1. TRIGGERING EXCIMER LASERS BY PHOTOIONIZATION FROM A CORONA DISCHARGE* Zhongmin Xiong and Mark J. Kushner University of Michigan Ann Arbor, MI 48105 USA zxiong@umich.edu mjkush@umich.edu Thomas Duffey and Daniel Brown Cymer, Inc. San Diego, CA 92127 Tom_Duffey@Cymer.com October 2009 * Work supported by Cymer, Inc.

2. AGENDA University of Michigan Institute 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

3. 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* + F 2 Ar, F ArF* R E(R) ArF University of Michigan Institute for Plasma Science & Engr. ANDY_GEC2009  e + Ar  Ar* + e  e + Ar  Ar + + 2e  e + F 2  F + F-  Coherent, short wavelengths have made ArF (193 nm) the source of choice for photolithography for micro-electronics fabrication.

4. PLASMA DISCHARGE and PRE-IONIZATION  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/F 2 /Xe = 96.4/3.5/0.1/0.001 Cathode Anode Insulator Metal Corona Bar (grounded) Dielectric Insulator 0.25mm  5 cm 12 cm P = 2625 Torr T = 338K University of Michigan Institute for Plasma Science & Engr. ANDY_GEC2009 Insulator

5. 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 Michigan Institute for Plasma Science & Engr. ANDY_GEC2009

6. REACTION MECHANISM  Reaction mechanism contains 35 species, 12 charged species, 300+ reactions for Ne/Ar/F 2 /Xe mixtures.  Operating pressures of  3 atm emphasize 3-body reactions leading to rapid dimerization. e + Ne  Ne + + e + e Ne + Ne + + M  Ne 2 + + M e + Ne  Ne* + eNe + Ne * + M  Ne 2 * + M e + Ar  Ar + + e + e Ar + Ar + + M  Ar 2 + + M e + Ar  Ar* + eAr + Ar * + M  Ar 2 * + M Ne 2 + + Ar  Ar + + Ne + Ne Ne 2 * + Ar  Ar + + Ne + Ne + e e + F 2  F - + F  Ion-Ion neutralization Ar 2 + + F -  ArF* + Ar Ar + + F - + M  ArF* + M University of Michigan Institute for Plasma Science & Engr. ANDY_GEC2009

7.  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 Ne 2 * 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  Ne 2 * + Ne  Ne 2 *  Ne + Ne + h  (15.5 eV, 800 A) University of Michigan Institute for Plasma Science & Engr. ANDY_GEC2009  h  + Xe  Xe + + e  Ionization potential: 12.13 eV  [Xe] = 7.5 x 10 14 cm -3  = 10 -16 cm 2

8. RADIATION TRANSPORT Emission species j Ionized Species i Absobers k AEinstein coefficient Photo-ionization cross section Photo-absorption cross section University of Michigan Institute 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

9. COMPUTATIONAL MESH  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 Michigan Institute for Plasma Science & Engr. ANDY_GEC2009

10. ELECTRICAL POTENTIAL  Ne/Ar/F 2 /Xe = 96.4/3.5/0.1/0.001  2625 Torr, 338K  Time: 0-35ns : University of Michigan Institute for Plasma Science & Engr. ANDY_GEC2009  Cathode pulsed to -40 kV  Avalanche breakdown collapsed potential in gap.

11. University of Michigan Institute for Plasma Science & Engr. ANDY_GEC2009 CORONA POTENTIAL  Ne/Ar/F 2 /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.

12. University of Michigan Institute for Plasma Science & Engr. ANDY_GEC2009 CORONA E-FIELD  Ne/Ar/F 2 /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 Corona Bar

13. University of Michigan Institute for Plasma Science & Engr. ANDY_GEC2009 CORONA [e]  Ne/Ar/F 2 /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 > 10 15 cm -3 penetrates through gaps.  Photoionization seeds electrons in remote high field regimes, initiating local avalanche.

14. University of Michigan Institute for Plasma Science & Engr. ANDY_GEC2009 Ne 2 * - VUV SOURCE  Ne/Ar/F 2 /Xe = 96.4/3.5/0.1/0.001  2625 Torr, 338K  Time: 0-35ns :  Electron impact from surface avalanche produces Ne*  Ne 2 *.  Densities in excess of 10 12 cm -3 produce photon sources of 10 18 cm -3 s -1.  Untrapped VUV is penetrates through to discharge gap.

15. University of Michigan Institute for Plasma Science & Engr. ANDY_GEC2009 PHOTOIONIZATION  Ne/Ar/F 2 /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 >10 9 cm -3 prior to avalanche.  During avalanche, “internal” VUV- accounts for > 10% of ionization.

16. University of Michigan Institute for Plasma Science & Engr. ANDY_GEC2009 ELECTRON DENSITY  Ne/Ar/F 2 /Xe = 96.4/3.5/0.1/0.001  2625 Torr, 338K  Time: 0-35ns :  Electron density > 10 15 cm -3 in mid gap – spreading from narrow anode to broad cathode.  Photoelectrons seed avalanches in all high field regions.

17. University of Michigan Institute for Plasma Science & Engr. ANDY_GEC2009 ArF* DENSITY  Ne/Ar/F 2 /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 10 14 /cm 3.  ArF*  Ar + F produces laser output

18.  = 5  Pre-ionization electron density at t=25ns University of Michigan Institute 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

19. CONCLUDING REMARKS University of Michigan Institute 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 Ne 2 * are sufficient to produce preionization densities > 10 9 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.

20. BACKUP V-I Curves University of Michigan Institute 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.