Extraordinary adhesion of a simple niobium metal-bonding structure
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Extraordinary adhesion of a Nb-based/Au-Sn metallization structure on a SiO 2 /Si substrate has been observed by a tensile pull test. The metallization bond fails mostly via fracture through either the Si substrate or epoxy adhesive. The intrinsic adhesion pull strength of the metallization scheme is concluded to be greater than 10 kpsi, and the adhesion exceeds the Si-Si cohesive strength. The Nb-Si and N b - 0 bonds at the interface are thermodynamically stable.
I. INTRODUCTION Ti-based metallization is currently employed to bond a semiconducting laser device and/or a silicon-integrated circuit die to a submount with Au-Sn solder.1 Ti is known as an excellent adhesion layer to Si and its oxides and nitrides, but not as barrier against the diffusion of Au-Sn solder. Therefore, it is necessary in practice to add a "diffusion barrier" such as Pt or Ni. However, during bonding when the solder is heated above its eutectic temperature, the Au-Sn solder reacts in seconds with the Pt or the Ni layer, resulting in depletion of Sn and a drastic increase in the melting temperature of the solder. This causes premature freezing of the solder. In order to provide an inert barrier, a complex metallization scheme involving five or more different layers, e.g., Ti/W/W(Ni-Sn)/Ni 3 Sn 4 /Au-Sn, has been used in today's integrated circuit packaging.2 In a recent paper3 we reported on a simple Nb-based metallization structure which shows promise for use in bonding semiconductor devices and integrated circuits. The Nb layer has been shown to act both as an effective adhesion layer and as a perfect diffusion barrier to Au-Sn solder, and to exhibit appropriate adhesion to the substrate. In this paper, we present a more extensive study of the bonding properties, the mechanical adhesion strength, and failure modes of a number of Nb-based bonding structures. We will show that the Nb layer bonds strongly to SiO 2 , with an adhesion strength greater than 10 kpsi exceeding the Si-Si cohesive strength. The Nb-Si and N b - O bonds are shown to be thermodynamically stable.
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