Quantum-mechanical modeling of accumulation layers in MOS structure

An original method is used for the quantum-mechanical modeling of n-type silicon accumulation layers. Unlike previous methods, which were only valid near 4.2 K, the approach is valid up to room temperature and beyond. The self-consistent results obtained are compared with those of the standard classical model for the accumulation layer, and the differences between them are found to be relevant for the modeling of important device applications. The dependences of the accumulation voltage drop and effective F-N (Fowler-Nordheim) barrier height on oxide electric field and substrate dopine are reported. Experimental F-N current-voltage characteristics of production-quality MOS capacitors are used to validate the quantum results and to show that the standard classical model is not adequate even if the barrier height is considered as a fitting parameter. Approximate analytical expressions giving the semiconductor voltage drop and the effective F-N barrier height as a function of oxide field and substrate doping are derived for <100> and <111> n-type silicon at 77 and 300 K     

Advanced electrical-level modeling of EEPROM cells

Conventional modeling of floating-gate electrically erasable programmable read-only memory (EEPROM) cells is shown to be inadequate to correctly evaluate the tunnel current flowing through the MOS injector during programming, essentially because of relevant quantum phenomena taking place at the cathode semiconductor-oxide interface. An electrical-level model incorporating numerical analysis of such effects is developed and discussed. The model is validated by comparing results of the simulations with experimental data obtained with EEPROM cells. The model of the MOS injector has been implemented in the circuit simulator SPICE     

Dynamic effects in the detection of bridging faults in CMOS ICs

Dynamic effects in the detection of bridging faults in CMOS circuits are taken into account showing that a test vector designed to detect a bridging may be invalidated because of the increased propagation delay of the faulty signal. To overcome this problem, it is shown that a sequence of two test vectors < T ₀, T ₁ >, in which the second can detect a bridging fault as a steady error, can detect the fault independently of additional propagation delays if T₀ initializes the faulty signal to a logic value different from the fault-free one produced by T ₁. This technique can be conveniently used both in test generation and fault simulation. In addition, it is shown how any fault simulator able to deal with FCMOS circuits can be modified to evaluate the impact of test invalidation on the fault coverage of bridging faults. For any test vector, this can be done by checking the state of the circuit produced by the previous test vector.     

Elements concentration analysis in groundwater from the North Serra Geral aquifer in Santa Helena-Brazil using SR-TXRF spectrometer

In this work the analysis of elements concentration in groundwater was performed using the synchrotron radiation total-reflection X-ray fluorescence (SR-TXRF) technique. A set of nine tube-wells with serious risk of contamination was chosen to monitor the mean concentration of elements in groundwater from the North Serra Geral aquifer in Santa Helena, Brazil, during 1 year. Element concentrations were determined applying a SR-TXRF methodology. The accuracy of SR-TXRF technique was validated by analysis of a certified reference material. As the groundwater composition in the North Serra Geral aquifer showed heterogeneity in the spatial distribution of eight major elements, a hierarchical clustering to the data was performed. By a similarity in their compositions, two of the nine wells were grouped in a first cluster, while the other seven were grouped in a second cluster. Calcium was the major element in all wells, with higher Ca concentration in the second cluster than in the first cluster. However, concentrations of Ti, V, Cr in the first cluster are slightly higher than those in the second cluster. The findings of this study within a monitoring program of tube-wells could provide a useful assessment of controls over groundwater composition and support management at regional level.     

Fabrication of High-Aspect Ratio Nanogratings for Phase-Based X-Ray Imaging

Diffractive optical elements such as periodic gratings are fundamental devices in X-ray imaging – a technique that medical, material science, and security scans rely upon. Fabrication of such structures with high aspect ratios at the nanoscale creates opportunities to further advance such applications, especially in terms of relaxing X-ray source coherence requirements. This is because typical grating-based X-ray phase imaging techniques (e.g., Talbot self-imaging) require a coherence length of at least one grating period and ideally longer. In this paper, the fabrication challenges in achieving high-aspect ratio nanogratings filled with gold are addressed by a combination of laser interference and nanoimprint lithography, physical vapor deposition, metal assisted chemical etching (MACE), and electroplating. This relatively simple and cost-efficient approach is unlocked by an innovative post-MACE drying step with hexamethyldisilazane, which effectively minimizes the stiction of the nanostructures. The theoretical limits of the approach are discussed and, experimentally, X-ray nanogratings with aspect ratios >40 are demonstrated. Finally, their excellent diffractive abilities are shown when exposed to a hard (12.2 keV) monochromatic X-ray beam at a synchrotron facility, and thus potential applicability in phase-based X-ray imaging.     

Microcomputer system for ion microscopy digital imaging and processing

Analytical ion microscopy is a powerful tool for biological tissue analysis as it allows direct chemical distribution imaging, even at low element concentrations. A microcomputer based digital imaging system achieving acquisition at low light level is presented. It includes a high sensitivity video camera connected to a specialized image processor subsystem. Acquired images consist of 512 times 512 pixels with 8 bits accuracy. Real-time image processing software has been implemented so that image processing may be performed on-line. Image processing software allows off-line image manipulation and correlation for biological interpretation of elemental mapping images. System capabilities are illustrated by a study of stable and radio iodine mapping in rat thyroid tissue.     

Four modified sodium alginate/carboxymethylcellulose blends for prednisone delivery

Polysaccharides have been widely used for the development of drug delivery systems. These systems can be physicochemically modified to enhance their stabilities and control their drug release profiles. However, such modifications cannot alter their biocompatibility or toxicity. Herein, four structurally modified sodium alginate/carboxymethylcellulose blends are synthesized and loaded with prednisone, and the effects of the modifications on their hydrolytic degradation rates, biocompatibilities, toxicities, and drug release profiles are investigated. All the blends are ionically cross-linked with Ca²⁺ and Fe³⁺. Blend 1 is not modified further, blend 2 is additionally reinforced with 8% w/w of cellulose nanocrystals, blend 3 is treated with epoxidized linseed oil to develop a hydrophobic layer, and blend 4 is chemically cross-linked with epichlorohydrin. Blends 2 and 4 exhibit similar physicochemical characteristics, appropriate hydrolytic degradation rates and drug release patterns, as well as biocompatibility and non-toxicity. In-vitro studies using the osteoblasts and CAM assay demonstrate that blends 2 and 4 are also biocompatible and non-toxic. In contrast, blend 1 exhibits the highest drug release rate, followed by blend 3.     

Improving performance and reliability of NOR-Flash arrays by using pulsed operation

Pulsed Operation (PO) is a new writing methodology based on a sequence of high voltage ultra-short pulses allowing of improving both performance and reliability of Flash memories. Measurements performed on 4Mb NOR-Flash test chips show the features of this methodology. Reliability, and in particular data retention, is shown to improve significantly for a whole array of cells with respect to standard FN writing operations that are based on sequences of box-shaped low voltage long pulses. Results show that PO allows for obtaining very compact threshold voltage distributions improving read margins and reducing the use of soft-programming techniques.