Large Area Image Sensors Research Articles

Optical fingerprint sensor with FBI certification

Isorg (Limoges, France), a pioneer in organic photodetectors (OPDs) and large area image sensors, announces the FBI certification of Bio1Print30, a large-area optical fingerprint sensor for applications where enhanced mobile security and nomad ID authentication are key

Carrier Blocking Layer Materials and Application in Organic Photodetectors.

As a promising candidate for next-generation photodetectors, organic photodetectors (OPDs) have gained increasing interest as they offer cost-effective fabrication methods using solution processes and a tunable spectral response range, making them particularly attractive for large area image sensors on lightweight flexible substrates. Carrier blocking layers engineering is very important to the high performance of OPDs that can select a certain charge carriers (holes or electrons) to be collected and suppress another carrier. Carrier blocking layers of OPDs play a critical role in reducing dark current, boosting their efficiency and long-time stability. This Review summarizes various materials for carrier blocking layers and some of the latest progress in OPDs. This provides the reader with guidelines to improve the OPD performance via carrier blocking layers engineering.

Transparent phototransistor with high responsivity, sensitivity, and detectivity from heterojunction metal oxide semiconductors

In this work, the high-performance transparent Al:InZnSnO/InZnO/Al:InZnSnO tri-layer thin-film phototransistors are reported. They show a high field-effect mobility of 40.1 cm2/V·s and an excellent high photoresponsivity of 25 000 A/W, a photosensitivity of 3.3 × 107, a specific detectivity of 4.3 × 1017 cm·Hz1/2·W−1 under the illumination at 460 nm with an intensity of 140 μW/cm2. The persistent photoconductivity inherent in phototransistors made of oxide semiconductors overcome by a pulsed gate bias with 1 μs, which accelerates the recombination of photogenerated holes with electrons. This demonstrates that the transparent photosensor array can be integrated monolithically on the display without appreciable loss of transparency for high functionality such as large area image sensor.

Organic Photodetectors and their Application in Large Area and Flexible Image Sensors: The Role of Dark Current

AbstractOrganic photodetectors (OPDs) have gained increasing interest as they offer cost‐effective fabrication methods using low temperature processes, making them particularly attractive for large area image detectors on lightweight flexible plastic substrates. Moreover, their photophysical and optoelectronic properties can be tuned both at a material and device level. Visible‐light OPDs are proposed for use in indirect‐conversion X‐ray detectors, fingerprint scanners, and intelligent surfaces for gesture recognition. Near‐infrared OPDs find applications in biomedical imaging and optical communications. For most applications, minimizing the OPD dark current density (Jd) is crucial to improve important figures of merits such as the signal‐to‐noise ratio, the linear dynamic range, and the specific detectivity (D*). Here, a quantitative analysis of the intrinsic dark current processes shows that charge injection from the electrodes is the dominant contribution to Jd in OPDs. Jd reduction is typically addressed by fine‐tuning the active layer energetics and stratification or by using charge blocking layers. Yet, most experimental Jd values are higher than the calculated intrinsic limit. Possible reasons for this deviation are discussed, including extrinsic defects in the photoactive layer and the presence of trap states. This provides the reader with guidelines to improve the OPD performances in view of imaging applications.

Biology microscopy using well-distributed sphere digital in-line holography

Traditional pinhole spherical wave digital in-line holography has proved to be powerful imaging tools. Image quality is affected by uncertain round of pinhole. Here, we propose a well-distributed sphere wave generation method and it demonstrates wide field of view and high resolution microscopy. The laser focuses into an infinitesimal spot through laser beam expander and microscope objective. Pinhole permutation with different sizes is utilized to match the focal point, and emerges an ideal spherical wave. Interference fringes pattern, formed by reference sphere wave and scattered sphere wave of object, is collected by large area image sensor. The influence of dirty in image sensor and parasitic light is eliminated through subtraction with and without object. Fresnel inverse transformation reconstruction algorithm presents the object information. Biology microscopy experiments demonstrate that the proposed techniques increase the flexibility in producing well-distributed point light source and improve the image quality. Field of view is 3.22 mm×3.22 mm and resolution is 5.09 μm. Furthermore, adjustable field of view with magnification, fast, no-contact make it to be a promising tool in optical element measurement, material identification, biology and medicine.

Cadmium Sulfide Nanocrystal Sensitized Vertically Aligned Titanium Dioxide Rods for Large Area Image Sensors on 3-D Substrates

We discuss here a semiconductors assembly comprising of titanium dioxide (TiO2) rods sensitized by cadmium sulfide (CdS) nanocrystals for potential applications in large area electronics on three dimensional (3-D) substrates. Vertically aligned TiO2 rods are grown on a substrate using a 150° C process flow and then sensitized with CdS by SILAR method at room temperature. This structure forms an effective photoconductor as the photo-generated electrons are rapidly removed from the CdS via the TiO2 thereby permitting a hole rich CdS. Current-voltage characteristics are measured and models illustrate space charge limited photo-current as the mechanism of charge transport at moderate voltage bias. The stable assembly and high speed are achieved. The frequency response with a loading of 10 pF and 9 MΩ shows a half power frequency of 100 Hz.

Updates from the SPADnet project (fully digital, scalable and networked photonic component for Time-of-Flight PET applications).

SPADnet is aimed at a new generation of fully digital, scalable and networked photonic components to enable large area image sensors, with primary target gamma-ray and coincidence detection in (Time-of-Flight) PET. The SPADnet photonic module, which lies at the heart of the concept, is built around an array of tessellated single-photon TSV sensor chips, manufactured in standard CMOS technology. The resulting sensor tile is connected on the back to an FPGA-based data processing and communication unit, whereas its front size is glued to scintillator crystals. The resulting modules are then connected in a token ring structure to form the actual PET system. Coincidence detection occurs directly in the ring itself, in a differed and distributed manner to ensure scalability. We have fabricated and tested the first version of the SPADnet photosensor, a fully digital CMOS SiPM with 8×16 pixels individually capable of photon time stamping and energy accumulation, together with the corresponding sensor tiles. The sensor also provides a real-time output of the total detected energy at up to 100Msamples/s and on-chip discrimination of gamma events. These events can then be routed to the SPADnet ring network, which operates at 2 Gbps providing real-time processing and coincidence determination; this architecture simplifies the construction of the overall system and allows the scaling of the system to larger arrays of detectors. This may result in better and faster image reconstruction. SPADnet will not only impact PET scalability but also performance robustness and cost; another advantage is the capability of being compatible with magnetic resonance imaging (MRI), thus prompting advances in multimodal imaging and medical diagnostics as a whole. SPADnet is being designed with scalability in mind, with the idea of being able to redeploy at reduced effort the SPADnet photonic module in other configurations such as brain PET.

SPADnet: Embedded coincidence in a smart sensor network for PET applications

In this paper we illustrate the core technologies at the basis of the European SPADnet project (www.spadnet.eu), and present the corresponding first results. SPADnet is aimed at a new generation of MRI-compatible, scalable large area image sensors, based on CMOS technology, that are networked to perform gamma-ray detection and coincidence to be used primarily in (Time-of-Flight) Positron Emission Tomography (PET). The project innovates in several areas of PET systems, from optical coupling to single-photon sensor architectures, from intelligent ring networks to reconstruction algorithms. In addition, SPADnet introduced the first computational model enabling study of the full chain from gamma photons to network coincidence detection through scintillation events, optical coupling, etc.

Preliminary Results on Large Area X-ray a-SiC:H Multilayer Detectors with Optically Addressed Readout

AbstractThis paper investigates a feasibility of using a large area image sensor with an optically addressed readout for medical X-ray diagnostic imaging. A device prototype comprises a multilayer glass/ZnO:Al/p (a-SiC:H)/i (a-Si:H)/ n (a-SiC:H)/ i(a-Si:H)/p (a-SiC:H)/ a SiNx/ITO structure coupled to a scintillator layer. Here, the p-i-n-i-p structure works in both sensing and switching modes depending on the biasing conditions. A numerical simulation is used to optimize the semiconductor layer thicknesses in order to achieve a photocurrent matching between back-to-back diodes in switching mode. The charge carrier transport within the p-i-n-i-p structure is also analyzed under different electric and optical biasing conditions. A physical model supports the results.

Tuning the spectral distribution of p–i–n a-SiC:H devices for colour detection

ZnO:Al/p (SiC:H)/i (Si:H)/n (SiC:H) large area image and colour sensor are analysed. Carrier transport and collection efficiency are investigated from dark and illuminated current–voltage ( I– V) dependence and spectral response measurements under different optical and electrical bias conditions. Results show that the carrier collection depends on the optical bias and on the applied voltage. By changing the electrical bias around the open circuit voltage it is possible to filter the absorption at a given wavelength and so to tune the spectral sensitivity of the device. Transport and optical modelling give insight into the internal physical process and explain the bias control of the spectral response and the image and colour sensing properties of the devices.

Large area single and stacked p-i-n photodiodes as a color image sensors

AbstractSingle and stacked p-i-n sensing elements for image recognition and color extraction applications are presented. The aim of this work is to optimize the performance of the a-SiC:H thin films layers in order to enhance its performance when making part of the structure of large area image and color sensors. The efforts are focused mainly on doped n- and p-type layers at high and low doping levels with and without carbon. The structural and optoelectronic properties of the single layers were determined through infrared and visible spectroscopy, temperature-dependent conductivity, and were complemented by CPM measurements. Junction properties, carrier transport, photogeneration and collection efficiency are investigated from dark and illuminated current-voltage characteristics and spectral response measurements, with and without additional background illumination and under different light bias conditions. The spectral response dependence on the applied voltage and on optical bias was also studied. Results show that the spectral sensitivity is strongly dependent on the applied voltage, namely the maximum spectral sensitivity shifts with the voltage, and at certain wavelengths the spectral response goes down to zero, which allows a different selectivity, and enables color recognition.

Micro-electro-mechanical system fabrication technology applied to large area x-ray image sensor arrays

Micromachining has potential applications for large area image sensors and displays, but conventional micro-electro-mechanical system (MEMS) technology based on crystalline silicon wafers cannot be used. Instead, large area devices use deposited films on glass substrates. This presents many challenges for MEMS, both with regard to materials for micromachined structures and to integration with large area electronic devices. We are exploring the novel thick photoresist SU-8 as well as plating techniques for the fabrication of large area MEMS. As an example of its application, we have employed this MEMS technology to improve the performance of an amorphous silicon based x-ray image sensor array. SU-8 is explored as the structural material for the x-ray conversion screen and as a thick interlayer dielectric for the thin film readout electronics of the imager. Plating techniques are employed to metallize the deep contact vias in SU-8. Processing challenges that are particularly important for large area fabrication will be addressed.

Large Area Mems: Materials Issues and Applications

AbstractConventional MEMS devices are based on silicon micro-machining and their maximum size is limited by the wafer. In contrast, we are exploring micro-machining for large area applications on substrates such as glass using polymeric materials. Our research is focused on the photopolymer SU-8, and we apply the MEMS fabrication technology to large area image sensors and displays. There are many challenges concerning the materials and processes, since large area compatibility is essential and integration with large area electronics may be required. The adhesion of SU-8 to the underlying layers as well as stress in the SU-8 are important issues and surface treatments have been investigated. Two applications of SU-8 MEMS are discussed to illustrate large area applications:. First, in the fabrication of an X-ray imager, high aspect ratio SU-8 walls form a micro-patterned phosphor screen to increase the image resolution. Second, a similar approach of patterning SU-8 into arrays of micro-cells is applied to an electrophoretic display.

Color characterization of large area polymer image sensors

Abstract Large area image sensors made with semiconducting polymers are demonstrated to have true-color resolution (24-bits). The red, green and blue color primaries are achieved by coupling a set of color filters with the polymer sensor; the color-sensing pixels are fabricated from a single polymer with photoresponse that covers the visible spectral range from 400 to 700 nm. A process for recovering the image from the pixelated photocurrent data is developed; the image recovery process is general and applicable to image arrays with power-law light intensity dependence and with finite pixel dark current. Large size, true-color images were processed by scanning with a linear array of polymer photodiodes.

MEMS Materials and Fabrication Technology on Large Areas: The Example of an X-ray Imager

ABSTRACTMicromachining has potential applications for large area image sensors and displays, but conventional MEMS technology, based on crystalline silicon wafers cannot be used. Instead, large area devices use deposited films on glass substrates. This presents many challenges for MEMS, both as regards materials for micro-machined structures and the integration with large area electronic devices. We are exploring the novel thick photoresist SU-8, as well as plating techniques for the fabrication of large area MEMS. As an example of its application, we have applied this MEMS technology to improve the performance of an amorphous silicon based image sensor array. SU-8 is explored as the structural material for the X-ray conversion screen and as a thick interlayer dielectric for the thin film readout electronics of the imager.

A Fully Self-Aligned Amorphous Silicon Tft Technology for Large Area Image Sensors and Active-Matrix Displays

ABSTRACTWe have developed a fully self-aligned amorphous silicon TFT technology, which is suitable for large area image sensors and active matrix displays. Self-alignment is achieved by defining the top nitride by back exposure and then forming source and drain contacts by ionimplantation and silicidation. We incorporate a low resistance gate metallisation process, by using Al metal, capped by Cr. We have compared the process of forming the silicide after the ion-implantation step, with a new process of forming the silicide first and then implanting through the formed silicide. We find a significant advantage to the latter method, where we can achieve a higher doping level and reduced contact resistance. We have therefore optimised our process based on this method. Transistor characteristics as a function of channel length for both methods show the improved contact resistance, obtained with the latter method. We obtain field effect mobilities of 0.7cm2V−1s−1, measured in the saturated region, for a channel length of 8μm.

Transient behavior of adjustable threshold a-Si:H/a-SiC:H three-color detector

In this paper, we investigate the transient behavior of a-Si:H/a-SiC:H adjustable-threshold three-color detectors (ATCDs) for applications in bidimensional large area image sensors. Red, green, and blue color sensing in the charge integration regime is demonstrated, and the transient mechanisms of charge and discharge of the equivalent capacitance are discussed by means of load curve diagrams. The possibility of driving 2-D arrays of ATCDs is discussed by means of a test circuit simulating the row select TFT and the data line capacitance by discrete components. Readout times on the microsecond timescale and saturation times three orders of magnitude greater have been obtained under 0.1 mW/cm/sup 2/ illumination, resulting in the possibility of scanning 1000 rows. Finally, an equivalent circuit is introduced and solved by AIM-SPICE, and simulations of the static and dynamic behavior are presented.

Microlens arrays for 2D large area image sensors

We report on the fabrication and use of microlenses in large area image sensor arrays. The microlenses were made using photoresist reflow, which is eminently suited to fabrication in a research environment. Microlenses of 190 mu m diameter were made in an orthogonal array at 200 mu m pitch and with focal lengths of 200-450 mu m. These microlenses were placed on top of a 192*96 imaging array made with amorphous silicon devices (sensors and switches) at identical pitch. Good images were obtained from text placed up to 7 mm away from the image sensor/microlens combination.