Direct polymer patterning by high energy reactive ion beam through stencil masks
Polytetrafluoroethylene (PTFE) is a polymer with remarkable physico-chemical properties which has been used for membrane and biomedical applications . This fluorpolymer is however difficult or cost- inhibiting to pattern with conventional methods with high aspect-ratios, especially at the micrometer scale [2-5]. The present work introduces an innovative efficient technique for direct micro-patterning of PTFE using a high energy un-focalized O + ion beam. A nanostencil  with features appropriately designed to be opaque to the un-focalized ions is placed between the large-area beam and the substrate. This allows the ions which pass though the mask to directly and unisotropically dry etch the polymer. The results prove this approach has a good reproducibility and is a cost-efficient way for local micro-structuring of high aspect- ratio structures in PTFE. Bulk, commercially available 500 m thick PTFE was used as the substrate. The stencil mask was based on low-stress LPCVD SiN membranes. Its local selectivity to the ion beam was obtained by having the membranes patterned with apertures and covered by a thick Au layer, as seen in the schematic from Fig. 1. The 500 nm SiN covered by 800 nm au was stopping the ions, while the beam was passing right through the membrane openings. The un-focused beam of O + ions was accelerated to 1 MeV in a Tandetron 1.7 MV particle accelerator. From simulations using SRIM software, the longitudinal straggling of the ions for 500 nm SiN and 530 nm Au was 1 m (Fig 2). Thus the 800 nm Au was expected to be a very good mask for stopping all the ions. For counter-acting thermal effects, the PTFE was heat-sunk in a customized frame, held at a distance of 2 mm from the stencil, and was irradiated in a pulsed mode. After two sequences of 5 seconds of beam on separated by a 30 second pause, the etched polymer depth reached around 10 m. This patterning speed of about 1 m/s revealed a good match to our simulations for the stencil mask materials and the high energy ion beam. The stencil was able to be reused tens of times without visible damages (Fig. 3). The smallest feature reproduced at the integral etch depth was a 1 m x 1 m square (Fig. 4). Outgassing was measured in-situ by mass spectroscopy. The results showed the emergence of combination molecules, indicating chemical reactions were taking place in PTFE under irradiation. We thus showed the viability of using an un-focused 1 MeV high-energy O + ion beam for direct etching of micrometer-size features in PTFE through a stencil mask. The mask opaqueness to the ions was optimized via simulations and good results were obtained using 800 nm Au as a stopping layer coating the SiN membrane. An aspect ratio of 10 was obtained for micro-patterns and further investigations are undergoing for optimizing the parameters which will allow reproducible nanopatterning in PTFE.