Reliable far-UVC LEDs despite hermetic sealing
Fig. 1: Relative optical power of far-UVC LEDs in different package variants versus operation time. The hermeticity causes an additional degradation effect that leads to a stronger reduction of the emission power (red). Improvements in chip assembly technology strongly reduces this effect (orange) by reducing the carbon sources in the package.
Fig. 2: Measured transmittance and Carbon layer thickness for LEDs with different hermeticity related power loss.
Hermetic packages protect UV LEDs from moisture and reactive gases, but can also accelerate optical power reduction. FBH has identified the underlying mechanism and developed a chip assembly approach that prevents it, enabling stable long‑term operation of UV LEDs.
LEDs with wavelengths below 235 nm (far‑UVC LEDs) are already employed in gas sensing and DNA detection applications. Moreover, they are promising candidates for skin‑friendly disinfection on human skin and for solar‑blind optical communication in outdoor environments. In many of these application environments, the LEDs are exposed to moisture or chemically reactive gases, making hermetic packages essential, as they shield the chips from the ambient atmosphere. Long‑term tests at FBH showed that hermetically sealed far‑UVC LED chips operated reliably for two years without catastrophic failures, demonstrating the strong protective effect of an hermetic package. The remaining question was how hermetic sealing influences the gradual optical degradation of the LEDs.
To investigate this question, we mounted LED chips emitting at 233 nm on ceramic cavity packages and operated them either openly or hermetically sealed using soldered quartz lids. All LEDs showed the typical gradual decrease in emission power during long‑term operation. However, the hermetically sealed LEDs degraded significantly faster (Fig. 1, red and blue data points). Depending on the operating temperature and current, the additional loss reached up to 60 % after 100 hours compared to open devices. To understand this behavior, we examined the sapphire backside of the LED chips using EDX (energy-dispersive X-ray spectroscopy) and spectroscopic ellipsometry. These measurements revealed that a carbon-containing layer forms on the chip’s sapphire light extraction surface when the LEDs are operated inside a hermetic package. This layer absorbs part of the emitted light and therefore reduces the effective output power (Fig. 2). The contamination became more pronounced at higher temperatures and operating currents. By contrast, there was no evidence of contamination on hermetically sealed LEDs kept at elevated temperature without electrical operation, indicating that the mechanism requires active LED operation.
Once the underlying mechanism was understood, reducing carbon sources in the package emerged as the evident mitigation strategy. Accordingly, LED chips soldered with flux‑free AuSn preforms showed only minimal hermeticity‑related degradation (Fig. 1, orange data points), as carbon-containing flux was not used for the soldering. Furthermore, we observed that the degradation can partially reverse when hermetically sealed LEDs are opened and operated further, indicating a UV‑induced self‑cleaning process of the chip surface that requires oxygen.
Overall, the results show that hermetic packaging can introduce an additional optical degradation mechanism in UV LEDs, but that this effect can be effectively prevented through an optimized chip assembly approach developed at FBH. This enables more stable long‑term operation of far-UVC LEDs with hermetic packages and significantly improves their applicability in demanding environments.
Publication
J. Ruschel, C. Stölmacker, N. Lobo-Ploch, J. E. Boschker, N. Kemf, J. Glaab, T. Filler, T. Musengezi, J. Rass, H. K. Cho, M. Schilling, T. Wernicke, S. Knigge, M. Kneissl, S. Einfeld, „Impact of hermetic package and UV-induced self-cleaning on the reliability of UV LEDs”, Journal of Physics D: Applied Physics, Vol. 59, Nr. 7 (2026).