Interface structure of photonic films created by plasma enhanced chemical vapor deposition

Plasma polymerized (PP-) films offer several advantages over conventionally deposited polymeric films including good adhesion to a variety of substrates and bulk cross-linked structures. However, several facets of the structure of these commercially important materials have received almost no systematic study. There were three key objectives for this work. The first was to understand how the formation of the interfaces of a single layer by plasma enhanced chemical vapor deposition (PECVD) impacts the width and type of interface formed between layers in a multilayer system. The second objective was to determine whether the cross-link density in PECVD deposited films is uniform through the depth of the film and, if not, how it varies with depth. The final objective was to map out how both interface structure and cross-link density uniformity are impacted by the important parameters of monomer feed position, plasma power, and reactor pressure. In pursuing these objectives, PP-homopolymer films, PP-bilayer films, and PP-copolymer single layer films were studied.Changes in the structure of PP-homopolymer films with processing parameters were investigated using X-ray reflectivity (XR) and neutron reflectivity (NR). For ultrathin PP-homopolymer films (less than 200 A) deposited under 45 W power and 0.6 and 0.05 torr reactor pressures, variations in structure with depth near the substrate and near the surface were much more readily apparent than in thick films. Thin PP-homopolymer films (greater than 250 A) deposited at 30 W power and 0.6 torr reactor pressure showed no variations in structure with depth. A transition region next to the substrate in which the structure varies markedly, over a very small depth, was also observed for both PP-octafluorocyclobutane (OFCB) and PP-benzene (BZ) films. Variations in the cross-link density with processing pressure were much stronger for PPBZ films than for PP-OFCB films.Measurements of films swollen with solvent vapor revealed that each film had a thin layer (1-1.5 nm) of lower cross-link density next to the substrate and a layer (1.5-3.0 nm) of lower cross-link density next to the air where the vast majority of the swelling took place. The thickness of the region of lower cross-link density at the surface reflects the extent of a reaction zone that moves with the deposition and is responsible for dictating the width of interfaces that are formed when a layer of different precursor is deposited atop the first layer.Changes in film structure caused by "copolymerization" with a second precursor to form PP-copolymer films were investigated using X-ray reflectivity (XR). Copolymer films made from comonomers BZ, OFCB, and hexamethyldisiloxane (HMDS) showed extremely sharp interfaces and scattering length density depth profiles that were uniform with depth. Addition of either benzene or HMDS as a comonomer in the deposition of OFCB diminished the transient deposition behavior at the silicon oxide interface that occurs when using only OFCB. For the B-OFCB copolymer films, a facile control of refractive index with monomer feed composition was achieved. A peak in the X-ray scattering length density and nonlinear visible light refractive index (633nm) with composition was observed for the HMDS-OFCB copolymer film.