A. Scarpa

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.

Emerging oxide degradation mechanisms: stress induced leakage current (SILC) and quasi-breakdown (QB)

Abstract The reliability of thin oxides is a primary concern for the qualification of advanced CMOS, DRAMS and non volatile memory technologies. With the scaling down process the thickness of active dielectrics in CMOS, DRAMs and memory devices has steadily been reduced during the past years. The reduction of the oxide thickness has given rise to the onset of new phenomena for the viewpoint of reliability such as stress induced leakage current (SILC) and quasi-breakdown (QB). This paper intends to present a brief review of these emerging degradation processes which affect the reliability of ultra-thin oxides. The main characteristics and the physics underlying the SILC and QB will be discussed and illustrated with recent experimental results obtained on advanced technologies featuring thin gate dielectrics.

Stress induced leakage current in ultra-thin gate oxides after constant current stress

Abstract Constant current stress induced leakage currents are studied in very thin oxide devices, for both stress polarities. This current has been investigated for both positive and negative gate voltage measurements. Stress induced leakage current (SILC) physical nature has been studied and an interpretation has been proposed, considering local oxide weak spots with barrier height close to 1eV. The increasing rate of the SILC versus the injection dose has been studied and compared with the degradation positive charge build-up rate, observed in the same oxide, indicating that hole trapping and stress induced leakage current could have the same physical origin.

Electrically and radiation induced leakage currents in thin oxides

Abstract Stress Induced Leakage Current (SILC) has been recognized as a topic of concern in flash memory reliability. It is a veritable failure mechanism, occurring long before oxide breakdown and, hence, limiting oxide lifetime. The physical origin and mechanisms of SILC have not yet been clearly understood and several open points to discussion remain. In this work the electrical characteristics of SILC have been studied and an empirical reliability model for ultra-thin gate oxide has been proposed. Moreover, ionizing radiation effects in leakage current generation have been analyzed and compared to electrical SILC.

Modifications of Fowler-Nordheim injection characteristics in /spl gamma/ irradiated MOS devices

In this work we have investigated how gamma irradiation affects the tunneling conduction mechanism of a 20 nm thick oxide in MOS capacitors. The radiation induced positive charge is rapidly compensated by the injected electrons, and does not impact the gate current under positive injection after the first current-voltage measurement. Only a transient stress induced leakage current at low gate bias is observed. Instead, a radiation induced negative charge has been observed near the polysilicon gate, which enhances the gate voltage needed for Fowler-Nordheim conduction at negative gate bias. No time decay of this charge has been observed. Such charges slightly modify the trapping kinetics of negative charge during subsequent electrical stresses performed at constant current condition.

Degradation of electron irradiated MOS capacitors

Abstract We have investigated the degradation of MOS structure due to high energy electron irradiation as a function of radiation dose and gate bias applied during the irradiation. Devices have been characterized by current–voltage measurements, in order to study charge accumulation also at the gate interface. Three types of oxide charge have been observed: the unstable positive charge, due to trapped holes induced by the electron irradiation; the negative charge in the oxide bulk, deriving from capture of electrons injected during electrical measurements in radiation generated traps; and border traps, at both oxide interfaces.

Low-field current on thin oxides after constant current or radiation stresses

Abstract Stress induced leakage current after electrical stress and radiation induced leakage current after irradiation have been studied on ultra-thin gate oxides, 4 nm and 4.4 nm thick. Differences between positive and negative leakage currents, which is very large (almost one order of magnitude) if taken at a given gate-to-substrate voltage, almost disappears when currents are plotted as a function of the oxide electric field, if this field is correctly evaluated. We suggest that residual asymmetries are due to a non-homogeneous distribution of traps across the oxide. These asymmetries of stress induced leakage current and radiation induced leakage current have been studied for electrical stresses at different polarities and for positive and negative gate bias voltages applied during irradiation, respectively. Electrical stresses have been performed by using Fowler–Nordheim injection under positive or negative gate voltages. Radiation stresses have been performed by using an 8 MeV electron beam and devices have been biased at positive or negative gate-to-substrate voltages during irradiation.

Instability of post-Fowler-Nordheim stress measurements of MOS devices

Abstract MOS test device electrical measurements (C-V, charge pumping), performed after accelerated ageing stress based on Fowler-Nordheim injection, strongly modify the net oxide charge. This can lead to an incorrect evaluation of the oxide long-term stability. We ascribe the observed instabilities to partial annealing of unstable slow states and trapped holes which lie close to the Si SiO 2 interface. Finally we propose and compare different experimental approaches to get more “stable” measurements.

On the degradation kinetics of thin oxide layers

Abstract After Fowler–Nordheim stress, since SiO 2 charge trapping occurs up to final dielectric breakdown, oxide layers degrade. The degradation features of very thin gate oxide films have been studied, after constant current stress. Furthermore charge build up in the oxide bulk and in the anodic/cathodic oxide regions has been investigated. For oxides thicker than 5 nm it was possible to provide a physical interpretation of the experimental results, showing a power law of charge trapping kinetics as a function of stress level. For thinner oxides, it has been shown that the degradation mechanisms can be very different and the proposed interpretation is no longer valid.

A new model of tunnelling current and SILC in ultra-thin oxides

We propose a new double-box model of the oxide conduction band of MOS devices, aiming to quantitatively predict the gate current across ultra-thin oxides. Simulations nicely fit the experimental quantum oscillations of the gate current in the Fowler-Nordheim tunnelling regime. The oscillation period of the gate current has been empirically correlated with the oxide thickness. The low-field stress induced leakage current can be fitted by our model as well, by inserting oxide traps mediating an inelastic trap-assisted tunnelling.