G. Ghidini

Radiation induced leakage current and stress induced leakage current in ultra-thin gate oxides

Low-field leakage current has been measured in thin oxides after exposure to ionising radiation. This Radiation Induced Leakage Current (RILC) can be described as an inelastic tunnelling process mediated by neutral traps in the oxide, with an energy loss of about 1 eV. The neutral trap distribution is influenced by the oxide field applied during irradiation, thus indicating that the precursors of the neutral defects are charged, likely to be defects associated with trapped holes. The maximum leakage current is found under zero-field condition during irradiation, and it rapidly decreases as the field is enhanced, due to a displacement of the defect distribution across the oxide towards the cathodic interface. The RILC kinetics are linear with the cumulative dose, in contrast with the power law found on electrically stressed devices.

A model of radiation induced leakage current (RILC) in ultra-thin gate oxides

An analytical model of Radiation Induced Leakage Current (RILC) has been developed for ultra-thin gate oxides submitted to high dose ionizing radiation. The model is based on the solution of the Schrodinger equation for a simplified oxide band structure, where RILC occurs through electron trap-assisted tunneling. The values of the model parameters have been calibrated by comparing the transmission probabilities obtained in this model with those obtained through the WKB method in the actual oxide band structure. No free fitting parameter has been introduced, and all physical constant values have been selected within the values found in literature. Different trap distributions have been considered as candidates, but the comparison between simulated and experimental curves have indicated that a double gaussian distribution in space and in energy grants the best fit of the experimental results for different ionizing particles, oxide fields during irradiation, radiation doses, and oxide thickness. Excellent matching has been found for both positive and negative RILC by using a single trap distribution. The trap density linearly increases with the radiation dose and decreases with the oxide field during irradiation. The trap distribution is spatially symmetrical in the oxide, centered in the middle of the oxide thickness, and is not modified as the cumulative dose increases.

Giant Random Telegraph Signals in Nanoscale Floating-Gate Devices

The magnitude of a random telegraph signal (RTS) in nanoscale floating-gate devices has been experimentally investigated as a function of carrier concentration. Discrete current switching, which is caused by a single trap, has been found to be almost one order of magnitude higher with respect to what was predicted by the classical theory of carrier number and correlated mobility fluctuations. Nevertheless, the trap signature well fits the typical SiO2 trap spectroscopy. In addition, the rigid shift between the transfer curves related to filled- and empty-trap state, together with the normalized current fluctuation dependence on the channel carrier density, suggests that a pure number fluctuation is the correct theoretical interpretative framework. Thus, we propose a possible physical explanation for such a giant RTS on the basis of a quasi-1-D current filamentation.

Accelerated wear-out of ultra-thin gate oxides after irradiation

We have investigated how ultra-thin gate oxides subjected to heavy ion irradiation react to a subsequent electrical stress performed at low voltages. Even in devices exhibiting small (or even no) increase of the gate current after irradiation, the time-to-breakdown is substantially reduced in comparison with unirradiated samples due to the onset of a soft or hard breakdown, in contrast with previous results found on thicker oxides. In fact, we have demonstrated that the radiation damage acts as a seed for further oxide degradation by electrical stress during the device operating life. The accelerated oxide wear-out depends on the linear energy transfer (LET) coefficient of radiation source.

A model of the stress induced leakage current in gate oxides

A new quantitative model of the stress induced leakage current (SILC) in MOS capacitors with thin oxide layers has been developed by assuming the inelastic trap-assisted tunneling as the conduction mechanism. The oxide band structure has been simplified by replacing the trapezoidal barrier with two rectangular barriers. An excellent agreement between simulations and experiments has been found by adopting a trap distribution Gaussian in space and in energy. Only minor variations of the trap distribution parameters were observed by increasing the injected charge during electrical stress, indicating that oxide neutral defects with similar characteristics are generated at any stage of the stress.

Drain current decrease in MOSFETs after heavy ion irradiation

In this work, we have focused our attention on MOSFETs, which are the real basic elements of all CMOS applications. We have studied the immediate and latent effects produced by heavy ion irradiation on MOSFETs with ultrathin gate oxide, even after electrical stresses subsequent to irradiation. We found that a single ion can generate a physically damaged region (PDR) localized in the Si-SiO/sub 2/ interface, which may hamper the surface channel formation. In order to generate a PDR the ion hit must be close enough to MOSFET borders, i.e., in correspondence with the STI or the LDD spacer. Consequently, if both MOSFET W and L are large enough only few ion hits may give place to a PDR, mitigating the radiation damage. Finally we have developed an original model to describe the impact of the PDR on channel conductance in the ohmic linear region. On the basis of this model, we predict a PDR size around 0.2-1 /spl mu/m.

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.

A new model of gate capacitance as a simple tool to extract MOS parameters

This paper tackles the difficult task to extract MOS parameters by a new model of the gate capacitance that takes into account both poly-Si depletion and charge quantization and includes temperature effects. A new fast and iterative procedure, based on this simplified self-consistent model, will be presented to estimate simultaneously the main MOS system parameters (oxide thickness, substrate, and poly-Si doping) and oxide field, surface potentials at the Si/SiO/sub 2/ and at the poly-Si/SiO/sub 2/ interfaces. Its effectiveness will be demonstrated by comparing oxide field and oxide thickness to those extracted by other methods proposed in the literature. Moreover, these methods are critically reviewed and we suggest improvements to reduce their errors. The agreement between CV simulation and experimental data is good without the need of any free parameter to improve the fitting quality for several gate and substrate materials combinations. Finally, a simple law to estimate substrate and poly-Si doping in n/sup +//n/sup +/ MOS capacitors from CV curves is proposed.

Experimental method for the determination of the energy distribution of stress-induced oxide traps

An experimental procedure for the determination of the energy distribution of oxide neutral traps is presented, showing the evolution of the stress-induced damage as a function of Fowler-Nordheim stress fluence and field. It is shown that the traps are mainly distributed around 2 eV from the oxide conduction band. Results are presented for different oxide technologies, investigating the effect of oxide nitridation and growth conditions on the trap energy distribution.

Collapse of MOSFET drain current after soft breakdown

Gate-oxide soft breakdown (SB) can have a severe impact on MOSFET performance even when not producing any large increase of the gate leakage current. The SB effect on the MOSFET characteristics strongly depends on the channel width W: drain saturation current and MOSFET transconductance dramatically drop in transistors with small W after SB. As W increases, the SB effect on the drain current fades. The drain saturation current and transconductance collapse is due to the formation of an oxide defective region around the SB spot, whose area is much larger than the SB conductive path. Similar degradation can be observed even in heavy ion irradiated MOSFETs where localized damaged oxide regions are generated by the impinging ions without producing any increase of gate leakage current.