Multispectral sensors such as Ambient Light sensors (ALS) are becoming increasingly popular due to growing concerns for health, environment, and safety. These sensors provide non-intrusive quantitative and qualitative information on the photonic footprints of interest. To meet the demand for mass production, CMOS image sensors (CIS) are a good basis for these devices. Commercial hyperspectral cameras use a generalization of Bayer-like matrix of Fabry-Perot cavities (FPC) as multispectral filters embedded onto a CIS. However, the delicate fabrication of these filters is tedious and leads to a pronounced surface topology. In this study, we demonstrate experimentally that multispectral sensing can be achieved using a hybrid FPC (h-FPC) which is an improved version of the regular FP cavity, that consists of two silicon mirrors, SiO2 spacer, and a sub-wavelength silicon grating at the center of the cavity. These structures were fabricated using a CMOS compatible process and can be integrated into an imager process flow. The h-FPC optical response can be tuned in the near-infrared region (750-950nm) by changing the filling factor of the grating inside the cavity without varying its height, unlike planar FPC. This feature makes the hybrid FPC a more versatile and efficient option for agile multispectral sensing.
Next-generation BSI CMOS Imager Sensors are strongly driven by novel applications in depth sensing, mainly operating in the NIR (940nm) spectrum. As a result, the need for higher pixel sensitivity while shrinking pixel pitch is more present than ever. In this work, we present a new technology platform based on ad-hoc nano diffractor geometries, integrated in the Back Side of BSI CIS that allow to drastically improve the QE of the sensor for pitches varying from 10 μm down to 2.2 μm, co-optimized for both optical and electronic pixel performance.
Due to their low-cost fabrication process and high efficiency, silicon-based Complementary Metal Oxide Semiconductor (CMOS) image sensors are the reference in term of detection in the visible range. However, their optical performances are toughly degraded in the Near Infrared (NIR). For such wavelengths, Silicon has a small absorption coefficient, leading to a very poor Quantum Efficiency (QE). A solution to improve it is to implement structures like pyramids that are etched in the Silicon layer. This will lead to diffraction inside the photodiode, enhancing the light path and therefore the absorption. Using Finite Difference Time Domain (FDTD) simulations, we demonstrated a huge QE enhancement at 940nm on real pixels, by implementing this kind of diffractive structures and we finally confirmed these results by characterizations. We obtained QE values up to 47% at 940nm for our 3.2μm pixel, corresponding to a gain of 2 comparing to a pixel without any diffractive structures. We also measured the Modulation Transfer Function (MTF), to evaluate how this figure of merit is impacted by the addition of these structures. As expected, the MTF was degraded when we added these diffractive patterns but were still high looking at the values. We indeed demonstrated MTF values going up to 0.55 at Nyquist/2 frequency and 0.35 at Nyquist frequency. Looking not only at QE values but also at MTF ones, these are very promising results that could be used in many different NIR applications like face recognition, Light Detection and Ranging (LIDAR) or AR/VR.
In the field of uncooled Long Wave Infra Red (LWIR) imaging, CMOS compatible bolometers technology is being more and more popular, exhibiting precise temperature measurement at moderate cost. The price of this technology is proportional to the number of components produced per wafer, leading to a shrinkage of the pixel. Enhancing the resolution level of the focal plane array (FPA) requires an improvement of the point spread function (PSF) of the optical system, leading to more and more complex aspheric lenses, and an increased cost of imaging systems. We propose to add a sub-wavelength blade to the existing parts of the imaging system to ease the overall improvement of the image quality in applications with a constraint budget. The main function of such a subwavelength blade should be to control the phase of the light into an optical system to compensate optical aberrations. A cost effective solution consists to make such devices using microelectronics based collective fabrication process. The main difficulty is to predict the subwavelength blade behavior within an optical system that is to say combining millimeter sized optical components that are modeled using ray-tracing or electromagnetic simulations. In this paper we present the results obtained from an effort to simulate, fabricate and characterize all-dielectric subwavelength blade. In an imaging system, our devices will have to deal with non-flat wavefronts. Our method is based on Fourier Modal Method and Angular Spectrum Method to simulate subwavelength optics into such an optical system. Finally, we have compared our simulations results to experiments on basic examples, like spherical aberration correction of a commercial lens.
HgCdTe avalanche photodiodes offers a new horizon for observing spatial or temporal signals containing only a few infrared (IR) photons, enabling new science, telecommunication and defence applications. The use of such detectors for free space optical communications is particularly interesting for both deep space and high data rate links as it enables wide field of view free space optical coupling to the detector at high sensitivity, down to single photon level and with a close to negligible loss of the information contained in the strongly attenuated photon flux. Measurement of the response time and dark current shows that such devices can be operated at room temperature with bandwidths up to 10 GHz in a back-side illuminated configuration. This configuration allows to use micro-lenses fabricated directly into the APD substrate and enables to use a large photosensitive area while maintaining a high bandwidth, low dark current and /or high operating temperature. We report on the expected performance 4-quadrant APD detector demonstrator with single photon sensitivity, which is currently developed to be used in deep space telecommunications by ESA and present the potential use for high data rates links of 10 Gbits/s.
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