Advanced analytical approaches to characterize ion mobility and structural changes in polyimides

Abstract

High-performance polymers are usually indicated by their extraordinary material properties like high heat and oxidative resistance, chemical inertness, high dimensional stability and corrosion resistance. Because of their ability to maintain their mechanical and physical properties over a wide temperature range, they are widely used in applications that involve high temperature and extreme environment. Polyimides are one important class of high-performance polymers, characterized by an imide group as a building block of the polymer backbone. Since they can be processed as films, fibers, porous membranes, etc. they have found extensive implementations in the automotive, aerospace and electronics industry. Due to their low dielectric constant and very good electrical insulation properties, they are particularly interesting for electronics and electrical engineering applications and are extensively used as isolation and protective medium. Their purpose is to ensure reliable operation under harsh conditions, for instance in the presence of high temperatures, humidity or corrosive gases. To prevent device failure and improve the reliability of microelectronic devices, two interdependent questions regarding the use of polyimides arise: (1) the uptake and diffusion behaviour of corrosive species within polymers, especially polyimides are of great interest and (2) techniques to obtain structural information about the polymer and monitor alterations both laterally and depth-resolved are needed for thorough investigations.In this work, we aimed to implement LIBS (Laser induced breakdown spectroscopy) and LA-ICP-MS (Laser ablation inductively coupled plasma mass spectrometry) based measurement protocols to investigate the migration pattern of harmful substances like Cl and K while gaining information about the polymer structure. Especially the usage of LIBS for polymer analysis could be a promising strategy to perform elemental as well as structural analysis since characteristic regions in the LIBS spectrum can be used to gain molecular information.In order to keep the system as simple as possible and limit the number of influencing parameters, industry relevant polyimides were synthesized and used as a model substance. First, the capabilities of LIBS to monitor possible structural alterations were tested and the imidization reaction was chosen as an illustrative example. This reaction is especially important because the shaping of polyimides is typically done using a soluble precursor which is cyclodehydrated in a thermal or chemical process called imidization. The degree of imidization is one parameter, in general obtained by IR (infrared) spectroscopy representing the conversion of the amic acid group into the imide and it is closely related to the final properties of the polymer. Using LIBS, the determination of the imidization degree as a bulk parameter was sucessfully implemented and similar results compared to IR spectroscopy were obtained. Additionally, depth resolved polymerization studies of layered polymer structures were conducted enabling a deeper insight into the reaction process and the optimization of curing parameters during production. Furthermore, the robustness of the procedure was demonstrated in the presence of additives and by analyzing industry-relevant polymer samples. Subsequently, the investigations were continued using an in-situ measurement setup to study the reaction process within polymer thin films in more detail. With this approach, sample preparation time was minimized and the reaction progress could be tracked continuously on the same sample. Furthermore, the implemented LIBS method was transferred to other instrumental setups to ensure independence from hardware conditions like the laser wavelength.In order to investigate failure mechanisms, the uptake and distribution of harmful substances is equally important as changes in polymer structure. Determining the degree of imidization based on LIBS was an important step in this direction. Subsequently, the methodology was optimized and applied in order to quantify the chlorine (Cl) and potassium (K) uptake in polyimides as examples for corrosion-inducing substances in an electronic device. Therefore, commercially available polyimide films were exposed to various aqueous solutions and voltage in a migration cell to simulate environmental stress testing. Especially halogens, such as chlorine pose a challenge in elemental analysis. As a result, different approaches to obtain maximum sensitivity were employed: (1) byreducing the ambient pressure during LIBS measurements and (2) by implementing a LA-ICP-MS measurement protocol. Both imaging and depth-profiling experiments of Cl and K distribution profiles within the stressed polyimide samples were conducted and revealed an even distribution of both elements over the sampled area. The penetration profiles were dependent on the exposure conditions and Cl uptake only occurred after a certain time period. Compared to the initial LIBS experiments, where a limit of detection (LOD) of 0.71 m% was calculated for Cl, a clear improvement in sensitivity (LOD = 0.015 m%) and depth resolution (approx. 200 nm) was achieved by implementing a LA-ICPMS method although imaging experiments were not feasible due to severe problems regarding the sample transport. As an alternative, LIBS measurements under reduced pressure still enabled imaging while obtaining an LOD of 0.12 m% for Cl.In combination, the newly developed analytical approaches using LIBS and LA-ICPMS proved to be a powerful toolbox to investigate migration patterns and track the imidization reaction within a high-performance polymer.