Calibrating an optical scanner for quality assurance of large area radiation detectors

A gas electron multiplier (GEM) is a particle detector used in high-energy physics. Its main component is a thin copper-polymer-copper sandwich that carries empty set = 70 +/- 5 mu m holes. Quality assurance (QA) is needed to guarantee both long operating life and reading fidelity of the GEM. Absence of layer defects and conformity of the holes to specifications is important. Both hole size and shape influence the detector's gas multiplication factor and hence affect the collected data. For the scanner the required lateral measurement tolerance is +/- 5 mu m. We calibrated a high aspect ratio optical scanning system (OSS) to allow ensuring the quality of large GEM foils. For the calibration we microfabricated transfer standards, which were imaged with the OSS and which were compared to corresponding scanning electron microscopy (SEM) images. The calibration fulfilled the ISO/IEC 17025 and UKAS M3003 requirements: the calibration factor was 1.01 +/- 0.01, determined at 95% confidence level across a 950 x 950 mm(2) area. The proposed large-scale scanning technique can potentially be valuable in other microfabricated products too.