Environmental Effects on Crack Characteristics for OSG Materials
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Environmental Effects on Crack Characteristics for OSG Materials Jeannette M. Jacques, Ting Y. Tsui, Andrew J. McKerrow, and Robert Kraft Silicon Technology Development, Texas Instruments Inc., Dallas, Texas 75243 ABSTRACT To improve capacitance delay performance of the advanced back-end-of-line (BEOL) structures, low dielectric constant organosilicate glass (OSG) has emerged as the predominant choice for intermetal insulator. The material has a characteristic tensile residual stress and low fracture toughness. A potential failure mechanism for this class of low-k dielectric films is catastrophic fracture due to channel cracking. During fabrication, channel cracks can also form in a timedependent manner due to exposure to a particular environmental condition, commonly known as stress-corrosion cracking. Within this work, the environmental impacts of pressure, ambient, temperature, solution pH, and solvents upon the channel cracking of OSG thin films are characterized. Storage under high vacuum conditions and exposure to flowing dry nitrogen gas can significantly lower crack propagation rates. Cracking rates experience little fluctuation as a function of solution pH; however, exposure to aqueous solutions can increase the growth rate by three orders of magnitude. INTRODUCTION The integration of low-k dielectric films is required to maintain and improve device performance in future technologies. For intermetal dielectrics, the class of materials known as organosilicate glass (OSG) has emerged as the principal candidate. During the manufacturing process, catastrophic fracture due to channel cracking is a potential failure mechanism. 1 The driving force for channel cracking is dependent on several material properties, with the film modulus, density, and residual tensile stress serving as key factors. 2-3 Channel cracks can also form in a time-dependent fashion due to exposure to specific environmental conditions. 2 These mechanisms are commonly known as environmentally-assisted or stress-corrosion cracking. A typical cohesive failure mode for thin films under stress is channel cracking. Channel cracks are defined as extending through the film thickness and propagating perpendicular to the substrate-film interface. 2 Studies by Cook et al. 3, 4 showed that the crack propagation rate V in the reaction-limited regime can be expressed by Equation 1. where σ represents the residual film stress, h the film thickness, and E the plane-strain modulus. The variable k is Boltzmann’s constant, T is temperature, and n denotes the area density of bonds fractured at the crack tip during crack propagation. Z is a constant defined by crack geometry and Vo is a fitting constant. The density of bonds fractured during crack separation n is analogous to the average film density ρ. 5
V = Vo e
πZ ⎛⎜ σ 2 h ⎞⎟
4 nkT ⎜⎝ E ⎟⎠
(1)
The majority of studies 6-13 regarding the cracking and failure of silicon-based structures have employed double cantilever beam geometries and cleavage techniques. Limited studies 2-3 have utilized channel
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