Direct absorption and melting of 0.2, 0.5 and 1.1 μm polystyrene particles on a Si substrate irradiated by 248 nm excimer laser radiation was found to contribute to their dry laser removal via a  hopping  mechanism at cleaning thresholds of 0.05, 0.1, and 0.16 J/cm 2, respectively. Ablation of these particles, which starts near the beginning of substrate deceleration at fluences above 0.4-0.5 J/cm 2, suppresses particle removal due to ablative recoil momentum. At fluences above a second cleaning threshold of 0.7 J/cm 2 particles are completely evaporated without any visible surface damage of the Si substrate.
© 2002 American Institute of Physics

Allen, Susan D.

Kudryashov, Sergey I.

English

Embry-Riddle Aeronautical University

Mechanical Engineering - Daytona Beach College of Engineering 11-1-2002 Removal Versus Ablation in KrF Dry Laser Cleaning of Polystyrene Particles from Silicon 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. (2002). Removal Versus Ablation in KrF Dry Laser Cleaning of Polystyrene Particles from Silicon. Journal of Applied Physics, 92(9). https://doi.org/10.1063/1.1503854 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. Removal versus ablation in KrF dry laser cleaning of polystyrene particlesfrom siliconSergey I. Kudryashova)Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390Susan D. Allenb)Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390and Department of Electrical and Computer Engineering, FAMU-Florida State University Collegeof Engineering, Tallahassee, Florida 32310-6046~Received 29 March 2002; accepted 9 July 2002!Direct absorption and melting of 0.2, 0.5 and 1.1 mm polystyrene particles on a Si substrateirradiated by 248 nm excimer laser radiation was found to contribute to their dry laser removal viaa ‘‘hopping’’ mechanism at cleaning thresholds of 0.05, 0.1, and 0.16 J/cm2, respectively. Ablationof these particles, which starts near the beginning of substrate deceleration at fluences above0.4–0.5 J/cm2, suppresses particle removal due to ablative recoil momentum. At fluences above asecond cleaning threshold of 0.7 J/cm2 particles are completely evaporated without any visiblesurface damage of the Si substrate. © 2002 American Institute of Physics.@DOI: 10.1063/1.1503854#I. INTRODUCTIONReduction of particulate contaminants is a basic require-ment in the rapidly growing field of nanotechnology, espe-cially in micro- and nanoelectronics. Different cleaning strat-egies ~ultrasonic cleaning, wiping, brush scrubbing, high-pressure jet spraying, etching, and megasonic cleaning!1 arecurrently being tested for removal of dust particles one fifthto one tenth the size of current 130 nm integrated circuitslinewidths2 from critical ~predominantly silicon and litho-graphic mask! surfaces. An environmentally friendly, cost-effective noninvasive cleaning process with a high through-put will be required.Dry laser cleaning ~DLC! is a well-recognized techniquefor micron and submicron particle removal from semicon-ductor devices ~e.g., Si wafers or lithography masks!1–5 andhigh density memory devices.6 Although DLC is frequentlydescribed as a ‘‘trampoline’’1 or ‘‘hopping’’ effect7 that oc-curs during fast thermal expansion of a nanosecond laser-heated substrate or an absorbing particle, respectively, a va-riety of optical, thermal, acoustic, mechanical,thermochemical, and mass transport phenomena are involvedin the DLC process,3–5 complicating its mechanism and de-velopment of a general theory of this phenomenon.5 Laserbeam characteristics ~wavelength, pulse width, and intensity!and intrinsic material properties are key parameters in DLC,and they determine the predominance of optical or thermalphenomena.4 For nanosecond laser pulses thermal aspects oflaser–matter interaction such as, in order of increasing laserfluence, thermal expansion, melting, and ablation, seem to bemore important. We report here the negative interference be-tween DLC and laser ablation under nanosecond KrF exci-mer laser irradiation for micron and submicron absorbingpolystyrene ~PS! particles on Si substrates, and measure therelative density and actual size of PS particles as a functionof incident laser fluence.II. EXPERIMENTIn this work we used an excimer laser ~Lambda Physik,LPX 210! with the following characteristics: wavelength of248 nm, energy of 0.6 J/pulse ~63%!, nearly rectangular andGaussian fluence, F, distribution in horizontal and verticaldirections, respectively, with characteristic dimensions of x55 mm and sy51.5 mm and pulse width t @full width athalf maximum ~FWHM!# of 20 ns. The laser beam with a 1cm wide vertical slit aperture in the center, was attenuated byUV transmitting color filters ~Corning Glass Works! and wasfocused on the Si~100! wafer sample at normal incidence bya fused silica lens with a focal length f 58 cm. The Si waferwas mounted on a three-dimensional stage and irradiated in apattern of spots using single laser shots. Part of the beam wasdirected by a beam splitter to a pyroelectric detector ~GentecED-500! for energy measurement of each pulse using aLeCroy 9360 storage oscilloscope. The excimer laser andoscilloscope were triggered manually in single-shot modeusing a pulse generator ~Stanford Research Systems DG535!.Single-shot DLC was performed in ambient atmospherefor Si wafers covered with monodisperse PS particles of di-ameters, D 5 0.2, 0.5, and 1.1 mm ~Surf-Cal™ grade, den-sity of 1.05 g/cm3, relative standard deviation in D less than1%! from Duke Scientific Corp. Particles were deposited onthe Si wafer samples from a suspension of monodisperse PSparticles in a water/ethanol mixture maintained at 55 °C us-ing an airbrush.8 Typical particle densities and average ag-gregation numbers for 0.2, 0.5, and 1.1 mm PS particles werea!Permanent address: Institute for High Energy Densities, Joint Institute ofHigh Temperatures of Russian Academy of Science, 127412 Moscow, Rus-sia; electronic mail: sergeikudryashov@hotmail.comb!Author to whom correspondence should be addressed; electronic mail:sdallen@mailer.fsu.eduJOURNAL OF APPLIED PHYSICS VOLUME 92, NUMBER 9 1 NOVEMBER 200251590021-8979/2002/92(9)/5159/4/$19.00 © 2002 American Institute of Physicsabout 104, 105, and 105 cm22 and 3–4 particles/cluster, re-spectively. Since the particle deposition and removal experi-ments were done in normal laboratory air, the samples wereoccasionally contaminated by large dust particles several mi-crons in size.Analysis of laser-irradiated spots ~including measure-ments of the size of the area cleaned, particle size, and den-sity! was made using dark-field optical microscopy ~Mitu-toyo WH and Olympus BH-2 microscopes! and scanningelectron microscopy ~SEM! ~JSM 5900!. The right half ofeach irradiated spot was treated with acetone to dissolve thePS particles in order to test the Si substrate for surface dam-age underneath the particles. Digitized dark field images ofscattered light from the irradiated spots were processed ~in-cluding inversion and subtraction of background! usingSCION IMAGE software ~Scion Corp.!. Spatial profiles of thenormalized scattering intensity ~the ratio of scattered lightintensity taken vertically across laser-irradiated spots and anarbitrary unirradiated area! were used to measure the degreeof DLC for different irradiated regions.III. RESULTS AND DISCUSSIONThe Gaussian fluence distribution in the vertical direc-tion of the excimer laser beam and a single-shot exposureallowed the study of the phenomenological nature of DLCand measurement of the cleaning efficiency directly as afunction of the fluence at one laser-irradiated site. As shownin Fig. 1, different surface topologies like the maximum lat-eral clean area ~I!, band-like area with particles ~II!, andcenter clean area ~III! were revealed vertically across theGaussian direction of the laser-irradiated spot for all threewafers covered by 1.1, 0.5, and 0.2 mm PS particles. Size-dependent DLC thresholds, F1 , measured at the distinct ex-ternal edges of area I, are equal to 0.04960.006, 0.160.01,and 0.1660.02 J/cm2 for 1.1, 0.5, and 0.2 mm PS particles,respectively, and are consistent with data in the literature forambient conditions but at a laser wavelength of 532 nm anda pulse length of 7 ns ~0.05060.005, 0.07460.009, and0.15460.008 J/cm2 for the same particle sizes!.4 The agree-ment with prior experiments is quite good, although it is notexpected that the thresholds will be the same for differentlaser irradiation conditions. More importantly, the expectedtrend toward higher removal thresholds for smaller particlesis observed. Two prominent axially symmetric bands of PSparticles ~area II! nearer to the spot center and exhibitingwell-defined, nearly size-independent thresholds F2 of 0.43J/cm2 @0.3760.04 ~1.1 mm!, 0.4260.08 ~0.5 mm!, and 0.4960.06 ~0.2 mm! J/cm2# were observed ~Fig. 1!. Finally, anapparently clean ~area III! was found in the center of theirradiated spots at fluences above a size-independent thresh-old F3 of 0.7 J/cm2 ~the corresponding F3 values for 1.1, 0.5,and 0.2 mm PS particles are 0.70, 0.72, and 0.67 J/cm2!. Thefeatures observed were reproducible at five different peakfluences over the range of 0.45–2.4 J/cm2.Close inspection of the irradiated spots with SEM andoptical microscopy has shown shrinking of the PS particlesin areas II and III @Fig. 2~c! and 2~d!# relative to the initialparticle size in Fig. 2~a!. This effect was especially pro-nounced for separate particles observed in area III where thelaser fluence was much higher. The blue spectral shift forvisual white light scattering of particles ~predominantly for1.1 mm PS particles! that was observed in areas II and III isan additional indication of a postirradiation particle size de-crease. At the same time, optical microscopy and SEM ex-amination of acetone cleaned samples have not exhibited anymicron-scale ~>0.1 mm! damage in areas I–III @Fig. 2~b!# atfluences up to 2 J/cm2, i.e., well above the Si melting thresh-old at 248 nm of 0.5–0.75 J/cm2.9 However, at the highestfluence of 2.4 J/cm2 several nearly centrosymmetrical dam-aged spots 10–20 mm in size with radiating star-like cracksFIG. 1. Dark field optical microscopy images of the upper half of the axiallysymmetric, Gaussian lateral profile DLC spots: ~a! 1.1, ~b! 0.5, and ~c! 0.2mm PS particles on Si substrates.FIG. 2. SEM images ~frame size 13310 mm2! of a ~a! nonirradiated spotand ~b!–~d! DLC spots of areas I–III, respectively for 1.1 mm PS particleson a Si substrate. The arrows point to partially ablated PS particles in ~c! and~d!.5160 J. Appl. Phys., Vol. 92, No. 9, 1 November 2002 S. I. Kudryashov and S. D. Allenwere observed; they probably originated on large separatedust particles initially present on the substrate.The dependence of cleaning efficiency on the laser flu-ence ~Fig. 3! for PS particles of 0.2–1.1 mm was obtainedfrom the spatial profiles of the normalized scattering inten-sity, considering an absence of light scattering as the result ofcomplete cleaning. This assumption is not valid in area IIwhere particle shrinkage influences light scattering consider-ably ~Fig. 1!. High DLC efficiency ~near 90%! in a singleshot was observed in area I for all particle sizes. The clean-ing efficiency in area III was comparable. Additional artifactminima of these curves marked by the ‘‘A’’ resulted from thepresence of single large dust particles @cf. Figs. 1~a!–1~c!#deposited occasionally during handling of the samples.The main difference between the experimental condi-tions of this and previous work4 on DLC of PS particles liesin the significant PS absorption at the 248 nm laser wave-length @aPS(248 nm)56.33103 cm21#3,10 due to the S0→S1 electronic transition in a PS phenyl group11 comparedto negligible PS absorption at 532 nm.4 In contrast to theobservations at 532 nm of enhanced Si substrate damageunder the PS particles,4 no damage was observed on theparticle-contaminated substrates irradiated by the 248 nm la-ser beam even well above the Si melting threshold. At 532nm, the transparent PS particles can serve to ‘‘focus,’’ i.e.,locally enhance, the laser electromagnetic field, resulting insubmicron-size damage ~local melting and evaporation of theSi substrate! underneath particles.Because of the PS absorption at 248 nm one may expectdirect laser heating to result in the melting of PS particles attemperatures of 110–125 °C.12 The 248 nm surface meltingthreshold for bulk polystyrene in air, FM , is about 0.03J/cm2.10 Size-dependent melting thresholds FM(D) of 0.05~1.1 mm!, 0.03 ~0.5 mm!, and 0.03 ~0.2 mm! J/cm2 as well asa value of FM’0.03 J/cm2 were estimated from calculationsof the maximum rise in temperature at the end of a excimerlaser pulse on the front and back sides of PS particles usinga simple linear absorption model,13 heat capacity CpPS’1.6 J/cm3K, and thermal diffusivity xPS;1023 cm2/s.12The effects of local scattering and thermal transport to the Sisubstrate were neglected in these rough temperature calcula-tions. The large thermal coefficient of expansion for moltenPS particles ~typically 1024 K21 relative to 1026 K21 forSi!12 may decrease their DLC thresholds because of an en-hanced ‘‘hopping’’ effect. On the other hand, the motion ofmolten particles may be significantly damped by viscoplasticdeformation of the molten particles. This last phenomenonhas been used to explain difficulties in removal of moltenmetal particles from Si by DLC.1 Nevertheless, because the‘‘trampoline’’ effect of a Si substrate is weaker at 248 nmrelative to that at 532 nm because of the lower substrateabsorbance ~0.4 for 248 nm, 0.63 for 532 nm! and longerexcimer laser pulse ~20 ns! compared to 7 ns in previouswork,4 the hopping effect for molten or near-molten PS par-ticles seems to be significant enough to affect some DLCthresholds at these wavelengths.Another consequence of PS absorption at 248 nm isshown in the laser ablation and of PS particles decomposi-tion to styrene monomers14 competing with DLC at fluencesabove the ablation threshold of the material in air of 0.1–0.12 J/cm2.9,13 For such a low thermal conductivity materialas polystyrene, the onset of ablation near the threshold flu-ence occurs at the end of the pulse, even for 20 ns pulselengths, since the material effectively stores the incident la-ser energy. For increasing total fluences, the onset of ablationshifts toward the beginning of the laser pulse. For a temporalprofile of the excimer laser pulse at a total fluence of 0.4J/cm2, the onset of laser ablation is calculated to occur nearlysimultaneously with the deceleration of the Si substrate.4,5Ablation may suppress DLC completely, and produce a highenough counterforce from the recoil momentum of ablationproducts from the top surface of the particles, particularly forthe inhomogeneously heated large PS particles. Thus, theappearance of PS particles in area II at total fluences F2above 0.37–0.49 J/cm2 may correspond to the ablative sup-pression of DLC. This ablation effect may also be influencedby Si substrate surface melting, which should occur at nearlythe same fluence of 0.5 J/cm2.9,15 Si surface melting and theresulting contraction9 would also decrease the maximum ac-celeration of the substrate and PS particles or increase thedeceleration of the Si substrate,9,15 depending on whether theSi melt threshold is reached in the acceleration or decelera-tion phase.The second cleaning threshold, F3 , observed in area III,when multiplied by the effective absorption coefficient,@1-RPS(248 nm)#aPS(248 nm), where RPS(248 nm) is the re-flectivity and aPS(248 nm) the absorption coefficient of PS,respectively, corresponds to the volume energy density nec-essary to cause ablation. The energy needed to heat polysty-rene to an ablation temperature of 1.23103 °C,16 and toevaporate it in the form of a styrene monomer ~evaporationenthalpy DHevap’2.93103 J/cm3) is about 4.63103 J/cm3.The corresponding absorbed energy density calculated usingthe threshold fluence F3’0.7 J/cm2, the previously refer-enced absorption coefficient, and a reflectivity value charac-teristic for other polymers at 248 nm ~6%!,17 is 4.23103 J/cm3 for all particle sizes used. These estimates areconsistent with the observed ablation of PS particles inarea III.FIG. 3. DLC efficiency as a function of the laser fluence for the spots in Fig.1 ~solid curve, dashed curve, and open circles correspond to vertical scansacross Figs. 1~a!–1~c!, respectively!. Areas I–III, artifact features, and the‘‘first’’ DLC thresholds are marked by symbols, I, II, III, A and arrows,respectively.5161J. Appl. Phys., Vol. 92, No. 9, 1 November 2002 S. I. Kudryashov and S. D. AllenIV. CONCLUSIONSDirect laser heating and melting of absorbing 0.2–1.1mm PS particles on a highly absorbing Si substrate at a laserwavelength of 248 nm were found to be significant for theDLC process, decreasing the dry cleaning thresholds due tothe higher thermal expansion of molten particles. Laser ab-lation of PS particles seems to suppress DLC at the nearlysize-independent threshold ~0.37–0.49 J/cm2 for 0.2–1.1 mmPS particles! via ablative recoil momentum. At fluencesabove the second cleaning threshold of 0.7 J/cm2 nearlycomplete ablation of PS particles is observed. The ‘‘nega-tive’’ interference effect of laser ablation on DLC for PSparticles on a Si substrate may provide a better understand-ing of the DLC dynamics. DLC and laser ablation of PSparticles at 248 nm as well as surface melting of the Si sub-strate were found to occur without damage to the substrateup to a laser fluence of 2 J/cm2.ACKNOWLEDGMENTSThe authors acknowledge assistance with experimentsby P. S. Davis and R. S. 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Removal Versus Ablation in KrF Dry Laser Cleaning of Polystyrene Particles from Silicon

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Removal Versus Ablation in KrF Dry Laser Cleaning of Polystyrene Particles from Silicon

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