Water for Immersion Lithography M. Switkes, V. Liberman, and M. Rothschild Lincoln Laboratory Massachusetts Institute of Technology Lexington, MA 02420
Outline Water treatment The symptoms Differences among DI waters Water storage Residue on optical surfaces The causes Total organic carbon Other organics Gas content Particulates MIT/LL water treatment Water optic interaction Bubbles Alternative fluids
DI is Not Enough: Transmission Variations Source Storage
DI is Not Enough: Residue on Optical Surfaces
Total Organic Carbon
Other Water Quality Issues Gas content Bubble control Flow in small channels Particulates Deposition on wafer Flare Gas/Vacuum Temperature control Index stability 300 nm
Outline Water treatment The symptoms The causes MIT/LL water treatment testbed Components Troubleshooting Water optic interaction Bubbles Alternative fluids
Water Treatment Testbed Type II 10 15 MΩ Ultra-pure 10 30 ppb O 2 17.6 MΩ 1.5 ppb OOC 17.6 MΩ < 1 ppb OOC After Milli-Q, all fluorocarbon construction (PTFE & PFA) Control of Particles to 0.05 µm Ions Total organic carbon Dissolved gas No temperature control Mykrolis
Water Treatment Testbed
Water Treatment Issues nonvolatile liquid droplets Bare fused silica after 48 h in water flowing at 3.5 l/h residence time ~1.5 s
Water Treatment Solutions? Close collaboration with Mykrolis Careful measurements of resistivity, OOC, and nonvolatile residue (NVR) at each stage of the purification One component seems to have been the major source of contamination NVR and OOC now < 1 ppb each 48 h in flowing water leaves windows clean
Outline Water treatment Water optic interaction Dark interaction Water and bare CaF 2 Protective coating Long-term exposure testbed Bubbles Alternative fluids
Water Optic Interaction Bare CaF 2 pre-soak 2 hours in H 2 O 7 days in H 2 O 2 nm RMS 7 nm RMS 10 nm RMS
Water Optic Interaction 100 nm SiO 2 on CaF 2 pre-soak 2 hours in H 2 O 7 days in H 2 O 0.2 nm RMS 0.3 nm RMS 0.7 nm RMS
Water Optic Interaction Testbed contaminant (optional) Metrology in-situ ex-situ actinic transmission microscopic/ photographic inspection Lambda Physik A4030 4 khz at 3 5 mj/cm 2 /pulse 300x10 6 pulses/day 1 2 MJ/cm 2 /day spectroscopic transmission profiling AFM XPS FTIR small-spot spectroscopic ellipsometry with profiling
Water Optic Interaction Testbed laser output exposure/ metrology
Exposure/Metrology Chamber UV-Vis lamp ellipsometer in water in/out from laser ellipsometer out cell to spectrometer
Cell for Long Term Exposure 38x3 mm windows 2 mm water gap adapted from a design by John Burnett, NIST and from Harrick Scientific Corp. all stainless steel construction teflon-coated o-rings teflon (PTFE & PFA) fittings solvent and O2 plasma cleaned
First Results on Lincoln Coating 200 nm SiO 2 coating on CaF 2 deposited in house ~750 M pulses at ~3 mj/cm 2 /pulse N 2 H 2 O
Outline Water treatment Water optic interaction Bubbles Nanobubbles Resist outgassing Alternative fluids
Nano-Bubbles underwater AFM J. W. G. Tyrrell and P. Attard, Phys. Rev. Lett. 87, 176104/1 (2001). aerial image numerical simulation Michael Yeung Boston University
Rapid Cryofixation/Freeze Fracture gas-saturated water hydrophobic surface degassed water hydrophobic surface with J. Ruberti Cambridge Polymer Group gas-saturated water hydrophilic surface
Resist Outgassing in Immersion Lens contamination less likely than in conventional lithography Outgassing products diffuse across the same lens wafer gap ~1000 times slower in water than in gaseous ambient With 10x smaller gap, diffusion time still 10x greater in water Outgassed resist components not likely to induce significant changes in liquid index (< 1 ppm) Peak transient outgassing rates could exceed solubility limits in the immersion fluid Hydrophobic organics exhibit solubility of 0.1 500 µg/cm 3 Calculations show outgassing could exceed 100 µg/cm 3 for high end of current outgassing rates Outgassing requirements will be different No longer focused on photocontamination Driven by flux and product solubility
Impact of Microbubbles on Lithography 20 µm 10 µm 5.5 µm simulations by Michael Yeung Boston University
Modeling Outgassing Product Concentration Isobutene in Resist Isobutene in Water t = 10-5 s t = 10-5 s IBM V2 Peak Rate (mol/cm 2 -sec) Contaminant Diffusion Coefficient in Resist (cm 2 /sec) 10-12 10-11 10-10 10-9 10-8 10 12 0.0025 0.0073 0.025 0.073 0.25 10 13 0.025 0.073 0.25 0.73 2.5 10 14 0.25 0.73 2.5 7.3 25 10 15 2.5 7.3 25 73 250 modeling by Rod Kunz MIT/LL Values are concentration in µg/cm 3 ; isobutene s solubility limit is 340 µg/cm 3
Cumulative Resist Outgassing Resist thickness: 200 nm Diffusion in resist: 5x10-9 cm 2 /s Diffusion in water: 1x10-6 cm 2 /s Exposure duration: 1 pulse 1 >95% outgassing within 2 s modeling by Rod Kunz MIT/LL
Outgassing Into a Liquid Ideal Dissolution The Fickian Interface Phase Segregation Bubble Formation Minimal impact on refractive index Likely outcome for polar compounds or any case where the concentration is well below the solubility limit Phase inhomogeneities might be liquid or gaseous Driven by nucleation thermodynamics Phase Segregation Surface Nucleation Boundary Reflection No Net Mass Loss Phase inhomogeneities might be liquid or gaseous Driven by nucleation thermodynamics Precedent exists for this to occur for hydrophobic products due to the observed partition functions seen in solid-phase extraction Partitioning INTO solid polymer can occur
Looking for Outgassing-Induced Bubbles APEX resist high bubble generation - = 1 µs post-exposure experimental image with Tim Shedd University of Wisconsin 5 s post-exposure reference image processed image reveals bubbles
Outgassing Induced Bubbles 2 mj/cm 2 /pulse model immersion resist 2700 images (~ 235 mm 2 ) 500 controls with no resist 1000 images taken 1 µs after each isolated pulse (~ 1 Hz) 1000 images taken 10 µs after each isolated pulse (~ 1 Hz) 200 images taken 1 µs after a 5 pulse burst (100 Hz) Only 1 in focus particle detected Persistence makes it unlikely to be a bubble
Outline Water treatment Water optic interaction Bubbles Alternative fluids
Alternative Fluids Some alternatives to pure water may be desired Probably not at first Solutions to problems which arise Improved flow dynamics Increased index of refraction Decreased corrosion or optic contamination Improved resist performance etc. Air Products OptiFluids
Conclusions Water treatment is non-trivial Careful analysis of construction materials required Ions, organics, and fluorocarbons must be controlled Water optic interaction CaF 2 dissolves quickly (hours) in water SiO 2 coatings protect for days in the dark Infrastructure in place for long-term durability testing Bubbles No bubbles seen in 2200 resist outgassing images with TOK model resist One bubble candidate but probably a particle in the water Infrastructure in place to test more/different resists
Ongoing/Future experiments Multi-pronged analysis of water treatment system to determine source of residual contaminants In situ monitoring of nonvolatile residue GCMS analysis of nonvolatile residue Fluorocarbon analysis Durability testing of immersion coatings In-house Vendor supplied Resist outgassing images Automated collection of a large number of post-exposure images Machine-vision search for bubbles Different resists TOK models eventually vendor supplied
Acknowledgments This work was performed under Cooperative Research and Development Agreements between and SEMATECH and between and Air Products. Opinions, interpretations, conclusions, and recommendations are those of the authors, and do not necessarily represent the view of the United States Government. We would like to thank J.Curtin, D.Hardy, and S.Palmacci for expert technical assistance, R. Kunz for analysis of the residual water contamination, J. Ruberti at Cambridge Polymer for freeze-fracture work, H. Burnett and T. Shedd at UW for work on the resist outgassing/bubbles experiment, J. Smith and colleagues at Mykrolis for help with the water purification system, M. Yeung at BU for simulation data, P. Zhang at Air Products for OptiFluid samples, A. Grenville at Intel and SEMATECH for useful discussion, and A.G. Malus at WaveMetrics Inc. for software support.