Development and operation of laser machined microwell detectors

Arrays of 100 μm diameter cylindrical wells were laser micromachined on a 200 micrometer Cartesian grid, producing MicroWell Detectors (MWD). The substrate was 125 μm thick polyimide foil, more than twice as thick as a typical GEM or WELL detector. An advantage of the laser micromachining process is that the wells are produced with nearly vertical sidewalls, in contrast to the sloping sidewalls characteristic of conventional chemical etching processes. With the steeper sidewall, active elements may be more closely packed than is possible with wet etching techniques. Thicker substrates can be patterned, increasing the length of the charge multiplication region and reducing the internal capacitance per unit element. A series of prototypes have been produced and tested in a counting gas composed of 85% argon and 15% carbon dioxide, with a maximum measured gas gain of approximately 12 000.