Paper link: https://opg.optica.org/oe/fulltext.cfm?uri=oe-34-4-6880
In a recently published paper in Optics Express titled "Highly curved large-format sensors for infrared imaging", O'Masta et al of HRL Laboratories LLC write:
A curved, rather than flat, photoreceptive surface requires fewer optical elements and enhances illumination and sensitivity uniformity for wide-field vision. While such designs are common in vertebrate eyes, creating manmade curved sensing surfaces using high-performance sensors has been technologically challenging. Here, we surpassed previous practices by manipulating the strain within a die using a ductile metal layer. This enabled scalability to large-format sensors, which are valuable for capturing wide viewing angles with high resolution. We showcased this methodology using two types of III-V compound semiconductor photodetectors for mid-wave infrared (MWIR) imaging. The radiometric response of a hybridized fanout chip curved to a solid angle of 0.24 sr – over 300% beyond the limit of standard methods – was evaluated. A fully imaging focal plane array curved to 0.11 sr exhibited >97% operability. This demonstration of a spherically curved, cryogenically cooled MWIR imaging sensor highlights the feasibility of curving sensors comprised of heterogeneous semiconductor layering, which are commonly used in infrared, visible, and ultraviolet imagers.
Pushing the limits for spherically curving existing imaging sensors, as demonstrated with an infrared focal plane array (FPA). (a) A simple lens naturally focuses light onto a curved rather than flat image surface. (b) Cross-sectional schematic showing heterogenous layering of a cryogenically-cooled, MWIR FPA, following post-processing steps to enhance curving potential. (c) Historical [2–10] and present chip solid angle as a function of chip size. Details are provided in Supplement 1, Table S2. The dashed line represents the predicted limit for curving bare semiconductor die developed here. (d) Image of MWIR hybridized fanout chip curved to a 0.24 sr solid angle.Mechanical failure modes when spherically curving bare semiconductor chips. Data is shown for a 37 mm side length, square Si die. (a) Image sequence of 60 µm thick Si die being curved to 0.08 sr, with the pneumatic pressure noted along top. Striped pattern is from reflection of a poster board to aid visualization of curvature. Die is imaged through a transparent Mylar film. (b) Fracture of an 80 µm thick die curved to 0.08 sr. (c) Buckling of a 40 µm thick die when curving to 0.12 sr. (d), (e) FEA simulations of the dies curved in (b) and (c), respectively. (f) Fracture (contour map) and buckling (greyed in region) predictions. Symbols show experimental values, with measured probabilities of fracture from 10 replicate specimens indicated by color. (g) Predicted maximum solid angle for square Si dies, assuming Pf = 30%.
Maps of the minimum thickness hcr to suppress buckling of a bare Si die. (a) Comparison of buckling criterion predictions (dashed lines) compared to FEA results (circles) for square die of side length L. Thicknesses below a given line are predicted to buckle. (b) Comparison of predictions (dashed line) to experimental values (squares) for an L = 37 mm square die.
A 16 megapixel, 10 µm pitch InAsSb bulk alloy based FPA curved to 0.11 sr. (a) Picture of the curved FPA after wire-bonding to the PCBA for testing. (b) Measured responsivity across the curved FPA. (c) Histogram of the responsivity.
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