Fracture on thin layer systems with realistic feature geometry

Date of Completion

January 2001


Applied Mechanics|Engineering, Mechanical




Thin films are commonly employed in many applications, varying from thermal barrier coatings to microprocessor computer chips. Reliability is an important issue in such applications, yet for some situations the quantitative details of mechanics of failure are not well understood. This paper investigates several different thin film fracture problems, with the goal of establishing a mechanics framework to quantify critical flaw sizes, configurations, stresses and material properties that lead to fracture. ^ The common feature among the problems investigated is that all incorporate some sort of realistic feature geometry. The first situation studied is an interface crack in a three-dimensional corner or edge geometry. The role of the crack front shape is investigated and propagation characteristics are determined. A second fracture scenario studied is finite length film channeling cracks, which are explicitly modeled in three-dimensions. It is determined when plane analyses are applicable, and different methods of calculating the energy release rate are compared. Plastic deformation of the substrate is also considered in this situation. Finally, a realistic model of microprocessor chip architecture is modeled. A plane model of a periodic repeating layered structure is created, and channel cracking between the sections is examined. The effects of section width, material properties, and residual stresses are considered. ^ Several important conclusions are drawn from the research presented in this dissertation. For interface cracks, the likelihood of crack propagation increases dramatically near the free edge, and the plane strain conditions are reached in the center of a crack when the aspect ratio (width to depth) of the crack front is greater than 4. For the channeling cracks emanating from a free edge, steady state conditions are reached more slowly than for contained channeling cracks, requiring crack lengths greater than 15 times the film thickness. In addition, traditional plane methods of calculating the energy release rate are inappropriate when a residual stress in the substrate causes plastic deformation. For the periodic architecture, decreasing the width of low-modulus sections may in some cases increase the risk of cracking, and residual stresses in adjacent layers significantly affects cracking (contrary to blanket film results). ^