Initiated Chemical Vapor Deposition of Polymer Thin Films for Photolithography Applications

The use of initiated chemical vapor deposition (iCVD) to synthesize conformal layers of photoresist is reported.  iCVD films are functionalized with a photosensitive species post-deposition using a dip-coating process.  Exposure of the functionalized films to UV light induces photopolymerization, making the exposed regions insoluble in common organic solvents.  Functionalization and photopolymerization are confirmed using Fourier Transform Infrared spectroscopy and UV-visible spectroscopy, respectively.

The use of iCVD affords unique attributes to the photoresist.  In the iCVD 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.  Non-planar surfaces can be coated with iCVD polymer; these surfaces are easily post-functionalized to achieve a uniform layer of photoresist.  Importantly, the resolution (3-4 μm) and sensitivity (0.071 ± 0.010 J/cm²) of the iCVD resist are comparable to those of commercial products used for planar lithography.

Micro-scale features have been patterned on a variety of non-planar surfaces.  Glass rods (D=2-6 mm) were coated with the iCVD layer and post-functionalized with photosensitive material.  TEM grids (Cu, 45 μm parallel bars, spacing 80 μm) were used as contact masks and were wrapped around the rods.  The patterns observed after exposure and developing 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. 

The resist can also be used to fabricate multifunctional surfaces.  200 nm of the iCVD thin film described above was deposited on top of 35 nm of iCVD fluoropolymer.  Functionalization of the top film with the photosensitive species, 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 photosensitive layer is removed during the developing process, which utilizes a non-solvent for the underlying fluoropolymer.  Use of the iCVD photoresist for metal patterning has also been demonstrated.  Patterns of Au/Cr have been fabricated on planar silicon and glass substrates; fabrication of these structures on curved substrates is the subject of current work.