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Liquid film enhanced laser cleaning
Microelectronic Engineering 17 (1992) 4731178 Elsevier
LIQUID W.
*IBM
FILM
ENHANCED
413
LASER
Zapka, A.C. Tam l, G. Ayers
CLEANING
*, W. Ziemlich
IBM GMTC, Max-Eyth-Str. 6, D-7032 Sindelfingen Almaden Research Center, San Jose, CA 95120,
USA
Abstract ‘Liquid Film Enhanced Laser Cleaning’ is a technique 1;) remove particulate contamination from surfaces. A thin liquid film deposited onio the contaminated surface is flood irradiated with a short laser pulse. The resulting sudden evaporation of the liquid film leads to high transient explosive forces, large enough to expel even submicron particles from the surface. For instance O.l/Jm alumina particles could elficiently be removed from a silicon surface with KrF excimer laser irradiation of 16 ns pulse length and 120 mJ/cm* energy density. 1. Introduction Manufacturing yield of microelectronic devic:es and the performance of data storage devices are all seriously affected by particulate contamination. Small particles of size 0.2 - 0.1 p.rn can cause fatal damage with current products. With the continuous trend towards smaller device dimensions we will have to be concerned about particulates smaller 0.1 /Lm. Because of the nature of the adhesion forces these smaller particles are especially difficult to remove /I/. Since traditional cleaning techniques using wiping, scrubbing, etching, pressurized gas, liquid, or fine-particle jets, ultrasonic/megasonic, and various liquid immersion techniques are considered to be ineffective for removing small particulates (especially submicron ones) /2,3,4/. There is a need to develop alternative techniques, specially suited for removal of micron and submicron particles. We have previously reported on ‘laser cleaning’, a new technique, which makes use of short pulse excimer laser radiation to remove micron and submicron particles horn silicon surfaces /5,6/. Recently we have considerably enhanced the cleaning efficiency of our laser technique by removing a thin liquid film - as a booster - together with the particulate contamination /7/. This ‘liquid film enhanced laser cleaning’ will be described in the following. 2. Liquid
film enhanced
laser cleaning
When a thin liquid film coating is present on the sample surface during short pulse laser irradiation the sudden heating of,the liquid filrn leads to its ‘ablation’ or explosive evaporation. The resulting large transient expansion forces can overcome lhe particle-to-substrate adhesion forces of particulate contamination present on the surface. Liquid film ablation can be obtained by both, short pulse UV- or IR-laser radiation. Since we have observed best cleaning efficiencies with UV-excimer lasers we will concentrate on this approach. and briefly mention laser cleaning results with l,R-lasers later. Our cleaning technique involves a shott pulse excimer laser, that is strongly absorbed by the substrate surface, coupled with the controlled deposition of a thin layer of a suilable liquid (typically water; of thickness on the order of microns) onto the particle-contaminated surface before the pulsed laser irradiation. This technique is schematically indicated in fig. 1. The pulse length we have used so far (typically about 16 nsec) corresponds to a thermal diffusion length of about 1 micron in Si and about 0.1 micron in water; this produces very efficient heating of the liquid/substrate interface to produce superheating and explosive evaporation of the liquid film initiating at this interface. In principle, the shorter is the pulse duration T, the shorter is the thermal diffusion length 4&l’* in the liquid and in the substrate, where D is the thermal diffusivity in the medium concerned. Shorter diffusion length means less incident energy needed to achieve the explosive evaporatior: of the liquid film within its thermal diffusion length. More importantly, shorter T
0167-9317/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved.
W. Zapka et al. I Liquid
filmenhanced laser cleaning
means that higher superheating of this thin liquid interface can be attained to produce higher explosion pressure. The lower limit to T is due to material damage caused by the high intensity associated with very short pulses. The upper limit to z is due to the decreasing capability to superheat the liquid film at the interface when T exceeds - 1 jcsec. There is some experimental evidence that laser cleaning is less effective for psec laser pulse and very ineffective for msec laser pulse under similar conditions (laser fluence and wavelength) as the 10 nsec laser pulse. 3. Experiments
and results
of ‘laser
cleaning’
with
excimer
laser
radiation
Our experimental set-up is shown in Fig. 2. A KrF excimer laser (Lumonix Hyperex 400) delivering up to 400 mJ per pulse in 16 ns pulses at 248 nm irradiates the sample surface. Angles of incidence can be varied between 0 and 90”. An aperture, which is projected onto the sample defines the target area. Liquid films of micron thickness are intrinsically unstable at ambient air, so that we apply such liquid films in a pulsed fashion onto the surface suitably preceeding the laser pulse. As is shown in fig. 2 this heater contains the certain liquid at a ternperature typically some 15” C above room temperature. A burst of gas (air, nitrogen or helium) drives a small volume of saturated vapor out of a heated nozzle onto the sample surface. Here it condenses and forms the desired thin liquid film, which lasts a few thenths of a second before it evaporates. The timing generator allows to vary the time interval between the liquid film deposition and the laser irradiation and thereby to vary the liquid film thickness at the laser irradiation. We typically work with time intervals of 0.15 s giving liquid film thicknesses of estimated a few microns. Best cleaning results were achieved with a liquid consisting of mainly water (- 90 %) with the remainder being e.g. ethanol or isopropanol to enhance wetting. Particulates of various material (e.g. gold, alumina, silicon, latex) were deposited out of a liquid solution (typically a water-ethanol mixture) onto substrates like silicon wafers or silicon membrane masks. As shown in fig. 3 we were able to almost completely clean a silicon wafer surface from 0.2/Lm gold spheres. 4 KrF excimer laser pulses of 200 mJ/cm2, and a liquid iIlm of condensed water of estimated micron thickness were used. With ‘dry’ laser cleaning, i.e. without the presence of a thin liquid film during laser irradiation, we could not remove the gold particles at this laser fluence. Increasing the laser fluence did not help either, but resulted in melting and coalescing of the gold spheres. In another experiment we cleaned a silicon random sh;,pe. 20 KrF excimer laser pulses applied. Cleaning efficiency was estimated
wafer surface from O.l/Lm alumina particles of of 120 mJ/cm* and water films of micron size were well above 90%, as seen in fig. 4.
It is of importance to note that ‘liquid film enhanced technique’. Since after each pulsed laser irradiation sample is essentially dry after treatment. 4. Discussion
of cleaning
laser cleaning’ indeed is a ‘DRY cleaning the liquid film is completely removed, the
mechanism
How can the liquid film provide the enormous boost to detach and expel particles? Taking into account the optical absorptivity, density, specific heat and thermal diffusion lengths of e.g. silicon and water, a 40 mJ/cm* KrF excimer laser pulse should heat up the water film at the interface to 375OC, ihe critical temperature of water, within nanoseconcls. Assuming such strong superheating before evaporation, this would result in explosive evaporation at pressures up to about 200 bar. Pressure in this range can produce forces of approximately 2 dynes perpendicular to the surface on a lprn particle, thus exceeding the strong adhesion forces of approximately IO million times the weight of such particle. Such an ‘explosive force’ scales ad d2, where d is the particle diameter, and so it decreases as a higher order of d than the adhesion forces which scale as d. We estimate that such explosive forces should be adequate to overcome adhesion forces at least down to d M O.l/Am. The high explosive forces can as well explain our observation that the steam - and probably the detached particles as well - are ejected from the surface at supersonic speed in a narrow jet perpendicular to the surface. The jet comes to rest and forms a stationary cloud at a distance some IO to 15 mm from the surface as seen in fig. 5.
W. Zapka et al. I Liquid jilm enhanced
5. Results
on IR-laser
usage for ‘liquid
film enhanced
475
laser cleaning
laser cleaning’
‘Liquid film enhanced laser cleaning’ with pulsed IR-lasers was reported by S. Allen and coworkers /8,9/. Contrary to UV-lasers being almost completely transmitted through water, the 10.6pm radiation of CO, lasers is partially absorbed in water films of micron thickness, and the 2.9jLm radiation of an Er : YAG laser is completely absorbed already in the upper layers of a 1 micrometer thick water film (penetration depth O.Qm). Though the direct heating of the water film may appear of advantage in view of ‘liquid film enhanced laser cleaning’ we experimentally find its performance inferior to UV excimer laser cleaning, especially when dealing with substrates like silicon. In fact, ‘liquid film enhanced laser cleaning’ with Er : YAG laser pulses of 10 ns pulse length did not yield any significant removal of 0.2iLm gold particles on silicon wafers (compare fig. 3 for excimer laser cleaning). While no effect on particle density was observed at low fluences, higher irradiation fluences resulted in sirong melting and coalescing of the gold spheres on the silicon wafer surface. 6. Conclusions We report here on our recent improvements on laser cleaning for removal of particles from surfaces. Particulate size range of optimum performance of this technique appears to be between IOpm down to O.l/tm size. Such particles adhere with enormous forces and are difficult to remove with traditional cleaning techniques. We observe that the most efficient laser cleaning of small particles, down to 0.1 micrometer, from a silicon wafer surface is obtained by using a UV-laser with pulse duration of 16 ns and a wavelength strongly absorbed in the surface, and a water film pulse deposited onto the surface prior to the pulsed laser irradiation. The laser fluence is chosen typically in the range of 40 - 300 mJ/cm”, depending on the types of contaminants to be removed and on the substrate damage threshold. The water film at the irradiated area is completely removed by the laser irradiation. carrying the particulate contamination with it, and leaving an essentially DRY surface. We therefore regard ‘liquid film enhanced laser cleaning’ a DRY cleaning technique. 7. Acknowledgements We thank our colleagues Wing P. Leung and Peter Leung for their valuable participation in this work. We also thank Alan D. Wilson of IBM T.J. Watson Research Center for his support, and Susan Allen of U. of Iowa and J.D. Kelley of McDonnell Douglas Res. Lab. for useful discussions.
8. References 1.
J.M.
Dt.:ffalo
2.
R.A. Bowling,
3.
M.B.
4.
P.E. Ross,
5.
W. Zapka, A.C. Tam, lishers, 1991, p. 547
6.
W. Zapka, surfaces’,
K. Asch, European
7.
W. Zapka,
W. Ziemlich
8.
K. Imen, S.J. Lee and S.D. Allen, ence on Lasers and Electra-Optics
‘Laser assisted micron scale particle CLEO’SO Conf. Digest, p. 228, 1991
9.
K. Imen,
Appl.
ranade,
and J.R. Monkowsku, in ‘Particles ‘Aerosol
‘Scientific
Solid
on Surfaces’,
Sci. Techn.‘, American’, W. Ziemlich,
State Vol.
Techn.,
p. 109, March
1, K.L. Mittal,
Plenum,
1988, p. 129
Vol. 7, p. 161, 1987
p. 88, June ‘Microcircuit
1990 Engineering
J. Keyser and K. Meissner. ‘Removal patent EP 0 297 506 A2, Jan. 4. 1989 and A.C. Tam,
S.J. Lee and S.D. Allen,
Editor,
1984
Appl.
Phys.
Phys.
Lett.,
Lett.,
90’, Elsevier
of particles
Science
from
Pub-
solid-state
Vol 58, p. 2217, 1991
Vol. 58, p. 203, 1991
removal’.
Confer-
W. Zapka et al. / Liquid film enhanced laser cleaning
476
Fig 1: Schematic of ‘liquid film enhanced laser cleaning’ based on the case of strong substrate absorption and a covering thin-film of transparent liquid. The density of black dots on the substrate (Si here) indicates the steep temperature gradient at the opaque substrate, producing explosive interfacial boiling.
_________________-~~~~ __________-_________-___--_______________---C _____________________________________________ f i 1m
Fig 2: Experimental arrangement for liquid-film enhanced pulsed-laser cleaning to remove particulates on a surface
Later
Beam Hcmoganirer
52 t
Fig. 3 SEM photos of laser cleaned Si wafer (left), and reference area still covered with 0.2jLm gold parlicles (right). 4 KrF excimer laser pulses at 200 mJlcm2 ‘Nere used for laser cleaning with a water film of micrometer size thickness. l/m marker irj SFM photo
Gas Valve
Pulrs
Control
W. Zapka et al. / Liquid film enhanced laser cleaning
411
Fig. 4a and 4b: SEM photos showing ‘liquid film enhanced laser cleaning’ of a silicon wafer with KrF laser irradiation (20 pulses, 120 mJ/cm*). Particle contaminaiion was O.l/rm alumina polishing powder. Fig. 4a shows border line of irradiated area (SE’M marker Ir)O/rm), while fig. 4b shows cleaned area (SEM marker llrm).
Fig. 5 Photo and schematic of material
of ‘liquid
filrn enhanced
laser
cleaning’
shows
ejection
of fast, narrow
jet
LIQUID W.
*IBM
FILM
ENHANCED
413
LASER
Zapka, A.C. Tam l, G. Ayers
CLEANING
*, W. Ziemlich
IBM GMTC, Max-Eyth-Str. 6, D-7032 Sindelfingen Almaden Research Center, San Jose, CA 95120,
USA
Abstract ‘Liquid Film Enhanced Laser Cleaning’ is a technique 1;) remove particulate contamination from surfaces. A thin liquid film deposited onio the contaminated surface is flood irradiated with a short laser pulse. The resulting sudden evaporation of the liquid film leads to high transient explosive forces, large enough to expel even submicron particles from the surface. For instance O.l/Jm alumina particles could elficiently be removed from a silicon surface with KrF excimer laser irradiation of 16 ns pulse length and 120 mJ/cm* energy density. 1. Introduction Manufacturing yield of microelectronic devic:es and the performance of data storage devices are all seriously affected by particulate contamination. Small particles of size 0.2 - 0.1 p.rn can cause fatal damage with current products. With the continuous trend towards smaller device dimensions we will have to be concerned about particulates smaller 0.1 /Lm. Because of the nature of the adhesion forces these smaller particles are especially difficult to remove /I/. Since traditional cleaning techniques using wiping, scrubbing, etching, pressurized gas, liquid, or fine-particle jets, ultrasonic/megasonic, and various liquid immersion techniques are considered to be ineffective for removing small particulates (especially submicron ones) /2,3,4/. There is a need to develop alternative techniques, specially suited for removal of micron and submicron particles. We have previously reported on ‘laser cleaning’, a new technique, which makes use of short pulse excimer laser radiation to remove micron and submicron particles horn silicon surfaces /5,6/. Recently we have considerably enhanced the cleaning efficiency of our laser technique by removing a thin liquid film - as a booster - together with the particulate contamination /7/. This ‘liquid film enhanced laser cleaning’ will be described in the following. 2. Liquid
film enhanced
laser cleaning
When a thin liquid film coating is present on the sample surface during short pulse laser irradiation the sudden heating of,the liquid filrn leads to its ‘ablation’ or explosive evaporation. The resulting large transient expansion forces can overcome lhe particle-to-substrate adhesion forces of particulate contamination present on the surface. Liquid film ablation can be obtained by both, short pulse UV- or IR-laser radiation. Since we have observed best cleaning efficiencies with UV-excimer lasers we will concentrate on this approach. and briefly mention laser cleaning results with l,R-lasers later. Our cleaning technique involves a shott pulse excimer laser, that is strongly absorbed by the substrate surface, coupled with the controlled deposition of a thin layer of a suilable liquid (typically water; of thickness on the order of microns) onto the particle-contaminated surface before the pulsed laser irradiation. This technique is schematically indicated in fig. 1. The pulse length we have used so far (typically about 16 nsec) corresponds to a thermal diffusion length of about 1 micron in Si and about 0.1 micron in water; this produces very efficient heating of the liquid/substrate interface to produce superheating and explosive evaporation of the liquid film initiating at this interface. In principle, the shorter is the pulse duration T, the shorter is the thermal diffusion length 4&l’* in the liquid and in the substrate, where D is the thermal diffusivity in the medium concerned. Shorter diffusion length means less incident energy needed to achieve the explosive evaporatior: of the liquid film within its thermal diffusion length. More importantly, shorter T
0167-9317/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved.
W. Zapka et al. I Liquid
filmenhanced laser cleaning
means that higher superheating of this thin liquid interface can be attained to produce higher explosion pressure. The lower limit to T is due to material damage caused by the high intensity associated with very short pulses. The upper limit to z is due to the decreasing capability to superheat the liquid film at the interface when T exceeds - 1 jcsec. There is some experimental evidence that laser cleaning is less effective for psec laser pulse and very ineffective for msec laser pulse under similar conditions (laser fluence and wavelength) as the 10 nsec laser pulse. 3. Experiments
and results
of ‘laser
cleaning’
with
excimer
laser
radiation
Our experimental set-up is shown in Fig. 2. A KrF excimer laser (Lumonix Hyperex 400) delivering up to 400 mJ per pulse in 16 ns pulses at 248 nm irradiates the sample surface. Angles of incidence can be varied between 0 and 90”. An aperture, which is projected onto the sample defines the target area. Liquid films of micron thickness are intrinsically unstable at ambient air, so that we apply such liquid films in a pulsed fashion onto the surface suitably preceeding the laser pulse. As is shown in fig. 2 this heater contains the certain liquid at a ternperature typically some 15” C above room temperature. A burst of gas (air, nitrogen or helium) drives a small volume of saturated vapor out of a heated nozzle onto the sample surface. Here it condenses and forms the desired thin liquid film, which lasts a few thenths of a second before it evaporates. The timing generator allows to vary the time interval between the liquid film deposition and the laser irradiation and thereby to vary the liquid film thickness at the laser irradiation. We typically work with time intervals of 0.15 s giving liquid film thicknesses of estimated a few microns. Best cleaning results were achieved with a liquid consisting of mainly water (- 90 %) with the remainder being e.g. ethanol or isopropanol to enhance wetting. Particulates of various material (e.g. gold, alumina, silicon, latex) were deposited out of a liquid solution (typically a water-ethanol mixture) onto substrates like silicon wafers or silicon membrane masks. As shown in fig. 3 we were able to almost completely clean a silicon wafer surface from 0.2/Lm gold spheres. 4 KrF excimer laser pulses of 200 mJ/cm2, and a liquid iIlm of condensed water of estimated micron thickness were used. With ‘dry’ laser cleaning, i.e. without the presence of a thin liquid film during laser irradiation, we could not remove the gold particles at this laser fluence. Increasing the laser fluence did not help either, but resulted in melting and coalescing of the gold spheres. In another experiment we cleaned a silicon random sh;,pe. 20 KrF excimer laser pulses applied. Cleaning efficiency was estimated
wafer surface from O.l/Lm alumina particles of of 120 mJ/cm* and water films of micron size were well above 90%, as seen in fig. 4.
It is of importance to note that ‘liquid film enhanced technique’. Since after each pulsed laser irradiation sample is essentially dry after treatment. 4. Discussion
of cleaning
laser cleaning’ indeed is a ‘DRY cleaning the liquid film is completely removed, the
mechanism
How can the liquid film provide the enormous boost to detach and expel particles? Taking into account the optical absorptivity, density, specific heat and thermal diffusion lengths of e.g. silicon and water, a 40 mJ/cm* KrF excimer laser pulse should heat up the water film at the interface to 375OC, ihe critical temperature of water, within nanoseconcls. Assuming such strong superheating before evaporation, this would result in explosive evaporation at pressures up to about 200 bar. Pressure in this range can produce forces of approximately 2 dynes perpendicular to the surface on a lprn particle, thus exceeding the strong adhesion forces of approximately IO million times the weight of such particle. Such an ‘explosive force’ scales ad d2, where d is the particle diameter, and so it decreases as a higher order of d than the adhesion forces which scale as d. We estimate that such explosive forces should be adequate to overcome adhesion forces at least down to d M O.l/Am. The high explosive forces can as well explain our observation that the steam - and probably the detached particles as well - are ejected from the surface at supersonic speed in a narrow jet perpendicular to the surface. The jet comes to rest and forms a stationary cloud at a distance some IO to 15 mm from the surface as seen in fig. 5.
W. Zapka et al. I Liquid jilm enhanced
5. Results
on IR-laser
usage for ‘liquid
film enhanced
475
laser cleaning
laser cleaning’
‘Liquid film enhanced laser cleaning’ with pulsed IR-lasers was reported by S. Allen and coworkers /8,9/. Contrary to UV-lasers being almost completely transmitted through water, the 10.6pm radiation of CO, lasers is partially absorbed in water films of micron thickness, and the 2.9jLm radiation of an Er : YAG laser is completely absorbed already in the upper layers of a 1 micrometer thick water film (penetration depth O.Qm). Though the direct heating of the water film may appear of advantage in view of ‘liquid film enhanced laser cleaning’ we experimentally find its performance inferior to UV excimer laser cleaning, especially when dealing with substrates like silicon. In fact, ‘liquid film enhanced laser cleaning’ with Er : YAG laser pulses of 10 ns pulse length did not yield any significant removal of 0.2iLm gold particles on silicon wafers (compare fig. 3 for excimer laser cleaning). While no effect on particle density was observed at low fluences, higher irradiation fluences resulted in sirong melting and coalescing of the gold spheres on the silicon wafer surface. 6. Conclusions We report here on our recent improvements on laser cleaning for removal of particles from surfaces. Particulate size range of optimum performance of this technique appears to be between IOpm down to O.l/tm size. Such particles adhere with enormous forces and are difficult to remove with traditional cleaning techniques. We observe that the most efficient laser cleaning of small particles, down to 0.1 micrometer, from a silicon wafer surface is obtained by using a UV-laser with pulse duration of 16 ns and a wavelength strongly absorbed in the surface, and a water film pulse deposited onto the surface prior to the pulsed laser irradiation. The laser fluence is chosen typically in the range of 40 - 300 mJ/cm”, depending on the types of contaminants to be removed and on the substrate damage threshold. The water film at the irradiated area is completely removed by the laser irradiation. carrying the particulate contamination with it, and leaving an essentially DRY surface. We therefore regard ‘liquid film enhanced laser cleaning’ a DRY cleaning technique. 7. Acknowledgements We thank our colleagues Wing P. Leung and Peter Leung for their valuable participation in this work. We also thank Alan D. Wilson of IBM T.J. Watson Research Center for his support, and Susan Allen of U. of Iowa and J.D. Kelley of McDonnell Douglas Res. Lab. for useful discussions.
8. References 1.
J.M.
Dt.:ffalo
2.
R.A. Bowling,
3.
M.B.
4.
P.E. Ross,
5.
W. Zapka, A.C. Tam, lishers, 1991, p. 547
6.
W. Zapka, surfaces’,
K. Asch, European
7.
W. Zapka,
W. Ziemlich
8.
K. Imen, S.J. Lee and S.D. Allen, ence on Lasers and Electra-Optics
‘Laser assisted micron scale particle CLEO’SO Conf. Digest, p. 228, 1991
9.
K. Imen,
Appl.
ranade,
and J.R. Monkowsku, in ‘Particles ‘Aerosol
‘Scientific
Solid
on Surfaces’,
Sci. Techn.‘, American’, W. Ziemlich,
State Vol.
Techn.,
p. 109, March
1, K.L. Mittal,
Plenum,
1988, p. 129
Vol. 7, p. 161, 1987
p. 88, June ‘Microcircuit
1990 Engineering
J. Keyser and K. Meissner. ‘Removal patent EP 0 297 506 A2, Jan. 4. 1989 and A.C. Tam,
S.J. Lee and S.D. Allen,
Editor,
1984
Appl.
Phys.
Phys.
Lett.,
Lett.,
90’, Elsevier
of particles
Science
from
Pub-
solid-state
Vol 58, p. 2217, 1991
Vol. 58, p. 203, 1991
removal’.
Confer-
W. Zapka et al. / Liquid film enhanced laser cleaning
476
Fig 1: Schematic of ‘liquid film enhanced laser cleaning’ based on the case of strong substrate absorption and a covering thin-film of transparent liquid. The density of black dots on the substrate (Si here) indicates the steep temperature gradient at the opaque substrate, producing explosive interfacial boiling.
_________________-~~~~ __________-_________-___--_______________---C _____________________________________________ f i 1m
Fig 2: Experimental arrangement for liquid-film enhanced pulsed-laser cleaning to remove particulates on a surface
Later
Beam Hcmoganirer
52 t
Fig. 3 SEM photos of laser cleaned Si wafer (left), and reference area still covered with 0.2jLm gold parlicles (right). 4 KrF excimer laser pulses at 200 mJlcm2 ‘Nere used for laser cleaning with a water film of micrometer size thickness. l/m marker irj SFM photo
Gas Valve
Pulrs
Control
W. Zapka et al. / Liquid film enhanced laser cleaning
411
Fig. 4a and 4b: SEM photos showing ‘liquid film enhanced laser cleaning’ of a silicon wafer with KrF laser irradiation (20 pulses, 120 mJ/cm*). Particle contaminaiion was O.l/rm alumina polishing powder. Fig. 4a shows border line of irradiated area (SE’M marker Ir)O/rm), while fig. 4b shows cleaned area (SEM marker llrm).
Fig. 5 Photo and schematic of material
of ‘liquid
filrn enhanced
laser
cleaning’
shows
ejection
of fast, narrow
jet