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This study proposes a simple and cost-effective method of fabricating a double-layer polymeric optical waveguide, using two hot-embossing processes with a single stamp and template for passive alignment between the top and bottom layers. The two hot-embossing processes were conducted sequentially on the top layer and the bottom layer of the polymer layer. The second hot-embossing process was conducted after fabricating the buffer layer on the surface of the polymeric channel structure to control deformation and destruction of the previously fabricated polymeric channel structure. Passive alignment of the channel structure for the top layer and the bottom layer was automatically performed by simple insertion of the stamp and polymer layer using a metal template with the same dimensions (width x length) as the stamp. Regarding the polymer layer, the buffer layer on the side with the channel structure was coated, whereas the layer contacting the stamp did not have a buffer layer. For the purposes of this study, a 2 x 50 channel polymeric multimode optical waveguide was fabricated using a stamp with 50 straight ribs, without any coupling between the layers. The fabricated optical waveguide was controlled within positional tolerances of less than ± 5 μm between layers; propagation loss of below 0.2 dB/cm at 850 nm; and channel uniformity of below 0.5 dB.
©2011 Optical Society of America
1. Introduction
Hot-embossing [1,2] technology, which directly transfers nano/micro patterns by physically contacting a nano-/microstructure stamp with a polymer, is gaining popularity as a next-generation technology due to its simplicity, speed, and low cost. In particular, many studies have been conducted on the fabrication of optical devices using hot-embossing technology [3–5]. As regards the fabrication of optical devices, the need for multi-channel optical waveguides as a means of processing the high capacities of data at high speeds has been increasing, and single-layer waveguides are being replaced by double-layer optical waveguides. In addition, the need for and importance of smaller-sized multi-channel optical waveguides in integrated optical printed-circuit boards (OPCBs) has increased.
Pattern transfer principally depends on the stamp, which makes it possible to achieve nano-level resolution. However, hot-embossing technology requires temperatures in excess of the glass transition temperature (Tg) of the thermoplastic polymer, as well as a certain level of pressure to replicate the stamp pattern on the polymer. Thus, conducting multiple processes on the same polymer layer or replicating the pattern on both sides of the polymer layer has certain limitations, primarily because the initial pattern fabricated by the hot-embossing process entails the possibility of deformation due to the high pressure and high temperature required by the multiple processes. In previous studies on the application of multiple hot-embossing processes to the same polymer layer and the fabrication of patterns on both sides, Zhang et al., Choi et al., and others reported using the process of fabricating patterns by first embossing the pattern using a stamp, then using another stamp on the embossed pattern surface, and the process of embossing both sides at the same time [6,7]. To produce a pattern on both sides, two expensive stamps, which are manufactured by a multiple process such as UV exposure/etching or a metal electroplating process, should be used. For the alignment of each layer, an active optical alignment system with a transparent substrate and stamp is also required.
Therefore, the aim of this study is to fabricate a passively aligned, precise pattern on the double-layers of the polymer layer with a single stamp. The effectiveness of this study was proved by the actual fabrication and testing it with the double-layer polymeric multimode optical waveguide.
2. Concepts of passive alignment with a single stamp
The double-layer multimode optical waveguide was designed as a stable structure so that the coupling of optical signals would not occur between the layers. For the low-cost OPCB, a vertical cavity surface emitting laser (VCSEL) with a wavelength of 850 nm was used as the optical source. The refractive index difference between the core and the clad was 0.75% and the waveguide size was designed as 50 μm x 50 μm by considering the aperture size of the VCSEL and photodiode (PD). Structural optimization to prevent the coupling of optical signals between the layers was simulated using the beam propagation method (BPM). Figure 1 shows the simulated optical coupling losses at different waveguide distances; there was no coupling when the distance between the waveguide on each layer was 150 μm or more.
In the fabrication of the double-layered pattern, hot embossing with a single stamp should be performed twice. To align both the up- and down-sided patterns of the waveguides easily and precisely, passive alignment with the Ni stamp and metal disk template were used, as shown in Fig. 2 . As the dimensions of the Ni stamp and the rectangular hole in the metal disk template are the same, the Ni stamp could be placed exactly in the cavity of the metal disk. Therefore, the position repeatability of the Ni stamp could be obtained, and simple passive alignment with high accuracy was possible. The stamp was fabricated using the ultra-precision machining (UPM) process [8]. The Ni stamp was of the positive type, with a protruding pattern and dimensions of 20.0 mm (width) x 140.0 mm (length) x 3.1 mm (thickness). It also has 50 ribs, each measuring 50 μm x 50 μm in the cross-sectional area with pitch of 250 μm. The metal template, fabricated using an ultra-precision laser process, consisted of a 6-inch disk with a thickness of 3.6 mm, with a rectangular hole with a width of 20.0 mm and a length of 140.0 mm, thus having the same dimensions as the Ni stamp.
Figure 3 shows the fabrication procedure for the double-layer polymeric optical waveguide, which will be described in Section 3 in detail. In the second hot embossing process, the edge of the first fabricated pattern risked being easily broken and deformed due to the processing pressure. To prevent damage to the first fabricated pattern, we used a buffer layer that was coated on the first fabricated pattern. As the size of the coated buffer layer was the same as that of the polymer layer, the polymer layer with the buffer layer could be inserted into the metal template along with the Ni stamp. Therefore, the first fabricated pattern and the Ni stamp for the second hot embossing process were automatically aligned. The buffer layer was fabricated in order to protect the initially fabricated polymeric channel pattern, using elastic polydimethylsiloxane (PDMS). Because of its high thermal stability, the thermally cured PDMS layer was also used to fabricate the metal stamp [9]. Furthermore, it has been reported that PDMS improves mechanical strength when extra curing agent is added during thermal curing [10].
3. Fabrication of the double-layer polymeric optical waveguide
NIL (Obducat Co., NIL6) equipment was used to fabricate the double-layer polymeric multimode optical waveguide. Based on the structural design of the double-layer optical waveguide, the hot-embossing process was carried out on a polymethylmethacrylate (PMMA) sheet with a thickness of 250 μm. Figure 3 shows the fabrication procedure for the double-layer polymeric optical waveguide, while Table 1 summarizes the process conditions and materials used in this study.
Table 1. List of the parameters used in the systematic study
In the first hot-embossing process, the pressure was applied step by step to improve the accuracy of the replication. The pattern could be reproduced with the dimensional change maintained at below 1%. The buffer layer was fabricated by coating a PDMS (Dow Corning Co., Sylgard 184) solution onto the polymer pattern surface to a thickness of about 500 μm and by conducting thermal curing for 60 min at 80 °C. To improve the mechanical property (elastic modulus) of the PDMS layer, an elastomer precursor and a curing agent were mixed at a ratio of 2:1. For the secondary hot-embossing, compared with the process conditions of the first hot-embossing, the temperature was reduced by 20 °C while the process time was increased by 60 s to maintain the same dimensional accuracy. After the second hot-embossing process, the PDMS layer on one layer of the middle-clad layer was easily separated from the middle-clad because of its low surface energy and elasticity. Finally, the double-layer optical waveguide was fabricated using the middle-clad. A UV curable resin was used as the core (Chem Optics Co., WIR30-500). To minimize the slab’s thickness and improve the core’s degree of cure, liquid core resin was dropped into the middle-clad, and the PMMA sheet was brought into contact with the middle-clad from one specific point until they were in full contact with each other. The UV exposure process was performed for 5 min at 15 bar pressure in N2 atmosphere. Such an optical waveguide can be used for all wavelengths such as 850 nm, 1,330 nm, and 1,550 nm. However, in this study, it was fabricated using a core material with the lowest loss at a wavelength of 850 nm.
4. Results and discussion
The accuracy of the polymer layer fabricated using the hot-embossing process was measured. Figure 4(a) shows the image of the Ni stamp with 50 straight ribs used in the hot-embossing process. Since the stamp could be transferred precisely to the polymer layer, the single polymer layer was fabricated in the same size as the stamp area. The overall length and width of the reproduced pattern, as shown in Fig. 4(b), were measured, and were found to be the same as the Ni stamp, at 140.0 mm and 20.0 mm respectively. The overall length and width of the polymer layer affected the precision of the position of the middle-clad, which was produced in the next step, between the layers. Figure 4(c) shows the magnified cross-sectional image of the first hot-embossed polymer structure using optical microscopy of 200 magnifications (Mitutoyo Co., MF). The tolerance of the reproduced pattern was ± 0.5 μm in the square channel measuring 50 μm x 50 μm with a pitch of 250 μm, and the dimensional accuracy was measured to within less than 1% of the dimensions of the Ni stamp.
Fig. 4 Fabrication of a single-layer polymer structure by hot-embossing: (a) Ni stamp for the hot-embossing process; (b) hot-embossed polymer structure;(c) cross-sectional image of a hot-embossed polymer structure.
We measured the precision of the channel-pattern positioning on both sides of the polymer layer and the PDMS pattern after the second hot-embossing process. Figure 5 shows the results of the double-layer fabricated by the second hot-embossing process. Figure 5(a) shows an image obtained by scanning electron microscopy (SEM, Raith Co., Raith 150) of the middle-clad fabricated by the two hot-embossing processes with the channel patterns on both sides. The positional tolerance between each of the layers, which were passively aligned by a metal disk template as shown in Fig. 5(b), was found to be less than ± 5 μm. On both sides, channel patterns measuring 50 μm x 50 μm with pitch of 250 μm were observed without destruction or deformation, and the channel distance between the top layer and the bottom layer was 150 μm. Figure 5(c) shows a cross-sectional image of the disassembled PDMS layer after secondary hot-embossing process. As there was no destruction or deformation after the hot-embossing process, as shown in Fig. 5(c), the PDMS could be used as the buffer layer.
Fig. 5 Fabrication of polymer middle-clad: (a) fabricated middle-clad SEM image; (b) metal disk template for passive alignment between the top layer and the bottom layer; (c) PDMS structure image for the buffer layer.
Finally, the dimensions and the optical characteristics of the fabricated double-layer polymeric multimode optical waveguide were measured. Figure 6 shows a cross-sectional image of the fabricated double-layer multimode polymeric optical waveguide. The residual slab layer was measured at below 1 μm from the optical microscopy of 1,000 magnifications (Mitutoyo Co., MF). Insufficient filling or micro void of the core resin was not observed.
The optical uniformity of the fabricated polymeric multimode optical waveguide with 100 channels is shown in Fig. 7 . The light source used for the measurement was an 850 nm VCSEL, whose laser beam was directed at the waveguide’s core using a multimode fiber with a core of 50 μm; this was measured using a multimode fiber with a core of 62.5 μm. The laser beam was a random polarization of power of −26.1 dBm.
Channels #1 ~#50 were on the top layer, while channels #51~#100 were on the bottom layer. Channel uniformity of 0.5 dB was measured, and there was no coupling between each layer. The measurement of the optical waveguide was conducted over a length of 110 mm. Propagation losses of 0.1 dB/cm - 0.2 dB/cm were observed by the cut-back method, and a polarization dependent loss (PDL) of below 0.2 dB was measured. These results offer an evaluation of the effectiveness of the double-layer polymeric optical waveguide fabricated for this study.
5. Conclusion
In this study, a simple method of fabricating a double-layer polymeric optical waveguide using a single stamp, with passive alignment on both sides of the polymer, is proposed. The proposed passive alignment method proved to be simple and accurate, and fabrication with a single stamp proved to be cost-effective. If the optical waveguide were to be redesigned for different light sources of various wavelengths, this simple fabrication method could also be used.
The polymeric multimode optical waveguide with 2 x 50 channels was designed to prevent coupling between layers, and the fabricated positional tolerance was less than ± 5 μm between the layers. The channel uniformity was less than 0.5 dB, and the propagation loss was below 0.2 dB/cm at 850 nm. We therefore conclude that the proposed method is effective for the fabrication of double-layer optical waveguides.
Acknowledgments
This work was supported by the KSEF, grant R01-2008-000-10585-0, and the World Class University Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (grant no. R31-20004).
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