commercial helicon sources inject plasma into a field-free region
Post on 09-Feb-2016
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Commercial helicon sources inject plasmainto a field-free region
To pump
PROBE 1PROBE 2
The MORI source A helicon injection expt.
We wish to optimize a source which could be part of a large array
ELECTROSTATIC CHUCK
WAFER
MULTI-TUBE HELICON PLASMA SOURCE
PE
RM
AN
EN
T M
AG
NE
T A
RR
AY
Conceptual RF plasma source for etching and depostion of semiconductor wafers and flat-panel display substrates.
Metal or ceramic endplate
Arbitrary antenna
We can use a low-field density peak seen in at B 30G for n 1012 cm-3
The low-B peak has been seen in many different experiments
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 50 100 150 200B (G)
n (1
013 c
m-3
)
Uniform B4 mTorr Ar2.2kW @ 27.12MHz2cm diameter
The field at the peak can range between 10 and 50G, and the peak is much larger in some experiments than in others.
1989, 1" tube
The low-B peak is also seen in a 7-tube array of m = 0 sources
0.0
0.6
1.2
1.8
0 20 40 60 80 100 120 140B (G)
n (1
011 c
m-3
)
200135115857045
Prf (W)
8 mTorr, 13.56 MHz
Computations using Don Arnush's HELIC code predict this peak and show how it varies
a
b
c
Distantconducting shell
antenna
plasma
LLc
dEndplates havearbitrary reflectivity
HELIC calculates waves and loading with• Radial density profile n(r)• Collisional damping• Different (thin) antenna configurations• Antenna coupling• Trivelpiece-Gould modes• Endplates (arbitrary reflectivity)
It requires• Uniform B-field• Uniform n(z)• Cold-plasma dielectric
The plasma loading R() shows a low-B peak which moves with density
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 50 100 150 200B (G)
R (o
hms)
2E+114E+116E+118E+111E+12
n (cm-3)
1011-1012 range
Single-loop m = 0 antenna, 10 cm from endplate, 2 mTorr, Te = 4eV
At high densities, the low-B peak is gone
1012-1013 range
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 50 100 150 200B (G)
R (o
hms)
1.0E+121.5E+122.0E+124.0E+126.0E+1210 E+12
n (cm-3)
Single-loop m = 0 antenna, 10 cm from endplate, 2 mTorr, Te = 4eV
The low-B peak vs. density at constant B
0.0
0.5
1.0
1.5
2.0
2.5
3.0
1E+11 1E+12 1E+13n (cm-3)
R (o
hms)
3050100150200300
B (G)
Single-loop m = 0 antenna, 10 cm from endplate, 2 mTorr, 13.56 MHz
What is the cause of the low-B peak?
• It is not the lower hybrid resonance (1450 G)
• Is it due to a resonance between the helicon mode and the Trivelpiece-Gould mode, when they have comparable radial wavelengths and can interfere constructively?
• Is it due to constructive interference by the wave reflected from the endplate?
Computations show that it is probably the latter.
"Standard" conditions for numerical tests
200 cm
10 12
10Single loop
Half helical
0.0
0.2
0.4
0.6
0.8
1.0
1.2
-1.0 -0.5 0.0 0.5 1.0r / a
n / n
0
B = 10-300 Gno = 1012 cm-3
p = 10 mTorr ArInsulating endplateKTe = 4 eV (affects
collisions only)
f = 13.56 MHz
Radial density profile
The low-B peak sharpens at lower pressure
0
1
2
3
4
5
6
0 50 100 150 200 250 300
B (G)
R (o
hms)
2mTorr10mTorr
p
The peak is sensitive to the density profile
0
1
2
3
4
5
6
0 50 100 150 200 250 300
B (G)
R (o
hms)
UniformFlat with rolloffParabolic
The peak depends on the boundary condition
0
1
2
3
4
5
6
7
8
0 50 100 150 200 250 300
B (G)
R (o
hms)
InsulatingConducting
The peak depends on distance from endplate
0
1
2
3
4
5
6
0 50 100 150 200 250 300
B (G)
R (o
hms)
5 cm10 cmNo bdy
d
The peak depends on the type of antenna
0
1
2
3
4
5
6
0 50 100 150 200 250 300
B (G)
R (o
hms)
Loop, d = 10 cmHH10, d = 10 cmNagoya III
Single loop: m = 0, bidirectionalHH (half-wavelength helical): m = 1, undirectionalNagoya Type III: m = 1, bidirectional
Axial deposition profile of HH10 antenna depends on the sign of the helicity
0.0
0.5
1.0
1.5
2.0
2.5
-1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2z (m)
P(z
)
m = -1m = +1
R = 1.085
R = 0.739
Direction of propagation: (m = +1), (m = -1)
This explains previous data on density enhancement by an aperture limiter
TO PUMP
Block can be solid or have a 1.2-cm diam hole.It can be conducting (carbon) or insulating (boron nitride).It can be moved to various positions behind or under the antenna.
Radial density profile shows enhancement whenever there is a limiter
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0r (cm)
n (1
013 c
m-3
)
C limiterNo holeBN limiterNo limiter
Uniform field
At 800G, the limiter position is not critical
0
2
4
6
-2 -1 0 1 2r (cm)
n (a
rb. u
nits
)
-4-6-8-12-18-28None
z (cm)800 G, 1.8kW, 8mTorr
Carbon block with 1.2 cm diam holez cm behind antenna midplane
The end coils can also be turned off or reversed to form a cusped B-field
to pump
END COILS
The field lines then end on the glass tube, which forms an insulting endplate. An aperture limiter can also be added.
The cusp configuration doubles the amount of plasma created
0.0
0.4
0.8
1.2
1.6
-3 -2 -1 0 1 2 3r (cm)
n
End coils reversedEnd coils offUniform B-field
B = 560G1.8kW@27.12MHz
4mTorr Ar 4 cm diameter
2.23 X
1 X
1.95 X
Integrated density enhancement
Limiter enhancement with and without a magnetic cusp
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0r (cm)
n (1
013 c
m-3
)
C limiterNo holeBN limiterNo limiter
Uniform field
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0r (cm)
n (1
013 c
m-3
)
C lim.BN lim.No lim.
Cusp field
The cusp field greatly enhances the density without a limiter, but adds little when a limiter is already in place.
Density increase with end coil current is larger with a bidirectional antenna
0.0
0.4
0.8
1.2
1.6
2.0
-40 -20 0 20 40End coil voltage
n (1
013 c
m-3
)
Right helicalNagoya III
Antenna
Cusp field Uniform field
CONCLUSION
For low-field, low-density helicon injection into a large chamber, reflection of waves from an endplate can be designed to optimize plasma production. This phenomenon is probably responsible for previously unexplained density increases with aperture limiters and cusped magnetic fields . This is a significant effect.
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