KEYWORDS: Light, Back end of line, Light sources, Geometrical optics, Semiconductors, Structural design, Ray tracing, Radio optics, Reflection, Interfaces
Previous research has shown that the use of back-end-of-line (BEOL) light directing structures with silicon hot carrier light sources in a complementary metal-oxide semiconductor results in improved light extraction efficiency. This work focuses on the design of an improved back-end-of-line structure for improving light extraction efficiency when using substrate-based silicon light emitters. With the use of FRED optical engineering ray-tracing software, it was found that a significant amount of generated light is lost at the material interfaces of the optical structure, including losses due to significant internal reflections. Therefore, an optimized optical structure was designed to reduce internal reflections at the base of the structure. Simulation results show a 33.6% improvement in light extraction efficiency over the previously designed parabolic optical structure, over the visible spectrum. The light sources were tested using a parameter analyzer, radiometer, spectrometer, and goniometer. It was calculated that the luminance exiting the optimized optical structure had a 55.66-factor improvement over the control structure and a 1.35-factor improvement over the parabolic structure. Furthermore, the optimized structure had a 1.38-factor improvement in light extraction efficiency over the parabolic structure. Overall, the improved designed pipe-like BEOL light directing structure helped to improve the device luminescence and light emission direction from the light source, which invariably increased the light extraction efficiency.
Light emission from silicon is possible in CMOS through hot carrier electroluminescence. Low conversion and low extraction efficiency remains a challenge. By using existing back-end-of-line interconnect structures it is possible to improve the extraction efficiency. Such light directing structures were analysed with the use of a focused ion beam and scanning electron microscope. It was found that it is possible to improve light extraction efficiency and directionality of the light sources through a combination of back-end-of-line structures and field oxide manipulation resulting in an improved optical path for emitted photon radiation. However, further analysis indicates that total internal reflections, scattering and electromagnetic absorption from the via plugs and metal interconnects in the back-end-of-line stack are some of the key contributors to the inefficient light extraction efficiency.
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