AGENDA

DEVELOPMENT OF ION ENERGY DISTRIBUTIONS 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, MI 48109([email protected] , [email protected])

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

([email protected] , [email protected] )November 16, 2011

* Work supported by National Science Foundation, Semiconductor Research Corp. and the DOE Office of Fusion Energy Science

AGENDA

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

Description of the model IEADs and plasma properties for 2 MHz Ar/O2

Uniformity and Edge Effect O2 Percentage

Pressure Plasma properties for dual-frequency Ar/O2

Concluding Remarks

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University of MichiganInstitute for Plasma Science & Engr.

DUAL FREQUENCY CCP SOURCES

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

High driving frequencies produce 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.

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

Institute for Plasma Science & Engr.

Tegal 6500 series systems high-density plasma etch tools featuring the HRe–™ capacitively coupled plasma etch reactor and dual-frequency RF power technology.

Coupling between the dual frequencies may interfere with independent control of plasma density, ion energy and produce non-uniformities.

ION ENERGY AND ANGULAR DISTRIBUTIONS (IEAD)

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Control of the ion energy and angular distribution (IEAD) incident onto the substrate is necessary for improving plasma processes.

A narrow, vertically oriented angular IEAD is necessary for anisotropic processing.

Edge effects which perturb the sheath often produce slanted IEADs.

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


Ion velocity trajectories measured by LIF (Jacobs et al.)

IEADs THROUGH SHEATHS

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

Investigation addresses the motion of ion species in the RF pre-sheath and sheath as a function of position in the sheath and 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, pressure of operation, dual-frequency effects.


HYBRID PLASMA EQUIPMENT MODEL (HPEM)

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Monte Carlo Simulation f(ε) or Electron Energy Equation

Electron Magnetic Module (EMM): Maxwell’s equations for electromagnetic inductively coupled fields.

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’s

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.

Maxwell Equation

CircuitModule

I,V(coils) E

EMM E(r,θ,z,φ)B(r,θ,z,φ)

REACTOR GEOMETRY

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Inductively coupled plasma with multi-frequency capacitively coupled bias on substrate.

2D, cylindrically symmetric. Base case conditions

ICP Power: 400 kHz, 480 W Substrate bias: 2 MHz Pressure: 2mTorr

Ar plasmas: Ar , Ar*, Ar+, e

Ar/O2 plasmas: Ar , Ar*, Ar+, e O2 ,O2

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

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 from Δλ=λ0-λL=v//λ0/c

•B. Jacobs, PRL 105, 075001(2010)YZHANG_GEC2011_07


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PLASMA PROPERTIES Majority of power

deposition that produces ions comes from inductively coupled coils.

Te is fairly uniform in the reactor due to high thermal conductivity - peaking near coils where E-field is largest.

Small amount of electro- negativity [O2

-] /[M+] =0.0175, with ions pooling at the peak of the plasma potential.


MIN MAX Log scale

MIN MAX MIN MAX Log scale Ar/O2=80/20, 2mTorr, 50 SCCM

Freq=2 MHz, 500 V DC Bias=-400 V

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Ar+ IEAD FROM BULK TO SHEATH In the bulk plasma and

pre-sheath, the IEAD is essentially thermal and broad in angle. Boundaries of the pre-sheath are hard to determine.

In the sheath, ions are accelerated by the E-field in vertical direction and the angular distribution narrows.

Note: Discontinuities with energy increase caused by mesh resolution in collecting statistics.


MIN MAX Log scale

MIN MAX MIN MAX Log scale

Ar/O2=80/20, 2mTorr, 50 SCCM Freq=2 MHz, 500 V DC Bias=-400 V

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IEAD NEAR EDGE OF WAFER IEADs are separately

collected over wafer middle, edge and chuck regions.

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

Focus ring helps improve uniformity.

Maximum energy consistent regardless of wafer radius.

University of MichiganInstitute for Plasma Science & Engr.MIN MAX

Log scaleMIN MAX MIN MAX

Log scale

Ar/O2=0.8/0.2, 2mTorr, 50 SCCM Freq=2 MHz VRFM=500 Volt DC Bias=-400 Volt

0.5 mm above wafer

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IEAD vs RF PHASE: PRESHEATH

MIN MAX Log scale


B. Jacobs (2010) Ar/O2 = 0.8/0.2, 0.5 mTorr, 50 SCCM LF= 600kHz, 425W HF=2 MHz, 1.5kW Sheath ~3.6 mm LIF measured 4.2

mm above wafer Phase regard to HF

• Ar/O2 =0.8/0.2, 2mTorr, 50 SCCM• Freq=2 MHz, 500 V• DC Bias =-400 V• IEAD 4.2 mm above wafer

IEADs near presheath boundary are independent of phase, and slowly drifting.

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

Experimental result shows the same trend.


Phase

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IEAD UNDER DIFFERENT RF PHASES Due to periodic acceleration in

sheath, 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.

Phase

B. Jacobs (2010) Ar/O2 = 0.8/0.2, 0.5 mTorr, 50 SCCM LF= 600kHz, 425W HF=2 MHz, 1.5kW Sheath ~3.6 mm LIF measured 4.2

mm above wafer Phase regard to HF

• Ar/O2 =0.8/0.2, 2mTorr, 50 SCCM• Freq=2 MHz, 500 V• DC Bias =-400 V• IEAD 0.5 mm above wafer

MIN MAX Log scale

MIN MAX MIN MAX Log scale University of Michigan


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IEAD vs PHASES FROM BULK TO SHEATH


MIN MAX Log scale


• Ar/O2 =0.8/0.2, 2mTorr, 50 SCCM,Freq=2 MHz, 500 V• DC Bias =-400 V ,IEAD 0.5 mm above wafer

3.3 mm

2.6 mm

1.9 mm

1.2 mm

0.5 mm

Phase

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O2 ADDITION TO Ar

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

With increasing O2 in Ar/O2, negative ion ( O-) formation decreases fluxes to substrate for fixed power.

Sheath potential only moderately increases - for up to 20% O2, IEADs are only nominally affected since negative ions are limited to core of plasma.


MIN MAX Log scale


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IEADs vs PRESSURE With decreasing pressure and increasing

mean free path, trajectories are more ballistic - ions still drift into wafer at low energy during anodic part of cycle.

With higher pressure, lower plasma density increases thickness of sheath . Thicker sheath, more collisions, longer transit time – more distributed ion trajectories through sheath.


MIN MAX Log scale


Ar+ IEAD on wafer 5/10/20mTorr, 75/150/300 SCCM. Freq=2 MHz, 500 V DC Bias =-400 V

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IEADs vs HIGH FREQUENCY

MIN MAX Log scale



If high frequency (10 MHz) is close to low frequency (2 MHz), they will interfere each other and contribute to multiple peaks in IEADs.

When high frequency is largely separated from the low frequency (2 MHz) since they changes so fast that ion fail to response, 30 MHz and 60 MHz show similar properties for ion distribution function.

Ar/O2=0.8/0.2, 2mTorr, 50 SCCM HF = 10/30/60 MHz, 100 V LF = 2 MHz 400 V DC BIAS = -100 V, IEAD on wafer

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DUAL-FREQ IEAD vs PHASES

High frequency produces additional peaks in IEADs compared to single low frequency – structure is phase dependent.

Experiments show similar trend.

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

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

SCCM LF=600kHz, 425W HF=2MHz, 1.5kW Phase refers to HF

MIN MAX Log scale

MIN MAX MIN MAX Log scale University of Michigan


Ar/O2=0.8/0.2, 2mTorr, 50 SCCM HF = 30 MHz, 100 V LF = 2 MHz, 400 V DC BIAS = -100 V, Phase refers to LF IEAD 0.5mm above wafer

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SHEATH vs HIGH FREQUENCY

MIN MAX Log scale



Ar/O2=0.8/0.2, 2mTorr, 50 SCCM HF = 10/60 MHz, 100 V LF = 2 MHz, 400 V DC BIAS = -100 V, Phase refers to LF IEAD 0.5mm above wafer

The sheath and pre-sheath thickness are nearly independent of HF on substrate (for fixed voltage).

Higher frequencies add modulation onto IEADs as a function of phase.

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.

Multiple peaks in IEADs come from IEADs alternately accelerated by rf field during the whole RF period.

Sheath and Pre-sheath thicknesses are both increased with the pressure. On the other hand, higher pressure bring more collisions and ions reach low energy and broad angular distribution.

Dual Frequency enhance electron and ion densities, provide flexibility of control of ion distribution while adding modulation to IEAD.


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