These webpages relate to research carried out between 2001 and 2004 into applying new micromachining techniques to the development of gas-avalanche type radiation detectors. Three-dimensional microstructures fabricated with SU-8 photosensitive epoxy showed promise in overcoming electrical breakdowns and other problems encountered in high radiation fluxes with the early generation of gas-avalanche micropattern radiation detectors. The motivation behind the work was to produce stable detectors with fast response principally for X-ray imaging and synchrotron diffraction applications. The work was funded by a Marie Curie Fellowship awarded by the European Community.

SU-8 Photosensitive Epoxy

SU-8 was originally developed for the microelectronics industry to provide a high resolution negative imaging resist for use in the fabrication of advanced semiconductor devices. However, the resist has several attributes which make it suitable for micromachining applications and prompt consideration of the material for microstructure radiation detector fabrication. The photosensitivity of SU-8 is 300-400 nm, a region accessible with conventional photolithography equipment and the high transparency in the near UV allows structures with high aspect ratios to be fabricated with near-vertical side walls. Also, because of the highly cross-linked matrix in the exposed material, it is thermally stable (up to 200°C) and chemically stable after development. Finally, its solubility in a variety of organic solvents allows solutions with high solids contents to be formulated, which means that a substrate can be coated with a relatively thick film in a single spin. In terms of radiation hardness, the high cross-linkage density in the cured material, resulting from the exceptionally high epoxy functionality of 8, provides a high tolerance to absorbed radiation dose (see publications).

Microgap detector fabrication process (click to enlarge) using SU-8 epoxy-based photoresist

Earlier work on gas avalanche detector fabrication with SU-8 (at the University of Surrey, UK, 1998-99) produced microgap detectors with the photoepoxy used to raise anode strips above the cathode. The microgap devices in the optical micrographs below have 10 micron wide gold anode strips deposited on top of 20 micron wide supporting pillars patterned in a 10 micron thick SU-8 film. The substrate in this case is indium-tin oxide coated borosilicate glass, which forms the cathode. Previously, the anode-cathode separation in microgap detectors was limited by the material used to construct the supporting pillars (usually silicon dioxide).The graphic on the right shows the structure of the microgap gas amplification region, with the SU-8 pillars represented in blue.

Non-planar microstrip (microtrench) detectors, micrographs and 3-D model. Anode thickness is 10 microns.

Below are some images from an exposure test for a 40 micron thick SU-8 film. The SEM image on the left shows SU-8 strips of 10, 12 and 14 micron widths; in the centre is a optical image of the 14 micron wide strips; the SEM image on the left is a close up of the 10 micron wide strips (note that the cross-section was made using a wafer saw). Further images showing detector devices fabricated using SU-8 can be seen on the microstructure detector page.

Test structures in a 40 micron thick SU-8 film

The resist is supplied as a liquid consisting of an epoxy resin, a solvent (GBL or cyclopentanone depending on formulation) and a photo-acid generator. The substrate is coated using a conventional photoresist spinner, with the film thickness controlled by the spin speed and the solids content of the epoxy solution. A baking stage removes excess solvent from the layer. Upon exposure to UV radiation, a strong acid (HSbF6) is generated which causes the epoxy resin to form a ladder-like structure with a high cross-linking density when heated above a critical temperature provided in a post-exposure bake. The unexposed material is then removed with a solvent in the development process.

SU-8 molecular structure

In gas microstructure radiation detectors, it is important that construction materials should not outgas impurities that could accelerate detector aging and degredation. Tests performed with a residual gas analyser on a 100 mm diameter wafer coated with a fully cured 10 micron thick layer of SU-8 have shown no outgassing above the detection limit of 10-10 torr in measurements taken between 30 minutes to 48 hours after pumpdown.

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