UHV CAFM characterization of high-k dielectrics: Effect of the technique resolution on the pre- and post-breakdown electrical measurements

In this work, a Conductive Atomic Force Microscopy (CAFM) working in contact mode has been used to compare the measured electrical properties and breakdown (BD) on ultra thin high-k dielectrics, when different environmental conditions are used. In particular, the effect of the environment on the conductivity measurements, the lateral resolution in current images and the lateral propagation of the BD event will be analyzed in air, dry nitrogen (N₂) and Ultra High Vacuum (UHV).     

Local current fluctuations before and after breakdown of thin SiO2 films observed with conductive atomic force microscope

Very thin SiO₂ films (3–6 nm) have been characterized with a conductive atomic force microscope (C-AFM). The set-up allows the electrical characterization of 30–50 nm² areas, which are of the order of single breakdown spots. Voltage ramps have been repeatedly applied to induce the degradation. On these spots, the phenomenology observed is quite similar to that during conventional electrical tests. In particular, on–off fluctuations before and after breakdown are reported on single breakdown spots. The results confirm the C-AFM as a suitable tool for the analysis of the gate oxide electrical properties and degradation dynamics at a nanometer scale.     

Pre-breakdown noise in electrically stressed thin SiO2 layers of MOS devices observed with C-AFM

A conductive atomic force microscope (C-AFM) has been used to analyse the degradation stage (before breakdown, BD) of ultrathin (<6 nm) films of SiO₂ at a nanometer 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 analysed areas, several features, which can be masked by the current that flows through the overall test structure during standard electrical tests, are observed. In particular, switching between different conduction states and sudden changes of conductivity have been measured during ramped voltage tests, which have been related to the trapping and detrapping of single electronic charges in the defects generated during the electrical stress. This phenomenon, which has been observed during constant voltage stresses in the form of random telegraph signals, has been associated to the pre-breakdown noise measured in poly-gated structures. The C-AFM has also allowed to directly measure the I–V characteristics of the fluctuating spot.     

New insights on the post-BD conduction of MOS devices at the nanoscale

We have combined standard electrical tests with conductive-atomic force microscopy experiments to investigate the conduction of MOS devices after the dielectric breakdown (BD) of the SiO/sub 2/ gate oxide. In particular, the influence of the conduction nanometer scale parameters on the overall device post-BD current-voltage characteristics has been analyzed. The results show a nonuniform conductivity of the oxide at the BD area and that the total current flowing through the device is mainly driven by a very small fraction of that region.     

Subsectional-polynomial E-pulse synthesis and application to radartarget discrimination

A new family of extinction-pulses (E-pulses), called subsectional-polynomial E-pulses, is presented. This new type of E-pulse is constructed by choosing polynomials of degree Q as subsectional basis-functions in the E-pulse expansion. The main feature of this family of E-pulses is that the waveforms are continuous and smooth. Several topics concerning the E-pulse technique are investigated, such as: insensitivity to the exact number of natural modes present in the target response; aspect-angle independence; and effects of additive white Gaussian noise. Numerical results, using the response of a thin cylinder and a sphere, show that the subsectional-polynomial E-pulses improve the results obtained using subsectional-rectangular E-pulses     

Time-varying electromagnetic-media modelling with TLM method

A new TLM node is presented for modelling of media with time-varying electromagnetic properties. The node is derived by using a modified Thevenin's theorem that deals with variations in the node parameters. The successful behaviour of the node is shown by its application to a highly time-varying system.     

Effects of the Localization of the Charge in Nanocrystal Memory Cells

In this paper, we present a peculiar characteristic of nanocrystal (NC) memory (NCM) cells: The programming (P) windows measured in linear and subthreshold regions are different. A floating-gate flash memory cell with a similar structure does not show the same behavior, and the P window (PW) is independent of the current level of the extrapolation, as expected. By performing 2-D TCAD simulations, we demonstrated that this characteristic of NCM cells is due to the localization of the charge into the NCs. We investigate the correlation between the difference of the PWs in linear and subthreshold regions and the number, width, and position of the NCs.     

Characterising the surface roughness of AFM grown SiO2 on Si

The reliability of AFM grown SiO₂ as a gate oxide needs to be examined if nanodevices fabricated from the oxide are to be integrated into standard microelectronic technology. In this article we present our preliminary results on AFM fabrication and topographical characterisation of large area oxide, electrical characterisation is to follow. Roughness is the central issue of this work due to its importance in relation to the quality of ultra thin dielectrics.