Transcript Mesh
TRIGGERING EXCIMER LASERS BY
PHOTOIONIZATION FROM A CORONA
DISCHARGE*
Zhongmin Xiong and Mark J. Kushner
University of Michigan
Ann Arbor, MI 48105 USA
[email protected] [email protected]
Thomas Duffey and Daniel Brown
Cymer, Inc. San Diego, CA 92127
[email protected]
October 2009
* Work supported by Cymer, Inc.
AGENDA
Excimer discharge excited lasers for photolithography
Preionization schemes
Description of Model
Discharge triggering sequence
Dependence on corona bar properties
Concluding Remarks
ANDY_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
EXCIMER LASERS FOR PHOTOLITHOGRAPHY
Discharge excited excimer lasers operate in the UV on bound-free
transitions of rare-gas halogens
Typical conditions: many atms, a few cm gap, pulsed 10s kV in 10s ns.
Ar+ + FAr* + F2
ArF*
E(R)
Laser
ArF
e + Ar Ar* + e
e + Ar Ar+ + 2e
e + F2 F + FAr, F
R
Coherent, short
wavelengths have
made ArF (193 nm) the
source of choice for
photolithography for
micro-electronics
fabrication.
(www.spie.org)
(Cymer Inc.)
ANDY_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
PLASMA DISCHARGE and PRE-IONIZATION
Gas mixtures contain highly attaching
halogens which places premium on high
preionization density for optimizing gain.
Frame 001 02 Oct 2009 CYMER_01E NE/AR/F2/XE (cymer_nlist_01b.nlist)
Insulator
e
Preionization provided by UV illumination
from corona bar.
Frame 001 02 Oct 2009 CYMER_01E NE/AR/F2/XE (cymer_nlist_01b.nlist)
Investigate preionization mechanisms.
Metal Corona
Bar (grounded)
0.25mm
Dielectric
Insulator
Cathode
5 cm
Ne/Ar/F2/Xe =
96.4/3.5/0.1/0.001
P = 2625 Torr
T = 338K
Anode
Insulator
12 cm
ANDY_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
DESCRIPTION OF MODEL
Discharge chamber and plasma kinetics modeled using nonPDPSIM
Poisson’s Equation:
e N j q j s
j
Continuity equation for charged and neutral species:
N j
t
j S j
s
q j ( j S j ) ( ())
Surface charge balance
t
j
nee
5
Bulk electron temperature:
j E ne Ni i e Te ,
t
2
i
j qe
Radiation transport for photons (more on this later)
Secondary electron emission (ion and photons) from surfaces.
Transport and rate coefficients obtained from solution of
Boltzmann’s equation for electron energy distribution.
ANDY_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
REACTION MECHANISM
Reaction mechanism contains 35 species, 12 charged species,
300+ reactions for Ne/Ar/F2/Xe mixtures.
Operating pressures of 3 atm emphasize 3-body reactions
leading to rapid dimerization.
e + Ne Ne+ + e + e
e + Ne Ne* + e
Ne + Ne+ + M Ne2+ + M
Ne + Ne* + M Ne2* + M
e + Ar Ar+ + e + e
e + Ar Ar* + e
Ar + Ar+ + M Ar2+ + M
Ar + Ar* + M Ar2* + M
Ne2+ + Ar Ar+ + Ne + Ne
Ne2* + Ar Ar+ + Ne + Ne + e
e + F2 F- + F
Ion-Ion neutralization
Ar2+ + F- ArF* + Ar
ANDY_GEC2009
Ar+ + F- + M ArF* + M
University of Michigan
Institute for Plasma Science & Engr.
PHOTOIONIZATION
Excited stats generated by corona discharge produce VUV photons
which propagate to main discharge gap to photo-ionize low
ionization potential species for pre-ionization.
Many species likely contribute to VUV flux – here we used Ne2* as
VUV source.
Sufficient density and short enough lifetime to account for VUV
flux required to produce observed preionization densities –
radiation is not trapped.
Xe has the lowest ionization potential in mixture and is the
photoionized atom.
e + Ne Ne* + e
Ne* + 2Ne Ne2* + Ne
Ne2* Ne + Ne + h
(15.5 eV, 800 A)
ANDY_GEC2009
h + Xe Xe+ + e
Ionization potential: 12.13 eV
[Xe] = 7.5 x 1014 cm-3
= 10-16 cm2
University of Michigan
Institute for Plasma Science & Engr.
RADIATION TRANSPORT
Emission species j
Radiation transport modeled using
propagator or Greens function
approach which relates photo flux at r
to density of excites states at r’.
Absobers k
Includes view-factors.
Rate of ionization
N (r )
3
Ni (r ) N j (r ' ) Aj G j (r , r ' ) ij d r '
t
j
i
Ionized
Species i
r
exp N k (r " ) kj dr "
k r '
G j (r , r ' )
2
4 | r r ' |
ANDY_GEC2009
A
Einstein coefficient
ij
Photo-ionization cross section
kj
Photo-absorption cross section
University of Michigan
Institute for Plasma Science & Engr.
COMPUTATIONAL MESH
e 001 05 Oct 2009 CYMER_01E NE/AR/F2/XE (cymer_nlist_01b.nlist)
Unstructured mesh used to resolve
chamber geometry and large
dynamic range in dimensions.
Total number of nodes: 9,336
Plasma nodes:
5,607
ANDY_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
ELECTRICAL POTENTIAL
Cathode pulsed to -40 kV
Avalanche breakdown
collapsed potential in gap.
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001
2625 Torr, 338K
Time: 0-35ns :
ANDY_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
CORONA POTENTIAL
Probe from cathode to corona
dielectric surface initiates
surface discharge.
Charging of surface occurs
around the circumference.
ANDY_GEC2009
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001
2625 Torr, 338K
Time: 0-35ns :
University of Michigan
Institute for Plasma Science & Engr.
CORONA E-FIELD
Electric field in surface
avalanche propagates around
circumference.
Remaining charge produces
radial fields in corona bar.
Surface charges on insulator
produce large sheath fields.
Cathode
ANDY_GEC2009
Corona
Bar
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001
2625 Torr, 338K
Time: 0-35ns :
University of Michigan
Institute for Plasma Science & Engr.
CORONA [e]
Small [e] seeded near probe from
cathode.
Avalanche along surface to > 1015
cm-3 penetrates through gaps.
Photoionization seeds electrons
in remote high field regimes,
initiating local avalanche.
ANDY_GEC2009
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001
2625 Torr, 338K
Time: 0-35ns :
University of Michigan
Institute for Plasma Science & Engr.
Ne2* - VUV SOURCE
Electron impact from surface
avalanche produces Ne* Ne2*.
Densities in excess of 1012 cm-3
produce photon sources of 1018
cm-3s-1.
Untrapped VUV is penetrates
through to discharge gap.
ANDY_GEC2009
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001
2625 Torr, 338K
Time: 0-35ns :
University of Michigan
Institute for Plasma Science & Engr.
PHOTOIONIZATION
VUV from all sources seeds
electrons by photoionization.
Preionization density in gap >109
cm-3 prior to avalanche.
During avalanche, “internal” VUVaccounts for > 10% of ionization.
ANDY_GEC2009
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001
2625 Torr, 338K
Time: 0-35ns :
University of Michigan
Institute for Plasma Science & Engr.
ELECTRON DENSITY
Electron density > 1015 cm-3 in mid
gap – spreading from narrow anode
to broad cathode.
Photoelectrons seed avalanches in
all high field regions.
ANDY_GEC2009
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001
2625 Torr, 338K
Time: 0-35ns :
University of Michigan
Institute for Plasma Science & Engr.
ArF* DENSITY
The density of the excimer ArF*
produced in the discharge exceeds
1014 /cm3.
ArF* Ar + F produces laser output
ANDY_GEC2009
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001
2625 Torr, 338K
Time: 0-35ns :
University of Michigan
Institute for Plasma Science & Engr.
CORONA BAR e
2.E+10
1.E+10
1.E+10
The capacitance of the corona bar
increases with e.
Longer charging time produces
more VUV, increasing [e] in gap.
1.E+10
8.E+09
6.E+09
4.E+09
2.E+09
0.E+00
0
Pre-ionization electron density at t=25ns
e=5
ANDY_GEC2009
e = 20
20
40
60
80
Corona Bar e/e0
e = 60
University of Michigan
Institute for Plasma Science & Engr.
CONCLUDING REMARKS
Preionization by VUV photons from a corona bar was
investigated in an ArF excimer discharge laser.
Photons emitted by Ne2* are sufficient to produce
preionization densities > 109 cm-3 in mid gap.
VUV produces photoionization electrons in all high field
regions, seeding avalanche there.
Degree of photoionization is controllable by dielectric
constant of corona bar.
ANDY_GEC2009
University of Michigan
Institute for Plasma Science & Engr.
BACKUP V-I Curves
Peak voltage difference across the gap reaches
40KV. Avalanche starts and decreases the voltage
difference.
Peal current exceeds 40KA before starting to decay
due to the drop of voltage.
ANDY_GEC2009
University of Michigan
Institute for Plasma Science & Engr.