A1/A1-Oxide Interface Diffusion During Electromigration
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ABSTRACT This paper presents new results concerning diffusion at the interface between Al and its oxide, due to electromigration in Al interconnects. The results show that the phenomenon is an important and general one. Significant large-scale modification of the surface of Alalloy conductors was observed after electromigration stress under a wide range of different conditions: 1) in a number of different alloys (Al-Si, Al-Cu, Al-Si-V & Al-Si-V-Pd), 2) with and without passivation, 3) over an extended temperature range, 4) over an extended range of current density, and 5) with a number of different underlayers (Si0 2 , W-Ti (no vacuum-break before Al deposition) and W-Ti (oxidized surface before Al deposition)). After electromigration stressing, the surface showed height variations: at certain locations the original thickness remained, while at adjacent locations the thickness was reduced; as much as 150nm in 2500 hrs, for Al-Si on oxidized W-Ti metallization. Damage by thinning has received little attention in the past. Based on the results presented, a model for the underlying mechanism is proposed. In this model Al atoms diffuse concurrently along the interface between the Al-metal and its oxide, and through the grain boundaries, even at low stress conditions and in non-bamboo lines.
INTRODUCTION
Over the last 25 years it has been demonstrated experimentally that damage due to electromigration in thin Al films is strongly related to the microstructure of the film (see for example [1 - 3] ). Such films are used as the interconnect layer(s) in integrated circuits (ICs). It is now accepted that diffusion along the boundaries between the crystal grains of these films is the dominant mechanism whereby Al migrates [4], at least in cases where the width of the interconnect line is greater than the average grain size of the film, and where the temperature is low (< 200 'C). Nevertheless, with the continuing miniaturization of ICs, interconnect lines have reached a width which is less than or equal to the average grain size, and the lines have a "nearbamboo" grain structure [5,6]. In the case of near-bamboo lines, a continuous grain boundary diffusion path only exists in short lengths, consequently grain boundary diffusion is suppressed. It is now believed that diffusion along the interface between the Al-metal and its native oxide is the dominant diffusion path leading to damage in near-bamboo lines [7 11]. However, this paper presents results which suggest that interface diffusion can also be observed under conditions where the grain structure is not bamboo or near-bamboo and where the electromigration stress is relatively low (approx. 10' A/cm2 and 175 'C), i.e. conditions under which grain boundary diffusion has been thought to dominate. The results 259 Mat. Res. Soc. Symp. Proc. Vol. 391 01995 Materials Research Society
Figure 2: High tilt (450) SEM micrograph. Same metallization and test conditions as in Figure 1. Electron flow is from bottomleft to top-right. The pattern of outlines of the thinned regions was
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