electron and ion energy distributions in 2-frequency capacitively coupled plasma tools

ELECTRON AND ION ENERGY DISTRIBUTIONS IN 2-FREQUENCY CAPACITIVELY COUPLED

PLASMA TOOLS CONSIDERING WAVE EFFECTS*

Yang Yanga) and Mark J. Kushnerb)

a)Department of Electrical and Computer Engineering Iowa State University, Ames, IA 50011, 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

October 2008

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* Work supported by Semiconductor Research Corp., Applied Materials and Tokyo Electron Ltd.

AGENDA

Introduction to wave effects in 2-frequency capacitively coupled plasma (2f-CCP) sources

Description of the model Scaling of 2f-CCPs in Ar/CF4 mixture with

High frequency (HF) Low frequency (LF) power

Concluding remarks

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University of MichiganInstitute for Plasma Science

and Engineering

WAVE EFFECTS IN hf-CCP SOURCES

Wave effects in CCPs become important with increasing frequency and wafer size.

Wave effects (i.e., propagation, constructive and destructive interference) can significantly affect the spatial distribution of power deposition and reactive fluxes.

G. A. Hebner et al, Plasma Sources Sci. Technol., 15, 879(2006)YY_MJK_GEC2008_03


and Engineering

Relative contributions of wave and electrostatic edge effects determine plasma distribution.

Plasma distribution ultimately depends on How electrons are accelerated by electric fields Electron energy distributions Electron impact reactions with feedstock gases and their

fragments. In this talk, results from a computational investigation of

plasma properties in two-frequency CCPs will be discussed : Spatial variation of electron energy distributions (EEDs) Radial variation of ion energy and angular distributions

(IEADs) onto wafer

GOALS OF THE INVESTIGATION

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and Engineering

Full-wave Maxwell solvers are challenging due to coupling between electromagnetic (EM) and sheath forming electrostatic (ES) fields.

EM fields are generated by rf sources and plasma currents while ES fields originate from charges.

We separately solve for EM and ES fields and sum for plasma transportation.

Compatible boundary conditions (BCs) defined for EM and ES fields: BCs for EM field: Determined by rf sources. BCs for ES field: Determined by blocking capacitor (DC bias)

or applied DC voltages.

mEE

METHODOLOGY OF THE MAXWELL SOLVER

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and Engineering

tH

rE

zE zr

0

tEJ

zH r

rr

0

tEJ

rrH

rz

rz

0

1

Launch rf fields where power is fed into the reactor. For cylindrical geometry, TM mode gives Er , Ez and H . Solve EM fields using FDTD techniques with Crank-Nicholson

scheme on a staggered mesh:

Mesh is sub-divided for numerical stability.

ji ,

1,1 ji

ji ,1jiEr ,

jiEr ,1

jiEz , jiEz ,1jiB ,

1, ji

THE FIRST PART: EM SOLUTION

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and Engineering

Solve Poisson’s equation semi-implicitly:

Boundary conditions on metal: self generated DC bias by plasma or applied DC voltage.

Implementation of this solver: Specify the location that power is fed into the reactor. Addressing multiple frequencies in time domain for arbitrary

geometry. First order BCs for artificial or nonreflecting boundaries (i.e.,

pump ports, dielectric windows).

tdt

tttdttt

,)(

THE SECOND PART: ES SOLUTION

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and Engineering

HYBRID PLASMA EQUIPMENT MODEL (HPEM)

Electron Energy Transport Module: Electron Monte Carlo Simulation

provides EEDs of bulk electrons Separate MCS used for secondary,

sheath accelerated electrons Fluid Kinetics Module:

Heavy particle and electron continuity, momentum, energy

Maxwell’s Equation Plasma Chemistry Monte Carlo Module:

IEADs onto wafer

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E, N

Fluid Kinetics ModuleFluid equations

(continuity, momentum,

energy)Maxwell

Equations

Te,S,μ

Electron Energy Transport

Module

Plasma Chemistry Monte Carlo

Module University of MichiganInstitute for Plasma Science

and Engineering

REACTOR GEOMETRY

2D, cylindrically symmetric. Ar/CF4 = 90/10, 50 mTorr, 400 sccm Base conditions

HF upper electrode: 10-150 MHz, 300 W

LF lower electrode: 10 MHz, 300 W Specify power, adjust voltage.

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Main species in Ar/CF4

mixture Ar, Ar*, Ar+

CF4, CF3, CF2, CF, C2F4, C2F6, F, F2

CF3+, CF2

+, CF+, F+

e, CF3-, F-


and Engineering

AXIAL EM FIELD IN HF SHEATH HF = 50 MHz, Max = 410 V/cm

HF = 150 MHz, Max = 355 V/cm

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|Ezm| = Magnitude of axial EM field’s first harmonic at HF. No electrostatic component in Ezm: purely electromagnetic. 150 MHz: center peaked due to constructive interference of plasma

shortened wavelengths. 50 MHz: Small edge peak.

Ar/CF4=90/10, 50 mTorr, 400 sccm HF: 10-150 MHz/300 W LF: 10 MHz/300 W


and Engineering

AXIAL E-FIELD IN HF AND LF SHEATH: 10/150 MHz

|EZ| in LF(10 MHz) Sheath, Max = 1700 V/cm

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Significant change of |Ez| across HF sheath as evidence of traveling wave.

HF source also modulates E-field in LF sheath.


and Engineering

|EZ| in HF (150 MHz) Sheath, Max = 1500 V/cm

Ar/CF4=90/10 50 mTorr, 400 sccm

HF: 150 MHz/300 W LF: 10 MHz/300 W

ANIMATION SLIDE-GIF

LF CYCLE AVERAGEDAXIAL E-FIELD IN HF AND LF SHEATH: 10/150 MHz

|EZ| in LF(10 MHz) Sheath, Max = 750 V/cm

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Significant change of |Ez| across HF sheath as evidence of constructive interference.


and Engineering

|EZ| in HF (150 MHz) Sheath, Max = 450 V/cm


HF: 150 MHz/300 W LF: 10 MHz/300 W

1 2 3

EEDs IN HF SHEATH

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150 MHz 50 MHz

1 : r = 0.3 cm 2 : r = 7.5 cm 3 : r = 15 cm

Ar/CF4=90/10 50 mTorr, 400 sccm HF: 10-150 MHz/300 W LF: 10 MHz/300 W

150 MHz: elevated EEDs in the center where sheath field is largest. 50 MHz: populated tails for r 7 cm due to edge effect. From 150 MHz to 50 MHz: 2 temperature distribution transits to 1

temperature distribution. University of MichiganInstitute for Plasma Science

and Engineering

1 2 3

EEDs IN LF SHEATH

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HF modulation extends to the LF sheath at 150 and 50 MHz.

150 MHz 50 MHz

1 : r = 0.3 cm 2 : r = 7.5 cm 3 : r = 15 cm



and Engineering

1 2 3

EEDs IN BULK PLASMA

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150 MHz 50 MHz

1 : r = 0.3 cm 2 : r = 7.5 cm 3 : r = 15 cm


150 MHz: does not show strong radial variation. 50 MHz: edge effect affects EEDs in the bulk plasma.


and Engineering

ELECTRON AND NEGATIVE IONS DENSITY: Ar/CF4 = 90/10

[e]

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HF: 10-150 MHz/300 W LF: 10 MHz/300 W

[CF3- + F-]


and Engineering

Spatial variation of EEDs translates to plasma uniformity through electron impact reactions.

100 MHz: [e] is edge peaked. 150 MHz: [CF3

- + F-] peaked in the center and flattens local plasma

potential, so [e] escaping from the center and peaked at r = 9.5 cm.

ION FLUXES INCIDENT ON WAFER

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CF3+ Flux Total Ion Flux

HF: 10-150 MHz/300 W LF: 10 MHz/300 W University of Michigan

Institute for Plasma Scienceand Engineering

Plasma spatial distribution determines local sheath thickness, potential and ion mixing ratio…

Thereby determining radial uniformity of ion fluxes and their IEADs onto wafer.

Relative uniform fluxes at 100 MHz.

Center Edge

TOTAL ION IEADs INCIDENT ON WAFER

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HF=150 MHz Center Center Edge Edge

IEADs are separately collected over center&edge of wafer.

Radial non-uniformity increases from 100 MHz to 150 MHz.

Results from increasing radial variation of sheath thickness, potential...

Ar/CF4=90/10, 50 mTorr, 400 sccm HF: 150 MHz/300 W LF: 10 MHz/300 W

HF=100 MHz


and Engineering

EFFECT OF LF POWER ON EEDSIN HF SHEATH: 10/150 MHz

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HF: 150 MHz/300 W LF: 10 MHz

LF: 1500 W 1 : r = 0.3 cm 2 : r = 7.5 cm 3 : r = 15 cm

Increasing LF power increases [e] and so shortens plasma wavelength.

Strengthens finite wavelength effect and so gives large sheath potential in the center.

Results in more prominent tails for local EEDs.


and Engineering

LF: 300 W

ELECTRON IMPACT IONIZATION SOURCE FUNCTION: 10/150 MHz

University of MichiganOptical and Discharge Physics

LF: 300 W, Max = 3.9 x 1016 cm-3s-1

LF: 1500 W, Max = 3.3 x 1016 cm-3s-1

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Source from bulk and secondary electrons.

LF power mainly enhances bulk ionization.

Contribution to ionization from electrons accelerated by HF sheath is still significant at 1500 W.


HF: 150 MHz/300 W LF: 10 MHz

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With increasing LF power More energetic electrons are available in the HF sheath, thereby

enhancing ionization in the center. [e] becomes increasingly center enhanced, which increases

plasma non-uniformity.

SCALING WITH LF POWER: 10/150 MHz


HF: 50-150 MHz/300 W LF: 10 MHz University of Michigan

Institute for Plasma Scienceand Engineering

[e] [CF3- + F-]

Center Edge

TOTAL ION IEADs INCIDENT ON WAFER

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300 W Center Center Edge Edge

Increasing LF power increases plasma non-uniformity.

As such, radial uniformity of sheath thickness and potential decreases.

Translates to non-uniformity of IEADs across wafer.

Ar/CF4=90/10, 50 mTorr, 400 sccm HF: 150 MHz/300 W LF: 10 MHz

1500 W


and Engineering

CONCLUDING REMARKS

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A full Maxwell solver separately solving for EM and ES fields was developed and incorporated into the HPEM.

Wave and electrostatic coupling produces spatial variation of EEDs, which, in turn, contributes to plasma non-uniformity.

For 2f-CCPs sustained in Ar/CF4=90/10 mixture, at HF = 150 MHz, Non-uniform IEADs across the wafer due to plasma non-

uniformity. With increasing LF power, tails of EEDs are enhanced in the

center (HF sheath) thereby producing [e] profile which is increasingly center enhanced.

Increasing LF power does not improve radial uniformity of IEADs onto wafer.


and Engineering

electron and ion energy distributions in 2-frequency capacitively coupled plasma tools

Documents

plasma distribution

plasma sources

es fields

em fields

plasma transportation

plasma tools

plasma currents

frequency ccps