Polymers in Microlithography - American Chemical Society

Chapter 24

Molecular Studies on Laser Ablation Processes of Polymeric Materials by Time-Resolved Luminescence Spectroscopy 1

Downloaded by UNIV OF ARIZONA on October 8, 2015 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch024

Hiroshi Masuhara , Akira Itaya, and Hiroshi Fukumura Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan

By fluorescence analyses just upon laser ablation and of ablated surface, molecular aspects of ablation mechanism were elucidated and a characterization of ablated materials was performed. Laser fluence dependence of poly(N-vinylcarbazole) fluorescence indicates the inportance of mutual interactions between excited singlet states. As the fluence was increased, a plasma-like emission was also observed, and then fluorescence due to diatomic radicals was superimposed. While the polymer fluorescence disappeared mostly during the pulse width, the radicals attained the maximum intensity at 100 ns after irradiation. Fluorescence spectra and their rise as well as decay curves of ablated surface and its nearby area were affected to a great extent by ablation. This phenomenon was clarified by probing fluorescence under a microscope. There are a number of reports concerning laser ablation of polymeric materials i n r e l a t i o n to microelectronics technology (1-8). Most of the polymers studied were polyimide, poly(methyl methacrylate), poly(ethylene terephthalate) and some photoresists. They have carbonyl as w e l l as amino groups and hetero-atoms. In view of photophysics and photochemistry, t h i s means that fluorescence l i f e t i m e i s very short, intersystem crossing occurs with high quantum y i e l d , and they are photochemically r e a c t i v e (9). On the other hand, there are other s e r i e s of polymers having π-electronic chromophore such as N-carbazolyl and 1-pyrenyl groups, whose photophysical properties are quite d i f f e r e n t from the above polymers and whose laser chemistry i s studied i n d e t a i l . A r e l a t i o n among interchromophoric i n t e r a c t i o n , s p e c t r a l shape and geometrical structure i n the excited s i n g l e t , t r i p l e t , c a t i o n i c and anionic 1

Current address: ERATO, JRDC (1988-1993), 15 Morimoto-cho, Shimogamo, Sakyo-ku, Kyoto 606, Japan 0097-6156/89/0412-0400$06.00/0 ο 1989 American Chemical Society

In Polymers in Microlithography; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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Molecular Studies on Laser Ablation Processes

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states of these compounds has been elucidated i n most d e t a i l by using fluorescence and t r a n s i e n t absorption spectroscopic methods (10, 11). I t i s also confirmed that an incree.se of e x c i t a t i o n i n t e n s i t y of l a s e r pulse i n e v i t a b l y r e s u l t s i n an e f f i c i e n t i n t e r a c t i o n between excited s i n g l e t states even i n d i l u t e s o l u t i o n . This process i s one of the main f a c t o r s leading to non-linear e f f e c t s i n fluorescence r i s e and decay curves, fluorescence y i e l d , intersystem crossing y i e l d , i o n i z a t i o n , photochemical reactions, etc. (12) Therefore, laser a b l a t i o n study on these polymers i s expected to be very f r u i t f u l from mechanistic viewpoint, however, such a report i s quite scarce (13-16). Another advantage to examine these polymers i s that characterizations of ablated materials can be made possible by fluorescence spectroscopy. Fluorescence i s very s e n s i t i v e , and such surrounding microenvironmental conditions around the κ-chromophore a« p o l a r i t y and v i s c o s i t y and chromophore aggregation can be probed. This indicates that a new c h a r a c t e r i z a t i o n which could not be achieved by ESCA and FTIR i s expected. In the present work, we have examined poly(N-vinylcarbazole) (abbreviated hereafter as PVCz) and pyrene-doped poly(methyl methacrylate) (PMMA) f i l m s by using a time-resolved fluorescence spectroscopic method. Fluorescence spectra and t h e i r dynamic behavior of the former f i l m were elucidated with a high i n t e n s i t y laser pulse and a streak camera, which makes i t possible to measure dynamics j u s t upon laser a b l a t i o n . This method reveals molecular and e l e c t r o n i c aspects of laser a b l a t i o n phenomena (17). For the l a t t e r f i l m a l a s e r pulse with weak i n t e n s i t y was used f o r c h a r a c t e r i z i n g the ablated and masked areas. On the basis of these r e s u l t s , we demonstrate a high p o t e n t i a l of fluorescence spectroscopy i n molecular studies on laser a b l a t i o n and consider i t s mechanism. Experimental Materials. PVCz (Takasago International Co. Ltd.) was p u r i f i e d by several r e p r e c i p i t a t i o n s from benzene-methanol s o l u t i o n . 1Ethylpyrene (EPy) was r e c r y s t a l l i z e d and sublimed i n vacuo before use. PMMA was r e p r e c i p i t a t e d twice from tetrahydrofuran s o l u t i o n with methanol. PVCz f i l m s were prepared by spin-coating a 10 wt% anisole s o l u t i o n of the polymer on a quartz p l a t e . PMMA and EPy were dissolved i n chlorobenzene and cast on quartz or sapphire p l a t e s . Each f i l m was d r i e d under vacuum i n several hours. Film thickness before and a f t e r a b l a t i o n was measured with a Dektak 3030 or a Tencor Alpha 2000. Instruments. A schematic diagram of the microcomputer-controlled system f o r the fluorescence measurement j u s t upon a b l a t i o n i s shown i n Figure 1. The e x c i t a t i o n l i g h t source was a 351 nm laser (Lumonies He-400, 15 ns) and fresh surface of PVCz f i l m was examined i n a i r . The laser beam was focused onto a 2 χ 1 mm spot by using a quartz lens with a 250 mm f o c a l length and an aperture. A copper mesh mask made p h o t o - l i t h o g r a p h i c a l l y was used i n a contact mode. The laser fluence was measured with a Gentec ED-200 power meter. The fluorescence j u s t from the ablated area was led to a polychromator (Jobin-Yvon HR-320). Time-resolved fluorescence was measured by using a streak camera system (Hamamatsn C2830, M2493). The spectra), data were averaged over 30 measurements. 2



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POLYMERS IN MICROLITHOGRAPHY

COMPU' ER MONITORl

Γ

EXICIMER LASER

INTERFACE

2D-CCD CAMERA^sTRËÂK CAMERA

ι MIRROR POWER METER STORAGE OSCILLOSCOPE

POLYCHROMATOR

Figure 1. A schematic diagram of the streak casera system for laser a b l a t i o n .


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Molecidar Studies on Laser Ablation Processes 403

For EPy-doped PMMA f i l m , a 308 nm excimer l a s e r (Lumonics TE 430T-2, 6ns) was used as as exposure source. We used a timecorrelated s i n g l e photon counting system (18) f o r measuring fluorescence spectra and r i s e as well as decay curves of a small ablated area. The e x c i t a t i o n was a frequency-doubled laser pulse (295 nm, lOps) generated from a synchronously pumped cavity-dumped dye laser (Spectra Physics 375B) and a CW mode-locked YAG laser (Spectra Physics 3000). Decay curves under a fluorescence microscope were measured by the same system as used before (19).


Laser Ablation Dynamics of PVCz Film Fluence-Dependent Luminescence Spectra. The surface of the present f i l m was homogeneous and very smooth, while l a s e r i r r a d i a t i o n resulted i n morphological changes. The i r r a d i a t e d volume was removed to some extent and a hole was l e f t , showing laser ablation behavior. An etched depth brought about with one shot of e x c i t a t i o n was p l o t t e d against the logarithm of l a s e r fluence. I f the laser pulse penetrates into the f i l m according to the Lambert-Beer equation, l o g ( I o / I ) = ε cd, and the depth region where I i s larger than the threshold i s ablated, the etched depth should be proportional to the logarithm of the fluence. However, no l i n e a r r e l a t i o n was obtained. This i s reasonable, since the energy of the 351 nm photon i s lower than any bond energy of the polymer and a p l u r a l i t y of e x c i t e d states and/or multi-photon absorption processes should be involved i n bond cleavage. The ablation threshold was estimated to be a few tens of mJ/cm . The fluorescence spectra measured j u s t upon ablation are given in Figure 2A as a function of l a s e r fluence. The contribution below 370 nm was suppressed, as a Hoya L37 f i l t e r was used i n order to cut off the laser pulse. Fluorescence spectra of t h i s polymer f i l m consist of sandwich (max. 420 nm, l i f e t i m e 35 ns) and p a r t i a l overlap (max. 370 nm, l i f e t i m e 16 ns) excimers (20). The l a t t e r excimer i s produced from the i n i t i a l l y excited monomer s t a t e , while the sandwich excimer from the p a r t i a l overlap excimer and the monomer excited states. Since these processes compete with e f f i c i e n t i n t e r a c t i o n s between i d e n t i c a l and d i f f e r e n t excimers (Si - S i a n n i h i l a t i o n ) (12), the sandwich excimer i s quenched to a greater extent compared to the p a r t i a l overlap one under a high e x c i t a t i o n . A c t u a l l y the fluencedependent spectral change around the threshold can be interpreted i n terms of S i - S i a n n i h i l a t i o n . With increasing laser fluence, an a d d i t i o n a l t a i l i n the long wavelength region was detected. Further increases above 1 J/cm resulted i n the structured bands superimposed on the broad spectra. The new t a i l might be a plasma-like emission and suggests that a new ablation process i s involved. This type of emission has been confirmed also f o r polyimides, PMMA, graphite, and b i o l o g i c a l t i s s u e s (2, 21, 22). The Bremsstrahlung and recombination processes may respond to the continuum i n problem. The structured bands at high fluence can be assigned to C2 (Swan band) and CN r a d i c a l s , i n d i c a t i n g fragmentation of the polymer. 2

2

Temporal C h a r a c t e r i s t i c s of Luminescence. In order to reveal dynamics of these emissions, we adjusted the gate width of the streak




404

Figure 2. (A) Luminescence spectra of PVCz film j u s t upon laser ablation. (B) Fluorescence r i s e and decay curves of C2 r a d i c a l (a) and p a r t i a l overlap excimer of PVCz film (b).


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Molecular Studies on Laser Ablation Processes 405

camera to -15 ~~ 72 ns, where the o r i g i n of the time a x i s was set to the maximum of the laser pulse. We were unable to detect any emission other than the two kinds of excimer, independent of laser i n t e n s i t y . Only the r e l a t i v e i n t e n s i t y of two excimer fluorescence was confirmed to change i n the region of 3 mJ/cm ^ 1.55 J/cm . The new emission i n the long wavelength region and the structured bands became d i s t i n c t at l a t e stages a f t e r e x c i t a t i o n . One of the t y p i c a l examples i s also given i n Figure 2B. Compared to the p a r t i a l overlap excimer, the maximum i n t e n s i t y of the r a d i c a l emission was observed at 100 ns a f t e r i r r a d i a t i o n . Temporal c h a r a c t e r i s t i c s at e a r l y stages were elucidated by measuring fluorescence i n t e n s i t y with the gate time of 1.74 ns as a function of the delay time. Compared to the laser pulse, the time where the maximum i n t e n s i t y i s attained s h i f t s to the e a r l y stage as the laser fluence becomes high. Of course, we could not f i n d out any decay component with i n t r i n s i c fluorescence l i f e t i m e of 17 and 35 ns. I t i s concluded that an S i - Si a n n i h i l a t i o n occurs q u i t e e f f i c i e n t l y during the pulse width.


2

Si - Si A n n i h i l a t i o n and Ablation Mechanism.

2

The Si - Si

a n n i h i l a t i o n process i s responsible to laser a b l a t i o n , which was supported by the following experiment. Total fluorescence i n t e n s i t y and the r e l a t i v e i n t e n s i t y of excimer emissions (-15 *** 72 ns gate width) were p l o t t e d against the fluence i n Figure 3. I t i s i n t e r e s t i n g that the r e l a t i v e c o n t r i b u t i o n of excimers showed a s i m i l a r change to that of t o t a l fluorescence i n t e n s i t y . This indicates that the Si - Si a n n i h i l a t i o n has an important r o l e i n the primary processes of laser a b l a t i o n phenomena, since the r e l a t i v e contribution of excimers i s determined by the degree of S i - S i a n n i h i l a t i o n , and the suppressed fluorescence i n t e n s i t y corresponds to the enhanced a b l a t i o n . Fluorescence i n t e n s i t y increases with the laser fluence, while i t s change was quite smooth even around the threshold. I f a new process leading to a b l a t i o n was involved i n addition to the Si - S i a n n i h i l a t i o n , the r e l a t i v e fluorescence i n t e n s i t y of two excimers would not change furthermore above the threshold. Although the d e t a i l s are beyond our current knowledge, we conclude that the Si Si a n n i h i l a t i o n i s the o r i g i n of laser a b l a t i o n i n t h i s fluence range· The t o t a l fluorescence i n t e n s i t y saturated around a few hundreds of mJ/cm which corresponds to the i r r a d i a t i o n condition where the new plasma-like emission was observed. Above t h i s value fluorescence i n t e n s i t y decreased, which i s accompanied with the recovery of the r e l a t i v e i n t e n s i t y of excimer emissions. This means that a quite e f f i c i e n t deactivation channel of e x c i t a t i o n i n t e n s i t y opens i n t h i s energy range, and the c o n t r i b u t i o n of Si - S i a n n i h i l a t i o n i s depressed. This suggests that fragmentation reactions to diatomic r a d i c a l s are not induced by the a n n i h i l a t i o n process. Multi-photon absorption processes v i a the Si states and chemical intermediates should be involved, although no d i r e c t experimental r e s u l t has as yet been obtained. 2

Characterization of Ablated PMMA Film with EPy Fluorescence Spectra. Fluorescence spectra of PMMA f i l m s doped with a high concentration of EPy are composed of a structured monomer and



406

a red-shifted broad excimer bands, and t h e i r r e l a t i v e c o n t r i b u t i o n i s modified by l a s e r a b l a t i o n . Figure 4 shows normalized fluorescence spectra of the ablated area of EPy-doped PMMA f i l m s i r r a d i a t e d with s i n g l e and t r i p l e l a s e r shots with the fluence of 4.3 J/cm . For comparison, the spectrum of the u n i r r a d i a t e d f i l m i s also shown. An i n t e n s i t y r a t i o of the excimer fluorescence to the monomer one ( I E / I M ) decreased with the number of l a s e r shots. I t was also confirmed that t h i s value of I E / I M decreases with increase of the laser fluence. This indicates that the high fluence leads to the larger change as compared with the low fluence, which i s quite reasonable. An e f f e c t i v e thickness of the layer where the fluorescence i s observed i s assumed to be the depth where the e x c i t a t i o n l i g h t i n t e n s i t y i s 1/e of the i n i t i a l value. The thickness was c a l c u l a t e d to be 1.4 μ m from an absorption c o e f f i c i e n t of the f i l m at 295 nm ( e x c i t a t i o n wavelength). Therefore, the observed fluorescence spectral change i s due to that of aggregate states of EPy i n the depth region of 1.4 jum from the ablated surface. A c t u a l l y , i t i s w e l l known i n a PMMA matrix that the excimer band i s due to the ground state dimer of the dopant (23).


2

Fluorescence Rise and Decay Curves. Both monomer and excimer fluorescence decay curves of the unirradiated f i l m are nonexponential and the excimer fluorescence shows a slow r i s e component. This behavior i s quite s i m i l a r to the r e s u l t reported f o r the PMMA f i l m doped with pyrene. (23) A delay i n the excimer formation process was interpreted as the time taken f o r the two molecules i n the ground state dimer to form the excimer geometry. Dynamic data of the ablated area observed at 375 nm (monomer fluorescence) and 500 nm (excimer fluorescence) are shown i n Figure 5. When the l a s e r fluence increased, the monomer fluorescence decay became slower. The slow r i s e of the excimer fluorescence disappeared and the decay became faster. One explanation i s due to a change of EPy aggregation i n the area l e f t upon the l a s e r a b l a t i o n . Some of EPy dimers c o n s t i t u t e the non-fluorescent quenching s i t e , and others form the dimer which are converted to excimer more e a s i l y compared to before i r r a d i a t i o n . In the fluorescence studies on vacuum-deposited f i l m s of ω-(1pyrenyl)alkanoic a c i d , we reported an important r o l e of aggregation of pyrenyl chromophores (24). During l a s e r annealing of these f i l m s , t h e i r fluorescence s p e c t r a l shape changed and f i n a l l y i t s i n t e n s i t y decreased. This c h a r a c t e r i s t i c behavior was interpreted by introducing a non-fluorescent aggregate i n the mechanism. We consider that the s i m i l a r quenching s i t e s are responsible f o r the present r e s u l t . Another possible explanation i s that some of EPy molecules i n the l e f t area are evaporated by the l a s e r a b l a t i o n , which seems to be consistent with the prolonged decay time of the monomer fluorescence. However, accelerated excimer formation process can not yet be explained. Fluorescence Characterization of Ablated Polymeric M a t e r i a l s . In order to produce sharply etched patterns, the f i l m was ablated with a photo-lithographically prepared mesh mask i n the contact mode. The ablation was conducted with two l a s e r shots with the l a s e r fluence of 0.2 J/cm . The decay curves of the ablated f i l m was measured by a 2


MÀSUHARA ET AL.



24.

Number of laser pulse

ι

400

ι

ι

500 WAVELENGTH/nm

Figure 4. Fluorescence spectral change of EPy in PMMA induced by laser ablations.




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dynamic fluorescence microprobe apparatus with a two-dimensional resolution of 5 /im, which was constructed by us (19). The fluorescence behavior of the masked area was different from that of the ablated one. Furthermore, i t i s worth noting that the decay curve of the former area i s not i d e n t i c a l with that of the untreated film. We measured excimer fluorescence dynamics as a function of the horizontal position of the fluorescence microscope. The time when the intensity became one-twentieth of the i n i t i a l one was used as a measure, because the decay curves were non-exponential. The values below 80 ns and around 100 ns correspond to the masked and the ablated areas, respectively. From these data, i t was concluded that the width of the masked area was 37 jum, which i s in agreement with the mesh dimension. These results indicate that aggregation states of EPy in the region of about 20 μ m around the ablated area were affected by the laser ablation with low fluence of 0.2 J / c m . Since the excitation energy cannot migrate up to 20 /im, another effect which causes t h i s change of EPy aggregation may be transmitted through the PMMA matrix from the ablated area. When materials are ablated by the l a s e r , a fast thermal elevation and a volume expansion occur simultaneously with the explosive desorption of the ablated materials. These expansions are transmitted through the PMMA matrix to unirradiated and unablated areas, and lead to different d i s t r i b u t i o n s of the dopant and to changes in the physico-chemical properties of the PMMA. These morphological changes have been probed here for the f i r s t time by the fluorescence c h a r a c t e r i s t i c s of EPy aggregation. 2

Conclusion The time-resolved fluorescence spectroscopic approach i s very f r u i t f u l in elucidating electronic and molecular aspects of laser ablation of fluorescent polymers. While a plasma-like emission and fluorescence behavior of ablated r a d i c a l s have been reported (21, 22), fluorescence dynamics of the polymer i t s e l f has been considered for the f i r s t time in the present work. A t y p i c a l temporal c h a r a c t e r i s t i c of PVCz film at 1.3 J/cm fluence was as follows. Only excimer emissions were observed during laser pulse, a broad plasma-like emission was detected l a t e r , and fragmented r a d i c a l s became d i s t i n c t . Ablation behavior can be interpreted in terms of photophysical and photochemical processes, including Si - Si annihilation. It i s confirmed that the polymer matrix around ablated area was also affected strongly by laser a b l a t i o n . The change of the matrix properties are brought about over a few tens of μ*. This type of information i s b a s i c a l l y important and indispensable for p r a c t i c a l applications such as excimer laser lithography. The time-resolved fluorescence spectroscopy i s one of the powerful characterization methods for ablated polymer matrix. 2

Acknowledgments The authors wish to express t h e i r sincere thanks to Messrs. A. Kurahashi and S. Eura for t h e i r experimental e f f o r t s . Thanks are also due to Prof. I. Yamazaki and Dr. N. Tamai who helped us with the single photon counting measurements. The present work i s p a r t l y


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supported by the Grant-in-Aid on Special Project Research for Photochemical Processes (63104007) and on P r i o r i t y Area for Macromolecular Complexes (63612510), and the Grant-in-Aid for S c i e n t i f i c Research (63430003) from the Japanese Ministry of Education, Science, and Culture.

Literature Cited 1.


2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Srinivasan, R.; Leigh, W.J. J. Am. Chem. Soc. 1982, 104, 6784. Yeh, J. T. C. J . Vac. Sci. Tech. A 1986, 4, 653. Srinivasan, R.; Braren, B.; Seeger, D.E.; Dreyfus, R.W. Macromolecules 1986, 19, 916. Danielzik, D.; Fabricius, N.; Rowekamp, M.; Von der Linde, D. Appl. Phys. Lett. 1986, 48, 212. Srinivasan, R.; Braren, B.; Dreyfus, R.W. J . Appl. Phys. 1987, 61, 372. Larciprete, R.; Stuke, M. Appl. Phys. Β 1987, 42, 181. Koren, G. Appl. Phys. Lett. 1987, 50, 1030. Estner, R.C.; Nogar, N.S. Appl. Phys. Lett. 1986, 49, 1175. For example, Turro, N.J. Modern Molecular Photochemistry; Benjamin: New York, 1978. Masuhara, H. Makromol. Chem. Suppl. 1985, 13, 75. Masuhara, H. In Photophysical and Photochemical Tools in Polymer Science. Winnik, M. A. Ed.; Reidel: Dordrecht, 1986; p.65. Masuhara, H.; In Photophysical and Photochemical Tools in Polymer Science, Winnik, M. A. Ed.; Reidel: Dordrecht, 1986; p.43 Masuhara, H.; Hiraoka, H.; Domen, N. Macromolecules 1987, 20, 450. Masuhara, H.; Hiraoka, H.; Marinero, E.E. Chem. Phys. Lett. 1987, 135, 103. Hiraoka, H.; Chuang, T . J . ; Masuhara, H. J. Vac. Sci. Tech. Β 1988, 6, 463. Srinivasan, R.; Braren, B. Appl. Phys. A 1988, 45, 286. Eura, S.; Itaya, Α.; Masuhara, H. Polym. Preprints Japan 1988, 37, E591. Yamazaki, I.; Kume, H.; Tamai, N.; Tsuchiya, H.; Oka, K. Rev. Sci. Instr. 1985, 56, 1185. Itaya, Α.; Kurahashi, Α.; Masuhara, H.; Tamai, N.; Yamazaki, I. Chem. Lett. 1987, 1079. Itaya, Α.; Sakai, H.; Masuhara, H. Chem. Phys. Lett. 1987, 138. 231. Koren, G.; Yeh, J.T.C. J . Appl. Phys. 1984, 56, 2120. Gauthier, T.D.; Clarke, R.H.; Isner, J. M. J . Appl. Phys. 1988, 64, 2736. Avis, P.; Porter, G. J . Chem. Soc. Faraday Trans. 2 1974, 70, 1057 Itaya, Α.; Kawamura, T.; Masuhara, H.; Taniguchi, Y.; Mitsuya, M. Chem. Phys. Lett. 1987, 133, 235.

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