What’s special about helicon discharges?

-1

0

1

2

3

4

5

6

3.0 3.5 4.0 4.5 5.0 5.5 6.0

log (wp2/w2)

log

(c/

)

f = 13.56 MHz

n = 1E12 cm-3

B = 100 G

lower hybrid

Wci

wc

(wcW

c)1/2

Wpi

Density

B-f

ield

Helicon waves are whistler waves confined to a cylinder.Helicon discharges are made by exciting these waves.

The boundary has a large effect on theionization efficiency

Trivelpiece-Gould mode

Helicon mode

The H mode peaks at the center, but its currents or charges at the boundary mode-couples to an electron cyclotron wave (TG mode) at the edge. The TG wave is electrostatic and travels slowly inward, efficiently depositing the RF energy into the electrons.

k1k2

k0

k0 = helicon wave, k1 = ion acoustic wavek2 = Trivelpiece-Gould mode

This was verified experimentally.

0 1 2

0 1 2

0 1 2 1 20,

w w w

k k k

k k k k k

The deposition occurs via parametric instability

UCLA

As Prf is raised, the sidebands get larger due to

the growth of the LF wave.

Krämer et al. detected the ion wave B. Lorenz, M. Krämer, V.L. Selenin, and Yu.M. Aliev, Plasma Sources Sci.Technol. 14, 623 (2005)

Thus, helicon research links several disciplines

1. Low-temperature plasma physics

2. Space physics (whistler waves)

3. Magnetic fusion (B-field, RF power, Bohm diffusion)

4. Laser fusion (parametric instabilities)

A helicon discharge in a straight cylinder can produce densities up to 1014 cm3 with only 1-2 kW.

How can we use this dense source?

This is a commercial helicon source made by PMT, Inc. and successfully used to etch semiconductor wafers. It required two large and heavy electromagnets and their power supplies.

Computer chips are now etched with simpler sources without a DC B-field.

New applications require larger area coverage.

Possible uses of large-area plasma processing

Roll-to-roll plastic sheets

Smart windowsOLED displays

Solar cells, mass production Solar cells, advanced

Distributed helicon source: proof of principle

ROTATING PROBE ARRAY

PERMANENT MAGNETS

3"

DC MAGNET COIL

18"

Power scan at z = 7 cm, 5 mT A, 20 G, 13.56 MHz,

0.0

0.5

1.0

1.5

2.0

0 5 10 15 20 25 30R (cm)

N (

101

2 cm

-3) 3.0

2.5

2.0

1.5

1.0

P(kW)

7-tube m=0 array

ARGON

PROBE

Achieved n > 1.7 x 1012 cm-3, uniform to 3%, but large magnet is required.F.F. Chen, J.D. Evans, and G.R. Tynan, Plasma Sources Sci. Technol. 10, 236 (2001)

The problem with small magnets

-10

0

10

20

30

z (c

m)

QUARTZ TUBE

PVC PIPE

ANTENNA

MAGNET WINDING

7 cm

5 cm

13 cm

BNC connector

5 mm

17 mm

1 cm

1 cm

10 cm

Internal field

External field

Internal field

External fieldA small solenoid Field lines diverge

too rapidly

Annular permanent magnets have same

problem

However, the external field can be used

Note that the stagnation point is very close to the magnet

Place plasma in the external field, and eject downwards

Internal field

External field

Internal field

External field

Gate Valve

To Turbo Pump

34 cm

36 cm

D

Z1

Z2

-300

-250

-200

-150

-100

-50

0

50

100

150

0 5 10 15 20 25 30

z (cm)

Bz

(G)

Calculated

Measured

External field

Internal field

0

1

2

3

4

5

6

7

-5 0 5 10 15 20r (cm)

n (

101

0cm

-3)

Z2, 40

Z2, 35

Z2, 30

Z2, 21

Z2, 1

D (cm)

500W, 1 mTorr

The bottom curve is when the tube is INSIDE the magnet

PM helicons: proof of principle

Evolution of a multi-tube PM helicon source

1. Antenna design

2. Discharge tube geometry

3. Permanent magnets

4. RF circuitry

Next: construction and testing of Medusa 2

Medusa Medusa 1

Helicon m = 1 antennas

--

+

E

B

Only the RH polarized wave is strongly excited

Nagoya Type III antenna:symmetric, so RH wave is driven in both directions.

RH helical antenna:RH wave is driven only in the direction matching the antenna’s helicity.

This antenna has the highest coupling efficiency

Why we use an m = 0 antenna

A long antenna requires a long tube, and plasma goes to wall before it gets out.

An m = 0 loop antenna can generate plasma near the exit aperture. Note the “skirt” that minimizes eddy currents in the flange.

Now we have to design the diameter and length of the tube.

The low-field peak: an essential feature

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1E+11 1E+12 1E+13n (cm-3)

R (

oh

ms)

100.0

63.1

39.8

25.1

15.8

10.0

B(G) L=2", 1mTorr, conducting

Low-field peak

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1E+11 1E+12 1E+13n (cm-3)

R (

oh

ms)

100.0

63.1

39.8

25.1

15.8

10.0

B(G) L=2", 1mTorr, conducting

Low-field peak

The peak occurs when the backward wave is reflected to interfere constructively with the forward wave.

R is the plasma resistance, which determines the RF power absorbed by the plasma,

Designing the tube geometry

H

2a

CONDUCTING ORINSULATING ENDPLATE

1

Z

n

a k B

w

Adjust a, H, and wRF so that n and B are in desired range.

This is done with the HELIC codeD. Arnush, Phys. Plasmas 7, 3042 (2000).

a

b

c

Distant conducting shell

antenna

plasma

Lc

a b

h

Loop antenna

Helical antenna

B0

Lc is made very large to simulate injection into a processing chamber.

The code computes the wave fields and the plasma loading resistance Rp vs. n and B

Choose a peak at low B, mid 1012 cm-3 density

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1E+11 1E+12 1E+13n (cm-3)

R (

oh

ms)

100.0

63.1

39.8

25.1

15.8

10.0

B(G) L=2", 1mTorr, conducting

Low-field peak

0.0

0.5

1.0

1.5

2.0

2.5

1E+11 1E+12 1E+13n (cm-3)

R (

oh

ms)

1000

464

215

100

46

22

10

B (G) d = 3", H = 2", 13.56MHz

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1E+11 1E+12 1E+13n (cm-3)

R (

oh

ms)

d = 4 in.

d = 3 in.

d = 2 in.

100G, H = 2", 13.56 MHzTube diameter

0.0

0.5

1.0

1.5

2.0

2.5

1E+11 1E+12 1E+13n (cm-3)

R (

oh

ms)

H = 3 in.

H = 2 in.

H = 1 in.

100G, d = 3", 13.56 MHz

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1E+11 1E+12 1E+13n (cm-3)

R (

oh

ms)

f = 27.12 MHz

f = 13.56 MHz

f = 2 MHz

Typical R(n,B) curves at the low-field peak

Vary the B-field Vary the tube length

Vary the tube diameter Vary the RF frequency

Final tube design for 13.56 MHz

5.1 cm

10 cm

5 cmANTENNA

GAS INLET (optional)

Material: Pyrex or quartzWith aluminum top

Reason for maximizing Rp: circuit loss Rc

pin rf

p c

RP P

R R

: pp c in rf p

c

RR R P P R

R

:p c in rfR R P P

10

100

1000

1E+11 1E+12 1E+13n0 (cm-3)

Pin

(W

)

1000

500

200

100

Loss

Prf (W)

No helicon ignition

Unstable equilibrium

Stable equilibrium

Rc = 1.0 W

10

100

1000

1E+11 1E+12 1E+13n0 (cm-3)

Pin

(W

)

1000

500

200

100

Loss

Prf (W)

Stable equilibria

Rc = 0.1 W

Magnet design for 60-100 G

Vary the outside diameter

Vary the vertical spacing

Final magnet design

12.7 cm

7.6 cm

PLASMA

NdFeB material, 3”x 5”x1” thickBmax = 12 kG

-16

-14

-12

-10

-8

-6

-4

-2

0

2

4

6

8

-10 -8 -6 -4 -2 0 2 4 6 8 10

0

50

100

150

200

250

300

0 2 4 6 8 10 12z (in.)

Bz (G

)

0.0

0.52

0.92

r (in.)

D

RF circuitry

R, L

R, L

R, L

R, L

PS

N loads

Z2 - short cables

Distributor

Z1Z2

Z1 - long cable

C1C2

Matching ckt. 50W

For equal power distribution, the sources are connected in parallel with equal cable lengths. The problem is that the cable lengths, therefore, cannot be short.

The length Z2 and the antenna inductance L are the most critical.

C1, C2 for N=8, L = 0.8H, Z1 = 110 cm, Z2 = 90 cm(unless varied)

0

200

400

600

800

1000

1200

1400

1600

0 50 100 150 200Z2 (cm)

C (

pF)

C1(S)

C2(S)

0

200

400

600

800

1000

1200

1400

1600

0 0.5 1 1.5 2 2.5 3L (uH)

C (

pF)

C1(S)

C2(S)

Allowable values of C1, C2 in match circuit

There is an upper limit to each antenna’s inductance L.

The range of Z2 can be restrictive for large arrays

Current and voltage in CW operation

Coax connectors cannot take the startup voltage or the CW current. All joints have to be soldered or have large contact area.

Junction box Connection to water-cooled antenna

A low-Rc, 50-Wcooled, rectangular transmission line

Layout of 8-tube test module, Medusa 2

165 cm

53.3 cm

17.8

17.8

17.8

17.8 cm73.7 cm

8.9 cm

x

y

Compact configurationStaggered configuration

The spacing is determined from the single-tube density profiles to give 2% uniformity

Side view

165 cm

30 cm

15 cm

Probe ports

Aluminum sheet

Adjustable height

The source requires only 6” of vertical space above the process chamber

Z1

Z2

Wooden frame for safety and movability

Medusa 2 in operation at 3 kW CW

Radial profile between tubes at Z2

0

0.5

1

1.5

2

2.5

3

3.5

-25 -20 -15 -10 -5 0 5 10 15 20 25r (cm)

n (1

01

1 c

m-3

) n

KTe

UCLA

0 3.5”

Compact configuration, 3kW

Side Langmuir probe

Density profiles across the chamber

<< 4” below tubes

<< 7” below tubes

0

2

4

6

8

10

-8 -6 -4 -2 0 2 4 6 8y (in)

n (1

01

1cm

-3)

Z1, x = 0

Z1, x = 3.5

Z2, x = 0

Z2, x = 3.5

Compact 3kW, D=7", 20mTorr

UCLA

Density profiles across the chamber

0 7-7 14”

Staggered configuration, 3kW

Bottom probe array

0

1

2

3

4

5

-8 -6 -4 -2 0 2 4 6 8y (in.)

n (

10

11 c

m-3

)

-7

07

14

x (in.)Staggered3kW, D=7",

20mTorr

Density profiles along the chamber

Staggered configuration, 2kW

Bottom probe array

0

1

2

3

4

5

-8 -6 -4 -2 0 2 4 6 8 10 12 14 16x (in.)

n (1

011 c

m-3

)

-3.5

0

3.5

Staggered, 2kW, D=7", 20mTorr

y (in.)

UCLA

Density profiles along the chamber

Compact configuration, 3kW

Bottom probe array

0

2

4

6

8

10

-8 -6 -4 -2 0 2 4 6 8 10 12 14 16

x (in.)

n (

10

11 c

m-3

)

3.5-03.5

Compact, 3kW, D=7", 20mTorr

y (in)

Data by Humberto Torreblanca, Ph.D. thesis, UCLA, 2008.

CONCLUSIONS

We’ve found a sweet spot where the tube, the antenna, the magnet, and the matching circuit can all work together.

There’s a large step between laboratory physics and a practical device.