We report transmissive color filters based on subwavelength dielectric gratings that can replace conventional dye-based color filters used in backside-illuminated CMOS image sensor (BSI CIS) technologies. The filters are patterned in an 80-nm-thick poly-silicon film on a 115-nm-thick SiO 2 spacer layer. They are optimized for operating at the primary RGB colors, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 exhibit peak transmittance of 60-80%, and an almost insensitive response over a ±20 • angular range. This technology enables shrinking of the pixel sizes down to near a micrometer. KeywordsSubwavelength grating, high-index contrast, color filters, CMOS image sensor Scalability of complementary metal oxide transistor (CMOS) technology has improved the performance of CMOS image sensors (CIS) in the past decade. One of the trends in the CIS technology, driven mainly by portable devices with small form-factors, is to decrease the pixel size, which will lead to improved spatial resolution for digital imaging. 1 The scalability of CIS technology has enabled lateral pixel size to be reduced from more than 10 µm to less than 2 µm in the last decade. 2,3 Along with scaling of the pixel size, there have been considerable efforts to redesign color filters, 4-6 microlenses, 7 and infrared filters 8 to prevent the degradation of the optical performance. Dye-doped polymers have been conventionally used for RGB color filters in digital color imaging. However, when the pixel size gets smaller, the optical crosstalk among pixels becomes significant because of the small absorption coefficient of the organic dyes. Besides, the dye-doped polymers are essentially photoresists that degrade under ultraviolet illumination or high temperature environments. To address these issues, plasmonic color filters made of metallic thin films with subwavelength patterning have been studied for CIS technology. 5,9,10 The plasmonic color filters have several advantages such as flexible color tunability across visible spectrum and compatibility with CMOS processes. However, the absolute efficiency of the plasmonic color filters is relatively low (40-50% range) compared to conventional organic dye-doped filters. Here, we show that dielectric subwavelength gratings can be used to achieve highly efficient transmission color filters with close to angular insensitive properties. Furthermore, compared to dye-based filters, our dielectric-based filters possess better reliability under ultraviolet illumination and at high temperature. For the optimized filter designs, we take the advantage of the backside-illumination (BSI) CIS technologies, where the color filter layer can be placed in close proximity to the photodiodes 2 16 anti-reflection coating, 17,18 and reflective-transmission filters for displays. 19 In this letter, we demonstrate designs for polarizatio...

We have demonstrated a compact and efficient metasurface-based spectral imager for use in the near-infrared range. The spectral imager was created by fabricating dielectric multilayer filters directly on top of the CMOS image sensor. The transmission wavelength for each spectral channel was selected by embedding a Si nanopost array of appropriate dimensions within the multilayers on the corresponding pixels, and this greatly simplified the fabrication process by avoiding the variation of the multilayer-film thicknesses. The meta-spectral imager shows high efficiency and excellent spectral resolution up to 2.0 nm in the near-infrared region. Using the spectral imager, we were able to measure the broad spectra of LED emission and obtain hyperspectral images from wavelength-mixed images. This approach provides ease of fabrication, miniaturization, low crosstalk, high spectral resolution, and high transmission. Our findings can potentially be used in integrating a compact spectral imager in smartphones for diverse applications.

AbstractOwing to the recent developments of dielectric metasurfaces, their applications have been expanding from those pertaining to the thickness shrinkage of passive optical elements, such as lenses, polarizers, and quarter-wave plates, to applications pertaining to their integration with active optical devices, such as vertical-cavity surface-emitting lasers. Even though directional lasing and beam shaping of laser emission have been successfully demonstrated, the integration of metasurfaces with random light sources, such as light-emitting diodes, is limited because of function and efficiency issues attributed to the fact that metasurfaces are basically based on the resonance property of the nanostructure. To control the direction of emission from colloidal quantum dots, we present a dielectric metasurface deflector composed of two asymmetric TiO2 nanoposts. TiO2 deflector arrays were fabricated with a dry etching method that is adaptive to mass production and integrated with a colloidal quantum dot resonant cavity formed by sandwiching two distributed Bragg reflectors. To ensure the deflection ability of the fabricated sample, we measured the photoluminescence and far-field patterns of emission from the resonant cavity. From the obtained results, we demonstrated that the colloidal quantum dot emission transmitted through our deflector arrays was deflected by 18°, and the efficiency of deflection was 71% with respect to the emission from the resonant cavity. This integration of dielectric metasurfaces with a resonant cavity shows the possibility of expanding the application of visible metasurfaces in active devices and may help to develop next-generation active devices with novel functions.

Area-selective deposition (ASD) using a precursor inhibitor (PI) is a promising alternative to self-assembled monolayer inhibitors due to a wide range of material selection and high process compatibility. In this study, bis­(ethylcyclopentadienyl)ruthenium [Ru­(EtCp)2] is introduced as a homometallic PI for the ASD of Ru. The chemical reactivity and steric hindrance between Ru­(EtCp)2, the Ru precursor, and H2O are theoretically calculated using density functional theory calculations and Monte Carlo simulations. The blocking property is related to the packing density of Ru­(EtCp)2 on the surface, and unoccupied sites degrade the blocking property. An additional H2O pulse is used to hydrolyze and remove the Et groups of Ru­(EtCp)2 to create more space for the additional adsorption of Ru­(EtCp)2. As a result, the packing density of Ru­(EtCp)2 PI increases, leading to an improvement in the blocking property. A single pulse of Ru­(EtCp)2 inhibits the growth of the Ru atomic layer deposition (ALD) film for 200 cycles, whereas Ru­(EtCp)2 with an additional H2O pulse inhibits the growth of the Ru ALD film for up to 300 cycles. Transmission electron microscopy results show that the Ru ASD thin films are purely metallic even after the degradation of Ru­(EtCp)2. This highlights the possibility of using homometallic PIs in future applications of metal ASD processes.

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