Mechanism of a Trapezoidal Magnetic Field Enhanced Helium Plasma Jet for Improving the Surface Modification Effect on Polyimide Films

Atmospheric pressure plasma jets can directly act on the surface of materials and improve the surface properties of the materials. However, the concentration of active species generated by plasma jets is low, and the treatment area is small, which makes it difficult to realize the ideal material surface modification effect within a short processing time. In this paper, a method was proposed to increase the concentration of active species and treatment area of a helium plasma jet by applying a steady-state trapezoidal magnetic field, thus enhancing the surface modification effect on polyimide (PI) films. Compared to the case without a magnetic field, the plasma jet under a 0.25 T trapezoidal magnetic field reduced the surface water contact angle of the PI film from 71.5 to 22 degrees and increased the surface modified area by about 2.5 times, from 0.5 to 1.77 cm2, which corresponded to a reduction of the processing time from 120 to 80 s. The reason for this was that the particle velocity produced by the plasma jet bombarding the PI film increased by about 42% under a 0.25 T trapezoidal magnetic field, resulting in a rougher surface. Meanwhile, the concentration of active species deposited on the surface of PI film increased; especially the concentrations of OH and O atoms increased by about 40 and 46%, respectively, resulting in the grafting of more hydrophilic functional group C-O on the surface of the PI film. Moreover, the plasma deposited on the surface of the PI film under the trapezoidal magnetic field did not disappear gradually but unfolded along the surface of the PI film in all directions, which significantly increased the surface modification area of the PI film.