Influence of a Capping Layer on the Mechanical Properties of Copper Films

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ABSTRACT The substrate curvature technique was employed to study the mechanical properties of 0.6 ýtm and 1.0 jim Cu films capped with a 50 nm thick Si3 N4 layer and to compare them with the mechanical properties of uncapped Cu films. The microstructures of these films were also investigated. Grain growth, diffusional creep and dislocation processes are impeded by the cap layer. This is evident in the form of high stresses at high temperatures on heating and at low temperatures on cooling. At intermediate temperatures on heating and cooling, stress plateaus a relatively low stresses exist. This can be explained by the so-called Bauschinger effect. A film thickness dependence of the stresses in the film could not be observed for capped Cu films.

INTRODUCTION Copper is a potential alternative to aluminum as an interconnect material because of its higher electrical and thermal conductivity. The melting temperature of Cu is considerably higher than that of Al, which decreases diffusional processes, due to electromigration for example, at service temperature. However, the product of elastic modulus and thermal expansion coefficient of Cu (2.14 MPa/ 0 C) is similar or higher (for 111-texture) than that of Al, which can cause high thermal stresses and severe reliability problems. There are already several publications [1-3] about the mechanical properties of continuous Cu films. In this paper we concentrate on the mechanical properties of continuous films covered by a thin capping layer. In future microelectronic applications, the Cu interconnect lines will always be encapsulated by passivation and/or diffusion barrier layers. Therefore it is important to know how the mechanical properties are affected by a capping layer.

EXPERIMENTAL DETAILS Thermally oxidized, (100) oriented, 100 mm diameter silicon wafers were used as substrates. Because Cu diffuses very quickly through Si0 2 and reacts with Si, a 50 nm thick Si 3 N 4 diffusion barrier was deposited prior to Cu deposition. Si3 N4 was used because it is known as a thermally stable barrier material for Cu [4] and Cu grows on Si 3N4 with a strong (111) texture [2], which increases electromigration resistance [5]. The Si 3 N4 was deposited by LPCVD and then densified at 950'C for one hour. Cu films of 0.6 gim and 1.0 gm thickness were deposited by sputtering. The capping layer, consisting of 50 nm of Si3 N4 , was sputtered in order to avoid the high temperature anneal necessary for LPCVD Si 3N 4 , which would have caused Cu diffusion through the barrier layer and changed the microstructure of the Cu film. The base pressure prior to sputtering was below lxl0-6 Torr. Cu and Si3 N4 were sputtered at 1020 A/min and 15 A/min, respectively, in 2 mTorr Ar. 453 Mat. Res. Soc. Symp. Proc. Vol. 356 01995 Materials Research Society

The mechanical properties of these films were studied by measuring the biaxial stress in the film as a function of temperature using the substrate curvature method [6]. This technique is based on the fact that a stressed thin film bends its substrate e