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Removal mechanisms of micro-scale particles by surface wave in laser cleaning
ARTICLE IN PRESS
Optics & Laser Technology 38 (2006) 544–551 www.elsevier.com/locate/optlastec
Removal mechanisms of micro-scale particles by surface wave in laser cleaning Shin-Chun Hsu, Jehnming Lin Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan Received 21 November 2003; received in revised form 10 October 2004; accepted 29 November 2004 Available online 3 February 2005
Abstract This paper is to investigate the mechanisms of micro-scale particle removal by surface wave, which was induced by a short pulse laser in a cleaning process. The authors analyzed the adhesive forces of particles on substrate surface and the clearance force produced by surface wave in laser cleaning. The physical model of particle removal by laser-induced surface wave was established to predict the removal area and the processing conditions of laser cleaning. In this research, a KrF excimer laser was applied to irradiate 304 stainless steel specimen distributed with copper particles ðo45 mmÞ to generate surface wave for copper particle removal. Considering that a time-varying and uniformly distributed heat source irradiates on material surface with thermao-elastic behavior, the displacement and acceleration of substrate induced by a pulsed laser were solved by an uncoupled thermal–mechanical analysis based on the finite element method. The processing parameters such as laser energy, laser spot size are discussed, respectively. A series of laser cleaning experiments were designed to compare with computation results. The results show that the removal area by surface wave beyond the laser spot increases with the laser energy and that, the surface acceleration decreases with the increase of the laser spot size. r 2005 Elsevier Ltd. All rights reserved. Keywords: Laser cleaning; Surface elastic wave; Particle; Finite element simulation
1. Introduction With the rapid development of the semi-conductor industry in recent years and the trend towards the microelectronics industry, manufacturing smaller integrated circuits makes the particle pollutions of growing importance in affecting quality and yields of the products. Hence, the clearance of particles becomes a critical issue in aforesaid processing. In current semiconductor industry, the clearance of silicon wafer is mainly achieved by wet cleaning method or dry cleaning method. The wet cleaning method needs a great amount of de-ionized water or chemical agents and causes further environmental pollutions. Among the numerous Corresponding author. Tel.: +886 6 2757575x62100; fax: +886 6 2352973. E-mail address: [email protected] (J. Lin).
0030-3992/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2004.11.021
dry cleaning methods, the laser cleaning technique achieved a remarkable cleaning result when the particles range below micrometer and sub-micrometer [1–3]. In recent decade, many investigators devoted themselves to the research of laser cleaning technique. In accordance with the difference of mechanisms, particle removal mechanisms can be divided into (a) steam laser cleaning mechanism [1–4], (b) extra thermal detachment mechanism for particle removal [5–8], (c) the mechanism of the surface acoustic wave for particle removal [9], and (d) plasma shock wave [10,11]. In dry laser cleaning, when a short pulse from a laser irradiates a substrate surface with a low-energy pulse, it will produce a thermal effect in the irradiated zone. Because the thermal conduction of the short pulse laser is negligible outside the laser irradiation region, the thermal expansion of specimen surface thus produces a strong elastic wave, such as acoustic wave beyond the
ARTICLE IN PRESS S.-C. Hsu, J. Lin / Optics & Laser Technology 38 (2006) 544–551
laser spot [9]. Generally speaking, the elastic wave produced on the surface of thick substrate can be divided into compression wave, shear wave, and Rayleigh wave as discussed in details in Ref. [9]. Some investigators used laser interferometer to measure surface waves at different positions from laser source and compared the results with numerical solutions [12], or used piezoelectric transducer, PZT, to measure surface waves in different positions and discussed the influence of laser spot size to surface waves [13]. Most researches restricted the removal areas within the spot of laser beams. The use of laser ablation was promoted to induce nonlinear surface wave and enlarge the removal areas. However, it is very possible to damage the substrate surface [9]. In this study, the laser cleaning experiment was performed with UV laser energy below the damage threshold of substrate material. On this condition, a removal area beyond laser spot was produced. Considering the thermao-elastic behavior of the material, the software ABAQUS based on finite element method was adopted to analyze surface acceleration. The numerical simulation was compared with related experiments of substrate heating by short-pulse laser. The physical model of particle removal by laser-induced surface wave was established, and the influences of processing parameters on the particle removal were discussed.
d
545
particle
zf substrate (a)
d
particle liquid
substrate (b)
d
particle
substrate (c) Fig. 1. Three types of adhesive force between substrate and particles: (a) Van der Waals force, (b) capillary force, and (c) electrostatic force.
2. Physical model 2.1. Surface adhesive force In general terms, the adhesive forces between particles and substrate surface mainly come from Van der Waals force, capillary force, and electrostatic force [14]. The illustrations of the corresponding force models are shown in Fig. 1. It was pointed out in Ref. [14] that when the radius of particles on substrate surface is less than 50 mm; the major acting adhesive force between particles and substrate surface is Van der Waals force, and its value is several times greater than the gravity of particles. Thus, traditional clearance techniques using gravitational effect become ineffective. The Van der Waals force between spherical particles and substrate surface can be described as follows [14]: FV ¼
hd hd 2c þ : 16pz2f 32pz3f
(1)
In Eq. (1), the first term is the Van der Waals force between spherical particle and substrate surface under point contact. While, the second term is the extra adhesive force when particles or substrate surface deforms and the contact area increases, where d is the
diameter of particles, d c is the diameter of contact area, zf is the distance between particles and atoms of substrate surface (about 4 1010 m), h is a Lifshitz– Van der Waals constant and it can be calculated with Hamaker coefficient A by the following equation [15,16]: h¼
4pA : 3
(2)
The Hamaker coefficient of copper is about 40 1020 J and of iron is 21:2 1020 J: Since the major composition of the substrate material is iron, by Eq. (3), the Hamaker coefficient of copper particle at iron substrate is approximately 2:912 1019 J: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ACu2Fe ¼ ACu AFe : (3) 2.2. Surface clearance force Laser beams produce surface wave with significant displacement and acceleration on the substrate surface, and it will remove the particle from the substrate surface mainly due to the inertia effects. Since the vertical displacement of surface wave is about 1.5 times that in horizontal displacement [17], the shear effect is therefore
ARTICLE IN PRESS S.-C. Hsu, J. Lin / Optics & Laser Technology 38 (2006) 544–551
546
z
1E+009 Relation between partice size and cleaning acceleration Cu on Fe without deformation Contact diameter=1% of particle diameter Contact diameter=2% of particle diameter Contact diameter=5% of particle diameter
Critical acceleration (m/s2)
1E+008
Laser beam
4mm 1E+007
r
0
Fig. 3. Geometric domain of specimen. 1E+006
z 1E+005
1E+004 0
10
20
30
40
50
Particle diameter (µm)
Fig. 2. The relationship between critical acceleration and particle diameter.
neglected in the analysis. When the surface clearance force is larger than surface adhesive force, the particle is removed. Thus, the clearance force is expressed as f ¼ man ;
(4)
where m is the mass of the particle, and an is the acceleration in vertical direction. From (1) and (4), the critical acceleration to remove a particle is obtained as acr ¼
A Ad 2c þ : 2prz2f d 2 4prz3f d 3
(5)
In Fig. 2, the relations between critical acceleration and particle diameter are shown. It is clearly found that the value of contact area will significantly affect the critical acceleration. With the decrease of particle diameter, the critical acceleration increases dramatically. Thus, it is difficult to remove the particles in small diameters.
3. Numerical simulation The surface wave induced by laser is a nonlinear physical phenomenon, which includes interactions among mechanics, heat transfer, and microstructure. To simplify the analysis, an uncoupled thermal–mechanical scheme was adopted to divide the simulation into heat transfer and mechanism models. This means that the stress result will not influence the temperature field. Because the area of laser clearance is much smaller than the specimen area, the boundary of the specimen was neglected in the simulation. As shown in Fig. 3, it is considered as a laser beam irradiate on an infinite disc
r
0
Fig. 4. Finite element mesh of specimen.
with thickness of 4 mm. The analysis was simplified as a two-dimensional problem due to the symmetry of the computation domain. Four-node elements DCAX4 [18] were used in the heat transfer analysis for a 2D cylindrical domain divided by 363 36 nodes at r- and z-axis, respectively. The mesh distributes over the area of 18.1 mm in diameter and 4 mm in depth, as shown in Fig. 4. Because there is an intense temperature gradient near the laser beam, the grid structure must be fine meshed in the heating area. In the mechanical analysis, the grid structure is similar to that of heat transfer analysis. However, CAX4R [18] stress/displacement elements were adopted. Infinite elements CINPE4 [18] were used to treat the boundary condition. Therefore the influence of wave reflection on the boundary was neglected in the simulation. A SUS304 stainless steel was used as substrate material. The nonlinear effects of temperature dependent properties such as thermal expansion coefficient, heat conduction coefficient, specific heat, density, Poisson’s ratio, and Young’s modulus have been considered in the simulation [19]. 3.1. Heat transfer analysis In heat transfer analysis, the following assumptions were adopted to simplify the simulation: 1. The skin depth of laser absorption was neglected and the laser energy is in a form directly heating the specimen surface. 2. The energy distribution of the laser beam is uniform.
ARTICLE IN PRESS S.-C. Hsu, J. Lin / Optics & Laser Technology 38 (2006) 544–551
3. Heat loss occurred by convection effect and the thermal radiation was neglected. 4. The temperature variations due to the phase change and deformation were neglected. The time-varying function I s ðtÞ of the laser power density absorbed by the specimen surface was expressed as Eq. (6) [20] OE 0 ð1 Rf Þ 8t3 2t2 =t2 e I s ðtÞ ¼ ; (6) t4 pR2 where I s ðtÞ is the time-varying heat flux ðW=m2 Þ on specimen surface, Rf is the reflectivity coefficient of specimen material, R is the radius of laser beam (m), E 0 is the laser energy (J/pulse), O is the percentage of surface area irradiated by laser beam (%), and t is the period of pulse (s). The reflectivity of the specimen with polished surface was assumed as a constant reflectivity of 0.38 in this study [21]. 3.1.1. Initial and boundary conditions Assuming the initial temperature is the ambient temperature, namely, Tðx; y; z; 0Þ ¼ 25 C: In heat transfer analysis, since the heat affected area due to short pulse laser is very small and the thermal diffusion length is only several micrometers for stainless steel [22]. Thus, it was assumed that there is only heat convection on the laser radiation surface and adiabatic on all other surfaces. That is,
547
Since the specimen was clamped in the processing and the beam center is aligned to the cylindrical axis of the domain, there is no displacement in r direction and other surfaces are unconstrained. The boundary conditions were simplified as ur ðr; z; tÞ ¼ 0; ur ðr; z; tÞ ¼ 0;
r ¼ 0 mm; r ¼ 0 mm; z ¼ 0 mm;
uz ðr; z; tÞ ¼ 0;
r ¼ 0 mm; z ¼ 0 mm:
4. Numerical results 4.1. Computation verification The simulation of laser-induced surface wave was first verified by experimental results from Ref. [22]. In the numerical analysis adopting the aforesaid assumptions and grid structure, the results shown in Fig. 5 are for the displacement at 10 mm on the irradiated surface from the center of laser beam spot. It shows that the surface wave velocities in the simulation and experiment are in a good agreement. The process parameters in the related cases are: the specimen thickness is 4 mm; the surface area occupancy rate of copper particles is 25%; laser energy is 4 mJ/ pulse; the laser spot size is 1 mm; the pulse period is 30 ns. Fig. 6 shows the vertical components of the surface displacement and acceleration at 5 mm from the center of laser beam spot. Since the numerical fluctuation has
qT ¼ hc ðT T 1 Þ; z ¼ 4 mm ðtop surfaceÞ; qn qT ¼ 0; on other surfaces except for z ¼ 4 mm; k qn k
3.2. Stress analysis In the analysis of the displacement and acceleration of the substrate, it was assumed as follows: 1. The material is isotropic. 2. The material is stress free initially. 3. There is no influence of strain rate on the material properties. 4. The material is under thermao-elastic behavior and the force is within elastic limit.
Experiment ABAQUS simulation
1
Normalized displacement
where the heat convection coefficient hc ¼ 21 W=m2 C [23]. The temperature at each node of the domain was calculated in according to the heat input, material properties and boundary conditions. The transient temperature fields were then substitutes as the thermal loading into mechanical model for the stress analysis.
2
0
-1
-2
-3 0
1e-006
2e-006
3e-006
4e-006
5e-006
Time (s)
Fig. 5. Displacement results at 10 mm from the center of the laser beam by simulation and experiment [22].
ARTICLE IN PRESS S.-C. Hsu, J. Lin / Optics & Laser Technology 38 (2006) 544–551
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5
Displacement before filtering Displacement after filtering Acceleration after filtering
1e-009
Energy=4mJ/pulse FWHM=30ns Beam diameter=1mm Acceleration Critical acceleration
6.0E+005
4
2.0E+005
0
0.0E+000 -5e-010 -2.0E+005 -1e-009 -4.0E+005
-1.5e-009
Max-acceleration (×105m/s2)
4.0E+005
Acceleration (m /s2)
Displacement (m)
5e-010
-6.0E+005 0
1e-006
2e-006
3
2
1
3e-006
Time (s)
Fig. 6. The vertical displacement and acceleration at 5 mm from the center of laser beam.
0 0
1
2
3
4
5
Distance from the laser heated area (mm)
Fig. 7. Maximum acceleration at different positions compared with the critical acceleration of copper particles with a particle diameter of 20 mm and 1% diameter at the contact surface.
1.2 FWHM= 30ns Beam diameter=1mm
0.8
L (mm)
caused a difficulty in the analysis, the fluctuations were thereby deleted with a cut-off frequency. It is found that the maximum acceleration occurred at the largest negative displacement of surface wave. If the acceleration is larger than the critical acceleration ac ; then the particle removal is achieved. If the processing conditions and critical acceleration for particle removal are specified, the effective cleaning area will be predicted. By Eq. (5) with the conditions of copper particles in a diameter of 20 mm and 1% of the particle diameter at the contact area with substrate, the critical and maximum accelerations corresponding to different positions are shown in Fig. 7. It can be seen that the threshold distance with a surface acceleration equal to the critical acceleration is about 0.75 mm from the laser beam center. Thus, besides the particles within the laser spot, the particles could be moved by elastic surface wave in the area beyond the laser spot.
0.4
4.2. Parametric analysis The processing parameters such as laser energy and laser spot size might dominate the characteristics of surface wave, thus these two parameters were analyzed in details. The laser energy was set at 2.5–4.5 mJ/pulse, and rest of the parameters were constants. Fig. 8 gives the relationship between different laser energy and removal area in computation. L is the distance beyond laser spot and defined as follows: L ¼ Rc R; where Rc is the radius of the removal area. It is evident in Fig. 8 that the energy of laser pulse increases with the laser spot size and removal area.
0 2.5
3
3.5
4
4.5
Pulse energy (mJ /pulse)
Fig. 8. Length of removal area beyond laser spot under various laser energies.
Considering the removal area with laser spot sizes set at 0.6–1.0 mm at the same laser energy density, the effective distance of removal was obtained by the same method of prediction. The ratios of removal distance beyond laser spot to the radius of laser spot with various laser spot are shown in Fig. 9, where the definition of J is
ARTICLE IN PRESS S.-C. Hsu, J. Lin / Optics & Laser Technology 38 (2006) 544–551 Attenuator
8
549
Homogenizer
Mask Mirror
Excimer laser Lens group
6
Condenser lens
J
10X Lens
:Particle
Extractor
4
Sample Computer
2
X-Y-Z-U controller
X-Y-Z-U Stage
Vacuum Chuck
Fig. 10. Apparatus setup of laser cleaning experiment.
Table 1 Specimen dimension and processing parameters
0 0.6
0.7
0.8
0.9
1
Beam diameter (mm)
Fig. 9. The ratio of length of removal area beyond laser spot to the radius of laser beam under various laser beam sizes.
expressed as L : R It can be found that the removal mechanism on the region beyond the laser spot is caused by the propagation of elastic surface wave and the removal mechanism within the laser spot is mainly caused by direct thermal expansion. As shown in Fig. 9, the smaller the laser spot the larger the ratio J. It means that elastic surface wave has a significant influence on the removal area outside the laser spot, especially for a small spot size. J¼
5. Experiments The experimental setup is shown in Fig. 10. A KrF excimer laser was used as the laser source (wavelength of 248 nm). The pulse duration is about 30 ns with the maximum energy output of 400 mJ/pulse. The laser beam was shaped by a mask with a center hole of different radii and passed to a lens set to focus it on the surface of specimen with a round uniform distribution of the laser power density. The substrate material is annealed 304 stainless steel, which has been polished, dried and distributed with copper particles with sizes between 10–45 mm on the top surface of substrate. Table 1 shows the dimensions of the specimen and processing parameters in the experiments. In Fig. 11, the results of cleaning with laser energy 4 mJ and 1 mm radius of laser spot are shown. The
Specimen dimensions Length Width Thickness
100 mm 60 mm 4 mm
Processing parameters Laser energy Pulse duration Spot size Percent area of particle
2.5–4.5 mJ/pulse 30 ns 0.6–1 mm 25%
bright spots on the picture are copper particles and the dark region is stainless steel substrate. It is clearly shown that the removal area is not restricted by laser spot area and the removal effects are beyond the spot area. Fig. 12 is the enlarged image of Fig. 11. It can be seen that the larger copper particles are located at the outer ring from the center of the irradiated zone. Thus, to remove small particles needs large surface acceleration. Also, it can be seen that a tendency of the acceleration caused by surface wave decreases with the increase of distance from the beam center. Fig. 13 shows the comparison of experiment and numerical simulation results of removal areas under various laser energies. Both have consistent tendencies: the removal areas increase with laser energy. However, the errors might come from the roughness of substrate surface and the irregular shapes of copper particles. Since the contact area is much greater than the assumed, the larger removal acceleration is needed for the real cases. Also the drag forces due to the ambient air and moisture may cause the difficulties in particle removal. Fig. 14 shows the experiment and numerical simulation results of removal areas under various laser spot sizes. The ratio of removal distance beyond laser spot to the radius of laser spot increase with the decrease
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Beam diameter=1mm Experiment Simulation
L (mm)
1.2
0.8
0.4
0 2.5
3
3.5
4
4.5
Pulse energy (mJ/pulse)
Fig. 13. Comparison of experimental and numerical results of the length L under various laser energies.
Fig. 11. The cleaning result with laser energy of 4 mJ/pulse and laser diameter of 1 mm: (a) before cleaning, and (b) after cleaning.
J
Experiment Simulation
1
0.1 0.6
0.7
0.8
0.9
1
Beam diameter (mm)
Fig. 14. Experimental and numerical results of the ratio J under various laser beam diameters.
6. Conclusions
Fig. 12. Enlarged image of Fig. 11(b).
of spot radii for both approaches. The influences of surface wave on removal area are significant for small beam sizes.
According to the predications of the removal surface, the areas beyond the laser spot increase with the laser energy. The clearance effect due to thermal expansion on substrate is not only the consideration, but the wave propagation on the areas beyond the laser spot is also significant, especially for small laser beam sizes. The
ARTICLE IN PRESS S.-C. Hsu, J. Lin / Optics & Laser Technology 38 (2006) 544–551
efficiency of cleaning was mainly affected by laser energy, size of laser beam, and the material properties. References [1] Imen K, Lee SJ, Allen SD. Laser-assisted micron scale particle removal. Appl Phys Lett 1991;58:203–5. [2] Zapka W, Ziemlich W, Tam AC. Efficient pulsed laser removal of 0:2 mm sized particle from a solid surface. Appl Phys Lett 1991;58:2217–9. [3] Tam AC, Leung WP, Zapka W, Ziemlich W. Laser-cleaning techniques for removal of surface particulates. J Appl Phys 1992;71:3515–23. [4] Imen K, Lee SJ, Allen SD. Shock wave analysis of laser assisted particle removal. J Appl Phys 1993;74:7044–7. [5] Kelley JD, Hovis FE. A thermal detachment mechanism for particle removal from surfaces by pulsed laser irradiation. Microelectron Eng 1993;20:159–70. [6] Lu YF, Song WD, Ye KD, Lee YP. A cleaning model for removal of particles due to laser-induced thermal expansion of substrate. Jpn J Appl Phys 1997;36:1304–6. [7] Lu YF, Song WD, Ang BW, Hong MH, Chan DSH, Low TS. A theoretical model for laser removal of particles from solid surfaces. Appl Phys A 1997;65:9–13. [8] Lee JM, Curran C, Watkins KG. Laser removal of copper particles from silicon wafers using UV, visible and IR radiation. Appl Phys A 2001;73:219–24. [9] Kolomenskii AA, Schuessler HA, Mikhalevich VG, Maznev AA. Interaction of laser-generated surface acoustic pulses with fine particles: surface cleaning and adhesion studies. J Appl Phys 1998;84:2404–10. [10] Curran C, Watkins KG, Lee JM. Shock pressure measurements for removal of particles of sub-micron dimensions from silicon wafers. Section D-ICALEO 2002.
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[11] Lee JM, Cho SH, Park JG, Lee SH, Kang YJ, Han YP, Kim SY. Laser induced shock cleaning of silicon wafers. Section D-ICALEO 2002. [12] Scruby CB, Moss BC. The launching of Rayleigh waves from surface point sources. In: Ash EA, Paige EGS, editors. Rayleigh wave theory and application, 1985. p. 102–9. [13] Scala CM, Doyle PA. Time- and frequency-domain characteristic of laser-generated ultrasonic surface waves. J Acoust Soc Am 1989;85:1569–76. [14] Bowling RA. A theoretical review of particle adhesion. In: Mittal KL, editor. Particles on surface 1: detection, adhesion, and removal, 1988. p. 129–42. [15] Osborne-Lee IW. Calculation of Hamaker coefficients for metallic aerosols from extensive optical data. In: Mittal KL, editor. Particles on surface 1: detection, adhesion, and removal, 1988. p. 77–90. [16] Ranade MB. Adhesion and removal of particles: effect of medium. In: Mittal KL, editor. Particles on surface 1: detection, adhesion, and removal, 1988. p. 179–91. [17] Viktorov IA. Rayleigh and lamb waves. New York: Plenum Press; 1967. [18] ABAQUS User’s Manual Volume II, Pawtuckett, RI: H.K.S. Inc., 1998. [19] Peckner D, Bernstein IM. Handbook of stainless steels. New York: McGraw-Hill; 1977. [20] Lee JH, Burger CP. Finite element modeling of laser generation lamb waves. Comput Struct 1995;54:499–514. [21] Ready JF. Industrial applications of lasers. New York: Academic Press; 1978. [22] Scruby CB, Drain LE. Laser ultrasonics: techniques and applications. New York: Adam Hilger; 1990. [23] Lee KC, Lin J. Transient deformation of thin metal sheets during pulsed laser forming. Opt Laser Technol 2002;34: 639–48.
Optics & Laser Technology 38 (2006) 544–551 www.elsevier.com/locate/optlastec
Removal mechanisms of micro-scale particles by surface wave in laser cleaning Shin-Chun Hsu, Jehnming Lin Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan Received 21 November 2003; received in revised form 10 October 2004; accepted 29 November 2004 Available online 3 February 2005
Abstract This paper is to investigate the mechanisms of micro-scale particle removal by surface wave, which was induced by a short pulse laser in a cleaning process. The authors analyzed the adhesive forces of particles on substrate surface and the clearance force produced by surface wave in laser cleaning. The physical model of particle removal by laser-induced surface wave was established to predict the removal area and the processing conditions of laser cleaning. In this research, a KrF excimer laser was applied to irradiate 304 stainless steel specimen distributed with copper particles ðo45 mmÞ to generate surface wave for copper particle removal. Considering that a time-varying and uniformly distributed heat source irradiates on material surface with thermao-elastic behavior, the displacement and acceleration of substrate induced by a pulsed laser were solved by an uncoupled thermal–mechanical analysis based on the finite element method. The processing parameters such as laser energy, laser spot size are discussed, respectively. A series of laser cleaning experiments were designed to compare with computation results. The results show that the removal area by surface wave beyond the laser spot increases with the laser energy and that, the surface acceleration decreases with the increase of the laser spot size. r 2005 Elsevier Ltd. All rights reserved. Keywords: Laser cleaning; Surface elastic wave; Particle; Finite element simulation
1. Introduction With the rapid development of the semi-conductor industry in recent years and the trend towards the microelectronics industry, manufacturing smaller integrated circuits makes the particle pollutions of growing importance in affecting quality and yields of the products. Hence, the clearance of particles becomes a critical issue in aforesaid processing. In current semiconductor industry, the clearance of silicon wafer is mainly achieved by wet cleaning method or dry cleaning method. The wet cleaning method needs a great amount of de-ionized water or chemical agents and causes further environmental pollutions. Among the numerous Corresponding author. Tel.: +886 6 2757575x62100; fax: +886 6 2352973. E-mail address: [email protected] (J. Lin).
0030-3992/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2004.11.021
dry cleaning methods, the laser cleaning technique achieved a remarkable cleaning result when the particles range below micrometer and sub-micrometer [1–3]. In recent decade, many investigators devoted themselves to the research of laser cleaning technique. In accordance with the difference of mechanisms, particle removal mechanisms can be divided into (a) steam laser cleaning mechanism [1–4], (b) extra thermal detachment mechanism for particle removal [5–8], (c) the mechanism of the surface acoustic wave for particle removal [9], and (d) plasma shock wave [10,11]. In dry laser cleaning, when a short pulse from a laser irradiates a substrate surface with a low-energy pulse, it will produce a thermal effect in the irradiated zone. Because the thermal conduction of the short pulse laser is negligible outside the laser irradiation region, the thermal expansion of specimen surface thus produces a strong elastic wave, such as acoustic wave beyond the
ARTICLE IN PRESS S.-C. Hsu, J. Lin / Optics & Laser Technology 38 (2006) 544–551
laser spot [9]. Generally speaking, the elastic wave produced on the surface of thick substrate can be divided into compression wave, shear wave, and Rayleigh wave as discussed in details in Ref. [9]. Some investigators used laser interferometer to measure surface waves at different positions from laser source and compared the results with numerical solutions [12], or used piezoelectric transducer, PZT, to measure surface waves in different positions and discussed the influence of laser spot size to surface waves [13]. Most researches restricted the removal areas within the spot of laser beams. The use of laser ablation was promoted to induce nonlinear surface wave and enlarge the removal areas. However, it is very possible to damage the substrate surface [9]. In this study, the laser cleaning experiment was performed with UV laser energy below the damage threshold of substrate material. On this condition, a removal area beyond laser spot was produced. Considering the thermao-elastic behavior of the material, the software ABAQUS based on finite element method was adopted to analyze surface acceleration. The numerical simulation was compared with related experiments of substrate heating by short-pulse laser. The physical model of particle removal by laser-induced surface wave was established, and the influences of processing parameters on the particle removal were discussed.
d
545
particle
zf substrate (a)
d
particle liquid
substrate (b)
d
particle
substrate (c) Fig. 1. Three types of adhesive force between substrate and particles: (a) Van der Waals force, (b) capillary force, and (c) electrostatic force.
2. Physical model 2.1. Surface adhesive force In general terms, the adhesive forces between particles and substrate surface mainly come from Van der Waals force, capillary force, and electrostatic force [14]. The illustrations of the corresponding force models are shown in Fig. 1. It was pointed out in Ref. [14] that when the radius of particles on substrate surface is less than 50 mm; the major acting adhesive force between particles and substrate surface is Van der Waals force, and its value is several times greater than the gravity of particles. Thus, traditional clearance techniques using gravitational effect become ineffective. The Van der Waals force between spherical particles and substrate surface can be described as follows [14]: FV ¼
hd hd 2c þ : 16pz2f 32pz3f
(1)
In Eq. (1), the first term is the Van der Waals force between spherical particle and substrate surface under point contact. While, the second term is the extra adhesive force when particles or substrate surface deforms and the contact area increases, where d is the
diameter of particles, d c is the diameter of contact area, zf is the distance between particles and atoms of substrate surface (about 4 1010 m), h is a Lifshitz– Van der Waals constant and it can be calculated with Hamaker coefficient A by the following equation [15,16]: h¼
4pA : 3
(2)
The Hamaker coefficient of copper is about 40 1020 J and of iron is 21:2 1020 J: Since the major composition of the substrate material is iron, by Eq. (3), the Hamaker coefficient of copper particle at iron substrate is approximately 2:912 1019 J: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ACu2Fe ¼ ACu AFe : (3) 2.2. Surface clearance force Laser beams produce surface wave with significant displacement and acceleration on the substrate surface, and it will remove the particle from the substrate surface mainly due to the inertia effects. Since the vertical displacement of surface wave is about 1.5 times that in horizontal displacement [17], the shear effect is therefore
ARTICLE IN PRESS S.-C. Hsu, J. Lin / Optics & Laser Technology 38 (2006) 544–551
546
z
1E+009 Relation between partice size and cleaning acceleration Cu on Fe without deformation Contact diameter=1% of particle diameter Contact diameter=2% of particle diameter Contact diameter=5% of particle diameter
Critical acceleration (m/s2)
1E+008
Laser beam
4mm 1E+007
r
0
Fig. 3. Geometric domain of specimen. 1E+006
z 1E+005
1E+004 0
10
20
30
40
50
Particle diameter (µm)
Fig. 2. The relationship between critical acceleration and particle diameter.
neglected in the analysis. When the surface clearance force is larger than surface adhesive force, the particle is removed. Thus, the clearance force is expressed as f ¼ man ;
(4)
where m is the mass of the particle, and an is the acceleration in vertical direction. From (1) and (4), the critical acceleration to remove a particle is obtained as acr ¼
A Ad 2c þ : 2prz2f d 2 4prz3f d 3
(5)
In Fig. 2, the relations between critical acceleration and particle diameter are shown. It is clearly found that the value of contact area will significantly affect the critical acceleration. With the decrease of particle diameter, the critical acceleration increases dramatically. Thus, it is difficult to remove the particles in small diameters.
3. Numerical simulation The surface wave induced by laser is a nonlinear physical phenomenon, which includes interactions among mechanics, heat transfer, and microstructure. To simplify the analysis, an uncoupled thermal–mechanical scheme was adopted to divide the simulation into heat transfer and mechanism models. This means that the stress result will not influence the temperature field. Because the area of laser clearance is much smaller than the specimen area, the boundary of the specimen was neglected in the simulation. As shown in Fig. 3, it is considered as a laser beam irradiate on an infinite disc
r
0
Fig. 4. Finite element mesh of specimen.
with thickness of 4 mm. The analysis was simplified as a two-dimensional problem due to the symmetry of the computation domain. Four-node elements DCAX4 [18] were used in the heat transfer analysis for a 2D cylindrical domain divided by 363 36 nodes at r- and z-axis, respectively. The mesh distributes over the area of 18.1 mm in diameter and 4 mm in depth, as shown in Fig. 4. Because there is an intense temperature gradient near the laser beam, the grid structure must be fine meshed in the heating area. In the mechanical analysis, the grid structure is similar to that of heat transfer analysis. However, CAX4R [18] stress/displacement elements were adopted. Infinite elements CINPE4 [18] were used to treat the boundary condition. Therefore the influence of wave reflection on the boundary was neglected in the simulation. A SUS304 stainless steel was used as substrate material. The nonlinear effects of temperature dependent properties such as thermal expansion coefficient, heat conduction coefficient, specific heat, density, Poisson’s ratio, and Young’s modulus have been considered in the simulation [19]. 3.1. Heat transfer analysis In heat transfer analysis, the following assumptions were adopted to simplify the simulation: 1. The skin depth of laser absorption was neglected and the laser energy is in a form directly heating the specimen surface. 2. The energy distribution of the laser beam is uniform.
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3. Heat loss occurred by convection effect and the thermal radiation was neglected. 4. The temperature variations due to the phase change and deformation were neglected. The time-varying function I s ðtÞ of the laser power density absorbed by the specimen surface was expressed as Eq. (6) [20] OE 0 ð1 Rf Þ 8t3 2t2 =t2 e I s ðtÞ ¼ ; (6) t4 pR2 where I s ðtÞ is the time-varying heat flux ðW=m2 Þ on specimen surface, Rf is the reflectivity coefficient of specimen material, R is the radius of laser beam (m), E 0 is the laser energy (J/pulse), O is the percentage of surface area irradiated by laser beam (%), and t is the period of pulse (s). The reflectivity of the specimen with polished surface was assumed as a constant reflectivity of 0.38 in this study [21]. 3.1.1. Initial and boundary conditions Assuming the initial temperature is the ambient temperature, namely, Tðx; y; z; 0Þ ¼ 25 C: In heat transfer analysis, since the heat affected area due to short pulse laser is very small and the thermal diffusion length is only several micrometers for stainless steel [22]. Thus, it was assumed that there is only heat convection on the laser radiation surface and adiabatic on all other surfaces. That is,
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Since the specimen was clamped in the processing and the beam center is aligned to the cylindrical axis of the domain, there is no displacement in r direction and other surfaces are unconstrained. The boundary conditions were simplified as ur ðr; z; tÞ ¼ 0; ur ðr; z; tÞ ¼ 0;
r ¼ 0 mm; r ¼ 0 mm; z ¼ 0 mm;
uz ðr; z; tÞ ¼ 0;
r ¼ 0 mm; z ¼ 0 mm:
4. Numerical results 4.1. Computation verification The simulation of laser-induced surface wave was first verified by experimental results from Ref. [22]. In the numerical analysis adopting the aforesaid assumptions and grid structure, the results shown in Fig. 5 are for the displacement at 10 mm on the irradiated surface from the center of laser beam spot. It shows that the surface wave velocities in the simulation and experiment are in a good agreement. The process parameters in the related cases are: the specimen thickness is 4 mm; the surface area occupancy rate of copper particles is 25%; laser energy is 4 mJ/ pulse; the laser spot size is 1 mm; the pulse period is 30 ns. Fig. 6 shows the vertical components of the surface displacement and acceleration at 5 mm from the center of laser beam spot. Since the numerical fluctuation has
qT ¼ hc ðT T 1 Þ; z ¼ 4 mm ðtop surfaceÞ; qn qT ¼ 0; on other surfaces except for z ¼ 4 mm; k qn k
3.2. Stress analysis In the analysis of the displacement and acceleration of the substrate, it was assumed as follows: 1. The material is isotropic. 2. The material is stress free initially. 3. There is no influence of strain rate on the material properties. 4. The material is under thermao-elastic behavior and the force is within elastic limit.
Experiment ABAQUS simulation
1
Normalized displacement
where the heat convection coefficient hc ¼ 21 W=m2 C [23]. The temperature at each node of the domain was calculated in according to the heat input, material properties and boundary conditions. The transient temperature fields were then substitutes as the thermal loading into mechanical model for the stress analysis.
2
0
-1
-2
-3 0
1e-006
2e-006
3e-006
4e-006
5e-006
Time (s)
Fig. 5. Displacement results at 10 mm from the center of the laser beam by simulation and experiment [22].
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5
Displacement before filtering Displacement after filtering Acceleration after filtering
1e-009
Energy=4mJ/pulse FWHM=30ns Beam diameter=1mm Acceleration Critical acceleration
6.0E+005
4
2.0E+005
0
0.0E+000 -5e-010 -2.0E+005 -1e-009 -4.0E+005
-1.5e-009
Max-acceleration (×105m/s2)
4.0E+005
Acceleration (m /s2)
Displacement (m)
5e-010
-6.0E+005 0
1e-006
2e-006
3
2
1
3e-006
Time (s)
Fig. 6. The vertical displacement and acceleration at 5 mm from the center of laser beam.
0 0
1
2
3
4
5
Distance from the laser heated area (mm)
Fig. 7. Maximum acceleration at different positions compared with the critical acceleration of copper particles with a particle diameter of 20 mm and 1% diameter at the contact surface.
1.2 FWHM= 30ns Beam diameter=1mm
0.8
L (mm)
caused a difficulty in the analysis, the fluctuations were thereby deleted with a cut-off frequency. It is found that the maximum acceleration occurred at the largest negative displacement of surface wave. If the acceleration is larger than the critical acceleration ac ; then the particle removal is achieved. If the processing conditions and critical acceleration for particle removal are specified, the effective cleaning area will be predicted. By Eq. (5) with the conditions of copper particles in a diameter of 20 mm and 1% of the particle diameter at the contact area with substrate, the critical and maximum accelerations corresponding to different positions are shown in Fig. 7. It can be seen that the threshold distance with a surface acceleration equal to the critical acceleration is about 0.75 mm from the laser beam center. Thus, besides the particles within the laser spot, the particles could be moved by elastic surface wave in the area beyond the laser spot.
0.4
4.2. Parametric analysis The processing parameters such as laser energy and laser spot size might dominate the characteristics of surface wave, thus these two parameters were analyzed in details. The laser energy was set at 2.5–4.5 mJ/pulse, and rest of the parameters were constants. Fig. 8 gives the relationship between different laser energy and removal area in computation. L is the distance beyond laser spot and defined as follows: L ¼ Rc R; where Rc is the radius of the removal area. It is evident in Fig. 8 that the energy of laser pulse increases with the laser spot size and removal area.
0 2.5
3
3.5
4
4.5
Pulse energy (mJ /pulse)
Fig. 8. Length of removal area beyond laser spot under various laser energies.
Considering the removal area with laser spot sizes set at 0.6–1.0 mm at the same laser energy density, the effective distance of removal was obtained by the same method of prediction. The ratios of removal distance beyond laser spot to the radius of laser spot with various laser spot are shown in Fig. 9, where the definition of J is
ARTICLE IN PRESS S.-C. Hsu, J. Lin / Optics & Laser Technology 38 (2006) 544–551 Attenuator
8
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Homogenizer
Mask Mirror
Excimer laser Lens group
6
Condenser lens
J
10X Lens
:Particle
Extractor
4
Sample Computer
2
X-Y-Z-U controller
X-Y-Z-U Stage
Vacuum Chuck
Fig. 10. Apparatus setup of laser cleaning experiment.
Table 1 Specimen dimension and processing parameters
0 0.6
0.7
0.8
0.9
1
Beam diameter (mm)
Fig. 9. The ratio of length of removal area beyond laser spot to the radius of laser beam under various laser beam sizes.
expressed as L : R It can be found that the removal mechanism on the region beyond the laser spot is caused by the propagation of elastic surface wave and the removal mechanism within the laser spot is mainly caused by direct thermal expansion. As shown in Fig. 9, the smaller the laser spot the larger the ratio J. It means that elastic surface wave has a significant influence on the removal area outside the laser spot, especially for a small spot size. J¼
5. Experiments The experimental setup is shown in Fig. 10. A KrF excimer laser was used as the laser source (wavelength of 248 nm). The pulse duration is about 30 ns with the maximum energy output of 400 mJ/pulse. The laser beam was shaped by a mask with a center hole of different radii and passed to a lens set to focus it on the surface of specimen with a round uniform distribution of the laser power density. The substrate material is annealed 304 stainless steel, which has been polished, dried and distributed with copper particles with sizes between 10–45 mm on the top surface of substrate. Table 1 shows the dimensions of the specimen and processing parameters in the experiments. In Fig. 11, the results of cleaning with laser energy 4 mJ and 1 mm radius of laser spot are shown. The
Specimen dimensions Length Width Thickness
100 mm 60 mm 4 mm
Processing parameters Laser energy Pulse duration Spot size Percent area of particle
2.5–4.5 mJ/pulse 30 ns 0.6–1 mm 25%
bright spots on the picture are copper particles and the dark region is stainless steel substrate. It is clearly shown that the removal area is not restricted by laser spot area and the removal effects are beyond the spot area. Fig. 12 is the enlarged image of Fig. 11. It can be seen that the larger copper particles are located at the outer ring from the center of the irradiated zone. Thus, to remove small particles needs large surface acceleration. Also, it can be seen that a tendency of the acceleration caused by surface wave decreases with the increase of distance from the beam center. Fig. 13 shows the comparison of experiment and numerical simulation results of removal areas under various laser energies. Both have consistent tendencies: the removal areas increase with laser energy. However, the errors might come from the roughness of substrate surface and the irregular shapes of copper particles. Since the contact area is much greater than the assumed, the larger removal acceleration is needed for the real cases. Also the drag forces due to the ambient air and moisture may cause the difficulties in particle removal. Fig. 14 shows the experiment and numerical simulation results of removal areas under various laser spot sizes. The ratio of removal distance beyond laser spot to the radius of laser spot increase with the decrease
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Beam diameter=1mm Experiment Simulation
L (mm)
1.2
0.8
0.4
0 2.5
3
3.5
4
4.5
Pulse energy (mJ/pulse)
Fig. 13. Comparison of experimental and numerical results of the length L under various laser energies.
Fig. 11. The cleaning result with laser energy of 4 mJ/pulse and laser diameter of 1 mm: (a) before cleaning, and (b) after cleaning.
J
Experiment Simulation
1
0.1 0.6
0.7
0.8
0.9
1
Beam diameter (mm)
Fig. 14. Experimental and numerical results of the ratio J under various laser beam diameters.
6. Conclusions
Fig. 12. Enlarged image of Fig. 11(b).
of spot radii for both approaches. The influences of surface wave on removal area are significant for small beam sizes.
According to the predications of the removal surface, the areas beyond the laser spot increase with the laser energy. The clearance effect due to thermal expansion on substrate is not only the consideration, but the wave propagation on the areas beyond the laser spot is also significant, especially for small laser beam sizes. The
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efficiency of cleaning was mainly affected by laser energy, size of laser beam, and the material properties. References [1] Imen K, Lee SJ, Allen SD. Laser-assisted micron scale particle removal. Appl Phys Lett 1991;58:203–5. [2] Zapka W, Ziemlich W, Tam AC. Efficient pulsed laser removal of 0:2 mm sized particle from a solid surface. Appl Phys Lett 1991;58:2217–9. [3] Tam AC, Leung WP, Zapka W, Ziemlich W. Laser-cleaning techniques for removal of surface particulates. J Appl Phys 1992;71:3515–23. [4] Imen K, Lee SJ, Allen SD. Shock wave analysis of laser assisted particle removal. J Appl Phys 1993;74:7044–7. [5] Kelley JD, Hovis FE. A thermal detachment mechanism for particle removal from surfaces by pulsed laser irradiation. Microelectron Eng 1993;20:159–70. [6] Lu YF, Song WD, Ye KD, Lee YP. A cleaning model for removal of particles due to laser-induced thermal expansion of substrate. Jpn J Appl Phys 1997;36:1304–6. [7] Lu YF, Song WD, Ang BW, Hong MH, Chan DSH, Low TS. A theoretical model for laser removal of particles from solid surfaces. Appl Phys A 1997;65:9–13. [8] Lee JM, Curran C, Watkins KG. Laser removal of copper particles from silicon wafers using UV, visible and IR radiation. Appl Phys A 2001;73:219–24. [9] Kolomenskii AA, Schuessler HA, Mikhalevich VG, Maznev AA. Interaction of laser-generated surface acoustic pulses with fine particles: surface cleaning and adhesion studies. J Appl Phys 1998;84:2404–10. [10] Curran C, Watkins KG, Lee JM. Shock pressure measurements for removal of particles of sub-micron dimensions from silicon wafers. Section D-ICALEO 2002.
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[11] Lee JM, Cho SH, Park JG, Lee SH, Kang YJ, Han YP, Kim SY. Laser induced shock cleaning of silicon wafers. Section D-ICALEO 2002. [12] Scruby CB, Moss BC. The launching of Rayleigh waves from surface point sources. In: Ash EA, Paige EGS, editors. Rayleigh wave theory and application, 1985. p. 102–9. [13] Scala CM, Doyle PA. Time- and frequency-domain characteristic of laser-generated ultrasonic surface waves. J Acoust Soc Am 1989;85:1569–76. [14] Bowling RA. A theoretical review of particle adhesion. In: Mittal KL, editor. Particles on surface 1: detection, adhesion, and removal, 1988. p. 129–42. [15] Osborne-Lee IW. Calculation of Hamaker coefficients for metallic aerosols from extensive optical data. In: Mittal KL, editor. Particles on surface 1: detection, adhesion, and removal, 1988. p. 77–90. [16] Ranade MB. Adhesion and removal of particles: effect of medium. In: Mittal KL, editor. Particles on surface 1: detection, adhesion, and removal, 1988. p. 179–91. [17] Viktorov IA. Rayleigh and lamb waves. New York: Plenum Press; 1967. [18] ABAQUS User’s Manual Volume II, Pawtuckett, RI: H.K.S. Inc., 1998. [19] Peckner D, Bernstein IM. Handbook of stainless steels. New York: McGraw-Hill; 1977. [20] Lee JH, Burger CP. Finite element modeling of laser generation lamb waves. Comput Struct 1995;54:499–514. [21] Ready JF. Industrial applications of lasers. New York: Academic Press; 1978. [22] Scruby CB, Drain LE. Laser ultrasonics: techniques and applications. New York: Adam Hilger; 1990. [23] Lee KC, Lin J. Transient deformation of thin metal sheets during pulsed laser forming. Opt Laser Technol 2002;34: 639–48.