Status of Uncooled Infrared Detector Technology at ULIS, France

The high level of accumulated expertise by ULIS and CEA/LETI on uncooled microbolometers made from amorphous silicon enables ULIS to develop uncooled IRFPA with 17 µm pixel-pitch to enable the development of small power, small weight and power (SWaP) and high performance IR systems. Key characteristics of amorphous silicon based uncooled IR detector is described to highlight the advantage of this technology for system operation. A full range of products from 160 x 120 to 1024 x 768 has been developed and we will focus the paper on the ¼ VGA with 17 µm pixel pitch. Readout integrated circuit (ROIC) architecture is described highlighting innovations that are widely on-chip implemented to enable an easier operation by the user. The detector configuration (integration time, windowing, gain, scanning direction), is driven by a standard I²C link. Like most of the visible arrays, the detector adopts the HSYNC/VSYNC free-run mode of operation driven with only one master clock (MC) supplied to the ROIC which feeds back pixel, line and frame synchronisation. On-chip PROM memory for customer operational condition storage is available for detector characteristics. Low power consumption has been taken into account and less than 60 mW is possible in analogue mode at 60 Hz. A wide electrical dynamic range (2.4V) is maintained despite the use of advanced CMOS node. The specific appeal of this unit lies in the high uniformity and easy operation it provides. The reduction of the pixel-pitch turns this TEC-less ¼ VGA array into a product well adapted for high resolution and compact systems. Noise equivalent temperature difference (NETD) of 35 mK and thermal time constant of 10 ms have been measured leading to 350 mK.ms figure of merit. We insist on NETD trade-off with wide thermal dynamic range, as well as the high characteristics uniformity and pixel operability, achieved thanks to the mastering of the amorphous silicon technology coupled with the ROIC design. This technology node associated with advanced packaging technique, paves the way to compact low power system.Defence Science Journal, 2013, 63(6), pp.545-549, DOI:http://dx.doi.org/10.14429/dsj.63.5753

A highly digital microbolometer ROIC employing a novel event-based readout and two-step time to digital converters

Uncooled infrared imaging systems are a light weight and low cost alternative to their cooled counterparts. Uncooled microbolometer IR focal plane arrays (IRFPAs) for applications such as medical imaging, thermography, night vision, surveillance and industrial process control have recently been under focus. These systems have small pixel pitches ( 250 K). Low NETD demands excellent microbolometer and readout noise performance. If sensitive analog circuits, driving long metal interconnects, are part of the predigitization readout channel, this necessitates the use of power consuming buffers, potentially in conjunction with noise cancellation circuits that result in power and area overhead. Thus re-thinking at the architectural level is crucial to meet these demands. Accordingly, in this thesis a column-parallel readout architecture for frame synchronous microbolometer imagers is proposed that enables low power operation by employing a time mode digitizer. The proposed readout circuit is based on a bridge type detector network with active and reference microbolometers and employs a capacitive transimpedance amplifier (CTIA) incorporating a novel two-step integration mechanism. By using a modified reset scheme in the CTIA, a forward ramp is initiated at the input side followed by the conventional backward integrated ramp at the output. This extends the measurement interval and improves signal-to-noise ratio (SNR). A synchronous counter based TDC measures this interval providing robust digitization. This technique also provides a way of compensating for self-heating effects. Being highly digital, the proposed architecture offers robust frontend processing and achieves a per channel power consumption of 66 µW, which is considerably lower than the most recently reported designs, while maintaining better than 10mK readout NETD

Analysis and comparison of resistive, ferroelectric and pyroelectric uncooled bolometers for electronic imaging systems

The performance parameters (responsivity (Rv). detectivity (D*), total noise and response time) of resistive, pyroelectric and ferroelectric bolometer detectors are dependent on a large number of key variables including chopping frequercy, the input impedance and voltage noise of the readout circuitry, the structure dependent parameters (particularly thermal conductance and thermal capacitance), and material properties such as dielectric constant, pyroelectric coefficient, loss tangent and thin film thickness. The interrelationship between the key variables and their influence on performance is often complex and not easily discerned for the three major types of thermal detectors: resistive, pyroelectric and ferroelectric bolometers. In this thesis research, the dependence of Rv, D* and total noise on these key parameters were analyzed and written as equations from which computer calculations could easily be made. The analyzed results were used to compare the pertbrmance of the three types of sensors for present-day structure and material characteristics and also for material characteristics and structures that night be developed in the future

Readout electronics for microbolometer infrared focal plane array

Ph.DDOCTOR OF PHILOSOPH

Miniaturized Silicon Photodetectors

Silicon (Si) technologies provide an excellent platform for the design of microsystems where photonic and microelectronic functionalities are monolithically integrated on the same substrate. In recent years, a variety of passive and active Si photonic devices have been developed, and among them, photodetectors have attracted particular interest from the scientific community. Si photodiodes are typically designed to operate at visible wavelengths, but, unfortunately, their employment in the infrared (IR) range is limited due to the neglectable Si absorption over 1100 nm, even though the use of germanium (Ge) grown on Si has historically allowed operations to be extended up to 1550 nm. In recent years, significant progress has been achieved both by improving the performance of Si-based photodetectors in the visible range and by extending their operation to infrared wavelengths. Near-infrared (NIR) SiGe photodetectors have been demonstrated to have a “zero change” CMOS process flow, while the investigation of new effects and structures has shown that an all-Si approach could be a viable option to construct devices comparable with Ge technology. In addition, the capability to integrate new emerging 2D and 3D materials with Si, together with the capability of manufacturing devices at the nanometric scale, has led to the development of new device families with unexpected performance. Accordingly, this Special Issue of Micromachines seeks to showcase research papers, short communications, and review articles that show the most recent advances in the field of silicon photodetectors and their respective applications

A low-power CMOS readout IC with on-chip column-parallel SAR ADCs for microbolometer applications

A readout IC (ROIC) designed for high temperature coefficient of resistance (TCR) SiGe microbolometers is presented. The ROIC is designed for higher Ge content SiGe microbolometers which have higher detector resistance (∼1MΩ) and higher TCR values (∼%5.5/K). The ROIC includes column SAR ADCs for on-chip column-parallel analog to digital conversion. SAR ADC architecture is chosen to reduce the overall power consumption. The problem of resistance variation across the bolometers which introduce fixed pattern noise is addressed by setting a tunable reference resistor shared for each column which can be calibrated offline to set the common-mode level. Moreover, column non-uniformity has been reduced through comparator offset compensation in the SAR ADC. The columnwise architecture in this work reduces the number of integrators needed in the architecture and enables 17×17 μm2 pixel sizes. The prototype has been designed and fabricated in 0.25-μm CMOS process

Design and Fabrication of Vertically-Integrated CMOS Image Sensors

Technologies to fabricate integrated circuits (IC) with 3D structures are an emerging trend in IC design. They are based on vertical stacking of active components to form heterogeneous microsystems. Electronic image sensors will benefit from these technologies because they allow increased pixel-level data processing and device optimization. This paper covers general principles in the design of vertically-integrated (VI) CMOS image sensors that are fabricated by flip-chip bonding. These sensors are composed of a CMOS die and a photodetector die. As a specific example, the paper presents a VI-CMOS image sensor that was designed at the University of Alberta, and fabricated with the help of CMC Microsystems and Micralyne Inc. To realize prototypes, CMOS dies with logarithmic active pixels were prepared in a commercial process, and photodetector dies with metal-semiconductor-metal devices were prepared in a custom process using hydrogenated amorphous silicon. The paper also describes a digital camera that was developed to test the prototype. In this camera, scenes captured by the image sensor are read using an FPGA board, and sent in real time to a PC over USB for data processing and display. Experimental results show that the VI-CMOS prototype has a higher dynamic range and a lower dark limit than conventional electronic image sensors

Bolometers

Infrared Detectors and technologies are very important for a wide range of applications, not only for Military but also for various civilian applications. Comparatively fast bolometers can provide large quantities of low cost devices opening up a new era in infrared technologies. This book deals with various aspects of bolometer developments. It covers bolometer material aspects, different types of bolometers, performance limitations, applications and future trends. The chapters in this book will be useful for senior researchers as well as beginning graduate students

Uncooled Carbon Microbolometer Imager

The discovery of infrared radiation two centuries ago and the theory of blackbody radiation one century later have given birth to the field of thermal imaging. Since then, researchers have devised numerous ways to detect infrared radiation. From World War II to the 1980s, semiconductor-based cooled photon detector arrays have reigned over the field of thermal imaging. Albeit limited to expensive, bulky systems used for military applications due to their cooling requirement they have been . The emergence of micromachining techniques in the 1980s however, have allowed for the development of uncooled, thermal detector arrays. Uncooled systems are expected to find more and more applications, especially in the civilian world. Here we present a novel and simple way to fabricate uncooled infrared detectors suitable for integration into large-area arrays. The design is based on carbon obtained by means of polymer pyrolysis. We demonstrate how some electrical and thermal properties can be adjusted by process parameters, and then present the first micromachined carbon uncooled bolometer made of two-layers of self-supporting pyrolyzed-parylene carbon having different process-tuned properties. Finally, based on this unique design and fabrication process, we develop a carbon bolometer array and demonstrate the thermal imaging capability by taking thermal images. Measurements show that the sensitivity to target temperature can be as low as 31mK and 44mK for 100us and 12us electrical signal integration time, respectively. This matches the current state of the art which is very promising considering the fact that this is the first time pyrolytic carbon has been used to fabricate a microbolometer array.</p