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.