Laser-Induced Microstructural Modification of Polycrystalline Cu and Ag Films Encapsulated in SiO 2
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Laser-Induced Microstructural Modification of Polycrystalline Cu and Ag Films Encapsulated in SiO2 Rong Zhong, Jorg M.K. Wiezorek, and John P. Leonard Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, 15261 Excimer-laser-induced rapid lateral solidification was used to produce large grain microstructures in copper and silver thin films. These were multilayer thin film structures consisting of sputter deposited copper and silver thin films encapsulated by silica (SiO2/metal/SiO2/Si substrate). In this process, a single excimer laser pulse and projection imaging optics were used to melt a 60 micron wide line in the metal film. The resolidification of the melted lines is found to occur laterally in the plane of the film, resulting in grains greater than 20 um in length and 1 um wide. Electron diffraction analysis allowed identification of a strong texture in the growth direction along the major axis of the elongated grains as well as similar texture parallel to the film surface. Various dislocation and faulted defect structures are identified and examined in the context of the rapid solidification and potential application to interconnects. INTRODUCTION The ITRS technology roadmap[1] calls for line pitch scaling below 150 nm, increased speed, and higher current densities. Recent studies now indicate these goals may not be met without significant changes to the materials and processing[2,3]. Future interconnects must exhibit electrical resistivity at or lower than current copper technology ρ ≅ 2.5 µΩ·cm, with sufficient thermal conductivity for heat dissipation[4]. Copper (ρbulk = 1.7 µΩ·cm) still appears to be the best choice in the near term[6]. Resistivity increases are due to electron scattering from film interfaces[7], grain boundaries[8], impurities and localized defects[9,10], and extended defects such as dislocations[11]. Electromigration is controlled by mass transport in the bulk, at grain boundaries and the metal/liner interface. In the case of copper interconnects, the latter is found to be the dominant factor[6]. Ideally a single crystal of high conductivity material free from defects and possessing smooth ideal interfaces with its encapsulant material offers the best chance of meeting future roadmap goals. As a result, most research has focused on increasing crystallinity, creating larger defectfree grains and smooth, low energy interfaces that inhibit electromigration [12,13]. A significant limitation is the intrisnic, or as-deposited microstructure of metallization layers. Current deposition techniques can produce highly conformal polycrystalline Cu films with grain size distributions in the range of 20-200 nm[6]. These thin metal films are comprised of statistically distributed grain sizes, which may range from the surface diffusion length to the thickness of the film, and typically exhibit essentially random grain orientations or a fiber texture normal to the surface. Techniques to circumvent this limitation can be divided into two different general ap
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