Optoelectronic packaging on flexible substrates using flip chip-based optodic bonding

The optoelectronic industry has been rapidly developed in the past few decades. Whether in the field of science and technology or in the technical aspects of human social life, optoelectronics has a huge impact and is increasingly complementing or replacing electronic systems. Targeting the optoelectronic components, such as laser diodes, light emitting diodes (LED) or photo diodes, packaging plays a crucial role for their use in various applications. In addition, with the growing demand for flexible circuits or systems, flexible packaging attracts more research interest. In this contribution we introduce a novel approach for realizing the packaging of commercially available optoelectronic components on flexible substrates. Here, a bare chip of a laser diode with a dimension of 300 x 250 x 100 μm was employed. Its two electrodes are located in two opposite faces, thus generating a vertically flowing current. Considering the flexibility, we decided to use a polymer film as substrate material. In contrast to the state of the art for the packaging of laser diodes with the use of wire bonding technology, we fully adopted a new flip chip die bonding technique to accomplish the mechanical mounting and electrical connection. The key for realizing the entire packaging is employing a small-sized polymer carrier, which is partly metallized as a contact pad. The bare chip is firstly bonded on this polymer carrier with one electrode and then flipped down to be mounted on a large polymer substrate, which is designed as a circuit board to be connected with a power supply. Simultaneously, the other electrode mounted on this large substrate so far is electrically contacted as well. In this way, the bare chip of the laser diode is assembled on the flexible substrate by applying the new flip chip die bonding technique without any wire connections. The demand for developing this new flip chip bonding process instead of adopting the current flip chip technology derives from the selection of the used flexible substrate materials. By employing polymer films, we preferred to choose those with a higher transparency since their better optic characteristics enable much broader optical applications. However, those polymers have mostly a low glass transition temperature, thus a restricted thermal load is required during processing. In order to comply with this, we employed adhesives as bonding materials. For a further minimization of the thermal load we chose ultraviolet (UV)-curing adhesives and correspondingly designed an optode instead of the widely used thermode to cure the adhesives. This new process of adopting the optode is called optodic bonding, which was applied twice for contacting both electrodes of the laser diode to complete the packaging on the polymer substrate. We illustrate this novel approach and subsequently conduct a comparison of this new technique and the common technology applied for optoelectronic packaging. A first prototype of the packaged laser diode using this approach is produced and hereon presented.

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