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Experimental and theoretical analysis of the laser shock cleaning process for nanoscale particle removal
Applied Surface Science 253 (2007) 8322–8327 www.elsevier.com/locate/apsusc
Experimental and theoretical analysis of the laser shock cleaning process for nanoscale particle removal Dongsik Kim a,*, Bukuk Oh a, Deoksuk Jang a, Jeong-Wook Lee a, Jong-Myoung Lee b a
Department of Mechanical Engineering, POSTECH, Pohang 790-784, Republic of Korea b Laser Engineering Group, IMT Co. Ltd., Uiwang 449-860, Republic of Korea Available online 25 February 2007
Abstract The laser shock cleaning (LSC) process has been shown to be effective for removing submicron-sized contaminant particles from solid surfaces and thus bears strong potential in various applications. In this work, experimental and theoretical analysis are conducted to reveal the underlying physical mechanisms of the LSC process, with emphasis on the laser-induced hydrodynamics and the effect of external gas-jet injection through a nozzle. A two-dimensional theoretical model is proposed for rigorous simulation of the hydrodynamic phenomena occurring in the LSC process. The hydrodynamics computed by the model is in qualitative agreement with experimental observations and reveal the details of the physics involved in the cleaning process. The effect of gas blowing on the cleaning performance is analyzed both experimentally and theoretically. The results indicate that the gas flow can significantly change the hydrodynamics and increase the cleaning efficiency by reducing the chance of particle redeposition. # 2007 Elsevier B.V. All rights reserved. Keywords: Laser cleaning; Laser-induced breakdown; Particle removal; Shock wave
1. Introduction Control of particle contamination has long been a critical issue in the microelectronics industry and significant efforts have been made to develop effective laser-based cleaning techniques [1–6]. Recent studies on the laser shock cleaning (LSC) have shown that the LSC process is effective for removing small particles from solid surfaces as a non-contact, dry method [1,3–5]. In the process, laser-induced breakdown (LIB) occurs in the ambient gas over the surface to be cleaned, which results in strong shock wave formation. Accordingly, the LSC process is a dry process and the surface is not directly exposed to the laser beam. Since the dynamics of the laserinduced shock wave is central to the cleaning process, a series of investigations were performed to analyze the hydrodynamics associated with the LSC process [1,3]. The results revealed that the cleaning efficiency is proportional to the intensity of the shock wave [1]. However, as the previous studies employed the simple blast wave theory [6], several important aspects of the
* Corresponding author. Tel.: +82 54 279 2179; fax: +82 54 279 3199. E-mail address: [email protected] (D. Kim). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.119
process, especially the asymmetry of the shock wave shape and thus the inhomogeneous spatial distribution of the cleaning efficiency, could not be explained theoretically. In this regard, this work proposes a more refined two-dimensional theoretical model for the hydrodynamics involved in the LSC process, i.e., LIB, shock formation, propagation, and interaction with a solid surface and external flow. Though the LIB is relatively well understood [7–9], the hydrodynamics of the LIB flow and its interaction with a solid surface have hardly been investigated. Furthermore, this work examines the effect of gas-jet injection through a nozzle on the cleaning performance. It is evident that the gas-jet injection changes the flow field and eventually the cleaning efficiency since the hydrodyanmics and redepostion of the detached particles are strongly influenced by the flow. Both numerical and experimental analysis are conducted to uncover the effect of gas blowing. Fig. 1 depicts the problem this work is concerned with A Qswitched laser (wavelength l = 1064 nm, full width at half maximum FWHM =6 ns) induces optical breakdown of N2. The shock wave generated from the plasma core impinges onto the sample, removing the particles from the surface. Two different angles a = 08 and 308 are tested when the blowing effect is analyzed. It is noted that a spherical shock wave is
D. Kim et al. / Applied Surface Science 253 (2007) 8322–8327
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modeled by the plasma absorption coefficient ap [12]. Expansion of the high-temperature plasma is followed by shock wave generation. The subsequent hydrodynamic motion is modeled by two-dimensional transport equations, representing conservation of mass, momentum, and energy. The transport equations are simultaneously solved for atoms, ions, and electrons. In summary, the scalar transport equations are: @ ðrfi Þ þ r ðr~ Vfi Þ ¼ si @t @ ðrfe Þ þ r ðr~ Vfe Þ ¼ se @t Fig. 1. Schematic diagram for the LSC process.
formed in the actual LSC process and therefore a threedimensional model is required for quantitative analysis of the phenomena. Nevertheless, since the three-dimensional effect is secondary, changing only the number, this work employs a twodimensional model for computational efficiency to analyze the hydrodynamics of the LSC process.
@ne nn A K ¼ nne þ I ðtÞ; @t K 3=2
A¼
sK v2K1 hK ðK 1Þ!
for electrons
(2a) (2b)
where r is the density, fi the ion mass fraction, fe the electron mass fraction, ~ V the velocity vector, and si, se is the ionization source terms for ion and electron, respectively. And the mass, momentum, energy conservations equations and the equation of state are: @r þ r ðr~ VÞ ¼ 0 @t
(3a)
r
@~ V þ rð~ V rÞ~ V ¼ r p þ r ðti j Þ @t
(3b)
r
* @e þ rð~ V rÞe ¼ pðr ~ VÞ þ ap ðr ; tÞIðtÞ eap d @t
(3c)
2. Theoretical model The LIB can be modeled via the process of multiphoton ionization (MPI) and impact (avalanche) ionization by electron/ atom collisions because the laser irradiance I O (1013 W/ cm2) is high enough to activate the ionization mechanisms. Eq. (1) shows the ionization model utilized in the present work. The model considers both the avalanche ionization and MPI mechanisms [7] (ne, nn: electron and atom concentration, n: ionization collision frequency, K: multiphoton order, I(t): laser intensity, v: laser frequency, t: time, h: Planck’s constant):
for ions
p ¼ ðg 1Þre
(3d)
where e = cvT is the internal energy, cv the specific heat at * constant volume, p the pressure, r the position vector, tij the Cauchy stress tensor, d the absorption length, and g is the
(1)
In the above equation, the precise value of the photon crosssection s is generally unknown although it is typically of the order of 1016 cm2 [10,11]. Therefore, the photon cross-section s has been assumed to be s = 2.4 1016 cm2. This value minimizes the difference between the calculated and measured [3] shock wave velocities at a propagation distance r = 3 mm. Variation of the photon cross-section leads to changes in the computed hydrodynamics. For example, the maximum pressure changes from 0.8 to 2.0 MPa at r = 2 mm when the crosssection is varied from 1.0 1016 to 1.0 1015 cm2 at I = 1.34 1013 W/cm2. However, numerical calculation shows that the photon cross-section does not affect the qualitative structure of the flow, which justifies our use of the s value in the present study. The initial electron number density has been assumed to be zero. It is noted that presence of impurities with low ionization potential can affect the time necessary for the breakdown to develop but the change did not affect the overall computation result considerably. The incident laser pulse energy is absorbed by the inverse Bremsstrahlung process through electron–ion collisions. This absorption process is
Fig. 2. (a) Example of an optical micrograph showing typical particle distribution and (b) the result of image processing for particle counting.
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specific heat ratio. All the properties of the plasma are calculated by mass fraction averaging the values for ions, electrons, and neutrals. 3. Experiment Experiments were performed to reveal the effect of gas blowing on the cleaning efficiency using a Q-switched Nd:YAG laser (pulse energy E = 530 mJ, irradiance I = 2.29 1013 W/ cm2). The cleaning efficiency was measured using alumina particles of 1 mm in nominal diameter. The alumina particles were deposited on a Si wafer by spin-drying ethanol suspensions containing the particle at 3000 rpm. The flow rate of the nitrogen jet was controlled by a mass flow meter. After the cleaning experiment, the sample surfaces were inspected using an optical microscope (1000). The number of particles was counted by an image processing program. Fig. 2 shows a typical optical micrograph and the result of image processing for number counting, validating the use of the image processing program. Before measuring the cleaning efficiency by varying the blowing condition, the characteristics of particle redeposition in the LSC process has been measured. After particles were deposited in a square-shaped zone of 7 mm 7 mm located at the center of a Si wafer, a single laser pulse was applied 4 mm above the center. The height was controlled by a three-axis
micro-positioning stage with a vision system. To selectively deposit the particles only in the square region by the spindrying technique, the rest of the wafer surface was masked by a thin aluminum foil with an adhesive tape. The particle distribution was measured outside the square zone as a function of distance from the boundary. From the scattered pattern of the particles, the degree of redeposition and the particle-flight range were estimated. The cleaning efficiency was measured by varying the gas-jet velocity (0, 20, and 50 m/ s) and the incident angle a (08, 308). In all cases, experiments were repeated five times at the same condition and the results were averaged. 4. Results and discussion Numerical simulation was carried out for the geometry depicted in Fig. 1 for h = 2 mm. The overall results of the numerical simulation are in qualitative agreement with the experimental results. Fig. 3(a) shows the LIB pressure field calculated in the numerical simulation, indicating qualitative agreement with the experimental result obtained by the laser flash photography in our previous work [3]. Fig. 3(b) displays the collision and reflection of the shock wave, which also agrees reasonably with the experimental observation. In Fig. 4, the results of the pressure field computation are exhibited. Notable
Fig. 3. (a) Laser flash shadowgraph at 310 ns and calculated pressure distribution at 300 ns (I = 3.8 1012 W/cm2). (b) Shadowgraph at 1980 ns (310 mJ, I = 1.52 1013 W/cm2, h = 2.5 mm) and calculated pressure distribution at 2000 ns (310 mJ, I = 1.34 1013 W/cm2, h = 2 mm). The grey scale is in arbitrary units.
D. Kim et al. / Applied Surface Science 253 (2007) 8322–8327
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Fig. 4. Pressure distributions above the sample (I = 1.34 1013 W/cm2, E = 310 mJ, frame width: 20 mm).
in Figs. 3 and 4 is that the present model can compute the asymmetric flow features that the simple blast wave theory cannot predict. It has been found that the spatial distribution of the cleaning efficiency is not symmetric but relatively high in the upstream region of the laser beam [1]. Initially, the high temperature/pressure core is formed at the focal point. As the shock wave expands, however, the density and pressure around the focal point becomes smaller than those at the shock front. Correspondingly, the absorption coefficient has two peaks at the shock-laser intersections, one at the left-propagating shock front and the other at the right-propagating front. This bipolar distribution of the absorption coefficient leads to the eggshaped shock wave generation. The temperature of the plasma generated by LIB (I = 1.34 1013 W/cm2, and E = 310 mJ) increases up to 2 105 K at the end of the laser pulse. As the shock wave expands the temperature at the plasma core decreases to 7000 K at 2 ms, which is close to that reported in a similar numerical calculation of LIB (l = 1064 nm, FWHM = 7 ns, P0 = 0.8 atm, E = 300 mJ) [9]. In the LSC process, the gas temperature on the sample surface is important as it may cause a thermal damage
Fig. 5. Temperature distribution on the surface at 5 ms (I = 1.34 1013 W/cm2, thick solid line: no blowing; () 20 m/s, a = 08; (&) 50 m/s, 08; (*) 20 m/s, 308; (~) 50 m/s, 308).
on the cleaning sample. According to Fig. 5, the maximum gas temperature on the sample surface at t = 5 ms is approximately 1000 K without blowing. This seems a high value but it should be pointed out that the temperature of the solid surface is much lower than that of the gas as the thermal contact is very brief and the heat capacity of the solid is much greater that that of the gas. When two semi-finite materials at different initial temperatures are put into thermal contact, the interfacial temperature is pffiffiffiffiffiffiffi determined by a weighting factor krc (k is the thermal conductivity, r the density, and c is the specific heat) and the temperature will very soon approach pffiffiffiffiffiffiffithe initial temperature of the material having a greater krc value [13]. Fig. 5 also reveals that the gas-jet injected from an inclined nozzle (a = 308) results in higher surface temperature than that from a parallel nozzle. This is because the gas flow shifts the plasma core toward the sample surface. No remarkable change in temperature was obtained when a = 08. No significant temperature change is induced by the gas-jet injection regardless of the gas blowing condition. The effect of the gas-jet injection on the flow field was analyzed by numerical simulation. Fig. 6 exhibits the effect of gas blowing on the horizontal-velocity distribution at two instants. The figure discloses that the gas-jet injection has only a minor effect on the flow field, and thus on the particle removal force, in the initial stage O (1 ms). On the other hand, the long-term flow structures formed over O (100 ms) are strongly affected by the external flow. The blowing enhances the sweeping flow motion over the surface, suggesting a reduced chance of particle redeposition. Notable is that oblique injection of the gas-jet gives stronger enhancement in the horizontal flow than parallel injection, which is a consequence of complicated hydrodynamic phenomena depending on many parameters but largely because of the drift motion of the plasma core toward the sample, as in the case of gas temperature (see Fig. 5). The stagnation region is not completely removed with the gas blowing, which is because the injected gas-jet is to flow against the LIB flow in certain regions. Consequently, design of a blowing system that minimizes particle redeposition is a complicated and challenging task and further work is required to obtain the optimal jet-blowing condition.
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Fig. 8. Cleaning efficiency for various blowing conditions (I = 2.29 1013 W/ cm2, h = 4 mm).
Fig. 6. Horizontal velocity distributions on the surface at (a) 5 ms, (b) 100 ms (I = 1.34 1013 W/cm2, thick solid line: no blowing; () 20 m/s, a = 08; (&) 50 m/s, 08; (*) 20 m/s, 308; (~) 50 m/s, 308).
Figs. 7 and 8 summarize the experimental results concerning the blowing effect. In Fig. 7, the distribution of the redeposited particles outside the cleaning zone is plotted. It is clear that a significant portion of the detached particles are redeposited without aid of the external gas-jet, which explains why a multiple number of pulses are needed to scan a finite area for complete cleaning. Fig. 7 also discloses that the range of the
particle flight is of the order of 1 cm under typical conditions. In Fig. 8, the cleaning efficiency is illustrated for four different blowing conditions. As expected, enlarged gas flow increases the cleaning efficiency but the increase is ultimately saturated above a certain flow rate. It is also observed in Fig. 8 that the cleaning efficiency is higher at a = 308 than at a = 08. This dependence can be explained well by Fig. 6(b). The vertical component of the gas flow shifts the position of the plasma core toward the surface and can increase the sweeping flow velocity. Concerning the particle redeposition mechanism, it can be shown that the gravitation force is negligibly small in comparison with the hydrodynamic force acting on the particle. Furthermore, the terminal velocity of particle sedimentation by gravity would be too small for the gravity effect to be important. It can also be argued that Brownian motion of the particle cannot contribute to the particle redeposition significantly. The Stoke–Einstein relation gives the particle-diffusion constant D = kBT/3pmd = 2.4 1011 m2/s at T = 300 K (kB is the Boltzmann constant, m the air viscosity, and d is the particle diameter = 1 mm). The time required for a particle to move by Dx = 1 mm is thus estimated to be O (104 s), which means that the Brownian motion is too slow to explain the results in Fig. 7. Consequently, it is likely that the hydrodynamic flow is mainly responsible for the redeposition process. However, the time scales for the redeposition process and the effect of longrange attractive forces such as the electrostatic force are not yet clear and requires further investigation. 5. Conclusions
Fig. 7. Distribution h = 4 mm).
of
redeposited
particles
(I = 2.29 1013 W/cm2,
In this work, a two-dimensional theoretical model has been developed for elucidating the hydrodynamics taking place in the LSC process. Also, experiments were performed to analyze the effect of blowing-gas injection. Numerical computation of the LSC process shows qualitative agreement with experimental observations and explains the details of the hydrodynamic phenomena which could not be described by the simple blast wave theory. Experimental results reveal that a significant portion of the detached particles are redeposited in the range of O (1 cm) without gas blowing. Both the numerical computation and experiments reveal that gas-jet
D. Kim et al. / Applied Surface Science 253 (2007) 8322–8327
injection significantly enhances the long-term sweeping flow, decreasing the chance of particle redeposition. Oblique injection gives higher cleaning efficiency than parallel injection but increases the chance of thermal damage. Further study is on-going to determine the optimal blowing conditions.
[2] [3] [4] [5]
Acknowledgements
[7] [8]
Support for this work is greatly appreciated: KIMM Grant for Advanced Laser Microfabrication, KOSEF Basic Research Program and Micro Thermal System ERC. Major financial support came from IMT Co.
[9]
References
[6]
[10] [11] [12] [13]
[1] H. Lim, D. Jang, D. Kim, J.W. Lee, J.-M. Lee, J. Appl. Phys. 97 (2005) 054903.
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D. Kim, J. Lee, J. Appl. Phys. 93 (2003) 762. H. Lim, D. Kim, Appl. Phys. A 79 (2004) 965. J. Lee, K. Watkins, J. Appl. Phys. 89 (2001) 6496. J. Lee, S. Cho, J. Park, S. Lee, Y. Han, S. Kim, SPIE Proc. 4830 (2002) 287. L.I. Sedov, Similarity and Dimensional Methods in Mechanics, CRC Press, Boca Raton, 1991. T.X. Phuoc, Optics Communications 175 (2000) 419. C.L.M. Ireland, C.G. Morgan, J. Phys. D: Appl. Phys. 6 (1973) 720. H. Sobral, M. Villagra´n-Muniz, R. Navarro-Gonza´lez, A.C. Raga, Appl. Phys. Lett. 77 (2000) 3158. Y.E. Gamal, M.A. Harith, J. Phys. D: Appl. Phys. 14 (1981) 2209. C.G. Morgan, Rep. Prog. Phys. 38 (1975) 621. Z. Zhang, Z. Han, G.S. Dulikravich, J. Appl. Phys. 90 (2001) 5889. F.P. Incropera, D.P. DeWitt, Fundamentals of Heat and Mass Transfer, 5th ed., Wiley, New York, 2002.
Experimental and theoretical analysis of the laser shock cleaning process for nanoscale particle removal Dongsik Kim a,*, Bukuk Oh a, Deoksuk Jang a, Jeong-Wook Lee a, Jong-Myoung Lee b a
Department of Mechanical Engineering, POSTECH, Pohang 790-784, Republic of Korea b Laser Engineering Group, IMT Co. Ltd., Uiwang 449-860, Republic of Korea Available online 25 February 2007
Abstract The laser shock cleaning (LSC) process has been shown to be effective for removing submicron-sized contaminant particles from solid surfaces and thus bears strong potential in various applications. In this work, experimental and theoretical analysis are conducted to reveal the underlying physical mechanisms of the LSC process, with emphasis on the laser-induced hydrodynamics and the effect of external gas-jet injection through a nozzle. A two-dimensional theoretical model is proposed for rigorous simulation of the hydrodynamic phenomena occurring in the LSC process. The hydrodynamics computed by the model is in qualitative agreement with experimental observations and reveal the details of the physics involved in the cleaning process. The effect of gas blowing on the cleaning performance is analyzed both experimentally and theoretically. The results indicate that the gas flow can significantly change the hydrodynamics and increase the cleaning efficiency by reducing the chance of particle redeposition. # 2007 Elsevier B.V. All rights reserved. Keywords: Laser cleaning; Laser-induced breakdown; Particle removal; Shock wave
1. Introduction Control of particle contamination has long been a critical issue in the microelectronics industry and significant efforts have been made to develop effective laser-based cleaning techniques [1–6]. Recent studies on the laser shock cleaning (LSC) have shown that the LSC process is effective for removing small particles from solid surfaces as a non-contact, dry method [1,3–5]. In the process, laser-induced breakdown (LIB) occurs in the ambient gas over the surface to be cleaned, which results in strong shock wave formation. Accordingly, the LSC process is a dry process and the surface is not directly exposed to the laser beam. Since the dynamics of the laserinduced shock wave is central to the cleaning process, a series of investigations were performed to analyze the hydrodynamics associated with the LSC process [1,3]. The results revealed that the cleaning efficiency is proportional to the intensity of the shock wave [1]. However, as the previous studies employed the simple blast wave theory [6], several important aspects of the
* Corresponding author. Tel.: +82 54 279 2179; fax: +82 54 279 3199. E-mail address: [email protected] (D. Kim). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.119
process, especially the asymmetry of the shock wave shape and thus the inhomogeneous spatial distribution of the cleaning efficiency, could not be explained theoretically. In this regard, this work proposes a more refined two-dimensional theoretical model for the hydrodynamics involved in the LSC process, i.e., LIB, shock formation, propagation, and interaction with a solid surface and external flow. Though the LIB is relatively well understood [7–9], the hydrodynamics of the LIB flow and its interaction with a solid surface have hardly been investigated. Furthermore, this work examines the effect of gas-jet injection through a nozzle on the cleaning performance. It is evident that the gas-jet injection changes the flow field and eventually the cleaning efficiency since the hydrodyanmics and redepostion of the detached particles are strongly influenced by the flow. Both numerical and experimental analysis are conducted to uncover the effect of gas blowing. Fig. 1 depicts the problem this work is concerned with A Qswitched laser (wavelength l = 1064 nm, full width at half maximum FWHM =6 ns) induces optical breakdown of N2. The shock wave generated from the plasma core impinges onto the sample, removing the particles from the surface. Two different angles a = 08 and 308 are tested when the blowing effect is analyzed. It is noted that a spherical shock wave is
D. Kim et al. / Applied Surface Science 253 (2007) 8322–8327
8323
modeled by the plasma absorption coefficient ap [12]. Expansion of the high-temperature plasma is followed by shock wave generation. The subsequent hydrodynamic motion is modeled by two-dimensional transport equations, representing conservation of mass, momentum, and energy. The transport equations are simultaneously solved for atoms, ions, and electrons. In summary, the scalar transport equations are: @ ðrfi Þ þ r ðr~ Vfi Þ ¼ si @t @ ðrfe Þ þ r ðr~ Vfe Þ ¼ se @t Fig. 1. Schematic diagram for the LSC process.
formed in the actual LSC process and therefore a threedimensional model is required for quantitative analysis of the phenomena. Nevertheless, since the three-dimensional effect is secondary, changing only the number, this work employs a twodimensional model for computational efficiency to analyze the hydrodynamics of the LSC process.
@ne nn A K ¼ nne þ I ðtÞ; @t K 3=2
A¼
sK v2K1 hK ðK 1Þ!
for electrons
(2a) (2b)
where r is the density, fi the ion mass fraction, fe the electron mass fraction, ~ V the velocity vector, and si, se is the ionization source terms for ion and electron, respectively. And the mass, momentum, energy conservations equations and the equation of state are: @r þ r ðr~ VÞ ¼ 0 @t
(3a)
r
@~ V þ rð~ V rÞ~ V ¼ r p þ r ðti j Þ @t
(3b)
r
* @e þ rð~ V rÞe ¼ pðr ~ VÞ þ ap ðr ; tÞIðtÞ eap d @t
(3c)
2. Theoretical model The LIB can be modeled via the process of multiphoton ionization (MPI) and impact (avalanche) ionization by electron/ atom collisions because the laser irradiance I O (1013 W/ cm2) is high enough to activate the ionization mechanisms. Eq. (1) shows the ionization model utilized in the present work. The model considers both the avalanche ionization and MPI mechanisms [7] (ne, nn: electron and atom concentration, n: ionization collision frequency, K: multiphoton order, I(t): laser intensity, v: laser frequency, t: time, h: Planck’s constant):
for ions
p ¼ ðg 1Þre
(3d)
where e = cvT is the internal energy, cv the specific heat at * constant volume, p the pressure, r the position vector, tij the Cauchy stress tensor, d the absorption length, and g is the
(1)
In the above equation, the precise value of the photon crosssection s is generally unknown although it is typically of the order of 1016 cm2 [10,11]. Therefore, the photon cross-section s has been assumed to be s = 2.4 1016 cm2. This value minimizes the difference between the calculated and measured [3] shock wave velocities at a propagation distance r = 3 mm. Variation of the photon cross-section leads to changes in the computed hydrodynamics. For example, the maximum pressure changes from 0.8 to 2.0 MPa at r = 2 mm when the crosssection is varied from 1.0 1016 to 1.0 1015 cm2 at I = 1.34 1013 W/cm2. However, numerical calculation shows that the photon cross-section does not affect the qualitative structure of the flow, which justifies our use of the s value in the present study. The initial electron number density has been assumed to be zero. It is noted that presence of impurities with low ionization potential can affect the time necessary for the breakdown to develop but the change did not affect the overall computation result considerably. The incident laser pulse energy is absorbed by the inverse Bremsstrahlung process through electron–ion collisions. This absorption process is
Fig. 2. (a) Example of an optical micrograph showing typical particle distribution and (b) the result of image processing for particle counting.
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specific heat ratio. All the properties of the plasma are calculated by mass fraction averaging the values for ions, electrons, and neutrals. 3. Experiment Experiments were performed to reveal the effect of gas blowing on the cleaning efficiency using a Q-switched Nd:YAG laser (pulse energy E = 530 mJ, irradiance I = 2.29 1013 W/ cm2). The cleaning efficiency was measured using alumina particles of 1 mm in nominal diameter. The alumina particles were deposited on a Si wafer by spin-drying ethanol suspensions containing the particle at 3000 rpm. The flow rate of the nitrogen jet was controlled by a mass flow meter. After the cleaning experiment, the sample surfaces were inspected using an optical microscope (1000). The number of particles was counted by an image processing program. Fig. 2 shows a typical optical micrograph and the result of image processing for number counting, validating the use of the image processing program. Before measuring the cleaning efficiency by varying the blowing condition, the characteristics of particle redeposition in the LSC process has been measured. After particles were deposited in a square-shaped zone of 7 mm 7 mm located at the center of a Si wafer, a single laser pulse was applied 4 mm above the center. The height was controlled by a three-axis
micro-positioning stage with a vision system. To selectively deposit the particles only in the square region by the spindrying technique, the rest of the wafer surface was masked by a thin aluminum foil with an adhesive tape. The particle distribution was measured outside the square zone as a function of distance from the boundary. From the scattered pattern of the particles, the degree of redeposition and the particle-flight range were estimated. The cleaning efficiency was measured by varying the gas-jet velocity (0, 20, and 50 m/ s) and the incident angle a (08, 308). In all cases, experiments were repeated five times at the same condition and the results were averaged. 4. Results and discussion Numerical simulation was carried out for the geometry depicted in Fig. 1 for h = 2 mm. The overall results of the numerical simulation are in qualitative agreement with the experimental results. Fig. 3(a) shows the LIB pressure field calculated in the numerical simulation, indicating qualitative agreement with the experimental result obtained by the laser flash photography in our previous work [3]. Fig. 3(b) displays the collision and reflection of the shock wave, which also agrees reasonably with the experimental observation. In Fig. 4, the results of the pressure field computation are exhibited. Notable
Fig. 3. (a) Laser flash shadowgraph at 310 ns and calculated pressure distribution at 300 ns (I = 3.8 1012 W/cm2). (b) Shadowgraph at 1980 ns (310 mJ, I = 1.52 1013 W/cm2, h = 2.5 mm) and calculated pressure distribution at 2000 ns (310 mJ, I = 1.34 1013 W/cm2, h = 2 mm). The grey scale is in arbitrary units.
D. Kim et al. / Applied Surface Science 253 (2007) 8322–8327
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Fig. 4. Pressure distributions above the sample (I = 1.34 1013 W/cm2, E = 310 mJ, frame width: 20 mm).
in Figs. 3 and 4 is that the present model can compute the asymmetric flow features that the simple blast wave theory cannot predict. It has been found that the spatial distribution of the cleaning efficiency is not symmetric but relatively high in the upstream region of the laser beam [1]. Initially, the high temperature/pressure core is formed at the focal point. As the shock wave expands, however, the density and pressure around the focal point becomes smaller than those at the shock front. Correspondingly, the absorption coefficient has two peaks at the shock-laser intersections, one at the left-propagating shock front and the other at the right-propagating front. This bipolar distribution of the absorption coefficient leads to the eggshaped shock wave generation. The temperature of the plasma generated by LIB (I = 1.34 1013 W/cm2, and E = 310 mJ) increases up to 2 105 K at the end of the laser pulse. As the shock wave expands the temperature at the plasma core decreases to 7000 K at 2 ms, which is close to that reported in a similar numerical calculation of LIB (l = 1064 nm, FWHM = 7 ns, P0 = 0.8 atm, E = 300 mJ) [9]. In the LSC process, the gas temperature on the sample surface is important as it may cause a thermal damage
Fig. 5. Temperature distribution on the surface at 5 ms (I = 1.34 1013 W/cm2, thick solid line: no blowing; () 20 m/s, a = 08; (&) 50 m/s, 08; (*) 20 m/s, 308; (~) 50 m/s, 308).
on the cleaning sample. According to Fig. 5, the maximum gas temperature on the sample surface at t = 5 ms is approximately 1000 K without blowing. This seems a high value but it should be pointed out that the temperature of the solid surface is much lower than that of the gas as the thermal contact is very brief and the heat capacity of the solid is much greater that that of the gas. When two semi-finite materials at different initial temperatures are put into thermal contact, the interfacial temperature is pffiffiffiffiffiffiffi determined by a weighting factor krc (k is the thermal conductivity, r the density, and c is the specific heat) and the temperature will very soon approach pffiffiffiffiffiffiffithe initial temperature of the material having a greater krc value [13]. Fig. 5 also reveals that the gas-jet injected from an inclined nozzle (a = 308) results in higher surface temperature than that from a parallel nozzle. This is because the gas flow shifts the plasma core toward the sample surface. No remarkable change in temperature was obtained when a = 08. No significant temperature change is induced by the gas-jet injection regardless of the gas blowing condition. The effect of the gas-jet injection on the flow field was analyzed by numerical simulation. Fig. 6 exhibits the effect of gas blowing on the horizontal-velocity distribution at two instants. The figure discloses that the gas-jet injection has only a minor effect on the flow field, and thus on the particle removal force, in the initial stage O (1 ms). On the other hand, the long-term flow structures formed over O (100 ms) are strongly affected by the external flow. The blowing enhances the sweeping flow motion over the surface, suggesting a reduced chance of particle redeposition. Notable is that oblique injection of the gas-jet gives stronger enhancement in the horizontal flow than parallel injection, which is a consequence of complicated hydrodynamic phenomena depending on many parameters but largely because of the drift motion of the plasma core toward the sample, as in the case of gas temperature (see Fig. 5). The stagnation region is not completely removed with the gas blowing, which is because the injected gas-jet is to flow against the LIB flow in certain regions. Consequently, design of a blowing system that minimizes particle redeposition is a complicated and challenging task and further work is required to obtain the optimal jet-blowing condition.
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Fig. 8. Cleaning efficiency for various blowing conditions (I = 2.29 1013 W/ cm2, h = 4 mm).
Fig. 6. Horizontal velocity distributions on the surface at (a) 5 ms, (b) 100 ms (I = 1.34 1013 W/cm2, thick solid line: no blowing; () 20 m/s, a = 08; (&) 50 m/s, 08; (*) 20 m/s, 308; (~) 50 m/s, 308).
Figs. 7 and 8 summarize the experimental results concerning the blowing effect. In Fig. 7, the distribution of the redeposited particles outside the cleaning zone is plotted. It is clear that a significant portion of the detached particles are redeposited without aid of the external gas-jet, which explains why a multiple number of pulses are needed to scan a finite area for complete cleaning. Fig. 7 also discloses that the range of the
particle flight is of the order of 1 cm under typical conditions. In Fig. 8, the cleaning efficiency is illustrated for four different blowing conditions. As expected, enlarged gas flow increases the cleaning efficiency but the increase is ultimately saturated above a certain flow rate. It is also observed in Fig. 8 that the cleaning efficiency is higher at a = 308 than at a = 08. This dependence can be explained well by Fig. 6(b). The vertical component of the gas flow shifts the position of the plasma core toward the surface and can increase the sweeping flow velocity. Concerning the particle redeposition mechanism, it can be shown that the gravitation force is negligibly small in comparison with the hydrodynamic force acting on the particle. Furthermore, the terminal velocity of particle sedimentation by gravity would be too small for the gravity effect to be important. It can also be argued that Brownian motion of the particle cannot contribute to the particle redeposition significantly. The Stoke–Einstein relation gives the particle-diffusion constant D = kBT/3pmd = 2.4 1011 m2/s at T = 300 K (kB is the Boltzmann constant, m the air viscosity, and d is the particle diameter = 1 mm). The time required for a particle to move by Dx = 1 mm is thus estimated to be O (104 s), which means that the Brownian motion is too slow to explain the results in Fig. 7. Consequently, it is likely that the hydrodynamic flow is mainly responsible for the redeposition process. However, the time scales for the redeposition process and the effect of longrange attractive forces such as the electrostatic force are not yet clear and requires further investigation. 5. Conclusions
Fig. 7. Distribution h = 4 mm).
of
redeposited
particles
(I = 2.29 1013 W/cm2,
In this work, a two-dimensional theoretical model has been developed for elucidating the hydrodynamics taking place in the LSC process. Also, experiments were performed to analyze the effect of blowing-gas injection. Numerical computation of the LSC process shows qualitative agreement with experimental observations and explains the details of the hydrodynamic phenomena which could not be described by the simple blast wave theory. Experimental results reveal that a significant portion of the detached particles are redeposited in the range of O (1 cm) without gas blowing. Both the numerical computation and experiments reveal that gas-jet
D. Kim et al. / Applied Surface Science 253 (2007) 8322–8327
injection significantly enhances the long-term sweeping flow, decreasing the chance of particle redeposition. Oblique injection gives higher cleaning efficiency than parallel injection but increases the chance of thermal damage. Further study is on-going to determine the optimal blowing conditions.
[2] [3] [4] [5]
Acknowledgements
[7] [8]
Support for this work is greatly appreciated: KIMM Grant for Advanced Laser Microfabrication, KOSEF Basic Research Program and Micro Thermal System ERC. Major financial support came from IMT Co.
[9]
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