The Effects of Morphology on the Dielectric and Mechanical Properties of Parylene-C Microfibrous Thin Films

Silicon-based CMOS material set is rather restrictive for the realization of the full potential of NEMS/MEMS and, hence, a wide spectrum of new material compositions and morphologies are being explored nowadays. Because of its extensive applications in electronic and medical devices as well as because of its chemical inertness and thermal stability, a polymer called Parylene C was used to grow microfibrous thin films using physicochemical vapor deposition in a PDS2010 Parylene-C labcoater on p-type Si or brass substrates. The thin films are assemblies of parallel microfibers tilted at an angle χ_v with respect to the substrate plane. During deposition, a collimated vapor flux of Parylene C was directed at an angle χ_v with respect to the same plane. Values of χ_v used were 10°, 25^°, 45^°, 60^°, and 90^°. Both χ and the mass density increase with increasing χ_v. Using Parylene-C microfibrous thin films as the dielectric layers in metal-insulator–semiconductor (MIS) and metal–insulator–metal (MIM) capacitor structures, we were able to study their dielectric properties. The dependence of the “effective” relative permittivity e_eff on χ_v was observed to be controlled by the amplitude of the applied AC signal, and e_eff was found to decrease with increasing signal frequency. The MIS and MIM leakage current was used to study the dependence of hopping conduction on both the frequency and the temperature. These observations were interpreted relative to series/parallel networks of miniature resistors and capacitors. Also, we undertook a dynamic mechanical analysis, whereby a Parylene-C microfibrous thin film was clamped at two ends and subjected to cyclic loading. The storage and the loss moduli were measured as functions of frequency, in the frequency range 5 to 200 Hz, and temperatures up to 125 °C. The glass transition temperature was identified as 65 °C from the temperature maximum of the loss modulus.