L. Bandiera

Noise characteristics of radiation-induced soft breakdown current in ultrathin gate oxides

We have investigated new aspects of the gate leakage current due to radiation-induced soft breakdown (RSB) of thin oxides subjected to heavy-ion irradiation. Temperature and noise characteristics of RSB on MOS capacitors with 3- and 4- nm MOS oxides have been experimentally investigated. We have developed an empirical law to describe quantitatively the temperature dependence of the RSB current. A small activation energy has been found by using an Arrhenius relation, in agreement with the RSB tunneling conduction mechanism. The RSB variation at high temperature has been only estimated, as measurements of RSB oxides easily produced catastrophic breakdown. We have studied the RSB noise and identified different contributions to the characteristic random telegraph noise, correlated with the trapping and conduction characteristics of the RSB spots. An original model has been developed that successfully describes the different probability distributions of the current fluctuations that cannot be simulated by using previous models, such as those based on Levy or Gaussian distributions. Finally, a correlation was established between the shape of the fluctuation distribution and the degradation level of the oxide.

Soft breakdown current noise in ultra-thin gate oxides

In this work we studied the soft breakdown (SB) in ultra-thin gate oxides (<3 nm) subjected to constant current stress. SB current derives from the superposition of several random telegraph signals noises with different time constants and amplitudes. Such fluctuations derive from the conductance modulation of a damaged region inside the oxide layer, due to the electrical stress. We found that the current noise power density follows the 1/fα power law (with α between 1 and 2) over a wide range of frequency (1 Hz–100 kHz). Only at frequency smaller than 1–10 Hz a possible deviation from this low cannot be excluded. Moreover, the discrete fluctuations typical of SB are statistically independent events at least over time periods around hundreds of seconds, according to a Poisson process. This result suggest that electron trapping/detrapping in defect sites near or inside the SB conductive path can be claimed as responsible for such conductance modulation.

A novel approach to quantum point contact for post soft breakdown conduction

A new approach based on quantum point contact (QPC) is presented to describe the electrical characteristics of soft breakdown in electrically stressed MOS capacitors. By using the new analytical model it is possible to extrapolate the barrier profile of the SB spot from experimental Ig-Vg curves. Barrier height and width are the two representative parameters of the barrier. Results obtained for a variety of oxide thickness and gate areas show that the SB current can be controlled either by the barrier width or barrier height depending on the leakage intensity.

A Purely Electronic Method to Measure Transfection Efficiency in a Single-Cell Electroporation Biochip

Many fundamental researches in molecular biology deal with molecules which need to be inserted into the plasmatic membrane of living (cultured) cells. Working with single cells represents an important improvement for some of these studies, since it allows experiments where the behavior of a modified cell in a non-modified context can be studied. This result can be achieved with a Multi Electrode Array which can locally break down the cell membrane, allowing molecules to enter the cell and acting as a multi-site, single-cell, electroporation biochip. Electrochemical impedance spectroscopy (EIS) measurements, coupled with an accurate electrical model of the biochip and cell interface, are used to detect the presence of a cell above the biochip microelectrode. This allows fully automatic single cell electroporation.

Degradation of low frequency noise and DC characteristics on MOSFETs and its correlation with SILC

In this work we have addressed oxide and interface degradation as the mechanisms affecting MOSFET performance in terms of transconductance (g/sub m/), saturation current (I/sub ds,/SAT) and threshold voltage (V/sub th/) before SB (soft breakdown) and HB (hard breakdown). We focused our attention on three points: 1) how interface and oxide trap generation modify the DC MOSFET characteristics; 2) the drain current noise during stress; 3) the correlation between the degradation of DC and noise characteristics and the traps commonly related to SILC (stress induced leakage current) conduction.

Detrended fluctuation analysis of the soft breakdown current

We have addressed the problem of the gate current fluctuations after soft breakdown in thin gate oxides, mainly focused on the intrinsic instability of SB conductance. We found that heavily degraded oxides exhibit large instability and more complex fluctuations. Finally we have developed a model to describe the gate distribution of the gate current fluctuations by taking in account such conductance instability.

Using a cell manipulation biochip to investigate the adhesion characteristics of single mammalian cells

Single cultured cells can be genetically modified with a silicon Multi Electrode Array which can locally and transiently break down the cell membrane, allowing exogenous molecules to enter the cell. An accurate electrical model of the biochip-solution-cell system can be coupled to a statistical method in order to evaluate the probability of correctly detecting the presence (or absence) of a cell over a given electrode.

ELECTRICAL DETECTION OF CELL ADHESION IN A SINGLE-CELL ELECTROPORATION BIOCHIP

Working a Single cells represents an important type of analysis for the study of the living organism, so new technologies and new methodologies have been developed in these years with this aim. In this work, electrochemical impedance spectroscopy (EIS) measurements are used to detect the presence of a cell above a biochip microelectrode used for single-cell electroporation, thus allowing fully automatic, multisite single cell, electroporation.

Electrical measurement of adhesion and viability of living cells with a silicon chip

Silicon biochips are used to study, stimulate and manipulate single (mammalian) cells, whose viability and adhesion are usually studied via subjective, non quantitative, microscope observations. In this work an accurate equivalent model of the biochip/cell system has been developed and successfully applied to experiments, which have permitted to quantitatively correlate cell health status and changes in its membrane morphology to the measured impedance of such system.

Logistic modeling of progressive breakdown in ultrathin gate oxides

The sigmoidal behavior exhibited by the current-time characteristics of constant voltage stressed MOS capacitors with ultrathin oxides is ascribed to a self-constrained increase of the leakage sites population that assist the conduction process between the electrodes. To analytically describe this dynamical process we consider a classical model of population growth theories such as the Verhulst differential equation. The role played by the background tunneling current in the detection of the breakdown event is also discussed.