Novel two-step etching process for ion tracks in polyimide

Polyimide is of great technical importance because of its excellent material properties (including chemical inertness and radiation resistance) which are maintained even at low and at high temperatures. When thin polyimide foils are irradiated with energetic heavy ions, long nanometric tracks are formed. Under suitable conditions, the damaged material along these tracks is dissolved and developed into microor nano-sized pores of high aspect ratio. In early studies aq. KMnO4 and H2O2 were suggested as etchant [1,2]. Later, better results were obtained with NaOCl as oxidizing agent [3-5], but the situation was still not satisfying because this hypochlorite is unstable and decomposes when exposed to light and/or temperature. An additional difficulty is linked to the pH of the solution which strongly influences the etch rates of the bulk material as well as along the tracks. The controlled production particularly of highly cylindrical pores is problematic because it requires low pH values where the etch rates become extremely slow. The etching times are extended and accompanied by decomposition of NaOCl. In this work we suggest a two-step procedure combining the initially proposed treatment with H2O2 at high temperature followed by etching in NaOCl at high pH and moderate temperature. As material we used 25and 50μm thick foils of polyimide Kapton (DuPont) and irradiated them at the UNILAC with U ions of 11.1 Mev/u energy and a fluence of 4×10 and 10 ions/cm. The pre-etching was performed in an aqueous solution of 30% H2O2 at 90°C for different times between 1 and 3 hours. For the second etching step, the samples were immersed into a 60°C warm commercial solution of NaOCl of pH 12.5 and containing 13% active chlorine. Fig. 2 shows the resulting pore diameters as a function of etching time.