Sunday, November 15, 2009

Image Sensor Content at Electronic Imaging Conference

The annual Electronic Imaging Conference to be held on January 17-21 in San Jose has many image sensor related papers. Just to name a few out of many:

Fairchild Imaging presents its sCMOS sensor:

Wide dynamic range low light level CMOS Iimage sensor (Invited Paper)
Authors: Boyd A. Fowler, Xinqiao Liu, Stephen W. Mims, Janusz Balicki, Wang Li, Hung Do, Paul Vu, Fairchild Imaging

In this paper we present a CMOS image sensor technology suitable for the next generation of scientific cameras. We describe a 5.5Mpixel device based on this technology. The sensor features 5T pixels with pinned photodiodes on a 6.5um pitch. Each pixel also includes an integrated micro-lens. The 5T pixel architecture enables both rolling and global shutter operation. The measured peak quantum efficiency of the sensor is greater than 60% at 550nm, and the read noise is less than 1.5e- RMS at room temperature. The linear full well capacity is greater than 35ke-, the dark current is less than 3.8pA/cm2 at 20°C, and the MTF at 77 lp/mm is 0.4 at 600nm. The sensor also achieves an intra-scene linear dynamic range of greater than 87dB (23000:1) at room temperature.

This sensor has on-chip dual column level amplifiers and 11 bit single slope analog to digital converters (ADC) for high speed readout and wide optical dynamic range. The dual column level amplifier/ADC pairs have independent gain settings, and the final image is reconstructed by combining pixel readings from both the high and low gain readout channels to achieve a wide intra-scene dynamic range. The bandwidths of the column level amplifiers are programmable to optimize the sensor read noise for the selected sensor frame rate. The sensor can be readout in either rolling shutter or global shutter mode. The sensor can operate up to a line rate of 9us or a frame rate of 100Hz. The high speed digital readout electronics allow pixels to be scanned out at up to 290MHz.

Eliminating crosstalk in vertically integrated CMOS image sensors
Authors: Orit Skorka, Tyler Lucas, Dileepan Joseph, Univ. of Alberta (Canada)

Image sensors can benefit from 3D IC fabrication methods because photodetectors and electronic circuits may be fabricated using significantly different processes. When fabricating the tier that contains the photodetectors, it is desirable to avoid pixel level patterning of the light sensitive semiconductor. But without a physical border between adjacent photodetectors, lateral currents may flow between neighboring devices, which is called ''crosstalk''. These currents degrade the image quality because photo-generated charge carriers are sometimes collected in the ''wrong'' pixels. In this work, we present a method to reduce crosstalk in unpatterned photodetectors for vertically-integrated (VI) CMOS image sensors. Through feedback control of the electric potential at vertical interconnects, the crosstalk becomes negligible under normal imaging conditions. By maintaining a vertical electric field throughout the photodetector of sufficient uniformity and magnitude, lateral currents due to drift and diffusion are buried in the read noise. We illustrate the general method using the specific example of a VI-CMOS image sensor fabricated by flip-chip bonding a glass die with photodetectors to a CMOS die with active pixel sensor (APS) circuits. We present a logarithmic APS design with feedback control, which can be used to maintain an appropriate and suitably constant potential at the flip chip bonds. Simulation results are shown for a 0.18um CMOS process.

Kodak presents its W-RGB filter integrated onto CCD:

Improved sensitivity high-definition interline CCD using the Kodak TRUESENSE color filter pattern
Authors: James A. DiBella, Eastman Kodak Co. (United States); Marco Andreghetti, Kodak Japan Ltd. (Japan); Amy Enge, Eastman Kodak Co (United States); Doug A. Carpenter, Eastman Kodak Co. (United States); William Chen, Kodak (China) Ltd. (China)

NHK updates its organic photodiode state of the art:

Stacked color image sensor using wavelength-selective organic photoconductive films with zinc-oxide thin film transistors as a signal readout circuit
Author(s): Hokuto Seo, Satoshi Aihara, NHK Science & Technical Research Labs. (Japan); Masakazu Namba, Toshihisa Watabe, Hiroshi Ohtake, Misao Kubota, Norifumi Egami, NHK Science & Technology Research Labs. (Japan); Takahiro Hiramatsu, Tokiyoshi Matsuda, Mamoru Furuta, Hiroshi Nitta, Takashi Hirao, Kochi Univ. of Technology (Japan)

Our group has been developing a new type of image sensor overlaid with three organic photoconductive films, which are individually sensitive to only one of the primary color components (blue (B), green (G), or red (R) light), with the aim of developing a compact, high resolution color camera without the color separation optical systems used in current color cameras.
In this paper, we firstly revealed the unique characteristics of organic photoconductive films. Only choosing organic materials can tune photoconductive properties of the film, especially excellent wavelength selectivities which are good enough to divide the incident light into three primary colors. High-resolution of the organic photoconductive film sufficient for high-definition TV was also confirmed by shooting experiment using a camera tube. Secondly, as a step toward our goal, we fabricated a stacked organic image sensor with G- and R-sensitive organic photoconductive films, each of which had a readout circuit of zinc oxide (ZnO)- thin film transistor (TFT), and demonstrated the image pickup at a TV frame rate. A color image with a resolution corresponding to the pixel number of ZnO-TFT was obtained from the stacked image sensor, clearly indicating that color separation can be achieved using vertically stacked organic films.

Edoardo Charbon's group presents experimental results from its "Gigavision" sensor (the one that appears to be similar to Eric Fossum Digital Jot idea):

On pixel detection threshold in the gigavision camera
Author(s): Feng Yang, Ecole Polytechnique Fédérale de Lausanne (Switzerland); Luciano Sbaiz, Google Zurich (Switzerland); Edoardo Charbon, Technische Univ. Delft (Netherlands); Sabine Süsstrunk, Martin Vetterli, Ecole Polytechnique Fédérale de Lausanne (France)

Recently, we have proposed a new image device called gigavision camera whose most important characteristic is that pixels have binary response. The response function of a gigavision sensor is non-linear and similar to a logarithmic function, which makes the camera suitable for high dynamic range imaging. One important parameter in the gigavision camera is the threshold for generating binary pixels. Threshold T relates to the number of photo-electrons necessary for the pixel output to switch from "0" to "1". In this paper, a theoretical analysis of the influence of threshold in the gigavision camera is studied. If the threshold in the gigavision sensor is large, there will be a "dead zone" in the response function of a gigavision sensor. A method of adding artificial light is proposed to solve the "dead zone" problem. Through theoretical analysis and experimental results based on synthesized images, we show that for high light intensity, the gigavision camera with a large threshold and added light works better than one with unity threshold. Experimental results with a prototype camera based on a single photon avalanche diodes (SPAD) camera are also shown.

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