NSF Center for Micro and Nanoscale Contamination Control The Removal of Nanoparticles from Nanotrenches Using Megasonics Pegah Karimi 1, Tae Hoon Kim 1, Ahmed A. Busnaina 1 and Jin Goo Park 2 1 NSF Center for Micro and Nanoscale Contamination Control and NSF Nanoscale Science and Engineering Center for High-rate Nanomanufacturing Northeastern University, Boston, USA 2 Department of Materials and Chemical Engineering, Hanyang University, Ansan, Korea NSF Center for Micro and Nanoscale Contamination Control
Outline Overview of the NSF Center for Micro and Nanoscale Contamination Control Megasonic Cleaning Mechanisms Particle removal Mechanism Trench cleaning (prior results) Florescent Particles Localized and Global (wafer scale) measurements Experimental Procedure Fabrication Process of Si trench Particle Deposition Experimental Results Conclusions NSF Center for Micro and Nanoscale Contamination Control
Research Focus Fundamentals of surface cleaning and preparation Understanding of physical and chemical cleaning mechanisms such as megasonics, brush, laser shock, etc. Damage mechanisms and mitigation. Post-CMP cleaning applications. Cleaning of EUV reticles Particle adhesion Removal of Nanoparticles Cleaning of trenches and vias Laser Shock Cleaning Particle generation, transport and deposition Particulate Contamination in low pressure processes (LPCVD, Sputtering, ion implant, etc.) Contamination during wafer handling. NSF Center for Micro and Nanoscale Contamination Control
George Kostas Nanoscale Manufacturing Center The Kostas Center has 10,000 ft2 facility including a 7,000 ft2 cleanroom (class 10 and 100), with complete 6 wafer fabrication facility (for CMOS, MEMs, NEMs, etc.) including bulk micromachining, metal surface micromachining, CMP, and E-beam, optical and nanoimprint lithography. It also includes Field Emission Electron Microscope (FESEM), Raman spectroscopy, laser surface scanner, airborne and liquid counter, CNC particle counters (10 nm resolution), Zeta potential measurement down to 1 nm particles, several cleaning tools, several Atomic Force Microscopes, several batch and single wafer cleaning tanks (up to 12 inch).
Introduction The removal of nanoparticles from patterned wafers is one of the main challenges in the semiconductor industry. Megasonics utilizes acoustic streaming to reduce the acoustic boundary layer and the generated pulsating flow to remove nanoscale particles from trenches and other structures on the wafer. In this paper, we investigate the removal of 100nm polystyrene latex (PSL) particles from silicon trenches. The trenches used are 800 nm and 2 microns wide with an aspect ratio of one. NSF Center for Micro and Nanoscale Contamination Control
Removal Mechanism of Nanoparticles Removal Percentage Moment Ratio U M R Re m oval Pe rce ntage 100 90 80 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 Moment Ratio Removal Percentage vs. Moment Ratio (Silica Removal Experiment) The figure shows when RM >1, 80 % of particles are removed. 1.399R a RM = RM = δ Removal moment Adhesion resisting moment Fd ( 1. 399R δ ) + Fdl a F a a F elec. double layer O M A F Adhesion F drag Rolling removal mechanism NSF Center for Micro and Nanoscale Contamination Control
Megasonic Cleaning: Acoustic Streaming 2500 2000 Streaming Velocity vs. Acoustic Power 1 M H z 850k Hz 760k Hz 360k Hz v(cm/s) 1500 1000 500 0 0 5 10 15 20 25 30 Intensity (W /cm 2 ) NSF Center for Micro and Nanoscale Contamination Control
Acoustic Streaming; Boundary Layer Thickness u u Free stream δ(x) y x δ Velocity boundary layer Velocity boundary layer on a flat plate τ τ Acoustic boundary layer thickness in water for 850KHz is δ ac =0.61μm The hydrodynamic boundary layer thickness in water, u=4m/s, at center of the wafer, the boundary layer thickness is δ H =2570 μm Boundary layer thickness (micron) 6 4 2 Acoustic Flow Properties Acoustic, f=360khz Acoustic, f=760khz Acoustic, f=850khz Boundary layer thickness (micron) Streaming Velocity (m/s) 0 10 1 10 2 10 3 10 4 Frequency(kHz) I = 7.75 W/cm 2 u>0.3c 10 3 10 2 10 1 10 0 10-1 10-2 10-3 Streaming Velocity (m/s) NSF Center for Micro and Nanoscale Contamination Control
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. External oscillating flow stimulates the vortex destruction and regeneration. Contaminants are removed from cavity by the expanded vortex. The vortex oscillating mechanism significantly enhances mixing. Distance From Wafer Surface(um) Distance From Wafer Surface(um) 7500 7000 6500 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 Steady Rinse Flow: u s = 15 cm/s 0 0 500 1000 1500 2000 Distance Along Wafer Surface (um) time=1.0s 7500 7000 6500 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 0 500 1000 1500 2000 Distance Along Wafer Surface (um) time=0.5s 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 Oscillating Rinse Flow: u s = 0 cm/s u p =47cm/s u avg =15cm/s f = 2000 Hz 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) Distance From Wafer Surface(cm) 0.1 0.05 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 Streamlines and Concentration Contour 0 0.05 0.1 0.15 0.2 Distance Along Wafer Surface (cm) time = 3.9s 0 0 0.05 0.1 0.15 0.2 Distance Along Wafer Surface(cm) t/t= 1.50, time=.0579s Steady Flow u=4.3 cm/s OSCILLATING FLOW f=25.9hz u s =0 u p =13.5cm/s u Avg = 4.3 cm/s W=1mm, D=0.7mm 1E+12 1E+11 1E+10 1E+09 1E+08 1E+07 NSF Center for Micro and Nanoscale Contamination Control
Fluorescent Microscope Visualization Visualization of nanoscale particles Xenon Arc lamp G block fluorescent filter specs. NSF Center for Microcontamination Control (NEU, UA)
Visualization of Fluorescent Particles Nikon's OPTIPHOT200 optical microscope with auto stage Nikon G block filter 75 W Xenon arc lamp Extra N.D filter Fluorescent Cube NSF Center for Microcontamination Control (NEU, UA)
Images of Fluorescent Particles The nano-particle detection has been verified using scanning electron microscopy (SEM). Has proven to be effective for single particle detection down to 50 nm particles. Agglomerated particles can be eliminated from the counting procedure by filtering the count by diameter and aspect ratio values. 45K J. Microscopy Research and Technique, 4 May 2007. NSF Center for Micro and Nanoscale Contamination Control
Particle Removal Experiments Single wafer megasonic cleaning using 300nm PSL particles in DI water at 25 oc 100% 80% Efficiency 60% 40% At the surface 100 micron below 200 micron below 20% 0% 11min 3 min 2 5 min 3 8 min 4 Bottom of Trench Time (minutes) Moment Ratio 300, 800 nm PSL particles Single Wafer Megasonic Tank (760 khz). Trenches of 112 micron wide and 508 micron deep are used in the experiments. Moment Ratio 12 10 8 6 4 2 MR=1 0 3.5 10.1 3.43 3.29 3.36 1.2 1.15 1.18 At Surface 1 100mm 2 200mm 3 Bottom 4of Trench below below 300nm 800nm NSF Center for Micro and Nanoscale Contamination Control
Why does it take time to remove the particles? Busnaina, A., Bakhtari, K., Guldiken, G. and Park, J, Experimental and Analytical Study of Submicron Particle Removal from Deep Trenches, J. of Eletrochem. Soc., 153 (9) C603-C607, 2006 NSF Center for Micro and Nanoscale Contamination Control
Experimental & Computational Fluid Dynamics Simulation
Global Metrology of the Removal Efficiency of Submicron Particles from Structured Substrates There is a need for fast, quick metrology for evaluating cleaning process for many applications. There is a need for global (wafer-scale) metrology for measuring the removal efficiency of nanoparticles. Fluorometer offers a very fast and reliable evaluation of global removal efficiencies. The technique works for flat or structured substrates (trenches, etc.) NSF Center for Microcontamination Control
INTRODUCTION Fluorometer Picofluor handheld Fluorometer from Turner designs Dual channel fluorometer, Channel A for Green fluorescent while Channel B for Red fluorescent particles. Three solvent alternatives (Toluene, Methyl Ethyl Keton and Methylene Chloride) are considered. The techniques was evaluated for cleaning with DI water and dilute SC-1 Chemistry. Fluorescent Particles Green Fluorescent Excitation maxima (nm) 468 508 Emission maxima (nm) Fluorometer Excitation maxima (nm) Emission maxima (nm) Channel A 475 ± 15 515 ± 20 Red fluorescent 542 612 Channel B 525 ± 20 >570 NSF Center for Microcontamination Control
Solvent Selection Fluorometer 100 95 90 85 80 75 70 65 Toleune Methylene Chloride Metyl Ethyl Ketone (M.E.K) Acetone 60 55 50 Percent Yield Solvent Selection NSF Center for Microcontamination Control
Calibration Curves (Methylene Chloride) Fluorometer 0.3 micron Calibration Curve 80 70 60 Fluorometer Reading 50 40 y = 0.1105x + 0.6835 0.3 micron PSL fluorescent particles Real Particle Count Linear (0.3 micron PSL fluorescent particles) Linear (Real Particle Count) 30 20 100 200 300 400 500 600 700 Particle Number (0.3 micron particles) NSF Center for Microcontamination Control
Calibration Curves (Methylene Chloride) Fluorometer 0.5 micron Calibration Curve 350 300 Fluorescence Reading 250 200 150 y = 0.5283x - 0.61 0.5 micron PSL fluorescent particles Real Particle count Linear (0.5 micron PSL fluorescent particles) Linear (Real Particle count ) 100 50 100 200 300 400 500 600 Particle Number (0.5 micron particles) NSF Center for Microcontamination Control
Calibration Curves (Methylene Chloride) Fluorometer 0.8 micron Calibration Curve 1200 1000 Fluorescence Reading 800 600 400 y = 2.1478x + 4.6501 0.8 micron PSL fluorescent particles Real Particle Count Linear (0.8 micron PSL fluorescent particles) Linear (Real Particle Count) 200 0 0 100 200 300 400 500 600 Particle Number (0.8 micron) NSF Center for Microcontamination Control
Calibration Curves (Methylene Chloride) Fluorometer 1200 Fluorometer reading vs. volume of particle 1000 fluorescent reading 800 600 400 y = 8.0204x + 4.3084 0.5 micron particles 0.3 micron particles 0.8 micron particles Linear (0.8 micron particles) Linear (0.5 micron particles) Linear (0.3 micron particles) 200 0 y = 8.0698x - 0.5674 y = 7.8204x + 0.6632 0 20 40 60 80 100 120 140 160 Particle volume(micrometer3) NSF Center for Microcontamination Control
Removal from 500 micron deep trenches Fluorometer Efficiency (%) 100 98 96 94 92 90 88 86 84 82 80 1 minute 3 minute 5 minute 0.3 micron, DI water 25 C 0.3 micron, DI water 50 C 0.3 micron, SC-1 25 C 0.3 micron, SC-1 38 C 0.8 micron, DI water 25 C Single wafer tank cleaning, Fluorometer results (percentage) NSF Center for Microcontamination Control
Trench Fabriation Fabrication Process of Si trench
SEM Images of Trenches NSF Center for Microcontamination Control (NEU, UA)
Particle Deposition in Trenches Particle Deposition Dip Coater & Liquid Flow in trench driven by dip coating Front View Side View Deposition of particles using dip coater 100nm PSL particle DI water Si NSF Center for Microcontamination Control (NEU, UA)
Results Visualization of nanoscale particles Verification of nanoparticle counting technique NSF Center for Microcontamination Control (NEU, UA)
Results Megasonic cleaning of 100nm PSL particles in DI water at 25 C Before cleaning After cleaning Fluorescent Microscope images before and after cleaning of 100nm PSL particles in the trench with 800nm width The average number of particles deposited in trench: 300~400 NSF Center for Microcontamination Control (NEU, UA)
Results Megasonic cleaning of 100nm PSL particles in DI water at 25 C Removal of 100nm PSL particles after 4 min as a function of input megasonic power NSF Center for Microcontamination Control (NEU, UA)
Results Megasonic cleaning of 100nm PSL particles in DI water at 25 C Removal of 100nm PSL particles using 100% power input as a function of cleaning time NSF Center for Microcontamination Control (NEU, UA)
Effect of Time on the Removal of 100nm and 200nm Particles in 2 micron trenches at 100% power in DIW Removal Efficency (%) 100 80 60 40 20 0 100 nm PSL Particles 200 nm PSL Particles 1 min 4 min 8 min Cleaning Time NSF Center for Microcontamination Control (NEU, UA)
Effect of Power on the Removal of 200nm Particles in 2 micron Trenches in DIW Removal Efficiemcy (%) 100 90 80 70 60 50 40 30 20 10 0 30% 60% 100% 1 min Cleaning 2 min Cleaning % of Maximum Output Power NSF Center for Microcontamination Control (NEU, UA)
Conclusions The detection of fluorescence from dissolved PSL particles is demonstrated for obtaining global removal efficiency for particles. The obtained calibration curves enable the fluorometer user to get a very quick and reliable global removal efficiency. Particle removal efficiency increase in both 2µm and 800nm wide trenches as the input power is increased. Complete removal of 100 nm PSL particles is achieved from a 2µm wide trench using megasonics in DI water only after 4min. Cleaning of 200nm PSL particles requires less time than 100nm PSL particle in the same size trenches. It is more difficult to remove particles from trenches smaller that 2 microns using megasonics and DI water only. In order to effectively remove 100nm PSL particles from a 800nm trench, megasonic cleaning will need to be applied for more than 8 min. NSF Center for Micro and Nanoscale Contamination Control
NSF Center for Micro and Nanoscale Contamination Control
Results Visualization of nanoscale particles Verification of nanoparticle counting technique Fluorescent Microscope image FESEM image NSF Center for Microcontamination Control (NEU, UA)