Toward flexible optoelectronics packaging

Flexible substrates are a trend in electronics packaging, optical components, and fibers. In data transmission applications, they enable compact, 180° optical coupling of racks and boards, 90° coupling of mother with daughter boards, hingelike optical interconnects in mobile phones, and onboard bended optical connections. In the sensing area, smart clothing is a targeted application, with optical sensors measuring vital body parameters. In structural engineering, sensor foils can be wrapped around irregularly shaped and moving objects or bodies, acting as an early warning system to alert engineers to stress or strain problems with dams, bridges, aircraft wings, and helicopter or windmill blades. Sensing foils could also be applied to robot feet to improve their walking behavior or to robot fingers.

Standard optoelectronics packages are very bulky and often use wire bonding, making them incompatible with flexible substrate technologies. We have developed and evaluated a new packaging technology for optoelectronic chips such as vertical-cavity surface-emitting lasers (VCSELs) and photodetectors. We have embedded the devices in a thin foil only 40μm thick.¹ The resulting foil contains a planar electrical fan-out of the embedded device. This way, the optoelectronics footprint is not larger than the chip plus the electrical fan-out, resulting in optimal compactness. The foil is highly mechanically flexible, which enables mounting on non-flat surfaces and use in hingelike applications. The flexibility also increases the package's resistance to vibrations and releases the embedded optoelectronics from stresses.

Figure 1. Fabrication steps of an ultrathin flexible optoelectronics (OE) device package. SU-8: Photoresist.

Figure 2. Cross-section of an embedded vertical-cavity surface-emitting laser (VCSEL) in an SU-8 photoresist performed on a fiber-reinforced epoxy (FR-4) board.

Our fabrication is a sheet-to-sheet process. Since the package is a complete planar foil, a roll-to-roll fabrication process could be applied, which would reduce the packaging cost significantly. Optoelectronic chips are commercially available with a typical thickness of 150μm. To realize a flexible package, the chip itself needs to bend along with the surrounding substrate. Therefore, the stiffness or bending resistance must be decreased. We achieved this by removing a significant part of the chip's backside substrate. This way, very thin optoelectronics can be obtained with a thickness of only 20μm. We developed a single-die mechanical thinning process. The dies are wax-mounted on a temporary carrier. They are then consecutively lapped with a large slurry grain to 50μm thick and polished with a small slurry grain to 20μm. The latter process removes the chips' backside roughness caused by the lapping process.

Figure 3. An ultrathin flexible package of a 1×4 VCSEL array.

Optoelectronic 20μm devices are so thin that they become mechanically bendable. However, the brittleness of such thin components limits the handling. As a result, the dies need to be embedded. Figure 1 shows the process flow for fabricating an ultrathin flexible optoelectronics package. We chose SU-8 (a negative, epoxy-type, near-UV photoresist) as the embedding material due to its excellent optical and mechanical properties. A free-standing SU-8 layer is fragile to handle, however, and can break easily under high applied forces. Consequently, we sandwiched the material between two polyimide layers for mechanical support.

The stack of polyimide, SU-8, and polyimide is made by consecutive coating and baking steps. On the bottom polyimide layer, a patterned small copper island acts as a heat sink. The chip is embedded by laser ablating a cavity in the SU-8 layer atop the copper island using copper as a laser-ablation stop. After the chip is glued into the cavity with thermal conductive glue, it is covered with another SU-8 layer. The galvanic interconnection and fan-out of the chip is done by laser-ablating micro via-in-pad technology and sputtering copper tracks. After finishing the stack with the final SU-8 and top polyimide layers, the galvanic fan-out pads are opened again by laser ablation. Figure 2 shows a cross-section of a packaged VCSEL fabricated atop an FR-4 substrate without polyimide sandwiching.

Figure 4. Schematic and photographic images of different applications of the ultrathin flexible OE package. From left to right: Shear sensor, tactile sensor, and parallel optical VCSEL-to-photodiode link.

We characterized the flexible package's optical, mechanical, and thermal behavior and showed that the original properties of the packaged optoelectronic devices remain unchanged. The package is highly flexible down to a bending radius of 2.5mm and withstands multiple accelerated aging tests.¹ Figure 3 shows the optoelectronic package being bent.

Finally, we demonstrated the ultrathin package in different applications. We built very thin, flexible optical shear and tactile sensors^{2, 3} based on a combination of flexibly packaged optoelectronics and a deformable silicone layer. In another application area, we built a complete flexible parallel optical link between an embedded array of VCSELs and photodiodes, including embedded 90° optical coupling components and polymer optical waveguides (see Figure 4).⁴

In summary, we developed a packaging technology for commercially available optoelectronics resulting in a final package just 40μm thick. The package's small size is realized by thinning the optoelectronics to only 20μm thick, followed by embedding the chips inside a hybrid SU-8/polyimide layer stack. The ultra-thin package has proven to be highly flexible and reliable during characterization. We demonstrated the optoelectronic package in different applications, proving the broad uses of the technology. Improvement of the flexible package by optimizing the its materials toward low internal stress is ongoing. As next steps, we plan also to optimize the demonstrated sensors by increasing their sensitivity and by adding orientation and more functionality.

This work is conducted partially within the framework of the Flexible Artificial Optical Skin project (funded by the Institute for the Promotion of Innovation by Science and Technology, Flanders, Belgium) and the Phosfos project (funded within the European Commission's Seventh Framework Programme). The author also acknowledges support from his colleagues Jeroen Missinne, Bram Van Hoe, Sandeep Kalathimekkad, Geert Van Steenberge, Jan Vanfleteren, and Peter Van Daele.