Alessandro Diligenti

Electromigration and low-frequency resistance fluctuations in aluminum thin-film interconnections

Low-frequency noise spectra originating from resistance fluctuations in Al films during electromigration were measured in the absolute temperature and current density intervals<tex>327 \leq T \leq 396</tex>K and<tex>1.34 \times 10^{6} \leq j \leq 2.22 \times 10^{6}</tex>A/cm². The values of S<inf>R</inf>, the resistance power spectral density, at 20 × 10^-3Hz allowed the construction of an Arrhenius plot from which a grain-boundary activation energy value of about 0.6 eV was deduced. This value lies in the range of values found by other authors using different techniques. A first attempt to model the observed dependence of S<inf>R</inf>on<tex>j</tex>and<tex>T</tex>is also described.

Electromigration detection by means of low-frequency noise measurements in thin-film interconnections

Low-frequency noise power spectral densities associated with displacements of atoms caused by electromigration were measured on Al-Si (1 percent) resistors at various current densities in the frequency range 5 × 10-2 ÷ 2Hz. The temperature of the resistors was the one set by the current density itself, all the samples having been tested at room temperature. After noise measurement, each resistor was subjected to the current density at which the measurement was performed up to failure. The total electromigration power, measured in the frequency interval 0.1 ÷ 1 Hz was plotted as a function of the failure time for three different series of resistors obtained by means of an RF sputtering process. The measurement system and the characteristic features of the electromigration spectra are described.

Micromachined silicon suspended wires with submicrometric dimensions

Abstract Silicon suspended wires with a cross section of about 0.25 μm 2 have been fabricated by means of a simple process, starting from a SOI n-type wafer. Standard lithography, isotropic and anisotropic etchings were used. The process requires only two masks with a minimum dimension of 4 μm. After the definition of the suspended structures a further oxidation step was performed in order to reduce the silicon core dimensions. Particular care was devoted to the rinsing step to avoid any sticking problem to the substrate.

A study of electromigration in aluminum and aluminum-silicon thin film resistors using noise technique

Abstract Aluminum and aluminum-silicon (1%) thin film resistors, with and without an Si 3 N 4 passivating layer, were characterized by means of noise measurements in a low-frequency range (10 mHz-1 Hz), during electromigration. Activation energies for Al test patterns were found to be E a =0.64 eV and E ap =0.88 eV for non-passivated and passivated samples respectively, whereas for AlSi (1%) test patterns, E a =0.94 eV and E ap =1.07 eV. After noise measurements the samples were subjected, to the same stress conditions, current density and temperature, as during the noise measurements for a period of 165 h. Then both percentage resistance variation and damage in the stripes were evaluated and correlated with the power spectral density of resistance fluctuations. As a result, by means of noise technique one is able to obtain activation energy values that are consistent with the subsequent modification undergone by the resistors, so that activation energies determined in this way can be used as “true” reliability parameters. The time interval necessary to obtain such parameters (about 10 h for each series) is much shorter than those required by traditional techniques.

Silicon micromachined periodic structures for optical applications at λ=1.55μm

In this letter, the authors report the design, fabrication, and characterization of a silicon micromachined periodic structure for optical applications at λc=1.55μm. The microstructure, which can be envisioned as a one-dimensional photonic crystal, is composed of a periodic array of 1-μm-thick silicon walls and 2-μm-wide air gaps, each one corresponding to a different odd number of quarter wavelength at λc (hybrid quarter wavelength). The fabrication is based on the electrochemical etching of silicon, yielding parallel trenches with depths up to 100μm. Preliminary reflectivity measurements show the presence of a band gap at λc=1.55μm, as theoretically expected.

Current transport in free-standing porous silicon

The electrical conduction of free‐standing porous silicon layers, obtained from n+ silicon with various anodization currents and illumination conditions, has been investigated in vacuum as a function of the temperature in the interval 300‐210 K. The two‐contact I‐V characteristic is determined by the metal/porous silicon rectifying interface, whereas, by using the four‐contact technique, a linear dependence of the current vs voltage was found. The resistance of free‐standing samples showed a thermally activated behavior, with activation energies ranging from 0.1 to 0.44 eV. It was found that the activation energy decreased if the light intensity during the anodization was reduced. Variations of activation energy were also observed if the anodization current was changed but, in this case, it was not possible to find any correlation over the parameter range investigated.

Noise measurements in thin‐film interconnections: A nondestructive technique to characterize electromigration

Electromigration in aluminum and aluminum‐silicon (1%) resistors was detected by means of a new technique based on the particular features of an ultralow‐noise amplifier, which made it possible to evaluate the stochastic resistance fluctuations associated with vacancy generation‐recombination processes. The power spectra of these fluctuations display a 1/f γ behavior with 2≤γ≤2.6, in the frequency range 10 mHz–1 Hz, and the analysis of these spectra, recorded at various current densities and temperatures, led to an evaluation of the activation energies characteristic of the process. Scanning electron microscope observation of the damage induced by stressing the resistors at the same current density and temperature as the spectra measurement but for a longer time, showed that what was being observed was the electromigration phenomenon in its early stages.

PdSi Schottky diodes as hydrogen sensing devices: Capacitance-voltage characteristics

Abstract C-V measurements on PdSi Schottky diodes without any intentionally grown SiO2 insulating layer have been carried out in ambient and in hydrogen atmosphere. Sensitivity to hydrogen has been detected; besides, from the behaviour of the barrier height as a function of time during the hydrogenation some evidence of a modification on the Si interfacial state density has been obtained.

Integrated porous-silicon light-emitting diodes: A fabrication process using graded doping profiles

A fabrication process, compatible with an industrial bipolar+complementary metal–oxide–semiconductor (MOS)+diffusion MOS technology, has been developed for the fabrication of efficient porous-silicon-based light-emitting diodes. The electrical contact is fabricated with a double n+/p doping, achieving a high current injection efficiency and thus lower biasing voltages. The anodization is performed as the last step of the process, thus reducing potential incompatibilities with industrial processes. The fabricated devices show yellow-orange electroluminescence, visible with the naked eye in room lighting. A spectral characterization of light emission is presented and briefly discussed.

Improved optical emission of porous silicon with different postanodization processes

Time integrated and time resolved photoluminescence measurements have been performed at room temperature on porous silicon samples prepared under identical conditions but processed after anodization in four different ways. New and simple treatments were made to reduce the structural damage due to the drying process. The results show that suitable conditions exist to substantially improve the optical emission of porous silicon samples. The observed modifications of the time integrated photoluminescence signal are related to dangling bonds produced during the drying process that are supposed to act as trap states in the electron–hole or excitons recombination mechanism. Time resolved measurements provide an evaluation of the relative density of trap states for each analyzed sample.