A. Scorzoni

Electromigration in thin-film interconnection lines: models, methods and results

Abstract Electromigration (EM) in thin-film interconnection lines is one of the major concerns for the development of ULSI devices, employing advanced design rules. Starting from the early sixties, several techniques have been used to characterize this phenomenon, producing a large, but frequently contradictory, amount of data. Different models have been proposed, but the complete comprehension of the basic physical mechanisms leading to EM is still unsatisfactory. In this work, well-established results and unsolved problems are reviewed. The physical model based on the general diffusion theory is used to describe the EM failure mechanism; the influence of the different stress parameters (temperature, current density, mechanical stress), of material properties (structural inhomogeneities, chemical composition) and line topography are taken into account. The accelerated methods employed to evaluate the EM resistance of the lines are classified into destructive and non-destructive, according to their effects on the samples under test. In the first group, a core position is occupied by the so-called median time to failure (MTF) technique, that has been extensively employed to gather results on many different materials and structures. Within the same group a survey is given of resistometric methods and faster techniques, based on a further acceleration of EM by means of high current densities and related Joule heating. The parameters extracted with these techniques are discussed in relation with the MTF results. The choice of a suitable statistical distribution, related to both the times to failure (TTFs) and the parameters used to estimate the EM performance with alternative methods, is also reviewed. More recently, an increasing importance has been achieved by non-destructive techniques able to give reliable information about the phenomenon without irreversibly damaging the samples. An important section of this work is devoted to the discussion of these techniques, which are mainly based on the accurate measurement of the resistance drift or low-frequency noise induced by the damage occurring at microscopic level. In the course of the discussion, particular emphasis is given to the comparison of the results obtained with the different techniques and to the improvement achievable by employing new materials and structures, including different aluminum alloys and Al/refractory metal sandwiches.

Automated defect detection in uniform and structured fabrics using Gabor filters and PCA

This paper describes an algorithm for texture defect detection in uniform and structured fabrics, which has been tested on the TILDA image database. The proposed approach is structured in a feature extraction phase, which relies on a complex symmetric Gabor filter bank and Principal Component Analysis (PCA), and on a defect identification phase, which is based on the Euclidean norm of features and on the comparison with fabric type specific parameters. Our analysis is performed on a patch basis, instead of considering single pixels. The performance has been evaluated with uniformly textured fabrics and fabrics with visible texture and grid-like structures, using as reference defect locations identified by human observers. The results show that our algorithm outperforms previous approaches in most cases, achieving a detection rate of 98.8% and a false alarm rate as low as 0.20-0.37%, whereas for heavily structured yarns misdetection rate can be as low as 5%.

Electromigration testing of integrated circuit interconnections

The electromigration phenomenon has been one of the most intriguing physical problems in the semiconductor device reliability. The models to explain the phenomenon are here revised, together with the influence of materials and their microstructure. The various measuring techniques are described, including the design of special test patterns, and statistical data analysis is briefly reviewed.

Interaction between electromigration and mechanical-stress-induced migration; New insights by a simple, wafer-level resistometric technique

A wafer-level resistometric technique was used as an indirect way to detect the combined effect of mechanical stress migration and electromigration (EM). A technique was developed to perform reliable high-resolution resistance measurements. In this technique, the compensation of small thermal instabilities is achieved by means of an additional measurement on a reference device. EM tests performed at constant temperature and current on Al-1%Si stripes exhibit an initial nonlinear resistance vs. time behavior, probably due to the simultaneous action of the accumulated mechanical stress and the high current density, followed by linear behavior. An activation energy is extracted by means of an original statistical analysis of the experimental data, and its meaning is discussed, taking into account the influence of temperature- and time-dependent mechanical stress. It is concluded that kinetics of stress relaxation should be known more deeply in order to perform reliable operating temperature extrapolations from the calculated activation energies. >

Electromigration in thin-films for microelectronics

Abstract Electromigration (EM) is one of the major concerns for the development of ULSI divices, but not all the aspects of the phenomenon are presently well understood. In this paper well established results and unsolved problems are reviewed and discussed. First, the physical model and in particular the influence of the mechanical stress on EM is considered. Then, the various techniques used to characterize electromigration are analyzed, making distinction between traditional techniques (median time to failure technique and resistometric methods) and more recently developed methods (high-resolution resistometric techniques and low-frequency noise measurement), also considering the fast techniques used for metallisation testing in the industrial environment. Finally, a section is devoted to the problem of test-structure and test-procedure standardisation in EM experiments.

A System for the Dynamic Control and Thermal Characterization of Ultra Low Power Gas Sensors

This paper describes a system for the simultaneous dynamic control and thermal characterization of the heating and cooling phases of an ultralow-power (ULP) micromachined sensor, featuring thermal characteristics that are quite similar to those of innovative ULP semiconducting metal-oxide gas sensors. A pulsewidth-modulated (PWM) excitation system has been realized using a microcontroller featuring an ARM7 core to characterize the thermal behavior of a device formed by a Pt microheater and a Pt temperature sensor, over an insulating membrane. Three operating modes, i.e., constant target heater resistance, constant heating power, and cooling phase monitoring, were implemented. Objectives of the research were to analyze the relation between the time period and duty cycle of the PWM signal and the operating temperature of such ULP micromachined systems, to observe the thermal time constants of the device during the heating and cooling phases, and to measure the total thermal conductance. Experiments indicated that an approximately constant heater temperature in the constant target heater resistance regime (i.e., after the initial thermal transient due to the heating algorithm) can only be obtained if the time period of the heating signal is smaller than 50 μs, i.e., much faster than the time constant of the device. Constant power experiments show quantitatively a unique time constant τ for both the heater and the temperature sensor (T-sensor) in the heating phase (with a known applied power) and the cooling phase (with zero power). This time constant decreases during heating in a range of 2.3-2 ms as a function of an increasing temperature rise ΔT between the ambient and the operating temperature. Moreover, we observed that, in the chosen operating temperature range, the thermal conductance is a linear function of ΔT. Finally, repeatability of experimental results was assessed by guaranteeing that the standard deviation of the controlled temperature was within ±5.5°C in worst-case conditions.

Improved electrical characterization of Al–Ti ohmic contacts on p-type ion implanted 6H-SiC

This paper deals with the electrical characterization of low resistance Al–Ti 72/28 wt% ohmic contacts to a p-type ion implanted 6H-SiC layer. Transmission line model (TLM) structures were realized on the top of MESA islands defined in this ion implanted layer. A metal scheme composed of Al-1%Si(350 nm)/Ti(80 nm) was deposited by sputtering, photolithography defined and annealed at 1000 °C in Ar for 2 min. TLM structures were measured as a function of the temperature in the range 25–290 °C. The TLM data were mainly analysed by a two-dimensional finite difference simulation tool that takes into account the current crowding effect at the contact periphery. Extracted contact resistivity values fall in the low range of data from the literature. The sheet resistance values computed from the TLM data agreed with those measured using Van der Pauw devices realized next to the TLM structures.

A resistometric method to characterize electromigration at the wafer level

Abstract Electromigration resistometric measurements at the wafer level cannot be carried out with adequate precision using standard DC techniques, due to high, non-stable thermoelectric voltages arising at the pad/probe-tip interface. In this work an AC resistometric technique is proposed, which allows the detection of very small resistance changes in the early stages of electromigration process. Measurements were carried out at different temperatures and an activation energy was extracted. The validity of the Matthiessens rule for the interpretation of electromigration data was studied. Its applicability was not proved: experimental data showed that the resistance changes can be explained in terms of geometrical variations even in the first stages of electromigration.

A Programmable Interface Circuit for an Ultralow Power Gas Sensor

Abstract-An electronic system based on a microcontroller architecture, devoted to interfacing a three-terminal, ultralow power (ULP) Metal OXide (MOX) gas sensor is presented. The sensor features a novel three-terminal configuration where the microheater is not galvanically isolated with respect to the MOX sensor. The system provides both control of the operating temperature and management of the acquired data. A Pulse Width Modulation (PWM) signal with variable duty cycle is used to provide power to the heating resistor in order to set the desired operating temperature. The heating resistance value is measured in the range (100-300) Ω with a relative error of less than 1%. The circuit devoted to measuring the gas concentration is based on a logarithmic amplifier which measures the current flowing in the sensing layer of the sensor. The measurand range is 30 nA to 60 mA and the relative error of the measured current is less than 0.6%. The data acquisition system was successfully tested by acquiring data of a three-terminal ULP gas sensor located in an automatically controlled environmental chamber under benzene and NO2 flow.

Biased resistor network model for electromigration failure and related phenomena in metallic lines

Electromigration phenomena in metallic lines are studied by using a biased resistor network model. The void formation induced by the electron wind is simulated by a stochastic process of resistor breaking, while the growth of mechanical stress inside the line is described by an antagonist process of recovery of the broken resistors. The model accounts for the existence of temperature gradients due to current crowding and Joule heating. Alloying effects are also accounted for. Monte Carlo simulations allow the study within a unified theoretical framework of a variety of relevant features related to the electromigration. The predictions of the model are in excellent agreement with the experiments and in particular with the degradation towards electrical breakdown of stressed