The Effect of Oxygen on Adhesion of Thin Copper Films to Silicon Nitride
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THE EFFECT OF OXYGEN ON ADHESION OF THIN COPPER FILMS TO SILICON NITRIDE Mengzhi Pang, Monika Backhaus-Ricoult, and Shefford P. Baker Cornell University, Department of Materials Science and Engineering Ithaca, NY 14853 U.S.A. ABSTRACT The effect of oxygen on adhesion and chemistry at interfaces between Cu thin films and SiNx barrier layers on Si substrates was investigated. Films were deposited in an ultra-high-vacuum sputter deposition chamber with good control of oxygen content. Adhesion was measured using a high throughput driver film method. Microstructure and bonding were investigated using transmission electron microscopy and electron energy loss spectroscopy, respectively, on sample cross sections. For films to which a small amount of oxygen was added during deposition, oxygen segregated to Cu/SiNx interfaces during thermal cycling, where it induced charge transfer and the presence of Cu+ interfacial states, and significantly reduced the work of adhesion. INTRODUCTION Silicon nitride barrier and passivation layers are often used to isolate copper films and conductor lines in integrated circuits. As is true with most thin films on substrates, high stresses may arise due to differential thermal expansion between the Cu film and Si substrate. These stresses can lead to failure by a number of mechanisms including void formation, cracking and delamination. Thus, it is important to understand the origins and magnitudes of such stresses in order to develop ways to improve device reliability. Recently, several studies have shown that the thermomechanical behavior of thin Cu films sandwiched between nitride (SiNx, AlNx, TaNx) barrier and passivation layers can be strongly affected by small amounts of oxygen added to the Cu during deposition [1, 2, 3, 4]. The shape of the thermomechanical stress-temperature hysteresis is dramatically changed and shows such “anomalous” behaviors as negative yielding, plastic deformation at zero applied stress, and increasing flow stress with temperature [4]. The oxygen was shown to segregate to the interfaces during thermal cycling [3], and it was suggested that this oxygen induced the changes in thermomechanical behavior by changing the adhesion of the copper/nitride interfaces [4]. In particular, experimental evidence suggests that the anomalous behaviors are associated with a reduction in adhesive strength of copper/nitride interfaces [4]. The effect of oxygen on adhesion at metal/nitride interfaces is not well characterized, but the effects of oxygen on metal/oxide interfaces have been studied both experimentally and theoretically. A model based on Gibbs’ adsorption at metal/oxide interfaces [5, 6] suggests that the interface energy is diminished in presence of excess oxygen. Segregation of excess oxygen to metal/oxide interfaces was theoretically predicted and experimentally shown for several systems [7, 8]. In metal/oxide systems, it was always associated with a decrease in the interfacial energy [9]. Transposition of those findings to metal-nitride interfaces suggests a s
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