Transcript No Slide Title

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
([email protected] , [email protected])
(b) Department
of Physics, University of California,
Los Angeles, 90095
([email protected] , [email protected] )
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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DUAL FREQUENCY CCP SOURCES
 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 quasiindependent 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)
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ION ENERGY AND ANGULAR DISTRIBUTIONS (IEAD)
• 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.
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•S.-B. Wang and A.E. Wendt,
• J. Appl. Phys., Vol 88, No.2
•B. Jacobs, PhD Dissertation
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Institute for Plasma Science & Engr.
GOALS
 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 submeshing 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.
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HYBRID PLASMA EQUIPMENT MODEL (HPEM)
EETM
Monte Carlo Simulation f(ε)
or Electron Energy
Equation
FKM
Se(r)
N(r)
Es(r)
Continuity, Momentum,
Energy, Poisson equation
PCMCM
Monte
Carlo
Module
 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
•M.Kushner, J. Phys.D: Appl. Phys. 42(2009)
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REACTOR GEOMETRY
 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+, OYZHANG_MIPSE2011_06
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PLASMA PROPERTIES
Ion Density (cm-3)
 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.
•
•
•
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Ar/O2 =0.8/0.2,
20mTorr, 300 SCCM
Freq=2 MHz, 300 Watt
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radiation to the etch tool (located in the Plasma Processing Lab, down the hall from t
Room). A Gaussian intensity profile is ideal for uniformly filling a multi-mode optic
was necessary when operating near the damage threshold of the fiber.
PULSED LASER-INDUCED
FLUORESCENCE (LIF)
The Intevac Corporation donated a state of the art plasm a etch tool for proces

industry-standard 30 cm diameter silicon wafer substrates for use in this experiment
(see Fig. 3) was approximately cylindrical, with a maximum inside diamete
A non-invasive opticalchamber
technique
and a height of 40 cm. The experimental bulk plasma was produced by a low freque
kHz, 425 W) inductively coupled plasma (ICP) discharge. A higher frequency (2.2 M
for measuring the ion velocity
capacitively coupled RF bias was applied simultaneously through a metal electr
distribution function. kW)
ceramic support structure holding the substrate; the vacuum chamber walls served as
currents to return to groun d.
 Ions moving along the direction of
laser propagation will have the
FIG. 2. Schematic diagram of the Laser Room optics showing the lasers, iodine calibration sy
absorption wavelengths
Dopplershaping optics.
The abbreviations are BD for beam dump, BS beam splitter, L lens, M mirror, N
filter and PMT photomultiplier tube. The upper optical system conditions the beam profile and
shifted from λ0,
into the optical fiber. The iodine vapor system provides an absolute wavelength calibration o
 Ion velocity parallel to the
The laser
dye laser spectrum was measured with a Fabry-Perot interferometer t
Gaussian with a FWHM bandwidth of only 0.0013 nm (correspon
obtained fromΔλ=λapproximately
0-λL=v//λ0/c
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temperature of T L = 0.033 eV), however there is a broadband (as wide as the lase
598-636 nm), unpolarized component to the radiation output from a phenomenon
Amplified Spontaneous Emission (ASE) [34]. Although ASE can be minimized t
proper alignment of the dy e laser resonance cavity, it cannot be completely elimi
case, the estimated fraction of ASE is less than 1% of the total radiation output , b
to reduce it as much as possible as it causes a constant background LIF signal at a
wavelength tunings. The dye laser output energy was also very high (typically 90
much higher than the damage threshold for the optical fiber (approximately 1 mJ
FIG.
3. Chamber
Schematic:
laserhigh
beampower
is incident
from the top of the machine
such that
the ver
address
both
of these
issues,the
two
Glan-Thompson
polarizing
beam
sp
BS2) were employed in series. BS1 is aligned such that its polarization axis was
University
of Michigan
dye laser's polarization. Half of the unpolarized
ASE radiation
was thus split off
•B. Jacobs, PRL 105, 075001(2010)
for of
Plasma
beam, and sent to a beam dump. Institute
The other half
the ASEScience
radiation&
waEngr.
s subsequ
Ar+ IEAD FROM BULK TO SHEATH
 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 Efield in z direction and
the angle narrows.
 Ar, 20mTorr, 300 SCCM
 HF=30 MHz 100Watt
 LF=2 MHz 300Watt
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IEAD NEAR EDGE OF WAFER
Center
Middle
 IEADs are separately
collected over center,
middle and edge
regions.
Edge
 Non-uniformity near the
edge region - IEAD has
broader angular
distribution.
０.5 mm above wafer
 Maximum energy
consistent regardless of
radius.
•
•
•
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Ar, 20mTorr, 300 SCCM
HF=30MHz 100Watt
LF=2 MHz 300Watt
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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.
•
B.Jacobs,
W.Gekelman,
PRL 105,
075001(2010)
 Ar/O2=0.8/0.2,
 0.5 mTorr, 50
SCCM
 LF600kHz, 425W
 HF=2MHz, 1.5kW
 Phase refers to
HF
•
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•
Ar, 20mTorr, 300 SCCM
HF=30MHz 100Watt
LF=2 MHz 300Watt
Phase refer LF
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FIG. 14. Time-averaged vertical ion energy distribution (IED) measured by LIF at z = 1.06 mm. The energy spltting
of the two peaks is ΔEi = 380 eV at this location. The expected ΔEi at z = 0, the substrate surface is 1100 V.
Fig. 15 shows a family of vertical IVDFs taken at z = 42 mm above the substrate. All 8
IEAD UNDER
DIFFERENT RF PHASES
phases of LIF data are shown. The IVDFs are independent of the RF phase as the bias voltages
do not penetrate far into the pre -sheath region[38].
 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.
•
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
•
•
•
•
FIG. 15. Collection of vertical IVDFs measured by LIF at z = 42 mm.
•
•
•
•
Fig. 16 shows a family of IVDFs taken at z = 1.06 mm above the substrate. The shape of
the IVDFs depend strongly on the RF phase; the observation of very high energy ions (π/4 and
π/2) is delayed by about a quarter of the RF period from the phase of maximum ion acceleration
(the most negative sheath potential). This has been predicted by Bose et al.[5] to be due to the
ion inertia.
Ar/O2
=0.8/0.2,
20mTorr,
300 SCCM
Freq=2 MHz
IEAD 4 mm
above wafer
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IEAD UNDER DIFFERENT RF PHASES
•
B. Jacobs, W.
 Due to periodic acceleration in
Gekelman, PRL
sheath, development of IEAD
105, 075001(2010)
depends on phase.
 Ar/O2=0.8/0.2,
 0.5 mTorr, 50
 During low acceleration
SCCM
phases, IEAD drifts in sheath.
 HF600kHz, 425W
 LF=2 MHz, 1.5kW
 During high acceleration
 Sheath ~3.6 mm
phase, IEAD narrows as
 LIF measured 1
perpendicular component of
mm above wafer
velocity distribution increases.FIG. 16. Collection of IVDFs measured by LIF at z = 1.06 mm.
Figure 17 shows 4 3-D surface contours of IVDF’s; each contour shows data taken at a
different phase of the RF bias.
 Ar/O2
=0.8/0.2,
 20mTorr, 300
SCCM
 Freq=2 MHz
 IEAD 0.5 mm
above wafer
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O2 ADDITION TO AR
 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.
 Ar+ IEAD on wafer
 20 mTorr, 300 SCCM.
 Freq=2 MHz, 300 W.
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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.
 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%.
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