Optimizing quantum efficiency of image sensors, whether CCD or CMOS, has usually required backside thinning to
bring the photon receiving surface close to the charge generation elements. A new CMOS sensor architecture has been
developed that permits high-fill-factor photodiodes to be placed at the silicon surface without the need for backside
thinning. The photodiode access provided by this architecture permits application of highly-effective anti-reflection
coatings on the input surface and construction of a mirror inside the silicon below the photodiodes to effectively double
the optical thickness of the silicon charge generation volume. Secondary benefits of this architecture include prevention
of light from reaching the CMOS circuitry under the photodiodes, improvement of near-infrared quantum efficiency, and
reduction in optical artifacts caused by reflections from the sensor surface.
Utilizing these techniques, a sensor is being constructed with 4096 x 4096 pixels 4.8 μm square with 95% fill factor
backed by a mirror tuned to the 400-700 nm visible band and a front-surface anti-reflectance coating. The quantum
efficiency is expected to exceed 80% through the visible and the global shutter extinction ratio should exceed 106:1. The
sensors have been fabricated and first test data is due in February 2011.
Recently introduced CMOS sensors using three layers of photodiodes for color separation1 can also function well in the
near ultraviolet and infrared bands. Ultraviolet sensitivity results from the close proximity of the top (blue) photodiode
junctions to the surface of the silicon and the lack of any significant UV-absorbing materials above them. Infrared
sensitivity extending nearly to the silicon band-gap cutoff results from depletion of the bottom (red) phodiodes into the
substrate. Preliminary measurements indicate that the layered structure has high quantum efficiency over most of the
200-1100nm band covered by silicon photodiodes. Uniquely, these devices can be switched rapidly between
narrowband monochrome imaging and full-color imaging in the visible band by the introduction of a visible pass filter.
The response of the three photodiode layers is broad enough to permit stable false color encoding using two or three
channels in conjunction with a redefinable 3 x 3 color transform matrix. Images have been acquired in the 300-400 nm
UV band and for broad and narrow infrared bands out to 1064 nm. Thermal images of objects in the range of 600C have
also been acquired demonstrating color-encoding of various UV, visible and IR bands and applications for particular
High-performance color image acquisition has heretofore relied on color video cameras using multiple image sensors mounted on spectral separation prisms to provide geometrically accurate color data free of reconstruction artifacts. Recently, a CMOS image sensor has been developed that incorporates three complete planes of photodiodes in a single device to provide color separation without the need for external optical elements. The first commercial device based on this technology has 1512 x 2268 three-color photosites on 9.12 micron centers and includes provisions for combining pixels in X and Y, region-of interest selection and sparse scanning. The camera described in this paper operates the sensor in a variety of scan modes offering tradeoffs between resolution, coverage and speed. In this camera, a 128x128 raster of either a matrix of this size or binned from a large area can be scanned at nearly 150 frames per second and a single 2048-element line can be scanned at 7 KHz. At full resolution, the image sensor will acquire four frames per
second. The scan configuration can be reloaded in less than 50 microseconds permitting mod e changes on the fly.
A silicon image sensor has been developed and placed in production using standard 0.18 μm CMOS process having three stacked photodiodes per pixel location to provide full-color imaging without external color filters. With a fill factor exceeding 50%, this image sensor achieves approximately 45% peak quantum efficiency in the mid range visible and provides usable response extending from the near-ultraviolet to the near-infrared. Initial results from a commercial digital still camera indicate that this device can produce excellent color reproduction with equivalent ISO film speeds from 100 to 400 and that it produces images free from color artifacts common in images made with sensors incorporating color filter arrays. Development is now underway on camera equipment designed to operate this image sensor in a variety of scan modes.
As a payload of KITSAT-2, also known as KO-25, the CCD earth imaging system is equipped with a single chip color CCD camera system as well as a monochrome camera system. The black and white narrow angle camera has a mean ground pixel resolution of about 200 m and the color wide angle camera has a meteorological mean ground pixel resolution of 2 km. Brief test results of the narrow angle camera are presented. The paper also presents image processing activities, including image correction and the vegetation index comparison of narrow images with readily available satellite data such as AVHRR. General information on the color imager is also given.