Title: DEVELOPMENT OF ION ENERGY DISTRIBUTIONS THROUGH THE PRE-SHEATH AND SHEATH IN DUAL-FREQUENCY CAPACITIVELY COUPLED PLASMAS*
1DEVELOPMENT 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
2AGENDA
- 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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3DUAL 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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4ION 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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5IEADs 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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6HYBRID 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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7REACTOR 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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8PULSED 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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9PLASMA 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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10Ar 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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11IEAD 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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12IEAD 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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13IEAD 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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14IEAD 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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15O2 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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16IEADs 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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17IEADs 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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18DUAL-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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19SHEATH 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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20CONCLUDING 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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