein the dielectric tube is of a shape which limits cable expansion and helps provide a relatively constant distance between the antenna tip and the dielectric tube.
67 . The apparatus of claim 64 , wherein the corner of the coaxial cable abuts against the dielectric tube, and the antenna tip extends into a portion of the dielectric tube with reduced diameter.
68 . The apparatus of claim 65 , wherein the corner of the coaxial cable abuts against the dielectric tube, and the antenna tip extends into a portion of the dielectric tube with reduced diameter.
69 . The method of claim 66 , wherein the corner of the coaxial cable abuts against the dielectric tube, and the antenna tip extends into a portion of the dielectric tube with reduced diameter.Join the waitlist — get patent alerts
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