Enabling integration of vapor-deposited polymer thin films

Abstract

Initiated Chemical Vapor Deposition (iCVD) is a versatile, one-step process for synthesizing conformal and functional polymer thin films on a variety of substrates. This thesis emphasizes the development of tools to further enable the use of iCVD for industrial applications. The ability to pattern polymer thin films is a prerequisite for device fabrication. Two methods were developed for patterning iCVD polymers. The first technique facilitated patterning of nano- and microscale features of any iCVD thin film on planar surfaces. Retention of polymer functionality was demonstrated by incorporating the features into high-resolution resistive sensors. The second method adapted photolithographic techniques to achieve patterning on highly curved surfaces. Non-planar substrates were coated with a uniform layer of a functionalized, photoreactive iCVD polymer and exposed to ultraviolet light through a flexible mask. Exposed regions became insoluble in a developing solvent. The resolution and sensitivity of this iCVD-based negative photoresist were comparable to those of commercial products. Additionally, the patterned polymer was used as a mask for patterning metal on planar and curved surfaces. iCVD is typically a semi-continuous process. A batch process was investigated in order to minimize the use of expensive and corrosive reactants. The chemical functionality and conformality of the films were unaffected by the change in processing mode. Reaction yield was improved by one to two orders of magnitude for several film chemistries. iCVD is also unique in that it enables the deposition of cross-linked polymer films, which are difficult to create using conventional, solution-based methods. To potentially enhance durability, cross-linked poly(divinylbenzene) and poly(4-vinylpyridine-co-divinylbenzene) films were synthesized via iCVD. This is the first vapor-phase synthesis of the copolymer, which is a major component of many commercial ion exchange membranes. The degree of cross-linking was quantified using spectroscopic methods and was tightly controlled by adjusting the flow rate of divinylbenzene. Corresponding changes in the elastic moduli of the films were confirmed using nanoindentation. The first vapor-phase synthesis of poly(vinyl cinnamate) was also demonstrated. The cross-linking density of this polymer increases upon exposure to ultraviolet light and is readily quantifiable. Vinyl cinnamate was incorporated into a copolymer with N-isopropylacrylamide, yielding a temperature and light-responsive thin film.