Composition and morphology of two types of bilayers of plasma‐enhanced chemical vapor deposition hydrogened silicon nitride (SiN x :H) and polyimide (PI), as effcient barrier against moisture in SiC‐based power devices, are investigated. Two types of silicon nitrides are obtained by changing the flow ratios of the SiH 4 and NH 3 precursors. Rutherford Backscatterered analyses show that the Si/N ratio varies from 0.6 to 0.8. Elastic recoil detection analyses show that the sample with higher nitrogen content has a higher total bound hydrogen content of 7.8 × 10 17 cm −2 with respect to the 7.1 × 10 17 cm −2 . Fourier‐transform infrared spectroscopy characterizations show Si–H group concentrations of 0.96 × 10 17 and 6.86 × 10 17 cm −2 and NH groups of 4.82 × 10 17 and 2.28 × 10 17 cm −2 , respectively. Silicon nitride films with higher concentration of N–H groups show higher reactivity and permeability to water, making them less effective as a barrier layer. Atomic force microscopy analyses of a PI layer deposited on the nitride layer, SiN x :H/PI show for both type of samples a similar roughness, indicating planarization that can increase the adhesion of SiN x :H/PI and resistance to moisture. The delamination mechanism of the bilayer under pressure pot test conditions is proposed.