Invited Paper TF-MoM3
Conformal Polymeric Thin Films via Initiated Chemical Vapor Deposition

Monday, October 15, 2007, 8:40 am, Room 613/614

Conformal surface modification of high aspect ratio micro- and nano-structures is desirable for a range of applications ranging from microelectronics to textiles. In this work, initiated chemical vapor deposition (iCVD) is demonstrated to conformally deposit thin polymer films (<500 nm) at high growth rates (up to 100 nm/min). The iCVD technique is a low energy process (0.01 W/cm²) that fully retains the pendent functionality in the deposited polymer films, a result that has been demonstrated with more than 40 different vinyl monomers. The iCVD method is particularly valuable for creating ultrathin layers of insoluble polymers. For the process, initiator and monomer vapors simultaneously flow into a vacuum where decomposition of the initiator results in free-radical polymerization of the monomer on the substrate. As iCVD relies on neutral chemistry, directional effects on deposition typically associated with the electric fields in plasma processes are avoided. Step coverage of trenches in silicon substrates by polymeric iCVD coatings will be shown to depend on the ratio of monomer to initiator. A kinetic model highlights the surface-driven nature of iCVD polymerization and that is analogous to free radical polymerization in the bulk phase, albeit on the surface. The iCVD process is often absorption limited and hence cool (~25 ºC) substrates are essential for rapid film growth. From an applications perspective, low substrate temperatures are favorable for the coating of thermal sensitive materials, including paper, plastics, fabrics, and membranes. The iCVD process can be used to functionalize membranes with high aspect ratio (80:1) pores with a hydrophobic polymer coating. X-ray photoelectron microscopy data confirmed the presence of the iCVD coating on the topside and backside of the membranes and electron microprobe analysis confirmed the presence of the coating along the pore wall. Additionally, the iCVD process has been demonstrated conformally to coat carbon nanotube forests and microparticles. Superhydrophobic surfaces (contact angles >175 degrees) result from surface modification of nanostructured substrates by iCVD PTFE and polymerized perfluoroalkyl ethyl methacrylate.