Curved CMOS Image Sensors: Packaging Issues, Applications and Roadmaps
Authors: Chambion Bertrand , Moulin G. , Caplet S. , Gaschet C. , Getin S. , Vandeneynde A. , Jahn W. , Henry D. , Hugot E. 
Published: September 2017

Since few years, there has been an increasing interest and demand in flexible electronics. Standard imaging system consists of an optical module (set of lenses) and an image sensor. For wide field of view applications, and due to the curved shape of lenses and mirrors, the flat image after being propagated through the optical system is not flat but curved, i.e. the off-axis light focuses in a curved manner. This problem is called Petzval Field Curvature Aberration (Petzval FCA). It is generally fixed by additional complex lenses to “flatten” the image plane. We propose another approach with a hemispherical curved sensor technology. It allows eliminating FCA directly at the sensor level and thus makes it possible to drastically simplify, and hence miniaturize, the optical system architecture.

First, a brief state of the art on curved detectors will be detailed for different application fields. Bendable capacities of hydrid detectors (included interconnection layer) were fully investigated and tested in the past [1, 2]. Moreover, a hemi-spherically curved visible image sensor with better optical characteristics (image quality) was realized and patented by Sony Company in 2014 [3]. Recently, a tunable curving packaging technology, with new optical functions possibilities has been presented in Electronic Component and Technology Conference 2016 [4].

Then, CEA-LETI curving technologies will be explained to address fixed and tunable curvature packaging applications, included modeling and technical process steps. Characterization of curved sensors prototypes have been performed to understand mechanical and electro-optical bending limits and will be also presented in the paper. Based on an existing fisheye flat sensor optical design, a curved focal plane will be described, showing that it's possible to simplify the standard system from 14 lenses (11 types of optical glass) with 2 aspheric lenses, to only 9 lenses (−35%), 3 types of optical glasses, without aspheric surfaces. The benefits of a curved sensor will be summarized into two categories: those related to the optical system design and those related to the quality of images produced by a camera with curved sensor.

Optical system:» Miniaturization of optical devices (volume, weight);» Simplification of the lenses alignment process (due to reduced number of lenses);» Suppression of aspheric lenses;» Wide field of view enhancement.

Image quality:» More homogeneous image quality (reduced image noise);» Similar or improved resolution and higher sensitivity;» Corrected distortion occurring along the image edges.

Finally, curved CMOS image sensor roadmaps and perspectives will be discussed: from a market point of view, application field surveys have been done on mass market applications (mobile, consumer…), photography, automotive… From a technical aspect, a curving technologies roadmap will be proposed, leaded by applications needs, on single chip, collective, and wafer level processes.