Aqueous-Derived Thin Films and their Interfacial Interactions with Semiconductor Surfaces: A Spectroscopic Study
Thomas, Milana Cherie
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Metal oxide systems are well known for their high dielectric constants, which are important for advanced microelectronics applications. The microelectronics industry currently employs vacuum-based techniques, such as chemical vapor deposition (CVD), to deposit metal oxide films. These vapor-phase deposition techniques suffer due to their slow deposition rates and their use of expensive equipment. Additionally, these processes sometimes require the use of harmful source gases and/or generate corrosive by-products. On the other hand, solution-processed thin films fabricated by spin-coating are advantageous because the process is simple, low cost, and scalable. Aqueous solution deposition is particularly attractive because it offers a green alternative to vapor-phase deposition and has been shown to produce uniform thin films by spin coating on hydrophilic silicon surfaces. However, it has been shown that silicon’s native oxide can degrade device performance due to its electronic interfacial states. In addition, aqueousderived thin films suffer from poor electrical performance due to mobile water and hydroxyl protons, often requiring very high temperature anneals to mitigate. Such anneals compromise the interface between the film and the silicon substrate, hence the electrical performance. One effective method to control the interface, and thus improve device performance, is to functionalize the semiconductor surface using wet chemistry. Here, we address the concerns of aqueous thin film deposition and present a method for alleviating the issues associated with current silicon-silicon oxide devices. We use wet chemical functionalization to graft selfassembled monolayers (SAMs) onto oxide-free silicon, then spin-coat an aqueous thin film on top of the SAM layer. The chemical stability of the SAM and the changes that occur at the interfaces between the Si/SAM/film stack during film deposition and dehydration are monitored by in situ Fourier transform infrared spectroscopy (FTIR) and ex situ X-ray photoelectron spectroscopy (XPS). The modification of the Si/SAM interface is studied as a function of annealing temperature, with electrical measurements used as a metric to quantify the effectiveness of the SAM layer to alleviate issues of interfacial defects observed for films on silicon oxide. The results are presented in three parts: (1) a dehydration study of aqueous-derived thin films deposited on silicon oxide, (2) the synthesis of a novel SAM interfacial layer tailored to accommodate aqueous, Al-based precursors and (3) a study to quantify the effectiveness, if any, on the SAM interfacial layer through electrical characterization methods. In the first part, we investigate the mechanism for dehydration of aqueous thin films and present a method to enhance the removal of water from the films. Using in situ FTIR, we find that the addition of a protective capping layer can enhance the dehydration of the thin film and prevent water reabsorption for a period of up to 14 days. In the second part, we present hydrosilylation methods to graft SAMs onto oxide-free silicon surfaces. The results show that it is possible to covalently attach the SAMs to silicon, evidenced by the formation of Si-C (detected by XPS) at the interface between the Si and the SAM. Four phosphonic acid-terminated SAMs are prepared and contact angle measurements are used as a metric for evaluating which can best accommodate aqueous spin-coater solutions. To conclude, we investigate the interface between the SAM layer and an aluminum-based thin film derived from aqueous precursor solutions. Current-voltage and capacitance-voltage measurements are used to quantify the effectiveness of the SAM layer.