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Prerequisites for Building Long-Term Stable, Reliable Optical Probes with Hermetically Packaged Discrete Components
von Linda RudmannIn the recent years, optical probes with integrated discrete components, e. g., LEDs, gained more and more importance mainly due to the requirements in the field of optogenetics. To date, no probe concept has been presented that uses miniaturized, hermetic packages whose outlines specifically match the component sizes.
Leak paths along material interfaces that extend into the interior of the package play an increasingly important role as internal volumes become smaller. Therefore, in the context of the presented work, package components which reduce the number of interfaces to a minimum were developed.
The majority of leak paths occur in conjunction with electrical feedthroughs. A diffusion-based feedthrough approach enables their fabrication without introducing additional interfaces into the silicon substrate. By using this approach LEDs could be successfully driven vertically through the silicon substrate.
Refractive lenses were integrated directly into the transparent package lid. Based on photoresist melting and subsequent transfer of the profile into the substrate, lenses could be fabricated. The sphericity of the etched structures is responsible for the good agreement between calculated and measured focal lengths.
Even with a maximum reduction of leak paths, a completely hermetic packaging cannot be guaranteed. A novel capacitive humidity monitoring system, which is particularly suitable for the use in the very low relative humidity range < 8 %RH, circumvents this problem.
In addition to the detection of the current moisture content inside the cavity, a lifetime extension of several orders of magnitude due to the desiccant-based dielectric could be determined. This is a unique feature of the developed sensor.
By using non-hermetic encapsulation of the optical components, the functionality of the presented probes could be demonstrated by means of near-infrared spectroscopy measurements.
Leak paths along material interfaces that extend into the interior of the package play an increasingly important role as internal volumes become smaller. Therefore, in the context of the presented work, package components which reduce the number of interfaces to a minimum were developed.
The majority of leak paths occur in conjunction with electrical feedthroughs. A diffusion-based feedthrough approach enables their fabrication without introducing additional interfaces into the silicon substrate. By using this approach LEDs could be successfully driven vertically through the silicon substrate.
Refractive lenses were integrated directly into the transparent package lid. Based on photoresist melting and subsequent transfer of the profile into the substrate, lenses could be fabricated. The sphericity of the etched structures is responsible for the good agreement between calculated and measured focal lengths.
Even with a maximum reduction of leak paths, a completely hermetic packaging cannot be guaranteed. A novel capacitive humidity monitoring system, which is particularly suitable for the use in the very low relative humidity range < 8 %RH, circumvents this problem.
In addition to the detection of the current moisture content inside the cavity, a lifetime extension of several orders of magnitude due to the desiccant-based dielectric could be determined. This is a unique feature of the developed sensor.
By using non-hermetic encapsulation of the optical components, the functionality of the presented probes could be demonstrated by means of near-infrared spectroscopy measurements.