Transcript Slide 1

Consequences of Implanting and Surface Mixing
During Si and SiO2 Plasma Etching*
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
36th ICOPS, June 2009, San Diego, CA
*Work supported by Tokyo Electron Ltd. Micron Inc. and the SRC.
MINGMEI_ICOPS09_01
AGENDA
 Implanting, mixing and photoresist (PR) erosion
 Processes
 Molecular ion dissociation on surfaces
 Small ion penetration and mixing
 PR cross-linking
 Description of Model
 Scaling of Mixing and Implantation
 Concluding Remarks
MINGMEI_ICOPS09_02
University of Michigan
Institute for Plasma Science & Engr.
IMPLANTATION AND MIXING DURING PLASMA ETCHING
Bulk Plasma
Diamond structure of Si
CxFy+
Cx-1Fy-1
+
Polymer
Ar+
+
Si +
O
+
O2 + F
2
+
+
O,F
F
C
Si Ar O,F
O
Substrate (SiO2, Si or PR)
C O F Si
Ar
 Small ions accelerated by the sheath implant into the wafer
surface forming weakly bonded or interstitially trapped species.
 Implanting causes surface mixing which produces damage
during plasma etching.
MINGMEI_ICOPS09_03
University of Michigan
Institute for Plasma Science & Engr.
PHOTORESIST (PR) EROSION
Bulk Plasma
Ions
C H O
PR
PR
 Photoresist defines the features to be etched
– usually a hydrocarbon polymer.
SiO2
 PR is sputtered by energetic ions. Fragmented PR segments are more easily eroded.
 Profile of high aspect ratio (HAR) features
can be modified due to PR erosion.
MINGMEI_ICOPS09_04
University of Michigan
Institute for Plasma Science & Engr.
APPROACHES AND GOALS OF INVESTIGATION
 Consequences of implantation and mixing are poorly
characterized in modeling of plasma etching.
 Usually only included in compute-intensive molecular
dynamics simulations.
 Incorporate implantation, mixing, and sputtering into
Monte-Carlo Feature Profile Model coupled to equipment
scale plasma models.
 Investigate:
 Scaling of implantation, mixing, and etching selectivity.
 Degradation and cross-linking of PR surface due to
energetic bombardment.
 Goals
 Characterize mixing damage during etching.
 Develop strategies to preserve pattern transferring
while minimizing damage.
MINGMEI_ICOPS09_05
University of Michigan
Institute for Plasma Science & Engr.
DESCRIPTION OF MODEL
Hybrid Plasma
Equipment Model
(HPEM)
Sources
Fields
Transport coefficients
Si
SiO2
Plasma Chemistry
Monte Carlo Model
(PCMCM)
Fluxes
Energy angular distributions
Sputtering
Yields
Range of Ions
Monte Carlo
Feature Profile Model
(MCFPM)
Implantation / Mixing
MINGMEI_ICOPS09_06
University of Michigan
Institute for Plasma Science & Engr.
IMPLANTATION MODEL
Ar+,F+,Si+
C+,O+

Pushed
out
n<N*
in
Stopping
range
 = f(in)
Mixing
Yes
4
Move to
next cell
Implant
2
3
4
No
5
Within one cell: out= in exp(-a/)
Where in = incident energy; out = left energy.
a = Actual length that the particle travels.
 = Calculated stopping range f(in).
*n = mixing step; N = allowed maximum mixing step.
MINGMEI_ICOPS09_07
Implant
Mixing
End
SiO2,Si or PR
i=1
No
a
3
5
Surface
reaction
Yes
out
2
No
Implant
Start
a
j=1
Gas-solid
surface
interaction
/in  R*
Yes
Exchange
identity
*R = Random number
University of Michigan
Institute for Plasma Science & Engr.
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.
 Example reaction of surface dissociation.
 M(s) + CxFy+(g)
 M(s) + Cx-1Fy-1(g) + C(g) + F(g)
 Sputtering of PR and redeposition.
 PR(s) + Ar+(g)
 PR2(s) + Ar(g) + H(g) + O(g)
 PR(s) + CxFy+(g)  PR(s) + CxFy(g)
 PR(g) + SiO2CxFy(s)  SiO2CxFy(s) + PR(s)
*PR2 = cross-linked PR
MINGMEI_ICOPS09_08
University of Michigan
Institute for Plasma Science & Engr.
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_ICOPS09_09
University of Michigan
Institute for Plasma Science & Engr.
ION ENERGY ANGULAR DISTRIBUTIONS (IEADs)
 Peak of ion energy ranges from 300 to 1200 eV
for 1 – 4 kW bias power.
 Angle distribution spreads from -10 to 10 degree .
 Stopping range ranges from 0 to 70 Å.
MINGMEI_ICOPS09_10
University of Michigan
Institute for Plasma Science & Engr.
STOPPING RANGE IN PR, POLYMER, Si, AND SiO2
 Stopping range increases with increasing energy.
 At specific energy, implanting depth:
PR(PMMA*) > SiO2 > Si > Polymer
*PMMA = Polymethylmethacrylate
 Data from SRIM (the Stopping and Range of Ion
in Matter)
Stopping range  = f(incident)
MINGMEI_ICOPS09_11
University of Michigan
Institute for Plasma Science & Engr.
IMPLANTING AND MIXING DEPTH vs BIAS POWER
 After 5 s
(a)
(b)
MINGMEI_ICOPS09_12
 Etch rates, degree
of mixing and depth
of implantation
increase with bias
power.
 After same etch
level
University of Michigan
Institute for Plasma Science & Engr.
IMPLANTING AND MIXING DEPTH vs ENERGY
 Only polymer deposition occurs at 1 eV.
 Sputtering, implanting and deposition coexist at 10 eV.
 Depth of implantation and mixing increases with increasing ion
energy (100 eV~10 keV).
MINGMEI_ICOPS09_13
University of Michigan
Institute for Plasma Science & Engr.
ETCHING SELECTIVITY
 Etch stop occurs at
Si surface due to
low reaction rate
with CFx.
3 nm
 Etching selectivity
for SiO2/PR(PMMA)
is around 10.
30 nm
 The roughness of
SiO2 surface is due
to non-uniform
polymer deposition.
Micro-masking
Micro-trenching
 Ar/C4F8/O2 = 80/15/5, 4 kW, 300sccm, 40mTorr, DC 200W/-250V.
MINGMEI_ICOPS09_14
University of Michigan
Institute for Plasma Science & Engr.
MECHANISM FOR DEGRADATION AND
CROSS-LINKING OF PR
Structure of PR(PMMA)
CH3
…CH2
CH3
C
C
CH2
O
CH3
C
C
CH2
O
CH2…
C
C
O
O
O
CH3
CH3
CH3
O
 PR molecule is degraded due to energetic ion sputtering.
 Degraded PR segments are more easily eroded.
 Newly formed dangling bonds are recombined (cross-linking),
and cross-linked PR forms a “hard crust”.
 Sputtering yield is calculated using SRIM.
MINGMEI_ICOPS09_15A
University of Michigan
Institute for Plasma Science & Engr.
DEGRADATION OF PR (SPUTTERING)
CH3
…CH2
CH3
C
C
CH2
O
CH3
C
C
CH2
O
CH2…
C
C
O
O
O
CH3
CH3
CH3
O
Broken Bond
MINGMEI_ICOPS09_15B
University of Michigan
Institute for Plasma Science & Engr.
DEGRADATION OF PR (SPUTTERING)
H
CH3
…CH2
CH2
C
C
CH2
O
CH3
C
C
CH2
O
CH2…
C
C
O
O
O
CH3
CH3
CH3
O
Broken Bond
MINGMEI_ICOPS09_15C
University of Michigan
Institute for Plasma Science & Engr.
INTER-MOLECULE CROSS-LINKING
CH3
…CH2
CH3
C
C
CH2
O
O
C
C
O
CH2
H
CH3
CH2
CH2
O
CH2…
C
C
O
O
CH3
H
Broken Bond
New Bond
MINGMEI_ICOPS09_15D
University of Michigan
Institute for Plasma Science & Engr.
INTER-MOLECULE CROSS-LINKING
H
H
CH3
…CH2
CH2
C
C
CH2
O
O
C
C
O
CH2
H
CH2
CH2
CH2
O
CH2…
C
C
O
O
CH3
H
Broken Bond
New Bond
MINGMEI_ICOPS09_15E
University of Michigan
Institute for Plasma Science & Engr.
INTER-MOLECULE CROSS-LINKING
H
H
CH3
…CH2
O
C
CH2
CH2
C
C
CH2
CH2
O
C
CH2
O
H
C
CH2
O
CH2…
C
O
O
H
CH3
Broken Bond
New Bond
MINGMEI_ICOPS09_15F
University of Michigan
Institute for Plasma Science & Engr.
INTRA-MOLECULE
CROSS-LINKING
CH3
…CH2
C
C
Molecule A
CH3
CH2
O
Broken Bond
New Bond
MINGMEI_ICOPS09_15G
CH2
O
CH2…
C
C
O
O
O
H
CH2
H
CH2
CH2
H
H
CH2
H
CH2
CH2
H
…CH2
C
C
Molecule B
C
C
O
CH3
CH2
O
C
C
CH2
O
CH2…
C
C
O
O
O
CH3
CH3
CH3
O
INTRA-MOLECULE
CROSS-LINKING
Molecule A
CH3
…CH2
C
C
CH3
H
Broken Bond
New Bond
…CH2
MINGMEI_ICOPS09_15H
CH2
O
CH2
H
C
CH2
O
CH3
C
C
CH3
O
C
Molecule B
CH3
CH2
O
C
O
O
CH2
CH2
C
C
CH2
O
CH2…
C
O
CH3
H
CH2…
C
C
O
O
O
CH3
CH3
CH3
O
ETCHING SELECTIVITY vs ENERGY
(a)
(b)
(c)
 Etching rate for SiO2 increases with increasing ion energy.
 Balance between sputtering and cross-linking (more resistive to etching)
on PR(PMMA) surface results in similar etching rate for all energies.
 Surface roughness of SiO2 increases as etching proceeds due to micromasking.
 Etching selectivity (SiO2/PR): 100 eV, 6; 500 eV, 18; 1000 eV, 23.
MINGMEI_ICOPS09_16
University of Michigan
Institute for Plasma Science & Engr.
ETCHING SELECTIVITY vs RF BIAS POWER
4 kW
1 kW
4 kW
4 kW
No mixing, no sputtering
PR
Si
 At similar etching level of PR,
aspect ratio (AR) of the trench
increases with bias power.
SiO2
 Etching selectivity increases
when PR is cross-linked.
Si
AR = 9
14
13
 Ar/C4F8/O2 = 80/15/5, 300 sccm, 40 mTorr,
10 MHz, DC 200 W/-250 V.
MINGMEI_ICOPS09_17
 Si is damaged during overetch by implantation and
mixing.
University of Michigan
Institute for Plasma Science & Engr.
CONCLUDING REMARKS
 Algorithms have been incorporated into the MCFPM to predict
implanting and mixing.
 Depth of implanting and mixing increases with increasing
bias power and ion energy.
 More damage is obtained at higher etching rate.
 PR surface is cross-linked due to sputtering.
 At higher bias power and ion energy, etching selectivity
for SiO2/PR is better.
 Strategies to be developed for high power processing:
 Protect pattern transferring at higher etching rate.
 Reduce damage during over-etch.
MINGMEI_ICOPS09_18
University of Michigan
Institute for Plasma Science & Engr.