Yixiao Wang

Optodic bonding of optoelectronic components in transparent polymer substrates-based flexible circuit systems

In the field of modern information technology, optoelectronics are being widely used, and play an increasingly important role. Meanwhile, the demand for more flexible circuit carriers is rapidly growing, since flexibility facilitates the realization of diverse functions and applications. As a potential candidate, transparent polymer substrates with a thickness of about a hundred micrometers by virtue of their low cost and sufficient flexibility are getting more attention. Thus, accomplishing an integration of optoelectronic components into polymer based flexible circuit systems increasingly is becoming an attractive research topic, which is of great significance for future information transmission and processing. We are committed to developing a new microchip bonding process to realize it. Taking into account the fact that most economical transparent polymer substrates can only be processed with restricted thermal loading, we designed a so-called optode instead of a widely adopted thermode. We employ UV-curing adhesives as bonding materials; accordingly, the optode is equipped with a UV irradiation source. An investigation of commercial optoelectronic components is conducted, in which their dimensions and structures are studied. While selecting appropriate transparent polymer substrates, we take their characteristics such as UV transmission degree, glass transition temperature, etc. as key criterions, and choose polyethylene terephthalate (PET) and polymethyl methacrylate (PMMA) as carrier materials. Besides bonding achieved through the use of adhesives cured by the optode, underfill is accordingly employed to enhance the reliability of the integration. We deposit electrical interconnects onto the polymeric substrate to be able to bring the optoelectronic components into electrical operation. In order to enlarge the optical coupling zone from component to substrate within the proximity of the adhesive or underfill, we employ transparent interconnects made of indium-tin-oxide. We present the results of the performance tests, including the contact resistances, mechanical tests and environmental tests.

Chip-Level Packaging of Edge-Emitting Laser Diodes Onto Low-Cost Transparent Polymer Substrates Using Optodic Bonding

Nowadays, the optoelectronic industry increasingly occupies a pivotal position in modern communication fields. Packaging of optoelectronic components and modules ensures the reliable performance in mechanical, electrical, and optical properties. As the demand for a mechanical flexibility of optoelectronic packaging and also for a lower manufacturing budget is continuously growing, cost-effective transparent polymer foils are becoming more favored. They are employed for establishing novel planar optronic systems, which are applied as high-integrated sensing networks. In this contribution, we choose a bare edge-emitting laser diode as the light source for the optronic sensor systems and mainly investigate its optical performance in the chip-level packaging directly onto polymeric carrier substrates. Since these materials with low glass transition temperatures require thermal-restricted processes, we adopt previously developed optodic bonding utilizing light energy instead of heat energy to accomplish the chip-level packaging of the laser diode. However, the heat dissipation of the active optoelectronic components constitutes one of the most critical issues in their packaging technologies, even more severe because of the extreme low thermal conductivity of the employed polymer foils. Addressing this challenge, packaging of the laser diode die directly on the conventional substrate FR4 for printed circuit boards without any thermal management is attempted as a reference for packaging onto flex polymeric foils. The optical output power and the heat accumulation behavior in the form of the temperature of the active laser diode onto both substrates are measured and continuously monitored. Heat dissipation approaches based on accelerating the dissipation speed and reducing the heat accumulation are investigated.

Automated Misalignment Compensating Interconnects Based on Self-Written Waveguides

Optical interconnects are the key components for integrated optics to link photonic integrated circuits or to connect external elements such as light sources and detectors. However, misalignment of the optical elements contained and its compensation is a remaining challenge for integrated optical devices. We present a novel method to establish rigid interconnects based on a 2-wavelength self-written waveguide process which automatically compensates for misalignment. We exemplarily demonstrate the capability of our process by writing interconnects between two multimode fibers as well as hot-embossed integrated polymer waveguides and a bare laser diode chip. The coupling efficiency of the interconnects obtained is analyzed with respect to misalignment. We found that coupling losses are as low as 1.3dB if a lateral misalignment lies within a 10μm interval, which is achieved by commercially available pick-and-place machines. Our approach is easily combined with high-throughput techniques such as hot embossing and enables low-cost production of interconnects even for mass fabrication in future applications.

Simulative Investigations of the Mechanical Reliability of the Flexible Optoelectronic Packaging Using Optodic Bonding

With the constant innovation and development of the advanced materials and technologies in the field of optoelectronics, a variety of applications adopting the benefits of optoelectronics is increasingly penetrating into our daily routines. The way of packaging has a huge impact on the performance of optoelectronics in mechanical, electrical as well as optical properties. Particularly, flexible packaging is becoming a trend thanks to its compatibility to any irregular surfaces in diverse applications. By employing transparent polymer foils, such as polymethyl methacrylate (PMMA) and polyethylene terephthalate (PET), as carrier substrates for optoelectronic packaging, a greater mechanical flexibility can be achieved. Their transparency expands the range of application, for instance in optical sensing areas. In addition, they are mostly cost-effective, which enables an economical roll-to-roll manufacturing process. However, the challenge of using them is their low glass transition temperatures. In previous works, a novel optodic bonding process employing UV curable adhesives for processing these thermally sensitive polymer foils was introduced and verified as a promising technology for flexible optoelectronic packaging. In this work, we conduct further investigations of its mechanical reliability with the focus on the aspect of flexibility. FEM-based simulations for emulating various mechanical loadings including shear, bending and tensile stresses are implemented. We perform analyses of the influencing factors, particularly their degree of efficiency on the mechanical stability, e.g. the material properties of the optoelectronic components, the employed polymers as well as the bonding adhesives. These comprehensive investigations establish a constructive guideline for choosing materials for flexible optoelectronic packaging. In addition to this, we attain a convincing statement for the reliable feasibility of the optodic bonded packages with respect to a sufficient mechanical flexibility and stability.

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 m 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.