Thursday, August 12, 2021

Plasmonic Anti-Reflection Layer

OSA Optics Express publishes a paper "Broadband responsivity enhancement of Si photodiodes by a plasmonic antireflection bilayer" by Jongcheol Park, Il-Suk Kang, Gapseop Sim, Tae Hyun Kim, and Jong-Kwon Lee from National NanoFab Center and Cheongju University, Korea.

"Randomly distributed plasmonic Ag nanoparticles (NPs) with various sizes were fabricated by a reflow process to an island-shaped Ag thin-film deposited on a Si photodiode. These NPs conformally enclosed by an antireflective (AR)-type SiNx/SiO2 bilayer reveal significantly diminished reflectance in a broad wavelength (500 nm - 1100 nm) as compared to the cases of Ag NPs or SiO2 layer enclosing Ag NPs on the Si substrate. Accordingly, the forward scattering and the total reflection along with wide-angle interference in between the dielectric bilayer incorporating the Ag NPs induce highly increased light absorption in the Si substrate. The fabricated Si photodiode adopting the plasmonic AR bilayer shows the responsivity peak value of 0.72 A/W at 835 nm wavelength and significant responsivity enhancement up to 40% relative to a bare Si photodiode in a wavelength range of 500 nm to 1000 nm."


  1. Probably interesting for RGB-IR CFA sensors, but it seems comparable to SiNx/SiO2 anti-reflective layer performance for RGB sensors which already include IRCF filters where >650nm light is cut off and there is no benefit to low reflectance from this process.

  2. The thickness of the SiO/SiN reference lambda/4 ARC does not seem to be adjusted correctly. When optimizing it for a wavelength within the measured range it would show much better performance.+

    Looking closely at the green curve in the middle image, the reflactance looks exactly like an ARC optimized for ~800nm.

    Is this a clean experiment?

    1. The thicknesses are actually correct since it is SiO2 first on the silicon. It matches the purple dashed curve in Fig 4.

    2. Yes, the thickness measurement shown in the SEM cross section is correct, but it is not the correct thickness to reduce reflection within the visible range. The thicknesses used for the SiN/SiO layer (17.7/33.3 nm) optimize it for a reflectance minimum at 400 nm.

      The sample with the Ag nanoparticles uses a completely different thickness. By adjusting the SiO/SiN layers to 17.7/65.8 nm you actually get a far better result than with Ag nanoparticles. The samples are simply not compareable due to a maladjusted experiment.

      In addition, the reflectance results are not adjusted for absorption in the SiN.

      You can try here:

      This is not good science, and this paper was not reviewed properly.


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