Investigation of Self-Heating Phenomenon in Small Geometry Vias Using Scanning Joule Expansion Microscopy

This paper reports the use of a novel thermometry technique, scanning Joule expansion microscopy (SJEM), to study the steady state and dynamic thermal behavior of small geometry vias under sinusoidal and pulsed current stress for the first time. Spatial distribution of temperature rise surrounding a sub-micron via has been determined and the corresponding temperature contour image has been extracted. The thermal time constant of the via structure has been determined from the measured AC frequency dependence of the temperature rise. Furthermore, the average (DC) and peak temperature rise under pulsed stress condition has been estimated from the measured first harmonic temperature rise. INTRODUCTION The continuous scaling of VLSI circuits has resulted in an increase in the aspect ratio of the vias (connection between adjacent metallization levels) and increases in the current density and associated thermal effects, namely self-heating. Current crowding and localized heating [1], [2], [3] in deep sub-micrometer vias are known to strongly impact reliability of VLSI interconnects. The magnitude and spatial distribution of the temperature rise in the via are important to accurately estimate interconnect lifetime under electromigration (EM), which is temperature dependent. Localized temperature rise can also cause stress gradients inside the via structures and can also lead to melting under short-duration high current stress conditions, such as electrostatic discharge (ESD) events [4]. Hence, measurements of the magnitude and spatial distribution of the temperature rise in deep sub-micrometer vias are important to accurately model their reliability and provide thermal design guidelines for various via technologies. In general, interconnect thermometry based on temperaturedependent electrical resistivity of the interconnect metal is used to calculate a spatially averaged temperature rise along the interconnects [5]. However, this does not provide local temperature rise which may be much higher around vias. The spatial resolution of far-field optical techniques, such as scanning thermoreflectance thermometry [6], infrared thermography [7], and liquid crystal thermography [8], is diffraction-limited to about 1 μm. This is insufficient to probe deep sub-micrometer vias in the size range of 0.1-0.5 μm. Near-field optics can be employed to overcome the diffraction limit [9], but it is still in its infancy as far as accurate measurements of all desired parameters are concerned. The SJEM, has recently been developed with spatial resolution in A B Laser