1. Introduction

Optical interconnection techniques can be one of the solutions for high-speed interconnection between systems to overcome the limits of electrical interconnects in operating speed, dense packaging and power dissipation [1]. Recent efforts have concentrated on combining a chip-to-chip optical interconnection system with conventional electrical printed circuit boards (PCB) [2]-[6]. These papers have reported the implementation of an optical interconnection techniques based on rigid PCB embedded with separately fabricated waveguide due to the stability of chips mounting. Furthermore, the additional layers as a chip-bonding platform have to be used to avoid the thermal damage of polymeric optical waveguide having a low thermal stability subjected to chip bonding process. In recent years, the result of an optical interconnection using a flexible opto-electrical PCB (FOE-PCB) for high-speed mobile devices has been reported [7]. In this paper, the rigid PCB layers were used as the platform to mount the electrical and the optical devices. In these cases, the additional optical loss and optical crosstalk can occur due to the increase of distance between a vertical cavity surface emitting laser (VCSEL)/photodiode (PD) and the optical waveguide embedded in PCB layers.

In order to solve some difficulties mentioned above, it is desirable to directly integrate the electrical and the optical device on the polymeric optical waveguide. To do this, it is very important to select the waveguide materials having an excellent thermal stability and to design the architecture of an optical waveguide suitable for a chip-to-chip optical interconnection.

In this paper, the intra-system optical interconnection module for flexible-type boards was suggested. Here the simplified fabrication processes of an optical waveguide suitable for a mass production and a cost-effective process were also proposed. The intra-system optical interconnection was achieved by direct integrating the electrical/optical devices on the polymeric optical waveguide, so this approach was developed in an effort to realize a compact and slim interconnection module without bulky optics or chip-mounted platforms.

2. Architecture of intra-system optical interconnection module

Figure 1 illustrates an intra-system optical interconnection module directly integrated on a polymeric optical waveguide. Optical components and electronic integrated circuit (IC) chips required for optical interconnection, such as VCSEL/PD devices and driver/receiver IC chips, are integrated on the top surface of a polymeric waveguide. To couple the beam between the VCSEL (or PD) and the waveguide without a collimating micro-lens or a chip-mounting platform, a thin polymeric waveguide with 150-μm thickness was just used. The VCSEL and the PD chips are flip-chip bonded on the waveguide. It is important to note that this architecture is expected to make the alignment procedure quite simple and cheap because all the alignment steps between optical components are achieved on the waveguide with a flip-chip bonding technique.

 

Fig. 1. Schematic of an inter-chip optical interconnection module.

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Another key feature of our architecture is that a micro-SubMiniature A (μ-SMA) connector is mounted on the polymer platform. Therefore, our interconnection module can be attached at any place on system boards that has connector-to-connector type for the intra-system interconnection.

3. Fabrication of an intra-system optical interconnection module

3.1 Fabrication of a polymeric optical waveguide as an interconnection platform

 

Fig. 2. (a) Fabrication process of a polymeric waveguide as an intra-system optical interconnection module platform.

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In order to directly mount various devices on the polymeric waveguide, the selection of a waveguide material is important. Especially, the thermal stability of the waveguide is required, considering the packaging of chips under high temperature. Here we selected polyether sulfone (PES) for the waveguide core, which has the thermal stability of more than 200°C and a refractive index of 1.65. An epoxy adhesive film with a refractive index of 1.625 was selected for the waveguide clad. To investigate the thermal stability of waveguide materials, we placed both core and clad sheets in a high temperature situation using a furnace chamber. It was not observed any optical degradation of the polymeric waveguide under the exposure condition of 300°C for 10 seconds.

Figure 2 shows the fabrication process of a polymeric waveguide as an intra-system optical interconnection module platform in more detail. The steps in this process can be outlined as follows.

We first prepared film-type cores and clads with the thickness of 100 μm and 25 μm, respectively. The clad films include the polyimide film of 10 μm thick as a protective layer on the one side. The core and clad films are laminated under the conditions of 180°C of temperature, 539 N/cm², and 90 minutes of holding time. Next, to make optical waveguide sheets with 45° mirrors, a V shaped-blade sawing technique was applied. After sawing, the surface roughness of the 45°-ended waveguide facet was measured to be approximately 40 nm (rms), which was measured with an interferometer. Due to the fine surface roughness, another additional surface-fining process of the waveguide facet was not required. In the following, using an e-beam evaporation process, a Ni adhesive layer (20 nm) and a Ag reflecting layer (300 nm) were deposited sequentially as a thin metal mirror on the 45°-ended facets through a locally opened thin mask. To make the optical channel in the waveguide film, the waveguide film is set to perpendicular to the 45°-ended facets, followed by sawing the waveguide core film for an optical channel with the width of 100 μm. We achieved an average of 30 nm (rms) of the sidewall roughness using a diamond blade with 2-μm grains under a 5-mm/sec speed and a 40000rpm. Then, an under-clad sheet is covered, being thermally compressed with the same condition with the over-clad lamination process. Finally, the polymeric waveguide as an intra-system optical interconnection module platform was completed after filling the clad epoxy into channel gaps during covering the under-clad sheet. In this work, the adhesive material used in PCB lamination plays a role in waveguide clad and thus, the lamination of protecting films are simultaneously accomplished with the fabrication of a waveguide. The fabricated waveguides are able to play a role in a module platform because they have protecting layers. Figures 3(a) and 3(b) show the cross-sections of the waveguide channels and the 45°-ended facet of waveguide, respectively. The channel size of the optical waveguide is 100 μm × 100 μm and the angular deviation of 45°-mirror was observed to be within ±1°. The fabricated polymer platform was only 150 μm thin including a 100-μm core, a 30-μm epoxy clad and a couple of 10-μm polyimide layer.

 

Fig. 3. (a) Cross-sectional views of waveguide channels and (b) 45°-ended facet of waveguide made by a sawing machine.

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3.2 Fabrication of FOE-PCB

The steps in FOE-PCB fabrication can be outlined as follows. We first dug via-holes on the fabricated waveguide film using a CO₂ laser and the mirror facet plays a role in a base point. The formed via-holes are used to align circuit patterns to the waveguide. Next, to make circuit patterns to be matched with the position of waveguide core, electroless copper plating on the both sides of waveguide and lithography patterning were performed. And gold plating was also accomplished for the flip-chip bonding of the optical components. Finally, the FOE-PCB was completed after coating coverlay with the window opening of the electrode pads and the waveguide mirror facets, as shown in Fig. 4. Via-holes shown in the inset of Fig. 4 play a role in an electrical connection between both side grounds as well as in align-marks for high alignment between the waveguide and the metal pads. Considering the alignment between the waveguide core and the circuit patterns including via-holes, the misalignment between the designed and the fabricated boards were measured to be an average of 10 μm in the z direction and an average of 5 μm in the x direction of the waveguide.

 

Fig. 4. Photographs of the completed FOE-PCB embedded with the polymeric waveguide.

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3.3 Packaging of electrical/optical components

For the fabrication of an intra-system interconnection module, Optical components, driver/receiver IC chips, radio frequency (RF) connectors, and electrical passive components were packaged on an intra-system optical interconnection module platform. For an optical interconnection module, a single channel VCSEL and PD with a wavelength of 850 nm operating by 5 Gb/s from ULM photonics were used and the driver/receiver IC chips operating by 4.25 Gb/s from Zarlink were used. All the details of the packaging process of the electrical/optical components are as follows.

First, VCSEL and PD were aligned to the mirror plane of the waveguide ends and were flip-chip-bonded using a Suss MicroTec Triad05AP flip-chip bonder optimized for optoelectronics modules with the post-bonding accuracy of less than ±0.5 μm. We aligned a VCSEL/PD aperture to the waveguide core and bonded to metal pads formed on the waveguide sheet. Next, driver/receiver ICs were die-bonded and wire-bonded to metal pads. Finally, electrical passive components and RF connector were packaged by soldering and thus the intra-system interconnection module was completed. We used a μ-SMA connector as a RF connector on the interconnection module.

Figure 5 shows the completed FOE-PCB interconnection module. Because an optical transmission line region of the module is very flexible, our module can be applied to the various architectures such as a fold-type mobile and a laptop to need a flexible interconnection through a hinge part. Furthermore, our module can be applied to the interconnection between system boards because it is compact and pluggable.

 

Fig. 5. Photographs of the completed the flexible optical interconnection module.

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4. Experimental results and discussions

 

Fig. 6. Measured (a) 3.25 Gb/s and (b) 5 Gb/s eye diagrams through the interconnection module.

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We measured the propagation loss and bending loss of a waveguide as an intra-system optical interconnection platform. The propagation loss of the fabricated optical waveguide was measured to be 1.3 dB/cm at 850-nm wavelength using a cut-back method. The measured propagation loss of our waveguide was somewhat larger than results reported in [2, 3, 6, 7]. This reason is that we used the currently commercial PCB materials as the core and clad of a waveguide, which has low optical transmission properties. The bending loss of the waveguide was measured by comparison between insertion losses of a straight structure and a bent structure. The bending losses of the waveguide were added to be less than 2 dB under a 360°-turned and 3-mm radius bending conditions, compared with the loss of a straight waveguide.

To measure the total link loss from VCSEL to PD of the interconnection module, a DC voltage of 3.3 V was supplied into the transmitter and the current generated in receiver was measured. The output power of VCSEL was 3 dBm and input power into PD was -12 dBm. Therefore, the total link loss from VCSEL to PD of the interconnection module was measured to be 15 dB, which included the VCSEL-to-waveguide coupling loss, waveguide-to-PD coupling loss and waveguide propagation loss. Finally, the optical transmission characteristics of the intra-system optical interconnection module were measured. An electric signal of clear 3.25 Gb/s to 5 Gb/s generated by a pulse pattern generator were supplied to the transmitter module through the μ-SMA connector and the signals transmitted to the receiver module through the waveguide were monitored by a digital oscilloscope. As a result, the measured clear eye pattern was observed at a data rate of 3.25 Gb/s and 5 Gb/s, as shown in Figs. 6. In addition, we successfully transmitted 3.25-Gb/s and 5-Gb/s with signals error free and with signals error of 10^-11, respectively, at a bit error rate test.

5. Conclusions

We suggested a new chip-to-chip optical interconnection module directly integrated on a polymeric optical waveguide. The proposed FOE-PCBs and interconnection module were successfully fabricated with a slim and compact size using sawing techniques and the flip-chip bonding technique. And the module was successfully demonstrated with a 5-Gb/s data transmission rate. The optical interconnection module based on the optical waveguide platform is expected to make a great contribution to the next generation OE-PCB for a fold-type mobile and a laptop to need a flexible optical interconnection through a hinge part.