Transcript Plasma Etching of Extremely High Aspect Ratio Features: Twisting

PLASMA ETCHING OF EXTREMELY HIGH
ASPECT RATIO FEATURES:
TWISTING EFFECTS*
Mingmei Wanga), Ankur Agarwalb), Yang Yanga) and
Mark J. Kushnera)
a)Iowa
State University, Ames, IA 50011, USA
[email protected]
[email protected]
b)University
of Illinois, Urbana, IL 61801, USA
http://uigelz.ece.iastate.edu
60th Gaseous Electronics Conference, October 2007
*Work supported by Micron Technology Inc., SRC and NSF
AGENDA
 High Aspect Ratio Contact (HARC) Etching
 Approach and Methodology
 Charging of features
 Fluorocarbon etching of HARC
 SiO2-over-Si etching
 Potential
 Effect of open field
 Concluding Remarks
MINGMEI_GEC07_AGENDA
Iowa State University
Optical and Discharge Physics
HARC ETCHING: ISSUES
 As aspect ratio (AR) of features increases, complexity of plasma
etching increases.
 Aspect Ratio Dependent Etching
 Etch rate decreases with increasing AR.
 Charging of features due to ion and
electron bombardment.
 Electric field variations affect ion
trajectories; deviation from ideal profile.
 Non-uniform ion flux despite uniform bulk
plasma.
 As AR increases, the cross-sectional
area of each via is smaller.
 Increasingly random nature of incident
ions and radicals.
MINGMEI_GEC07_01
Ref: Micron Technology, Inc.
Iowa State University
Optical and Discharge Physics
OBJECTIVES AND APPROACH
 Objectives
 Computationally investigate consequences of charging of
high aspect ratio features in SiO2.
 Approach
 Reactor scale: Hybrid Plasma Equipment Model.
 Feature scale: Monte Carlo Feature Profile Model.
 Poisson’s equation is solved for electric potentials.
 Acceleration of ions and electrons due to electric fields in
feature.
 Dissipation of charge through material conductivity.
MINGMEI_GEC07_02
Iowa State University
Optical and Discharge Physics
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
MINGMEI_GEC07_04
Iowa State University
Optical and Discharge Physics
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.
 Ion and radical fluxes at selected
wafer locations.
 Maxwellian electron fluxes with
Lambertian distribution
 Fluxes and distributions from HPEM.
MINGMEI_GEC07_05
Iowa State University
Optical and Discharge Physics
MCFPM: CHARGING ALGORITHMS
 The electric potential is solved using
the method of Successive Over
Relaxation (SOR).
 Large mesh sizes pose computational
challenges to solve for potential after
launch of each particle.
 Electric field is being updated after the
launch of every 30 charged particles.
Charged particle
Mask
+
SiO2
+
+
+
+
-
+
 Particles are a few nm on a side.
 Total particles launched (ions and
radicals): 150,000-300,000.
- -
+
+
+ +
Si
 The charge of pseudo-particles mesh
is adjusted to account for finite sized
particles.
MINGMEI_GEC07_03
Iowa State University
Optical and Discharge Physics
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
Passivation
Layer
 As SiO2 consumes the polymer,
thicker layers on Si slow etch rates
enabling selectivity.
MINGMEI_GEC07_06
CxFy
Polymer
Passivation
Layer
Ion
I*,+CF 2
++
IonIon
CO
CO2
+ 2
Ion
CFx
CxFy
Plasma
CO2
Ion +,F
Ion +
SiF3
CO2
Polymer
SiO2CxFy
SiO2
SiO2
SiOCFy
SiO2CxFy
SiF3
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
SiF3
Ion +,F
SiF3
CxFy
Passivation
Layer
CxFPolymer
y
Passivation
Layer
SiF
SiF2
Si
Si
SiF
Polymer
SiF3
SiF2
SiF3
Iowa State University
Optical and Discharge Physics
Ion
FLUOROCARBON ETCH OF HARC
 Dual frequency capacitivelycoupled (CCP) reactor
geometry.
 Base case conditions:
 Ar/C4F8/O2 = 80/15/5, 300
sccm
 40 mTorr
 500 W at 25 MHz
 4000 W at 10 MHz
 Low frequency: Substrate
High Frequency: Showerhead
MINGMEI_GEC07_07
Iowa State University
Optical and Discharge Physics
REACTANT FLUXES
 10 mTorr, HF 500 W, LF 4 kW,
Ar/C4F8/O2 = 80/15/5, 300 sccm
 Dominant Ions: Ar+, CF2+, C2F4+, CF+
 Dominant Neutrals: CF2, C2F4, CF,
CF3, F
 Polymer clearing fluxes
 O = 3  1016 cm-2.s-1
 O+ = 3  1014 cm-2.s-1
MINGMEI_GEC07_08
Iowa State University
Optical and Discharge Physics
ION ENERGY ANGULAR DISTRIBUTIONS (IEADs)
 IEADs for sum of all ions.
 Peak in ion energy increase
with increasing bias power.
 High ion energies required for
etching of HAR features.
 Narrow angular distribution
reduce sidewall impacts.
 10 mTorr, Ar/C4F8/O2 = 80/15/5,
300 sccm, LF 10 MHz, HF 500 W.
MINGMEI_GEC07_09
Iowa State University
Optical and Discharge Physics
SiO2-over-Si HARC ETCH: NO CHARGING
 Etch profile evolution without
charging.
 Etch rate higher at higher bias
powers owing to high ion
energies.
 No charging:
 Generally straight profiles.
 High ion energies  low
polymer coverages.
 Some evidence of randomness
due to small contact area
 10 mTorr, Ar/C4F8/O2 = 80/15/5,
300 sccm, 10 MHz, HF 500 W.
Aspect Ratio = 1:10
MINGMEI_GEC07_10
Iowa State University
Optical and Discharge Physics
SiO2-over-Si HARC ETCH: EFFECT OF CHARGING
 Charging effects are considered:
 Charge buildup on polymer
affects plasma potential.
 Ion trajectories influenced by
electric-field.
 Electrons neutralize charge
deep in trench.
 Lower ion energies (due to buildup
of charge)
 Lower etch rates.
 Deviation from “ideal” anisotropic
etch profiles.
 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, 10
MHz, HF 500 W.
Animation Slide
Aspect Ratio = 1:10
MINGMEI_GEC07_11a
Iowa State University
Optical and Discharge Physics
SiO2-over-Si HARC ETCH: EFFECT OF CHARGING
 Charging effects are considered:
 Charge buildup on polymer
affects plasma potential.
 Ion trajectories influenced by
electric-field.
 Electrons neutralize charge
deep in trench.
 Lower ion energies (due to buildup
of charge)
 Lower etch rates.
 Deviation from “ideal” anisotropic
etch profiles.
 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, 10
MHz, HF 500 W.
Aspect Ratio = 1:10
MINGMEI_GEC07_11b
Iowa State University
Optical and Discharge Physics
SiO2/Si HARC ETCH: PLASMA POTENTIAL
Max
Min
213 V 116 V 151 V 110 V
-5
-3
-6
-4
 Charge deposition on polymer affects
plasma potential.
 Small depths:
 Electrons effectively neutralize
charge buildup.
 Potential essentially maintained at
zero.
 Large depths:
 Trapping of charge in polymer
perturbs ion trajectories.
 Electrons are “pulled” into bottom
of trench by large positive potential
and neutralizes.
AR = 1:10
Increasing Power
MINGMEI_GEC07_12a
Animation Slide
-6
0
213
Iowa State University
Optical and Discharge Physics
SiO2/Si HARC ETCH: PLASMA POTENTIAL
Max
Min
213 V 116 V 151 V 110 V
-5
-3
-6
-4
 Charge deposition on polymer affects
plasma potential.
 Small depths:
 Electrons effectively neutralize
charge buildup.
 Potential essentially maintained at
zero.
 Large depths:
 Trapping of charge in polymer
perturbs ion trajectories.
 Electrons are “pulled” into bottom
of trench by large positive potential
and neutralizes.
AR = 1:10
Increasing Power
MINGMEI_GEC07_12b
-6
0
213
Iowa State University
Optical and Discharge Physics
SiO2-over-Si HARC ETCH: RANDOMNESS?
 Monte Carlo modeling utilizes
random number generator to
simulate a physical process.
 Different seed numbers
 All other conditions are same.
 Is it reproducible?
 No charging effects:
 Etch profiles vary little
 Anisotropic etch
 No anomalies observed
 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300
sccm, LF 4 kW, HF 500 W.
Different seed numbers
Aspect Ratio = 1:10
MINGMEI_GEC07_13
Iowa State University
Optical and Discharge Physics
SiO2/Si HARC ETCH: RANDOMNESS OF CHARGING?
 Different seed numbers
 All other conditions are same.
 Is it reproducible?
 Charging effects:
 Stochastic nature of incident ion
fluxes reflected in profiles.
 Twisting observed
 Etch direction shifts which
reinforces anomoly.
 Some unphysical behavior also
observed (last trench)
Different seed numbers
 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300
sccm, LF 4 kW, HF 500 W.
Aspect Ratio = 1:10
MINGMEI_GEC07_14
Iowa State University
Optical and Discharge Physics
SiO2/Si HARC ETCH: RANDOMNESS OF CHARGING?
 6 Trenches receiving
“same fluxes.
 Stochastic nature of
fluxes produces random
twisting.
 Similar behavior
observed experimentally.
 10 mTorr, Ar/C4F8/O2 =
80/15/5, 300 sccm, LF 4
kW, HF 500 W.
Ref: Micron Technology, Inc.
Aspect Ratio = 1:10
MINGMEI_GEC07_15
Iowa State University
Optical and Discharge Physics
EFFECT OF OPEN FIELD: NO CHARGING
 4 trenches followed by a
“plasma-only” region with hard
mask.
 Open field has large sidewall
polymerization.
 Charging not considered
 Trenches have some
randomness in profiles
owing to non-uniform ion
fluxes.
 No effect due to plasma-only
region.
Aspect Ratio = 1:10
Animation Slide
MINGMEI_GEC07_16a
Iowa State University
Optical and Discharge Physics
EFFECT OF OPEN FIELD: NO CHARGING
 4 trenches followed by a
“plasma-only” region with hard
mask.
I
II
 Open field has large sidewall
polymerization.
 Charging not considered
 Trenches have some
randomness in profiles
owing to non-uniform ion
fluxes.
III
Aspect Ratio = 1:10
MINGMEI_GEC07_16b
IV
 No effect due to plasma-only
region.
Iowa State University
Optical and Discharge Physics
EFFECT OF OPEN FIELD: EFFECT OF CHARGING
 Open field can impact etch of
adjacent trenches by trapping
of charge in polymer.
 Transverse electric fields from
external charge significantly
affects adjacent trenches.
 Inner trenches less affected by
charging.
 10 mTorr, Ar/C4F8/O2 = 80/15/5,
300 sccm, LF 4 kW, HF 500 W.
Animation Slide
Aspect Ratio = 1:10
MINGMEI_GEC07_17a
Iowa State University
Optical and Discharge Physics
EFFECT OF OPEN FIELD: EFFECT OF CHARGING
I
II
 Open field can impact etch of
adjacent trenches by trapping
of charge in polymer.
 Transverse electric fields from
external charge significantly
affects adjacent trenches.
 Inner trenches less affected by
charging.
III
Aspect Ratio = 1:10
MINGMEI_GEC07_17b
IV
 10 mTorr, Ar/C4F8/O2 = 80/15/5,
300 sccm, LF 4 kW, HF 500 W.
Iowa State University
Optical and Discharge Physics
OPEN FIELD EFFECT
ON CHARGING
 Open field impacts adjacent
trenches by transverse electric
field from trapped charged in
polymer.
I
 Isolating open field by making
spacer of SiO2 thicker reduces
transverse fields and
perturbation of etch profiles.
 Smaller deviation for the
adjacent trench.
II
 Note effect of stochastic ion
fluxes in second trench.
 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300
sccm, LF 4 kW, HF 500 W.
MINGMEI_GEC07_18
Aspect Ratio = 1:10
Iowa State University
Optical and Discharge Physics
COMPUTATIONAL ASPECTS: DISSIPATION OF CHARGE
 Dissipation of charge accounted for
through material conductivity
 I: Static charge
 II: Only electron charges move
 III: Both ion and electron charges move
 Positive charges inside materials leads to
high potentials inside the trench
 Lower ion energies  polymer
deposition
 Etch stop observed
 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, LF 4
kW, HF 500 W.
MINGMEI_GEC07_20
Aspect Ratio = 1:10
Iowa State University
Optical and Discharge Physics
ELECTRIC FIELDS: BOUNDARY CONDITIONS
 Boundary conditions for Poisson’s
equation: Zero potential at mesh
boundaries.
 Both electron and ion charges move
 Small mesh:
 Unphysical high gradients in fields
 Leads to etch stops
 Wide mesh:
 Gradients in fields relaxed
 Etch progresses to completion
 Higher conductivity  less effect
of charging
 10 mTorr, Ar/C4F8/O2 = 80/15/5,
300 sccm, LF 4 kW, HF 500 W.
MINGMEI_GEC07_21
Aspect Ratio = 1:10
Iowa State University
Optical and Discharge Physics
CONCLUDING REMARKS
 Etching of high aspect ratio contacts (HARC) has been
computationally investigated in fluorocarbon plasma.
 Charging of features has been included to investigate anomalies
such as twisting observed during etching of HARCs.
 Charge buildup in/on polymer layer decreases etch rates and
deviates the etching profile.
 Ultimately a stochastic process for small features.
 Various factors affect etching profiles:
 Special structures like open field.
 High energy ions may mitigate the effect of charging.
 Charge dissipation due to material conductivity.
MINGMEI_GEC07_22
Iowa State University
Optical and Discharge Physics