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Deformation and fracture of thin films on compliant substrates are key factors  constraining the performance of emerging flexible substrate devices. [1-3] These  systems often contain layers of thin polymer, ceramic and metallic films and  stretchable interconnects where differing properties induce high normal and  shear stresses. [4] As long as the films remain bonded to the substrates, they  may deform far beyond their freestanding form. Once debonded, substrate  constraint disappears leading to film failure. [3] Experimentally it is very  difficult to measure properties in these systems at sub-micron and nanoscales.  Theoretically it is very difficult to determine the contributions from the  films, interfaces, and substrates. As a result our understanding of deformation  and fracture behavior in compliant substrate systems is limited. This motivated  a study of buckle driven delamination of thin hard tungsten films on pure PMMA  substrates.  The films were sputter deposited to thicknesses of 100 nm, 200 nm, and 400 nm  with a residual compressive stress of 1.7 GPa. An aluminum oxide interlayer was  added on several samples to alter interfacial composition. Buckles formed  spontaneously on the PMMA substrates following film deposition. On films without  the aluminum oxide interlayer, an extensive network of small telephone cord  buckles formed following deposition, interspersed with regions of larger  telephone cord buckles. (Figure 1) On films with an aluminum oxide interlayer,  telephone cord buckles formed creating a uniform widely spaced pattern.  Through-substrate optical observations revealed matching buckle patterns along  the film-substrate interface indicating that delamination occurred for large and  small buckles with and without an interlayer. The coexistence of large and small  buckles on the same substrate led to two distinct behaviors as shown in Figure 2  where normalized buckle heights are plotted against normalized film stress. The  behaviors deviate significantly from behavior predicted by rigid elastic  solutions.  To address this issue we developed a finite element analysis technique that  employed a cohesive zone model to simulate interfacial crack growth. Specifying  the traction-separation relationship, cohesive strength, and work of separation  along with film thickness, film stress, and film and substrate properties,  buckle width and height were determined as a function of interfacial toughness.  The simulations indicate that an analysis based on rigid substrate solutions  significantly underestimate toughness for prescribed buckle widths: a result  consistent with an analysis by Yu and Hutchinson [5] that pieced together a  solution based on non-linear plate theory with a solution for the linear film on  substrate problem. More importantly, the results defined a lower limiting bound  to seemingly disparate buckle deflection data (Figure 2). The variance from  linear elastic behavior, especially for the small buckles, indicates more than  substrate compliance is controlling behavior. Comparison of the experimental  results with cohesive zone simulations suggests that the two buckle behaviors  are associated with different levels of substrate yielding. In this presentation  we will use the results to show how substrate compliance and deformation affect  delamination and buckling of films on compliant substrates and provide a means  to predict device performance

Buckle Driven Delamination in Thin Hard Film Compliant Substrate Systems