Pore geometry of etched ion tracks in polyimide

Tracks of energetic heavy ions in the polyimide Kapton were etched in a NaOCl solution. It was found that the pH value of the etchant plays a crucial role for the selectivity of track etching. The bulk etching rate upsilon(b) increased exponentially with pH. From the temperature dependence of the bulk etching rate an activation energy of E(a) = 0.74 eV was deduced. In contrast to upsilon(b), only a slight and linear increase with pH value was observed for the track etching rate upsilon(t). As a consequence, the etch ratio upsilon(t)/upsilon(b) can be adjusted over one order of magnitude by controlled varying the pH of the etchant. This is a new way to tailor the pore geometry from nearly cylindrical to funnel-shaped pores.