Electroplated Cu Recrystallization in Damascene Structures at Elevated Temperatures
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ABSTRACT Transformations in electroplated Cu films from a fine to course grain crystal structure (average grain sizes went from -0.1 pm to several microns) were observed to strongly depend on film thickness and geometry. Thinner films underwent much slower transformations than thicker ones. A model is proposed which explains the difference in transformation rates in terms of the physical constraint experienced by the film since grain growth in thinner films is limited by film thickness. Geometrical constraints imposed by trench and via structures appear to have an even greater retardation effect on the grain growth. Experimental observations indicate that it takes much longer for Cu in damascene structures to go through grain size transformations than blanket films. INTRODUCTION It has been reported that electroplated Cu undergoes self-annealing after plating [1-4], in which the grains can grow significantly from -0. 1 pim to several microns. During and after selfannealing, both the physical and electrical film parameters are significantly changed. The selfannealing of blanket films was found to depend strongly on the film thickness. With thinner films, longer times were required to achieve an equivalent degree of self-annealing. In view of the theory proposed earlier [21 that physical constraint affects the self-anneal, electroplated Cu in damascene lines would take even longer to undergo transformation. In this paper, we report resistance transformations of electroplated Cu in blanket films and damascene lines. EXPERIMENT AND RESULT Electroplating of 1.6 ptm Cu blanket films were performed using a sulfate based plating bath with a small amount of proprietary additives. Prior to electroplating, a nominal 250A collimated PVD Ta barrier layer was deposited followed by a 1000 A collimated PVD Cu seed layer. The seed layer was deposited in situ after barrier deposition without a vacuum break. The plating bath temperature was regulated at 25 'C by a heat exchanger. The sheet resistances of films with 3 different electroplated Cu thicknesses of 0.8 jim, 1.6 pjm, and 3.3 pjm were monitored over time. Fig. 1a displays a 49-point contour map average and standard deviation of the film bulk resistivity as a function of time. The data indicates that although the film bulk resistivity declines from -2.2 jiQ-cm to -1.8 pO-cm during selfannealing for all films, the time it takes to go through the transformation is very different. Thinner films take much longer than thicker ones to transform. The transformation times were 429
Mat. Res. Soc. Symp. Proc. Vol. 564 ©1999 Materials Research Society
140 hours, 36 hours and 15 hours respectively for 0.8 jim, 1.6 jim and 3.3 jim films accordingly. Fig. lb displays the corresponding standard deviation in bulk resistivity for all 3 films. A similar trend t
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