Advanced comparator method for measuring ultra-small aperture areas

An advanced method is presented, which allows the measurement of aperture areas with diameters smaller than 50 mum. The method follows the comparator technique and compares the photocurrent of a detector covered with a reference aperture of known area with the photocurrent of the same detector equipped with the aperture under investigation. Using typical reference apertures with diameters of some millimetres. the very small measured aperture area results in extreme large ratios of measured photocurrents. This requires sophisticated detector and measurement electronics to obtain high accuracy. Significant improvement with respect to common set-ups has been achieved by applying a silicon trap detector instead of a single photodiode. The advanced set-up allows areas with diameter below 50 mum to be measured with uncertainties in the order of 10(-3). Application of the method to four ultra-small apertures with nominal diameters in the range from 300 mum down to 50 mum is presented.