Generation of Air Microplasma Jet and Its Application to Local Etching of Polyimide Films

An air microplasma jet (air µ-PJ) was generated at the tip of a stainless steel surgical needle with outer diameters of 0.4–0.7 mm at air flow rates of 0.3–1.0 l/min and RF (13.56 MHz) powers of 4–14 W, in which the air µ-PJ was operated without generating arc discharge at atmospheric pressure. The needle acts as both a powered electrode and a narrow gas nozzle. The peak-to-peak voltages V pp were 1.5–1.7 kV and the temperatures of the plasma-irradiated spot (φ<1 mm) were 70–160 °C. The air µ-PJ was applied to the local etching of polyimide films with a thickness of 0.025 mm. A polyimide etch rate of approximately 5 µm/s was attained using a 0.4-mm-φ needle electrode at a RF power of 8 W. The etching mechanism was not based on a thermal effect but on a chemical reaction of oxygen atoms. Furthermore, the local removal of a polyamide–imide insulator film coated on a copper winding wire (φ<0.25 mm) was demonstrated and a good selectivity of the insulator film to a copper wire was achieved at a RF power of 7 W and plasma irradiation times of 5–20 s.