(530e) Initiated Chemical Vapor Deposition of Polymer Thin Films for Photolithography Applications

The use of initiated chemical vapor deposition (iCVD) to synthesize thin films of poly(4-vinylpyridine) (P4VP) is reported.  The films are functionalized post-deposition with carboxylic acid-containing diacetylene monomers.  Exposure of the functionalized films to UV light (λ=254 nm) polymerizes the diacetylene, making the exposed regions insoluble in common organic solvents. 

Functionalization can be achieved by dissolving the diacetylene monomer in toluene and using a dip-coating process. Fourier Transform Infrared spectroscopy confirms the presence of hydrogen bonding between the carboxylic acid moieties and the nitrogen atoms of the pyridine rings.  For P4VP films with thicknesses less than 35 nm, sublimation can be used; use of a solvent-less process results in improved pattern resolution and uniformity and reduction of the polydiacetylene crystalline domains observed as a result of the dip-coating process.  Polymerization of the diacetylene is verified using UV-visible spectroscopy.  Patterns with features smaller than 5 μm have been defined on planar substrates using a contact photolithography mask. 

The use of iCVD affords some unique attributes to the photosensitive film.  In this process, a thermally-labile gas phase initiator is decomposed into radicals as it travels through an array of heated filaments. Free radical polymerization occurs as the initiator radicals and monomer adsorb onto a cooled substrate located beneath the filament array. iCVD is a low-energy technique that results in excellent control over film thickness and conformality. The library of iCVD polymers is substantial and includes a wide range of chemical functionalities, as well as several insoluble or infusible polymers that cannot be easily synthesized using other methods. 

The conformality attainable using iCVD makes it possible to pattern micro-scale features on a variety of non-planar surfaces.  Glass rods (D=2-6 mm) were coated with iCVD P4VP and functionalized using the methods described above.  TEM grids (Cu, 45 μm parallel bars, spacing 80 μm) were used as contact masks; the films were exposed for three minutes, then developed in ethanol.  The resulting patterns were well-defined—the 2 mm rod exhibited the entirety of the grid pattern (wrapped halfway around its circumference), with excellent agreement observed between the mask and the patterned polymer. 

This process can also be used to develop multifunctional surfaces.  200 nm of iCVD P4VP was deposited on top of 35 nm of iCVD poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA).  Functionalization of the P4VP, followed by application of a mask, UV exposure, and developing, yielded a patterned surface with regions of contrasting hydrophobicity.  This method is facilitated by the fact that the unexposed P4VP is removed during the developing process, which utilizes a non-solvent for the underlying PPFDA layer.