MULTI-FREQUENCY OPERATION OF RIE AND ICP...

Optical & Discharge PhysicsUniversity of Illinois

AVS98_P1_F1

45th International Symposium of the American Vacuum SocietyBaltimore, MarylandNovember 2-6, 1998.

MULTI-FREQUENCY OPERATION OF RIE AND ICP SOURCES*

Shahid Rauf and Mark J. KushnerDepartment of Electrical & Computer Engineering

University of Illinois, Urbana-Champaign

*Work supported by AFOSR/DARPA, SRC and NSF.

AGENDA


AVS98_P1_F2

• Introduction

• Computational Model

• Asymmetric Capacitively Coupled Discharge

• Electrode Currents and Voltages

• Effect of Frequency, Voltage Waveform and Source Interaction

• Inductively Coupled Plasma

• Electrode Currents and Voltages

• Effect of Frequency and Source Interaction

• Conclusions

INTRODUCTION


AVS98_P1_F3

• Source frequency has a strong influence on plasma characteristics in radio frequency (rf) discharges.

• Multiple sources at different frequencies are often simultaneously used to separately optimize the magnitude and energy of ion fluxes to the substrate.

• The sources can, however, nonlinearly interact if the frequencies are sufficiently close, and resulting plasma and electrical characteristics can be different than due to individual sources.

• A plasma equipment model has been used to investigate the interaction of multiple frequency sources in both capacitively and inductively coupled discharges.

• In this talk, we discuss the effect of frequency on plasma and electrical characteristics, and describe the consequences of source interactions.

THE COMPUTATIONAL MODEL


AVS98_P1_F4

The Hybrid PlasmaEquipment Model

s(r,z), I (coils), J(r,z,φ)N(r,z) Es(r,z,φ)

S(r,z),Te(r,z),

µ(r,z)

Eθ(r,z,φ)

B(r,z,φ)ELECTRO-MAGNETICS

MODULE

CIRCUITMODULE

I,V (coils) Eθ

ELECTRONMONTECARLO

SIMULATION

ELECTRONENERGY

EQN./BOLTZMANN

MODULE

NON-COLLISIONAL

HEATING

FLUID-KINETICS

SIMULATION

HYDRO-DYNAMICSMODULE

CIRCUITMODEL

Vrf, Vdc

• Our computational platform consists of the coupled Hybrid Plasma Equipment Model (HPEM) and a circuit model.

• The circuit model uses intermediate results from the HPEM to compute voltages (dc, fundamental and harmonics) at electrodes.

ne, Te,

vi

SHEATH-CIRCUIT MODEL


AVS98_P1_F4a

• The reactor and circuitry are replaced by the following equivalent circuit.

Sheath 1 Sheath 2 Sheath N

RC1

CC1

LC1

RB1

CB1

VS1

LB1

RA1

CA1

LA1

RB2

CB2

VS2

LB2

RA2

CA2

LA2

RBN

CBN

VSN

LBN

RAN

CAN

LANRCN-1

CCN-1

LCN-1

Reactor

GEC REFERENCE CELL


AVS98_P1_F5

• We first explore the effects of frequency and source interactions in the capacitively coupled GEC reference cell with Ar at 100 mTorr.

• Sources and blocking capacitors have been connected to both electrodes.

Radius (cm)0.0 10.2

0.0

10.9[e] (Max = 1.18 × 1010 cm-3)

107 4

Electrode 1

Electrode 2

Pump Port

Showerhead

1

Dark SpaceShield

E1

E2

G

C2V2

C1V1

• Ar, 100 mTorr, V1 = 100 V, V2 = 0 V.

SHEATH VOLTAGES AND CURRENTS


AVS98_P1_F6

• A negative dc voltage appears across the capacitor C1 (dc bias) to balance

currents through the powered and grounded surfaces.

• The sheath currents are fairly nonlinear with large higher harmonics.

58.84 59.00Time (µs)

0

-150

50

VE1+VC1

VG

VE1

VC1

-50

-100

-20058.84 59.00Time (µs)

-0.15

0.15

0.0

IE1

IE2IG

• Ar, 100 mTorr, V1 = 100 V, V2 = 0 V, 13.56 MHz.

EFFECT OF SOURCE FREQUENCY


AVS98_P1_F7

• Total current through electrodes and walls increases with frequency because of enhancement of displacement current.

• Larger current leads to more electron heating and larger electron densities.

10 20 30Frequency (MHz)

0.10.20.30.40.50.60.7

0.0

IE1

IE2

IG

10

5

0

Center×0.1

Above E1

-50

-60

-70

-80

Bias (C1)Sheath (E1)


• Electron temperature decreases with frequency, resulting in better electron confinement between electrodes and smaller dc biases.

• Ar, 100 mTorr, V1 = 100 V, V2 = 0 V.

INTERACTION OF MULTIPLE FREQUENCY SOURCES - I


AVS98_P1_F8

• In these results, a 13.56 MHz source (V1=100 V) is connected to E1 while a

27.12 MHz source is connected to E2.

• Electron density increases with V2 (27.12 MHz) due to the enhancement of

displacement currents.

10

5

0

Center×0.1

Above E1

0 10020 806040V2(27.12 MHz) (V)

10

-10

-30

-50

-70Electrode E2

Bias (C2)

Sheath

-40

-50

-80

-60

-70

Electrode E1

Bias (C1)

Sheath

0 10020 806040V2(27.12 MHz) (V)

• The dc bias magnitude on E1 decreases with increasing V2 (27.12

MHz) due to source interactions.

• Ar, 100 mTorr, V1 (13.56 MHz) = 100 V.

INTERACTION OF MULTIPLE FREQUENCY SOURCES - II


AVS98_P1_F9

• In these results, we show the sheath voltages and currents for only the 13.56 MHz source, 27.12 MHz source and their combination.

• Sheath voltages at E1 and E2 are primarily governed by the sources connected to them.

• The sheath voltage at the grounded wall is, however, in the linear regime and the two sources interact increasing the sheath voltage drop.

• DC bias at E1, which is the difference between the dc sheath voltage at E1 and wall, therefore decreases in magnitude.

2

0

-2

1.2

0

-1.2

0

-200

-100

0

-1600

-60

13.56

Both

13.56

Both

(a)

(c)

27.12

Both (b)

13.56

27.12

Both

(e)

27.12

Both (d)

-30

-80

79.0 80.0Time (1/13.56 µs)

79.5


AVS98_P1_F17

ARBITRARY VOLTAGE WAVEFORMS

0

100

-100

0

100

-100

0

100

-100

0

100

-100

(a)

(b)

(c)

(d)

0 2πωt (rad)

0 2πωt (rad)

π 0 2πωt (rad)

π

-90

0

-180

-90

0

-180

-80

0

-160

-75

0

-150

0.00

0.25

-0.25

0.00

0.70

-0.70

0.00

0.25

-0.25

0.00

1.70

-1.70

! By varying the rf bias voltage waveform, one can control the dc bias, sheathvoltage, plasma characteristics and the ion energy distribution at the substrate.

EFFECT OF WAVEFORM ON PLASMA PARAMETERS


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• Waveforms which have higher first harmonic lead to larger dc biases.• Higher first harmonics also lead to enhanced power deposition in the plasma and higher electron densities.

10.9

0.00.0 10.2 0.0 10.2 0.0 10.2

Radius (cm) Radius (cm) Radius (cm)

0.0

0.9

0.6

0.3

0.0

1.2

0.8

0.4

1.5

0.0

0.5

1.0

Triangular Wave Sinosoidal Wave Square Wave

[e] (1010 cm-3) [e] (1010 cm-3) [e] (1010 cm-3)

• Since displacement current increases with frequency, waveforms with larger amplitudes at higher harmonics result in larger plasma densities.

INDUCTIVELY COUPLED PLASMA (ICP) SOURCE


AVS98_P1_F10

• We next consider the effects of rf bias frequency and rf source interaction in an ICP reactor.

• For circuit simulation, the dielectric window is replaced by effective capacitors.

Radius (cm)0.0 17.0

0.0

15.0

[e] (Max = 5.4 × 1011 cm-3)

107 4

PumpPort

Coils Showerhead

Electrode (S1)S3S2

1

C1V1

1

2

3

4 5

CW4 CW5

• Ar, 20 mTorr, 500 W, V1 (13.56 MHz) = 100 V.

TYPICAL SHEATH VOLTAGES AND CURRENTS


AVS98_P1_F11

• Most of the rf current flows out through the grounded surfaces.

0 2π 4πωt (radians)

0

-20

-40

-60

-80

-100

-120

-140

1.5

0.0

-1.5

I5I4

6

4

2

0

-2

-4

-6

-8

0 2π 4πωt (radians)

• Sheath voltage is larger near the grounded walls at 10 MHz because of low plasma density (i.e., high impedance) adjacent to them.

• Ar, 20 mTorr, 500 W, V1 (10 MHz) = 100 V.

V1

V2,3

V4 V5

10 MHz

I1

I3

I2

10 MHz

EFFECT OF RF BIAS SOURCE FREQUENCY


AVS98_P1_F12

• Since plasma is generated by the inductive source, rf bias frequency does not significantly affect the electron density.

• Displacement current through the sheaths increases with bias frequency, enhancing the total sheath current.


30

0

-60

0

5

10

-30 Sheath

Bias (C1)

8

6

4

2

010 20 30

Frequency (MHz)

• Ar, 20 mTorr, 500 W, V1 = 100 V.

WHY DOES RF BIAS FREQUENCY STRONGLY EFFECT DC BIAS?


AVS98_P1_F13

• Most of the current through the substrate is conduction while that at grounded surfaces is displacement.

• An increase in rf bias frequency, therefore, decreases the sheath impedance more strongly at grounded surfaces than at the substrate.

• The resulting disproportionate change in sheath voltage at different surfaces modifies the dc bias at the substrate.

0 4πωt (radians)

2π

0

-20

-40

-60

-80

-100

-120

-140

10

5

0

-5

-10

-15

-200 4π

ωt (radians)2π

• Ar, 20 mTorr, 500 W, V1 (30 MHz) = 100 V.

V1

V2,3

30 MHzI1

I3

I2

30 MHz

DEPENDENCE OF DC BIAS ON INDUCTIVE POWER DEPOSITION


AVS98_P1_F14

• Dc bias was positive at low frequencies (in the previous results) because electron density is much larger near the powered electrode than the grounded surfaces.

• By powering only the outer two coils, electron density profile can be shifted towards the grounded wall (S3).

• This changes the dc bias at the powered substrate (S1) from 10.8 V

to -21.9 V.

• Ar, 20 mTorr, 500 W, V1 (13.56 MHz) = 100 V.

Radius (cm)0.0 17.0

0.0

15.0

[e] (Max = 3.2 × 1011 cm-3)

10 74

1S1

S3

RF SOURCE INTERACTION IN ICP REACTOR


AVS98_P1_F15

• In these results, a 13.56 MHz source (V1 = 100 V) is connected to S1 while a

27.12 MHz source is connected to S3.

30 11050 9070VS3(27.12 MHz) (V)

45

25

15

12

8

4

0

-4

35

Electrode S1

Bias (C1)

-VSheath

6.0

5.8

5.6

5.4

5.2

5.030 11050 9070

VS3(27.12 MHz) (V)

• Sheath voltage at S1 is mainly

governed by the rf bias source.

• The two rf sources, however, interact at the grounded surface S2 and

change the sheath voltage there.

• Dc bias at S1 is therefore modified.

• Ar, 20 mTorr, 500 W, V1 (13.56 MHz) = 100 V.

CONCLUSIONS


AVS98_P1_F16

• The effect of rf bias source frequency and source interactions have been discussed in both capacitively and inductively coupled sources.

• In the capacitively coupled GEC reference cell, frequency had a significant effect on currents and electron density, but not dc bias.

• On the other hand, rf bias source frequency appreciably modified the dc bias in the inductively coupled plasma reactor, but not plasma density.

• Multiple rf sources at different frequencies were found to interact with each other in both inductively and capacitively coupled reactors.

• This interaction was strong near surfaces where sources were not attached and very weak in sheaths connected to the sources.

• Due to this inhomogeneous and nonlinear response of different sheaths to multiple sources, the electrical characteristics of the discharge were significantly modified.

MULTI-FREQUENCY OPERATION OF RIE AND ICP...

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