(3an) Design of Functional, Patternable Vapor-Deposited Polymer Thin Films

Initiated Chemical Vapor Deposition (iCVD) is a low-energy process for synthesizing functional polymer thin films. Since surface tension and de-wetting effects are absent during vapor-phase processing, iCVD thin films conform to the geometry of the underlying substrate. Additionally, the benign reaction conditions inherent to the iCVD process allow for complete retention of polymer functionality, as well as the ability to deposit on delicate substrates.  

iCVD can be used to conformally coat templates defined via electron-beam lithography.  The template is removed using a non-solvent for the iCVD layer, yielding well-defined polymeric features that range in size from the micro to the nanoscale.  The process is chemically-nonspecific, and can be used with any iCVD polymer, provided an appropriate non-solvent can be determined.  The features retain their functionality; this has been confirmed by incorporating poly(4-vinylpyridine) microstructures into sensors for detecting nitroaromatic explosives.

iCVD can also be used to synthesize thin films of conformal photoresist.  After deposition, the photosensitivity of the film--typically poly(4-vinylpyridine) or poly(4-aminostyrene)--is enhanced by functionalizing the iCVD polymer with a photoactive diacetylene. Exposure of the functionalized films to UV light induces 1,4 addition photopolymerization of the diacetylene, making the exposed regions insoluble in common solvents. The use of iCVD affords unique attributes to the process, including the ability to pattern bifunctional surfaces as well as polymeric and metal microstructures on non-planar substrates.  Importantly, the resolution and sensitivity of the iCVD resist are comparable to those of commercial products. 

The properties and chemical composition of iCVD films are easily tuned by adjusting deposition parameters.  Divinylbenzene (DVB) is an excellent cross-linking molecule for iCVD copolymers due to its high vapor pressure and vinyl bond reactivity.   A quantitative study of DVB vinyl bond reactivity and incorporation of DVB into iCVD copolymers has been performed using FTIR spectroscopy.  Experimentally-determined sticking probabilities and monomer reactivity ratios and the effect of DVB incorporation on mechanical properties such as Young’s modulus lend insight into the utility of this material.