Effects of Confinement on Plastic Deformation in Passivated Al Films
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EFFECTS OF CONFINEMENT ON PLASTIC DEFORMATION IN PASSIVATED AL FILMS S.G.H. ANDERSON*, I.-S. YEO*, P.S. HO*, S. RAMASWAMI**, AND R. CHEUNG** *Materials Laboratory for Interconnect and Packaging, University of Texas at Austin, BRC/MER Mail Code 78650, Austin, TX 78758-1100
"**Advanced Micro
Devices, 901 Thompson Place, P.O. Box 3453, Sunnyvale, CA 94088-
3000 ABSTRACT
Wafer curvature measurements of a trilayer (SiO 2 / AISiCu / Si) structure are compared to that predicted by a weighted sum of individual measurements of Si0 2 and AlSiCu films on Si, and significant differences are found to exist for temperatures above 200'C. A straightforward analysis of the stresses in each layer has been modeled using an extension of a model by Feng et al. which assumes uniform plastic deformation throughout the Al. The modeling results suggest a straightforeward method for determining stresses in deformable thin films that are confined by elastic overlayers. A comparison of the stress-temperature behavior for unpassivated and passivated AlSiCu films reveals that the confined films exhibit less plastic deformation and both higher tension and compression during thermal cycling. INTRODUCTION
to the measured bending of a beam, as The stresses in a thin film on a plate can be related 2 1 derived by Stoney and amended by Hoffman as: (1)
Etl 02=6Rt2 (I_-l)
where El/(I-•I) is the biaxial modulus of the substrate, t1 and t 2 are the substrate and film thicknesses, and R is the radius of curvature. Several authors have developed solutions for bilayer strips with different boundary conditions, and have extended the analysis to multilayers of elastic materials. 3 -6 Some of these studies have concluded that the development of stresses in multilayer materials can be adequately described by a weighted average of measurements on individual films.5,6 Unfortunately, the modeling of multilayer structures has been limited to elastic cases where no flow, or plastic deformation, of any of the films occurs. Significantly, several practical applications are influenced by the deformation of confined materials. For example, Si0 2 sandwiched between two elastic media (Si and SiN) reduces the stress buildup and dislocation production at the edge of pad structures because it deforms at high temperatures. 7 Metal lines encapsulated by a dielectric medium exhibit stress-temperature dependencies that are distinct from those studied for thin films because the deformation of the metal is constrained, underscoring the importance of confinement on the development and relaxation of thermal stresses in these structures.8-11 In addition, confinement by vacuum or oxide interfaces has been shown to affect thin film stresses. 12-14 In this paper, we compare the results of a weighted sum of measurements of two individual thin films (Al and SiO 2 ) deposited on Si to those of a separate measurement of a trilayer structure composed of SiO 2/AI/Si. We show that the stress-temperature relationships are significantly different, suggesting that the presence of the oxide
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