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Fabrication of MMI optical power splitter by UV embossing with PDMS mold
Microelectronic Engineering 84 (2007) 1231–1234 www.elsevier.com/locate/mee
Fabrication of MMI optical power splitter by UV embossing with PDMS mold Chul-Hyun Choi, Min-Woo Lee, Jun-Ho Sung, Bo Soon Kim, Beom-Hoan O
*
Optics and Photonics Elite Research Academy (OPERA), School of Information and Communication Engineering, INHA University, 253 YongHyun-Dong, Namg-Gu, Incheon 402-751, South Korea Available online 2 February 2007
Abstract We report on UV embossing process for realizing an optical power splitter using a UV curable polymer material. The polydimethylsiloxane (PDMS) mold transferred from the master was used for forming the micro-scale structure and patterning repeatedly it. We experimented also on the durability of the PDMS mold in casting a UV curable polymer. Residual layer thickness on the replica was observed to be below 0.5 lm. 2007 Elsevier B.V. All rights reserved. Keywords: Embossing; PDMS; MMI; Waveguide; Polymer
1. Introduction Optical power splitters based on multimode interference (MMI) are very important to photonic device for optical communication systems, such as wavelength division multiplexing (WDM) and fiber to the home (FTTH) systems [1]. Also, various functional devices using MMI device, such as demultiplexers, polarization splitters, and Mach– Zehnder interferometers have been extensively investigated [2–4]. The polymeric photonic devices have been widely fabricated using traditional semiconductor processes, such as photolithography and etching. These methods involve many processing steps, and can lead to a long fabrication time and low yield [5]. However, embossing techniques using a patterned master or mold are low in capital cost, easy to learn, straightforward to apply, and accessible to a wide range of users [6]. Silicon, nickel, and polymer materials are generally used for the master or mold of embossing process. Especially, the polydimethylsiloxane (PDMS) polymer material is usually used for the elastomer mold. *
Corresponding author. E-mail address: [email protected] (B.-H. O).
0167-9317/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.090
The PDMS mold provides good chemical stability, optically transparency, flexibility and easy release from the rigid masters or replicas. In this paper, we report on UV embossing technique for the fabrication of 1 · 4 power splitter for optical communication systems and optical interconnections. PDMS mold was used for the fabrication of the device. We experimented also on the durability of the PDMS mold when used in a UV curable polymer casting process. In order to get the thickness of residual layer to acceptable level, we tried reducing the droplet volume. 2. Fabrication and results 2.1. 1 · 4 optical power splitter We designed the 1 · 4 MMI optical power splitter as shown in Fig. 1. The widths of the single mode waveguide and the MMI device are set to 4 and 60 lm, respectively. The splitting length is 855 lm and the height of the core (n = 1.47 at 1550 nm) on the substrate is 1.8 lm. The substrate material is fused silica glass (n = 1.444 at 1550 nm).
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Fig. 1. Schematic diagrams of: (a) 1 · 4 MMI optical power splitter and (b) cross-section.
2.2. Fabrication process The optical power splitter was fabricated by PDMS mold with UV curable polymer. Fabrication process consists of manufacturing the master, mold, and replica. Fig. 2 shows the schematic diagram of the fabrication process for the master and mold. The master was produced from the photoresist (AZ 1518, Clariant) and the structures were formed using a photolithography process. The photoresist was coated onto a silicon substrate. Here, the height of the photoresist is determined by that we whish to give to the core of the optical power splitter. After the soft baking, the photoresist layer was illuminated by UV light through a photomask to define the master pattern and developed.
Fig. 2. Schematic diagram of the fabrication process for photoresist master and PDMS mold.
The fabrication of a master was completed by a hard bake (3 min at 110 C). This process can improve the hardness and the sidewall roughness of the master. The mold is prepared by casting molding using liquid PDMS (Sylgard 184, Dow Corning). This liquid, that is a mixture of a liquid silicon rubber and a curing agent, is poured over a master. After 48 h at room temperature, the PDMS mold is cured to solid and peeled away carefully. Optical power splitter is fabricated by UV embossing and this process is shown in Fig. 3. Before the UV embossing process, a thin adhesion promoter (ZAP 1020, Chemoptics) is coated onto a fused silica substrate and cured at 110 C for 3 min. For the fabrication of a power splitter, polymer droplet of several micro-liters (ZPU 12– 47, Chemoptics) was applied to the patterned surface of the PDMS mold. This mold filled with the polymer was placed in contact with the surface of the fused silica substrate coated adhesion promoter. The polymer, then, was cured to solid by illuminating the mold with UV light (>2500 mJ/cm2). After curing, the mold was peeled away carefully, and a patterned micro-scale structure was left on the surface of that substrate. The fabrication of the optical power splitter was completed by a cleaving process. 2.3. Fabrication results The size of the substrate is 30 · 30 mm2 and the length of the patterned structure is about 23 mm. The microstructure
Fig. 3. Schematic diagram of UV embossing process for optical power splitter.
C.-H. Choi et al. / Microelectronic Engineering 84 (2007) 1231–1234
was uniformly formed without noticeable deformation on the substrate. The MMI region of the device was shown in Fig. 4. We experimented also on the durability of PDMS mold in repeated casting process. There was no change of the mold after the first replica. Also, there was difficult to distinguish the different between replicas, and the SEM image view of the 5th replica is shown in Fig. 5. But, the mold started to change after the 17th replica or more. Replicas observed in these cases have local discontinuities and partial collapses of pattern. In order to analyze the problem, we measured the change of the contact angle on the surface of the PDMS mold. Previously to replication the contact angle of the PDMS mold was 114, but that of the mold after the 17th replica was decreased to 87. Because of some impurity adhesion problem, some ZPU 12–47 polymer was left on the surface of the PDMS mold, and the contact angle was changed. Therefore, the PDMS mold was changed from hydrophobic to hydrophilic. Fine-structures fabricated by UV embossing on a flat surface usually have an unwanted background residual layer [5]. High residual thickness can impair the performance of photonic devices. It is important to reduce the residual layer to acceptable level. This level in our device is designed at about 0.5 lm. In order to get the residual layer thickness for 0.5 lm or less, we tried reducing the droplet volume. As the droplet volume decreases, the resid-
1233
Fig. 6. SEM image of a cleaved polymer waveguide. Residual layer is observed that it has the thickness below 0.5 lm.
ual layer thickness usually decreases. However, if the droplet volume decreases less than the limit level, the pattern is not properly formed. Because liquid polymer has the limit length that can spread through a mold or substrate, a very small volume of the polymer cannot pattern the whole of a substrate. In our experiment, it was observed the partial pattern in the replica using the droplet volume of about 7 lL or less. Fig. 6 shows SEM image of the cross-section of the replica which is fabricated on the droplet of volume of 8 lL. The residual layer observed in this case has a thickness below 0.5 lm. 3. Conclusions
Fig. 4. Optical microscope image of optical power splitter.
We have described soft lithography technique for the fabrication of 1 · 4 MMI optical power splitter with UV curable polymer. We fabricated up to 17 replicas with one PDMS mold. In order to get the residue layer thickness to acceptable levels, we tried reducing the droplet volume. The UV embossing technique is expected to reduce the fabrication cost of photonics devices and photonic integrated circuits for mass production. Acknowledgements This work was supported by the Engineering Research Center Grant No. R11-2003-022-03002-0 (2006) for Optics and Photonics Elite Research Academy (OPERA), and in part by INHA University through a special program to promote the information and communication science and engineering education and research.. References
Fig. 5. SEM image of output waveguides in 5th replica.
[1] Lucas B. Soldano, Erik C.M. Pennings, J. Lightwave Technol. 13 (1995) 615–627. [2] David S. Levy, Robert Scarmozzino, Richard M. Osgood, IEEE Photon. Technol. Lett. 10 (1998) 830–832.
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C.-H. Choi et al. / Microelectronic Engineering 84 (2007) 1231–1234
[3] M.R. Paiam, R.I. MacDonald, Electron. Lett. 33 (1997) 1219–1220. [4] N.S. Lagali, M.R. Paiam, R.I. MacDonald, IEEE Photon. Technol. Lett. 11 (1999) 665–667.
[5] N. Bogdanski, M. Wissen, A. Ziegler, H.-C. Scheer, Microelectron. Eng. 78–79 (2005) 598–604. [6] Y. Xia, G.M. Whitesides, Angew. Chem., Int. Ed. 37 (1998) 550–575.
Fabrication of MMI optical power splitter by UV embossing with PDMS mold Chul-Hyun Choi, Min-Woo Lee, Jun-Ho Sung, Bo Soon Kim, Beom-Hoan O
*
Optics and Photonics Elite Research Academy (OPERA), School of Information and Communication Engineering, INHA University, 253 YongHyun-Dong, Namg-Gu, Incheon 402-751, South Korea Available online 2 February 2007
Abstract We report on UV embossing process for realizing an optical power splitter using a UV curable polymer material. The polydimethylsiloxane (PDMS) mold transferred from the master was used for forming the micro-scale structure and patterning repeatedly it. We experimented also on the durability of the PDMS mold in casting a UV curable polymer. Residual layer thickness on the replica was observed to be below 0.5 lm. 2007 Elsevier B.V. All rights reserved. Keywords: Embossing; PDMS; MMI; Waveguide; Polymer
1. Introduction Optical power splitters based on multimode interference (MMI) are very important to photonic device for optical communication systems, such as wavelength division multiplexing (WDM) and fiber to the home (FTTH) systems [1]. Also, various functional devices using MMI device, such as demultiplexers, polarization splitters, and Mach– Zehnder interferometers have been extensively investigated [2–4]. The polymeric photonic devices have been widely fabricated using traditional semiconductor processes, such as photolithography and etching. These methods involve many processing steps, and can lead to a long fabrication time and low yield [5]. However, embossing techniques using a patterned master or mold are low in capital cost, easy to learn, straightforward to apply, and accessible to a wide range of users [6]. Silicon, nickel, and polymer materials are generally used for the master or mold of embossing process. Especially, the polydimethylsiloxane (PDMS) polymer material is usually used for the elastomer mold. *
Corresponding author. E-mail address: [email protected] (B.-H. O).
0167-9317/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.090
The PDMS mold provides good chemical stability, optically transparency, flexibility and easy release from the rigid masters or replicas. In this paper, we report on UV embossing technique for the fabrication of 1 · 4 power splitter for optical communication systems and optical interconnections. PDMS mold was used for the fabrication of the device. We experimented also on the durability of the PDMS mold when used in a UV curable polymer casting process. In order to get the thickness of residual layer to acceptable level, we tried reducing the droplet volume. 2. Fabrication and results 2.1. 1 · 4 optical power splitter We designed the 1 · 4 MMI optical power splitter as shown in Fig. 1. The widths of the single mode waveguide and the MMI device are set to 4 and 60 lm, respectively. The splitting length is 855 lm and the height of the core (n = 1.47 at 1550 nm) on the substrate is 1.8 lm. The substrate material is fused silica glass (n = 1.444 at 1550 nm).
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C.-H. Choi et al. / Microelectronic Engineering 84 (2007) 1231–1234
Fig. 1. Schematic diagrams of: (a) 1 · 4 MMI optical power splitter and (b) cross-section.
2.2. Fabrication process The optical power splitter was fabricated by PDMS mold with UV curable polymer. Fabrication process consists of manufacturing the master, mold, and replica. Fig. 2 shows the schematic diagram of the fabrication process for the master and mold. The master was produced from the photoresist (AZ 1518, Clariant) and the structures were formed using a photolithography process. The photoresist was coated onto a silicon substrate. Here, the height of the photoresist is determined by that we whish to give to the core of the optical power splitter. After the soft baking, the photoresist layer was illuminated by UV light through a photomask to define the master pattern and developed.
Fig. 2. Schematic diagram of the fabrication process for photoresist master and PDMS mold.
The fabrication of a master was completed by a hard bake (3 min at 110 C). This process can improve the hardness and the sidewall roughness of the master. The mold is prepared by casting molding using liquid PDMS (Sylgard 184, Dow Corning). This liquid, that is a mixture of a liquid silicon rubber and a curing agent, is poured over a master. After 48 h at room temperature, the PDMS mold is cured to solid and peeled away carefully. Optical power splitter is fabricated by UV embossing and this process is shown in Fig. 3. Before the UV embossing process, a thin adhesion promoter (ZAP 1020, Chemoptics) is coated onto a fused silica substrate and cured at 110 C for 3 min. For the fabrication of a power splitter, polymer droplet of several micro-liters (ZPU 12– 47, Chemoptics) was applied to the patterned surface of the PDMS mold. This mold filled with the polymer was placed in contact with the surface of the fused silica substrate coated adhesion promoter. The polymer, then, was cured to solid by illuminating the mold with UV light (>2500 mJ/cm2). After curing, the mold was peeled away carefully, and a patterned micro-scale structure was left on the surface of that substrate. The fabrication of the optical power splitter was completed by a cleaving process. 2.3. Fabrication results The size of the substrate is 30 · 30 mm2 and the length of the patterned structure is about 23 mm. The microstructure
Fig. 3. Schematic diagram of UV embossing process for optical power splitter.
C.-H. Choi et al. / Microelectronic Engineering 84 (2007) 1231–1234
was uniformly formed without noticeable deformation on the substrate. The MMI region of the device was shown in Fig. 4. We experimented also on the durability of PDMS mold in repeated casting process. There was no change of the mold after the first replica. Also, there was difficult to distinguish the different between replicas, and the SEM image view of the 5th replica is shown in Fig. 5. But, the mold started to change after the 17th replica or more. Replicas observed in these cases have local discontinuities and partial collapses of pattern. In order to analyze the problem, we measured the change of the contact angle on the surface of the PDMS mold. Previously to replication the contact angle of the PDMS mold was 114, but that of the mold after the 17th replica was decreased to 87. Because of some impurity adhesion problem, some ZPU 12–47 polymer was left on the surface of the PDMS mold, and the contact angle was changed. Therefore, the PDMS mold was changed from hydrophobic to hydrophilic. Fine-structures fabricated by UV embossing on a flat surface usually have an unwanted background residual layer [5]. High residual thickness can impair the performance of photonic devices. It is important to reduce the residual layer to acceptable level. This level in our device is designed at about 0.5 lm. In order to get the residual layer thickness for 0.5 lm or less, we tried reducing the droplet volume. As the droplet volume decreases, the resid-
1233
Fig. 6. SEM image of a cleaved polymer waveguide. Residual layer is observed that it has the thickness below 0.5 lm.
ual layer thickness usually decreases. However, if the droplet volume decreases less than the limit level, the pattern is not properly formed. Because liquid polymer has the limit length that can spread through a mold or substrate, a very small volume of the polymer cannot pattern the whole of a substrate. In our experiment, it was observed the partial pattern in the replica using the droplet volume of about 7 lL or less. Fig. 6 shows SEM image of the cross-section of the replica which is fabricated on the droplet of volume of 8 lL. The residual layer observed in this case has a thickness below 0.5 lm. 3. Conclusions
Fig. 4. Optical microscope image of optical power splitter.
We have described soft lithography technique for the fabrication of 1 · 4 MMI optical power splitter with UV curable polymer. We fabricated up to 17 replicas with one PDMS mold. In order to get the residue layer thickness to acceptable levels, we tried reducing the droplet volume. The UV embossing technique is expected to reduce the fabrication cost of photonics devices and photonic integrated circuits for mass production. Acknowledgements This work was supported by the Engineering Research Center Grant No. R11-2003-022-03002-0 (2006) for Optics and Photonics Elite Research Academy (OPERA), and in part by INHA University through a special program to promote the information and communication science and engineering education and research.. References
Fig. 5. SEM image of output waveguides in 5th replica.
[1] Lucas B. Soldano, Erik C.M. Pennings, J. Lightwave Technol. 13 (1995) 615–627. [2] David S. Levy, Robert Scarmozzino, Richard M. Osgood, IEEE Photon. Technol. Lett. 10 (1998) 830–832.
1234
C.-H. Choi et al. / Microelectronic Engineering 84 (2007) 1231–1234
[3] M.R. Paiam, R.I. MacDonald, Electron. Lett. 33 (1997) 1219–1220. [4] N.S. Lagali, M.R. Paiam, R.I. MacDonald, IEEE Photon. Technol. Lett. 11 (1999) 665–667.
[5] N. Bogdanski, M. Wissen, A. Ziegler, H.-C. Scheer, Microelectron. Eng. 78–79 (2005) 598–604. [6] Y. Xia, G.M. Whitesides, Angew. Chem., Int. Ed. 37 (1998) 550–575.