Improvement of the polyimide surface wettability using SiOx films deposited in a DECR reactor from HMDSO/O2 mixtures

Polyimide (PI) foils were coated with thin SiOx films for surface wettability improvement. The deposition was carried out in a microwave DECR (Distributed Electron Cyclotron Resonance) plasma reactor from a mixture of HMDSO/O-2. During deposition, the PI substrate was located downstream, exposed to long-lived reactive precursors and to VUV radiation (position I) or located again downstream but shielded from VUV radiation (position II). Deposition time was varied in order to observe different stages of the film growth. The surfaces of the deposited films were investigated by means of contact angle, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) measurements. According to contact angle measurements, the coated PI surface became hydrophilic already at a thickness of about 2.8 nm and stayed nearly at the same level still for thicker coating. The plasma deposition of the ultra-thin SiO, layer increased significantly the Folar component of the surface energy from 9 to 53 mJ(.)m(-2) and consequently enhanced the wettability of the material. The treated surfaces were only moderately affected after eight weeks storage time in air, indicating stable surface treatments. AFM images revealed that during deposition, the PI surface topography remains relatively smooth when coated in position I. However, for PI coated in position II, the surface roughness becomes more pronounced as thickness is increased. XPS analysis showed that VUV irradiation of the PI surface leads to the formation of a high proportion of polar species which enhance the wettability of the surface. Curve fitting of the Si2p peak showed that, as the film thickness increases, the deposited film becomes more oxidized.