M. Saka

Electromigration failure of metal lines

Abstract.With the scaling down process of microcircuits in semiconductor devices, the density of electric current in interconnecting metal lines increases, and the temperature of the device itself rises. Electromigration is a phenomenon that metallic atoms constructing the line are transported by electron wind. The damage induced by electromigration appears as the formation of voids and hillocks. The growth of voids in the metal lines ultimately results in electrical discontinuity. Our research group has attempted to identify a governing parameter for electromigration damage in metal lines, in order to clarify the electromigration failure and to contribute to circuit design. The governing parameter is formulated based on the divergence of the atomic flux by electromigration, and is denoted by AFD. The prediction method for the electromigration failure has been developed by using AFD. The AFD-based method makes it possible to predict the lifetime and failure site in universal and accurate way. In the actual devices, the metal lines used in the integrated circuit products are covered with a passivation layer, and the ends of the line are connected with large pads or vias for current input and output. Also, the microstructure of metal line distinguishes the so-called bamboo structured line from polycrystalline line depending on the size of metallic grains relative to the line width. Considering the damage mechanisms depending on such line structure, our research group has made a series of studies on the development of the prediction method. This article is dedicated to make a survey of some recent achievements for realizing a reliable circuit design against electromigration failure.

NDE of a Crack by Using Closely Coupled Probes for DCPD Technique

In order to enhance the sensitivity of the d-c potential drop technique, the principle of a technique using the closely coupled probes is proposed. A calibration equation for this technique is derived, where the path-independent integral j e for electric crack problems is utilized. Experiments on laboratory specimens are discussed which validate the use of the calibration equation for the nondestructive evaluation of cracks. It is shown that the sensitivity of the present method using closely coupled probes is higher than that of the usual method using a uniform current flow in the region far from the crack.

A Method to Predict Electromigration Failure of Metal Lines

A new calculation method of atomic flux divergence (AFDgen) due to electromigration has recently been proposed by considering all the factors on void formation, and AFDgen has been identified as a parameter governing void formation by observing agreement of the numerical prediction of the void with experiment. In this article, a method to predict the electromigration failure of metal lines was proposed by using AFDgen. Lifetime and failure site in a polycrystalline line were predicted by numerical simulation of the processes of void initiation, its growth to line failure, where the change in distributions of current density and temperature with void growth was taken into account. The usefulness of this prediction method was verified by the experiment where the angled aluminum line was treated. The failure location was determined by the line shape and the operating condition. The present simulation accurately predicted the lifetime as well as the failure location of the metal line.

A Sensitive Ultrasonic Approach to NDE of Tightly Closed Small Cracks

A sensitive ultrasonic technique for the quantitative nondestructive evaluation of small closed cracks is developed. The use of an oblique longitudinal wave with small angle of incidence upon the specimen surface is emphasized. Firstly, ultrasonic testing is performed with open cracks in order to predict the optimum angle of evaluation, and an empirical calibration equation is then derived for the same as a basis to deal with the analysis of closed cracks. From the measurement on real fatigue cracks, the parameters of interest, specifically the crack size and the closure stress, are determined simultaneously by analyzing the inverse problem The method developed here is fully automated and computer-controuea. Evaluated results are found to be in good conformity with the destructive measurements, and the corresponding superiority of the method is realized, especially for tightly closed small fatigue cracks, when compared with the method using a beam of longitudinal wave incident normally and/or with a large angle.

Simplified NDE of a closed vertical crack using ultrasonics

Ultrasonic testing using a normally incident longitudinal wave is used for sizing a closed crack. The crack exists vertically on the back-wall of the inspected plate. In order to evaluate the closed crack, the relationship between the first back-wall echo height and the crack depth for an open crack as the basis for evaluating the closed crack is determined experimentally and is introduced to the analysis of echo height for the closed crack. The unknown parameters in the evaluation of the closed crack, such as crack depth and the stress closing the crack, are determined by comparing the first back-wall echo height obtained through both analysis and experiment on the closed crack.

NDE of a 3-D Surface Crack Using Closely Coupled Probes for DCPD Technique

A procedure of applying the d-c potential drop technique using the closely coupled probes to NDE of a 3-D surface crack is newly developed. The calibration equation for three sensors which differ in the distance between the probes is derived. Experiments validated the use of the calibration equation for the NDE of cracks. The method to use the three sensors properly based on the measuring sensitivity is shown.

Mechanism of electrical conduction through anisotropically conductive adhesive films

Anisotropically conductive adhesive films (ACFs) which had different amount of electrically conductive particle contents and insulating particle contents were offered for measurement of electrical resistance between electrically conductive glass substrates and flexible printed circuits (FPCs) connected by using these ACFs. As a result, the electrical resistance for samples using ACFs containing electrically conductive particles was proved to he low, and also that for samples using ACFs containing insulating particles or no particles was proved to be considerably lower than we had expected. On the other hand, after high temperature and high humidity treatment or temperature cycle treatment, the electrical resistance for samples using ACFs containing electrically conductive particles did not change so much, but that for samples using ACFs containing insulating particles or no particles changed significantly, that is, it increased remarkably. From these facts, we presumed that there were two kinds of electric current passing through ACFs: one passing through electrically conductive particles and the other not passing through them; the current passing through electrically conductive particles did not change so much even after high temperature and high humidity treatment or temperature cycle treatment, but the current not passing through electrically conductive particles significantly reduced after the treatment mentioned above. On the basis of the above result, the role of conductive particles in ACFs was considered.

Experimental verification of prediction method for electromigration failure of polycrystalline lines

A prediction method for electromigration failure in polycrystalline lines has been proposed using the governing parameter of electromigration damage, the atomic flux divergence (AFDgen), and the usefulness has been verified by experiment where various line shapes were treated under various operating conditions. In the prediction method, lifetime and failure site in a metal line have been predicted by numerical simulation of the processes of void initiation, its growth, to line failure. The simulation has predicted accurately the lifetime as well as the failure site of the metal line. In the verification, however, the metal lines treated had the same grain size, that is, the same microstructure. In this article, the prediction method for the electromigration failure of polycrystalline lines was verified in more detail, by comparing the prediction results of lifetime and failure site with the results of experiments using not only various shaped lines but also lines whose microstructures were different.

Fracture strength of chemically vapor deposited diamond on the substrate and its relation to the crystalline structure

Abstract The mechanical strength of chemically vapor deposited diamond is characterized in terms of fracture toughness. The toughness of film and adhesion interface to the substrate were independently measured using techniques recently developed by the authors. In this paper, diamond was deposited by microwave plasma chemical vapor deposition using a variety of deposition conditions. The fracture toughness of the interface initially increased and then decreased with increasing amounts of methane in the source gas mixture. On the contrary, the toughness of the films was simply found to decrease with respect to the increment of methane concentration. This interesting trend of toughness was explained by the crystalline structure of the deposited diamond.

Nde of a 3-D surface crack using magnetic field induced by DC current flow

Abstract A method is proposed for nondestructive evaluation of a 3-D surface crack by means of measurement of a magnetic field induced in the air by DC current flow in a material. A semielliptical crack is considered. The crack length is assumed to be known in advance and the crack depth is to be found. The current is applied between two close points on both sides of the crack and the change in magnetic: flux density in the air due to the crack is measured in non-contact with the surface of the material. An approximate equation relating the magnetic flux density to the depth and length of the crack is derived numerically and some experimental results show that the evaluated crack depth is in good agreement with the actual one.