The effect of the ablation mechanism on surface morphology changes during an ablation process was studied by comparing three different polymers: a triazene polymer, a polyimide and poly(methylmethacrylate) (PMMA) with nanosecond surface interferometry. The triazene polymer, for which only indications for a photochemical ablation mechanism had been detected in previous studies, revealed no surface swelling, which could be attributed to a thermal ablation mechanism. For polyimide, a photothermal ablation mechanism is usually used to describe the ablation process at irradiation wavelengths ≥248nm. However, the interferometric measurements do not show any surface swelling, which would be a clear indication for a thermal ablation mechanism. A surface swelling was only detected for PMMA with irradiation at 248nm and fluences below the threshold of permanent surface modification. The detected phase shift, which is proportional to the change of the film thickness and the refractive index, can be explained by the opposite signs of the thermal expansion coefficient and the thermal refractive-index coefficien

Hauer, M.

Lippert, T.

Wokaun, A.

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DOI: 10.1007/s00339-004-2721-zAppl. Phys. A 79, 1215–1218 (2004)Materials Science & ProcessingApplied Physics Am. hauert. lipperta. wokaunNanosecond surface interferometrymeasurements on designed and commercialpolymersPaul Scherrer Institut, 5232 Villigen PSI, SwitzerlandReceived: 25 September 2003/Accepted: 24 February 2004Published online: 26 July 2004 • © Springer-Verlag 2004ABSTRACT The effect of the ablation mechanism on surfacemorphology changes during an ablation process was studied bycomparing three different polymers: a triazene polymer, a poly-imide and poly(methylmethacrylate) (PMMA) with nanosec-ond surface interferometry. The triazene polymer, for whichonly indications for a photochemical ablation mechanism hadbeen detected in previous studies, revealed no surface swelling,which could be attributed to a thermal ablation mechanism.For polyimide, a photothermal ablation mechanism is usuallyused to describe the ablation process at irradiation wavelengths≥ 248 nm. However, the interferometric measurements do notshow any surface swelling, which would be a clear indicationfor a thermal ablation mechanism. A surface swelling was onlydetected for PMMA with irradiation at 248 nm and fluencesbelow the threshold of permanent surface modification. The de-tected phase shift, which is proportional to the change of the filmthickness and the refractive index, can be explained by the oppo-site signs of the thermal expansion coefficient and the thermalrefractive-index coefficient.PACS 52.38.Mf; 42.87.Bg; 71.20.Rv1 IntroductionLaser ablation is a promising tool to create mi-crostructures in polymers. Due to several drawbacks (i.e. highablation thresholds, low ablation rates, redeposition of debris)it is only used in a limited number of industrial applications(i.e. drilling of nozzles for ink-jet printers [1] and drillingof holes for multi-chip modules [2]). Various aspects of theablation mechanism and laser processing in general are dis-cussed in current reviews [3, 4]. It has been repeatedly em-phasized that a better monitoring of the temporal evolution ofthe ablation process is important to understand the physicaland chemical processes that characterize ablation. A more de-tailed understanding will help to improve the technique andto overcome some of the above-mentioned drawbacks. A var-iety of different time-resolved spectroscopy methods, suchas absorption [5], emission [6, 7], infrared [8, 9] or Ramanspectroscopy [10, 11], have been applied to probe the abla-tion process. Other techniques are used to analyze the ablation Fax: +41-56/310-2199, E-mail: thomas.lippert@psi.chproducts [12–15] or to measure the velocity of the shock wavereleased in air [16, 17].In this study, nanosecond (ns) surface interferometry isused to observe the morphology changes of the polymer sur-face during the ablation process. A variation of this techniquehas been used in previous studies to observe the ablation pro-cess of different polymers. In some cases surface swellingprior to material removal was observed [18, 19], while in othercases the material removal started and ended with the laserpulse [19–21]. Pronounced surface swelling has been used asan indication for a photothermal process, where thermal ex-pansion is observed prior to ablation.Three polymers, i.e. a triazene polymer, a polyimideand poly(methylmethacrylate) (PMMA), were selected tostudy the influence of the ablation mechanism on the sur-face changes and to evaluate the appearance of differentmechanisms in the interferometry. The triazene polymer be-longs to a group of specially designed polymers with superiorlaser ablation properties (sharp ablation edges, no debris, lowthreshold fluence and high etch rates at low fluences [22])and a high absorption coefficient at all applied wavelengths.Previous studies of this polymer gave indications for a pho-tochemical ablation process at an irradiation wavelength of308 nm [12]. It has been reported for KaptonTM, a polyimidewhich is very similar to the polyimide used in this study, thatthe irradiation wavelength has a pronounced influence on theablation mechanism, i.e. photochemical at short wavelengthsand photothermal at longer (≥ 248 nm) wavelengths [23].Polyimide should therefore be a good example of a polymerwhere a transition from a photochemical to a photothermalablation process can be observed. The polyimide and thetriazene polymers are studied at three different irradiationwavelengths (193, 248 and 308 nm). The surface expansionof PMMA after 248-nm irradiation was studied as a referencefor a clear thermal process at fluences below the threshold ofpermanent surface modification.2 ExperimentalThe triazene polymer was prepared by a methoddescribed elsewhere [24]. The 1- to 2-µm-thick polymer filmswere spin coated from a chlorobenzene solution (10% poly-mer and 0.05% of a surfactant) of the polymer and driedfor 24 h at 40 ◦C. The polyimide films (1–3 µm thickness)1216 Applied Physics A – Materials Science & Processingwere prepared by spin coating a precursor solution and bak-ing the films for 1 h at 250 ◦C (under nitrogen atmosphere).It is noteworthy mentioning that this polymer is not identicalwith KaptonTM, but it is the same polymer as used in anotherinterferometric study [19]. The PMMA films were preparedby spin coating of a 15% solution of PMMA in chloroben-zene (with 0.05% surfactant) and drying the film in vacuum at60 ◦C for 12 h.The ns surface interferometry setup has been described indetail elsewhere [25]. Briefly, in a pump–probe setup an ArF(LPX 300, Lambda Physik, FWHM 25 ns), a KrF (LPX 100,Lambda Physik, FWHM 20 ns) or a XeCl (Compex 100,Lambda Physik, FWHM 30 ns) excimer laser was used to ir-radiate the polymer surface. The second harmonic of a Nd :YAG laser (Brilliant B, Quantel, FWHM 6 ns) was used ina Michelson interferometer to measure the thickness of thepolymer film. To avoid the influence of the released shockwave on the interferometer, the thickness of the polymer wasmeasured from the back side. Changes of the polymer thick-ness result in a change of the fringes from the Michelson in-terferometer. The data evaluation converts this fringe shift intoa phase shift, which is proportional to the thickness change ofthe polymer film:∆p(∆d) = 4π ∆dnDλ. (1)From this equation the phase shift depends only on the changeof the film thickness, ∆d, the refractive index, nD, and thewavelength, λ, of the probe beam. Within the same measure-ment series a linear relation between the ablation depth andthe resulting phase shift is obtained, but the coefficient that re-lates both ((1) shows that this should only depend on λ andnD, which are both constant) changes from measurement se-ries to measurement series. Possible origins for this effect maybe the multiple reflection of the laser beam within the thin filmor slight changes of the angle of the substrate within the inter-ferometer. At the moment, it is not clear which source is moreimportant.3 Results and discussion3.1 Triazene polymerThe changes of the phase shift of the triazene poly-mer during and after irradiation with 193-, 248- and 308-nmlaser light are shown in Fig. 1. The time t = 0 is defined asthe time where the laser pulse starts (the laser intensity ex-ceeds 1% of its maximum intensity). The positive phase shiftcorresponds to a decrease of the polymer thickness. The timeprofiles of the corresponding laser pulses and deposited en-ergies are also included in Fig. 1. The scaling of the laserintensity and the deposited energy was adjusted to the begin-ning and the end of the phase shift of the polymer.The changes of the surface morphology start and end withthe laser pulse for all irradiation wavelengths and follow inall cases the deposited laser energy quite well. Small devi-ations are observed in the initial part of the deposited laserenergy, i.e. slightly delayed for 248- and 308-nm irradiationand maybe faster for 193-nm irradiation. This behavior is alsoobserved at other irradiation fluences (not shown) and is prob-ably due to an ill-defined surface position (i.e. due to denseFIGURE 1 The phase shift versus time data of the triazene polymer sur-face during and after 193-, 248- and 308-nm irradiation. The laser intensitiesand the deposited energies are included in arbitrary units for each irradiationwavelength. The positive phase shift corresponds to ablation, i.e. surfaceremovalgaseous products) during the beginning of the ablation pro-cess. It is therefore not possible to attribute this delay directlyto a delay in the surface-removal process.The ablation of the polymer occurs without any sign ofthermal expansion. This suggests that for all irradiation wave-lengths a photochemical ablation mechanism is most im-portant. This has also been supported by the detection ofvery similar ablation products for all irradiation wavelengths,which suggests identical reaction pathways, and the presenceof excited nitrogen species that could not be explained bya thermal process [12].3.2 PolyimideThe phase shift of the polyimide during and afterirradiation at the three wavelengths is shown in Fig. 2. Thelaser intensity and the deposited energy are again included inthe figure; the positive phase shift corresponds to material re-moval.The changes of the phase shift occur at all wavelengthswithin the laser pulse. The changes of the phase shift for 193-and 248-nm irradiation are faster than the deposition of thepulse energy and are even more pronounced than in the caseof the triazene polymer at 193 nm. For 308-nm irradiation thephase shift seems to follow the deposited energy quite well.As mentioned above it is not possible to relate this behavior di-rectly to the material removal. However, it is apparent that thechanges of the surface morphology are directly related to thelength of the laser pulse (i.e. the 308-nm laser pulse has a longtail and surface changes are observed till the end of the laserpulse). No signs of a thermal swelling were detected, whichHAUER et al. Nanosecond surface interferometry measurements on designed and commercial polymers 1217FIGURE 2 The phase shift versus time data of the polyimide surface dur-ing and after 193-, 248- and 308-nm irradiation. The laser intensities and thedeposited energies are included in arbitrary units for each irradiation wave-length. The positive phase shift corresponds to ablation, i.e. surface removalwould be an indication of a photothermal ablation process ofthis polyimide.For KaptonTM it is normally assumed that the ablationprocess at wavelengths ≥ 248 nm is mainly thermal [23]. Upto now only the polyimide used in this study was meas-ured with interferometry. For an irradiation wavelengths of351 nm a thermal swelling prior to ablation was previouslyreported [19]. It seems that the ablation process changes sig-nificantly when the irradiation wavelength changes from 308to 351 nm. The differences between the interferometric stud-ies and other measurements may be caused by three differenteffects which can not be separated at the moment: the changeof the dominant part in the mechanism from photochemicalto photothermal is not observed between 193-nm and 248-nmirradiation, but between 308-nm and 351-nm irradiation; sur-face swelling is up to now only observed for polymers witha low absorption coefficient at the irradiation wavelength; thepolyimide used in the interferometric studies has a differentchemical structure and therefore different properties.3.3 Poly(methylmethacrylate)PMMA was applied as a reference polymer fora true thermal process. Irradiation below the threshold ofpermanent surface modifications (i.e. reactions) should onlybe due to thermal heating of the polymer. PMMA exhibitsa quite high threshold of permanent surface modifications atan irradiation wavelength of 248 nm (900 mJ cm−2, ablationthreshold 1400 mJ cm−2 [21]).The phase changes of the PMMA film after 380 mJ cm−2at an irradiation wavelength of 248 nm are shown in Fig. 3.The laser intensity and the deposited laser energy are also in-cluded in the graph. The positive phase shift in this graph nowrepresents a surface expansion. The transient expansion of thepolymer starts with the laser pulse and is observed for morethan 10 µs. Images taken several seconds after the laser pulsedo not reveal a permanent swelling of the polymer surface. Anoscillatory behavior of the polymer surface after irradiation at248 nm has also been reported [21] and was attributed to tran-sitions over the glass-transition temperature Tg of the PMMA.Our measurements do not reveal such an oscillatory behav-ior, and we suggest that the observed variations are most likelydue to noise in the measurements related to the small phaseshifts.There is one experimental difference between the meas-urements reported earlier [21] and those reported in thispaper. Our measurements are performed with the probe beamcoming from the back side of the polymer, while the meas-urements of Masabuchi et al. [21] were performed fromthe front side of the polymer. Our measurements are in-fluenced by the index of refraction of the polymer, whichchanges with temperature. The change of the index of refrac-tion with temperature (T < Tg : dn/dT = −1.1 ×10−4 K−1,T > Tg : dn/dT = −2.1 ×10−4 K−1) [26] has the same orderof magnitude as the expansion coefficient of the polymer(T < Tg : dn/dT = 2.4–2.7 ×10−4 K−1, T > Tg : dn/dT =5.6–5.8 ×10−4 K−1) [26], but with the opposite sign. Thisresults in the effect that both effects counteract each other, re-sulting in a small change of the phase and only minor changeswhen the temperature exceeds Tg. We were therefore not ableto detect the previously described oscillations.PMMA has a low absorption coefficient at 248 nm andalso reveals a high threshold of permanent surface modifi-cations. It noteworthy that the counteracting effect of thethermal expansion and the temperature change of the refrac-tive index can be rationalized with the temperature-dependentvalues that have been obtained for low heating rates. In thecase of the interferometric studies, heating rates of up to2.5 ×109 K s−1 are obtained, and it has always been a matterFIGURE 3 The phase shift versus time data of PMMA during and after ir-radiation with 380 mJ cm−2 at 248 nm. The laser intensity and the depositedenergy are included in arbitrary units. The positive phase shift corresponds toa surface expansion1218 Applied Physics A – Materials Science & Processingof discussion whether these reference values will also hold forhigh heating rates. Our studies suggest that this may be thecase, which has also been observed by Masabuchi et al. [21].It is also interesting to emphasize that PMMA has at248 nm a very low absorption coefficient (< 200 cm−1),which supports the suggestion above that expansion is ob-served for polymers with low absorption coefficients at theirradiation wavelength. The correlation of the absorption co-efficient with material properties such as thermal expansion isan interesting tool for further modeling.4 ConclusionThe measurements of the different polymers re-veal a clear influence of the ablation mechanism on the time-resolved surface changes of the polymer. The triazene poly-mer reveals no surface swelling, which would be an indicatorfor a thermal process, after laser irradiation at different wave-lengths with fluences above the ablation threshold. This isin agreement with previous studies of the triazene polymer,where mainly indications for a photochemical ablation pro-cess were found.A transition from a photochemical to a photothermal abla-tion mechanism was expected to be observed for polyimide,but the results reveal no indication of a photothermal abla-tion mechanism at irradiation wavelengths of 248 or 308 nm.Whether this result is due to the different chemical structuresof the polyimide of this study and KaptonTM, for which a pho-tothermal ablation mechanism has been assigned for wave-lengths ≥ 248 nm, or if thermal expansion is only observed forpolymers with a low expansion coefficient at the irradiationwavelengths, is the subject of an ongoing study.A surface swelling was detected for PMMA with irradi-ation at 248 nm and fluences below the threshold of permanentsurface modification. The detected small phase shift can beexplained by the opposite signs of the thermal expansion coef-ficient and the thermal refractive-index coefficient. This sug-gests that these indices, obtained at low heating rates, behavesimilarly at high heating rates (2.5 ×109 K s−1).ACKNOWLEDGEMENTS The authors would like to thankD. Funk for helpful discussions. This work has been supported by the SwissNational Science Foundation.REFERENCES1 H. Aoki: US Patent No. 5 736 999 (1998)2 R.S. Patel, T.A. Wassick: Proc. SPIE 2991, 217 (1997)3 D. Bäuerle: Laser Processing and Chemistry (Springer, Berlin 2000)4 T. Lippert, J.T. Dickinson: Chem. Rev. 103, 453 (2003)5 H. Fukumura, E.-I. Takahashi, H. Masuhara: J. Phys. Chem. 99, 750(1995)6 H. Fujiwara, H. Fukumoto, H. Fukumura, H. Masuhara: Res. Chem.Intermed. 24, 879 (1998)7 K.H. Wong, T.Y. Tou, K.S. Low: J. Appl. Phys. 83, 2286 (1998)8 T. Lippert, A. Koskelo, P.O. Stoutland: J. Am. Chem. Soc. 118, 1151(1996)9 T. Lippert, P.O. Stoutland: Appl. Surf. 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Nanosecond surface interferometry measurements on designed and commercial polymers

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Nanosecond surface interferometry measurements on designed and commercial polymers

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