Transcript Slide 1

SURFACE MODIFICATION OF POLYMER
PHOTORESISTS TO PROTECT PATTERN TRANSFER
IN FLUOROCARBON PLASMA ETCHING*
Mingmei Wanga) and Mark J. Kushnerb)
a)Iowa
State University, Ames, IA 50011 USA
[email protected]
b)University
of Michigan, Ann Arbor, MI 48109 USA
[email protected]
http://uigelz.eecs.umich.edu
62th GEC, October 2009, Saratoga Springs, NY
*Work supported by Tokyo Electron Ltd. and Semiconductor Research Corp.
MW_GEC2009
AGENDA
 Consequences of ion induced cross-linking on etch rates and
photoresist (PR) CD control.
 Description of the model
 Scaling of mixing and implantation with Ion Energy
Distributions
 Strategies to control PR erosion
 VUV induced degradation and cross-linking of PR
 Si extraction (SiFx) and deposition on PR and CFx polymer
 Concluding Remarks
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
IMPLANTATION and MIXING DURING
PLASMA ETCHING
Ions
Bulk Plasma
CxFy+
Cx-1Fy-1
+
Polymer
Ar+
+
Si +
O
+
O2 + F
2
+
PR
+
O,F
F
C
Si Ar O,F
O
SiO2
Substrate (SiO2, Si or PR)
 Small ions accelerated by the sheath implant into the wafer surface
forming weakly bonded or interstitially trapped species causing mixing
and damage during plasma etching.
 PR sputtering and ion-induced composition changes change PR facets
which affect profile during high aspect ratio (HAR) etching.
 Develop computational infrastructure to investigate implantation effects.
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
MOLECULAR DYNAMICS SIMULATION on MIXING
D. Humbird, D. B. Graves et. al.,
J. Vac. Sci. Technol. A, Vol. 25, 2007
 Mixing of Si crystal due to Ar+ bombardment was investigated
using MD simulation.
 Scaling of amorphous layer thickness with ion energy showed a
good correlation.
 Amplification faces difficulties due to huge amount of calculations.
MW_GEC2009
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
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
IMPLANTATION MODEL
Ar+,F+,Si+
C+,O+

in
a
Gas-solid
surface
interaction
Pushed
out
n<N*
No
Implant
Surface
reaction
Yes
Stopping
range
 = f(in)
Start
out
Mixing
No
a
Implant
Yes
Implant
SiO2,Si or PR
MCFPM Mesh
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.
MW_GEC2009
Mixing
Move to
next cell
End
No
/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)
 Ions on PR sputter, produce cross-linking and redeposit PR.
 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
MW_GEC2009
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.
MW_GEC2009
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 in surface materials ranges from 0 to 70 Å.
MW_GEC2009
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).
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
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.
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
PR EROSION vs ASPECT RATIO
 Cross-linking of PR due to
ion bombardment protects
PR.
 Selectivity to SiO2 is  10.
 As AR increases, PR is
eroded slowly.
 For AR>16, PR is depleted.
 Other strategies are
needed to better retain CD.
(AR = 7
12
16
22)
Animated Slide-GIF
MW_GEC2009
 Ar/C4F8/O2 = 80/15/5, 300 sccm,
40 mTorr, 10 MHz, DC 200 W/-250
V, RF 4 kW.
University of Michigan
Institute for Plasma Science & Engr.
STRATEGY to ELIMINATE PR EROSION
 In DC-CCP, large fluxes of
Si (in addition to VUV
fluxes) may be incident on
wafer and PR.
 Deposition of Si and
formation of Si-C layers
may improve PR
selectivity.
 Si easily extracts one or
two F from polymer CxFy to
promote further polymer
deposition.
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
STRATEGY to ELIMINATE PR EROSION
 Step 1: PR and CFx Activation
PR(s) + VUV
CxFy(s) + Si(g)
 PR*(s) + PR(g)
 CxFy*(s) + SiFx(g)
 Step 2: Deposition of Si, CFx, Passivation
PR*(s)
PR*(s)
Si(s)
Si(s)
+ Si(g)
+ CxFy(g)
+ CxFy(g)
+ F(g)
 PR(s) + Si(s)
 PR(s) + CxFy(s)
 Si(s)
+ CxFy (s)
 SiFx(s)
 Step 3: VUV Photoablation, activation
CxFy(s) + VUV
 CxFy*(s) + CxFy(g)
 Step 4: Further Deposition
SiFx(s) + CxFy(g)  SiFx(s) + CxFy(s)
CxFy*(s) + CxFy(g)  CxFy(s) + CxFy(s)
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
VUV BOND BREAKING and PHOTOLYSIS
Average Bond Energy*
Polymer (CxFy)
Bond
ΔH
(eV/bond)
C-F
C-F
C-F
(methyl)
(ethyl)
(i-propyl)
4.77
4.77
4.73
PR (PMMA)
C-C
C-C
3.60
3.60
C-H
C-H
C-H
(methyl)
(ethyl)
(i-propyl)
4.47
4.25
4.12
Si or SiO2
C=O
Si-Si
Si-O
7.72
2.25
4.77
* Organic Chemistry, Michigan State University
 VUV resonant radiation from Ar produces lines at ~105 nm (11.8 eV).
 Photon energy able to break all “first bonds” in PMMA, polymer, Si,
SiO2.
 Isotropic VUV fluxes are onto and absorbed in top layers of features.
 Interactions of VUV with PR are important in PR erosion and surface
activation.
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
IEADs on TOP and BOTTOM ELECTRODES
(Top electrode)
(Bottom electrode)
 Ion energy increases with increasing DC power on top electrode.
 On bottom electrode, ion energy is almost unchanged when
varying DC power.
 AR, HF 500 W at 60 MHz, LF 4 kW at 5 MHz, 40 mTorr, 300 sccm.
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
Photon Flux / Total Ion Flux (%)
20
30
Flux (x 1015 cm-2s-1)
Si Flux / Total Ion Flux (%)
FLUXES at WAFER CENTER
25
20
15
10
0
500
1000
1500
Total Ion
10
5
Si
0
2000
500
1000
1500
2000
DC Power (W)
DC Power (W)
 At wafer center Si/Ion flux
increases with DC power.
0.4
 Photon/ion flux does not have
clear correlation with DC power.
0.2
0.0
15
0
500
1000
1500
2000
•
AR, HF 500 W, LF 4 kW, 40 mTorr, 300 sccm.
DC Power (W)
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
PROTECTING PR WITH VUV and Si FLUXES
 Without Si and VUV exposure, PR is slowly etched (~8 nm/min).
 Cross linking by VUV flux has a small effect.
 Si flux ultimately increases polymer deposition and Si-C rich layer.
 Combination of VUV and Si induces more polymer deposition.
•
Ar/C4F8/O2=80/15/5, HF 500 W, LF 4 kW, DC 2 kW, 40 mTorr, 300 sccm.
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
VUV FLUX vs PR ETCHING
 Increasing VUV flux induces more cross-linking and activated
surface sites.
 Cross-linked PR is more resistive to etch.
 With highly cross-linked PR at high VUV flux, polymer
deposition dominates.
•
Ar/C4F8/O2=80/15/5, HF 500 W, LF 4 kW, DC 2 kW, 40 mTorr, 300 sccm.
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
Si FLUX vs PR ETCHING
 Si deposition and its promotion of polymer deposition protects
PR from sputtering and erosion.
 Sensitivity of balance of PR etching and deposition with respect
to Si flux is being investigated.
•
Ar/C4F8/O2=80/15/5, HF 500 W, LF 4 kW, DC 2 kW, 40 mTorr, 300 sccm.
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
CONCLUDING REMARKS
 Implantation has been investigated as damage mechanism and
hardening of PR through cross linking.
 PR hardening scales similarly to sputtering – weak effect.
 Mixing at interfaces increases with ion energy.
 Consequences of Si fluxes sputtered from dc electrodes studied
in concert with VUV fluxes.
 High VUV fluxes (~1014 cm-2s-1) produce highly cross-linked PR
surface.
 Si fluxes produce Si-C hardened surface and promote CFx
deposition.
 Net effect is preservation of PR.
MW_GEC2009
University of Michigan
Institute for Plasma Science & Engr.