Advanced comparator method for measuring ultra-small apertureareas

Abstract

An advanced method is presented, which allows the measurement ofaperture areas with diameters smaller than 50 µm. The method follows thecomparator technique and compares the photocurrent of a detector covered witha reference aperture of known area with the photocurrent of the same detectorequipped with the aperture under investigation. Using typical referenceapertures with diameters of some millimetres, the very small measured aperturearea results in extreme large ratios of measured photocurrents. This requiressophisticated detector and measurement electronics to obtain high accuracy.Significant improvement with respect to common set-ups has been achieved byapplying a silicon trap detector instead of a single photodiode. The advancedset-up allows areas with diameter below 50 µm to be measured withuncertainties in the order of 10-3. Application of the method to fourultra-small apertures with nominal diameters in the range from 300 µm downto 50 µm is presented.