Deposition of Wave-shaped Layered Low-k Dielectric a-C:F Films by C8F18 Plasma CVD

We have composed amorphous fluorocarbon (a-C:F) polymer films, which are low-k insulators, by plasma-enhanced chemical vapor deposition using C8F18 as the feedstock. In our previous attempts to form multi-layered a-C:F films under various combinations of plasma power density P and deposition time T, it was found that a-C:F films deposited at a high P (typically 2.0 W/cm3) in a short T (∼1 min) became wavy when they are deposited on other a-C:F films deposited at a low P (0.2 W/cm3). A heating experiment for the films showed that their thermal tolerance was dependent on P; films deposited at 0.2 W/cm3 started to melt at 250-270°C, while those deposited at 2.0 W/cm3 did at 350-370°C. It is considered that species of the precursors for the a-C:F film deposition are determined by P through the degree of C8F18 decomposition. The waving is induced by heating the lower layer during the deposition of the upper layer. The following conditions are necessary for the waving; the lower layer is deposited at a low P, the upper layer is deposited at a temperature at which the lower layer expands but does not melt, and the upper layer is not thickened excessively to keep its flexibility.