Types of RF plasma sources

Types of RF plasma sources

• Old RIE parallel plate etcher (GEC reference cell)

• Inductively coupled plasmas (ICPs)

• New dual frequency capacitively coupled plasmas (CCPs)

• Helicon wave sources (HWS)

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Schematic of a capacitive discharge

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Plasma

Sheath

Sheath

Gas inlet

Gas outlet

Main RF

He coolant

Chuck

Bias RF

Powered electrode

Wafer

Grounded electrode

The GEC Reference Cell

In the early days of plasma processing, the Gaseous Electronics Conference standardized a capacitive discharge for 4-inch wafers, so that measurements by different groups could be compared.

Brake et al., Phys. Plasmas 6, 2307 (1999)

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Problems with the original RIE discharge

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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

Dual-frequency CCPs are better

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W. Tsai et al., JVSTB 14, 3276 (1996)

One advantage of a capacitive discharge

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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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Inductive coupling: The original TCP patent

US Patent 4,948,458, Ogle, Lam Research, 1990

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The Lam TCP (Transformer Coupled Plasma)

UCLASimulation by Mark Kushner

Top and side antenna types

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US Patent 4,948,458, Fairbairn, AMAT, 1993

Applied Materials' DPS (Decoupled Plasma Source)

UCLAUS Patent 4,948,458, Fairbairn, AMAT, 1993

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What the DPS looks like

Outside

Inside

Other antennas in AMAT patent

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US Patent 4,948,458, Fairbairn, AMAT, 1993

B-field pattern comparison (1)

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-30

-20

-10

0

10

20

30

-30 -20 -10 0 10 20 30

-30

-20

-10

0

10

20

30

-30 -20 -10 0 10 20 30

Horizontal strips Vertical strips


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-30

-20

-10

0

10

20

30

-30 -20 -10 0 10 20 30

-30

-20

-10

0

10

20

30

-30 -20 -10 0 10 20 30

3 close coils 2 separate coils


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-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

How do ICPs really work?

0

z (c

m)

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

In MEMs etcher by Plasma-Therm (now Unaxis), density is uniform well outside skin depth

UCLAIn the plane of the antenna, the density peaks well

outside the classical skin layer

0

1

2

3

0 5 10 15r (cm)

n (1011 cm-3)

KTe (eV)

RF Bz field skin depth

Data by John Evans

Anomalous skin effect (thermal motions)

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x

x

x

x

x

Jo

J

B

skin wall

antenna

E.g., Kolobov and Economou, Plasma Sources Sci. Technol. 6, R1 (1997).Most references neglect collisions and curvature.

Nonlinear effects have been observed

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Collisionless power absorption

(Godyak et al., Phys. Rev. Lett. 80, 3264 (1998)

Second harmonic currents

Smolyakov et al., Phys. Plasmas 10, 2108 (2003)

Ponderomotive force

Godyak et al., Plasma Sources Sci. Technol.

10, 459 (2001)

Electron trajectories are greatly affected by the nonlinear 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

Without FL, electrons are fast only in skin

0

20

40

60

80

0 360 720 1080 1440Phase (degrees)

E (

eV)

with V x B

no V x B

Argon ionization threshold

Reason: The radial FL causes electrons to bounce off the sheath at more than a glancing angle.

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Electrons spend more time near center

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UCLA

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 5 10 15r (cm)

Re

lativ

e d

en

sity

Density profile in four sectors of equal area

Points are data from Slide 5

Disadvantages of stove-top antennas

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• 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


UCLA

-30

-20

-10

0

10

20

30

-30 -20 -10 0 10 20 30

-30

-20

-10

0

10

20

30

-30 -20 -10 0 10 20 30

3 close coils 2 separate coils

Coupling can be improved with magnetic cover

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H H

H H

B B

E

H = J

B = H

Four configurations tested

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

The dielectric is inside the vacuum

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

Iron 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

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


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

Godyak: All RF lamps use iron cores

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Philips QL Lamp: 2.65 MHz, 85W (equiv. to 350W lamp)






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A LAM Exelan oxide etcher


A dual-frequency CCP

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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

Most of volume is sheath

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• Electrons are emitted by secondary emission

• Ionization mean free path is shorter than sheath thickness

• Ionization occurs in sheath, and electrons are accelerated into the plasma

• Why there is less oxide damage is not yet known

Large electrode

Small electrode

PLASMA

Sheath

Sheath

E

E

(a) (b)

(c) (d)

0 20 40 600

10

20

30

40

50

60

70

Pla

sma

den

sity

(p

eak

valu

e), (

1016

m-3)

Frequency (MHz)

0 20 40 60

0,2

0,4

0,6

0,8

1,0

Sh

eah

wid

th (

cm)

Frequency (MHz)

The density increases with frequency squared

Density Debye length

Reason: The rf power is I2R, where I is the electron current escaping through the sheath. Since one bunch of electrons is let through in each rf cycle, <Irf> is proportional to .


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


Co

nce

ntr

atio

n (

m-3)

r (m)

40 MHz

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

1013

1014

1015

1016

1017


Co

nce

ntr

atio

n (

m-3)

r (m)

60 MHz


Effect of frequency on IEDF at the smaller electrode

13.56 MHz27 MHz

40 MHz 60 MHz

(a) (b)

(c) (d)


IEDF at Wall – Pressure Variation

10 mTorr10 mTorr 20 mTorr20 mTorr

50 mTorr50 mTorr30 mTorr30 mTorr







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A helicon source requires a DC magnetic field..

U. Wisconsin

...and is based on launching a circularly polarized wave in the plasma

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+

_+

_ _

+

+ +

_ _

_

B

k

(a)

(b)

(c)

+

• Much higher density at given power than ICPs

• Density peak occurs downstream from the antenna

• Magnetic field provides adjustment for uniform density

Axial density and temperature profiles

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Density increases greatly as B-field is added.

The density peak is detached from the source.

Two commercial helicon reactors

UCLAThe Boswell source

The PMT (Trikon) MØRI source

The Coil Current Ratio shapes the plasma

UCLAThe MØRI source

How do helicon source really work?

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A cyclotron (TG) wave at the surface rapidly damps the RF energy

0

1000

2000

3000

4000

5000

0.00 0.01 0.02 0.03 0.04 0.05r (m)

P(r

)

0

2

4

6

8

10

12

14

-6 -4 -2 0 2 4 6r (cm)

J z (a

rb. u

nits

)

4.2

4.2

no TG

n (1011)

40G(b)

Typical radial deposition profile

Direct detection of the TG peak in the RF current

There are actually 2 types of helicon discharges

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The Big Blue Mode The Low Field Peak

B > 800G, n > 1013 cm-3

Due to an neutral depletion instability

No important application yet

Low density, low B-fieldIdeal for plasma processing

Reflection from end causes the L.F. peak

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A 7-tube array of stubby helicon sources

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UCLA

ROT AT ING PROBE ARRAY

PERMANENT MAGNETS

3"

DC MAGNET COIL

18"

Gives good uniformity and high density

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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

2-D density scans show no m = 6 asymmetry

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-20 -10 0 10 20-20

-10

0

10

20

Helicon tools have been modeled

MØRI tool: Kinder and Kushner, JVSTA 19, 76 (2001)

TG mode is seen

Bose, Govindan, and Meyyappan, IEEE Trans. Plasma Sci. 31, 464 (2003)

Power deposition

Plasma density

What next for RF sources?

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• Control of KTe, species production, ion velocities

— Electron filtering, pulsed plasmas, gas feed and pumping, additive gases to absorb electron groups, shaped bias voltage, electronegative optimization. etc.

• Understanding and eliminating oxide damage

• Large area sources for FPDs, not wafers

• Eventual widespread adoption of helicon sources

Documents

Types of RF plasma sources