Atomic Force Microscopy Studies of Fracture Surfaces From Oxide / Polymer Interfaces
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Atomic Force Microscopy Studies of Fracture Surfaces From Oxide / Polymer Interfaces
Maura Jenkins, Jeffrey Snodgrass, Aaron Chesterman, Reinhold H. Dauskardt, and John C. Bravman Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, U.S.A. ABSTRACT Atomic Force Microscopy (AFM) is used to characterize fracture surfaces in silicon oxide / silane adhesion promoter / BCB polymer systems. Fatigue striations were found on some samples, and these were correlated with the crack growth rate per fatigue cycle. X-ray Photoelectron Spectroscopy (XPS) was used to identify the species present on each surface, and it was found that striations only form when the fracture path is through the polymer. INTRODUCTION Fatigue striations are deformation bands that appear on fatigue fracture surfaces. They can be seen as evidence of plastic deformation during the fatigue process. When bands are seen on a metallic surface, they can be reliably identified as fatigue striations. In polymers, however, there are several deformation mechanisms that may be operating, for example shear banding. In order for bands to be positively identified as fatigue striations, it must be verified that a single striation is formed during each fatigue cycle1. Many studies have focused on fatigue striations in metals2 and bulk polymers3-4. Few have investigated the phenomenon in thin film polymer structures. In this work, several multilayer structures were produced and fractured using various fatigue debond growth rates. The resulting fracture surfaces were then examined using atomic force microscopy (AFM) to determine whether striations were formed. The AFM software allows for measurement of features, allowing the striation sizes to be recorded. X-ray photoelectron spectroscopy (XPS) was used to determine the fracture path formed in each case, and thus explain the appearance of fatigue striations in some samples but not others. EXPERIMENTAL DETAILS This study focuses on fatigue striations in a silicon dioxide / silane adhesion promoter / polymer system. The thin film sandwich structure is used to study the mechanical properties of these materials for use in microelectronic packaging. The two adhesion promoters are γaminotriethoxysilane and vinyltriethoxysilane (AP8000 and AP3000, Dow Chemical Co., Midland, MI), and their structures are given elsewhere5. The polymer is benzocyclobutene, or BCB (Dow Chemicals Cyclotene 3022-35), which is also discussed in depth elsewhere6. The BCB polymer serves to protect the completed integrated circuit, and the adhesion promoter increases the interfacial bonding effectiveness in this system. Samples were prepared by creating a sandwich structure in which a single layer of BCB polymer is bonded to two native oxide-covered silicon wafers using an adhesion promoter. Fourinch silicon wafers were cleaned in 9:1 H2SO4:H2O2 at 120°C for 20 minutes before being rinsed AA2.7.1
with deionized water and dried. The wafers were then spin-coated with either AP8000 or AP3000 solution to produce an adhes
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