or sulfur-containing polymers revealed by time-resolved luminescence spectroscopy

Volume 194, number 3

CHEMICALPHYSICSLETTERS

26 June 1992

Laser ablation dynamics of silicon- and/or sulfur-containing polymers revealed by time-resolved luminescence spectroscopy H i r o s h i F u k u m u r a a, K o u j i H a m a n o a, Shigeru E u r a ~, Hiroshi M a s u h a r a a,~, H i r o n o b u Ito b, Taro Sakakibara b and Minoru Matsuda b a Department•fP••ymerScienceandEngineering•Ky•t•Institute•fTechn•l•gy.Matsugasaki'Ky•t•6•6.Japan b InstituteforChemicalReactionScience, Tohoku University, Katahira, Sendai 980, Japan

Received 5 February 1992; in final form 31 March 1992

Poly(p-tert-butylstyrene), poly(p-tert-butylstyrenesulfone), poly(p-trimethylsilylstyrene),and poly(p-trimethylsilylstyrene sulfone) films were ablated with a 248 nm excimer laser. Emission spectra of decomposed products such as Si atoms and C2 as well as CN moleculeswere measured, while no information of S atoms was obtained. The effects of contained Si and S atoms upon laser ablation were examined, and the ablation dynamicswas discussed,

1. Introduction

When a polymer film is irradiated by an intense UV-laser pulse, it can be etched instantaneously. This phenomenon termed as "laser ablation" [ 1 ] involves electronic excitation of chromophores, fragmentation to small molecules, and ejection of products from surface. Ablation behaviour is strongly dependent on photochemical properties of chromophores, chemical structure of the solid film, wavelength of laser light, its pulse width and intensity, and so on [2 ]. Although the ablation mechanism is complicated, its systematic study is one of the important subjects of solid state photochemistry. To understand the mechanism, molecular aspects of laser ablation should be clarified as a first step, namely, what kinds of excited states, molecular species, and products are generated in each stage of the ablation process. Spectroscopic approach is indispensable without any doubt, however, only few reports have been given yet. Guest Professor of Institute for Chemical Reaction Science, Tohoku University (August l, 1991-March 31, 1992). Correspondence to." H. Fukumura, Department of Applied Physics, Osaka University, Suita 565, Japan.

Recently we have applied nanosecond time-resolved luminescence spectroscopy to study laser ablation of poly (N-vinylcarbazole) and have shown that mutual interactions between excited singlet states (SI-SI annihilation) play an important role in photochemical primary processes [ 3 ]. The spectroscopy was also applied to study laser ablation of superconducting materials [4,5]. Ionic species in the electronic ground state are generated at the initial stage of the ablation, while neutral atoms are produced later by collisions in the ablation plume [ 5 ]. Thus, time-resolved luminescence spectroscopy is quite effective to elucidate the molecular mechanism of laser ablation. In this Letter, we report time-resolved luminescence spectra just upon laser ablation of polymers which contain sulfur and/or silicon. The bond energy of C-S and C-Si is 2.83 and 2.74 eV, respectively, and smaller than that of a C - C bond of the polymer (3.59 eV) [6]. The introduction of such low energy bonds to polymer matrices is expected to induce efficiently bond cleavage upon photoexcitation. The results clearly show that Si atom as well as C2 and CN molecules are generated at a few tens of nanoseconds after the laser pulse, but no information on S atom was obtained. The dynamic behav-

0009-2614/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

203

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iour of the fragmented Si, C2, and CN is presented, and laser ablation mechanism for these polymers is discussed.

2. Experimental Samples ofpoly(p-tert-butylstyrene) (PB), poly(ptert-butylstyrene sulfone) (PBS), poly(p-trimethylsilylstyrene) (PTMSi), and poly (p-trimethylsilylstyrene sulfone) (PTMSiS) were prepared from the corresponding styrene derivatives and optimum amount of SO2 by free-radical polymerization. Synthesis and characterization of these polymers have been reported previously [ 7 ]. The structures of these polymers are shown in scheme 1. Polymers were dissolved in chlorobenzene and spin coated on quartz plates to a thickness of 1 ktm. The films were cured for 10 min at 60°C and evacuated for more than 3 h at room temperature. The films were irradiated at normal incidence in air by an excimer laser (Lambda Physik EMG101MSC; 248 nm, 18 ns fwhm). The laser intensity was adjusted using a set of partially reflecting mirrors and monitored with a Gen-tec ED200 joulemeter. Depth profiles of ablated surfaces were measured by a depth gauge (Sloan Dektak 3030 or Tencor a-Step 200). O

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26 June 1992

Threshold laser intensity for ablation was estimated by plotting the etch depth against the laser intensity. An optical arrangement for measuring the time-resolved luminescence spectra just upon laser ablation was described elsewhere [5]. The luminescence passed through a polychromator (Jovin Yvon HR320) was detected by a streak camera system (Hamamatsu C2830) with a two-dimensional chargecoupled device (CCD) camera (Hamamatsu C3140). The time resolution and the spectral resolution were 0.6 ns and 1.6 nm, respectively, which were determined by an entrance slit width of the streak camera and that of the polychromator as well as the pixel size of the CCD camera. The origin of the time axis, t= 0, was defined to be the time for the laser pulse to take a maximum intensity at the detector.

3. Results of discussion The penetration depth of a laser pulse is determined by the absorption coefficient of a polymer film, and the absorbed energy per volume near the surface increases with an increase of the absorption coefficient. Hence, ablation threshold of a polymer film generally decreases with an increase of the absorption coefficient [8]. Since the absorption coefficients of PBS, PTMSi, and PTMSiS at the laser wavelength are smaller than that of PB, ablation threshold for the latter should be higher than the formers. However, experimental values for PBS, PTMSi, and PTMSiS were estimated to be all less than 20 m J / c m 2, while that for PB was around 80 m J / c m 2. This means that the introduction of weak bonds such as C-S and C-Si initiates decomposition with rather low energy and reduces the ablation threshold. It is expected, therefore, that various decomposed products are formed immediately after intense irradiation. Fig. 1 shows the time-resolved luminescence spectra upon ablation of PBS. A broad luminescence around 320 nm at an early stage of the irradiation could be assigned to fluorescence from the aromatic ring (fig. la). The fluorescence decayed rapidly within the laser pulse and instead a weak broad-band emission emerged (fig. lb). This emission was clearly observed when the surface was irradiated with an intense laser pulse, however, the emission gave no dis-

Volume 194, number 3

CHEMICAL PHYSICS LETTERS CN

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26 June 1992

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Fig. 1. Time-resolved luminescence spectra upon the laser ablation of poly(p-tert-butylstyrene sulfone). The spectra were recorded at the gate time of (a) - 9.9 to - 2.5 ns, (b) - 1.9 to 5.6 ns, (c) 6.2-50.0 ns, and (d) 149.5-197.7 ns. The origin of the time axis is set to the maximum of the laser pulse (248 nm, 18 ns, 1.34 J/cm2). tinct structure in the region from 270 to 470 rim. As time goes on after laser pulse (figs. 1c a n d 1d ) , several peaks were distinguished clearly a n d assigned to the emission from C N molecule (358 a n d 386 n m ) a n d that from C2 molecule (436 and 468 n m ) according to the reference spectral d a t a [9]. The rise a n d decay profiles o f the p o l y m e r fluorescence a n d the radical emission are shown in fig. 2. It is worth noting that the C N radical was f o r m e d rather slowly i n d e p e n d e n t o f the laser intensity. The t e m p o r a l beh a v i o u r o f the fluorescence d i d not change essentially when the laser intensity was reduced. T h e intensity o f C2 a n d C N b a n d s as well as that of.the b r o a d b a n d were reduced with a decrease o f the laser intensity, a n d only the p o l y m e r fluorescence was observed at the laser intensity just a b o v e the ablation threshold. The emission from bare silicon a t o m s was observed when P T M S i a n d P T M S i S were ablated with a laser intensity o f m o r e than 0.5 J / c m 2. Fig. 3 shows the time-resolved spectra for PTMSi. The bands at 288 a n d 391 n m are assigned to the transition tp~ ~ ~D2 and ~P~ --, t So o f Si atom, respectively [ 10 ]. The generation o f ~P~ state requires the energy o f 5.08

0

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Time(ns) Fig. 2. Luminescence rise and decay curves upon the laser ablation of poly(p-tert-butylstyrene sulfone). Observation wavelength and laser intensity were (a) 300.7-313.5 nm, 0.68 J/cm2; (b) 300.7-313.5 nm, 1.34 J/cm2; (c) 380.2-389.0 nm, 0.68 J/ era2; and (d) 380.2-389.0 nm, 1.34 J/cm 2.

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Fig. 3. Time-resolved luminescence spectra upon the laser ablation of poly(p-trimethylsilylstyrene sulfone). The spectra were recorded at the gate time of (a) -9.9 to -2.5 ns, (b) - 1.9 to 5.6 ns, (c) 6.2-50.0 ns, and (d) 149.5-200.2 ns. The origin of the time axis is set to the maximum of the laser pulse (248 nm, 18 ns, 1.37 J/cm2). 205

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CHEMICAL PHYSICS LETTERS

eV in addition to that for C-Si bond cleavages, which indicates that multiphoton excitation and/or mutual interaction between excited states are involved. The rise of the Si emission, however, was not instantaneous and almost similar to those of C2 and CN as shown in fig. 4. The observed time lag implies that photoexcitation does not yield directly the bare silicon atoms, but a similar reaction producing C2 and CN does. The decay constants of those bands can be evaluated by fitting each decay curve to a single exponential function, and 33 ns for Si, 111 ns for CN, and 106 ns for C2 are obtained. The rise and decay dynamics of the broad emission described above is considered first from viewpoints of plasma. Generally, broad continuum emission spectra are found in a laser-produced plasma and can be ascribed to the bremsstrahlung and/or recombination emissions [ 11 ]. The spectral peak should depend on the kinetic energies of free electrons in the plasma [4]. In the present experiment, however, we could not observe the peak shift of the continuum spectra with the laser intensity. The absence of the emission at 427 nm from carbon ion (C + ) also suggests that the plasma generation is unlikely around the intensity of 1 J/cm 2. Although sev,

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Fig. 4. Temporal behaviour of luminescence intensity upon the laser ablation of poly(p-trimethylsilylstyrene). Observation wavelength was (a) 300.7-313.5 nm, (b) 284.6-292.3 nm, (c) 380.2-389.0 nm, and (d) 463.7-472.5 nm. The laser fluence was 1.36 J/em 2.

206

26 June 1992

eral peaks came into prominence and the spectral shape changed later, the continuum emission lasted for several hundreds of nanoseconds. Since the lifetime is too long for an ordinary laser-induced plasma emission, the weak continuum is probably ascribed to the emission from hot polyatomic molecules. A similar idea was given by Koren and Yeh for unresolved continuum emission upon laser ablation of polyimide [ 12 ]. The line spectra of C2 and CN were often observed upon laser ablation of polymers in air, although the dynamics is not well elucidated [ 12-15 ]. In our results, the observed bands for C2 can be assigned to the high pressure bands of the Swan system (A 3ng_ X 3~,) [9 ]. This means that the v= 6 vibrational level in the excited state of C2 is selectively generated. We consider that free C atoms (3p) are produced during the laser ablation and followed by the inverse predissociation [ 16 ]. This explanation is quite reasonable since the CN emission is always observed although the polymers do not contain N atom. Nitrogen molecule in air should react with the free C atom. It should be noted that the rise of C2 and CN does not correspond to the decay of the fluorescence. An induction period is involved in the process before generating small molecular fragments such as C2 and CN. The present decay constants of these bands are also longer than the literature values which were measured in collision free conditions (4.1 ns for Si, 62 ns for CN) [ 10 ]. A possible explanation for the slow rise and the slow decay is that molecular fragments ejected from the polymer surface are decomposed in air giving the luminescent species. It was reported that irradiation o f SO2 gas o r HESO4 aerosol with a 248 nm laser gave rise to strong emission from SO and SO2 in the wavelength region from 250 to 450 nm [17,18]. However, the features of time-resolved spectra for PBS and those for PTMSiS were quite similar to those for PB and those for PTMSi, respectively. Namely, neither the emission from S atom nor those from sulfur-containing molecules were found during the laser ablation, although carbon-sulfur bond is confirmed to dissociate and reduce the ablation threshold. It is, therefore, quite possible that the scissions of all bonds connected to sulfur atom is not necessary to cause laser ablation. The mechanism is also supported by the fact that only the intrinsic fluorescence of the polymers were ob-

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CHEMICAL PHYSICS LETTERS

served upon ablation by a laser intensity o f less than 100 m J / c m 2.

4. Conclusion Time-resolved luminescence spectroscopy has revealed the t i m e scale o f the generation o f decomposed products for PB, PBS, P T M S i , and P T M S i S upon laser ablation. While the i n v o l v e m e n t o f weak b o n d s in p o l y m e r chains m a d e the ablation threshold decrease, the emission from radicals a n d a t o m s was not the requisite for the ablation with a laser intensity just a b o v e the threshold. Even with a laser intensity o f m o r e than 500 m J / c m 2, the generation o f radicals and a t o m s was not i m m e d i a t e after the laser pulse. This implies that the a t o m s a n d radicals are produced concomitantly by the reaction o f ejected molecular fragments in air.

Acknowledgement The authors wish to express their sincere thanks to Professor Isao Y a m a d a a n d Dr. H i r o a k i Usui o f K y o t o U n i v e r s i t y for the use o f the depth profiler. The authors are also i n d e b t e d to Mr. Hiroshi N a k a m i n a m i a n d Mr. M a k o t o H a y a s h i for the software to analyze the results o b t a i n e d by a two-dimensional C C D camera. The present work is supp o r t e d b y a G r a n t - i n - a i d from the Japanese M i n i s t r y o f Education, Science and Culture (63430003, 63104007, 02640369).

26 June 1992

References [ 1] R. Srinivasan and W.J. Leigh, J. Am. Chem. Soc. 104 (1982) 6784. [2] R. Srinivasan and B. Braren, Chem. Rev. 89 (1989) 1303, and references therein. [ 3 ] H. Masuhara, S. Eura, H. Fukumura and A. Itaya, Chem. Letters (1989) 446; H. Masuhara, A. Itaya and H. Fukumura, Am. Chem. Soc. Symp. Set. 412 (1989) 400. [4] H. Fukumura, H. Nakaminami, S. Eura, H. Masuhara and T. Kawai, Japan. J. Appl. Phys. 28 (1989) L412. [ 5 ] H. Fukumura, H. Nakaminami, S. Eura, H. Masuhara and T. Kawai, Appl. Phys. Letters 58 (1991 ) 2546. [6] J.A. Dean, ed., Lange's handbook of chemistry, I lth Ed. (McGraw Hill, New York, 1973). [7]T. Sakakibara, H. Ito, M. Matsuda, S. Ito and H. Ono, Polymer Prep. Japan 38 ( 1989 ) 2262. [8] H. Fukumura, N. Mibuka, S. Eura and H. Masuhara, Appl. Phys. A 53 (1991) 185. [9] R.W.B. Pearse and A.G. Gaydon, The identification of molecular spectra, 4th Ed. (Wiley, New York, 1976). [ 10] A.A. Radzig and B.M. Smirnov, Reference data on atoms, molecules, and ions (Springer, Berlin, 1985 ). [ l l ] J.F. Ready, Effects of high-power laser radiation (Academic Press, New York, 1971 ). [ 12] G. Koren and J.T.C. Yeh, J. Appl. Phys. 56 (1984) 2120. [ 13 ] G.M. Davis, M.C. Gower, C. Fotakis, T. Efthimiopoulos and P. Argyrakis, Appl. Phys. A 36 (1985) 27. [ 14] M. Golonbok, M.C. Gower, S.J. Kirkby and P.T. Rumsby, J. Appl. Phys. 61 (1987) 1222. [ 15 ] P.E. Dyer and J. Sidhu, J. Appl. Phys. 64 ( 1988 ) 4657. [ 16] G. Herzberg, Spectra of diatomic molecules, 2nd Ed. (Van Nostrand, Princeton, 1950). [ 17 ] C. Fotakis, Chem. Phys. Letters 82 ( 1981 ) 68. [ 18 ] Y.G. Jin, H.C. Zhou, M. Suto and L.C. Lee, J. Photochem. Photobiol. A 52 (1990) 255.

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