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Development of skills and tools for micro opto-electrical integration on wafer level
Keywords: opto-electrical integration , RGB-light source, laser diode assembly
The challenge in the production of small form factor RGB-laser light engines are the hermetic optical packaging. The package form factor should enable a straight forward integration into a projection system with an overall size that matches typical glasses frames. Light weight, ultra-low power consumption and ruggedness are further demands. Wafer Level Packaging (WLP) as a new concept realizes assemblies with good thermal properties high optical precision and low inductance for ultra-high speed laser emission control. The possible high bandwidth will be particularly important for 2k to 8k video resolution at more than 50 images per second. The beam characteristics of the light engine may be customized to the projection system requirements. In this paper we describe a novel lateral emission based glass-silicon wafer level assembly and packaging technology for laser diodes and the functionality of a customer specific precision opto-device mounter. A miniaturized hermetic laser diode package can be realized by a Wafer Level Packaging approach. A transparent glass cap with an integrated vertical window protects especially the blue and green laser against premature failure. Hydrocarbons have been identified to be a root cause of laser diode degradation [1]. Our hermetic packaging concept relies on joining material selections that reduce possible hydrocarbon outgassing’s within the sealed housing. Specific filler gas compositions may be enclosed within the cavity to further improve the environmental conditions for robust laser performance. The lateral emission enables the beam shaping by low cost optics placed outside of the hermetic laser package area. To further reduce the efforts for lens mounting, a hybrid FAC/SAC lens is in planning for second generation light engines, that will also help to reduce the package width. The choice of glass as a capping material is based on its low permeability for water and gases and by the high optical transparency. The good optical quality of the emission window in respect to roughness and flatness is achieved by fusing two glasses with different softening points. Our glass cap production technology is based on hot temperature viscous glass micromachining with glass and silicon wafers [2]. First simplified demonstrators were built on a silicon wafer with AuSn metallized solder pads and metal seal frames to prove the feasibility. Eutectic AuSn bonding is a joining technology widely used in opto-electronic packaging. While we used contact heating for first demonstrator assemblies, our aim is the development of in-situ laser soldering technologies for both the submount and the laser diode soldering. The demand for a miniaturized multi-color laser engine requires further improvements in assembly technologies. An investment into a new ultra-precision assembly machine as part of the “Forschungsfabrik Mikroelektronik Deutschland” (FMD) is going on. Considering a number of possible joining techniques, we prefer in-situ laser soldering with direct heating of selected mounting positions to reduce the heat spreading. This will reduce thermal effects on neighboring metallization’s and mounted components, prevent pre-mature alloying and increase the eutectic soldering throughput. The aim is to reduce the chip gap to around 100 µm with a chip position accuracy of around 0.5 µm. The laser focus will be programmable within a spot size of 400 µm to 3000 µm. Most of the assembly job should to performed in a passive alignment strategy and finalize the assembly with only a limited number of active aligned components. This will balance the demand for throughput with the requirement to achieve well defined beam properties. Experiences have to be gained, which optical components have important influence on the optical performance and therefore define the yield. The machine platform, built by ficonTEC Service GmbH in Germany, incorporates a beam characterization unit to allow active alignment jobs. The machine is in the specification process right now and will be available for research activities in early 2019. Our new hermetic glass-silicon packaging platform enables miniature laser packages. Going from a single laser die assembly to a full optical multi-color system is a challenge in a number of technological topics. The actual work is focused on the machine specification, the definition of optical elements based on ZEMAX simulation and purchasing of these optical components. The development of a low temperature sealing technique for hermetic glass cap sealing applied to our first generation light engine concept is planned. In future, we will integrate vertical vias in the substrate wafer to achieve SMD-compatible packages.
Saskia Schröder, Team Leader Modul Services
Fraunhofer Institute for Silicon Technology
Itzehoe, Schleswig-Holstein

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