IEEE TRANSACTIONS ON SEMICONDUCTOR...

IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. 22, NO. 4, NOVEMBER 2009 579

Onset of Material Alterations Due to Laser-InducedPlasma Exposure in Nanofilms Deposited on

PhotomasksIvin Varghese, Dong Zhou, M. D. Murthy Peri, and Cetin Cetinkaya

AbstractDamage-free removal of sub-100 nm particles fromphotomasks with deposited nanofilms is a challenge in lithog-raphy. Laser-induced plasma (LIP) is an emerging noncontact,chemical-free, dry, and selective nanoparticle removal technique.Investigation of the onset of material alterations on bondednanofilms for optimizing LIP particle removal process is theobjective of this paper. Shockwave thermomechanical excitationand radiation heating from the plasma core are major potentialsources of damage. Computational analyses reported here indicateradiation heating as the chief damage source. Damage thresholdsfor the critical nanofilm surface temperature rise and radial stresscomponent have been identified for a given nanosecond pulsedlaser.

Index TermsCleaning of thin films, damage threshold, mate-rial alteration, nanofilm, nanoparticle removal, photomasks, radi-ation intensity from LIP, shockwave.

I. INTRODUCTION

S UB-100-nm particle removal from photomasks, whichconsist of layers of deposited nanofilms on substrates, in adamage-free manner is one of the current issues in lithography.Even nanometer-range defects on the surfaces of these maskswould result in optical scattering and, as a result, increasethe possibility of fabricating a faulty integrated circuit (IC).Laser-induced plasma (LIP) particle removal technique isan emerging dry, rapid, noncontact, chemical-free, selectivecleaning technology in which a beam of a pulsed laser isfocused to a point above the surface of the substrate to createplasma due to dielectric breakdown of air at a firing distancefrom the substrate surface. This plasma core expands and aftera while saturates, resulting in shockwaves propagating out, thatapplies a thermomechanical (pressure and thermal) field ontothe substrate to initiate removal of the particles subjected to theshockwave front by rolling, sliding, or lifting (or a combinationof these). Excluding direct contact of the LIP with the substratesurface, the two possible sources of damage are recognized as i)thermomechanical excitation (due to the thermal expansion ofthin films and substrate) from the LIP shockwaves from gases

Manuscript received June 16, 2008; revised August 05, 2008. First publishedSeptember 09, 2009; current version published November 04, 2009. This workwas supported by Intel Corporation.

The authors are with the Department of Mechanical and Aeronautical Engi-neering, Center for Advanced Materials Processing, Wallace H. Coulter Schoolof Engineering, Clarkson University, Potsdam, NY 13699-5725 USA (e-mail:[email protected]; [email protected]; [email protected];[email protected]).

Digital Object Identifier 10.1109/TSM.2009.2024872

(convection heating) and ii) radiation intensity heating from theLIP plasma core and resulting thermal expansion of the thinfilms. The objective of this paper is to investigate the onset,rather than the extent, of material alterations on nanofilmsdeposited on photomasks during LIP particle removal process,and thus determine LIP experimental damage thresholds. Dueto their large ratios surface area to volume, strong opticalabsorption, weak bonding with substrates, low melting temper-ature, and reduced mechanical strength, bonded nanofilms areoften considered more susceptible to damage than their bulkmaterial counterparts [1].

According to the 2007 edition of the International Tech-nology Roadmap for Semiconductors, for optical and extremeultraviolet lithography mask requirements, the specified defectsizes and the substrate defect sizes are 36 and 35 nmfor 2010; 20 and 27 nm for 2015; and 11 and 18 nm for 2020,respectively [2]. It is therefore required to ensure that thesemasks containing nanofilms/layers are free of contamination,utilizing an effective cleaning technique in a damage-freemanner. Current sub-100-nm particle removal techniques suchas brush scrubbing [3][5], megasonic cleaning [6][8], andcryogenic cleaning [9], [10] may suffer from various problemssuch as postcleaning damage (substrate damage incurred by thecleaning process), particle redeposition [4], [9], and cleaningefficiency of nanoscale particles [8]. Therefore, there is stilla need for improved, efficient, precision, and/or selectivecleaning techniques for sub-100-nm particles. LIP applicationis capable of selective area cleaning as well as entire masksand wafers. Shockwaves generated by LIP [11][18] have ex-perimentally been demonstrated to be an effective, noncontact,and dry method for micro- and nanoparticle detachment (e.g.,1040 nm and 60 nm polystyrene latex (PSL) [11], [12]). Aswith other cleaning techniques, damage is a concern in LIPsub-100-nm particle removal; therefore a safe window of LIPprocess parameters is determined in order to prevent onset ofmaterial alteration of Cr nanofilms on quartz substrates.

II. MATERIAL ALTERATONS IN NANOFILMS UNDERTHERMOMECHANICAL EXCITATIONS

Different material alteration/damage modes for nanofilms onsubstrates due to thermomechanical excitationssuch as sur-face cracks, channeling, substrate damage, spalling, delamina-tion, tunneling cracks, decohesion, peeling, melting, and partial/complete removalhave been previously reported. Peeling-off,mosaic patterned cracks, melting, and partial/complete removal

1For materials properties of PSL particles, see http://www.matweb.com.

0894-6507/$26.00 2009 IEEE

580 IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. 22, NO. 4, NOVEMBER 2009

Fig. 1. Shockwave pressures measured on the surface in air at firing distances ranging from to mm with an interval of 1 mm. Inset shows thepressure transducer measuring the LIP shockwave pressure. The curve fitted for the peaks of the various shockwave pressure amplitudes is also shown.

due to excimer laser ablation have been observed on chromium(Cr) nanofilms [19]. Single-shot direct laser ablation of 248 nmCr film on fused silica substrate resulted in rupture of the Crfilm at low fluence followed by melting and vaporization for in-creasing fluence [20]. Thermal cycling in air (293 to 594 to 293K in 1 h) of 100-nm Cr film on soda-lime glass resulted in cracknucleation [21].

Surface cracks, channeling, substrate damage, spalling, anddebonding for pretensioned films, buckle-driven delaminationof thin films resulting in blisters, and tunneling cracks occur-ring in brittle adhesive layers under residual compression havebeen reported [22]. Decohesion of Cr films from glass substratesresulted in crack extension into the substrate with a steady-state trajectory parallel to the interface [23]. In multilayer sys-tems (with at least one brittle layer) thermomechanical damagemodes identified are i) interface decohesion, ii) tunnel crackingor crazing, and iii) spalling or delamination [24]. Design rulesdeveloped for multilevel interconnect test structures can avertchanneling cracks due to differential thermal expansion [25].Parametric study of laser-induced thin-film spallation was con-ducted, and development of a decompression shock in the fusedsilica substrates resulting in very high tensile stresses at the in-terface was observed [26]. Based on nanofilm/substrate materialproperties and the interface bonding, material alterations can beaverted by investigating safe excitation levels.

III. LIP REMOVAL TECHNIQUE

In LIP cleaning, the two key interdependent and criticalprocess parameters that have been identified are the firingdistance (see Fig. 1) and the laser pulse energy . TheLIP shockwaves and filmsubstrate interaction being in mi-croseconds, its effects are experienced much later compared

to those of radiation heating from the plasma core, which isin nanoseconds for a nanosecond pulsed-laser irradiation dueto their propagation speeds. The transient pressures of theLIP shockwaves in air decrease as the firing distance isincreased, as shown in Fig. 1. A dynamic pressure transducer[27], [28] with a resonant frequency of 500 kHz (i.e., a rise timeof 1 s), a surface diameter of 5.54 mm (Kistler 603B1), anda Spectra Physics Nd:YAG INDI-series pulsed-laser (450 mJrated pulse energy, 58 ns pulse width, 10 Hz repetitive rate,8.510 mm beam diameter, and 1064 nm wavelength) wereutilized for the transient pressure measurement experiments[29] (see inset of Fig. 1 for the experimental setup). Variousshockwave characteristics [29][33] (the location of the shock-wave front as a function of the arrival time) along with theobtained experimental results compared in [18] indicate slightoffsets due to the different laser pulse energies utilized.

Removal of smaller particles requires higher pressure levels[12] attainable at lower firing distances for a given pulse en-ergy but increases substrate damage risk as the plasma core getsin close proximity to the surface. The threshold limit, i.e., thecritical (minimum) firing distance at which damage-freeLIP particle removal can be attained, is to be determined. Exper-imentally optimized parameters for 60 nm PSL particle removalfrom silicon (Si) substrates were determined as firing distanceof mm and ten laser pulses for a 370 mJ, 1064 nmNd:YAG pulsed-laser [12]. For a 60 nm PSL particle on a Crfilm/substrate, work of adhesion of 35.65 mJ/m , contactradius of 2.72 nm, pulloff force of 5.04 nN, and re-quired removal pressure of 96.3 kPa were calculated basedon the formulae (JKR model for rolling detachment) reported in[12]. LIP shockwave pressure exerted on the Cr film surfaceat various firing distances and the corresponding minimum

VARGHESE et al.: ONSET OF MATERIAL ALTERATIONS DUE TO LASER-INDUCED PLASMA EXPOSURE 581

Fig. 2. Extreme cases of material alterations (surface cracks, channeling, discoloration, peeling, film stripped off) of 100 nm Cr film on a quartz substrate inten-tionally created at LIP firing distance of mm (plasma in contact with nanofilm surface). (a) Optical microscope images at 100 magnification and (b) SEMimages at (from left to right) 500, 1000, and 5000 magnifications.

PSL particle diameter that can be removed with that pres-sure level (calculated based on the formula reported in [12])are plotted in [18]. Pressure amplification techniques [12], [14],[15], [18] can be utilized to remove particles smaller than thatpossible at the critical firing distance .

Radiation heating and subsequent thermal stresses is identi-fied [31] as the dominant damage source compared to thermo-mechanical excitation from the LIP shockwave (heat convec-

tion) during LIP particle removal. Thermocouples in the kilo-hertz range [34], and optical fibers (high frequency range) withrise time of approximately 1 s [35], are not suitable for tem-perature estimation in the current study. Radiation energydeposited on the surface of the nanofilm, that leads to radia-tion heating of the substrate, was approximated from indirectmeasurement for single-shot laser pulses utilizing a volume ab-sorber-type radiant power meter [27]. The transient radiation in-


Fig. 3. Optical microscope image at 50 magnification depicting slight mate-rial alterations (surface cracks and channeling) of 100-nm Cr film on a quartzsubstrate at LIP firing distance of mm for a single laser shot.

tensity (radiation intensity , where is the ampli-tude and is the temporal radiation intensity profile obtainedfrom [27]) can be applied as thermal load for a one-dimensionalthermoelastic model to approximate the transient temperature

experienced by the film surface due to this radiation ex-posure. Radiation intensity magnitude was obtained in theform of

(1)

where GW/m , GW/m , andmm for the 450 mJ Nd:YAG pulsed-laser at any firing distance

. At the firing distance mm, radiation intensitymagnitude GW/m was calculated from [18, (1)].

IV. MATERIAL ALTERATION ANALYSIS FOR NANOFILMS

For a given laser pulse energy, experimental investigation ofthe critical firing distance for the inception of material al-terations/damage on a specific photomask (rather than the ex-tent of damage) due to LIP exposure (shockwave and radiationheating) is crucial for damage-free particle removal. For anyfiring distance , the load experienced by the film wouldresult in temperature greater than the sustainable surface tem-perature (melting point) and/or stress components larger thanthe dynamic yield/rupture stress of the film, thus resulting inthermomechanical material alterations such as melting, cracks,channeling, peeling, and discoloration.

Experimental damage was investigated by applying LIP onthe surface of a photomask, 100-nm Cr film on quartz substrate,utilizing a Nd:YAG pulsed-laser (370 mJ rated pulse energy, 5ns pulse width, 10 Hz repetitive rate, 5 mm beam diameter, and1064 nm wavelength) at a range of firing distances to determine

Fig. 4. Computational results of the (a) surface temperature rise , (b)radial stress component , (c) shear stress component , and (d) axialstress component of the 100-nm Cr film on a 500-nm quartz substratedue to radiation intensity level heating at a firing distance of mm .


the critical firing distance (onset of the material alteration), andto determine material alterations that occur. Extreme cases ofmaterial alterations, such as surface cracks, channeling, peeling,film stripped off, melting and discolorations, are observed fromthe optical microscope images [Fig. 2(a)] and scanning electronmicroscope (SEM) images [Fig. 2(b)] on the photomask whenintentionally subjected to 2 min of continuous LIP shooting( 1200 shots) at a firing distance of mm (plasma hit-ting the film surface). For mm, with a single laser shot,some material alterations, i.e., surface cracks and channeling,were observed as shown in Fig. 3. At LIP firing distance of

mm, no material alteration/damage was observed;therefore mm has been predicted to be the criticalfiring distance for 450-mJ Nd:YAG laser. The first ma-terial alteration modes that occur are possibly surface cracksand channeling (Fig. 3), since the other material alterations ob-served at a firing distance of mm were not observed at

mm [Fig. 2(a) and (b)]. Using the formula reported in[12], nm is the smallest predicted PSL particle that canbe removed in a damage-free manner from the photomask uti-lizing LIP shockwave pressure of 138 kPa in air at mm[18] for Re 450-mJ Nd:YAG laser.

V. COMPUTATIONAL STUDIES FOR DETERMINATION OF ONSETOF MATERIAL ALTERATIONS IN BONDED NANOFILMS

Computational studies were required to investigate the effectsof the LIP thermomechanical shockwave and radiation heat ex-citations to determine the major damage source. The initiationof material alterations in bonded nanofilms was studied in orderto determine safe LIP excitation conditions for damage-freeparticle removal from nanofilms on lithography photomasks.Shockwave pressure (mechanical) and temperature (thermal)fields, as well as the radiation intensity (thermal) field fromthe LIP core, need to be approximated. To characterize theLIP shockwaves, transient pressure measurements (Fig. 1)conducted in air at various firing distances were utilized,while the shockwave temperature was estimated using this pres-sure and gas dynamics relations [29]. A linear, fully coupledthermal stress analysis using finite-element (FE) method for anaxisymmetric model of a 100-nm Cr film on a quartz substratewas conducted using the commercial FE analysis packageABAQUS to study the effect of the shockwave thermomechan-ical excitation (shockwave pressure and temperature). The peakvalues of temperature rise , radial stress component ,axial stress component , and shear stress componenton the Cr film surface due to thermomechanical excitation ofLIP shockwaves were obtained as 152 K, 649 MPa, 195 kPa,and 172 kPa, respectively, for the 450 mJ Nd:YAG pulsed laserutilized. It was observed that the axial stress component wasdominated by the mechanical excitation (shockwave pressure),while the radial and shear stress components were dominatedby the thermal excitation (shockwave temperature). For a Crnanofilm on a quartz substrate, assuming both the film andthe substrate are isotropic and linearly elastic, there is a con-siderable mismatch of the coefficient of thermal expansions(CTE), i.e., the biaxial misfit stress. The thermal (radial) stress

component generated due to a temperature elevationcan be analytically predicted by [22]

(2)

where is the CTE mismatch (difference) between thesubstrate and the film , respectively; the Poissonratio and Youngs modulus of Cr film, respectively; and theresultant temperature elevation taken to be 152 K from the FEanalysis of the thermoelastic shockwave excitation [29]. The ob-tained radial stress is compressive if , as in thepresent case since the CTE of quartz is lower than that of Cr. Themagnitude of the radial stress component estimated using (2),i.e., 622 MPa, is close to the FE simulation result of 649 MPa[29]. This indicates good agreement of the results of the FE sim-ulations for thermoelastic shockwave excitation with theoreti-cally expected stress levels.

Radiation heating (from the LIP core) leading to thermal ex-pansion of the nanofilm is reported as the dominant factor fordamage [18], [29]. It is less likely that thermomechanical exci-tation of the film (LIP shockwaves) can cause damage. A ther-moelastic investigation of the effect of the LIP temporal radia-tion intensity excitation on a 100-nm Cr film on quartz substrateis explored by a dynamic and fully coupled linear thermal-stressanalysis (using explicit integration scheme) conducted usingABAQUS. For Cr, the thermal skin thickness is 849 nm.Since the Cr film thickness is much smaller, the quartz (thermalskin thickness is 168 nm) substrate thickness would havea significant effect on the Cr film. Several simulations wereconducted to study the effect of the substrate thickness and themodel size on the film surface temperature and stresses. An ax-isymmetric FE mesh for a 100-nm-thick Cr film with radius100 m on a 500-nm-thick quartz substrate, with infinite el-ements at the ends, was utilized for the simulations. Elementsizes of 50 50 nm in Cr and 100 100 nm in quartz wereemployed in modeling. Radiation intensity amplitude of

GW/m , corresponding to the critical firing distancemm, was utilized as the amplitude for the normalized ra-

diation intensity profile [27]. This radiation intensity level wasuniformly applied as heat flux (distributed surface flux for heattransfer analysis) on the entire Cr film surface to model the LIPradiation exposure. Axisymmetric solid element coupled tem-perature displacement was utilized for both the film and the sub-strate, while infinite (reflection-free) element type was used tomodel the free boundary of the model. The resultant Cr film sur-face temperature rise of 450 K, the maximum radial stresscomponent of 1.93 GPa, axial stress component of46.4 kPa, and shear stress component of of 0.8 kPa wereobtained (Fig. 4). It is observed that the radial stress compo-nent [Fig. 4(b)] dominates the axial [Fig. 4(c)] and shear stresscomponents [Fig. 4(d)]. It is to be determined whether the ra-dial stress component (i.e., the most critical stress componentin this case) exceeds the yield strength of the Cr film and/or thesurface temperature reaches the melting point, to predictwhether damage/material alteration of the Cr film would occurdue to LIP radiation intensity heating. The film surface tempera-ture and the radial , axial , and shear stress


Fig. 5. Effect of the firing distance on (a) the radiation intensity magnitude and (b) the maximum surface temperature as well as the radialstress component amplitudes experienced by the 100-nm Cr film ona 500-nm quartz substrate from FE simulations.

components are linearly proportional to the laser intensity ,since the FE material and analysis type adopted in this paper islinear.

From the resultant temperature and stress levels, corre-sponding to the firing distance of mm, the film damagethresholds can be predicted. The temperature and stresses expe-rienced for the permissible window of safe firing distancesthat can be utilized for particle removal experiments can be ob-tained. The Cr film surface maximum temperature andradial stress component amplitudes for the differentfiring distances are obtained. Utilizing K [fromthe obtained FE results for radiation intensity heating Fig. 4(a)]in (2), the magnitude of the thermal (radial) stress component

that is generated due to this temperature elevationis estimated to be 1.84 GPa, which is in agreement with thevalue of 1.93 GPa that was obtained from the FE simulations.Reflectivity of the nanofilms restricts the amount of the inci-dent radiation transmitted to the film surface, therebyincreasing the experimental damage threshold and implyingthat LIP application at slightly lower firing distances is possiblewithout damage.

In LIP exposure, first the Cr film surface (with optical pene-tration) is exposed to irradiative transient heat. The quartz sub-strate is then subjected to the heat through the thermal skin of thefilm. Since the CTE of quartz is substantially lower than Cr, thethermomechanical expansion of the thin film is predominantlyresponsible for the induced film radial stress (compressive) .Similarly, during LIP shockwave thermomechanical excitation,the Cr film is exposed to transient convective heat from the hotgas (shockwave front) resulting in compressive stresses on thefilm, in addition to the mechanical (shockwave pressure) excita-tion that results in initial film compression in the axial directionand follow-on expansion in the radial direction.

The material failure under high-strain-rate thermomechan-ical excitations is significantly complex compared to thoseunder quasi-static loads. The strain-rate sensitive yield stress(dynamic) of the Cr film is expected to be a few times the actualyield stress [29], due to the strain rate of the LIP appli-cation (shockwave propagation is in the range of 10 10 s[36]), similar to stainless steel in compression [37] and 1100-0aluminum [38]. The firing distances and maximum surfaceradial stress corresponding to dynamic yield stress limits of3 and 4 (assumed) experienced by the film are shown inFig. 5. For no reflectivity of the Cr film, the yield stress shouldbe at least 5.32 for no material alteration/damage. For 20%reflectivity of Cr film, it is 4.25 (20% of 5.32 ).

Table I summarizes the firing distances ; radiation inten-sity levels for 3 4 ; experimental damage threshold(for nm), maximum radial stress component );and temperature ) for the Cr film surface. The availableshockwave pressure at these firing distances and the cor-responding minimum size of the PSL particle that can be re-moved from the Cr film, based on the JohnsonKendallRoberts(JKR) adhesion model [12], [18], are also listed in Table I. Atpredicted critical firing distance of 2.5 mm , the damagelimits for maximum radial stress component and surface tem-perature are 1.93 GPa and 725 K for no reflectivity. It is ob-served that the thermomechanical responses of the Cr film sur-face (temperature rise and stresses) due to laser radiation inten-sity level heating is approximately three times higher inmagnitude compared to those due to the LIP shockwave ther-momechanical load. Film surface temperature rise is a damageconcern, as surface melting could occur. As previously reportedin [29], the film radial stress component and the surfacetemperature rise are important parameters of damage con-cern in LIP shockwave excitation as well as LIP radiation expo-sure.

VI. CONCLUSION AND REMARKS

Laser-induced plasma is an emerging technique for fast, dry,chemical-free, noncontact precision, and selective cleaning ofsub-100-nm particles. With decreasing feature sizes, smallerparticles need to be removed, for which higher loads are re-quired, resulting in increased surface damage concerns in LIPcleaning as well as in conventional techniques (such as brushscrubbing, megasonic, and cryogenic cleaning). Determinationof the primary causes for material alterations and damage dueto LIP application in nanofilms deposited on substrates utilizedin lithography masks (photomasks), as well as investigation


TABLE ITHE MAXIMUM RADIAL STRESS COMPONENT AND TEMPERATURE EXPERIENCED BY THE CR FILM DUE TO THE LASER RADIATIONINTENSITY LEVEL AT AND DIFFERENT FIRING DISTANCES

ALONG WITH THE AVAILABLE SHOCKWAVE PRESSURES AT THESE FIRINGDISTANCES AND THE CORRESPONDING MINIMUM REMOVABLE PSL PARTICLE

SIZE THAT CAN BE REMOVED DUE TO THE SHOCKWAVES FOR THE CASES OFNO REFLECTIVITY AND 20% REFLECTIVITY OF THE CR FILM

of the onset of these material alterations, were the objectivesof this paper. The two possible sources of damage (excludingdirect contact of the LIP with the nanofilms/substrate surface)are identified as i) the thermomechanical excitation from theLIP shockwaves from gases (convection heating) and ii) thelaser radiation heating (due to the very high plasma tempera-ture) from the plasma core. As a result of the study describedin this paper, it can be concluded that though there is a lowprobability that thermomechanical excitation of the propa-gating LIP shockwaves can cause damage, irradiative heatingleading to thermal expansion of the nanofilm seems to be themajor source of damage. The radiation intensity magnitudeestimated from the experimentally conducted radiation energy

measurements was utilized as loading condition in thecomputational studies to obtain the effect of radiation heatingon the transient thermoelastic response of the substrate. It isfound that, for damage concerns, the radial stress component

is the most critical stress component on the film surfacefor both the thermomechanical LIP shockwave excitation andthe LIP radiation exposure. The critical firing distance of

mm was predicted for LIP application on 100-nm Crfilm on a quartz substrate (as slight damage was detected at

mm but not at mm) based on experimentalobservations. Surface cracks and channeling were the firstexperimentally observed material alteration modes on the Crnanofilm (as the firing distance is made smaller than ).The smallest removable PSL particle is 46 nm from the Crfilm utilizing LIP at mm, with the available shock-wave pressure of 138 kPa based on the JKR model for rollingdetachment of spherical particles from a flat substrate. It isobserved in the experiments that the Cr film surface responses(temperature rise and stress) due to laser radiation intensitylevel heating dominate the thermomechanical load due to theLIP shockwave. For the Cr film, the actual dynamic yield stress(for ) is approximated to be at least two to four times of thematerial yield stress for bulk Cr (i.e., 362 MPa) due to thehigh strain rate experienced in the LIP radiation excitation. Ifthe Cr film radial stress component exceeds the dynamicyield stress (5.32 for no reflectivity) and/or the surfacetemperature rise leads to melting, material alteration/damagewould occur. The maximum Cr film radial stress componentamplitude and surface temperature are GPa

and K (at ). As a potential solution to damagerisk, residual radial tension in the film might help extend thedamage threshold by lowering the resultant exerted stresses,since compressive stresses are applied due to LIP excitation.

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Ivin Varghese received the B.Tech. degree from theIndian Institute of Technology Madras, Chennai, in2002 and the M.S. and Ph.D. degrees from ClarksonUniversity, Potsdam, NY, in 2004 and 2007, respec-tively, all in mechanical engineering.

He is currently a Research and Development Sci-entist with Eco-Snow System L.L.C., Linde Group,Livermore, CA.

Dong Zhou received the B.S. and M.S. degrees inengineering mechanics from Tsinghua University,Beijing, China, in 1990 and 1999, respectively, andthe Ph.D. degree in mechanical engineering fromClarkson University, Potsdam, NY, in 2004.

He was a Postdoctoral Research Associate fora year at Clarkson University. He is currently anProfessor in the Institute of Civil Engineering,Chongqing Jiaotong University, Chongqing, China.

M. D. Murthy Peri received the B.Tech. degree fromJawaharlal Nehru Technological University, Hyder-abad, India, in 2001 and the M.S. and Ph.D. degreesfrom Clarkson University, Potsdam, NY, in 2004 and2007, respectively, all in mechanical engineering.

He is currently a Senior Applications Engineerwith FSI International, Chaska, MN.

Cetin Cetinkaya received the B.Sc. degree inaerospace engineering from Istanbul TechnicalUniversity, Istanbul, Turkey, in 1986 and the M.Sc.and Ph.D. degrees in aeronautical and astronauticalengineering from the University of Illinois at Ur-bana-Champaign in 1991 and 1995, respectively.

He is currently a Professor of mechanical en-gineering in the Department of Mechanical andAeronautical Engineering, Clarkson University,Potsdam, NY. His research interests are in the areasof ultrasonic testing and monitoring, laser ultra-

sonics, thermoelastic wave propagation, nanoadhesion, and computer algebra.

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