High-sensitivity Image Sensor Research Articles

Dispersion-engineered metasurfaces for high-sensitivity color image sensors

AbstractIncreasing the sensitivity of image sensors is a major challenge for current imaging technology. Researchers are tackling it because highly sensitive sensors enable objects to be recognized even in dark environments, which is critical for today’s smartphones, wearable devices, and automobiles. Unfortunately, conventional image-sensor architectures use light-absorptive color filters on every pixel, which fundamentally limits the detected light power per pixel. Recent advances in optical metasurfaces have led to the creation of pixelated light-transmissive color splitters with the potential to enhance sensor sensitivity. These metasurfaces can be used instead of color filters to distinguish primary colors, and unlike color filters, they can direct almost all of the incident light to the photodetectors, thereby maximizing the detectable light power. This review focuses on such metasurface-based color splitters enabling high-sensitivity color-image sensors. Their underlying principles are introduced with a focus on dispersion engineering. Then, their capabilities as optical elements are assessed on the basis of our recent findings. Finally, it is discussed how they can be used to create high-sensitivity color-image sensors.

Full-color-sorting metalenses for high-sensitivity image sensors

Image sensors play a critical role in current technologies ranging from smartphones to autonomous vehicles. In these technologies, high-sensitivity image sensors are highly desired because they enable dark-scene/ultra-fast imaging. Unfortunately, a conventional sensor architecture that employs color filters on every pixel fundamentally limits the detected light power per pixel because of the filtering, which has been a long-standing barrier to sensitivity improvement. Here, we demonstrate polarization-insensitive metasurface lenses (metalenses) that sort primary colors on high-density pixels without the use of color filters. The metalenses simultaneously act as pixel-scale color splitters and lenses and are compatible with complementary metal–oxide-semiconductor sensor technology. An image sensor with such metalenses significantly enhances the detected light power, while affording high image quality, incident angle tolerance, and sub-micrometer spatial resolution. The demonstrated architecture opens the way to the development of high-sensitivity color image sensors that exceed current limits while maintaining consistency with state-of-the-art sensor technology.

高感度・高精細・リアルタイム近接容量イメージセンサ

高感度・高精細・リアルタイム近接容量イメージセンサ

NINscope, a versatile miniscope for multi-region circuit investigations.

Miniaturized fluorescence microscopes (miniscopes) have been instrumental to monitor neural signals during unrestrained behavior and their open-source versions have made them affordable. Often, the footprint and weight of open-source miniscopes is sacrificed for added functionality. Here, we present NINscope: a light-weight miniscope with a small footprint that integrates a high-sensitivity image sensor, an inertial measurement unit and an LED driver for an external optogenetic probe. We use it to perform the first concurrent cellular resolution recordings from cerebellum and cerebral cortex in unrestrained mice, demonstrate its optogenetic stimulation capabilities to examine cerebello-cerebral or cortico-striatal connectivity, and replicate findings of action encoding in dorsal striatum. In combination with cross-platform acquisition and control software, our miniscope is a versatile addition to the expanding tool chest of open-source miniscopes that will increase access to multi-region circuit investigations during unrestrained behavior.

Three-dimensional integral imaging in photon-starved environments with high-sensitivity image sensors.

Imaging in poorly illuminated environments using three-dimensional (3D) imaging with passive imaging sensors that operate in the visible spectrum is a formidable task due to the low number of photons detected. 3D integral imaging, which integrates multiple two-dimensional perspectives, is expected to perform well in the presence of noise, as well as statistical fluctuation in the detected number of photons. In this paper, we present an investigation of 3D integral imaging in low-light-level conditions, where as low as a few photons and as high as several tens of photons are detected on average per pixel. In the experimental verification, we use an electron multiplying charge-coupled device (EM-CCD) and a scientific complementary metal-oxide-semiconductor (sCMOS) camera. For the EM-CCD, a theoretical model for the probability distribution of the pixel values is derived, then fitted with the experimental data to determine the camera parameters. Likewise, pixelwise calibration is performed on the sCMOS to determine the camera parameters for further analysis. Theoretical derivation of the expected signal-to-noise-ratio is provided for each image sensor and corroborated by the experimental findings. Further comparison between the cameras includes analysis of the contrast-to-noise ratio (CNR) as well as the perception-based image quality estimator (PIQE). Improvement of image quality metrics in the 3D reconstructed images is successfully confirmed compared with those of the 2D images. To the best of our knowledge, this is the first experimental report of low-light-level 3D integral imaging with as little as a few photons detected per pixel on average to improve scene visualization including occlusion removal from the scene.

A theoretical study of digital silicon photomultiplier utilization in diffuse optical imaging systems.

Digital silicon photomultiplier (dSiPM) is introduced for diffuse optical imaging (DOI) applications instead of conventional photomultiplier tubes and avalanche photodiodes (APDs) as a state-of-the-art detector. According to the low-level light regime in DOI applications, high sensitivity and high dynamic range (DR) image sensors are needed for DOI systems. dSiPM is proposed as a developing detector which can detect low-level lights. Also, an accurate equation is obtained for calculating the DR of dSiPMs. Different dSiPMs and the corresponding benefits are studied for DOI applications. Furthermore, a 120 dB DR dSiPM is chosen for use in DOI systems. It is shown that dSiPMs can be utilized in DOI configurations such as time domain (TD), frequency domain (FD) and continuous wave (CW) systems. Ultimately, by utilizing dSiPM in DOI systems, the DOI method can be used for thoracic imaging due to the high DR and signal-to-noise ratio (SNR) of the detector.

Effect of drain current on appearance probability and amplitude of random telegraph noise in low-noise CMOS image sensors

Random telegraph noise (RTN), which occurs in in-pixel source follower (SF) transistors, has become one of the most critical problems in high-sensitivity CMOS image sensors (CIS) because it is a limiting factor of dark random noise. In this paper, the behaviors of RTN toward changes in SF drain current conditions were analyzed using a low-noise array test circuit measurement system with a floor noise of 35 µVrms. In addition to statistical analysis by measuring a large number of transistors (18048 transistors), we also analyzed the behaviors of RTN parameters such as amplitude and time constants in the individual transistors. It is demonstrated that the appearance probability of RTN becomes small under a small drain current condition, although large-amplitude RTN tends to appear in a very small number of cells.

A Low Noise and High Sensitivity Image Sensor with Imaging and Phase-Difference Detection AF in All Pixels

A Low Noise and High Sensitivity Image Sensor with Imaging and Phase-Difference Detection AF in All Pixels

Paper] A Low Noise and High Sensitivity Image Sensor with Imaging and Phase-Difference Detection AF in All Pixels

In this paper, we describe a device structure and optical design for a CMOS image sensor with phase-difference detection photodiodes (PD) for autofocus (AF) function. The individual pixel of this image sensor is composed of two horizontally displaced PDs separated by a PN junction. All the effective pixels function as both the imaging and the phase-difference detection AF (PDAF). We have realized a low dark random noise (1.8e- at 1PD, 2.5e- at 1pixel) and high sensitivity (78Ke-/lx.sec at 1green pixel) image sensor with the imaging and the PDAF functions in all the effective pixels.

Interview

Dr Zhang Yixin of Nanjing University, China, talks about the work behind the paper ‘Portable true random number generator for personal encryption application based on smartphone camera’, page 1841. Dr Zhang Yixin My major was more related to radio communication as an undergraduate. About eight years ago, I started a PhD at Nanjing University, that was the beginning of my research in optical. In 2011, I joined NTRC at Nanyang Technological University in Singapore as a postdoctoral research fellow. The lab had just begun a joint military project on unconditional encrypted optical communication. That is where my current research work really began. Two years later I returned to Nanjing University as a lecturer and my current main field of research is single photon level optical signal detection and its applications, such as the true random number generator (TRNG) based on photon distribution. True random numbers are the key to secure communication, even if quantum technology is included. A TRNG based on an off-the-shelf image sensor, rather than a thousands-of-dollars photon counter, would be much more attractive for portable personal encryption applications, such as E-payment, privacy call and cryptographic data transmission on a smartphone or laptop. In the near future, the security level of personal devices has to be enhanced with new encryption technology to stand against the technological advances of eavesdroppers; this is the goal of our current project. Present TRNGs are mainly based on specialised, expensive hardware like single photon detectors, chaotic lasers and radioactive nuclei, which are more suited to commercial or academic applications than personal usage. We present a portable TRNG configuration simply based on the camera of a smartphone. The randomness of the output bit sequence has been proved with NIST tests, and all necessary processing functions could be fully integrated within Android software in the near future. This approach offers a promising solution for portable personal encryption, since no equipment is needed except the smartphone itself. Back in my PhD days, working on micro-structure imaging, our group confirmed by experiment that in certain conditions, shot noise other than thermal noise can dominate the overall noise characteristics of a high-sensitivity CCD image sensor. Commonly speaking, shot noise of photocurrent is believed to be a quantum process and true random number could be generated accordingly. However, the performance of image sensors on smartphones were not as good then. Two or three years later, I saw a picture taken by my colleague's latest phone, and thought maybe it was time to realise the portable TRNG with smartphone camera. I see two main trends in recent TRNG research. The first is realising higher bit rates; speeds of gigabits per second were reported several years ago. The second is looking for new physical random phenomena that are more reliable. Arguments have always existed on whether chaotic process is as random as quantum process. However, I believe that the integration of TRNGs is very likely to be the main topic in the future. TRNGs are still too big and expensive for most practical applications. Integration of optics and electronics components on a single chip is necessary for portable and low-cost TRNGs. Meanwhile, TRNGs only solve the random number generation problem, a communication protocol which employs true random numbers as secret keys for encryption also needs to be developed. Here at the Institute of Optical Communication Engineering at Nanjing, my group will try to improve the Android App for TRNGs to automatically adapt to different types of smartphone, since the camera setting is essential to the overall performance of output random bits. We are also working on high-speed single photon counting technology based on avalanche photodiodes to improve the detection speed if higher rates are required. Another research area we are involved in is fibre optical sensing, with the goal of locating eavesdroppers in time, while they are intercepting optical signals from fibre communication links. Probe pulse coding based on TRNGs will be used for the improvement of measuring speed, spatial resolution and dynamic range.

Nano-Crystalline Oxide Semiconductor Materials for Display and Semiconductor Device Applications

Ever evolving advances in oxide semiconductor materials and devices continue to fuel leading edge developments in display technology, and transparent electronics, thanks to new integration processes, enabling large area processing on rigid and flexible substrates. Nanocrystalline oxide semiconductor offer a host of advantages such as low cost and high scalability, in addition to seamless heterogeneous integration with a host of other inorganic and organic materials in view of its low thermal budget in processing which provides integration flexibility. This has spawned a wealth of applications ranging from high frame rate interactive displays with embedded imaging to flexible electronics, where speed and transparency are essential requirements Therefore, transparent electronic systems, which have been once viewed as science fiction, can now become a reality. In semiconductor device applications, interest in oxide semiconductors stem from a number of attributes primarily their ease of processing, and high field effect mobility, resulting in stackable process nature on silicon circuits. In this talk, various semiconductor device applications via nanocrystalline oxide semiconductor materials will be presented.In the first part of our presentation, as seen in Figure 1, we present a photo-transistor embedded in a display pixel, in which gate operation is used to accelerate recovery from photocurrent level to the dark state. We describe the origin of ultra-high quantum efficiencies in photo-sensors based on nanocrystalline oxide hetero-junction thin film transistor (TFT). In order to understand the origin of high photocurrent of a device, we evaluated the influence of a light spot from source to drain side on the photoconductive gain of photosensor array. This work here demonstrates high sensitivity image sensor along with quantitative analysis of the quantum efficiency in the hetero-junction TFT taking into account the optical absorption, electron lifetime, and transit time.The integration of electronically active oxide components onto silicon circuits represents an innovative approach to improving the functionality of novel devices. Like most semiconductor devices, complementary-metal-oxide-semiconductor image sensors (CISs) (see Figure 2) and memory devices have physical limitations when progressively scaled down to extremely small dimensions. In the 2nd part of presentation, we propose a novel hybrid CIS architecture and memory architectures that are based on the combination of nanometer-scale oxide thin-film transistors (TFTs) and a conventional Si device. The results demonstrate how our stacked hybrid device could be the starting point for new device strategies in image sensor architectures. Furthermore, we expect the proposed approach to be applicable to a wide range of micro- and nanoelectronic devices and systems.

Proposal of Micro-Trichroic Structures for High-Sensitivity Color Image Sensors

Proposal of Micro-Trichroic Structures for High-Sensitivity Color Image Sensors

Proposal of Micro-Trichroic Structures for High-Sensitivity Color Image Sensors

We propose a micro-trichroic structure based on multiplexed Bragg reflectors. Employing such a structure on each image sensor (IS) cell can enhance the ability of color ISs. Numerical calculations confirm the feasibility of the proposed structure. We also emulated a structure with several layers, and reduced the number of layers substantially by decreasing the diffraction angles.

E2v contract to supply a set of 16-Megapixel, high-sensitivity CCD image sensors for the Very Large Telescope in Chile

e2v contract to supply a set of 16-Megapixel, high-sensitivity CCD image sensors for the Very Large Telescope in Chile

Development of a super-high-sensitivity image sensor using 640×480 pixel active-matrix high-efficiency electron emission device

An extremely high-sensitivity image sensor has been developed with a combination of an active-matrix high-efficiency electron emission device (HEED) array and a high-gain avalanche rushing amorphous photoconductor (HARP) target. Following the description of the requirements for the use of the electron emitter as an image-sensing probe, the characteristics of a fabricated 640×480 pixel active-matrix HEED image sensor with a HARP target are presented by image-pickup experiments under a dark condition. The emission current density of the experimental HEED under an active-matrix drive was 4 A/cm2 corresponding to a practical level. A clear image observed in the prototype compact camera demonstrates that the surface-emitting HEED is useful for image sensing based on HARP with high sensitivity and sufficient definition.

고감도 이미지 센서용 실리콘 나노와이어 MOSFET 광 검출기의 제작

We fabricated Si nano-wire MOSFET by using the conventional photolithography with a <TEX>$1.5{\mu}m$</TEX> resolution. Si nano-wire was fabricated by using reactive ion etching (RIE), anisotropic wet etching and thermal oxidation on a silicon-on-insulator (SOI) substrate, and its width is 30 nm. Logarithmic circuit consisting of a NMOSFET and Si nano-wire MOSFET has been constructed for application to high-sensitivity image sensor. Its sensitivity was 1.12 mV/lux. The output voltage swing was 1.386 V.

A high-sensitivity CCD image sensor using source follower circuit with actively controlled gain characteristics

A high-sensitivity charge coupled device (CCD) image sensor that utilizes a new output structure has been developed. The structure utilizes an enhancement inverter, a coupling capacitor, and resistors in addition to the conventional source follower. The output of the source follower is inverted and ac component of that signal is fed back to the gate of the load transistor of source follower to modulate the impedance. The overall gain improvement can be estimated from simple small-signal analysis and the results match well to the circuit simulation. A 1/4 in. 250,000 pixels color CCD image sensor was fabricated employing the new output circuit. The fabrication process is same to that for conventional CCD image sensors since the inverter in this circuit consists of enhancement mode transistors. It is observed that 14% of sensitivity improvement in the CCD image sensor is achieved by the new structure and the linearity shows no distortion compared to conventional image sensors in the full range of imaging illumination.

高感度固体撮像素子の実現に向けたＭＯＳＦＥＴの高耐圧化

A high-sensitivity solid-state image sensor consisting of an avalanche multiplication film and a CMOS read-out circuit is currently under investigation. The image sensor requires a high-voltage MOSFET to prevent the destruction of the read-out circuit by the high voltage required to achieve high sensitivity with avalanche multiplication in a photoconductive film. The destruction mechanism was revealed and various necessary conditions for a high-voltage MOSFET applicable to a 2/3-inch format image sensor were clarified. The new device has an n- electric field reduction layer surrounding an n+ drain contact. The distance between the gate electrode end and the drain contact end was set to 3μm. The simulation results show that the breakdown voltage increases as the impurity dosage is decreased and the highest breakdown voltage is obtained under implantation energy of 400 keV. The destruction of the new device was mainly due to the breakdown of a pn junction caused by current through the drain to the substrate rather than due to punch-through in which the current through the drain to the source crosses over an npn structure. The on-state resistance of the MOSFET increased abruptly when the Si surface in the electric reduction layer was nearly intrinsic. The breakdown voltage reached 54 V which was about five times as high as that of a conventional device. The device has a simple structure and is fabricated by means of a slightly modified process.

A new CCD architecture of high-resolution and sensitivity for color digital still picture

A new charged-coupled device (CCD) architecture developed for building high-resolution and high-sensitivity image sensors suitable for color digital still picture applications is presented. The sensor is based on the interline CCD structure. Both the interline pixels and the vertical charge transfer lines are utilized as light-sensing elements to improve simultaneously the resolution and sensitivity. This device is named the sea-of-photosensor array (SPA-CCD). A camera and supporting digital system were designed and built specifically to evaluate the device. Digital picture processing for white balance adjustment, chromatic correction, and high-frequency luminance was performed to improve color reproduction and picture resolution. An increased light sensitivity and limiting resolutions of 550 horizontal lines and 400 vertical lines on a TV screen were confirmed with an SPA-CCD of the same chip size as a conventional 190-k pixel IT-CCD. The new design of the SPA-CCD overcomes both the sensitivity and the resolution limitations of previous approaches. >

Heterostructures with hydrogenated amorphous silicon for use in imaging devices

Heterostructures with hydrogenated amorphous silicon (a Si:H) and crystalline semiconductor materials such as silicon (c-Si:H), germanium (c Ge), cadmium telluride (CdTe), etc. have been investigated for application to imaging devices. The large valence-band offset ofc-Si/a-i:H heterojunctions was compensated by boron doping in the deposition of a-Si:H and imaging devices with high resolution were demonstrated in the visible and near-infrared region. Inc-Ge/a-Si:H heterojunctions the offset is too large to be compensated by boron doping and to realize IR imaging devices. Finally,c-CdTe/a-Si:H heterojunctions indicated no band offset at the valence side. Using the CdTe heterojunction, a high-sensitivity X-ray image sensor was constructed and demonstrated utilizing its high absorption coefficient. It was shown under the band offset consideration that heterostructures with a Si:H and some kinds of crystalline semiconductors provide promising image sensors covering wide spectral ranges.