TRIGGERING EXCIMER LASERS BY PHOTOIONIZATION FROM A CORONA DISCHARGE* Zhongmin Xiong and Mark J....
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Transcript of TRIGGERING EXCIMER LASERS BY PHOTOIONIZATION FROM A CORONA DISCHARGE* Zhongmin Xiong and Mark J....
TRIGGERING EXCIMER LASERS BY PHOTOIONIZATION FROM A CORONA
DISCHARGE*
Zhongmin Xiong and Mark J. KushnerUniversity of Michigan
Ann Arbor, MI 48105 [email protected] [email protected]
Thomas Duffey and Daniel BrownCymer, Inc. San Diego, CA 92127
October 2009
* Work supported by Cymer, Inc.
AGENDA
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
Excimer discharge excited lasers for photolithography
Preionization schemes
Description of Model
Discharge triggering sequence
Dependence on corona bar properties
Concluding Remarks
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.
(www.spie.org)
(Cymer Inc.)
Laser
Ar+ + F-Ar* + F2
Ar, F
ArF*
R
E(R)
ArF
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
e + Ar Ar* + e e + Ar Ar+ + 2e e + F2 F + F-
Coherent, short wavelengths have made ArF (193 nm) the source of choice for photolithography for micro-electronics fabrication.
PLASMA DISCHARGE and PRE-IONIZATION
Frame 001 02 Oct 2009 CYMER_01E NE/AR/F2/XE (cymer_nlist_01b.nlist)
Frame 001 02 Oct 2009 CYMER_01E NE/AR/F2/XE (cymer_nlist_01b.nlist)
Gas mixtures contain highly attaching halogens which places premium on high preionization density for optimizing gain.
Preionization provided by UV illumination from corona bar.
Investigate preionization mechanisms.
Ne/Ar/F2/Xe =
96.4/3.5/0.1/0.001
Cathode
Anode
Insulator
Metal CoronaBar (grounded) Dielectric
Insulator
0.25mm
5 cm
12 cm
P = 2625 TorrT = 338K
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
Insulator
DESCRIPTION OF MODEL Discharge chamber and plasma kinetics modeled using nonPDPSIM Poisson’s Equation:
Continuity equation for charged and neutral species:
Surface charge balance
Bulk electron temperature:
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.
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
j
sjjqN
jjj St
N
jjjj
s Sqt
))(()(
e
ieiie
e qjTNnEjt
n
,2
5
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 Ne + Ne+ + M Ne2+ + M
e + Ne Ne* + e Ne + Ne* + M Ne2* + M
e + Ar Ar+ + e + e Ar + Ar+ + M Ar2+ + M
e + Ar Ar* + e 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 Ar+ + F- + M ArF* + M
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
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.
PHOTOIONIZATION
e + Ne Ne* + e
Ne* + 2Ne Ne2* + Ne
Ne2* Ne + Ne + h (15.5 eV, 800 A)
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
h + Xe Xe+ + e Ionization potential: 12.13 eV [Xe] = 7.5 x 1014 cm-3
= 10-16 cm2
RADIATION TRANSPORT
Emission species j
Ionized Species i
Absobers k
')',()'()()( 3rdrrGArNrN
t
rNijjj
jji
i
2'
|'|4
")"(exp
)',(rr
rdrN
rrGkj
k
r
r
k
j
A Einstein coefficient
ij Photo-ionization cross section
kj Photo-absorption cross section
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
Radiation transport modeled using propagator or Greens function approach which relates photo flux at r to density of excites states at r’.
Includes view-factors.
Rate of ionization
COMPUTATIONAL MESHFrame 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
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
ELECTRICAL POTENTIAL
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 2625 Torr, 338K Time: 0-35ns :
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
Cathode pulsed to -40 kV Avalanche breakdown
collapsed potential in gap.
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
CORONA POTENTIAL
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 2625 Torr, 338K Time: 0-35ns :
Probe from cathode to corona dielectric surface initiates surface discharge. Charging of surface occurs
around the circumference.
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
CORONA E-FIELD
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 2625 Torr, 338K Time: 0-35ns :
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 CoronaBar
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
CORONA [e]
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 2625 Torr, 338K Time: 0-35ns :
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.
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
Ne2* - VUV SOURCE
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 2625 Torr, 338K Time: 0-35ns :
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.
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
PHOTOIONIZATION
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 2625 Torr, 338K Time: 0-35ns :
VUV from all sources seeds electrons by photoionization. Preionization density in gap >109
cm-3 prior to avalanche. During avalanche, “internal” VUV-
accounts for > 10% of ionization.
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
ELECTRON DENSITY
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 2625 Torr, 338K Time: 0-35ns :
Electron density > 1015 cm-3 in mid gap – spreading from narrow anode to broad cathode. Photoelectrons seed avalanches in
all high field regions.
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
ArF* DENSITY
Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 2625 Torr, 338K Time: 0-35ns :
The density of the excimer ArF* produced in the discharge exceeds 1014 /cm3. ArF* Ar + F produces laser
output
0.E+00
2.E+09
4.E+09
6.E+09
8.E+09
1.E+10
1.E+10
1.E+10
2.E+10
0 20 40 60 80
= 5
Pre-ionization electron density at t=25ns
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
CORONA BAR
The capacitance of the corona bar increases with .
Longer charging time produces more VUV, increasing [e] in gap.
= 20 = 60
Corona Bar /0
CONCLUDING REMARKS
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
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.
BACKUP V-I Curves
University of MichiganInstitute for Plasma Science & Engr.ANDY_GEC2009
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.