Near-Field Optical Imaging of Electromigration Damages in Passivated Metal Stripes
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"**INFM and Dip. Scienza dei Materiali, Universitd di Milano Bicocca, via Cozzi 53, 20125 Milano, Italy ***STMicroelectronics, via C. Olivetti 2, 20041 Agrate B.za (Milano), Italy
Abstract Electromigration is one of the main failure mechanisms limiting the miniaturization of microelectronicdevices. As a consequence of the high current densities in the interconnections, hillocks and voids are formed and their evolution can modify the electrical performances of devices till failure. To characterize electromigration damages,. today's failure analysis techniques require removal of the protection passivation to allow scanning electron microscope or focused ion beam microscope imaging, but the removal process itself can damage the surface of the metal stripes. Due to the optical transparency of the passivation, near-field scanning optical microscopy can be used to overcome this problem. We succeeded in obtaining the first near-field images in resolution < 150 nm of electromigrationdamaged metal structures without complete removal of the passivation. The latter was thinned to 100 - 200 nm to allow the evanescent waves to reach the metal structures and illuminate a subwavelength zone of the sample. Near-field images show the presence of hillocks and voids of dimensions down to 250 nm which can
be due only to electrornigration, and, in this sense, they are more reliable than the usual scanning electron microscope images.
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INTRODUCTION
Electromigration[1] is a degradation phenomenon induced by the motion of electrons in a metal subjected to high current densities. When an electric field is applied, every ion is subjected to a force which is due mainly to the electromagnetic driving force acting on the ion itself and to several collisions with electrons moving towards the anode. The resultant
101 Mat. Res. Soc. Symp. Proc. Vol. 571 © 2000 Materials Research Society
can be schematized in terms of the effective charge Z of the ion, which can be split in two contributions: the electrostatic charge of the ion Zm and the so called electron wind term Z1W, so that the force F equals (Zh + Zew)E, where E is the applied electric field. For example, for aluminum Z is approximately -10. Near-field scanning optical microscopes (SNOM)[2] offer the possibility of controlling and positioning a monochromatic 50 nm - wide light source with a precision of a few nanometers. Due to the evanescent waves the electromagnetic field in close proximity of the source is significantly higher than the field far from the source. Thus, by placing the source near the sample, it is possible to perform optical analysis with evanescent waves, which allow a resolution beyond the classical diffraction barrier of half a wavelength A of the impinging wave. In this work, this property was used to investigate damages in metal lines produced by electromigration avoiding the complete removal of the passivation, a process which could itself damage the metal structures.
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EXPERIMENTALS
The experimental setup was based on the commercially available SNOM Auror
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