TRIGGERING EXCIMER LASERS BY PHOTOIONIZATION FROM A CORONA DISCHARGE*

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

[email protected]

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


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


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.


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


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)


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


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


ELECTRICAL POTENTIAL

Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 2625 Torr, 338K Time: 0-35ns :


Cathode pulsed to -40 kV Avalanche breakdown

collapsed potential in gap.


CORONA POTENTIAL


Probe from cathode to corona dielectric surface initiates surface discharge. Charging of surface occurs

around the circumference.


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 CoronaBar


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.


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.


PHOTOIONIZATION


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.


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.


ArF* DENSITY


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


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


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


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.

TRIGGERING EXCIMER LASERS BY PHOTOIONIZATION FROM A CORONA DISCHARGE*

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