Download Aspect Ratio Dependent Twisting and Mask

Aspect Ratio Dependent Twisting and Mask
Effects During Plasma Etching of SiO2 in
Fluorocarbon Gas Mixture*
Mingmei Wang1 and Mark J. Kushner2
1Iowa
State University, Ames, IA 50011 USA
[email protected]
2University
of Michigan, Ann Arbor, MI 48109 USA
[email protected]
http://uigelz.eecs.umich.edu
55th AVS, October 2008, Boston, MA
*Work supported by the SRC, Micron Inc. and Tokyo Electron Ltd.
AGENDA
 Issues in high aspect ratio contact (HARC) etching.
 Approaches and Methodologies
 Electric field buildup due to charge deposition.
 Feature twisting; trench to trench variation when
etching at critical dimension (CD).
 High energy electron (HEE) effects on feature twisting
in SiO2 etching over Si.
 Varied mesh resolution due to computing limitation.
 Photo resist sputtering and redeposition.
 Twisting and bowing during etch in features patterned
with photo resist (PR) and hard mask (HM).
 Concluding Remarks
MINGMEI_AVS08_AGENDA
University of Michigan
Institute for Plasma Science
and Engineering
CHALLENGES IN HARC ETCHING
Ref: Oxford
Instruments
Mask
Erosion
Bowing
Ref: JJAP, 46, p7873 (2007)
Ref: ULVAC
Technologies
Twisting
 Etched features for advanced micro-electronic devices have aspect
ratios (AR) approaching 100.
 Twisting, bowing and consequences of mask erosion challenge
maintaining CD.
 In this poster, results from a computational investigation of these
processes are presented.
University of Michigan
MINGMEI_AVS08_01
Institute for Plasma Science
and Engineering
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.
 Plasma Chemistry Monte Carlo
Module:
 Ion, Higher Energy Electron
(HEE) and Neutral Energy and
Angular Distributions.
 Fluxes for feature profile model.
MINGMEI_AVS08_02
University of Michigan
Institute for Plasma Science
and Engineering
MONTE CARLO FEATURE PROFILE MODEL
 Monte Carlo techniques address plasma
surface interactions and evolution of surface
profiles.
Ions, HEE,
radicals and
neutrals
 Electric potential is solved using Successive
Over Relaxation (SOR) method.
Mask
Charged particles
- +
+
+
+
+
SiO2
Polymer
+
+
+
+ +
Si
-6
MINGMEI_AVS08_03
0
151
University of Michigan
Institute for Plasma Science
and Engineering
SURFACE REACTION MECHANISM
 Etching of SiO2 is dominantly through a formation of a
fluorocarbon complex.
 SiO2(s) + CxFy+(g)
 SiO2*(s) + CxFy#(g)
 SiO2*(s) + CxFy(g)
 SiO2CxFy(s)
 SiO2CxFy (s) + CxFy+(g)  SiFy(g) + CO2 (g) + CxFy#(g)
 Further deposition by CxFy(g) produces thicker polymer
layers.
 Sputtering of photo resist and redeposition.
 PR(s) + CxFy+(g)
 PR(g) + CxFy#(g)
 PR(g) + SiO2CxFy(s)
 SiO2CxFy(s) + PR(s)
MINGMEI_AVS08_04
University of Michigan
Institute for Plasma Science
and Engineering
FLUOROCARBON ETCHING OF SIO2
 DC augmented single frequency
capacitively coupled plasma (CCP)
reactor.
 DC: Top electrode
RF: Substrate
 Plasma tends to be edge
peaked due to electric field
enhancement.
 Plasma densities in excess
of 1011 cm-3.
 Ar/C4F8/O2 = 80/15/5, 300 sccm,
40 mTorr, RF 1 kW at 10 MHz,
DC 200 W/-250 V.
MINGMEI_AVS08_05
University of Michigan
Institute for Plasma Science
and Engineering
10 MHz LOWER, DC UPPER: PLASMA POTENTIAL
 LF electrode passes rf current. DC electrode passes combination of
rf and dc current with small modulation of sheath potential.
 Ar, 40 mTorr, LF: 10 MHz, 300 W, 440V/dc=-250V
 DC: 200 W, -470 V
MINGMEI_AVS08_06
ANIMATION SLIDE-GIF
University of Michigan
Institute for Plasma Science
and Engineering
HIGH ENERGY ELECTRON (HEE) FLUXES
 HEE fluxes increase with
increasing RF bias power due
to increase in plasma density.
 40 mTorr, RF 10 MHz, DC 200 W/-250 V,
Ar/C4F8/O2 = 80/15/5, 300 sccm
 HEE flux increases with
increasing DC voltage.
 HEE is naturally generated by
RF oscillation (when VDC=0 V).
 40 mTorr, RF 4 kW/1.5 kV at 10 MHz,
Ar/C4F8/O2 = 80/15/5, 300 sccm
MINGMEI_AVS08_07
University of Michigan
Institute for Plasma Science
and Engineering
ION ENERGY ANGULAR DISTRIBUTIONS (IEADs)
 IEADs for sum of all ions.
 Peak in ion energy increases
with increasing rf bias power
while IEAD narrows.
 Higher energy ions increase
maximum positive charging
of feature.
 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300
sccm, RF 10 MHz, DC 200 W/-250 V.
MINGMEI_AVS08_08
University of Michigan
Institute for Plasma Science
and Engineering
HEE ENERGY ANGULAR DISTRIBUTIONS
 HEE energy increases with
increasing rf bias power.
 Narrower angular
distribution (-2０~ 2０) than
for ions.
 Peak at maximum energy
with long tails.
 40 mTorr, Ar/C4F8/O2 = 80/15/5,
300 sccm, RF 10 MHz, DC 200 W/250 V.
MINGMEI_AVS08_09
University of Michigan
Institute for Plasma Science
and Engineering
HEE EFFECTS ON TWISTING: FINE MESH
 Atomic scale mesh size (~3 Å).
 Ions hitting the surface deposit charge. Electrons may scatter.
Statistical composition of fluxes into small features produces
occasional twisting.
 Twisting occurs randomly without considering HEE (3/20).
 HEE neutralizes charge effectively deep into the trench.
 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 1 kW at 10 MHz, DC 200 W.
Different random seeds
Without HEE
Different random seeds
With HEE
Aspect Ratio = 1:25
MINGMEI_AVS08_10
HEE EFFECTS on TWISTING:
COARSE MESH
 Coarse mesh (~5 nm) with photo
resist erosion on the top.
Without HEE
 Bowing occurs at later stage of
etching due to reflection from
sloped profile of eroded PR.
 HEE fluxes improve feature profiles.
 Trench to trench differences due to
small opening (75nm) to the plasma
and statistican nature of fluxes.
 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF
5 kW at 10 MHz.
Aspect Ratio = 1:20
With HEE
MINGMEI_AVS08_11
University of Michigan
Institute for Plasma Science
and Engineering
HEE ENERGY ANGULAR DISTRIBUTIONS
 HEE energy increases with
increasing DC voltage.
 Narrower angular distribution
is obtained at high voltage
with longer tails.
 At low energy region (<500
eV), low DC voltage causes
broader angular distribution
and lower particle density.
 40 mTorr, Ar/C4F8/O2 = 80/15/5,
sccm, RF 1.5 kV at 10 MHz.
MINGMEI_AVS08_12
300
University of Michigan
Institute for Plasma Science
and Engineering
TWISTING ELIMINATION: DC VOLTAGE
Different random seeds
 Two group of profiles are selected from
21 cases with different random seed
number generators.
 HEE neutralizes positive charge deep
into the trench.
 Higher HEE energy and flux produce
better profiles and higher etch rates:
 VDC=0 V,
twisting probability=7/21.
 VDC=500 V, twisting probability=5/21.
 VDC=750 V, twisting probability=3/21.
 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm,
RF 1.5 kV at 10 MHz.
MINGMEI_AVS08_13
Aspect Ratio = 1:20
University of Michigan
Institute for Plasma Science
and Engineering
PHOTO RESIST SPUTTERING and PROFILE BOWING
 Time sequence of feature etching.
 Photo resist is eroded during
process broadening view-angle to
plasma.
 Bowing occurs at later stage of
etching as view-angle and slope of
PR increases.
 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 5
kW at 10 MHz.
Aspect Ratio = 1:30
MINGMEI_AVS08_14
ANIMATION SLIDE-GIF
University of Michigan
Institute for Plasma Science
and Engineering
PHOTO RESIST SPUTTERING and PROFILE BOWING
 Time sequence of feature etching.
 Photo resist is eroded during
process broadening view-angle to
plasma.
 Bowing occurs at later stage of
etching as view-angle and slope of
PR increases.
 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 5
kW at 10 MHz.
Aspect Ratio = 1:30
MINGMEI_AVS08_14
University of Michigan
Institute for Plasma Science
and Engineering
MASK MATERIAL EFFECTS
 Hard mask is not etched or
sputtered easily.
 PR has an etching selectivity of
~10 over SiO2.
 Bowing occurs at the middle
height of trench with the hard
mask.
 Bowing occurs right under the PR
layer.
 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm,
RF 5 kW at 10 MHz.
(AR=30)
MINGMEI_AVS08_15
(AR=30)
(AR=40)
University of Michigan
Institute for Plasma Science
and Engineering
BOWING MECHANISM
Ions & HEE
 With hard mask, as etch
depth increases, ions
with a small incident
angle hit the side wall.
 Statistical deposition of
charge produces
deflection of narrow
angle ions.
 With photo resist
etching, ions hitting PR
surface reflect to the side
wall of trench.
 40 mTorr, Ar/C4F8/O2 = 80/15/5,
300 sccm, RF 5 kW at 10 MHz.
MINGMEI_AVS08_16
E-Field
University of Michigan
Institute for Plasma Science
and Engineering
PROPOSED METHODS OF BOWING ELIMINATION
 Many methods have been proposed to address bowing.
 Deposit a protective
layer onto PR.
 Sputtering protective
layer away at later
stage of etching.
PR
HM
 Multiple layers of
mask materials
(upper PR, lower
hard mask).
 Increase HEE flux and energy to further neutralize positive charge
on trench bottom and side walls.
 Control ion energy as the etch proceeds to utilize selectivity
difference between PR and SiO2 etching.
MINGMEI_AVS08_17
University of Michigan
Institute for Plasma Science
and Engineering
CONCLUDING REMARKS
 HEE effects on eliminating twisting in HARC etching have been
computationally investigated in fluorocarbon plasmas.
 Statistical nature of ion fluxes into small features produce
lateral electric fields which deflect ions.
 HEE neutralizes positive charge deep into the trench to
eliminate ion trajectory change and accelerate etching.
 Photo resist sputtering leads to bowing at top of feature profile.
 Bowing occurs at middle of feature in HARC (AR~40) etching.
MINGMEI_AVS08_18
University of Michigan
Institute for Plasma Science
and Engineering