UV-induced porosity using photogenerated acids to catalyze the decomposition of sacrificial polymers templated in dielectric films

Abstract

A UV-initiated process for selective incorporation of porosity and tuning of refractive index in a dielectric film is presented. The formation of porosity via radiation offers the advantages of lower processing temperatures and shorter processing times than most processes that rely on thermally-induced porosity. The ability to pattern porosity via the use of a photomask allows structures such as porous direct-write waveguides to be made. The direct-write process presented here used UV-generated acids to catalyze the decomposition of sacrificial polymers templated in a crosslinked matrix, resulting in the formation of porosity and decrease of refractive index and dielectric constant after subsequent removal of the generated acid. In unexposed areas, no acid molecules were generated, thus these areas remained non-porous and their refractive index remained relatively high compared to porous regions. Base-catalyzed gelation was used to crosslink the matrix at temperatures well below the glass transition temperature of the matrix in order to avoid thermal activation of the photoacid generators (PAGs) in the direct-write formulation. After crosslinking the matrix, deep ultraviolet (DUV) radiation was used to initiate the direct-write reactions thus creating a refractive index profile within the film. Neutralization or volatilization of the photogenerated acid was used to deactivate the system. The lowest interline dielectric constant achieved by direct-written films was 2.36 after the volatilization step compared to a value of 2.29 for porous films formed via the traditional template process. At optimum conditions, the average amount of UV-induced porosity was estimated to be as high as 21%. Transmission electron microscopy (TEM) imaging showed that elongated mesopores (10 nm by about 20–30 nm) and elongated macropores (100 nm by about 200 nm in size) were produced within the internal structure of direct-written films. The porous films formed via the traditional template technique had internal structures with large collections of spherical mesopores (5–10 nm) located in close proximity to one another due to phase segregation of the polymer at high temperatures.