DEVELOPMENT OF ION ENERGY ANGULAR DISTRIBUTION THROUGH THE PRE-SHEATH

AND SHEATH IN DUAL-FREQUENCY CAPACITIVELY COUPLED PLASMAS*

Yiting Zhanga, Nathaniel Mooreb, Walter Gekelmanb and Mark J. Kushnera

(a) Department of Electrical and Computer Engineering,University of Michigan, Ann Arbor, 48109

(yitingz@umich.edu , mjkush@umich.edu)

(b) Department of Physics, University of California, Los Angeles, 90095

(moore@physics.ucla.edu , gekelman@physics.ucla.edu )September 2011

* Work supported by National Science Foundation and Semiconductor Research Corp.

AGENDA

Introduction to dual frequency capacitively coupled plasma (CCP) sources and Ion Energy Angular Distributions (IEAD)

Description of the model

Plasma properties for 2 MHz / 30 MHz

Ar Plasma properties

Ar/O2 Plasma Properties

Uniformity and Edge Effect

Concluding Remarks

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University of MichiganInstitute for Plasma Science & Engr.

DUAL FREQUENCY CCP SOURCES

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Capacitively coupled discharges (CCPs) are widely used for etching and deposition of microelectronic industry.

High driving frequency achieve higher electron densities at moderate sheath voltage and higher ion fluxes with moderate ion energies.

A low frequency contributes the quasi-independent control of the ion flux and energy.

However, the non-uniformity problems arise with increases of the driving frequency.

A. Perret, Appl. Phys.Lett 86 (2005)University of Michigan

Institute for Plasma Science & Engr.

ION ENERGY AND ANGULAR DISTRIBUTIONS (IEAD)

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• Control of the ion energy and angular distribution (IEAD) at the substrate provides the potential for improving plasma processes.

• A narrow angular IEAD at the substrate with the majority ion flux perpendicular to the substrate is desired for anisotropic processing.

• Edge effects produce slanted IEADs.

•S.-B. Wang and A.E. Wendt,• J. Appl. Phys., Vol 88, No.2•B. Jacobs, PhD Dissertation

University of MichiganInstitute for Plasma Science & Engr.

GOALS

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Results from a computational investigation of ion transport through RF sheaths will be discussed.

Investigate the motion of ion species in the RF pre-sheath and sheath region of CCPs using sub-meshing technique to provide finer resolution at different phase of RF source.

Comparison to experimental results from laser induced fluorescence (LIF) measurements by Low Temperature Plasma Physics Laboratory at UCLA.

Assessment of O2 addition to Ar plasmas.

University of MichiganInstitute for Plasma Science & Engr.

HYBRID PLASMA EQUIPMENT MODEL (HPEM)

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Monte Carlo Simulation f(ε) or Electron Energy Equation

Electron Energy Transport Module　（ EETM): Electron Monte Carlo Simulation provides EEDs of bulk electrons. Separate MCS used for secondary, sheath accelerated electrons.

Fluid Kinetics Module (FKM): Heavy particle and electron continuity, momentum, energy and

Poisson equations.

Plasma Chemistry Monte Carlo Module (PCMCM): IEADs in bulk, pre-sheath, sheath, and wafers Recorded phase, submesh resolution

EETM

Continuity, Momentum, Energy, Poisson equation

FKM

Monte Carlo Module

PCMCMSe(r)

N(r)Es(r)

•M.Kushner, J. Phys.D: Appl. Phys. 42(2009) University of MichiganInstitute for Plasma Science & Engr.

REACTOR GEOMETRY

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University of MichiganInstitute for Plasma Science & Engr.

Inductively coupled with 2-freq CCP on substrate

2D, cylindrically symmetric.

Base conditions ICP Power: 400kHz,300 Watt High Freq RF: 10 MHz

300 Watt 300 Volt Low Freq RF: 2MHz

100 Watt 150 Volt Specify power, adjust

voltage.

Main Species in Ar Ar , Ar*, Ar+, e

Main Species in Ar/O2

Ar , Ar*, Ar+, e O2 ,O2

*, O2+, O, O*,O+, O-

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PLASMA PROPERTIES

• Ar/O2 =0.8/0.2, • 20mTorr, 300 SCCM• Freq=2 MHz, 300 Watt

Majority of power deposition that produces ions comes from inductively coupled coils.

Ion acceleration is produced by capacitive coupling.

Plasma distribution determines local sheath thickness, potential and ion mixing ratio at wafer.

Te peaks near coil where E-field is largest.

Electro-static waves due to double layers.

University of MichiganInstitute for Plasma Science & Engr.MIN MAX

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Log scale

Ion

De

ns

ity

(c

m-3)

PULSED LASER-INDUCED FLUORESCENCE (LIF)

A non-invasive optical technique for measuring the ion velocity distribution function.

Ions moving along the direction of laser propagation will have the absorption wavelengths Doppler-shifted from λ0,

Ion velocity parallel to the laser obtained fromΔλ=λ0-λL=v//λ0/c

•B. Jacobs, PRL 105, 075001(2010)YZHANG_MIPSE2011_08

University of MichiganInstitute for Plasma Science & Engr.

YZHANG_MIPSE2011_09

Ar+ IEAD FROM BULK TO SHEATH

Ar, 20mTorr, 300 SCCM HF=30 MHz 100Watt LF=2 MHz 300Watt

IEAD changes significantly through sheath from bulk plasma.

In the bulk plasma and pre-sheath, the IEAD is essentially thermal and broad in angle.

In the sheath, ions are accelerated by the E-field in z direction and the angle narrows.

MIN MAX Log scale

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University of MichiganInstitute for Plasma Science & Engr.

Center Edge

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Middle

• Ar, 20mTorr, 300 SCCM• HF=30MHz 100Watt• LF=2 MHz 300Watt

IEAD NEAR EDGE OF WAFER

０ .5 mm above wafer

IEADs are separately collected over center, middle and edge regions.

Non-uniformity near the edge region - IEAD has broader angular distribution.

Maximum energy consistent regardless of radius.

University of MichiganInstitute for Plasma Science & Engr.MIN MAX

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Log scale

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PEAKS IN ION ENERGY DISTRIBUTION vs PHASE

IEAD properties differ during the RF period.

Argon ions are most energetic shortly after the maximum in accelerating field.

Experiments show similar trend.

• Ar, 20mTorr, 300 SCCM• HF=30MHz 100Watt• LF=2 MHz 300Watt• Phase refer LF

Ar/O2=0.8/0.2, 0.5 mTorr, 50

SCCM LF600kHz, 425W HF=2MHz, 1.5kW Phase refers to

HF

• B.Jacobs, W.Gekelman, PRL 105, 075001(2010)

MIN MAX Log scale

MIN MAX MIN MAX Log scale University of Michigan

Institute for Plasma Science & Engr.

YZHANG_MIPSE2011_12

IEAD UNDER DIFFERENT RF PHASES

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• B. Jacobs, PhD Dissertation (2010)

• Ar/O2=0.8/0.2,• 0.5 mTorr, 50

SCCM• HF600kHz, 425W• LF=2 MHz, 1.5kW• Sheath ~3.6 mm• LIF measured 4.2

mm above wafer

• Ar/O2 =0.8/0.2,

• 20mTorr, 300 SCCM

• Freq=2 MHz• IEAD 4 mm

above wafer

IEADs far above wafer are independent of phase, and slowly drifting.

In the pre-sheath, small ion drifts cause the IEAD to slightly change vs phase.

University of MichiganInstitute for Plasma Science & Engr.

YZHANG_MIPSE2011_13

IEAD UNDER DIFFERENT RF PHASES

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• B. Jacobs, W. Gekelman, PRL 105, 075001(2010)

Ar/O2=0.8/0.2, 0.5 mTorr, 50

SCCM HF600kHz, 425W LF=2 MHz, 1.5kW Sheath ~3.6 mm LIF measured 1

mm above wafer

Ar/O2 =0.8/0.2,

20mTorr, 300 SCCM

Freq=2 MHz IEAD 0.5 mm

above wafer

Due to periodic acceleration in sheath, development of IEAD depends on phase.

During low acceleration phases, IEAD drifts in sheath.

During high acceleration phase, IEAD narrows as perpendicular component of velocity distribution increases.

University of MichiganInstitute for Plasma Science & Engr.

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O2 ADDITION TO AR

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Ar+ IEAD on wafer 20 mTorr, 300 SCCM. Freq=2 MHz, 300 W.

With increasing O2, negative ion ( O2-, O-) formation increases

the sheath potential for fixed power.

IEAD for Ar+ extends in energy and narrows in angle.

University of MichiganInstitute for Plasma Science & Engr.

CONCLUDING REMARKS

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In the pre-sheath, IEAD is thermal and broad in angle. When the ion flux is accelerated through the sheath, the distribution increases in energy and narrows in angle.

Edge Effect can be observed clearly by using the high resolution afforded by sub-meshing. Multiple peaks in IEADs come from IEADs alternately accelerated by rf field during the whole RF period.

Increasing O2 changes the sheath properties – a narrower IEAD achieved when percentage of O2 increase from 5% to 20%.

University of MichiganInstitute for Plasma Science & Engr.