Thin film growth into the ion track structures in polyimide by atomic layer deposition

Abstract

High-aspect ratio porous structures with controllable pore diameters and without a stiff substrate can be fabricated using the ion track technique. Atomic layer deposition is an ideal technique for depositing thin films and functional surfaces on complicated 3D structures due to the high conformality of the films.In this work, we studied Al2O3 and TiO2 films grown by ALD on pristine polyimide (Kapton HN) membranes as well as polyimide membranes etched in sodium hypochlorite (NaOCl) and boric acid (BO3) solution by means of RBS, PIXE, SEM-EDX and helium ion microcopy (HIM). The focus was on the first ALD growth cycles.The areal density of Al2O3 film in the 400 cycle sample was determined to be 51±3×1016at./cm2, corresponding to the thickness of 55±3nm. Furthermore, the growth per cycle was 1.4Å/cycle. The growth is highly linear from the first cycles.In the case of TiO2, the growth per cycle is clearly slower during the first 200 cycles but then it increases significantly. The growth rate based on RBS measurements is 0.24Å/cycle from 3 to 200cycles and then 0.6Å/cycle between 200 and 400cycles. The final areal density of TiO2 film after 400 cycles is 148±3×1015at./cm2 which corresponds to the thickness of 17.4±0.4nm. The modification of the polyimide surface by etching prior to the deposition did not have an effect on the Al2O3 and TiO2 growth.