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TRIGGERING EXCIMER LASERS BY PHOTOIONIZATION FROM A CORONA DISCHARGE*

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.

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TRIGGERING EXCIMER LASERS BY PHOTOIONIZATION FROM A CORONA DISCHARGE*

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  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. University of Michigan Institute for Plasma Science & Engr. • Excimer discharge excited lasers for photolithography • Preionization schemes • Description of Model • Discharge triggering sequence • Dependence on corona bar properties • Concluding Remarks AGENDA ANDY_GEC2009

  3. University of Michigan Institute for Plasma Science & Engr. • 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. EXCIMER LASERS FOR PHOTOLITHOGRAPHY Ar+ + F- Ar* + F2 • Coherent, short wavelengths have made ArF (193 nm) the source of choice for photolithography for micro-electronics fabrication. • e + Ar  Ar* + e • e + Ar  Ar+ + 2e • e + F2 F + F- ArF* E(R) Laser ArF Ar, F R (www.spie.org) (Cymer Inc.) ANDY_GEC2009

  4. University of Michigan Institute for Plasma Science & Engr. • 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. Insulator PLASMA DISCHARGE and PRE-IONIZATION e 0.25mm Metal Corona Bar (grounded) 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

  5. University of Michigan Institute for Plasma Science & Engr. • 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. DESCRIPTION OF MODEL ANDY_GEC2009

  6. University of Michigan Institute for Plasma Science & Engr. • 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 REACTION MECHANISM ANDY_GEC2009

  7. University of Michigan Institute for Plasma Science & Engr. • 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 • h + Xe  Xe+ + e • Ionization potential: 12.13 eV • [Xe] = 7.5 x 1014 cm-3 •  = 10-16 cm2 • e + Ne  Ne* + e • Ne* + 2Ne  Ne2* + Ne • Ne2*  Ne + Ne + h (15.5 eV, 800 A) ANDY_GEC2009

  8. University of Michigan Institute for Plasma Science & Engr. 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’. • Includes view-factors. • Rate of ionization RADIATION TRANSPORT Absobers k Ionized Species i A Einstein coefficient Photo-ionization cross section Photo-absorption cross section ANDY_GEC2009

  9. University of Michigan Institute for Plasma Science & Engr. • Unstructured mesh used to resolve chamber geometry and large dynamic range in dimensions. • Total number of nodes: 9,336 • Plasma nodes: 5,607 COMPUTATIONAL MESH ANDY_GEC2009

  10. University of Michigan Institute for Plasma Science & Engr. • Cathode pulsed to -40 kV • Avalanche breakdown collapsed potential in gap. ELECTRICAL POTENTIAL • Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 • 2625 Torr, 338K • Time: 0-35ns : ANDY_GEC2009

  11. University of Michigan Institute for Plasma Science & Engr. • Probe from cathode to corona dielectric surface initiates surface discharge. • Charging of surface occurs around the circumference. CORONA POTENTIAL • Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 • 2625 Torr, 338K • Time: 0-35ns : ANDY_GEC2009

  12. University of Michigan Institute for Plasma Science & Engr. • Electric field in surface avalanche propagates around circumference. • Remaining charge produces radial fields in corona bar. • Surface charges on insulator produce large sheath fields. CORONA E-FIELD Cathode Corona Bar • Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 • 2625 Torr, 338K • Time: 0-35ns : ANDY_GEC2009

  13. University of Michigan Institute for Plasma Science & Engr. • 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. CORONA [e] • Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 • 2625 Torr, 338K • Time: 0-35ns : ANDY_GEC2009

  14. University of Michigan Institute for Plasma Science & Engr. • 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. Ne2* - VUV SOURCE • Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 • 2625 Torr, 338K • Time: 0-35ns : ANDY_GEC2009

  15. University of Michigan Institute for Plasma Science & Engr. • 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. PHOTOIONIZATION • Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 • 2625 Torr, 338K • Time: 0-35ns : ANDY_GEC2009

  16. University of Michigan Institute for Plasma Science & Engr. • Electron density > 1015 cm-3 in mid gap – spreading from narrow anode to broad cathode. • Photoelectrons seed avalanches in all high field regions. ELECTRON DENSITY • Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 • 2625 Torr, 338K • Time: 0-35ns : ANDY_GEC2009

  17. University of Michigan Institute for Plasma Science & Engr. • The density of the excimer ArF* produced in the discharge exceeds 1014 /cm3. • ArF*  Ar + Fproduces laser output ArF* DENSITY • Ne/Ar/F2/Xe = 96.4/3.5/0.1/0.001 • 2625 Torr, 338K • Time: 0-35ns : ANDY_GEC2009

  18. University of Michigan Institute for Plasma Science & Engr. CORONA BAR  • The capacitance of the corona bar increases with . • Longer charging time produces more VUV, increasing [e] in gap. Corona Bar /0 • Pre-ionization electron density at t=25ns e = 5 e = 60 e = 20 ANDY_GEC2009

  19. 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

  20. University of Michigan Institute for Plasma Science & Engr. • 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. BACKUP V-I Curves ANDY_GEC2009

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