Calibration of high aspect ratio quality control optical scanning system

Gas electron multiplier (GEM) detectors are widely used in contemporary high-energy physics experiments. The GEM is a detector containing a densely pierced polymer foil, coated with a thin metal layer on one or both sides. They are able to achieve high amplification gains and performance at low cost, even under harsh radiation conditions. The holes in the foils have a nominal diameter of 70 +/- 5 mu m and 140 mu m pitch distance between the centers of the holes.

High-quality assurance is needed to guarantee a long lifespan for the detectors in the severe radiation environment. Mapping of the defects connecting two or more holes is important phase when determining the usability of a foil for detector application.

The commercial optical scanning system (OSS) with a scanning area of 950 x 950 mm was further developed in the Detector Laboratory at Helsinki Institute of Physics for controlling the quality of GEM foils.

Microfabricated transfer standard containing sets of 10 x 10 numbered etched cavities with a nominal diameter of 70 +/- 5 mu m was produced for system calibration. The cavity dimensions and the expanded uncertainty were calculated with the 95% confidence level, as is required by the ISO Guide for Expression of Uncertainty in Measurement.

The transfer standard was examined with the OSS in nine different positions of the scanning area. The results were analyzed, the uncertainties were calculated and the corrections were made according to the ISO requirement.