Transcript Document

COMPUTATIONAL INVESTIGATION OF DUALFREQUENCY POWER TRANSFER IN
CAPACITIVELY COUPLED PLASMAS*
Yiting Zhang and Mark J. Kushner
Department of Electrical and Computer Engineering,
University of Michigan, Ann Arbor, 48109
([email protected], [email protected])
Sang Ki Nam and Saravanapriyan Sriranman
Lam Research Corp., Fremont, CA 94538
([email protected],
[email protected])
June 17, 2013
*
Work supported by Semiconductor Research Cooperation, National Science
Foundation and the DOE Office of Fusion Energy Sciences.
AGENDA
 Dual frequency Capacitively Coupled Plasma (CCPs) and
Ion Energy Angular Distributions (IEADs)
 Description of the model
 IEADs and plasma properties for dual-frequency Ar
plasma
 Voltage control vs. power control
 Ratio of dual frequencies
 Phase shift
 Concluding remarks
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DUAL FREQUENCY CCP SOURCES
 Dual frequency capacitively coupled discharges (CCPs) are
widely used for etching and deposition in the
microelectronics industry.
 High frequencies produce higher electron densities at
moderate sheath voltage and higher ion fluxes with
moderate ion energies.
 Low frequencies contribute to 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.
 LAMRC 2300 Flex dielectric
etch tool
 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.
 Ion velocity trajectories measured
by LIF (Jacobs et al.)
 A narrow angle, vertically
oriented IEAD is necessary
for anisotropic processing.
 Edge effects which perturb
the sheath often produce
slanted IEADs.
● S.-B. Wang and A.E. Wendt, JAP
● B. Jacobs, PhD Dissertation
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ICOPS_2013
CONTROL OF IEADs
 As the size of transistors shrink, the critical dimension
requirements of semiconductor fabrication become more
stringent, and therefore more precise control of IEADs
becomes important.
 This computational
investigation addresses
plasma dynamics and control
of IEADs onto wafers in dual
frequency CCPs.
 Controlling the voltage,
power and phase difference
between dual-frequencies to
customize IEADs will be
discussed.
 SEM of a high aspect ratio profile
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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)
PCMCM
Continuity, Momentum,
Energy, Poisson
equation
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’s
equations.
 Plasma Chemistry Monte Carlo Module (PCMCM):
 IEADs in bulk, pre-sheath, sheath, and wafers.
 Recorded phase, submesh resolution.
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REACTOR GEOMETRY
 Capacitively coupled plasma
with multi-frequency rf biases
on bottom electrode.
 2D, cylindrically symmetric.
 Ar plasma: Ar, Ar(1s2,3,4,5),
Ar(4p), Ar+, e
 Base case conditions:
 Ar, 30 mTorr, 1000 sccm
 Voltage Control (VC):
2 MHz, 300 V; 60 MHz, 300 V
 Power Control (PC):
2 MHz, 300 W; 60 MHz, 300 W
(Note: Y:X = 2:1)

Real geometry aspect ratio
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ELECTRON DENSITY, TEMPERATURE
 VC: 300 V, 2 MHz; 300 V, 60 MHz
 PC: 300 W, 2 MHz; 300 W, 60 MHz
 HF mainly contributes to ionization, and thus the current
generated by HF is larger than the LF current.
 With voltage control, larger HF power (2926 W total) generates
higher ne. With power control, relatively low HF (51 V) due to
higher efficiency of electron heating generates lower electron
density.
 Uniformity increases with higher ionization.
● Ar, 30 mTorr, 1000 sccm
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MIN
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Institute for Plasma Science & Engr.
MAX
Ar+ IEAD FROM BULK TO SHEATH
 VC :300 V, 2 MHz; 300 V, 60 MHz
DC bias -256 V
 The lower ne with PC produces
a thicker time averaged sheath
thickness.
 Longer ion transit time in thick
sheath makes the IED curve
smoother and lower in energy.
 PC :300 W, 2 MHz, 300 W 60 MHz
 IED on wafer
DC bias -123 V
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MIN
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Institute for Plasma Science & Engr.
MAX
HIGH FREQUENCY VOLTAGE
 The plasma impedance changes little, total power increases
with VHF2.
 With higher HF voltage, plasma density increases significantly.
 Higher power correlates with better uniformity.
● Ar, 30 mTorr, 1000 sccm
ICOPS_2013
MIN
University of Michigan
Institute for Plasma Science & Engr.
MAX
HIGH FREQUENCY VOLTAGE
 Changing HF/LF voltage ratio will strongly affect the IEADs.
With higher HF, self generated DC bias becomes more
negative, and the total IEAD shifts to higher level due to
higher sheath potential during anodic LF cycle.
 Broadening in high energy peaks due more HF modulation.
The energy width is almost independent of HF amplitude.
DC bias
- 256 V
● Ar, 30 mTorr, 1000 sccm
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MIN
DC bias
- 355 V
DC bias
- 459 V
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MAX
LOW FREQUENCY VOLTAGE
 Increasing LF (300 V to 600 V) has little effect on ne and Te
since electron heating scales with 2.
 Majority of additional power results in ion acceleration.
● Ar, 30 mTorr, 1000 sccm
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MIN
University of Michigan
Institute for Plasma Science & Engr.
MAX
LOW FREQUENCY VOLTAGE
 Increasing LF voltage shapes IED (e.g., ΔE) with little change in
plasma properties.
 During the cathodic LF cycle, increase in sheath potential
accelerates ions to higher energy.
 During anodic LF cycle, sheath potential is dominated by HF
which is unchanged – and so modulation of IED persists.
- 256 V, DC - 307 V, DC - 371 V, DC
● Ar, 30 mTorr, 1000 sccm
ICOPS_2013
MIN
University of Michigan
Institute for Plasma Science & Engr.
MAX
HIGH FREQ POWER
 With increase in HF
power 300 W to 900 W,
HF voltage changes by
only 30 V.
 Amplitude is always
small compared to LF,
and so width of IEAD
does not significantly
change.
 Increase in ne with power
reduces sheath width
which then modulates
IEAD at HF.
● Ar, 30 mTorr, 1000 sccm
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MIN
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Institute for Plasma Science & Engr.
MAX
LOW FREQ POWER
 The plasma density and
uniformity change little with
LF power.
 At low frequency, LF
voltage increases nearly
linear with power.
 The LF power is mainly
deposited into sheath – the
width of IED increases with
LF power.
● Ar, 30 mTorr, 1000 sccm
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MIN
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Institute for Plasma Science & Engr.
MAX
IED DEPENDENCE ON ΔPHASE
 Energy of HF modulated peaks in IED depend on relative phase
between LF and HF.
 Shift of energy of peaks depends on value of high frequency due
in part to change in sheath thickness.
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● Ar, 30 mTorr, 1000 sccm
MIN
MAX
SHEATH vs ΔPHASE
 Phase difference between
LF and HF modulates
sheath potential and
electron dynamics during
rf period.
 The sheath thickness
(scales with [e]-1/2 ) is larger
in cathodic LF cycle, and
so brings about less
modulation in high energy
peak of IED.
 By dynamically controlling
phase difference, a smooth
time averaged IED can be
produce without HF
modulation.
● Ar, 30 mTorr, 1000 sccm
ICOPS_2013
300V 2 MHz
300V 60 MHz
CONCLUDING REMARKS
 For dual frequency CCPs sustained in Ar plasma,
 With higher frequency , the current generated by HF is
greater than the LF current.
 Increasing HF voltage will increase the plasma density
as well as shift the total IEADs to higher energies.
 Increasing LF voltage will mainly deposit power within
sheath, and therefore extend the IEAD energy width
with little change in composition of fluxes.
 Changing phase between HF and LF in high density, thin
sheath plasma will modify time averaged IEADs
significantly.
 With knowledge of the relationship between IEADs and
settings of dual frequency rf biases, precise customization
and control of IEADs can be achieved.
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ICOPS_2013