Spectroscopic Studies on the Molecular Structural Changes of Polymer Substrates and Polymer Adhesives Relevant to the Microelectronics Industry

Abstract

Microelectronic devices affect us every day; they are ubiquitously utilized in automotives, personal electronics, and biomedical devices. Adhesives are used in microchip assemblies to attach the chip to the printed circuit board, which greatly increases the lifetime of the microelectronic device. The focus of this thesis is to elucidate molecular interactions key to adhesion, how to improve adhesion, and how the molecular orientation at buried interfaces can affect macroscopic adhesion properties. In this work the intrinsically surface sensitive technique, sum frequency generation (SFG) vibrational spectroscopy, was utilized to noninvasively investigate buried interfaces in situ. By probing systems in their natural unperturbed state, we can deduce information that enables better design of adhesion systems. In this work, adhesion promoters, plasma treatment, real electronic devices, and flux residues were characterized and the molecular details of the buried interfaces were correlated to the macroscopic properties. Epoxy based adhesives were modified with small amounts of adhesion promoters and they drastically improved the adhesion strength following accelerated stress testing; they were also capable of preventing interfacial water. This research demonstrates that molecular structural studies of buried epoxy interfaces during hygrothermal aging using SFG spectroscopy can greatly contribute to the overall understanding of moisture-induced failure mechanisms of adhesives found in microelectronic packaging. Another method to improve the adhesion strength is plasma treatment, which is utilized to clean and activate the substrates prior to solder reflow and applying epoxy underfill. Two projects are outlined in this work, (1) plasma treatment of covered surfaces, and (2) plasma-based adhesion promotion. For (1), polymer surfaces were protected with a cover and exposed to various plasmas, to simulate the plasma processing steps found within the microelectronics industry. It was demonstrated that the middle and edge regions of the covered polymer surface behaved differently when exposed to various plasmas, which was determined both qualitatively and quantitatively. Next, project (2), focused on both the surface and the buried polymer/epoxy interface, and how plasmas treatment can improve the adhesion strength. The mechanism of this increase in adhesion strength has not been thoroughly investigated at the molecular level in situ previously, because it is difficult to probe a buried interface where the adhesion occurs. To understand how plasma changes the surface and the corresponding buried interface, polymer surfaces were plasma treated and then put in contact with epoxy. It was found that the molecular structure of the buried interface of the pristine polymer/epoxy interface is drastically different from the plasma treated polymer/epoxy interface. The buried interface with plasma treated polymer surface was found to be very disordered and had much higher adhesion strength. The main mechanism for plasma-based adhesion when using He plasma was found to be disordering of the interface. This research elucidates the plasma treatment effects on structures and properties of buried polymer/epoxy interfaces, providing in-depth understanding on the mechanism of adhesion strength increase facilitated by plasma treatment.