Characterization of laser ablation of polymethylmethacrylate at different laser wavelengths

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Characterization of laser ablation of polymethylmethacrylate at differentlaser wavelengthsL. Torrisi ab; A. Lorusso cd; V. Nassisi cd;A. Picciotto ab

a Department of Physics of Messina, Messina, Italy b INFN - Laboratori Nazionali del Sud (LNS),Catania, Italy c Department of Physics of Lecce, Laboratorio di Elettronica Applicata e Strumentazione(LEAS), d INFN of Lecce, Lecce, Italy

To cite this Article Torrisi, L. , Lorusso, A. , Nassisi, V. andPicciotto, A.(2008) 'Characterization of laser ablation ofpolymethylmethacrylate at different laser wavelengths', Radiation Effects and Defects in Solids, 163: 3, 179 — 187To link to this Article: DOI: 10.1080/10420150701259172URL: http://dx.doi.org/10.1080/10420150701259172

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Radiation Effects & Defects in SolidsVol. 163, No. 3, March 2008, 179–187

Characterization of laser ablation of polymethylmethacrylate atdifferent laser wavelengths

L. Torrisia,b*, A. Lorussoc,d, V. Nassisic,d and A. Picciottoa,b

aDepartment of Physics of Messina, 98128 Messina, Italy; bINFN - Laboratori Nazionali del Sud (LNS),Via Santa Sofia, 95124 Catania, Italy; cDepartment of Physics of Lecce, Laboratorio di Elettronica

Applicata e Strumentazione (LEAS); d INFN of Lecce, Via per Arnesano, 73100 Lecce, Italy

(Received 12 January 2007; in final form 3 February 2007 )

Pulsed laser ablation of polymethylmethacrylate (PMMA) in high vacuum was investigated by usingdifferent laser beams. A Nd:Yag laser, working at fundamental (1064 nm) and second harmonics (532 nm)with 9 ns pulse duration and 800 mJ maximum pulse energy was used at INFN-LNS of Catania. Twoexcimer UV lasers working at 308 nm (XeCl) and 248 nm (KrF) with 20 ns pulse width and 600 mJmaximum pulse energy was used at INFN-LEAS of Lecce. The laser ablation effects on the PMMA wereinvestigated by using similar experimental set-ups and fluences ranging between 2 and 20 J/cm2. A 200amu mass quadrupole spectrometer was employed to detect ‘on line’ the atom and molecular speciesemitted in vacuum by the laser ablation process. The ablation yield increases with the laser wavelength,demonstrating that IR radiation induces photothermal effects. The molecular fragmentation increases withthe photon energy, demonstrating that photochemical effects become predominant for UV radiation.

An investigation about the possible laser ablation mechanisms of PMMA, depending on the laserparameters, will be presented and discussed.

Keywords: laser ablation; polymethylmethacrylate; photothermal effects; photochemical effects

1. Introduction

The laser ablation of polymers is an interesting research field, especially due to the high control onthe etching profile. Since the pioneering work by Srinivasan and Leigh (1), the pulsed laser abla-tion of polymer materials has been the object of growing interest due to the potential applicationsin chemistry and electronic technology (2). It represents, also, a versatile technique for fabri-cation of complex 3-D surface relief structures using programmed mask scanning or diffractiveoptics techniques as well as improvement of micro-optical components such as diffractive lenses,photolithography, electrical conductive polymers, micromechanical properties and preparation ofpeculiar deposited films (3). Nowadays, the laser ablation of polymethylmethacrylate (PMMA)is widely studied in the field of engineering, semiconductor packaging, microelectronics and thinfilm medical applications (4–6).

*Corresponding author. Email: [email protected]

ISSN 1042-0150 print/ISSN 1029-4953 online© 2008 Taylor & FrancisDOI: 10.1080/10420150701259172http://www.informaworld.com

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180 L. Torrisi et al.

It is well known in literature that the laser ablation of hydrogenated polymers is a function ofthe deposited energy and that the ablation yield depends on the materials absorption coefficientat the applied incident laser wavelength (7).

The mechanisms responsible for the ablative photodecomposition of the irradiated polymerare the involved photochemical and photothermal effects. Depending on the properties of theirradiated material and the irradiation conditions, such as wavelength and pulse duration, one ofthem may become dominant. Generally, the photochemical (1) ablation occurs when the incidentphotons have enough energy to bring the molecule in the excited electronic state directly breakingmain chain bonds. Energy absorbed in this bond-breaking process would restrict the temperaturerise and the extent of thermal damage to the substrate. Instead, generally, the thermal ablationoccurs when the incident single photon energy is not sufficient to break a single chemical bond. Inthis case the polymer chains bond breaking occurs for collective photon–molecule interactions.The thermal process can produce effective polymer ablation in which the photons couple tovibrational modes of the molecule (8, 9).

This work summarizes results about the ablation effects induced by nanosecond laser ablationof PMMA chains by using different laser fluences and wavelengths. The ablation effects wereinvestigated in order to understand the chemical modification of the polymer submitted to theenergetic interaction with fast and energetic laser pulses.

2. Material and methods

PMMA is a vinyl-polymer produced by free radical vinyl polymerization from the monomermethylmethacrylate (MMA). Table 1 shows the structural formula of the monomer (C5H8O2)n

and the polymeric structure of the PMMA.The PMMA samples, utilized in our measurements, were provided by Goodfellow©. They

were pure polymers with 2 cm × 2 cm surface and 2 mm thickness.At INFN-LNS of Catania, a Nd:Yag laser of 9 ns Q-switch pulse duration working at 1064 nm

(fundamental) or 532 nm (second harmonic) and a maximum pulse energy of 800 mJ and 500 mJ,respectively, was employed in order to irradiate the target in the infrared and visible region at

Table 1. Chemical scheme of the PMMA polymerization(a) and main physical properties of the polymer (b).

Properties PMMA

Density 1.16 g/cm3

Stoichiometry C5H8O2Water absorption rate 0.3%Elongation 48%Tensile strength 7000 psiCompression strength 11500 psiMelting point 101◦CExpansion coefficient 5.5 × 10−5 ◦C−1

Dielectring strength 17 kV/mmRefractive index 1.49VIS transmittance (3 mm) 92%Hardness (Rockwell) 90Molecular weight of the monomer 100


Radiation Effects & Defects in Solids 181

Figure 1. Catania and Lecce set-up schemes.

a laser fluence ranging between 2 and 20 J/cm2. At INFN-LEAS of Lecce, a XeCl and a KrF(Lambda Physics) excimer laser operating at 308 nm (maximum energy of 200 mJ) and 248 nm(maximum energy of 600 mJ), respectively, with 20 ns pulse width, were employed to ablatePMMA at the same laser fluence range of Nd:Yag. All lasers operated in single pulse mode or atthe repetition rate of 1–30 Hz.

The PMMA laser ablation was performed at room temperature in a vacuum chamber of10−7 mbar. A mass quadrupole spectrometer (MQS), Pfeifer PRISMA 200, was employed ‘online’ to the laser irradiation, in order to detect the ablated particles produced, as a gas developed inthe vacuum chamber, during the ablation process. The MQS is characterized by a sensibility valuebelow 1 ppm for mass 40 amu and it is equipped with a Secondary Electron Multiplier (SEM)detector to analyse the masses ranging between 1 and 200 amu. The instrumental sensor did notsee the direct emission from the ablated target and it is placed at about 50 cm distance from thetarget. Figure 1 shows the experimental set-up schemes employed at Catania and Lecce for thelaser irradiation of PMMA targets. The laser was focused on the polymer surface by a lens (50 cmfocal length) in order to have a spot dimension of about 1 mm2. The laser incidence angle withrespect to the target normal was 45◦.

‘Off line’ measurements of the ablated polymer were performed with a stylus surface profiler(Tencor Instruments ALPHA-STEP 200) and with a traditional optical microscope, in order tocompare the crater profiles at different laser wavelength and to determine the ablation yield(removed mass/pulse) for different irradiation conditions.

3. Results

A comparison of 1–200 amu mass spectra of chemical species obtained by the ablation processinduced by different lasers operating at about 10 J/cm2 laser fluence is reported in Figure 2. Twotypical spectra obtained with excimer lasers at 248 nm and 308 nm wavelengths are shown inFigure 2a and 2b, respectively and two typical spectra obtained with the Nd:Yag laser, operatingat 532 nm and 1064 nm wavelength, are shown in Figure 2c and 2d, respectively. The spectracomparison, reported with subtracted background, indicates that an extensive fragmentation ofthe main chain of PMMA occurred by decreasing the photon wavelength. The number of molecular



Figure 2. Mass spectra produced by laser ablation of PMMA at 248 nm (a), 308 nm (b), 532 nm (c) and 1064 nm (d).

species produced by the ablation process in the range 1–200 amu, in fact, increases strongly withthe photon energy. This result is in agreement with the photothermal and photochemical processesinvolved in the ablation mechanism. Infrared and visible radiations, in fact, induce mainly thermaleffects, producing a low fragmentation of the main chain of PMMA (C5H8O2)n, the yield of whichincreases with the laser intensity more than with the photon energy. On the contrary, ultravioletradiation induced an extensive fragmentation, which is proportional to the photon energy. Theseresults confirm that the UV laser energy is sufficient to break the chemical bonds of the polymerchain resulting in the production of a large range of light molecular species (10, 11).

The mass spectrum, obtained at 248 nm, shows the production of the main chemical speciessuch as CO (28 amu), CO2 (44 amu), COH3 (31 amu), COOCH3 (59 amu), CH3OH (32 amu),CH4 (16 amu), HCOOCH3 (60 amu), according to the literature (12), and also the presence ofmeta-stable groups, e.g. C2H2- (acetylene groups), CH3- (methyl groups) and C6H6- (benzenegroups) and of the other groups due to the photochemical dissociation. At 5 eV (248 nm) photonenergy, the chemical bond of C−H (3.5 eV bond strength) can be broken by a single photon whilethe chemical bonds C−C (6.3 eV bond strength) and C−O (11.2 eV bond strength) can only bebroken by multiple photon interactions. The consequence of such interaction is the formation ofa high rate of atomic species and radical formation.

Moreover, Figure 2 shows that the MMA production (C5H8O2, 100 amu monomer weight) wasabsent at 248 nm, whereas it was present, as a small amount, in the 308 nm spectrum due to thedepolymerization reactions as a consequence of the thermal effects which start to be evident atmore longer wavelengths.



Furthermore, it is noteworthy to mention that when IR or visible laser irradiation was applied,the induced thermal effects are dominant and in this case the temperature increasing may alsoinduce additional decomposition of the main thermal product, MMA.

Besides, from the spectra of Figure 2 it is also possible to observe that a lot of common lightchemical species are produced at the different wavelengths.

Furthermore, the heavier species detected during the laser ablation had mass 169 at 1064 nmand mass 165 at 532 nm, while mass 128 was detected at 308 nm and at 248 nm wavelength.These considerations confirm that the photochemical reactions are dominant with respect to thephotothermal ones in the polymer fragmentation at shorter wavelengths.

The PMMA laser ablation at different wavelengths was controlled ‘off line’ by measuring thecrater depth profiles with a stylus surface profiler. Figure 3 reports a comparison of the crater depthprofiles obtained by 248 nm (a) (80 shots at a laser fluence of 3 J/cm2), 308 nm (b) (70 shots at alaser fluence of 3 J/cm2), 532 nm (c) (60 shots at a laser fluence of 7 J/cm2) and 1064 nm (100shots at a laser fluence of 20 J/cm2). The crater volume was calculated assuming a trunk of coneshape and the removed mass was calculated by considering the PMMA density of 1.16 g/cm3.Thus the ablation yield can be measured in terms of removed mass (µg) per pulse.

Figure 4 shows a comparison of ablation yields vs. laser fluence for the four different wave-lengths. Results indicate that, for each wavelength, the ablation yield increases linearly with thelaser fluence. The ablation yield is low at lower wavelengths, at which it is of the order of µg/pulsefor UV radiation, and increases at higher wavelengths, becoming higher than 100 µg/pulse for IRradiation, according to literature data (12). Moreover, the ablation yield shows ablation thresh-olds depending on the laser fluence value under which no ablation occurs. The experimentalablation thresholds are about 0.2 J/cm2 at 248 nm, 0.5 J/cm2 at 308 nm, 1.7 J/cm2 at 532 nm and1.85 J/cm2 at 1064 nm.

The results indicating that the ablation yield is low at lower wavelengths, so as for UV radiation,and increases at higher wavelengths, so as for IR radiation, which is in agreement with the

Figure 3. Crater depth profiles obtained by laser ablation at: 248 nm (a), 308 nm (b), 532 nm (and 1064 nm (d).



Figure 4. PMMA ablation yield vs. laser fluence for the different used wavelengths.

Figure 5. PMMA absorption coefficient vs. wavelength.

absorption coefficient of PMMA. This coefficient is high at low wavelength, where low-lightpenetration occurs and it is low at high wavelengths, where high-light penetration occurs, asreported in Figure 5 (13). The absorption coefficient dependence on the wavelength explains thedifferent ablation thresholds obtained at different wavelengths.

It is interesting to notice that the craters obtained with the shorter wavelength radiation(UV) were characterized by a regular shape, well-defined crater aperture with a size corre-sponding to the laser spot diameter, well net-defined edges, no change in the colour of thepolymer crater and absence of edge debris around the cone aperture, in agreement with atypical photochemical etching process. Instead, the craters produced with longer wavelengths(IR) were characterized by irregular shape with a crater aperture larger than the laser spot



dimension, indicating possible thermal diffusion processes. These last crater profiles shownalso the presence of edge regions, due to an amount of re-solidified material ejected duringthe ablation process, which increased proportionally to the number of laser shots. Moreover, at1064 nm wavelength the crater edges showed a black coloured region, typical of a carbonizedmaterial. Figure 6 compares two optical microscope photos relative to the crater produced at1064 nm (IR) and at 308 nm (UV) obtained by 4 J/cm2 pulse ablation and 1 mm spot diam-eter of PMMA. The first photo shows clear indication of thermal etching processes, due tothe larger spot size, to the irregular crater edges due to re-solidified fuse polymer and to theblack carbon present in the crater surface; the second photo shows a net edges crater anda clear crater surface without carbon debris or re-solidified parts, typical of photochemicalprocesses.

A data comparison between the PMMA ablation yield vs. wavelength, at different laser fluence,permits to plot the trends reported in Figure 7. The plot indicates that, although two differentmechanisms of ablation occur (photochemical and photothermal), the resulting ablation yieldincreases proportionally with the laser wavelength and with the laser fluence. In conditions ofphotochemical processes the dependence is almost linear, while in presence of photothermalprocesses the dependence is almost exponential. The etching yields produced by the visibleradiation (532 nm) lie intermediately in between the low-UV values and the high-IR values andseem to be due to both effects.

4. Conclusions

The main bond strengths in the polymer molecules are due to the chemical bonds C−H, C−C,C−O and H−H, which have bonding energies of the order of 3–4 eV. Thus the used IR and

Figure 6. Comparison of crater images for ablation at 4 J/cm2 using 1064 nm (IR) and 308 nm (UV).



Figure 7. Ablation yield vs. wavelength for different laser fluences.

visible photons transporting 1.16 eV and 2.32 eV, respectively, have insufficient energy to breakchemical bonds by direct impact. Only the collective absorption of the radiation photons canproduce molecular dissociation with heat transfer to the reticule structure, i.e. though thermaleffects. The used UV photons, transporting 4.02 eV at 308 nm and 4.99 eV at 248 nm, instead,have sufficient energy to break chemical bonds and to induce a direct molecular dissociationproducing scissions, free radicals ad free atoms and molecules, i.e. their effect is mainly due tophotochemical processes.

The ablative photodecomposition of PMMA is strongly dependent on the irradiation conditions,especially concerning the wavelength and the pulsed energy values. At 1064 nm and 532 nmradiation the thermal ablation process is dominant which is responsible for the production ofcraters characterized by an irregular shape and it transforms a component of the polymer intocarbon layers. At these wavelengths the polymeric chains are less fragmented with respect to theUV radiations. On the contrary, in the UV laser ablation photochemical processes are predominant,which are intrinsically capable to etch the polymer with a higher spatial resolution because thermaldamage to the surrounding material is negligible.

Mass spectra of the ablated products show that the chemical species produced during the ablationprocess depends on the laser wavelength; in particular, by UV ablation an extensive fragmentationof the main chain of PMMA occurs, especially with the shorter wavelength radiation.

In conclusion, the PMMA laser ablation in vacuum can be controlled mainly by the energyand wavelength parameters. In order to produce micrometric holes in PMMA for a microfiltersheet realization, for example, high-focalized UV laser beams can be employed (14). To obtainthin deposited films of PMMA on different substrates, instead, the pulsed laser deposition tech-nique needs to be employed by using visible or IR laser radiation, which ensures lower chainfragmentation and removing of whole monomers from the irradiated target.

Acknowledgements

The authors thank the INFN Gr. V Commission for the financial support given to the PLAIA (Plasma Laser Ablationfor Ion Acceleration) and PLATONE (Pulsed Laser Ablation for Transient ObtaiNable Electric-field) Projects, and toProfessor M. Di Giulio for his assistance for the crater profile measurements.



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(10) Blanchet, G.C.; Cotts, P.; Fincher Jr., C.R. J. Appl. Phys. 2000, 88, 2975.(11) Kuper, S.; Stuke, M. Appl. Phys. 1987, B 44, 199.(12) Pham, D.; Tonge, L.; Cao, J.; Wright, J.; Papiernik, M.; Harvey, E.; Niclau, D. Smart Mat. Struct. 2002, 11, 668.(13) Torrisi, L.; Borrielli, A.; Margarone, D. Nucl. Instrum. Method 2007, B 255, 373.(14) Torrisi, L. Bio-Med. Mat. Eng. 1994, 4(1), 17.


Characterization of laser ablation of polymethylmethacrylate at different laser wavelengths

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