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Ionizing radiation effects on MOSFET thin and ultra-thin gate oxides
No full textWe briefly review the most important degradation mechanisms affecting ultra-thin gate oxides after exposure to ionizing radiation. One of most crucial issues for device lifetime is the gate leakage current measured after irradiation, that may appear as radiation induced leakage current (RILC) and radiation soft breakdown (RSB). Other important issues are the early breakdown of irradiated oxides, even when stressed at low gate voltages, and the decrease of the drain current on irradiated ultra-thin gate oxide MOSFETs. Finally, we show the effects of irradiation on the thin tunnel oxides of floating gate memory arrays
Data retention of irradiated FG memories
No full textFloating Gate (FG) memories, and in particular Flash,
are the most important player in nowadays nonvolatile
memory arena [1]. The basic structure of all FG
memories relies on an insulated polysilicon layer (that is,
the FG) interposed between the substrate and the Control
Gate, CG (see Fig.1). By accumulating a net charge in
the FG we can modify the threshold voltage VTH of the
FG transistor, thus storing a bit of information or more.
Electrons or holes in the insulated FG cannot escape
from it (at least in principle), hence granting a permanent
storage of information. Electrons and holes can be
injected (emitted) in (from) the FG by using Channel Hot
Electron (CHE) injection or Fowler-Nordheim (F-N)
tunneling [1][2]. Flash memories feature extremely high
density, while maintaining good speed and reliability, but
also a complex control circuitry.
Single Event Effect (SEE) performances of Flash
memories has been studied by several authors [3][4][5].
The most radiation sensitive part of commercial Flash
memories is the complex circuitry external to the FG
memory cell array [3]. For converse, the loss of the
charge stored in the floating gate of a programmed cell
and the consequent threshold voltage shift ÄVTH have
been less frequently investigated in literature [6][7][8].
By using specially designed instrumentation and
devices, we have recently shown that FG charge loss
upon heavy ion irradiation is not negligible [7][8], even
when it does not lead to a read error at the circuit output.
The charge loss subsequent to a single heavy ion strike
appears to be due to two parallel mechanisms. The first is
a prompt one, taking place in times shorter than those
elapsed between irradiation and measurement, and it
appears as the responsible for the main part of the charge
loss [8]. The second mechanism, which is the main
subject of this paper, is active over long times (days and
weeks) and is responsible for the slow discharge of some
of the hit FGs [8]. In this context, the aim of this paper is
to investigate the long-term retention issues in two
advanced Flash memory technologies submitted to heavy
ion irradiation
Variability in FG memories performance after irradiation
No full textEvery time a heavy ion crosses a programmed Floating Gate (FG) in a nonvolatile memory array, it quickly discharges the FG and it produces a number of defects in the tunnel oxide, depending on its linear energy transfer (LET). These defects can build up a multitrap assisted tunneling path which allows electrons stored in the FG to escape it, every time the FG is programmed after irradiation [this phenomenon is called radiation-induced leakage current (RILC)]. We are showing by using retention and gate stress experiments that RILC has peculiar erratic characteristics, similar to those found for stress-induced leakage current (SILC). The erratic behavior is due to changes in the occupation states on defects, resulting in orders-of-magnitude changes in the tiny (≪1 fA) current responsible for the FG discharge
Micro breakdown in small-area ultra-thin gate oxide
No full textPurpose of this work was to study the gate oxide leakage current in small area MOSFETs. We stressed about 300 nMOSFETs with an oxide thickness tOX=3.2 nm by using a staircase gate voltage. We detected the oxide breakdown at an early stress stage, by measuring the leakage current at low fields during the stress. The gate leakage of stressed devices is broadly distributed, but two well-defined current regimes appear, corresponding to currents larger than 1mA or smaller than 100pA, respectively. We focused our attention on the small current regime, which shows all the electrical characteristics typical of the soft breakdown, with the noticeable exception of the current intensity that is much smaller than usually reported in literature, being the average leakage around 40pA at VG=+2V. For this reason, we introduce the oxide micro breakdown. The leakage kinetics during stress, the gate-voltage characteristics of stressed devices, and the breakdown statistical distributions are in agreement with the formation of a single conductive path across the oxide formed by few oxide defects. Just two positively charged traps can give rise to a gate leakage comparable to those experimentally found, as evaluated by using an new original model of Double Trap Assisted Tunneling (D-TAT) developed ad hoc
Subattoampere current induced by single ions in silicon oxide layers of nonvolatile memory cells
No full textA single ion impinging on a thin silicon dioxide layer generates a number of electron/hole pairs proportional to its linear energy transfer coefficient. Defects generated by recombination can act as a conductive path for electrons that cross the oxide barrier, thanks to a multitrap-assisted mechanism. We present data on the dependence of this phenomenon on the oxide thickness by using floating gate memory arrays. The tiny number of excess electrons stored in these devices allows for extremely high sensitivity, impossible with any direct measurement of oxide leakage current. Results are of particular interest for next generation devices
Radiation damage on dielectrics: Single event effects
No full textSingle event effects are due to ionizing particles impinging on sensitive circuit regions, directly or as a by-product of nuclear reactions. Atmospheric neutrons originating from the interaction of cosmic rays with the outer layers of the atmosphere, alpha particles from radioactive contaminants at sea level, and heavy ions in space continuously hit electronic devices, causing a plethora of effects, some of which are governed by the properties and geometry of the dielectric layers used in the struck devices. This contribution will review these effects, focusing first on the thin gate dielectrics used in complementary metal-oxide semiconductor circuits and then on tunnel oxide of floating-gate memories
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A fully electronic sensor for the measurement of cDNA hybridization kinetics
Full text linkIon sensitive field effect transistors (ISFET) are candidates for a new generation of fully electrical DNA sensors. To this purpose, we have modified ISFET sensors by adsorbing on their Si(3)N(4) surface poly-L-lysine and single (as well as double) stranded DNA. Once coupled to an accurate model of the oppositely charged layers adsorbed on the surface, the proposed sensor allows quantitatively evaluating the adsorbed molecules densities, as well as estimating DNA hybridization kinetics
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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