NSF Center for Micro and Nanoscale Contamination Control Removal of Nanoscale Particles from EUV Mask Layers by Laser Induced Shockwaves and Megasonics Ahmed Busnaina* and J. G. Park** * W.L.Smith Professor and Co-director NSF Center for Microcontamination Control, Northeastern University, Boston, MA 02115-5000 Tel: 617 373-2992, Fax: 617 373-2921 Email: a.busnaina@neu.edu, URL: www.cmc.neu.edu ** Dept. of Metallurgy & Materials Engineering, Hanyang University, Ansan, 425-791, Korea
Outline Introduction Megasonic Cleaning Mechanism Nanoparticle Removal Particle Removal from Deep Trenches Laser cleaning technique overview Conventional laser wafer cleaning Laser shock cleaning (LSC) Experimental test and results Pros and Cons of Megasonics, LSC, Supercritical CO2 and Brush Cleaning Conclusions
Introduction EUV lithography needs to operate without a pellicle. The reticle may be subject to particulate and chemical contamination, in the absence of a pellicle. There is a need to develop a cleaning process that would remove all contaminants according to EUV cleaning requirements. EUV cleaning requirements are: Removal of all particles larger than 30nm Removal of organic contamination Cleaning should not change the ARC reflectivity by more than 1%, The process should be damage free and should adhere to the environmental safety standards.
Acoustic Boundary Layer Thickness y Acoustic boundary layer thickness: in water, f=850khz, δ ac =0.61µm f=760khz, δ ac =0.65µm f=360khz, δ ac =0.94µm The hydrodynamic boundary layer thickness: δ δ δ ac u f H, Turbulent H, La min ar 2ν = ω ν = 0.16 Ux ν = 5.0 Ux 1 2 1 7 x x x x x 1 2 6 7 G u f τ τ Free stream Velocity boundary layer Velocity boundary layer on a flat plate 1 2 in water, u=4m/s, at center of an 8 wafer, δ H =2570 µm G(x) y(micron) y(micron) 2500 2000 1500 1000 500 Velocity Profile Laminar flow Turbulent flow Acoustic Flow ( f = 800kHz ) 0 0 1 2 3 4 10 Velocity Profile 9 8 7 6 5 4 3 2 1 Velocity (m/s) x = 4 inch, U = 4m/s x = 4 inch, U = 4m/s Laminar flow Turbulent flow Acoustic Flow ( f = 800kHz ) 0 0 1 2 3 4 Velocity (m/s)
Acoustic Streaming Boundary layer thickness (micron) 6 I = 7.7 5 W /cm A co u stic, f= 3 6 0 K H z A co u stic, f= 7 6 0 K H z A co u stic, f= 8 5 0 K H z B o u n d a ry la ye r th ickn e ss (m icro n ) S tre a m in g V e lo city (m /s) u > 0.3 c 4 2 10 3 10 2 10 1 100 1 0-1 2 0 101 102 103 104 10-2 10-3 Streaming Velocity (m/s) A co u stic F lo w P ro p e rtie s F re q u e n cy ( k H z ) f=760khz 400 S tre a m in g V e lo c ity v s. A c o u s tic P o w e r 2500 1 M Hz 850k H z 760k H z 360k H z 300 v(cm/s) v(cm/s) 2000 1500 I=.77W/cm2 2 I=1.55W/cm 2 I=2.33W/cm 2 I=3.10W/cm 2 I=3.88W/cm 2 I=4.65W/cm 2 I=5.43W/cm 2 I=6.20W/cm 2 I=6.97W/cm 2 I=7.75W/cm 200 1000 100 500 0 0 5 10 15 20 2 In te n s ity ( W /c m ) 25 30 0 5 10 15 20 DistanceFromTankWall(cm)
Removal/Adhesion Moment Ratio (RM) Removal Efficiency vs. Moment Ratio (MR) Colloidal Silica (0.3-0.7micron) Removal Experiments U Removal Percentage Moment Ratio M R 100 90 80 Removal Percentage 70 60 50 40 30 20 10 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 MR = MR = F Moment Ratio Re moval moment Adhesion resisting moment D ( 1.399R δ ) F a a When MR >1, most particles are removed. 1.399R a O δ M A F Adhesion F Drag Rolling removal mechanism
Removal/Adhesion Moment Ratio (RM) vs. D 10 4 Removal Moment Ratio (RM) vs. Particle Diameter DI water, electrical double layer force is negligible U = 4 m/s (Acoustic Flow, 800kHz, 7.75W/cm 2 ) Removal Moment RM = Adhesion Resisting Moment Removal Moment Ratio (RM) vs. Particle Diameter SC-1, electrical double layer force is repulse U = 4 m/s (Acoustic Flow, 800kHz, 7.75W/cm 2 ) 10 4 Removal Moment RM = Adhesion Resisting Moment 10 3 10 3 10 2 10 1 RM > 1 Removal > Adhesion Particle will be removed 10 2 10 1 RM > 1 Removal > Adhesion Particle will be removed RM 10 0 10-1 10-2 10-3 10-4 10-5 SiO 2 particle on SiO 2,Acoustic(800kHz) PSL particle on SiO 2, Acoustic ( 800 khz ) SiO 2 particle on SiO 2, Hydrodynnamic PSL particle on SiO 2, Hydrodynnamic 10-2 10-1 10 0 10 1 Particle Diameter (micron) RM = 1 RM < 1 Removal < Adhesion Particle can not be removed 10-5 10-2 10-1 10 0 10 1 Particle Diameter (micron) Hard particles (silica) are easier to remove than soft particles (PSL). RM Particles larger than 20nm can be removed, but when electrical double layer force is considered, removal of 10 nm silica particle is possible. Hydrodynamic flow (at 4m/s) did not remove PSL particles smaller than 10 um. 10 0 10-1 10-2 10-3 10-4 SiO 2 particle on SiO 2, Acoustic (800kHz)+SC1 PSL particle on SiO 2, Acoustic (800kHz)+SC1 SiO 2 particle on SiO 2, Hydrodynnamic+SC1 PSL particle on SiO 2, Hydrodynnamic+SC1 RM = 1 RM < 1 Removal < Adhesion Particle can not be removed
Single Wafer Megasonic Cleaning 200mm Wafer
Silicon Complete Nitride-D.I. removal Water, of silicon Temperature Nitride particles at 38 C ( 200nm) Using DI water only Frame 001 20 May 2003.28-1um Si3N4 Removal Power Vs Time @38 120 Removal Efficiency For Silicon Nitride Particles Ranging From 0.26 to 1.18 um in diameters, Using D.I. Water & Bottom Megasonic Transducer, Temperature at 38 C Time (Sec) 110 100 90 80 70 60 50 40 64.4 61.3 67.4 70.4 76.5 73.4 79.5 82.5 85.5 88.6 91.6 94.6 100.0 100.0 efficiency 100.0 94.6 91.6 88.6 85.5 82.5 79.5 76.5 73.4 70.4 67.4 64.4 61.3 58.3 30 30 40 50 60 70 80 90 100 Power (%)
Introduction Visualization of Fluorescent Particles ¾ ¾ ¾ Nikon G block filter 75 W Xenon arclamp Extra N.D filter 542 nanometers Fluorescent Cube G block fluorescent filter specs.
Visualization of Fluorescent Particles 98 nm PSL fluorescent particles 500 X Fluorescent Field 1000 X Fluorescent field 28 nm PSL fluorescent particles
Nano-Particle Removal with Megasonics Has been Demonstrated Using Fluorescence Microscopy Removal of nanoscale PSL Florescent Particles 90, 63 and 28 nm PSL particles were removed from bare silicon wafers using the Single wafer megasonic cleaning tank 63 nm Before 63 nm After
Nano-Particle Removal from EUV Substrates with Megasonics and DI Water 100.0% 98.0% 96.0% removal efficieny (DI water) 94.0% 92.0% 90.0% 88.0% Bare Silicon wafer+di water EUV 4 nm Si_Cap ML wafer+di water EUV 11 nm Si_Cap ML wafer+di water 86.0% 84.0% 82.0% 2 3 4 5 6 7 8 Time
Nano-Particle Removal from EUV Substrates with Megasonics and SC1 100% Removal efficiency (SC1) 99% 98% Bare Silicon wafer+sc1 EUV 4 nm Si_Cap ML wafer+sc1 EUV 11 nm Si_Cap ML wafer+sc1 97% 2 3 4 5 6 7 8 Time
Physical Cleaning Of Submicron Trenches Mixing and Cleaning in Steady and Pulsating Flow Steady flow induces a vortex inside the cavity. There is no convection between the vortex and the main flow. The transport of contaminant happens by diffusion only, which may take a long time depending on the trench size. Distance From Wafer Surface(um) 7500 7000 6500 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 0 500 1000 1500 2000 Steady Rinse Flow: u s = 15 cm/s Geometry: D/W = 5 :1 W=1mm D=5mm C-ion #/cm 3 1.5E+12 1.4E+12 1.2E+12 1E+12 8E+11 6E+11 4E+11 2E+11 1E+11 1E+10 1E+09 1E+08 1E+07 1E+06 Distance From Wafer Surface(cm) Streamlines and Concentration Contour 0.14 Steady Flow u=4.3 cm/s 0.12 0.1 0.08 0.06 0.04 0.02 0 0 0.05 0.1 0.15 0.2 Distance Along Wafer Surface (um) time=1.0s Distance Along Wafer Surface (cm) time = 3.9s External oscillating flow stimulates the vortex destruction and regeneration. Contaminants are dragged out of cavity by the expanded vortex. The vortex oscillating mechanism significantly enhances the mixing. Distance From Wafer Surface(um) 7500 Oscillating Rinse Flow: 7000 u s = 0 cm/s u p =47cm/s u avg =15cm/s 6500 f = 2000 Hz 6000 Geometry: D/W = 5 :1 5500 W=1mm D=5mm 5000 C-ion #/cm 3 4500 1.5E+12 1.4E+12 1.2E+12 4000 1E+12 8E+11 3500 6E+11 4E+11 3000 2E+11 1E+11 1E+10 2500 1E+09 1E+08 2000 1E+07 1E+06 1500 1000 500 0 0 500 1000 1500 2000 Distance Along Wafer Surface (um) time=0.5s Distance From Wafer Surface(cm) 0.1 0.05 0 0 0.05 0.1 0.15 0.2 Distance Along Wafer Surface(cm) t/t= 1.50, time=.0579s OSCILLATING FLOW f=25.9hz u s =0 u p = 13.5 cm/s u Avg = 4.3 cm/s W=1mm, D=0.7mm 1E+12 1E+11 1E+10 1E+09 1E+08 1E+07
Particle Removal in Trenches 2000 1750 1500 1250 1000 750 500 4m/s Without Deformation, 2m/s (I =25W/cm2, f =850kHz,Up=14.89m/s) Without Deformation, 0.8m/s (I =7.5W/cm2, f =800kHz, Up=4m/s) With Deformation, 2m/s (I =25W/cm2, f =850kHz, Up=14.89m/s) With Deformation, 0.8m/s (I =7.5W/cm2, f =800kHz, Up=4m/s) 1000 250 2000 1750 0 0 500 1000 1500 2000 t/t=1.8 RM 100 10 1 RM = 1 1500 1250 1000 750 500 4m/s 0.1 0.01 10 100 1000 PSL Particle Size (nm) 250 0 0 500 1000 1500 2000 t/t=2.0
Submicron Particle Removal from Deep Trenches Cleaning for 1 minutes 3 minutes 5 minutes 8 minutes At the Surface (0.3 µm) (0.8 µm) 100 Pm below surface (0.3µm) (0.8 µm) 80 % removal 200 Pm below surface (0.3 µm) (0.8 µm) 70 % removal 90 % removal 300 Pm below surface (0.3 µm) (0.8 µm) 50 % removal 70 % removal 70 % removal Bottom of the trench (0.3 µm) (0.8 µm) 30 % removal 50 % removal 50 % removal 80 % removal 90 % removal
Introduction To Laser Shock Cleaning Laser Shock Cleaning (LSC) is a room temperature physical cleaning process that has been shown to be effective in the removal of particles down to 200 nm from silicon wafers. It has been shown to work in post CMP applications and on patterned wafers without damage. Laser shock cleaning is a very promising technique for reticle cleaning because of the similarity of the substrates (Silicon, quartz, etc.) and physical nature of the LSC technique. Experiments are underway to use LSC to remove nanoparticles from EUV reticles.
Conventional Laser Wafer Cleaning Mostly UV Excimer laser (KrF) used Effective for the removal of organic contaminants by photochemical interactions Related Patents: > Radiance => licensed by AMAT > Oramir => bought by AMAT O 3, NF 3
Disadvantages of Conventional LC Slow speed of cleaning Small laser spot size => Slow process and significant High probability of substrate damage Focusing the laser beam on the surface => possible damage Poor cleaning performance for inorganic particles Inorganic submicron particles are not removed in a dry process >> More effective cleaning techniques are required not only to enhance the cleaning speed and cleaning power but also to clean the surface safely without damage.
Oramir Laser Cleaning PR residue removal after poly-si RIE process (at Motorola) > Effective for organic Problem: W particle removal before cleaning after cleaning at 0.28 J/cm 2 Si substrate damage threshold = 0.3 J/cm 2 Æ Not feasible for inorganic Æ Inorganic particle shows critical killer effect: more than 85% of device failure
Laser Shock Cleaning Technique (LSC) A technique to remove the contaminants on the surfaces using laser-induced plasma shock waves (multi-photon ionization) 6KRFN:DYH)URQW 6KRFN:DYH)URQW *DS 3XOVHG/DVHU 3XOVHG/DVHU %HDP %HDP 3DUWLFOHV 3DUWLFOHV :RUNLQJ7DEOH :DIHU Cleaning condition: Shock wave force > Adhesion force
Visualization of Shock Wave Laser shadow-graphic photography XeCl excimer laser used for back-light source Parametric investigation of the shock wave: laser power, pulse width, wavelength, type of gas, gas pressure, particle behavior and shock propagation through the hole (patterned wafer).
Laser Shock-wave Cleaning Moment Ratio at a shock wave velocity of 10,000 m/s 1.E+07 Moment Ratio MR vs. Particle Diameter for SiO2 on SiO2 when shock wave velocity =10000m/s 1.E+05 Moment Ratio MR vs. Diameter for PSL on SiO2 when shock wave velocity =10000m/s 1.E+06 1.E+05 X=1mm X=6mm X=100mm 1.E+04 1.E+03 X=1mm X=6mm X=100mm 1.E+04 1.E+02 MR 1.E+03 MR 1.E+01 1.E+02 1.E+00 1.E-08 1.E-07 1.E-06 1.E-05 1.E+01 1.E-01 1.E+00 1.E-08 1.E-07 1.E-06 1.E-05 1.E-01 Particle Diameter (m) 1.E-02 1.E-03 Particle Diameter (m) Silica particles on SiO 2 PSL particles on SiO 2
LSC Removal of W Particles from Wafers Just 2 laser pulses irradiated Very effective for inorganic particles Huge cleaned area Æ high cleaning speed Æ high throughput
Effect of Gap Distance on Cleaning ¾ Significant increase of the residual particles with increasing the gap distance ¾ Which is due to the attenuation of the shock wave with distance i.e I ~ I 0 exp(-ax) ¾ Gap distance => strong cleaning factor No. of Residual Particulates 200 160 120 80 40 0 Wafer #1 Wafer #2 Wafer #3 8 mm 7 mm 6 mm 5 mm Gap Distance
Silica Particles on Silicon Wafers Hybrid: UV irradiation + Laser shock wave application 6LOLFD5HPRYDO(IILFLHQF\ XP PLF URQ PLF URQ XPDW+\ EULG
Alumina Particles on Silicon Wafers Hybrid: UV irradiation + Laser shock wave application $O25HPRYDO(IILFLHQF\ PLF URQ PLF URQ XP XP XP XPDW +\ EULG
Pattern Damage Issue Target: Gate patterned wafers (DR = 0.12 Pm feature, Aspect Ratio = 3) PP No pattern damage was found which is superior to some megasonic based wet cleaning processes.
Nanoparticle Removal Using LSC Wafers Cleaning Recipe Removal Efficiency (%) 11nm Si_CAP UV light(3 cycle) + LSC(1 cycle) 96.6 4nm Si_CAP UV light(3 cycle) + LSC(1 cycle) 97.4
Particle Removal in Supercritical CO2 Pros: Supercritical CO2 has many advantages. First, the CO2 acts as a solvent for the removal of organic contaminants. Second, It could also be used use with co-solvents (that could be easily separated from CO2 after cleaning). Third, it also has a very low surface tension that allows the cleaning of nano and microscale trenches and vias. Fourth, it eliminates the need for a drying process since no liquids are used. Cons: Physical removal of contaminants such as particles, although likely, has yet to be demonstrated. The high cost of capital for the equipment because of high pressure (3000-6000 psi) and the high cost of high purity CO2 unless recycling is used.
Megasonic Cleaning Pros and Cons Pros: Megasonic cleaning has been introduced in the mid seventies by Werner Kern and since then it has been used in every fab. It has and still been used exclusively for FEOL cleaning. It has been recently been used for the BEOL cleaning applications such post-cmp cleaning. It has been shown to effectively remove particles down to 100 nm. It could be used with different cleaning chemistry, surfactants or just DI water. The technique has also been shown to clean trenches and vias. Cons: The only reported disadvantage is possible damage on patterned wafers (such as polysilicon lines) as mentioned in section 7.4. No scientific experimental study has been performed on the damage to identify the mechanism and show possible cleaning without damage.
Brush Cleaning Pros and Cons Pros: Brush cleaning has been for decades. It has and still been used recently been exclusively used for BEOL cleaning. If used properly (right pressure, rotational speed and water flow rate) the technique is very effective in the removal of particles down to 100 nm and it could be used with DI water or a cleaning chemistry. The best cleaning conditions is high pressure, high speed, and short cleaning time (15-30 seconds). Cons: This technique could cause scratching (surface damage) or particle re-deposition from a loaded brush if the water flow is low or the pressure is high (for a long cleaning time).
Laser Shock Cleaning Pros and Cons Pros: Laser shock cleaning is a new techniques that for the first time allow the removal of submicron inorganic particles (down to 200 nm) using a completely dry process. The technique has been show to effectively remove silica and alumina particles without substrate damage. The process throughput is also high at 1 minute per 8 wafer due to the large cleaning are per pulse (a few square centimeters). Cons: This technique is not very effective in the removal of organic particles. However, it is has been shown to be effective when combined with a UV laser (hybrid laser). Damage is possible if the laser is too close to the wafer. However, no damage has been observed on patterned wafers down (down to a separation gap of 5 mm between the laser focus point and the laser).
Conclusions ¾ The removal of nanoscale particles (63 nm) using megasonic and laser shock cleaning is investigated experimentally in this study. ¾ The following substrates were used in the megasonic cleaning experiments; 4 nm Si_cap ML wafers, 11 nm Si_cap ML wafers. The particles used were 63 nm PSL red fluorescent particles that were deposited using a nebulizer. ¾ The results show that that complete removal can be obtained in a short time for the bare Si wafer and 4 nm Si_cap ML wafers. ¾ However, when megasonics is used with dilute SC1 complete removal of 63 nm particles is achieved under most cleaning conditions.
Conclusions ¾ Laser shock cleaning is an effective cleaning technique with unique characteristics compared with the conventional laser cleaning. ¾ The Laser induced shock wave has a velocity of 2-10 km/s that provides a very large hydrodynamic removal force. ¾ The removal of nano-size silica particles (10 nm for 10 km/s velocity and 100 nm for 2km/s) can be accomplished. ¾ Silica, metallic and alumina particles were successfully removed from silicon and oxide wafers ¾ The Hybrid design of LSC was very effective to remove organic contamination ¾ The laser cleaning results for the EUV 11 nm and 4 nm Si_cap ML wafers show that the measured removal efficiency obtained for both substrates were in the high nineties.