Transcript No Slide Title

DEVELOPMENT OF ION ENERGY
DISTRIBUTIONS THROUGH THE PRESHEATH 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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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.
 A. Perret, Appl. Phys.Lett 86 (2005)
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 Tegal 6500 series systems high-density plasma etch tools
featuring the HRe–™ capacitively coupled plasma etch
reactor and dual-frequency RF power technology.
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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.
 Ion velocity trajectories measured
by LIF (Jacobs et al.)
 A narrow, vertically oriented
angular IEAD is necessary for
anisotropic processing.
 Edge effects which perturb the
sheath often 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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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)
EMM
E(r,θ,z,φ)
B(r,θ,z,φ)
Maxwell
Equation
I,V(coils)
E
Circuit
Module
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 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.
• 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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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 from Δλ=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
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 electronegativity [O2-] /[M+]
=0.0175, with ions
pooling at the peak of
the plasma potential.
MIN
Log scale
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MAX
 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 presheath 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
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 Ar/O2=80/20, 2mTorr, 50 SCCM
 Freq=2 MHz, 500 V
 DC Bias=-400 V
Log scale
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MAX
IEAD NEAR EDGE OF WAFER
 IEADs are separately
collected over wafer
middle, edge and chuck
regions.
0.5 mm above wafer
 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.
 Ar/O2=0.8/0.2, 2mTorr, 50 SCCM
 Freq=2 MHz VRFM=500 Volt
 DC Bias=-400 Volt
YZHANG_GEC2011_10
MIN
Log scale
MAX
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IEAD vs RF PHASE: PRESHEATH
 IEADs near presheath
boundary are independent of
phase, and slowly drifting.
Phase
 In the pre-sheath, small ion
drifts cause the IEAD to
slightly change vs phase.
 Experimental result shows
the same trend.







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
•
MIN
Log scale
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MAX •
•
•
Ar/O2 =0.8/0.2, 2mTorr, 50 SCCM
Freq=2 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.
Phase
 During low acceleration phases,
IEAD drifts in sheath.
 During high acceleration phase,
IEAD narrows as perpendicular
component of velocity distribution
increases.







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
MIN
Log scale
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MAX
•
•
•
•
Ar/O2 =0.8/0.2, 2mTorr, 50 SCCM
Freq=2 MHz, 500 V
DC Bias =-400 V
IEAD 0.5 mm above wafer
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IEAD vs PHASES FROM BULK TO SHEATH
MIN
Phase
Log scale
MAX
3.3 mm
2.6 mm
1.9 mm
1.2 mm
0.5 mm
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•
•
Ar/O2 =0.8/0.2, 2mTorr, 50 SCCM,Freq=2 MHz, 500 V
DC Bias =-400 V ,IEAD 0.5 mm above wafer
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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.
MIN
Log scale
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MAX
 Ar+ IEAD on wafer
 2 mTorr, 300 SCCM.
 Freq=2 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.




MIN
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Log scale
MAX
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

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.
MIN
YZHANG_GEC2011_16




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
Log scale
MAX
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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.
MIN
Log scale
MAX
YZHANG_GEC2011_17




 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
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

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.
MIN
YZHANG_GEC2011_18




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
Log scale
MAX
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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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