RF Plasma Sources

and

How to Use HeliconsFrancis F. Chen

Professor Emeritus, UCLA

Semes Co., Ltd., Chungnam, Korea, February 15, 2012

Plasma is necessary for etching

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SHEATH

Three kinds of RF discharges

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• Capacitively Coupled Plasmas (CCPs), formerly called Reactive Ion Etchers (RIEs)

• Inductively Coupled Plasmas (ICPs)

• Helicon Wave Sources (HWS)

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Capacitive Discharges (CCPs)

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Schematic of a capacitive discharge

Plasma

Sheath

Sheath

Gas inlet

Gas outlet

Main RF

He coolant

Chuck

Bias RF

Powered electrode

Wafer

Grounded electrode

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Sheaths keep a plasma neutral

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Sheaths are very thin

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De

e

T eV

n 7 4

1018 3.( )

( )mmnumerically,

11 3 17 310 cm (10 m ), 4 eVe en T Let

Then 50 mD

Debye sheaths are approximately 5D thick

5 200 m = 0.2mmD

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The Child-Langmuir Law

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The Debye (normal) sheath has a voltage drop of about 5KTe.

If additional voltage is applied, a CL sheath forms with only ions.

ions and electrons

ions only

quasi-neutral

+ +++

d V 3/4

Most of the volume is sheath

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• Electrons are emitted by secondary emission ( and modes)

• Ionization mean free path is shorter than sheath thickness ()

• Ionization occurs in sheath, and electrons are accelerated

into the plasma (gamma mode)

• Why there is less oxide damage is not yet known

Large electrode

Small electrode

PLASMA

Sheath

Sheath

E

E

Effect of frequency on plasma density profiles

0,030 0,035 0,040 0,045

1013

1014

1015

1016

1017

45 mTorr 13.56 MHz 800 VC

on

cen

trat

ion

(m

-3)

r (m)

13.56 MHz

0,030 0,035 0,040 0,045

1013

1014

1015

1016

1017

45 mTorr 27 MHz 800 V

Co

nce

ntr

atio

n (

m-3)

r (m)

27 MHz

0,030 0,035 0,040 0,045

1013

1014

1015

1016

1017

45 mTorr 40 MHz 800 V

Co

nce

ntr

atio

n (

m-3)

r (m)

40 MHz

0,030 0,035 0,040 0,045

1013

1014

1015

1016

1017

45 mTorr 60 MHz 800 V

Co

nce

ntr

atio

n (

m-3)

r (m)

60 MHz

Plasma ApplicationModeling GroupPOSTECH

Problems with the original CCP discharges

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• The electrodes have to be inside the vacuum

• Changing the power changes both the density and the sheath drop

• Particulates tend to form and be trapped

• Densities are low relative to the power used

• In general, too few knobs to turn to control the ion and electron distributions and the plasma uniformity

Ion velocity distribution can be adjusted by applying low frequencies to substrate

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27 MHz

2 MHz

Thin gap. Unequal areas to increase sheath drop on wafer

High frequency controls plasma density

Low frequency controls ion motions and sheath drop

A LAM Exelan oxide etcher

Plasma ApplicationModeling GroupPOSTECH

CCPs are great for large gas feed

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GAS INLETS

HOLESRFGLASS SUBSTRATE

Fast and uniform gas feed for depositing amorphous silicon on very large glass substrates for displays (Applied Komatsu)

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Inductively Coupled Plasmas (ICPs)

Inductively Coupled Plasmas (ICPs)

The antenna can be on top or on the side

TCP (Transformer Coupled Plasma)PlasmaTherm etcher

Or it can be a combination

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Applied Materials patent

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RF B-field pattern comparison

-30

-20

-10

0

10

20

30

-30 -20 -10 0 10 20 30

Lam type AMAT type

-20

-15

-10

-5

0

5

10

15

20

-20 -15 -10 -5 0 5 10 15 20

Simulation of the plasma in an ICP

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Early AMAT system

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The electrostatic chuck is an essential part

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How can the RF energy get inside?

0

2

4

6

8

10

12

-5 0 5 10 15 20r (cm)

n (1

010

cm

-3)

800

240

200

Prf(W)3 mTorr, 1.9 MHz

The Plasma-Therm etcher

The density is high where RF is low!

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0

1

2

3

0 5 10 15r (cm)

n (1011 cm-3)

KTe (eV)

RF Bz field skin depth

Explanation 1: electron path with Lorentz force

0

180

360

540

720

900

1080

1260

RF phase(degrees)

Skin depth

with FL

without FL

dm e

d t

vE v B

F L UCLA

Electrons spend more time near center

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Explanation 2: Sheaths at endplates

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HIGH DENSITY

LOWER DENSITY

SHEATH

B

+

+

e

e

ION DIFFUSION+

-

E

(b)

1

2

The Simon short-circuit effect at endplates causes an electric field to

develop that drives the ions inwards, and the electrons can “follow”

even across magnetic fields.

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Disadvantages of stove-top antennas

• Skin depth limits RF field penetration. Density falls rapidly away from antenna

• If wafer is close to antenna, its coil structure is seen

• Large coils have transmission line effects

• Capacitive coupling at high-voltage ends of antenna

• Less than optimal use of RF energy

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Proposed enhancements of ICPs

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Coupling can be improved with a magnetic cover

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H H

H H

B B

E

H = J

B = H

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The ferrite is inside the vacuum

Meziani, Colpo, and Rossi, Plasma Sources Science and Technology 10, 276 (2001)

Fluxtrol F improves both RF field and uniformity(Meziani et al.)

0 1 2 3 4 5 6 7 8 9 10 11 120

1

2

3

4

5

6

7

700 W

100 W

700 W

100 W

w.o. magnetic pole (C1) w. magnetic pole (C2)

Br (

Ga

uss)

Irms

(A) 0 2 4 6 8 102

10

20

40Argon30 mtorr, 600 W

2 turn coil 2 turn coil + mag. pole Spiral MaPE

Ji (

mA

/cm

2 )

r (cm)

Magnetic material

2 loops in //

2 serpentines in //

3 loops in //

1 2 3 4 5 6 7 8 9

X (cm)

Y (cm)

110 mm 800 x 800 mm

750 x720 mm

Magnets are used in Korea (G.Y. Yeom)

SungKyunKwan Univ. KoreaSungKyunKwan Univ. Korea

Both RF field and density are increased

4006008001000120014001600180020002200

2.0x1010

4.0x1010

6.0x1010

8.0x1010

1.0x1011

1.2x1011

Without magnetic fields With magnetic fields

Ni (Ion Density /cm

3 )

RF power(Watts)

0 500 1000 1500 2000

100

200

300

400

500

600

700 Antenna type=serpentine(7m)Operating pressure=15mTorr

Vrm

s (V

olts

)

Input power (W)

No multipolar magnetic fields With multipolar magnetic fields

SungKyunKwan Univ. KoreaSungKyunKwan Univ. Korea

Serpentine antennas(suggested by Lieberman)

Plasma ApplicationModeling GroupPOSTECH

Magnets

Density uniformity in two directions

0 10 20 30 40100

150

200

250

300

350

400

450

500

550

600

1000W RF Input power 1500W RF Input power 2000W RF Input power

Ion

Satu

ratio

n C

urr

ent (1

0-6A

)

Position, Parallel to the antenna (cm)

-30 -20 -10 0 10 20 3050

100

150

200

250

300

350

400

450

500

Ion S

atu

ration

Curr

ent(

10

-6A

)

Probe Position (cm)

1000W RF Input Power 1500W RF Input Power 2000W RF Input Power

G.Y. Yeom, SKK Univ., Korea

Effect of wire spacing on density (calc.)

7.2cm

7.8cm

9cm

10.2cm

11.4cm

13.2cm

0

2 E + 0 1 0

4 E + 0 1 0

6 E + 0 1 0

8 E + 0 1 0

1 E + 0 1 1

1 E + 0 1 1

1 E + 0 1 1

Plasma ApplicationModeling GroupPOSTECH

Park, Cho, Lee, Lee, and Yeom, IEEE Trans. Plasma Sci. 31, 628 (2003)

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Helicon Wave Sources (HWS)

--

+ --

+

--

+ --

+

(a)

(b)

(c)

B

In helicon sources, an antenna launches wavesin a dc magnetic field

The RF field of these helical waves ionizes the gas.The ionization efficiency is much higher than in ICPs.

Why are helicon dischargessuch efficient ionizers?

Trivelpiece-Gould mode

Helicon mode

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 1 2 3 4

r (cm)

P(r

) (

/ cm

2 )

Parabolic Profile: R = 1.47 Ohms

Square Profile: R = 1.71 Ohms

The helicon wave couples to an edge

cyclotron mode, which is rapidly

absorbed.

A helicon discharge at Wisconsin

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A commercial helicon etcher (PMT MØRI)

It required two heavy electromagnets with opposite currents.

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Replace heavy electromagnet with small permanent magnet

and

Design of an array source with small tubes

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.

Antenna must be short so that a short tube can be used

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The low-field peak: constructive interference

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

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

The tube length is designed to maximize rf energy absorption

Final design of the discharge tube

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5.1 cm

10 cm

5 cmANTENNA

GAS INLET (optional)

Use the far-field of the magnet

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

The magnet position sets the B-field magnitude

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10 cm

5 cmANTENNA

Nd Permanent Magnet

7.6 cm

12.7 cm

A quartz tube with 3-turn antenna

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An 8-tube staggered array in operation

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The magnet tray

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The magnets are dangerous!

Array fed by a 50 water-cooled transmission line

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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.)

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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.

Conclusion

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• Helicon sources can produce high densities over large areas.

• Permanent-magnet helicon arrays are cheap and compact.

• Their design was made possible by extensive theory.

• To penetrate the industry, other gases must be tried.

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THE END

Thank you