Tuesday, July 03, 2018

MTA Special Section on Advanced Image Sensor Technology

Japanese ITE Transactions on Media Technology and Applications publishes a Special Section on Advanced Image Sensor Technology with many interesting papers, all in open access:

Statistical Analyses of Random Telegraph Noise in Pixel Source Follower with Various Gate Shapes in CMOS Image Sensor
Shinya Ichino, Takezo Mawaki, Akinobu Teramoto, Rihito Kuroda, Shunichi Wakashima, Tomoyuki Suwa, Shigetoshi Sugawa
Tohoku University

Random telegraph noise (RTN) that occurs at in-pixel source follower (SF) transistors and column amplifier is one of the most important issues in CMOS image sensors (CIS) and reducing RTN is a key to the further development of CIS. In this paper, we clarified the influence of transistor shapes on RTN from statistical analysis of SF transistors with various gate shapes including rectangular, trapezoidal and octagonal structures by using an array test circuit. From the analysis of RTN parameter such as amplitude and the current-voltage characteristics by the measurement of a large number of transistors, the influence of shallow trench isolation (STI) edge on channel carriers and the influence of the trap location along source-drain direction are discussed by using the octagonal SF transistors which have no STI edge and the trapezoidal SF transistors which have an asymmetry gate width at source and drain side.

Impacts of Random Telegraph Noise with Various Time Constants and Number of States in Temporal Noise of CMOS Image Sensors
Rihito Kuroda, Akinobu Teramoto, Shigetoshi Sugawa
Tohoku University

This paper describes the impacts of random telegraph noise (RTN) with various time constants and number of states to temporal noise characteristics of CMOS image sensors (CISs) based on a statistical measurement and analysis of a large number of MOSFETs. The obtained results suggest that from a trap located relatively away from the gate insulator/Si interface, the trapped carrier is emitted to the gate electrode side. Also, an evaluation of RTN using only root mean square values tends to underestimate the effect of RTN with large signal transition values and relatively long time constants or multiple states especially for movie capturing applications in low light environment. It is proposed that the signal transition values of RTN should be incorporated during the evaluation.

Quantum Efficiency Simulation and Electrical Cross-talk Index Development with Monte-Carlo Simulation Based on Boltzmann Transport Equation
Yuichiro Yamashita, Natsumi Minamitani, Masayuki Uchiyama, Dun-Nian Yaung, Yoshinari Kamakura
TSMC and Osaka University

This paper explains a new method to model a photodiode for accurate quantum efficiency simulation. Individual photo-generated particles are modeled by Boltzmann transport equation, and simulated by Monte-Carlo method. Good accuracy is confirmed in terms of similarities of quantum efficiency curves, as well as color correction matrices and SNR10s. Three attributes - "initial energy of the electron", "recombination of electrons at the silicon surface" and "impurity scattering" - are tested to examine their effectiveness in the new model. The theoretical difference to the conventional method with drift-diffusion equation is discussed as well. Using the simulation result, the relationship among the cross-talk, potential barrier, and distance from the boundary has been studied to develop a guideline for cross-talk suppression. It is found that a product of the normal distance from the pixel boundary and the electric field perpendicular to the Z-axis needs to be more than 0.02V to suppress the probability of electron leakage to the adjacent pixel to less than 10%.

A Multi Spectral Imaging System with a 71dB SNR 190-1100 nm CMOS Image Sensor and an Electrically Tunable Multi Bandpass Filter
Yasuyuki Fujihara, Yusuke Aoyagi, Maasa Murata, Satoshi Nasuno, Shunichi Wakashima, Rihito Kuroda, Kohei Terashima, Takahiro Ishinabe, Hideo Fujikake, Kazuhiro Wako, Shigetoshi Sugawa
Tohoku University

This paper demonstrates a multi spectral imaging system utilizing a linear response, high signal to noise ratio (SNR) and wide spectral response CMOS image sensor (CIS), and an electrically tunable multi bandpass optical filter with narrow full width at half maximum (FWHM) of transmitted waveband. The developed CIS achieved 71dB SNR, 1.5x107 e- full well capacity (FWC), 190-1100nm spectral response with very high quantum efficiency (QE) in near infrared (NIR) waveband using low impurity concentration Si wafer (~1012 cm-3). With the developed CIS, diffusion of 5mg/dl glucose into physiological saline solution, as a preliminary experiment for non-invasive blood glucose measurement, was successfully visualized under 960nm and 1050nm wavelengths, at which absorptions of water molecules and glucose appear among UV to NIR waveband, respectively.

Single Exposure Type Wide Dynamic Range CMOS Image Sensor With Enhanced NIR Sensitivity
Shunsuke Tanaka, Toshinori Otaka, Kazuya Mori, Norio Yoshimura, Shinichiro Matsuo, Hirofumi Abe, Naoto Yasuda, Kenichiro Ishikawa, Shunsuke Okura, Shinji Ohsawa, Takahiro Akutsu, Ken Wen-Chien Fu, Ho-Ching Chien, Kenny Liu, Alex YL Tsai, Stephen Chen, Leo Teng, Isao Takayanagi
Brillnics Japan

In new markets such as in-vehicle cameras, surveillance camera and sensing applications that are rising rapidly in recent years, there is a growing need for better NIR sensing capability for clearer night vision imaging, in addition to wider dynamic range imaging without motion artifacts and higher signal-to-noise (S/N) ratio, especially in low-light situation. We have improved the previously reported single exposure type wide dynamic range CMOS image sensor (CIS), by optimizing the optical structure such as micro lens shape, forming the absorption structure on the Si surface and adding the back side deep trench isolation (BDTI). We achieved high angular response of 91.4%, high Gr/Gb ratio of 98.0% at ±20°, 610nm, and high NIR sensitivity of QE 35.1% at 850nm, 20.5% at 940nm without degrading wide dynamic range performance of 91.3dB and keeping low noise floor of 1.1e-rms.

Separation of Multi-path Components in Sweep-less Time-of-flight Depth Imaging with a Temporally-compressive Multi-aperture Image Sensor
Futa Mochizuki, Keiichiro Kagawa, Ryota Miyagi, Min-Woong Seo, Bo Zhang, Taishi Takasawa, Keita Yasutomi, Shoji Kawahito
Shizuoka University

This paper demonstrates to separate multi-path components caused by specular reflection with temporally compressive time-of-flight (CToF) depth imaging. Because a multi-aperture ultra-high-speed (MAUHS) CMOS image sensor is utilized, any sweeping or changing of frequency, delay, or shutter code is not necessary. Therefore, the proposed scheme is suitable for capturing dynamic scenes. A short impulse light is used for excitation, and each aperture compresses the temporal impulse response with a different shutter pattern at the pixel level. In the experiment, a transparent acrylic plate was placed 0.3m away from the camera. An objective mirror was placed at the distance of 1.1 m or 1.9m from the camera. A set of 15 compressed images was captured at an acquisition rate of 25.8 frames per second. Then, 32 subsequent images were reconstructed from it. The multi-path interference from the transparent acrylic plates was distinguished.

CMOS Image Sensor with Pseudorandom Pixel Placement for Image Measurement using Hough Transform
Junichi Akita, Masahi Toda
Kanazawa University, Kumamoto University

The pixels in the conventional image sensors are placed at lattice positions, and this causes the jaggies at the edge of the slant line we perceive, which is hard to resolve by pixel size reduction. The authors have been proposing the method of reducing the jaggies effect by arranging the photo diode at pseudorandom positions, with keeping the lattice arrangement of pixel boundaries that are compatible with the conventional image sensor architecture. In this paper, the authors discuss the design of CMOS image sensor with pseudorandom pixel placement, as well as the the evaluation on image measurement accuracy of line parameters using Hough transform.

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