control of electron energy distributions and flux ratios in pulsed capacitively coupled plasmas *

CONTROL OF ELECTRON ENERGY DISTRIBUTIONS AND FLUX RATIOS IN PULSED

CAPACITIVELY COUPLED PLASMAS*Sang-Heon Songa) and Mark J. Kushnerb)

a)Department of Nuclear Engineering and Radiological Sciences University of Michigan, Ann Arbor, MI 48109, USA

[email protected]

b)Department of Electrical Engineering and Computer ScienceUniversity of Michigan, Ann Arbor, MI 48109, USA

[email protected]

http://uigelz.eecs.umich.edu

Oct 2010 AVS

* Work supported by DOE Plasma Science Center and Semiconductor Research Corp.

AGENDA

Motivation for controlling f() Description of the model Typical Ar pulsed plasma properties Typical CF4/O2 pulsed plasma properties

f() and flux ratios with different PRF Duty Cycle Pressure

Concluding Remarks

University of MichiganInstitute for Plasma Science & Engr.

SHS_MJK_AVS2010_02

CONTROL OF ELECTRON KINETICS- f() Controlling the generation of reactive species for technological

devices benefits from customizing the electron energy (velocity) distribution function.


, , , , ,, , ,

df v r t qE r t f v r tv f r v f v r t

dt m tx ve c

1 2

0

2, , ,ije

k r t f r t dm

,

,,k

e ij ji j

dN r tn k r t N

dt

e + SiH4 SiH3 + H + ek

LCD Solar Cell Need SiH3 radicals*

* Ref: Tatsuya Ohira, Phys. Rev. B 52 (1995)

SHS_MJK_AVS2010_03

HYBRID PLASMA EQUIPMENT MODEL (HPEM)

Fluid Kinetics Module: Heavy particle and electron continuity, momentum,

energy Poisson’s equation

Electron Monte Carlo Simulation: Includes secondary electron transport Captures anomalous electron heating Includes electron-electron collisions

E, Ni, ne, Ti

Fluid Kinetics ModuleFluid equations

(continuity, momentum, energy)Poisson’s equation

Te, S, kElectron Monte Carlo Simulation


SHS_MJK_AVS2010_04

REACTOR GEOMETRY

2D, cylindrically symmetric Ar, CF4/O2, 10 – 40 mTorr, 200 sccm

Base conditions Lower electrode: LF = 10 MHz, 300 W, CW Upper electrode: HF = 40 MHz, 500 W, Pulsed


SHS_MJK_AVS2010_05

PULSE POWER

Time = 1/PRF

Duty Cycle

Power(t)

Pmin

0

1 dttPPave

Pmax


Use of pulse power provides a means for controlling f(). Pulsing enables ionization to exceed electron losses during a portion

of the period – ionization only needs to equal electron losses averaged over the pulse period.

Pulse power for high frequency. Duty-cycle = 25%, PRF = 100 kHz, 415 kHz Average Power = 500 W

SHS_MJK_AVS2010_06

Ar

SHS_MJK_AVS2010_07

PULSED CCP: Ar, 40 mTorr


Pulsing with a PRF and moderate duty cycle produces nominal intra-cycles changes [e] but does modulate f().

LF = 10 MHz, 300 W HF = 40 MHz, pulsed 500 W PRF = 100 kHz, Duty-cycle = 25% [e]

Te

SHS_MJK_AVS2010_08

MIN MAX

f()VHF 226 VVLF 106 V

ANIMATION SLIDE-GIF

PULSED CCP: Ar, DUTY CYCLE Excursions of tail are more extreme with lower duty cycle – more

likely to reach high thresholds.


Duty cycle = 25% Cycle Average Duty cycle = 50%

SHS_MJK_AVS2010_09

LF 10 MHz, pulsed HF 40 MHz PRF = 100 kHz, Ar 40 mTorr

VHF 128 VVLF 67 V

VHF 226 VVLF 106 V

ANIMATION SLIDE-GIF

PULSED CCP: Ar, PRESSURE Pulsed systems are more sensitive to pressure due to differences in

the rates of thermalization in the afterglow.


10 mTorr Cycle Average 40 mTorr

SHS_MJK_AVS2010_10

LF 10 MHz, pulsed HF 40 MHz PRF = 100 kHz

VHF 226 VVLF 106 V

VHF 274 VVLF 146 V

ANIMATION SLIDE-GIF

CF4/O2

SHS_MJK_AVS2010_11

ELECTRON DENSITY CW

At 415 kHz, the electron density is not significantly modulated by pulsing, so the plasma is quasi-CW.

At 100 kHz, modulation in [e] occurs due to electron losses during the longer inter-pulse period.

The lower PRF is less uniform due to larger bulk electron losses during longer pulse-off cycle.


PRF=415 kHz

PRF=100 kHz

MIN MAX

ANIMATION SLIDE-GIF

40 mTorr, CF4/O2=80/20, 200 sccm LF = 10 MHz, 300 W HF = 40 MHz, 500 W (CW or pulse)

SHS_MJK_AVS2010_12

ELECTRON SOURCES BY BULK ELECTRONS


CW

PRF=415 kHz

PRF=100 kHz

The electrons have two groups: bulk low energy electrons and beam-like secondary electrons.

The electron source by bulk electron is negative due to electron attachment and dissociative recombination.

Only at the start of the pulse-on cycle, is there a positive electron source due to the overshoot of E/N.

MIN MAX

40 mTorr, CF4/O2=80/20, 200 sccm LF 300 W, HF 500 W

ANIMATION SLIDE-GIFSHS_MJK_AVS2010_13

ELECTRON SOURCES BY BEAM ELECTRONS


CW

PRF=415 kHz

PRF=100 kHz

MIN MAX

40 mTorr, CF4/O2=80/20, 200 sccm LF = 10 MHz, 300 W HF = 40 MHz, 500 W (CW or pulse)

The beam electrons result from secondary emission from electrodes and acceleration in sheaths.

The electron source by beam electron is always positive.

The electron source by beam electrons compensates the electron losses and sustains the plasma.

ANIMATION SLIDE-GIFSHS_MJK_AVS2010_14

TYPICAL f(): CF4/O2 vs. Ar

Less Maxwellian f() with CF4/O2 due to lower e-e collisions.

Enhanced sheath heating with CF4/O2 due to lower plasma density.

Tail of f() comes up to compensate for the attachment and recombination that occurs at lower energy.

CF4/O2 Ar


SHS_MJK_AVS2010_15

40 mTorr, 200 sccm LF = 10 MHz, 300 W HF = 40 MHz, 500 W (25% dc)

VHF 226 VVLF 106 V

VHF 203 VVLF 168 V

ANIMATION SLIDE-GIF

In etching of dielectrics in fluorocarbon gas mixtures, the polymer layer thickness depends on ratio of fluxes. Ions – Activation of dielectric etch, sputtering of polymer CFx radicals – Formation of polymer O – Etching of polymer F – Diffusion through polymer, etch of dielectric and polymer

Investigate flux ratios with varying PRF Duty cycle Pressure

RATIO OF FLUXES: CF4/O2


Flux Ratios: Poly = (CF3+CF2+CF+C) / Ions O = O / Ions F = F / Ions

SHS_MJK_AVS2010_16

f(): CF4/O2, PRF Average

The time averaged f() for pulsing is similar to CW excitation.

Extension of tail of f() beyond CW excitation during pulsing produces different excitation and ionization rates, and different mix of fluxes to wafer.


PRF = 100 kHz

40 mTorr, CF4/O2=80/20, 200 sccm LF = 10 MHz, 300 W HF = 40 MHz, 500 W (25% dc)

ANIMATION SLIDE-GIF

SHS_MJK_AVS2010_17

VHF 203 VVLF 168 V

0.0

1.0

2.0

3.0

4.0

5.0

6.0

F O POLY

Ave

rage

Flu

x R

atio

CW100

415 kHz

CW100415

CW

100

415


RATIO OF FLUXES: CF4/O2, PRF Ratios of fluxes are tunable using pulsed excitation. Polymer layer thickness may be reduced by pulsed excitation

because poly to ion flux ratio decreases.

F O Poly 40 mTorr, CF4/O2=80/20, 200 sccm, Duty-cycle = 25% LF = 10 MHz, 300 W HF = 40 MHz, 500 W

SHS_MJK_AVS2010_18

f(): CF4/O2, DUTY CYCLE Control of average f() over with changes in duty cycle is limited if

keep power constant. ANIMATION SLIDE-GIF


40 mTorr, CF4/O2=80/20, 200 sccm LF 10 MHz, Pulsed HF 40 MHz, PRF = 100 kHz

Duty cycle = 25% Cycle Average Duty cycle = 50%

SHS_MJK_AVS2010_19

VHF 191 VVLF 168 V

VHF 203 VVLF 168 V

0.0

1.0

2.0

3.0

4.0

5.0

6.0

F O POLY

Ave

rage

Flu

x R

atio

RATIO OF FLUXES: CF4/O2, DUTY CYCLE Flux ratio control is limited if keep power constant. With smaller duty cycle, polymer flux ratio is more reduced

compared to the others.


F O Poly

50%25%

50%25%

50%

25%

SHS_MJK_AVS2010_20

LF 10 MHz, Pulsed HF 40 MHz, PRF = 100 kHz 40 mTorr, CF4/O2=80/20, 200 sccm

CW

CW

CW

f(): CF4/O2, PRESSURE Pulsed systems are sensitive to pressure due to differences in the

rates of thermalization in the afterglow. ANIMATION SLIDE-GIF


CF4/O2=80/20, 200 sccm, PRF = 100 kHz LF 10 MHz, Pulsed HF 40 MHz

10 mTorr Cycle Average 40 mTorr

SHS_MJK_AVS2010_21

VHF 191 VVLF 168 V

VHF 233 VVLF 188 V

0.0

1.0

2.0

3.0

4.0

5.0

6.0

F O POLY

Ave

rage

Flu

x R

atio

RATIO OF FLUXES: CF4/O2, PRESSURE Flux ratios decrease as pressure decreases. Polymer layer thickness may be reduced with lower pressure in

the pulsed CCP.


F O Poly

10

40 mTorr

1040

10

40

CF4/O2=80/20, 200 sccm LF = 10 MHz, 300 W HF = 40 MHz, 500 W

SHS_MJK_AVS2010_22

CW

CW

CW CW

CW

CWP

P

P

P

P

PRF = 100 kHz, Duty-cycle = 25%

P

P: Pulsed excitation CW: CW excitation

CONCLUDING REMARKS

Extension of tail of f() beyond CW excitation produces different mix of fluxes.

Ratios of fluxes are tunable using pulsed excitation. Different PRF provide different flux ratios due to different

relaxation time during pulse-off cycle. Duty cycle is another knob to control f() and flux ratios, but it is

limited if keep power constant Pressure provide another freedom for customizing f() and flux

ratios in pulsed CCPs.

SHS_MJK_AVS2010_23

control of electron energy distributions and flux ratios in pulsed capacitively coupled plasmas *

Documents

electron continuity

electron densitycwat

equal electron losses

lower duty cycle

larger bulk electron

moderate duty cycle

cycle averageduty cycle

pulsed hf