Organic Antireflective Coatings for Photomask Fabrication using Optical Pattern Generators Benjamen M. Rathsack 1, Cyrus E. Tabery 1, Cece Philbin 2, and C. Grant Willson 1 September 15, 1999 1 Department of Chemical Engineering, The University of Texas at Austin 2 DPI Reticle Technology Center LLC, 2011 Greenhill Dr., Round Rock, TX 78664
High Resolution Resist Optimization Goal: Improve resolution and process latitude for photomask fabrication Method: Line-edge optimization of exposure image and resist development response Challenge: Standing waves impact resist performance Solution: PEB and/or organic antireflection coatings (ARCs)
Basis of Line Edge Optimization (0.5 µm Space in Resist) Aerial Image Relative PAC (m) Exposure Dose + Kinetics (A, B and C) Relative Intensity -1000-500 0 500 1000 Horizontal Direction (nm) m Develop Time Contours Output: Resist Profile Sidewall angle θ and feature size w θ PROLITH 2 v6.05 w
Lithographic Imaging Equation High Resolution Imaging 1. Image Transfer Position steepest slope (inflection point) of the aerial image at the feature edge 2. Dissolution contrast (γ R ) Maximize dissolution change with dose through R(m) analysis 3. Dissolution Threshold Position dissolution threshold at the inflection point of the image 1-Dimensional Analysis dr dx R x m γ R x * γ x * R = = γ R dr dm dm dx = Dissolution rate = Horizontal position = Relative PAC Concentration = Resist contrast = Nominal edge of resist feature
Simulated Optimum Exposure Dose for Photomask Lithography Determined exposure dose that resulted in the maximum m Best Doses IP3600 210 mj/cm 2 PAC gradient (m -1 ) 3.2 2.8 2.4 2.0 IP3600 100 150 200 250 300 Exposure Dose (mj/cm 2 ) Determined m at the edge of the resist feature at the dose giving maximum m m(x,z) of IP3600 with a 210 mj/ cm 2 dose Target m IP3600 = 0.3
Optimal Dissolution Notch Location Lower developer concentration shifts notch toward the target m Optimal resist for mask lithography has Large dissolution Notch near Target m Diss. Rate (nm/s) 80 60 40 20 Low Developer Conc. 0.26N Model(0.26N) 0.23N Model(0.23N) 0.20N Model(0.20N) 0 0 0.2 0.4 0.6 0.8 Relative PAC (m) Target m IP3600 Notch Location m 0.35 (developed w/ 0.20 N TMAH)
Optimal Development Rate Function (R(m)) High contrast resists have a large dissolution notch that resolves standing waves Dissolution Rate (nm/sec) 40 35 30 25 20 15 10 5 0 Largest standing waves 0.4 0.5 0.6 0.7 0.8 Relative PAC Concentration PFI88A i120 ip3600
Standing Waves in Photomask Resists Measured oscillation in dissolution rate shows clear evidence of interference Lower developer concentration increases influence of standing waves Dissolution Rate (nm/s) 25 20 15 10 5 0 IP3600 developed with 0.20 N TMAH (NMD-W) 0 200 400 600 Thickness (nm)
High Resolution Resists Resolve Standing Waves No Standing Waves High Resolution 0.27 micron space Lower Developer Concentration Standing Waves PFI88A3/ 0.26N TMAH/ 300 sec. 120 C PEB PFI88A3/ 0.26N TMAH/ 300 sec. 120 C PEB PFI88A3/ 0.23N TMAH/ 300 sec. 120 C PEB
Organic Antireflection Coatings (ARCs) for Laser Photomask Fabrication Resist (1) ARC (2) Photomask (3) Rtotal = ρ total 2 = ρ 12 + ρ 23 τ D 2 2 1+ ρ12ρ23τ D 2 Where and n ~ j ρ ij = n~ n~ i i + n~ n~ is the complex index of refraction j j 2 ρ 12 + ρ23τ D = 0 Minimize reflectivity (R total ) τ e i2πn ~ 2D / λ D = ARC index of refraction (n 2 ) ARC thickness (D)
ARCs on Photomasks Coating Process Spin coat diluted Barli II at 2000 rpm Bake 15 minutes at hot plate set point of 200 C Successfully coated 55 nm ARC thickness ARC Evaluations No gross defects at 55 nm Gross defects at 35 nm No resist beading or intermixing
Simulated Organic ARC for Iline 0.030 Simulated Optimal ARC thickness is 46nm Resist-ARC Reflectivity 0.025 0.020 0.015 0.010 0.005 0.000 0 25 50 75 100 125 150 ARC Thickness (nm) Resist swing curve dramatically reduced with an ARC Air-Resist Reflectivity 0.14 0.12 0.10 0.08 0.06 0.04 0.02 Reflectivity w/ ARC Reflectivity w/o ARC 0.00 400 500 600 700 800 Resist Thickness (nm)
Simulated PAC distributions PAC distribution in resist (IP3600) without ARC PAC distribution in resist ARC (46 nm) Minimize standing waves
Photomask Resist Profiles on an Organic ARC Standing waves are reduced ARC film thickness needs to be optimized to remove more of the reflections i120/ 0.20 N/ ARC/ PEB PFI88A3/ 0.23 N/ ARC/ PEB
I-line ARC and Resist Thickness Optimization Simultaneous optimization of both ARC and resist thickness is required ARC Thickness (nm)
Organic ARCs for 257 nm Laser Photomask Fabrication NCA resist shows similar substrate reflectivity to I-line resists 0.035 0.030 0.025 CA resist shows more reflectivity at substrate interface than NCA resist High contrast of CA resists will increase impact of standing waves Reflectivity 0.020 0.015 0.010 0.005 0.000 CA Resist NCA Resist 0 50 100 150 ARC Thickness (nm)
ARC Thickness and Swing Curves at 257 nm CA resists display more severe swing ratio without an ARC Simultaneous ARC and resist thickness optimization required Resist Reflectivity 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 NCA w/ ARC NCA w/o ARC CA w/ ARC CA w/o ARC 300 350 400 450 500 550 600 Resist Thickness (nm) ARC Thickness (nm)
Conclusions High resolution I-line resists resolve larger standing waves Photomask resist profiles have been successfully imaged on organic ARCs to minimize reflections High contrast resists (CA resists) for 257 nm optical pattern generators may require ARCs ARC and resist thickness need to be simultaneously optimized to reduce reflections
Acknowledgements ETEC Systems Inc. SRC/ Texas Instruments Fellowship Shipley and Clariant FINLE Technologies J. A. Woollam (Ron Synowicki) N&K Technologies