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Fabrication of high efficiency SOI taper structures
Microelectronic Engineering 86 (2009) 1117–1119
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Fabrication of high efficiency SOI taper structures Thorsten Wahlbrink a,*, Wan Shao Tsai c, Michael Waldow b, Michael Först b, Jens Bolten a, Thomas Mollenhauer a, Heinrich Kurz a,b a
AMO GmbH, AMICA, Otto-Blumenthal-Straße 25, D-52074 Aachen, Germany Institut für Halbleitertechnik, RWTH Aachen University, D-52074 Aachen, Germany c Department of Applied Materials and Optoelectronic Engineering, National Chi-Nan University, Nantou 54561, Taiwan b
a r t i c l e
i n f o
Article history: Received 30 September 2008 Received in revised form 28 November 2008 Accepted 14 January 2009 Available online 29 January 2009 Keywords: Si-photonic Spot size converter SU8 cladding Electron beam lithography
a b s t r a c t A taper technique is presented which utilizes micrometer-scale SU8 waveguides to couple light into nanometre-scale silicon waveguides with high conversion efficiency. The conversion efficiency between the fibre and the silicon waveguide is investigated for different taper lengths and silicon waveguide taper tip widths. Maximum coupling efficiency is ascertained to be about 60% per coupler. The 3 dB bandwidth of the presented structure is determined to be well above 100 nm. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon-on-insulator (SOI) has proven to be an ideal material system for the realization of integrated nanophotonic components. Its material properties and the availability of a mature process technology allow the fabrication of a wide variety of photonic devices [1]. A key issue is the coupling between silicon photonic chips and the outside world, mostly in form of fibre-to-chip coupling. Due to the high refractive index contrast between silicon and silica the waveguide size on SOI can be scaled down to several hundreds of nm to support a single guiding mode at 1550 nm. However, the mode size of optical fibres is typically several orders of magnitude larger, also the effective index difference between silica fibre and silicon waveguide is quite large. This mode and index mismatches cause high coupling losses. Different techniques have been proposed to overcome this issue, ranging from simple adiabatic tapers to sophisticated taper structures [2–5] and complex grating coupler designs [6]. The taper technique investigated in this work, suitable for TE and TM polarizations and almost no bandwidth limitation, is schematically depicted in Fig. 1. The light is first coupled from a micrometer-scale fibre core into a micrometer-sized waveguide with approximately the same effective refractive index. A nanoscale tapered silicon strip waveguide is embedded into this larger cladding waveguide. The light mode is continuously transferred from the cladding waveguide to the silicon strip waveguide with * Corresponding author. Tel.: +49 2418867206. E-mail address: [email protected] (T. Wahlbrink). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.01.047
the effective index gradually changing from that of silica to that of silicon. 2. Experimental To prove the potential of the above-described taper technique for efficient fibre-to-chip coupling the coupling mechanism has been simulated using the commercial finite element software tool HFSS. Different coupler designs with varying taper lengths Ltaper, taper tip widths wtip and different dimensions of the cladding waveguide have been analyzed numerically and experimentally. Test structures have been fabricated on SOI material with a top silicon thickness of tsi = 340 nm and a buried oxide thickness of tox = 2 lm. In a first lithographic process step, the silicon waveguide structure including the thin taper tip and alignment markers for further lithographic steps have been defined using electron beam lithography. A Vistec EBPG5000 tool operated at 100 kV has been used to expose a 200 nm thick Hydrogen Silsesquioxane (HSQ) layer used as resist material. The high contrast of the lithographic process used [7] allows the definition of taper tips with widths down to 40 nm. The HSQ resist mask has been transferred into the top silicon by means of an ICP-RIE process [8] using an Oxford Plasmalab100 tool and HBr-based chemistry. Due to its optical properties and good processability a 3 lm thick SU8 layer has been chosen as material for the fibre-scale cladding waveguide. Aligned to the markers fabricated in the first process steps, this layer has been exposed with the same electron beam tool. Development conditions have been chosen according to the supplier’s recommendations. It is worth to note that the alignment between the silicon
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T. Wahlbrink et al. / Microelectronic Engineering 86 (2009) 1117–1119
waveguide and the SU8 cladding waveguide and the lithographic definition of smooth SU8 waveguides with low losses in a 3 lm thick polymer layer by means of electron beam lithography is a demanding task. With the marker trapped between the isolating silica layer and the non-conducting SU8 polymer a precise prealignment of the samples and a fine-tuning of the marker search algorithm of the electron beam writer are needed for a reliable high-precision alignment with an overlay accuracy of about 50 nm. In Fig. 2 a SU8-cladded silicon waveguide taper is depicted in a SEM micrograph. No significant misalignment between both layers can be seen. The sidewalls of the SU8 are relatively smooth, guaranteeing low linear losses in the cladding waveguides. The test structures have then been optically characterized in a continuous wave (c.w.) measurement setup. Measurements have been carried out with a polarization maintaining lensed optical fibre with a
Fig. 1. Schematic of the fabricated coupler structure.
Fig. 2. SEM micrograph of a silicon waveguide embedded into 3 lm thick SU8 waveguide.
mode field diameter of 2.5 lm and maximum transmission for TE and TM polarizations at a wavelength of 1550 nm has been determined. 3. Results and discussion First the coupling efficiency between the input fibre and the SU8 cladding waveguide has been determined experimentally, utilizing the ‘cutback method’. The determined coupling efficiencies are plotted as a function of the SU8 waveguide width for 1.6 lm (SU8-2) and 3 lm high (SU8-5) SU8 layers in Fig. 3. Additionally the dependency for TE (black) and TM (grey) polarisation is presented. Fig. 3 reveals an increase of the efficiency of about 10% when the SU8 thickness increases from 1.6 to 3 lm. Further the coupling efficiency increases slightly with increasing SU8 waveguide width. Highest coupling efficiencies, mainly limited by facet preparation, of around 70% are achieved for both polarizations for a 5 lm wide and 3 lm high SU8 waveguide. Consequently this geometry has been used for the following experiments. Second the transformation process of the SU8 waveguide mode to the mode of silicon waveguide is investigated. Fig. 4 visualizes this process in a cross-sectional view along the propagation direction of the light. Our simulations show that the theoretical limit of the conversion efficiency is 85%. The highest conversion efficiencies can be achieved at narrow silicon taper tip widths as predicted by simulations. The decisive influence of the taper tip width on the achievable conversion efficiency has been investigated experimentally. The conversion efficiency as a function of the taper tip width is shown in Fig. 5. In contrast to Fig. 3, which only shows the transfer efficiency from the fibre to the SU8 cladding, here the total coupling efficiency from the fibre to the silicon waveguide is presented. A maximum has been measured for a tip width of about 60 nm for both TE and TM polarisation. For thinner tips the roughness of the structure increases, reducing slightly the conversion efficiency. For larger tip widths the conversion efficiency drops as predicted by simulations. In Fig. 6 the measured conversion efficiency is plotted as a function of taper length. For both polarizations the conversion efficiency reaches its maximum for taper lengths of about 200 lm. For TM polarization; however, the conversion efficiency remains nearly constant for longer SU8 cladding. A slight decrease of the conversion efficiency has been observed for longer SU8 tapers in case of TE polarization, which might be attributed to linear waveguide losses within the cladding waveguide.
Fig. 3. Experimental determined coupling efficiency from an optical fibre to SU8 waveguides for different SU8 waveguide width, for TE and TM polarization and for two different SU8 heights: 1.6 lm SU8 thickness (SU8-2) and 3 lm SU8 thickness (SU8-5).
T. Wahlbrink et al. / Microelectronic Engineering 86 (2009) 1117–1119
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Fig. 4. Simulation of the transformation process of the SU8 waveguide mode to the mode of the silicon waveguide in a cross-sectional view along the propagation direction of the light.
the silicon waveguide of above 85% per coupler for both TE and TM polarization over a 3 dB bandwidth of more than 100 nm have been experimentally determined. This transformation efficiency corresponds to a total coupling efficiency from the fibre to the silicon waveguide of up to 60% per coupler, as additional losses during coupling from the fibre to the SU8 waveguide have to be taken into account, mainly caused by a not optimized facet preparation. 4. Conclusion
Fig. 5. Experimentally determined total conversion efficiency vs. silicon taper tip width for 3 lm high SU8 waveguides.
The investigated coupling structure has been intensively studied. Suitable test structures have been successfully fabricated and characterized, revealing the expected characteristics. The 3 dB bandwidth of the devices tested is well above 100 nm, allowing broadband coupling. The maximum conversion efficiency from fibre to waveguide has been measured to be up to 60% at telecom wavelength. Potential for further optimization is seen in the facet preparation. With an optimized facet preparation even higher conversion efficiencies are expected. Acknowledgements The authors acknowledge financial support by the EU project Circles of Light (FP6-034883). This research is partly supported by NSC of Taiwan and DAAD of Germany under grant 96-2911-I002-026-2. The authors wish to thank H. Hahn for his skilful help during sample preparation and processing as well as B. Hadam for SEM microscopy. References
Fig. 6. Experimentally determined total conversion efficiency vs. taper length for 3 lm high SU8 waveguides.
Both numerical and experimental results show an optimum transfer efficiency of the taper structure for small taper tip widths and long taper lengths [2,5,7]. In our experiments, we found that for a taper tip width of 60 nm and a taper length of 200 lm, efficiencies for the modal transformation from the SU8 cladding to
[1] B. Jalali, S. Fathpour, J. Lightwave Technol. 24 (2006) 4600–4615. [2] S.J. McNab, N. Moll, Y.A. Vlasov, Opt. Express 11 (2003) 2927–2939. [3] Y. Shani, C.H. Henry, R.C. Kistler, K.J. Orlowsky, D.A. Ackerman, Appl. Phys. Lett. 55 (1989) 2389–2391. [4] V.R. Almeida, R.R. Panepucci, M. Lipson, Opt. Lett. 28 (2002) 1302–1304. [5] T. Tsuchizawa, K. Yamada, H. Fukuda, T. Watanabe, J. Takahashi, M. Takahashi, T. Shoji, E. Tamechida, S. Itabashi, H. Morita, IEEE J. Sel. Quantum Electron 11 (2005) 232–240. [6] F. Van Laere, G. Roelkens, M. Ayre, J. Schrauwen, D. Taillaert, D. Van Thourhout, T.F. Krauss, R. Baets, J. Lightwave Technol. 25 (2007) 151–156. [7] W. Henschel et al., J. Vac. Sci. Technol. B 21 (5) (2003) 2018. [8] T. Wahlbrink et al., Microelectron. Eng. 78–79 (2005) 212.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Fabrication of high efficiency SOI taper structures Thorsten Wahlbrink a,*, Wan Shao Tsai c, Michael Waldow b, Michael Först b, Jens Bolten a, Thomas Mollenhauer a, Heinrich Kurz a,b a
AMO GmbH, AMICA, Otto-Blumenthal-Straße 25, D-52074 Aachen, Germany Institut für Halbleitertechnik, RWTH Aachen University, D-52074 Aachen, Germany c Department of Applied Materials and Optoelectronic Engineering, National Chi-Nan University, Nantou 54561, Taiwan b
a r t i c l e
i n f o
Article history: Received 30 September 2008 Received in revised form 28 November 2008 Accepted 14 January 2009 Available online 29 January 2009 Keywords: Si-photonic Spot size converter SU8 cladding Electron beam lithography
a b s t r a c t A taper technique is presented which utilizes micrometer-scale SU8 waveguides to couple light into nanometre-scale silicon waveguides with high conversion efficiency. The conversion efficiency between the fibre and the silicon waveguide is investigated for different taper lengths and silicon waveguide taper tip widths. Maximum coupling efficiency is ascertained to be about 60% per coupler. The 3 dB bandwidth of the presented structure is determined to be well above 100 nm. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon-on-insulator (SOI) has proven to be an ideal material system for the realization of integrated nanophotonic components. Its material properties and the availability of a mature process technology allow the fabrication of a wide variety of photonic devices [1]. A key issue is the coupling between silicon photonic chips and the outside world, mostly in form of fibre-to-chip coupling. Due to the high refractive index contrast between silicon and silica the waveguide size on SOI can be scaled down to several hundreds of nm to support a single guiding mode at 1550 nm. However, the mode size of optical fibres is typically several orders of magnitude larger, also the effective index difference between silica fibre and silicon waveguide is quite large. This mode and index mismatches cause high coupling losses. Different techniques have been proposed to overcome this issue, ranging from simple adiabatic tapers to sophisticated taper structures [2–5] and complex grating coupler designs [6]. The taper technique investigated in this work, suitable for TE and TM polarizations and almost no bandwidth limitation, is schematically depicted in Fig. 1. The light is first coupled from a micrometer-scale fibre core into a micrometer-sized waveguide with approximately the same effective refractive index. A nanoscale tapered silicon strip waveguide is embedded into this larger cladding waveguide. The light mode is continuously transferred from the cladding waveguide to the silicon strip waveguide with * Corresponding author. Tel.: +49 2418867206. E-mail address: [email protected] (T. Wahlbrink). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.01.047
the effective index gradually changing from that of silica to that of silicon. 2. Experimental To prove the potential of the above-described taper technique for efficient fibre-to-chip coupling the coupling mechanism has been simulated using the commercial finite element software tool HFSS. Different coupler designs with varying taper lengths Ltaper, taper tip widths wtip and different dimensions of the cladding waveguide have been analyzed numerically and experimentally. Test structures have been fabricated on SOI material with a top silicon thickness of tsi = 340 nm and a buried oxide thickness of tox = 2 lm. In a first lithographic process step, the silicon waveguide structure including the thin taper tip and alignment markers for further lithographic steps have been defined using electron beam lithography. A Vistec EBPG5000 tool operated at 100 kV has been used to expose a 200 nm thick Hydrogen Silsesquioxane (HSQ) layer used as resist material. The high contrast of the lithographic process used [7] allows the definition of taper tips with widths down to 40 nm. The HSQ resist mask has been transferred into the top silicon by means of an ICP-RIE process [8] using an Oxford Plasmalab100 tool and HBr-based chemistry. Due to its optical properties and good processability a 3 lm thick SU8 layer has been chosen as material for the fibre-scale cladding waveguide. Aligned to the markers fabricated in the first process steps, this layer has been exposed with the same electron beam tool. Development conditions have been chosen according to the supplier’s recommendations. It is worth to note that the alignment between the silicon
1118
T. Wahlbrink et al. / Microelectronic Engineering 86 (2009) 1117–1119
waveguide and the SU8 cladding waveguide and the lithographic definition of smooth SU8 waveguides with low losses in a 3 lm thick polymer layer by means of electron beam lithography is a demanding task. With the marker trapped between the isolating silica layer and the non-conducting SU8 polymer a precise prealignment of the samples and a fine-tuning of the marker search algorithm of the electron beam writer are needed for a reliable high-precision alignment with an overlay accuracy of about 50 nm. In Fig. 2 a SU8-cladded silicon waveguide taper is depicted in a SEM micrograph. No significant misalignment between both layers can be seen. The sidewalls of the SU8 are relatively smooth, guaranteeing low linear losses in the cladding waveguides. The test structures have then been optically characterized in a continuous wave (c.w.) measurement setup. Measurements have been carried out with a polarization maintaining lensed optical fibre with a
Fig. 1. Schematic of the fabricated coupler structure.
Fig. 2. SEM micrograph of a silicon waveguide embedded into 3 lm thick SU8 waveguide.
mode field diameter of 2.5 lm and maximum transmission for TE and TM polarizations at a wavelength of 1550 nm has been determined. 3. Results and discussion First the coupling efficiency between the input fibre and the SU8 cladding waveguide has been determined experimentally, utilizing the ‘cutback method’. The determined coupling efficiencies are plotted as a function of the SU8 waveguide width for 1.6 lm (SU8-2) and 3 lm high (SU8-5) SU8 layers in Fig. 3. Additionally the dependency for TE (black) and TM (grey) polarisation is presented. Fig. 3 reveals an increase of the efficiency of about 10% when the SU8 thickness increases from 1.6 to 3 lm. Further the coupling efficiency increases slightly with increasing SU8 waveguide width. Highest coupling efficiencies, mainly limited by facet preparation, of around 70% are achieved for both polarizations for a 5 lm wide and 3 lm high SU8 waveguide. Consequently this geometry has been used for the following experiments. Second the transformation process of the SU8 waveguide mode to the mode of silicon waveguide is investigated. Fig. 4 visualizes this process in a cross-sectional view along the propagation direction of the light. Our simulations show that the theoretical limit of the conversion efficiency is 85%. The highest conversion efficiencies can be achieved at narrow silicon taper tip widths as predicted by simulations. The decisive influence of the taper tip width on the achievable conversion efficiency has been investigated experimentally. The conversion efficiency as a function of the taper tip width is shown in Fig. 5. In contrast to Fig. 3, which only shows the transfer efficiency from the fibre to the SU8 cladding, here the total coupling efficiency from the fibre to the silicon waveguide is presented. A maximum has been measured for a tip width of about 60 nm for both TE and TM polarisation. For thinner tips the roughness of the structure increases, reducing slightly the conversion efficiency. For larger tip widths the conversion efficiency drops as predicted by simulations. In Fig. 6 the measured conversion efficiency is plotted as a function of taper length. For both polarizations the conversion efficiency reaches its maximum for taper lengths of about 200 lm. For TM polarization; however, the conversion efficiency remains nearly constant for longer SU8 cladding. A slight decrease of the conversion efficiency has been observed for longer SU8 tapers in case of TE polarization, which might be attributed to linear waveguide losses within the cladding waveguide.
Fig. 3. Experimental determined coupling efficiency from an optical fibre to SU8 waveguides for different SU8 waveguide width, for TE and TM polarization and for two different SU8 heights: 1.6 lm SU8 thickness (SU8-2) and 3 lm SU8 thickness (SU8-5).
T. Wahlbrink et al. / Microelectronic Engineering 86 (2009) 1117–1119
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Fig. 4. Simulation of the transformation process of the SU8 waveguide mode to the mode of the silicon waveguide in a cross-sectional view along the propagation direction of the light.
the silicon waveguide of above 85% per coupler for both TE and TM polarization over a 3 dB bandwidth of more than 100 nm have been experimentally determined. This transformation efficiency corresponds to a total coupling efficiency from the fibre to the silicon waveguide of up to 60% per coupler, as additional losses during coupling from the fibre to the SU8 waveguide have to be taken into account, mainly caused by a not optimized facet preparation. 4. Conclusion
Fig. 5. Experimentally determined total conversion efficiency vs. silicon taper tip width for 3 lm high SU8 waveguides.
The investigated coupling structure has been intensively studied. Suitable test structures have been successfully fabricated and characterized, revealing the expected characteristics. The 3 dB bandwidth of the devices tested is well above 100 nm, allowing broadband coupling. The maximum conversion efficiency from fibre to waveguide has been measured to be up to 60% at telecom wavelength. Potential for further optimization is seen in the facet preparation. With an optimized facet preparation even higher conversion efficiencies are expected. Acknowledgements The authors acknowledge financial support by the EU project Circles of Light (FP6-034883). This research is partly supported by NSC of Taiwan and DAAD of Germany under grant 96-2911-I002-026-2. The authors wish to thank H. Hahn for his skilful help during sample preparation and processing as well as B. Hadam for SEM microscopy. References
Fig. 6. Experimentally determined total conversion efficiency vs. taper length for 3 lm high SU8 waveguides.
Both numerical and experimental results show an optimum transfer efficiency of the taper structure for small taper tip widths and long taper lengths [2,5,7]. In our experiments, we found that for a taper tip width of 60 nm and a taper length of 200 lm, efficiencies for the modal transformation from the SU8 cladding to
[1] B. Jalali, S. Fathpour, J. Lightwave Technol. 24 (2006) 4600–4615. [2] S.J. McNab, N. Moll, Y.A. Vlasov, Opt. Express 11 (2003) 2927–2939. [3] Y. Shani, C.H. Henry, R.C. Kistler, K.J. Orlowsky, D.A. Ackerman, Appl. Phys. Lett. 55 (1989) 2389–2391. [4] V.R. Almeida, R.R. Panepucci, M. Lipson, Opt. Lett. 28 (2002) 1302–1304. [5] T. Tsuchizawa, K. Yamada, H. Fukuda, T. Watanabe, J. Takahashi, M. Takahashi, T. Shoji, E. Tamechida, S. Itabashi, H. Morita, IEEE J. Sel. Quantum Electron 11 (2005) 232–240. [6] F. Van Laere, G. Roelkens, M. Ayre, J. Schrauwen, D. Taillaert, D. Van Thourhout, T.F. Krauss, R. Baets, J. Lightwave Technol. 25 (2007) 151–156. [7] W. Henschel et al., J. Vac. Sci. Technol. B 21 (5) (2003) 2018. [8] T. Wahlbrink et al., Microelectron. Eng. 78–79 (2005) 212.