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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.