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UV–laser-assisted modification of the optical properties of polymethylmethacrylate
Applied Surface Science 154–155 Ž2000. 706–711 www.elsevier.nlrlocaterapsusc
UV–laser-assisted modification of the optical properties of polymethylmethacrylate C. Wochnowski ) , S. Metev, G. Sepold Department of Laser- and Plasma Microtechnology (LPMT) BIAS, Bremer Institute for Applied Beam Technology Klagenfurter Str. 2, 28359 Bremen, Germany Received 1 June 1999; accepted 5 August 1999
Abstract Polymethylmethacrylate ŽPMMA. has been irradiated by UV–laser light with different wavelengths Ž193, 248, 308 nm. in order to modify photolytically its refractive index. The photoinduced chemical reactions at the polymer surface have been investigated by QMS, XPS and FTIR and the mechanism of the structure modification has been clarified. It was shown that depending on the experimental conditions, an increase of the refractive index in the modified zones could be achieved. The controllable local increase of the refractive index was used to prepare passive integrated-optical waveguides in PMMA chips. q 2000 Elsevier Science B.V. All rights reserved. Keywords: UV–laser; Polymethylmethacrylate; Norrish reaction; Refractive index modification; Integrated waveguide
1. Introduction Treatment of polymers by UV–laser radiation is a fast developing area of modern research in applied physics. The UV radiation performs electronic excitation of some organic bonds and so initiates photochemical reactions. These modify the polymeric structure in a controllable way to obtain specific functional properties in the irradiated area w1–3x. The aim of this paper is to present recent results on the UV–laser-assisted modification of Polymethyl-
) Corresponding author. Tel.: q49-421-218-5078; fax: q49421-218-5095. E-mail address: [email protected] ŽC. Wochnowski..
methacrylate ŽPMMA. and the influence of this modification on its optical properties.
2. Experimental set-up The experimental set-up consists of an excimer laser as an UV source for different wavelengths Ž193, 248 and 308 nm.; a vacuum chamber containing the holder for the PMMA sample and an optical system which guarantees the irradiation of the PMMA probe under controllable conditions in a specific gas atmosphere. A four-pole mass spectrometer was integrated in the vacuum chamber to examine the volatile defragmentation products ablated from the irradiated polymeric surface by an in situ rest gas analysis Žin situ RGA..
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 4 3 5 - 3
C. Wochnowski et al.r Applied Surface Science 154–155 (2000) 706–711
707
Additionally the irradiated PMMA samples have been examined by XPS and FTIR-spectroscopy. The determination of the refractive index of the irradiated area was done by an Abbe-Refractometer.
3. Analytical measurements During the UV irradiation with the wavelength of 248 nm, the in situ QMS spectrum shown in Fig. 1 has been recorded. At a relatively high fluence Ž45 mJrcm2 . the mass spectrum clearly indicates the existence of methyl formate, methanol, carbon dioxide and other molecule fragments of the polymer side chain. It is known that the complete side chain separation Ža hydrogen abstraction from the main chain. occurs thereby generating hydrogen radicals w4,5x. This effect was also observed in our measurements. After irradiation, the PMMA sample has been investigated by XPS and FTIR spectroscopy. The FTIR spectrum shown in Fig. 2 confirms the existence of methanol by detecting the OH-peak at 3500 cmy1 . The existence of water to which the OH-peak could also be assigned can be excluded because in the mass spectrum in Fig. 1, no peaks related to water molecule or its fragments were observed. The
Fig. 2. FTIR spectrum of irradiated PMMA Ž l s 248 nm, I s 30 mJrcm2 ..
peak at 1650 cmy1 can be identified as a double bonding between two carbon atoms in the main chain. In Fig. 3 the XPS spectra of PMMA samples are presented. After a short irradiation dose Ž500 pulses. at a relatively low fluence Ž15 mJrcm2 ., the XPS spectra of carbon in Fig. 3a already shows a
Fig. 1. Mass spectrum of irradiated PMMA Ž l s 248 nm, I s 45 mJrcm2 ..
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C. Wochnowski et al.r Applied Surface Science 154–155 (2000) 706–711
chemical shift of the right peak to higher energies which can be interpreted as the existence of a free methyl formate radical. At high irradiation doses Ž20 000 pulses. at the same fluence, the XPS spectra of oxygen in Fig. 3b shows the merging of the two oxygen peaks indicating the generation of carbon dioxide as a degradation product of the free methyl formate radical. The PMMA samples treated by 193-nm-UV–laser radiation have also been analysed by XPS and FTIR-spectroscopy. The XPS spectra of oxygen indicate formation of methanol radicals as indicated by the peak shift. In the FTIR spectrum a weak OH-peak of methanol at 3500 cmy1 is identified but no peak of the double bonding at 1650 cmy1 was observed. The XPS and FTIR spectra of the PMMA probe at a 308-nm irradiation do not differ from the spectra of unirradiated PMMA. The measured values of the refractive index of the areas irradiated with 193 and 248 nm go through a
Fig. 3. XPS spectra of irradiated PMMA Ž l s 248 nm, I s15 mJrcm2 . Ža. Carbon XPS spectra of PMMA, Žb. Oxygen XPS spectra of PMMA.
Fig. 4. Refractive index modification of irradiated PMMA Ž l s193 and 248 nm, I s17 mJrcm2 ..
maximum with increasing irradiation dose as shown in Fig. 4.
4. Discussion The spectrometric investigations yield the scheme of the main laser-induced photochemical reactions in PMMA as shown in Fig. 5. By UV irradiation with a wavelength of 248 nm and of a relatively low fluence Ž15 mJrcm2 ., the complete side chain is separated from the main chain. This photochemical reaction of Norrish Type I occurs left from the carbonyl group w6x. The dissociative excitation energy is theoretically calculated to a corresponding wavelength of 222 nm w7x. This generates a free methyl formate radical detectable in the XPS and the QMS measurements. Continuous irradiation at high fluence Ž30 mJrcm2 or more. degrades the free esther radical either into methane and CO 2 as indicated by the XPS measurements or into CO and methanol as indicated by the FTIR and the QMS measurements. The complete separation of the side chain of PMMA causes the formation of a radical electron at the a-C-atom w4,5x. This implies destabilization of the polymer main chain leading to its scission at high irradiation doses w8,9x and double bonding formation as identified in the FTIR measurements. As indicated by XPS and FTIR spectra, UV irradiation with wavelength of 193 nm, mainly a partial separation of the side chain occurs at the right side from the carbonyl group. This also is a photochemical reaction of Norrish Type I w6x. In this case, the
C. Wochnowski et al.r Applied Surface Science 154–155 (2000) 706–711
709
Fig. 5. Simplified degradation scheme of irradiated PMMA.
theoretical dissociative excitation energy corresponds to a wavelength of 172 nm w7x. Since only a partial separation of the side chain takes place, there is almost no creation of a radical electron at the a-Catom, consequently almost no main chain scission occurs and thus no observable generation of double bonding occurs as confirmed by FTIR measurements. The increase of the refractive index ŽFig. 4. is due to the complete or partial separation of the side chain from the PMMA molecule. According to Ref. w10x this implies a volume contraction by Van der Waals interactions leading to a local increase of the mechanical density and consequently of the refractive index. Because of a limited separation of the side chain at a 193-nm irradiation, the volume contraction is weaker hence, the refractive index modification effect is inferior to that achieved by 248 nm. As seen from the XPS and FTIR spectra of PMMA irradiated by UV–laser light with a wavelength of 308 nm, it does not show any modification of the polymer structure due to the relatively low photon energy at this wavelength.
The dependence of the modification mechanism on the wavelength is a clear indication that photochemical reactions, rather than photothermal occur in the polymer molecule. The drop of the refractive index at high irradiation doses Žespecially at 248 nm. can be explained with the observed degradation of the main polymer structure. Because of the limited separation of the side chain at a 193-nm irradiation, the main polymer chain retains its stability longer w11x and the rela-
Fig. 6. UV–laser-assisted manufacturing of integrated-optical waveguide in polymeric device.
710
C. Wochnowski et al.r Applied Surface Science 154–155 (2000) 706–711
visible light region. The waveguide for red light at 632.8 nm is 3 dBrcm. This value increases for infrared light due to the enhanced absorption of the C`C and C`H bonds in this region. Waveguides generated by 193 nm UV radiation are inferior to those generated by 248 nm due to geometrical reasons: the lower refractive index requires a bigger diameter for the waveguide. Additionally, the strong absorption behaviour of the n–pU transition of the carbonyl group at 212 nm w6,12,13x is due to the small depth of modification at the PMMA surface at an incident UV irradiation of 193 nm. Consequently, wave modes are inhibited from propagating.
6. Conclusions
Fig. 7. Linear waveguide structure. Ža. Žtop. Strip waveguide in polymeric device. Žb. Žbottom. Light exit at the front side of a polymeric integrated waveguide.
In this paper, it is shown that the UV–laser-assisted photochemical modification of the polymer structure of Polymethylmethacrylate ŽPMMA. allows a local controllable modification of optical properties such as the refractive index. Hence, by UV lithography, integrated-optical waveguides in polymer devices can be generated. The loss rate is quite high but it is expected to improve by optimization of the process parameters.
Acknowledgements tively low maximum of refractive index occurs at higher irradiation doses as compared to 248 nm.
5. Application The controllable local increase of the refractive index of PMMA permits the manufacturing of integrated-optical waveguides in polymer integrated devices. By UV–laser lithography as shown in Fig. 6, one can generate passive integrated-optical components like strip waveguides or multiplexers. Fig. 7a shows the top view of a strip waveguide whose light exit at the front side of the polymer device can be seen in Fig. 7b. The waveguide generated by UV– laser light of 248 nm has a diameter of around 10 mm and a refractive index hub of about 0.9%, which makes it applicable for monomode waveguides in the
The author would like to thank the BMBF, Bundesministrium fur ¨ Bildung und Forschung for the financial support of this work ŽProject 13N6928r0. and acknowledge the Institute for Applied Material Science ŽIFAM., Bremen, for the recording of the XPS spectra.
References w1x J. Breuer, S. Metev, G. Sepold et al., Appl. Surf. Sci. 46 Ž1990. 336–341. w2x J. Breuer, S. Metev, Las. U. Optoelec. 28 Ž1996. 48–54. w3x J. Breuer, S. Metev, G. Sepold, Proc. of Int. Symp. High Power Lasers and Applications V, Wien, SPIE 2207 Ž4. Ž1994. 566–576. w4x M. Dole, Academic Press NY, London, Vol. II, 1973, 97– 117.
C. Wochnowski et al.r Applied Surface Science 154–155 (2000) 706–711 w5x R.M. Tarro, J.T. Warden, J.A. Moore et al., in: Microcircuit Engineering 84, Inter. Conf. Proceed, Academic Press, London, 1985, pp. 537–544. w6x Fozza, Roch, Kruse et al., Nucl. Instrum. Methods Phys. B 131 Ž1997. 205–210. w7x Private conversation: Author–Prof. Dr. M. Braun, Department Environment Analytics at the University of Karlruhe in BIAS, 1999. w8x N. Ueno, T. Mitsuhata, K. Sugita, K. Tanaka, Am. Chem. Soc. Symp. Ser. 412 Ž1989. 424.
711
w9x J.O. Choi, J.A. Moore, J.C. Corelli, J.P. Silverman, H. Bakhru, J. Vac. Sci. Technol., B 6r6 Ž1988. 2286–2289. w10x A. Schmoldt, Dissertation, TU Berlin, 1994. w11x R.C. Estler, N.S. Nogar, Appl. Phys. Lett. 49 Ž18. Ž1986. 1175–1177. w12x J.G. Calvert, J.N. Pitts, Jr., Wiley, Ž1966. 435 w13x E. Sutcliffe, R. Srinivasan, J. Appl. Phys. 60 Ž9. Ž1986. 3315–3321.
UV–laser-assisted modification of the optical properties of polymethylmethacrylate C. Wochnowski ) , S. Metev, G. Sepold Department of Laser- and Plasma Microtechnology (LPMT) BIAS, Bremer Institute for Applied Beam Technology Klagenfurter Str. 2, 28359 Bremen, Germany Received 1 June 1999; accepted 5 August 1999
Abstract Polymethylmethacrylate ŽPMMA. has been irradiated by UV–laser light with different wavelengths Ž193, 248, 308 nm. in order to modify photolytically its refractive index. The photoinduced chemical reactions at the polymer surface have been investigated by QMS, XPS and FTIR and the mechanism of the structure modification has been clarified. It was shown that depending on the experimental conditions, an increase of the refractive index in the modified zones could be achieved. The controllable local increase of the refractive index was used to prepare passive integrated-optical waveguides in PMMA chips. q 2000 Elsevier Science B.V. All rights reserved. Keywords: UV–laser; Polymethylmethacrylate; Norrish reaction; Refractive index modification; Integrated waveguide
1. Introduction Treatment of polymers by UV–laser radiation is a fast developing area of modern research in applied physics. The UV radiation performs electronic excitation of some organic bonds and so initiates photochemical reactions. These modify the polymeric structure in a controllable way to obtain specific functional properties in the irradiated area w1–3x. The aim of this paper is to present recent results on the UV–laser-assisted modification of Polymethyl-
) Corresponding author. Tel.: q49-421-218-5078; fax: q49421-218-5095. E-mail address: [email protected] ŽC. Wochnowski..
methacrylate ŽPMMA. and the influence of this modification on its optical properties.
2. Experimental set-up The experimental set-up consists of an excimer laser as an UV source for different wavelengths Ž193, 248 and 308 nm.; a vacuum chamber containing the holder for the PMMA sample and an optical system which guarantees the irradiation of the PMMA probe under controllable conditions in a specific gas atmosphere. A four-pole mass spectrometer was integrated in the vacuum chamber to examine the volatile defragmentation products ablated from the irradiated polymeric surface by an in situ rest gas analysis Žin situ RGA..
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 4 3 5 - 3
C. Wochnowski et al.r Applied Surface Science 154–155 (2000) 706–711
707
Additionally the irradiated PMMA samples have been examined by XPS and FTIR-spectroscopy. The determination of the refractive index of the irradiated area was done by an Abbe-Refractometer.
3. Analytical measurements During the UV irradiation with the wavelength of 248 nm, the in situ QMS spectrum shown in Fig. 1 has been recorded. At a relatively high fluence Ž45 mJrcm2 . the mass spectrum clearly indicates the existence of methyl formate, methanol, carbon dioxide and other molecule fragments of the polymer side chain. It is known that the complete side chain separation Ža hydrogen abstraction from the main chain. occurs thereby generating hydrogen radicals w4,5x. This effect was also observed in our measurements. After irradiation, the PMMA sample has been investigated by XPS and FTIR spectroscopy. The FTIR spectrum shown in Fig. 2 confirms the existence of methanol by detecting the OH-peak at 3500 cmy1 . The existence of water to which the OH-peak could also be assigned can be excluded because in the mass spectrum in Fig. 1, no peaks related to water molecule or its fragments were observed. The
Fig. 2. FTIR spectrum of irradiated PMMA Ž l s 248 nm, I s 30 mJrcm2 ..
peak at 1650 cmy1 can be identified as a double bonding between two carbon atoms in the main chain. In Fig. 3 the XPS spectra of PMMA samples are presented. After a short irradiation dose Ž500 pulses. at a relatively low fluence Ž15 mJrcm2 ., the XPS spectra of carbon in Fig. 3a already shows a
Fig. 1. Mass spectrum of irradiated PMMA Ž l s 248 nm, I s 45 mJrcm2 ..
708
C. Wochnowski et al.r Applied Surface Science 154–155 (2000) 706–711
chemical shift of the right peak to higher energies which can be interpreted as the existence of a free methyl formate radical. At high irradiation doses Ž20 000 pulses. at the same fluence, the XPS spectra of oxygen in Fig. 3b shows the merging of the two oxygen peaks indicating the generation of carbon dioxide as a degradation product of the free methyl formate radical. The PMMA samples treated by 193-nm-UV–laser radiation have also been analysed by XPS and FTIR-spectroscopy. The XPS spectra of oxygen indicate formation of methanol radicals as indicated by the peak shift. In the FTIR spectrum a weak OH-peak of methanol at 3500 cmy1 is identified but no peak of the double bonding at 1650 cmy1 was observed. The XPS and FTIR spectra of the PMMA probe at a 308-nm irradiation do not differ from the spectra of unirradiated PMMA. The measured values of the refractive index of the areas irradiated with 193 and 248 nm go through a
Fig. 3. XPS spectra of irradiated PMMA Ž l s 248 nm, I s15 mJrcm2 . Ža. Carbon XPS spectra of PMMA, Žb. Oxygen XPS spectra of PMMA.
Fig. 4. Refractive index modification of irradiated PMMA Ž l s193 and 248 nm, I s17 mJrcm2 ..
maximum with increasing irradiation dose as shown in Fig. 4.
4. Discussion The spectrometric investigations yield the scheme of the main laser-induced photochemical reactions in PMMA as shown in Fig. 5. By UV irradiation with a wavelength of 248 nm and of a relatively low fluence Ž15 mJrcm2 ., the complete side chain is separated from the main chain. This photochemical reaction of Norrish Type I occurs left from the carbonyl group w6x. The dissociative excitation energy is theoretically calculated to a corresponding wavelength of 222 nm w7x. This generates a free methyl formate radical detectable in the XPS and the QMS measurements. Continuous irradiation at high fluence Ž30 mJrcm2 or more. degrades the free esther radical either into methane and CO 2 as indicated by the XPS measurements or into CO and methanol as indicated by the FTIR and the QMS measurements. The complete separation of the side chain of PMMA causes the formation of a radical electron at the a-C-atom w4,5x. This implies destabilization of the polymer main chain leading to its scission at high irradiation doses w8,9x and double bonding formation as identified in the FTIR measurements. As indicated by XPS and FTIR spectra, UV irradiation with wavelength of 193 nm, mainly a partial separation of the side chain occurs at the right side from the carbonyl group. This also is a photochemical reaction of Norrish Type I w6x. In this case, the
C. Wochnowski et al.r Applied Surface Science 154–155 (2000) 706–711
709
Fig. 5. Simplified degradation scheme of irradiated PMMA.
theoretical dissociative excitation energy corresponds to a wavelength of 172 nm w7x. Since only a partial separation of the side chain takes place, there is almost no creation of a radical electron at the a-Catom, consequently almost no main chain scission occurs and thus no observable generation of double bonding occurs as confirmed by FTIR measurements. The increase of the refractive index ŽFig. 4. is due to the complete or partial separation of the side chain from the PMMA molecule. According to Ref. w10x this implies a volume contraction by Van der Waals interactions leading to a local increase of the mechanical density and consequently of the refractive index. Because of a limited separation of the side chain at a 193-nm irradiation, the volume contraction is weaker hence, the refractive index modification effect is inferior to that achieved by 248 nm. As seen from the XPS and FTIR spectra of PMMA irradiated by UV–laser light with a wavelength of 308 nm, it does not show any modification of the polymer structure due to the relatively low photon energy at this wavelength.
The dependence of the modification mechanism on the wavelength is a clear indication that photochemical reactions, rather than photothermal occur in the polymer molecule. The drop of the refractive index at high irradiation doses Žespecially at 248 nm. can be explained with the observed degradation of the main polymer structure. Because of the limited separation of the side chain at a 193-nm irradiation, the main polymer chain retains its stability longer w11x and the rela-
Fig. 6. UV–laser-assisted manufacturing of integrated-optical waveguide in polymeric device.
710
C. Wochnowski et al.r Applied Surface Science 154–155 (2000) 706–711
visible light region. The waveguide for red light at 632.8 nm is 3 dBrcm. This value increases for infrared light due to the enhanced absorption of the C`C and C`H bonds in this region. Waveguides generated by 193 nm UV radiation are inferior to those generated by 248 nm due to geometrical reasons: the lower refractive index requires a bigger diameter for the waveguide. Additionally, the strong absorption behaviour of the n–pU transition of the carbonyl group at 212 nm w6,12,13x is due to the small depth of modification at the PMMA surface at an incident UV irradiation of 193 nm. Consequently, wave modes are inhibited from propagating.
6. Conclusions
Fig. 7. Linear waveguide structure. Ža. Žtop. Strip waveguide in polymeric device. Žb. Žbottom. Light exit at the front side of a polymeric integrated waveguide.
In this paper, it is shown that the UV–laser-assisted photochemical modification of the polymer structure of Polymethylmethacrylate ŽPMMA. allows a local controllable modification of optical properties such as the refractive index. Hence, by UV lithography, integrated-optical waveguides in polymer devices can be generated. The loss rate is quite high but it is expected to improve by optimization of the process parameters.
Acknowledgements tively low maximum of refractive index occurs at higher irradiation doses as compared to 248 nm.
5. Application The controllable local increase of the refractive index of PMMA permits the manufacturing of integrated-optical waveguides in polymer integrated devices. By UV–laser lithography as shown in Fig. 6, one can generate passive integrated-optical components like strip waveguides or multiplexers. Fig. 7a shows the top view of a strip waveguide whose light exit at the front side of the polymer device can be seen in Fig. 7b. The waveguide generated by UV– laser light of 248 nm has a diameter of around 10 mm and a refractive index hub of about 0.9%, which makes it applicable for monomode waveguides in the
The author would like to thank the BMBF, Bundesministrium fur ¨ Bildung und Forschung for the financial support of this work ŽProject 13N6928r0. and acknowledge the Institute for Applied Material Science ŽIFAM., Bremen, for the recording of the XPS spectra.
References w1x J. Breuer, S. Metev, G. Sepold et al., Appl. Surf. Sci. 46 Ž1990. 336–341. w2x J. Breuer, S. Metev, Las. U. Optoelec. 28 Ž1996. 48–54. w3x J. Breuer, S. Metev, G. Sepold, Proc. of Int. Symp. High Power Lasers and Applications V, Wien, SPIE 2207 Ž4. Ž1994. 566–576. w4x M. Dole, Academic Press NY, London, Vol. II, 1973, 97– 117.
C. Wochnowski et al.r Applied Surface Science 154–155 (2000) 706–711 w5x R.M. Tarro, J.T. Warden, J.A. Moore et al., in: Microcircuit Engineering 84, Inter. Conf. Proceed, Academic Press, London, 1985, pp. 537–544. w6x Fozza, Roch, Kruse et al., Nucl. Instrum. Methods Phys. B 131 Ž1997. 205–210. w7x Private conversation: Author–Prof. Dr. M. Braun, Department Environment Analytics at the University of Karlruhe in BIAS, 1999. w8x N. Ueno, T. Mitsuhata, K. Sugita, K. Tanaka, Am. Chem. Soc. Symp. Ser. 412 Ž1989. 424.
711
w9x J.O. Choi, J.A. Moore, J.C. Corelli, J.P. Silverman, H. Bakhru, J. Vac. Sci. Technol., B 6r6 Ž1988. 2286–2289. w10x A. Schmoldt, Dissertation, TU Berlin, 1994. w11x R.C. Estler, N.S. Nogar, Appl. Phys. Lett. 49 Ž18. Ž1986. 1175–1177. w12x J.G. Calvert, J.N. Pitts, Jr., Wiley, Ž1966. 435 w13x E. Sutcliffe, R. Srinivasan, J. Appl. Phys. 60 Ž9. Ž1986. 3315–3321.