Effects of Size, Humidity, and Aging on Particle Removal

Transcription

1 LEVITRONIX Ultrapure Fluid Handling and Wafer Cleaning Conference 2009 February 10, 2009 Effects of Size, Humidity, and Aging on Particle Removal Jin-Goo Park Feb. 10, 2009 Department t of Materials Engineering, i Hanyang University, Korea +82 (0) jgpark@hanyang.ac.kr

2 Theoretical Background of Particle Adhesion For small uncharged particles (diameter, d<50 µm) on uncharged substrates, van der Waals interaction will be the predominant adhesion force considered. Van der Waals Force between Particles and Surfaces Simple expression for the interaction between particle (1) of radius R and surface (2) immersed in a medium (3) as a function of distance of separation, D. 2 D 3 R 1 F VDW = A DD R (N) Attractive in nature Hamaker Constant, A (Combining Law: Frequently used for obtaining approximate values for unknown Hamaker constant in terms of known ones.) A A A in vacuum or air A ( A A )( A A ) in third medium where A 11, A 22, and A 33 are Hamaker constant of materials 1, 2, 3 respectively. Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas

3 Adhesion Induced Deformation When particle deformed, van der Waals force would increase as increased contact area AR Fadhesion = Fvdw + Fdeformation = D 2 a RD A: The Hamaker constant R: The particle radius D: The distance between particle and substrate (Usually it is assumed as 4 Ǻ) a : The contact radius (obtained from elastic, DMT, JKR, or plastic model, MP) For silica, PSL particle on Si surface, The MP model is applicable. a 2WR = A WA = 2 γγ 1 2 H W A is work of adhesion between particle and surface. γ 1 and γ 2 are surface free energy of two contact materials. (γ =70 2 = SiO2 mj/m, γ PSL mj/m ) H is the deformation part s hardness. (H SiO2 =750 Mpa, H PSL =32.4 MPa) Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas

4 Capillary Force Since samples exposes in Air, condensed liquid layer have been trapped between particle and substrate. γ L Fcapillary = (2 πrd)( ) = 4πRγ L cosθ r μm particle (10-8 N) Capillary force ( γ Water : mn/m γ IPA : mn/m Surface tension (dynes/cm) The capillary force as a function of the surface tension of liquid. Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas

5 Experimental Setup Laser System for Characterization of Laser Shock Wave and Its Cleaning Performance Laser Source Q-switched Nd:YAG laser (1800E, IMT Inc., FWHM=10 ns) Wavelength: 1064 nm Maximum Energy: 2.0 J Repetition Frequency: up to 10 Hz Focal Lens: 150 mm focal length with an anti-reflection coating Motion of Wafer Scan: Spiral scan (r-θ-z axis) Probe Laser for Characterization ti CW He-Ne Laser (1.5mW, Thorlabs) Wavelength: nm Samples Substrate: Precleaned Si wafer Particles: Silica (1.0, 0.5, and 0.3 μm, Duke Scientific) c) Laser Shock Cleaning System, 1800E, IMT Inc Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas 5

6 Experimental Procedure Sample Preparation Measurement Analysis Wafer precleaned Particle contamination 1.Particles were suspended in IPA. 2.Wafers were dip into solution for 5 min. 3.Dried out by N 2. Store samples ; Samples were stored in sealed boxes at different humidity and times. Characterize laser induced plasma (LIP) shock wave. LIP shock wave was performed at a laser energy of 1.8 J, a gap distance of 3 mm, and a radiation frequency of 10 Hz. Particles were counted by optical microscope with counting software. Particle removal efficiency was calculated by below equation. nd -ni -nc PRE (%) = 100 n - n where n i : the initial particle count before particle deposition, n d : the particle count after particle deposition, n c : the particle count after performing cleaning. d i Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas 6

7 Simple Model for Laser-Induced Shock Wave The pressures of shock waves were calculated from the propagation speed of shock wave based on the Blast-wave theory. Effective in Predicting Pressure Temperature Energy-conversion efficiency Shock Wave Pressure P = P + (1 ρ / ρ) ρ ( U a) Density Increase in the Compressed Air Layer 2 ρ/ ρ0 = ( γ + 1)/( γ 1+ 2 M s ) where P 0 : ambient pressure. γ: adiabatic coefficient ( γ=1.4 for air) ρ 0 : density of air (ρ 0 =1.169 g/l for air) U: shock wave propagation speed a: sound speed M s : Mach number Reference: A. M. Azzeer, et al, Applied Physics B 63, 307 (1996) Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas 7

8 Properties of Shock Wave 3500.) Intensity (a.u mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm (m/s) Sh hock Wave Velocity ( J 1.5 J 1.8 J Shock Wave Press sure (MPa) Shock Wave Propagation Time (ns) 1.2 J 1.5 J 1.8 J Gap Distance (mm) Gap Distance (mm) Propagation speed and pressure were measured and calculated based on the deflection signals due to propagation of shock wave front. Propagation speed of shock wave decreased rapidly as a function of gap distance. There are not much differences in shock pressures at the long gap distance. Gap distance is more effective parameter than laser energy. Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas 8

9 Particle Trajectory due to Shock Wave From Side View Point Center This work was contributed by D. Kim from POSTECH. From Front View Point Center Dominant Direction of Particle Movement: Parallel to laser incident direction Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas 9

10 Particle Removal Apart from Center Incident Laser Laser Induced Plasma Cleaned Area [Top View] Sweep mark due to propagation of shock wave showed that the drag force was dominant particle removal mechanism apart from center of cleaned area. Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas 10

11 Particle Removal Mechanism Shadowgraphy of Shockwave F Lift F Drag 1. Shock wave was generated by laser induced plasma and propagated and contacted sample surface. 2. Lifting force was dominant at the center of plasma due to reflected shock wave 3. Drag and lifting force was dominant apart from the center by propagation of shock wave and reflected wave. Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas 11

12 Variation of The Cleaning Moment and Resisting Moment Assumed that the drag force was dominant particle removal force Cleaning Moment, M R M A with F capillay and F vdw Resisting Moment, M A M A with F vdw M = F sin θ h R s y M A = Fscos θ ( hx + a) + Fa a v v F = ( P P) n da s A Ap 2 1 P: The pressure of LIP shock wave Capillary force would reduced the particle removal moment and followed by low particle removal efficiency. Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas 12

13 Silica particle suspended in IPA on Si substrate / 1.8 J, 3.0 mm, 10 Hz ency (%) Particle Removal Effici um R.H: 100% 1.0 μm Aging Time (days) PRE as a function of Aging Time, Relative Humidity, and Particle Size le Removal Effic ciency (%) Particl μm Humidity 0 % 50 % 100 % Humidity 0 % 50 % % After 5 days Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas le Removal Effic ciency (%) Partic Aging Time (days) 0.3 μm Humidity 0 % 50 % 100 % Aging Time (days) 16

14 Conclusions Particle removal mechanism of laser induced plasma (LIP) shock wave: Lift force was dominant at the center of plasma by reflected shock wave Drag and lifting force was dominant apart from the center by propagation of shock wave and reflected wave. Present capillary force between particle and substrate was key factor to reduce the particle removal moment. A bigger than 1 μm size of particle was not sensitive to humidity and aging time. As particle size decreased, Particle removal decreased rapidly even though particles were aged for short period. High condensed liquid layer might caused the generation of chemical bonding between particle and substrate. As a consequence, humidity control is much more important for high PRE as cleaning target particle size is being sub-micron. Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas 14

15 Acknowledgements This work has been supported by The Medium-term Strategic Technology Development Program The EUV R&D Program of Ministry of Commerce, Industry and Energy (MOCIE) The fostering project of the Lab of Excellency Post BK21 program. Sematech Surface Preparation and Cleaning Conference, Mar. 31-Apr. 2, 2008, Austin, Texas 15