Harry A. Schafft

Thermal analysis of electromigration test structures

Analytical expressions are derived for estimating the temperature profile along a straight-line resistor test structure due to the joule heating generated by a high current-density stress, such as is used in accelerated stress tests to characterize metallizations for electromigration. It is shown how an improved estimate of the mean metallization stress temperature may be made and how the thickness and thermal conductivity of the underlying electrical insulator affect the temperature profile of the metallization. Recommendations for the design of electromigration test structures are developed that will promote reduced temperature gradients in the metallization during stress testing and improved reproducibility of electromigration characterizations.

Second breakdown—A comprehensive review

This paper is a comprehensive review of the published literature dealing with the phenomenon of second breakdown in semiconductor devices and the problems it creates in the design, fabrication, testing, and application of transistors.

Reproducibility of electromigration measurements

The reproducibility of median-time-to-failure (t50) measurements was determined in an interlaboratory experiment in which 11 laboratories and a reference laboratory took part. Each laboratory used a method of its choosing to test equivalent samples under the same conditions of current density and oven temperature. The between-laboratory reproducibility of t50measurements normalized to one metallization temperature was dependent on current-density stress: at 1.0 MA /cm2it was within 15 percent, while at 2.5 MA/cm2it was generally within 50 percent. The primary source for variability is in estimating the temperature rise of the test metallization due to joule heating. Recommendations are given for the design and test of electromigration test structures to improve the reproducibility of t50measurements.

Improved estimation of the resistivity of pure copper and electrical determination of thin copper film dimensions

Abstract Improved values for the resistivity, ρ, of pure, bulk copper from 50 to 1200 K, and their confidence intervals, are developed by extending the analysis of Matula. A recommended value for dρ/dT and its confidence interval in the temperature range of 290–425 K is developed for use with Matthiessens rule to calculate the electrical thickness of thin copper films and the cross-sectional area of copper lines from resistance measurements at two temperatures. Error analyses are used to estimate the uncertainty with which the electrical thickness and cross-sectional area can be determined. Values for the temperature coefficient of resistance of pure, bulk copper are also provided.

The Measurement, Use, and Interpretation of the Temperature Coefficient of Resistance of Metallizations

Abstract A review is given of the measurement and use of the temperature coefficient of resistance (TCR) of Al-based metallizations used as electrical interconnects in microelectronic circuits. The TCR has various reliability-related applications in the measurement of temperature and the characterization of a metallization for its susceptibility to electromigration failure and for the magnitude of its residual resistivity. Applications reviewed include the characterization of wafer-level test stations.

Thermal conductivity measurements of thin-film silicon dioxide

Measurements of the thermal conductivity of micrometer-thick films of silicon dioxide are reported for the first time. Results show that the thermal conductivity is much lower than the values reported for bulk specimens, decreases with increasing temperature, and decreases with decreasing film thickness. This means that heating effects may be much larger than expected in accelerated stress tests and in other cases where joule heating can be a concern.<<ETX>>

Modeling and Simulation of Resistivity of Nanometer Scale Copper

Abstract A highly versatile simulation program was developed and used to examine how the resistivity of thin metal films and lines is increased as their dimensions approach and become smaller than the mean free path of electrons in metals such as copper and aluminum. The simulation program is flexible in that it can include the effects of surface and grain-boundary scattering on resistivity either separately or together, and it can simulate the effect on resistivity where each surface of a film or line has a different value for the scattering parameter. The simulation program (1) provides a more accurate calculation of surface scattering effects than that obtained from the usual formulation of Fuchs’ theory, (2) calculates grain-boundary effects that are consistent with the theory of Mayadas and Shatzkes, (3) shows that surface and grain-boundary scattering effects are interdependent, and (4) shows that the change in resistivity with temperature begins to increase as dimensions approach the bulk mean free path of the electrons in the metal.

Building-in reliability: making it work

Aggressive reliability and market-entry demands will require the use of a building-in approach to reliability. The objectives are (1) to review the essential features of this new approach and contrast them with those of the traditional approaches, (2) to identify obstacles in accepting the building-in-reliability approach and suggest ways to overcome them, and (3) to suggest a way to facilitate the implementation of this approach. The approach requires that significant breaks be made from the traditional ways of improving and appraising reliability. The nature of these breaks is discussed in the context of describing the basic elements of the approach of building-in reliability and the obstacles that hinder its adoption. To help visualize how the approach can be implemented, initial steps in making the transition and some specific examples of its use are described.<<ETX>>

Statistics for electromigration testing

A comprehensive statistical basis is given for the design and conduction of electromigration stress tests that allows for the efficient use of test parts, equipment, and test time. It shows how to select the size of the sample, the required control of the stress conditions, and the number of failures required before halting the test in order to characterize metallization interconnects with a quantifiable level of confidence. The results are applicable to any failure mechanism for which the failure times obey a normal or log-normal distribution. >

Electromigration and the Current Density Dependence

The empirical expression used to predict metallization resistance to electromigration failure involves the current density raised to the power n. A value for n of 1.5 was obtained from stressing unpassivated Al 1% Si metallization test structures over a range of current densities from 0.5 to 2.5 MA/cm2. The steps taken to ensure an accurate estimate of the metallization stress conditions of temperature and current density to obtain this value are described in detail.