IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 7, NO. 4, JULY/AUGUST 2001 567
Surface Microstructuring of Aluminum Alloy 2024Using Femtosecond Excimer Laser Irradiation
Kai Dou, Robert L. Parkhill, Jack Wu, and Edward T. Knobbe
Abstract—Surface microstructuring and modification of alu-minum alloy 2024-T3 were investigated using a femtosecond pulselaser irradiation. The surface micrographs of the scanning elec-tron microscopy were characterized as a function of incident laserfluence. The surface features ranging from submicrometer to mi-crometers were developed through the variation in laser fluenceand count of laser pulse. Results indicated that surface patternswere uniquely constructed in a controlled use of pulsed laser pa-rameters for the ablation. Two ablation regimes were found in thelogarithmic dependence of the ablated depth on laser fluence andtwo components were attributed to the optical and energy penetra-tion. A thermal dynamic model was adopted to analyze the ablationprocesses, and the theoretical analysis agreed well with the exper-imental results.
Index Terms—Aluminum alloys, excimer laser, femtosecondpulse laser, laser ablation, laser micromachining and microstruc-turing, scanning electron microscopy (SEM), surface modification.
I. INTRODUCTION
T HE USE of pulsed lasers in material processing hasgenerated excellent works in the fields of laser drilling,
cutting, welding, and annealing. Significant works aimed atthe modification of surface compositions and/or morphologies,such as thin-film deposition, surface alloying, melting andcladding, ablation, surface cleaning and hardening, phase trans-formations, and micromachining and microstructuring, haveall been demonstrated using pulsed laser sources to irradiatesolid-state specimens [1]–[33].
Temporally short pulse lasers, such as those characteristics ofexcimer lasers, provide a mechanism for the alteration of sur-face morphology or structure without significant damage to orperturbation of the underlying material. Unique materials sur-face modification or surface structuring effects may be achievedthrough the controlled use of the available irradiation parame-ters of pulse laser, such as wavelength, pulse energy, fluence,repetition rate, and pulse count. Physically, these correlate topenetration depth, thermal flux, localized heating, thermal gra-dient structure, and quenching rates. Thus, it is possible to de-velop unique phases, surface compositions, and/or surface mi-crostructures through the appropriate combination of laser pro-cessing parameters.
Manuscript received January 8, 2001; revised April 16, 2001. This work wassupported by the Air Force Office of Scientific Research, the Air Force ResearchLaboratory/Materials Directorate, and the National Science Foundation.
K. Dou and E. T. Knobbe are with the Environmental Institute and the Uni-versity Center for Laser and Photonics Research, Oklahoma State University,Stillwater, OK 74078 USA (e-mail: [email protected]; [email protected]).
R. L. Parkhill is with CMS Technetronics, Inc., Stillwater, OK 74075 USA.J. Wu is with the Department of Plant and Soil Sciences, Oklahoma State
University, Stillwater, OK 74078 USA.Publisher Item Identifier S 1077-260X(01)09945-2.
In previous studies of aluminum substrates, surface struc-turing or modification has shown to be affected by parametersincluding fluence, ambient pressure, and count of pulses.Studies on aluminum alloys [4]–[6] and pure aluminum [9]showed that plastic deformation, associated with thermallyinduced stresses, results from repetitive pulsed irradiation atfluence levels well below the ablation threshold of 1.25 J/cm[4], [9]. In studies where the ablation threshold is exceeded,wavy fluidic melt morphologies have been reported [18].Such features were attributed to rapid surface quenching andsolidified particle redeposition processes that occur duringincomplete ablation. The cited literatures lead to the conclusionthat surface topographies may be modified, in a controlledmanner, through the use of laser processing methods.
A challenging problem in the laser surface modification andmicrostructuring of metals, such as aluminum alloys with rel-atively low melt temperature, is of high thermal conductivitycompared to semiconductors and polymers [24], [27]–[29]. Theablation of this kind of metal is thus always accompanied withthe formation of larger heat-affected zones and the appearanceof the melt specimen. One is able to take advantage of the heat-affected zones to effectively produce the desired patterns. Onthe other hand, to achieve the high precision and quality ofthe desired structures, the heat-affected zone needs to be inten-sively controlled during laser treatment. The extent of the heat-affected zone varies based on the materials properties, such asmelt point, thermal conductivity, and experimental conditions,including laser fluence, pulse width, count of pulse, and repeti-tion rate. Unique surface modification and microstructuring canbe achieved through the controlled use of these available ultra-short-pulse laser irradiation parameters.
Rapid advances in the development of ultrashort laser pulseshave created many new aspects in laser–matter interaction andmaterial processing [14]–[21] More recently, ultrashort pulselasers with high output energy have become available andoffer many new opportunities in the field of laser processingof materials via very short pulses ranging from picoseconds tofemtoseconds. Significant works in laser material processing,such as thin-film deposition, ablation, microstructuring, laseretching, and surface cleaning, have been demonstrated inrecent years. The extremely short pulsewidth facilitates theachievement of a very high peak laser intensity with low pulseenergy. The ultrashort pulse duration fundamentally changesmechanisms of the laser–matter interaction in some aspects.With such a short pulse duration, the hydrodynamic motion ofthe matter under laser irradiation can be ignored, and no fluiddynamics is essentially to be considered during the laser–matterinteraction. Electrons are driven to a much higher temperature
than the ions, and thus the interaction is highly nonequilibrium.Subsequent electron-lattice energy exchange takes place on amuch longer time scale than the laser–matter interaction time.In this paper, these features of femtosecond pulses will bedemonstrated for laser surface modification on the aluminumsubstrates, and the physics of the laser–matter interactionleading to materials modification will be discussed.
Ultrashort pulse laser ablation of solid materials, such asglass, polymers, ceramics, and semiconductors [11], [13], [15],[17], [27], [33], exhibits potential applications in microm-eter feature sizes in microelectronics and materials industry.Submicrometer feature sizes have been achieved in laser modi-fication and micromachining with femtosecond lasers becauseultrashort laser pulses have precise breakdown thresholds andnegligible thermal diffusion. For transparent materials, the non-linear processes causing ablation or breakdown are avalancheionization and multiphoton ionization. For absorbing materials,such as metals and semiconductors, there is large density offree electrons and valence electrons with an ionization potentialless than the photon energy. Two ablation processes wereidentified in ceramics [33] using femtosecond pulse laser, andthe melt zone induced by laser pulses was significantly reducedcompared to the metal ablation. In this paper, two different laserwavelengths with a femtosecond pulse duration are employedto ablate aluminum alloy 2024-T3, and two ablation regimesare observed. A series of fluence dependence of the scanningelectron microscopy (SEM) images provides a precise ablationthreshold in order to compare the proposed model with theexperimental observations.
In this work, femtosecond pulse lasers are in use for studyinglaser microstructuring and laser-induced surface modificationsof aluminum alloy 2024-T3, and the surface micrographs of theSEM have been characterized as a function of incident laserintensity. Aluminum alloy 2024 was chosen for this study be-cause aluminum alloys not only are widely used for the air-craft and mechanical industry but also exhibit special featuresof high thermal conductivity and low melt point, which cause
a considerable heating impact on the material performance andlaser processing. Aluminum alloy substrates were irradiated ata laser fluence ranging from well below (0.01 J/cm) to abovethe ablation threshold (10.0 J/cm). Modifications to a surfacestructuring and the evolution of surface texturing are shown withthe SEM images. Surface features ranging from nano- to mi-crodimension have been constructed and are found to appearunder the ten-pulse irradiation at a lower fluence, whereas the200-pulse irradiation generates surface-bound particles in mi-crodimension. In addition to the surface texturing effect, the flu-ence dependence of ablation of metals has been studied, and twoablation regimes are found in the logarithmic dependence of ab-lation depth on the laser fluence. The physics of ultrashort pulselaser–matter interaction leading to material ablation will be dis-cussed.
II. EXPERIMENTAL
Aluminum alloy 2024-T3 substrates were directly obtainedfrom Reynolds Aluminum. The X-ray analysis for the surfacecomposition confirms that the substrate contains 4.4 wt% ofcopper, 0.98 wt% of magnesium, and 0.96 wt% of manganese.Substrates were polished by using silicon carbide paper and thendiamond suspensions. The substrates were cleaned in methanoland ethanol prior to laser irradiation.
Three laser wavelengths at 496, 248, and 308 nm were em-ployed to demonstrate the surface structuring and modificationin aluminum alloys. Two irradiation sources for generating500-fs pulses were a laser system of a Lambda Physik ModelLPD 500-fs dye laser delivering the 496-mn light and a LambdaPhysik EMG150 of a dual-cavity excimer laser at 248 nm, asshown in Fig. 1. The 308-nm pulses at a nanosecond scalewere directly generated by an excimer laser with the workingmedium of xenon chloride. A Model LPD 500-fs dye laser waspumped by a 308-nm beam with a pulse width of 15 ns fromthe dual-cavity excimer laser, and a 496-nm laser beam with thepulse duration of 500 fs was generated at a maximum output of
DOU et al.: SURFACE MICROSTRUCTURING OF ALUMINUM ALLOY 2024 569
0.4 mJ. The amplified 496-nm beam was frequency-doubled tobe 248 nm through a crystal. The 248-nm pulse was sent to theamplifier filled with krypton fluoride, and a final energy outputat 248 nm with a pulse width of 500 fs was generated. The pulseoutput energy was measured by a Molectron J3-09 joulemeterprobe connected to a Tektronics TDS640A oscilloscope.Maximum energy output was obtained at 15 mJ at 248 nm. Ananosecond pulse had maximum energy output of 200 mJ witha pulse duration of 15 ns. A fused silica lens with focal lengthof 30 cm was used to focus a laser beam on the sample. Eachsample was irradiated with 10–1000 pulses at a repetition rateof 10 Hz. Variable spot sizes were used to produce irradiationfluence ranging from 0.01 to 10.0 J/cm. The fluence wasvaried by moving the focal position of the lens mounted on the
– translation stage that is controlled by a computer.The morphology of the ablated area was characterized by a
Leica–Cambridge Stereoscan 360 FE electron microscope op-erating at 15 kV with a working separation of 10 mm. An opticalmicroscope or interferometer was used to measure the ablateddepth.
III. T HERMAL DYNAMIC ANALYSIS
Thermal effect in laser processing of metals is thought to bean important factor responsible for the precision and quality ofthe produced structures. Thermal response of metals to an ul-trashot-pulse laser excitation mainly relates to two processes:thermal conductivity and thermal exchange with the lattice. Theelectron-lattice exchange is competing with the thermal conduc-tivity in the laser–metal interaction. The electron-lattice cou-pling transferring their heat to the lattice provides a routine tocool hot electrons heated by short pulses. Thermal conductionpasses energy from the metal surface to the underlying material.The mechanism of the laser–matter interaction thus depends notonly on laser intensity but also on laser pulse duration.
The laser energy is assumed to be initially deposited intothe electrons at the surface during the interaction of ultrashortpulses with metals. The laser pulses create nonequilibriumelectron distribution, leaving the lattice temperature essentiallyunchanged. Then the hot electrons interact with the lattice toelevate the lattice temperature with electron-lattice coupling,causing an equilibrium distribution of the excess energy be-tween the electron and lattice subsystem. Finally, heat diffusiondominates the relaxation process.
Assume that the thermalization in the electron subsystem isvery fast and the electron and lattice subsystems can be char-acterized with their temperatures and . The heat transportinside the metal can be described with the following one-dimen-sional two-temperature-components model [21]:
(1)
(2)
is the laser intensity, and are the heat capacities ofthe electron and lattice subsystems,is the constant of the elec-tron–phonon coupling, is the laser heating source, is theabsorption coefficient, is the surface transmission,is thecoordinate axis at the direction perpendicular to the surface,is the heat energy, is the electron thermal conductivity, and
W/mK. The electronic heat capacity is pro-portional to the electron temperature, J/m K( is a constant, is material dependent, and falls in a range of10–100 for aluminum alloys). The first term on the right-handside of (1) stands for thermal conductivity losses. The secondterm in (1) represents electron–phonon coupling. The coeffi-cient of heat transfer between the electrons and the latticeisobtained by a fit of the model to the experimental data. Let
W/m K in use in Fig. 2. The third term in (1) rep-resents the laser-heating source. The present calculation uses avalue of 2.5 10 J/m K for the lattice heat capacity and 100nm for the inverse absorption coefficient (the penetration depth)
. In (2), a thermal conductivity for the lattice subsystem isneglected.
Equations (1) and (2) can be solved numerically. Time evo-lution of the electron and the lattice temperatures for differentvalues of electron–phonon coupling constantand at the in-termediate laser fluence of 0.1 J/cmis shown in Fig. 2(a) and(b). Fig. 2(c) shows the penetration dependence of the latticetemperature, and the parameters in use for the simulation arethe same as those used for Fig. 2(a) and (b). The laser pulse isassumed to be Gaussian with a pulse width of 0.5 ps. Valuesof electron–phonon couplingare taken 0.8, 1.4, and 2.0( W/m K). It is obviously found that the elec-tron–phonon relaxation time is strongly subject to thevalue,and it increases with a decrease in the electron–phonon cou-pling constant. The model predicts a peak electron tempera-ture of 17 105 K and an equilibrium electron–phonon temper-ature of 2505 K at a fluence of 0.1 J/cm. As a result, electronsare heated to very high transient temperature because the elec-tron heat capacity is much less than the lattice heat capacity. Inthe experiment, the ultrafast pulse of laser light heats electronsin the metal, and for a very short time (1 ps), the electronsare driven out of equilibrium with the host lattice because thephonon emission rate is not large enough to maintain equilib-rium. Consequently, the electron temperature rises far above thatof the lattice. The electrons transfer energy to the lattice throughphonon emission subsequent to the excitation, and the electronand the lattice temperatures equilibrate at a rate governed by thestrength of the electron-lattice coupling. These characteristicswill provide a physical insight for understanding the ultrashortpulse structuring or machining to be discussed in the followingsections.
IV. RESULTS
A. Fluence Dependence of the Ablated Depth
Fig. 3 shows the laser fluence dependence of the ablated depthper pulse for femtosecond pulse irradiation at both 496 and248 nm. Two different logarithmic dependence regimes for thefemtosecond pulse ablation can be identified in both excitationconditions. Different physical mechanisms may be involved in
570 IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 7, NO. 4, JULY/AUGUST 2001
(a)
(b)
(c)
Fig. 2. Numerical time dependence of (a) electron and (b) lattice temperaturein variety of coupling constants of 0.8, 1.4, and 2.0� G (= 10 W/m K) inaluminum alloy 2024-T3 with a 500-fs pulse irradiation. The lattice temperatureversus penetration depth is shown in (c). The Gaussian laser pulse is in use forthe numerical calculation.
two ablation regimes, as discussed in detail in the next sec-tion. At a fluence of less than 0.6 J/cm, the expression
is used for describing the process dominated bythe optical penetration depth, whereis the ablated depth each
(a)
(b)
Fig. 3. Fluence dependence of ablation depth per pulse for aluminum alloy2024-T3 at (a) 496 nm and (b) 248 nm. The ablated depth per pulse versusincident fluence for the 500-fs pulse ablation by using both 496- and 248-nmlasers shows the two-component structure. Triangles indicating the ablationresult for the 308 nm/15 ns irradiation are plotted in both (a) and (b).
pulse, is the absorption coefficient, andand are the laserfluence and the ablation threshold fluence. nm and
mJ/cm are obtained by a fit to the experimentaldata of aluminum alloy 2024-T3 irradiated with a 496-nm pulse.
nm and mJ/cm for the 248-nmirradiation are also measured. Eleven or 7 nm for is in goodagreement with the optical penetration depth of nmfor metals.
At fluences exceeding 0.6 J/cm, the second regime for thefemtosecond pulse ablation is found in Fig. 3. This part is de-scribed by the process known as the high energy penetrationdefined by the expression , where is the en-ergy penetration depth and is the threshold fluence for usein the higher fluence range. The penetration depthnm for 496 nm and nm for 248 nm are obtainedby using to fit to the experimental data inthe second regime. The same data fitting to the second ablationregime provides the threshold fluence mJ/cmfor the 248-nm ablation and mJ/cm for the
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Fig. 4. SEM images of AA2024-T3 irradiated at 248 nm/500 fs with ten pulses at a fluence (a) 0.0, (b) 0.02, (c) 0.06, (d) 0.2, (e) 0.4, and (f) 1.0 J/cm.
496-nm ablation. These values are consistent with the thresholdvalues of 200 mJ/cmat 248 nm and 100 mJ/cmat 496 nmobtained in the experimental surface modification (as shown inFigs. 4 and 5). The results suggest that different ablation be-havior originates from the difference between two laser wave-lengths and that the 496-nm light more efficiently ablates themetal than the 248-nm irradiation. This is because nearly res-onant absorption for the 496-nm laser beam takes place duringthe ablation process [32].
B. Surface Structure Evolution as a Function of Laser Fluence
1) Surface Structuring With a Ten-Pulse Irradiation:Thefollowing description applies to the results using the ten-pulseirradiation at 248 nm, as shown in Fig. 4, and is also suitable tothe experimental results performed by ten pulses at 496 nm, asplotted in Fig. 5.
Low fluence irradiation yields no notable surface modifica-tion. Fig. 4(a) illustrates the surface of the untreated substrate.Two types of defects, scratches and secondary phases, can be
identified from the SEM images. A small amount of elements,such as copper, zinc, manganese, and magnesium, incorporatedin alloys, plays an important role in materials reinforcement.Clusters resulting from these incorporated elements in alloys forreinforcement are known as the second phase or second phases.The scratches are thought to be a side-effect produced by the me-chanical polishing process, and the visible segregation of sec-ondary phases randomly distributed results from the alloyingand heat treatment processes. Irradiation intensities of less than0.01 J/cm produce no remarkable surface modification. Ob-servable surface modification induced by the laser irradiationappears at a low fluence of 0.02 J/cmin Fig. 4(b), having asize ranging from submicrometer to a micrometer.
Notable surface modification appears at a fluence around 0.06J/cm , as shown in Fig. 4(c), and small isolated surface partic-ulates in submicrometer and cotton-like fragments are formedcompared to unirradiated surface. At this fluence irradiation,surface ablation becomes feebly visible by view of plasma oc-currence.
572 IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 7, NO. 4, JULY/AUGUST 2001
Fig. 5. SEM images of AA2024-T3 irradiated at 496 nm/500 fs with ten pulses at a fluence (a) 0.0, (b) 0.02, (c) 0.06, (d) 0.1, (e) 0.4, and (f) 1.0 J/cm..
Significant surface texturing comes into view in Fig. 4(d)while laser irradiation intensity approaches a crucial fluence of0.2 J/cm at 248 nm. As a laser fluence exceeds 0.2 J/cm, thesurface texturing undergoes widespread expansion along witha small melt formation, as shown in Fig. 4(e). The laser flu-ence of 0.2 J/cmis thought of as the ablation threshold. How-ever, Fig. 5 indicates that the ablation threshold at 496 nm is0.1 J/cm. At this fluence irradiation, surface ablation becomesvisibly apparent, accompanying a plasma formation during theinteraction of laser with metal. Studies on the surface modifi-cation using laser fluence below or above the threshold haveshown the surface structure transition, at this turning point, fromthe cotton-like to texturing structure or from the texture to wide-spread particles with a melt structure. The produced structuresare thus much more sensitive to the variation of the laser flu-ence.
At a higher laser fluence exceeding 0.3 J/cm, surfacetopology is poorly defined and the surface becomes substan-tially less fine-featured in Fig. 4(e). The surface feature atan ablation threshold disappears and specimen-removed meltformation appears. At a much higher fluence above 0.8 J/cm,
more specimens are removed from the laser focus spot and adrilling hole is found to appear with a melt zone around thehole, as shown in Fig. 4(f).
2) Surface Structuring With a 200-Pulse Irradiation:Arepetitive pulse interaction with a surface yields novel struc-tures to the surface because surface properties associatedwith reflectivity, anisostropy, and stress field are substantiallyinvolved in multipulse, repetitive ablation. The evolution of thesurface feature irradiated below the ablation threshold, whichdevelops as a result of localized anisotropic surface reflectivity,alters the nature of laser–surface interactions. Such effects leadto considerable variations in localized absorbance and lead tostrong differential thermal loading. Highly localized heatingand large thermal gradients cause the development of isolatedmelt formations and the development of severe strain fields,finally resulting in isolated clusters, as shown in Fig. 6(b) and(c).
As the irradiating fluence approaches the fluence value forablation in aluminum, surface features range in several mi-crometers, as shown in Fig. 6(d). Surface-bound clusters appearto serve as nucleation sites for subsequent resolidification,
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Fig. 6. SEM images of AA2024-T3 irradiated at 248 nm/500 fs with 200 pulses at a fluence (a) 0.0, (b) 0.02, (c) 0.06, (d) 0.1, (e) 0.4, and (f) 1.0 J/cm
melt formation, and greater volumetric heating for successivelyrepetitive pulse irradiation. At fluences above the ablationthreshold, the surface melt zone undergoes rapid thermalexpansion. The kinetic energy associated with the localizedrecoil pressure results in splashing of the melt layer in Fig. 6(f).
Atan irradiation fluencemuchhigher thantheablationfluence,surface features became coarse and poorly defined. The dramaticloss in surface particle features is postulated to occur as a resultof extensive ablation, widespread melt formation, greater volu-metric heating, and substantially reduced quenching rates, andsurface particles dissolve into the melt before resolidification.
The surface structuring, similar to the results performed byusing the 248-nm irradiation, is displayed in Fig. 7 by using the496-nm ablation. The examination in Fig. 7 indicates the sameconclusion in both cases: that the fine texturing at the ten-pulseexcitation disappears and the surface-bound particle structure isfound to appear at the repetitive pulse interaction. As a result, the
variation of pulse number is able to produce a profound effect onthe surface structure. The unique, fine structure on the surfacecan be precisely constructed by the controlled use of fluence andcount of pulses.
An examination of results in surface modifications indicatesthat there is a difference between two laser wavelengths of 248nm (5.0 eV) and 496 nm (2.5 eV), and the 496-nm pulses seemto work better than the 248 nm pulses. In other words, a lowerfluence at 496 nm is able to achieve the same surface modifica-tion as at 248 nm. Considering the absorption and compositionof aluminum alloy 2024-T3 (AA2024-T3) would be helpful tounderstand the difference. Our previous research [5]–[7] indi-cated that the ablation started with the copper sites or copperclusters in the alloy, and then developed into the alloy becausethe X-ray analysis pointed out that AA2024-T3 contains cop-pers in 4.1–4.8 wt% for the purpose of reinforcement. There-fore, the absorption associated with coppers or copper particles
574 IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 7, NO. 4, JULY/AUGUST 2001
Fig. 7. SEM images of AA2024-T3 irradiated at 496 nm/500 fs with 200 pulses at a fluence (a) 0.0, (b) 0.02, (c) 0.06, (d) 0.1, (e) 0.4, and (f) 1.0 J/cm.
will produce a notable effect on the surface abaltion. The ab-sorption in the alloy results from the transition of the d-bandelectrons of coppers, and the absorption band is about 2.15 eV[32]. As a result, the 496-nm photons are referred to as nearlyresonant absorption, whereas the 248 nm is far off-resonance inthe energy compared to the copper band absorption. The sur-face-bound clusters or particles consequently appear at lowerfluence of 496 nm.
3) Surface Structuring With a Nanosecond-Pulse Irradia-tion: Our studies on the surface modification indicated that200 pulses at 308 nm with a pulse width of 15 ns are not ableto produce a significant modification to the surface, as shownin Fig. 8(a), (c), and (e). The surface feature develops as aresult of the ablation startup at the second phases, and the meltstructure develops as laser fluences increase. However, the1000-pulse irradiation generates significant surface modifica-tion, as manifested in Fig. 8(b), (d), and (f), that are found to
exhibit the threshold behavior as shown in the results of surfacestructuring using the femtosecond-pulse ablation.
As the irradiating fluence approaches the fluence value forablation in aluminum, surface texturing effects are observedand surface features ranging from several micrometers downto submicrometer dimensions become widespread, as shown inFig. 8(c). At fluences exceeding the ablation threshold, the sur-face melt zone is postulated to undergo rapid thermal expansion.The kinetic energy associated with the localized recoil pressureresults in splashing of the melt layer. Fine-textured surface tex-turing effects were observed to occur over a small fluence range(approx. 1.0–1.4 J/cm). Surface texturing effects appear defi-nitely controlled through the selection of appropriate combina-tion with pulse count, repetition rate, and fluence.
At irradiation fluences above the threshold, as shown inFig. 8(f), surface features become poorly defined. The dramaticloss in surface texture features results from extensive ablation,
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Fig. 8. SEM images of AA2024-T3 irradiated at 308 nm/15 ns by using a 200-pulse ablation at a fluence (a) 0.8, (c) 1.2, and (e) 2.7 J/cmand by using a1000-pulse ablation at a fluence (b) 0.8, (d) 1.2, and (f) 2.7 J/cm.
formation of the widespread melt area, volumetric heating, andsplashing of the melt region, as discussed in Section IV-B2.
V. DISCUSSION
Femtosecond optical pulses interacting with the metal pro-duce the following features that will be taken into account in thelaser structuring or machining. Electrons are driven to a muchhigher temperature than the ions or the lattice, and these hotnonequilibrium electrons interacting with the lattice through theelectron–phonon coupling elevate the lattice temperature. Fi-nally, an equilibrium distribution of the excess energy betweenthe electron and lattice is achieved. Thus, the electron and lat-tice can be separately described by their characteristic temper-atures and . Three characteristic time constants, , and
are thus able to be introduced in (1) and (2).is the electron cooling time, is the lattice heatingtime ( ), and is the laser pulse width. Accordingto the relative relation between, , and , we classify thelaser-metal interaction in different regimes from femtosecondto nanosecond.
A. Nanosecond Pulse Laser Treatment
For the nanosecond pulse laser ablation, is satisfiedand the electron temperature is equal to the lattice temperature,
. Equation (1) becomes
576 IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 7, NO. 4, JULY/AUGUST 2001
Long pulses in nanosecond scale first heat the metal surface tothe melting temperature and then accumulate the laser energyto heat the metal to the vaporization temperature. The thermalconduction dominates this process. The heat penetration depthis given by , and is usually fulfilledfor the long-pulse interaction with metals, whereis the heatdiffusion coefficient. The metal absorbs the energy given by
, where is the material density. Significant evapo-ration occurs while the accumulation of the deposited energy asa function of time exceeds the heat evaporation threshold.The laser fluence for a strong evaporation can be obtained at
This relation indicates clearly that the threshold fluence causingthe strong evaporation with long pulses is proportional to .
During the long pulse laser ablation, the thermal transport intothe metal takes a laser pulse duration, which is enough for thethermal conduction, and causes a relatively large area of meltedmetals. As a result, the evaporation starts from the liquid phase,which makes a complicatedly precise processing of metals witha nanosecond-pulse laser.
B. Femtosecond Pulse Laser Treatment
The femtosecond pulse duration is assumed to be muchshorter than the electron cooling time, [19], [20].Assume that the electron and lattice have the same temperaturebefore the laser interaction; thus the initial condition is givenby . Electrons are heated to a much highertemperature at the laser pulse end, i.e., .
The excitation source term is removed after the pulse irradi-ation ends, and (1) and (2) develop as follows:
(3)
(4)
The fast energy transfer to the lattice cools the electrons rapidly.A general solution to (3) and (4) can be found
(5)
with the following initial and boundary conditions:
(6)
is the heat penetration depth and isthe optical penetration depth. is the absorption fluence ofthe surface. The ablation process is thought to be a combinationof the optical and the heat penetration. The optical penetrationdominates the ablation at a low fluence, and then the heat pene-tration rules the ablation process and spreads deeply and widely
as laser fluence increases. Equation (5) for these two cases de-velops to the following expressions:
(7)
(8)
Consequently, the ablation depth per pulse for the specific casesdevelops as a function of fluence
(9)
(10)
Two components are found in the logarithmic fluence depen-dence of the ablated depth for the femtosecond pulse ablationin Fig. 3. They are well understood by considering the ablationprocesses in connection with the optical penetration depth andthe electronic heat penetration. The former explains the surfacemodification produced under the low laser fluence, and the latteris responsible for the deep layer ablation with the higher fluenceirradiation.
The ablation process can be considered as a solid-vapor phasetransition because femtosecond laser pulses ablate metals invery short time duration. Rapidly heating lattice in a picosecondtime scale directly creates vapor or plasma in the metal surface.The thermal conduction to the metal can be neglected in thermaldynamic processes. Therefore, femtosecond laser pulses are sig-nificant to achieve very precise laser processing of metals. How-ever, during the long pulse laser ablation, the thermal transportinto the metal takes about laser pulse duration, which is enoughfor the thermal conduction, as discussed in previous sections,and causes a relatively large area of melted metals. As a result,the evaporation starts from the liquid phase, which makes pre-cise processing of metals with a nanosecond-pulse laser verycomplicated.
The breakdown threshold becomes precisely controlled atan irradiation of ultrashort pulses compared to long pulsesbecause the heating effect is much reduced. When the plasmaof free electrons is generated, irreversible material breakdownand ablation occur. The electrons transfer their energy tothe ions or the lattice while the electrons are heated up to amuch higher temperature by absorbing laser energy throughelectron–ion coupling. The energy transfer is then heatingmaterial. The heating process depends on the laser pulseduration. The material volume at the laser focal spot absorbslaser energy, and subsequently the absorbed energy transfersto a larger volume that is heated by heat conduction due tothermal gradient. The energy exchange from electrons to latticefor the long pulse during laser–matter interaction is strong, anda much larger volume can be involved in heat diffusion andmelted. This results in a relatively small layer’s being heatedto the vaporized temperature. For the ultrashort laser pulse, theinteraction time is much shorter and the laser field strength isvery strong. The electrons are driven to a very high temperature,and then the ions or the lattice will be rapidly heated to hightemperature through electron–ion energy exchange comparedto the long pulse interaction. The short interaction time and thehigh laser intensity make the focal part of the interacted layermelt and vaporize immediately and synchronously. Therefore,
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the breakdown threshold is precise, and the fine structuring canbe achieved using the ultrashort pulse irradiation.
C. Considerations Under a Repetitive Femtosecond-PulseIrradiation at a Higher Fluence
The ablated surface evolution is subjected to the successivepulse irradiation. Subsequently, the surface properties associ-ated with surface reflection, absorption, stress, quenching, andso on have to be taken into account. The evolution of surface fea-ture irradiated with the repetitive pulse ablation alters the natureof laser–surface interactions. Such effects resulting from local-ized anisotropic surface reflectivities lead to considerable vari-ations in localized absorbance and strong differential thermalloading. Highly localized heating and large thermal gradientcause the development of isolated particulate formations andthe development of severe strain fields. Large numbers of irra-diating pulses eventually cause the stresses to exceed the localelastic limit of the alloy, resulting in melt plastic deformation.
As the irradiating fluence approaches the fluence value forablation in aluminum, surface texturing effects are observedand surface features ranging several micrometers become wide-spread. The resulting topography is thought to form from a com-bination of several anisotropic factors, including differential lo-calized surface absorbance; localized surface ablation; spatiallyconfined particulate formations; and thermally induced strainregions produced by rapid quenching of the melt regions.
At fluences above the ablation threshold, the surface-boundparticles in the melt zone are postulated to undergo rapidthermal expansion under the successive pulse irradiation. Thekinetic energy associated with the localized recoil pressureresulted partly in splashing of the melt specimens. Under theinfluence of rapid quenching rates, the melt resolidificationcreates large thermally induced stresses, which are associatedwith extensive plastic deformation. Fine-textured surfaceeffects were observed to occur over a comparatively smallfluence range. It is apparent that large anisotropy is inducedover this fluence regime, causing spatially confined stressesand related surface effects. Surface features are controlledthrough the selection of appropriate pulse count, repetitionrate, and fluence. At a high irradiating fluence, surface featuresbecame coarse and poorly defined. The dramatic loss in finesurface features is postulated to occur as a result of extensiveablation, widespread melt formation, greater volumetric heatingand substantially reduced quenching rates, and reflow of themelt region. Especially important are the comparatively shortquenching times, which promote stress relaxation and therebyprompt plastic deformation of the surface.
VI. SUMMARY
Surface structuring of metal substrates using a femtosecondpulsed laser irradiation is characterized with SEM images as afunction of laser fluence ranging from 0.02 to 10.0 J/cm. Twoablation regimes are identified as the optical and energy pene-tration by examining the fluence dependence of ablated depthper pulse. Laser ablation provides a mechanism to facilitate anachievement of nano- and microstructuring on the metal sur-face. The surface texturing in nanodimension is observed for
aluminum alloy at a lower fluence excitation using a ten-pulsetreatment with a pulse width of 500 fs. A surface feature of sur-face-bound particles can be developed by the controlling use oflaser ablation parameters. The results indicate that the ablatedsurface evolution is subjected to the successive pulse irradiationand the subsequent surface properties, such as reflection, ab-sorption, stress, quenching, and so on. The repetitive pulse abla-tion alters the nature of laser–surface interactions. Based on ourresults, an ultrashort pulse provides a wide competitive range ofapplications in surface structuring and patterning from nano- tomicrodimensional scales.
ACKNOWLEDGMENT
The authors would like to thank Dr. J. J. Song and Dr. S. J.Hwang at the OSU Center for Laser and Photonics Research;E. McTernan, R. Ray, V. Pogue, J. Jobe, and J. Todd at the En-vironmental Institute; and Dr. K. Church, B. Irwin, D. Moon,H. Church, and S. Coleman at CMS Technetronic Inc. for theirhelpful discussions and assistance.
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Kai Dou received the B.Sc. degree in physics from Jilin University, Changchun,China, in 1981 and the M.Sc. and Ph.D. degrees in physics and nonlinear opticsfrom Changchun Institute of Physics, Chinese Academy of Sciences, in 1986and 1992, respectively.
He is currently a Research Scientist with the University Center for Laser andPhotonics Research, the Center for the Energy Research, and the EnvironmentalInstitute, Oklahoma State University, Stillwater. From 1983 to 1992, he was anAssistant Research Fellow and Associate Research Fellow with Changchun In-stitute of Physics, where he worked on the electron and phonon coupling ofIII–V semiconductors with intense laser excitation, laser processing of mate-rials, optical nonlinearity, and ultrafast processes of the excited states in ultrafinesemiconductor particles and organic molecule materials. In 1992–1993, he wasa Visiting Research Scientist at Laboratory Aime Cotton, the National Centerfor Scientific Research, France, where he conducted the research on how toapply spectral hole burning to optical data storage and the analysis of ultrafastoptical signals by the field cross-correlation. From 1993 to 1996, he was anAssociate Professor and then a Professor of physics at the Changchun Instituteof Physics. His research focused on the application of nanosize semiconduc-tors to photonics and the study of photophysical properties of low-dimensionalsemidonductor and organic materials. He has published more than 60 papers inprofessional journals. Since 1996, he has been as a Research Scientist with Ok-lahoma State University. His current research interests involve nonlinear opticaleffects and ultrafast processes in chromophore-doped sol-gels and nanomate-rials for sensors and optical limiters; laser processing of materials; and surfacepatterning and structuring in nanodimension with femtosecond pulsed lasers.
Prof. Dou is a member of the International Society for Optical Engineering,the Optical Society of America, and the Materials Research Society of America.
Robert L. Parkhill was born in Ardmore, OK, on February 13, 1971. He re-ceived the B.S. degree from the University of Central Oklahoma, OklahomaCity, OK, and the Ph.D. degree from Oklahoma State University, Stillwater,both in chemistry.
He was with the Air Force Material Directorate at Wright Patterson Air ForceBase, Dayton, OH, from 1998 to 1999. He is currently with CMS Technetronics,Inc., Stillwater, where he is working on developments in sol-gel systems andlaser processing of materials. His research interests include sol-gel thin-film ap-plications for corrosion resistance and laser surface texturing of metal surfaces.
Dr. Parkhill is a member of the Americal Chemical Society, International So-ciety for Optical Engineering, and Materials Research Society of America.
Jack Wu received the B.Sc. degree in civil engineering and the M.Sc. degree insoil physics and groundwater hydraulics from Wuhan University of Hydraulicand Electric Engineering, Wuhan, China, in 1983 and 1986, respectively, andthe Ph.D. degree in soil science from the University of Wyoming, Laramie, in1996.
He is currently a Research Associate at the Department of Plant and Soil Sci-ence, Oklahoma State University, Stillwater. His main research areas includewater movement and solute transport in soil media, plant–water relationships,water resources management, animal waste management, and the fate and trans-port of agrochemicals. His current research focuses on the development of math-ematical and numerical models to predict ammonia volitization from swine-ef-fluent irrigation, the development of GUI-oriented web-based interactive com-puter programs for soil physics teaching and research, and the development ofmultipurpose decision support systems for agrochemical management and agri-cultural production.
Dr. Wu is a member of the Soil Science Society of America, the Crop ScienceSociety of America, and the American Society of Agronomy
Edward T. Knobbe received the B.Sc. degree in engineering, the M.Sc. degreein materials, and the Ph.D. degree in engineering from the University of Cali-fornia, Los Angeles, in 1982, 1986, and 1990, respectively.
From 1981 to 1990, he was a Member of Technical Stuff at Hughes AricraftCompany. He has received three patents. He joined the Department of Chem-istry, Oklahoma State University, Stillwater, in 1990 as an Assistant Professorand is now a full Professor. He is currently Associate Dean of the Graduate Col-lege of Arts and Sciences, Director of the Environmental Institute, and Directorof the Center for the Energy Research, Oklahoma State University. His currentresearch interests involve metal alkoxide and sol-gel chemistry, thin-film mate-rials preparation and processing methods, corrosion-resistant coatings and cor-rosion science, and material preparation and processing using UV pulsed fastersources.
Prof. Knobbe is a member of the American Chemical Society, the Interna-tional Society for Optical Engineering, and the Materials Research Society.He received the Hughes Aircraft Company Technical Acnievement Award, theHughes Aricraft Company-Radar Systems Group Achievement Award, and theOSU College of Arts and Sciences Scholarly Research Excellence Award.
