Abstract: Low−temperature diffusion in metallic systems
Authors: Rosenberg Robert 
Published: July 2002

Mass transport in metallic components in microelectronic circuits at temperatures typical of processing and use condition is dominated by defect structures such as grain boundaries and dislocations. For example, many investigators have shown that grain boundary transport is controlling in reactions and interdiffusion between thin film metallic layers 1 and in degradation of metallization by electromigration. 2 A major difficulty in obtaining a detailed understanding of the mechanisms by which interactions occur lies in the difficulty of measuring diffusion parameters in thin film samples and in defect paths. Without the availability of such quantitative data, it becomes impossible to perform the needed analyses of given situations in which serious reliability and degraded device performance characteristics may become prevalent.

The work reported here concerns self−diffusion studies performed on gold and gold−tantalum alloys in which lattice, dislocation, and grain boundary parameters are simultaneously measured. Solute effects on self−diffusion are of special interest in light of improved electromigration behavior achieved by adding copper to aluminum, 3 for example. The gold system was chosen because of preliminary indications that tantalum improves electromigration lifetime 4 and because of the availability of long−lived radioactive tracer isotopes and lattice data for gold. Because of the comparatively low−diffusion temperatures used (204−395°C) and shallow diffusion profiles, an ion−sputtering technique was developed 5 to section samples subsequent to the tracer diffusion in units of about 20−30 Å per section. In this temperature range a three stage profile for penetration of the gold tracer resulted, corresponding to lattice, subboundary (or dislocation), and grain boundary contributions in order of appearance. Using standard analyses for calculating diffusion coefficients for the lattice region and the whipple analysis for dislocation and grain boundary regions resulted in the data in the Table. (A more detailed description is published elsewhere 6: Several significant observations can be made. The effect of tantalum addition on lattice self−diffusion of gold is to cause an enhancement as indicated by the lowering of activation energy from 1.81 to 1.48 eV. In the same samples, however, the grain boundary diffusion energy is increased from 0.88 to 1.26 eV, a clear inhibiting effect. This could be explained by invoking solute−defect interaction arguments (3) and could help to quantify the electromigration information becoming available. It should be noted that the diffusivity in dislocation subboundaries is not altered by the solute. Dislocations in the gold samples are dissociated into partials which apparently do not promote solute segregation to the less−distorted core regions. Activation energies for dissociated dislocations are known to be higher than undissociated dislocations even in pure samples. Utilization of lattice diffusion parameters and solute effects for predicting grain boundary behavior must be considered, in light of these results, to be a complicated procedure. Direct measurements, such as those presented here, are more desirable and should be actively pursued.