## 3 Results and Discussion
Figure 3: **(a)** Fourier transform infrared (FTIR) spectra of the films as a function of the ammonia gas flow ratio in the glow discharge and **(b)** a highlight of the spectra at wavenumbers ranging from 700 to 1300 cm\({}^{-1}\). from the film synthesized using the TMS-O\({}_{2}\)-NH\({}_{3}\) gas mixture at a gas flow ratio of 0.5 (Figure 6b) which also resulted in the lowest surface roughness. As quoted from the previous reports [39; 40; 41], the coverage of the voids resulted from the terminated bonds in the Si-O-Si networks and/or the excess oxygen contents to cause the compressive stress residing in the deposited film which was most likely responsible for the appearance of these protrusions. In addition, the additive terminated bonds associated with the Si(CH\({}_{3}\))\({}_{x}\) species and/or the introduced nitrogen contents in the film deposited using the TMS-NH\({}_{3}\) gas mixture were ascribed to result in the dot-sized protrusions. The inset figures in Figure 6a,b give the tape-peeling test results of the films deposited onto the PET substrates. A significant peel-off area was observed from the SiO\({}_{x}\) film deposited onto the PET, whereas the film synthesized from the TMS-O\({}_{2}\)-NH\({}_{3}\) gas mixture at a gas flow ratio of 0.5 completely adhered to the PET substrate after the tape-peeling test. According to the ASTM D3359 standard tape-peeling test, the adhesion levels of these two films were ranked at about 3B and 5B, respectively. The relationship between the surface roughness and internal stress residing in the deposited film as a function of the gas flow ratio is presented in Figure 7. All these films synthesized from the TMS-O\({}_{2}\)-NH\({}_{3}\) gas mixture accumulated compressive stress after deposition onto the silicon substrates as the protrusions attributed to the film coverage on the voids/gas atoms appeared on the surface. Combined with their AFM and SEM measurements, the evolutions on the residual internal stress of the films were closely linked to the surface roughness and morphologies. The fewer the protrusions that appeared on the film surface, the lower the compressive stress and the smoother the surface obtained. A large compressive stress of about 455 MPa resided in the film synthesized without the ammonia gas which led to large protrusions and a rough surface. Moreover, since the inlet ammonia gas facilitated the reduction of the oxygen content for stress relaxation and introduced limited Si-C bonds for inducing tensile stress in the deposited film, the compressive stress that resided in the film was effectively released as the ammonia gas flow ratio increased and also resulted in a flattened surface with a decrease in the protrusions. The lowest internal compressive stress of about 123 MPa was obtained from the film deposited using the gas flow ratio of 0.5 while the surface showed almost no protrusions. However, as the introduced ammonia gas reached a gas flow ratio of 0.75, the dot-like protrusions originating from the incorporation of nitrogen atoms led to the increase in the residual compressive stress of the deposited film. The highest compressive stress of about 651 MPa was then measured from the film synthesized using the TMS-NH\({}_{3}\) gas mixture which contained a mass of dot-like protrusions on the film surface. Table 1 summarizes the WVTR of these films as a function of the gas flow ratio in the TMS-O\({}_{2}\)-NH\({}_{3}\) glow discharge deposited onto the PET substrates (the corresponding surface morphology and residual internal stress when these films were deposited onto the silicon substrate also are listed for comparison). A TMS-O\({}_{2}\)-synthesized SiO\({}_{x}\) film with a thickness of 100 nm deposited onto the PET substrate could slightly strengthen the barrier property of the bare PET substrate to water vapor permeation from 3.67 to 1.65 g/m\({}^{2}\)/day.