CMOS image sensors for sensor networks

We report on two generations of CMOS image sensors with digital output fabricated in a 0.6 μm CMOS process. The imagers embed an ALOHA MAC interface for unfettered self-timed pixel read-out targeted to energy-aware sensor network applications. Collision on the output is monitored using contention detector circuits. The image sensors present very high dynamic range and ultra-low power operation. This characteristics allow the sensor to operate in different lighting conditions and for years on the sensor network node power budget. Eugenio Culurciello (S’97–M’99) received the Ph.D. degree in Electrical and Computer Engineering in 2004 from Johns Hopkins University, Baltimore, MD. In July 2004 he joined the department of Electrical Engineering at Yale University, where he is currently an assistant professor. He founded and instrumented the E-Lab laboratory in 2004. His research interest is in analog and mixed-mode integrated circuits for biomedical applications, sensors and networks, biological sensors, Silicon on Insulator design and bio-inspired systems. Andreas G. Andreou received his Ph.D. in electrical engineering and computer science in 1986 from Johns Hopkins University. Between 1986 and 1989 he held post-doctoral fellow and associate research scientist positions in the Electrical and Computer engineering department while also a member of the professional staff at the Johns Hopkins Applied Physics Laboratory. Andreou became an assistant professor of Electrical and Computer engineering in 1989, associate professor in 1993 and professor in 1996. He is also a professor of Computer Science and of the Whitaker Biomedical Engineering Institute and director of the Institute’s Fabrication and Lithography Facility in Clark Hall. He is the co-founder of the Johns Hopkins University Center for Language and Speech Processing. Between 2001 and 2003 he was the founding director of the ABET accredited undergraduate Computer Engineering program. In 1996 and 1997 he was a visiting professor of the computation and neural systems program at the California Institute of Technology. In 1989 and 1991 he was awarded the R.W. Hart Prize for his work on mixed analog/digital integrated circuits for space applications. He is the recipient of the 1995 and 1997 Myril B. Reed Best Paper Award and the 2000 IEEE Circuits and Systems Society, Darlington Best Paper Award. During the summer of 2001 he was a visiting professor in the department of systems engineering and machine intelligence at Tohoku University. In 2006, Prof. Andreou was elected as an IEEE Fellow and a distinguished lecturer of the IEEE EDS society. Andreou’s research interests include sensors, micropower electronics, heterogeneous microsystems, and information processing in biological systems. He is a co-editor of the IEEE Press book: Low-Voltage/Low-Power Integrated Circuits and Systems, 1998 (translated in Japanese) and the Kluwer Academic Publishers book: Adaptive Resonance Theory Microchips, 1998. He is an associate editor of IEEE Transactions on Circuits and Systems I.     

Merocyanine/C60 Planar Heterojunction Solar Cells: Effect of Dye Orientation on Exciton Dissociation and Solar Cell Performance

In this study the charge dissociation at the donor/acceptor heterointerface of thermally evaporated planar heterojunction merocyanine/C₆₀ organic solar cells is investigated. Deposition of the donor material on a heated substrate as well as post‐annealing of the complete devices at temperatures above the glass transition temperature of the donor material results in a twofold increase of the fill factor. An analytical model employing an electric‐field‐dependent exciton dissociation mechanism reveals that geminate recombination is limiting the performance of as‐deposited cells. Fourier‐transform infrared ellipsometry shows that, at temperatures above the glass transition temperature of the donor material, the orientation of the dye molecules in the donor films undergoes changes upon annealing. Based on this finding, the influence of the dye molecules’ orientations on the charge‐transfer state energies is calculated by quantum mechanical/molecular mechanics methods. The results of these detailed studies provide new insight into the exciton dissociation process in organic photovoltaic devices, and thus valuable guidelines for designing new donor materials.     

Preparation of Ring-Shaped Thermoelectric Legs from PbTe Powders for Tubular Thermoelectric Modules

Waste heat recovery—for example, in automotive applications—is a major field for thermoelectric research and future application. Commercially available thermoelectric modules are based on planar structures, whereas tubular modules may have advantages for integration and performance in the field of automotive waste heat recovery. One major drawback of tubular generator designs is the necessity for ring-shaped legs made from thermoelectric material. Cutting these geometries from sintered tablets leads to considerable loss of thermoelectric material and therefore high cost. Direct sintering of ring-shaped legs or tubes of thermoelectric material is a solution to this problem. However, sintering such rings with high homogeneity and density faces some difficulties related to the mechanical properties of typical thermoelectric materials such as lead telluride (PbTe)—particularly brittleness and high coefficient of thermal expansion. This work shows a process for production of thermoelectric rings made of p- and n-doped PbTe. Long tubes of PbTe have been sintered in a current-assisted sintering process with specially designed sintering molds, coated with a diffusion barrier, and finally cut into ring-shaped slices. To demonstrate the technology, a tubular thermoelectric module has been assembled using these PbTe rings.     

Thermoelectric Coolers with Sintered Silver Interconnects

The fabrication and performance of a sintered Peltier cooler (SPC) based on bismuth telluride with sintered silver interconnects are described. Miniature SPC modules with a footprint of 20 mm² were assembled using pick-and-place pressure-assisted silver sintering at low pressure (5.5 N/mm²) and moderate temperature (250°C to 270°C). A modified flip-chip bonder combined with screen/stencil printing for paste transfer was used for the pick-and-place process, enabling high positioning accuracy, easy handling of the tiny bismuth telluride pellets, and immediate visual process control. A specific contact resistance of (1.4 ± 0.1) × 10^?5 Ω cm² was found, which is in the range of values reported for high-temperature solder interconnects of bismuth telluride pellets. The realized SPCs were evaluated from room temperature to 300°C, considerably outperforming the operating temperature range of standard commercial Peltier coolers. Temperature cycling capability was investigated from 100°C to 235°C over more than 200 h, i.e., 850 cycles, during which no degradation of module resistance or cooling performance occurred.     

Nanocomposite Microcontainers with High Ultrasound Sensitivity

A water suspension of nanocomposite microcapsules with embedded ZnO nanoparticles in the capsule shell is reported. The microcapsule morphology is characterized by confocal microscopy, TEM, SEM, and AFM before and after ultrasound treatment. A remarkably high capsule sensitivity to ultrasound is evidenced, and it is observed to grow with increasing number of ZnO nanoparticle layers in the nanocomposite shell. This effect is correlated with the mechanical properties of microcapsules measured with AFM.     

Environmental monitoring aware routing: making environmental sensor networks more robust

Wireless Sensor Networks (WSNs) have a broad application range in the area of monitoring and surveillance tasks. Among these tasks, disaster detection or prevention in environmental scenarios is one typical application for WSN. Disasters may for example be forest fires, volcano outbreaks or flood disasters. Here, the monitored events have the potential to destroy the sensor devices themselves. This has implications for the network lifetime, performance and robustness. While a fairly large body of work addressing routing in WSNs exists, little attention has been paid to the aspect of node failures caused by the sensed phenomena themselves. This paper presents a proactive routing method that is aware of the node’s destruction threat and adapts the routes accordingly, before node failure results in broken routes, delay and power consuming route re-discovery. The performance of the presented routing scheme is evaluated and compared to OLSR based routing in the same scenario.     

Composition, structure, and mobility of water-acetonitrile mixtures in a silica nanopore studied by molecular dynamics simulations

To investigate the effect of the nanoscale confinement on the properties of a binary aqueous-organic solvent mixture, we performed molecular dynamics simulations of the equilibration of water-acetonitrile (W/ACN) mixtures between a cylindrical silica pore of 3 nm diameter and two bulk reservoirs. Water is enriched, and acetonitrile is depleted inside the pore with respect to the bulk reservoirs: for nominal molar (~volumetric) ratios of 1/3 (10/90), 1/1 (25/75), and 3/1 (50/50), the molar W/ACN ratio in the pore equilibrates to 1.5, 3.2, and 7.0. Thus, the relative accumulation of water in the pore increases with decreasing water fraction in the nominal solvent composition. The pore exhibits local as well as average solvent compositions, structural features, and diffusive mobilities that differ decidedly from the bulk. Water molecules form hydrogen bonds with the hydrophilic silica surface, resulting in a 0.45 nm thick interfacial layer, where solvent density, coordination, and orientation are independent of the nominal W/ACN ratio and the diffusive mobility goes toward zero. Our data suggest that solute transport along and across the nanopore, from the inner volume to the interfacial water layer and the potential adsorption sites at the silica surface, will be substantially different from transport in the bulk.     

Pummelos as Concept Generators for Biomimetically Inspired Low Weight Structures with Excellent Damping Properties

Natural materials often exhibit excellent mechanical properties. An example of outstanding impact resistance is the pummelo fruit (Citrus maxima) which can drop from heights of 10 m and more without showing significant outer damage. Our data suggest that this impact resistance is due to the hierarchical organization of the fruit peel, called pericarp. The project presented in the current paper aims at transferring structural features from the pummelo pericarp to engineering materials, in our case metal foams, produced by the investment casting process. The transfer necessitates a detailed structural and mechanical analysis of the biological model on the one hand, and the identification and development of adequate materials and processes on the other hand. Based on this analysis, engineering composite foam structures are developed and processed which show enhanced damping and impact properties. The modified investment casting process and the model alloy Bi57Sn43 proved to be excellent candidates to make these bio‐inspired structures. Mechanical testing of both the natural and the engineering structures has to consider the necessity to evaluate the impact of the different hierarchical features. Therefore, specimens of largely varying sizes have to be tested and size effects cannot be ignored, especially as the engineering structures might be upscaled in comparison with the natural role model. All in all, the present results are very promising: the basis for a transfer of bio‐inspired structural hierarchical levels has been set.     

Solid-phase epitaxy of undoped amorphous silicon by in-situ postannealing

The solid phase epitaxy (SPE) of undoped amorphous Si (a-Si) deposited on SiO₂ patterned Si(001) wafers by reduced pressure chemical vapor deposition (RPCVD) using a H₂-Si₂H₆ gas system was investigated. The SPE was performed by applying in-situ postannealing directly after deposition process. By transmission electron microscopy (TEM) and scanning electron microscopy, we studied the lateral SPE (L-SPE) length on sidewall and mask for various postannealing times, temperatures and a-Si thicknesses. We observed an increase in L-SPE growth for longer postannealing times, temperatures and larger Si thicknesses on mask. TEM defect studies revealed that by SPE crystallized epi-Si exhibits a higher defect density on the mask than at the inside of the mask window. By introducing SiO₂-cap on the sample with 180 nm Si thickness following postannealing at 570 °C for 5 h, the crystallization of up to 450 nm epi-Si from a-Si is achieved. We demonstrated the possibility to use this technique for SiGe:C heterojunction bipolar transistor (HBT) base layer stack to crystallize Si-buffer layer to widen the monocrystalline region around the bipolar window and to improve base link resistivity of the HBT.