Abstract
Compared with conventional planar optical image sensors, a curved focal plane array can simplify the lens design and improve the field of view. In this paper, we introduce the design and implementation of a segmented, hemispherical, CMOS-compatible silicon image plane for a 10-mm diameter spherical monocentric lens. To conform to the hemispherical focal plane of the lens, we use flexible gores that consist of arrays of spring-connected silicon hexagons. Mechanical functionality is demonstrated by assembling the 20-μm-thick silicon gores into a hemispherical test fixture. We have also fabricated and tested a photodiode array on a silicon-on-insulator substrate for use with the curved imager. Optical testing shows that the fabricated photodiodes achieve good performance; the hemispherical imager enables a compact 160 ° field of view camera with >80% fill factor using a single spherical lens.
Highlights
Curved image planes that mitigate spherical aberration are essential for wide-angle, monocentric imaging technology to be generally useful[1]
Sony has developed a curved CMOS imager by controlling bending stress[4], which shows a 1.4 × improvement in sensitivity and a 5 × reduction of dark current; the radius of the curvature is too large for full hemispherical coverage of a miniature spherical lens
Rogers et al.[5,6,7,8] demonstrated a hemispherical camera, consisting of isolated sensor clusters attached to an elastic material; the sensor area has a low fill factor, and the pixel density is nonuniform because of stretching
Summary
Curved image planes that mitigate spherical aberration are essential for wide-angle, monocentric imaging technology to be generally useful[1]. Crosstalk and the mode volume of the fibers constrain the pixel size and fill factor These limitations motivate an alternative approach—a curved imaging plane. We seek to address the challenge of developing a lowcost, CMOS-compatible process for silicon-based flexible electronics by demonstrating a miniature hemispherical curved silicon substrate for monocentric imagers. The top-right sketch of Figure 1 indicates such an aberrationfree monocentric imager design with a hemispherical focal plane; the single ball lens can achieve a wide field of view with a significant reduction of the imager size. To demonstrate the manufacturability of this design, we have fabricated and tested photosensitive diodes on the hexagon cells and assembled the tessellated planar silicon into a hemispherical holder with a ball lens
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