Effect of Aqueous Solution Chemistry on the Accelerated Cracking of Lithographically Patterned Arrays of Copper and Nano
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Effect of Aqueous Solution Chemistry on the Accelerated Cracking of Lithographically Patterned Arrays of Copper and Nanoporous Thin-Films E. P. Guyer and R. H. Dauskardt Department of Materials Science and Engineering, Stanford University 416 Escondido Mall, Stanford, CA 94305 ABSTRACT The effect of moisture and aqueous solution chemistry on the rate of crack growth in lithographically patterned arrays of copper and low dielectric constant (LKD) materials is reported. Crack growth in the direction orthogonal to the features is demonstrated to fail at significantly increased loads compared to parallel cracking. Decreasing feature width is also shown to increase the structures resistance to fracture, particularly at reduced loads. The chemical interactions and mechanisms of energy dissipation are discussed. INTRODUCTION Considerable research is being directed at integrating nanoporous LKD layers into the interconnect structures of high-density integrated circuits. The reliable fabrication of devices containing these extremely fragile materials is, however, a significant technological challenge due to their high propensity for mechanical failure and susceptibility to stress corrosion cracking in moist environments. Survival through chemical mechanical planarization (CMP) and subsequent device packaging is of particular concern as the device structures are subjected to additional loads in the presence of highly reactive solution chemistries [1]. Although significant efforts have been directed at formulating CMP slurries that optimize polishing rates and minimize dishing, little consideration is given to the affect these harsh solutions have on accelerating the rate of stress corrosion cracking [2, 3]. The synergistic effect of mechanical loads and reactive aqueous solutions was recently demonstrated to have a significant effect on the rate of crack growth in blanket thin-films of nanoporous methylsilsesquioxane (MSSQ) [4, 5]. Interconnect devices, however, consist of a number of dissimilar materials that are arranged in lithographically patterned arrays typically containing copper, LKDs, and various diffusion barriers (e.g. TiN, SiC, SiCN, SiO2). Although the interfacial adhesion of patterned films has been investigated [6], virtually nothing is known about the effect aqueous solution chemistry has on accelerating crack growth in these structures. An advanced understanding of this fundamental mode of failure is necessary for MSSQ and other LKDs to be considered viable candidates for next generation interconnect devices. EXPERIMENTAL TECHNIQUES The patterned structures selected for study are shown schematically in Figure 1 (a). The MSSQ and copper lines were approximately equal in width and were 300 nm in height. Features widths of 300 nm and 500 nm were examined. Blanket thin-films of MSSQ capped with a technologically relevant SiC barrier layer were also investigated. The MSSQ (JSR 5109) was ~20% porous, with pores of ~1.5 nm in diameter, and a dielectric constant of 2.2. Double cantilever beam (DCB) sample geo
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