DEVELOPMENT OF ION ENERGY DISTRIBUTIONS THROUGH THE PRE-SHEATH AND SHEATH IN DUAL-FREQUENCY CAPACITIVELY COUPLED PLASMAS*

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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 (yitingz_at_umich.edu ,
mjkush_at_umich.edu) (b) Department of Physics,
University of California, Los Angeles, CA
90095 (moore_at_physics.ucla.edu ,
gekelman_at_physics.ucla.edu ) November 16, 2011
Work supported by National Science Foundation,
Semiconductor Research Corp. and the DOE Office
of Fusion Energy Science
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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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DUAL FREQUENCY CCP SOURCES

• 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.

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

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

? A. Perret, Appl. Phys.Lett 86 (2005)
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ION ENERGY AND ANGULAR DISTRIBUTIONS (IEAD)

• 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.

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

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

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IEADs THROUGH SHEATHS

• 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.

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HYBRID PLASMA EQUIPMENT MODEL (HPEM)
EETM
FKM
EMM
E(r,?,z,f) B(r,?,z,f)
PCMCM
Monte Carlo Simulation f(e) or Electron Energy
Equation
Se(r)
Maxwell Equation
Continuity, Momentum, Energy, Poisson equation
Monte Carlo Module
N(r) Es(r)
I,V(coils) E
Circuit Module

• Electron Magnetic Module (EMM)
• Maxwells 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 Poissons equations.
• Plasma Chemistry Monte Carlo Module (PCMCM)
• IEADs in bulk, pre-sheath, sheath, and wafers.
• Recorded phase, submesh resolution.

• M. Kushner, J. Phys.D Appl. Phys. 42 (2009)

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REACTOR GEOMETRY

• 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-

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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 ?0,
• Ion velocity parallel to the laser obtained from
  ???0-?Lv//?0/c

• B. Jacobs, PRL 105, 075001(2010)

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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.

• Ar/O280/20, 2mTorr, 50 SCCM
• Freq2 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.

• Ar/O280/20, 2mTorr, 50 SCCM
• Freq2 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.

0.5 mm above wafer

• Ar/O20.8/0.2, 2mTorr, 50 SCCM
• Freq2 MHz VRFM500 Volt
• DC Bias-400 Volt

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IEAD vs RF PHASE PRESHEATH

• 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

• B. Jacobs (2010)
• Ar/O2 0.8/0.2,
• 0.5 mTorr, 50 SCCM
• LF 600kHz, 425W
• HF2 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
• Freq2 MHz, 500 V
• DC Bias -400 V
• IEAD 4.2 mm above wafer

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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
• HF2 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
• Freq2 MHz, 500 V
• DC Bias -400 V
• IEAD 0.5 mm above wafer

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IEAD vs PHASES FROM BULK TO SHEATH
3.3 mm
Phase
2.6 mm
1.9 mm
1.2 mm

• Ar/O2 0.8/0.2, 2mTorr, 50 SCCM,Freq2 MHz, 500 V
• DC Bias -400 V ,IEAD 0.5 mm above wafer

0.5 mm
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O2 ADDITION TO Ar

• 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.

• Ar IEAD on wafer
• 2 mTorr, 300 SCCM.
• Freq2 MHz, 300 W.

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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.

• Ar IEAD on wafer
• 5/10/20mTorr, 75/150/300 SCCM.
• Freq2 MHz, 500 V
• DC Bias -400 V

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IEADs vs HIGH FREQUENCY

• 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/O20.8/0.2, 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/O20.8/0.2,
• 0.5 mTorr, 50 SCCM
• LF600kHz, 425W
• HF2MHz, 1.5kW
• Phase refers to HF

• Ar/O20.8/0.2, 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

• 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.

• Ar/O20.8/0.2, 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

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CONCLUDING REMARKS

• 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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