VOC Emissions Reduction from Photolithographic Processes Dr. Thomas W. Peterson Chemical and Environmental Engineering, University of Arizona 1999 Arizona Board of Regents for The University of Arizona Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 1 Photolithography • Application and development of photoresist with the use of light and heat to write a pattern of microstructures on a wafer • Photoresist – Protective material used to form a resistant film on the wafer surface – Reacts chemically with areas exposed (not covered by a patterned mask) to become more or less soluble with respect to a solvent Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 2 Basic Photo Process Steps Incoming Wafer Soft Bake Adhesion Cool Plate Cool Plate Stepper 1 Interface Post Exposure Bake Cool Plate Develop Resist Application Align and Expose Post-Dev Inspection Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 3 Spin-Coat Resist Usage/Waste ~ 5 ml Dispense on 200 mm Dia Wafer 1 - 2 Microns Thick +/- 0.01 Microns Initial Wet Volume: ~ 5 ml Final Wet Volume: ~ .06 ml Approx 99% Waste Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 4 Chemical Usage in the Photolithography Process • Standard photoresist chemicals – Resin (e.g. Cresol-formaldehyde {Novolac} ) – Solvents • Ethyl lactate (EL) (b.p 154C) • Ethyl 3-ethoxypropionate (EEP) • Cellosolve Acetate – Photo-active Compounds • Developer • Edge Bead Remover • Solvents Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 5 Strategies for Reducing VOC Emissions • Switch to Water-Based Resists – Advantage: no VOC emissions – Disadvantage: line width resolution, “speed” • Optimize Use of Current Solvent-Based Resists – Advantage: utilize current formulations, equipment – Disadvantage: no hope for eliminating VOC emissions Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 6 Benefits to Minimizing Resist Usage • Resist cost (~$500/gallon) • Environmental Impact – Most of the Resist is “wasted”, in the sense that it is spun off the wafer. The high “waste” is required to achieve uniformity in thickness and proper coverage. – Because of precise resist formulations, solvent evaporation during spinning, and contamination concerns, it is impossible to recycle resist that is spun from the wafer. – Typically, Organic Solvents are used to clean the photoresist “cups” into which the excess resist is spun. Less solvent usage leads to less emissions of Volatile Organic Compounds (VOCs). Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 7 Photoresist Dispense Optimization • Objective – Coverage of given thickness – Coverage of given uniformity • Control Variable(s) – Resist properties (viscosity, solvent mass fraction, amount dispensed) – Spin speed – Spin recipe – Temperature, RH Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 8 Three Steps to Coating Process Surface Tension Viscosity Solvent Evaporation Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 9 Spin-Coat Operating Parameters • • • • • • • • Spin Speed (rpm) Spin Time (sec) Surface Temperature Room Temperature Room Humidity Resist Volume Resist Viscosity Resist Solid Mass Fraction Ambient T, RH Resist Volume, Solids Wt %, surface T t, Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 10 Properties of Photoresist • Initial Viscosity: 5 to 100 cP • Solvent Mass Fraction (SMF): 70-90% • Photoresist viscosity for the experiments presented: – 560 Resist - 17 cP – 500 Resist - 40 cP • Typical Viscosity Changes: 0.01 to 10,000 Poise as SMF goes from 1.0 to 0.0 • Typical Diffusivity Changes: 10-6 to 10-12 cm2/sec as SMF goes from 1.0 to 0.0 Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 11 Coating Process Overview • Current Practice – At slow speed (usually ~2000 rpm) dispense 3-5 cc resist and spin for less than 2 seconds – Ramp up speed (~3000 rpm) and spin for 20-30 seconds – Apply Edge Bead Remover – Fraction Resist “spun off”: typically 97-99% • Ultra Casting Pre-dispense (UCP) – At very high speed (6000 rpm) dispense 1 cc resist and spin for 1 second – Decelerate to 2000 rpm and apply second dispense of 1 cc – Accelerate to 3000 rpm and spin for 20-30 seconds – Apply Edge Bead Remover – Fraction Resist “spin off”: typically 95-97% Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 12 How Fast Does the Solvent Evaporate? Solvent Mass Fraction vs time 0.8 Solvent Mass Fraction 0.7 0.6 0.5 0.4 0.3 6000 RPM 0.2 4000 RPM 2000 RPM 0.1 0 0 5 10 15 20 25 30 Time, sec Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 13 Solid and Solvent Mass Remaining on Wafer Solids Remaining on Wafer Solvent Mass Remaining on Wafer 60 180 160 2000 rpm 140 4000 rpm 120 6000 rpm Solvent Remaining, mg Solids remaining, mg 50 40 30 20 2000 rpm 4000 rpm 10 100 80 60 40 6000 rpm 20 0 0 0 5 10 15 Time, sec 20 25 30 0 5 10 15 20 25 30 Time, sec Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 14 Flow Characteristics: Resist Dispense Nozzle Flow Rate through Resist Nozzle Water Flow through 2mm x 6mm Resist Nozzle 6 Flow Rate, ml/sec 5 4 3 2 Flow Rate = 0.4125 (Inches H20) + 1.4911 1 2 R = 0.9834 0 0 1 2 3 4 5 6 7 8 9 10 Pressure Drop, In H2O Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 15 Resist Flows vs. Pressure Drop in TRACK Pump Resist Flows in TRACK Pump 4 560 @ 21.7C y = 0.2659x - 0.3795 R2 = 1 3.5 500 @ 22.0C Linear (560 @ 21.7C) Flow Rate, gr/sec 3 Linear (500 @ 22.0C) 2.5 2 y = 0.1078x - 0.154 R2 = 1 1.5 1 0.5 0 0 2 4 6 8 10 12 14 16 Pressure Drop, PSI Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 16 Dispense Rate vs. Temperature Dispense Rate vs Temperature 3 2.5 g/sec 2 1.5 1 0.5 0 30 C 5 psi 560 25 C 22 C 10 psi 560 18 C 10psi 500 16 C 15psi 500 Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 17 Temperature Dependence of Resist Viscosity Viscosity vs Temperature Visc = V0 exp(K/T) 1 Resist Viscosity, Poise 560 500 560 500 Resist Resist model model 560: K=710.58 Deg K V0 = 0.0157 Poise 500: K=1220.8 Deg K V0 = 0.0067 Poise 0.1 3.28 3.30 3.32 3.34 3.36 3.38 3.40 3.42 3.44 3.46 1000/T (Deg K) Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 18 Theory of Spin Coating Conservation of solvent xA xA xA ( D( xA) )0 t z z z Radial momentum/continuity 2 2 (h z') w dz ' 0 [ x A ( z', t )] z z 0 Boundary Conditions ( 1 x ) D( x A ) A k ( x A x A ) 0 at z = h 1 xA z xA 0 at z = 0 z dh w( z h) k ( x A ( z h) x A ) 0 dt w 0 and where xA mass fraction of solvent in resist D Diffusivity of solvent density of resist spin speed h Can show theoretically that h h ( , ) K 0.25 / 1/ 2 resist thickness viscosity of resist Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 19 Effect of Evaporation on Film Height During Spinning 1000 RPM, NO evap 2000 RPM 4000 RPM 8000 RPM 1000 RPM W/ evap 2000 RPM 4000 RPM 8000 RPM 2000 rpm 4000 rpm 8000 rpm 1 100 80 Solvent Mass Fraction Film Height vs Spin Time Effect of Solvent Evaporation 90 Film Height, um 1000 rpm 70 60 50 40 30 20 10 Solvent Mass Fraction Due to Evaporation 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0 5 10 15 Spin Time, Sec 20 25 30 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Spin Time, sec Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 20 Final Film Thickness: Single Dispense Single Dispense Data, 500 and 560 Resists 2 1.9 hf = 10^1.38 Nu^0.4 / RPM^0.52 Final Height, um 1.8 1.7 1.6 1.5 1.4 1.3 1.2 model data 1.1 1 0.04 0.045 0.05 0.055 0.06 0.065 0.07 0.075 0.08 0.085 Nu^0.4/RPM^0.52 Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 21 Ultra Casting Predispense Technique UCP 6000 6000 1cc dispense 3-5 cc dispense 4000 4000 casting rpm casting rpm EBR 2000 EBR 2000 time[sec] conventional dispense time[sec] < 1 sec UCP dispense Note: All dispenses achieve complete coverage of the wafer with coating solution (Resist) ; the process that forms the final thickness and uniformity is at the same rpm in both cases ! Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 22 UCP Resist Coating - 2 Layer Model Solvent Evaporation Layer 2 Viscous Flow g2 wafer Layer 1 g1 FORCE g2 <<< g1 Step 1: (1cc) Overcome surface tension Use radial force (ultra casting rpm, 5000-6000 rpm) Create interface Step 2: (1cc) Dispense at or below casting rpm, Minimal resist needed because very little resistance from surface tension Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 23 Resist Thickness vs. RPM 560 Resist Thickness vs. RPM Angstrom 21500 19500 17500 15500 UCP 13500 5cc 1cc 11500 9500 7500 0 500 1000 1500 2000 2500 3000 3500 4000 4500 rpm * Once surface is wetted, only a small additional amount is needed to built thickness. * Key concept: initial wetting at ultra casting rpm, second dispense at or below casting Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 24 Resist Thickness vs. RPM ALL DATA Film Thickness vs RPM 5 17 cP 40 cP, UCP 50 cStk 4.5 Film Thickness, um 4 40 cP 20 cStk 75 cStk 17 cP, UCP 35 cStk 125 cStk 3.5 3 2.5 2 1.5 1 0.5 0 1000 2000 3000 4000 5000 6000 Spin Speed, RPM Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 25 Final Film Thickness: Double Dispense Data Double Dispense 500 and 560 Data 3 hf = 10^1.349 * Nu^0.4/RPM^0.514 Final height, um 2.5 2 1.5 model 1 0.5 0.04 0.05 0.06 0.07 0.08 data 0.09 0.1 0.11 0.12 0.13 Nu^0.4 / RPM^0.514 Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 26 Final Height: Single and Double Dispense Single and Double Dispense 500 and 560 Resists 3 4 data,single Measured Hf 500 and 560 Resists Resists 20-125 cStk 3.5 data,double 2.5 ALL DATA 7 Resists, 4 Operating Conditions 3 Model 2.5 2 2 1.5 1.5 r^2 = 0.9995 r^2 = 0.96 1 1 0.5 hf = 10^1.36 ^ NU^0.4 / RPM^0.514 Hf = 10^1.43 * Nu^0.48 / RPM ^ 0.56 0 0.5 0.5 1 1.5 2 Predicted Hf 2.5 3 0 1 2 3 Predicted Hf 4 Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 27 Conclusions • Resist usage: – ~5ml for single dispense – ~2ml for UCP method • Resist thickness and uniformity: identical for both methods • Comparison to theoretical models: identical for both methods; slightly stronger viscosity dependence than predicted theoretically • Resist and solvent usage reduction yields substantial reduction in VOC emissions Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 28 Acknowledgments • Motorola MOS 12 – Tom Roche – Melissa Masteller – Frank Fischer – Michelle Demumbrum • University of Arizona – Chris Doty Peterson NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing 29