High-resolution CMOS Image Sensors Research Articles

Characterization of Partial Discharges in Dielectric Oils Using High-Resolution CMOS Image Sensor and Convolutional Neural Networks.

In this work, an exhaustive analysis of the partial discharges that originate in the bubbles present in dielectric mineral oils is carried out. To achieve this, a low-cost, high-resolution CMOS image sensor is used. Partial discharge measurements using that image sensor are validated by a standard electrical detection system that uses a discharge capacitor. In order to accurately identify the images corresponding to partial discharges, a convolutional neural network is trained using a large set of images captured by the image sensor. An image classification model is also developed using deep learning with a convolutional network based on a TensorFlow and Keras model. The classification results of the experiments show that the accuracy achieved by our model is around 95% on the validation set and 82% on the test set. As a result of this work, a non-destructive diagnosis method has been developed that is based on the use of an image sensor and the design of a convolutional neural network. This approach allows us to obtain information about the state of mineral oils before breakdown occurs, providing a valuable tool for the evaluation and maintenance of these dielectric oils.

Anti-Aliasing and Anti-Color-Artifact Demosaicing for High-Resolution CMOS Image Sensor

Anti-Aliasing and Anti-Color-Artifact Demosaicing for High-Resolution CMOS Image Sensor

Improved Correlated Multiple Sampling by Using Interleaved Pixel Source Follower for High-Resolution and High-Framerate CMOS Image Sensor

This article describes an improvement in the noise reduction performance of a column correlated multiple sampling (CMS) readout circuit using interleaved pixel source follower for high-resolution and high-framerate CMOS image sensors (CISs). In this architecture, the time-interleaved operation of the two pixel source followers reduces the restrictions imposed by the settling time of the pixel source followers and extends the time for multiple sampling. The noise analysis indicates that this method has an advantage of enhanced noise reduction not only for thermal noise but also for 1/ f noise when a high-speed readout operation is required. The measurement of the noise performance of the 8K image sensor using the CMS with the interleaved pixel source follower method exhibits a low input-referred noise of 3.2 e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> at 8K 120 frames per second, while 4.6 e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> with the conventional single-source follower readout method. The measurement results match reasonably well with the analysis presented in this article, demonstrating the effectiveness of the interleaved pixel source follower method for high-resolution and high-framerate CISs.

0.8 um Color Pixels with Wave-Guiding Structures for Low Optical Crosstalk Image Sensors

Low optical-crosstalk color pixel scheme with wave-guiding structures is demonstrated in a high resolution CMOS image sensor with a 0.8um pixel pitch. The high and low refractive index configuration provides a good confinement of light waves in different color channels in a quad Bayer color filter array. The measurement result of this back-side illuminated (BSI) device exhibits a significant lower color crosstalk with enhanced SNR performance, while the better angular response and higher angular selectivity of phase detection pixels also show the suitability to a new generation of small pixels for CMOS image sensors.

Front-Inner Lens for High Sensitivity of CMOS Image Sensors.

Due to the continuing improvements in camera technology, a high-resolution CMOS image sensor is required. However, a high-resolution camera requires that the pixel pitch is smaller than 1.0 μm in the limited sensor area. Accordingly, the optical performance of the pixel deteriorates with the aspect ratio. If the pixel depth is shallow, the aspect ratio is enhanced. Also, optical performance can improve if the sensitivity in the long wavelengths is guaranteed. In this current work, we propose a front-inner lens structure that enhances the sensitivity to the small pixel size and the shallow pixel depth. The front-inner lens was located on the front side of the backside illuminated pixel for enhancement of the absorption. The proposed structures in the 1.0 μm pixel pitch were investigated with 3D optical simulation. The pixel depths were 3.0, 2.0, and 1.0 μm. The materials of the front-inner lens were varied, including air and magnesium fluoride (MgF2). For analysis of the sensitivity enhancement, we compared the typical pixel with the suggested pixel and confirmed that the absorption rate of the suggested pixel was improved by a maximum of 7.27%, 10.47%, and 29.28% for 3.0, 2.0, and 1.0 μm pixel depths, respectively.

A Method to Reduce the Effect on Image Quality Caused by Resistance of Column Bus

In this paper, a method to reduce the effect on image quality caused by the parasitic resistance of column bus in high-resolution cMOS image sensors is presented. Through the mathematical model of the column bus, the effect on image quality caused by the resistance is analyzed. The resistance would cause two nonideal factors: dynamic swing reduction and nonlinearity. To reduce the effect of the nonideal factors, this paper utilizes a layout design by putting the readout circuit and column bias circuit on different sides of the pixel array. The relationship among the resistance, light intensity of input, and standard deviation of output gray levels is analyzed. In a 0.13- $\mu \text{m}$ CMOS process and a 12 cm $\times$ 12 cm pixel array, the simulation result shows that the resistance would cause column stripes or gradient under a traditional method. Under the proposed method, the peak standard deviation under uniform illumination decreases from 16.25 to 1.44. The peak signal-to-noise ratio increases from 64.66 to 91.64 dB and the structural similarity index increases from 0.79 to 0.97. The proposed method can enhance image quality for high-resolution image sensors significantly.

Tuning of Plasmonic Resonances in the Near Infrared Spectrum Using a Double Coaxial Aperture Array

Plasmonic filters are excellent candidates for spectral filters in the near infrared spectrum. Such filters require only a single nanoscale thickness perforated metal film that can be tailored to tune the location of the transmission maximum. Here, we demonstrate using computational methods a double coaxial aperture array (DAA) in a hexagonal geometry that can be tuned to produce plasmon resonances accompanied by strong transmission in the near-infrared spectrum and at telecommunication wavelengths with a peak wavelength insensitive to angle of incidence. The DAA consists of two concentric annular apertures and radial distance between the two apertures is varied to tune the resonant wavelength. The presented geometry finds potential applications in modulators for telecommunication, spectroscopy, solar cells, high-resolution CMOS image sensors, chemical sensors, and biosensors.

An apparatus and method for directly measuring the depth-dependent gain and spatial resolution of turbid scintillators.

Turbid (powder or columnar-structured) scintillators are widely used in indirect flat panel detectors (I-FPDs) for scientific, industrial, and medical radiography. Light diffusion and absorption within these scintillators is expected to cause depth-dependent variations in their x ray conversion gain and spatial blur. These variations degrade the detective quantum efficiency of I-FPDs at all spatial frequencies. Despite their importance, there are currently no established methods for directly measuring scintillator depth effects. This work develops the instrumentation and methods to achieve this capability. An ultra-high-sensitivity camera was assembled for imaging single x ray interactions in two commercial Gd2 O2 S:Tb (GOS) screens (Lanex Regular and Fast Back, Eastman Kodak Company). X ray interactions were localized to known depths in the screens using a slit beam of parallel synchrotron radiation (32keV), with beam width (~20μm) much narrower than the screen thickness. Depth-localized x ray interaction images were acquired in 30μm depth-intervals, and analyzed to measure each scintillator's depth-dependent average gain and modulation transfer function MTF(z,f). These measurements were used to calculate each screen's expected MTF(f) in an energy-integrating detector (e.g., I-FPD). Calculations were compared to presampling MTF measurements made by coupling each screen to a high-resolution CMOS image sensor (48μm pixel) and using the slanted-edge method. Both and MTF(z,f) continuously increased as interactions occurred closer to each screen's sensor-coupled surface. The Regular yielded 1351±66 and 2117±54 photons per absorbed x ray (42-66keV-1 ) in interactions occurring furthest from and nearest to the image sensor, while the Fast Back yielded 833±22 and 1910±39 photons (26-60keV-1 ). At f=1mm-1 , MTF(z,f) varied between 0.63 and 0.78 in the Regular and 0.30-0.76 in the Fast Back. Calculations of presampling MTF(f) using and MTF(z,f) showed excellent agreement with slanted-edge measurements. The developed instrument and method enable direct measurements of the depth-dependent gain and spatial resolution of turbid scintillators. This knowledge can be used to predict, understand, and potentially improve I-FPD imaging performance.

Efficiency enhancement in a backside illuminated 1.12 μm pixel CMOS image sensor via parabolic color filters.

The shrinkage of pixel size down to sub-2 μm in high-resolution CMOS image sensors (CISs) results in degraded efficiency and increased crosstalk. The backside illumination technology can increase the efficiency, but the crosstalk still remains an critical issue to improve the image quality of the CIS devices. In this paper, by adopting a parabolic color filter (P-CF), we demonstrate efficiency enhancement without any noticeable change in optical crosstalk of a backside illuminated 1.12 μm pixel CIS with deep-trench-isolation structure. To identify the observed results, we have investigated the effect of radius of curvature (r) of the P-CF on the efficiency and optical crosstalk of the CIS by performing an electromagnetic analysis. As the r of P-CF becomes equal to (or half) that of the microlens, the efficiencies of the B-, G-, and R-pixels increase by a factor of 14.1% (20.3%), 9.8% (15.3%), and 15.0% (15.7%) with respect to the flat CF cases without any noticeable crosstalk change. Also, as the incident angle increases up to 30°, the angular dependence of the efficiency and crosstalk significantly decreases by utilizing the P-CF in the CIS. Meanwhile, further reduction of r severely increases the optical crosstalk due to the increased diffraction effect, which has been confirmed with the simulated electric-field intensity distribution inside the devices.

Novel source follower transistor structure without lightly doped drain for high performance CMOS image sensor

To realize high-resolution pixels in the CMOS image sensor, it is necessary to reduce low-frequency noise, particularly random telegraph signal (RTS) noise of the source-follower transistor (SFT). To achieve less relative variation of drain noise current, ΔID/ID, a metal–oxide–semiconductor field-effect transistor structure without the lightly doped drain (LDD) for the SFT transistor is proposed. Then, a comparison of RTS noise characteristics between the proposed SFT structure without LDD and the conventional SFT structure with LDD was conducted. Although the RTS noise occurrence probability of the proposed SFT structure without LDD is somewhat greater than that of the conventional SFT structure with LDD, the amplitude of relative variation of drain noise current of the proposed SFT structure is significantly less than that of the conventional SFT. Despite changes in several factors in the proposed SFT, such as effective channel length, trap depth profile in gate oxide, and random dopant fluctuation (RDF), it is believed that the change of trap depth profile is a primary factor for the improved RTS characteristic. Therefore, the proposed SFT is highly desirable for the high-resolution CMOS image sensor.

A CURRENT SOURCE WITH IMMUNITY FROM IR-DROP FOR HIGH-RESOLUTION CMOS IMAGE SENSORS

A current source circuit for high-resolution CMOS image sensors is introduced in this paper, which can provide a stable current with immunity from IR-drop along the power and ground wires. In the circuit, two pre-charged floating capacitors controlled by two-phase non-overlap clock are connected between the gate and source of a MOS transistor alternately to maintain a constant V GS of the MOS transistor. In this way, the current flowing through the MOS transistor will be stable and has nothing to do with power level. The post simulation results show that the current supplied by the proposed circuit decreases only by 1.6% when the voltage of the power supply decreases by 30%. The simulations have proved that the proposed circuit can improve the performance of high-resolution CMOS image sensors.

4+1-transistor pixel architecture for high-speed, high-resolution CMOS image sensors

A pixel architecture is introduced which allows a drastic reduction of the column capacitance of a monolithic pixel array. It consists of a classic 4T pixel architecture together with an extra switch added at regular positions in the column array and shared by a group of pixels of the column. In this way, each pixel will see an output capacitance proportional to the number of pixels sharing the extra switch and the total number of extra switches.