Water plume velocities were measured in air by optical transmission as a function of laser fluence using a KrF laser for explosive boiling and liftoff of a layer of micron-scale waterdroplets from a laser-heated Si substrate of interest for laser particle removal. The thickness of the superheated water layer near the water/Si interface determines acceleration and removal of the waterdroplets from the Si substrate.
© 2003 American Institute of Physic

Allen, Susan D.

Kudryashov, Sergey I.

English

Embry-Riddle Aeronautical University

Mechanical Engineering - Daytona Beach College of Engineering 4-1-2003 Optical Transmission Measurements of Explosive Boiling and Liftoff of a Layer of Micron-Scale Water Droplets from a KrF Laser-Heated Si Substrate Sergey I. Kudryashov Florida State University Susan D. Allen Florida State University, allens17@erau.edu Follow this and additional works at: https://commons.erau.edu/db-mechanical-engineering  Part of the Physics Commons Scholarly Commons Citation Kudryashov, S. I., & Allen, S. D. (2003). Optical Transmission Measurements of Explosive Boiling and Liftoff of a Layer of Micron-Scale Water Droplets from a KrF Laser-Heated Si Substrate. Journal of Applied Physics, 93(7). https://doi.org/10.1063/1.1555272 < href="http://scitation.aip.org/content/aip/journal/jap/93/7/10.1063/1.1555272">Full-text article This Article is brought to you for free and open access by the College of Engineering at Scholarly Commons. It has been accepted for inclusion in Mechanical Engineering - Daytona Beach by an authorized administrator of Scholarly Commons. For more information, please contact commons@erau.edu. Optical transmission measurements of explosive boiling and liftoffof a layer of micron-scale water droplets from a KrF laser-heatedSi substrateSergey I. Kudryashova)Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390Susan D. Allenb)Department of Chemistry and Biochemistry, Florida State University and Department of Electrical andComputer Engineering, FAMU/FSU College of Engineering, Tallahassee, Florida 32306-4390~Received 13 May 2002; accepted 6 January 2003!Water plume velocities were measured in air by optical transmission as a function of laser fluenceusing a KrF laser for explosive boiling and liftoff of a layer of micron-scale water droplets from alaser-heated Si substrate of interest for laser particle removal. The thickness of the superheatedwater layer near the water/Si interface determines acceleration and removal of the water dropletsfrom the Si substrate. © 2003 American Institute of Physics. @DOI: 10.1063/1.1555272#INTRODUCTIONDeposition of a thin, submicron to several micron-thickliquid layer as an energy transfer medium ~ETM!1 on con-taminated critical surfaces ~e.g., Si masks,1 telescope optics2!was found in the early 1990s to enhance laser cleaning ofsuch critical surfaces.1–3 Since then, explosive boiling of aliquid layer near a substrate surface and evaporation of thewhole liquid layer together with contaminants in the form ofa vapor-droplet plume was determined as the basic mecha-nism of this steam laser cleaning ~SLC! technique. Thresholdconditions ~temperature, pressure and laser fluence! for theonset of boiling were experimentally measured for water,different alcohols and their mixtures using ‘‘bulk’’ ~severalmm thick! liquid layers.4,5 In these experiments, the liquidlayer is sufficiently thick that removal of the liquid does notoccur. Many important experimental parameters of SLC,e.g., vaporization thresholds and plume velocities, have notyet been studied for different laser heating conditions and thethin liquid layers of variable thickness or water droplet layersactually used in laser particle removal,5,6 where the liquid ordroplet layer deposited is completely evaporated in the pro-cess. In this work velocities of a water plume produced in airby KrF laser heating of a Si substrate with a pre-depositedlayer of micron-scale water droplets were measured at vari-ous laser fluences using an optical transmission technique.EXPERIMENTThe beam from a 248 nm, 20 ns KrF excimer laser~Lambda Physik, LPX 210! with a 1 cm wide vertical slitaperture in its center was imaged by a cylindrical lens( f 510 cm) at normal incidence onto a 0.25 mm thickSi~100! wafer with a pre-deposited thin layer of water ~Fig.1!. The laser beam has nearly rectangular and Gaussian flu-ence, F, and distributions in the horizontal ~X! and vertical~Y! directions ~see the insets in Fig. 1!, respectively, withcharacteristic dimensions of x55 and sy1/e51.5 mm. Laserenergy @0.2 J/pulse ~63%! after the aperture# was attenuatedby color filters ~Corning Glass Works! and was measured bysplitting off part of the beam to a pyroelectric detector ~Gen-tec ED-500!. The water dosing system used3 consisted of asource of pressurized nitrogen with a trigger valve, con-nected to a bubbler immersed in a glass flask filled withheated de-ionized water and directed onto the Si surfacethrough a heated output nozzle. The dosing system ~gas pres-sure of 0.7 bar, flask water and nozzle temperatures of 40 °C,dosing pulse of 0.5 s! was employed to deposit a layer ofwater droplets several microns in diameter on the Si waferplaced 5 cm from the nozzle. Micron-scale water droplets areformed because the native oxide film present on a Si sub-strate stored under ambient conditions does not wet.1,3,7 Theexcimer laser was fired 0.05 s after the end of each liquidfilm deposition step. Deposition and removal of the waterlayer were measured in real time by observing the opticalreflectance/scattering of a continuous wave ~cw! HeNe laserfocused on the center of the irradiated area. Since themicron-scale water droplets are relatively dense and the ex-cimer laser beam is much larger than the droplet size, theplume that results from explosive boiling of the water drop-lets behaves as if it originated from a thin continuous layer ofwater. Propagation of the water plume along the normal tothe Si surface was investiged using another cw HeNe laserbeam tightly focused ~waist diameter of 0.08 mm! with a 5cm focal length lens at various distances Z above the centerof the KrF laser spot on the wafer. The HeNe laser andfocusing lens were mounted on a one-dimensional stage andshifted perpendicular to the Si wafer plane in 0.5 mm steps,along with the corresponding fast photodiode detector. Aa!Permanent address: Institute for High Energy Densities, Joint Institute ofHigh Temperatures, Russian Academy of Sciences, 127412 Moscow, Rus-sia; present address as of 5/3: Laser Diagnostics Laboratory, General Phys-ics Institute, Vavilove Str., 38, 119991 Moscow, Russia; electronic mail:sergeikudryashov@hotmail.comb!Permanent address: Vice Chancellor for Research and Academic Affairs,P.O. Box 179, Akansas State University, State University, AR 72467; elec-tronic mail: sdallen@astate.eduJOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 7 1 APRIL 200343060021-8979/2003/93(7)/4306/3/$20.00 © 2003 American Institute of PhysicsLeCroy 9360 storage oscilloscope triggered by an electricalpulse from a fast photodiode, which detected part of the ex-cimer beam scattered directly from the slit, was used torecord water plume transmission transients delayed by thetime of flight, tD , of the plume to the HeNe probing distanceand to measure the laser pulse energy in each pulse. The gasvalve and excimer laser were triggered manually in single-shot mode with corresponding delays using a pulse generator~Stanford Research Systems DG 535!.RESULTS AND DISCUSSIONThe water plume transmission transients for constantwater dosing conditions, different probing distances and peaklaser fluences of Fmax50.32– 0.76 J/cm2, i.e., above the ex-plosive boiling threshold of water on a Si surface ~0.2J/cm2!,8 but below the ablation threshold of Si ~about 1.4J/cm2!,8,9 are presented in Fig. 2. The maximum probe dis-tance, Zmax , is approximately 3 mm, where detection of theplume transmission transients becomes very unstable be-cause of three-dimensional rarefaction, dispersion, vaporiza-tion, turbulence and other instabilities in the plume5 as wellas the decelerating effect of viscous air drag. As shown inFig. 2, the plume transmission varies much less at largerdistances (Z52.5 mm) than it does near the Si surface (Z50.5 mm).A characteristic parameter of the transmission transientsis the plume arrival time at the HeNe probe position, tD ,depending on Z and defined in Fig. 2 as the onset of the dropin HeNe transmission. The experimental data as Z – tD trajec-tories for several Fmax values are given in Fig. 3~a!. TheseZ – tD curves were fit with an exponential functionf (t)5V0Tdamp@12exp2t/Tdamp# . The initial plume velocity atZ50, V0 , and the viscous drag velocity damping time,Tdamp , which takes into account the decelerating viscousdrag force in air for the water vapor plumes were used asvariable parameters. In order to have a simple analytical de-scription of the vapor plumes we employed a one-dimensional model of continuous viscous flow assumingthin, homogeneous initial liquid layers.The f (t) functions employed fit the Z – tD curves quitewell @Fig. 3~a!#. At fluences of 0.32, 0.45 and 0.76 J/cm2good agreement is observed between the calculated maxi-mum plume propagation distance V0Tdamp (2.8860.14,2.8760.14 and 2.8960.03 mm, respectively! and the experi-mentally observed value Zmax’3 mm. The correspondingTdamp values are different (0.1460.02, 0.2160.03 and 0.2560.01 ms, respectively! and the calculated V0 values de-crease with an increase in Fmax @Fig. 3~b!#. The differentTdamp values that characterize viscous drag damping of waterplume motion may mean that the simple one-dimensionalviscous continuous flow plume propagation model used isnot really applicable in this case. For example, we have as-sumed that the droplet structure on the surface does not af-fect vaporization and that the plume always consists of thesame size and density droplet and vapor mixture. A morecomplex model would include these and other effects such asthree-dimensional rarefaction, dispersion, turbulence andother instabilities in liquid plumes. The V0 values obtained,however, seem to be a good estimate of plume initial veloci-ties, at least for small propagation distances where Z – tDcurves can be linearized and viscous drag and plumeinstabilities neglected. The resulting initial velocities,V0’10– 20 m/s, are very low relative to the supersonic ve-locities expected for condensation products formed duringFIG. 1. Experimental setup for transient plume transmission studies: BS—beam splitter, L—focusing lenses, PD—fast photodiodes, target—Si waferon a three-dimensional stage. Insets: ~a! Laser power temporal dependenceof the KrF laser and ~b! Gaussian distribution of fluence F in the vertical ~Y!direction.FIG. 2. Characteristic water plume transmission transients at different flu-ences Fmax : Z5(a) 0.5 and ~b! 2.5 mm. The delay time values, tD , areshown for different fluences by several arrows.4307J. Appl. Phys., Vol. 93, No. 7, 1 April 2003 S. I. Kudryashov and S. D. Allenadiabatic expansion of a molecular vapor, implying that lift-off of the entire water layer occurs as a result of explosiveboiling of the near-surface water layer. Qualitatively similarresults were obtained using the same experimental setup onseveral micron-thick, homogeneous layers of 2-propanol,which does wet the native oxide on Si.10The observed trend of the V0(Fmax) curve might be ex-plained by the simultaneous explosive boiling and expansionof a superheated water layer near the water/Si interface ineach droplet under spinodal conditions,11 that acts as a vapor‘‘piston’’ and imparts mechanical momentum to the top‘‘cold’’ liquid layer of the droplet. The onset of explosivewater boiling, tboil , due to the instantaneous spinodal decom-position shifts gradually with an increase in Fmax to the be-ginning of the heating laser pulse. This shortening of theexplosive boiling delay time reduces the heat-affected zone,Ldep;(xtboil)1/2;0.01 mm, in the water droplets ~height L’1.5 mm), while the water thermal diffusivity, x, slowlydecreases as a function of the increase in water temperaturenear the water/Si interface. The velocity V0 of the entire coldliquid layer that gives rise to the plume can be calculatedusing momentum conservation in the form of Ldep3Vexp’L3V0 , where Vexp;Cl3(DV/V0) is the thermal expansion ve-locity of the superheated water layer under spinodal condi-tions, Cl’1.5 km/s is the longitudinal sound velocity in wa-ter at ambient conditions,12 and the increase in water molarvolume (DV/V0)’0.5– 1.5 at positive pressures along thespinodal curve up to the critical point.11 Thus, V0 shouldscale as Vexp3Ldep /L , in qualitative agreement with theoret-ical predictions13 and the experimental results of this work.The decrease of V0(Fmax) with an increase in Fmax is shownin Fig. 3~b! with lift-off ~plume! velocities of 10–20 m/s forour experimental conditions.CONCLUSIONSWater plume velocities were measured in air by an opti-cal transmission technique as a function of laser fluence forexplosive boiling and liftoff of a thin layer of water dropletson a Si substrate heated by a KrF laser. The results are coun-terintuitive in that the initial velocity of the vapor/dropletplume decreases with an increase in laser fluence. The thick-ness of the near-surface superheated water layer in the drop-lets determines the acceleration and removal of the entirewater droplet layer from the Si substrate. At higher fluences,the water in contact with the hotter laser-heated Si substratereaches the critical temperature faster, and liftoff limits addi-tional heating by the substrate. For lower fluences, the longerheating time results in a thicker heated layer and more totalenergy in the water droplets. These results help in under-standing and optimization of laser particle removal via ex-plosive boiling of a thin deposited liquid or droplet film.1 K. Imen, J. Lee, and S. D. Allen, Appl. Phys. Lett. 58, 203 ~1991!.2 K. Mann, B. Wolff-Rottke, and F. Muller, Appl. Surf. Sci. 96–98, 463~1996!.3 A. C. Tam, W. P. Leung, W. Zapka, and W. Ziemlich, J. Appl. Phys. 71,3515 ~1992!; S. J. Lee, K. Imen, and S. D. Allen, Microelectron. Eng. 20,145 ~1993!.4 O. Yavas, P. Leiderer, H. K. Park, C. P. Grigoropoulos, C. C. Poon, W. P.Leung, N. Do, and A. C. Tam, Appl. Phys. A: Solids Surf. 58, 407 ~1994!;H. K. Park, C. P. Grigoropoulos, C. C. Poon, and A. C. Tam, Appl. Phys.Lett. 68, 596 ~1996!; H. K. Park, D. Kim, C. P. Grigoropoulos, and A. C.Tam, J. Appl. Phys. 80, 4072 ~1996!; O. Yavas, A. Schilling, J. Bischof, J.Boneberg, and P. Leiderer, Appl. Phys. A: Mater. Sci. Process. 64, 331~1997!; M. Mosbacher, M. Bertsch, H.-J. Muentzer, V. Dobler, B.-U.Runge, D. Baeuerle, J. Boneberg, and P. Leiderer, Proc. SPIE ~to be pub-lished!.5 M. She, D. Kim, and C. P. Grigoropoulos, J. Appl. Phys. 86, 6519 ~1999!.6 P. T. Leung, N. Do, L. Klees, W. P. Leung, F. Tong, L. Lam, W. Zapka, andA. C. Tam, J. Appl. Phys. 72, 2256 ~1992!; N. Do, L. Klees, A. C. Tam, P.T. Leung, and W. P. Leung, ibid. 74, 1534 ~1993!.7 S. J. Lee, K. Imen, and S. D. Allen, Appl. Phys. Lett. 61, 2314 ~1992!; J.B. Heroux, S. Boughaba, I. Ressejac, E. Sacher, and M. Meunier, J. Appl.Phys. 79, 2857 ~1996!; Y. P. Lee, Y. F. Lu, D. S. H. Chan, T. S. Low, andM. S. Zhou, Jpn. J. Appl. Phys., Part 1 37, 2524 ~1998!; W. H. Lawnik, U.D. Goepel, A. K. Klauk, and G. H. Findenegg, Langmuir 11, 3075 ~1995!,and references therein.8 S. I. Kudryashov, and S. D. Allen ~unpublished results!.9 X. Xu, C. P. Grigoropoulos, and R. E. Russo, Appl. Phys. Lett. 65, 1745~1994!; S. Zhu, Y. F. Lu, and M. H. Hong, ibid. 79, 1396 ~2001!.10 S. I. Kudryashov, and S. D. Allen, Appl. Phys. Lett. ~to be published!.11 V. P. Skripov, E. N. Sinitsyn, P. A. Pavlov, G. V. Ermakov, G. N. Muratov,N. V. Bulanov, and V. G. Baidakov, Thermophysical Properties of Liquidsin the Metastable State ~Gordon and Breach, New York, 1988!.12 I. S. Grigor’ev and E. Z. Meilikhov, Fizicheskie Velichini ~Physical Quan-tities, Energoatomizdat, Moscow, 1991!, Chaps. 5–15 ~in Russian!.13 Y. Dou, L. V. Zhigilei, N. Winograd, and B. J. Garrison, J. Phys. Chem.105, 2748 ~2001!; Y. Dou, L. V. Zhigilei, Z. Postawa, N. Winograd, and B.J. Garrison, Nucl. Instrum. Methods Phys. Res. B 180, 105 ~2001!.FIG. 3. ~a! Z – tD trajectories and their exponential curve fits at differentfluences Fmax ; ~b! dependence of the initial water plume velocity V0 onFmax .4308 J. Appl. Phys., Vol. 93, No. 7, 1 April 2003 S. I. Kudryashov and S. D. Allen

Optical Transmission Measurements of Explosive Boiling and Liftoff of a Layer of Micron-Scale Water Droplets from a KrF Laser-Heated Si Substrate

Crossref

https://commons.erau.edu/cgi/viewcontent.cgi?article=1016&amp;context=db-mechanical-engineering

Optical Transmission Measurements of Explosive Boiling and Liftoff of a Layer of Micron-Scale Water Droplets from a KrF Laser-Heated Si Substrate

Abstract

Similar works

Full text

Available Versions

Embry-Riddle Aeronautical University

Crossref