Low temperature rf-plasma was used to test the changes in the chemical and physical properties of a plasma-treated polymeric membrane made from C₇H₃F₃ and SiH₄, two materials that are very thermally, chemically, and mechanically stable. C₇H₃F₃ and SiH₄ plasma polymers were efficiently deposited to an aluminum oxide substrate with a pore size of 0.02㎛. Even though the amount of the monomer C₇H₃F₃ increased, there was no significant change in the permeability of oxygen. However, selectivity of oxygen over nitrogen increased noticeably. Also, when the plate distance of the supporter from the cathode head within the reactor was 4 cm, the concentration gradients of C₇H₃F₃ and SiH₄ were the greatest. This resulted in the maximum value of permeability and selectivity and allowed the creation of a plasma- treated polymeric membrane with such characteristics when the composite permeation coefficient ranged from 4,500-7,500 GJ·s/kg.