Physical properties and microelectronic applications of low permittivity fluoromethylene cyanate ester resins

Abstract

A series of fluoromethylene cyanate ester resins have been synthesized and studied as potential low dielectric resins for microelectronics. Physical properties were observed to change with the fluoromethylene segment length in the monomer. The polymer can be formed from a stable oligomer that is thermally converted in a single step to a crosslinked thermoset. The complex permittivity decreases with the number of fluoromethylenes in the monomer. The dielectric constant changes significantly with the n=6 monomer and appears to show very little change for the n=8, 10 compounds. The loss tangent appears to have a downward trend with increasing fluoromethylene chain length. Complex permittivities of select systems were studied up to frequencies as high as 40 GHz. Differential scanning calorimetry was used to determine melting points of the monomer series and show them increasing with the fluoromethylene chain length. With a broad exothermic curing reaction between 200 and 300 °C, the process window is sequentially shortened as the number of fluoromethylenes increases. This makes the processing of the larger monomers (i.e., n=8, 10) very difficult. A comparison of physical properties and ease of processing shows the n=6 compound to be an optimum compromise. A prepolymer of the n=6 system (F6Cy) was made for spin coat processing with a conversion of 30%–40% determined by F19 NMR. A 2 h cure at 175 °C was found to crosslink the material sufficiently for potential microelectronic applications. Excellent gap fill was observed via SEM of spin coated test wafers. Contact angle measurements and ASTM tape tests indicate good surface wetting and adhesion.