Creating Mircofluidic Devices using PBW

Dipl.-Phys. René Feder

The creation of microfluidic devices in polymethylmeth- acrylat (PMMA) using Proton Beam Writing (PBW) for diffusion measurements in organic crystals and zeolites is in progress. PBW is a direct writing technique using a focused beam of MeV protons to create three-dimen- sional structures in different resist and semiconductor materials down to the sub-micrometer range. Because the height of the structures is given by the irradiation range of the protons, the farbricated channels are up to ca. 90 µm deep. After developing the PMMA the structures were coupled on a substrate by thermal bonding. A microfluidic test structure with ink flowing through the channels only powered by capillary forces is shown in figure 1:

Figure 1: Ink flowing through enclosed microchannel only powered by capillary forces.

The chamber for the diffusion experiments is linked with two capillaries for evacuation and for Ar gas inlet, to establish a defined atmosphere surrounding the crystal. Installing this construction into the LIPSION nanoprobe irradiation chamber allows in situ PIXE and RBS measurements of the diffusion process. A possible effect of the inner boundaries given by the crystal structure to the diffusion can be determined by these experiments.

Figure 2: shows one of the microfluidic chambers, sealed with a Si₃N₄ window.

Another task of the experiments was to analyse the characteristics of PMMA influenced by different irradiation fluences. PMMA is usually a positive resist, so irradiated arrays dissolve during the development.
At fluences which are about ten times higher than necessary for normal exposure (0.5 pC/µm2) PMMA crosslinks. This crosslinked PMMA is resistant against aceton and can now also be used as a high resolution negative resist..