Hochauflösende Röntgenlithografie zur Herstellung polymerer Submikrometerstrukturen mit großem Akspektverhältnis

Abstract

In this work the process for X-ray lithography in PMMA was further developed to generate structures in PMMA layers with lateral dimensions in the submicron range and heights of several micrometers resulting in aspect ratios of 10 and higher. PMMA films of 2 ÷ 10 μm could be structured with aspect ratios up to more than 12 using Synchrotron radiation with λ c = 0.4 nm after the systematic analysis and optimization of the development- and exposure process. An X-ray mask technique using a free suspended membrane of 1 μm thick silicon nitride with 2 pm high gold absorbers was introduced. After optimizing the entire process for mask production, the limits of electron beam lithography could be determined. Due to the proximity effect, CAD data of lateral dimensions need to be shrunken by -100 ÷ 150 nm per edge in order to receive nominal values in the PMMA structures produced via lithography. Locally minimized walls of resist may be used as templates for electroplating to reproducible create minimal slit sizes down to 75 nm in 2 μm high gold absorbers with tolerances of ± 25 nm on the same substrate. To spincoat the requested PMMA films, resist systems offered for electron beam lithography were used. Both examined resists, MicroChem 950k PMMA A11 and All Resist AR SX-P 6540, have a higher contrast in the dose regime above 1 kJ/cm 3 compared to the dose regime below this value. Surface tension during drying as part of the development process limits the achievable aspect ratio as function of the actual structure height for walls and columns. Rising the pre-bake temperature of MicroChem 950k PMMA A11 from 111°C to 180°C results in more stable structures. Adding 10 ppm fluoride tenside to the rinse bath during the wet development process reduces the structure collapse. Using an adhesion layer out of polyimide avoids cracks in the resist even for lateral dimensions of several 10 pm. The proximity gap during X-ray exposure influences the structure quality via diffraction effects significantly. Reducing this gap from 100 μm to 15 μm minimizes the influence of this effect. The process optimized in this study could be used to generate submicron structures for fluidic applications with aspect ratios more than 10. It enables the creation of polymer moulds and masks for the production of submicron structures in metals. By electroforming, e.g. gold structures with aspect ratios more than 12 and lateral dimensions around 500 nm were fabricated. This allows for batch-fabrication of SAW-filters for frequencies above the presently used ones. In addition the optimized process can be used to build metallic filters with high aspect ratios for the use as band-pass filters with sharp cut-off-frequencies in the infrared.