Microlenses with high relative aperture can be fabricated in PMMA by irradiation with high energetic proton beams and diffusion of monomer vapor into the irradiated domains. Some important process parameters had already been characterized and their effect on the shape of the lenses had also been discussed in detail . However, for good reproducibility of the experiments stable conditions regarding the temperature and pressure during diffusion have to be guaranteed and the deposited dose in the PMMA must be controlled exactly. A diffusion apparatus has been designed and constructed in order to monitor and precisely control the diffusion parameters. Particularly, the setup includes a water jacket for keeping the diffusion temperature constant and a sensor for monitoring the pressure of the monomer vapor in the vessel during the diffusion process. The fabrication process itself has been further developed by the introduction of a thermal annealing process before the diffusion step at a slightly higher temperature than the diffusion temperature. This additional step in the fabrication process was found to be crucial since it relaxes the internal stress and helps to avoid cracks in the substrate. Furthermore the curing of the microlenses has been done successfully with a wavelength of 254 nm, which is highly absorbed by styrene, but not by polystyrene. With the new apparatus and the modified fabrication process microlenses have been fabricated on PMMA substrates with a thickness of 1 mm. The substrates have been irradiated through a metal mask with circular apertures of 250 mm diameter, which corresponds with the diameter of the microlenses. A proton beam of 3 MeV was used resulting in a penetration depth of about 120 mm. The deposited dose was varied in a range between 1x1012 cm-2 and 2x1013 cm-2. Limitations of the range for the temperature in the diffusion apparatus and for the diffusion time have been determined experimentally as a function of the deposited dose. For example, at a temperature of 85(C the lens shape was distorted and it wasn't possible to keep spherical shapes for any dose as the swelling region flowed due to the low viscosity. At lower temperatures diffusion time and deposited dose become the critical parameters. For example, at a dose of 5x1012 cm-2 even for 70 min. diffusion time lenses with nearly hemispherical shape have been fabricated.
1. Maria Kufner, Stefan Kufner, Michael Frank, Jorg Moisel and Markus Testorf, Pure Appl. Opt. 2 (1993) 9-19.
Micro & Nano Engineering 2000, 17-21, Sep. Jena, German
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