Contact holes: A challenge for signal collection efficiency and measurement algorithms

As critical features become smaller, metrology becomes more challenging; this is especially the of contact holes and trenches with diameters less than 0.25 um. Reduced collection efficiency of secondary electrons from such contact holes can provide false edge sharpening and may interfere with obtaining critical information from the bottom of the contact hole. Whether the contact hole is open or closed is a key concern and can be determined from the waveform in many cases. This paper introduces a measure of the collection efficiency by comparing signal strength from the contact hole with a blanked beam signal and a method for determining whether the contact hole is open or closed, The collection efficiency measure can be used to determine whether information from the bottom of the contact hole is meaningful. When found to be meaningful, the information is then evaluated to flag the contact hole's status, Results will be based on saved image analysis and lead into a discussion on the value of these parameters. Reporting collection efficiency and whether the contact hole is open or closed, along with the diameter measurement, can provide a measure of quality not yet achieved in CD metrology.