Transcript ICOPS_agarwal_2007_v6

RECIPES FOR PLASMA
ATOMIC LAYER ETCHING*
Ankur Agarwala) and Mark J. Kushnerb)
a)Department
of Chemical and Biomolecular Engineering
University of Illinois, Urbana, IL 61801, USA
[email protected]
b)Department
of Electrical and Computer Engineering
Iowa State University, Ames, IA 50011, USA
[email protected]
http://uigelz.ece.iastate.edu
34th IEEE ICOPS, June 2007
*Work supported by the SRC and NSF
AGENDA
 Atomic Layer Processing
 Plasma Atomic Layer Etching (PALE)
 Non-sinusoidal Bias Waveforms
 Tailored Bias PALE Recipes
 SiO2 using Ar/c-C4F8
 Self-aligned contacts
 Concluding Remarks
ANKUR_ICOPS07_Agenda
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ATOMIC LAYER PROCESSING
 Advanced microelectronics structures
require extreme selectivity in etching
materials with nm resolution.
 Atomic layer plasma processing may
allow for this level of control.
 Double Gate MOSFET
 Current techniques employ
specialized ion beam equipment.
 The high cost of atomic layer
processing challenges its use.
 Plasma Atomic Layer Etching (PALE)
is potentially an economic alternative.
 Tri-gate MOSFET
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Refs: AIST, Japan; Intel Corporation
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Optical and Discharge Physics
PLASMA ATOMIC LAYER ETCHING (PALE)
 In PALE etching proceeds monolayer by monolayer in a cyclic, self
limiting process.
 First step: Top monolayer is passivated in non-etching plasma.
 Passivation makes top layer more easily etched compared to
sub-layers.
 Second step: Remove top layer (self limiting).
 Exceeding threshold energy results in etching beyond top layer.
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PLASMA ATOMIC LAYER ETCHING (PALE)
 PALE has been computationally and experimentally
investigated using conventional plasma equipment.
 Inductively coupled plasma (ICP)
 Capacitively coupled plasma (CCP)
 Since the equipment is already in fabrication facilities, no
additional integration costs are incurred.
 The low speed of PALE processes hinder its integration into
production line.
 Speed can be increased but only at the cost of losing control
of CD (critical dimensions) or damaging material interfaces.
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INCREASING SPEED
OF PALE … HOW?
 Conventional PALE
 Different gas mixtures for each step.
 Although self-limiting, purge steps
increase process time.
 Conventional PALE
 Tailored bias PALE
 Create nearly mono-energetic ion
distribution.
 Control ion energies via changes in
voltage amplitude.
 Single gas mixture for both steps
eliminates purge and reduces time.
 Tailored Bias PALE
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NON-SINUSOIDAL BIAS WAVEFORMS: IEADs
Vp-p
 Custom waveform produces nearly
constant sheath potential resulting
in narrow IEAD.
 Peak energy of IEAD is controlled
by amplitude.
 IED broadens at higher biases due
to thickening of sheath and longer
transit times.
  = 10%; Vp-p = 200 V
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Ref: A. Agarwal and M.J. Kushner, J. Vac.
Sci. Technol. A, 23, 1440 (2005)
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HYBRID PLASMA EQUIPMENT MODEL (HPEM)
 Electromagnetics Module:
Antenna generated electric and
magnetic fields
 Electron Energy Transport
Module: Beam and bulk generated
sources and transport
coefficients.
 Fluid Kinetics Module: Electron
and Heavy Particle Transport,
Poisson’s equation
 Plasma Chemistry Monte Carlo
Module:
 Ion and Neutral Energy and
Angular Distributions
 Fluxes for feature profile model
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MONTE CARLO FEATURE PROFILE MODEL
 Monte Carlo techniques address
plasma surface interactions and
evolution of surface morphology
and profiles.
 Inputs:
 Initial material mesh
 Surface reaction mechanism
 Ion and neutral energy and
angular distributions
 Fluxes at selected wafer
locations.
 Fluxes and distributions from
equipment scale model (HPEM)
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FLUOROCARBON PLASMA ETCHING OF SiO2/Si
 CFx radicals produce polymeric
passivation layers which regulate
delivery of precursors and activation
energy.
 Chemisorption of CFx produces a
complex at the oxide-polymer
interface
I*, CF 2
Plasma
CxFy
Passi vation
Layer
 As SiO2 consumes the polymer,
thicker layers on Si slow etch rates
enabling selectivity.
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CxFy
Polymer
Passi vation
Layer
Ion
I*,+CF 2
++
IonIon
CO
CO 2
+ 2
Ion
CFx
CxFy
Plasma
CO 2
Ion +,F
Ion +
SiF 3
CO 2
Polymer
SiO 2CxFy
SiO2
SiO2
SiOCFy
SiO 2CxFy
SiF 3
SiOCFy
Ion +,F
 Low energy ion activation of the
complex produces polymer.
 Polymer complex sputtered by
energetic ions  etching.
+
CFx Ion
CxFy
Plasma
F
CFx
Plasma
F
CFx
SiF 3
Ion +,F
SiF 3
CxFy
Passi vation
Layer
CxFPolymer
y
Passi vation
Layer
SiF
SiF 2
Si
Si
SiF
Polymer
SiF 3
SiF 2
SiF 3
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Ion
MAIN ETCH-PALE FOR
VERY HIGH ASPECT RATIO FEATURES
 PALE will always be slow compared to
conventional etching.
 Selectivity of PALE is only needed at end of
etch at material interface.
 Combine:
 Rapid “main etch” to reach material
interface
 PALE to clear feature with high
selectivity.
 Feature to be investigated is SiO2-over-Si
trench with an aspect ratio of 1:10.
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 10:1 Trench
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Ar/c-C4F8 ICP FOR SiO2 ETCHING
 Test system is inductively coupled
plasma with 5 MHz biased substrate.
 Ar/C4F8 = 75/25, 100 sccm, 15 mTorr,
500 W ICP
 Main etch is conventional sinusoidal
waveform.
 PALE uses tailored bias waveform:
 Passivate: 50 V (peak-to-peak)
 Etch: 100 V (peak-to-peak)
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MAIN ETCH OF SiO2-over-Si
Mask
 Main etch performed using a
sinusoidal bias waveform.
 Micro-trenching at sides of feature
due to specular reflection off walls.
 Central SiO2 remains when
underlying Si is exposed.
 Significant etching into Si during
over-etch to clear feature.
SiO2
 Ar/C4F8 = 75/25, 100 sccm, 15 mTorr, 500
W, 100 V at 5 MHz
Si
Aspect Ratio = 1:10
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ANIMATION SLIDE-GIF
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Ar/c-C4F8 TAILORED BIAS PALE: IEADs
 PALE of SiO2 using ICP Ar/C4F8 with
variable bias.
 Step 1
 Vp-p = 50 V
 Passivate single layer with SiO2CxFy
 Low ion energies to reduce etching.
 Step 2
 Vp-p = 100 V
 Etch/Sputter SiO2CxFy layer.
 Above threshold ion energies.
 Narrow IEADs enable discrimination
between threshold energies of undelying
SiO2 and polymer complex.
 Ar/C4F8 = 75/25, 100 sccm, 15 mTorr, 500 W
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Optical and Discharge Physics
SiO2-over-Si: PALE vs CONVENTIONAL ETCH
SiO2
Si
 5 cycles of PALE
 Conventional Etching
 Narrow IEAD enables etching of rough initial profile at bottom.
 Redeposition of etched products and polymer cover exposed Si
and sidewall; avoids notching and damage.
 High speeds (~ 4 ML/cycle) with high etch selectivity.
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ANIMATION SLIDE-GIF
 1 cell = 3 Å
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PALE: ROUGHNESS vs STEP 2 ION ENERGY
 Speed of PALE can be increased
via change in ion energies.
 110 eV
 At high ion energies, distinction
between threshold energies is
lost.
 Final etch profile is rough.
 Already exposed underlying Si
vulnerable at high ion energy.
 120 eV
 140 eV
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 Surface roughness scales
linearly with ion energies.
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PALE: ETCH RATE vs STEP 2 ION ENERGY
 Number of PALE cycles required
to clear feature decrease with
increasing ion energy.
 Etch rate saturates at high ion
energies due to the rough initial
feature profile.
 Trade-off between high etching
rates and selectivity.
 Etching of already exposed
underlying Si leads to roughness.
 Initial
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 Final
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PALE: CONVENTIONAL vs TAILORED BIAS
SiO2CxFy
Plasma
SiO2
Si
 Tailored: 5 cycles
 Conventional: 20 cycles
 Conventional PALE scheme utilizes 20 cycles.
 High speeds (~ 3-4 ML/cycle) and extreme selectivity of PALE
enable fast etching of self-aligned contacts.
 Final etch profile is smooth even at high etching rates.
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ANIMATION SLIDE-GIF
 1 cell = 3 Å
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Optical and Discharge Physics
CONCLUDING REMARKS
 Atomic layer control of etch processes will be critical for 32 nm
node devices.
 PALE using conventional plasma equipment makes for an
more economic processes.
 Slow etching rates of conventional PALE need to be optimized:
trade-off between high selectivity and etch rate
 PALE of SiO2 in Ar/c-C4F8 plasma investigated using custom
bias waveforms,
 Non-sinusoidal bias waveforms enable:
 Precision control of IEADs
 Elimination of purge step to increase process speeds
 High selectivity at high etching rates (~ 4 ML/cycle)
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Iowa State University
Optical and Discharge Physics