X. Aymerich

On the breakdown statistics of very thin SiO2 films

In accordance with the idea that the degradation of the SiO2 network and the dielectric breakdown are intimately related, a new model to describe the breakdown statistics of thin SiO2 films is presented. The obtained distribution of failures has been found to provide very good fits of the experimental statistical data that correspond to both constant-current and constant-voltage stress experiments. The fundamental properties of the extreme-value distributions are preserved by the presented model and, what is more important, the two involved statistical parameters have a natural physical interpretation. Finally, the Monte Carlo method has been applied to simulate the degradation of the SiO2 film until breakdown, and this has been demonstrated to be a powerful technique for introducing second-order effects into the study of the breakdown statistics.

Electrical characterization of stressed and broken down SiO2 films at a nanometer scale using a conductive atomic force microscope

A conductive atomic force microscope (C-AFM) has been used to investigate the degradation and breakdown of ultrathin (<6 nm) films of SiO2 at a nanometric scale. Working on bare gate oxides, the conductive tip of the C-AFM allows the electrical characterization of nanometric areas. Due to the extremely small size of the analyzed areas, several features, which are not registered during macroscopic tests, are observed. In particular, before the oxide breakdown, switchings between different conduction states and sudden changes of conductivity have been measured, which have been related to the prebreakdown noise observed in conventional metal–oxide–semiconductor structures. Moreover, similar switchings have been also measured after the oxide breakdown, which have been related to the opening or closure of conduction channels between the electrodes. The C-AFM has also allowed the determination of the areas in which the degradation and breakdown take place. The results have shown that, although degradation takes...

A function-fit model for the soft breakdown failure mode

An empirical one parameter-based power law model for the leakage current through one or more soft breakdown spots in ultrathin (<5 nm) gate oxides is presented. Good fit to data can be obtained in nearly five decades of current from 0.5 to 5 V. In addition, it is shown that there exists a slight correlation between the parameters which describe the soft breakdown conduction characteristic and the stressing condition which triggers it.

Nanometer-scale oxidation of Si(100) surfaces by tapping mode atomic force microscopy

The nanometer‐scale oxidation of Si(100) surfaces in air is performed with an atomic force microscope working in tapping mode. Applying a positive voltage to the sample with respect to the tip, two kinds of modifications are induced on the sample: grown silicon oxide mounds less than 5 nm high and mounds higher than 10 nm (which are assumed to be gold depositions). The threshold voltage necessary to produce the modification is studied as a function of the average tip‐to‐sample distance.

Nondestructive multiple breakdown events in very thin SiO2 films

Several breakdown events and multilevel current fluctuations have been observed when ultrathin SiO2 films are subjected to constant‐voltage stresses. These breakdown events are sometimes reversible, and consist in a local change of conduction mechanism. This reversibility shows that no catastrophic thermal effects occur, and that the breakdown is only a local switching between two oxide conduction states of very different conductivities.

Point contact conduction at the oxide breakdown of MOS devices

Experiment and simulation are combined to demonstrate that, provided that thermal effects are limited, the dielectric breakdown of SiO/sub 2/ films in MOS devices opens atomic-size conduction channels (with radius in the 1 to 10 nm range) which behave as point contacts. Depending on the size of the breakdown spot, the conduction properties are either those of a Sharvin point contact or those of a quantum point contact.

Correlation between the nanoscale electrical and morphological properties of crystallized hafnium oxide-based metal oxide semiconductor structures

The relationship between electrical and structural characteristics of polycrystalline HfO2 films has been investigated by conductive atomic force microscopy under ultrahigh vacuum conditions. The results demonstrate that highly conductive and breakdown (BD) sites are concentrated mainly at the grain boundaries (GBs). Higher conductivity at the GBs is found to be related to their intrinsic electrical properties, while the positions of the electrical stress-induced BD sites correlate to the local thinning of the dielectric. The results indicate that variations in the local characteristics of the high-k film caused by its crystallization may have a strong impact on the electrical characteristics of high-k dielectric stacks.

SOFT BREAKDOWN FLUCTUATION EVENTS IN ULTRATHIN SIO2 LAYERS

When an ultrathin (<5 nm) oxide is subjected to electrical stress, several soft-breakdown events can occur prior to the final dielectric breakdown. After the occurrence of such failure events, the current–voltage (I–V) characteristic corresponds to the superposition of highly conductive spots and background conduction through the undegraded capacitor area. In this conduction regime, the application of a low constant voltage gives rise to large leakage current fluctuations in the form of random telegraph signal. Some of these fluctuations have been identified with ON/OFF switching events of one or more local conduction spots, and not with a modulation of their conductance. The experimental soft-breakdown I–V characteristics are shown to be better understood if the spot conduction is considered to be locally limited by the silicon electrodes and not by the oxide.

Modification of HF‐treated silicon (100) surfaces by scanning tunneling microscopy in air under imaging conditions

The modification of HF‐etched silicon (100) surface with a scanning tunneling microscope(STM) operated in air is studied for the first time in samples subjected to the standard HF etching without the follow‐up rinsing in H2O. The modifications are produced in air under normal STM imaging conditions (V t =−1.4 V and I t =2 nA). The simultaneous acquisition of topographical, current image tunneling spectroscopy and local barrier‐height images clearly shows that the nature of the modification is not only topographical but also chemical. The features produced with a resolution better than 25 nm are attributed to a tip‐induced oxidation enhanced by the presence of fluorine on the surface.

Nanometer-scale electrical characterization of stressed ultrathin SiO2 films using conducting atomic force microscopy

A conductive atomic force microscope has been used to electrically stress and to investigate the effects of degradation in the conduction properties of ultrathin (<6 nm) SiO2 films on a nanometer scale (areas of ≈100 nm2). Before oxide breakdown, switching between two states of well-defined conductivity and sudden changes of conductivity were observed, which are attributed to the capture/release of single charges in the defects generated during stress.