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A thermally stable polymeric optical waveguide has been fabricated using ultraviolet (UV)-curable epoxy resins for the core and clad materials. A simple and cost-effective fabrication method that uses reusable polydimethylsiloxane (PDMS) masters has been developed. The 12-channel under-clad layer of the UV-cured epoxy was prepared using a PDMS master whose embossed channels had been fabricated by a polycarbonate (PC) secondary master. The thermal stability of the fabricated waveguide was tested at 200 °C for one hour. The optical waveguide was not damaged physically by thermal stress. Propagation losses detected by a cut-back method were 0.16 dB/cm and 0.26 dB/cm, respectively, before and after the thermal stability test at 850 nm. Loss increase after the thermal treatment can be attributed to the formation of the absorbing and scattering sources. This waveguide can be applied for areas that require thermal stability such as an optical printed-circuit board.
©2008 Optical Society of America
1. Introduction
Polymeric materials have been used widely for optical waveguides in optical interconnection systems. They have excellent characteristics for application in optical interconnection media, such as their flexible processing capability, low processing cost, flexibility to form the routes for the designing of passive optical components, and good compatibility with electronic circuits and components [1–3].
Thermoplastic polymers such as poly methyl methacrylate (PMMA) and polycarbonate (PC) show outstanding optical properties and are very easy to process. PMMA has an appropriate refractive index of 1.45 at a 1350 nm wavelength when used compatibly with other polymeric materials as a polymeric waveguide. PC has a slightly higher refractive index of 1.586 at the 1350 nm wavelength. PMMA has a transparency of around 92 %, while that of PC is around 82 %. Thus PMMA- or PC-based optical waveguides have been extensively tested and applied for optoelectronic systems. Even though they have good optical characteristics, most of the thermoplastic polymers have a relatively low glass transition temperature (T g) compared with thermosetting polymers. The T g of PMMA is 100 °C, and that of PC is 150 °C. The relatively low T gs of PMMA and PC prevent their versatile usages for thermal-stability-required components, especially for embedding them in a printed-circuit board (PCB) in order to fabricate optical printed-circuit boards (OPCBs) [4].
On the other hand, most of the ultraviolet (UV)-cured epoxies are thermosetting polymers, thus they do not have glass transition temperatures, but instead have degradation temperatures (Tds) of around 300 °C. Furthermore, their optical properties are as good enough as thermoplastic polymers to be used as optical waveguides. Therefore, a polymeric waveguide using UV-curable epoxy resins can be a potent solution to fabricate a thermally stable optical waveguide [5].
Many kinds of fabrication processes of polymeric waveguides have been introduced, and they can be categorized by three main processes such as lithography, hot-embossing, and laser writing [6–8]. Most of the processes for the fabrication of the polymeric waveguides have high costs and are complex because they require expensive equipment and precise control. A simpler and more cost-effective process is highly necessary in order to come closer to a wide commercialization of the polymeric waveguide.
In this paper, we fabricated a thermally stable polymeric waveguide using UV-curable epoxy resins. We carried out the thermal stability test using a fabricated epoxy polymeric optical waveguide, and compared its physical state and optical property changes before and after the thermal stability test. We also developed a simple and low-cost fabrication process for this thermally stable polymeric waveguide. The process uses polydimethylsiloxane (PDMS) rubber masters that can be made using PC or PMMA masters fabricated through a hot-embossing technique. Once the PDMS rubber masters are made, it becomes economical since they can be reused for the fabrication of the polymeric waveguides several times [9].
Although several similar methods of the replication of polymer waveguide by PDMS molds have been proposed [10, 11], our process has differences in specific techniques from those processes. As an example, we fabricated the over-clad layer by PDMS molds, as well as the under-clad layer. It can make cost down since we fully use the repeatability of the PDMS molds for preparation of the cladding layers. Detailed descriptions on the optical and thermal properties and fabrication process follow in the next sections.
2. Fabrication of the waveguide
The fabrication procedure for an optical waveguide using UV-curable epoxy resins can be classified into two sub steps. The first step is for the fabrication of the reusable PDMS rubber masters of the under-clad and over-clad layers. The second step is for the fabrication of the optical waveguide using UV-curable epoxy resins and PDMS rubber masters.
Figure 1 shows the first step of the fabrication procedure. The photograph in Fig. 1 is the embossing equipment used (Jenoptik Mikrotechnik, Jena, Germany). In the first step, three kinds of masters were made step-by-step, primary Ni and Si masters, secondary PC (or PMMA) masters, and tertiary PDMS masters. A conventional Ni master that has 12 embossed channels and a Si flat master were used respectively as the primary masters to conduct the hot-embossing for the PC secondary masters of the under-clad and over-clad layers. The 12 embossed channels of the Ni primary master were formed through conventional photolithography of a Si wafer and a Ni plating process on it. The dimension of the Ni master was 100 mm×15 mm×0.5 mm with a channel pitch of 250 µm. We used the Si wafer part for over-clad fabrication of the primary master. The Ni and Si primary masters were slightly coated with perfluorootyltrichlrosilane to increase the release ability with the PC secondary masters.
For the hot-embossing of the PC, a 0.5 mm-thick PC sheet was put under the Ni (or Si) primary master, and they were then both sandwiched between two glass plates. The hot-embossing temperature of the PC was 190 °C, and 0.35 MPa of pressure was given for 400 seconds. The total processing time was 10 minutes [7]. Then we could obtain a 12-channel engraved PC secondary master for the under-clad and a flat PC secondary master for the over-clad. Their thicknesses are h=150 µm for the under-clad and h’=150 µm for the over-clad, each with a length of 100 mm. After preparing the PC secondary masters, they were cut into a proper size to be set in the inner bottom area of a rectangular plastic box. Then, liquid-state PDMS was poured onto the prepared PC secondary masters after removing bubbles inside with a vacuum pump. The PDMS was cured at 60 °C for 12 hours. Releasing the PC secondary masters, we could attain the 12-channel embossed PDMS tertiary rubber masters for the under-clad and over-clad. The depths of the PDMS rubber master for the under-clad layer and for the over-clad layer are identical with the thicknesses of the PC secondary masters, i.e., h=h’=150 µm. It is preferable to use the Ni primary master that has 12 engraved channels instead of 12 embossed channels in order to fabricate the PDMS rubber master by omitting the process for the PC secondary masters. However, to acquire the result as early as possible and to make it convincing that PC secondary masters also work well enough, we used the conventional embossed Ni master.
Fig. 1. The first step of the fabrication procedure: Fabrication of the PDMS rubber masters. The photograph shows the embossing equipment (Jenoptik Mikrotechnik, Jena, Germany).
Figure 2 shows the second fabrication step for the multi-channel waveguide by UV-curable epoxy resins. In the second fabrication step, PDMS rubber masters fabricated through the procedure described in Fig. 1 were used. After pouring the UV-curable epoxy resin for under-clad in the PDMS rubber master, we covered it with another flat PDMS rubber layer. Then it was exposed to a UV light of 100 mW/cm2 for 10 minutes for curing. We used DYMAX 2000-EC as the UV light source, for curing the epoxy resins. A longer curing time than normal was given since the UV light has to penetrate the PDMS rubber set on the epoxy resin. Releasing the PDMS rubber, we could obtain the UV-cured-epoxy under-clad having 12 embossed channels. The UV-cured-epoxy over-clad was prepared in the same way. The UV-curable epoxy resin for the core was poured onto the under-clad layer. Covering the under-clad layer with over-clad layer, and curing it under a UV light for 2 min with proper pressure in the jig, we could fabricate the optical waveguide made of UV-cured epoxies. We note that the sequential procedure, pouring the core resin and pressing the over-clad on the resin, can reduce the possibility of micro-bubble formation in the core layer, while the typical procedure, injecting the core resin into the empty core channels after attaching the over-clad and the under-clad, maybe suffered from micro-bubble formation as tried in other work [10].
Fig. 2. The second step of the fabrication procedure: Fabrication of a multi-channel waveguide using UV-curable epoxy resins.
Figure 3 shows photographs of the masters and fabricated waveguide. Figure 3(a) shows the Ni primary master with 12 embossed channels for the under-clad and the Si primary master for the over-clad. Figure 3(b) shows the fabricated PC secondary masters; the under-clad PC secondary master was engraved using the Ni primary master and the over-clad PC secondary master was formed using the Si primary master. Figure 3(c) shows the fabricated PDMS tertiary masters using the PC secondary masters and the finally formed optical waveguide by UV-cured epoxies. The upper left-side of the photograph is a cross-sectional view of the PDMS rubber master, and the bottom of the photograph shows the clearly embossed channels of the finally formed waveguide. The perfect rectangular shape of the embossed channels of the PDMS master and clear channels of the waveguide can guarantee the feasibility of this waveguide fabrication process using PDMS masters. The dimension of the UV-cured epoxy waveguide was 75 mm×8 mm×300 µm. Because the UV-cured epoxies can shrink a little after curing, certain careful treatment is required to maintain the dimension of the waveguide during the curing process. When the cured hot waveguide is exposed to room temperature directly, partial areas are cooled down by the air at different cooling speeds, which makes the waveguide shrink severely. To prevent this shrinkage effect, we cooled the waveguide down enough after curing in the holding jig until it lowered to room temperature. The light-emission of the fabricated waveguide is shown in Fig. 3(d). The microscope captured this light-emitting photo by irradiating a white beam onto the fabricated waveguide.
Fig. 3. Photographs of the masters and fabricated waveguide: (a) The Ni primary master for under-clad and Si primary master for over-clad, (b) PC secondary masters, (c) PDMS tertiary masters and final optical waveguide, and (d) light emitting image from a cross-section of the fabricated waveguide.
The physical and optical characteristics of the UV-curable resins are shown in Table 1. Because thermosetting polymeric materials including epoxy have cross-linked chains, they do not melt as heated, which means they do not have glass transition temperatures. UV-cured epoxies do not have glass transition temperatures (Tg) either, but they do have degradation temperatures (Td) higher than 280 °C. Because UV-cured epoxies have good thermal stability, they are suitable for the fabrication of optical waveguides especially where thermal resistance characteristics are required in areas as in an OPCB [12]. The refractive index of the core UV-cured epoxy is 1.6, and that of the clad UV-curable epoxy is 1.512 at a 1550 nm wavelength. The birefringence of the UV-cured epoxies is 0.001 ± 0.0005 at the 1550 nm wavelength, which is small enough to make a polarization-independent multimode optical waveguide for optical interconnections [13]. Because of its excellent optical property and adhesion characteristic, we used the UV-curable epoxy already for bonding of the bottom-emitting VCSEL on the fiber connector in order to remove the air gap between the VCSEL device and the connector in the fabrication of a low-loss optical transmitter module [14]. The UV-cured epoxy layer between the VCSEL and the fiber provides reduction of the divergence angle and scattering of the light beam, the same as the index matching fluid works [14].
Table 1. Specific characteristics of the UV-curable epoxy resins for clad and core materials.
3. Optical and thermal characteristics of the fabricated waveguide
The UV-cured epoxy waveguide has a good enough thermal resistance to stand up to around 300 °C. To obtain the certified stability against thermal stresses during the embedding process of the OPCB manufacturing and soldering processes, waveguides should endure a severe heat treatment above at least 180 °C since the standard lamination of an FR4 PCB is processed at 180 °C for 1–2 hours under about 25 kg/cm2 of pressure [15, 7]. To test the thermal stability of the fabricated optical waveguide, the waveguide was put on a hot-plate heated at 200 °C for an hour. Then, the physical states and optical properties of the waveguide such as propagation loss were investigated.
Figure 4 shows the comparison of the cross-sectional views of the optical waveguide fabricated using UV-cured epoxy layers before and after a thermal stability test. As identified in Figs. 4(a) and 4(b), there is no physical damage causable by the thermal stress to the fabricated waveguide. After the thermal stability test, the length and width were not changed within measurement error ranges, and all the pitches of the 12 channels were kept with 250 µm. From the magnified cross-sectional features of the waveguide seen in the bottom of Figs. 4(a) and 4(b), it is clear that there is no observable damage from thermal stress.
Fig. 4. Comparison of the states of the waveguide between (a) before and (b) after the thermal stability test
The optical propagation losses of both before and after the thermal stability test were measured by the cut-back method and compared as shown in Fig. 5. From the slopes of Fig. 5, the propagation loss increased from 0.16 dB/cm to 0.26 dB/cm after the thermal stability test. The optical loss can be improved by applying UV-curable epoxy resins with better optical properties [17].
The increase of the propagation loss after thermal treatment of our waveguide might be originated from the increase of the absorbing and scattering sources. The acryl-based epoxy resins containing unreacted monomers and molecules react each other during thermal treatment since they obtain activation energy for the reactions. Thus, various kinds of bonds (O-H, C-H, C-O, and etc) and molecules such as water (H2O) and hydroxyl ion (OH-) can be formed [16]. These bonds and particles can work as the absorbing and scattering sources, increasing optical losses [17].
The total optical loss for the typical-length waveguide from the Fig. 5 can be within the acceptable power budget range for the short distance optical interconnections systems [18]. Thus the polymeric waveguide using UV-curable epoxies could be a good candidate for the application where waveguides should endure high temperature and high pressure processes, particularly, for the fabrication of the OPCB. For the next study, we are trying to fabricate an OPCB by embedding this optical waveguide.
4. Conclusions
We have fabricated a thermally stable optical waveguide using UV-curable epoxy resins. We developed a cost-effective and simple optical waveguide fabrication process that uses reusable PDMS rubber masters. A thermal stability test for the fabricated waveguide at 200 °C for one hour showed no physical defects caused by thermal stress. The measured optical propagation loss increased 0.1 dB/cm after the thermal stability test, which could be within an acceptable range for short distance optical interconnection systems. The optical loss increase after the thermal treatment can be attributed to the formation of the absorbing and scattering sources. This waveguide can be used for areas where thermal resistance is required such as an optical PCB.
Acknowledgments
This work was supported by the national program Tera-level nanodevices as a 21st Century Frontier R & D Project funded by the Korean Ministry of Education, Science and Technology (MEST) and it was also supported in part by the IT R & D program of the Korean Ministry of Knowledge Economy (KME).
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