sio 2 etch property control using pulse power in capacitively coupled plasmas *

SiO2 ETCH PROPERTY CONTROL USING PULSE POWER IN 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

Nov. 2011 AVS

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

AGENDA

Motivation for controlling f()

Description of the model

Typical Ar/CF4/O2 pulsed plasma properties

Etch rate with variable blocking capacitor

Etch property with different PRF

Etch rate, profile, and selectivity

Concluding Remarks

University of MichiganInstitute for Plasma Science & Engr.

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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, , ,ij

ek r t f r t d

m

,

,,k

e ij ji j

dN r tn k r t N

dt

e + CF4 CF3 + F + ek

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ETCH RATE vs. FLUX RATIOS


Ref: D. C. Gray, J. Butterbaugh, and H. H. Sawin, J. Vac. Sci. Technol. A 9, 779 (1991)

Flux Ratio (F/Ar+) Flux Ratio (CF2/Ar+)

Etc

hin

g Y

ield

(S

i/Ar+

)

Etc

hin

g Y

ield

(S

i/Ar+

)

Large fluorine to ion flux ratio enhances etching yield of Si.

Large fluorocarbon to ion flux ratio reduces etching yield of Si.

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Ref: K. Ono, M. Tuda, H. Ootera, and T. Oomori, Pure and Appl. Chem. Vol 66 No 6, 1327 (1994)

Large chlorine radical to ion flux ratio produces an undercut in etch profile.

Etch profile result in ECR Cl2 plasma after 200% over etch with different flux ratios

p-Si p-Si


ETCH PROFILE vs. FLUX RATIOS

Flux Ratio (Cl / Ion) = 0.3 Flux Ratio (Cl / Ion) = 0.8

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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

Fluid Kinetics ModuleFluid equations

(continuity, momentum, energy)Poisson’s equation

Te, Sb, Seb, kElectron Monte Carlo Simulation


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MONTE CARLO FEATUREPROFILE MODEL (MCFPM) The MCFPM resolves the surface

topology on a 2D Cartesian mesh.

Each cell has a material identity. Gas phase species are represented by Monte Carlo pseuodoparticles.

Pseuodoparticles are launched with energies and angles sampled from the distributions obtained from the HPEM

Cells identities changed, removed, added for reactions, etching deposition.

PCMCM

Energy and angular distributions for ions

and neutrals

MCFPM

Etch rates and profile


Poisson’s equation solved for charging

HPEM

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REACTOR GEOMETRY: 2 FREQUENCY CCP

2D, cylindrically symmetric

Ar/CF4/O2 = 75/20/5, 40 mTorr, 200 sccm

Base conditions

Lower electrode: LF = 10 MHz, 500 W, CW

Upper electrode: HF = 40 MHz, 500 W, Pulsed


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PULSE POWER

Time = 1/PRF

Duty Cycle

Power(t)

Pmin

0

1dttPPave

Pmax


Use of pulse power provides a means for controlling f().

Pulsing enables ionization to exceed electron losses during a portion of the ON period – ionization only needs to equal electron losses averaged over the pulse period.

Pulse power for high frequency.

Duty-cycle = 25%, PRF = 50, 100, 200, 415, 625 kHz

Average Power = 500 W

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VARIABLE BLOCKING CAPACITOR

Due to the different area of two electrodes, a “dc” bias is produced on the blocking capacitor connected to the substrate electrode.

The temporal behavior of “dc” bias is dependent on the magnitude of the capacitance due to RC delay time.


We investigated variable blocking capacitor of 10 nF, 1 F, and 100 F

100 F of blocking capacitor results in NO “dc” bias on the substrate.

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Typical Plasma Properties

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PULSED CCP: Electron Density & Temperature

Pulsing with a moderate PRF duty cycle produces nominal intra-cycles changes in [e] but does modulate Te.

Electron Density (x 1011 cm-3) Electron Temperature (eV)


MIN MAX

40 mTorr, Ar/CF4/O2=75/20/5

PRF = 100 kHz, Duty-cycle = 25%

HF = 40 MHz, pulsed 500 W

LF = 10 MHz, 250 VSHS_MJK_AVS

ANIMATION SLIDE-GIF

PULSED CCP: ELECTRON SOURCES

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

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

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

by Bulk Electrons (x 1014 cm-3 s-1) by Secondary Electrons


MIN MAX

40 mTorr, Ar/CF4/O2=75/20/5 LF 250 V, HF 500 W

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ANIMATION SLIDE-GIF

PULSED CCP: E-SOURCES and f()


40 mTorr, Ar/CF4/O2=75/20/5 PRF = 100 kHz, Duty-cycle = 25% LF = 10 MHz, 250 V HF = 40 MHz, pulsed 500 W

Rate coefficient of e-sources is modulated between electron source (electron impact ionization) and loss (attachment and recombination) during pulsed cycle.

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ANIMATION SLIDE-GIF

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Etch Properties:Variable Blocking Capacitor

PULSED CCP: PLASMA POTENTIAL & dc BIAS A small blocking capacitor allows the “dc” bias to follow the

change during the pulse period.

Maximum ion energy gain = Plasma Potential – “dc” Bias


PRF = 100 kHz, Duty-cycle = 25% LF = 10 MHz, 250 V HF = 40 MHz, pulsed 500 W

1 F 10 nF

ETCH PROFILE IN SiO2 & IEAD: 1 F

With constant voltage, bias amplitude is constant but blocking capacitor determines “dc” bias.

Cycle Average IEAD Etch Profile (600 sec)


Angle (degree)Width (m)ANIMATION SLIDE-GIF

Pulsed HF 40 MHz 500 W LF 10 MHz 250 V, Blocking Cap. = 1 F

En

erg

y (

eV

)

He

igh

t (

m)

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ETCH PROFILE IN SiO2 & IEAD: 10 nF

With smaller blocking capacitor, “dc” bias begins to follow the rf power and so produces a different IEAD.


Pulsed HF 40 MHz 500 W LF 10 MHz 250 V, Blocking Cap. = 1 nF

SHS_MJK_AVSAngle (degree)Width (m)

ANIMATION SLIDE-GIF


En

erg

y (

eV

)

He

igh

t (

m)

ETCH PROFILE IN SiO2 & IEAD: NO dc BIAS

In absence of dc bias and for constant voltage, pulse power and is effect on f() in large part determine etch properties.


Pulsed HF 40 MHz 500 W LF 10 MHz 250 V, Blocking Cap. = 100 F

SHS_MJK_AVSAngle (degree)Width (m)

ANIMATION SLIDE-GIF


En

erg

y (

eV

)

He

igh

t (

m)

POWER NORMALIZED ER: Blocking Capacitor

Power normalized etch rate is dependent not only on the pulse repetition frequency (PRF), but also the value of the blocking capacitor on the substrate at lower PRF.


Pulsed HF 40 MHz 500 W LF 10 MHz 250 V

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CW 250 100 50 kHz0.0

1.0

2.0

3.0

4.0

5.0

NO dc 10 nF 1 uF

A

B

C

A B C

F to Poly Flux ratio

Electron source rate coefficient is modulated with f() by pulse power.

Modulation is enhanced with smaller PRF.

E-SOURCES and FLUX RATIO: PRF


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Pulsed HF 40 MHz 500 W LF 10 MHz 250 V Blocking Cap. = 1 F

F to Poly Flux ratio

0.0

1.0

2.0

3.0

4.0

5.0

6.0

CW 250 100 50 kHz

Power normalized etch rate is large at 250 kHz with ion distribution extending to higher energies.


Pulsed HF 40 MHz 500 W LF 10 MHz 250 V Without DC Bias on LF electrode

ETCH RATE: POWER NORMALIZED

CW 250 100 50 kHz

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Cycle Average IEAD Normalized Etch Rate

Angle (degree)

En

erg

y (

eV

)

1



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EPD + Over Etch 50%

ETCH PROFILE: CRITICAL DIMENSION

2

(1/A)

(2/A)

CW 250 100 50 kHz

CD is compared at the middle and bottom of feature.

CW excitation produces bowing and an undercut profile.

Pulse plasma helps to prevent the bowing and under-cutting.

Smaller PRF has a tapered profile.

A

ETCH SELECTIVITY: Between SiO2 and Si



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CW 250 100 50 kHz

Silicon damage depth is compared in 2-D etch profile.

Pulsed operation helps to prevent the silicon damage.

Lower damage appears to be correlated with smaller F flux ratio at 250 kHz.

EPD + Over Etch 50%

CONCLUDING REMARKS

Extension of tail of f() beyond that obtained with CW excitation produces a different mix of fluxes to substrate.

Etch rate can be controlled by pulsed operation with different pulse repetition frequencies.

Blocking capacitor is another variable to control ion energy distributions and etch rates. Smaller capacitance allows “dc” bias to follow the plasma potential in pulse period more rapidly.

Etch rate is enhanced by pulsed power operation in CCP.

Etch profile is improved with pulsed operation preventing undercut.

Etch selectivity of SiO2 to Si is also improved with PRF of 250 kHz with a smaller fluorine flux ratio.

University of MichiganInstitute for Plasma Science & Engr.SHS_MJK_AVS

sio 2 etch property control using pulse power in capacitively coupled plasmas *

Documents

ion flux ratio

flux ratiosflux ratio

electron continuity

flux ratiosref

equal electron losses

etching yield of si

monte carlo pseuodoparticles

pulse period